GB814723A - Improvements in or relating to apparatus for forming thin-walled tubes or tubular films of flexible, thermoplastic material - Google Patents

Abstract

<PICT:0814723/IV (a)/1> Apparatus for forming continuous lengths of tubular film of flexible thermoplastic material comprises an annular extrusion die, an extruder, a first pair of rotatable nip-rollers disposed in fixed spaced relationship to the die, means for rotating the first pair of nip rollers continuously so as to draw off the tube from the die, a second pair of nip rollers capable of being fixed at a variable position in the path of the tube passing from the extrusion die to the first pair of nip rollers and adapted to trap within the portion of the tube travelling from the extrusion die to the second pair of nip rollers a substantially constant volume of a gaseous inflation medium for inflating the tube and a valvecontrolled pressure line extending through the die through which line a suitable gaseous medium is introduced, initially to expand the portion of the tube passing between the pressure die and the second pair of nip rollers and subsequently to maintain the desired pressure within this portion of the tube by introduction of further gaseous medium when required. The second pair of nip rollers are normally undriven, but may be driven if desired. The line of their nip should preferably be at right angles to the axis of the tube and parallel to the line of the nip of the first pair of nip rollers. As shown in the drawing, hot thermoplastic material is continuously extruded upwardly by the extruder 11 through the annular die 12 to form the tubular film or thin-walled tube 13. The tube 13, after having been subjected to known cooling and shaping treatments, is drawn vertically upwards through air at room temperature by a pair of nip rollers 14, 141 each 8 inches in diameter and made of rubber-covered metal and disposed in fixed spaced relationship to the die 12 at a distance of 12 feet vertically above the latter so that the walls of the tube 13 have sufficiently solidified and cooled not to adhere to one another during the passage of the tube through the nip of the rollers 14, 141. The nip rollers 14, 141 are driven by an electric motor 15 through gearing 16. The tubular film 13, flattened by the nip rollers 14, 141, passes in lay-flat form to the driven wind-up 17. Compressed air controlled by the tap 19 may be introduced, when required, into the interior of the tube 13 through a conduit 18 passing through the die 12 and terminating in the inside of the tube 13. Between the die 12 and the nip rollers 14, 141 there is provided a second pair of rubber-covered metal nip rollers 20, 201 which are each four inches in diameter and are disposed in adjustable spaced relationship to the die 20 and adapted to trap a substantially constant volume of air in the section of the tube 13 extending from the die 12 to the nip rollers 20, 201. When manufacturing tubular film which is 6 inches wide in the lay flat form it is found necessary to locate the nip rollers 20, 201 at about 8 feet away from the die 12 in order to maintain the bubble of air trapped between the nip of the rollers 20, 201 and the die 12 dimensionally stable. The invention enables tubes of constant width to be obtained even in the smaller sizes (e.g. up to 5 inches lay flat width) with increases in production rates which may be up to 3 to 5 times prior art production rates.