A HEATING ELEMENT, A HEATABLE BODY AND A METHOD FOR MAKING SUCH A HEATABLE BODY

Abstract: This invention relates to a heating element (13) for a heatable body (1), to a method of making a heatable body (1), having a plurality of elongate cavities (7) with openings (7a, 7b) accommodating such heating elements (13), and to a heatable body (1) comprising heating elements (13). The heating element (13) comprises an elongate rod-shaped carrier (18, 23) which is at least partially surrounded by an electrically insulative material (20) having embedded therein resistance heating means (19); the method comprises coating a respective heating element (13) with a curable liquid adhesive material (30) at the opening (7a) of a respective elongate cavity (7), inserting it into said cavity (7) after coating with said liquid adhesive material (30) and curing said adhesive material (30), in order to provide a body (1) wherein the electrically insulative material (20) of each respective heating element (13) is fixed with its outer surface to the inner wall of each cavity (7) by the adhesive material (30).

Description: A Heating Element, a Heatable Body and a Method for Making such a Heatable Body This invention relates to a heating element for a body having a temperature controllable outer surface. Further, the present invention relates to a heatable body having a temperature controllable surface and comprising a plurality of open-end elongate cavities, each of said cavities being provided with at least one electrical heating element. The invention also relates to a method of making a body having a temperature controllable outer surface, the body having a plurality of elongate cavities with openings at either end to receive heating elements . The invention relates particularly, though not exclusively, to rolls or bowls for calendering sheet materials such as for example paper, steel, plastics, board, cloth, foodstuffs, etc.. Rolls having a temperature controllable surface are extensively used in paper making, and are often called Thermo calender rolls. Such rolls are usually cylindrical, having a typical axial length of less than 0.5 meters to about 15 meters. A typical 12 meter calender roll weighs over 80 tons, and is formed by a casting process in which the cooling rate is controlled such that a very hard, smooth martensite surface forms on a nodular iron core. Much research and development has gone into the manufacturing process to provide the current state of the art. The surface of a typical roll will be ground to a mirror-like finish, having a surface roughness of less than 0.1 micrometers . The performance of such rolls tends to be limited by their large mass. In use, a typical paper calendering process will require a roll surface temperature of between room temperature and 220°C, a pressure of 10 bar against the paper (although higher pressures are used for some applications), and a surface speed of 30 m/s. The nodular iron core has a larger coefficient of thermal expansion than that of the hard martensite surface layer, and thus in use this surface layer is under considerable tensile stress. This surface layer will crack if a sufficiently large temperature gradient is formed. Temperature variations along the axis of the roll, combined with pressure variations along the axis of the roll, can result in variations in paper web thickness known as "soft spots" along the axis of the calendered, wound paper roll. When this calendered wound-up paper roll is subsequently unwound the "soft spots" can cause tearing. There are currently several known ways to heat such rolls . The most common way to control the surface temperature of the roll is to use a hot fluid such as oil or steam which is fed into peripheral channels provided in the roll . A somewhat modified system is disclosed in EP 471 655 A, where a fluid, in particular water, is used which is heated directly within channels in the heatable roll by electrical resistors arranged in the channels. Rolls can have as many as 56 channels formed, usually by drilling cylindrical bore holes through the roll along the axis. These holes must be drilled fairly close to the surface. The drilling of holes as deep as 6 meters (starting from either end in the case of a 12 meter bore) into such steel is a difficult process, and the holes drilled from the ends of the roll will not necessarily always meet exactly in the middle, sometimes causing a stepped bore profile. Disadvantages with heating fluid systems include the fact that considerable maintenance is required, and expensive valves must be used. There is the problem of corrosion of the heated body if hot water or steam is used, and the body must be constructed as a pressure vessel. There are safety difficulties if hot oil is used, since oil is hazardous environmentally since if/when it leaks from the valves contaminates factory. There can also be temperature variations of up to 40 °C along the axis of the roll, and up to 8°C along the length of the calender nip, resulting in variations in roll diameter along axis of roll, which in turn causes variations in calendered paper thickness as mentioned before. Systems using a hot fluid lack good temperature control, and typically require air jets to cool larger surface areas not in contact with the product in order to minimise damage to plastic coated non-heated bodies facing the heated body in use. In addition this non-heated body may also require air jet cooling to reduce the temperature differential across the axial length of the surface cover of the non-heated body in use. Said lack of good temperature control also causes the roll's journals to operate at too high a temperature, which in turn, even with bearing cooling through increased circulatory lubrication, reduces the operating life of the journal bearings. Another system is described in DE 296 15 219 Ul . In this system, a heating mat made of silicone and heating wires is rolled up into the shape of an expandable pipe, and inserted into the bore holes. The mat is fixed in place either by using a layer of adhesive on the outer surface of the rolled up heating mat, and/or by vulcanising. During the attachment process an inflatable inner tube is inserted into the expandable pipe, and this tube is inflated to provide good contact between the pipe and the bore surface whilst the adhesive material is curing or the silicone is vulcanised. However, manufacture of this known system is difficult, as insertion of the adhesive coated pipe causes the adhesive to be stripped from the end first inserted, so that thermal contact between the body of the roll and the mat may be patchy; this can also result in hot spots which can lead to the wire burning out the mat. Also, vulcanising the mat can cause the pipe to lose resilience after attachment, resulting in cracking in use; there are also degassing and ventilation difficulties. In operation, variations in the operating temperature of the heated roll body cause the pressure in the inflatable inner tube to vary, so creating oscillatory pressure variations on the heating mat therefore changing 1 ) the internal pressure and so the separation between the heating mat and the inside surface of the peripheral bore which has the effect of changing the effective thermal resistance and effectiveness of heat transfer, 2) the loading on critical parts of the heater such as junctions connecting the heating wire to the electrical supply cable, even where both are made of the same material, 3) where the blown-up tube presses on the electrical cables, such pressure can damage the heating mat. Further, at temperatures above 180°C - 220°C all silicone and rubber compounds are in a state of slow degradation, particularly if also exposed to excess pressure. The general consequence of such a transient change in chemical properties of the silicone is a slow hardening of the silicone material. A by-product of this process is the slow release of gases which in the case of the inflatable inner tube are prevented from escaping which may result in damage, most severely a small explosion (this depends on the silicone used, the gases liberated, and the degree to which ventilation is impared by the inflatable inner tube e.g. length of roll bore). Another drawback with this system is that the resistive heating wires produce electromagnetic fields which can induce electrical currents in adjacent equipment, resulting in malfunction or early failure, or generation of unwanted radio frequency electromagnetic waves . According to a first aspect of the invention, it is an object of