Experimental Investigation of the Cooling Flow in the Film Blowing Process

Abstract

Abstract The film blowing process relies on external air cooling of the extruded polymer hose. The air is supplied by a ring nozzle surrounding the die. We investigate the flow as it emanates from a commercial nozzle by a laser light sheet method which reveals the instantaneous rather than the mean flow structure. Flow separation in the nozzle is diagnosed as well as a high degree of disturbances affecting the thermal boundary layer. To improve the flow a modified nozzle is investigated in the same way, which shows how separation is avoided and how the regular structure of a wall jet is attained. The limits of this approach are highlighted. The optical measurements are amended by pressure measurements inside and outside the nozzle. They show what the flow disturbances mean in terms of normal forces on the film.