WO2004040380A1

WO2004040380A1 - Manufacture of heat fuser roll

Info

Publication number: WO2004040380A1
Application number: PCT/SE2003/001675
Authority: WO
Inventors: Per Sundström
Original assignee: Hot Tech Ab
Priority date: 2002-10-31
Filing date: 2003-10-29
Publication date: 2004-05-13
Also published as: AU2003274875A1; SE0203212D0; SE0203212L

Abstract

Method for the manufacture of a fuser roll, and a fuser roll manufactured according to the method and useful for a copying machine and a laser printer, whereby an electrically conducting resistance paint (9) is applied onto a base tube (8), whereby said resistance paint contains metal powder, mixed with glass powder or a high temperature strength polymeric material, and in which the resistance paint is solved in thinner to provide an easily applicable paint which, during the manufacture of the heat fuser roll, is brought to melt together with the base tube (8).

Description

MANUFACTURE OF A HEAT FUSER ROLL

The present invention relates to a method for the manufacture of a heat fuser roll for fusing of toner in a copying machine or a laser printer. The invention also relates to a fuser roll manufactured according to said method.

PRIOR ART

Fuser rolls for this purpose have conventionally been made as hollow tubes of a heat resistant material, for instance an aluminium tube which is black oxidized at the interior surface thereof for quickly absorbing heat, and which may be heated by means of a heating bulb, generally a halogen bulb which is mounted at the axial centre of said hollow tube. The exterior surface the roll has usually been formed with a covering of Teflon^® or a similar material for preventing toner particles from becoming burnt to, or stick to the tube at the same time as they are being fused to a paper passing above/underneath said roll.

An apparatus of said type is shown for instance in the patent US 5,286,948. Such a fuser roll is disadvantages in several respects. For instance it takes long time for transferring of sufficient fusing heat to the exterior side of the roll, and manufacturing process therefore is relatively expensive and time consuming.

Products like laser printers and copying machines are considered to be the most high class technique for printouts from computers. In spite thereof the said technique is apt to some disadvantages, for instance that it takes a considerable time to heat the fuser roll to operation temperature (about 150°C to 170°C). Upon starting a printout it may be necessary to wait for up to 30 seconds for the fuser unit to become ready for start if it is standing in the "sleeping mode". If, on the contrary, starting from room temperature it may take substantially longer time. By "sleeping mode" meant that the fuser roll is continuously kept at a temperature of 70°C to 100°C for reducing the time for the roll to reach its operation temperature, whereby current is continuously consumed for the roll to be maintained at said increased sleeping mode temperature. The most part of the energy consumption for a laser printer or a copying machine is consumed in the "sleeping mode", in part depending on how frequently the machine is in use. Further said fuser technique involves costs which do not appear in other printing techniques, and finally the fuser roll has to be serviced at even time intervals.

The problem that it takes a considerable time to warm up the roll is based on the technical construction of the fuser roll. Thus, normally there is used a heating bulb (of the type halogen bulb) which is mounted at the centre of a tube made of aluminium which is black oxidized at the interior thereof for quickly absorbing heat. At the exterior side of the tube there is a covering of Teflon^® manufacturing process for preventing toner particles from becoming fused (annealed) to the tube at the same time as they are being fused to the paper. Many other fuser units additionally are formed with means for lubricating the Teflon^® surface with Teflon^® oil thereby preventing wear of the Teflon^® surface considering the fact that the paper has a great wearing effect on the Teflon^® surface which is relatively soft. It can be understood that it takes a relatively long time, perhaps 30 seconds, for the fuser roll to reach operation temperature starting from its sleeping mode since the halogen bulb must first be heated, whereupon the heat thereof, via the air, is transported to the aluminium roll, and said roll becomes heated and finally the Teflon^® surface reaches the correct temperature before the fuser process can be started. It may be contemplated to provide a larger halogen bulb, or two halogen bulbs inside an aluminium tube, but in such case also the tube has to be larger, whereby en envelope surface of the tube becomes larger and the available energy per area unit is substantially constant. Presently most fuser rolls have en effect of about 3 W/cm² and it is difficult to increase said effect figure, since the available space is restricted. It is also obvious that such a fuser roll would necessitate a rather expensive mechanism since an aluminium tube has to be rotated about a stationary halogen bulb.

