GB1592958A

GB1592958A - Method of and apparatus for producing an insulating covering of cross-linked insulating material

Info

Publication number: GB1592958A
Application number: GB48330/77A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1977-02-19
Filing date: 1977-11-21
Publication date: 1981-07-15
Also published as: IT1088040B; ATA698977A; SE7801480L; AT353872B; FR2361730A1; FI773574A; DE2707297B1; NL7800477A

Description

(54) IMPROVEMENTS IN OR RELATING TO A METHOD OF AND APPARATUS FOR PRODUCING AN INSULATING COVERING OF CROSS-LINKED INSULATING MATERIAL (71) I, KLAUS HERMANN SUTTERLIN, of Dusseldorfer Strasse 45, 5000 Koln 80, Germany, a German Citizen, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of and apparatuse for producing an insulating covering of cross-linked insulating material.

A known method of this kind is described for example in West German Offenlegungsschrift 24 54 478, in which the cross-linking temperature of a mouldable mass of heat-setting polymeric material is attained by strong mechanical working of the mass by means of a rotatable worm during or immediately after mixing and homogenising or plasticising treatment of the material, and the material thus heated is conveyed continuously progressing through a moulding tool substantially at the cross-linking temperature, and is finally cross-linked by the application of further heat from an external source. In this case, a method is involved which may also be employed for the production of cables or cable cores with an insulation of cross-linkable polyethylene wherein the progressively conveyed, settable polymeric material is moulded concentrically around the conductor and is finally cross-linked immediately after its moulding by further application of heat. In this known method, the strong mechanical working of the material occurs by means of extrusion wqrms in two separate steps, wherein in the first working step the material is plasticised and homogenised and is supplied in this state to the second step where it is simultaneously conveyed to the moulding stage and heated approximately to the cross-linking temperature, and is moulded at the latter and guided through a heating device in which the cross-linking of the article previously initiated in the second working step, for example a cable insulation, is completed.

The apparatus used for performing this method, comprises two mutually connected extruders, the production outputs of which must be so matched to each other that in the second step the cross-linking temperature of the mass which depends on the reaction temperature of the cross-linking agent can be obtained solely or preponderantly by the shearing heat produced in the mass during the working thereof by means of the worm, so that the mass leaves the second step in an already largely cross-linked state and the linking process must merely be finalised in the subsequent after-heating stage. However, it follows therefrom that, for a given worm geometry of the two steps, in spite of differently controllable rotary speeds of the extruder worms, the operating conditions which ensure a satisfactory cross-linking throughout the product can be obtained only at a certain predetermined product output of the apparatus.

In other words, thoroughly cross-linked extrusion products can be produced optimally only with a certain predetermined cross-section. If the supply power of the first step is increased, the supply power of the second step must also be increased simultaneously for reasons of continuity of the material flow, in which case, the necessary cross-linking temperature can no longer be attained. However, a reduced feed output, e.g. in the production of a thin insulation, may lead to overheating of the cross-linkable mass in the second working step, because then the feed quantity of the mass no longer corresponds to that for which the second working step is designed to effect the desired cross-linking temperature by shearing heat. However, this imports the risk of premature cross-linking and setting of this mass.even in the second working step and thus of operational failure of the apparatus.

In consequence of the low throughput power parameter, this known method, as well as the apparatus constructed therefor, is uneconomical. Furthermore, since the extrusion pressure at which the product is moulded depends upon the cross-linking temperature to be attained and thus upon the rotary speed of the worm in the second working step, the moulding of the extrusion product preferably by means of a flexible tube tool but alternatively by means of a pressure moulding tool, which is considerably more complicated for production reasons but is necessary for certain products, is difficult or even entirely impossible to effect because of the difficulty of maintaining a constant moulding pressure which is independent of the cross-linking temperature. Moreover it has been found that even if the predetermined operating conditions are strictly maintained and in spite of the efforts to effect de-gassing of the previously plasticised mass after its entry into the second working step, a product perfectly free of bubbles, as is absolutely necessary for the insulation of electrical cables, cable cores and conductors, cannot be ensured. It has also been found that, when this method is used for the production of the covering of electrical conductors, the cross-linking of the insulation is defective in the vicinity of the conductor and cannot satisfy the generally accepted requirement of a crosslinking degree of at least 75%, measured in accordance with the solvent extraction test. It is believed that this effect is produced inter alia by the condition of the conductor surface, by the relatively high heat loss due to the electrical conductor to be covered, and the formation of bubbles occurring particularly frequently in the vicinity of the conductor.

Furthermore, the known apparatus required for performing this known method is complicated because of the complicated construction comprising two complete separately controllable extruders which are fitted with different additional devices and which are combined to form a unit, as well as special worm constructions, so that the advantage aimed at thereby of a considerable reduction of the investment costs by the omission of the long cross-linking pressure tubes and associated devices is realised only to a modest extent.

On the other hand, for example West German Offenlegungsschrift 22 04 655, discloses a method of and apparatus for prodicing electrical cables or conductors with an insulation based on a cross-linked polyethylene, wherein likewise polyethylene in the non-linked state is extruded concentrically around the electrical conductor and is cross-linked continuously in the same operating step by means of peroxides under the effect of heat. This known method, too, aims at a reduction of the high investment costs, which were unavoidable heretofore in the usual production methods for cables with an insulation of a cross-linkable polyethylene, for the necessary apparatus including an extruder followed by a tube, the length of which corresponds to the required dwell period and in which the cross-linking occurs with the application of heat and pressure by means of a heat transferring medium, for example, steam. Production facilities which are more economical and production-technically more favourable, and which at the same time ensure good dielectric properties of the insulation thus produced and cross-linked, are required.

This known method requires that a polyethylene, from which non-soluble components and moisture have been removed and which has predetermined properties, be used which, after extrusion upon the conductor which has a temperature of at least 80"C, is cross-linked in a salt bath or similar fluidised bed at or nearly at atmospheric pressure. It is also required that the metal surface of the conductor on to which the material to be cross-linked is extruded be cleaned prior to the extrusion and under certain circumstances metal oxide products be removed therefrom, the conductor being pre-heated to a temperature of from 80 to 200 C. The conductor surface is further coated with an adhesion aid prior to the deposition of the cross-linkable synthetic resin.

