In recent years, Leistritz has equipped a great many systems for the production of thermoforming films with twin screw extruders. The reason for this is that increasing numbers of film producers in this field rely on the special vacuum technology which makes it possible to process bottle flakes directly without predrying. Up to 100% recycled material can thus be processed without adding any new material. Since most thermoforming films are used for food packaging, the films are furnished with cover layers by means of co-extrusion during the production process.
For the production of calender films, the pellets are first melted in the extruder. A film die then directs the melt onto a calender stack. This procedure allows for high production speeds and high transparency of the film. The polymers which can be processed in this manner include, among others: PET, PP, PS, PC ABS, PMMA, PLA.
These films are produced on a chill roll system and are biaxially stretched, in both a longitudinal and transverse direction, directly afterwards. Certain properties such as strength, barrier effect and transparency can thus be deliberately influenced. In order to achieve the most finely crystalline condition of the polymer possible, it is cooled quickly after extrusion. Further processing is then carried out towards oriented films in a stretching procedure. The crystallinity of the film is thus significantly increased. Further crystallization is achieved through reheating, which fixes the film structure.