US 3413167 A
Description (OCR text may contain errors)
S. W. TRILL Nov. 26, 1968 MANUFACTURE OF PLASTIC-INSULATED ELECTRICAL CABLE Filed July 9. 1965 INVENTOR SIDNEY w rmu.
United States Patent 3,413,167 MANUFACTURE OF PLASTIC-INSULATED ELECTRICAL CABLE Sidney W. Trill, Scarsdale, N.Y., assignor to Phelps Dodge Copper Products Corporation, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 339,252, Jan. 21, 1964. This application July 9, 1965, Ser. No. 473,562
6 Claims. (Cl. 156-51) This invention relates to a method for the improvement in quality of thermosetting and thermoplastic insulated electric cable's by the elimination of voids between an electrical conductor and the insulation on the conductor. This present application is a continuationin-part of applicants prior and co-pending application Ser. No. 339,252 filed Jan. 21, 1964 now abandoned.
It is well known in the manufacture of insulated electrical cables to form a cable having a thermoplastic or thermosetting insulation covering the conductor. In the customary way of forming such cable the insulation is continuously extruded at a high temperature in plastic condition to form a coating about the conductor. Typically, the conductor of this type cable consists of several copper strands surrounded by a thin semi-conductor layer. After the extrusion the insulating material must be cooled to solidify it. The cooling step has presented difiiculties in the past. In order to provide a durable and efficient cable it is necessary to have the insulation and the conductor in continuous contact without the presence of gaps or voids between the two. The cooling step used in the past has resulted in the formation of such voids and gaps. The prior-art process of cooling consists of taking the high-temperature coated cable and immersing it in hot water. The hot-water step is followed by several immersion steps utilizing water of decreasing temperature. The sudden initial temperature drop, from about 350 F. in the case of typical thermoplastic polyethylene, to a temperature below 212 F. results in a case hardening of the outer surface of the thermoplastic insulating material. The insulation then tends to cool toward the outer hardened surface resulting in cavities between the surface of the conductor and the insulation and mechanical stresses in the polyethylene. With a high voltage over the conductor, corona discharge may occur across the voids. The corona effect, when it occurs over an extended period of time, leads to an erosion of the insulation at the void, producing a larger gap and continued erosion until the cable fails or is otherwise rendered defective for service. With the elimination of voids the insulated conductor may be subjected to higher voltages without a corona discharge.
The problem of voids in the formation of polyethylene or polypropylene insulation is particularly acute due to their high co-efficient of expansion and poor thermal conductivity. The prior art process of hot-water cooling is a slow process and reduces the speed at which cable may be manufactured in addition to having limitations on the minimum void size that can be achieved.
The invention of applicant reduces the presence of voids by the use of a high-temperature pressurized steam system for initial cooling of the thermoplastic or thermosetting insulation on the conductor. After the insulation is extruded on to the conductor, it emerges from the extrusion machine and is then passed into a chamber containing live steam under pressure. The temperature and pressure of the steam may be varied according to material being extruded, the speed of extrusion and the 3,413,167 Patented Nov. 26, 1968 'desired cooling to be achieved. Since the steam is under pressure, temperatures in excess of 212 F. may be achieved resulting in a more moderate first stage of cooling. After passing through the steam section the insulated conductor then passes into hot water followed by cool water, both being under pressure, where fi-nal cooling is achieved. Since the cooling is controlled to achieve a more gradual change from the temperature of extrusion the insulation tends to contract inwardly against the conductor resulting in a more satisfactory and durable product because of the substantial absence of voids between the insulation and conductor and the reduction of mechanical stress in the insulation material. Also, about 20% higher rate of production is obtained than with the prior hot-water cooling process.
The invention of applicant is described in greater detail in the description below and in the drawings in which:
FIG. 1 is a simplified flow diagram showing the passage of the insulated cable through the various processing steps; and FIIIGI. 2 is a cross-section of the steam chamber of Referring to FIGS. 1 and 2 a conductor 1 having an extruded outer thermoplastic coating 2 leaves the extrusion machine 3, shown schematically, and passes into a chamber 5 larger than the finished cable diameter and which is sealed by mechanical means 4 against the extrusion head to retain steam pressure. The chamber shown in the drawing is catenary-shaped to prevent contact between the walls of the chamber and the hot thermoplastic insulation. If it is desired the chamber may be vertical and the hot thermoplastic insulation will pass downwardly through the steam chamber without contacting the walls, or it may be horizontal, for a smaller limited range of conductor and insulation sizes.
Pressurized live steam 6 at a temperature above 212 F. and below the temperature of the thermoplastic insulation is introduced into the chamber through an inlet 7. The pressurized steam causes a decrease in the temperature of the insulation while inducing an inward contraction toward the conductor.
The partially cooled cable passes from the steam chamber through a steam-water interface 8 into a pressurized water section which continues the cooling of the cable. The water between exit pipe 11 and interface 8 is heated by the contact at the interface between the steam and the water and provides an intermediate temperature to further reduce the temperature of the cable. The water in this section is essentially stagnant. The length of the section between interface 8 and pipe 11 is adjustable by means of water level control 9, shown schematically, which controls the position of the steam-water interface 8. The location of the interface can be adjusted to compensate for the use of dilferent materials and temperatures.
The cable, after travelling through the interface 8, passes through the intermediate temperature section and then into a cool-water section located between outlet pipe 11 and water seal 17. Cool water 10 is continuously fed to this section by a high-pressure water pump 14 through inlet pipe 16 at a pressure equal to the steam pressure. The temperature of the water entering through pipe 16 may be ambient or any other selected cool temperature.