By means of this known method, even electrical conductors with non-circular cross-section, for example pre-twisted sector conductors, may be insulated by means of the socalled "tube stretching method", wherein first a tube is extruded which surrounds the conductor at a spacing, the conductor is drawn off at a higher speed than the exit speed of the tube and is caused by stretching to lie on all sides on the conductor. Admittedly one of the conditions for this is that the subsequent cross-linking of the synthetic resin occurs without the application of external pressure, but, for instance, by the effect of a heat-transferring pressure-free liquid medium of the kind referred to above. Also, pre-heating of the conductor to be insulated is required, for which purpose the apparatus known from the said specification is provided with a heatable mandrel which projects into the extrusion head of the extruder.

Admittedly, this known method permits a pressure-free cross-linking of the insulating material and thus the use of the more economical tube extrusion methdd. However, the costs of and the space requirement for such an installation are high because of the need for supplying the reac tion heat necessary for cross-linking the insulating material after the extrusion thereof. In spite of the pre-heating of the conductor and because of the long dwell period, a correspondingly long treatment path of the liquid heat transfer medium is absolutely necessary in this case for completely cross-linking the insulation throughout to the desired high degree of crosslinking of at least 75%. The costs of and the space required for, for example, a hot fluidised bath of appropriate length are hardly less than in the pressure crosslinking installations usual heretofore.

There is a further disadvantage that this known method can be performed only when certain sorts of high pressure polyethylene are used which fulfil the stated conditions wherein additionally supporting measures are necessary in order to maintain the cable or conductor insulation thus produced free of bubbles during the pressure-free crosslinking. A further disadvantage is the relatively high operation energy costs because of the requirement of heating and maintaining hot a large quantity of the heat transfer medium in the liquid bath or fluidised bed which necessarily has a considerable length.

An essential disadvantage of this known method, however, results from the fact that the considerable ammount of heat necessary for the cross-linking process is supplied, as usual heretofore, as late as after the extrusion of the cross-linkable polyethylene, and not as in the method previously described, during the plasticising or homogenising of the insulating material in the extrusion head of the extruder by controlled production of friction heat in the mass itself. In spite of the possibility obtained thereby of pressure-free cross-linking, this method is unable to improve considerably the economy of the production of conductors, cables or cable cores with an insulation of cross linked polyolefine compared with the methods usual heretofore, and thus does not constitute a true alternative therefor.

On the other hand, West German Offenlegungsschrift 20 59 469, discloses apparatus for plasticising synthetic resins having a feed device and two devices operating independently of one another for producing plasticising heat or shearing heat, wherein a shearing device is provided which is effective independently of the feed device and which is formed by a mandrel which is rotatable in a ring-shaped operating space.

In the vicinity of its outlet opening for the synthetic resin material which has been brought to the necessary treatment temperature by such rotation of this mandrel and the shearing hea mechanically imported thereby, this apparatus is provided with additional heating devices for fine adjustment of the temperature of the synthetic resin flow. This known apparatus is suitable for the various kinds of synthetic resin treatment, for example extrusion moulding, but may also be used in a considerably less complicated form, for plasticising synthetic resins during continuous extruding. For this purpose the mandrel is arranged, for example, longitudinally non-displaceably and a continuously operating conveyor means, for example feed screw, is provided.

This apparatus provides the possibility of heating the synthetic resin to any desired temperature, for example the reaction temperature of a cross-linking means previously added to the synthetic resin, solely by changing the rotary speed of the shear mandrel disposed in the extrusion head independently of the output of the feed device i.e. in the case of continuous extrusion, the extrusion worm, under certain circumstances in co-operation with the heat supplied simultaneously by the additional external heating device, in order to produce in this way elongate articles of plastifiable or cross-linkable synthetic resin in a continuous process. However, nothing can be found in the said specification as to how such apparatus permits the production of an insulating covering of an electrical conductor to be effected which fulfils the high demands, independently of the fact that a conductor insulation of non-cross-linked or cross-linked synthetic resin may be involved therein.

According to one aspect of the invention, there is provided a method of producing an insulating covering of cross-linked insulating material by extrusion around an electrical conductor, comprising the steps of providing the insulating material with a of heat-activatable cross-linking agent, mechanically working the insulating material to raise its temperature and pressure and to homogenise and plasticise it, conveying the insulating material to a die head under extrusion pressure, heating the insulating material immediately prior to extrusion from the die head to above its crosslinking temperature without changing the extrusion pressure by means of rotational shearing heat produced by mechanical friction, supplying the conductor in a preheated and surface treated state wherein the conductor is heated to above the reaction temperature of the cross-linking agent and the conductor surface is treated with meansfor inhibiting the formation of bubble nuclei, extruding the insulating matterial at the said temperature above the cross-linking temperature in the form of a sleeve around the conductor with simultaneous evacuation of at least a region between the sleeve and the conductor surface, maintaining the insulating material for a short period of time substantially at its cross-linking temperature without external application of pressure to cause cross-linking at constant temperature of an outermost zone or outer layer of the conductor sleeve formed thereby, and cooling the insulating material externally while allowing cross-linking of the conductor sleeve throughout the entire cross-section thereof to be completed by the heat previously supplied to the insulating material.

In a preferred method of producing an insulating covering of cross-linked insulating material, in particular polyethylene, the required reaction heat for the cross-linking means is introduced into the cross-linkable synthetic resin substantially during the manipulation thereof by shearing heat produced thereby, wherein the quantity, manipulated in unit time, of the cross-linkable material is controllable independently of the desired exit temperature of the latter, the cross-linkable mass is heatable by shearing heat to any desired extent and, after extrusion, is cross-linkable free of bubbles up to a high degree of cross-linking of at least 75%, and wherein the economy of the production is considerably increased compared with the cross-linking method usual heretofore, and the space requirement and theinvestment expenditure required for performing the method are considerably reduced, as well as also to provide apparatus suitable for performing this method.

