Creep Model on Simulating In-mold Stress in Injection Molded Polymers

In-mold stress may occur for injection molded parts during the cooling stage. It not only affects the shrinkage of the part, but also has great influence on its mechanical properties. In this paper, stress-strain response was studied under the action of temperature and packing pressure in the process of injection molding. Furthermore, a new model was set to simulate the level of in-mold stresses in injection molded parts, and its creep constitutive equation was obtained. Instead of dealing with viscoelastic parameters of the polymer with WLF equation, the viscous factor was calculated with inversion method and the formula of elastic module was directly derived from P-V-T equation. The in-mold stresses in injection molded plates were simulated with the model. The results revealed that the new approach is reliable and accurate.