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Plastic PartManufacturing

Dr. Sunil J ha

Extrusion

z Two main components of extruder are Barrel and Screwz Die is not a component of extruder; it is a special tool that is

fabricated for the particular profile to be produced

z Extruder rotates around 60 rpmz Internal diameter of barrel typically ranges from 25 to 150 mm

and L/D ratio ranges from 10 to 30.

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Extrusion

Extrusion

zThe screw performs three functions and isdivided into sections:

z Feed Section: Stock is moved from the hopperport and is preheated.

z Compression section: Polymer is transformedinto liquid consistency, air entrapped amongst thepallets is extracted from the melt, and the materialis compressed

z Metering section: Melt is homogenized andsufficient pressure is developed to pump itthrough die opening.

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Extrusion

z Function of Breaker Plate

z Filter containment and hard lumps from the melt.

z Build pressure in the metering section

z Straighten the flow of polymer and remove itsmemory of circular motion imposed by screw.

Extrusion

Typical pressure gradient in an extruder Extruder and die characteristics

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ExtrusionSide view cross-section of die for coating electrical

wire by extrusion

Injection Moulding

In this process polymer is heated to a highly plastic state and forced toflow under high pressure into a mould cavity, where it solidifies.

z Complex and intricate shapes are possible to produce, however thechallenge is to design mould so that the part can be ejectedsuccessfully

z Process is economical in large production as the cost of mould is veryhigh.

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Injection Moulding

Mould is closed Melt is injected in cavity

Screw is retracted Mould opens and part is ejected

Typical Moulding Cycle

Injection Moulding

Two Plate Mould

Closed Open

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Injection Moulding

Three Plate Mould

Closed Open

Compression Molding

Charge is loadedCharge is compressed

and cured

Part is ejected

and removed

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Pot transfer molding

Plunger transfer molding

Blow Moulding

(1) Extrusion of parison

(2) Parison is pinched at the top and sealed at the bottom around a metal bowpin 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 Moulding

Stretch Blow Moulding

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Vacuum Thermoforming

A fl at sheet is so ftened by heat ing.

The softened sheet

is kept over a

concave cavity

A vacuum draws the sheet into cav it y

The plastic hardens on

contact with the cold

mold surface and the

part is removed andthen trimmed

Vacuum Thermoforming

With Positive Mould

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Pressure Thermoforming

Mechanical Thermoforming

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Defects in Injection Moulding

z Short Shots:

z solidified before completely filling the cavity. The defect canbe corrected by increasing temp. and/or pressure.

z Flashing:

z the polymer melt is squeezed into the parting surfacebetween mold plates; it can also occur around ejectionpins. The defect is usually caused by

z (1) vents and clearances in the mold that are too large;

z (2) Injection pressure too high compared to clamping force;

z (3) melt temperature too high;

z (4) excessive shot size.

Defects in Injection Moulding

z Sink marks and voids:

z Occur in thick molded section. When the outer skinsolidifies and the and inner core remains mushy. Laterwhen it solidifies a depression is seen on the outer surfacedue to shrinkage.

z A void is also caused by the same phenomenon.

z These defects can be addresses by increasing packingpressure following injection.

z A better solution is to design a part having uniform section.

z Weld l ines:

z It is formed when the melt comes from two sides and formsweld. The properties of weld line are inferior.