HEAT ROLL FOR ELECTROPHOTOGRAPHY

Abstract: A heat roll for electrophotography of a surface heating type having a heating resistor is arranged such that a bonding layer (3), an insulating layer (4), a resistance layer (5), and a surface insu­ lating layer (8) are consecutively provided on an outer surface of a hollow cylindrical core (2), and a bearing member for fitting with a bearing is fitted at each opposite end of the heat roll. A pipe (1) or a round bar formed of aluminum, an aluminum alloy, copper, a copper alloy, or the like which has a greater coefficient of thermal conductivity than that of mild steel is disposed inside the core (2) to permit a unform distri­ bution of the temperature. In addition, a gap of 0.2 mm or more is provided between the inner surface of the core (2) and an outer peripheral surface of the member formed of the high-temperature conductive material. As a result, effects that are attributable to a difference in the coefficients of thermal conductivity can be alleviated, and a temperature rise of the heat roll up to a predetermined temperature can be accelerated.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to improvements of a heat roll for electrophotography in which a bonding layer, an insulating layer, a resistance layer, and a surface insulating layer are provided sequentially on an outer surface of a core formed into a hollow tubular shape, and heat is generated by energization of the resistance layer so as to fix a toner image on a recording medium. Description of the Prior Art: A conventionally known heat roll used in an electrophotographic printing system for thermally fixing a toner image transferred onto a recording medium such as copying paper is arranged such that a halogen lamp is provided in a hollow tubular core as a heat source to effect heating. With a heat roll of this type, however, there are drawbacks in that the rate of power consumption is large, and that a warming-up time required until the start of copying after energization is long. In addition, there is another drawback in that its outside diameter cannot be made small since the lamp is provided inside it. As a means of overcoming this drawback, a direct-heating heat roll is known in which a heating resistor is arranged on the outer surface of the core. An example of such a prior construction is shown in our earlier EP-A-0 240 730 upon the disclosure of which is based the prior art portion of claim 1. Figure 5 is a partial cross-sectional view illustrating one example thereof, in which an insulating layer 4 and a resistance layer 5 are provided on the outer surface of a core 2 formed into a hollow tubular shape via a bonding layer 3. Electrode rings 6 are respectively fixed to opposite end portions of the resistance layer 5 and are electrically connected to the resistance layer 5. A feeder brush 7 is disposed such as to slidably abut against the outer periphery of each of the electrode rings 6. A surface insulating layer 8 is disposed on the outer periphery of the resistance layer 5 to electrically protect the insulating layer 5 and prevent the insulating layer 5 from becoming damaged by external force. Generally, a ceramic is used for the insulating layer 8 and the resistance layer 5. Incidentally, in cases where the core is formed of an insulating material, the bonding layer 3 and the insulating layer 4 may not be provided. If a ceramic is used for the insulating layer or the resistance layer, it is necessary to use a material for the core having a coefficient of thermal expansion which is close to that of the ceramic (generally 5 to 10 x 10⁻⁶/°C. If an aluminium alloy which has a large coefficient of thermal expansion is used as the core, cracks occur in the ceramic owing to repetition of thermal load during production or usage thereof, resulting in deterioration of its electrical properties and breakage of the resistor. In terms of economic efficiency, a ferrous ally (e.g. mild steel, ferrite-based stainless steel or martensite-based stainless steel) is most desirable. In addition, there are cases where an insulating ceramic formed of alumina or the like is used. However, if alumina or the like is used, as insulating ceramic, since its coefficient of thermal expansion is smaller than those of aluminium, steel and an alloy thereof, the temperature of each surface portion of the roll does not become uniform, so that there has been drawbacks in that faulty fixing occurs, and that the temperature of certain portions rises high. Accordingly, an object of the present invention is to provide a heat roll for electrophotography which is capable of alleviating effects which are attributable to a difference in the coefficients of thermal expansion and in its preferred form, of accelerating a rise in the surface temperature of the heat roll up to a predetermined temperature, thereby overcoming the above-described drawbacks of the prior art. To this end, according to the