The following account describes investigations of dimensional and stiffness characteristics of representative polyacrylic and silicone rubbers over the approximate temperature range 20 °C–175 °C. The results have led to a quantitative understanding of distinctive features in the behaviour of these elastomers, encountered during the course of their repeated use as tool materials in composite component manufacture, conforming with an explanation in terms of thermally induced cross-linking. It is concluded that silicone rubber offers more promise than polyacrylic rubber in applications where behavioural stability and a reasonable degree of control are required.