Manufacture of insulated electric cables

Claim 1

In the manufacture of an extruded covering on a continuously advancing core by means of an extrusion machine having a core tube extending through the machine, an inner die or point mounted on the front end of the core tube and an outer die at the outlet end of the machine which defines with the inner die or point an annular extrusion orifice having an internal diameter substantially greater than the external diameter of the extruded covering and by a float-down process in which the core is drawn through the core tube of the extrusion machine, a continuous covering is applied to the core at the extrusion orifice and a pressure difference is maintained across the extruded covering at the extrusion orifice sufficient to cause the extruded covering to collapse firmly on to the core as it emerges from the extrusion machine but insufficient to force the extruded covering back along the core tube, the radial thickness of the extruded covering being controlled by varying the speed at which the core is drawn through the extrusion machine, the improvement comprising a. continuously treating the covering so formed with a fluid medium at super-atmospheric pressure by passing the covered core through a chamber hermetically sealed to the outlet end of the extrusion machine and containing the fluid medium at superatmospheric pressure; b. injecting fluid under pressure into the interior of the core tube, and c. maintaining the fluid in the core tube at a pressure which is less than that of the fluid medium by an amount such that the said pressure difference across the extruded covering at the extrusion orifice is maintained.