Patent GB-738354-A (GB)
Title: Apparatus for coating cores by extrusion
IPC: B29C48/06
Filed: 1953-08-25
Abstract: 738,354. Coating cores by extrusion. WESTERN ELECTRIC CO., Inc. Aug. 25, 1953 [Aug. 28, 1952; Aug. 28, 1952; Aug. 28, 1952], No. 23434/53. Class 87 (2). Apparatus for extruding a layer of plastic compound in expanded or cellular form upon a filamentary core comprises an extrusion die through which the plastic compound is applied to the filamentary core and wherein a plurality of flow-restricting passageways are provided for the plastic compound which extend into or'opposite the entrance to the die and are spaced from each other circumferentially around the filamentary core so as to create in the plastic compound 'being fed into the die a sufficiently high back pressure to prevent or materially reduce undesired expansion thereof 'before it passes through the die on to the core. Besides control of the expansion of the material, compensation is required for the unbalanced flow created by the 90, degrees bend in the usual type of apparatus in which the die passage is transverse to the axis of the extrusion screw. The plastic material 14, of which a suitable example is granular polyethylene with dinitroso pentamethylene tetramine as a heat-decomposable blowing agent is forced by the extrusion screw 10, Fig. 1, through a strainer 16 into a chamber 22 behind the die 24. The wire 34 to be coated is drawn through a core tube 28 and thence through the die by means not shown. The core tube holder 26 has an inclined face as shown to urge the material towards the die. The end of the core tube is tapered and enters a similarly tapered aperture 30 in the die, and this tapered end 38 has longitudinal grooves. The amount by which the end 38 enters the aperture 30 is variable by moving the core tube axially, and so the area of the passage for the material to the die orifice, and thus the back pressure, may be adjusted, the least area being presented when the end 38 seats in the aperture 30 and only the grooves allow the material'to pass. The movement of the core tube is brought about by screwing a nut 44 in and out of a bushing 48, the nut being constructed with a head 40 so that the core tube does not rotate. Screws 26 prevent rotation of the core tube holder. The angle of taper of the core tube end and the number and size of grooves may be designed to suit particular conditions, as discussed in the Specification. In one example the grooves do not extend the full length of the taper, but a plain conical portion is left near the tip of the core tube end. In another example the grooves further from the screw 10 are wider than those near it, to present a larger passage for the material and compensate for the 90 degree bend referred to. Instead of the grooves being on the core tube they may be on the surface of the core tube holder, Figs. 9 and 10, which show a core tube holder 26 with a cylindrical plug portion 140 which blocks the outlet from the chamber 22 but have longitudinal grooves 142 leading to radial grooves 144 in its face which allow the plastic material to pass while setting up back pressure. The tip of the core tube projects beyond the face of the holder into the die aperture, being conical at 137 as shown. The core tube holder 26 is secured by a bushing 27 and the core tube is fixed in it by a pin 29. The cylindrical core tube holder is reduced in diameter at 146 and the material flows round this smaller portion and into the grooves, and compensation for the longer distance which the material has to travel when furthest from the extrusion screw, due to the 90 degree bend in its path, may be made by chamfering the portion joining the large and small diameter of the core tube holder helically, at 156, resulting in the grooves 142 on that side being shorter and the zone of the chamber 22 being larger. Instead of the die having the shape shown in Fig. 9 it may be as in Fig. 8, that is, quite short, and with a bell-mouthed entrance and exit to its orifice 30. The tip of the core tube which co-operates with this type of die orifice is composed of two frusto-conical sections, one, nearest the face 128, with a wider apex angle and the other, next the tip, with a narrower apex angle, the more gradual taper entering the die orifice. As with the grooves in the core tube, the grooves 142, 144 in the core tube holder may be wider at the side furthest from the extrusion screw to compensate for the 90 degree bend in the path of the material. Fig. 12 shows an arrangement in which the core tube has a cylindrical head 238 which abuts the face 245 at the entrance end of the die 24. The opposite face of the head 238 has a shallow conical depression 246 which directs the material into perforations 240 which pass through the head and provide the resistance to give the required back pressure. The core tube 28 is held in the core tube holder 26 by a pin 248 and is pressed hard against the die by the pressure of the plastic material, which may be directed by an inclination given to the core tube holder as shown. The perforations may be all the same size and on a circle half way between'the axis and the periphery of the head, or they may be arranged to compensate for the bend through which the material flows by making those furthest from the extrusion screw of larger diameter, as shown in Fig. 14.