GB891336A - Improvements in or relating to methods and apparatus for the production of polytetrafluoroethylene articles by dry powder ram extrusion - Google Patents

Abstract

891,336. Ram-extruding polytetrafluoroethylene. BAILEY, F. T. June 4, 1958 [June 4, 1957], No. 17929/58. Class 87(2). An apparatus for ram-extruding dry polytetrafluoroethylene powder of all kinds, including reclaimed sintered powder, comprises a powder hopper 31, a die 32, a ram 62 adapted to enter the die 32 and compact the powder therein, means 37 for reciprocating the ram 62, heating means 33 to raise the temperature of the die 32 to the coalescing temperature of the powder and aerating means 65 to provide upward generally circumferentially directed gas streams through the hopper 31 to aerate and agitate the powder. The apparatus may extrude tubes, solid sections or coat one or more wires or electrical cables. The aeration of the material, Figs. 3 and 4, is accomplished by periodic bursts of short duration of air through passages 79,81 in the aerating means 65, the air being admitted when the ram 62 is withdrawn. Feeding. To ensure, when extruding small sections, an adequate supply of material for the ram to compact the material may be blown into the die by an air jet from a pipe 86, by a circumferentially directed series of air jets arranged in a ring surrounding the ram, Fig. 5 (not shown), or when the ram 62 is withdrawn and after creation of the material, Fig. 11, a tube 111 surrounding the ram may be released by a trigger mechanism, actuated by the withdrawal of the ram, to be urged forward by springs 126 to enclose a " cylinder " of material which is forced into the die by the ram. Cooling. To avoid lines of weakness, in the extruded product, caused by poorly-united slugs of material advanced by the ram the first portion of the die may be cooled by fluid circulation, Fig. 5 (not shown), so that the compressed powder reaches the heated portion of the die in a continuous rod. The tip 63 of the ram 62, Fig. 28 (not shown), may be similarly cooled. When extruding tubes the mandrel, over which the ram moves, Fig. 27 (not shown), may be cooled by fluid circulation in a position adjacent to the cooled die. Both the die and mandrel may also be cooled by fluid circulation in a region after the heated portion of the die, Fig. 24 and Fig. 28 (not shown). Heating. The die is heated by band heaters 33 controlled by a thermostat 34 with an indicator 35. For extruding tubes the mandrel may be similarly heated, Fig. 28 (not shown), the heating being arranged between the two fluid cooled zones. The ram To avoid contamination of the extrusion by " smeared " particles of the powder the ram may have an enlarged tip 63 which is a close fit in the die 32 and, where the ram passes through the hopper 31 a large clearance 162 and various arrangements of wipers 164 may be provided. When the passage of a wire, to be coated, through the ram necessitates a clearance hole in the ram, particles of powder passing through that hole may be removed by a retractable device arranged to reciprocate within the ram, Figs. 20, 21 (not shown). One or more wires may be passed through the ram for coating Fig. 13, 14 (not shown), and the wires may be precoated with a different or differently coloured p.t.f.e. coating. Brake. The passage of the extrudate from the die 32 may be retarded, Fig. 24, by a fluidly expanded collar 246 which is held between plates 241, 242 which are adjustably spring loaded to move with the extrudate 50 when the extrusion force is sufficient to overcome the force exerted by the springs 252. Reciprocating mandrel. To lessen the effort required to force the ram 62 into the die the mandrel, Fig. 22 (not shown), may be arranged to be withdrawn through the ram by a hydraulic cylinder when the ram is stationary at the forward end of its stroke. The next forward stroke of the ram may be sufficient to cause the frictional pull exerted by the material on the mandrel to move the mandrel forward with the ram or the mandrel may be advanced, with the ram, by the second hydraulic cylinder. The extruded tube may have one or more bores through it, Fig. 19 (not shown).