Analysis of Shrinkage Development of Injection Moulded PS Samples

Abstract

Abstract Modern approaches to the phenomenon of dimensional accuracy in injection moulding link the evolution of shrinkage from the moment of first solidification to a force balance between restraining and constraining forces which sets in inside the mould before ejection. Such an approach needs a complete understanding of what happens inside the cavity during the moulding cycle. In a recent paper [9], a new technique has been presented, by which it is possible to follow the shrinkage development of an injection moulded sample from the moment it starts solidifying to soon after ejection. In this work, with reference to a simple rectangular cavity, shrinkage curves measured by strain gages in different moulding conditions are analysed by means of a simple thermomechanical model recently proposed. Results show that the model satisfactorily predicts the moment shrinkage starts inside the mould and reveal the existence of a restraining force not due to pressure, which builds in gradually in a few seconds from the instant of first solidification. Knowing the value of this force—which can be drawn by Hooke's law from the strain jump at mould opening—the model gives a good description of both shrinkage development inside the mould and ejection shrinkage.