Note: Descriptions are shown in the official language in which they were submitted.
1049719
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
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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
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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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1049719
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
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cooling medlums are installed and worked separately from the pump
for sending silicone oil into the curina tube under pressure, the
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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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