Patent GB-1053224-A (GB)
IPC: B29C48/08
Abstract: 1,053,224. Films. NATIONAL DISTILLERS & CHEMICAL CORPORATION. Sept. 19, 1963 [Sept. 19, 1962], No. 36973/63. Heading B5B. A method for the production of a film of thermoplastic material comprises extruding a molten film of the material with beaded side edges and longitudinally stretching the molten film by drawing under longitudinal tension from the extruder to a chilling zone wherein it is chilled to below the softening point of the thermoplastic material, the beaded side edges of the molten film being chilled below the softening point of the thermoplastic material and restrained and guided during the longitudinal stretching by physical engagement with respective parallel or divergent guides to prevent simultaneous transverse shrinkage of the film. Preferably, the beaded and chilled side edges of the molten film are restrained and guided along paths diverging from the extruder to the chilling zone, whereby the molten film is simultaneously stretched longitudinally and transversely before being chilled to below its softening point. Suitably the side edges of the molten film are guided so as to retain the width of the film in the range of from one to six times hat at the line of extrusion, and the film is tensioned laterally at a substantially uniform rate between the line of extrusion and the chilling zone, which latter suitably comprises one or more rotatable chill rolls around which the film is drawn. Apparatus as shown comprises an extruder 1 for extruding a molten film 2 of material having beaded side edges 2a, 2b, a rotatable chill roll 3 spaced from and parallel to the extrusion orifice of the extruder, and adjustable towards and away from the extruder according to the degree of divergence between the guide members, said chilling roll being spaced preferably at least 16 inches from the extruder and desirably about 48 inches therefrom, said chilling roll chilling the molten film to below its softening point, means for drawing the molten film under longitudinal tension directly from the extruder to the chilling roll, and means disposed between the extruder and the chilling roll for chilling the beaded side edges below the softening point of the thermoplastic material and for restraining and guiding the chilled beaded side edges of the molten film to prevent transverse shrinkage of the film. (Clamps 41, 42, 43, 44, provided with internal cooling means, e.g. longitudinal guide channel 80, tubing 81, 82, 83, 84, for circulating a heat exchange liquid therethrough, which may be supplemented by water supply tubes 90 and 91.) As shown, the guide members are mounted so as to be adjustable between relative dispositions in which they run parallel to one another and in which they diverge from one another in the direction of the chilling roll. Means for holding and adjusting the guide members comprises a pair of spaced parallel shafts, 36, 38, a support 60 at one end of the shafts engaging the shafts for pivotal movement about the axis of the support and for reciprocal movement along the axis of the shafts, a pair of clamping blocks 52, 54 pivotally mounted on the opposite ends of the shaft with one of the pair adapted for reciprocal movement in the pivot mount relative to the upper one of said pair, means for reciprocally moving the movable blocks 56, 56a, 56b, means for mounting the respective guide members on the respective blocks, means for reciprocally moving the shafts in the pivot support therefore, and means for moving the shafts and pivot support pivotally, including the web guides. Suitably, the ratio of the spacings between the guide members at the chilling roll and extruder is adjustable in the range of from 1 : 1 to 6 : 1. A range of from about 2À5 : 1 to about 3À5 : 1 is preferred. Typically, a flat film web is extruded from a die orifice having a width of about 13 inches, and adjusted to discharge a web having a thickness of about 0À025 inches. The ends of the die orifice are formed to provide beaded edge portions on the extruded web having a thickness in the range of from 0À050 to 0À070 inch, this thickness reducing as the bead is drawn from the die to from about 0À045 to about 0À065 inch.