In an extremely economical manner, and with very small space requirement, a preferred method permits the production of insulated electrical conductors of any desired cross-sectional size and shape with an insulation of cross-linked insulating material, in particular polyethylene, in which case solid conductors as well as stranded conductors may be involved, for example pre-formed segment conductors, and wherein, dependently upon the desider thickness of the insulation, the known tube extrusion method (for thin insulations as for example in the case of 1 kV segment conductor cable cores) as well as the usual kind of concentric moulding of the insulation by means of a pressure moulding tool, for example in the case of thick insulations for circular conductor cables of high voltage, is usable. The high economy of the method results on the one hand, from the low installation costs, since in this case the late heating for effecting the cross-linking of the insulating sleeve is omitted, and on the other hand, from the high production speed rendered possible in this case in particular by the independent controllability of the mass temperature, wherein the freedom from bubbles and the high quality of the insulation as well as the necessary thorough cross-linking up to the desired high degree of cross-linking are ensured by the measures precribed for this purpose prior to and during the extrusion process as well as the directed control of the cross-linking process during and after the extrusion of the cross-linking insulating material.

Preferably, taking into account its crosssectional area, the production speed and the draw-off speed and the insulating material and cross-linking agent used, the conductor to be insulated is preheated to a temperature between 150 and 200"C, the temperature of the conductor is measured immediately prior to the extrusion covering thereof and the pre-heating of the conductor is controlled dependently upon the measured value, the plasticised insulating material being heated prior to the extrusion to a temperature between 160 and 220 C, the extruded insulating sleeve which is provided with up to four % by weight of the cross-linking agent being maintained without the application of external pressure at the same temperature only until a lower-cross-linking degree than that of the already cross-linked outer layer of the insulating sleeve, measured in accordance with the so-called "solvent extraction test", is effected, the insulating sleeve finally being cooled in at least two, preferably three, cooling stages starting with at most 1200C, to be cross-linked throughout thereby and finally cross-linked up to at least 75%.

Numerous tests which were performed after overcoming great difficulties have shown that the best results may be obtained with a preferred method when the conditions stated above are observed. The difficulies which had to be overcome thereby resided above all in the suppression of the bubble formation without the application of an external over-pressure as well as in obtaining the necessary cross-linking throughout up to the high cross-linking degree aimed at, in spite of the high production speed which exceeds the cable production speed obtainable by means of the usual cross-linking methods. In addition to the high proportion of cross-linking, the extrusion of the latter in an already slightly cross-linked state on to a conductor heated to the reaction temperature is of considerable significance in this case in such a manner that the cross-linking to the desired degree of cross-linking occurs during and immediately after the extruding of the material proceeding substantially from the outside to the inside of the sleeve, so that the outer layers are already finally crosslinked and may be cooled, whereas the cross-linking of the insulating material occurs in the direction of the conductor when the outer layer of the insulation is already hardening and cooled to a certain degree, because of the residual heat present in the mass or delivered to the latter by the conductor.

In view of the control of the cross-linking process, for instance for moulding relatively thick insulating sleeves such as are employed in cables or cable cores designed for high voltages, the extruded insulating sleeve may be maintained at approximately the same temperature as before the extrusion of the insulating material by external heat radiation, is subsequently pre-cooled by a liquid having a high boiling point, for example brine heated to 110 or water whose toiling point is raised by increased pressure, to a temperature below the softening point of the insulating material, is thereby finally cross-linked owing to the residual heat present in the insulating material or delivered by the pre-heated conductor within the cross-linked outer layer, which is sufficiently rigid to maintain the pressure forming in the interior of the outer layer as a result of cross-linking, and is finally cooled in at least two further different temperature stages preferably by water.

In contrast to the known cross-linking methods, no cross-linking heat is supplied in this case, but instead the cooling of the previously cross-linked insulating sleeve is slowed down within the meaning of the preceding statements, e.g. in overheated water, to such an extent that, in spite of the increased producion speed, the final cross-linking occurs in the interior of the previously cross-linked outer layer of the insulating sleeve which, after cooling has been effected, forms a pressure resistant outer skin, pressure forming in the sleeve, which pressure substantially prevents any bubble formation by the gas released during the cross-linking process or delivered by the cross-linking means. The same kind of temperature control and cooling from outside of the extruded sleeve is advisable even for tube-extruded thin insulating sleeves, for example of low voltage cable cores, in which case, however, the sleeve may also be cooled in a first cooling stage without pressure by liquid having a high boiling point or by water of below 100 C, in order to effect first the cross-linking and hardening of its outer layer with the smallest possible loss of heat and so as not to interrupt prematurely the cross-linking process which continues therebelow to the conductor surface, at the same time not damaging or compressing the sleeve which is initially still soft also on the outside.

In order to substantially avoid bubble formation during the cross-linking process, prior to the extrusion of the insulating material, a heat-resistant thin foil which is compatible with the conductor and the insulating material, for instance of a polytherephthalic acid ester, is wrapped around the conductor to compensate the irregularities of the surface thereof.

Alternatively, prior to the extrusion of the insulating material, a viscous heat-resistant mass compatible with the conductor and the insulating material, for example silicone oil, is deposited on the surface of the conductor.

These measures are based on the recognition that the irregularities of the conductor surface may favour the formation of bubbles during the cross-linking process and moreover impart the advantage that the insulation can be easily removed from the conductor end for assembly purposes, for example for producing a conductor connection or a conductor joint.

Advantageously, for forming the insulating material, a granulate of a polymer or copolymer, preferably an ethylene polymer or copolymer, is provided with a dry, powder-like or granulated cross-linking agent, such as dicumyl peroxide or 1.3this (tert. butyl peroxyisipropyl)-benzol, under the exclusion of air with simultaneous evacuation, one to 5% by weight of a filler substance being added thereto as additional bubble initiation inhibitor, for example powdered chalk, aluminium oxide, or titanium dioxide, and a cross-linking accelerator, such as triallyl cyanurate.

The tests performed have shown that such insulating materials in conjunction with the stated cross-linking means are suitable in an excellent manner not only for performing the preferred method, but also have a good storage ability, are relatively inexpensive and above all can be easily admixed, complicated installations for preparation or introduction being unnecessary.

Advantageously, a liquid cross-linking agent, for example a solution or a mixture of dicumyl peroxide in or with ditertiary butyl peroxide and/or tertiary butyl cumyl peroxide, under certain circumstances in conjunction with a cross-linking accelerator, such as triallyl cyanurate, is added to the insulating material immediately before or during its manipulation. For the economy of the performance of the preferred method, a quantity of at least two % by weight of the insulating material may be added to the latter as cross-linking agent or as crosslinking agent in combination with a crosslinking accelerator to allow the extrusion speed- or the conductor drawing-off speed to be increased. This quantity of crosslinking agent or cross-linking agent accelerator which is excessively large in comparison with the usual cross-linking methods permits, in contrast- to the conventional opinion of the experts, a considerable increase of the production or the draw-off speed without disadvantageous effect on the quality of the product, under the condition that the other production conditions of the preferred method explained above are carefully observed.

