The residual stresses in a molded product can be mainly classified into two types according to the mechanism of their generation. One is what is called flow induced residual stress and the other is called thermal stress. In the present paper, the mechanism on the generation of these residual stresses and the method for predicting them in our PDC CAE system for the injection molding are explained. The flow induced residual stress is due to freezing of flow stress in its relaxation process and is concerned with molecular behavior such as an orientation. In our system the Leonov model representing viscoelastic behavior of a molten polymer is used for simulating the relaxation and freezing of flow stress. The thermal stress is caused by non-uniformity temperature distribution in a molding product. In our system, this thermal stress and creep strain and shrinkage/warpage deformation are calculated using the thermodynamic equations of equilibrium under the load of the thermal strain which arises in proportion as temperature lowers in the cooling process. By the simulation results, the flow induced residual stress is smaller than the thermal stress. The flow induced stress, however, is not able to be neglected, because it becomes the cause of anisotropy of mechanical properties and optical properties of a molded product.
