Patent GB-1456506-A (GB)
Title: Machine for and a method of forming pellets of plastics material
IPC: B29B9/06
Filed: 1972-12-23
Abstract: 1456506 Pellitizing die FARREL BRIDGE Ltd 19 Dec 1973 [23 Dec 1972] 59731/72 Heading B5A A readily degradable plastics material, e.g. polyvinyl chloride, is pelletized by cutting the extrudate at the downstream face of a plurality of extrusion die orifices of a die plate by a rotary cutter having a blade position which sweeps across the holes, the material and the blade being cooled by a blast of cooling air directed along a path generally parallel with the plate. The extrusion rate is less than four lbs. per hour through each orifice, and preferably 1 to 3 lbs. per hour. The extrusion pressure is not more than 2000 lb/sq. inch, and preferably 500 to 1500 lbs./sq. inch. The ratio of the number of orifices to the square of the diameter of the extrusion screw in inches is at least 15 : 1 and preferably greater than 20 to 1, e.g. 30 to 1. Each orifice is preferably not more than # inch in diameter, and 1000 orifices may be provided, and the thickness of the die plate is preferably greater than ¢ inch and less than 1 inch. The pellets are cut from the orifices by a pair of blades 104, the cutting edges of which rotate against the outlet face of a die plate (30), Fig. 3 (not shown). The cutting blades 104 form part of a cutter assembly 14 which is secured to a clamp ring by the threaded ends 52 of four bolts 50, the other ends of which are secured between bolt shoulders and lock nuts 60 to a bridge member 54. A tubular member 64, provided with an end 81 for engagement with a spanner, threadedly engages through the axis of the bridge member 54 to bear upon the centre part of the die plate, to bow it in the upstream direction by an amount which is restored to a flat surface by extrusion pressure. A heater rod 74 extends through the tubular member 64, for a heater portion thereof to be recieved as a push fit in a bore within a conical torpedo 46 extending upstream from the die plate. The cutter blades 104 are carried for rotation by a sleeve 82 which is rotated by a pulley 98 around bearings on the tubular member 64, the bearings being provided with dirt shields. The knives 104 are each screw clamped to a holder 112 which is pivotable, on an axle extending from a block 108 secured to the sleeve 82, to urge its blade 104 against the die plate face under the action of a torsion spring 118, acting between the holder 112 and a collar 116 on the axle. Pivotable movement of the holder 112 on the axle, is limited by a pair of screws circumferentially spaced and threadedly engaging through the holder 112 to engage a flat on the axle. The granules being cut, and the blades 104, are cooled by an air blast at ambient temperature delivered from a duet 150, inwardly across the face of the die, via a peripheral annular duct 155 which tapers inwardly and contains deflector blades 154. The air and the pellets are drawn from the cutter unit at a duct 152, by the partial vacuum created by air being delivered along a pipe 168, past an adjustable baffle, by means of a fan 170 which also supplies the cooling air to the duct 150. The die head 12 carrying the die plate is clamped, together with a screen plate 29, to the extruder barrel 16 by means 22, and is pivotable away from the barrel together with the cutter unit 14 upon a hinge pin 158. The cutter unit 14 is pivotable away from the die head, on a hinge member 166 secured to the clamp ring in which the bolts 50 engage, and which is pivotable about a die head hinge pin 164. The die head is provided with pressure sensing means indicating on an indicator 116, and is provided with passages for circulation of heating or cooling fluid, and the extruder barrel 16 may also be heated or cooled. In the embodiment, one thousand orifices are formed in an annular region of a die plate.