Injection moulding modified

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2.1 Introduction 2.1.1 Historical Background

2.1.2 Advantages of Injection Moulding Process

2.1.3 Limitations of Injection Moulding

2.1 Introduction

One of the most common methods of converting plastics from the raw material form to

an article of use is the process of injection moulding. This process is used for

thermoplastic materials and other polymeric materials which may be successively melted,

reshaped and cooled. Injection moulded components are a feature of almost every

functional manufactured article in the modern world, from automotive products through

to food packaging. This versatile process allows us to produce high quality, simple or

complex components on a fully automated basis at high speed with materials that have

changed the face of manufacturing technology over the last 50 years or so. Injection moulding is suitable processing method for following materials:

Thermoplastics

Elastomers

Rubbers

Thermo sets

Composites Foamed plastics

2.1.1 Historical Background

The process of injection moulding began as a result of extensive research into

substitution products, mainly for ivory and rubber. Development began in the late 19th

century with a modified form of existing die-casting machines used for the manufacture

of lengths of rod and tube. A single-action hydraulic injection machine was designed in

the U.S.A. in 1870 by Hyatt. This process was adapted for covering metal and wood

parts. The material was cellulose nitrate and was extruded in its plastic state round a

core contained in a cavity and allowed to cool in the cavity.

Cellulose acetate was first used to make a solid part by an injection moulding process in

the early 1920's; where the process of injection moulding known today began. Initially

the progress was slow, but by the early 1930's, in addition to private development in

Great Britain, commercial horizontal machines for injection moulding were available in

Germany and injection moulding was starting to reduce the greater lead held by

compression moulding. Heating-cylinder design was first recognised in a patent issued to Adam Gastron in 1932.

Large-scale development of injection moulding machinery design towards the machines

we know today did not occur until the 1950's in Germany. Earlier machines were based

on a simple plunger arrangement to force the material into the mould, although these

machines soon became inadequate as materials became more advanced and processing

requirements became more complex. The main problem with a straightforward plunger

arrangement was that no melt mixing or homogenisation could be readily imparted to the

thermoplastic material. This was exacerbated by the poor heat transfer properties of a

polymeric material. One of the most important developments in machine design to

overcome this problem, which still applies to modern processing equipment today, was

the introduction to the injection barrel of a plunging helical screw arrangement. The

machine subsequently became known as a 'Reciprocating Screw' injection moulding machine.

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2.1.2 Advantages of Injection Moulding Process

Parts can be produced at high production rates.

Large volume production is possible.

Relatively low labour cost per unit is obtainable.

Parts require little or no finishing.

Many different surfaces, colours, and finishes are available.

Good decoration is possible.

For many shapes this process is the most economical way to fabricate.

Process permits the manufacture of very small parts which are almost impossible

to fabricate in quantities by other methods.

Minimal scrap loss result as runners, gates, and rejects can be reground and

reused.

Same items can be moulded in different materials, without changing the machine

or mould in some cases.

Close dimensional tolerances can be maintained.

Parts can be moulded with metallic and non-metallic inserts.

Parts can be moulded in a combination of plastic and such fillers as glass,

asbestos, talc and carbon.

The inherent properties of the material give many advantages such as high

strength-weight rates, corrosion resistance, strength and clarity.

2.1.3 Limitations of Injection Moulding

Intense industry competition often results in low profit margins.

Mould costs are high.

Moulding machinery and auxiliary equipment costs are high.

Lack of knowledge about the fundamentals of the process causes problems.

Lack of knowledge about the long term properties of the materials may result in

long-term failures.

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2.2 Glossary – Technical terms used in Injection Moulding Process

ADDITIVE - A substance added to a plastic compound to alter its characteristics.

Examples are plasticizers, and flame retardants.

ALLOY - A combination of 2 or more plastics which form a new plastic. See BLEND.

AMORPHOUS - A plastic material in which the molecular structure is random and becomes mobile over a wide temperature range. See CRYSTALLINE.

ANNEAL - To heat a moulded part up to a temperature just below its melting point and

slowly cooling it back; down to room temperature. This relieves moulded stresses. See CONDITIONING.

ANISOTROPIC SHRINKAGE - Shrinkage that occurs more in one direction (usually the

direction of flow; reinforced materials shrink more across the direction of flow) than another.

AUTOMATIC OPERATION - The term used to define the mode in which a moulding machine is operating when there is no need for an operator to start each cycle.

BARREL - A metallic cylinder in which the injection screw (or plunger) resides in the moulding machine. Also called CYLINDER.

BLEND - A mixture of 2 or more plastics.

BOSS - A projection of the plastic part, normally round, which is used to: strengthen an

area of a part; provide a source of fastening; or to provide an alignment mechanism during assembly.

CARTRIDGE HEATERS - Pencil-shaped electrical heater devices sometimes placed in

moulds to raise the temperature level of the mould. Especially beneficial when moulding high-temperature crystalline materials.

CAVITY - A depression or female portion of the mould which creates the external plastic

part surface.

CHECK RING - A ring shaped component that slides back and forth over the tip end of

the screw. The check ring eliminates the flow of molten material backwards over the screw during the injection process.

CLAMP FORCE - The force, in tons, that the clamp unit of a moulding machine exerts to keep the mould closed during the injection process.

CLAMP UNIT - That section of the moulding machine containing the clamping

mechanism. This is used to close the mould and keep it closed against injection pressure created by the injection process. The clamp unit also contains the ejection mechanism.

COLD SLUG WELL - A depression (normally circular) in the ejection half of an injection

mould, opposite the sprue, designed to receive the first front, or "cold" portion, of molten plastic during the injection process.

COMPRESSION RATIO - A factor that determines the amount of shear that is imparted to

plastic material as it travels through the barrel. It is determined by dividing the depth of

the screw flight in the feed section by the depth of the screw flight in the metering section.

CONDITIONING - Exposing a moulded part to a set of conditions (such as hot oil) which impart favorable characteristics to the product. See ANNEAL.

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COOLING CHANNELS - Drilled holes or channels machined into various plates or

components of an injection mould providing a flow path for cooling medium (such as water) in order to control the temperature of the mould.

CORE - a) An extended or male portion of the mould which creates the internal plastic

part surface. b) A pin or protrusion designed to produce a hole or depression in the

plastic part.

COUNTERBORE - A recessed circular area. Commonly used to fit the head of an ejector pin (return pin, sucker pin, etc.) in the ejector plate.

CRYSTALLINE - A plastic material in which the molecular structure becomes mobile only after being heated above its melting point. See AMORPHOUS.

CUSHION - A pad of material left in the barrel at the end of the injection stroke. It is

excessive to the amount needed to fill the mould and acts as a focus point for holding

pressure against the cooling melt.

CYCLE - The total amount of time required for the completion of all operations needed to

produce a moulded part. Sometimes referred to as the "gate-to-gate" time, meaning the

time from when an operator first closes the gate until the time the operator closes the gate again for starting the next cycle.

CYLINDER - (See BARREL)

DECOMPRESSION - A method of relieving pressure on the melt after preparing it for

injection during the upcoming cycle. This minimizes the drooling that occurs when a shutoff nozzle is not utilized.

DEFECT - An imperfection in a moulded part that results in the product not meeting original design specifications. These defects can be visual, physical, and/or hidden.

DRAFT - An angle (or taper) provided on the mould to facilitate ejection of the moulded part.

EJECTOR HALF - That half of the mould which is mounted to the moving platen of the

injection machine. Sometimes called the "live" half or the "moveable" half because it

moves. This half of the mould usually contains the ejection system:

EJECTOR PIN - A pin, normally circular, placed in either half of the mould (usually the

ejector half) which pushes the finished moulded product, or runner system, out of a mould. Also referred to as a "knockout" pin, for obvious reasons.

FEED THROAT - The area at the rear end of the injection unit that allows fresh plastic to fall from the hopper into the heating barrel.

FEED ZONE - That area of the screw that is at the rear and receives fresh material from

the feed throat.

FILLER - Specific material added to the basic plastic resin to obtain particular chemical, electrical, physical, or thermal properties.

FLASH - A thin film of plastic that tends to form at parting line areas of a mould. May

also be found in vent areas and around ejector pins. Flash is caused by too great a clearance between mating metal surfaces, which allows plastic material to enter.

FLIGHT - The helical metal thread structure of the injection screw.

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GATE - An opening found at the entrance of a cavity (end of the runner system) which allows material to enter.

GRANULATOR - A machine designed to grind up rejected pre-moulded plastic (products or runners). The material generated by this process is called REGRIND.

GUIDE PINS - A pin (usually circular) which normally travels in a bushing in order to

provide alignment of two unattached components, such as the two halves of an injection

mould. Also called LEADER PINS.

HEATING CYLINDER - That section of the injection moulding machine in which the plastic resin is heated to the proper moulding temperature prior to injection into the mould.

HEATING ZONE - An area of the heating barrel that is controlled by a temperature

controller attached to a set of heater bands. There are four major zones: rear, center, front, and nozzle.

HEATER BANDS - Bracelet-shaped electrical heaters that are placed around the outside

circumference of the heating barrel.

HOPPER - A funnel-shaped container mounted over the feed throat of a moulding

machine. It holds fresh material to be gravity fed into the feed zone of the heating

barrel. Hoppers are normally designed to hold an average of 2 hours worth of material for a given machine size.

HYDRAULIC CLAMP - A term used to describe the use a large hydraulic cylinder to open

and close the clamp unit of a moulding machine.

HYGROSCOPIC - A term applied to those plastics (such as ABS and NYLON) which absorb

moisture from the atmosphere.

INJECTION CAPACITY - A rating of the maximum amount of plastic material, in ounces, a

machine is capable of injecting in a single stroke of the injection screw or plunger. It is based on the specific gravity of polystyrene as a standard.

INJECTION MOULDING - The process of pushing a molten plastic material into a relatively cooled mould in order to produce a finished product.

INJECTION PRESSURE - That pressure which performs the initial filling of the mould. It is

supplied by the injection screw or plunger as it pushes material out of the heating barrel and into the mould.

INJECTION UNIT - That section of the moulding machine which contains the injection

components, including the hopper, heating cylinder, screw (or plunger), nozzle, and heater bands.

ISOTROPIC SHRINKAGE - Shrinkage that occurs equally in all directions. See

ANISOTROPIC SHRINKAGE.

KNOCKOUT PIN - See Ejector Pin

LAND - A term used to describe the area in which the gate, or vent, resides. It can also

be thought of as the "length" dimension in the "L, W, H" terminology used for describing the dimensions of the gate or vent. See also SHUTOFF LAND.

L/D RATIO - The result of a calculation which divides the entire length of flighted area on a screw by its nominal diameter.

LEADER PIN - See Guide Pin

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MANUAL OPERATION - The term used to define the mode in which a moulding machine

is operating when there is a need for an operator to start and finish each phase of the total cycle.

MECHANICAL CLAMP - See TOGGLE CLAMP

MELT - A term given to describe the condition of molten plastic prior to injection into a mould. A proper melt has the consistency of warm honey.

METERING ZONE - That area of the screw at the front end which contains properly

melted plastic that is ready to inject.

MOULD - The term given to the entire tool (cavity, core, ejectors, etc.) needed to

produce moulded parts from molten plastic material.

MONOMER - A molecular unit of an organic substance, usually in the form of a liquid or gas. See POLYMER.

MOVING PLATEN - The platen of a moulding machine that travels (opens and closes). It

is connected to the clamp unit and is the mounting location for the "B", or traveling, half of the mould.

NON-RETURN VALVE - A mechanism mounted in (or at) the nozzle of the injection

machine, which operates to shut off injection flow at the end of the injection cycle. This

eliminates material from the upcoming shot from drooling out of the nozzle when the mould opens to eject parts from the previous shot.

NOZZLE - A device mounted at the end of the heating barrel which focuses plastic material to flow from the machine into the mould.

PAD - See CUSHION

PARTING LINE - A plane at which two halves of a mould meet. Also applies to any other

plane where two moving sections come together and form a surface of a moulded part.

PLASTIC - A complex organic compound (usually polymerized) that is capable of being shaped or formed.

PLATENS - The flat surfaces of a moulding machine onto which the two halves of the

mould are mounted. One is stationary and the other travels. There is a third platen

(stationary) at the clamp end of the machine which serves as an anchoring point for the

clamp unit.

PLUNGER - The injecting member of a non-screw design moulding machine. Plungers do

not rotate, (auger) to bring material forward in preparation for the next cycle. Nor do they blend the material as a screw does.

POLYMER - A group of long chains of monomers, bonded together in a chemical reaction

to form a solid. This term is often used interchangeably with PLASTIC, but there can be

a difference.

PURGING - A process of injecting unwanted plastic material from the injection cylinder

into the atmosphere for the purpose of changing materials, changing colors, or removing degraded material. Also, the name given to the mass of material that is purged.

RECIPROCATING SCREW - A helical flighted, metal shaft which rotates within the heating

cylinder of a moulding machine, shearing, blending, and advancing the plastic material.

After rotating, the screw is pushed forward which injects the plastic into the mould. Also, simply referred to as "the Screw".

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REGRIND - Plastic material formed by granulating pre-moulded material. Regrind is virgin material that has been exposed to at least one heating cycle.

RUNNER - Grooves or channels cut into either or both halves of the injection mould to

provide a path for the molten plastic material to be carried from the sprue to the gate(s) of the cavity.

SCREW - See RECIPROCATING SCREW.

SCREW SPEED - The rotating speed of the screw as it augers new material towards the

metering zone. It is expressed in RPM (revolutions per minute).

SECONDARY OPERATION - Any activity performed after the moulding process required to

produce a finished product suitable for its designed purpose.

SEMI-AUTOMATIC OPERATION - The term used to define the mode in which a moulding machine is operating when there is a need for an operator to start each cycle.

SHOT - A term given to the total amount of plastic material that is injected (or shot) into a mould in a single cycle.

SHOT CAPACITY - See INJECTION CAPACITY.

SHUTOFF LAND - A raised area of the mould surface surrounding the cavity image. This

area is usually between 0.002 and 0.003 inch high, approximately 1/2 inch wide and is

used to focus clamping pressure on the mould. The use of a shutoff land reduces the amount of tonnage required to keep a mould closed against injection pressure.

SLIDE - A section of the mould which is made to travel at an angle to the normal movement of the mould. Used for providing undercuts, recesses, etc.

SPRUE - The plastic material that connects the runner system to the nozzle of the

heating cylinder of the moulding machine. It is formed by the internal surface of a

bushing that joins the mould to the machine's nozzle.

SPRUE BUSHING - A hardened bushing that connects the mould to the moulding machine nozzle and allows molten plastic to enter the runner system.

STATIONARY PLATEN ("A") - The platen at the injection end of the moulding machine

that does not travel. It contains the "A" half of the mould and locates the mould to the

nozzle of the injection unit. The moving platen travels between this platen and

stationary platen "B".

STATIONARY PLATEN ("B") - The platen at the clamp end of the moulding machine that does not travel. The moving platen travels between this platen and stationary platen "A".

STRESS - A resistance to deformation from an applied force. Moulded plastic products

tend to contain stresses moulded in as a result of forces applied during the injection

process. These stresses may result in fractures, cracks, and breakage if they are

released during use of the product.

SUCK BACK - See DECOMPRESSION.

SUPPORT PILLAR - A circular rod mould component used to support the ejector half of

the mould. It is required because of the tremendous amount of pressure exerted against the "B" plate by the injection phase of the moulding process.

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THERMOCOUPLE - A device made of two dissimilar metals which are used to measure

the temperature of a heated area such as a barrel or nozzle. It sends a signal to a controller which then turns off or on to control the temperature of that area.

THERMOPLASTIC -A plastic material which, when heated, undergoes a physical change. It can be reheated, thus reformed, over and over again. See THERMOSET.

THERMOSET - A plastic material which, when heated, undergoes a chemical change and

"cures". It cannot be reformed, and reheating only degrades it. See THERMOPLASTIC.

TIE BARS - Large diameter rods that connect stationary platen "A" to stationary platen

"B". The moving platen contains bushings which are used for sliding over the tie bars, allowing the moving platen to travel between the 2 stationary platens.

TOGGLE CLAMP - A term used to describe the use of a mechanical "scissors action"

system to open and close the clamp unit of a moulding machine. It is operated by a

relatively small hydraulic cylinder.

TRANSITION ZONE - That area in the center of the screw (between the feed zone and

metering zone) This section has a tapering flight depth condition which compresses the plastic material in preparation for injection.

UNDERCUT - A recess or extension on the moulded part, located in such a way as to prevent or impede ejection of the part by normal moulding machine operation.

VENT - A shallow groove machined into the parting line surface of a mould in order to

allow air and gases to escape from the cavity, or runner, as the molten plastic is filling the mould. Sometimes also located on ejector and core pins.

VENTED BARREL - A heating barrel designed with an automatic venting port which allows moisture and gases to escape from molten plastic prior to being injected into a mould.

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2.3 Types of Injection Moulding Machines – Hand, Plunger, Screw, Plunger-

Plunger, Screw-Plunger etc with their parts and functions.

There are various type of Injection Moulding Machine available based on their functions

and utilizations. The cost of the machines is increased depending on their functions and

size. The simplest Injection Moulding machine is a hand injection moulding machine with

a very low cost and the costliest machine may be a screw type automatic microprocessor

injection moulding machine.

2.3.1 Hand Injection Moulding Machine

2.3.2 Plunger Type Injection Moulding Machine

2.3.3 Screw Type Injection Moulding Machine

2.3.4 Plunger-Plunger Injection Moulding Machine

2.3.5 Screw-Plunger Injection Moulding Machine

2.3.1 Hand Injection Moulding Machine

This type of Injection Moulding Machines are the simplest vertical machine consists of

Barrel, Plunger, Band Heaters along with energy regulator, Rack & Pinion system for

Injecting the material by the plunger, a torpedo and nozzle. The clamping is done

manually on a working table. The machine is fitted on the working table. Heating is set

manually. The capacity of the machine is available from 0.5 Oz to 2 Oz. Once heating is

achieved the production starts manually. The quality of the product is completely depend

upon the skill ness of the operator. The heating set point is achieved by heat & trial

method. Although Temperature controller may be fitted on the machine, but the set point

is completely depend on the quantity of product produced by the operator. The cycle

time is completely variable and it depends on the competence of the operator. The

function of the torpedo is to help the material for proper melting and crate back pressure

for help in mixing.

2.3.2 Plunger Type Injection Moulding Machine

Vertical & Horizontal Plunger Type Injection Moulding Machine

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The Plunger type injection moulding machine is available horizontal or vertical type and

operated pneumatically or hydraulically. The clamping and Injection may be Semi or

automatic. In a semi Automatic type the clamping cylinder & the Injection cylinders are

operated by levers which is connected the pneumatic or hydraulic cylinders. In Automatic

type the cylinders are actuated automatically to a set timers.

2.3.3 Screw Type Injection Moulding Machine

The Screw type Injection Moulding Machine consists of a hopper, a reciprocating screw

and barrel assembly, and an injection nozzle. This system confines and transports the

plastic as it progresses through the feeding, compressing, degassing, melting, injection, and packing stages.

A single screw injection Moulding machine for thermoplastics, showing the plasticizing

screw, a barrel, band heaters to heat the barrel, a stationary platen, and a movable platen.

2.3.4 Plunger-Plunger Injection Moulding Machine

As the plastic industry developed a second type plunger machine appeared, known as

two stage plunger. This type of equipment involved two plunger units set on top of other

the one to plasticize the material and feed it to another cylinder that consist of a

chamber to heat the plastic material by conduction and plunger that operates as a

shooting plunger and push the plasticize material into the mould.

Advantages:

Faster than conventional machine (single plunger)

No pressure loss encountered in compacting the granules.

Allows larger parts with more projected area.

Disadvantage:

Extra construction for pre plasticizing so cost is more.

2.3.5 Screw-Plunger Injection Moulding Machine

Later still another variation appeared in which first plunger stage (also known as a pre

plasticizer) was replaced by a rotating screw. In this case the action of the screw serves

to work and melt (plasticize) the resin and feed it into the second plunger unit where the

injection ram forces it forward into mould.

Advantages:

Better mixing and shear action of the plastic melt.

Broader range of stiffer flow

Heat sensitive material can be processed

Colour change can be handled in shorter time.

Lower stresses are obtained in the moulded form.

Disadvantages:

Higher cost in extra construction for pre plasticising unit.

Longer cycle time than reciprocating screw type machine.

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2.4 Screw – Nomenclature, types of screw, Ring-Plunger assembly, Screw drive

etc

2.4.1 Feed Zone

2.4.2 Transition Zone

2.4.3 Metering Zone

2.4.4 L/D Ratio

2.4.5 Compression Ratio

2.4.6 Helix Angle

2.4.7 Screw Plasticizing

2.4.8 Screw Operation

2.4.9 Back Pressure

2.4.10 Injection Speed

2.4.11 Screw Rotation Speed

2.4.12 Cushion

2.4.13 Screw Design Variables

2.4.14 What are the symptoms of wear in cylinders, screws and non-return

valves?

2.4.0 Screw Used in Injection Moulding Machines

SCREW PROFILE

The standard metering screw has three zones with a ring-plunger assembly. The feed

zone, where the plastic first enters the screw and is conveyed along a constant root

diameter; the transition zone, where the plastic is conveyed, compressed and melted

along a root diameter that increases with a constant taper; and the metering zone,

where the melting of the plastic is completed and the melt is conveyed forward along a constant root diameter reaching a temperature and viscosity to form parts.

The screw profile is described as the length, in diameters or flights, of each of the three

sections of the screw. A 10-5-5 profile indicates 10 diameters in feed, 5 diameters in

transition and 5 diameters in meter. General purpose screws typically use a 10-5-5 profile.

2.4.1 Feed Zone

The feed section is located under the hopper and in the rear section of the screw. The

flight depth is at its maximum, and the material from the hopper fills the flight of the

screw. The feed section has a constant channel depth throughout its entire length. Since

the conveying action is caused by the difference in friction between the plastic and the

barrel wall and the plastic and the screw, the screw is always more highly polished than

the barrel. Normally, the barrel temperature is higher than the screw temperature.

Consequently, the material adheres to the barrel as it softens and slip upon the cooler

screw. The material is then compacted in the feed section and begins to melt. The

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majority of the melting occurs in the compression or transition section. In most moulding

metering screws, the feed section is approximately half the screw length.

2.4.2 Transition Zone

The transition or compression section where the channel depth is continually

decreased completes the compacting and heating of the plastic granules. Here all the

remaining air is released as a result of heat supplied by the cylinder heaters and

mechanical energy supplied by the rotation of the screw.

Normally a transition zone encompasses approximately 25% of the screw length.

When the material leaves the area under the hopper, it only partially fills the screw. After

about four turns the material is fully compacted. The material touching the barrel melts

by conduction.

Two types of heating are occurring. One is the convection of heat from the heater

barrel. The second is the conversion of mechanical energy from turning the screw, into

the energy, by shearing the plastic.

Since the same weight of material per unit cross section must flow through the

whole length of the extruder, and since the bulk density of the unmelted portion is less

than that of the melted portion, the unmelted portion must be moving at a faster rate.

2.4.3 Metering Zone

The metering section is also known as mixing zone helps for homogeneous mixing

of the materials and acts as a pump, removing the material plasticized in the transition

zone.

2.4.4 L/D Ratio

The L/D ratio is the ratio of the flighted length (Effective Length) of the screw to its

outside diameter. The ratio calculation is calculated by dividing the flighted length of the

screw by its nominal diameter. Although several injection moulding machine

manufacturers now offer a choice of injection units, most injection screws use a 20:1 L/D

ratio. But it may range from 18:1 to 24:1 and in the case of thermoset it may range from 12:1 to 16:1.

The effect of changing the L/D ratio can be summarized in the following manner ... the larger the L/D Ratio (longer flighted length), the -

More shear heat can be uniformly generated in the plastic without degradation;

Greater the opportunity for mixing, resulting in a better homogeneity of the melt;

Greater the residence time of the plastic in the barrel possibly permitting faster cycles of larger shots.

2.4.5 Compression Ratio

The ratio of the first feed zone channel depth to the last meter zone channel depth, or

first flight depth of feed zone to last flight depth of metering zone, referred to as

"compression ratio", typically ranges from 1.5:1 to 4.5:1 for most thermoplastic

materials. Most injection screws classified as general purpose have a compression ratio

of 2.5:1 to 3.0:1. Thermoset screws have a 1:1 ratio.

