Injection Moulding

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http://www.youtube.com/watch?v=eUthHS3MTdA (Embed)

Name a specific polymer suitable for the manufacture of the chair. [2]

Polypropylene.

Explain why this polymer is suitable for the chair. [6]

Polypropylene is a thermoplastic and therefore suitable for injection moulding.

It is stiff but will flex slightly to prevent fracture under shock load, for example when someone sits on it or leans back.

It is available in a range of colours to suit different settings.

It does not require any finishing/future maintenance.

It is weather resistant but the colour and surface tend to degrade over time if left outside.

Use notes and diagrams to explain how the chair is manufactured. [9]

Stage 1

Plastic granules are inserted into the hopper along with a colour pigment.

Stage 2

Granules are moved along the injection moulder by an Archimedean screw, which compresses the plastic.

Stage 3

Heaters melt the plastic. When sufficient plastic is melted, the screw retracts and a hydraulic ram forces a charge of plastic into the mould.

Stage 4

The moulding is cooled by water coolers within the mould sections. When the item has set, the mould splits. Ejector pins/and or compressed air force the moulding outould.

Stage 5

Complex mouldings may be placed in a suitable jig whilst the polymer is still slightly flexible to maintain dimensional accuracy. Sprue pins and flash would be removeed.

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