Stationary and stability analysis of the film casting process

Abstract

Film casting process is widely used to produce polymer film: a molten polymer 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 a inhomogeneous decrease of the thickness. Thickness as well as width instabilities may be observed above a critical draw ratio. An isothermal and time dependent two-dimensional (2D) membrane model is proposed and compared to a non-constant width 1D model. Newtonian and viscoelastic constitutive equations have been tested. The influence of the processing parameters (draw ratio and aspect ratio) and of the rheology of the polymer (Deborah number) on the film geometry is first determined. The onset of the draw resonance instability is finally studied by linear stability analysis and through the dynamic response to small perturbations. A critical curve splitting the processing conditions into a stable and an unstable zone is derived. It is shown that an increase of the air-gap between the die and the roll improves the stability of the process. Numerical results concerning periodic fluctuations of the flow in unstable conditions are compared with previous experimental results.