The higher the compression ratio, the greater the:

Shear heat imparted to the resin

Heat uniformity of the melt

Potential for creating stresses in some resins

More energy consumption

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2.4.6 Helix Angle

The helix angle is the angle of a screw flight relative to a plan perpendicular to the screw

axis. Although the helix angle is not commonly altered from the standard square pitch, such a change can have a significant impact on processing.

The standard helix angle for most injection and extrusion screws is 17.6568º and is calculated as follows (a square pitch screw):

Helix Angle = arctangent of Lead

π x Diameter

A change to a smaller helix angle, hence more flight turns per diameter:

Reduces the axial melting length

Conveys stiffer materials with greater ease (and less torque) Reduces the rate at which material is conveyed.

2.4.7 Screw Plasticizing

In screw, the melting of the plastic is caused by the shearing action on the polymer

between the barrel and root of the screw. As the polymer molecules slide over each other

they convert the mechanical energy of the screw drive into the heat energy. The heat is

applied directly to the material. This process and the mixing action of the screw

contribute to its major advantages as a plasticizing method. These advantages are as

follows:

High shearing rates are obtained. Theses high rates lower the viscosity of the melt

making the material flow easier.

Good mixing is developed resulting in a homogenous melt. This usually means lower

injection pressure and hence lower clamp pressure.

Flow is non laminar.

Residence time in the cylinder is much less than in a plunger machine.

Most of the heat is supplied directly to the material.

Since relatively little heat is supplied from the heating bands compared to the

plunger machine the cycle can be delayed for a longer period before purging,

since the screw is not turning and little heat is being generated.

The action of the screw reduces chances of material hold-up and subsequent

degradation.

Machine can be used with heat sensitive materials, such as PVC.

The screw is easier to purge than the plunger.

2.4.8 Screw Operation

The reciprocating screw is used to plasticize the plastic pellets using various RPMs, inject

the molten plastics as a plunger at various speeds and shot volumes, and control the

pressure level in the molten plastic charge in front of the screw.

2.4.9 Back Pressure

Back pressure is the amount of pressure exerted on the material volume ahead of the

screw, as the screw is pushed back in preparation for the next shot.

Setting the maximum back pressure

Typically, all machines have an adjustment for the maximum back pressure. This "screw-

back" stage stops when the screw reaches a preset position. The stop position is

manually set, based on the amount of material required to fill the mould's cavity and

runner system. When the machine is ready to inject the shot, the screw then plunges the

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material ahead of the screw forward, injecting it into the mould. While the injected

material is cooled in the mould after the injection, the screw-back stage is re-initiated

and the moulding cycle repeated.

2.4.10 Injection Speed

The injection speed (or ram speed) is the forward speed of the screw during its injection

operation.

Setting the injection speed

For most engineering resins, the ram speed should be set to the fastest setting that the

part design and process will allow for technical and economic reasons. However, slower

injection speed at the beginning of injection may be necessary to avoid turbulent flow

and Jetting, as material passes through the restrictive areas such as the gates. The

injection speed should be reduced again toward the end of injection to avoid flashing at

the end of stroke, and to enhance the formation of homogenous weld lines after a

divided flow.

2.4.11 Screw Rotation Speed

The screw rotation speed (RPM) is the rate at which the plasticizing screw rotates. The

faster the screw rotates, the faster the material is compressed by the screw flights,

increasing the amount of shear heating.

2.4.12 Cushion

The cushion is the difference in the final forward position of the screw and its maximum

allowable forward position. If the screw were allowed to travel its full stroke and stop

mechanically against the nozzle, the cushion would be zero. Typically a cushion of 3 to 6

mm is used.

2.4.13 Screw Design Variables

In order to select the proper screw design for a particular resin (or resins), a basic

understanding of screw design variables and their effect on processing is essential.

Excluding the use of multiple flights (as used in barrier screws) and mixing sections,

there are five variables that may be utilized in the design of a metering screw that have

a bearing on how a solid resin becomes a melt. In addition to these variables, there are

barrier and mixing screw designs that can significantly affect processing results.

2.4.14 What are the symptoms of wear in cylinders, screws and non-return

valves?

In an injection moulding environment, the symptoms of wear and typical causes are:

SYMPTOM TYPICAL CAUSE

Screw rotating during injection Worn or damaged non-return valve and/or cylinder

allows screw to move forward without pushing the

plastic causing the screw to rotate

Inability of screw to hold a

cushion

Worn or damaged non-return valve and/or cylinder

permits screw to move forward without pushing and

packing the plastic in the mould

Longer than normal recovery

time

Reduced plasticizing efficiency resulting from worn

cylinder and/or screw

Increase in overall cycle time Caused by all of the above. Worn cylinder, screw and

non-return valve result in slower and inefficient

plasticizing

- 15 -

Defective, streaked or non-

uniform appearing parts

Poor melt quality resulting from worn cylinder and/or

screw

Metal particles in parts Flaking or peeling of screw plating or cylinder lining

Dark or burned specks in parts Cylinder and/or screw worn or pitted, allowing plastic to

gather in wear spots and degrade

Difficulty in achieving a colour

change

Plastic hangs up in worn areas of cylinder or screw

requiring repeated cycles to clear out

- 16 -

2.5 Moulds – Nomenclature of Moulds, Types of moulds used etc.

2.5.0 Nomenclature of Injection Mould

2.5.1 Two-Plate mould

2.5.2 Three-Plate mould

2.5.3 Cooling channels (circuits)

2.5.4 Moulded system

2.5.5 Cold runners

2.5.6 Hot runners

2.5.7 Gates

2.5.8 Ejection systems

2.5.0 Nomenclature of Injection Mould

FIGURE 1. A typical (three-plate) moulding system.

The mould system consists of tie bars, stationary and moving platens, as well as

moulding plates (bases) that house the cavity, sprue and runner systems, ejector pins,

and cooling channels, as shown in Figure 1. The mould is essentially a heat exchanger in

which the molten thermoplastic solidifies to the desired shape and dimensional details

defined by the cavity. A mould system is an assembly of platens and moulding plates

typically made of tool steel. The mould system shapes the plastics inside the mould

cavity (or matrix of cavities) and ejects the moulded part(s). The stationary platen is

attached to the barrel side of the machine and is connected to the moving platen by the

tie bars. The cavity plate is generally mounted on the stationary platen and houses the

injection nozzle. The core plate moves with the moving platen guided by the tie bars.

Occasionally, the cavity plate is mounted to the moving platen and the core plate and a hydraulic knock-out (ejector) system is mounted to the stationary platen.

2.5.1 Two-plate mould

The vast majority of moulds consist essentially of two halves, as shown below. This kind

of mould is used for parts that are typically gated on or around their edge, with the

runner in the same mould plate as the cavity.

- 17 -

FIGURE.2 . A two-plate mould.

2.5.2 Three-plate mould

The three-plate mould is typically used for parts that are gated away from their edge.

The runner is in two plates, separate from the cavity and core, as shown in Figure 5

below.

FIGURE.3 A three-plate mould.

2.5.3 Cooling channels (circuits)

Cooling channels are passageways located within the body of a mould, through which a

cooling medium (typically water, steam, or oil) circulates. Their function is the regulation

of temperature on the mould surface. Cooling channels can also be combined with other

temperature control devices, like bafflers, bubblers, and thermal pins or heat pipes.

2.5.4 Moulded system

A typical moulded system consists of the delivery system and the moulded part(s), as shown in Figure 4.

FIGURE 4. The moulded system includes a delivery system and moulded parts.

- 18 -

2.5.5 Cold runners

After moulding, the cold-runner delivery system is trimmed off and recycled. Therefore,

the delivery system is normally designed to consume minimum material, while maintaining the function of delivering molten plastic to the cavity in a desirable pattern.

2.5.6 Hot runners

The hot-runner (or runner less) moulding process keeps the runners hot in order to

maintain the plastic in a molten state at all times. Since the hot-runner system is not

removed from the mould with the moulded part, it saves material and eliminates the secondary trimming process.

2.5.7 Gates

Gating can be done a number of ways

2.5.8 Ejection systems

Ejection systems will push the part out of the mould when it is opened.

- Knockout pins

- Blades

- Stripper rings

- Air

- Hard stripping

- 19 -

2.6 Material Selection – Types of Materials, Criteria for selection of materials,

Material handling, Special care for engineering plastics etc.

2.6.0 Materials Selection for injection moulding

2.6.1 Resin data table

2.6.2 ABS

2.6.3 PA 12

2.6.4 PA 6

2.6.5 PA 66

2.6.6 PBT

2.6.7 PC

2.6.8 PC-ABS

2.6.9 PC-PBT

2.6.10 PE-HD

2.6.11 PE-LD

2.6.12 PEI

2.6.13 PET

2.6.14 PETG

2.6.15 PMMA

2.6.16 POM

2.6.17 PP

2.6.18 PPE-PPO

2.6.19 PS

2.6.20 PVC

2.6.21 SAN

2.6.0 Materials Selection for injection moulding

Material selection depends to a large extent on the functional constraints of the part.

Both amorphous and crystalline thermoplastic resins are used in injection moulding.

Short glass fibers are commonly used as reinforcements

Thermoplastic compounds commonly used in injection moulding include:

Acrylonitrile butadiene styrene (ABS)

Acetal

Acrylic

Polycarbonate (PC)

Polyester

Polyethylene

Fluoroplastic

Polyimide

Nylon

Polyphenylene oxide

Polypropylene (PP)

Polystyrene (PS)

Polysulphone Polyvinyl chloride (PVC)

The physical properties of the materials (density, thermal conductivity, melting

temperature, etc.) must be considered in light of the required mechanical properties of the finished part (strength, stiffness, hardness, etc.)

- 20 -

This schematic illustrates the performance spectrum of a variety of plastic materials.

Various additives may be added to injection moulding compounds to accomplish various

purposes. This table summarizes some of them.

Additive Function Examples

Filler increase bulk density calcium carbonate, talc, limestone

Plasticizer improve processability, reduce

product brittleness

phthalate esters, phosphate esters

Antioxidant prevent polymer oxidation phenols, aromatic amines

Colorant provide desired part application

color

oil-soluble dyes, organic pigments

Flame retardant reduce polymer flammability antimony trioxide

Stabilizer stabilize polymer against heat or UV

light

carbon black,

hydroxybenzophenone

Reinforcement improve strength E-glass, S-glass, carbon, Kevlar

fibers

Processing temperature is an issue in material selection for injection moulding.

Polymer physical properties dictate temperature processing window (Tg < Tproc < Tdeg)

The operating temperature must lie in the range between the glass transition temperature and the degradation temperature of the polymer.

Example: Nylon 6,6 has its glass transition at 240°C - 265°C, but its suggested

processing temperature range is 270°C - 305°C. Degradation occurs at temperatures

above 350°C or 400°C. The maximum use temperature of nylon 6,6 parts is about

150°C. (Note that this is considerably below the glass transition temperature.)

- 21 -

Example: Material Selection Guidelines for Radiator End Cap

Material Selection Guidelines

Fatigue Strength Semi-crystalline e.g. Nylon 66 is superior

Impact Strength Polycarbonate is good but it has poor fatigue strength

High Temperature Phenolics have good high temperature performance, but they have

poor impact strength

Solvent Resistance Phenolics, PPS, Nylon

Cost Phenolics < Nylon < PPS

Semi-crystalline polymers like nylons are superior to amorphous polymers in fatigue

strength. Some amorphous polymers like polycarbonate have good impact strength. But

polycarbonate has very poor fatigue strength and it is notch sensitive. For high

temperature environments, phenolics offer a good option: however the impact strength

of phenolics is very low. For solvent environments, phenolics, polyphenylene sulfide and

nylon are good choices. Finally, from the cost point of view, phenolics are cheaper than

nylon and nylon is cheaper than polyphenylene sulfide. Nylon is chosen as the material

for the radiator end cap.

2.6.1 Resin data table

Generic

Name

Melt Temperature

(ºC)

Mould Temperature

(ºC)

Min. Rec. Max. Min. Rec. Max.

ABS 200 230 280 25 50 80

PA 12 230 255 300 30 80 110

PA 6 230 255 300 70 85 110

PA 66 260 280 320 70 80 110

PBT 220 250 280 15 60 80

PC 260 305 340 70 95 120

PC-ABS 230 265 300 50 75 100

PC-PBT 250 265 280 40 60 85

PE-HD 180 220 280 20 40 95

PE-LD 180 220 280 20 40 70

PEI 340 400 440 70 140 175

PET 265 270 290 80 100 120

PETG 220 255 290 10 15 30

PMMA 240 250 280 35 60 80

POM 180 225 235 50 70 105

PP 200 230 280 20 50 80

PPE-PPO 240 280 320 60 80 110

PS 180 230 280 20 50 70

PVC 160 190 220 20 40 70

SAN 200 230 270 40 60 80

Commonly used Plastics Materials and their parameters for Injection Moulding

2.6.2 ABS (Acrylonitrile-Butadiene-Styrene)

Typical Applications

Automotive (instrument and interior trim panels, glove compartment doors, wheel

covers, mirror housings, etc.)

Refrigerators, small appliance housings and power tools applications (hair dryers,

blenders, food processors, lawnmowers, etc.)

Telephone housings, typewriter housings, typewriter keys Recreational vehicles such as golf carts and jet skis.

- 22 -

Injection Moulding Processing Conditions:

Drying: ABS resins are hygroscopic and drying is required prior to processing.

Suggested drying conditions are 80 - 90ºC for a minimum of 2 hours.

Resin moisture content should be less than 0.1%

Melt

Temperature:

210 - 280ºC;

Mould

Temperature:

25 - 70ºC. (Mould temperatures control the gloss properties; lower

mould temperatures produce lower gloss levels)

Injection

Pressure:

500 - 1,000 bar (7,250 - 14,500 psi)

Injection

Speed:

Moderate - high

2.6.3 PA 12 (Polyamide 12 or Nylon 12)


Gear wheels for water meters and business machines

Cable ties

Cams

Slides

Bearings

Injection Moulding Processing Conditions

Drying: The moisture content must be below 0.1% prior to processing. If the

material is exposed to air, drying in a hot air oven at 85ºC for 4 -5 hours is

recommended (3-4 hours in a desiccant dryer). If the container is

unopened, it may be used directly for moulding after 3 hours of

equilibration to shop floor temperature.

Melt

Temperature:

240 - 300ºC; Not to exceed 310ºC for standard grades and 270ºC for flame

retardant grades

Mould

Temperature:

30 - 40ºC for unreinforced grades; for thin walled or large surface area

components, 80 - 90ºC may be used; 90 - 100ºC for reinforced grades.

Increasing the mould temperature increases the crystallinity level. It is very

important to precisely control the mould temperature.

Injection

Pressure:

Up to 1,000 bar (14, 500 psi) Low hold pressures and high melt

temperatures are recommended.

Injection

Speed:

High (high speeds give better finish on glass-filled grades)

Runners and Gates

Runner diameters for unfilled grades may be as small as 3 - 5 mm because of the

material's low viscosity. Reinforced grades require larger diameters (5 - 8 mm). The

runner shape should be the full round type. Sprues should be as short as possible.

A variety of gates may be used. Small gates for large parts should be not be used, in

order to avoid highly stressed components or excessive shrinkage. The thickness of the

gate should preferably be equal to the part thickness. When using submarine gates, the minimum recommended diameter is 0.8 mm.

Hot runner moulds may be used effectively but precise temperature control is necessary

to prevent material drooling or freezing off at the nozzle. When hot runners are used, the

size of the gates may be smaller than in the case of cold runners.

- 23 -

2.6.4 PA 6 (Polyamide 6, Nylon 6, or Polycaprolactam)

Applications

Used in many structural applications because of its good mechanical strength and

rigidity. It is used in bearings because of its good wear resistance.

Injection Moulding processing conditions

Drying: Since PA 6 absorbs moisture readily, care should be taken to ensure its

dryness prior to moulding. If the material is supplied in watertight

packaging, the containers should be kept closed. If the moisture content is

>0.2%, drying in a hot air oven at 80ºC for 16 hours is recommended. If

the material has been exposed to air for more than 8 hours, vacuum drying

at 105ºC for more than 8 hours is recommended.

Melt

Temperature:

230 - 280ºC; 250 - 280ºCfor reinforced grades

Mould

Temperature:

80 - 90ºC. Mould temperature significantly influences the crystallinity level

which in turn affects the mechanical properties. For structural parts, a high

degree of crystallization is required and mould temperatures of 80 -

90ºCare recommended. High mould temperatures are also recommended

for thin-wall parts with long flow lengths. Increasing the mould temperature

increases the strength and hardness, but the toughness is decreased. When

the wall thickness is greater than 3 mm, a cold mould is recommended (20

- 40ºC), which leads to a higher and more uniform degree of crystallinity.

Glass reinforced resins are always processed at mould temperatures greater

than 80ºC

Injection

Pressure:

Generally between 750 - 1,250 bar (~11,000 - 18,000 psi) (depends on

material and product design)

Injection

Speed:

High (slightly lower for reinforced grades)

Runners and Gates

The gate location is important because of very fast freeze-off times. Any type of gate

may be used; the aperture should not be less than 0.5*t (where "t" is the thickness of

the part). When hot runners are used, the size of the gates can be smaller than when

cold runners are used, because premature freeze-off is prevented. When using

submarine gates, the minimum diameter of the gate should be 0.75 mm.

2.6.5 PA 66 (Polyamide 66, Nylon 66, or Poly (hexamethylene adipamide))

Applications

Competes with PA 6 for most applications. PA 66 is heavily used in the following:

The automotive industry

Appliance housings Where impact resistance and strength are required

Injection Moulding Processing conditions

Drying: Drying is not required if the material is sealed prior to moulding; however,

if the containers are left open, drying in a hot air oven at 85ºC is

recommended. If the moisture content is > 0.2%, vacuum drying at 105ºC

for 12 hours is recommended.

Melt

Temperature:

260 - 290ºC ; 275 - 280ºC for glass filled grades; melt temperatures above

300ºC should be avoided

Mould 80ºC suggested. Mould temperature affects crystallinity level which in turn

- 24 -

Temperature: affects physical properties. In the case of thin walled parts, crystallinity

changes with time if mould temperatures of less than 40ºC are used. In

such cases, annealing may be needed to retain dimensional stability.

Injection

Pressure:

Generally between 750 - 1,250 bar (~11,000 - 18,000 psi), depends on

material and product design

Injection Speed:

High (slightly lower for reinforced grades)

Runners and Gates

The gate location is important because of very fast freeze-off times. Any type of gate

may be used; the aperture should not be less than 0.5*t (where "t" is the thickness of

the part). When hot runners are used, the size of the gates can be smaller than when

cold runners are used, because premature freeze-off is prevented. When using

submarine gates, the minimum diameter of the gate should be 0.75 mm.

2.6.6 PBT (Polybutylene Terephthalates)


Household appliances (e.g., food processor blades, vacuum cleaner parts, fans,

hair dryer housings, coffee makers)

Electronics (e.g., switches, motor housings, fuse cases, key caps for computer

keyboards, connectors, fiber optic buffer tubing)

Automotive (e.g., grilles, body panels, wheel covers, and components for doors

and windows)


Drying: This material is sensitive to hydrolysis at high temperatures. It is therefore

important to dry the material prior to moulding. Suggested drying

conditions (in air) are 120ºC for 6 - 8 hours (or 150ºC for 2 - 4 hours).

Moisture levels must be below 0.03%. When using a desiccant dryer, drying

at 120ºC for 2.5 hours is recommended.

Melt

Temperature:

225 - 275ºC ; aim: 250ºC

Mould

Temperature:

40 - 60ºC for unreinforced grades. For other grades, a wide range of

temperatures can be used, depending on the grade (15 - 120ºC). Cooling

channels should be properly designed to minimize part warpage. The heat

removal must be fast and uniform. Cooling channels of 12 mm diameter are

recommended.

Injection

Pressure:

Moderate (up to maximum of 1500 bar / 21750 psi).

Injection Speed: Fastest possible speeds should be used (due to fast solidification of PBTs)

Runners and Gates

Full round runners are recommended to impart maximum pressure transmission (rule of

thumb: runner diameter = part thickness + 1.5 mm). A wide variety of gates may be

used. Hot runners may also be used, taking care to avoid drool and material degradation.

Gate diameters or depths should preferably be between 0.8 - 1.0 * t where "t" is the part

thickness. When using submarine gates, the minimum recommended diameter is 0.75

mm.

- 25 -

2.6.7 PC (Polycarbonate)


Electronic and business equipment (e.g., computer parts, connectors)

Appliances (e.g., food processors, refrigerator drawers) Transportation (e.g., head lights, tail lights, instrument panels)


Drying: PC resins are hygroscopic and pre-drying is important. Recommended

drying conditions are 100 - 120ºC for 3 to 4 hours. Moisture content must

be less than 0.02% prior to processing.

Melt

Temperature:

260 - 340ºC ; higher range for low MFR resins and vice-versa

Mould

Temperature:

70 - 120ºC ; higher range for low MFR resins and vice-versa

Fill Pressure: As high as possible for rapid moulding

Injection Speed: Slow injection speeds when small or edge gates are used; high speeds for

other types of gates

2.6.8 PC|ABS (Polycarbonate-Acrylonitrile-Butadiene-Styrene Blend)


Computer and business machine housings

Electrical applications

Cellular phones

Lawn and garden equipment Automotive components (instrument panels, interior trim, and wheel covers)

Injection moulding processing information

Drying: Drying is required prior to processing. Moisture content should be less than

0.04 % to ensure stable processing parameters. Drying at 90 - 110ºC for 2

to 4 hours is recommended.

Melt Temperature: 230 - 300ºC

Mould

temperature:

50 - 100ºC

Injection Pressure: Part dependent

Injection Speed: As high as possible

2.6.9 PC|PBT (Polycarbonate | Polybutyleneterephthalate Blend)


Gear cases and automotive (bumpers)

Applications that require chemical and corrosion resistance, high heat resistance,

high impact strength over wide temperature ranges, and high dimensional stability


Drying: 110 - 135ºC for approximately 4 hours

Melt Temperature: 235 - 300ºC ; Depends on specific grade

Mould Temperature: 37 - 93ºC

- 26 -

6.1.10 PE-HD (High Density Polyethylene)


Major use is in blow-moulding (packaging) applications such as:

Containers in refrigeration units

Storage vessels

Household goods (kitchenware)

Seal caps Bases for PET bottles

Injection Moulding Processing conditions

Drying: Not normally necessary if stored properly.

Melt temperature: 220 - 260ºC ; for high molecular weigh resins, the suggested melt

temperature range is 200 - 250ºC

Mould

temperature

50 - 95ºC (higher temperatures for wall thickness of up to 6 mm;

lower temperature for wall thicknesses greater than 6 mm.)

The cooling rate should be uniform to minimize shrinkage variations. For optimum cycle

times, the cooling channel diameters should be at least 8 mm and must be within a

distance of 1.3 d from the mould surface (where "d" is the diameter of the cooling

channel).

Injection

pressure:

700 - 1,050 bar (10,000 - 15,000 psi)

Injection speed Fast injection speeds are recommended; profiled speeds reduce

warpage in the case of components with a large surface area.

Runners and Gates

Diameters of runners range from 4 - 7.5 mm (typically 6 mm). Runner lengths should be

as short as possible. All types of gates may be used. Gate lands should not exceed 0.75

mm in length. Ideally suited for hot runner moulds; an insulated hot tip runner is

preferred when there are frequent color changes.

2.6.11 PE-LD (Low Density Polyethylene)


Closures

Bowls

Bins

Pipe couplings


Drying: Not usually necessary

Melt

Temperature:

180 - 280ºC

Mould

Temperature:

20 - 40ºC

For uniform and economic heat removal, it is recommended that the

cooling channel diameters be at least 8 mm and the distance from the

surface of the mould to the edge of the cooling channel be not more than

1.5 times the diameter of the cooling channel.

Injection

Pressure:

Up to 1,500 bar (21,750 psi)

Pack Pressure: Up to 750 bar (10,850 psi)

Injection

Speed:

Fast speeds are recommended; profiled speeds can limit warpage problems

of large surface area parts.