the invention to provide a heating element for a body having a temperature controllable outer surface, which is easy to insert into channels of the body, and which will not undergo any unpredictable degradations when the heating element gets hot. According to a second aspect of the invention, it is an object of the invention to provide a heatable body which is easy to construct and has an outer surface which may be reliably controlled as to its temperature. Further, regulation of the temperature control shall be easy, and maintenance costs shall be minimized. According to a third aspect of the invention, it is an object of the invention to provide an efficient method for making such a body, said method allowing to provide for extremely thin gaps between the heating elements and the cavity walls, which are reliably filled with heat-conducting material. The heating element according to this invention comprises an elongate rod-shaped carrier which is at least partially surrounded by an electrically insulative material having embedded therein resistance heating means. By providing the electrically insulative material on a stiff rod-shaped carrier, the heating element as a whole becomes rigid, and will stay in its shape even when it gets hot. Further the heating element will be easy to handle, and to insert in a cavity of a heatable body due to its rigidity. Advantageously, the carrier is surrounded by the electrically insulative material by an angle of substantially 315°, in order to facilitate the insertion of the heating element by providing a passage for an adhesive material used for fixing the heating element in the cavity of the heatable body. Further the material-free gap (360° - 315°) provides a short passage for release of excess adhesive material and acetoxy vapors released during cure of the adhesive layer. If the resistance heating means comprises a heating foil, which is either pressed or etched from film or sheet material, the resistance heating means is advantegously relativly flat. The heater foil may be chosen dependent on the necessary voltage, current and resistance and will therefore be made from an electrically conductive material, like cooper, steel, nickel, nickel chromium, aluminium, etc.. In some special arrangements , it may be of advantage if the resistance heating means comprises a heating wire. For ease of heating element manufacture, it is advantageous if the electrically insulative material comprising said resistance heating means forms at least one heating mat attached to the rod-shaped carrier. The heating mat may be prefabricated, and may be attached to the rod-shaped carrier e.g. by a silicone adhesive. To obtain a continuously adhering mat, and a smooth surface thereof, uniform radial pressure should be exerted during attaching the heating mat to the carrier rod, e.g. by winding a foil around the mat and/or using a die to apply a uniform, radial, compressive force. Preferably the rod-shaped carrier defines at least one inner passage for accommodating connection leads for the resistance heating means. This provides a convenient and secure connection of the resistance heating means with a power supply, where the connection leads are protected by the carrier's outer body. In a particularly preferred embodiment, the rod-shaped carrier is simply a tube. In order to prevent a blockage during the insertion pf the heating element, it is advantageous if the rod-shaped carrier comprises a plurality of openings provided in an electrically insulative material-free area and extending from the outer surface of the rod-shaped carrier to the inner passage. Therefore the adhesive material, which is used for fixing the heating element in a cavity, can escape to the inside of the rod-shaped carrier and a blockage of the heating element during insertion is avoided. Acetoxy vapors released during cure of the adhesive layer do not react chemically with steels, but can permeate through the relatively limited volume of the heating element and the adhesive material comprising the chemical and/or mechanical bond between the rod-shaped carrier and the cavity surface respectively, and the excess adhesive material filling the gap in the heating element on the rod-shaped carrier and penetrating the openings in the rod-shaped carrier to protrude slightly into (but not filling) the rod-shaped carrier, compared with the axial length of the heating element and/or the axial length of chemical and or mechanical bond between said heating element and cavity surface (after insertion of the heating element) . In order to facilitate the insertion of the heating element into a cavity of a heatable body, it is advantageous if a deflector is provided which extends from one front face of the rod-shaped carrier to the free space between the longitudinally extending edges of the electrically insulative material. If the rod-shaped carrier tube comprises a removable plug or bung at one open end, the heating element can be advantageously inserted into a cavity of a heatable body without clogging the inner passage of the carrier tube. When the plug or bung is removed after insertion of the heating element, air can pass through the inner passage when a second heating element is inserted into the respective cavity from the opposite side thereof. For a high form-stability, the rod-shaped carrier preferably is made of metallic material, for example copper or steel. Such material is suitable since it is both electrically and thermally conductive. Steel is particularly suitable since its thermal expansivity is very similar to silopren and other silicone materials. Further the thermal conductivity of a metallic carrier helps to reduce the operating temperature of the underside of the heater mats in operation, by conducting the heat away from the underside of the heater mats. Since the heating element is intended to be provided in a body which is rotating in operation, it is advantageous if the resistance heating means is arranged within said electrically insulative material so as to minimize external electromagnetic fields in use, which is particularly relevant for use with modern direct current, fuel cell electricity generation units. Preferably, for avoiding the creation of external electromagnetic fields, the resistance heating means is arranged such, that in use, electric current in adjacent conductors flows in opposite directions. By this arrangement the electromagnetic fields of adjacent conductors will interfere, and therefore there will be substantially no external electromagnetic fields. If the heating element comprises a plurality of separate heating mats to define different heating zones along said rod-shaped carrier, the heating element can advantageously be used to provide different temperatures (e.g. different heat fluxes) in different zones. To achieve different temperatures in different zones independently of each other, the electric current in a respective heating mat can be controlled independently of those in other heating mats . For easy manufacturing of the heating element and reliable electrical insulation, it is advantageous if the electrically insulative material comprises silicone. For even lower manufacturing costs it is also possible to use an electrically insulative material comprising polyimide. For the use of the heating mat at very high temperatures e.g. up to 500°C, it is advantageous if the electrically insulative material comprises compressed glass, ceramic, metal oxide, and/or PAN (Polyacrylonitrile) fibres; or mica material. For the highest operating temperatures the heater mat may be sintered under pressure so eliminating the need for chemical and mechanical binder additives like resins, silicones and PAN fibre additives. Such sintering under pressure causes the component materials to bond chemicaly at elevated temperatures, typically 700°C - 900°C. For a reliable attachment of the heating mat to the carrier, it is advantageous if the at least one heating mat is attached to the carrier by an adhesive material. However, the connection patch of the heater mat, since it protrudes through the carrier tube does act as an anchor within the heating element to prevent movement, particularly during attachment to the carrier tube. In order to control the temperature of the heating element, and of the body having this heating element, the, or at least one, respectively, heating mat may comprise at least one temperature sensor. However, not all heating elements must comprise a temperature sensor, since it may be sufficient to control the temperature in one or