6.7 Compression Moulding

Compression moulding is used to manufacture both thermoplastic and thermoset products. The process sequence is:

• The moulding compound is loaded.

• The mould is closed.

• The compression force and the heat of the mould cause flow.

• The mould opens.

• The part is ejected.

All the processes up to now have used materials in granular form, compression moulding however, often utilises raw materials in sheet form. Sections called blanks, of the correct weight, are pre-heated and then placed in the compression moulding tool. This is then closed to form the component. Because of the use of sheets rather than granules the raw material costs are much higher, as it is more expensive to manufacture sheets than granules. One common type of sheet is known as glass mat transfer (GMT); it consists of polypropylene and high levels of glass fibre. The sequence of moulding is shown in Figure 6.19.

Figure 6.19: Compression moulding. (i) Blank placed in (ii) Tool closed, mould, blank compressed (iii) Part ejection

Any recyclate can be used to make further GMT sheets. However the glass levels and flow characteristics of the sheet may be different. As these are generally provided for the manufacturers by a supplier, recycling of GMT by the compression moulders themselves is not common.

Compression moulding of mixed plastics is possible. Pre-melting materials into a suitable weighted blank and placing it into the mould is necessary. Moulding is then carried out in a normal way. Thick...