JP2006017955A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device constituted so that temperature distribution in a width direction can be uniformized by restraining temperature rise at the end of a paper passing area, regardless of the difference in the paper passing widths, and also temperature rise cannot be impaired in uniformizing the temperature distribution. <P>SOLUTION: The fixing device 7 is equipped with an endless fixing belt 70 trained about a heating roller 71 and a fixing roller 72 and capable of coming into contact with a recording medium P being a fixing object, a tension roller 73 coming into contact with the fixing belt 70 and applying tension, and a pressure member 74 forming a fixing nip part by pressing the fixing belt 70, and fixes an unfixed image carried on the recording medium by applying heat and pressure when the recording medium passes through the fixing nip part. In the fixing device 7, the tension roller 73 has anisotropy in thermal conductivity, and its heat conductivity in a shaft direction is higher than that in the thickness direction. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fixing device and an image forming apparatus, and more particularly to a temperature distribution correction mechanism of a fixing device using a belt as a fixing member.

  A typical method for forming a color image includes a direct transfer method in which toner images of different colors formed on a plurality of photoconductors are transferred while being directly superimposed on a transfer sheet, and a different color formed on a plurality of photoconductors. There is an intermediate transfer method in which a toner image is transferred while being superimposed on an intermediate transfer body, and then transferred onto a transfer sheet at once (for example, Non-Patent Document 1).

  Since a plurality of photoconductors are arranged side by side facing the transfer paper or intermediate transfer body, this is called a tandem method, and yellow (Y), magenta (M), cyan (C), and black (K) for each photoconductor. For each color, an electrophotographic process such as electrostatic latent image formation and development is performed, and the image is transferred onto a running transfer sheet in the direct transfer method, or onto a running intermediate transfer member in the intermediate transfer method (for example, Non-patent document 1).

  In the tandem color image forming apparatus using each of these methods, the direct transfer method receives an endless belt that runs while supporting transfer paper, and the intermediate transfer method receives an image from a photoconductor. Generally, an endless belt to be carried is employed. Then, an image forming unit including four photoconductors is arranged side by side on one running side which is one of the belt's extending surfaces.

  The image for each color formed in each image forming unit is superimposed and transferred onto a recording medium such as a recording sheet, and then is fixed by applying heat and pressure by a fixing device to melt and penetrate the toner in the image. This is a copy.

  By the way, in the fixing device, a belt using a structure using a heat roller fixing system in which a heating roller and a pressure roller are brought into contact with each other or a belt wound around a heating roller and a fixing roller facing the pressure roller is used as a fixing member. There is a fixing method (for example, Patent Document 1).

  The belt fixing method uses a belt having a smaller heat capacity than the roller used in the heat roller fixing method as a fixing member, which is advantageous in that the fixing time can be shortened and the fixing time can be shortened. Since it can be deformed following the peripheral surface of the pressure roller facing the fixing roller that is wound around, the fixing nip portion that is a fixing region can be set wide, and thus there is an advantage that fixing efficiency can be improved. The structure shown is provided.

  In FIG. 5, a fixing device (indicated by reference numeral 7 used in the embodiment of the present invention for convenience) is wound around a heating roller 71 and a fixing roller 72, which are a pair of opposed rollers, on a recording medium to be fixed. An endless fixing belt 70 that can come into contact with the corresponding transfer paper P, a halogen heater 71A built in the heating roller 71 as a means for heating the fixing belt 70, and a tension applied by contacting the fixing belt 70. The main portion includes a tension roller 73 and a pressure roller 74 as a pressure member for pressing the fixing belt 70 to form a fixing nip portion.

  The fixing belt 70 is wound around the heating roller 71 and the fixing roller 72, respectively, and can move in a direction in which the transfer paper can be conveyed (in the direction of the arrow in the drawing), and is a halogen heater disposed inside the heating roller 71. It is heated to a temperature necessary for fixing through a heating roller 71 heated by 71A.

Below, the detail of each member is demonstrated.
As the fixing belt 70, an endless belt-like substrate formed of a metal such as SUS or nickel or a heat-resistant resin is used.
As the material of the heat resistant resin, polyimide, polyamideimide, polyetherketone (PEEK) or the like can be used. Further, the thickness of the substrate is 30 to 100 μm from the balance between heat conduction and strength.

