Numerical simulation of the film casting process

The film casting process is widely used to produce polymer film: a molten p olymer is extruded through a flat die, then stretched in air and cooled on a chill roll. This study is devoted to the extensional flow between the die and the chill roll. The film shows a lateral neck-in as well as an inhomog eneous decrease of the thickness. An isothermal and Newtonian membrane mode l, constituted of an elastic-like equation for velocity coupled to a transp ort equation for thickness and a free surface computation, is used. These e quations are solved via the finite element method (continuous Galerkin for velocity and discontinuous Galerkin for thickness). Both tracking and captu ring strategies are used to determine the position of the free surface (lat eral neck-in). The influence of the processing parameters (Draw ratio and A spect ratio) on the film geometry is first determined. The onset of the Dra w Resonance instability is then studied through the dynamic response of the process to small perturbations. A critical curve splitting the processing conditions into a stable and an unstable zone is derived. It is shown, cons istently, with results of a 1D model, that an increase of the air-gap betwe en the die and the roll improves the stability of the process. Numerical re sults concerning periodic fluctuations of the flow in unstable conditions a re compared with previous experimental results. Copyright (C) 1999 John Wil ey & Sons, Ltd.