Numerical Simulation Of Residual Stress During The Postfilling Stage In Injection Molded Products

Injection molding is one of the most widely used methods of polymer processing, it has many specialities such as high precision of products,lots of polymer species in processing, and high automatization degree. The injection- molded products,due to the influence of various factors such as viscoelasticity of the polymer ,non-linear of process parameters ,flow orientation, non-uniform cooling and pressure gradient in cavity,and during the injection molding the polymer undergoing the melt,the glass condition physical change after under -going the filling ,packing ,cooling and ejection stage ,its'quality is hard to control. The key question is that the polymer has the viscoelasticity, in the non-isothermal change environment, the molding product will have the resi -dual stress. The residual stress could cause the product deformed, has the very tremendous influence to the product optical quality. If we can discover the distribution and the size of the residual stress , theoretically we may avoid the influence which the residual stress brings.The developed in-mould residual stresses mainly arise from two effects, which is namely the flow-induced stress due to flow, the thermally induced stress due to cooling. These stresses create product deformed. The flow-induced stresses during the filling stage are in the order of magnitude smaller than the thermally induced stresses. So the productive residual is the thermally induced stress.In view of the injection molding formation characteristic, by using the linear viscoelasticity model this article computed the thin wall product stress variation. First we computed the temperature field of the postfilling stage. Then according the temperature field, we computed the product stress field. The temperature histories in the postfilling stage can be determined to three stage. The model divides the thickness into two regions, maded by liquid and solid. Residual stress calculation is applied in the solid region, while the liquid region is kept at the local melt pressure. The residual stress was simulated in wall thickness direction and flow direction distribution and in formation change process. According to the analogue result, we discussed mechanismly the residual stress and the evolution situation. It was found that there is a high tensile stress on the surface, a compressive peak value close to the surface, and a parabolic tensile peak at the core region. Residual stress distribution along the thickness is almost the same in the same flow path .