  Since the surface of the fixing belt 70 is in pressure contact with the transfer paper P and the toner T, a surface release layer such as a fluororesin is coated as a surface release layer having excellent heat resistance and durability. Furthermore, in order to obtain image uniformity, an elastic layer made of heat-resistant rubber such as silicone rubber or fluorine rubber having a thickness of about 100 to 300 μm is provided under the surface release layer.

  The heating roller 71 has a diameter of 30 mm, for example, and the core metal is a thin metal such as aluminum or iron having a good thermal conductivity. The thickness of the heating roller 71 is considered from the mechanical strength such as bending and crushing and the shortening of the rise time. It is set to about 0.3 to 0.8 mm. In order to improve heat absorption from the halogen heater 71A on the inner surface of the cored bar, black body coating may be performed.

The fixing roller 72 has a configuration in which, for example, a diameter of 40 mm and a heat insulating layer is provided on the outer side of a core metal such as aluminum or iron. As the heat insulating layer, silicone rubber, porous ceramics, or the like that provides good heat resistance can be used. used.
The lower the thermal conductivity of the material used for the heat insulating layer, the more effective the material is, and approximately 0.2 W / m / K or less is selected.

  The pressure roller 74 is configured by forming a heat-resistant elastic layer such as a fluorine-based resin or silicone rubber on the outer periphery of a cored bar and a surface release layer made of a fluorine resin or the like. Further, in order to improve the peelability of the transfer paper P from the fixing belt 70, the surface hardness of the pressure roller 74 is made higher than that of the fixing roller 72, and the fixing belt is interposed between the fixing belt 72 and the pressure roller 74. A downward-facing fixing nip portion 70 is formed in a state where 70 is pressed to the peripheral surface side of the fixing roller 72 and is depressed. The thickness of the elastic layer of the pressure roller 74 is about 1 mm to several mm.

  Since the tension roller 73 is used as a tension applying member of the fixing belt 70, for example, an aluminum core bar having a diameter of 14 is used. In addition, in order to speed up the rise, a heat insulating layer having a thickness of about 0.2 to 2 mm may be formed on the outer periphery of the core bar to make it difficult to take heat of the fixing belt 70.

  In the fixing device 7 as described above, when the heating roller 71 is heated by energizing the halogen heater 71A in the heating roller 71, the fixing belt 70 wound around the heating roller 71 is heated. The surface temperature of the heating roller 71, in other words, the temperature control of the fixing belt 70 is performed using a temperature detection sensor 75 facing the fixing belt 70 that is wound around the heating roller 71.

  The fixing belt 70 is stretched between the heating roller 71 and the fixing roller 72, and the surface facing the pressure roller 74 among the stretched surfaces is in contact with the transfer paper P. The transfer sheet P guided by the guide member 76 with respect to the fixing belt 70 moves toward the fixing roller 72 side with the toner T carrying surface, and the pressure roller 74 is pressed against the fixing roller 72. When the toner T is introduced into the formed fixing nip portion, it is fixed by receiving heat from the fixing belt 70 and melting and permeating the toner T.

“Continued Fundamentals and Applications of Electrophotographic Technology” (published November 15, 1996, edited by the Electrophotographic Society, pages 35-36) JP 2001-92282 A (paragraph “0023” column)

  Conventionally, a major problem in a fixing device is that it takes a long time to rise to a predetermined fixing temperature. Therefore, as disclosed in Patent Document 1, a measure for increasing the winding angle of the fixing belt around the heating roller has been taken.

  However, as a problem of the fixing device, the temperature distribution in the direction corresponding to the width of the transfer paper in the width direction of the fixing belt, that is, the direction perpendicular to the moving direction is different depending on the size of the transfer paper. Is mentioned.

The width of the fixing belt is set to be larger than the maximum sheet passing width of the transfer sheet, but when the transfer sheet having the maximum sheet passing width is continuously conveyed for fixing, the temperature of the sheet passing part is lowered. The temperature on both sides of the non-sheet passing will rise. For this reason, when a transfer sheet having a larger sheet passing width than the transfer sheet that has completed the sheet passing immediately after the end of the continuous sheet passing is passed, In some areas, hot offset, that is, molten toner may reversely transfer to the fixing member.
In particular, when the transfer paper is thick or the transfer paper is moved at a high speed in the fixing operation performed earlier, the temperature difference between the paper passing portion and the non-paper passing portion becomes significant, and the subsequent Depending on the size of the transfer paper used for the fixing operation performed in this manner, the toner at the position in contact with the non-sheet passing portion tends to cause hot offset.

