Patent GB-1426180-A (GB)
Title: Manufacture of insulated electric cables
IPC: B29C48/34
Filed: 1972-11-24
Abstract: 1426180 Sheathing stranded conductors BICC Ltd 23 Nov 1973 [24 Nov 1972] 54385/72 Addition to 1201528 Heading B5A [Also in Divisions D1 and H1] In the Specification, a cable sheathing is extruded from an annular extrusion orifice to collapse around a sector-shaped conductor under the pressure within a treatment chamber containing a fluid medium at superatmospheric pressure, and upstream displacement of the sheathing around the cable is prevented by maintaining a predetermined pressure differential between the pressure of the treatment chamber and of fluid e.g. nitrogen, within a core tube of the extrusion die through which the conductor passes. The improvement of the present Specification is directed to minimizing leakage of fluid from the core tube along the interstices of a stranded conductor member, by filling at least the outer interstices with a filling medium as a barrier to the fluid. The filling medium may be introduced by vacuum impregnation as a final step in the manufacture of the stranded member, or in stages intermediate the formation of strand layers, but in the described embodiment is introduced during travel of the stranded member to the extrusion die. Numerous curable sheathing materials, and filling media are specified, which are optionally rendered semi-conductive by carbon black. The described embodiments relate to a filling medium comprising low density polyethylene, microcrystalline petroleum wax, organic peroxide curing agents, and carbon black, the sheathing comprising polyethylene which is cross-linked during passage through an inclined treatment chamber containing steam, and cooling water. In the Fig. 4 embodiment, the filling is applied by a die 31 having a bore 32 through which the stranded conductor passes and the outlet of which is of the same or greater diameter than that of the conductor. The filling medium is applied to the conductor from three passages 36 equally spaced around the bore 32, supplied via a die inlet 39 and annular passage 40. The die 31 is located above a filling medium tank into which falls surplus filling medium from the die. The tank is provided with heated coils to maintain the filling in a liquid state, and filling is delivered therefrom to the die passage 39, via a pump and filter, a by-pass to the tank being provided between the filter and die for use in starting-up. In the Fig. 5 and 6 embodiment, the filling is applied by countercurrent flow, at above atmospheric pressure, during passage of the conductor through chamber 61 of a member 50 having an inlet die 65, and an outler die 66 of slightly larger diameter. The dies 65 and 66 are cooled by water circulating through chambers 67, 68, and the chamber 61 is heated by a heater 62, with heat insulant 69 being provided being the chamber 61 and the dies 65, 66. The member 50 is located above a filling medium tank 51, from which filling is circulated through the chamber 61, via a pump 52 and chamber inlet and outlet 63, 64. Guiding dies 53 are provided for the conductor upstream and downstream of the member 50. A gauge measures pressure in the chamber 61.