MANUFACTURE OF ELECTRIC CABLES AND THE LIKE

Abstract: An electric cable or like elongate body comprises a core and, surrounding the core, at least one layer of a hard polymeric material (having at ambient temperature a flexural modulus of at least 1500 MPa) divided by helically extending divisions into a plurality of parts that are capable of relative movement on flexing of the body. The body is made by simultaneously extruding the parts of at least one layer of the hard polymeric material and applying them helically to the said core; preferably the parts are extruded separately from one another using a rotating die to establish the required helical relationship and are cooled sufficiently before they are brought together around the core to avoid their merging on contact with one another. Alternatively they may be formed by initially extruding a tubular sheath of the hard polymeric material around the core and then subdividing it into the required parts by helically-working dividing tools.

Description: Manufacture of Electric Cables and the like This invention relates to the manufacture of electric cables and other like elongate bodies (such as optical communication cables and flexible pipes) that require a high degree of resistance to mechanical forces. Typically such bodies incorporate one or two (rarely more) layers of "armour" wires of steel or other high-strength material, helically applied to a "core" comprising the functional part of the cable or other body and usually (but not always) covered by a layer of plastics material to protect the wires from corrosion. The use of metal wires in this way is expensive because massive rotating machinery is necessary to apply them (as relative rotation between the core and the supply of wires is needed) , and for some applications the electrical and/or magnetic properties of the wires may be undesirable or even totally unacceptable. An alternative is to use a tubular sheath of a particularly hard plastics material, but for a given level of machanical protection this increases stiffness much more than wire armouring does. It has been suggested (EP-A-0,342,855 and EP-A- 0,412,726) that cables might in special cases be made with a layer of arcuate preformed segments of resin- bonded fibrous or other tough non-metallic material; this avoids the disdvantages of metallic armour wires, but still needs massive rotating machinery to apply the preformed segments. In accordance with the invention, an electric cable or like elongate body comprising a core and, surrounding the core, at least one laysr of a hard polymeric material divided by helically extending divisions into a plurality of parts that are capable of relative movement on flexing of the body is made by simultaneously forming the said parts by an extrusion process and applying them helically to the core of the cable. For avoidance of uncertainty, a "hard" polymeric material is defined as one having at ambient temperature a flexural modulus of at least 1500MPa; polymeric materials with flexural moduli up to around 10,000MPa are available and can be used; we prefer to use materials with flexural moduli in the range from 1500 to 3000MPa. If there is more than one layer of hard polymeric material, preferable the divisions in (at least) one layer extend in a right-hand helix and those in (at least) one other layer in a left-hand helix. The divisions between the parts of the layer of hard polymeric material may be total, so that the parts are separate and the layer may be regarded as a layer of non-metallic "wires", or it may be incomplete so that the parts remain joined together by bridges that are either flexible or frangible so as to allow relative movement of the parts when the cable or other body is flexed. The parts may be of any convenient cross-section, including circular wire-like form, arcuate segments, and various interlocking shapes. Preferably the parts are extruded separately from one another using a rotating die to establish the required helical relationship and are cooled sufficiently before they are brought together around the core to avoid their merging on contact with one another. Alternatively they may be formed by initially extruding a tubular sheath of the hard polymeric material around the core and then subdividing it into the required parts by helically-working dividing tools. Preferably the dividing tools are indenting tools that separate or nearly separate the parts without removing any material while the temperature of the material is such that it is easily divided but can be prevented from rejoining by welding on contact (with or without the assistance of a parting agent applied in the vicinity of the parting tools) . Alternatively the parting tools may be cutting tools, such as saws or reamers, acting on the tubular sheath when it has been cooled to the extent that it is essentially solid (but preferably before it has achieved its full strength) . When rotating dies are used, two (or more) layers may be formed by concentric rotating die rings (preferably rotating in opposite directions) in the same extrusion machine; alternatively, and in the alternative methods, each layer can be formed separately by its own extruder, preferably operating "in tandem" as the core advances. In some cases (for example when the layer made up of parts of hard polymeric material are intended primarily to protect the cable from attack by rodents or termites) it is likely to be preferable that the said layer is the outermost layer of the cable (so that all the other layers are protected) ; since the layer is non- metallic, the precautions that must usually be taken to avoid corrosion of metallic armour wires can be dispensed with. However, if preferred an outer extruded sheath or a tape layer (for example) may surround the layer of hard polymeric parts. If resistance to termites is desired, the polymeric material preferably has a hardness of at least 65 Shore D and the hard polymeric parts should be smoothly curved with dimensions large ineough to avoid giving purchase to the mandibles of any termite species likely to be encountered. The invention will be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic cross-section through one form of electric cable that can be made in accordance with the invention; Figure 2 is a diagram of a production line for the manufacture of the cable of figure 1; Figure 3 is a diagrammatic cross-section of an alternative form of cable; and Fi ure 4 is a diagram of a production line for the manufacture of the cable of Figure 3. In the cable of Figure 1, the cable core 1 is simply enclosed in a layer made up of a number of parts 2 each of circular cross-section, which are made of a hard polymeric material, such as the one sold by G E Plastics Ltd under the designation Xenoy TPM 3000, which is believed to be a secondary (recovered scrap) blend of polycarbonate and polybutylene terephthalate and has a flexural modulus of about 1900MPa, tensile strength at yield 43MPa, Izod impact value (notched, 23°C) 400J/m and MFI (250 β C/5kg) of 16g/10min. This cable can be made (Figure 2) by a modified extrusion line receiving the cable core 1 from any suitable source (not shown) to the left of the figure; this may be a pay-off reel but is preferably manufacturing plant performing at least the last step in the manufacture of the core. The line includes a cross- head extrusion machine 3, cooling trough 4 and take-up 5, which are conventional except that the extrusion die, instead of having a single annular orifice surrounding the cable core 1, has a plurality of small circular orifices 6 each forming one of the wire-like parts 2 and is rotatable about the axis of the core by a drive mechanism 7 in order that the parts 2 will be applied helically around the core 1. The parts 2 are spaced apart to avoid contact between them until they are cool enough to avoid any major degree of adhesion between them; a coolant spray 8 may be applied before they enter the cooling trough 4 and/or a parting agent (such as silicone oil or talc) may be applied for extra security against adhesion. The cable of Figure 3 is similar except that the parts 2 of the outer layer are in the form of arcuate strips of hard polymeric material which are joined together by thin webs 9 which are weak enough to buckle or fracture if the cable is flexed and therefore make no s ^ gnificant contribution to the stiffness of the cable. This cable can be made using a conventional cross- head extruder 10 (Figure 4) that initially extrudes the outer layer as a tubular sheath 11; this is passed through two cooling troughs 12,13 before reaching take- up 5. Between the two troughs is an assembly 14 comprising an annular tool unit 15 and means 16 for rotating it about the axis of the cable. The tool unit (inset diagram) resembles a multi-start thread-cutting die and includes a plurality of parting tools 17 (only a few of which are shown in the diagram) which engage the tubular sheath at a place where it is partly cooled and indent it to form helically-extending grooves 18 that separate the parts 2 of the outer layer. The length or temperature of the first cooling trough 12 may be adjustable to obtain the optimum degree of cooling for easy indenting without undue risk of the parts becoming re-joined by deformation and welding across the grooves 18.

