INCORPORATION OF DENSITY RELAXATION IN THE ANALYSIS OF RESIDUAL-STRESSES IN MOLDED PARTS

Thermo-rheologically / piezo-theologically simple viscoelastic constit utive equations are adopted for the material behavior of a generic pol ystyrene, in both the deviatoric and dilatational domains, in order to investigate the effect of density relaxation on the development of th e thermal residual stresses in a thin injection-molded strip. A prelim inary study is undertaken to assess the ability of the proposed dilata tional viscoelastic constitutive equations to capture some of the dens ity-relaxation behavior such as the isobaric volume relaxation followi ng a sudden quench from above the glass-transition temperature and upo n constant rates of cooling at different temperatures and pressures. I n this preliminary study, different combinations of relaxation functio ns and shift factors are investigated. An appropriate combination is s elected and used for the residual-stress analysis. The numerical simul ation of the development of the stresses in a one-dimensional cavity q ualitatively predicts the correct stress profile across the thickness of the molded part, as well as the dependency of this profile on some of the material properties and molding conditions. In general, the inv estigation presented in this paper suggests that density relaxation pl ays an important role in the development of residual stresses in molde d parts.