Patent GB-664412-A (GB)
Title: Process and apparatus for manufacturing thin wall tubing from thermoplastic materials by extrusion
Inventor: JAMES BAILEY
IPC: B29C48/09
Filed: 1949-01-04
Abstract: <PICT:0664412/IV (a)/1> <PICT:0664412/IV (a)/2> <PICT:0664412/IV (a)/3> <PICT:0664412/IV (a)/4> Plastic material such as polyethylene extruded as tubing T from a die D and expanded by air under pressure introduced through the die is cooled by a ring of air jets 10 fed from a manifold 13 and individually adjustable by valves 12 to enable local variations in thickness of the tube to be countered by regulation of the cooling effect of the adjacent jet.  The expanded tube next passes through a cooling wind tunnel comprising three annular manifolds 14a, 14b, 14c, with ports 18 individually controlled by dampers 18a.  The manifolds are fed from a common duct 15 supplied with air by a blower 16, dampers 17 controlling the air supply to each manifold.  In its passage through the wind tunnel the tubing is supported by sets of collars R, R1, extending round the lower part of its periphery.  The tubing then passes between two sets of rollers 20 mounted on converging frames 28, 29, the separation and inclination of which to each other can be varied by chain and sprocket arrangements 30, 31 and 35, 36.  Further cooling is provided by blowers 38, 39 and hoods 38a, 39a directing air on to the tubing between the frames 28, 29.  The flattened tubing drawn by pulling rollers 44, 45, the pressure of which on the tubing can be varied by devices 48, passes between idler rolls 40, 41, 42, which eliminate wrinkles, and a device C which includes a pivoted arm which moves in accordance with variations in width of the flattened tubing.  The tubing is finally wound on reels on a shaft 53 after passing through several sets of smoothing rolls.  Two winding shafts are provided, the tubing being transferred from one to the other as the reels become full.  The device C, which is adjustable for a range of desired widths of tubing, controls the pressure within the extruded tube.  If the tube width exceeds a certain size the pivoted arm closes a pair of contacts energizing a circuit to a solenoid controlling a valve in a pressure control device to cause the expanding pressure in the extruded tube to fall to a fixed value such that the tube will slowly decrease in size.  The contacts consequently open, the valve closes and the expanding pressure increases to a second fixed value such that the tube slowly increases in size until the contacts close again. Fig. 3 shows one form of extrusion die which includes a threaded inlet 2 for receiving heated plastic P from the extruder outlet 1 and an annular composite extrusion orifice 3 formed between an inner circular member 3b and an outer annular member 3a which may be adjusted to be coaxial with the member 3b by screws 3c.  The die includes electric heating elements 4 and 5 the current to the latter being controlled by a thermoswitch 5a.  An outlet 8 for the introduction of air into the extruded tube is provided, fed by pipe 9.  Figs. 9 and 10 show a modified die designed to eliminate "grain" or "spider" lines in the extruded tubing.  The die includes a cylindrical housing 94, plastic material 98 from the extruder 96 being fed through a conical passageway to a homogenizing spider 99 held in the housing.  The material passes through a large number of holes 100 arranged in two concentric rows in the spider 99 (Fig. 10), and through an annular screen 101 held in a recess in the spider by washers 101a, 101b, and an outer nozzle member 103.  An inner nozzle member 102 diverts the plastic outwards to an annular passage between the members 102 and 103 and provides sufficient back pressure to prevent disruption of the screen 101. The passage between the members 102 and 103 first increases and then decreases in size to the final orifice provided between the member 103 and a replaceable tip 108 to the member 102.  The thickness of the extruded tube may be changed by replacing the tip 108 with one of different size.  The die is heated by electric elements located in passages 112 in the housing 94 and 111 in an element 110 pressed on to the member 103.