Commercial Processes For Polymers (AQA GCSE Design & Technology): Revision Note
Exam code: 8552
Injection moulding
Injection moulding forces molten thermoplastic into a closed metal mould under pressure
How does injection moulding work?
Polymer granules enter a heated barrel from a hopper
A rotating screw melts and moves the polymer towards the nozzle
Molten polymer is injected into the mould cavity
The part cools, the mould opens and ejector pins release it
Typical uses
Casings, clips, toys, bottle caps, chairs and electrical fittings
Quality and production
Accurate temperature, pressure and cooling time reduce defects such as sink marks, short shots and warping
Expensive tooling suits mass production
Worked Example
Why are ejector pins needed?
[1 mark]
Answer
They push the cooled moulding out of the mould without the operator having to pull it out by hand. [1]
Extrusion
Extrusion continuously forces softened polymer through a shaped die to make a constant cross-section
How does extrusion work?
Granules are fed into a heated barrel
A screw softens and pushes the polymer forward
The polymer passes through a die with the required profile
The continuous section is cooled and cut to length
Typical uses
Pipes, window frames, rails, sheet, film and cable insulation
Quality and production
Die shape, temperature, screw speed and cooling control the dimensions and surface finish
The process is efficient for long continuous products
Worked Example
State one limitation of extrusion.
[1 mark]
Answer
The made product cannot vary, it must have the same cross-section along its length. [1]
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