A NUMERICAL MODEL FOR NON-ISOTHERMAL FLOW INDUCED CRYSTALLIZATION IN THERMOPLASTIC POLYMERS

摘要

In industrial forming processes such as extrusion or injection molding, polymeric materials experience severe thermomechanical conditions: high pressure, high deformation rates, very fast cooling kinetics and important temperature gradients. In semi-crystalline thermoplastics, such as polypropylene, these phenomena have a major influence on the crystallization occurring during cooling, which determines the final microstructure. Predicting the solidified part properties by numerical simulation requires the implementation of a crystallization kinetics model including both the thermally and flow induced effects. In this work, a numerical model simulating polymer crystallization under non-isothermal flows is developed. The model is based on the assumption that the polymer melt elasticity, quantified by the first normal stress difference, is the driving force of flow-induced extra nucleation. Two sets of Schneider equations are used to describe the growth of thermally and flow induced nuclei. The model is then coupled with the momentum equations and the energy equation. As an application, a simple shear flow configuration between two plates (Couette flow) is simulated. The relative influence of the mechanical and thermal phenomena on the crystallization development as well as the final morphology distribution is finally analyzed as a function of the shearing intensity, in terms of nucleation density and crystallite mean sizes.