Patent GB-763472-A (GB)
Title: Extrusion and melt-spinning apparatus
IPC: B29C48/395
Filed: 1954-01-29
Abstract: Apparatus for extruding thermoplastic material, as in melt-spinning, comprises a chamber, a shaping means (such as a die or spinning jet) fed directly from the chamber, a heated passageway leading into the chamber, a rotatable conveying screw in the passageway mounted in a fixed axial position and adapted to convey <PICT:0763472/IV(a)/1> thermoplastic material through the passageway and to deliver it into the chamber under pressure, and means responsive to the pressure of the thermoplastic material in the chamber for varying the capacity of the chamber, without interrupting the passage of the material through it, in such a way that when the pressure decreases the capacity of the chamber is decreased, and vice versa.  As shown, comminuted filament-forming thermoplastic material is fed into the feed hopper 10 around the conveying screw 21, and is carried down and compressed in intimate contact with the heated inner walls 16 of the cylinder block 12 by the threads 22 of the screw when the latter is rotated. Because the inner walls 16 are tapered while the body 23 of the screw is cylindrical, the thermoplastic material is considerably compressed.  Desirably, screw 21 is so proportioned that, for each revolution of the screw, the ratio of the volume of material moved by the upper end of the screw into the block 12 to the volume of material moved out of the block by the lower end of the screw, is equal to the ratio of the absolute density of the solid thermoplastic material to the bulk or apparent density of the material in the comminuted form in which it is fed to the hopper 10.  The block 12 is kept hot by means of a heating element 14, which may be an electric heater or a circulating stream of a hot heat-transfer fluid, the inner walls 16 of the block being thus heated to a temperature sufficient to melt the thermoplastic material.  The feed hopper 10 can be cooled by a cooling medium circulating through a cooling coil 11 in its wall, which is separated from the cylinder block 12 by insulation 13.  The molten thermoplastic material emerges under pressure from the aperture 19, at the bottom of the block 12, into a chamber 31 formed in an adapter ring 32 secured to the lower end of the block 12.  The molten material then passes through the apertures 33 of a spinning jet 34 which is held at the bottom of the adapter ring 32 by a retaining ring 36 secured to the adapter ring.  The rings 32, 36 are heated by a band heater 37.  The screw 21 is provided at its lower end with a cylindrical torpedo pin 38 which can slide axially in the bore 39 of the screw.  The pin 38 and bore 39 are lapped to a fit sufficiently close to maintain an air pressure of 100 p.s.i. gauge for 24 hours.  The pin 38 is provided with circumferential grooves 40, for the reception of a lubricant resistant to high temperatures, a portion 41 of slightly decreased diameter between the grooves, and an enlarged head 42 slidably fitted in an enlarged bore 43 of the screw 21.  Abutting against the head 42 is a helical compression spring 44, the upper end of which abuts against an adjustable threaded plug 45 mounted in an internally threaded portion 46 of the screw 21 and provided with a slot 47 adapted to receive an adjusting tool such as a screwdriver.  The slot 47 can be reached for adjustment of the position of the plug 45 by unscrewing the nipple 28 of the drive shaft 29 from the end of the screw 21.  The spring 44 urges the pin 38 downwards towards the position shown in dotted lines, further downward movement of the pin being prevented by the engagement of its head 42 with an internal shoulder 48 of the screw 21.  The initial loading of the spring 44 is so adjusted, by movement of the plug 45, that the pressure of the thermoplastic material in the chamber 31 during normal operation forces the pin 38 upwards to the position shown in solid lines.  In this position the pin 38 serves to maintain the material, coming from the screw, in a thin film in contact with the walls of the heating block 12. If the flow of material from the screw decreases, due to temporary jamming or severe local compression of the material, the resulting decrease in pressure in the chamber 31 allows the pin to be urged downwards into the chamber, thus largely maintaining the pressure in this chamber and overcoming any tendency for the formation of voids in the chamber through an unduly large pressure drop therein.  When the rate of flow of material from the screw returns to its normal value and the pressure in the chamber 31 builds up again, the torpedo pin is forced back to its original position.