Computer Modeling for Injection Molding: Simulation, Optimization, and Control

Chapter 6: Residual Stress and Warpage Simulation

Fen Liu

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong, China

Lin Deng and Huamin Zhou

State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, Hubei, China

During the molding process, shrinkage and warpage of products are generally constrained by the surrounding walls of mold within the cavity. Meanwhile, residual stresses are built up within the mold. After the ejection, the product is removed from the cavity and exposed to a completely different set of thermal and mechanical boundary conditions. As a result, part of the generated stress will be relieved by deformation, and the product is allowed to contract without any restrictions or external constraints, which leads to shrinkage or warpage.

Flow stresses during the filling stage will deteriorate the surface optical property of products,¹ while warpage and deformation due to thermal stresses produced in postfilling stages are detrimental to the dimension of products and even result in local cracking. With an increasing demand on dimension stability and quality of products in plastic industry, warpage has become an important criterion in evaluating the quality of injection-molded products. Prediction of residual stresses and warpage therefore provides a starting point for enhancing the quality of molded products through the optimization of processing conditions. ...