We studied in detail the effects of pressure and shear stress on the crystallization behaviors during injection molding process. Crystallization of isotactic -polypropylene (iPP) was simulated for various injection flow rates. The crystallization started during the filling stage under long filling condition. Moreover, the crystallization in some layers showed two -step growth behavior, while in other layers the crystallization does not progress during the packing -cooling stage. Since pressure becomes very high during molding process under long filling time condition, the change of pressure is the most important factor governing the crystallization.
We have developed a simulation method for injection molding which takes account of the dependency of material properties on crystallinity. The crystallinity is calculated during the simulation, and the thermal conductivity and viscosity of the resin are updated at each time step. We also take account of the latent heat of crystallization and pressure dependence of the thermal conductivity. The equation of state obtained during the crystallization simulation under actual cooling rate is used. We have applied this method to the molding of polypropylene in a rectangular mold. The results show that pressure in the mold rapidly decreases during the packing stage , in good agreement with experiment. It is found that taking account of the crystallinity dependence of viscosity and use of correct equation of state are most important in improving the accuracy of the simulation.
Injection molding processes of crystalline polymers were simulated with a crystallization kinetics program. Crystallinity and spherulite size were evaluated at various points of a product model, by using pressure, shear stress, and temperature data obtained with a computer aided engineering (CAE) program. It was found that crystallinity and spherulite size have maximum values at the same depth between the surface and center layers, suggesting that they are closely related to each other. It was also confirmed that the variation of crystallinity with depth qualitatively agrees with experimental data in the literature.
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