In chill roll or cast film extrusion, the molten polymer is cast through a narrow slot die and drawn down and formed onto a rotating chill roll with the aid of a web forming device. One or more additional chill rolls can be provided fro improved film cooling and higher outputs. A heated roll can also be provided for annealing the film.
The principal advantages of the film casting method compared with the quench-tank method are substantial improvements in the control of optical properties of the film, potentially increased output with larger diameter chill rolls, production of films with a higher modulus, improved thickness and profile control.
The orientation of the die can vary greatly depending on the machinery manufacturer or process conditions. A die that extrudes vertically downward is generally preferred. However, dies which extrude horizontally or at any angle between vertical and horizontal can be used. Horizontal orientation is typically used in the production of rigid sheet over ten mils in thickness. Some European producers use angled dies in an attempt to minimize the distance from the die to the casting roll and to prevent die lip drag. The most common film casting arrangement uses the die in the vertical position.
In cast film extrusion, the molten polymer drops onto the chill roll and contacts tangentially or above the tangent line. The alignment or parallelism of this roll to the die is critical in relation to the falling film. Whenever wrinkling of the film occurs on the casting roll surface, the first roll must be carefully repositioned in relation to the die lips.
The principal advantages of the film casting method compared with the quench-tank method are substantial improvements in the control of optical properties of the film, potentially increased output with larger diameter chill rolls, production of films with a higher modulus, improved thickness and profile control.
The orientation of the die can vary greatly depending on the machinery manufacturer or process conditions. A die that extrudes vertically downward is generally preferred. However, dies which extrude horizontally or at any angle between vertical and horizontal can be used. Horizontal orientation is typically used in the production of rigid sheet over ten mils in thickness. Some European producers use angled dies in an attempt to minimize the distance from the die to the casting roll and to prevent die lip drag. The most common film casting arrangement uses the die in the vertical position.
In cast film extrusion, the molten polymer drops onto the chill roll and contacts tangentially or above the tangent line. The alignment or parallelism of this roll to the die is critical in relation to the falling film. Whenever wrinkling of the film occurs on the casting roll surface, the first roll must be carefully repositioned in relation to the die lips.