Plastic Injection Moulding

Question 1

Injection Moulding

Answer 1

 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.

Question 2

Mould temperatures

Answer 2

 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.

Question 3

Sprue

Answer 3

leads from the nozzle into the mould.

Question 4

Runners

Answer 4

lead from the sprue to the cavity.

Question 5

Gates

Answer 5

constrict the flow of plastic into the cavity.

Question 6

Ejection System

Answer 6

eject moulded part at the end of the cycle.

Question 7

Cooling System

Answer 7

removal of heat from the hot plastic.

Question 8

Air Vents

Answer 8

to evacuate the air as the polymer gets injected.

Question 9

Two-plate mould

Answer 9

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

Question 10

Three-plate mould

Answer 10

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

Question 11

Hot-runner mould (runnerless mould)

Answer 11

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

Question 12

Short shots

Answer 12

Solidification of the polymer before completely filling the mould.

Question 13

Flashing

Answer 13

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

Question 14

Sink marks

Answer 14

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

Question 15

Voids

Answer 15

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

Question 16

Weld lines

Answer 16

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

Question 17

Warpage

Answer 17

Deformation of the workpiece after removing it from the mould.

Question 18

Advantages

Answer 18

 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.

Question 19

Limitations

Answer 19

 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.

Question 20

Blow Moulding

Answer 20

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).

Question 21

Extrusion Blow Moulding

Answer 21

 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.

Question 22

Injection Blow Moulding

Answer 22

 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.

Question 23

Stretch Blow Moulding

Answer 23

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