During the injection moulding process, the polymer undergoes a complex thermomechanical history, which influences the mechanical properties and the final dimensions (with respect to the corresponding mould dimensions) of the part (Titomanlio and Jansen 1996; Viana, Cunha et al. 2001). The process encompasses four stages: filling, packing, cooling and the demoulding. Demoulding is critical when complex geometry parts are produced and distortion or denting may be caused by the ejectors (Araújo and Pouzada 2002).