- 27 -

Runners and Gates

All conventional types may be used; PE-LD is well suited for hot runner moulds. Insulated

hot tip runners are preferred for frequent colour changes.

2.6.12 PEI (Polyetherimide)


Automotive (engine components: temperature sensors, fuel and air handling

devices

Electrical/electronics (connector materials, printed circuit boards, circuit chip

carriers, explosion proof boxes)

Packaging applications

Aircraft (interior materials) Medical (surgical staplers, tool housings, non-implant devices)

Injection moulding processing information

Drying: PEI absorbs moisture and can cause material degradation. Moisture

content should be less than 0.02%. Suggested drying conditions are 150ºC

for 4 hours in a desiccant dryer (6 hours for reinforced and blended

grades)

Melt Temperature: 340 - 400ºC unreinforced grades

340 - 415ºC reinforced grades

Mould

Temperature:

107 - 175ºC ; Aim: 140ºC

Injection Pressure: 700 - 1500 bar (~10,000 - 22,000 psi) Typical

Injection Speeds: As high as possible

2.6.13 PET (Polyethylene Terephthalate)


Automotive (structural components such as mirror backs, and grille supports,

electrical parts such as head lamp reflectors and alternator housings)

Electrical applications (motor housings, electrical connectors, relays, and switches,

microwave oven interiors) Industrial applications (furniture chair arms, pump housings, hand tools)

Injection moulding processing conditions

Drying: Drying is essential prior to moulding. PETs are very sensitive to

hydrolysis. Recommended drying conditions are 120 - 165ºC(248 - 329

F) for 4 hours. The moisture content should be less than 0.02%.

Melt

Temperature:

For unfilled grades:265 - 280ºC

For glass reinforced grades:275 - 290ºC

Mould

Temperature:

80 - 120ºC

(preferred range: 100 -110ºC;

Injection

Pressure:

(300 - 1,300 bar ; 4,350 - 19,000 psi)

Injection speed: High speeds without causing embrittlement

- 28 -

Runners and Gates

All conventional types of gates may be used; gates should be 50 - 100% of the part

thickness.

2.6.14 PETG (Glycol-modified PET; Copolyesters)


PETGs offer a desirable combination of properties such as clarity, toughness, and

stiffness. Applications include:

Medical devices (test tubes and bottles)

Toys

Displays

Lighting fixtures

Face shields Refrigerator crisper pans

Injection moulding processing conditions

Drying: Drying is essential for PETG prior to injection moulding. The moisture

level must be below 0.04%. Drying temperature is not to exceed 66ºC.

Drying at approximately 65ºC for 4 hours is recommended.

Melt Temperature: 220 - 290ºC; The melt temperature is grade specific

Mould

Temperature:

10 - 30ºC; Recommended: 15ºC

Injection Pressure: 300 - 1,300 bar (4,350 - 19,000 psi)

Injection speed: High speeds without causing embrittlement

2.6.15 PMMA (Polymethyl Methacrylate)


Automotive (signal light devices, instrument panels)

Medical (blood cuvettes)

Industrial (video discs, lighting diffusers, display shelving) Consumer (drinking tumblers, stationery accessories)


Drying: PMMA is hygroscopic and must be dried prior to moulding. Drying at

90ºC for 2-4 hours is recommended.


Mould

Temperature:

35 - 70ºC

Injection Speed: Moderate

2.6.16 POM (Polyacetal or Polyoxymethylene)

Applications

Acetals have a low coefficient of friction and good dimensional stability. This makes it

ideal for use in gears and bearings. Due to its high temperature resistance, it is used in

plumbing (valve and pump housings) and lawn equipment.

- 29 -


Drying: Not usually required but resin should be stored in a dry atmosphere.

Melt Temperature:

190 - 230ºC for homopolymer; 190 - 210ºC for copolymer

Mould

Temperature:

80 - 105ºC; Higher mould temperatures are preferred for precision

moulding which reduce post-moulding shrinkage

Injection

Pressure:

700 - 1,200 bar (~10,000 - 17,500 psi)

Injection Rate: Medium - High

Runners and Gates

Any type of gate may be used. When using tunnel gates, the short type is preferred.

Insulated, hot tip runners are preferred for homopolymers; both internally and externally

heated hot runners may be used in the case of copolymers.

2.6.17 PP (Polypropylene)


Automotive (mostly mineral-filled PP is used: dashboard components, ductwork,

fans, and some under-hood components)

Appliances (door liners for dishwashers, ductwork for dryers, wash racks and lids

for clothes washers, refrigerator liners)

Consumer products (lawn/garden furniture, components of lawn mowers, sprinklers)


Drying: Not normally necessary if proper storage is used

Melt Temperature: 220 - 275ºC ; not to exceed 275ºC

Mould Temperature:

40 - 80ºC; suggested: 50ºC The crystallinity level is determined by the

mould temperature.

Injection Pressure: Up to 1,800 bar (26,000 psi)

Injection Speed

Typically, fast injection speeds are used to minimize internal stresses; if surface defects

occur, slow speed moulding at a higher temperature is preferred. Machines capable of

providing profiled speed is highly recommended.

Runners and Gates

In the case of cold runners, typical diameters range from 4 - 7 mm. Full round sprues

and runners are recommended. All types of gates can be used. Typical pin gate

diameters range from 1 - 1.5 mm, but diameters as low as 0.7 mm may be used. In case

of edge gating, the minimum gate depth should be half the wall thickness and the width

should be at least double the thickness. Hot runners can readily be used for moulding PP.

2.6.18 PPE|PPO (Polypropylene Ether Blends)


Household appliances (dishwasher, washing machine)

Electrical applications, such as control housings, fiber-optic connectors

- 30 -


Drying: Recommend drying before moulding for approximately 2 - 4 hours at

100ºC. PPOs have low levels of moisture absorption can typically be

moulded as received.

Melt Temperature: 240 - 320ºC (higher ranges for resins with higher levels of PPO)

Mould

Temperature:

60 - 105ºC

Injection pressure:

600 - 1,500 bar (8,700 - 21,750 psi)

Runners and Gates

All gates can be used; tab and fan gates are preferred

2.6.19 PS (Polystyrene)


Packaging

House wares (tableware, trays) Electrical (transparent housings, light diffusers, insulating film)


Drying: Not usually required unless stored improperly. If drying is needed, the

recommended conditions are 2-3 hours at 80ºC

Melt Temperature: 180 - 280ºC ; upper limit is 250ºCfor flame retardant grades

Mould Temperature: Suggested: 40 - 50ºC

Injection Pressure: 200 - 600 bar (3,000 - 8,700 psi)

Injection Speed: Fast speeds are recommended

Runners and Gates

All types of conventional gates may be used.

2.6.20 PVC (Polyvinyl Chloride)


Water distribution piping

Home plumbing

House siding

Business machine housings

Electronics packaging

Medical apparatus

Packaging for foodstuffs


Drying: Not usually necessary as PVC absorbs very little water.


Mould Temperature: 30 - 50ºC

Injection Pressure: Up to 1,500 bar (21,750 psi)

Packing Pressure: Up to 1,000 bar (14,500 psi)

Injection Speed: Relatively slow, to avoid material degradation

- 31 -

Runners and Gates

All conventional gate types may be used; pin-point and submarine gates are used for

moulding small components and fan gates are typically used for thick sections. The

minimum diameter of pin-point or submarine gates should be 1 mm and the thickness of

fan gates should not be less than 1 mm.

Sprue should be as short as possible; typical runner sizes are 6 - 10 mm and should have

a full round cross-section. Insulated hot runners and certain types of hot sprue bushings may be used with PVC.

2.6.21 SAN (Styrene Acrylonitrile)


Electrical (receptacles, mixer bowls, housings, etc. for kitchen appliances,

refrigerator fittings, chassis for television sets, cassette boxes)

Automotive (head lamp bodies, reflectors, glove compartments, instrument panel

covers)

Household appliances (tableware, cutlery, beakers)

Cosmetic packs


Drying: Under improper storage conditions, SAN absorbs moisture; it is

recommended that it be dried at 80ºC for 2-4 hours prior to

moulding.

Melt

Temperature:

200 - 270ºC; 230 - 260ºCfor most applications; lower end of the

range is used for moulding thick wall components

Mould

Temperature:

40 - 80ºC; SAN solidifies rapidly at higher temperatures; in case of

reinforced grades, the mould temperatures should not be less than

60ºC

The cooling system must be well designed because the mould

temperature affects the parts appearance and shrinkage and

warpage.

Injection

Pressure:

350-1,300 bar (5,000 - 20,000 psi)

Injection Speed: High speeds are recommended

Gates

All conventional gate types may be used. The gates must be of proper size which aid in

processing and do not cause streaks, burn marks, or voids.

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2.7. Clamping Systems – Manual, Toggle, Hydraulic, Hydro-Mechanical & Tie-bar

less clamping etc.

2.7.1 Manual Clamping

2.7.2 Toggle Type Clamping

2.7.3 Hydraulic Clamping

2.7.4 Hydro-Mechanical

2.7.5 Tie-bar less Clamping

The function of clamping unit is to clamp the Injection Mould. The clamping Pressure is

set more than the Injection Pressure so that the mould does not open during plastic melt

Injection.

2.7.1 MANUAL CLAMPING

Manual clamping in only seen in the case of manual or Hand Injection Moulding Machine

or some time in pneumatic Injection Moulding Machine. The proper clamping is depend

on the skill of the operator. The Clamping force is direct and not measured. The position

of the sprue with respect to nozzle axis is critical. The ejection of the part from the mould

is difficult.

2.7.2 TOGGLE TYPE CLAMPING

A toggle is mechanically device to amplify force. In a moulding machine, which consists

of two bars joined, together end to end with a pivot .The end of one bar is attached to a

stationary platen, and the other end of a second bar is attached to the movable platen.

When the mould is open, the toggle is in the shape of a V. When pressure is applied to

the pivot, the two bars form a straight line.

ADVANTAGE

1. Low cost and lower horsepower needed to run.

2. Positive clamp of the mould

DISADVANTAGE

1. Do not read the clamp force.

2. Clamping is more difficult.

3. Higher maintenance as lubricant is provided.

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2.7.3 HYDRAULIC CLAMPING

A clamping unit actuated by hydraulic cylinder, which is directly connected to the

moving, closed the mould. In this case ram of hydraulic system is attached to moving

platen. There are two halves in hydraulic cylinder, which is actually inlet and outlet of oil.

When oil goes to the cylinder with pressure oil pushes the ram to forward direction by

which moving platen moves and mould closed and when oil comes from the cylinder the

ram come back and mould is open.

ADVANTAGE

Clamp speed easily controlled and stopped at any point.

Direct read out of clamp force.

Easy adjustment of clamped force and easy mould set up.

Low maintenance as part is self lubricated.

DISADVANTAGE

1. It is higher cost and more expensive than toggle system.

2. None positive clamp.

2.7.4 HYDRO-MECHANICAL

This Clamping System is combination of Toggle & Hydraulic Clamping System. To move

the toggle a hydraulic cylinder is operated.

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2.7.5 TIE-BAR LESS CLAMPING

Tie-Bar less clamping system is basically Hydraulic clamping system without any tie bar.

The platen is moved on a rail system.

The main advantage of this system there is no limitation of mould platen size. As there is

no tie bar so the mould dimension is not so important. Also mounting of the mould is easy and it is very useful when products eject from the mould is manual.

Advantages Of Tie-Bar-Less Machines:

Much larger mould mounting area.

Larger stroke compared to the toggle type machines.

Full machine capacity can be utilised.

Smaller machines can mould larger components.

Saves floor space.

Saves electrical energy because of reduction in the size of machine.

Has the capacity to reduce weight of the moulded component because tie-bar

stretching is not there.

Machine becomes very flexible for future modification.

Easy access to mould cavity's because of the absence of the tie bars.

Robotic arm movement becomes easy.

Fewer moving parts so lesser wear and tear so longer life for machines.

Lower lubrication required.

Removal of mould plates much simple.

Greater stability.

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2.8. Hydraulic Machine operation Sequences

Injection moulding is a cyclic process. During the injection moulding process, the

machine undertakes a sequence of operations in a cyclic fashion. A process cycle is one

complete operation of an injection moulding machine.

Process cycle

The basic injection moulding machine operations are shown in the series of diagrams

below.

1

The mould closes and the screw begins

moving forward for injection.

2

The cavity fills as the reciprocating screw

moves forward, as a plunger.

3

The cavity is packed as the screw

continuously moves forward.

4

The cavity cools as the gate freezes off and

the screw begins to retract to plasticize

material for the next shot.

5

The mould opens for part ejection.

6

The mould closes and the next cycle begins

- 36 -

2.9. Machine selection – Based on Shot weight, Clamping, Injection &

Theoretical calculation

2.9.0 How to Select a Plastic Injection Moulding Machine

2.9.1 Shot weight

2.9.1.1 Definition of shot weight

2.9.1.2 Shot weight in terms of the resin to be used

2.9.1.3 Relation of shot weight to injection volume

2.9.1.4 Selecting a machine with sufficient shot weight

2.9.2 Selecting a machine which is not too big

2.9.3 Barrel residence time

2.9.4 Clamping force

2.9.5 Determining Projected Area & Clamp Force

2.9.6 Injection pressure

2.9.7 Injection volume

2.9.8 Injection speed

2.9.9 Accumulator

2.9.10 Injection rate

2.9.11 Screw rotary speed

2.9.12 Screw motor torque

2.9.13 Plasticizing capacity

2.9.14 Mould opening stroke

2.9.15 Mould height (thickness)

2.9.16 Minimum Mould Height

2.9.17 Maximum daylight

2.9.18 Space between Tie bars

2.9.19 Platen size

2.9.20 Platen thickness

2.9.21 Tie bar diameter

2.9.22 Dry cycle time

2.9.23 Electric motor rating

2.9.24 Electric heater rating

2.9.25 Total power

2.9.26 Number of heating zones

2.9.27 Oil tank capacity

2.9.28 Hopper capacity

2.9.29 System pressure

2.9.30 Screw and Barrel

2.9.31 Cold start interlock

2.9.32 Low pressure mould protection

2.9.33 Nozzle type

2.9.34 Hydraulic oil temperature control

2.9.35 Hydraulic oil contamination control

2.9.36 Safety features

2.9.37 Metal detector option

2.9.0 How to Select a Plastic Injection Moulding Machine

2.9.1 Shot weight

Shot weight is an important attribute of the injection unit of an Injection Moulding

Machine. Expressed in ounces or grams, this is by far the most commonly used single attribute to select a plastic injection moulding machine.

The reason is simple. A moulder has an article at hand to be moulded. Once the plastic

material is selected, it has a weight. An injection moulding machine with sufficient shot weight is then selected.

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2.9.1.1 Definition of shot weight

The shot weight is the measured (therefore actual) weight of the plastic 'injected' when

the nozzle is free-standing (not held against the mould). The plastic used is usually

polystyrene (PS) with a specific gravity (S.G.) of 1.05. This is specified in the specification as PS.

2.9.1.2 Shot weight in terms of the resin to be used

If the article to be moulded is made of a resin different than PS, then the shot weight in

the specification could not be used immediately, but must be calculated as follows:

Shot weight in terms of a resin =ºC* b/1.05

where b = S.G. of the resin,

ºC= shot weight in terms of PS (S.G. = 1.05)

Table 1. Specific gravity of resins at room temperature

Example 1: POM has an S.G. of 1.42. It is to be moulded in an Injection Moulding

Machine with a shot weight of 80 gms (in PS). This machine has a shot weight of 80 * 1.42 / 1.05 = 108.19 gms of POM.

Example 2: PP has an S.G. of 0.90. It is to be moulded in an Injection Moulding Machine

with a shot weight of 80 gms (in PS). This machine has a shot weight of 80 * 0.90 / 1.05 = 68.57 gms of PP.

2.9.1.3 Relation of shot weight to injection volume (Swept Volume)

Shot weight is not equal to injection volume (Swept Volume). Injection volume (Swept

Volume) multiplying with the S.G. of PS, Shot weight is measured. Injection volume is

theoretical. Injection volume multiplying with the S.G. of PS provides a higher value than

actual shot weight due leakage pass the screw during injection. Also, the non-return

valve at the tip of the screw moves backward a little before it reaches the closed position.

Resin Abbreviation S.G. at room

temperature

General Purpose Polystyrene GPPS (PS) 1.04 - 1.09

High Impact Polystyrene HIPS 1.14 - 1.20

Acrylonitrile Butadiene Styrene ABS 1.01 - 1.08

Acrylonitrile Styrene AS (SAN) 1.06 - 1.10

Low Density Polyethylene LDPE 0.89 - 0.93

High Density Polyethylene HDPE 0.94 - 0.98

Polypropylene PP 0.85 - 0.92

Plasticized Polyvinyl Chloride (soft) PPVC 1.19 - 1.35

Unplasticized Polyvinyl Chloride (rigid) UPVC 1.38 - 1.41

Polyamide-6 PA-6 1.12 - 1.15

Polyamide-66 PA-66 1.13 - 1.16

Polymethyl Methacrylate PMMA 1.16 - 1.20

Polycarbonate PC 1.20 - 1.22

Polyoxymethylene (Polyacetal) POM 1.41 - 1.43

Polyethylene Terephthalate PET 1.29 - 1.41

Polybutylene Terephthalate PBT 1.30 - 1.38

Cellulose Acetate CA 1.25 - 1.35

Polyphenylene Oxide, modified PPO-M 1.04 - 1.10

Polyphenylene Sulfide PPS 1.28 - 1.32

- 38 -

Some manufacturers prefer to use injection volume as the starting point to state the shot weight of their machines, instead of using measured shot weight.

2.9.1.4 Selecting a machine with sufficient shot weight

Shot weight should not be equal to the combined weight of the article (or articles for a

multi cavity mould) plus runners that could be injection moulded. The latter is set at 85%

of the shot weight for articles with low requirement, 75% of shot weight for articles with

high requirement, e.g. crystal parts. The discrepancy is due the much higher injection pressure when there is a mould. High requirement moulding uses high injection pressure.

Example 3: Figurines made of UPVC (S.G. 1.38) with a combined weight of figurine plus runners of 40 gms. are to be moulded. What size of machine is sufficient?

Shot weight in terms of PS = 40 * 1.05/1.38 = 30.43 gms. Using the 85% guide line, the machine shot weight needed = 30.43/0.85 =35.80 gms.

2.9.2 Selecting a machine which is not too big

An injection moulding machine of a specified shot weight can be used to mould article(s)

including the runners weighing from 35% to 85% of the shot weight. The lower limit

comes from bending on the platens, barrel resident time of the resin and electric power consumption per kg of processed material.

A small article using a small mould puts undue bending on the mould platens, causing them to deflect (which affects product quality), and to break in the extreme.

If a big machine is used to mould small articles, the melt in the barrel could degrade due to unduly long residence time. Barrel residence time could be estimated as follows.

2.9.3 Barrel Residence Time

= (weight of melt in barrel * cycle time) / (actual shot weight)

Weight of melt in the barrel is estimated to be the weight in two times the injection

volume.

Moulding small parts with a big machine is inefficient in energy usage per kg of material processed, also known as specific power consumption.

Example 4: The same figurine in example 3 is to be moulded in a big machine. What is the biggest machine that could be used?

Using the 35% rule, the biggest machine that could be used has a shot weight = 30.43/0.35 = 86.94 gms.

Example 5: What is the residence time of UPVC (S.G. 1.38) in a machine with screw

diameter of 55 mm, injection stroke of 250 mm, shot weight (PS) of 567 g, and a cycle

time of 10 s moulding shots weighing 260 g?

Volume of melt in the barrel is estimated to be two times the injection volume = 2 * 3.1416 * 5.5 * 5.5 * 25 / 4 = 1188 cm3

Barrel residence time = 1188 * 1.38 * 10 / 260 = 63 s

Having multi cavities per mould to increase the articles' weight and to increase the mould

size are solutions to using bigger machines. Alternatively, lowering the barrel temperature would help avoid degradation due to long residence time.

- 39 -

2.9.4 Clamping force

Clamping force is an important attribute of the clamping unit of an Injection Moulding

Machine. It is the maximum force the machine is capable of to keep the mould closed

against the cavity pressure during injection. Insufficient clamping force gives rise to flash

at the mould joint. Most Injection Moulding Machines today use their clamping force (in

tonnes) in their model name.

It is advisable to use a sufficient clamping force below the maximum. The sufficient clamping force is proportional to the projected area of the cavity.

How Much Force is required?

The answer to this question depends on how much injection pressure is required to inject

a specific plastic material into a mould. This is determined by the viscosity (the thickness

value) of the material. Viscosity is a value that must be correctly understood, and an

explanation follows.

Thicker materials require greater injection pressures and are difficult to flow. There are

flow ranges in which each material will fall, and these can be classified as "high flow",

"average flow", and "low flow". The Melt Flow Index test determines the flow rate of any

plastic, and the material suppliers make this information readily available on their

material information sheets. These index numbers may range, for instance, from 5 to 20.

The lower numbers signify that the specific material does not flow easily so would be

classified as low flow. The higher numbers signify a material that flows very easily so would be classified as high flow.

It is not as important to remember a specific flow number as it is to know in what range

a material falls; high flow, average flow, or low flow. Then, understanding that it requires

more injection pressure to inject a low flow material than a high flow material, it is

understood that a low flow material will require much more clamp force to keep the

mould closed against that higher injection pressure.

A comparison of two materials will show the difference. A product moulded of

polycarbonate (a low flow plastic) may require an injection pressure of 15,000 psi, while

that same product moulded of acetal (a high flow plastic) may require only 5,000 psi.

Therefore, the polycarbonate product will require a clamp force on the mould that is

approximately three times (3) that for the acetal product.

The method used for determining the required clamp force is to take the

projected area of the part to be mounded and multiply that number by a factor of from 2 to 8.

2.9.5 Determining Projected Area & Clamp Force

Projected area is calculated by multiplying length times width. The sketch that follows is an example.

The projected area of this part is found by multiplying the length dimension (6.00") times

the width dimension (also 6.00"). The depth dimension ( no dimension) is only important if it is more than 1 inch. This will be explained later.

- 40 -

So, for this particular product, the Projected Area is determined by multiplying 6" times 6". The result is an area of 36 square inches.

Clamp Force requirements can now be calculated by multiplying the 36 square inches

by a factor of between 2 and 8 tons per square inch. The lower numbers can be used for high flow materials and the higher numbers can be used for low flow (stiff) materials.

For this example, polycarbonate has been selected as the material for mouding.

Polycarbonate is fairly stiff and a lower flow material, so the clamp factor used must be

towards the high side. Experience has shown that a clamp factor of 5 tons per square

inch is adequate for polycarbonate. That means that the 36 square inch projected area

found above must be multiplied by the clamp factor of 5 tons per square inch, to result in

a total clamp tonnage requirement of 180 tons (36 x 5 = 180). There should be a safety

factor of 10% added, so the final clamp force needed is 198 tons. The machine with the closest rating for this product would be a 200 ton machine.

It must be noted that these numbers are only correct if there is a shutoff land

surrounding the part. If that land does not exist the clamp tonnage will have to be

doubled or tripled, or more! This may result in mould damage, machine damage, and longer cycle times.

Summarizing, the total clamp force required for a specific product is determined by finding the projected area of that product

[Projected area = length x width]

and multiplying that area by a clamp factor of between 2 and 8. If in doubt, use 5.

[Projected Area x 5 = Clamp Force required]

What about that "D" dimension?

The "D" dimension only becomes important if the plastic part is more than 1 inch deep.

That is not the thickness of the wall, but the total depth of the part. For every inch of

depth over 1 inch the total clamp force must be increased by 10%. So, if the part shown

above was 2 inches deep the clamp force would be increased by 18 tons (10% increase

for every inch over 1") to a total of 198. Add 10% for safety factor and the required force

increases to 217.8 tons. The nearest machine size to that requirement would probably be

a 225 ton machine.

The clamping force needed could be estimated in several ways.

The conservative method is to multiply the projected cavity area by a constant which is

different for each material. For example, for GPPS, the constant is 1.0 to 2.0 tonnes/in2

for thick wall articles, 3.0 to 4.0 tonnes/in2 for thin wall articles. 1.0 tonne/in2 = 0.155 tonne/cm2 = 15.4 MN/m2.

