| 摘要 |
656,300. Moulding polytetrafluorethylene. BRITISH MECHANICAL PRODUCTIONS, Ltd. Oct. 1, 1948, No. 25684. [Class 87 (ii)] Rods, tubes and like elongated articles are produced by preparing a polytetrafluorethylene pellet of rod or tube-like form by compacting the material in powdered, granulated or flocculent form, sintering the compacted material at above 327‹ C., and then extruding the pellet to decrease its cross-sectional dimension with a corresponding increase in length, To avoid disintegration of the pellet, the reduction in cross-sectional dimension may be limited to one-half. The extrusion may be carried out at a temperature below that at which the material becomes substantially elastic, e.g. normal atmospheric temperature, or at a temperature higher than that at which the material becomes elastic, in which latter case the material is constrained in the extruded form during cooling. The articles produced by the invention are under internal stress, and by releasing this stress they can be restored in part to the original shape of the pellet. The release may be effected successively along the length of the article, to produce in effect one or more local increases in cross-section dimension. The release may be effected by heating or by mechanical means such as shock, vibration or bending. The drawing shows an apparatus for extruding tubes, comprising a body 1 having a socket 2 in which the pellet is placed, a bottom die 3 having a bore 4 into which the pellet is to be extruded, and a top tool 5 which carries a plunger 6 and mandrel 7. The plunger and mandrel are removably held in the tool by a spring-pressed pin 8 which is forced inwardly by a ramp 9 as the body and tool are brought together. In use, a succession of pellets are placed in socket 2, the mandrel 7 is pushed through them, and the pellets are successively extruded into bore 4 by pressure of plunger 6 on the uppermost pellet. To extract the tube, the top tool is removed, carrying with it the mandrel and tube, the pin 8 releases the mandrel, the mandrel is withdrawn carrying the tube, and the tube is removed from the mandrel. Specification 592,127 is referred to.
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