  Therefore, in order to prevent hot offset, it is possible to control the temperature of the non-sheet passing part to lower the temperature of the non-sheet passing part when passing the transfer paper that is less than the maximum sheet passing width. When passing a transfer sheet having a sheet passing width, a cold offset, that is, transfer of unfixed toner may occur in a non-sheet passing portion until then, and this toner may adhere to the fixing member. Such an offset may cause a transfer to transfer paper to be conveyed next, which may be fouled.

  Instead of controlling the temperature to the non-sheet passing portion, it is conceivable to provide a plurality of halogen heaters having a length substantially equal to the width of the plurality of transfer sheets to be passed. Since there are various sizes up to the plate size, too many halogen heaters are installed to accommodate all transfer paper widths, and the heat efficiency increases due to the increase in the roller thickness and the increase in roller thickness due to the increase in size. A new problem of reduction occurs. From these results, it is difficult to provide halogen heaters corresponding to all transfer paper widths.

  Actually, when the temperature rise occurs at both ends of the non-sheet passing portion, the transfer speed of the transfer paper is reduced to relax the temperature distribution in the width direction, or a fixing roller that contacts the fixing belt In many cases, the thickness of the pressure roller and further the tension roller is increased to increase the heat capacity, thereby relaxing the temperature distribution in the axial direction of the roller.

  However, when a realistic measure is selected, the thickness of the roller, which is a member that contacts the fixing belt, increases, resulting in an increase in heat capacity. As a result, the rise to the fixing temperature is deteriorated, and the user This gives the impression that the waiting time until the start of image formation becomes longer.

  In view of the problems in the conventional fixing device, the object of the present invention is to suppress the temperature rise at the end of the sheet passing area regardless of the difference in the sheet passing width and to uniform the temperature distribution in the width direction. An object of the present invention is to provide a fixing device and an image forming apparatus having a configuration capable of preventing a rise in temperature.

  According to the first aspect of the present invention, an endless fixing belt that is wound around a pair of opposed rollers and can contact a recording medium to be fixed, a means for heating the fixing belt, and a contact with the fixing belt. A tension roller that applies tension and a pressure member that forms a fixing nip portion by pressing the fixing belt, and recording is performed by applying heat and pressure when the recording medium passes through the fixing nip portion. In a fixing device capable of fixing an unfixed image on a medium, the tension roller has anisotropy in thermal conductivity, and the thermal conductivity in the axial direction is higher than the thermal conductivity in the thickness direction. It is characterized by.

  According to a second aspect of the present invention, in the fixing device according to the first aspect, the material having anisotropy in thermal conductivity is a fiber or a fiber reinforced resin, and the direction of the fiber and the axial direction of the tension roller It is characterized by being arranged in parallel.

  According to a third aspect of the present invention, in the fixing device according to the first or second aspect, graphite is used as the material having anisotropy in the thermal conductivity.

  According to a fourth aspect of the present invention, the fixing device according to any one of the first to third aspects is used in an image forming apparatus.

  According to the first and fourth aspects of the present invention, the tension roller in contact with the fixing belt has anisotropy in thermal conductivity, and the thermal conductivity in the axial direction is higher than that in the thickness direction. Therefore, it is possible to balance the temperature in the width direction of the fixing belt. Accordingly, it is possible to prevent the occurrence of hot offset by suppressing the temperature rise on both sides of the non-sheet passing portion due to the size of the recording medium. Moreover, since the conductivity in the thickness direction of the tension roller is low, the heat propagation in the radial direction of the tension roller is suppressed, so that the outflow of heat can be suppressed and the rise in temperature can be prevented from being hindered. .

  According to the second aspect of the present invention, since the material having anisotropy in the thermal conductivity in the tension roller has the fiber orientation parallel to the axial direction of the roller, the thermal conductivity in the axial direction is very high. While it can be increased, the thermal conductivity in the thickness direction can be kept low. As a result, it is possible to quickly perform heat propagation in the axial direction and eliminate the temperature gradient to achieve balance.

  According to the invention described in claim 3, since graphite is used as the material having anisotropy in thermal conductivity, the roller base material can be made thin by ensuring the mechanical strength. Thereby, by reducing the thickness of the roller, the heat capacity can be reduced to suppress the heat outflow from the fixing belt, and the time to rise to the fixing temperature can be shortened.

  The best mode for carrying out the present invention will be described below with reference to the embodiments shown in the drawings.

