1999-06-15
11:15 at CAB B.9

Dimensional changes and residual stresses in solidifting thermoplastic melts

Vijay K. Stokes

GE Corporate Research and Development, Schenectady, NY 12301

 Large volumetric changes during the solidification of thermoplastic melts in injection molding can affect the final shape and dimensions of a part, and nonhomogeneous cooling can result in residual stresses that can affect part performance. To counteract the effects of shrinkage in a mold, a "packing pressure" that continually pushes in molten material is applied to the solidifying melt. Any model for this process must account for volumetric shrinkage and stress relaxation. This talk will focus on a simple model for the molding process-in which flow effects are neglected, packing pressure effects are accounted for, and the solidifying melt is modeled as a thermorheologically simple thermoviscoelastic material-to illustrate how material properties, the melt and mold temperatures, and the constraints imposed by the mold affect the final size, shape, and residual stresses in injection molded parts. The packing-pressure history will be shown to have a dominant effect. The model predictions will be compared with the results of controlled molding experiments. The significance of these results for interpreting the injection molding process-including the effects of gate freeze-off and the annealing of molded parts-will also be discussed. REFERENCES 1. W. C. Bushko and V. K. Stokes, Polymer Engineering and Science, 35, 351 (1995). 2. W. C. Bushko and V. K. Stokes, Polymer Engineering and Science, 35, 365 (1995). 3. W. C. Bushko and V. K. Stokes, Polymer Engineering and Science, 36, 322 (1996). 4. W. C. Bushko and V. K. Stokes, Polymer Engineering and Science, 36, 658 (1996). 5. W. C. Bushko and V. K. Stokes, SPE ANTEC Technical Papers, 43, 682 (1997).