Plastic Part Manufacturing-ppt

8/17/2019 Plastic Part Manufacturing-ppt

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Two main components of extruder are Barrel and Screw

Die is not a component of extruder; it is a special tool that is fabricated for the particular

profile to be produced

Internal diameter of barrel typically ranges from 25 to 150 mm and L/D ratio ranges from

10 to 30.

Higher L/D is used for thermoplastics and lower for elastomers

Extruder rotates around 60 rpm


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The screw performs three functions and is divided into sections:

Feed Section: Stock is moved from the hopper port and is preheated.

Compression section: Polymer is transformed into liquid consistency, air entrapped amongst the pallets is extracted from the melt, and the material is

compressed

Metering section: Melt is homogenized and sufficient pressure is developed

to pump it through die opening.

Function of Breaker Plate

Filter containment and hard lumps

from the melt.

Build pressure in the metering

section

Straighten the flow of polymer and

remove its memory of circular

motion imposed by screw.


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Typical pressure gradient in an extruder Extruder and die characteristics

Side view cross-section ofdie for coating electrical

wire by extrusion


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Die configuration and extruded products


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Melt fracture

(a) Sharkskin (b) Bambooing


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In this process polymer is heated to a highly plastic state and forced to flow under

high pressure into a mould cavity, where it solidifies.

The process produces discrete components that are always net shape.

Complex and intricate shapes are possible to produce, however the challenge is to

design mould so that the part can be ejected successfully

Process is economical in large production as the cost of mould is very high.


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Mould is closed Melt is injected cavity

Screw is retracted Mould opens and part is ejected

Typical Moulding Cycle


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Details of two plate mould for thermoplastic Injection

Moulding (a) Closed (b) Open mould


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Three plate mould (a) Open (b) Close


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Two alternative type of Injection Moulding machines

Screw preplasticizer Plunger type (Older machine)

Shrinkage Considerations

High thermal expansion, which is up to 10% for typical thermoplastics

Contraction of crystalline polymer is more than than amorphous

Shrinkage is expressed as reduction in linear size (mm/mm) when cooled

from moulding temperature to room temperature.

Fillers in the plastic tend to reduce shrinkage


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Defects in Injection MouldingShort Shots: solidified before completely filling the cavity. The defect can

be corrected by increasing temp. and/or pressure.

Flashing: the polymer melt is squeezed into the parting surface between mold

plates; it can also occur around ejection pins. The defect is usually caused by

(1) vents and clearances in the mold that are too large; (2) Injection pressure

too high compared to clamping force; (3) melt temperature too high; (4)

excessive shot size.

Sink marks and voids: Occur in thick molded section. When the outer skin

solidifies and the and inner core remains mushy. Later when it solidifies a

depression is seen on the outer surface due to shrinkage. A void is also caused

by the same phenomenon. These defects can be addresses by increasing packing pressure following injection. A better solution is to design a part

having uniform section.

Weld lines: It is formed when the melt comes from two sides and forms

weld. The properties of weld line are inferior.


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Compression Molding

(1)Charge is loaded (2) and (3) Charge is compressed and cured and

(4) part is ejected and removed


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Port transfer molding

Plunger transfer molding


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Blow Molding

(1) Extrusion of parison (2) Parison is pinched at the top and sealed at the

bottom around a metal bow pin as the two halves of the mold come together

(3) the tube is inflated so that it takes the shape of the mold cavity and

(4) mold is opened and part is removed


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Injection Blow Molding

Stretch Blow

Molding


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Rotational Molding or Rotomolding


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Vacuum Thermoforming 1. A flat sheet issoftened by

heating.

2. The softened

sheet is kept over

a concave cavity

3. A vacuum draws

the sheet into

cavity

4. The plastic

hardens on

contact with thecold mold surface

and the part is

removed and then

trimmed


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Pressure Thermoforming

Vacuum Thermoforming with positive mold


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Pre-stretching the

sheet prior todraping and

vacuuming it

MechanicalThermoforming


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Plastic Part Manufacturing-ppt