present invention, there is provided a heat roll for electrophotography as defined in claim 1. The provision of the pipe or round bar formed of aluminium or aluminium alloy, copper, a copper alloy or the like which has a greater coefficient of thermal conductivity than that of mild steel inside the core assists even heating of the next roll and maintenance of the working surface of the roll at a level temperature. In the preferred form of the present invention, a gap of 0.2 mm or more is provided between the inner surface of the core and the outer surface of an insert member formed of a high-thermal conductivity material so as to alleviate effects that are attributable to a difference in the coefficients of thermal expansion during a temperature rise. In addition, an arrangement is preferably provided in such a manner as to substantially prevent the axial movement of the pipe or the round bar. The above-described arrangements produce the effect of preventing the exfoliation of the insulating film, the resistance film etc. In addition, in the present invention, since the high-temperature conductive material is provided inside the heat roll, there is an advantage of making the temperature uniform. Furthermore, if the gap of 0.2 mm or more is provided the temperature rise up to a predetermined temperature of the heat roll surface is accelerated as compared with a case where a smaller gap is provided. Brief Description of the Drawings Figure 1 is a cross-sectional view of a direct-heating-type roll in accordance with an embodiment of the present invention, the roll being shown in the heated condition; Figure 2 is a graph illustrating the temperature distribution of a heat roll in accordance with Example 1 and a comparative example; Figure 3 is a diagram illustrating the temperature distribution of the heat roll in accordance with Example 2 of the present invention and a comparative example; Figure 4 is a diagram illustrating temperature rise characteristics based on diametrical differences between the inside diameter of a core and the outside diameter of a member having a high coefficient of thermal conductivity in the present invention; and Figure 5 is a cross-sectional view illustrating a structure of a conventional heat roll. Figure 1 is a cross-sectional view of a heat roll embodying the present invention. In the drawing, a pipe 1 is made of a material having a high coefficient of thermal conductivity such as aluminium and has a thickness of, for example, 2 to 3 mm. However,the material may be an aluminium alloy, copper, a copper alloy or the like. If it is necessary, a round bar may be used in place of the pipe 1. A core 2 made of mild steel is formed into a hollow tubular shape and a gap of 0.2 mm or more is provided between the core 2 and the pipe 1 in the cool state of the heat roll. An outer surface of this core is sequentially coated with a bonding layer 3 made of Ni-Al-Mo with a thickness of 25 µm, an insulating layer 4 made of Al₂O₃ with a thickness of 300 µm or thereabout, a resistance layer 5, i.e. a heating resistor made of Al₂O₃ + NiCr with a thickness of 70 µm or thereabout, and a surface insulating layer 8, i.e. an insulating film, made of Al₂O₃ with a thickness of 100 µm or thereabout. An electrode ring 6 made of a conductive materia such as aluminium bronze is provided at each opposite end of the resistance layer 5 such as to project therefrom, and a feeder brush 7 is provided thereon such as to be slidable. Incidentally, an outer peripheral surface 9 of the surface insulating layer 8 is coated with Teflon with a thickness of approximately 30 µm, while the end surface portions of the insulating layer 4 that are outside the electrode rings 6 are provided with insulation by means of silicone resin. By virtue of the above-described arrangement, if a current for heating is supplied from the feeder brushes 7 to the resistance layer 5 via the electrode rings 6, the heat roll is capable of demonstrating its function. Example 1 A pipe made of aluminium (A 5056) and having an outside diameter of 36.4 mm, an inside diameter of 33.8 mm, and a length of 350 mm was inserted into a conventional heat roll using a core made of mild steel and having an outside diameter of 40 mm, an inside diameter of 37.2 mm, and a length of 330 mm between electrodes in such a manner as to substantially correspond with the distance between the electrodes. A comparison was made between the temperature distribution in this case and that of a conventional structure in which the pipe was not provided. The results are shown in Figure 2. In the graph, the dotted lines indicate temperature distributions obtained when the temperature of the heat roll in accordance with the invention became stable and immediately after 100 sheets of A4 size paper were continuously fed. Meanwhile, solid lines indicate temperature distributions in the case of a conventional heat roll which was not provided with the insert