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Compression Moulding Level 2

This is a common process for shaping thermosets.

Once formed, thermosets cannot be made to flow, so in any shaping process that involves flow, polymerisation must proceed during processing. Therefore, the starting materials for compression moulding are the mix of the molecular species that react to form the thermoset.

Basically the process is a simple one. See figure 1

diagram showing a compression moulding

The starting materials, as powder or pellets, are placed in the bottom half of a preheated mould. The material is made to melt and flow by the heat and pressure applied when the mould is closed. After it has set, the moulding is ejected. The excess material, the "flash", is then removed.

Thermosets made this way can also include other materials such as wood-flour or chalk for cheapness, or short glass fibres or beads for reinforcement. Such materials are called fillers.

Compression moulding is not suitable for the moulding of shapes of thick section, or with large changes of section. The resin possess low thermal conductivities, so that the centre portion of a thick section may not become fully heated and only partially cure. This problem is overcome in transfer moulding.


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Assessment:
Compression moulding is the process used to shape thermosetting plastics by and them in a mould. It produces items, which can resist temperature increases, such as handles and knobs.

A amount of powdered plastic is placed in the highly polished lower cavity of a mould, and a ram closes the mould compressing the material. The heated mould is then opened and the article is removed by an ejector mechanism.

The thermosetting plastic materials used in this process include phenol formaldehyde, urea formaldehyde and melamine formaldehyde. All of these are resins in powder form.
 

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