Week 5 Processing 3

8/4/2019 Week 5 Processing 3

1/33

Plastic Injection Molding


2/33

Injection Molding

3 major functional units; injection, mold, clamping
http://upload.wikimedia.org/wikipedia/commons/7/76/PlasticsInjectionMoulderJones.jpg


3/33

Plastic Injection Molding is a manufacturing technique for making parts

from thermoplastic and thermoset material s In contrast to the extrusion (which makes

continuous parts of constant cross section),injection molding make discrete parts (withcomplex and variable cross section)

Molten plastic is injected at high pressure into amold , which is the inverse of the desired shape.

The mold is made from metal, usually eithersteel or aluminium widely used for manufacturing a variety of parts,

from the smallest component to entire bodypanels of cars
http://en.wikipedia.org/wiki/Manufacturinghttp://en.wikipedia.org/wiki/Plasticshttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Molding_%28process%29http://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Automobilehttp://en.wikipedia.org/wiki/Automobilehttp://en.wikipedia.org/wiki/Aluminiumhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Molding_%28process%29http://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Plasticshttp://en.wikipedia.org/wiki/Manufacturing


4/33

The key to success in injection moldingare to have; Proper machine for good melting and injecting

of the resin The proper resin to appropriate part

performance A good mold for part definition and removal Proper operation for efficient molding cycle

(mold cycle depends on the design of themold and manufacturing parameters)

Plastic Injection Molding


5/33

The most commonly used thermoplasticmaterials are; polystyrene (low-cost, lacking the strength and

longevity of other materials) ABS or acrylonitrile butadiene styrene (a co-polymer

or mixture of compounds used for everything fromLego parts to electronics housings)

nylon (chemically resistant, heat-resistant, tough andflexible - used for combs)

polypropylene (tough and flexible - used forcontainers

PVC (more common in extrusions as used for pipes,window frames, or as the insulation on wiring where itis rendered flexible by the inclusion of a highproportion of plasticiser ).
http://en.wikipedia.org/wiki/Polystyrenehttp://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrenehttp://en.wikipedia.org/wiki/Nylonhttp://en.wikipedia.org/wiki/Polypropylenehttp://en.wikipedia.org/wiki/Plasticiserhttp://en.wikipedia.org/wiki/Plasticiserhttp://en.wikipedia.org/wiki/Polypropylenehttp://en.wikipedia.org/wiki/Nylonhttp://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrenehttp://en.wikipedia.org/wiki/Polystyrene


6/33

Injection Unit Purpose: to liquify the

plastic materials andthen inject the liquidinto mold Resin is introduced

through hopper Some machines can

have several hoppers (tofed filler, colorants, other

additives)-Injectionmolding act as mixer However, due to limited

size of barrel, mixingcapability is poor


7/33

From hopper hole (feed throat) Barrel made of heavy steel cylinder to

withstand the pressure and temperature

involved in melting the resin 2 types of system used in injection

molding;

Reciprocating screw- similar to extruder screwbut with unique reciprocating action

Ram injector

Injection Unit


8/33

Design of screw- similar to an extrusion screw 3 sections; Feed section- to advance the resin Compression section- to melt the resin Metering section- to homogenize the resin and pump it forward

The screw of injection molding machine is shorter thanextruder, L/D ratios are 12:1 and 20:1 Low L/D ratios suggest the mixing is less efficient in the

injection molding machine The compression ratio (diameter of root at feed zone to

the diameter of root at metering zone) often in the rangeof 2:1 and 5:1 Low compression ratio means less mechanical action is

added during melting process

Injection Unit


9/33

Important measure of the size of aninjection molding is weight of resin thatcan be injected, called shot size

Typical shot size range from 20g to 20 kg Since shot size depends on the density of

the plastic, PS has been chosed as thestandard for rating the machine

Injection Unit


10/33

Reciprocating Screw InjectionMolding Machine

Resin is melt by mechanical shear and thermalenergy from heaters

The molten resin is conveyed to a space at the

end of the screw- collects in a pool Here, the mold is closed


11/33

The entire screw move forward and pushes themolten resin out through the end of barrel

To ensure the resin does not flow backward, acheck valve

ornonreturn valve

is attached to theend of screw Normally the screw will stay in the forward

position, until resin began to harden in the mold



12/33

Retraction of the screw, create space at the end of thescrew

Cooling of the part in the mold, until it can be removed While the part is cooling, the screw turns and melts

additional resin



13/33

Advantages More uniform melting Improved mixing or additives and dispersion

throughout the resin Lower injection pressure Fewer stresses in the part

Faster total cycle



14/33

In this type of injection molding, the resin is fed from ahopper into the barrel, and heated through thermalenergy from the heaters

