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.
We have proposed a new method of calculating the crystallization process of flowing resin. A program for simulating the crystallization during the filling stage of injection molding process was developed, and applied to the molding process of polypropylene in a simple planar mold. For the case of short filling time (10s), only the resin near the surface of the mold started to crystallize during the filling stage, while for the case of long filling time (30s), crystallization started during the filling stage at all depths in the mold. For both cases, the crystallinity was highest at the melt front and lowest at the gate throughout the filling stage. For short filling time, the crystallization progressed in the packing-cooling stage, and the relative crystallinity reached 100% at the end of the cooling stage. For long filling time, however, the crystallization almost stopped during the packing-cooling stage due to high remaining pressure (about 60 MPa), and the relative crystallinity did not reach 100%.
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