guide the liquefied plastic material in the direction of the arrows 16 are cut into the outer periphery of the mandrel 10 in the top area of the mandrel 10 run the helical grooves 15 and feed the plastic material in the direction drawn by the curved arrows 16 into the slit 12 which widens in the processing direction between the mandrel 10 and the jacket 11 over the rest of the course not illustrated of the process whose direction is shown by the straight arrows 17 the plastic material flows first into the die slit then leaves the blown film die begins to solidify and finally forms a film bubble which is passed on for further direct processing or wound up fig2 to 5 show graphs where the speed of the extruders a b c d is plotted over the time that a process of the invention is being carried out at the start of time z 1 the production of a specific first film composite has already been running a long time at time z 1 the purging process starts in the illustrated embodiment with the following measures the speed of all of the extruders involved is decreased until the total mass throughput reaches at time z 2 a low level which avoids.