For providing the necessary mechanical pressure for mangling the semi- melted toner onto the paper fibres said fuser roll rolls against a rubber roll which forces the paper against the fuser roll. It is also obvious that the equipment must be equipped with means for supply of Teflon^® oil, or The fuser roll has to be exchanged for the reason that the Teflon^® surface has become worn. For accelerating the heating of the fuser roll it has also been suggested that the roll be made of an electrically insulating material, like ceramics, glass, high temperature resistant polymeric material or a similar material which over the surface thereof is formed with an electrically conducting covering providing a direct heating of the roll. Such a covering may for instance be a so called "thick-film", which is a paste that can be screen printed onto the outer surface of the roll. Such a thick-film is known in the art and is used in connection to the manufacture of resistances in the electronic industry.

A problem with thick-film on a heating roll is that the temperature of the roll becomes reduced at the ends thereof, and that the fusing of the layer of toner powder consequently becomes uneven.

U.S. patent 4,743,940 discloses a method of solving said problem, whereby a masking wire is wound in spiral manner on a fuser roll with varying spacing between the wires, namely a relatively large spacing between the wires at the axially central part of the roll and with a successively less spacing out towards the ends of the roll. Thereafter the roll is covered with a heat generating resistance material outside the masking wires. Finally the masking wires are torn off, whereby the resistance material on the roll forms a spirally formed resistance material having a large spacing at the central part of the roll and a successively reduced spacing towards the ends of the roll. Thereby the roll will get a relatively greater amount of heat adjacent the ends than at the more central parts, and the fuser heat becomes evenly distributed over the roll.

The described method is relatively complicated and the roll manufactured accordingly is disadvantageous in that is presents grooves without any resistance material extending in spiral formation, and this may makes it difficult to provide an even and unitary fusing of the toner powder of the copying machine or the laser printer.

In the document EP1 128 231 A3 there is described a method for the manufacture of a direct-heated roll for fusing of a toner image. This prior art heating roll comprises a roll body of an electrically conductive material, and an electrically insulating layer in contact with and covering the outer surface of the roll. A heat generating resistor layer is provided on top of said electrically insulating layer, which resistor layer consists of ruthenium and lead, and further an organic binder. Though the method, described in the document speeds up the heating of the roll to some extent, there are several drawbacks associated with such a method. For example, the application of the multiple layers on the roll is a time consuming and expensive step. Further, a protective layer is required for protecting the outer surface of the heat-generating resistor layer.

THE INVENTION

The present invention relates to simplifying improvements in the method of the manufacture of fuser rolls of the above mentioned type and improvements in the function of the fuser roll. The invention also relates to an improved fuser roll manufactured according to the new method.

Thus, the invention relates to a functionally improved type of fuser roll which can be used in available fuser units, especially of copying machines and laser printers, and which is intended for fusing of toner (dry ink in powder form) onto a suitable paper. The advantage of the new fuser roll is that, from room temperature, the roll reaches fusing operation temperature in a much shorter time than is possible in conventional fuser units. Thereby it is not necessary to keep the copying machine or the laser printer in "sleeping mode" for an especially long time while the fuser roll becomes heated up to fusing temperature. Since the machine consumes current while in the sleeping mode there is consumed less current than in conventional fuser units.

The patent application also relates to new, improved and less expensive methods of manufacturing of the suggested fuser roll.

DESCRIPTION OF FUSING OF TONER ONTO PAPER

The printing technique based on the use of toner, which may be described as a dry ink in powder form, must be completed with a method of fusing said toner powder onto the paper using heat and pressure. In technical terms the process may be described as a hot mangling of the paper with the toner thereon. What is done in this process is that toner is brought to become almost melted and to be pressed onto the paper. Toner comprises to about 90% of a polymeric powder having a typical glass temperature of 80°C to 100°C and a melting temperature of 150°C to 180°C.

The great advantage of having toner become fused onto the paper is that the printout, after the fuser process, adheres to the paper. There is no known method that can remove the printout from the paper, after having been fused, without the paper becoming destroyed. This is the reason why the method meets Swedish Archive Standard.

