Increased output of blown film extrusion lines by using a cooling sleeve

Production efficiency is one of the most important demands in blown film production. In many cases, the cooling power is the limiting factor for an increased output. A possible solution for a better cooling is the use of a cooling sleeve right after the outlet of the die in addition to the conventional air rings and internal bubble cooling (IBC). At the Institute of Plastics Processing (IKV), first tests were conducted to investigate the advantages of the use of a cooling sleeve. Therefore, the influence of several geometries of the cooling sleeve surface and different cooling sleeve temperatures on the process stability and the mechanical and optical film properties is investigated. The cooling sleeve surfaces differ in the tapping between inlet and outlet diameter from 0 % (cylindric) to 10 % (conical). The tests show that a high amount of tapping as well as too high resp. low cooling sleeve temperatures cause process instabilities and an uneven thickness profile of the film. While the mechanical film properties (E-modulus, elongation at break, tensile strength) of the films produced by the use of a cooling sleeve (cs-films) do not significantly differ from the values of the reference films, the haze of the cs-films was higher and therefore worse. A measurement of the bubble temperatures above the air ring shows that the use of a cooling sleeve can significant lower the bubble temperature at this point. Because of this and because of the results of the mechanical tests, the principle of a contact cooling is generally applicable. Further research and development on the geometry of the cooling sleeve surface has to be done to improve the process stability and the haze for a possible industrial application.