ABSTRACT

In the film casting process, polymer melt is extruded through a slit die to form a molten film. The film is then taken up by a chill roll, which quenches the molten film and solidifies it for windup. The process is shown schematically in Fig. 10.1. The distanceL between the die exit and the contact point at the chill roll is referred to as the drawing length. The ratio of the take-up velocity vL at the chill roll to the extrusion velocity v0 at the die exit is known as the draw ratio DR. Since the draw ratio is greater than unity, themolten film is stretched and drawn down in its cross section. The reduction in the film cross section involves the decrease of the film thickness and the reduction of the film width, usually referred to as neck-in. In addition, beads or regions of greater thickness are formed at the film edges. Typically, the die opening is in the order of 1 mm and the final film thickness ranges from 25 to 100 Am.