Defects in multilayer plastic films II: Streak formation in extruded films

The companion paper [Prakash, O., A. Moitra, Defects in multilayer plastic films I: Interface defects in Coextrusion (2004), companion paper.] dealt with prediction and control of defects that may form in multilayer plastic films as the constituent materials pass through a co-extrusion die. A methodology for developing a robust process window for elimination of these defects was presented. The defects considered were (a) zig-zag defects (b) parabolic patterns forming over the width of the film, and (c) viscous-finger formation at the interface. However, there can be other forms of defects that are widely encountered, for example formation of streaks (parallel to the machine direction) in the film. The streaks may be on the surface or sub-surface and can compromise not just the visual quality but also the adhesion performance of the films. present work, we consider the origin of one form of streak-formation in multilayer films. A mechanism for streak formation in relation to thermal conditions on cooling/calendaring rolls is proposed and it is shown that rapid solidification of the film must be achieved to prevent streak formation due to thermal issues.