UN PROCEDIMIENTO Y APARATO PARA EL TRATAMIENTO DE PELiCULAS

摘要

In the continuous production or treatment of oriented or orientable films of organic thermoplastic material, a film having a beaded or thickened portion at or near each edge and parallel therewith is held under or subjected to tension while at least each edge portion is in contact with a moving surface, the beaded or thickened portion along each edge being continuously brought into engagement with and held within a continuous groove in the moving surface. The edge portions of the film may thus be firmly held to prevent or reduce shrinkage when a transverse tension is applied or developed in the film, as for example in the quenching of cast film, in the longitudinal drawing of an orientable film, in the heat-setting of an oriented film or in the transverse drawing of a film. When the transverse tension is much lower than the longitudinal tension, the beadings may remain in the grooves without the provision of any further means to hold them in position, particularly when the surface is that of a drum or roller. When the transverse tension in the fil is sufficiently high to pull the beadings out of the grooves, means are provided over the grooves to hold the beadings in position, e.g. a series of small spring-loaded idle rollers over each groove, or a series of thrust race rollers acting along the inner edge of the groove, the axes of the rollers being parallel to each other and to the film and at right angles to the edges of the film, or a stationary metal plate or a series of stationary metal plates of suitable shape placed over and along the length of each groove. As shown in Figs. 1-4, both grooves may be in the surface of a single cylinder. In Fig. 1 a film 3 has beaded edges 4 which are received in two continuous peripheral grooves 2 of a cylinder 1 over which the film is passed. A series of stationary metal plates 5 prevents the underlying beading from slipping out of the groove. As shown in Figs. 2 and 3 a series of spring-loaded rollers 6 arranged as shown in Fig. 3 (which is a section through A-A of Fig. 2), each roller being shaped in accordance with the contour of the upper surface of the beading 4, prevents the beading from slipping from the grooves 2 of the cylinder 1. In Fig. 4 the rollers 7 prevent the beading 4 from slipping out of the grooves 2 of the cylinder 1 by exerting an axial thrust against the inner edge of the beading. Figs. 5 and 6 show an apparatus for drawing film longitudinally while reducing transverse shrinkage of the film to a minimum. In Fig. 5, 8 represents a casting roller which also functions as the slow roller for drawing the film longitudinally, and 9 is the fast roller, set very close to the roller 8. Both rollers have a pair of continuous peripheral grooves, spaced apart on the slow roller in accordance with the initial width of the film and on the fast roller in accordance with the desired width after drawing, these widths being shown as equal. The grooves on the roller 9 are smaller than those on the roller 8, in order to allow for the reduction in the cross-section of the beadings after the film has been longitudinally drawn. A series of small spring-loaded rollers 10 over each groove on each of the rollers 8, 9 prevents the beadings from slipping from the grooves, and nip rolls 11 prevent the film from slipping on the rollers. These nip rolls extend only to the inner edges of the grooves, as shown in Fig. 6, which is a part section through the line B-B of Fig. 5. In operating the longitudinal drawing process illustrated in Figs. 5 and 6, the molten film is received and chilled to a suitable drawing temperature upon the roller 8, which in the case of polyethylene terephthalate film may be maintained at 90 DEG C. The film, now solidified, passes beneath the nip roll 11, and from this point the beadings are held within the grooves by the rollers 10. The film then passes to the roller 9, which is preferably cooled and is rotated at a higher speed than the roller 8, and the beadings are received into and retained in the grooves of the roller 9, longitudinal drawing occurring between the fast and slow rollers. The drawn film passes beneath the second of the nip rolls 11 and then to a transverse drawing or heat setting apparatus. As shown in Fig. 7, a film is drawn longitudinally as a separate operational step from the casting operation, without the use of nip rolls. The film passes over a series of slow rolls 12, heated to a suitable temperature for drawing the film and separated from each other by narrow gaps, and a series of fast rolls 13, which are preferably cooled and also separated from each other by narrow gaps. All the rolls have continuous peripheral grooves to receive the edge beadings of the film and spring-loaded rollers above these grooves. Fig. 8 shows an apparatus for heat-setting oriented films in which the film 3 having beaded edges 4 is supplied from a drawing apparatus to a composite drum having a central section 14 heated to a temperature approaching the melting-point of the film material and side sections 15 thermally insulated from the central section and provided with grooves 2, and a series of springloaded rollers 6 to prevent the edge beadings 4 of the films from slipping from the grooves. The central section may be interchangeable with sections of the same diameter but of different width, adapted for the treatment of film of various widths. Fig. 9 shows apparatus for transversely drawing orientable film in which the film, which may have been previously oriented longitudinally, is supplied at the point 17 to two wheels 16, each provided with a peripheral groove 2 and a series of spring-loaded rollers, 6; the wheels are set at an angle to each other so that the distance between corresponding points on the two wheels increases in the direction of travel of the film. At the point 17 the width between the wheels is adjusted in accordance with the width of the film, and the edge beadings are received into the grooves. The film is carried by the wheels over a distance sufficient to give the desired transverse draw ratio, and the beadings are then released from the grooves and the film is taken off. In each of the apparatuses shown in Figs. 1-8 the centre portions of the cylinders may be omitted so that only the side portions of the film are supported by two wheels (as shown in Fig. 9, but in this case parallel with one another), any required temperature changes in the film being obtained, for example, by spraying the film with or immersing it in a liquid, or by supplying radiant heat above or below the film. The single cylinders shown in any of Figs. 1-7 may be replaced by composite cylinders as shown in Fig. 8, to allow the main part of the film to be heated without heating the beadings, and/or to allow for the treatment of films of various widths. The cylinder shown in any of the Figures may be replaced by a continuous travelling band. The thickened edge portions of the film may be the beadings that are normally formed at the edges of a film, by surface tension effects, when it is extruded through a slot orifice. The ends of the latter may be modified in shape to enlarge or to modify the shape of the beadings. The beadings may also be formed by extruding the molten film-forming material on to a moving support provided with two grooves in positions corresponding with the edges of the extruded film. Alternatively a preformed monofilament, preferably an unoriented monofilament of the same material as the film, may be fed to each edge of the film close to the extrusion orifice so that the extruded molten film material will join with the monofilaments to form the beadings.