Isothermal and Anisothermal Models for Cast Film Extrusion

Abstract

Abstract In cast film extrusion, the polymer is extruded through a slit die, stretched in air and then cooled on a chill roll. Isothermal and non-isothermal models have been developed to predict the polymer melt behavior between the die exit and the chill roll. The mechanical resolution is based on the assumption of a uniform thickness in the film cross section. It takes gravity and inertia forces into account and is performed in two steps: the equilibrium equation for a small film element is first written in a curvilinear coordinate system and the stress tensor is then calculated in a local Cartesian system. The temperature field in the film thickness is determined using local heat transfer coefficients, whose calculation is based on a precise description of the radiation and convection phenomena. This local description requires the measurement of air temperature close to and far from the film. The coupling between mechanical and thermal equations is achieved by introducing the temperature dependence of the viscosity. An iterative scheme is used to adjust the stretching force, the length of the film and the contact point on the roll. The theoretical predictions (film profile, velocity, width, temperature) are compared with experimental measurements performed on a pilot line. The agreement is always good, except for the width, whose computed linear variation is in contradiction with the experimental observations.