THE INVENTIONAL IDEA The idea of the invention is to provide a new type of fuser roll which still rolls against a rubber roll localized underneath the paper for providing a sufficient mechanical pressure for mangling the semi-melted toner into the paper fibres. In order to reduce the time for heating the fuse roll from room temperature to about 10 seconds the source of heat must be moved closer to the paper, and according to the invention it is therefore suggested that the fuser tube should be covered with an electric resistance film which becomes heated when connected to current. Therefore the tube must be made of an electrically insulating material, so that the tube does not short-circuit the resistance film lying on the envelope surface of the tube. The advantage thereof is that most materials which are electrically insulating also have a good heat resistance, and this increases the possibilities of quickly raising the temperature of the resistance film. The tube onto which the resistance film is applied ought to be as thin as possible in order to reduce the mass that has to be heated. Also the resistance film ought to be thin for minimizing the mass thereof. When dimensioning the thickness of the wall of the tube it must be considered that it has to stand the pressure created by the rubber roll. Since said fuser roll should be capable of standing varying pressures for many millions of turns it also must be capable of withstanding the fatigue that is built up in the material. This also relates to the thin resistance film which is applied to the envelope surface of the fuser roll. It has to be secured to the envelope surface so that it does not become peeled off for the reason that the fuser roll is subjected to continuous pressure changes and temperature changes from the rubber roll.

The outer surface of the resistance film should have a high surface finish and a surface structure which is substantially finer than the smallest toner particles for avoiding toner particles from becoming pressed into the resistance film. If the resistance film is harder than wearing particles in the paper a substantial wear of the resistance film is avoided. Thereby it is possible to eliminate the previously necessary covering of the roll with Teflon^® and the application of Teflon^® oil, and this means that it is possible to minimize the necessary service of the fuser unit.

For making it possible to check the temperature of the fuser roll said roll is equipped with a heat sensing means providing a signal for starting a control of the voltage of the resistance film when said resistance film has reached an operation temperature, for keeping the operation temperature as stable as possible. For obtaining the operation temperature in the shortest possible period of time a continuous voltage of 220 VAC is applied. After about 8 seconds the resistance film has reached about 85% of the operation temperature, and at that moment the voltage is switched off, but the temperature proceeds rising to operation temperature depending on the thermal inertia of the system. Now the temperature starts dropping, and when the temperature has dropped about 2%, or 5% at a maximum, of the operation temperature the voltage is once again switched on for a very short period of time, whereby the temperature rises and slightly passes the operation temperature level. The period of time that the voltage has to be switched on is about one second but is depending on the material and the size of the system and of the energy which is used per area unit.

MANUFACTURE OF THE FUSER ROLL

A typical manufacture of a fuser roll according to the above description is illustrated in figures 2 and 3. Thereby there is used a standard type tube 8 preferably made of a ceramic material like alumina, aluminium silicate or silicon oxide etc., but also a tube made of glass or high temperature resistant polymers can be a possible choice. The material of the tube should have as low heat transfer ability as in the order of 5 W/m°C or less, and a high tensile strength so that the thickness of the wall can be made small. If a tube 8 of a ceramic material is used it is possible to cover same with a resistance material 9 comprising a base of silver, palladium, chromium, titanium, molybdenum and other similar metal particles, or mixtures thereof. The said metal particles are mixed with a glass binding substance like a phosphate or boron silicate, or a polymer binding substance like Teflon^® (polyamid resin), whereupon a solvent (hydrocarbure) for the resistance means, generally referred to as thinner, or another hydrocarbure, is added to such extent that the resistance material gets a liquid consistency. The said resistance paint now preferably can be sprayed onto the ceramic tube 8 in the form of a mist of particles. The spraying of the resistance paint can be made using a conventional spray head 16 in which the paint is kept in a container 18 and is forced out through a spray nozzle 17 by means of compressed air 19. The spraying equipment can be mounted on a conveyor unit 21 which keeps the spray head on a suitable distance from the tube 8 and which moves the spray head 18 along the tube 8. The tube 8 is mounted in a carrier 22 having a motor 14 which makes the tube rotate. Since both the speed of the conveyor unit 21 and the rotational speed of the tube can be checked and controlled by means of conventional equipment like a step indicator and stepper motors 14, 15 it is possible, by using a computer, to maintain a great accuracy when spraying the resistance film onto the roll. It is preferred that a sweeping movement gives a thickness of the resistance layer of 1 to 2 μm, and by sweeping the spray head several times it is possible to build up a good total thickness of the resistance film. The resistance film should not be thicker than as to provide an even surface covering of metal and the glass particles. A covering resistance surface is obtained by a layer thickness of about 15 to 20 μm.

