Patent GB-959450-A (GB)
Title: Improvements in or relating to methods and apparatus for stretching of thermoplasticfilms
IPC: B29C55/16
Filed: 1960-08-15
Abstract: 959,450. Stretching films. GEVAERT PHOTO-PRODUCTEN N.V. Aug. 15, 1960 [Aug. 14, 1959], No. 28160/60. Heading B5B. An apparatus for stretching melt extruded films of organic thermoplastic material comprises feed rollers 1, 2 and 3 (Figs. 1 and 2), which are maintained at the softening temperature of the film and are rotated at a peripheral speed equal to or higher than the linear extrusion speed. The web of film after having been passed in a sinuous path through the nips of the feed rollers is conveyed into zone II<SP>1</SP> of the stretching apparatus where the film edges are gripped by two series of gripping devices moving parallel to each other and the film is heated to the desired stretching temperature by means such as an infra-red radiation heater. The grippers are independent of each other and slide along the operative portions of the endless metal guide rails 4 and 5, being moved along these by the travelling web of film itself. When it is desired to stretch the film transversely at the same time that it is stretched longitudinally, the operative portions of the guide rails 4 and 5 are arranged to diverge in zone II in the direction of travel of the film, the angle between these portions of the guide rails being adjustable up to a maximum of 45 degrees. In the production of webs of film such as magnetic recording tape, in which the web is only to be stretched longitudinally, the operative portions of the rails 4 and 5 are set parallel to one another, and the function of the rails and grippers is then confined to preventing the transverse shrinkage of the web of film which would otherwise occur as the result of the longitudinal stretching of the web. As the web of film leaves the zone II and enters the zone III, the grippers are automatically disengaged from the edges of the film web and travel along a return portion of the guide rails 4 and 5 to the beginning of zone II<SP>1</SP>. Each of the gripping devices comprises a gripping member 6 (Fig. 3), pivotably mounted at 7 on the frame 8 in such a manner that when it is caused to pivot into a vertical position, the edge 9 of the web of film is gripped between, the top of the member 6 and the underside of the upper flange of the frame 8. The latter is provided with freely-rotatable wheels running along the guide rail 4. When each gripper enters the zone II<SP>1</SP>, a locking lever 12 pivotably mounted at 13 in the lower flange of the frame 8 is automatically tripped so that a projection 14 on the lever releases a pin 15 projecting from the gripping member 6, thereby allowing a spring 16 to urge the top of the gripping member 6 into contact with the edge of the film web. As each gripper enters the zone III, the gripping jaws are automatically opened by means acting upon the portion of the gripping member below the pivot 7, e.g. by the movable core of a solenoid energized at the right moment by a photo-electric system or a microswitch. As shown in Fig. 2, the endless guide rails 4 and 5 are disposed in vertical planes, and the grippers are caused by the movement of the film web to move up into and along the upper run of each guide rail until they reach the driven wheel 17 which is provided with projections which engage the grippers and carry them down into the beginning of the zone II<SP>1</SP>. In a second embodiment of the apparatus of the invention, the endless guide rails 18 along which the grippers move lie in a horizontal plane (Figs. 4 and 5). The frame 8<SP>1</SP> of the gripping device in this case has pivotably attached thereto a bracket 19 (Fig. 6), carrying two rotatably mounted rollers 20 which can roll along the sloping sides of a guide rail 18<SP>1</SP>. The bracket 19 itself slides with its flat and polished underside over the lubricated flat top face of the guide rail 18<SP>1</SP>. The gripper frame 8<SP>1</SP> is so mounted on the bracket 19 that it can pivot in a horizontal plane about the axis 34, and comprises all the parts of the gripper frame 8 shown in Fig. 3. As each gripper 8<SP>1</SP> reaches the end of the stretching zone, i.e. the point 21 of the guide rails 18 (Fig. 4), it enters the magnetic field of a series of magnets 22 which are carried around a circular path inside a metallic drum 26 (Figs. 4 and 5), by arms 24 attached to a rotating shaft 25. One half 27 of the base of the drum 26 consists of a non-ferrous metal and the other half 28 consists of a ferrous metal such as iron or steel. In the path of travel of the magnet 22 over the half 27 of the drum base the grippers are attracted by the magnets 22 and are forced to travel half way round the drum until they reach the point 23 on the guide rail 18 when they are released since the lines of force of the magnets 22 are intercepted by the magnetic half 28 of the drum base. The grippers are then conveyed back to the beginning of zone II<SP>1</SP> by mechanical means. Specification 959,449 is referred to.