According to another aspect of the invention, there is provided an apparatus for performing the method of the invention, including a worm extruder, in the extrusion head of which is provided a rotatable mandrel drivable at a speed which is controllable independently of the rotary speed of the worm, the rotatable mandrel being arranged so that, in consequence of its rotation in the material surrounding it and located in the extrusion head, it can heat the material in a controllable manner by the shearing heat produced thereby and delivered to it, there being further provided a hopper, an inlet to the extrusion head for the conductor to be insulated, and an extrusion head outlet provided with an extrusion nozzle, the inlet to the extruder head being associated with a pre-heating device for pre-heating the conductor, a tubular after-heating device being disposed downstream of the extrusion head outlet and at least one cooling device being disposed downstream of the after-heating device, the length of the pre-heating device being substantially equal to half to twice the length of the exruder, the length of the after-heating device being substantially equal to five to ten times the extruder length, and the total length of the cooling device being substantially equal to two to three times the length of the after-heating device, the after heating device comprising a tubular body provided with peripheral heater elements which are controllable in sections and connected to a heat regulating device which is arranged to be controlled by temperature sensors distributed over the length of the after-heating device.

Such a relatively simple apparatus which is fitted with all devices necessary for a successful performance of the method according to the invention, may be constructed with a comparatively low expenditure in a relatively small space, since it does not comprise a vulcanising or cross-linking tower, nor a cross-linking pressure tube which is usually up to 150 m long and is preferably curved in chain form, nor the steam generators necessary therefor inclusive of accessories, pressure cooling water pumps and other devices usually required. In spite of its compactness and relative simplicity of construction, this apparatus is extremely effective, as will be explained below in detail with reference to the drawings.

The Invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a production installation for performing a preferred method; Figure 2 is an enlarged view partly in section of an extruder inlet with devices associated therewith, a pre-heating device for the conductor having been omitted for reasons of clarity and; Figure 3 and 4 are longitudinal sections of various sections of a cooling device.

As may be seen from Figure 1, a production installation arranged for performing a preferred method comprises, at the beginning of the production path, an unwinder 17 and at the end thereof a winder 18, The unwinder 17, which may be drivable under certain circumstances, is associated with a pushing device 19 which feeds the conductor 1 to be coated through a pre-heating device to a worm extruder 4. The winder 18 is associated with a pulling device 20 feeding the conductor 1 provided with the attached insulating sleeve 2 of cross-linked insulating material, for example a cable core. A straightener 21 for straightening the unwound conductor 1 may be disposed between the pushing device 19 and a conductor pre-heating device 14 which is associated with an extruder inlet 7 and is preferably combined with the extruder 4 to form a transportable unit. The length of the pre-heating device is equal to half to twice the length of the extruder. A suction device 26 as well as a foil inlet device or a wetting agent depositing device 31 layer 3, can be deflected for return guidance to the winder 18 which is arranged adjacent the unwinder 17, with simultaneous passage through the cooling duct 13 as the last cooling phase.

The pre-heating device 14, which has an external heat insulating layer, serving' for pre-heating the conductor as well as the tubular after-heating device 15 may be combined with the worm extruder 4 to form a transportable unit and may preferably be directly connected to the respective inlets and outlets 7 and 8 of the extrusion shearing head 5 of the extruder. For reasons of better accessibility as well as for reasons of the space requirement of additional devices, such as the wetting agent depositing device 31, spacings may alternatively be provided without deleterious effect on the production method between the said devices constituting parts of preferred apparatus, as well as between the after-heating device 15 and the subsequent stages of the cooling device 10. These spacings, which are very useful under certain circumstances, are inessential for the total length of the apparatus which is very short in comparison with the length necessary heretofore of the heating and cooling paths of the production installations usual heretofore for the production of conductors and cables with an insulation of cross-linked polyolefine. Since, in a preferred production installation, depending upon the kind of the selected devices and the parameters provided for the travelling-through speeds and the extruder output powers, the length of the pre-heating device may amount to approximately 1.5 to 7 m, that of the after-heating device 17.5 to 35 m, and that of the total cooling path between 30 and 60 m, for a worm extruder length of from 3.5 to 5.5 m, an advantage of the preferred apparatus resides in the comparatively small space requirement thereof. This is particularly small in the arrangement illustrated in Fig. 1 of the cooling path for parallel return guidance of the covered conductor, cable, or cable core to the winder 18 disposed adjacent the unwinder 17. These guide values, given merely by way of example, of the length may be reduced further when certain operative conditions are observed. However, they may also be exceeded under certain circumstances for certain production requirements.

The conductor pre-heating device 14 is so contructed that it is capable of heating the conductor to a temperature of at least 150 C, and preferably to above 200"C, at all required travelling-through speeds. The after-heating device 15 through which the covered conductor 1 travels immediately after the moulding of the sleeve 2, which is already at the cross-linking temperature and which is partly cross-linked, is provided with heater elements 23, which are controllable in sections, and with an outer heat insulation 22, in order to maintain the temperature in the interior as constant as possible and to prevent it from dropping. For accurately controlling the temperature course in the after-heating device 15, the heater elements 23 are coupled to a heat regulating device which may be arranged in an operation and control desk 25 for the entire apparatus and which is automatically controllable by temperature sensors 24 distributed over the length of the after-heating device 15.

As may be seen particularly clearly from Fig. 2, a further embodiment of the invention comprises a suction device 26 which is arranged at the shearing extrusion head 5 of the extruder 4 in the vicinity of the extrusion head inlet 7 and which is connected preferably by way of a control valve 27, to a vacuum source 28, for example a vacuum vessel and/or a vacuum pump.

