Patent GB-1301628-A (GB)
IPC: B29C48/515
Filed: 1971-04-19
Abstract: 1301628 Extruder screw MIDLAND-ROSS CORP 19 April 1971 [27 March 1970] 24696/71 Heading B5A In a screw extruder for thermoplastic materials comprising a screw melting section, within an e.g. electrically, heated barrel, having a screw channel of constant width but of decreasing depth, the discharge from the end of the melting section channel is substantially prevented by barrier means, and molten material forced towards the leading side of the channel thread is separated from unmelted material by passage through openings spaced throughout the melting section, extending radially through the screw and connecting with a passage extending axially of the screw to bypass the barrier means. In the Fig. 11 embodiment, the radially extending passages are located at the junction of the thread leading edge and of the screw core 9, and the axially extending screw passage is cylindrical and extends through the barrier means comprising a cylindrical enlargement at the end of the screw providing clearance with the barrel sufficient only to prevent stagnation of molten material, the radially extending passages being also provided circumferentially spaced of the channel adjacent the downstream end of the enlargement. In the Fig. 12 embodiment, the screw thread is thickened throughout the melting section and the radially extending passages 51 extend from a continuous groove 45 provided along the thread and to which the molten material passes over a leading part of the thread 48, which is of smaller diameter than the trailing part 53. In this embodiment, the barrier means comprises a cylindrical screw enlargement 33, Fig. 7 provided around the periphery thereof with axially extending and circumferentially spaced alternate grooves 57, 58, the grooves 57 extending from the downstream end of and terminating prior to the upstream end of the enlargement 33, and functioning to entrap unmelted particles and for transferring molten material therefrom, to the adjacent grooves 58 which communicate at openings 59 with the screw axial passages 35 and do not extend to the ends of the enlargement 33. In a modification of the Fig. 11 embodiment, Fig. 8, (not printed) a tapering mandrel is located within the cylindrical screw passage to define a passsage of increasing diameter towards the outlet thereof. In the Fig. 9 modification of the Fig. 11 embodiment, the barrier means comprises a stationary spider 79 to which is fixed a screw 78 of opposite hand 81 to that 62 of the extrusion screw 76 and which extends along the axially extending screw passage 84 throughout the melting zone. The core of the screw 78 tapers towards the spider 76 to define a passage 78 of increasing section, which discharges through the spider openings and to which molten material also passes from the end of the screw by radial grooves 86. In this embodiment, the axially extending screw passage extends throughout the extrusion screw downstream of the stationary screw for connection to a vacuum source. In the Fig. 10 modification of the Fig. 11 embodiment (not printed), the barrier means comprises a frusto-conical enlargement carried at the end of the extrusion screw, and a stationary screw of opposite hand extends along the axially extending extrusion screwpassageway and is supported downstream of the extrusion screw by a spider, from the upstream end whereof extends a tube defining mandrel; cooling or expanding air for the extruded tube being supplied by a passage extending throughout the stationary screw, spider, and mandrel. The Specification specifies various numerical screw parameters, including clearances with the barrel.