Manufacture of a flexible core for an optical cable

A method of manufacturing a flexible core of approximately circular transverse cross-section comprising a plurality of flexible elongate bodies of approximately sector-shaped transverse cross-section helically laid up together, each of which bodies throughout its length is made of a composition comprising a multiplicity of longitudinally stressed flexible elongate non-metallic reinforcing elements substantially encapsulated in extruded plastics material and each of at least some of which elongate bodies has in its outer arcuate surface a longitudinally extending groove, which method comprises the steps of:

• (a) drawing a multiplicity of flexible elongate non-metallic reinforcing elements under tension in the direction of their lengths through electrically insulating plastics material which is in a liquid or semi-liquid state;
• causing the multiplicity of non-metallic reinforcing elements encapsulated in electrically insulating plastics material in a liquid or semi-liquid state to pass through an elongate die having an outlet orifice having a transverse cross-section approximating to a sector of a circle to form a flexible elongate body of approximately sector-shaped transverse cross-section, which die, between its ends, is of such a shape that non-metallic reinforcing elements adjacent to the arcuate surface of the flexible elongate body so formed are travelling at a greater linear speed than non-metallic reinforcing elements remote therefrom, and which has means protruding inwardly of the die and forming in the arcuate surface of the body a longitudinally extending groove;
• (b) causing the flexible elongate body as it emerges from the die to follow a helical path within an imaginary cylinder of substantially circular transverse cross-section in such a way that the arcuate surface of the body is maintained outermost and so treating the helically advancing body that the plastics material in which the reinforcing elements are encapsulated sets;
• (c) at least partially filling the groove in the arcuate surface of the helically advancing flexible elongate body with a water-impermeable medium of a grease-like nature and continuously feeding into the water-impermeable medium in the groove at least one optical fiber;
• (d) laying up the helically advancing flexible elongate body housing in its groove at least one optical fiber with a plurality of helically advancing flexible elongate bodies of similar transverse cross-section and construction to form a flexible core of approximately circular transverse cross-section which has in its outer surface a plurality of circumferentially spaced, helically extending grooves in which optical fibers are loosely housed and which is rotating about its longitudinal axis;
• and
• (e) taking up the flexible core in such a way that any residual torsion in the core arising from said rotation is removed.