H. Ghoneim et Ca. Hieber, INCORPORATION OF DENSITY RELAXATION IN THE ANALYSIS OF RESIDUAL-STRESSES IN MOLDED PARTS, Polymer engineering and science, 37(1), 1997, pp. 219-227
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.