an overpressure forms between the films 3 4 this overpressure stretches the slack film 4 it is thereby assured that the film 4 is stretched between the roll 22 and the station 35 i e it cannot bulge it thereby becomes possible to accurately alight the two marks 12 13 next to the station 35 for the tack sealing if the distance a and a u2032 are exactly equal when the films 3 4 are not stretched and the films 3 4 are made of the same thickness the distance a u2032 will sometimes be slightly greater and sometimes slightly less than a for example on account of the different amount of material in the buffers 5 6 that occur during operation or the different stresses in the films 3 4 over the distance between the sensors 14 15 and the sealing station 35 consequently between the stations 35 40 sometimes the bottom film 4 as shown in fig1 and sometimes the top film goes u201c slack u201d i e it is pulled only by the pneumatic pressure the tension exerted by the buffer 43 therefore acts alternately on the film 3 and then on the film 4 at the transfer points this tension is transmitted only via tack welds.