Justify rotational moulding for a sit-on-top kayak.

Rotational moulding makes large, hollow, seamless parts with even walls and little built-in stress. The low-pressure moulds are relatively cheap, suiting the modest volumes in which kayaks sell.

What are the main limitations of rotational moulding?

Cycle times are long compared with injection or blow moulding. The range of suitable polymers is narrow, and fine detail and tight tolerances are hard to achieve.

Why can a rotational mould cost less than an injection mould?

Rotational moulding uses no injection pressure, so moulds can be thin cast aluminium or fabricated sheet steel. Injection moulds are usually machined from solid tool steel to resist high pressure.

CCEA A-Level Technology and Design 8900

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Processing Materials

Rotational moulding and vacuum forming

3 things to know
AS 1 Paper 1

Overview

Rotational moulding and its polymers and limits, the vacuum forming process, mould design, vents and webbing, and a break-even comparison with injection moulding.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe rotational moulding.

Key term

How is a rotationally moulded part cooled and removed?

Key term

Which polymer is most often rotationally moulded, and why?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe rotational moulding.
Model answer
A measured charge of polymer powder is sealed inside a hollow metal mould. The mould is heated while it turns slowly about two axes, so the melting powder coats the inside evenly.
2
How is a rotationally moulded part cooled and removed?
Model answer
The mould keeps turning while it is cooled by air or water spray, so the wall sets evenly. The mould is then opened and the hollow part lifted out.
3
Which polymer is most often rotationally moulded, and why?
Model answer
Polyethylene is the most common choice. The polymer melts and flows easily, is tough, and resists weather and chemicals, suiting tanks and kayaks.
4
Justify rotational moulding for a sit-on-top kayak.
Model answer
Rotational moulding makes large, hollow, seamless parts with even walls and little built-in stress. The low-pressure moulds are relatively cheap, suiting the modest volumes in which kayaks sell.
5
What are the main limitations of rotational moulding?
Model answer
Cycle times are long compared with injection or blow moulding. The range of suitable polymers is narrow, and fine detail and tight tolerances are hard to achieve.
6
Why can a rotational mould cost less than an injection mould?
Model answer
Rotational moulding uses no injection pressure, so moulds can be thin cast aluminium or fabricated sheet steel. Injection moulds are usually machined from solid tool steel to resist high pressure.

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