Other methods of applying the resistance paint are by dipping the roll into the paint, by rolling the paint onto the roll, by brush applying the paint, or by using a putty technics to apply said resistance paint.

As mentioned above the temperature is reduced towards the ends of the roll as compared with the parts of the fuser roll located axially more centrally. Therefore the resistance layer is preferably applied so as to form a thicker resistance layer at the (axially) more central parts of the roll than at the ends thereof. A thicker resistance layer gives less resistance, and thereby the temperature differences between the ends of the roll and the axially more central parts of the roll are equalized so that the temperature becomes more uniform over the entire roll.

After the spraying, or other methods for applying the resistance paint, the tube is moved into a hot air oven in which the thinner, and other liquids, are allowed to evaporate. The hot air is preferably kept at a temperature of 100°C to 200°C for some minutes, and thereafter the temperature is raised to 750°C to 850°C for some minutes, whereby the glass particles become softened and act as a binder against the ceramic surface and the particles which have been sprayed onto the tube melt together therewith. If polymer binders are used a lower binding temperature can be use, like approximately 300°C. The temperature of the oven ought to be controlled so that it can be increased linearly from 100°C up to 850°C, or 300°C for polymeric binders, in a controlled manner. If the temperature rice is made quickly in order that the resistance film does not become bubbly since the evaporization of moisture follows too quickly. Further, there may appear cracks in the resistance film if the temperature is lowered to quickly. Provided the temperature has been controlled in a correct way the outer surface is glass hard and very even, in the case that glass binder is used. During the melting process the glass and metal particles become integrated with the envelope surface of the ceramic material and there is obtained a good interconnection. It is important that there is used a type of glass that is adapted to the type of ceramic material of the tube. A correct resistance of the resistance film is not obtained until the above mentioned melting process if performed. If a polymeric binder is used on a polymeric roll the same assumptions can be applied as between glass and ceramics.

For creating a good electric contact for the resistance film said film is connected to contact ring 10 at each axial end of the roll. Said contact ring 10 also can be made of a film having a thickness of about 20 μm and which is applied in the same way as described above.

Preferably a contact ring 10 is sprayed onto each end of the tube after the intermediate part of the tube has been masked off. There is used a metal mixture providing a very low resistance (less than 1 Ohm per surface unit, e.g. per cm²). The contact rings 10 are thereafter melt connected in the hot air oven. Thereafter the resistance film 9 is spray applied and is masked off so as to overlap the contact rings by about 2 to 3 mm, and thereafter the resistance film 9 is melt connected in the hot air oven. Finally there is mounted an end piece 11 at each end of the tube so that the tube can be mounted in slide bearings. The said end pieces also can act as contact means for connection of the resistance surface to a voltage of 220 VAC.

There are several advantages with a fuser roll made according to the above description. Since the resistance film 9 provides a heat generating part which is in direct contact with the paper surface 5 and the toner particles, and since the roll base is of a ceramic material having a low heat conducting ability the roll quickly reaches fuse operation temperature. Further, much more heat energy can be withdrawn per surface unit from the said structure than what has previously been usual. In a conventional structure only about 3 W/cm² is withdrawn, whereas it is easily possible to withdraw three to four times higher effect, corresponding to 10 to 12 W/cm² from said new type of fuser roll. In total this makes it possible to reach an operation temperature, starting from room temperature, within 10 seconds, or probably still quicker, if desired. This is the explanation why the fuser roll does not need a "sleeping mode" in order that the printer of copying machine can be started quickly, and that a great amount of energy is thereby saved. Since the described technique eliminates the need of a "sleeping mode", and it is energy saving, also the need of the previously usual cooling is reduced for avoiding that other units of the equipment become over-heated. It is therefore likely that the need of fan cooling can be reduced, and this additionally saves energy.

Another problem is that radiation of heat is higher from the ends of the fuser roll, in particular if the roll is long as compared with the diameter thereof. This is the case for most fuser rolls for A4 and A3 format. Since the application of the resistance film is made by spraying it is possible, by using a computer, to control the rotational speed of the tube and the moving speed of the spray head, thereby making sure that a slightly thinner film is applied at the ends of the tube. Thereby the resistance at the ends can be reduced and it is possible to increase the energy output at the ends for compensating for the higher heat radiation.

