Residual stress and warpage models for complex injection molded parts

In the postfilling stage of an injection molding process, thermal induced residual stress usually develops during the solidification of the melt with non-uniform through thickness temperature distribution. Furthermore, the packing pressure is applied through the gate, and will be different in different locations. Since the local melt pressure varies with time due to the change of melt viscosity, the material solidifies under different frozen-in pressure. The stress relaxation behavior of the plastic materials also complicates the stress field. This study uses a simple thermoviscoelastic model to calculate the residual stress developed in the postfilling stage and a elastic model to determine the warpage after demolding for three dimensional complex parts. Based on the calculation of the injection molding process, the temperature and pressure histories in the postfilling stage can be determined, and used to calculate the developed stress field. The model divides the thickness into two regions, liquid and solid phases. Residual stress calculation is applied in the solid region, while the liquid region is kept at the melt pressure without any strain. The resulted in-mold stress fields are used to calculate the part warpage after demolding.