CA1049719A

CA1049719A - Apparatus for manufacturing cured polyolefin-insulated cables

Info

Publication number: CA1049719A
Application number: CA207,999A
Authority: CA
Inventors: Michio Takaoka; Hiroto Oshima; Toshiharu Katuhara; Motoyuki Ono; Hideo Sano; Masaichiro Seki; Masakazu Hasegawa
Original assignee: Fujikura Cable Works Ltd
Current assignee: Fujikura Cable Works Ltd
Priority date: 1973-08-30
Filing date: 1974-08-28
Publication date: 1979-03-06
Also published as: SE402371B; FR2242227A1; DE2441373B2; BR7407209D0; DE2441373C3; IT1020251B; FR2242227B1; SE7410668L; GB1479027A

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus for manufacturing cured polyolefin-insulated cables comprising a curable polyolefin-extruding device, a cur-ing tube partitioned into heating and cooling zones by way of a certain partition area, for example, an area composed of a seal packing, the two zones being filled with circulating silicone oil and connected to different circulating systems, through which the polyolefin-covered cable core is drawn, and a die provided at the other end of the curing tube and connected to a super-cooling device, where the pressurized oil is super-cooled to in-crease its density to such extent that the clearance between the outer surface of the cable core and the inner wall surface of the passage in the die is sealed.

Description

This invention relates to an apparatus for manufacturing electrical conducting cables covered with a cured polyolefin as the insulating material, and particularly relates to improvements in the construction and working of a curing tube where the insu-lating material is continuously cured.
In the known art, the usual apparatus for the manufacture of polyolefin-covered cables comprised the device of vulcanizing cable insulation composed of a mixture of a curable polyolefin and a cross-linking agent extruded over a conductor in saturated steam heated at about 200 C under the pressure of about 15 kg/cm2G~ followed by cooling the thus cured layer by water kept at about 20C under the same pressure. According to this device, the heated and pressured steam penetrates into the insulating ~ -material during the curing period and, as such penetrating steam is condensed by cooling, it naturally grows into the liquid form which would later volatilize, resulting in the formation of many voids within the vulcanized insulation. The formation of voids is known to have adversely affected the finished cable with res-pect to the wear resistant properties and the characteristic of aging in water, and such affected cables were not suitable for extra-high-voltage power transmission.
In order to remove the above disadvantage of the older con-ventional method, there has been proposed a method whereby the insulating material is heat-cured in a curing tube either by aid of heated wires together with an inert gas or by the circulation of a heated inert gas. These methods using no steam as the heat-ing medium have in fact been successful in eliminating the forma- -tion of voids during the heat~curing process, but they have not been able to remove any voids inherent in the pelletized insulat-ing material or prevent the formation of voids due to intrusion : - 1 -L ., '~
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of air during the extrusion of the insulating material onto the conductor. Besides, those methods are apt to impair the electric characteristics of finished product due to the presence of water - remaining after use as the cooling medium.
It is an object of the present invention to obviate or mitigate the above disadvantages.
According to the present invention there is provided an apparatus for manufacturing electric conducting cables insulated with a cured polyolefin which comprises a curing tube having a 10 first circulation system including a heating zone filled with a -~
heat transfer medium and a second circulation system including a ;-cooling zone filled with a cooling medium so that the heat trans-fer and cooling media are forced to follow separate circulation systemsj a pressure regulating means connected to each of the circulation systems, and a die arranged in alignment with and ad-jacent the outlet of the curing tube and filled with the cooling medium, the cooling medium in the die being super-cooled by super- ~ -cooling means so that the super-cooled medium works to seal the clearance between the outer surface of a passing insulated cable core and the inner wall surface of the passage in the die.
According also the present invention there is provided a method of curing a polyolefin-insulated cable comprising the steps of heating the cable in a heating zone of a curing tube by circulation of a heating medium therethrough, cooling the ~' cable in a cooling zone of the curing tube by circulation of a , cooling medium therethrough, passing the cable through a die , adjacent the outlet of the curing tube and forming a seal bet-' ween the die and the cable by circulating the cooling medium `, to the die and super-cooling the cooling medium adjacent the die to a temperature in the range -20C to -40C.