  FIG. 1 is a diagram showing a configuration of a color laser printer (hereinafter referred to as a laser printer) based on an electrophotographic direct transfer system as one of image forming apparatuses according to an embodiment of the present invention. FIG. 6 is a diagram for explaining a configuration of a tension roller used in the fixing device shown in FIG.

  In FIG. 1, a laser printer 2000 includes four toner image forming units 1Y, 1M, 1C, and 1K for forming images of each color of yellow (Y), magenta (M), cyan (C), and black (K). (Hereinafter, the subscripts Y, M, C, and K of the respective symbols indicate the members for yellow, magenta, cyan, and black, respectively) are the moving directions of the transfer paper 100 used as one of the recording media ( They are arranged in order from the upstream side in the direction in which the belt 60 travels along the arrow A in the figure.

  Each of the toner image forming units 1Y, 1M, 1C, and 1K includes photosensitive drums 11Y, 11M, 11C, and 11K as image carriers and a developing unit. Further, the arrangement of the toner image forming units 1Y, 1M, 1C, and 1K is set so that the rotation axes of the photosensitive drums are parallel to each other and can be arranged at a predetermined pitch in the transfer sheet moving direction.

A laser printer 2000 shown in FIG. 1 carries the optical image writing unit 2, paper feed cassettes 3 and 4, registration roller pair 5, and transfer paper 100 in addition to the toner image forming units 1Y, 1M, 1C, and 1K. A transfer unit 6 as a belt driving device having a transfer conveyance belt 60 as a transfer conveyance member that conveys the toner image forming unit so as to pass through a transfer position, a belt fixing type fixing unit 7, a paper discharge tray 8, and the like are provided. Yes. In addition, a manual feed tray MF and a toner supply container TC are provided, and a waste toner bottle, a duplex / reversing unit, a power supply unit, and the like (not shown) are also provided in a space S indicated by a two-dot chain line.
The optical writing unit 2 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and irradiates the surface of each of the photosensitive drums 11Y, 11M, 11C, and 11K while scanning the laser beam based on image data. To do.

The transfer conveyance belt 60 used in the transfer unit 6 is a high-resistance endless single-layer belt having a volume resistivity of 10 ⁹ to 10 ¹¹ Ωcm, and the material thereof is PVDF (polyvinylidene fluoride). The transfer / conveying belt 60 is wound around support rollers 61 to 65 so as to pass through the transfer positions that contact and face the photosensitive drums 11Y, 11M, 11C, and 11K of the toner image forming units.
Out of these support rollers, the outer peripheral surface of the transfer conveyance belt 60 is opposed to the entrance roller 61 on the upstream side in the transfer paper moving direction so as to face the electrostatic adsorption roller 80 to which a predetermined voltage is applied from a power source (not shown). Is arranged. The transfer paper 100 that has passed between the two rollers 61 and 80 is electrostatically attracted onto the transfer conveyance belt 60.

A roller 63 is a drive roller that frictionally drives the transfer conveyance belt 60, and is connected to a drive source (not shown) and rotates in the direction of the arrow.
As a transfer electric field forming means for forming a transfer electric field at each transfer position, a transfer bias applying member (typically 67Y in FIG. 1) is placed at a position facing the photosensitive drum so as to be in contact with the back surface of the transfer conveyance belt 60. Only are labeled). These are bias rollers provided with a sponge or the like on the outer periphery, and a transfer bias is applied to the roller mandrel from each transfer bias power source. Due to the action of the applied transfer bias, a transfer charge is applied to the transfer conveyance belt 60, and a transfer electric field having a predetermined strength is formed between the transfer conveyance belt 60 and the surface of the photosensitive drum at each transfer position. In addition, a backup roller 68 is provided in order to keep the contact between the transfer paper and the photoconductor in the transfer area and to obtain the best transfer nip.

  A cleaning device 85 composed of a brush roller and a cleaning blade is disposed on the outer peripheral surface of the transfer conveyance belt 60 wound around the driving roller 63. The cleaning device 85 removes foreign matters such as toner adhering to the transfer conveyance belt 60.

  A roller 64 is provided downstream of the driving roller 63 in the traveling direction of the transfer conveyance belt 60 in a direction to push the outer peripheral surface of the transfer conveyance belt, and a winding angle around the drive roller 83 is ensured. A tension roller 65 that applies tension to the belt with a pressing member (spring) is provided in a loop of the transfer conveyance belt 60 further downstream from the roller 64.