two cavities in the heatable body in which the heating elements will be inserted. For receiving a representative value of the temperature, it is advantageous if the, or at least one, respectively, heating mat comprises at least one temperature sensor in a middle region of the heating mat. This middle mat region is next to the body surface when the heating element has been inserted. If the, or at least one, respectively, heating mat comprises at least one temperature sensor in an edge region, this at least one temperature sensor can be used to watch and control the temperature of the body or the temperature of the foil at some distance from the body surface. Where measuring foil temperature it is possible to detect the onset of overheat and to prevent overheat occurance. If the, or at least one, respectively, temperature sensor comprises coated junctions, for example dipped in Teflon, the junctions of the temperature sensors are thermally conductive but electrically insulative, and capable of 400°C. Therefore the temperature sensors can be advantageously placed in direct contact with the resistance heating means in the heating mats, thus the temperature of the heating mats can be measured almost in "real time". According to the invention, at the heatable body as initially defined, the electrically insulative material of each respective heating element is fixed with its outer surface to the surface of a respective cavity by an adhesive material. Thereby a reliable connection between the heating element and the heatable body is achieved, so that the heating element is maintained in a fixed position in the heatable body even when the heating element and the heatable body get hot . If the adhesive material is an electrically insulative material, there is advantageously no risk that parts of the heating element could be short-circuited by the heatable body. Preferably the adhesive material comprises silicone, in order to achieve a particularly reliable attachment of the heating element to the heatable body. For regulating the temperature of the outer surface of the heatable body, it is advantageous if each heating element is provided with at least one temperature sensor which is in a position next to the outer surface of the body. Since the cavities of the heatable body are manufactured by drilling, it is easier do produce them only by drilling slightly more than half of the cavity from one side and slightly less than half from the other side of the body. Thus, any step (crack propagation sites) located at the mid point along the axial length of the roll is avoided which is important since this is the point of maximum bending stress. A slight step between the two drills may occur, and therefore it is advantageous if in each cavity two at least substantially coaxial heating elements are provided which are inserted from each open end of each cavity. However, two or more heating elements may nevertheless be inserted in a single cavity even if there is no step, depending on the axial length of the heatable body. For an easy construction of the cavities by drilling and a reliable attachment of the heating elements to the walls of the cavities, it is of advantage if the cavities and heating elements both are substantially cylindrical. If the heating element is provided in a position in said cavity in which the portion without said electrically insulative material is facing away from the body's outer surface, the stresses are advantageously reduced in the body's surface annular layer, if the body has an annular form, by heating the surface annular layer of the body (of inherently lower thermal expansivity) more than the body's core annulus (which is of higher thermal expansivity). In order to keep the heating element fixed in a cavity of the heatable body if the body is deflected during the production process, it is advantageous if the adhesive material has at least some resilience after curing. Preferably the body is a calender roll, as is used in the paper making industry. The method of making a body according to the invention comprises the steps: a) coating liquid curable adhesive material to the outer surface of a respective heating element with a thickness sufficient to fill the annular gap between the heating element and the inner surface of the cavity; b) inserting a respective heating element into said cavity, such that the heating element extends along the said cavity, and the gap between the heating element and the inner wall of the cavity is substantially filled by said liquid adhesive material; and c) curing said adhesive material to fix the heating element in place. By these steps an efficient method for mounting the heating elements of this invention in the body is provided which overcomes the difficulties of the insertion of heating elements and the attachment of the flexible heating mat elements by inflating in the body, as outlined above in connection with the prior art. For a reliable fixation of the heating element in the cavity it is advantageous if the diameter of the heating element before coating with liquid adhesive material is less than 2 mm smaller than the cavity's diameter into which it is to be inserted. In a somewhat modified form, there is provided a method as initially defined, with the steps: a) coating the inner wall of said cavities with a layer of liquid adhesive material with a thickness sufficient to fill the annular gap between the heating element and the inner surface of the cavity; b) providing a layer of a friction reducing material on the outer surface of a heating element; c) inserting said heating element into said cavity, such that the heating element extends along the said cavity, and the gap between the heating element and the inner wall of the cavity is substantially filled by said resilient adhesive material; and d) curing said adhesive material to fix the heating element in place. By this method, the insertion of the heating elements in the cavities is further facilitated by the use of the friction reducing material on the outer surface of the heating elements, which may be applied by a spray system. To assure that excess adhesive material is provided for fixing/bonding the heating element in the cavity, it is of advantage if all the methods comprise feeding adhesive material into said cavity to form a slug at the insertion end thereof, prior to insertion of the heating element. For a reliable attachment of the heating element in the cavity it is advantageous if the inner wall of said cavities is coated with a layer of an adhesive material prior to insertion of the heating element. In order to avoid the risk that some parts of the heating element are not adhered to the inner wall of the cavity by the adhesive material, it is advantageous if the layer of adhesive material provided is thicker than the final gap between the heating element and the inner wall of the cavity, the excess adhesive material being removed from the cavity prior to or by insertion of the heating element. If the adhesive material is partially cured to provide dimensional stability prior to the excess adhesive material being removed, advantageously the risk of creating areas where no adhesive material is provided is avoided. If the heating element is coated with adhesive material by guiding it through a reservoir of a liquid adhesive material which is provided at the opening of respective elongate cavity, the heating element can easily be fed through the reservoir and a layer of adhesive material is formed on the heating element. In order to avoid high excess amounts of adhesive layer, it is of advantage if the heating element is coated with adhesive material by spraying adhesive material on the heating element at the opening of the respective elongate cavity, by which a layer of adhesive material is formed quantitatively in relatively precise amounts. Also in order to form a layer of adhesive material without using much excess material, it is of advantage if the heating element is coated with adhesive material by passing the heating element along a loaded brush (or vice versa) which is provided at the opening of the respective elongate cavity. If the adhesive material comprises a silicone rubber, the heating element can be attached to the cavity easily and will be resiliently fixed in the cavity. Tests have shown that it is advantageous if the friction reducing agent is glycerine, which can be advantageously applied to the heating element by a spray system. If each heating element is supported along its length by a metal sleeve whilst being inserted