MANUFACTURE OF INSULATED ELECTRIC CABLES

1291880 Coating conductors by extrusion BRITISH INSULATED CALLENDERS CABLES Ltd 8 Dec 1970 [9 Dec 1969] 60011/69 Addition to 1201528 Heading B5A [Also in Division H1] The method and apparatus of the parent Specification, in which a covering extruded on to a conductor 1 is collapsed thereonto but is prevented from upstream displacement along the conductor by providing a predetermined pressure difference between the interior of a core tube 14 through which the core is drawn and a chamber 5 containing fluid at super-atmospheric pressure, is modified by coating the conductor with two peripherally continuous layers of covering material 8, 9, which come into contact upstream of the die outlet. In the described embodiment, an uncovered sector-shaped aluminum conductor 1, of solid section, is provided with inner 8 and outer 9 cross-linkable coverings, respectively, of semi-conducting and insulating polyethylene, in a cross-head 10 to which it is fed from a coil and in which it passes through a hermetically sealed core tube 14 to which nitrogen under pressure is injected, the coated conductor 1 being drawn from the annular die outlet by a haul-off unit through a hermetically sealed curing chamber 5 and subsequent cooling vessels. The apparatus is downwardly inclined so that the covering is cured by super-atmospheric pressure steam, e.g. at 190 to 210 p.s.i.g. at the upper end of chamber 5, and cooled by water at the lower end thereof; the chamber 5 may alternatively effect only cooling. The pressure difference across the covering is maintained constant, e.g. at from 0À03 to 0À07 atmospheres, by the control systems of the parent Specification. A vacuum pump may be employed at start-up. The nitrogen may be continuously leaked from the core tube 14, either througn a core tube valve or through the upstream seal 16 when it serves to reduce friction therethrough, with nitrogen being automatically continuously injected into the tube 14 by a differential pressure control device. The coverings may alternatively comprise natural rubber, neoprene (polychlorobutadiene), butyl rubber, silicone rubber, styrene-butadiene rubber, or ethylene-propylene polymers, and may also comprise an insulating layer sandwiched between semi-conductive layers of the same or different material, when the outer thereof has a weaker bond to the insulating layer to facilitate stripping for jointing or terminating purposes.