Forming, redistribution and addition processes Paper and board forming processes - Polymer processes — AQA A-Level Design and Technology: Product Design
Verified courseTeaching note checked against the official specification.
Forming, redistribution and addition processes Paper and board forming processes - Polymer processes explained
Rotational moulding (rotomoulding) forms hollow, seamless polymer products such as water tanks, kayaks and playground slides.
Read the full explanation
A measured charge of polymer powder, usually polyethylene, is placed in a cool female mould. The mould is closed, heated and rotated slowly about two axes so the powder tumbles and fuses into an even coating on the inner wall. Heating continues until the polymer sinters and melts; the mould is then cooled while still rotating so the skin solidifies evenly and the part does not sag. The mould is opened and the hollow product is removed. Because the mould is unpressurised and cheap compared with injection tooling, rotomoulding suits large, low-volume, one-piece hollow shapes, but cycle times are long and wall thickness control is less precise.
Your focus
- Describe the sequence of stages in rotational moulding.
- Explain why the mould rotates about two axes during heating and cooling.
- Justify the selection of rotational moulding for a given hollow polymer product.
Forming, redistribution and addition processes Paper and board forming processes - Polymer processes exam tips
Marking Points
- Identifies rotational moulding as a process for producing hollow, seamless polymer products, typically from powdered polyethylene.
- Describes the sequence: powder charge placed in a cool female mould, mould closed, then heated while rotated slowly about two axes.
- Explains that rotation distributes and fuses the powder into an even coating on the inner mould wall, forming the outer surface of the product.
- States that the mould is cooled while still rotating so the polymer solidifies evenly before the product is removed.
- Recognises the trade-off: low-cost tooling and suitability for large one-piece hollow items, but long cycle times and less precise wall thickness.
Examiner Tips
- 💡Link the process to a named product type, such as a water tank or kayak, to show understanding of why hollow seamless shapes suit it.
- 💡Sequence the stages in order: charge, close, heat and rotate, cool while rotating, demould.
- 💡Compare with blow moulding or injection moulding to justify when rotomoulding is the better choice for large low-volume hollow items.
Common Mistakes
- Assuming rotational moulding uses a rotating screw or ram to force molten polymer into a mould; correction: it uses a slowly biaxial-rotating mould with a powder charge that fuses on the inner wall.
- Thinking the mould is heated and rotated only after cooling; correction: heating and rotation happen together, and rotation continues through cooling to prevent sagging.
- Believing rotomoulding produces solid or high-precision parts; correction: it produces hollow, seamless parts with less precise wall thickness and long cycle times.