The moulding of thermoplastics in thick sections presents other shrinkage problems, especially where mould shrinkage is high (e.g. highly crystalline materials). In a thick section of an injection moulded product，for example, the outside layers in contact with the cold mould cool rapidly.
The inside layers remain hot for longer because polymers are poor thermal conductors. As the centre layers cool，shrinkage occurs (often to a greater extent than the quickly cooled outer layers) and in doing so pulls the outer layers away from the mould wall causing sinking at the surface .
In extreme cases (e.g. polypropylene) cavitation can occur as well as， or instead of，sinking. The inside of the cooling section shrinks away from itself producing voids or cavities which may result in essentially hollow centred sections which severely weakens the product (Figure 1.1b). Sinking is reduced by
- reducing part thickness;
- incorporating fillers;
- maintaining an internal pressure during cooling.
The most convenient way of producing an internal pressure is by the incorporation of a gas in the melt. This can be done by dispersing a blowing agent (e.g. an azo-carbamide) into the polymer which decomposes during processing to provide the gas (nitrogen).
The product is a slightly cellular moulding. An alternative is the gas (nitrogen) injection technique (e.g. Cinpress) which forms a tunnel of gas through the centre of thick sections. An advantage of the latter is that surface quality is not impaired through the escape of gas through the moulding surface as happens with blowing agents.