Morphology and structure development during injection molding with fast mold temperature evolution

The effect of temperature, pressure and flow on relaxation time, nucleation density, crystallites growth rate, the interrelation among these quantities and the distributions of deformation rate and cooling time during the process determine the morphology distribution in the final injection molded part. A model linking all these quantities was developed to describe morphology evolution during the process. The effect of flow on nucleation density and growth rate of an iPP was described on the basis of a molecular stretch parameter; the molecular stretch evolution was described by a nonlinear Maxwell model whose relaxation time was determined by temperature, pressure and crystallinity and by the molecular stretch. Following literature experimental indications the transition between spherulitic and fibrillar morphology was related to the local viscous dissipation, which in turn is related to the evolution of rheology during crystallization. In this work, the predictions of the model are compared with the cross section morphology distributions of samples obtained by injection molding, coupled with fast mold temperature evolution, of an iPP accurately characterized for rheology, quiescent crystallization, effect of flow on nucleation and spherulitic growth rates and spherulitic/fibrillar transition. The model predictions reproduce main characteristics of the experimental results for both pressure evolution and final morphologies distributions.