Table 2 lists the constants for commonly used resins.

Example 6: A GPPS cup of diameter 79 mm is to be moulded. The cup is 0.6 mm at its

thinnest section. Find a conservative clamping force which would be sufficient.

The projected area of the cup (and runner) is 3.1416 * 7.92 / 4 = 49 cm2. This cup

belongs to the thin wall domain. The conservative clamping force is 0.62 * 49 = 30.4 tonnes.

A more accurate method takes into account the flow path length and wall thickness. Flow

path is the length traveled by the resin from the sprue gate to the furthest point in the

mould cavity. This is also known as L/T Ratio.

- 41 -

See Figure 1. If the wall thickness of a part varies, take its minimum wall thickness.

Example 7: The same GPPS cup has a flow path length of 104 mm. Find a more accurate clamping force needed.

Flow path to thickness ratio (L/T Ratio) = 104 / 0.6 = 173. From Figure 2, at 0.6 mm wall thickness, the cavity pressure is 550 bar.

1 bar = 1.02 kg/cm2. The clamping force = 550 * 1.02 * 49 = 27,500 kg = 27.5 tonnes.

The above calculation has not accounted for viscosity. It turns out to be still correct as

the viscosity factor for GPPS is 1.0. The viscosity factor for common resins is listed in Table 3.

Example 8: The same cup as in the above example is to be made out of ABS. Find the

clamping force needed.

Using the viscosity factor of 1.5, the clamping force needed = 1.5 * 27.5 tonnes = 41.3 tonnes.

Table 2. Simple clamping force estimation

Resin tonnes/in2 tonnes/cm2 MN/m2

PS (GPPS) 1.0 - 2.0 0.155 - 0.31 15.4 - 30.9

PS (GPPS) (thin walls) 3.0 - 4.0 0.465 - 0.62 46.3 - 61.8

HIPS 1.0 - 2.0 0.155 - 0.31 15.4 - 30.9

HIPS (thin walls) 2.5 - 3.5 0.388 - 0.543 38.6 - 54.0

ABS 2.5 - 4.0 0.388 - 0.62 38.6 - 61.8

AS (SAN) 2.5 - 3.0 0.388 - 0.465 38.6 - 46.3

AS (SAN) (long flows) 3.0 - 4.0 0.465 - 0.62 46.3 - 61.8

LDPE 1.0 - 2.0 0.155 - 0.31 15.4 - 30.9

HDPE 1.5 - 2.5 0.233 - 0.388 23.2 - 38.6

HDPE (long flows) 2.5 - 3.5 0.388 - 0.543 38.6 - 54.0

PP (Homo/Copolymer) 1.5 - 2.5 0.233 - 0.388 23.3 - 38.6

PP (H/Co) (long flows) 2.5 - 3.5 0.388 - 0.543 38.6 - 54.0

PPVC 1.5 - 2.5 0.233 - 0.388 23.3 - 38.6

UPVC 2.0 - 3.0 0.31 - 0.465 30.9 - 46.3

PA6, PA66 4.0 - 5.0 0.62 - 0.775 61.8 - 77.2

PMMA 2.0 - 4.0 0.31 - 0.62 30.9 - 61.8

PC 3.0 - 5.0 0.465 - 0.775 46.3 - 77.2

POM (Homo/Copolymer) 3.0 - 5.0 0.465 - 0.775 46.3 - 77.2

PET (Amorphous) 2.0 - 2.5 0.31 - 0.388 30.9 - 38.6

PET (Crystalline) 4.0 - 6.0 0.62 - 0.93 61.8 - 92.6

PBT 3.0 - 4.0 0.465 - 0.62 46.3 - 61.8

CA 1.0 - 2.0 0.155 - 0.31 15.4 - 30.9

PPO-M (unreinforced) 2.0 - 3.0 0.31 - 0.465 30.9 - 46.3

PPO-M (reinforced) 4.0 - 5.0 0.62 - 0.775 61.8 - 77.2

PPS 2.0 - 3.0 0.31 - 0.465 30.9 - 46.3

- 42 -

Figure 1. Flow path length is measured from tip of sprue to an extremity of the article

Figure 2. Cavity pressure as a function of wall thickness and flow path length

Thermoplastics Viscosity

factor

GPPS (PS) 1

PP 1 - 1.2

PE 1 - 1.3

Nylons (PA6 or PA66),

POM

1.2 - 1.4

Cellulosics 1.3 - 1.5

ABS, ASA, SAN 1.3 - 1.5

PMMA 1.5 - 1.7

PC, PES, PSU 1.7 - 2.0

PVC 2

Table 3. Viscosity factor


As stated in an Injection Moulding Machine specification, injection pressure means the

maximum pressure in the barrel during injection, not the maximum hydraulic pressure.

The two are related by the ratio of the screw cross section area to the injection cylinders

area. Usually, injection pressure is higher than the maximum hydraulic pressure by about

10 times. Where there is a choice of screws for a given injection unit, the smaller

diameter screw produces the higher injection pressure. A high injection pressure helps in

moulding engineering thermoplastics. Material manufacturers publish minimum and maximum injection pressures in the specification of the materials.

Maximum Injection Pressure The injection pressure is required to overcome the

resistance to flow of melt in the mould. It depends on melt viscosity, flow ratio and

mould temperature. Set pressure is higher than actual pressure during filling phase. The

- 43 -

relief valve is actuated on reaching set pressure during pressure phase. Higher injection

pressure is required for processing high viscosity materials like PC, RPVC, TPU etc. At

present new generation machines are capable of giving maximum injection pressure of 2200 bar or even 2500 bar.

Injection units are normally offered with the option of three screws (A, B & C) of different

diameter.

A screw (lower dia) gives maximum injection pressure of around 1800 - 2400 bar.

- is recommended for moulding of Engineering parts of higher flow ratio with high viscous polymer & Commodity items of low flow ratio with commodity polymers.

B screw (medium dia) gives maximum injection pressure of around 1500 - 1800 bar with max. shot weight/volume higher than that of A.

- is recommended for moulding of Engineering Parts Of Lower Flow Ratio With High

Viscosity Polymer & Commodity Items Of Medium Flow Ratio With Medium Wall

Thickness In Commodity Polymer.

C screw (higher dia) gives maximum injection pressure of around 1300 - 1500 bar

with max. shot weight/volume higher than that of B.

- is recommended for moulding of Commodity Items Of Medium / High Flow Ratio With Thin / Thick Wall Thickness From Commodity Plastics.

It should be noted that the maximum flow ratio of the part to be moulded should be

lower than the maximum possible flow ratio of the polymer at the maximum injection

pressure of the injection unit. Refer the table for maximum flow ratio of various polymer.

2.9.7 Injection volume (Swept Volume)

Injection volume is theoretical. It equals the cross section area of the screw multiply with

the injection stroke.

Injection volume (cm3) = 3.1416 * (d2 / 4) * i

where d = diameter of screw , in cm

(~= diameter of barrel)

i = injection stroke, in cm

Due to leakage pass the screw tip and the backward movement of the non-return valve,

the actual injection volume is about 90% of the theoretical injection volume. To convert

the actual injection volume to shot weight, the resin S.G. at plasticizing temperature is used. See Table 4.

Resin Abbreviation S.G. at plasticizing

temperature

General Purpose Polystyrene GPPS (PS) 0.886 - 0.901

High Impact Polystyrene HIPS 0.895 - 0.917

Acrylonitrile Butadiene Styrene ABS 0.895 - 0.908

Acrylonitrile Styrene AS (SAN) 0.907 - 0.917

Low Density Polyethylene LDPE 0.730 - 0.740

High Density Polyethylene HDPE 0.752 - 0.772

Polypropylene PP 0.712 - 0.737

Plasticized Polyvinyl Chloride (soft) PPVC 1.050 - 1.389

Unplasticized Polyvinyl Chloride (rigid) UPVC 1.134 - 1.219

Polyamide-6 PA-6 0.958 - 0.995

Polyamide-66 PA-66 0.958 - 0.995

- 44 -

Polymethyl Methacrylate PMMA 0.996 - 1.012

Polycarbonate PC 1.018 - 1.037

Polyoxymethylene (Polyacetal) POM 1.187 - 1.214

Polyethylene Terephthalate PET 1.129 - 1.172

Polybutylene Terephthalate PBT 1.102 - 1.113

Cellulose Acetate CA 1.074 - 1.104

Polyphenylene Oxide, modified PPO-M 0.873 - 0.890

Polyphenylene Sulfide PPS 1.075 - 1.109

Table 4. Specific gravity of resins at plasticizing temperature

Instead of using shot weight and the 35% to 85% rule in selecting an Injection Moulding Machine, some manufacturers recommend using injection volume and the following rule.

For low requirement moulding, use between 20% to 80% of the injection unit injection volume. For high requirement, use between 40% to 60%.

2.9.8 Injection speed

As stated in an Injection Moulding Machine specification, injection speed is the maximum

speed of the screw the machine is capable of during injection. It is expressed in cm/s.

Injection speed affects the injection time. Moulding thin-walled articles requires high

injection speed so that the melt does not solidify before the cavity is completely filled.

Through controlling hydraulic oil flow, some machines have multiple injection speeds

available during injection. The constant melt front theory stipulates the best moulding

occurs when the leading edge of the melt (the melt front) moves in the cavity at constant

speed. Since the mould cavity varies in cross sectional area, this requires multiple injection speeds during injection. Some machines have as many as ten.

Maximum injection speed

Modern injection moulding machines are equipped with variable delivery or multiple

pumps which are capable of delivering sufficient oil to injection hydraulic cylinder to give high enough injection speed for filling mould cavities rapidly.

Further, higher injection speed can be achieved by the use of hydraulic accumulator

(normally offered as optional). Higher injection speed can push the melt to furthest part

of mould at shorter time before the freezing of melt (increase in melt viscosity) on account of lower mould temperature.

During filling phase, injection speed is required to be under control with out disturbing

the pressure setting. Therefore, set pressure has to be higher than actual pressure so

that the relief valve is not actuated. On actuation of relief valve, there would be no

control on injection speed. Thin walled commodity plastics items can be moulded with high injection speed.

Shear sensitive engineering polymers are likely to be overheated on account of excessive

shearing due to high melt velocity. Hence, injection speed will have to be limited to suit

the polymer characteristics. In modern machine injection speed can be set in multi stages - stroke controlled (up to 10 stages).

2.9.9 Accumulator

Some Injection Moulding Machines have an accumulator as an option to boost injection

speed. An accumulator is an energy storing device that stores up pressurized hydraulic

oil in a phase of low demand to be used in the injection (high demand) phase. It evens

out the load on the electric motor and reduces its overloading. While increasing the

electric motor and hydraulic pump sizes (available as an alternative by some

- 45 -

manufacturers) does increase injection speed by about 25%, an accumulator does so with about three times increase.

Figure 3. Accumulator

2.9.10 Injection rate

As an alternative to injection speed, some Injection Moulding Machine specifications use

injection rate. Injection rate is the maximum volume swept out by the screw per second during injection. It is expressed in cm3/s.

Injection rate = injection speed * 3.1416 * (d/2)2,

where d = screw diameter in cm.

Note that injection speed is independent of screw diameter, but injection rate is.

2.9.11 Screw rotary speed

Screw rotary speed is specified as a range in rpm. Screw rotary speed by itself is not as

critical as screw surface speed. The two are related by the screw diameter.

Screw surface speed (mm/s)

= 3.1416 * screw diameter (mm) * screw rotary speed (rpm) / 60

Each plastic material has a recommended maximum screw surface speed which must not

be exceeded. For example, UPVC should not experience a screw surface speed of higher than 200 mm/s.

Abbreviation Optimum surface

speed (mm/s)

Maximum surface

speed (mm/s)

GPPS (PS) 800 950

HIPS 850 900

ABS 550 650

AS (SAN) 400 450

LDPE 700 750

HDPE 750 800

PP 750 850

PPVC 150 200

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UPVC 150 200

PA-6 400 500

PA-66 400 500

PMMA 350 400

PC 400 500

POM (Copolymer) 200 500

POM (Homopolymer) 100 300

PET 300 400

PBT 300 350

CA 400 500

PPO-M 400 500

PPS 200 300

Table 5. Optimum and maximum surface speed of resins

Example 9: What is the maximum rpm for a 60 mm diameter screw injecting UPVC? Maximum rpm = 60 * 200 / (3.1416 * 60) = 64.

2.9.12 Screw Motor Torque

The hydraulic motor that turns the screw has a rated torque, expressed in Newton-meter

(Nm) in SI unit. It represents the maximum amount of turning moment the motor can

produce at the specified hydraulic pressure. A viscous material needs a high torque and a

low rotary speed, vice versa for an easy-flowing material.

A higher torque is needed for screw C (large diameter) than screw A (small diameter).

The proportional pressure valve is used to adjust the motor torque to the needed value during feeding.

2.9.13 Plasticizing Capacity

Plasticizing capacity is the amount of PS that an Injection Moulding Machine can

uniformly plasticize, or raise to a uniform moulding temperature, in one hour at

maximum screw rotary speed and zero back pressure. Since it is rated in PS, an

amorphous material, a higher plasticizing capacity is needed for semi-crystalline

materials. Although the barrel heaters also contribute to melt the plastic, their capacities

are not counted in plasticizing capacity.

To check if the plasticizing capacity of an Injection Moulding Machine is not being

exceeded, calculate the weight of component and sprue per shot W (g) divided by screw rotation time t (s), and convert the quotient to kg/hour:

W * 3600/(t * 1000).

This must be less than the plasticizing capacity of the machine.

Since cycle time is longer than screw rotation time, the shot weight S (g) of a machine and its plasticizing capacity G (kg/hr) set a lower limit on cycle time Tmin (s) as follows.

Tmin = S * 3600/(G * 1000).

It is particularly important to match shot weight and plasticizing capacity in the case of fast cycling machines producing thin walled or closed tolerance components.

Plasticizing capacity could be increased by a larger electric motor and hydraulic pump.

The next five attributes relates to the dimensions of the mould the machine could accommodate. They indirectly relate to the maximum dimension of the moulded part.

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2.9.14 Mould Opening Stroke

Mould opening stroke is the displacement of the moving platen from mould close to

mould open. Mould opening stroke determines the maximum height H of the moulded part the machine is capable of. The relationship is

Mould Opening Stroke >= 2H + Sprue Length L

In a hot runner system, L = 0.

The inequality allows for a clearance for gravity, the robot arm or human hand to remove the part.

Figure 4. Mould opening stroke

2.9.15 Mould Height (Mould Thickness)

Mould height is left over from the days when presses are vertical. In a horizontal press, a

more appropriate description is mould thickness.

Figure 5. Mould height, width and length

In a toggle clamp Injection Moulding Machine specification, mould height is expressed as

a range, from the minimum to the maximum mould height the machine could accommodate. The difference is the mould height adjustment the machine is capable of.

In a direct hydraulic clamp Injection Moulding Machine specification, mould height is

expressed as a number, the minimum mould height the machine could accommodate.

The actual mould height must be bigger than the machine minimum mould height for the

mould to be closed and clamped. Otherwise, a smaller machine (to be exact, a smaller clamping unit) is called for.

The actual mould height must be less than the machine maximum mould height for the mould to fit in. Otherwise, a bigger machine is called for.

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Figure 6. Mould height

2.9.16 Minimum Mould Height

With hydraulic clamp machine there can be only one specification of minimum mould

height. However, with toggle clamp machine, the shut mould height has to be between

minimum mould height and maximum mould height of machine.

If the shut mould height is lower than the minimum mould height of the machine then

the additional back plate of suitable thickness should be provided on the mould to

build up the shut mould height equal to more than the minimum mould height of the

machine.

2.9.17 Maximum Daylight

The maximum opening between the fixed and moving platens when the clamp is wide

open. It is related to mould opening stroke and minimum/maximum mould height as

follows.

For a Toggle Clamp Machine,

Maximum Daylight = Mould Opening Stroke + Maximum Mould Height.

For a direct hydraulic clamp machine,

Maximum Daylight = Mould Opening Stroke + Minimum Mould Height.

2.9.18 Space between Tie bars

The mould must fit within the space between tie bars. This space is expressed in horizontal and vertical dimensions.

Refer to Figures 5 and 7. The mould width must fit within the horizontal space between

tie bars if the mould is lowered from above. The mould length must fit within the vertical

space between tie bars if the mould is slit in from the side. It is advised that there is a

clearance of 25 mm on each side for a small mould, and 50 mm for a big mould. This is

to avoid banging of the heavy mould against the tie bars during loading, denting them and subsequently affecting the bearing in the moving platen which travels over them.

Figure 7. Space between tie bars

Tie bar less Injection Moulding Machines do not have this restriction.

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2.9.19 Platen size

The platens are thick steel plates to back up the moulds with. It is advisable that the

moulds do not protrude beyond the platen limits to avoid bending the moulds during

injection. Too small a mould would put undue bending stress on the platens, breaking

them in the extreme case. Some manufacturers offer a choice of platen sizes for machine

of a given clamping force. A car bumper is an example where a very wide platen is needed.

2.9.20 Platen thickness

The moving platen and fixed platen must have sufficient stiffness to transmit the forces

of the tie bars to the mould with minimum deflection. For a given geometry, a flat

platen’s deflection is proportional to the cube of its thickness. Especially for the moving

platen, a compromise has to be struck between weight and thickness.

Space between tie bars is related to platen size. If this space is increased without

increasing the platen thickness, the platen under the same load deflects more. In short,

one must not consider space between tie bars alone, but must consider it together with platen stiffness.

Platen deflection causes the mould to deflect which in turn changes the shape and

dimensions of the moulded article.

Figure 8. Platen deflection is affected by platen thickness and size

Some machine makers put ribs on a platen to increase its stiffness while minimizing its

weight. Since there is no standard rib patterns, comparison of platen stiffness across

manufactures is not easy.

Figure 9 Ribbed stationary platen

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2.9.21 Tie bar diameter

Most Injection Moulding Machines with tie bars have four of them, except small machines

below about 20 tonnes, which have two. Together, their tension forces hold the mould halves together against cavity pressure during injection.

If the tie bar tensions are even, the stress in each of them is given by

stress = clamping force * 1000/(3.1416 * (d2/4) * 4)

= clamping force * 1000/(3.1416 * d2),

where stress is in kg/mm2

clamping force is in tonnes,

diameter d in mm.

High tensile steel has a breaking stress of more than 90kg/mm2. Mild steel has a

breaking stress of 20kg/mm2. A tiebar breaks if its stress exceeds the breaking stress.

More often then not, a tiebar breakage is due to uneven tensions among them. This is

caused by

a. non-parallel mould faces,

b. non-symmetrical cavity with respect to the sprue,

c. misadjustment of the mould height adjustment

mechanism of a toggle clamp machine.

When the mould expands due to higher temperature, it stretches the tiebars more than

when the mould was set up when it was at room temperature.

Example 10: In a 125 Tonne Machine has four tie bars, each with diameter 75 mm. The

clamping force is 125 tonnes. High tensile steel is used. What is the safety factor built into tie bars of this machine?

Assuming even tension, each tie bar has stress

= 125 * 1000/(3.1416 * 752) = 7.07 kg/mm2.

The safety factor is 90/7.07 = 12.7.

Usually a safety factor of 10 or more is common in an industrial design. An example is

the stress in the cables hauling a fully loaded lift up and down. Tie bar breakage occurs at the root of a thread where the radius is smaller and there is stress concentration.

2.9.22 Dry Cycle Time

Dry cycle time is the mould closing time plus mould opening time plus idle time. Dry

cycle time is the ultimate cycle time as there is no cooling period. An alternative expression is cycle rate, the number of cycles per minute.

Running a machine at the maximum possible cycle rate is not desirable if the machine is

not running smooth and stable. This is another example why an attribute should not be evaluated by itself alone

2.9.23 Electric Motor Rating

The hydraulic system is driven by an electric motor. It converts electrical energy to

mechanical energy at certain efficiency. An electrical motor is rated in terms of kW or hp

which denotes its maximum power delivery under the specified conditions like

temperature of its windings. Some manufacturers offer a bigger pump size as an alternative. The motor size is also increased.

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It is important not to confuse the power rating of the electric motor to energy efficiency.

A lower power does not by itself mean an Injection Moulding Machine is more energy

efficient than another with a higher rating. It means it is overloaded more during the

moulding cycle. A three-phase motor is about 90% efficient over a wide range of power

rating.

The moulding cycle demands widely varying hydraulic power in its different phases. At

the electric motor, this translates to a similar demand in electrical power. Usually, the

injection phase is the most demanding phase of the cycle. An electric motor is rated at below that power, requiring it to run above its rating in the injection phase.

For an Injection Moulding Machine without an accumulator, the injection phase presents

an overload to the electric motor. Most motors could be overloaded to two times its rated

torque for short periods. Since a three phase motor runs at a relatively constant speed,

even at overload, the extra power comes from increased torque. Because power = rotary

speed * torque, the extra power comes from increased torque. Since motor current is

proportional to torque, an overloaded motor heats up (proportional to the square of

current) more than it is rated at, reducing its long-term reliability. A motor with a higher power is overloaded less.

The story is different if the Injection Moulding Machine has an accumulator which does

allow the electric motor to have a lower rating. Hydraulic energy is stored into the

accumulator in phases of low demand to be used in the injection phase. In short, it evens out the motor loading during the cycle and reduces its overloading.

A motor with a high rating does not use up more energy. How much energy is used

depends on the load (the work to be done) which in turn depends on the electric drive,

hydraulic drive and hydraulic circuit design.

The current per phase drawn by a three phase motor at its rated power is im (A)

= motor power rating (kW)*1000 / (3*single phase power voltage(V) *efficiency*power

factor)

= motor power rating (hp)*746 / (3*single phase power voltage(V) *efficiency*power factor)

For most three phase motors,

efficiency = 0.88 - 0.91,

power factor = 0.84 - 0.88.

Example 11: An Injection Moulding Machine is driven by a 30 hp three phase motor. Find the current per phase it draws when the single phase power voltage is 220 V.

Assume an efficiency of 0.91 and a power factor of 0.88. The current drawn per phase at

the rated power of 30 hp is im = 30*746/(3*220*0.91*0.88) = 42.3 A.

Figure 10 Power demand during the moulding cycle

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2.9.24 Electric Heater Rating

Electric band heaters along the barrel provides the initial heat up to the resin at start up.

It also supplements the heating by plastication (when the screw rotates) during the

moulding cycle. A higher rating per heater has the advantage of shortening the initial heat up time.

Usually, there are one to two band heaters per heating zone. As much as possible, the

heaters are evenly distributed among the three phases.

The maximum current drawn by the band heaters is

ih (A) = electric heater rating (kW)*1000 / (3*single phase voltage(V)).

Example 12: An Injection Moulding Machine has 6 band heaters each rated at 1.2 kW.

The 6 heaters are distributed 2 to a phase in the three phase electrical system. Find the maximum current per phase it draws when the single phase power voltage is 220 V.

ih = 6*1.2*1000 / (3*220) = 10.9 A.

2.9.25 Total Power

This equals the electric motor rating plus the electric heater rating. It is for planning the

current in the electric power connection. However, motor overloading is not accounted for in total power as the motor rating is used.

it = im + ih.

Example 13: What is the total current per phase needed when installing An Injection

Moulding Machine it = 42.3 + 10.9 = 53.2 A.

2.9.26 Number of Heating Zones

The number of heating zones is defined by the number of thermocouples installed on the

barrel. If discrete temperature controllers are used, it is the same as the number of

temperature controllers. Usually, a temperature controller controls one to two electric

band heaters.

More heating zones provide better control of temperature along the barrel length. Since a

bigger machine has a longer barrel, it also has more heating zones.

2.9.27 Oil Tank Capacity

Oil tank capacity has significance in cooling and number of barrels of oil to purchase.

More oil in a bigger tank reduces the temperature of the oil since the heat generated is spread out more. Furthermore, a bigger tank has a bigger cooling surface.

Hydraulic oil comes in 200 litre barrels. An oil tank of 220 litre capacity requires the user to purchase two barrels.

2.9.28 Hopper Capacity

Once a hopper is filled to capacity, for how long could it be left alone before refilling? A

bigger hopper capacity requires less attention by the operator.

