GB822821A - Improvements in or relating to a process and apparatus for forming tubular film of organic thermoplastic material - Google Patents

Abstract

<PICT:0822821/IV (a)/1> <PICT:0822821/IV (a)/2> Tubular film of organic thermoplastic material is formed by continuously and simultaneously extruding the material in a heated and plastic condition through a plurality of substantially concentric annular die orifices to form a plurality of separate and independent hot plastic tubular films, one within another, pressing and bonding successive adjacent portions of the films together into a single composite tubular film by introducing air or other gas into the inside of the inner or innermost film at superatmospheric pressure, and cooling and setting the bonded tubular film. As shown in Figs. 1 and 2, two thin-walled tubular films T1, T2 are continuously extruded from a die D which receives a plastic material, such as polyethylene, through pipe 7 from an extruder E which includes heating, homogenizing and pressure-producing components. The die D includes a generally annular outer die holder 1 (Fig. 2) to which a concentric outer die heater 2 is secured by studs 3. The heater 2 makes intimate contact with the outer radial and peripheral surfaces 4, 5 of the die-holder 1 and with the peripheral surface of an annular inlet shoulder portion 6 of the holder 1 through which plastic from discharge outlet 7 of the extruder E is fed into the inlet 8 of the die. An electric strip heater 9 secured by clips 10 to the periphery of the heater 2, and ring heaters 11, 12 secured by clamps 13 to the outer face of the heater 2, supply the heat required to maintain the plastic passing through the die at a desired temperature and consistency. An outer annular die lip member 14 is adjustably secured to the die-holder 1 by a retaining ring 15 fastened by bolts 16. Radial adjustment of the die lip 14 may be effected by a number of cap screws 17 disposed at spaced intervals about the circular die-holder 1. An intermediate die lip member 18 is located within the outer die member 14 on sleeves 19 threaded into the die-holder 1 and spaced from the latter by a distributer ring 20 so as to provide a passage 21 therebetween through which the plastic flows and is extruded through the annular orifice 25 to form the tubular film T1. An inner die-holder 30 is centred relative to the intermediate die member 18 by the sleeves 19 and spaced from the die member by a second distributer ring 20 so as to provide a passageway 31 therebetween through which the plastic flows and is extruded from the annular die outlet orifice 32 to form the tubular film T2. An inner annular die member 33 is adjusted with respect to the inner die-holder 30 by screws 34 and secured to the inner die-holder by bolts 35. An inner die-heater 36 housed in the casing 36a is secured to the inner faces of the die-holder 30 and the inner die 33 by bolts 37 and is provided with an electrical ring heater 38 and a blockhead thermostat 39 for regulating the temperature of the inner portion of the die. The electrical wiring connections for the ring heater and thermostat pass through one or more of the sleeves 19. Blowing air is introduced into the interior of the tubular film T2 through one of the sleeves 19 from a pressure line B controlled by valve B1. The films T1, T2 unite to form a single tubular film T in a plane adjacent to the die outlets, and the single film thereafter passes through an annular cooling ring 41 (Fig. 1), which consists of an annular chamber to which cooling air is supplied through inlet 43 (Fig. 3) and from which air is discharged against the tube T through an annular orifice 44 (Fig. 4), the size of which may be adjusted at spaced points about the inner circumference of the cooling ring by means of push and pull adjustment screws 45, 46. By local adjustment of the width of the orifice 44 the expansion of the film T about its periphery may be kept uniform. Thus in the event of a thin streak developing at one portion of the film periphery the expansion of this portion may be reduced by widening the portion of the discharge orifice 44 overlying the thin streak to reduce the expansion of the film in this area and increase its thickness. The film T next passes through a series of three annular wind boxes 47a, 47b, 47c (Fig. 1), which direct additional cooling air against it and are supplied with air from a common manifold 48 and blower 49. The rate of discharge of air from the cooling boxes on to the film may be controlled as described in Specification 670,708. The diameter to which the film is blown is limited by a series of freely rotatable horizontal rolls R and vertical rolls R1 arranged tangentially to the periphery of the film in spaced circles which p are coaxial with the wind boxes. The rolls R and R1 are mounted on brackets 51 secured to four members 52, each of which is secured to supporting rods 54 which extend radially from the film T. The rods 54 are slidably supported in individual brackets 55 secured to the wind boxes 47a, 47c and may be moved radially inward or outward to decrease or enlarge the cross-section of the passageway defined by the series of rollers R and R1. Each of the rods 54 can be locked in position by an adjustable clamp 56. After passing through the wind boxes the film T is partially collapsed by a series of metal rollers 58 located along two converging lines above and below the film, as described in Specification 664,412. From the rolls 58 the film may be drawn by a pair of driven pull rolls P through a collapsing gussetting mechanism described in Specification 670,707 and wound fully collapsed in a roll 74 upon a driven arbor 75. The blowing air introduced through pressure line B (Fig. 2), presses the inner tubular film T2, which it first expands, into firm bonding contact with the outer tubular film T1 (both films being in a plastic condition) and expands the composite tubular film T thus formed. In order to avoid the entraining of bubbles of air in the wall of the composite tubular film T in starting the process, a subatmospheric pressure is preferably maintained in the air space between the tubular films T1, T2 by connecting line 40 with suction means, after which valve 401 is closed to maintain this subatmospheric pressure. The distribution rings 20 (Figs. 2 and 5-7), located in the passages 21, 31, through which the film forming material flows to the annular extrusion orifices, are provided on their opposite sides with gear tooth-shaped projections 26 which tightly engage the die-holder 1 and intermediate die member 18, thereby spacing annular grooves 27 and flat lands 28 on opposite sides of the ring 20 from the adjacent faces of the die-holder 1 and die member 18 so as to form the annular chamber 23 and constricting annular passages 24. Between the projections 26 are spaced radially extending passageways 22 through which the plastic flows and then flows through chambers 23 and passages 24. Preferably the teeth 26 on opposite sides of the ring 20 are alternated so that a tooth 26 on one side is opposite a passage 22 on the opposite side (Fig. 6). Although pin holes may occasionally occur in one or other of the tubular films T1, T2 as a result of non-uniformity of the extruded plastic, the possibility of the pin holes in each tubular film being superimposed in the composite tubular film T is remote.