Plastic Injection Moulding Flashcards
Injection Moulding
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.
Mould temperatures
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.
Sprue
leads from the nozzle into the mould.
Runners
lead from the sprue to the cavity.
Gates
constrict the flow of plastic into the cavity.
Ejection System
eject moulded part at the end of the cycle.
Cooling System
removal of heat from the hot plastic.
Air Vents
to evacuate the air as the polymer gets injected.
Two-plate mould
standard mould with 2 plates, one that retracts to allow the part to be ejected.
Three-plate mould
uses 3 plates, 2 of which move. – runner system is separated from the part when the mould is opened.
Hot-runner mould (runnerless mould)
Molten plastic is kept in a heated runner plate eliminating sprue and runner waste. Shorter cycle times as no extra cooling or ejecting.
Short shots
Solidification of the polymer before completely filling the mould.
Flashing
Polymer melt is squeezed into the parting surfaces of the mould.
Sink marks
Outer surface solidifies but contraction of the internal material causes the skin to be depressed below its intended profile.
Voids
Surface material retains its form and the shrinkage is an internal void due to high tensile stress of the still molten polymer.
Weld lines
Boundary formed from polymer melt flowing around a core and meeting from opposite directions.
Warpage
Deformation of the workpiece after removing it from the mould.
Advantages
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.
Limitations
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.
Blow Moulding
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).
Extrusion Blow Moulding
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.
Injection Blow Moulding
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.
Stretch Blow Moulding
Blowing rod extends downwards and stretches the injection moulded parison inducing favourable stresses.
Stretched parison is more rigid, more transparent with better impact resistance.