As mentioned above the temperature will probably vary, for instance as a half sinus wave from left to right end depending on what type of contact ends are used, but if the energy output per surface unit is controlled according to the same pattern, but mirror symmetrically to the temperature curve, from end to end the temperature will be linearly.

Fuser rolls according to the above description include fewer parts than a conventional fuser roll, and the manufacture cost therefore is less.

Since the surface of the resistance film becomes glass hard and even after the melting process the surface finish is sufficiently good for preventing toner particles from becoming stuck to the surface. Therefore there is generally no need for covering the resistance film with Teflon. Also without Teflon the film is very resistant to wear, and in the best case there is no need for using Teflon oil. Therefore the fuser roll made according to the above principle does not need service with a high interval.

The described type of fuser roll which, in common with a rubber roll and two pressure springs, provides a fuser unit that can mechanically be applied to all laser printers and copying machines of different kind existing on the market and which make use of toner as a printer medium.

SHORT DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a fusing unit of priorly known type. Figure 2 is a perspective view of a fuser roll made in accordance with the present invention. Figure 3 diagrammatically, and in a perspective view, shows an apparatus for covering of a fuser roll with a resistance paint. Figure 4 is a temperature diagram showing the change of temperature along the fuser roll in a previously known fuser roll, and in a fuser roll according to the invention, respectively.

DETAILED DESCRIPTION

Figure 1 shows a fuser unit of priorly known type, in which a halogen bulb 1 is the heating source. At each end said bulb is connected to 220 VAC that can be controlled over the electronic circuit of the machine. Said bulb is placed at the centre line of a tube 2 of aluminium which is covered with a thin layer 3 of Teflon. The tube 2 rotates round the bulb, whereas said bulb is standing still. The tube is driven by a motor unit 7 and feeds the paper 4 which is pressed against a rubber roll 5 from the underside of the paper. The rubber roll 5 presses the paper upwards against the tube 2 by means of two (not illustrated) springs.

The function is such that the electronic circuit switches on 220 VAC to the halogen bulb 1 , whereby said bulb becomes warm and the heat is radiated to the inner surface of the tube 2, which is black oxidized over. the inner surface thereof. The tube 2 and the Teflon layer 3 becomes hot, and when a temperature sensor 6 indicates that the temperature has reached the fuser temperature, about 160°C to 170°C, the electronic circuit switches off the 220 VAC to the halogen bulb 1. During said heating phase the paper 4 is not introduced between the rolls but is located further away in the paper path (which is not shown in the drawing) and is waiting for the fuser roll to reach operation temperature. During the time that the fuser roll grows heated the tube is continuously rotating in contact with the rubber roll 5, and when the fuser roll 2, 3 has reached operation temperature the paper 4 with the text in the form of toner thereon is fed into the slot between the rolls 3, 5. The toner is melted by the fuser roll 3 and is pressed into the paper fibres by the rubber roll 5, whereby the text in the form of toner gets stuck in the paper.

Figure 2 shown the new fuser roll, which comprises a tube 8 of a ceramic or polymeric material and which is shown in partly cut open state in the figure for illustrating that there is question of a tube. Round the tube there is a thin layer of a resistance film 9, which is also shown in cut open mode for illustrating how the fuser roll is composed. At each end of the tube there is a contact ring 10 which is over-lapping the resistance film for creating a good electric contact with the resistance film 9 (here is only the ring 10 at one side shown). At the ends of the tube there is an end piece 11 made of for instance a high temperature stable polymeric material which is pressed into the tube. In said end piece 11 there is a contact piece 12 of metal which is in contact with the contact ring 10 and the inner side of the end piece 11 (as can not be seen from the figure). The paper is pressed between the fuser roll 8, 9 and the rubber roll 5 in the same way as in the conventional fuser unit according to figure 1.

The function is such that the electronic circuit is switched on at 220 VAC over the fuser roll 8, 9 in that the contact piece 12 at the right and left sides are in contact with two spring biased carbon contacts, which are not shown in the drawing. The contact pieces 12 are in contact with the contact rings 10 which are, in turn, in contact with the resistance film 9 at both sides. The film now becomes hot. The resistance film can generate much heat energy per surface unit, and if the ceramic or polymeric tube 8 has low heat transfer ability the resistance film 9 quickly reaches the intended temperature. The temperature is registered by a temperature sensor 6 which causes a switching off from 220 VAC shortly before the fuser roll 8, 9 has reached the operation temperature. During the said heating phase the fuser roll rotates against the rubber roll 5 whereas the paper is located further away in the paper path, exactly as in the conventional fuser unit. When the fuser roll 8, 9 has reached the operation temperature the paper is introduced between the rolls and is pressed against the fuser roll 8, 9 by means of the rubber roll 5, whereby the toner on the paper becomes fused. The fuser roll is driven over a (not illustrated) gear and is bearing mounted in two plastic bearings which are positioned approximately round the contact rings 10 of figure 2.