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. ~ - , - , ::. " . ~, 1049~719 Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:-FIG. 1 shows a diagrammatic cross-sectional view of one construction of an apparatus in accordance with the present invention, and FIG. 2 shows a diagrammatic cross-sectional view of a second construction.
Now the present invention will further be illus~
trated by reference to the drawings:
Referring to FIG. 1, a bare cable core denoted by numeral 1 is fed into the cross-head 3 of a semi-conduc-tive material extruder 2 to be coated with the extrudate as the inner layer. The coated cable core is sent into the cross-head 5 which is integrally formed with an in-sulating material extruder 4 and another semi-conductive material extruder 6 so that the core 2 is covered with the curable insulating material and thereafter with the semi-conductive material. The cable core thus covered is continuously passed through the curing tube, compris-ing a heating zone 10 filled with a heat transfer medium 9, a cooling zone 16 filled with a cooling medium 15 and an area composed of a seal packing 14 provided to parti-tion the heat transfer medium from the cooling medium.
As the suitable heat transfer or cooling medium a - completely degased and dried silicone oil is used which is stable against heat and harmless to polyolefins, In , order to promote its working efficiency, the silicone oil is recommended to have a high viscosity,preferably ranging from 5,000 to 100,000 cs. as well as a high heat coefficient.
As the suitable seal packing 14, p~ovided to prevent the heat transfer and cooling mediums from mixing together a sub-stance having good heat resistance and elasticity is used, for example, a cross-linked and foamed silicone rubber of the same nature as the heat transfer medium.
Heating zone 10 or cooling zone 16, separately and individu-ally, form a forced circulation heating or cooling medium system.
Heating medium 9 having been heated by a heater 7 and introduced thereinto by ~eans of a pump 8 is forced to circulate through the heating zone in the direction of the arrows. On the other hand, cooling medium 15 having been cooled by a cooling device 18 and introduced into the cooling zone by means of a pump 17 is forced to circulate in the direction of the arrows.
Both the curing and cooling zones 10 and 16 are put under pressure in the range of from 5 to 25 kg/cm G so that the covered ; core 22 may be prevented from foaming and the silicone oil may fill up any voids existing in the insulating polyolefin in an ac-celerated manner. Such pressure is supplied to both the curing and cooling zones 10 and 16 controlled automatically by a known pressure regulating device connected thereto comprising a vacuum pump 24, a silicone oil tank 13, a pressure pump 23, a check valve 11, and a set of relief valves 12. It is preferable to connect the pressure regulating device to the two zones at points near the center of the length of the curing tube to render the necessary even pressure distribution through each zone.
;~' The covered cable core 22 is passed through a die 20 which has been cooled down to a temperature between -20 and -40 C by means of a super-cooling device 19, then through a packing 21, and finally onto a reel (not shown).
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The silicone oil treated by the super-cooling device 19 when passed through the die 20 increases its viscosity thereby to pre-ssure-seal the clearance between the outer surface of covered core 22 and the wall surface of the passage in die 20. Packing 21 works to clean the surEace of the passing insulated cable core of any dirt or residues of the silicone oil. Incidently, it is designed that die 20 may be changed for another accordina to size of the cable core 22 to be passed through so that the passing clearance should be maintained at about 2 to about 5 mm.
FIG. 2 is a representation of a similar apparatus in which the so-called buffer area is provided between the heating and cooling zones in place of the partition area composed of a seal packing as in the embodiment shown by way of FIG. 1. Referring to FIG. 2, the curing tube comprising heating zone 10 filled with heat transfer medium 9 and cooling zone 16 filled with c,ooling medium 15 is provided with the buffer area 26 filled with the same medium thereby to keep the heat transfer and cooling mediums circulating in the heating zone and the cooling zone separately.
Surrounding the curing tube at this buffer area is installed a jacket 25 through which water is passed to cool the heat trans-fer and cooling media so effectively that the longitudinal length of the buffer area 26 can be as small as from 0.5 to 1 m. Within the buffer area the heat transfer and cooling mediums are in con-tact to each other, but these two mediums do not become mixed to ' worsen their heat efficiency. ~he provision of the buffer area ~ ' in liquid state according to the embodiment of FIG. 2 is advanta-geous over the provision of the partition area composed of solid ' material according to the embodiment of FIG. 1 as the latter in-; volves the risk of injuring the surface of the passing cable core " 30 while the former can preclude such possibility. ~ ~

