The effect of temperature, pressure and flow on viscosity, relaxation time, nucleation density, crystallites growth rate, the distributions of deformation rate and cooling time during the process determine the distribution of the molecular stretch at the solidification which give rise to the morphology distribution in the final object. The start of crystallization has a dramatic effect on viscosity, relaxation time and, consequently, on the evolution of molecular stretch which in its turn affects growth and nucleation rates. Each of the effects mentioned above was experimentally analyzed and described by a model for the iPP considered in this work. The ensemble of all specific models becomes a model for the morphology evolution during polymer processing and its application to the injection molding process was numerically developed into the UNISA code. The code was previously validated for predictions of evolutions during the process of the pressure distributions along the flow path and for the crystallinity at the sample surface. In this work, we propose a criterion for fibrillar crystallization. The criterion refers to local values of both the work done and the molecular stretch; at the crystallization they have to be higher than critical values and, in addition, the whole amount of work has to be done while the molecular stretch is above a critical value. thickness of both the shear and spherulite layers into injection molded samples are satisfactorily described by the criterion.
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