Figure 3 shows a spray head in an apparatus for the manufacture of a fuser roll according to the invention. The spray unit sprays a resistance paint onto the tube 8. The spray unit comprises a support 13, and in said support the tube 8 is clamped between two clamping means 22 which can release and clamp connect, respectively, the tube 8 by means of a clamping unit 20. The tube 8 now can be rotated be means of a step motor 14 which is formed with a step sensor for controlling the rotation. Along the tube 8 a spray head 16 is movable on a guide path belonging to a linear unit 21. The spray head 16 can be moved reciprocatingly by means of a step motor 15 which also has a step sensor for controlling the mode of the spray head. On the spray head there is a container for resistance paint which is fed into the spray head by means of compressed air over a hose connection 19. When the compressed air is connected the resistance paint is sprayed out of a spray nozzle 1 and a fine mist of resistance paint is sprayed against the tube. Since the ceramic tube 8 rotates at the same time as the spray head 16 is moved reciprocating by the stepper motors 14 and 15, respectively, which motors are controlled by step sensor means it is possible to vary the amount or resistance paint which is sprayed on using a predetermined computer program. This makes it possible to get a good controlling of the thickness of the resistance film and consequently also the final resistance of the resistance film. It is thereby possible to apply a successively thicker layer at the more central parts of the roll, so that the resistance at said more central parts become less, and the resistance becomes correspondingly higher at the ends of the roll, whereby the temperature of the resistance film 9 is equalized so the roll emits substantially the same temperature over the entire length thereof.

The full line 23 in the diagram of figure 4 shows how the temperature varies from the left to the right end of the tube 8 if the resistance film 9 has the same thickness along the entire tube. In this case an A4 tube is 230 mm long and can have a diameter of about 20 mm, and it has no end pieces. By forming the tube with end pieces the temperature profile can be made different. The temperature profile is substantially like a half sinus wave. If the thickness of the resistance film is varied in that the spray unit 16 is programmed so that the energy per surface unit of the resistance layer is varied according to a mirror symmetrical profile of the temperature profile the temperature will be substantially linear from left to right end of the ceramic tube, as shown with the dot-dash line 24 of figure 4. When the tube 8 is assembled with bearings, driving gears and end pieces this will, of course, affect the temperature profile, but since the spray technique is flexible it is possible to compensate for changes which appear depending on additional building parts round the fuser roll.

REFERENCE NUMERALS

1 halogen bulb 13 support

2 aluminum tube 14 stepper motor (rotation)

3 Teflon layer 15 stepper motor (linear)

4 paper 16 spray head

5 rubber roll 17 spray nozzle

6 temperature sensor 18 container, resistance paint

7 drive mechanism 19 hose, compressed air

8 tube 20 clamp unit

9 resistance film 21 linear unit 0 contact ring 22 clamp means 1 end piece 23 temperature profile, sinus 2 contact means 24 temperature profile, linear

Claims

C L A I M S

1. Method for the manufacture of a fuser roll for a copying machine or a laser printer, characterized in that an electrically conducting resistance paint is sprayed onto a tube (8) of a ceramic or polymeric material, which resistance paint contains a metal powder, like a powder of silver, palladium, chromium, titanium, molybdenum, mixed with a glass powder or a polymeric powder in the form of a phosphate, boron silicate or an amid plastic material (Teflon^®), and in which the resistance paint is solved into a liquid, sprayable form in paint thinner.

2. Method according to claim 1 , characterized in that the material of the tube (8) is selected of a type having a low heat conducting ability, which can be machined and which has a good bending strength, preferably aluminium silicate, silica or a high temperature polymer.

3. Method according to claim 1 or 2, characterized in that the resistance paint is sprayed onto the tube (8) by means of compressed air (19) by a so called air-sol-technique, so that the paint provides a spray mist after having passed a spray nozzle (17).

