Patent GB-1517487-A (GB)
Title: Screw-extruders
IPC: B30B11/24
Filed: 1976-09-17
Abstract: 1517487 Releasable extruder screw casing SOC NATIONALE DES POUDRES ET EXPLOSIFS 17 Sept 1976 [1 Oct 1975] 38706/76 Heading B5A The screw easing 4, of a twin screw extruder for a pyrotechnic composition, comprises sections 4a, 4b which are secured to a bed 34 to provide for pivotal movement of at least one section away from the other about a trunnion 17 which is perpendicular to the casing axis, and are releasably clamped together at surfaces 9, by elements 12 which are spaced apart longitudinally of the casing 4 and adapted to release at a predetermined pressure within the casing, to permit the at least one section to pivot away from the other, e.g. upon the outbreak of fire in the composition. The elements 12 each preferably comprise a bolt provided with weakened necks 12a which rupture upon the pressure being reached, i.e. at a bolt loading corresponding to 1À3 to 1À8, and preferably 1À5 times the maximum operating pressure required in the casing. The clamping by each element 12 is effected between a nut, and a bolt head which is either directly engaging, or comprises a hydraulic piston, Fig. 3 (not shown), effecting a bolt loading corresponding to the maximum operating pressure required. The bolts engage through openings extending from the lateral edges of plates 10 to which the sections 4a, 4b are attached, to facilitate closing and opening of the casing for cleaning. The downstream end of the casing 46 is supported by an adjustable prop 21 enabling adjustment of the casing around the screws prior to commencing extrusion, and which may be removed during extrusion to enable self centering of the casing around the screws, facilitated by the upstream ends of the portions 4a, 4b, abutting a bed support 19 at a seating which is part spherical, or inclined to provide vertical self centering. The easing parts are provided with channels 8 for thermal regulation fluid, and with apertures 22 for vacuum degassing, and drain wells 23 for liquid removal. The screws are bored for the circulation of different thermal regulation fluids, respectively peripherally and axially. The screws each comprise three successive gelatinizing and expansion zones, followed by a compression zone of fine thread pitch adjacent a converging portion 6 of the barrel. The gelatinizing zones each comprise threads and eccentric discs offset angularly, and the expansion zones comprise a coarse pitch thread. The first expansion zone is located adjacent the wells 23, and the 2nd and 3rd expansion zones are each located adjacent the vacuum apertures 22. The composition comprises nitrocellulose or a glycerine, with centralite, and is delivered to a casing inlet 24 from a spherical hopper provided with a rotary mixing arm having blades 29, by a metering screw 30 effecting delivery through a feedpipe 31 and a transparent pipe. A further hopper 33 communicates through a valve with the feedpipe 31, and from which inert matter, e.g. non-nitrated cellulose, is fed to the extruder, instead of the composition at the commencement and termination of extrusion. The casing and screws are heated during extrusion by water at 80 C., which may be replaced in the thermal regulation circuits by cold water which may also be injected into the apertures 22, 23 and feed hoppers. The screws may be rigidly connected to their driving motor at splines 3, or may be floatingly connected at universal joints.