GB1058090A - Process for continuous biaxial stretching of plastic film webs - Google Patents

Abstract

1,058,090. Films. KALLE A.G. Dec. 27, 1963 [Dec. 29, 1962], No. 51034/63. Heading B5B. In the continuous biaxial stretching of a synthetic plastic film, the film is first stretched transversely and then longitudinally between feed and draw roller systems with limitation of the resulting transverse shrinkage, the transversely stretched film being caused to pass through the feed system at a temperature at least 10‹ C. and preferably at least 30‹ C. below the longitudinal stretching temperature after which it is brought to the said temperature. After the longitudinal stretching, while maintaining the effect of means for limitation of transverse shrinkage but without further longitudinal stretching, the film may be heated to a temperature above the stretching temperature but below the melting point of the film, cooled, and the said limitation effect discontinued. The film may be heated above ambient temperature prior to passing through the feed roller system, and heating and/or cooling of the film before and after stretching respectively may be effected by air blasts at suitable temperatures, such heating being with or without additional radiant heating. Desirably, limitation of transverse shrinkage resulting from longitudinal stretching is effected by maintaining the film under transverse tension without essentially influencing any stage of the longitudinal stretching resulting from the co-action of the feed and draw-off systems and the heating of the film. If desired, however, the speed of forward movement of the clips, e.g. within the stretching zone, may be increased by externally acting advancing means which transmit only so much frictional force that the clips do not overrun the film. Stretching of polyethylene terephthalate films is described. Apparatus as shown comprises spaced feed and draw roller systems Z 1 and Z 2 , two series of non-interconnected clips (not shown), adapted to grip the margins of the film and carried in guide rails R 1 and R 2 , and return guide rails R 3 , R 4 , heating means for the film issuing from the feed roller system, cooling means for the film before entering the draw roller system, and heating means (pre-heating rollers W 1 , W 2 , W 3 and W 4 ) located in front of the entry to the feed roller system. Said heating and cooling means may include means for delivering air blasts. The guide rails are advantageously arranged so that they converge slightly from the beginning to the end of the longitudinal stretching zone, the angle of convergence being only a few degrees. The apparatus may be modified to permit subsequent heat-setting of the film by replacing the draw roller system Z 2 with one or two pairs of rollers so shaped that they engage only that part of the film surface not gripped by the clips, and also extending the clip guide rails, the clips being opened only after completion of the heatsetting step. In this case each pair of rollers consists advantageously of a rubber-sleeved roller and a steel roller.