4. Method according to claim 1 or 2, characterized in that the resistance paint is layed onto the tube (8) by dipping the roll into the paint, by rolling the paint onto the roll, by brush applying the paint, or by using a putty technics.

5. Method according to claim 3, characterized in that the resistance paint is sprayed onto the tube (8) sweeping several times there over for a good thickness controlling of the resistance film (9) which is formed on the surface.

6. Method according to claim 3, 5 or 6, characterized in that both the spray head (17) is moved along the tube with a speed that is controlled by computer, and, at the same time, the tube (8) is rotated with a speed that is controlled by a computer, whereby the thickness of the resistance film (9) can be varied along the ceramic tube.

7. Method according to claim 5, characterized in that the resistance film (9) is applied with a successively increased thickness layer in the direction towards the axially central parts of the roll (8) than the at the ends of said roll (8), so that the film covered roll (8, 9) issues a higher heat at the ends than at the more central parts thereof, whereby the temperature gradient along the roll is eliminated and the roll issues a lineary temperature between the ends (10) of the tube (8, 9).

8. Method according to any of the preceding claims, characterized in that contact rings (10) are spray applied onto the roll (8) before or after the application of the resistance film (9), which contact rings overlap the resistance film (9) for providing good electric contacts to the resistance film (9).

9. Method according to claim 8, characterized in that the fuser roll (8) comprising the resistance film (9) and the contact rings is treated in an hot air oven, whereby thinner and other existing liquids is/are evaporated.

10. Method according to claim 9, characterized in that the air of the hot air oven is given such heat that the glass powder, or the polymeric material of the resistance film (9) melts to stick to the roll (8).

11. Method according to claim 9 or 10, in which the roll is formed with glass powder, characterized in that the heat treatment is accomplished at a first temperature of 100-200°C for a couple of minutes, whereupon the temperature is raised to a second temperature of 750-850°C, which temperature is maintained for another few minutes.

12. Method according to claim 8 or 10, in which the roll is formed with a polymeric material, characterized in that the heat treatment is accomplished at a first temperature of 100-200°C for a couple of minutes, whereupon the temperature is raised to a second temperature of approximately 300°C, which temperature is maintained for another few minutes.

13. Method according to any of the preceding claims, characterized in that the fuser roll, at the ends thereof, is formed with end pieces (11) having contact means (12) for providing a simple voltage connection between a rotating fuser roll (8, 9) and a stationary carbon contact between said contact means (12) and the contact rings (10).

14. Fuser roll for a copying machine or a laser printer, manufactured according to the method of one or more of the preceding claims, comprising a base tube (8) having an electrically conducting surface covering (9) adapted to be capable of issuing heat for fusing of toner powder onto a material passing in contact with said fuser roll, characterized in that the surface covering provides an electrically conducting resistance film (9) on tube (8), and in which said resistance paint contains metal powder, like a powder of silver, palladium, chromium, titanium, molybdenum, mixed with a glass powder in the form of a phosphate or boron silicate, or mixed with a polymeric resin, and in which the resistance paint, when being applied on the tube (8), is solved in (paint) thinner to form a liquid, which can easily be applied onto the tube.

15. Fuser roll according to claim 14, characterized in that the surface covering electrically conducting resistance film is applied on the roll by being sprayed onto the tube (8), by dipping the roll into the paint, by rolling the paint onto the roll, by brush applying the paint, or by using a putty technics.

16. Fuser roll according to claim 14, characterized in that the base tube is a tube of a material, which is electrically insulating and has a good heat insulating property.

17. Fuser roll according to claim 14 or 15, characterized in that the electrically conducting resistance film (9) has been applied to the roll (8) in such a way that the temperature gradient between the ends (10) of the roll has been equalized.

18. Fuser roll according to claim 14, 15, 16 or 17, characterized in that the resistance film (9) is given a successively increased thickness from the ends (10) of the fuser roll in the direction towards the axially more central parts thereof.

19. Fuser roll according to any of claims 14-15, characterized in that the average thickness of the resistance film is 16-20 μm.

20. Fuser roll according to any of claims 14-19, characterized in that the resistance film contains particles of glass, or particles of a polymeric material, which have become melted during the heat treatment of the fuser roll, and which have melted together with the base tube (8) so that the glass, or polymeric particles provide a hard and lustre surface on the resistance film.