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As in the case of the FIG. 1 embodiment, the FIG 2 embodi-ment is free of foaming phenomena on the cable core 22 during the curing and cooling processes and also of any voids remaining un-filled with the silicone oil.
Further, according to the FIG. 2 embodiment in which the buffer area is provided, the pressure regulating device for the supply of silicone oil is connected to the curing tube at only one point since there is no boundary or partition between the heating and cooling zones.
In summation, the apparatus disclosed herein may ' be featured by the following: ;
(1) Since the heating and cooling zones in the curing tube are - separated from each other, the heat transfer and cooling mediums contained therein are prevented from mixing together and their heat efficiency can be improved.
(2) Since the heat transfer and cooling medium are independently circulated within their own respective zones, their heat conduc-tivity can be improved.

(3) Since pumpin~ means for circulating the heat transfer and .. . .
cooling medlums are installed and worked separately from the pump for sending silicone oil into the curina tube under pressure, the - , pressure regulation can be performed with ease.

(4) Since a completely degased and dried silicone oil is used as the heat transfer or cooling medium, there can be avoided the for-mation of voids in the insulating material or, even though a secon-dary formation of voids takes place, it can be dealt with by fill-ing those voids with the silicone oil, thus resulting in improve-ments in the wear property and the characteristic of aaing in water of the finished product.

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(5~ The pressure-sealing of the clearance between the passage walls in the die and the passing core surface is carried out with the silicone oil medium super-cooled and solidified.
(6) Since silicone oil as the heat transfer medium has approxi-mately the same specific gravity as the apparent specific gravity of polyolefin as the insulating material, the soft curable poly-olefin applied on the conductor is supported by the silicone oil as it is drawn through the heating zone to prevent distortion of its shape before it is fixed by vulcanization. Further, due to the buoyancy of the silicone oil to the conductor, the insulated cable core need not be kept at an extraordinarily high tension.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for manufacturing electric conducting cables insulated with a cured polyolefin which comprises a curing tube having a first circulation system including a heating zone filled with a heat transfer medium and a second circulation sys-tem including a cooling zone filled with a cooling medium so that said heat transfer and cooling media are forced to follow separate circulation systems, a pressure regulating means connected to each of said circulation systems, and a die arranged in alignment with and adjacent the outlet of said curing tube and filled with said cooling medium, said cooling medium in said die being super-cooled by super-cooling means so that the super-cooled medium works to seal the clearance between the outer surface of a pass-ing insulated cable core and the inner wall surface of the pass-age in said die.

2. The apparatus according to claim 1 wherein said heating and cooling zones are partitioned by means of a seal packing.

3. The apparatus according to claim 1 wherein said heating and cooling zones are partitioned by a buffer zone sur-rounded by a water jacket said heat transfer and cooling media being in contact with each other in said buffer zone.

4. Apparatus according to claim 1, 2 or 3 wherein said super-cooling means is connected to said die to super-cool the cooling medium therein.

5. Apparatus according to claim 1, 2 or 3 wherein said heating medium and cooling medium are a silicon oil.

6. A method of curing a polyolefin-insulated cable comprising the steps of heating said cable in a heating zone of a curing tube by circulation of a heating medium therethrough, cooling said cable in a cooling zone of said curing tube by cir-culation of a cooling medium therethrough, passing said cable through a die adjacent the outlet of said curing tube and forming a seal between said die and said cable by circulating said cool-ing medium to said die and super-cooling said cooling medium adjacent said die to a temperature in the range -20°C to -40°C.