Patent GB-691444-A (GB)
Title: Machines for feeding, working and mixing plastic material
IPC: B29C48/67
Filed: 1951-06-19
Abstract: 691,444. Screw extrusion machines. CLINEFELTER, J. C. June 19, 1951, No. 14574/51. Class 87 (ii) [Also in Group II] An impeller for receiving plastic material from a feed screw in an extrusion moulding machine comprises a hub tapering towards its delivery end and at least one blade integral with the hub and having its inner end of helical shape corresponding to that of a feed screw thread and its outer end at least as large in external diameter as its inner end, the outer end of the blade being shaped, graded and inclined to present a materialadvancing pressure surface terminating at its delivery end in a substantially radial cutting edge extending continuously from the axis to the periphery of the impeller. The impeller is for use particularly in extrusion machines as described in Specification 640,276, in which the material is forced by the feed screw through a perforated screen and is sheared off by impeller blades fixed to the delivery end of the screw so that material that has already passed through the perforations is severed from the rest of the batch that follows. The impeller according to the invention is preferably integral with the feed screw to obviate discontinuity, as well as to secure greater rigidity, but the impeller is not a mere extension of the feed screw as that would not result in a satisfactory cutting edge. With a double-thread feed screw 13, having threads 30 and 31, the operating surfaces 45 of the impeller blades constitute helical continuations of the feed screw surfaces 33 and the hub 37 of the impeller is a continuation of the hub of the feed screw, but the blades 38, 39 of the impeller progressively increase in depth towards the outer end, giving the hub a taper or bevel, and the surfaces 45 are graded to terminate in cutting edges 47, 48 radiating from a centre of minimum cross-sectional area. The ends of the blades are twisted at 49 so that the sides flanking the cutting edges are at right angles to the edges to increase the shearing action across the perforated screen 23. To reduce back pressure the extrusion chamber is enlarged at its delivery end 16 and the impeller is correspondingly flared outwards at 40, 41. For some purposes a two-bladed impeller is sufficient, but for additional cutting action the impeller may have additional blades 50, 51 midway between the main blades 38, 39 and similar to them. Each blade 50, 51 has its crest edge 54 inclined abruptly towards the hub of the feed screw 13 to merge into it, the edges 54 being bevelled to divide the streams of material from the full threads 30, 31. The impeller-extension and feed screw are first machined from a single length of cylindrical stock, and the impeller is then formed by further machining combined with weld deposition and welding on of the substantially triangular sections 40, 41. The additional blades 50, 51 may be welded in place and ground. With a single-thread feed screw one blade is formed as a continuation of the thread and another is welded on and ground to form a second blade. The possibility of using other numbers of threads is mentioned.