Shrinkage kinetics and thermal behaviour of injection moulded polymers

We measure the kinetics of shrinkage at the time of the cooling of injected polymers during a cycle of moulding. Dynamic measurement of the shrinkage of the castings after ejection is taken by the method of photomechanical in a thermally controlled environment representative of a workshop production one. The technique of photomechanical used allows measurement without contact of the shrinkage in the principal plan of the part. It is shown that the profile of temperature quickly becomes uniform in the thickness of the part after the phase of ejection and that the kinetics of shrinkage is controlled by the kinetics of cooling for the PS as for PP. One shows the influence of certain processing parameters on the evolution of the shrinkage, in particular, the dependence of the shrinkage according to the temperature of the mould and the packing pressure. Finally one notes the presence of an anisotropy of shrinkage in the case of PP.