Prediction of Residual Stresses in Injection-Molded Plates Using the Residual Temperature Field Concept

Injection molding is the most widely used technology for precision forming of thermoplastic products. However, high temperature and pressure gradients during solidification can be locked in as residual stresses, resulting in distortion of the product after ejection. Increasing demand for tight dimensional tolerances makes it increasingly important to predict such distortion. In this article, thermal-induced residual stresses generated during the filling, packing, and cooling stages of injection molding are estimated by implementing the residual temperature field concept to describe the relationship between the thermal history and the frozen-in strains. Although they are an order of magnitude lower than thermal stresses, it is shown that the approach can be extended to account for pressure-induced residual stresses, taking advantage of the orthogonal lines already used as integration paths for the residual temperature field. A crystallization model is coupled to the thermal analysis as a heat source to account for its effect on the thermal history of the material. Polymer plates were injection molded under symmetrical and asymmetrical cooling conditions, and the values of deflection were measured using image processing tools. Simulated and experimental results agreed within 7.5%. POLYM. ENG. SCI., 2019. (c) 2019 Society of Plastics Engineers