The cable finally passes out through water seal 17. The seal is adjusted to allow the passage of some water outward with the cable to lubricate the cable as it passes through. A caterpillar having elements 12 and 13 gripping the insulated conductor in controllable tension with a similar caterpillar, not shown, gripping the conductor element of the cable as it enters the extrusion machine, provide means to maintain the cable in catenary shape substantially clear of the wall of the enclosing chamber. After passing the caterpillars the cable is taken up on reel 18. The method of extrustion of the thermoplastic material and the manner of passing the conductor through the extruder, as well as the means for revolving the reel, are conventional and not shown.
In using my method, adjustments may be made in the pressure, and thus the temperature of the steam to vary cooling conditions according to the material extruded, the wall thickness and the size of conductor, and so obtain optimum product quality. In addition, my invention may be used with any thermoplastic material where intimate contact is desired between the plastic material and the conductor. Typical of the materials usable are polyethylene, polypropylene and mixtures of these plastics with polyisobutylene. The quality of polyvinyl chloride extrusion is also improved by the method described, although its use may not parallel that of polyethylene. Previously it has also been found that high-density polyethylene presents great difiiculty in utilization due to its high contraction rate but the method and apparatus of my invention facilitates its use. In addition, although the cable described previously is power cable, the apparatus and method of my invention may be used for the construction of coaxial cable.
My invention may also be used with termosetting materials, such as cross-linked polyethylene. in cooling cross-linked plastic insulation the steam chamber preferably has a steam pressure of 250 p.s.i. and 400 F. The temperature must be above 285 F. to cure the thermosetting plastic. The water section preferably has a temperature range of 200 F. at the entrance and 120 F. at
,the exit with the same pressure of 250 p.s.i. Higher pressures are also usable to obtain a faster cure, such as 400 p.s.i. and 575 F. These higher pressures require a longer cooling period and careful supervision of the sealing of the chamber.
My invention may also be used with a thermosetting or thermoplastic semi-conductive layer. The semi-conductivity is preferably due to the addition of semi-conductive carbon black to the plastic material. For example, a semi-conductive thermosetting polyethylene layer is extruded over the conductor, 1,-a layer of thermosetting polyethylene insulation extruded over the semi-conductive layer and the cable passed into the chamber 5 for initial cooling.
The following is a preferred manner of utilizing my invention.
EXAMPLE Seven strands of No. 2-AWG copper wire having a strand diameter of .0974 inch are woven into a conductor of .292 inch diameter, covered by a thin semi-conducting layer. Thermoplastic polyethylene insulation having a density of .918 and a melt index of between .15 and .30
a temperature between 200 F. at the entrance and 120 at least 50 kilovolts. A cable produced by conventional hot-water cooling, of similar length, as previously described, having the same characteristics as the cable of this example would have a corona level substantially lower and be produced at a lower speed.
1. A method for controlling the contraction of a plastic insulating material on an electrical conductor comprising the steps of enclosing an electrical conductor coated with a plastic material in a plastic state in a tube section of adjustable length filled with steam at a temperature less than the temperature of the plastic material and more than 212 F., allowing the thermoplastic material to cool to a lower temperature, passing said insulated conductor from said first section into a second section of adjustable length and immersing said insulated conductor in Water therein at a temperature below that of the steam at the exit of said first section and above room temperature, said water being pressurized to equal the pressure of the steam in the steam section, passing said insulated conductor out of said second section and into a third section and immersing said insulated conductor in water therein at a temperature approximately equal to room temperature and pressurized to equal the pressure in the second section, and removing said conductor from said third section, whereby an even contraction of the thermoplastic insulation occurs and substantially without the formation of voids between the conductor and the insulation.
2. A method according to claim 1 wherein the plastic insulating material is thermoplastic polyethylene.
3. A method according to claim 1 wherein the plastic insulating material is thermoplastic polypropylene.
4. A method for the continuous manufacture of a thermoplastic polyethylene-insulated electrical conductor comprising extruding polyethylene continuously on to an electrical conductor, passing the insulated conductor in a plastic state into a first tube section containing pressurized steam at a temperature between approximately 285 F. and approximately 240 F., passing the conductor from said first section through a steam-water interface into a second section and immersing the insulated conductor in water therein pressurized to the pressure of the steam and having a temperature between approximately 200 F. and approximately F, passing the coated conductor from said second section into a third section and immersing the conductor in water therein at room temperature and the same pressure as the second section and removing the cable from said third section, whereby an even contraction of the polyethylene occurs and substantially without the formation of voids between the conductor and the polyethylene.
5. A method for the continuous manufacture of a thermosetting polyethylene-insulated electrical conductor comprising extruding cross-linkable polyethylene continuously on to an electrical conductor, passing the insulated conductor in a plastic state into a first tube section containing pressurized steam at a temperature above 285 F., passing the conductor from said first section through a steam-water interface into a second section and immersing the insulated conductor in water therein pressurized to the pressure of the steam and having a temperature between approximately 200 F. and approximately 120 F., passing the coated conductor from said second section into a third section and immersing the conductor in water therein at a room temperature and the same pressure as the second section and removing the cable from said third section, whereby an even contraction of the thermosetting polyethylene insulation occurs and substantially without the formation of voids in the cross-linked polyethylene insulation and between the conductor and the cross-linked polyethylene insulation.
6. The method according to claim 5 wherein a layer of semi-conductive, cross-linkable, plastic material is ex- 5 6 truded on the conductor and the Said insulation extruded FOREIGN PATENTS over the seml-conductlve layer. 994,634 8/1951 France.
References Cited UNITED STATES PA 5 EARL M. BERGERT, Primary Examiner.
3, 4 ,673 7/1962 Goodwine 156-51 X T. R. SAVOIE, Assistant Examiner. 3,325,325 6/1967 Ward 15656