of high thermal conductivity material. As is apparent from the graph, the present invention displays a large effect in improving the temperature distribution and contributes greatly to the improvement of the fixing performance. Example 2 A round bar made of an aluminium alloy (A 5056) and having an outside diameter of 11.8 mm was inserted into a conventional heat roll using a core made of mild steel and having an outside diameter of 15 mm, an inside diameter of 12.6 mm and a length of 220 mm between electrodes in such a manner as to substantially correspond with the distance between the electrodes. A comparison was made between the temperature distribution in this case and that of a conventional structure in which the round bar was not provided. The results are shown in Figure 3. In the graph, the dotted lines indicate temperature distributions obtained when the temperature of the heat roll in accordance with the invention became stable and immediately after 100 sheets of A4 size paper were continuously fed. Meanwhile, solid lines indicate temperature distributions in the case of a conventional heat roll which was not provided with the insert of high thermal conductivity material. As is apparent from the graph, the present invention enables a substantial improvement in the temperature distribution. Example 3 A round bar made of an aluminium alloy (A 5056) and having an outer diameter of 16.8 mm was inserted into a conventional heat roll having an outside diameter of 20 mm, an inside diameter of 17 mm, and a length of 220 mm between electrodes in such a manner as to substantially correspond with the distance between the electrodes and a test was conducted in the same way as Example 2. Consequently, results similar to those of Example 2 were obtained. Example 4 Temperature rise characteristics up to 190°C were examined by varying the diametrical difference between the inner surface of the roll and the outer surface of the insert of high thermal conductivity material to 0.2 mm and 0.8 mm in the heat rolls of Examples 1, 2 and 3. The results are shown in Figure 4. As is apparent from this graph, a heat roll having a diametrical difference of 0.8 mm displayed a shorter rise time than the one having a diametrical difference of 0.2 mm. Particularly in cases where a roll diameter if 200 mm or less and the rise time is approximately 40 sec or less, the temperature distribution was excellent and the rise time was short, thus displaying good results. As a means of providing the material having a high thermal conductivity on the inner surface of a conventional heat roll, instead of a method of inserting the same by providing a gap, as described above, it is possible to adopt a method in which the high-temperature conductivity material is adhered by the use of an inner-surface flame spray gun, a method in which large gaps are provided and solder, adhesive or the like is used for the gaps, or a method in which shrinkage fit or expansion fit is carried out. Among these methods, the preferred method in which an insert of the material having a high thermal conductivity is inserted with the provision of a gap is most simple and since the occurrence of stress in the outer cylinder during heating is small, this method is most desirable. In addition, it goes without saying that the inner high-thermal conductivity material may be extended to the end portions of the roll, or a composite pipe may be used.

Claims: A heat roll for electrophotography of a surface heating type having a heating resistor (5) on an outer surface of a hollow cylindrical core (2) made of steel, characterised in that said core (2) is of mild steel and has an insert (1) comprising a material having a high thermal conductivity which is formed into the shape of a pipe or a round bar and has a greater coefficient of thermal conductivity than that of mild steel, said high thermal conductivity material insert (1) being provided within said hollow core (2). A heat roll for electrophotography according to claim 1, wherein said insert (1) high thermal conductivity material is aluminium or an aluminium alloy. A heat roll for electrophotography according to claim 1 or 2, wherein said insert (1) has an outside diameter smaller than the inside diameter of said core. A heat roll for electrophotography according to claim 3, wherein said difference between said inside and outside diameters is 0.2 mm or more, the coefficients of thermal expansion of the core and the insert being such that the inner surface of said core and the outer surface of said insert engage each other when the temperature is raised to 200°C or thereabout. A heat roll for electrophotography according to any preceding claim, wherein the diameter of said heat roll (9) is 20 mm or less.

Inventors: IIMURA, TSUTOMU, KINUGAWA, KIYOSHI, SAWANO, YASUO, SHIBATA, RYOICHI
Applicants: HITACHI METALS, LTD.
Application year: 1988
Main classification: G03G15
IPC codes: G03G15, H05B3