Simultaneous two-way stretching of film

摘要

A thermostretchable film is heated, gripped at closely-spaced positions along at least two of its edges, and stretched simultaneously in two directions perpendicular to one another by separating the gripping means longitudinally and laterally. The film may be gripped at uniform intervals at the two opposite longitudinal edges and the stretching may take place in a continuous manner. The apparatus employed may include two cone-shaped members disposed symmetrically opposite to one another with their bases outward, slots radiating outwardly in the surface of each member, and underneath these members cam cones with a continuous cam track in each which makes one circuit of the cam cone and leads from near the base of the cone to a portion of smaller diameter and back again, and film clips sliding in the slots with guide members fastened to the clips and moving in the cam tracks. Means are provided for opening the clips to release the film when the clips are in the portion of the cam track near the base of the cone, and for closing the clips to grasp the film fed thereto when the clips are near the narrower part of the cone. The film may be of rubber hydrochloride; a copolymer of vinyl chloride with vinyl acetate, vinylidene chloride, or an ester of maleic, fumaric, or other unsaturated acid; polyethylene or a modified polyethylene. After stretching, the film generally has a tendency to shrink when subsequently heated. In most cases the film must be hot while being stretched but rubber hydrochloride film after heating and then cooling may for a short time be stretched when cold. As shown in Figs. 1 and 2, truncated cones 5 keyed to stationary shaft 7 are provided. Outer conical drums 10 fit over these cones and are rotatably mounted on the shaft. They comprise tapering slats supported by ring 12 and cylindrical drum 13. Gear 15 causes rotation of both drums 10. The unstretched film 20 passes from roll 21 and around heated drum 23. It is grasped by clips 25 which are mounted in slots 26 between the tapered slats of outer drums 10. As shown in Fig. 3, each clip 25 is provided with ears 31 and a member 33 normally held by spring 34 in such a position as to grip the edge of film 20. A guide rail 35 bears against this member 33 to open or close the clip. The portion 30 of the clip passes between two slats of the drum 10 and carries a shaft 41 with roller 40 which fits into the cam track 42. The film 20 after leaving the heated drum 23 is gripped by clips 25a (Fig. 2). As the clips move round drums 10 the clips at each side of the film are caused to move laterally from the central part of the drum which is of smaller diameter to the outer part which is of greatest diameter. The width of the film is thus increased. At the same time the clips at each side of the film become spaced farther from each other and the film is thus stretched longitudinally. The shape of the cam track 42 and that of the cones 10 determine the degree of stretching in the two directions and also determine whether the rate of stretching is constant during the operation or whether it varies. At the completion of the stretching the clips 25b are opened by guide-rails 35 so as to release the film which then passes to roll 51. The stretching device may be enclosed in a heating chamber.