Instability Phenomena in Film Casting Process

This paper deals with instability phenomena in PET film casting. An experimental investigation makes it possible to confirm the occurrence of draw-resonance above a critical draw ratio, and to show interesting coupling effects between thickness and width fluctuations. A theoretical model is developed on the basis of some important assumptions: constant width, vertical film, isothermal regime and Newtonian behavior. It allows to calculate the response to a sudden increase of the chill roll velocity and to predict the critical draw ratio (D R = 20.3). Other types of perturbations of the roll velocity (sinusoidal oscillation, random fluctuations) are also treated using this model.