The inlet 7 of the extrusion head 5, which is provided with a special projection for the connection of the suction device 26, is effectively sealed from the atmosphere in this case by means of a resilient inlet seal, e.g. in the form of a felt disc 39 which is held against a sealing support 38 by means of a cover nut 40. The suction device 26 which is effective in the immediate vicinity of the conductor surface as well as, under certain circumstances, (e.g. in the case of standard conductors) in the interior of the conductor 1, provides in the extrusion head a vacuum which extends as far as the outlet 8 thereof and beyond it into the interior of the tube moulded there and contributes to or effects the tight abutment thereof on the conductor surface. At the same time, the suction device 26 removes from the plasticised insulating mass heated to the crosslinking temperature a considerable portion of the gases forming therein as by-product of the cross-linking agents added thereto, such as methane, acetophenone and the like, so that bubble formation in the crosslinked insulation can be largely prevented merely by the effect of the suction device 26.

Furthermore, a foil strip inlet or a wetting agent depositing device 31 may be disposed between the conductor heating device 14 and the extruder inlet 7. The device 31 may have a wetting device 32, for example in the form of. an apertured or slotted felt disc, which is fed from a storage container 33 and can be pressed against the conductor surface. Such a wetting agent depositing device 31 is likewise shown diagrammatically in Fig. 2. It serves for compensating unevennesses of the conductor surface and to smoothe them out, which measure contributes to the prevention of the formation of bubbles during the crosslinking process and moreover facilitates the removal of portions of the insulation for assembly purposes.

For obtaining the desired conductor temperature prior to the deposition of the insulating sleeve 2 to be cross-linked, the conductor pre-heating device 14 is fitted with at least three approximately uniformly distributed gas strip burners 29 (Fig. 1) which are connected in series and controllable and which irradiate star-like the central region of the heating device 14. This constructional form of the conductor pre-heating device 14 may'be produced at a low expenditure and, if its length corresponds to the dwell period or travelling-through speed of the conductor 1, attains the necessary heating power to a satisfactory extent. The degree of efficiency and thus the economy of the apparatus may be increased in that the gas strip burners 29 are disposed within a tubular housing 30 the inner surfaces of which are highly polished and / our provided with a mirror-like reflecting layer.

According to another more efficiency intensive and more space saving constructional form of the preferred apparatus, the conductor pre-heating device 14 is provided with an induction heater of medium frequency, wherein the length of the heating device 14 is at most equal to the length of the extruder 4. This constructional form, which may be so designed that its length does not amount to more than 1-1- to 2 m, is preferred for pre-heating conductors of large cross-section and/or for production at an increased travelling-through speed of cables or cable cores.

As mentioned before, very great significance is afforded to the arrangement and the construction of the cooling device 10 in respect of the quality of the insulating sleeves 2 produced as well as in respect of the attainable production speeds and the space required for the entire production installation. Accordingly the cooling device 10 comprises at least two cooling sections which are located under certain circumstances at a spacing from the outlet of the after-heating device 15 and which have provided therein or associated therewith guiding devices 36, e.g. support rollers. The first section 11 of these cooling sections following downstream of the heating device is heatable or adapted to be filled with a heated cooling medium.

The desired cross-linking degree of the insulating material of the conductor sleeve 2 is effected with the lowest possible expenditure in consequence of the cooling in the warmed up cooling medium of the first cooling section 11 of the partly cross-linked 'but not completely cross-linked conductor sleeve, after a passage through the afterheating device 15 corresponding to the dwell period necessary for at least the complete cross-linking of the outermost layer 3 of the insulating sleeve 2, i.e. the hardening of the outer layer 3 of the insulating sleeve 2 occurs in this case with such a small heat loss that upon continuous further conductance of the insulated conductor 1 at a high travelling-through or draw-off speed, thorough cross-linking of the insulating material of the conductor sleeve 2 through the entire crcss-section thereof as far as the conductor surface takes place owing to the residual heat still present in the plastic mass or delivered thereto by the heated conductor 1, without further heat supplied from outside. The previously completely cross-linked and sufficiently hardened outer layer 3 fulfils a double function in this process. On the one hand it provides a sufficiently load resistant rigid abutment surface of the conductor sleeve 2 for the guiding devices, illustrated in Figs. 3 and 4 as guide rollers, 36 which support the sleeve 2 in the partly hardened state. At the same time it also forms an outer skin which is relatively pressure resistant towards the interior and within which the complete cross linking of the insulating material of the sleeve 2 as far as to the conductor surface occurs without the application of any external pressure, but in the interior, under pressure conditions which substantially prevent any bubble formation.

In this case, the selection of the spacing of the first guiding device 36 from the outlet of the after-heating device 15 is of importance in that the outer layer 3 of the insulating sleeve 2 is already sufficiently solidified to exclude deformation thereof.

In order to attain this with even greater certainty, a cooling means receiving device 34, preferably in conjunction with a spraying device 35, is arranged in a preferred constructional form between the outlet of the after-heating device 15 and the first section 11 following the same of the cooling device 10, in which case the first cooling section 11 is constructed as a closed, pre ferably tubular container, under certain circumstances in the form of a pressure tube.

In this case the tubular first section 11 of the cooling device 10 is located at a small spacing from the outlet of the afterheating device 15, such as is illustrated in Fig. 3. Therein the cooling means receiving device 34 serves for receiving cooling means, such as leakage water or any other cooling liquid which may under certain circumstances issue from the inlet opening of the first section 11 of the cooling device 10; and for intercepting cooling means which flow out of the spraying device 35 over the surface 3 of the insulating sleeve 2 which is still soft at this location.

The covered conductor is guided through the cooling means receiving device 34 in a substantially free-floating manner, and the inlet opening of the tubular first section 11 of the cooling device 10 may be provided with a suitable longitudinal seal 37 which under certain circumstances has an excess dimension relative to the crosssection of the insulating sleeve 2, for example an approximately conically preformed, centrally apertured rubber sleeve, which lies against a sufficiently shape-stable sealing support 38, e.g. of metal. For the production of thick-walled insulating sleeves at high speed, the cooling of the previously completely cross-linked outer layer 3 of the insulating sleeve 2 may be effected not only at an increased temperature, but at the same time also at an increased pressure "oy means of cooling water the boiling point of which is appropriately raised to the desired cooling temperature of more than 1000 C. In a similar manner as at the inlet, the tubular first section il of the cooling device 10 may also be sealed at its outlet end by means of a longitudinal seal 37 with a conical support 38, and may also be fitted with a cooling means receiving device 34, as may be seen from Fig. 4.

