Patent GB-992598-A (GB)
Title: Method and apparatus for the production of films
IPC: B29C48/10
Filed: 1962-11-09
Abstract: 992,598. Making tubular films. W. R. GRACE & CO. Nov. 9, 1962 [Feb. 12, 19621, No. 42395/62. Heading B5B. Apparatus for making tubular films comprises a bell-shaped chamber open at both ends, adapted at its narrower end to receive a tube of freshly extruded hot thermoplastic material from an annular die coaxial with the chamber, and to form a vacuum-tight seal with the tube and adapted at its wider end to form a vacuumtight seal with the expanded plastic tube and to allow the tube to be withdrawn from the chamber by a collecting means, the chamber being provided with means for inducing a vacuum between the plastic tube and the walls of the chamber so as to cause the tube to expand as it passes through the chamber and form a tubular film, the tube during its passage through the chamber being at a temperature high enough to cause the tube to be in a plastic or formative state and cool enough to be set when it leaves the chamber. As shown in Fig. 2, the film-forming, material is melt extruded downwards from the hot jacketed extruder 1 provided with a conventional tapered helical feed screw 17 through the annular extrusion orifice 24 of the die 23 to form a tube 66 which passes through the centre of the lubricant applicator 2-whereupon lubricant is sprayed on to tube 66 from the orifices 34 in the hollow ring 33. Tube 66 is then drawn into the bell-shaped vacuum moulder 3 by way of flared opening 44. The diameter of tube 66 is such that a close contact is maintained between the tube and the inner surface of vacuum moulding throat 45. The purpose of applying lubricant to the outer surface of the tube 66 is to permit its smooth passage through the throat 45 and to improve the vacuum seal between the tube and the throat. Upon emerging from the throat and entering vacuum chamber 46 the tube is expanded as the result of negative pressure maintained within the chamber by means of vacuum ports 47 which communicate with a vacuum source. The tube 66 continues to expand as it passes through the chamber 46 to form a tubular film until it reaches the chamber outlet 48 the edge of which is slightly rounded to facilitate the smooth passage of the now expanded tube or tubular film over its surface. Again the lubricant applied previously serves to ensure this smooth passage and to maintain a vacuum seal between the edge of the outlet and the expanded tube 66. Upon emerging from the outlet 48 the expanded tube, now shown as film 67, is longitudinally slit by means of the knife 51 and is then collected on the driven roll 61. While the hot plastic tube 66 is drawn through chamber 46 it is cooled to a point which allows it to acquire a set by the time it reaches outlet 48. The film material may be polyethylene, a polypropylene or a polystyrene and/or may contain a cross-linking agent. In the latter case a cross-linking oven operating at about 150 to 300 C. may be provided between the extruder and the vacuum moulder. The lubricant used may be ethylene glycol, hot water, or, particularly, saturated steam. The apparatus facilitates the production of films of improved uniformity in thickness.