Co-axial cable

Abstract: Conventionally co-axial cable is made in a continuous extrusion machine by continuously extruding an aluminium tubular cladding (1) through an annular die and simultaneously continuously introducing a core (4), comprised of a conductive wire surrounded by insulation, through a bore in a mandrel (3). A gap is inevitably present between the outer surface of the core (4) and the tubular cladding (1). To eliminate the gap it is necessary to reduce the diameter of the tubular cladding by swagging or drawing. The present invention disposes of the swaqging or drawing step by compacting the insulation of the core before introduction to the mandrel (3). The insulation then gradually expands to recover its original diameter and fill the cladding which has been extruded to its final diameter.

Description: The present invention concerns a process for the manufacture of co-axial conductive cable, an apparatus for the process and an improved co-axial cable produced by the process. Conventionally, co-axial cable can be produced in a continuous extrusion machine sometimes known as a 'Conform' extrusion machine. This type of machine comprises a rotatably mounted wheel having an endless circumferential groove. A shoe is adapted to close part of the groove and mounts tooling which includes; an abutment arranged to at least partially block the groove and a passage leading to a die structure. Aluminium or other metal stock introduced into the groove is heated and pressurised by friction. The material engages the abutment in a condition in which it flows through the passage and is extruded through the die structure. To produce co-axial cable the aluminum is extruded as a tube through an annular die structure formed of an outer die part and a co-axial mandrel. An aperture is formed in the mandrel through which a core comprising a conductive wire coated in insulating material is passed. An annular space is formed between the core and the tube. To eliminate the space so that the core is tightly clad in a tubular sheath it is necessary to follow the extrusion stage by a step in which the tube is drawn or swagged as described in the specification of EP 0 125 788. To exemplify the problem experienced with the prior art method, it has been found that a cylindrical mandrel made of tungsten carbide or H13 tool steel must have an outside diameter at least 40% greater than the diameter of the aperture. Consequently to produce co-axial cable with a 12mm core diameter the tube extruded must have an inside diameter of at least 15mm. Subsequent to the extrusion step the tube must then be swagged or drawn down to an inside diameter of 12mm. This is inconvenient because of the apparatus required for the drawing or swagging step, the energy the step consumes and because the step work-hardens the cladding making the cable difficult to manipulate. It is an object of the present invention to provide a process and apparatus for the production of co-axial cable which alleviates the aforementioned problems. According to the present invention there is provided a process for the production of co-axial cable comprising the steps of: continuously extruding a tubular metal cladding and simultaneously continuously feeding an elongate core into the tubular cladding, the core comprising an electrical conductor coated with an insulating material characterised in that the insulating material is compacted to reduce the cross section of the core for introduction into the tubular cladding and is allowed to recover to fill the tubular cladding. According to a second aspect of the present invention there is provided apparatus for the production of co-axial cable comprising a continuous extrusion machine provided with a die structure for extruding tubular metal cladding around an elongate core comprised of an electrical conductor and an insulating coating characterised in that upstream of the die there is provided compacting means whereby the insulating coating can be continuously compacted from a cross section at least equal to the inside cross section of the tubular cladding to a cross section less than the inside cross section of the tubular cladding. It will be appreciated that the present invention depends on the discovery that cellular plastic insulating material can be compacted to reduce the cross-section (e.g., the diameter) of the core by the application of a compressive force in substantially the radial direction and, when the compressive force is relieved, the insulating material gradually recovers so the cross-section of the core tends to return to the original dimensions. Because the cross section of the core is temporarily reduced it can be fed through a mandrel dimensioned to extrude the tubular cladding to the finished dimensions required for the cable. The compacted core then expands to engage the inner surfaces of the tubular cladding so that the swagging or drawing step required in conventional methods and the apparatus for the swagging or drawing step is not required. Because the cladding is not swagged or drawn it is not work hardened and the co-axial cable produced is therefore advantageously more flexible. Recovery of the insulating material is not instantaneous. It has been found that the rate of recovery is temperature dependent and in consequence temperature control means may be installed downstream of the die structure to heat the co-axial cable in order to increase the rate of recovery. In an example of the process according to the present invention as illustrated in the figure, a die structure is provided in a continuous extrusion machine to extrude metal tubing 1 with an inside diameter of 12mm. An aperture 2 is formed co-axially in a mandrel 3 of the die structure and has a diameter less than or equal to about 60% of the outside diameter of the core so that in this case the aperture is approximately 8.5mm in diameter. An elongate 12mm diameter core 4 comprising a conductor surrounded by a cellular plastic insulating material is fed to compacting means provided by a conical drawing die 5 having a polished bore through which the core 4 is drawn to compress the insulating material to a diameter not greater than 8.5mm. The compacted core 4 is then fed through the mandrel aperture 2 into the tube 1 as it is being extruded. The core 4 is allowed to recover so that the spongy insulating material expands to fill the tubular cladding 1. The insulating material may be cellular polythene and the tubular cladding may be extruded aluminium having a proof stress of 50-60 N/mm2.

Claims: A process for the production of co-axial cable comprising the steps of: continuously extruding a tubular metal cladding (1) and simultaneously continuously feeding an elongate core (4) into the tubular cladding, the core (4) comprising an electrical conductor coated with an insulating material characterised in that the insulating material is compacted to reduce the cross section of the core for introduction to the tubular cladding (1) and is allowed to recover to fill the tubular cladding (1). A process according to claim 1 wherein the temperature of the core (4) is controlled to optimise the rate of recovery of the core cross section. Apparatus for the production of co-axial cable comprising: a continuous extrusion machine provided with a die structure for extruding tubular metal cladding (1) around an elongate core (4) comprised of an electrical conductor and an insulating coating characterised in that upstream of the die there is provided compacting means (5) whereby the insulating coating is continuously compacted from a cross section at least equal to the inside cross section of the tubular cladding to a cross section less than the inside cross section of the tubular cladding (4). Apparatus according to claim 3 wherein the compacting means comprises a conical drawing die (5). Apparatus according to claim 3 or claim 4 wherein temperature control means is provided to control the temperature of the core (4) to cause the insulation to recover at a desired rate. Co-axial cable produced by the process according to claim 1 or 2 and having an aluminium tubular cladding characterised in that the cladding has a proof stress in the range 50-60N/mm2 to improve the flexibility of the cable.

Inventors: MADDOCK BRIAN, MADDOCK, BRIAN
Applicants: HOLTON MACHINERY LTD, HOLTON MACHINERY LIMITED
Application year: 1992
Main classification: H01B13
IPC codes: H01B13, B21C23