(Invited) Modeling Morphology Distribution in Injection Molded Polypropylene Parts

Abstract

The UNISA code, a software for the simulation of injection molding developed at the University of Salerno, considers a series of specific models to consider several aspects of the mutual effect of flow and crystallinity: the effects of temperature, pressure and flow on relaxation times, the effect of crystallinity on viscosity, the effect of flow on nucleation density and spherulitic growth rate. Each of the effects mentioned above was experimentally analyzed and described by a model for the iPP grade considered in this work. The combination of all these specific models becomes a model for the morphology evolution during polymer processing. In order to verify the accuracy of the predictions, a series of injection molding tests were obtained applying a system which allows to control the cavity surface temperatures. This technique allows to keep, for assigned time intervals, the cavity surface temperature at intermediate values between injection and cooling channels temperatures. A modulation of the level of the cavity surface temperature and of the time, it was kept active, allowed to control the final sample morphology all the way from the complexity of a standard injection molded part down to a completely (skin and shear layers free) spherulitic structure. The fibrillar layer morphology was related to the achievement of critical values of both the molecular stretch and the mechanical work, the latter being performed after the achievement of the critical molecular stretch. The dependence of the morphological layers (skin, shear, spherulitic layers, the latter including the transition from the shear zone) thicknesses upon the heating conditions was satisfactorily described by the models adopted.