into the cavity, one end of the metal sleeve being located adjacent one end of the said cavity, the sleeve and cavity being located along a common axis during insertion of the heating element, the insertion of the heating element in the cavity is facilitated in an advantageous manner. In order to accept cavities in the body which are drilled in two parts from opposite ends, and to avoid very long heating elements which might be difficult to produce and especially difficult to insert in a cavity, it is advantageous if a respective heating element is inserted at each open end of each cavity. It also possible to insert a plurality of heating elements at each open end of each cavity in order to keep the length of the rod-shaped carriers relatively short, wherein for example only one heating mat may be attached to each rod-shaped carrier. Preferably the excess adhesive material is removed by passing a scraper having a cross section similar to that of the heating element through the cavity, in order to create a passage for the heating element which is coated with adhesive material for attaching the heating element to the body. For an easy insertion of the scraper in the cavity and for creating a passage which ensures reliable adhering of the heating element to the cavity inner wall, it is advantageous if the scraper takes the form of a rigid element having a tip which tapers in the direction of insertion along the cavity. In order to provide a particularly simple attachment of the heating element, the excess adhesive material may be removed simply by insertion of the heating element, the heating element having a temporarily sealed insertion end. For providing an inner passage through which air may flow, when inserting a heating element from the other end of a cavity, it is advantageous if the temporarily sealed insertion end is removed after insertion of the heating element. In order to prevent clogging of the cavity by excess adhesive material, it is advantageous if before the insertion of the respective heating element at one cavity end, a collector tube is provided in the cavity from the opposite end, for receiving excess adhesive material and discharging said excess adhesive material by removing the collector tube from the cavity after insertion of the heating element. For a reliable reception of the excess adhesive material the collector tube is advantageously rotated when being removed after insertion of the heating element for removing said excess adhesive material by discharging means provided inside said collector tube. As discharging means cross-shaped blades may be provided in the collector tube, wherein a plurality of cross-shaped blades may be arranged parallel, or a helical blade-structure may be provided. If the end of the first heating element is temporarily sealed by a bung during insertion, which bung is removed by pulling on a bung cable after having finished the insertion step, air can escape from the cavity through the passage in the carrier tube when a second heating element is inserted in the cavity . For discharging the excess adhesive material from the cavity, it is advantageous if the bung is provided adjacent to the inner end of the collector tube while removing the bung and the collector tube from said cavity. If the heating element is pushed into the respective cavity by use of a pin temporarily connected to the heating element during insertion, the pin can advantageously be used as a means against which a force is applied to the heating element to push it into the cavity against resistance of the adhesive material. The invention will be explained now in more detail, by way of example only, with reference to the accompanying drawings Figures in which: Figure 1 shows a schematic view of a calender roll together with a part of an adjacent roll and a manufactured paper sheet inbetween the two rolls ; Figure 2 shows a temperature profile of such a calender roll heated with a fluid medium and a temperature profile of calender roll heated with an electrical heating element according to the invention; Figure 3 shows an end view of a calender roll with an inner passage; Figure 4 shows a cross section of a heating element with a heating mat attached to a rod-shaped carrier; Figure 5 shows a cross sectional view according to line V-V in Fig. 6 of another, more preferred heating element including a tube as carrier for a heating mat; Figure 6 shows a side view of the heating element according to line VI in Fig. 5; Figure 7 shows a schematic view of part of another heating mat for attaching to a carrier; Figure 8 shows a cross-sectional view of another heating mat according to line VIII-VIII in Fig. 7; Figure 9 shows a temperature profile of a heatable roll using a zonal heating system; Figure 10 shows a view of the preferred embodiment of the heating element in detail; Figure 11 shows an end view according to line XI in Fig. 10; Figure 12 shows a view of a collector tube for removing adhesive material ; Figure 13 shows a view according to line XIII in Fig. 12; Figure 14 shows a block diagram of a method according to the invention; Figures 15 a-c show sectional views of a calender roll having a heating element inserted therein according to one embodiment of the method of the invention, at various stages during the insertion process; Figures 16 a-f show side views of a calender roll having a heating element inserted therein according to another, more preferred embodiment of the method of the invention, at various stages during the insertion process; Figure 17 shows a view of a liquid adhesive material reservoir according to line XVII in Fig. 18; and Figure 18 shows a side view of the liquid adhesive material reservoir. Fig. 1 shows a side view of a typical heatable calender roll 1 supported by journals 2, 2a on both sides of the roll 1. In order to produce a paper sheet 3 a small gap 4 is provided between the outer surface 5 of roll 1 and an adjacent polymer coated roll 6. The cavities 7 are provided next to the outer surface 5 of roll 1 and are substantially parallel to the axis of rotation 1 ' . Since the cavities 7 are manufactured by drilling from both sides of the roll 1 it may happen that the bores 7', 7'' are not exactly coaxial. As shown in Fig. 1 the bores 7 ' , 7 ' ' may not meet in the middle of roll 1 in order to keep stress concentration away from the point of maximum bending stress. Furthermore, there may be a slight misalignment between the bores 7 ' , 7 ' ' , leading to a step between that bores , as may be seen from Figure 1. In order to achieve a favourable temperature profile for the production of paper sheets 3 on the outer surface 5 of roll 1, it is advantageous if the outer surface 5 of roll 1 provides a plurality of independently controllable temperature zones, as shown in Fig. 9. Line 8 in Fig. 2 shows the surface temperature profile of a roll 1 with an oil or steam type heating system, according to the prior art, which is non-symmetrical, and where only one temperature zone can be created. In this case the heating medium (oil or steam) must pass through the journal 2, and therefore the operating temperature of bearings in the journal 2 on the inlet side 9 is significantly higher than the bearing operating temperature in roll 1 with an electrical type heating system as indicated by line 10 in Fig. 2. In case of the electrical heating system it is possible to control and power a plurality of independently heatable zones (cf. Fig. 9), without heating the journals 2, 2a. By an electrical heating system the heat flux profile 11 can be advantageously kept constant over paper sheet width 12, which is of highest importance for the manufacturing of paper sheets . Figure 3 shows a rather schematic view of a calender roll 1 having a plurality of cavities (channels) 7 near its outer surface 5, for insertion of heating elements 13 (as e.g. in Fig. 4 or 5), to allow for temperature control of the outer surface 5 of the roll body. Further bolt locations 14 for the connection to journals 2, 2a (cf. Fig. 1) are shown. Inside the roll 1 an internal bore 15 is provided which may be used for receiving cables 16 (cf. Fig. 5) of heating elements 13. Figure 4 shows a cross section of an embodiment of a heating element 13 with a heating mat 17 attached to a full-material rod-shaped carrier 18 and comprising resistance heating means 19. The heating element 13 is cylindrical, and has a diameter between 0 and 2 mm smaller than that of the corresponding cylindrical elongate cavity 7 (cf. Fig. 3) into which it is to be inserted. In the present example, the