Winding of plastic barrier films

Abstract

High quality plastic films are produced using the cast or blown film extrusion processes. Several kinds of resins are coextruded in a film of up to 11 layers to generate the best technical properties at competitive costs. A state-of-the-art film for food packaging has 5 layers including a 3-10 µm thin oxygen barrier layer of the high cost ethylene vinyl alcohol copolymer. The film is pre-treated for the subsequent processes of printing and lamination. The biggest challenge in this context is that some winding defects are able to destroy complete rolls. Two of the most common winding defects for barrier films are cross direction wrinkles and air knots.

Cross Direction (CD)-wrinkles are directly related to the blown film process, though they might be detected only after lamination. The core area is especially prone to these kind of wrinkles, but they can appear everywhere in a roll. A measurement device for the roll hardness is established and used to document the aging of the rolls. In addition to that, the core pressure is measured and analyzed. A high resolution thickness scan of a complete roll shows the web's thickness profile and how it is affected by the oscillation.

Air knots are small spot-like air entrapment zones between the web layers of the roll which can deform the film permanently in combination with the pressure build-up inside the roll. This defect can occur on film rolls of very different resins. Furthermore, the effect is influenced by the pre-treatment process that modifies the surface structure of the web. Experimental winding tests and analytical winding models are used to investigate the correlation between pre-treatment dosage and air knots.