Morphology distribution within injection molded parts, interpretation on the basis of stretch and work evolutions

Abstract

Isothermal experimentations carried out under steady shear clarified that, if the flow has adequate intensity, the polymer crystallization takes place into highly oriented morphological elements aligned along the flow direction. A criterion based on local critical values of both the shear rate applied, and the mechanical work spent was identified. However, rather than the shear rate, the molecular stretch is representative of the effect of flow on the molecules and it represents a more appropriate choice to adopt in a criterion. The aim of this paper is to find a criterion based on the critical values of both a molecular stretch, and the mechanical work and to apply it to the complete processing of the material (from pellets to a final product through an industrial process). Since polymer processing operations are carried out under unsteady conditions at least according to the Lagrangian sense, the interest to a criterion formulation which considers unsteady conditions automatically came in. It is assumed that if a critical amount of mechanical work is carried out while the stretch is above its critical value, there a fibrillar morphology will form; however, if once the critical stretch is reached it relaxes below its critical value, the accounting of the mechanical work has to start again. The criterion was applied to the analysis of a set of iPP injection molded samples obtained under a very wide range of thermal boundary conditions. The effects of temperature, pressure and flow on relaxation times, nucleation density, crystalline growth rate, as well as the interrelation among these quantities, were accounted for into a simulation software adopted to determine the evolutions of the molecular stretch and mechanical work. The results were combined into the criterion in order to obtain cross section morphology distribution into the final moldings. The dependence of the morphological layers widths upon the processing conditions was satisfactorily described.