18. Dimensional Stability and Residual Stresses

Most plastic part shaping processes – in particular injection molding and its variants – involve solidification of molten resins, during which they undergo very large decreases in volume, so that the final dimensions of a part can be significantly smaller than that of the cavity in which it was molded. This change in dimensions is generically referred to as shrinkage. As an example, on demolding, the length of a meter-long part can shrink by as much as 1 cm. Differences in shrinkage between the two surfaces of a part can result in the out of plane distortion – even in a nominally flat panel. Such distortions are referred to as warpage. Dimensional stability is used as a generic term for issues relating to shrinkage and warpage – both for the immediate, short term effects observed after demolding and for the smaller, longer term effects due to relaxation in the material.