Plastic Injection Moulding Flashcards

1
Q

Injection Moulding

A

 Polymer pellets or granules are fed from a hopper into a cavity and heated to a highly plastic state.
 Polymer melt is forced under high pressure into a mould cavity; where it solidifies.
 Injection unit is either a hydraulic plunger or a rotating screw of an extruder.
 The injection unit consists of a reciprocating screw which turns and mixes the polymer and also functions as a ram injecting the molten plastic into the mould.
 The clamping unit consists of two platens to open and close the mould, keep it aligned and apply clamping force.
 Single cavity or multiple cavities for each injection shot.
 Distribution channel to inject polymer melt.

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2
Q

Mould temperatures

A

 For thermoplastics the mould is kept at ~90°C

 Thermosets are moulded in a heated mould at ~200°C for curing and polymerisation to take place.

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3
Q

Sprue

A

leads from the nozzle into the mould.

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4
Q

Runners

A

lead from the sprue to the cavity.

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5
Q

Gates

A

constrict the flow of plastic into the cavity.

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6
Q

Ejection System

A

eject moulded part at the end of the cycle.

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7
Q

Cooling System

A

removal of heat from the hot plastic.

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8
Q

Air Vents

A

to evacuate the air as the polymer gets injected.

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9
Q

Two-plate mould

A

standard mould with 2 plates, one that retracts to allow the part to be ejected.

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10
Q

Three-plate mould

A

uses 3 plates, 2 of which move. – runner system is separated from the part when the mould is opened.

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11
Q

Hot-runner mould (runnerless mould)

A

Molten plastic is kept in a heated runner plate eliminating sprue and runner waste. Shorter cycle times as no extra cooling or ejecting.

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12
Q

Short shots

A

Solidification of the polymer before completely filling the mould.

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13
Q

Flashing

A

Polymer melt is squeezed into the parting surfaces of the mould.

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14
Q

Sink marks

A

Outer surface solidifies but contraction of the internal material causes the skin to be depressed below its intended profile.

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15
Q

Voids

A

Surface material retains its form and the shrinkage is an internal void due to high tensile stress of the still molten polymer.

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16
Q

Weld lines

A

Boundary formed from polymer melt flowing around a core and meeting from opposite directions.

17
Q

Warpage

A

Deformation of the workpiece after removing it from the mould.

18
Q

Advantages

A

 High volume production.
 Short cycle times.
 Complex and intricate part designs; net-shape.
 Excellent reproducibility.
 Wide range of materials can be used, thermoplastics, thermosets and elastomers.
 Multi-injection moulding allows different polymers to be moulded into one part.

19
Q

Limitations

A

 High volume production only.
 Expensive equipment ~£50-200k (2MN injection pressure).
 Expensive die costs ~£10-100k.
 Complicated process – controlling complex process parameters.
 Shrinkage and warpage problems.

20
Q

Blow Moulding

A

Fabrication of a starting tube of molten plastic (parison) by extrusion or injection moulding.
Inflation of the tube into a mould cavity using air pressure (350-700kPa) to get the desire shape (blow ratio ~7:1).

21
Q

Extrusion Blow Moulding

A

 For high production volumes; automated and integrated with any downstream operations.
 Extrusion of the parison.
 The parison is pinched at the top and sealed at the bottom around a metal blow pin as the two halves of the mold come together.
 The tube is inflated to take the shape of the mould cavity.
 Mould is opened to remove the solidified part.

22
Q

Injection Blow Moulding

A

 Lower production volumes and less widely used
 Stretch Blow Moulding, a variation of injection blow moulding creates favourable stress of the polymer for higher rigidity.
 Parison is injection moulded around a blowing rod.
 Parison is transferred to a separate blow mould.
 Soft polymer is inflated to conform to the blow mould.
 Blow mould is opened and blown product is removed.

23
Q

Stretch Blow Moulding

A

 Blowing rod extends downwards and stretches the injection moulded parison inducing favourable stresses.
 Stretched parison is more rigid, more transparent with better impact resistance.