Distortion, Voids and Frozen-in Stress

A characteristic feature of thermoplastics shaped by melt processing operations is that on cooling after shaping many molecules become frozen in an oriented conformation. Such a conformation is unnatural to the polymer molecule, which continually strives to take up a randomly coiled state. If the molecules were unfrozen a stress would be required to maintain their oriented conformation. Another way of looking at this is to consider that there is a frozen-in stress corresponding to a frozen-in strain due to molecular orientation. 

One consequence of this is that in a thermoplastic injection moulding the molecular orientations will vary from place to place according to the flow pattern. This may well set up sufficient stresses in the moulding to cause it to distort from its intended shape. Factors affecting the amount of orientation were discussed briefly in the previous chapter and by the author elsewhere. Sometimes the conditions required to minimise distortion may be such that the moulding cycle becomes too long to be economic. In these cases it may be necessary to modify the moulding or at least the position and number of feed 

Distortion in mouldings can be worse in crystalline polymers than in amorphous plastics. This is because additional stresses may be set up as a result of varying crystallinity from point to point in the moulding so that the shrinkage on cooling from the melt also varies from point to point. This uneven shrinkage sets up stresses which may lead to distortion. 

Another common problem on moulding is voids. These occur when a moulding or extrudate cools and hardens rapidly on the surface. On further cooling the moulding or extrudate cannot shrink inward because the outside dimensions are fixed and therefore the molecules are pulled towards the surface of the product, generating internal voids. At the same time molecules in the inner 

This chapter has attempted to describe briefly some properties of polymers relevant to product design in terms of molecular behaviour. For in depth consideration the reader should consult more detailed reviews (e.g. refs 4, 5 and 6). There also exist specialist monographs concerned with practical aspects of product design (e.g. refs 7 and 11). Mention should also be made of excellent booklets by materials suppliers (e.g. refs 2 and 8) concerned with design aspects. Some material manufactures now supply comprehensive data books backed by computer data bases for multi-point engineering data (e.g. ref. 12). 

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