The most productive processing of plastics is injection moulding. In order to keep cycle times short, processors try to cool the hot molten material injected into the mould as quickly as possible, but this means that the solidified moulded part does not reach theoretical maximum quality. In amorphous plastics, internal stresses remain, in partially crystalline plastics the degree of crystallinity will be at most around 90% of the optimum value. The ideal state can be approached by subsequent heat treatment, so-called tempering. Michael Sepe, owner of an American consultancy, reveals the secrets of tempering different plastics in a series of articles in the journal Plastics Technology. The following are his tips for tempering the commercial (amorphous and semi-crystalline) plastics he processes in the largest quantities.