resistance heating means 19 comprises the wires 21' which are made from niσhrome provided with an optional electroplated coating of copper to improve adhesion between the wire 21' and the silicone material 20 of the heating mat 17, where the wire 21' itself is not made from copper. Another and more preferred embodiment of the heating element 13 according to the present invention is shown in cross-section in Figure 5. In this embodiment, the resistance heating means 19 is a heating foil 19', which is provided in a heating mat 17 formed on or bonded to the outer surface 22 of a metal tube 23. The heating mat 17 surrounds the metal tube 23 by an angle , which may be substantially 315°, as shown in Figure 5. The resistance heating means 19 is welded to connection leads 24, and thermocouples 25, 26 (cf. Figures 7 and 8) are connected to connections leads 27 via at least one opening 28 in the tube-shaped carrier 23. Further, short intermittent slots 29 are provided along the carrier tube 23, s. also Figure 10, in order to allow adhesive material 30 (cf . Fig. 15a) which is used to fix the heating element 13 in a respective cavity 7 to escape into an inner passage 31 of the carrier tube 23. Thus, an unmanageable pressure rise due to accumulation of adhesive material 30 ahead of the carrier 23 during insertion of the heating element 13 is avoided. The material 20 of mat 17 may comprise silicone material, polyimide material or glass, ceramic, metal oxide, and/or PAN fibre containing material, which is electrically insulative when cured. The tube 23 may be cylindrical in cross section, and may comprise, for example, copper or steel. Both are suitable since they are both electrically and thermally conductive. The thermal conductivity of this heating element tube 23 helps to reduce the operating temperature of the underside 32 of the heater mats 17 in operation, by conducting the heat away from the underside 32 of the heater mats 17. The inner passage 31 may be filled with adhesive silicone material or a combination of silicone and a conductive metal oxide fibre after the insertion of the heating element 13 in the cavity 7. This stabilises the electrical cables 16, 24, 27 inside the heating element 13. A second series of bore holes may be drilled in the roll 1 in order to effect separate cooling. The holes may be used as a conduit for a cooling fluid in use if desired. This feature enables the roll 1 (cf. Fig. 1) to be cooled quickly to a different set temperature if a different batch of material to be processed (such as for example paper) is to be processed at a lower temperature. Conventional rolls 1 are usually allowed to cool by convection in such circumstances, which can take many hours or days during which time production is not possible. This feature therefore reduces dead time when changing process temperatures . Figure 6 shows a view of the heating element 13 according to line VI in Fig. 5. Although a plurality of heating mats 17 may be provided on the carrier tube 23, only one heating mat 17 is shown in Fig. 6, wherein the conductors 21 of the heating foils 19' are shown dotted. In order to prevent the creation of electro-magnetic forces, when a roll 1 in which the heating element 13 is inserted is rotating, the conductors 21 are provided in a meander-like shape. Since the current of each adjacent conductors 21 will flow in opposite direction the electromagnetic forces will interfere and therefore be substantially cancelled even when the roll is rotating. As shown in Figures 7 and 8, temperature sensing means such as thermocouples 25, 26 or resistance thermometers can be embedded at various points in the heating element 13 to enable temperature monitoring of the heating element 13 in-situ. Figures 7 and 8 show schematic views of a heater mat 17 for the attachment to a carrier 18, 23. The heater mat 17 comprises a first temperature sensor 25 in its middle region and a second temperature sensor or thermocouple 26 in an edge region of the heating mat 17. The first temperature sensor 25 is attached on the carrier 18, 23 and in the cavity 7 such that it is next to the outer surface 5 of a body 1 (cf. Figures 1 and 5), in order to sense and to control the actual temperature of the body 1 near its outer surface 5. The heating element 13 is inserted in the cavity 7 such that a region 33 where the metal tube 23 is not surrounded by the heating mat 17 is in a position facing away from the outer surface 5 of the roll body 1 (cf. Fig. 5). The gap between the cavity 7 and the outer surface of the heating element 13, including the region 33 where no heating mat 17 is provided, is filled by adhesive material 30, in order to fix the heating element 13 in the cavity 7 when cured. The thermocouple 26, in the mounted state, is provided more centrally in the cavity 7 and serves also to control the temperature of the roll body and on the other hand serves as security sensor to avoid overheating. Further, conductor connection leads 27 are shown in Figure 8. All connection leads 24, 27 are passed through a recess 28 in the tube 23, cf. Figure 5, to the interior of tube 23. According to Figure 8 showing a cross-sectional view of the heater mat 17 of Fig. 7, the heating foil 19 is provided between a top sheet 34 and a bottom sheet 35 both made of silicone comprising material 20. The temperature sensors 25, 26 are positioned so that the top of them is at the same level as the heating foil 19'. The heater mat 17 silicone preferably comprises a methyl vinyl silicone, in particular Rhodia Silicone MM40THT, of initial cured state Shore "A" hardness of greater than 60 after curing, but preferably 70 - 110. The thermal conductivity of the silicone materials comprising the heating mat 17 and the adhesive layer are preferably between 0.1 and 0.5 W m ~ 2 κ ~ l. This MM40THT silicone is capable of operating temperatures up to 315°C, and is available from Rhodia Silicones Ltd, Male Business Park, Leatherhead, Surrey, KT22 7BA, England. Though the properties of both may be altered by adding ceramic or metal oxide fibres to enhance the thermal conductivity, and the maximum operating temperature. Alternatively, the heater mat 17 may comprise a polyimide material, such as Kapton from Du Pont. Kapton can be used for heating mat temperatures up to 250°C and will be suitable for low to medium speeds of the roll body as silicone comprising material, too. It is also possible to use heater mats 17 comprising compressed glass fibres and/or other ceramic and/or metal oxide fibres and/or PAN (Polyacrylonitrile) fibres pressed together, in order to provide a heating element for high temperature use. Such heating mats may have a temperature up to 500 °C and can be used for roll bodies rotating at high speeds. As shown in Figure 9, the resistance heating means 19 may be arranged to produce a plurality of different zones 36 which can be set to respective temperatures. A resulting temperature profile 37 possible using such a zonal system is shown schematically in Fig. 9, in which the different zones 36 are labelled "zone 1" to "zone 9" respectively. In this Figure, the y axis denotes the temperature at the surface of the roll, and the x axis denotes the distance along the roll. The solid lines 38 indicate the nominal temperature settings in each respective zone, whilst the dotted line 39 indicates the resultant temperature profile. If many zones are present, it is possible to process more than one width of material on a roll at any one time. It is also possible to process two sheet materials simultaneously on different parts of the roll surface which require markedly different process temperatures, thus making each roll more versatile. Zone temperature control can be performed with conventional feedback systems commonly used for furnace zone temperature control. Figures 10 and 11 show in detail the insertion end 40 of heating element 13. In order to close the inner passage 31 of the carrier tube 23 for preventing adhesive material 30 (cf. Fig. 15a) from entering via other openings than slots 29 a bung 43 is provided, which is made from steel, plastic or rubber. The bung 43 is supported on an insert 44 which is accommodated in the inner passage 31 of element 13. To further facilitate the insertion of the heating element 13 a deflector 45 is provided which causes a pressure gradient in the direction of deflector taper so as to promote the passage (flow) of excess liquid silicone 30 through the gap in the heater mat 33, in a direction opposing the axial force applied to insert the heating element 13; this helps keep