The molten resin is collect in a pool in a barrel celledinjection chamber

The molten resin is then push forward by the action ofplunger (ram or piston)

To five better mixing, the molten resin is pushed past atorpedo/spreader, impart shear to the melt

Ram Injection- Injection Molding Machine


15/33

Molds Designing and making mold for injection

molding is more complicated than makingextrusion die

Mold Parts mold is placed in betweenstationary plate and the moveable plate


16/33

The connection from the injection unit to themold is through the nozzle

The channel that run through the stationary plate

of the mold is called the sprue channel (materialthat is in the channel is called the sprue) The solid sprue is removed from the finished

part assembly after the part is ejected from the

mold Resin flow from the sprue through the runner(connecting channel) to the mold cavities

Molds


17/33

Mold Bases Assembly of variousmold parts Mold bases can be

purchased as entireunits, then thecavities are cut fromA & B plates


18/33

Runners

Distribution system for the resin from the sprueto the cavities

Flow characteristics (viscosity), temperature and

other factors are important in determining therunner diameter and length If the diameter of the runner is too small or the

length is too long,the resin can freeze in the

runner before the mold is completely full If the runner system is too large, excess materialwould be ejected and too much regrind created


19/33

If the resins have a high viscosity, larger runnersare needed compared to low viscosity resin

The optimum flow of the resin through therunner system depends on the shape anddiameter of the channel

Round channel give the best flow characteristicsbut difficult to machine

Machining cost can be reduce by machining one

side of the mold plates Better shape where the depth of the channel isat least two-thirds the size of the width and thesides are tapered between 2 to 5.


20/33

Runner Channel Shapes


21/33

Secondary Runners

Secondary runnerchannel are used formulticavity molds

The flow into thesecondary channelshould bestreamlined (angle in

flow direction) The streamlined

minimizes shear onthe resin


22/33

Gates

The end of runner and the entry path intothe cavity

The gate shape can also affect the fillingof the cavity, dimension and properties ofthe parts

Gate is the most restricted point ininjection molding system, i.e. forreinforcement and filler + polymer systems


23/33

Gate Design

Small rectangular opening atthe end of the runner channel,connect to the edge of cavity

Edge gate can be below theparting line if the channel andpart are also below the partingline

Or it can be symetricaly aboutthe parting line, if the runnerchannel and part are at bothside of parting line

Edge gate


24/33

Submarine Gate Starts from the edge of the

runner, and goes into the cavityedge at an angle

It narrows to a point as it movesfrom the runner to the cavity

The advantage; separation ofthe parts and the runner isautomatic

Disadvantage; gate cannot beused for some resins becauseof high shear

Submarine Gate


25/33

Tab Gate

By connecting the runner directlyinto the cavity with no reductionin runner cross-section

Used for very large parts where areduction in flow would disturbthe resins flow pattern and mightresult inadequate flow into thecavity


26/33

Fan Gate

Made by reducing thethickness and not thediameter of the runner

channel as it goes into thecavity Used for intermediate size,

and when reinforcement inthe resin cannot flow throughthe edge gate


27/33

Ring Gate

Used to make hollowcylinder parts

The ring gate covers theentire top of the cylinderpart so that the resin flowis downward into the wallof the part


28/33

Cavities

Are actual molding locations Resin enter the cavities through gate, fills

the cavities, and cools to form the solid.The parts are ejected and finished

Cavities are the heart of the moldingprocess, and must be precisely prepared

The shape of the cavities determines theshape of the part


29/33

Materials & Product Consideration

Almost all thermoplastic can be injectionmolded

Resin with low melt viscosity is required;so that the flow through runner, gate,cavity easily done with minimuminjection pressure

Resin with injection molding grades havelow molecular weight and narrowmolecular weight distribution


30/33

Shapes

Hollow parts can be created by allowingthe moveable plate to protrude into thecavity of the stationary plate


31/33

Threads can be placed on the inside of apart by using a core pin that is insertedinto the cavity where the threads aredesired

Shapes


32/33

A hollow part with a hole on the side is evenmore complicated (the core pin is used)

The core pin slide into position after the mold isclosed

The core pin seals against the surface of themoveable plate, prevent flow of resin into thearea

Shapes


33/33

Documents

Week 5 Processing 3