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However, when moulding hygroscopic resin, a hopper must not be filled for the resin to

remain in the hopper for more than an hour. The weight of resin (kg) to be fed into the hopper should be less than

actual shot weight (g) * 3600 / (cycle time (s) * 1000).

Example 13: The weights of each component and the runners are 14g and 12g

respectively. The machine is producing 6 components per cycle with a cycle time of 24s.

How much should a hopper be filled so that the resin does not stay in the hopper for more than an hour?

The required weight = (6 * 14 + 12) * 3600 / (24 * 1000) = 14.4kg

Since plastic materials comes in 25-kg bags, half a bag would satisfy the requirement.

2.9.29 System pressure

The most common hydraulic system pressure used in An Injection Moulding Machine is

140 bars, which approximately equals to 140 kg/cm2. This is limited by the vane pump.

By its very design, vane pump has unbalanced pressure within, which limits it from reaching a higher pressure.

A higher system pressure of 170 bars or even 200 bars are used with piston pump, which

demands cleaner hydraulic oil to work with. At a high system pressure, either cylinder

diameter could be reduced to get the same force or higher force could be obtained from the same cylinder diameter. With a higher force, response to the control signals is faster.

2.9.30 Screw and Barrel

Nitrided screw and barrel

To protect the screw and barrel from wear and corrosion by the melt, especially acidic

plastic materials like PVC and acetate, nitride treatment of the screw and barrel is common. Nitriding hardens the screw and barrel surface.

Bimetallic screw and barrel

Glass fiber is getting popular as a material mixed with other resins. It is very abrasive.

Bimetallic screw and barrel are used in this case. For the barrel, an inner tube of

tungsten carbide (Xaloy 800) is used. For the screw, Colmony is sprayed onto the flight

and tungsten carbide onto the land to protect the metal below from abrasion. Naturally,

the non-return valve needs similar protection against abrasion. Bimetallic screw and

barrel is about 3 times more expensive than nitrided screw and barrel.

Figure 11. Bimetallic screw

Honed and chrome plated tie bars made of tensile steel

- 54 -

The moving platen slides on the tie bars back and forth every cycle. Having a honed and chrome plated surface reduces wear.

Held by the nuts at both ends of each tie bar, the tie bars provide the tensile force to the

clamping cylinder to hold the mould halves together. Tie bars made of high tensile steel could provide the tons of force needed. It has a higher breaking stress than mild steel.

2.9.31 Cold start interlock

When starting up the heaters and before the barrel temperature reaches the set

temperature, it is important that the screw does not turn to feed and to plasticize as the

screw and barrel could be damaged by the pellets. Many machines have such an interlock to prevent the screw from turning before the set temperature is reached.

2.9.32 Low pressure mould protection

A part moulded in the previous cycle that has not been properly ejected could damage

the mould when it closes again. Low pressure mould protection closes the mould at low

pressure. Opposed by the jammed article, the mould mould not close completely in the

preset time. This function would stop the closing and sound an alarm. It is not designed to protect the human body part, which is done by the interlocks at the safety gates.

2.9.33 Nozzle type

Simple nozzle, spring shut-off nozzle and hydraulic shut-off nozzle are the common

types. Simple nozzle is suited to plastic materials that degenerates with heat, e.g. PVC.

Being simple, it does not have stagnation points to accumulate stale plastic.

Spring shut-off nozzle is suited to plastics with low viscosity, e.g. nylon. The spring

action closes the nozzle during feeding. Springs tends to lose its elasticity over time when strained at high temperature.

Hydraulic shut-off valve provides a positive shut-off through actuating a hydraulic cylinder.

2.9.34 Hydraulic oil temperature control

Hydraulic oil must be maintained at between 40 and 50oC. This is done by control of the

cooling water flow.

Too high an oil temperature reduces the oil viscosity, and ages the rubber sealing rings

faster. For consistent product quality and to improve the Injection Moulding Machine's

reliability, it is worth investing in the closed loop temperature control of hydraulic oil, if it is available as an option.

2.9.35 Hydraulic oil contamination control

Contamination and metal filings from cylinder/piston wear degrade the hydraulic oil.

Hydraulic oil is filtered at the pump inlet and optionally filtered on return. A differential

pressure sensor across the filter raises an alarm when the oil is too contaminated and

must be replaced. Alternatively, an optical device immersed in the oil detects how dirty the oil is.

2.9.36 Safety features

The safety gate protects the human operator from mould closing. Once the safety gate is

opened, a mechanical stop is lowered and/or electrical and/or hydraulic circuits are

broken to prevent the mould from closing. The more methods of interlocking the safer is

- 55 -

the machine. Some manufactures only provide mechanical and/or hydraulic locks as options.

Some machines provide the same safety features at the front as well as the back safety gates.

2.9.37 Metal detector option

When a resin is recycled, it may be contaminated with pieces of metal. A magnetic

grating in the hopper prevents ferromagnetic metals from entering the barrel. Even

better, a metal detector signals even when non-ferromagnetic metal passes through the hopper. A pump then removes the contaminated pellets.

- 56 -

2.10. Melt Behavior – Plastic melt behavior in side the barrel and mould

2.10.0 How does plastic flow?

2.10.1 Material behaviour

2.10.2 Visco-elastic behavior

2.10.3 Melt shear viscosity

2.10.3.1 Newtonian fluid vs. non-Newtonian fluid

2.10.3.2 Shear-thinning behaviour

2.10.3.4 Shear rate distribution

2.10.3.5 Effects of temperature and pressure


2.10.4.1 Pressure drives the melt

2.10.4.2 Circular channel flow

2.10.4.3 Strip channel flow

2.10.4.4 Factors that influence injection pressure requirements

2.10.5 Pressure-driven flow

2.10.5.1 Pressure gradient and melt speed

2.10.6 Flow conductance

2.10.7 Melt flow length

2.10.8 Injection pressure vs. fill time

2.10.9 Flow instability

2.10.0 How does plastic flow?

2.10.1 Material behaviour

Molten thermoplastic exhibits viscoelastic behaviour, which combines flow characteristics

of both viscous liquids and elastic solids. When a viscous liquid flows, the energy that

causes the deformation is dissipated and becomes viscous heat. On the other hand, when

an elastic solid is deformed, the driving energy is stored. For example, the flow of water

is a typical viscous flow, whereas the deformation of a rubber cube falls into the elastic

category.

Deformation

In addition to the two types of material flow behavior, there are two types of

deformation: simple shear and simple extension (elongation), as shown in (a) and (b)

below. The flow of molten thermoplastics during injection moulding filling is

predominantly shear flow, as shown in (c), in which layers of material elements "slide"

over each other. The extensional flow, however, becomes significant as the material

elements undergo elongation when the melt passes areas of abrupt dimensional change

(e.g., a gate region), as shown in (d).

FIGURE 1. (a) Simple shear flow. (b)

Simple extensional flow. (c) Shear flow

in cavity filling. (d) Extensional flow in

cavity filling.

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2.10.2 Visco-elastic behavior

In response to an applied stress (force per unit area), molten thermoplastics exhibit

viscoelastic behavior, which combines characteristics of an ideal viscous liquid with those

of an ideal elastic solid. In other words, under certain conditions, molten thermoplastics

behave like a liquid, and will continuously deform while shear stress is applied, as shown

below. Upon the removal of the stress, however, the materials behave somewhat like an

elastic solid with partial recovery of the deformation, as shown in (b) and (c). This

viscoelastic behavior stems from the random-coil configuration of polymer molecules in

the molten state, which allows the movement and slippage of molecular chains under the

influence of an applied load. However, the entanglement of the polymer molecular chains

also makes the system behave like an elastic solid upon the application and removal of

the external load. Namely, on removal of the stress, chains will tend to return to the

equilibrium random-coil state and thus will be a component of stress recovery. The recovery is not instantaneous because of the entanglements still present in the system.

FIGURE 2. (a) Ideal viscous liquid deforms continuously under applied stress. (b) Ideal

elastic solid deforms immediately upon the application of stress, but fully recovers when

the stress is removed. (c) Molten thermoplastic deforms continuously under the applied

stress (like a viscous liquid), but it also recovers partially from the deformation upon

removal of the applied stress (like an elastic solid).

2.10.3 Melt shear viscosity

What is shear viscosity?

Melt shear viscosity is a material's resistance to shear flow. In general, polymer melts are

highly viscous due to their long molecular chain structure. The viscosity of polymer melt

ranges from 2 to 3,000 Pas (water 10-1, glass 1020). Viscosity can be thought of as the

thickness of a fluid, or how much it resists flow. Viscosity is expressed as the ratio of

shear stress (force per unit area) to the shear rate (rate change of shear strain), as

shown in the equation and diagram below:

where

FIGURE 3. The definition of polymer melt viscosity, illustrated by a simple shear flow

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2.10.3.1 Newtonian fluid vs. non-Newtonian fluid

For Newtonian fluids, viscosity is a temperature-dependent constant, regardless of the

shear rate. A typical example of Newtonian fluid is water. However, for non-Newtonian

fluids, which include most polymer melts, the viscosity varies, not only with temperature, but with the shear rate.

2.10.3.2 Shear-thinning behavior

When the polymer is deformed, there will be some disentanglement, slippage of chains

over each other, and molecular alignment in the direction of the applied stress. As a

result, the resistance exhibited by polymer to flow decreases with the deformation, due

to the evolution of its microstructure (which tends to align in the flow direction). This is

often referred to as shear-thinning behavior, which translates to lower viscosity with a

high shear rate. Shear-thinning behavior provides some benefits for processing the

polymer melt. For example, if you double the applied pressure to move water in an open-

ended pipe, the flow rate of the water also doubles, since the water does not have shear-

thinning behavior. But in a similar situation using a polymer melt, if the pressure is

doubled, the melt flow rate may increase from 2 to 15 times, depending on the material.

2.10.3.4 Shear rate distribution

Having introduced the concept of shear viscosity, let us look at the shear rate distribution

in the cavity during injection moulding. Generally speaking, the faster the adjacent material elements move over each other, the higher the shear rate is.

FIGURE 4. (a) A typical velocity profile with relative flow element movement and (b) the

corresponding shear rate distribution in injection moulding filling.

Therefore, for a typical melt flow velocity profile, shown in (a), it is clear that the shear

rate is highest at the mould-melt interface (or at the melt-solid interface if there is a

frozen polymer layer). On the other hand, the shear rate approaches zero at the center

line because there is no relative material element movement due to flow symmetry, as

shown in Figure 4 (b). Shear rate is an important flow parameter since it influences the

melt viscosity and the amount of shear (viscous) heating. The typical shear rate

experienced by the polymer melt during the injection moulding process ranges from 102 to 105 second-1.

2.10.3.5 Effects of temperature and pressure

Since the mobility of polymer molecular chains decreases with decreasing temperature,

the flow resistance of polymer melt also greatly depends on the temperature. As shown

in Figure 5, the melt viscosity decreases with increasing shear rate and temperature due

to the disentanglement and alignment of the molecules and enhanced mobility of

polymer molecules, respectively. In addition, the melt viscosity also depends on the

pressure. The higher the pressure, the more viscous the melt becomes.

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FIGURE 5. The viscosity of polymer melt

depends on the shear rate, pressure, and temperature.

Rheological material properties contains a mathematical description of the shear viscosity

as a function of shear rate, temperature, and pressure. For a discussion on how high pressure increases the level of viscosity, see Pressure dependence of viscosity.

Pressure-Volume-Temperature (PVT) Behaviour

PVT behavior refers to the change in specific volume with temperature and pressure

changes.

The specific volume is defined as volume per unit mass. The specific volume, v of a

polymer changes with variations in temperature and pressure.

Volumetric expansion data for polymeric materials are obtained under equilibrium. Such

data represent fundamental thermodynamic properties of the material and reflect the transitions as the material moves from glassy to crystalline to melt state.

In the figure, as the pressure and the temperature change from P and T to P ' and T', the

volume of the same mass m changes from V to V'.

P V T data can be measured using standard equipment. The specimen is heated in an

enclosed cell and the change in its volume is measured when it is subjected to a range of pressures.

Specimens may be either in the form of polymer pellets or they may be cut from the moulded plaques.

PVT behavior of materials plays a critical role in relating processing to the final part

performance.

- 60 -

The shrinkage of a moulded plastic part can be as much as twenty percent by volume

when measured between the processing temperature and the ambient temperature.

Semi-crystalline polymers have higher shrinkage than amorphous polymers because of

the ordering and folding of chains in a semi crystalline polymer below its freezing point.

This leads to a greater difference in specific volume ( v) between the melt phase and

the solid phase for semi-crystalline materials. The presence of fillers like talc or short

glass fibers reduces the difference in specific volume ( v) between the melt phase and the solid phase.

Viscosity

Viscosity behavior of materials is important in determining the flow length and the amount of viscous heating generated during the melt flow.

Most polymer melts exhibit shear-thinning behavior, which translates to lower, viscosity

with higher shear rate. Hence the viscosity of the melt varies across the thickness of the

part due to the variation in shear rate.

Melt viscosity decreases with temperature but the sensitivity varies among

thermoplastics. For example, the viscosity of polystyrene and polypropylene are

considerably more sensitive to temperature than that of polyethylene. At pressures of

several thousand p s i the viscosity increases with pressure. The presence of fillers

increases melt viscosity.

The viscosity is critical for determining the injection pressure with a given rate or the flow length with a given maximum pressure.

The viscosity of a polymer melt is adequately described by the Cross-model. This model treats viscosity as a function of temperature (T), pressure (P), and shear rate ( ).

- 61 -

This model handles both the Newtonian and the shear thinning flow regimes. The transition between the two regimes is characterized by t*.

t* is the shear stress at which shear thinning behavior begins to manifest itself. The

slope of the shear-thinning region can be characterized in terms of a shear thinning index, n.

This model is often adopted for simulating the filling stage of injection moulding.

Cooling time is a function of mould wall temperature, melt temperature, material

properties and part wall thickness.


2.10.4.1 Pressure drives the melt

Pressure is the driving force that overcomes the resistance of polymer melt (see

Pressure-driven flow), pushing the polymer to fill and pack the mould cavity. If you place

a number of pressure sensors along the flow path of the polymer melt, the pressure

distribution in the polymer melt can be obtained, as schematically illustrated in Figure 6

below.

FIGURE 6. Pressure decreases along

the delivery system and the cavity.

Equations

Based on a simplification of classic fluid mechanics theory, the injection pressure required

to fill the delivery system (the sprue, runner, and gate) and cavities can be correlated

with several relevant material, design, and processing parameters. In the following

equations, P is the injection pressure and n is a material constant (the power-law

coefficient), which typically ranges from 0.15 to 0.36 (with 0.3 being a good

approximation) for a variety of polymer melts. Figure 7 shows injection pressure as a

function of several of these parameters.

FIGURE 7. Injection pressure as a function

of melt viscosity, flow length, volumetric flow rate, and part thickness

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2.10.4.2 Circular channel flow

The melt flow in the sprue, runner, and cylindrical gates

2.10.4.3 Strip channel flow

Such as melt flow in a thin cavity

2.10.4.4 Factors that influence injection pressure requirements

The following diagrams illustrate the design and processing factors that influence injection pressure.

Variable Higher injection pressure

required

Lower injection pressure

required

PART DESIGN

Part thickness Thin Part Thickness Thick Part Thickness

Part surface area More Surface Area Less Surface Area

GATE DESIGN

Gate size Restrictive Gate Generous Gate

Flow length Long Flow Length Short Flow Length

PROCESSING CONDITIONS

Melt temperature Colder Melt Hotter Melt

Mould-wall Temp. Mould is Cool Mould is Hot

Injection Speed Improper Speed Optimized Speed

MATERIAL SELECTION

Melt flow index Low MFI High MFI

2.10.5 Pressure-driven flow

Flow of molten thermoplastics (in injection moulding filling) is driven by pressure that

overcomes the melt's resistance to flow. Molten thermoplastics flow from high pressure

areas to the low pressure areas, analogous to water flowing from higher elevations to

lower elevations. During the injection stage, high pressure builds up at the injection

nozzle to overcome the flow resistance of the polymer melt. The pressure gradually

decreases along the flow length toward the polymer melt front, where the pressure

reaches the atmospheric pressure, if the cavity is vented properly.

FIGURE 8. Evolution of pressure

distribution within the cavity during filling and early packing

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2.10.5.1 Pressure gradient and melt speed

The higher the pressure and resultant pressure gradient (pressure drop per unit flow

length) at the melt entrances, the faster the material flows. Therefore, increased flow

length requires increased entrance pressure, in order to generate the same pressure

gradient to maintain the polymer melt speed, as shown in Figure 9 below.

FIGURE 9. The relationship of melt velocity to the pressure gradient.

2.10.6 Flow conductance

The speed of the melt also depends on the flow conductance, an index of how easily the

melt can flow. Flow conductance, in turn, is a function of the geometry (e.g., wall

thickness, surface features) and the melt viscosity. The flow conductance increases with

increasing wall thickness and decreases with increasing melt viscosity, as shown below.

FIGURE 10. The relationship of flow

conductance to the wall thickness and

viscosity.

FIGURE 11. The melt flow length depends

on the part thickness and temperature.

2.10.7 Melt flow length

During injection moulding, the distance that the material can flow, with certain

processing conditions and wall thickness, is dependent on the thermal properties and

shear properties of the material. This behavior can be characterized by the melt flow length, as illustrated in Figure 11.

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2.10.8 Injection pressure vs. fill time

For injection moulding, if the injection pressure required to fill the cavity is plotted

against the fill time, a U-shaped curve typically results, with the minimum value of the

required injection pressure occurring at an intermediate fill time, as illustrated below. The

curve is U-shaped because, on the one hand, a short fill time involves a high melt

velocity and thus requires a higher injection pressure to fill the mould. On the other

hand, the injected polymer cools more with a prolonged fill time. This results in a higher

melt viscosity and thus requires a higher injection pressure to fill the mould. The shape

of the curve of injection pressure versus fill time depends very much on the material

used, as well as on the cavity geometry and mould design.

FIGURE 12. U-shaped curve of injection pressure vs. fill time.

2.10.9 Flow instability

Finally, it should be pointed out that the dynamics of cavity filling may sometimes

become quite complicated because of the interaction of the melt velocity (or,

equivalently, the shear rate), the melt viscosity, and the melt temperature. Recall that

the melt viscosity decreases with increasing shear rate and temperature. It is possible

that high shear rate and shear heating resulting from a higher melt velocity will drive the

viscosity down, so that the flow velocity actually increases. This will create a greater

shear rate and temperature rise, and is an inherent instability of highly shear-sensitive

materials.

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2.11. Machine Operation – Setting of Process parameters, Mould Setting, Semi

& Automatic Operation

Setting Machine Process Conditions

Before setting process conditions, you should make sure the moulding machine is in

proper working order, and that the mould you plan to use was designed for the particular

machine you plan to use. Follow the step-by-step procedure provided below to control the settings on machine.

Step 1 - Set the melt temperature

Melt temperature is one of the most important factors in moulding plastic parts. If it is

too low, the resin might not be completely melted or it might be too sticky to flow. If the

melt temperature is too high, the resin could degrade, especially if the resin is POM or

PVC. Suggested melt and mould temperatures for specific materials are available from

the resin supplier. Appropriate melt and mould temperatures for several materials are

listed in Resin data table. The resin table also contains links to descriptions of resins, their general properties, and typical applications.

Setting heater band temperatures

Most melting of the resin occurs because of the frictional heating from the screw rotation

inside the barrel. The barrel heater bands serve mainly to keep the resin at the

appropriate temperature. Typically there are three to five temperature zones or heater bands on the cylinder. The rules for setting the heater band temperatures are as follows:

The temperature should gradually decrease from the nozzle zone to the zone

nearest the hopper.

The last temperature zone, nearest the hopper, should be about 40º to 50ºC

lower than the calculated melt temperature, to give better transport of plastic pellets during plasticization.

The heater band at the nozzle zone should be set to the calculated melt temperature,

and should keep the temperature uniform. Improper heater band temperature settings

may cause drooling at the nozzle, and degradation or colour change, especially for PA

materials. Here is an example of PS having melt temperature 235ºC A process engineer

can use this melt temperature to set the heater band temperature as follows:

235ºC at the nozzle zone

235ºC at the front zone

210ºC at the first middle zone

195ºC at the second middle zone 180ºC at the rear zone

Step 2 - Set the mould temperature

Suggested melt and mould temperatures for specific materials are available from the resin supplier.

The mould temperature can be measured by using a thermometer. As illustrated below,

the average cavity surface temperature will be higher than the temperature of the

coolant during production. Thus, you should set the coolant temperature to be 10º to

20ºC lower than the required mould temperature. If the mould temperature is 40º to

50ºC or more, consider insulation plates between the mould and the clamping plates, for energy savings and process stabilization.

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FIGURE 2. Temperature-time curve at

various locations in the mould. a) Mould

cavity surface. b) Cooling channel wall. c)

Cooling channel outlet. d) Cooling channel inlet.

Use the lowest temperature setting to achieve the shortest cycle time. However, you

might try using higher temperatures to improve the appearance of the part. A higher mould temperature produces a higher gloss and more crystallization.

Considering temperature difference

For parts with a deep core, a lower coolant temperature is needed for the core (moving

plate) in order to minimize the temperature difference between the mould surfaces on

the core and cavity. A lower surface temperature difference will produce parts with

higher quality, at a lower cost. By a rule of thumb, the coolant temperature for fixed and

moving plates should not differ by more than 20ºC. This is related to thermal expansion,

which can be determined only by the user. A large temperature difference results in

differential mould plate thermal expansion, which may cause alignment problems in

guide pins, especially in large moulds. The mould will sometimes lock up for this reason. The cycle time can be increased to reduce the required coolant temperature difference.

Step 3 - Set the switch-over position

The switch-over position is the ram position where the filling (injection) stage switches to

the post-filling (packing or holding) stage. The cushion distance is the distance from the

switch-over position to the farthest position that the end of the screw can reach, as

shown in Figure 3. Thus, the switch-over position determines the cushion distance. The

cushion should contain adequate material for post-filling the part. An insufficient cushion

could cause sink marks. The typical cushion distance is about 5 to 10 mm.

At this step, set the switch-over position to fill about two-thirds of the mould. This prevents damage to the press or the mould.

FIGURE 3. Screw positions at various stages

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Step 4 - Set the screw rotation speed

Set the Screw rotation speed to the level required to plasticize the resin. Plasticizing

should not prolong the cycle time. If it does, increase the speed. The ideal speed causes

plasticizing to complete at the latest possible point in the cycle without prolonging the cycle time. Resin vendors supply the suggested screw rotation speed for specific resins.

Step 5 -Set the back pressure

The recommended Back pressure is about 5 to 10 MPa. Back pressure that is too low can

result in inconsistent parts. Increasing the back pressure will increase the frictional

contribution to the melt temperature and decrease the plasticization time. Use a higher

back pressure to achieve a shot volume that is a larger percentage of the injection

machine's capacity, in order to speed up plasticization. Use a lower back pressure for a

smaller percentage shot volume because the material will remain in the barrel longer (for many cycles) before it reaches the screw head.

Step 6 - Set the injection pressure to the machine maximum

The injection pressure is the pressure of the melt in front of the screw. The injection

pressure should be as low as possible to reduce part internal stress. On the machine, set

the injection pressure to the machine maximum. The purpose is to completely exploit the

injection velocity of the machine, so that the pressure setting valve does not limit the

velocity. Because the switch-over to holding pressure occurs before the mould is completely filled, no damage will be done to the mould.

Step 7 - Set the holding pressure at 0 MPa

For now, set the holding pressure at 0 MPa, so the screw will stop when it reaches the

switch-over position. This will prevent mould or press damage. In Step 17, the holding

pressure is increased to its final setting.

Step 8 - Set the injection velocity to the machine maximum

With the highest possible injection velocity, you can expect less flow resistance, longer

flow length, and improved strength in weld lines. However, you may need to create vents once you do this.

Proper venting minimizes defects

Insufficient venting causes compression of air trapped in the cavity. This results in very

high temperatures and pressures in the cavity, causing burn marks, material

degradation, and short shots. You should design a venting system to avoid or minimize the defects caused by trapped air in the mould.

A higher injection pressure requirement will slow the injection velocity, thus resulting in a longer filling time.

