To simulate buildup and relaxation of flow-induced stresses and molecularorientation in injection molding process, a mathematical model is derivedthat describes the unsteady and non-isothermal flow of compressibleviscoelastic polymer melts in the thin wall mold cavity on the base of thinfilm lubrication approximation. The compressible Leonov viscoelasticconstitutive equation and Tait state equation are used in the model todescribe the effects of compressibility of polymer melt which occur in thepost-filling stage. The model is applied to the injection molding processwhich is treated in terms of a filling, post-filling and cooling stage andis solved using a finite difference method. Stresses calculated with themodel are coupled to birefringence by means of the stress-optical rule.Birefrigence is used to characterize the molecular orientation. The finalresults are given in terms of residual stresses and associated birefringencein the molded part, as influenced by the processing conditions. The resultindicates that, for a given polymer, the main factors affecting flow-inducedresidual stresses and associated birefringence are flow rate and melttemperature, and with the incompressible case flow-induced residual stressesand associated birefringence are increased when the compressibility of themelt is considered. The result are compared with the birfriengencemeasurement in the literature and reasonable agreement is obtained.