Patent GB-1312074-A (GB)
Title: Extrusion of polymers
IPC: B29C48/445
Filed: 1970-04-20
Abstract: 1312074 Extruding high molecular weight material H F HANSON and W G J JAARSVELD 20 April 1970 18851/70 Heading B5A Degradation during extrusion of thermoplastics material of high molecular weight, e.g. 300,000 to 1,000,000, is avoided by softening the material in particulate form within the extruder, by conducted heat, after it has been compressed by and discharged from the extrusion screw or screws. In the Fig. 2 embodiment, the replaceably mounted screw 9, which is of constant pitch with an end 12 of tapering angle, of 3 to 30 degrees, is axially displaceable from a retracted position in alignment with a vacuum feed hopper 4, and whilst simultaneously rotating, to an intermediate position, shown in Fig. 2, to discharge material 15 against previously discharged material 16; and is then displaceable without rotation to an advanced position, adjacent the commencement of an external oil heating jacket 7, to advance the material 15 against the resistance of the material 16 which is extruded from the orifice as a rod 6. The screw 9 is then returned at a higher rate to its initial retracted position, during simultaneous rotation of the screw which prevents feeding back of the discharge 15. The extruded rod 6 is passed through a calibrator, cooled, and cut. In the Fig. 4 embodiment (not printed) the screw, which has a leading taper of 5 to 15 degrees, reciprocates along a mandrel which extends to the die to define a tubular orifice; is provided with electric heating elements in the region of the barrel heating jacket; and is cooled rearwardly thereof by internally circulated pressure air. In the Fig. 5 embodiment, the extruder is provided with a pair of concentric screws 102, 103 which do not axially reciprocate; are of progressively decreasing pitch; are of opposite hand and rotate in opposite directions, and the threads whereof may progressively decrease in thickness and/or height to zero. The material discharged from the screws 102, 103 is heated in an annular passage 129 defined between a barrel section 118 and a stationary mandrel 116, defining a tubular orifice, the barrel and mandrel each containing electric heating elements. The mandrel 116 is located upon a tube 106 passing through the screw 103 and which is non- rotatably mounted at 114, 115 and may be axially adjusted by a nut 113 to urge the mandrel 116 against the end of screw 103. The screw 102 is adjustably urged against the barrel section 118 by a nut 128. In the Fig. 6 embodiment, two co-operating concentric pairs of non-reciprocating screws 130, 131 are provided discharging on either side of a further annular heating element 137, and the material may be extruded as a rod or as the tube 135 illustrated which may have inner and outer walls of different materials discharged from each screw pair. Three or more concentric screw pairs may similarly be provided. In the modified Fig. 8 embodiment (not printed) the outer worms of the inner, and inner worm of the outer of a two pair arrangement, are formed integrally, and the annular heating element may also be integral therewith, and rotate, or may be stationary if suspended by radial wings from the external casing. The Fig. 8 arrangement may comprise five concentric pairs of worms. In the Fig. 7 embodiment (not printed) a thin blown foil is extruded from an orifice defined between outwardly flared extensions of unthreaded portions (wherein heating is effected) of a pair of concentric-rotating screws, the relative speeds of rotation whereof are adjustable to prevent rotation of the extruded film, and one whereof is axially adjustable, as in the Fig. 5 embodiment, to vary the extruded thickness. It is stated that one or both screws may be twostart, may be provided with cooling means, and the screws of a pair may rotate at different velocities.