Claims: CLAIMS 1 A process for making an electric cable or like elongate body comprising a core and, surrounding the core, at least one layer of a hard polymeric material divided by helically extending divisions into a plurality of parts that are capable of relative movement on flexing of the body comprising simultaneously forming the said parts by an extrusion process and applying them helically to the core of the cable. 2 A process as claimed in claim 1 in which the hard polymeric material is one having at ambient temperature a flexural modulus of at least 1500MPa. 3 A process as claimed in claim 1 in which the hard polymeric material has a flexural modulus in the range from 1500 to 10,000MPa. 4 A process as claimed in claim 1 in which the hard polymeric material has a flexural modulus in the range from 1500 to 3000MPa. 5 A process as claimed in any one of the preceding claims in which there is more than one layer of hard polymeric material and the divisions in (at least) one layer extend in a right-hand helix and those in (at least) one other layer in a left-hand helix. 6 A process as claimed in any one of the preceding claims in which the divisions between the parts of the layer of hard polymeric material are total. 7 A process as claimed in any one of claims 1-5 in which the divisions are incomplete and the parts remain joined together by bridges that are either flexible or frangible so as to allow relative movement of the parts when the body is flexed. 8 A process as claimed in any one of the preceding claims in which the said parts are circular in cross- section. 9 A process as claimed in cny one of claims 1-7 in which the said parts are arcuate segments. 10 A process as claimed in any one of Claims 1-7 or clai 9 in which the said parts interlock. 11 A method as claimed in any one of the preceding claims in which the parts are extruded separately from one another using a rotating die to establish the required helical relationship and are cooled before they are brought together around the core to avoid their merging on contact with one another. 12 A method as claimed in any one of claims 1 - 10 in which the parts are formed by initially extruding a tubular sheath of the hard polymeric material around the core and then subdividing it into the required parts by helically-working dividing tools. 13 A method as claimed in claim 12 in which the dividing tools are indenting tools that separate or nearly separate the parts without removing any material while the temperature of the material is such that it is easily divided but can be prevented from rejoining by welding on contact. 14 A method as claimed in claim 12 in which the parting tools are cutting tools acting on the tubular sheath when it has been cooled to the extent that it is essentially solid. 15 A method as claimed in claim 14 in which the cutting tools act on the sheath before it has achieved its full strength. 16 A method as claimed in claim 11 in which two (or more) layers are formed by concentric rotating die rings in the same extrusion machine. 17 A method as claimed in claim 16 in which the said concentric die rings rotate in opposite directions. 18 A method as claimed in any one of claims 1-15 in which two or more of the said layers are formed separately each by its own extruder. 19 A method as claimed in claim 18 in which the said extruders operate "in tandem" as the core advances.

Inventors: HOWELL BARRY GEORGE, POOLE MICHAEL JOHN, RENTON GEORGE, HOWELL, BARRY, GEORGE, POOLE, MICHAEL, JOHN, RENTON, GEORGE
Applicants: BICC PLC, BICC PUBLIC LIMITED COMPANY
Application year: 1992
Main classification: H01B13
IPC codes: H01B7, H01B13