The actual filling time on the shop floor may be shorter if there is a booster or

accumulator attached to the injection unit, or may be longer if the injection velocity is

not set to the maximum. Also note that the shop floor filling time often refers to "the

time while the screw is moving," which includes filling time and holding time. The actual filling time should stop at the switch-over position.

Step 9 - Set the holding time

The ideal holding time setting is the gate freezing (sealing) time or the part freezing

time, whichever is shorter. The gate and part freezing times can be calculated or

estimated.

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Step 10 - Set ample remaining cooling time

Cooling time can be calculated or estimated. The cooling time consists of the holding time

and the remaining cooling time, as shown in Figure 4. first estimate of the cooling time

can be 10 times the filling time. For example, if the predicted filling time is .85 seconds,

the initial holding time would be 8.5 seconds and the additional cooling time would be 8.5

seconds. This ensures that the part and runner system will be sufficiently solid for ejection.

FIGURE 4. Cycle time and its components

Step 11 - Set the mould open time

The mould open time is usually set at 2 to 5 seconds. This includes mould opening,

ejection of parts from the mould, then mould closing, as shown in Figure 4. The cycle

time is the sum of the filling time, cooling time, and mould open time.

Step 12 - Mould a short-shot series by increasing injection volume

For now, fill only two-thirds of the mould. The holding pressure should already be set at 0

MPa, so that mould filling stops when the screw reaches the switch-over position, thus

protecting the mould structure and the press. Next, increase the volume in increments of 5 to 10 percent, up to 95 percent of mould filling.

In order to prevent material from escaping from the open nozzle, relieve the back

pressure created during plasticizing by drawing back the screw a few millimeters, immediately after the rotation has stopped.

Step 13 - Switch to automatic operation

The purpose of an automatic operation is to obtain stability in the process.

Step 14 - Set the mould opening stroke

The mould opening stroke is comprised of the core height, the part height, and the

capsize space, as shown in Figure 5. You should minimize the mould opening stroke. The

mould opening speed should be slow at the very beginning, then accelerate, then slow

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down again at the end of the stroke. The sequence of the mould closing speed is similar to the mould opening speed: slow, fast, slow.

FIGURE 5. Required mould opening

Step 15 - Set the ejector stroke, start position, and velocity

Relieve any slides first. The ejector travel should be, at a maximum, the core height. If

the machine is equipped with a hydraulic ejector, set the start position at the point where

the part is clear of stationary mould parts. (When the ejector velocity is equal to the opening speed, the part remains where it was in relation to the stationary mould part.)

Step 16 - Set the injection volume to 99% mould filled

When the process has stabilized (when the same parts are produced each time), adjust

the switch-over position to 99 percent of filling. This will exploit the maximum injection speed in as large a part of the injection as possible.

Step 17 - Increase the holding pressure in steps

Increase the holding pressure in steps of approximately 10 MPa in the melt. If the first step does not fill the mould completely, increase the injection volume.

De-mould and remove the part. Write the holding pressure on it. This holding pressure

series forms a good basis for a more thorough examination. You can then discuss the

possibilities and limitations with the customer.

Choose the lowest acceptable holding pressure, as this minimizes the internal stresses in

the part and saves material, as well as operating costs. A high holding pressure can

cause excessive residual stresses that could warp the part. Moulded-in residual stresses

can be released somewhat by annealing at around 10ºC below the heat deflection

temperature.

If the material cushion is completely used (see Screw positions at various stages), the

last part of the holding pressure time will not be effective. This calls for a change in the injection stroke position, in order to increase the injection volume.

Calculating injection pressure

The hydraulic pressure in the injection cylinder can be read on the machine manometer.

However, the injection pressure in front of the screw is more important. To calculate the

injection pressure you will need to multiply the hydraulic pressure by the resin/hydraulic

pressure ratio. This ratio is usually found on the moulding machine near the injection unit

or in the instruction manual for the machine. The ratio is usually in the range of 7 to 15, as shown in below.

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FIGURE 6. Resin/hydraulic pressure ratio for a Ø 30 mm screw is 11.1

Step 18 - Minimize the holding time

A quick way to find the minimum holding time is by setting a longer holding time, then

reducing the holding time until sink marks appear.

If consistent part dimensions are essential, use the following more accurate

determination of the holding pressure time. From a curve of part weight versus holding

time, determine when the gate or the part freezes. For example, Figure 7 shows that the

holding pressure does not influence the part weight after nine seconds. This is minimum

holding time.

FIGURE 7. Determination of the gate/part freezing time by weighing parts manufactured

at various holding pressure times

Step 19 - Minimize the remaining cooling time

Reduce the remaining cooling time until the maximum surface temperature of the part

reaches the heat deflection temperature of the material. The heat deflection temperature can be provided by the resin supplier.

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2.12 Process controls – Process parameters with respect to product

quality

In brief, the injection moulding cycle can be broken down into four phases: fill, pack,

hold, and cooling/plastication. The process begins with the mixing and melting of

resin pellets. Molten polymer moves through the barrel of the machine and is forced

(injected) into a steel mould. As the plastic fills and packs the mould, the part takes

shape and begins to cool. The moulded part is then ejected from the mould, ready for finishing steps and assembly.

While machine selection, material properties, and part design all affect the outcome of

injection moulding, five processing variables specific to injection moulding can have as much or more impact on the success of this process. These variables are:

Injection Speed,

Plastic Temperature,

Plastic Pressure,

Cooling Temperature and Time.

Control of these variables during each of the four phases of the injection moulding

process can help improve part quality, reduce part variations, and increase overall productivity.

In Phase 1 Fill-the screw advances and plastic flows into the mould. Flow characteristics

are determined by melt temperature, pressure, and shear rate. Injection Speed--the rate

at which the ram (screw) moves--is the most critical variable during fill. A polymer flows

more easily as injection speed is increased. However, injection speed that is too high can

create excessive shear and result in problems such as splay and jetting. More

importantly, heat from a higher shear rate can degrade the plastic, which adversely

affects the properties of the moulded part.

The way in which plastics flow during fill is also affected by their viscosity, or resistance

to flow. Polymers with high viscosity are thick and taffy like; those with low viscosity are

thinner and flow more easily. Melt temperature affects viscosity and to achieve the best

results should be maintained within the temperature range recommended by the supplier.

Plastic pressure, another variable, increases sharply during fill. The molten plastic can, in

fact, be under much greater pressure than is indicated by hydraulic pressure. It is

important to understand the flow characteristics during fill of the material being used and

to operate the process consistently.

Phase 2 Pack-is when the plastic melt is compressed and more material is added to

compensate for any shrinkage during cooling. Approximately 95% of the total resin is added during fill, with the remaining 5% added during the pack phase.

Plastic pressure is the primary variable of concern during the pack phase. The screw

maintains pressure in the melt, compensating for shrinkage, which can cause sinks and

voids. Variations in cavity pressure are a primary cause of deviations in plastic parts.

It is important to completely fill the mould -avoiding over packing or under packing-since

packing pressure determines part weight and part dimensions. Over packing can cause

dimensional problems and difficulty in ejecting the part, while under packing can result in short shots, sinks, part-weight variations, and warpage.

Phase 3 Hold -is affected by all five of the process variables described earlier: injection

speed, plastic temperature, plastic pressure, and cooling temperature and time. After the

mould is packed, the plastic is held in the mould until it is partially solidified and the gate

freezes. The drop in plastic pressure reflects the amount of shrinkage that occurs from

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cooling. One way to optimize this phase is to decrease the hold time until the part weight

changes. At that point, the gate is no longer sealed and resin backflows out of the mould.

If hold continues after the gate seals, cycle time increases, using more time and energy to produce the part. The key is to maintain pressure on the plastic until the gate freezes.

Phase 4 Cooling and plastication -is generally the longest part of the moulding cycle--

up to 80% of the cycle time. Optimizing cooling time can yield substantial gains in

productivity. Because the gates are sealed during this phase, cooling temperature and

time are the only variables at work. The key to optimizing the cooling phase is to balance

the desire to cool quickly against the amount of moulded-in stress the final part can

withstand.

Cycle time

Typical process cycle time varies from several seconds to tens of seconds, depending on

the part weight, part thickness, material properties, and the machine settings specific to

a given process.

The injection moulding cycle can be divided

into several stages. They are mould closing,

filling, packing, cooling, mould opening and ejection as shown here.

Cooling Time is a major fraction of the total cycle time.

Cooling time is a function of :

Mould Wall Temperature

Melt Temperature

Material Properties

Part Wall Thickness

The equation shown here gives a rough

estimate of the minimum cooling time needed before part ejection.

is the thermal diffusivity of the material,

h, is the plate thickness, capital Tw is the

mould wall temperature, capital Tm is the

melt temperature, capital Te is the ejection temperature.

An example calculation has been shown

here with typical values for the different

variables. In the example, the minimum

cooling time for the part centerline to reach

the ejection temperature is calculated to be

23 seconds.

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The graph shows a typical pressure profile

for injection moulding. In this example, the

cycle time is 35 seconds. Filling requires just

a few seconds. A slight increase in pressure

occurs when the mould is closed. Filling

raises the pressure; during packing the

mould is held at this pressure. When gate

freeze off occurs, melt can no longer be

forced into the cavity and the pressure

drops. When the part cools below its glass

transition temperature, the mould can be

opened and the part ejected. At this stage, the pressure drops to ambient level.

This graph shows a typical temperature

profile for injection moulding of

thermoplastic ABS. Two curves are shown.

The solid line represents the temperature

inside the part, and the dashed line

represents the temperature near the gate.

The glass transition temperature for ABS is

100 °C to 110 °C. When the gate

temperature drops below this level, gate

freeze-off occurs. The inside of the part cools

more slowly than the gate section. When the

centerline temperature reaches the ejection

temperature, the part can be ejected.

This plot shows the variation of estimated

cooling time with the wall thickness of the part.

Cooling time increases in a non-linear fashion with increasing part wall thickness.

The cooling time for a semi-crystalline

material like Polybutylene Terephthalate is

always higher than that for an amorphous

material like a blend of Polycarbonate and

ABS.

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2.13 Microprocessor Control – Open Loop & Close Loop

2.13.1 Why Microprocessor Required?

2.13.2 Microprocessor Control

2.13.3 Open Loop Control

2.13. 4 Closed Loop Control

2.13. 5 Hydraulic Control Valves

2.13. 6 Stroke Control

2.13. 7 Temperature Control

2.13. 8 Position control

2.13. 9 Injection speed control

2.13. 10 Screw rotary speed control

2.13. 11 Hydraulic pressure control

2.13. 12 Back pressure control

2.13. 13 Nozzle pressure/temperature control

2.13. 14 Cavity pressure control

2.13.1 Why Microprocessor Required?

To ensure consistency of quality of injection moulded parts one has to look at the

controls like pressure, speed, stroke and temperatures. This is the repeatability of the

product quality with close tolerance on dimension and weight throughout long production

run. The injection moulding process control is quite complex because of the interplay

among the process parameters, i.e., melt temperature, viscosity, pressure, speed,

residence time etc.

The value of the parameters set can "drift" on account of

Any variation in the material

Density,

MFI,

Molecular Weight, etc.

Inaccuracies in repetition of set values of hydraulic valves like

Pressure relief valve and

Flow control valve.

This drift upsets the process balance and repeatability. In turn, this can result in

inconsistent quality of moulded part. Therefore, the parameters are required to be

"controlled" to get quality moulded part.

So, the desired control is wanted to be achieved, but the microprocessor alone is not

enough and it requires the matching support from the hydraulic control gears. The

microprocessor is brain and the hydraulic power is drawn for well-designed injection

moulding machines. Microprocessor control offers improved accuracy, efficiency

and versatility because of its high speed reasons, programmability and

computational ability.

2.13.2 Microprocessor Control

The advanced semi-conductor technology enables to get large scale integrated logic

circuit in a tiny silicon wafer enclosed in a small plastic case. This microprocessor has

replaced conventional hardwired contractor logic circuits.

The microprocessor based programmable logic controls include:

Microprocessor to perform logic operation

Memory to hold program and data

Input output device, i.e., alpha numeric keypad etc.

Peripheral device, i.e., limit switches, digital switches, proximity

Switches, thumbwheel switches.

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One can actually change the programmes of microprocessor to change the sequence or

programme at a later date quite easily in this machine. Normally core pulling ejector

programmes require alteration to suit the design of the mould.

In addition, the microprocessor in injection moulding machines has to perform very

complex and powerful operations involving logic, probability, computation and decision

making. In this operation all the relevant data are represented and processed in a digital

form precisely at high speeds.

The microprocessor is capable of accepting, processing and giving an output in both

analogue and digital signals. Analogue signal is comparable to time shown by two hands

of a mechanical watch; digital signal can be compared to the time shown on a digital

quartz watch.

Analogue input/output include temperature, pressure, speed distance, etc.

Digital input signal include those received from light switches or proximity switches push

button etc.

In injection moulding machine multiple monitoring is required and therefore, more than

one microprocessor are used within the control system. Besides, the microprocessors

communicate with each other with a common data memory, thus providing a common

pool of data for efficient coordination of mechanisms and processes through out of the

machines.

The developments of proportional valve (hydraulic) providing cut out of pressure and flow

proportional to the strength of input signal enabled the automatic setting of process

pressure or flow sequentially in a cycle.

The microprocessor can carry out following functions.

Sequence control

Process control

Monitoring control

Alarm function

Display of current as well as past status of actual process parameters

Graphic display

It would be found that the sequence control function is divided into sixty four steps. It

follows in logical progression. When all the conditions of one step have been fulfilled, the

command is passed on to next step. This guarantees interruption of the cycle, should an

electronic, hydraulic or operating fault develop. Visual digital indicators signal the

instantaneous operational condition of the machine thereby assisting in locating the

cause of the interruption.

2.13.3 Open Loop Control

At times, one may want the process to operate at a fixed value of temperature. The

control used is called open loop control. In open loop control the actuator is influenced

only by the set value for the process. There is no feedback on the effect of the input

signal. In open loop control the attempt is made successfully to hold process parameters

and environment at constant level so that the uniform repeatability is achieved in the

process. Open loop controls may be designed with analogue or digital operating devices.

Typical examples of open loop control are the setting of

Screw speed,

Clamp closing speed,

Cooling time etc.

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2.13.4 Closed Loop Control

There could also be occasions when one wants to conduct a sort of orchestra for

temperature control. That is, if there is a disturbance in process at any point, one wants

to take corrective action so that the process temperature is not disturbed. In this case

closed loop control system is used.

In closed loop control the effect of disturbance or deviation in the process is opposed

automatically by an error signal which is generated continuously by comparing the actual

value with set value. Therefore, it reduces a feed back sensor to measure the actual

value continuously.

The closed loop controller

Measures actual process value continuously.

Compares actual value with that of set value to generate feed back continuously.

And provides controlled output to actuator continuously.

Let us look at some typical applications for closed loop controls.

Set temperature of plasticizing cylinder.

Pressure transducer in mould cavity provides feed back to generate error signal

which in turn ensures correction on the valve setting.

Closed loop control of injection speed requires the monitoring of injection pressure

necessary to maintain the injection speed constant.

Linear transducer connected to screw measures injection speed and provides feedback to

generate error signal which in turn corrects the speed.

Normally following parameters are controlled in closed loop.

Cylinder temperature

Multiple stages of injection speed

Multiple stages of following pressure

2 stages of back pressure

2 stages of screw rpm

Sophistication of closed loop control

Nowadays, Injection Moulding Machine barrel temperature controls are always closed

loop. Occasionally, nozzle temperature control uses a simpler temperature controller and may even be open loop.

Time control is considered open loop, e.g. the control of injection time.

In the control of Injection Moulding Machine, there are many variables that could be

controlled in closed loop. By measuring the controlled variable and taking control action

to correct any deviation from the set value, closed loop control guarantees good

repeatability of the controlled variable despite changes in the uncontrolled variables, e.g. variation in the quality of reground materials, humidity of the plastic pellets.

2.13.5 Hydraulic Control Valves

By hydraulics, we mean the transmission and control of forces and movement (speed) by

means of fluid.

The conventional hydraulic valve includes on-off solenoid controlled directional valves,

manually setable pressure control valve and manually setable flow control valve.

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Proportional Valve for Pressure & Flow Control

It has a proportional solenoid which responds to the incoming variable signal and

provides smooth and proportional hydraulic output like pressure, flow, direction,

acceleration, deceleration, etc. There are proportional pressure control valves,

proportional flow control valves and proportional directional and flow control valves.

Contact less position transducer, if incorporated in the proportional valve can scan the

position of solenoid aperture and provide the feedback signal to reach the correct

position. Hence, position transducer improve the accuracies of proportional valve. The

linearisation of the characteristics can be incorporated in the (microprocessor) control

card.

2.13.6 Stroke Control

How the stroke of injection moulding machine is controlled. For stroke control Linear

Encoder is used which provides very precise control of stroke. It consists of a glass scale

with fine graduation and reference mark and a scanning unit. This is a little complicated

but worth knowing.

One would see that the scanning unit consists of a light source, a condenser lens,

scanning reticule with index graduation and photovoltaic cells. The scanning unit is fixed

in a position and the scale is moved. The lines of the scale coincide alternately with the

lines or spaces in the index graduation. The periodic fluctuation of light intensity is

converted by photovoltaic cell into electrical signals. These signals result from the

averaging of large number of lines. The two sinusoidal signals with 90 degree phase shift

and one reference signal are converted to square wave form. These signals are processed

to produce the counting pulse. The actual position is derived from counted pulse and the

calculated angle value. This result is transmitted to the machine controls.

2.13.7 Temperature Control

As an experienced moulder one should also know the importance of keeping temperature

in control. Thermocouple actuated temperature control systems are used in injection

moulding machine. Earlier version of ON-OFF temperature controllers are now obsolete

and are not used any more on injection moulding machine. This type of controller is

unable to control the overshooting of melt temperature which is due to process

dynamics.

To overcome this defect proportional temperature controllers were developed. It provides

anticipatory action as the temperature reaches the proportional band. The ON-OFF action

in the proportional band before reaching the set value is able to reduce the extent of

overshot and maintain the temperature within the proportional band. These controllers

were solid state discreet unit. The temperature deviation is indicated in a deviation

meter.

Further improvement in temperature controller resulted in Three Term Controller called

proportional, integral, derivative (PID) controller. This provides anticipatory action based

on the rate of change of process temperature (Derivative) and the corrective action

based on difference between set temperature and actual temperature (Error integrator)

in the proportional band. This provides very accurate control of temperature. For power

supply to heater, electromechanical contactor is replaced by semiconductor relay or

thyristor units. It is fast responding, virtually maintenance free noiseless in operation.

2.13.8 Position control

In an Injection Moulding Machine, screw position, mould position, ejector position and

mould height adjust position are measured, either by limit switches, proximity switches

or potentiometers. Potentiometers offer position measurement throughout the whole

stroke, while the former two only measure whether discrete positions have been reached.

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Depending on the stroke, a resolution of 0.1 mm is expected from potentiometer (the limit actually comes from the resolution of the A/D (analog to digital) converter).

Some manufacturers use a rotary encoder and rack and pinion to measure movement in

order to avoid the cost of the A/D converter. In this case, the resolution of the encoder and the rotary to linear conversion factor determine the resolution of the movement.

Screw position is measured to break down the injection stroke into stages each with a

different speed/pressure. It is also used to measure shot size during feeding and decompression.

Mould position is measured to break down the mould movement into slow-fast-slow

stages to reduce vibration in mould closing and mould opening. Mould position is also used in low pressure mould protection.

Ejector position is measured to short cycle the ejection stroke, especially in multiple

ejection.

In a toggle clamped Injection Moulding Machine, the stroke of the mould height adjust mechanism could be measured by a potentiometer.

2.13.9 Injection speed control

It is important that injection speed is controlled to obtain a high quality part. This could

be done in open loop, semi-closed loop or closed loop.

The open loop approach uses the ordinary proportional flow valve. A voltage proportional

to the desired flow rate is applied. Through the injection cylinder, the desired flow rate is mapped into the desired injection speed.

The semi-closed loop method uses the closed-loop proportional flow valve. The loop is

closed as far as the spool position is concerned. The movement of the spool within the valve controls the rate of oil flow through it.

The closed loop method uses the linear screw speed to close the loop. Either a velocity

transducer is used or the screw speed is derived from potentiometer readings in fixed

intervals of time. The proportional flow valve is adjusted to nullify any deviation from the

desired speed. Unless the control is done by dedicated electronics, closed loop speed control demands very much of the machine controller.

2.13.10 Screw rotary speed control

Screw rotary speed is monitored or controlled so as to control the screw surface speed to

below a value appropriate for the resin. A speedometer, the kind used in a bicycle, is the

usual analog measuring device. A chart converts screw rotary speed to screw surface

speed which is a function of screw diameter.

Figure 1. Screw rotary speed to screw surface speed chart

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2.13.11 Hydraulic pressure control

Closed loop hydraulic pressure control provides more consistent injection pressure,

holding pressure and back pressure from cycle to cycle. Note that hydraulic pressure control is not a good substitute for melt pressure control or cavity pressure control.

The signal from the pressure sensor adjusts the proportional pressure valve to nullify any deviation from the desired value

2.13.12 Back pressure control

As the screw rotates, it is forced backward by the melt at the tip of the screw. This

backward motion forces oil out of the injection cylinders through a flow control valve,

which creates a back pressure on the screw.

The back pressure sensor is mounted at the back of the injection cylinder. The same sensor is used for hydraulic pressure control.

Figure 2. Hydraulic/back pressure transducer location

2.13.13 Nozzle pressure/temperature control

Pressure and temperature are the two most important measurable process variables in

injection moulding. It could be used to control the injection fill, pack and hold pressures.

Figure 3. Nozzle pressure sensor

2.13.14 Cavity pressure control

Located where the action is, cavity pressure control provides the most accurate injection

fill, pack and hold pressures. In some cases, a temperature sensor is located within the same housing, providing temperature of the melt in the cavity as well.

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Figure 4. Cavity pressure sensor location

The cavity pressure curve clearly shows the injection fill, pack, and hold phases. In Figure 5, 1-2-3 is the injection phase, 3-4 is the pack phase and 4-5-6 is the hold phase.

Point 3 is when the mould is completely filled. As the screw advances beyond 3, cavity

pressure rises steeply as the melt is being compressed. At 4, injection pressure is

reduced to holding pressure which keeps the mould filled as it cools and shrinks. At 5, the melt at the gate is frozen and the hold pressure could be removed.

Figure 5. Cavity pressure curve

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2.14 Post Moulding Operation

2.14.1 Heat inserting

2.14.2 Chrome Plating

2.14.3 In Mould Insert Moulding

2.14.4 Post Mould Inserting

2.14.5 Drilling

2.14.6 Polishing

2.14.7 Assembly

Benefits of In House post moulding Operations

Reduced costs – by carrying out post moulding operations in house, and utilising lean

manufacturing tools, we can greatly reduce component costs and the complexity of work

that our customers would ordinarily undertake.

High level of quality – performing post-moulding operations on products helps ensure

that a high level of quality is maintained. By checking parts from the moment they leave

a press, to final assembly, quality levels can be maintained and ensure that components

are only assembled to the highest standards.

Reduction of Customer’s stock holding – Assembly of components will reduce the

cost of customers stock holding due to delivery of an assembly rather than a range of

components.

Reduced production times – post moulding operations mean there is very little time

between the production of components and their assembly. This means that a great deal

of time can be saved when components would normally be transported, or stored, in

between moulding and assembly operations.

2.14.1 Heat inserting

Example of a jig for heat inserting

Heat inserting is an example of the post mould process of inserting. The addition of

inserts into a part increases the functionality of a part by which components can be

assembled. The actual insert that is incorporated into the moulding can take a variety of

forms and be either metal or a pre-formed plastic insert. A good example for the

application of metal inserts is for instances where the part needs to be screwed to

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another part or to some other fixing. In this case the plastic might not be a strong

enough material to be threaded, or screwed into, so metal (usually brass) threaded

inserts are heat inserted into the part which can then provide the necessary strength.

The process of heat inserting a part is not very complicated but ensuring that each part

is inserted in exactly the same way is more complex. To ensure that there is no

variation in the location that the inserts are placed jigs are used to hold the work piece

firmly in place. Each insert will be located individually by using a heat gun that heats up

the insert and with it the plastic around it. This means the plastic becomes soft enough

for the insert to be pressed into place and when the plastic cools again the insert will be

held in place. The heat gun is also equipped with distance stops to ensure that every

insert is positioned consistently in the component.. To further ensure parts are inserted

correctly rigorous quality checks are carried out on all inserted parts to ensure a high

level of quality is maintained.

