The focus of this study is on thermally induced residual stress, which is the predominant cause for dimensional imperfections in unfilled injection-molded plastic products. A new viscoelastic phase-transformation model was proposed to simulate and predict the residual stress within injection-molded articles as induced during the cooling stage of the injection-molding cycle. The calculated results are in good agreement with the literature experimental data. Numerical simulation of a residual stress problem can be used to guide corrective measures if the problem arises and also to prevent a potential problem from occurring in the first place.