A UNIFIED K-BKZ MODEL FOR RESIDUAL-STRESS ANALYSIS OF INJECTION-MOLDED 3-DIMENSIONAL THIN SHAPES

The flow-induced and thermally induced residual stresses during inject ion molding of a thin part with complex geometries are predicted. The injection molding process was considered to consist of a filling and a post-filling stage (packing coupled with cooling). Additionally, the analyses were applied to successive stages of the process. The model t akes into account the viscoelasticity of the molding polymer, which ha s been neglected in most previous works, because of the complexity of its inclusion. A unified K-BKZ viscoelastic constitutive model, capabl e of modeling both the fluid-rubbery state and the glass state of amor phous polymers, was employed for simulating this problem. For the flow -induced residual stress predictions of the filling stage, a quasi-ste ady state approximation was employed for each element of the part, for the calculation of stress profile and subsequent stress relaxation af ter cessation of flow. Stress calculations were provided for the therm ally induced residual stress predictions of the post-filling stage. Th ese explicit calculations led to the results of pressure and temperatu re distributions of the part during the post-filling stage into the vi scoelastic constitutive model. Additionally, the pressure and asymmetr ic temperature profiles of the post-filling stage were based on finite element packing analysis coupled with a boundary element cooling anal ysis of the molding process. Finally, the total residual stress in the part was obtained via superposition of the flow-induced and thermally induced residual stresses. An example is provided to demonstrate the entire concept. The results indicate that thermally induced residual s tress is higher than the flow-induced residual stress by one to two or ders of magnitude.