Prediction of shrinkage and warpage in injection moulded components using computational analysis

Abstract

Injection moulding is a process by which molten polymer is forced into an empty cavity of the desired shape. At its melting point, polymers undergo a volumetric expansion when heated, and volumetric contraction when cooled. This volumetric contraction is called shrinkage. Once the mould cavity is filled, more pressure is applied and additional polymer is packed into the cavity and held to compensate for the anticipated shrinkage as the polymer solidifies. The cooling takes place via the cooling channels where the polymer is cooled until a specific ejection criterion is met. Heat from the polymer is lost to the surrounding mould, a part of this heat reaches the cooling channel surfaces, which in turn exchange heat with the circulating cooling fluid. Due to the complexity of injection moulded parts and the cooling channel layout, it is difficult to achieve balanced cooling of parts. Asymmetric mould temperature distribution causes contractions of• the polymer as it cools from its melting temperature to room temperature. This results in residual stresses, which causes the part to warp after ejection. Given the understanding of the mathematical model describing the heat transfer process during the cooling stage, the objectives of this study were three fold. Firstly, an alternative numerical model for the heat transfer process was developed. The proposed model was used to investigate the cooling stress build-up during the injection moulding process.