The subsequent second section 12 of the cooling device 10, as well as the third cooling section 13, is preferably constructed as an open cooling channel and is likewise provided with guiding devices 36, e.g. in the form of support rollers, as well as with longitudinal seals 37 at the ends, which seals may be constructed in the form of split discs of foamed material or felt or the like which are insertable between appropriately recessed support walls. However, the longitudinal sealing of the cooling sections 12 and 13, may be effected in another manner e.g. by means of sealing cords, for example asbestos cords, placed around the insulating sleeve 2, and loaded by weights at their ends.

The lengths of the individual sections 11, 12, and 13 of the cooling device 10 are such that the cross-linking of the insulating material of the insulating sleeve 2, which had been initiated already in the extruder 4 and which continued during the passage of the insulated conductor 1 through the after-heating device 15, is largely finished when the sleeve 2 issues from the cooling means acting on the latter at an increased temperature of the tubular first section 11 of the cooling device 10, so that the final solidification and cooling of the insulating sleeve down to room temperature may be effected in the subsequent cooling sections 12 and 13. In this case a division of the cooling device into a warm precooling track 11 of appropriately 15 m length and an after-cooling track of altogether, for example, 50 m length is of great importance for the successful performance of the preferred method, whereas the subdivision of the after-cooling track serves above all for the space saving arrangement of the production installation. However, it must be noted in this case that, considered in the movement direction of the conductor insulation 2 to be cooled, the leading section 12 of the after-cooling track 12 and 13 must comprise a sufficient length, for example 15 m, in front of the guide rollers 16, in order that the deflection of the insulated conductor can be effected without damage to its just-hardening sleeve 2 and the latter can'be cooled to room temperature for complete hardening in the channellike or trough-like last cooling section 13 which is sealed at each of the ends, for example, by means of a slit insert of foamed material 37. The length of the latter need not amount to more than approximately 20 m, the conductor 1, which is provided with the insulating sleeve 2 now finally crosslinked, hardened and cooled and which floats freely as far as the pulling device 20, being supplied to the winder 18 and deposited thereon.

For performing the preferred method, a granulate of a suitable insulating material, in particular a polymer, copolymer or propolymer, preferably an ethylene polymer or copolymer, together with a suitable crosslinking agent, is fed into the filler hopper 6 of the extruder 4. This cross-linking agent may be added in a liquir, powdered or likewise granulated form separately or may even be a component part of the appropriately prepared granulate of insulating material. In view of the short dwell period aimed at and the corresponding high production speed, a relatively large quantity of the cross-linking agent is essential.

The proportion thereof in the total quantity of the granulate supplied may amount to 4 % by weight, under certain circumstances in conjunction with cross-linking accelerators. At the same time the conductor 1 to be covered is supplied in the pre-heated and preferably surface-treated state to the inlet 7 of the extrusion shearing head 5 of the extruder 4 at a travelling-through speed which corresponds to the conveyance power of the extruder. The temperature of the conductor 1 supplied is arranged to be always higher than the reaction temperature of the cross-linking agent and thus amounts to more than 150"C. As the production speed increases, higher conductor temperatures up to more than 200"C are advisable, for example at a production speed of 30 m/min and an output power of the extruder of 200 kg/hour a conductor temperature between 160 and 1700C.

In the extruder specially adapted for this purpose, the granulate inserted is well worked, plasticised, homogenised and conveyed towards the outlet 8 of the extruder extrusion head 5 at a considerably rising pressure. In the vicinity thereof, the now plastic mass is brought to the respective optimum cross-linking temperature by the shearing heat produced in this mass by friction by means of the separately driven shearing mandrel. Although it is possible without difficulty to heat the mass in this way up to far beyond 200"C, the mass temperature is adjusted, for example to 180 C, in such a manner that the crosslinking of the insulating material starts already in the extruder, the importance determining quantities, such as conductor cross-section, draw-off speed, insulator thickness, length and temperature of the after-heating track 15 and the first warm cooling section 11, and other factors being taken into account. This imports the advantage of maximum utilisation of the heat energy and moreover ensures, in conjunction with the subsequent measures, the freedom from bubbles of the extruded product, and thus a very high quality of the insulating sleeve 2 thus produced. For this purpose, however, the facility provided by the extruder used therefor is required to control the mass temperature independently of the feed quantity and the feed pressure by means of controllable shearing heat.

As mentioned initially, the extrusion per se may be effected in any desired manner.

For reasons of increased economy, extrusion by means of a tube tool, in particular for covering sector conductors or the like, is prefcrred. The conductor 1 provided with the extruded sleeve 2 then travels through the after-heating device 15, taking into account the length thereof, at a speed which is adjusted to the dwell period necessary for completely cross-linking the outer edge layer 3 of the insulating sleeve 2 and which is decisive for the draw-off speed of the conductor. The dwell period depends upon the cross-linking speed of the cross-linking agent used, the quantity of cross-linking agent and the temperature in the interior of the after-heating device 15 which is usually made equal to the mass temperature in the extruder extrusion head 5 e.g.

to 190"C. In this case, the intensity of the cross-linking process progresses initially from the outside to the inside of the extruded sleeve, so that the outer layer 3 of the insulating sleeve is the first to attain a high cross-linking degree of a least 75%.

The cross-sectional dimensions of the after-heating device 15 are so selected that the conductor 1 provided with the sleeve 2 can be unsupported without damage until it rests on the next-following guiding device 36 of the cooling device 10. From the after-heating device 15, which is completely open at its end, the conductor 1, which is provided with the sleeve 2 now already partly cross-linked, is pulled continuously through the sections 11, 12 and 13 of the cooling device 10 which comprise at least two, preferably three different temperatures, at the draw-off speed resulting from the considerations stated above. It is guided therein by the guiding devices 36 provided therein, for example support rollers, and is maintained thereby at the desired height.

The temperature and length of the first cooling section 11 is of importance for the cross-linking process. Taking into account all remaining conditions, its length is so adjusted to that of the other parts of the apparatus that the already partly crosslinked insulating sleeve 2 is cross-linked in its totality during its passage through the cooling means in the section 11 which is maintained therein at a temperature of nearly or more than 100"C, and, after reversal of the direction of progress of the cross-linking i.e. from the conductor towards the outside of the sleeve, is now completely cross-linked up to a cross-linking degree of at least 75%.