the heater mat face against the side of the cavity 7 nearest the roll surface 5. The bung 43 provides a hook 46 for attaching a cable 47 (cf. Fig. 16a) by which the bung 43 may be removed from the cavity 7. For streamlining the flow of liquid adhesive silicone 30 over the insertion end 40 of the heating element 13 (cf. Fig. 10), an insertion protector 42 protects the first heating mat 17 on the carrier tube 23 from being damaged due to the flow of silicone 30 over the end of heating element 13 during insertion, and also straightens the flow of silicone 30 over the end of the heating element 13 by displacement of liquid silicone 30 during insertion process. Fig. 12 shows a side view of a collector tube 48 for collecting and carrying out excess adhesive material 30 from the cavities 7. On the inner side of the collector tube 48 a cross or screw 49 (cf. Fig. 13) is provided in an initial portion 50 of the collector tube 48, e.g. 10 cm. The cross 49 can have a helix like character, i.e. like a screw, or can consist of flat blades 51, as shown in Fig. 13. The collector tube 48 and the cross 49 can be made of plastic or metal, wherein the cross 49 can be fixed into position inside the tube 48 due by friction, weld or glue. The collector tube 48 has an outer diameter that fits well into the cavity 7 of which excess adhesive material 30 is to be carried out. In the center of the cross 49 an inner tube 52 is provided for the bung removal cable 47, by which the bung 43 can be removed after insertion of the heating element 13 is finished. In operation, the collector tube 48 is held inside the other end of the cavity 7 during insertion of the heating element 13. As the heating element 13 is fixed into position, the collector tube 48 fills with excess adhesive material 30. For aiding the filling of the collector tube 48 it is rotated, and the rotation is continued while the collector tube 48 is removed in order to detach the adhesive material and to carry the excess away. This leaves the rest of the cavity 7 free of excess adhesive material 30, and therefore unblocked, prior to insertion of the next heating element 13. The blocks in the process scheme of Figure 14 have the following significances. Block 53 denotes providing a reservoir, a spray or a loaded brush of a liquid adhesive material at an opening of an elongate cavity 7 in a body 1 having a temperature controllable outer surface, the body 1 having a plurality of elongate cavities 7 with openings at either end. Block 54, which is optional, denotes coating the inner wall of said cavities 7 with an adhesive material. Block 55 denotes inserting an elongate heating element 13, which carries one or more electrical heating mats 17, into said cavity 7 through said liquid adhesive material 30, such that the heating element 13 extends along the said cavity 7, and the gap between the heating element 13 and the inner wall of the cavity 7 is substantially filled by said liquid adhesive material 30. Block 56 denotes curing said adhesive material 30 to fix the heating element 13 in place. An example of a method according to the invention is schematically shown in Figures 15 a - c. Figure 15a shows a sectional view of a body 1, in the present example a thermo calender roll, having a temperature controllable surface 5 and a plurality of elongate cavities 7 with openings 7a, 7b at either end of each cavity 7. The elongate cavities 7 are preferably cleaned and decreased before the process commences. In addition a primer may be used in order to chemically clean the surface of the cavity 7 just prior to insertion. A reservoir 57 for a liquid adhesive material 30 is provided adjacent the opening 7a of one elongate cavity 7 , and is attached to the steel roll 1 by fixing means 58, such as for example magnetic clamps 58a, 58b. The inner passage 73 (cf. Fig. 17) provides a guide means 59 for the heating element 13 in addition to ensuring that a body of liquid can fill the gap between the heating element 13 and the cavity 7 in the roll 1. A long lead 60 is fed through the cavity 7, so that the heating element 13 (cf. Figures 15b and 15c) and an optional scraper 61 can be fixed to one end of the lead 60 and be pulled into the cavity 7. The lead 60 is fed into the cavity 7 before any liquid adhesive 30 is introduced. Support means 63, such as benches or aligned pipes are provided at either end of the cavity 7. The reservoir 57 is then filled with liquid adhesive material 30 such as, for example, a high temperature silicone rubber, and the scraper 61 is pulled into the cavity 7. The scraper 61 in the present example is shown as being in the shape of a spear, having a conical tip which tapers in the direction 64 of movement along the cavity 7. If desired, the elongate cavity 7 can be filled with a quantity of liquid adhesive material 30 in addition to that provided in the reservoir 55 prior to insertion of the scraper 61. The scraper 61 may be rotated as it is drawn through the cavity 7 if desired. Figure 15b shows what the cavity 7 looks like after the scraper 61 has moved through it, and before the heating element 13 has been inserted. A thin layer 65 of the liquid adhesive material 30 1-2 mm in thickness now evenly coats the interior surface of the cylindrical cavity 7 , having been dragged along the cavity 7 by the movement of the scraper 61 (cf. Figure 15a). It is possible to partly cure this layer 65 of adhesive material 30 before introducing the heating element 13, thereby promoting dimensional stability. As an alternative, the heating element 13 can be introduced immediately without this curing step. As a further alternative, the scraper 61 and heating element 13 can form a single two-part body, the scraper 61 part being removed after the heating element 13 has been inserted. The end 40 of the heating element 13 being first inserted into the cavity must be solid or closed to prevent the liquid adhesive material 30 accumulating inside any channels present in the heating element 13. The heating element 13 and scraper 61 are supported by the support means 63 prior to insertion. Figure 15c shows the cavity 7 and heating element 13 after insertion has been performed. If wished, the closed end 40 of the heating element 13 can now be removed. Although in the above example the scraper 61 and heating element 13 were pulled into the cavity 7, as an alternative they may be pushed in, or sucked in by reducing the air pressure in that part of the cavity 7 not yet filled. Combinations of such insertion methods may be used if desired. If a thin coating of adhesive material 30 is formed by the scraper 61 and partly cured prior to insertion of the heating element 13, the liquid adhesive material 30 may be replaced by a friction reducing fluid prior to insertion of the heating element 13. As an example, glyσerol may be used. Provided the partly cured layer 65 and heating element 13 are a tight fit, further curing of the layer 65 will anchor the heating element 13 to an acceptable extent. Alternatively, the reservoir 55 may be removed or emptied, and the surface of the heating element 13 coated with the friction reducing agent by, for example, spray coating or application from a coated cloth or loaded brush. The liquid adhesive material 30 is preferably a room temperature vulcanising silicone rubber which can withstand continuous temperatures of up to 270°C and peaks of up to 350°C, and which has a Shore "A" hardness of 20 to 60 when cured, very preferably 20 to 40. Such material is Dow Corning 736 Heat Resistant Sealant or Dow Corning Q3-1566 and is available from Dow Corning Corporation, Midland, Michigan 48686-0994. The adhesive material 20 is resilient when cured, so as to minimise the likelihood of cracking of the layer 30 or the heating element 3 in use. If the elongate cavity 7 in the roll has a rather large step in the middle (cf. Fig. 1), as may happen from time to time, it may not be possible to insert a single element 13 along the entire length of the cavity 7. In this case, two (or more) heating elements 13 may be inserted from different ends of the cavity 7. A particularly preferred example of the method according to the invention which refers to the insertion of two ore more heating elements 13 is shown in Figures 16 a-f. After cleaning and priming the cavity 7 and the collector tube 48 (cf. Figures 12, 13), the bung cable 47 is feed through cavity 7 and the inner tube 52 of the collector tube 48, as shown in Fig. 16a. The bung cable 47 is temporarily tied-off and the collector tube 48 is inserted into the open end 7b of the cavity 7 opposed