Properties of plastics in a liquid state

In both the solid and the liquid (molten) state, plastics have properties that differ from those of other materials. The way in which these differences are addressed when designing moulds, and also HR systems, and subsequently when running the process, is what governs product quality. The following material properties must therefore be appreciated  

The action of pressure on a plastic produces compression of it, and thus a diminution in specific volume through packing of the molecules. This means that, in contrast to typical fluids, plastics are compressible. 

Following injection of the plastic into the cavity (at a constant temperature, TJ its specific volume has diminished, and the compressed plastic is subject to further isobaric cooling at the holding pressure. The measure of compression of the plastic is the increase in its density. 

Shrinkage is caused by contraction of plastic. When the plastic cools, there is a diminution in the mobility of the molecules, and thermal contraction is associated with a diminution in the distance between molecules. 

Volume shrinkage of a plastic in the mould cavity is 5-7% for amorphous plastics, but 

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