As mentioned before, the completion of cross-linking occurs within the portion of the sleeve enclosed by the outer layer 3, which is already sufficiently hardened and pressure resistant and which, without the application of external pressure, substantially eliminates possible tendencies to form bubbles due to gases delivered thereby by the cross-linking agent or forming during the cross-linking process, and ensures a very good quality of the product. Finally the conductor 1 now provided with a finally cross-linked and externally hardened sleeve 2 travels through the successively disposed trough or channel-like after-cooling sections 12 and 13 which are filled with a cooling means, usually simple mains water maintained at room temperature or slightly above same, whereby the cross-linking process is terminated, the insulator sleeve is cooled to room temperature and is solidified throughout, before it is engaged by the pulling device and deposited on the winder.

WHAT WE CLAIM IS:- 1. A method of producing an insulating covering of cross-linked insulating material by extrusion around an electrical conductor, comprising the steps of providing the insulating material with a heat-activatable cross-linking agent, mechanically working the insulating material to raise its temperature and pressure and to homogenise and plasticise it, conveying the insulating material to a die head under extrusion pressure,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In the extruder specially adapted for this purpose, the granulate inserted is well worked, plasticised, homogenised and conveyed towards the outlet 8 of the extruder extrusion head 5 at a considerably rising pressure. In the vicinity thereof, the now plastic mass is brought to the respective optimum cross-linking temperature by the shearing heat produced in this mass by friction by means of the separately driven shearing mandrel. Although it is possible without difficulty to heat the mass in this way up to far beyond 200"C, the mass temperature is adjusted, for example to 180 C, in such a manner that the crosslinking of the insulating material starts already in the extruder, the importance determining quantities, such as conductor cross-section, draw-off speed, insulator thickness, length and temperature of the after-heating track 15 and the first warm cooling section 11, and other factors being taken into account. This imports the advantage of maximum utilisation of the heat energy and moreover ensures, in conjunction with the subsequent measures, the freedom from bubbles of the extruded product, and thus a very high quality of the insulating sleeve 2 thus produced. For this purpose, however, the facility provided by the extruder used therefor is required to control the mass temperature independently of the feed quantity and the feed pressure by means of controllable shearing heat. As mentioned initially, the extrusion per se may be effected in any desired manner. For reasons of increased economy, extrusion by means of a tube tool, in particular for covering sector conductors or the like, is prefcrred. The conductor 1 provided with the extruded sleeve 2 then travels through the after-heating device 15, taking into account the length thereof, at a speed which is adjusted to the dwell period necessary for completely cross-linking the outer edge layer 3 of the insulating sleeve 2 and which is decisive for the draw-off speed of the conductor. The dwell period depends upon the cross-linking speed of the cross-linking agent used, the quantity of cross-linking agent and the temperature in the interior of the after-heating device 15 which is usually made equal to the mass temperature in the extruder extrusion head 5 e.g. to 190"C. In this case, the intensity of the cross-linking process progresses initially from the outside to the inside of the extruded sleeve, so that the outer layer 3 of the insulating sleeve is the first to attain a high cross-linking degree of a least 75%. The cross-sectional dimensions of the after-heating device 15 are so selected that the conductor 1 provided with the sleeve 2 can be unsupported without damage until it rests on the next-following guiding device 36 of the cooling device 10. From the after-heating device 15, which is completely open at its end, the conductor 1, which is provided with the sleeve 2 now already partly cross-linked, is pulled continuously through the sections 11, 12 and 13 of the cooling device 10 which comprise at least two, preferably three different temperatures, at the draw-off speed resulting from the considerations stated above. It is guided therein by the guiding devices 36 provided therein, for example support rollers, and is maintained thereby at the desired height. The temperature and length of the first cooling section 11 is of importance for the cross-linking process. Taking into account all remaining conditions, its length is so adjusted to that of the other parts of the apparatus that the already partly crosslinked insulating sleeve 2 is cross-linked in its totality during its passage through the cooling means in the section 11 which is maintained therein at a temperature of nearly or more than 100"C, and, after reversal of the direction of progress of the cross-linking i.e. from the conductor towards the outside of the sleeve, is now completely cross-linked up to a cross-linking degree of at least 75%. As mentioned before, the completion of cross-linking occurs within the portion of the sleeve enclosed by the outer layer 3, which is already sufficiently hardened and pressure resistant and which, without the application of external pressure, substantially eliminates possible tendencies to form bubbles due to gases delivered thereby by the cross-linking agent or forming during the cross-linking process, and ensures a very good quality of the product. Finally the conductor 1 now provided with a finally cross-linked and externally hardened sleeve 2 travels through the successively disposed trough or channel-like after-cooling sections 12 and 13 which are filled with a cooling means, usually simple mains water maintained at room temperature or slightly above same, whereby the cross-linking process is terminated, the insulator sleeve is cooled to room temperature and is solidified throughout, before it is engaged by the pulling device and deposited on the winder. WHAT WE CLAIM IS:-

1. A method of producing an insulating covering of cross-linked insulating material by extrusion around an electrical conductor, comprising the steps of providing the insulating material with a heat-activatable cross-linking agent, mechanically working the insulating material to raise its temperature and pressure and to homogenise and plasticise it, conveying the insulating material to a die head under extrusion pressure,

heating the insulating material immediately prior to extrusion from the die head to above its cross-linking temperature without changing the extrusion pressure by means of rotational shearing heat produced by mechanical friction, supplying the conductor in a pre-heated and surface treated state wherein the conductor is heated to above the reaction temperature of the cross-linking agent and the conductor surface is treated with means for inhibiting the formation of bubble nuclei, extruding the insulating material at the said temperature above the cross-linking temperature in the form of a sleeve around the conductor with simultaneous evacuation of at least a region between the sleeve and the conductor surface, maintaining the insulating material for a short period of time substantially at its crosslinking temperature without external application of pressure to cause cross-linking at constant temperature of an outermost zone or outer layer of the conductor sleeve formed thereby, and cooling the insulating material externally while allowing crosslinking of the conductor sleeve throughout the entire cross-section thereof to be completed by the heat previously supplied to the insulating material.