Benefits of Heat Inserting

Increased functionality – by adding inserts to mouldings the part can more easily be

used for its designed purpose. For example by adding threaded inserts parts can be

easily be screwed to their fixings or other parts, increasing their functionality.

Low part degradation – the process of heat inserting means that the heating/melting

of the part is very localised to where the insert will be pressed in. this means that parts

do not suffer warping, or any other distortion effects, due to being heated again.

High level of quality – due to the known challenges with heat inserting extra

measures are taken to ensure the processes is repeated to as high a level as possible,

meaning part quality is kept very high.

2.14.2 Chrome Plating

Some examples of chrome plated mouldings

Chrome plating of plastic is significantly more difficult than performing the same

operation on a metal, but it will provide excellent results when the right process is

utilised. Due to the chrome plating process requiring the part to be electrically

conductive, a series of steps are required before the chrome can be deposited onto the

surface of the product.

The first step to be carried out is to etch the surface with a chemical so that the

subsequent layers of nickel and chromium will adhere. A large proportion of plastic parts

that will be chrome plated will be moulded from ABS as this gives a very good surface

finish to plate onto. ABS is also used because the butadiene molecules on the surface of

the material can be chemically removed. This removal of butadiene molecules leave

microscopic undercuts n the surface of the ABS and this acts as a very good key on to

which the first layer can be attached.

The next process that will be carried out is to attach a layer of nickel (with a catalyst)

onto the surface of the part. This layer of nickel will be what becomes electrically

conductive and allows the chrome to be electroplated to it. This layer is applied by

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means of dipping the product. The key that was put into the surface of the part will

ensure the nickel remains attached when the part is removed from the bath.

Once the layer of nickel has dried the part can be plated by electroplating. This involves

applying a negative charge to the part being plated and dipping it into a solution of the

metal it is to be plated with, which has a positive charge. The positively charged

metallic ions are attracted to the negatively charged part and once they come into

contact with the part they revert back to their metallic form again. The part is removed

from the solution and left to cool.

To ensure a good quality finish after chrome plating a part must be moulded to a very

high quality. Any defects that are on the surface of the part after injection moulding will

stand out after plating. Also, any stresses in the moulded components will show up as a

defect when chrome plated. Unlike other finishing methods chrome plating does not fill

in scratches or other defects. Instead the chrome will form a thinner layer over the

defects and, in effect, magnify the problem. For this reason rigorous quality checks are

carried out on all products so that money is not wasted plating a part that has a defect.

Benefits of Chrome Plating

Metal finish - Metal finishes can be very popular and, by coating plastics, advantage

can be taken of characteristics from both materials.

Wear resistant – as chrome is a metal rather than a plastic its wear resistance

properties are much greater than those of the plastic it covers. This means for

applications where a part might be handled repeatedly, such as a shower handset, a

chrome finish is likely to wear better than its plastic counterpart.

Electrically conductive parts – by chrome or nickel plating a part it is possible to

give a plastic component the ability to conduct electricity. This gives the advantage of

being able to create electrical components that are light weight and less costly to

produce than completely metal parts.

Attractive mouldings – by applying chrome finish to mouldings a

2.14.3 In Mould Insert Moulding

Moulded in speaker contacts

In mould insert moulding is the process by which a metal, or preformed plastic, insert is

incorporated in to the component during the moulding stage.

With in mould insert moulding the inserts will be very strongly attached to the rest of

the component. This happens because strong mechanical bonds are created between

the inserts and the mouldings. Inserts are designed so that when the polymer is

moulded around it, it will be securely fastened in place. This can be achieved by

incorporating plates that stick out from inserts, threads on the sides of the inserts or by

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the overall shape of the insert. By calculating the degree of shrinkage that will occur

during the cooling cycle designers can ensure there is a very tight fit of polymer around

the insert and give the strongest possible join.

When inserts are used during the moulding stage standard moulding machines can

make it difficult for operators to put inserts into their correct place in the tool. For this

process vertical press machines and vertical machines with a rotary table are most

effective.

Benefits Of In Mould Insert Moulding

Reduced post-moulding operations – With in mould insert moulding the need for

post moulding operations is greatly reduced. This helps with ease of assembly and

reduces the labour necessary for products.

Increased part consistency – Insert Moulding has major benefits in the consistency

of parts produced. As the inserts are placed in the same locations in tools for every

cycle each of the mouldings produced will be exactly the same. This helps reduce costs,

as rejected parts will be kept to a minimum.

Ease of assembly – Due to inserts being incorporated into parts during the moulding

stage this eases the assembly of the part. Instead of having to place fittings to attach

parts fittings can be incorporated during the moulding stage so that parts can be simply

clipped together.

Reduced production time – when vertical moulding machines, that are equipped with

a rotary table, are used for production there is the opportunity to have two halves of the

lower part of the tool. This means that production is almost constant with mouldings

being formed at the same time as fresh inserts are being loaded into the second half of

the tool. This lowers overall production times and can also reduce the amount of labour

needed.

2.14.4 Post Mould Inserting

Use of post moulded inserts A typical post moulding insert

Post mould inserting is the process by which a metal, or preformed plastic, insert is

incorporated into a moulding by means of a secondary process once the component has

already been moulded.

To place inserts into a part once it has already been moulded requires the use of

secondary operations equipment. This equipment includes heat guns, powered presses or

automated drills. To ensure every insert is positioned in exactly the same location,

custom jigs are used so that the moulding will be held in the same position for each

inserting procedure.

As with in-mould insert moulding the inserts are specially designed so that they will be

held securely in place after the inserting process. The inserts cannot be designed with

anchoring pieces extending from them but they can be designed so that they have

threads, or cuts, in the side of them. This means that when the plastic cools around the

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insert it will be securely held in place. For operations that do not use heat the insert will

be designed with deep threads on the sides so that they will be held tightly when

screwed into place.

Post mould inserting can be a time consuming additional process for some components,

but the time taken is more than offset when it goes onto final assembly.

Benefits of Post Mould Inserting

Ease of assembly – by adding inserts to a moulding the ease by which it can be

assembled is greatly increased. Inserts such as clips or screw bolts can be incorporated

into mouldings which greatly assist assembly operations and subsequent product

performance.

Increased part functionality – besides adding inserts to aid assembly inserts that

improve a parts functionality can also be used. For example, terminal fittings for wires, or

seals to make parts watertight.

Increased component value – any second operation carried out on a part will add

value to it. By adding inserts to help assembly or increase functionality, product value

will be raised. This helps to compensate for the extra time involved in second operations

and ensure products remain cost effective.

Good part consistency – to carry out post mould inserting jigs are used to hold

mouldings while they are inserted. This means that the repeatability of the operation is

very good and all parts inserted will be of the same quality.

2.14.5 Drilling

The drilling of parts is used to remove any unnecessary polymer that may have been

necessary in the moulding process. By removing this extra material in house it means a

ready-to-assemble moulding can be provided to the customer, or the part can be

assembled with other mouldings.

2.14.6 Polishing

For products that have a high quality gloss finish a post moulding polishing operation is

often a useful extra process. Even though the finish produced by the moulding tool may

be of a very high quality, a polishing operation to remove any dust from the product

before final packaging gives a part the high gloss finish that will have been specified..

Polishing operations are carried out on a soft-polishing wheel with high quality wax to

ensure that a part is polished to a perfect finish without leaving any marks.

2.14.7 Assembly

For products that require assembly we are able to carry out this operation in our

assembly facility. We can demonstrate examples of assemblies where we mould all the

separate components in house and assemble the parts either as a whole in the assembly

facility or as a step by step process on the press as each part is produced. By carrying

out assembly in house we can reduce costs for our customers while still producing

products to a high standard.

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2.15 Faults and Remedies 2.15.1 Air traps (voids and bubbles)

2.15.2 Black specks/black streaks

2.15.3 Brittleness

2.15.4 Burn marks

2.15.5 Delamination

2.15.6 Dimensional variation

2.15.7 Discoloration

2.15.8 Fish eyes

2.15.9 Flash

2.15.10 Flow marks

2.15.11 Jetting

2.15.12 Ripples

2.15.13 Short shot

2.15.14 Shrinkage and warpage

2.15.15 Silver streaks

2.15.16 Sink marks and voids

2.15.17 Weld lines and meld lines

2.15.1 Air traps (voids and bubbles)

An air trap is air that is caught inside the mould cavity. It becomes trapped by

converging polymer melt fronts or because it failed to escape from the mould vents, or

mould inserts, which also act as vents. Air-trap locations are usually in areas that fill last.

Lack of vents or undersized vents in these last-to-fill areas are a common cause of air

traps and the resulting defects. Another common cause is race-tracking (the tendency of

polymer melt to flow preferentially in thicker sections), caused by a large thickness ratio.

FIGURE : Air trap locations indicated by the computer-predicted melt-front

advancements.

Problems caused by air traps

Entrapped air will result in voids and bubbles inside the moulded part, a short shot

(incomplete fill), or surface defects such as blemishes or burn marks. To eliminate air

traps, you can modify the filling pattern by reducing the injection speed, enlarging venting, or placing proper venting in the cavity.

Remedies

Alter the part design

Reduce the thickness ratio.

This will minimize the race-tracking effect of polymer melt.

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Alter the mould design

Pay close attention to the proper placement of vents.

Place vents in the areas that fill last.

Vents are typically positioned at discontinuities of mould material, such as at

parting surfaces, between the insert and mould wall, at ejector pins, and at mould

slides.

Re-design the gate and delivery system.

Changing the delivery system can alter the filling pattern in such a way that the

last-to-fill areas are located at the proper venting locations.

Make sure the vent size is large enough so that the air present in the cavity

can escape during injection.

Be careful, however, that the vent is not so large that it causes flash at the edge

of the moulding. The recommended vent size is 0.025 mm for crystalline

polymers, and 0.038 mm for amorphous polymers.

Adjust the moulding conditions

Reduce the injection speed.

High injection speeds can lead to jetting, which causes air to become entrapped in

the part. Lowering the injection speed will give the air displaced by the melt

sufficient time to escape from the vents.

2.15.2 Black specks/black streaks

Black specks and black streaks are dark spots or dark streaks found on the surface or

throughout a moulded part. Brown specks or streaks refer to the same type of defect,

except the burning or discoloration is not as severe.

FIGURE . Black specks (left) and black streaks (right)

Causes of black specks/black streaks

Black specks and black streaks are caused by overheated (degraded, burned) material or by contamination of the resin.

Material degradation

Overheated materials can degrade and lead to black streaks. Material that stays in

the nicked rough surfaces of the barrel wall and screw surfaces for a prolonged period of time after heating will char and degrade, resulting in the defect.

Material contamination

Contaminants in the air or material, such as dirty regrind, foreign material,

different color material, or a lower melt-temperature material, are what most

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often lead to black specks and black streaks. Airborne dirt can also cause dark spots on the surface of a moulded part.

Other defects resulting from the same causes

Brittleness

Burn marks Discoloration

Remedies

Handle the material carefully

Make sure no contaminated materials, such as dirty regrind, are blended into the

original material.

Put the cover on the hopper and all bins of material. Airborne dirt can

contaminate the original material, leading to black spots.


Clean the ejectors and slides. The streaks could by caused by the grease or

lubricants on the slides or ejectors.

Improve the venting system. If the black specks are found at the end of flow

paths or blind spots, they are likely caused by a poor venting system.

Compressed air trapped in the cavity is sometimes ignited, leading to the

defect.

Clean or polish any nicked surface on the runner system to keep dirt from

lodging in these areas.

Clean the mould before moulding.

Select a proper machine

Size a proper injection machine for a specific mould.

The typical shot size should be between 20 and 80 percent of machine injection

capacity. For temperature-sensitive materials, the range should be narrowed

down more. Plastics simulation software can help you select the right size

injection machine for a specific mould. This will help avoid resin remaining in the

heated barrel for prolonged periods of time.

Check for scratched or dented barrel/screw surfaces that trap material.

This could lead to the material becoming overheated or burned.

Check for local overheating by a run-away heater band or a malfunctioning

temperature controller.


Lower the barrel and nozzle temperature.

Material degradation can result from a high melt temperature.

Purge and clean the injection unit.

The black streaks might be caused by contamination from the barrel wall or the

screw surface. When moulding with two materials, after switching from one

material to the other, the old material might not be purged from the barrel

completely. This could generate defects during the moulding of the second

material.

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Avoid recycling rejected parts with black specks and black streaks.

Recycling such parts could lead to further contamination, unless they'll be used for

parts that are in black or for which such defects are acceptable.

2.15.3 Brittleness

A brittle moulded part has a tendency to break or crack. Brittleness results from material

degradation leading to shorter molecular chain length ( thus lower molecular weight). As

a result, the physical integrity of the part is substantially less than the specification.

FIGURE . Degraded part tends to be brittle and break easily

Causes of brittleness

Brittleness is caused by material degradation due to

Improper drying conditions

Improper temperature setting

Improper runner system and gate design

Improper screw design Weld line weakness


Black specks/black streaks

Burn marks Discoloration

Remedies

Adjust the material preparation

Set proper drying conditions before moulding.

Brittleness can be caused by excessive drying time or drying temperature such as

at full heat for several days. Excessive drying either drives off volatiles in the

plastic, making it more sensitive to processing, or degrades the material by

reducing the molecular weight. Material suppliers can provide optimum drying

conditions for the specific materials.

Reduce regrind material. The brittleness could be caused by too much

reground material added into the original virgin material.

Change to a high-strength material since low-strength materials tend to

become more brittle if processed improperly.


Enlarge the sprue, runner, and/or gate.

Restrictive sprue, runner, gate, or even part design could cause excessive shear

heating that aggravates an already overheated material, causing material

degradation.

Select a proper machine/machine component

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Get a better screw design for the material you're using to achieve a better

mixed melt temperature.

Contact material suppliers to get the right screw design information to avoid

improper melt mix or overheating that leads to material degradation.

Adjust moulding conditions

Reduce the barrel temperature and nozzle temperature.

If the barrel and nozzle temperature are too high, the material in the barrel will be

overheated, leading to thermal degradation and the color change.

Reduce the back pressure, screw rotation speed, or injection speed. since

shear heating can result in material degradation.

Within the limit not to overheat the material, increase melt temperature,

mould temperature or injection pressure if the weld line has a tendency to

crack. See Weld lines and meld lines for more information.

2.15.4 Burn marks

Burn marks are small, dark or black spots that appear near the end of the flow path of a

moulded part or in the blind area where the air trap forms.

FIGURE . Burn marks

Causes of burn marks

Entrapped air

If the injection speed or injection pressure is too high, the air trapped in the

runner system and cavity cannot be released to the atmosphere through the

venting system properly within a very short filling time. Air traps also occur in

improperly vented systems when race-tracking behavior is significant.

Consequently, the air will be compressed, resulting in a very high pressure and

temperature, and which will cause the polymer to degrade on the surface near the

end of the flow path or the blind area.

Material degradation

Burn marks can also result from the degraded (charred) materials being carried

downstream and then appearing on the surface of the moulded part or near the

venting areas. Material degradation is caused by:

High melt temperature

Excessive melt temperature can be caused by improper barrel temperature setting, a broken thermocouple, or a malfunctioning temperature controller.

High screw rotation speed

If the screw speed is too high during the plasticization period, it will create too much frictional heat, which could degrade the material.

Restrictive flow path

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When the melt flows through restrictive nozzle, runner, gate, or part sections, it creates a lot of shear (frictional) heat, which could degrade the material.



Brittleness Discoloration

Remedies


Place an adequate venting system throughout the mould to help vent out the

entrapped air.

Vents are especially important near the end of the flow path and in the blind area.

The recommended venting size is 0.025 mm for crystalline polymers, and 0.038

mm for amorphous polymers.




degradation.


Reduce the likelihood of burn marks by avoiding excessive melt temperatures

during the moulding process:

Reduce the injection pressure.

Reduce the injection speed.

Reduce the screw rotation speed.

Decrease the barrel temperature.

Check the band heaters on the barrel and nozzle, and calibrate the

thermocouple.

2.15.5 Delamination

Delamination (sometimes called lamination or layering) is a defect in which the surface of

a moulded part can be peeled off layer by layer.

FIGURE . Delamination causes layer-wise peel-off on the surface of a moulded part

Causes of delamination

Delamination can be caused by several factors, including:

Incompatible materials blended together

Too much mould release agent being used during the moulding process

Low melt temperature in the cavity

Excessive moisture

Sharp corners at the gate and runner

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Remedies

Change the material preparation

Avoid using foreign material or contaminated regrind material in the moulding

process.


Smooth all of the corners at the gate and runner.

Sharp corners can tear apart melt flow and cause lamination.


Avoid using excessive mould release agent to fix the de-moulding problem.

Delamination can be caused by excessive use of mould release agent. You should

repair the ejection system or other problems to eliminate the difficulty of de-

moulding instead of over-using the mould release agent.

Follow the pre-dry instructions for the specific material and pre-dry the

material properly before moulding.

Excessive moisture heats up and forms steam, which results in lamination on the

surface.

Increase the barrel temperature and mould temperature.

If the melt temperature is too low, layers of material are formed because they

can't bond to each other. When ejected or subjected to stress, they separate from

each other.

2.15.6 Dimensional variation

Dimensional variation is a defect characterized by the moulded part dimension varying

from batch to batch or from shot to shot while the machine settings remain the same.

FIGURE . Dimensional variation is an unexpected change of part dimension

Dimensional variation can be caused by

Unstable machine control

A narrow process window

Improper process conditions settings

A broken check ring (within the injection unit)

Unstable material property

Remedies

Improve the material preparation

Contact the material vendor and change the material lot if the material has patch-to-

patch variation.

Pre-dry the material before moulding if the material is too wet.

Limit the percentage of regrind material added to the origin material.

The irregular particle size can cause different levels of mixed melt material,

and lead to unstable moulded part dimensional variation.

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Change a mould design/component

Fix or adjust the ejection system if the moulded part is bowed or distorted during

ejection.

Design a proper runner and gate system for a specific mould and material.

Use plastic injection moulding simulation software to optimize the runner system

dimension to assure a smooth melt flow into the cavity.

Change a machine component

Replace the check ring if it is broken or worn out.

Replace heater bands or the thermocouple if it is out of order and causes unstable

melt flow.


Increase the injection and packing pressure.

Make sure enough material is delivered into the cavity during the filling and

packing stages.

Increase the injection and packing time to be sure enough material is delivered

into the cavity during filling and packing stages.

Make sure the mould temperature is even by checking the cooling system.

Set-up screw metering and injection stroke, screw rotation speed, and back

pressure properly so that they fall within the process window.

2.15.7 Discoloration

Discoloration is a color defect characterized by a moulded part's color having changed

from the original material color.

Causes of discoloration

This defect can be caused by either material degradation or contamination from the following problems:

The material staying in the barrel too long.

The barrel temperature being too high, causing the color to change.

Contamination caused by reground material, different color material, or foreign material.



Brittleness Burn marks

Remedies


Maintain proper housekeeping for origin materials and regrind materials storage

to avoid contaminated materials.


Add an adequate venting system.

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To avoid discoloration (or burn mark) due to poor venting or air traps, use the

recommended venting size: 0.025 mm for crystalline polymers, and 0.038 mm for

amorphous polymers.

Select a proper machine

Use a different size injection moulding machine.

The typical shot size should be between 20 and 80 percent of machine injection

capacity. For temperature-sensitive materials, the range should be narrowed

down, depending on the material. Plastics simulation software can help you select

the right size machine for a specific mould. This will help avoid the resin

remaining in the heated barrel for prolonged periods of time.


Clean the hopper completely.

It's important to avoid foreign material or different color materials mixing

together before moulding.

Purge the injection unit completely if there is any material changing.

Reduce the barrel temperature and nozzle temperature.

If the barrel and nozzle temperature are too high, the material in the barrel will be

overheated, leading to thermal degradation and the color change.

2.15.8 Fish eyes

Fish eyes are a surface defect that results from unmelted materials being pushed with

the melt stream into the cavity and appearing on the surface of a moulded part.

FIGURE . Unmelted materials in the melt stream causes fish eyes

Causes of fish eyes

Fish eyes are caused by:

Low barrel temperature

If the barrel temperature is too low to melt the materials completely, the unmelted pellets will merge with the melt stream, marring the surface of the part.

Too much regrind

The shape and size of regrind is irregular compared with original material, and can trap more air and cause the material to blend unevenly.


If a high-melt-temperature material is blended into the original material, the

blended material may stay in pellet form, and cause fish eyes during the moulding process.

Low screw rotation speed and back pressure

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If the screw rotation speed and the back pressure setting are set too low, there

might not be enough frictional heating to melt the material completely in the barrel before the injection.

Remedies

Improve the material preparation

Limit or eliminate regrind for practical moulding, depending on part quality

requirements.

Adding ten percent of regrind is a good start, if regrind is allowed.

Store different materials separately and keep covers on the containers or bags to

avoid blending different materials.


Material suppliers usually provide the information about barrel temperature, back

pressure, and screw rotation speed for specific materials. If you've followed suppliers'

recommendations and are still experiencing problems, try making the following

adjustments.

Increase the barrel temperature.

Increase the back pressure to blend melt materials evenly.

Increase the screw rotation speed during the plasticization stage to create more

frictional heat to melt materials.

2.15.9 Flash

Flash is a defect where excessive material is found at locations where the mould

separates, notably the parting surface, movable core, vents, or venting ejector pins.

FIGURE . Flash

Causes of flash

Low clamp force

If the clamp force of the injection machine is too weak to hold the mould plates

together during the moulding process, flash will occur.

Gap within the mould

Flash will occur if the parting surface does not contact completely, due to a

deformed mould structure, parting surface defect, improper machine and mould set up, or flash or foreign material stuck on the parting surface.

Moulding conditions

Improper moulding conditions, such as a high melt temperature (which makes a thinner melt) or high injection pressure, will cause flash.

- 96 -

Improper venting

An improperly designed venting system, a very poor venting system, or a venting system that is too deep, will cause flash.

Remedies

Adjust the mould set-up

Set up the mould to seal properly. A mismatch or undesirable gap between the

cavity and core sides of the mould will result in flash.

Make sure the mould plates are strong enough to avoid deformation during

moulding.

Add pillar support or thicken the mould plates if there is any deformation of

the mould plate during the moulding process.

Check for adequate venting dimensions.



Clean the mould surface.

Flash can be caused by the mould surface not sealing well due to foreign

material remaining between the parting surfaces.

Mill out the surface to keep the sealing pressure of land area around the

cavities high enough.

Adjust the machine settings

Set up the machine and mould to seal properly. Flash can be caused by a poor

seal between the cavity and core sides of the mould, and machine platens that

are not parallel.

Increase the injection moulding machine size. Flash can result from insufficient

machine clamp force.

Adjust the clamp force if the machine capacity does have enough clamp force.


Decrease the barrel temperature and nozzle temperature.

A high melt temperature reduces the melt viscosity, making a thinner melt, which

causes flash. But beware: avoid melt temperatures so low such that the resulting

high injection pressure required causes flash.

Reduce the injection and packing pressure to reduce the clamp force

requirement.

Reduce the feed setting (stroke length) to reduce metering (over-fill).

Increase the injection time or slow down the injection speed.

2.15.10 Flow marks

A flow mark or halo is a surface defect in which circular ripples or wavelets appear near

the gate.

- 97 -

FIGURE . Flow marks

Causes of flow marks

Flow marks are caused by cold material near the gate or lack of compensated material during the packing stage. The problem can usually be attributed to:

Low melt temperature

Low mould temperature

Low injection speed

Low injection pressure Small runner stem and gate

According to a recent visual analysis using a glass-inserted mould, the flow mark defect

can also be due to cooling of the flow front portion on a cavity wall and the repeated

phenomena of "getting over" and cooling with the subsequent melt. This is discussed in

Ripples.

Remedies


Change the size of the cold well in the runner system to trap the cold material

during the filling stage.

The proper length of the cold well is usually equal to that of the runner diameter.

Increase the runner system and gate size for the specific mould and material.

