GB812387A - Film production - Google Patents

Abstract

<PICT:0812387/IV (a)/1> <PICT:0812387/IV (a)/2> A film of organic thermoplastic material that has been formed by extruding the material in tubular form, stretched by means of fluid pressure held within a section of the tubing, and thereafter reduced to a substantially flattened state, is treated to remove sag or creases by subjecting the film while held under tension in a substantially flattened state to a heat treatment sufficient to cause shrinkage of the film in the relaxed state. Heat treatment may be applied before winding, or during a rewinding process, and the film may be in a substantially flattened tubular form (see Fig. 2) or may be slit to form one or more lengths of flat film, as in Fig. 1. As shown, film 1 is held inflated between pairs of nip rolls 2, 3 and progressively flattened between one or two series of guide rollers 4, 9 respectively. Heat is applied by heaters 7 to the flattened tubular film in Fig. 2 or by heaters 7 and reflectors 8 to the two lengths of film 1a, 1b in Fig. 1, before the film passes to wind-up rollers 6. Where heat treatment is applied to the film in tubular form, the distance apart of the flattened tube walls is preferably no more than one tenth of the length of the major diameter of the film in this region. The film may be made of polythene, terephthalic acid linear polyesters, especially polyethylene terephthalate, vinyl polymers, for example polystyrene, polyvinyl chloride, and vinyl chloride/vinylidene chloride polymers, thermoplastic cellulose esters, or rubber hydrochloride. Preferably, heat treatment is applied by means of radiant or infra-red heaters immediately before the film is wound, and during treatment the film may be held under sufficient tension to prevent substantial overall shrinkage.