to the opening 7a into which the heating element 13 will be inserted, while measuring the insertion depth. As shown in Fig. 16b, some liquid adhesive material 30 is introduced into the open end 7a of the cavity 7 when the reservoir 57 for liquid adhesive material 30 is provided adjacent to the open end 7a of cavity 7 where the heating element 13 will be inserted. Then support means 63 for supporting and guiding the heating element 13 during insertion is prepared. Before the heating element 13 is introduced to the support means 63 the reservoir 57 is loaded with liquid adhesive material 30. After that the heating element 13 is brought into position for insertion by manoeuvering the support means 63 in the correct position, as shown in Fig. 16c. Before inserting the heating element 13 into the reservoir 57 the bung 43 is fit into the heating element 13, in order to close the open end of the carrier tube 23. With help of pole 68 (cf. Fig. 16d) which is connected to the heating element 13 via a hook, the heating element 13 is pushed, guided by the support means 63, into the cavity 7 starting with its closed end 40 through the reservoir 57 and the adhesive material 30 already provided in the cavity 7. The bung cable 47 is only reeled when it slackens. The adhesive material 30 is pushed along the length of the cavity 7 during insertion. The deflector 45 (cf. Figures 10 and 11) on the end of the heating element 13 aids positioning since the gap 33 between the heating mat 23 provides an escape path for the adhesive material 30 due to a pressure difference. As the heating element 13 approaches its final position, as shown in Fig. 16e, the end of the collector tube 48 is filled with excess adhesive material. While rotating the collector tube 48 and pulling the bung cable 47 to remove it from the end of the heating element 13, the collector tube 48 is pulled out of the cavity 7. The bung 43 is forced against one end of the collector tube 48, and is removed as the collector tube 48 is pulled out; so, the bung 43 leaves a hole in the end of the inserted heating element 13 and no excess adhesive material stays in the cavity 7. The open end in the inserted heating element 13 allows air to escape from ahead when inserting a second heating element 13 from the opposite side 7b (cf. Fig. 16a) of the cavity 7. After this the cables 16 with the connecting leads 24, 27 are fed through the journal 2 and guides are located inside the roll 1 and the journal 2. The insertion of the second (and possibly any further) heating element 13 succeeds in the same way, whereas, however, the end of the second heating element where one is inserted form either end of the cavity needs no bung 43 nor bung cable 47, since the end of the second heating element 13 can be sealed. Further, no collector tube 48 is required. Finally, after having achieved an arrangement as shown in Fig. 16f, the heating elements 13 inner passages 31 can be filled with adhesive material 30 in order to provide support for cables 16 with the connection leads 24, 27 during operation of the roll 1, if required. The cables 16 with the connection leads 24, 27 which emerge from the ends of the heating elements 13 in the roll 1 in use are collected in a protective ducting 69 and passed through a peripheral drilling 70, and are connected to a rotatable connection plate (not shown). The cables 16 emerging from the heating element 13 will preferably be insulated with a light thermal and good electrical insulation. The connection plate may be connected via a slip ring type coupling to a source of a.c. (or d.c.) power which is controlled by a temperature control system including for example thyristors and/or triacs. The connection plate may include junction boxes for connecting families of resistance heating means from a given zone or zones to a single or combination of slip rings. The connection leads 24, 27 may be connected directly to bolts connected to the slip ring itself if desired. A person skilled in the art will be aware that many different known methods of connection may be used. Figures 17 and 18 show in detail the reservoir 57 for liquid adhesive material 30 for fixation of the heating element 13 in the cavity 7. The reservoir 57 consists of two parts 71 that can be split, wherein the underside 72 of the reservoir 57 may be cambered to fit onto the journal 2. The reservoir 57 provides an inner passage 73 through which the heating element 13 is fed, while receiving liquid adhesive material 30 from the reservoir 57.

Claims: CLAIMS 1. A heating element (13) for a body (1) having a temperature controllable outer surface (5), said heating element (13) comprising an elongate rod-shaped carrier (18, 23) which is at least partially surrounded by an electrically insulative material (20) having embedded therein resistance heating means (19). 2. A heating element (13) according to claim 1, wherein the carrier (18, 23) is surrounded by the electrically insulative material (20) by an angle ( a ) of substantially 315°. 3. A heating element (13) according to claim 1 or 2 , wherein the resistance heating means (19) comprises a heating foil (19'). 4. A heating element (13) according to claim 1 or 2 , wherein the resistance heating means (19) comprises a heating wire (21'). 5. A heating element (13) according to any one of claims 1 to 4, wherein the electrically insulative material (20) comprising said resistance heating means (19) forms at least one heating mat (17) attached to the rod-shaped carrier (18, 23). 6. A heating element (13) according to any one of claims 1 to 5, wherein the rod-shaped carrier (23) defines at least one inner passage (31) for accomodating connection leads (24, 27) for the resistance heating means (19). 7. A heating element (13) according to claim 6, wherein the rod-shaped carrier is a tube (23). 8. A heating element (13) according to claim 6 or 7 , wherein the rod-shaped carrier (23) comprises a plurality of openings (29), provided in the electrically insulative material-free area (33) and extending from the outer surface of the rod-shaped carrier (23) to the inner passage (31). 9. A heating element according to any one of claims 2 to 8 , wherein a deflector (45) is provided which extends from one front face of the rod-shaped carrier (23) to the free region (33) between the longitudinally extending edges of the electrically insulative material (20). 10. A heating element according to any one of claims 7 to 9 , wherein the rod-shaped carrier (13) comprises a removable plug (43) at one open end (40). 11. A heating element (13) according to any one of claims 1 to 10, wherein the rod-shaped carrier (23) is made of metallic material . 12. A heating element (13) according to any one of claims 1 to 11, wherein the resistance heating means (19) is arranged within said electrically insulative material (20) so as to minimize external electromagnetic fields in use. 13. A heating element (13) according to claim 12, wherein the resistance heating means (19) is arranged such, that in use, electric current in adjacent electrical conductors (21) flows in opposite directions. 14. A heating element (13) according to any one of claims 5 to 13, wherein the heating element (13) comprises a plurality of separate heating mats (17) to define different heating zones (36) along said rod-shaped carrier (18, 23). 15. A heating element (13) according to claim 14, wherein the electric current in a respective heating mat (17) is controllable independently of those in other heating mats (17). 16. A heating element (13) according to any one of claims 1 to 15, wherein the electrically insulative material (20) comprises silicone. 17. A heating element (13) according to any one of claims 1 to 15, wherein the electrically insulative material (20) comprises polyimide. 18. A heating element (13) according to any one of claims 1 to 17, wherein the electrically insulative material (20) comprises compressed glass, ceramic, metal oxide, and/or PAN fibres. 19. A heating element (13) according to any one of claims 5 to 18, wherein the at least one heating mat (17) is attached to the carrier by an adhesive material. 20. A heating element (13) according to any one of claims 5 to 19, wherein the, or at least one, respectively, heating mat (17) comprises at least one temperature sensor (25, 26). 21. A heating element according to claim 20, wherein the, or at least one, respectively, heating mat (17) comprises at least one temperature sensor (25) in a middle region. 22. A heating element according to claim 20 or 21, wherein the, or at least one, respectively, heating mat (17) comprises at least one temperature sensor (26) in an edge region. 