Flow marks are sometimes caused by a restrictive runner system and gate size

that freeze-off prematurely so that the material cannot be compensated during

the packing stage.

Shorten the sprue length or use a hot runner design instead of a cold runner

design.


Increase the injection pressure and packing pressure.

Increase the barrel and nozzle temperature.

Increase the mould temperature.

2.15.11 Jetting

Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas,

such as the nozzle, runner, or gate, into open, thicker areas, without forming contact

with the mould wall. The buckled, snake-like jetting stream causes contact points to form

between the folds of melt in the jet, creating small-scale "welds".

- 98 -

FIGURE . Jetting vs. normal filling pattern.

Effects of jetting

Jetting leads to part weakness, surface blemishes, and a multiplicity of internal defects.

Contrast this with a normal filling pattern, in which melt advances in a progressive pattern from the gate to the extremities of the cavity, as illustrated above.

Remedies


You'll often find that the trouble lies with the gate design.

Direct the melt against a metal surface.

Use an overlap gate or a submarine gate as shown in Figure below.

FIGURE . Using an overlap gate to avoid jetting

Slow down the melt with a gradually divergent flow area.

A tab or fan gate provides a smooth transition from the gate to the cavity. This

reduces the melt shear stress and shear rate.

FIGURE . Using a tab gate (left) and a fan gate (right) to avoid jetting

Enlarge the size of the gate and runner or reduce the gate-land length.

You can also relocate or redesign the gate in one of the following ways to reduce

jetting.


Adjust the ram-speed profile.

- 99 -

Use an optimized ram-speed profile so that melt-front velocity is initially slow

when the melt passes through the gate, then increases once a dispersed,

"tongue" shaped material is formed near the gate. Figure below illustrates this

technique.

Adjust the barrel temperature to increase or decrease the melt temperature

incrementally.

The explanation for this is not well understood, but might be related to the die-

swell effect and the change of the melt properties (such as viscosity and surface

tension). For example, for most polymers, die swell increases as temperature

decreases, while some materials, such as rigid PVC, exhibit increasing die swell as

temperature increases.

FIGURE. Adjust ram-speed profile to avoid jetting.

2.15.12 Ripples

Ripples are the wavelets or small fingerprint-like waves near the edge or at the end of

the flow.

FIGURE . Ripples

Cause of ripples

According to a recent visual analysis using a glass-inserted mould, the ripple defect is

due to the flow front portion of the melt cooling on a cavity wall, and the repeated

phenomena of the subsequent melt "getting over" and cooling, as shown in the below.

Flow-front velocity and mould temperature have a stronger influence on the formation of ripples compared to the shape of the gates and the melt temperature.

- 100 -

FIGURE . (1) Normal filling with no ripples. (2) Generation of ripples with cold mould wall

and low melt-front velocity.

Remedies

Actions that increase the melt-front velocity or mould/melt temperature will help to eliminate the ripples.

Modify the part design

Increase the part thickness.

Change the mould design

Make sure the runner system, including the sprue, runners, and gates, is

adequate for the specific part.

Place an adequate venting system throughout the entire mould, especially around

the end of the flow path.

Make sure the venting system is large enough that the air present in the cavity

can escape during injection. Be careful, however, that the venting system is not

so large that it causes flash at the edge of the moulding. The recommended

venting size is 0.025 mm for crystalline polymers, and 0.038 mm for amorphous

polymers.


Increase the mould temperature.

Increase the injection speed.

This will create more viscous heating and reduce the melt viscosity.

Increase the injection pressure.

Be careful not to exceed the machines's capacity. The operating injection pressure

should normally be limited to 70 to 85 percent of the maximum injection pressure

to prevent accidental damage to the machine's hydraulic system.

Increase the melt temperature.

Be careful not to introduce material degradation due to prolonged exposure at an

elevated temperature.

2.15.13 Short shot

A short shot is a moulded part that is incomplete because insufficient material was

injected into the mould. In some cases, short shots are intentionally produced to

determine or visualize the filling pattern. But problematic short shots occur when the

- 101 -

polymer melt cannot fill the entire cavity (or cavities)-most commonly at thin sections or

extremities.

Causes of short shot

Any factors that increase the resistance of polymer melt to flow or prohibit delivery of sufficient material into the cavity can cause a short shot. These factors include:

Insufficiently-sized restrictive-flow areas, such as gates, runners, and thin

walls.

Low melt and/or mould-wall temperatures.

A lack of vents to bleed the air trapped inside the cavity.

Insufficient machine injection pressure (resulting from high melt resistance

and a restricted flow path), volume, and/or ram speed.

Machine defects such as an empty hopper, blocked feed throat, or a worn non-

return (check) valve that causes loss of injection pressure or leakage of

injection volume.

Premature solidification of the polymer melt due to Hesitation, poor filling

pattern, or prolonged injection time.

Remedies

Several factors influence the polymer's ability to fill the entire cavity. Proper remedial

actions can be taken when the cause of a short shot is pinpointed. Here are some

suggestions.


It's important to facilitate the flow of injected polymer melt; doing so can alleviate short

shots.

Strategically increase the thickness of certain wall sections (as flow leaders).


A properly designed delivery system (sprue, runner, and gate) will facilitate a

more balanced filling pattern. If needed, modify design in the following ways.

Fill the thick areas before filling the thin areas. Doing so will avoid hesitation,

which causes early solidification of polymer.

Increase the number and/or size of gates to reduce the flow length.

Increase the size of runner systems to reduce resistance.

Entrapped air inside the mould cavity can also lead to short shots

.

Place vents at the proper locations, typically near the areas that fill last.

This should help vent the displaced air.

Increase the size and number of vents.


Look closely at the factors that control how material is injected into the mould.

Increase the injection pressure.

Do not exceed the machine's capability. To prevent accidental damage to the

machine's hydraulic system, limit the operating injection pressure to 70 to 85

percent of the maximum injection pressure.

Increase the injection speed. Within the machine limits, this will create more

viscous heating and reduce the melt viscosity.

Increase the injection volume.

- 102 -

Increase the barrel temperature and/or the mould-wall temperature. Higher

temperatures will promote the flow of material through the cavity. Be careful

to avoid material degradation due to prolonged exposure at an elevated

temperature.

The moulding machine might also be the culprit if you're experiencing problematic

short shots.

Check the hopper for sufficient material supply or a clogged feed throat.

Inspect the non-return valve and barrel for excessive wear.

Wear can lead to loss of injection pressure and leakage of injection volume.

2.15.14 Shrinkage and warpage

Shrinkage is inherent in the injection moulding process. Shrinkage occurs because the

density of polymer varies from the processing temperature to the ambient temperature.

During injection moulding, the variation in shrinkage both globally and through the cross

section of a part creates internal stresses. These so-called residual stresses act on a part

with effects similar to externally applied stresses. If the residual stresses induced during

moulding are high enough to overcome the structural integrity of the part, the part will

warp upon ejection from the mould or crack with external service load.

The shrinkage of moulded plastic parts can be as much as 20 percent by volume, when

measured at the processing temperature and the ambient temperature. Crystalline and

semi-crystalline materials are particularly prone to thermal shrinkage; amorphous

materials tend to shrink less. When crystalline materials are cooled below their transition

temperature, the molecules arrange themselves in a more orderly way, forming

crystallites. On the other hand, the microstructure of amorphous materials does not

change with the phase change. This difference leads to crystalline and semi-crystalline

materials having a greater difference in specific volume ( ) between their melt phase

and solid (crystalline) phase. This is illustrated in Figure below. We'd like to point out that

the cooling rate also affects the fast-cooling pvT behavior of crystalline and semi-crystalline materials.

FIGURE. The pvT curves for amorphous and crystalline polymers and the specific volume

variation ( ) between the processing state (point A) and the state at room

temperature and atmospheric pressure (point B). Note that the specific volume decreases as the pressure increases.

- 103 -

Causes of excessive part shrinkage

Excessive shrinkage, beyond the acceptable level, can be caused by the following factors.

The relationship of shrinkage to several processing parameters and part thickness is

schematically plotted in Figure.

Low injection pressure

Short pack-hold time or cooling time

High melt temperature

High mould temperature

Low holding pressure.

Problems caused by part shrinkage

Uncompensated volumetric contraction leads to either sink marks or voids in the

moulding interior. Controlling part shrinkage is important in part, mould, and process

designs, particularly in applications requiring tight tolerances. Shrinkage that leads to

sink marks or voids can be reduced or eliminated by packing the cavity after filling. Also,

the mould design should take shrinkage into account in order to conform to the part

dimension.

FIGURE . Processing and design parameters that affect part shrinkage

Warpage

Warpage is a distortion where the surfaces of the moulded part do not follow the

intended shape of the design. Part warpage results from moulded-in residual stresses,

which, in turn, is caused by differential shrinkage of material in the moulded part. If the

shrinkage throughout the part is uniform, the moulding will not deform or warp, it simply

becomes smaller. However, achieving low and uniform shrinkage is a complicated task

due to the presence and interaction of many factors such as molecular and fiber orientations, mould cooling, part and mould designs, and process conditions.

Warpage due to differential shrinkage

Warpage in moulded parts results from differential shrinkage. Variation in shrinkage can

be caused by molecular and fiber orientation, temperature variations within the moulded

part, and by variable packing, such as over-packing at gates and under-packing at

remote locations, or different pressure levels as material solidifies across the part thickness. These causes are described more fully below.

Differences in filled and unfilled materials

Non-uniform mould cooling across the part thickness or over the part

Cooling rates that differ because of Part thickness variation Part geometry asymmetry or curvature

- 104 -

Differences in filled and unfilled materials

Differential shrinkage for filled and unfilled materials is shown in Figure below. When

shrinkage is differential and anisotropic across the part and part thickness, the internal

stresses created can lead to part warpage.

Filled materials

For fiber-filled thermoplastics, reinforcing fibers inhibit shrinkage due to their

smaller thermal contraction and higher modulus. Therefore, fiber-filled materials

shrink less along the direction in which fibers align (typically the flow direction)

compared to the shrinkage in the transverse direction. Similarly, particle-filled

thermoplastics shrink much less than unfilled grades.

Unfilled materials

On the other hand, if an unfilled moulded part contains high levels of molecular

orientation, shrinkage is anisotropic because aligned chains shrink to a greater extent in the direction of orientation.

FIGURE . Differential shrinkage for both unfilled and filled materials

Non-uniform mould cooling across the part thickness

Non-uniform cooling in the part and asymmetric cooling across the part thickness from

the mould cavity and core can also induce differential shrinkage. The material cools and

shrinks inconsistently from the mould wall to the center, causing warpage after ejection.

FIGURE . Part warpage due to (a) non-uniform cooling in the part, and (b) asymmetric

cooling across the part thickness.

Part thickness variation

Shrinkage increases as the wall thickness increases. Differential shrinkage due to non-

uniform wall thickness is a major cause of part warpage in unreinforced thermoplastics.

More specifically, different cooling rates and crystallization levels generally arise within

- 105 -

parts with wall sections of varying thickness. This causes differential shrinkage, resulting

in part warpage, as shown in Figure below.

FIGURE Larger volumetric shrinkage due to the high crystallization level in the slow

cooling areas (e.g., the thick sections) leads to differential shrinkage and thus part

warpage

Part geometry asymmetry or curvature

Geometric asymmetry (e.g., a flat plate with a large number of ribs that are aligned in

one direction or on one side of the part) will introduce non-uniform cooling and

differential shrinkage that can lead to part warpage, as shown in Figure below.

FIGURE . The poor cooling of the mould wall on the ribbed side causes a slower cooling of

the material on that one side, which can lead to part warpage

2.15.15 Silver streaks

Silver streaks are the splash appearance of moisture, air, or charred plastic particles on

the surface of a moulded part, which are fanned out in a direction emanating from the

gate location.

FIGURE . Silver streaks

Causes of silver streaks

Moisture

Plastic materials absorb a certain degree of moisture during storage. If the

material is not dried properly before moulding, the moisture residing in the

resin will turn into a steam during the injection process and splay on the

surface of the moulded part.

Air

During the plasticization period, a certain amount of gas can be trapped and

blended into the melt material. If the air does not escape during the injection

process, it could splay out on the surface of the moulded part.

- 106 -

Degraded (charred) plastic particles

There are a couple of reasons degraded (charred) plastic particles will splay on the

surface of a moulded part.


When moulding with two materials, as you switch from one material to another,

the residual particles left in the barrel could be charred if the second material is

being moulded at a higher temperature. In addition, contaminated, rejected parts and regrind will re-contaminate virgin material in the next batch of moulded parts.

Barrel temperature

Improper barrel temperature setting may degrade polymer molecules, and they will begin to char.

Shot volume

If the shot size is below 20 percent of the machine injection capacity, especially

for temperature-sensitive materials, the melt resin will remain in the barrel too

long and will begin to degrade.

Remedies


Dry the material properly before moulding, according to the resin supplier's

instructions.





degradation.

Check for adequate venting dimensions.




These precautions will deter material from degrading during the process.

Size a proper injection machine for a specific mould.

The typical shot size should be between 20 and 80 percent of the machine

injection capacity. For temperature-sensitive materials, the range should be

narrowed down, depending on materials. Plastics simulation software can help you

select the right size injection machine for a specific mould. This will help to avoid

a prolonged residence time for resin in the heated barrel.

Fully purge the older material from the barrel if switching material from one to the

other.

Old material particles left behind could be charred.

Increase the back pressure. This will help minimize air blending into the melt

material.

Improve the venting system.

It's important to allow air and steam to escape easily.

Decrease the melt temperature, injection pressure, or injection speed.

- 107 -

2.15.16 Sink marks and voids

A sink mark is a local surface depression that typically occurs in mouldings with thicker

sections, or at locations above ribs, bosses, and internal fillets. A void is a vacuum

bubble in the core.

Causes of sink marks and voids

Sink marks and voids are caused by localized shrinkage of the material at thick sections

without sufficient compensation when the part is cooling. A sink mark almost always

occurs in extrusion on a surface that is opposite to and adjoining a leg or rib. This occurs because of unbalanced heat removal or similar factors.

Factors that lead to sink marks and voids are:

Low injection and packing pressure

Short hold time or cooling time

High melt temperature or mould temperature

Localized geometric features

After the material on the outside has cooled and solidified, the core material starts to

cool. Its shrinkage pulls the surface of the main wall inward, causing a sink mark. If the

skin is rigid enough, as in engineering resins, deformation of the skin may be replaced by

formation of a void in the core. Figure illustrates this phenomenon.

FIGURE. Sink marks and voids are created by material shrinkage without sufficient

compensation.

Remedies

Sink marks and voids can usually be alleviated by fine-tuning some combination of part

and mould design and the conditions under which the part is moulded. Use the

suggestions below to pinpoint and fix the problem.


Conceal sink marks by adding a design feature, such as a series of

serrations on the area where they occur.

Figure illustrates this technique.

- 108 -

FIGURE . Sink marks can be eliminated by creating a design, rib, serrations.

Modify the part thickness design as suggested to minimize the thickness variation.

Re-design the thickness of the ribs, bosses, and gussets to be 50 to 80 percent of

the attached (base) wall thickness.

Figure shows the dimensions we prescribe.

FIGURE Recommended dimensions for ribs, bosses, and gussets


Increase the size of gates and runners to delay the gate freeze-off time.

This allows more material to be packed into the cavity.

Add more vents or enlarge the vents.

Vents allow air trapped inside the cavity to escape.

Relocate the gate to or near a thicker section.

This allows them to be packed before the thinner sections freeze off.


Increase the cushion at the end of the injection stroke.

You should maintain a cushion of approximately 3 mm.

Increase the injection pressure and the holding time.

- 109 -

Increase the screw forward time and decrease the injection rate.

Decrease the melt and mould-wall temperatures.

Increase the cooling time.

Check the non-return valve for possible material leakage.

2.15.17 Weld lines and meld lines

A weld line (also called a weld mark or a knit line) is formed when separate melt fronts

traveling in opposite directions meet. A meld line occurs if two emerging melt fronts flow

parallel to each other and create a bond between them. Weld and meld lines can be

caused by holes or inserts in the part, multiple gates, or variable wall thickness where

Hesitation or race tracking occurs. If weld or meld lines can't be avoided, position them

at low-stress and low-visibility areas by adjusting the gate position and dimension.

Improve the strength of weld and meld lines by increasing the local temperature and

pressure at their locations.

How to tell the difference between weld and meld lines

Traditionally, the "meeting angle" is used to differentiate weld lines and meld lines. As

illustrated in Figure below, a meeting angle, , smaller than 135º produces a weld line;

greater than 135º, a meld line. Note that the weld line surface mark disappears when the

meeting angle reaches 120º to 150º. Normally, weld lines are considered to be of lower

quality than meld lines, since relatively less molecular diffusion occurs across a weld line

after it is formed.

FIGURE Weld and meld lines

Problems caused by weld lines

Weld lines are generally undesirable when part strength and surface appearance are

major concerns. This is especially true with fiber-reinforced materials, because the fibers

do not bridge the weld lines and often are oriented parallel to them, as illustratedn Figure below.

- 110 -

FIGURE . Fiber distribution parallel to the weld line leads to a weaker bond

Strength of weld lines

The exact strength of the weld line depends on the ability of the flow fronts to weld (or

knit) to each other. The strength of the weld-line area can be from 10 to 90 percent as

strong as the pure material used. With such a wide range possible, the conditions that are favorable to better weld-line quality are worth examining:

High injection pressure and speed.

High melt and mould-wall temperature.

Formation of the weld lines closer to the gate.

A temperature difference of less that 10ºC between the two emerging melt fronts.

If a weld line forms before the filling is complete and is immediately subject to additional

packing pressure, the weld line will typically be less visible and stronger. For complex

part geometry, flow simulation helps to predict the weld/meld-line position with respect

to changes in the tool design, and to monitor the temperature difference.

FIGURE . Improving he weld-line position by modifying the delivery system.

- 111 -

Remedies


Increase the wall thickness.

This will facilitate the transmission of pressure and maintain a higher melt

temperature.

Adjust the gate position and dimension or decrease the part thickness ratio.


Increase the size of gate and runners.

Place a vent in the area of the weld/meld line.

This will eliminate entrapped air, which would further weaken the weld/meld-line.

Change the gate design to eliminate weld/meld lines or to form them closer to the

gate at a high temperature and under high packing pressure.


Increase the melt temperature, injection speed, or injection pressure.

- 112 -

2.16 Questions 1. Who invented the first screw-style injection-moulding machine, and when?

2. What are some of the advantages of using a screw injection machine over a

plunger machine?

3. What is the one major advantage to using a plunger-type machine?

4. What material is used as a standard for determining the capacity of an injection

cylinder? 5. What percentage of this capacity should be injected during any single cycle? 6. How do you calculate the weight of one material versus another, knowing the

specific gravity of both? 7. Name the three heater zones found in the injection barrel. 8. Where is the fourth zone? 9. How much pressure can the average moulding machine generate in the injection

cylinder? 10. What is the primary purpose of the clamp unit? 11. What is the formula for determining how much clamp force is required? 12. How is projected area determined? 13. What happens if:

i. Excessive clamp force is used? ii. Not enough clamp force is used?

14. What are the four groups into which all primary parameters are categorized?

15. What two methods are utilized for heating plastic in the injection barrel?

16. How can energy costs for heating the barrel be reduced by 25 percent?

17. What is the difference between injection pressure and hold pressure?

18. In your own words, how would you define back pressure?

19. List one advantage each for the hydraulic and mechanical clamp systems.

20. What is meant by the term gate-to-gate cycle?

21. Why should the mould open slowly?

22. Why is control of distance so critical to producing parts at low cost?

23. What information is needed for determining moulding costs?

24. What is meant by machine-hour rate?

25. What is meant by the term setup?

26. Why is it important to control as many parameters of the moulding process as

possible?

27. How are part quality requirements normally established?

28. List two property effects that result from:

i. Increasing injection pressure

ii. Decreasing injection pressure .

29. Why is it a good idea to have two different setup settings for one production run?

30. In your own words, how would you define bridging?

31. At what temperature should the nozzle heater normally be set?

32. What is the main advantage of using insulation jackets on the injection barrel?

33. How is the largest sprue diameter determined?

34. What are the two main advantages to using hot runner systems?

35. What is the definition of stress as used in this chapter?

36. What is the recommended minimum amount of draft required for injection

moulding?

37. Why is draft required?

38. Why should excessive moisture be removed from plastic materials before

moulding?

39. What is a hygroscopic plastic material?

40. What can be considered the most important component of the injection moulding

process?

41. What single item controls the consistency of a cycle?

42. Name the three items that should be inspected by the operator.

43. What is the one thing an operator should not do if anything seems different?

44. Why is housekeeping by the operator so important?

45. Why is input from the operator so important to the company?

46. What is the given definition of plastic?

47. Why is heat applied to the plastic for injection moulding?

48. What is the primary purpose for applying pressure to the plastic?

- 113 -

49. What is the primary purpose for applying cooling to the plastic?

50. As material performance requirements go up, what happens to processibility?

51. What is the primary reason for using a filler in a material?

52. What is the primary reason for using a reinforcement?

53. Why is a mould needed for the injection-moulding process?

54. How would you define the purpose of the sprue bushing?

55. Describe flash and list two causes of it.

56. What are the two major advantages of using hot runner systems?

57. Name the three parts of an ejector pin.

58. How is the ejector system typically actuated?

59. Where should the gate be located if at all possible?

60. Which shape is best for the cross section of a conventional runner?

61. What causes air to be trapped in a mould?

62. What can be done to a mould to allow trapped air to escape?

63. Why should the runner be vented?

64. Not considering initial product design, what are the four root causes of most

injection-moulded defects?

65. What percentage (range) of the barrel capacity should be emptied every cycle?

66. What is the term used to describe a material that absorbs moisture from the

atmosphere?

67. What happens to moisture in the material as it travels through the heating

cylinder of the machine?

68. What are the advantages by using Microprocessor Injection Moulding Machine.

69. Write four faults & their remedies in Injection Moulding.

70. Write Plastic flow in Injection Machine.

71. How you will calculate Clamping Tonnage for a given Mould.

72. Write down the criteria for selecting an Injection Moulding Machine.

73. For a given Mould How you will set the Machine.

74. Write down the advantages of Screw Type Machine over Plunger Type Machine.

75. Compare & Contrast between Toggle & Hydraulic Clamping.

- 114 -

2.17 References:

Internets

1. www.scudc.scu.edu

2. www.standardplasticcorp.com

3. www.manufacturingcenter.com

4. www.designfax.net

5. www.morganindustriesinc.com

6. www.westlandusa.com

7. https://american.redcross.org

8. www.zen8319.zen.co.uk

9. www.staffs.ac.uk

10. www.roboticsonline.com

11. www.internet-order.com

12. www.seattlerobotics.org

13. www.vectorsite.net

14. http://in.geocities.com/bolurpc

15. www.icyclopedia.com

16. www.vestaweb.com

17. http://islnotes.cps.msu.edu

18. www.sdplastics.com

19. www.pitfallsinmoulding.com

20. http://claymore.engineer.gvsu.edu

21. www.crtlabs.com

22. www.engr.uconn.edu

23. www.technologystudent.com

24. www.balasainet.com/pcbolur

25. www.tut.fi

26. www.bpf.co.uk

27. www.bpf.co.uk

28. http://kazmer.uml.edu

29. www.plastics-technology.com

30. www.azom.com

31. www.4spe.org

32. www.rcplastics.com

33. www.dow.com

34. www.tecrep.com

35. www.iplas.com

Books

1. Injection Moulding Handbook by Donald V. Rosato & Dominick V.Rosato

2. Injection Moulding Theory & Practice By Rubin I

3. Injection Moulding by A.S.Athalye

4. Injection Moulding Machines by F.Johannaber

5. Plastics Injection Moulding by Douglas M.Bryce

6. Handbook Of Thermoplastics Injection Moulding By Dyson R W

7. Plastics Engineering Handbook By Fradoes J

Paulson Training Programmes (CBT)

http://www.scudc.scu.edu/

http://www.standardplasticcorp.com/

http://www.manufacturingcenter.com/

http://www.designfax.net/

http://www.morganindustriesinc.com/

http://www.westlandusa.com/

https://american.redcross.org/

http://www.zen8319.zen.co.uk/

http://www.staffs.ac.uk/

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