2. A method as claimed in claim 1, in which, taking into account its cross-sectional area, the production speed and drawoff speed and the insulating material and cross-linking agent used, the conductor to be insulated is pre-heated to a temperature between 150 and 200"C, the temperature of the conductor is measured immediately prior to the extrusion covering thereof and the pre-heating of the conductor is controlled dependently upon the measured value, the plasticised insulating material being heated prior to the extrusion to a temperature between 160 and 220 C', the extruded insulating sleeve which is provided with up to four % by weight of the cross-linking agent being maintained without the application of external pressure at the same temperature only until a lowercross-linking degree than that of the already cross-linked outer layer of the insulating sleeve, measured in accordance with the so-called "solvent extraction test", is effected, the insulating sleeve finally being cooled in at least two, preferably three, cooling stages starting with at most 120on, to be cross-linked throughout thereby and finally cross-linked up to at least 75%.

3.- A method as claimed in claim 1 or 2, in which the extruded insulating sleeve is maintained at approximately the same temperature as before the extrusion of the insulating material by external heat radiation, is subsequently pre-cooled by a liquid having a high boiling point to a temperature below the softening point of the insu lating material, is thereby finally cross linked owing to the residual heat present in the insulating material, or delivered by the pre-heated conductor, within the cross linked outer layer, which is sufficiently rigid to maintain the pressure forming in the interior of the outer layer as a result of cross-linking, and is finally cooled in at least two further different temperature stages by water.

4. A method as claimed in any one of claims 1 to 3, in which, to provide the sur face treatment of the conductor prior to the extrusion of the insulating material, heat resistant thin foil which is compatible with the conductor and the insulating mat erial is wrapped around the conductor to compensate for the irregularities of the sur face thereof.

5. A method as claimed in any one of claims 1 to 3, in which, to provide the sur face treatment of the conductor prior to the extrusion of the insulating material, a viscous heat resistant mass compatible with the conductor and the insulating material is desposited on the surface of the conduc tor.

6. A method as claimed in any one of claims 1 to 5, in which for forming the insulating material, a granulate of an ethylene polymer or copolymer is provided with a dry, powder-like or granulated cross linking agent comprising dicumyl peroxide or -a 1.3-bis '(tern. butyl peroxyisipropyl) benzol, under the exclusion of air with sim ultaneous evacuation, one to five % by weight of a filler substance being added thereto as additional bubble initiation inhi bitor, the filler comprising powdered chalk, aluminium oxide, or titanium dioxide, and a cross-linking accelerator comprising triallyl cyanurate.

7. A method as claimed in any one of claims 1 to 5, in which a liquid cross-link ing agent comprises a solution or a mix ture of dicumyl peroxide in or with diter tiary butyl peroxide and/or tertiary butyl cumyl peroxide, in conjunction with a cross linking accelerator comprising triallyl cyanurate is added to the insulating material immediately before or during its working.

8. A method as claimed in any one of claims 1 to 7, in which a quantity of at least two % of the weight of the insulat ing material is added to the latter as cross linking agent or as cross-linking agent in 'combination with a cross-linking accelera tor to allow the extrusion speed or the con ductor drawing-off speed to be increased.

9. An apparatus for performing the method of claim 1, including a worm ex truder, in the extrusion head of which is provided a rotatable mandrel drivable at a speed which is controllable independently of the rotary speed of the worm, the rotatable mandrel being arranged so that, in consequence of its rotation in the material surrounding it and located in the extrusion head, it can heat the material in a controllable manner by the shearing heat produced thereby and delivered to it, there being further provided a hopper, an inlet to the extrusion head for the conductor to be insulated, and an extrusion head outlet provided with an extrusion nozzle, the inlet to the extruder being associated with a pre-heating device for pre-heating the conductor, a tubular after-heating device being disposed downstream of the extrusion head outlet and at least one cooling device being disposed downstream of the after-heating device, the length of the preheating device being substantially equal to half to twice the length of the extruder, the length of the after-heating device being substantially equal to five to ten times the extruder length, and the total length of the cooling device being substantially equal to two to three times the length of the afterheating device, the after-heating device comprising a tubular body provided with peripheral heater elements which are controllable in sections and connected to a heat regulating device which is arranged to be controlled by temperature sensors distributed over the length oft he after-heating device.

10. An apparatus as claimed in claim 9, in which a suction device is disposed at the shearing extrusion head of the extruder in the vicinity of the extrusion head inlet and is connected by way of a control valve to a vacuum source in the form of a vacuum container and/or a vacuum pump.

11. An apparatus as claimed in claim 9 or 10, in which the conductor pre-heating device is fitted with at least three substantially uniformly distributed gas strip burners which are connected in series and are controllable and which irradiate the central region of the heating device in a star-like manner.

12. An apparatus as claimed in claim 11, in which the gas strip burners are disposed within a tubular housing, the inner surface of which is highly polished and/or coated with a mirror-like reflecting layer and which is provided on the outside with a heat-insulating layer.

13. An apparatus as claimed in claim 9 or 10, in which the conductor pre-heating device is provided with an induction heater of medium frequency, wherein the length of the heating device is substantially equal to the length of the extruder.

14. An apparatus as claimed in any one of claims 9 to 13, in which a foil strip inlet or a wetting agent depositing device with a wetting device, which can be pressed against the conductor surface and fed from a storage container and which is in the form of an apertured or slit felt disc, is arranged between the conductor pre-heating device and the extruder inlet.

15. An apparatus as claimed in any one of claims 9 to 13, in which the cooling device comprises at least two cooling sections spaced from the outlet of the after-heating device and having guiding devices in the form of support rollers provided thereon or associated therewith, the first section of the cooling sections following the afterheating device being heatable or being arranged to be filled with a heated cooling medium.

16. An apparatus as claimed in claim 15, in which a cooling means intercepting device in conjunction with a spraying device is disposed between the outlet of the after-heating device and the said first section of the cooling device the first cooling section being in the form of a closed tubular container comprising a pressure tube.

17. A method of producing an insulating covering of cross-linked insulating material by extrusion around an electrical conductor, substantially as hereinbefore described with reference to the accompanying drawings.

18. An apparatus for producing an insulating covering of cross-linked insulating material by extrusion around an electrical conductor, substantially as hereinbefore described with reference to the accompanying drawings.

19. An electrical conductor provided with an insulating sleeve by a method as claimed in any one of claims 1 to 8 and 17 or by an apparatus as claimed in any one of claims 9 to 16 and 18.