(297g) Polypropylene Film Casting

During film casting of a polymer melt, the melt is extruded through a flat die before rapid cooling on a chill roll. The polymer film is stretched in an extension-dominated flow between the die and the chill roll since the chill roll is run at a higher velocity than the polymer melt exits the die. This induces some orientation in the film. In this work, we study the effects of material properties and process conditions on film formation of polypropylene in the region between the die exit and the chill roll (air gap). The temperature, film geometry and velocity profiles in the air gap are measured as a function of position within the film. Variables such as the polymer molecular weight (which impacts the polymer viscosity), the draw ratio and the die temperature are systematically varied. We find that the centerline velocity and strain rates increase with distance from the die. Draw direction velocities decrease towards the film edges. Neck-in increases with increasing die temperature and decreasing molecular weight (viscosity) due to the viscosity effects on edge beads. The centerline temperature decreases more rapidly as the draw ratio is increased. Experimental results are compared to 3-d finite element simulations.