23. A heating element according to any one of claims 20 to 22, wherein the, or each, respectively, temperature sensor (25, 26) comprises coated junctions. 24. A heatable body (1) having a temperature controllable surface (5) and comprising a plurality of open-end elongate cavities (7), each of said cavities (7) being provided with at least one electrical heating element (13) according to any one of claims 1 to 23, wherein the electrically insulative material (20) of each respective heating element (13) is fixed with its outer surface to the surface of a respective cavity ( 7 ) by an adhesive material (30). 25. A body (1) according to claim 24, wherein the adhesive material (30) is an electrically insulative material. 26. A body (1) according to claim 25, wherein the adhesive material (30) comprises silicone. 27. A body (1) according to any one of claims 24 to 26, wherein each heating element (13) is provided with at least one temperature sensor (25) which is in a position next to the outer surface of the body (1). 28. A body (1) according to any one of claims 24 to 27, wherein in each cavity (7) two at least substantially coaxial heating elements (13) are provided. 29. A body (1) according to any one of claims 24 to 28, wherein the cavities (2) and heating elements (13) both are substantially cylindrical. 30. A body (1) according to any one of claims 24 to 29, and comprising at least one heating element according to any one of claims 2 to 23, wherein the heating element (13) is provided in a position in said cavity (7) in which the portion (33) without said electrically insulative material (20) is opposed to the body's (1) outer surface (5). 31. A body (1) according to any one of claims 24 to 30, wherein the adhesive material (30) has at least some resilience after curing. 32. A body (1) according to any one of claims 24 to 31, wherein the body (1) is a calender roll. 33. A method of making a body (1) having a temperature controllable outer surface (5), the body (1) having a plurality of elongate cavities (7) with openings (7a, 7b) at either end to receive heating elements (13) according to any one of claims 1 to 23, the method comprising the steps: a) coating liquid curable adhesive material (30) to the outer surface of a respective heating element (13) with a thickness sufficient to fill the annular gap between the heating element (13) and the inner surface of the cavity (7); b) inserting the heating element (13) into said cavity (7), such that the heating element (13) extends along the said cavity (7), and the gap between the heating element (13) and the inner wall of the cavity (7) is substantially filled by said liquid adhesive material (30); and σ) curing said adhesive material (30) to fix the heating element (13) in place. 34. A method of making a body (1) having a temperature controllable outer surface (5), the body (1) having a plurality of elongate cavities (7) with openings (7a, 7b) at either end to receive heating elements (13) according to any one of claims 1 to 20, the method comprising the steps: a) coating the inner wall of said cavities (7) with a layer (65) of liquid adhesive material (30); b) providing a layer of a friction reducing material on the outer surface of an heating element (13) with a thickness sufficient to fill the annular gap between the heating element (13) and the inner surface of the cavity (7); c) inserting said heating element (13) into said cavity (7), such that the heating element (13) extends along the said cavity (7), and the gap between the heating element (13) and the inner wall of the cavity (7) is substantially filled by said resilient adhesive material (30); and d) curing said adhesive material (30) to fix the heating element (13) in place. 35. A method according to claim 33 or 34, which comprises feeding some adhesive material (30) into said cavity (7), to form a slug at the insertion end (7a) thereof, prior to insertion of the heating element (13). 36. A method according to claim 33 or 35 when dependent on claim 33, which comprises coating the inner wall of said cavities (7) with a layer (65) of an adhesive material (30) prior to insertion of the heating element (13). 37. A method according to any one of claims 33 to 36, wherein the layer (65) of adhesive material (30) provided is thicker than the final gap between the heating element (13) and the inner wall of the cavity (7), the excess adhesive material (30) being removed from the cavity (7) prior to or by insertion of the heating element (13). 38. A Method according to any one of claims 33 to 37, wherein the adhesive material (30) is partially cured to provide dimensional stability prior to the excess adhesive material (30) being removed. 39. A method according to any one of claims 33 to 38, wherein the heating element (13) is coated with adhesive material (30) by guiding it through a reservoir (57) of a liquid adhesive material (30) which is provided at the opening (7a) of the respective elongate cavity ( 7 ) . 40. A method according to any one of claims 33 to 38, wherein the heating element (13) is coated with adhesive material (30) by spraying adhesive material on the heating element (13) at the opening (7a) of the respective elongate cavity (7). 41. A method according to any one of claims 33 to 38, wherein the heating element (13) is coated with adhesive material (30) by passing the heating element (13) along a loaded brush which is provided at the opening (7a) of the respective elongate cavity ( 7 ) . 42. A method according to any one of claims 33 to 41, wherein the adhesive material (30) comprises a silicone rubber. 43. A method according to any one of claims 34 to 42, wherein the friction reducing agent is glycerine. 44. A method according to any one of claims 33 to 43, wherein each heating element (13) is supported along its length by a metal sleeve (63) whilst being inserted into the cavity, one end of the metal sleeve (63) being located adjacent one end of the said cavity (7), the sleeve (63) and cavity (7) being located along a common axis during insertion of the heating element (13). 45. A method according to any one of claims 33 to 44, wherein a respective heating element (13) is inserted at each open end (7a, 7b) of each cavity (7). 46. A method according to any one of claims 37 to 45, wherein the excess adhesive material (30) is removed by passing a scraper (61) having a cross section similar to that of the heating element (13) through the cavity (7). 47. A method according to claim 46, wherein the scraper (61) takes the form of a rigid element having a tip which tapers in the direction of insertion along the cavity (7). 48. A method according to any one of claims 37 to 45 , wherein the excess adhesive material (30) is removed by insertion of the heating element (13), the heating element having a temporarily sealed insertion end (40). 49. A method according to claim 48, wherein the temporarily sealed insertion end (40) is removed after insertion of the heating element (13). 50. A method according to any one of claims 37 to 45, wherein before the insertion of the respective heating element (13) at one cavity end (53) a collector tube (48) is provided in the cavity (7) from the opposite end (7b) for receiving excess adhesive material (30) and discharging said excess adhesive material (30) by removing the collector tube (48) from the cavity (7) after insertion of the heating element (13). 51. A method according to claim 50, wherein the collector tube (47) is rotated after during insertion of the heating element (13) for discharging said excess adhesive material by discharge means (49) provided inside said collector tube (48). 52. A method according to claim 50 or 51, wherein the end (40) of the first heating element (13) is temporarily sealed by a bung (43) during insertion, which bung (43) is removed by pulling on a bung cable (47) after having finished the insertion step. 53. A method according to claim 52, wherein the bung (43) is provided adjacent to the inner end of the collector tube (48) while removing the bung (43) and the collector tube (48) from said cavity ( 7 ) . 54. A method according to any one of claims 33 to 53, wherein the heating element (13) is pushed into the respective cavity (7) by use of a pin (68) temporarily connected to the heating element (13) during insertion.

Inventors: RACSTER NICHOLAS J, RACSTER, NICHOLAS, J.
Applicants: RACSTER NICHOLAS J, RACSTER, NICHOLAS, J.
Application year: 2000
Main classification: D21G1
IPC codes: D21G1, F28F5