Patent GB-981677-A (GB)
Title: Process and apparatus for the simultaneous biaxial stretching of webs of thermoplastic films
IPC: B29C55/16
Filed: 1962-09-14
Abstract: 981,677. Stretching films. KALLE A.G. Sept. 14, 1962 [Sept. 16, 1961], No. 35198/62. Heading B5B. Thermoplastic films are stretched simultaneously in the longitudinal and transverse directions, the two margins of the film being gripped by clips which are independent of each other, at intervals such that the distances between clips, measured from centre to centre, are less than 10%, preferably less than 5% of the width of the unstretched film, the two series of clips being caused to diverge laterally under the longitudinal tension of the film while the film is moving forward at a temperature such that the desired stretching takes place, the stretched film being released from the clips, at a point where they no longer diverge laterally when the film has cooled to a temperature at least 20‹ C. below the stretching temperature. preferably the margins of the film are gripped by the clips before entry into the stretching zone at a point where the film temperature is still at least 20‹ C. below the stretching temperature, the clips being approximately at the stretching temperature at the time of gripping the film, and the stretched film, while still gripped by the clips, is cooled throughout its width, suitablyby means of an air blast. The rate of feed of the film may be 5-10 metres/min. or more, using narrow clips (e.g. 15 mm. wide). Linear stretch ratios of 1: 3 to 1: 4 are preferred, the transverse stretch ratio being generally equal to the linear stretch ratio, although for film having a final width of 1 to 1À5 m., the stretching zone may be 1-5 m. long, the run-out area with substantially parallel guidance of the film margins being of the same order, and the run-in area somewhat shorter, e.g. under 2 m., for film of the stated width. The width of the run-out area may be reduced slightly towards its end, e.g. by 1-2% of the width of the fully stretched film, so as to minimize any transverse tension set up in the cooled film up to the point when the clips open. Specified materials include polyethylene terephthalate, polystyrene, polyvinyl chloride and polypropylene. Apparatus includes roller means for the supply of unstretched film and the take-up of stretched film, two laterally oppositely disposed endless means for supporting clips holding the film margins over part of the film path, a pair of divergent guide rails for the clips, means for heating the film and means for closing and opening the clips, the construction being such that the intervals between clips, measured from centre to centre parallel to the path of the film, are below 35 mm., preferably below 20 mm., the clips being caused to grip the margins of the film, to diverge along the guide rails under tension from the take-up roller whereby the film is simultaneously stretched longitudinally and transversely, and to release the stretched film at a point where it has travelled, without substantial stretching, through a distance of about 1 m. from the stretching zone. Preferably the clips are also caused to move, without substantial divergence, through about 1 mm. of the film path before the film enters the stretching zone. The clips may be adapted to grip over an area corresponding to 50 to 70% of their width and the gripping surfaces of the clips may be of different yielding capacity when gripping a film to be stretched. Guide rails may be of recumbent, inwardly headed T-section (Figs. 6a, 6b, 6d) or of H- section with reference to the film path (Fig. 6c), the clips being provided with coaxial rollers, as shown. In Figs. 6a, and 6c, distance strips 10 are provided to reduce the friction of the roller 2 on the centre portion 17 of the T-rail, while in Fig. 6b, the inside flanks of the flanges 18 of the T-rails may be slightly inclined for the same reason. For wider clips (e.g. 20 to 35 mm.) the guide unit of Fig. 6d, having three pairs of rollers, may be used. The clips holding the margins of the film may be assisted in their forward movement by additional advancing devices located outside the guide rails in front of, within and/or beyond the stretching zone, but acting only in a specific area of the stretching frame, e.g. in its run-in area, its stretching area or its run-out area. Thus endless belts provided with recesses or projections engaging respectively with projections or recesses of the clips may be used, or at the points where the clips are introduced into and guided out of the stretching frame, horizontally disposed wheels which within a specified angle, are engaging or disengaging individual clips from above or underneath may be used to cause the clips to move at the desired speed, whereupon the clips may be advanced by another advancing device operating by means of an endless belt. In Fig. 4a, conveyer belts 111, 11<SP>11</SP>, 11<SP>111</SP> and 11<SP>1111</SP> engage abutments 8a of clips of the type shown in Fig. 3a, as shown in Figs. 4b, 4c and 4d. Underneath the clip guides are provided wheels 12<SP>1</SP> and 12<SP>11</SP> from which, during the semicircular path of the clips, upwardly movable lugs are inserted into notches in the abutments 8b at the opposite ends of the clips and then become disengaged again. Figs. 4b-4d show conveyer belts with differently serrated surfaces, SCH being the guide rail, and closing and opening of the clips taking place in the vicinity of points 14<SP>1</SP> and 14<SP>11</SP>. Fig. 5 illustrates an advancing device suitable for use with clips of the type shown in Fig. 3b, acting from above on the lever of the clamping means 4. Serrated belts engage the clips at the point in the run-in area where they are closed and restrict their forward movement to the speed of the film. In the run-out area the clips are controlled by rigid belts having an external foam rubber layer which engages the lever of the clamping means 4 and conveys the clips past the clip opening device. The circular guiding out of the clips may be performed as in Fig. 4a.