VORRICHTUNG UND VERFAHREN ZUR HERSTELLUNG GESTRECKTER FILME

This invention relates to a tubular stretching method for producing a stretched film by passing a tubular thermoplastic resin film between nip rollers having mutually different peripheral speeds and inflating the tube by the internal pressure under heating and a stretching apparatus to be employed therefor. The nip rollers consist at least of three sets of nip rollers, i.e. nip rollers R1 on the introduction side, nip rollers R2 at fold end portion after stretching and take-up nip rollers R3. A slit-like compressed air jet nozzle is disposed in the gap between a pair of rollers R2 and compressed air is jetted from this nozzle into a stretch portion bubble generated between R1 and R2 so as to inflate and stretch the bubble under heating. In this instance the compressed air is controlled either manually or automatically by sensing the bubble diameter in order to keep a desired bubble diameter, whenever necessary. After stretching, the tubular film is folded by a folding apparatus and is then guided into the nip rollers R2 and its both ends are cut open into two flat films by a film opening knife disposed inside or outside the nozzle between the nip rollers R2. Each flat film is then guided to right and left guide rollers and is taken up by the nip roller R3 after the slit-like jet nozzle and knife are withdrawn. The apparatus of the present invention can adjust either manually or automatically the bubble diameter and the inner pressure during tube stretching operation and can be used advantageously for tubular stretching of porous films.