Investigations of film thickness variations in blown film extrusion when using air guiding systems

The mass throughput and thus the productivity of a blown film line strongly depends on heat transfer from the film. Existing cooling systems are therefore constantly being further developed. Usually, the film is convectively cooled by cooling rings, coupled with an internal bubble cooling (IBC) system to increase the surface area and the cooling rate. Convection has a major disadvantage which is a low cooling efficiency due to a low heat transfer coefficient. Against this background a flexible air guiding system was developed at the Institute for Plastics Processing (IKV), Aachen, Germany. This system encloses the bubble expansion zone and creates a flow gap between an air guiding flexible membrane and the film bubble. In this gap the cooling air velocity is increased, which leads to a higher cooling efficiency. The novel system can be adjusted to many bubble geometries during the extrusion process and increases the mass throughput by up to 62%. However, higher cooling air velocities lead to film movement in the bubble expansion zone and thus also to considerable film thickness variations. In this article the air guiding system influence on the thickness variations is investigated by quantifying the film movement versus the process parameters. For this purpose, a digital image processing system is used which corelates the film movement phenomena with the film thickness variations.