  A one-dot chain line in FIG. 1 indicates a conveyance path of the transfer paper 100. The transfer paper 100 fed from the paper feed cassettes 3 and 4 or the manual feed tray MF is transported by transport rollers while being guided by a transport guide (not shown), and is transported to a temporary stop position where the registration roller pair 5 is provided. The transfer paper 100 delivered at a predetermined timing by the registration roller pair 5 is carried on the transfer conveyance belt 60, conveyed toward the toner image forming units 1Y, 1M, 1C, and 1K, and passes through the transfer nips. .

  The toner images developed on the toner drums 11Y, 11M, 11C, and 11K of the toner image forming units 1Y, 1M, 1C, and 1K are superimposed on the transfer paper 100 at the transfer nips, respectively. It is transferred onto the transfer paper 100 under the action of pressure. By this superposition transfer, a full-color toner image is formed on the transfer paper 100. When forming a black toner image, although not shown, the transfer belt 60 is tilted so as to correspond only to the black image forming portion. Further, in FIG. 1, symbol B indicates the discharge direction of the transfer paper toward the abolished tray 8, and symbol C indicates the direction in which the conveyance direction is switched by the gate member.

Next, the configuration of the fixing device 7 will be described.
The fixing device 7 has the configuration shown in FIG. 5, and the tension roller 73 that applies tension to the fixing belt 70 has the configuration shown in FIG.
The tension roller 73 has anisotropy in thermal conductivity, and the thermal conductivity in the axial direction is set to be higher than the thermal conductivity in the thickness direction, that is, the radial direction.
Hereinafter, a specific example regarding the configuration of the tension roller 73 will be described.
(Example 1)
In this example 1, the tension roller 73 is provided with a graphite fiber 73B having a thickness of 2 mm on the outer peripheral surface of an aluminum solid core metal 73A having an outer diameter of φ10. The direction of the fiber is set to be parallel to the axial direction.
(Example 2)
In Example 2, the core metal 73A of the tension roller 73 is replaced with a solid hollow metal core of φ10 (thickness 0.3 mm) instead of the solid one shown in (Example 1), and the outer peripheral surface has a thickness. A 2 mm graphite fiber is provided. The direction of the fiber is set to be parallel to the axial direction.

The reason for providing graphite fibers as a material having anisotropy in thermal conductivity is that there is a large difference in thermal conductivity between the axial direction and the direction perpendicular to the axial direction, that is, the radial direction, and high mechanical strength. Is obtained. In other words, graphite has a Young's modulus of 750 (GPa), and has a strength ten times that of aluminum, which has a Young's modulus of 72 (GPa). For this reason, even when the cored bar is made thinner, it is possible to obtain sufficient strength with graphite, thereby reducing the heat capacity affected by the thickness of the cored bar and improving the temperature rise.
The material having anisotropy may be a fibrous or fiber reinforced resin, and examples of such a material include glass fiber and silicon nitride fiber in addition to graphite fiber. Among these fiber materials, graphite fiber is superior in terms of securing mechanical strength.

  Since the present embodiment is configured as described above, the temperature gradient in the width direction of the fixing belt 70 is eliminated by the fact that the thermal conductivity in the axial direction of the tension roller 73 is higher than the thermal conductivity in the thickness direction. Can be balanced. As a result, the temperature rise on both sides of the non-sheet passing portion that has not been passed is suppressed, and hot offset can be prevented.

  The inventor compared the temperature distribution in the fixing belt width direction and the rising characteristics up to the fixing temperature in the fixing device when the tension roller 73 according to this embodiment is used and when the conventional tension roller is used. The results shown in FIGS. 3 and 4 were obtained.

The conventional structure of the tension roller to be compared is as follows.
(Comparative Example 1)
A heat-insulating felt having a thickness of 2 mm is provided on the outer peripheral surface of a solid metal core made of φ10.
(Comparative Example 2)
Use a solid aluminum core with φ14.

  The tension rollers (Example 1) and (Example 2) according to the present embodiment and the conventional tension rollers (Comparative Example 1) and (Comparative Example 2) are all applied to the fixing device having the configuration shown in FIG. FIG. 3 shows the surface temperature in the axial direction of the fixing belt 70 immediately after 500 sheets of A4-size transfer paper (having the width indicated as the paper passing portion in FIG. 3) are continuously fed. FIG. 4 shows the time until the surface temperature of the fixing belt 70 reaches the control temperature. In addition, (Example 1) and (Example 2) shown in Examples in the experimental results shown in FIGS. 3 and 4 are conventional examples (Comparative Example 1) and (Comparison 1) indicated by reference numerals (1) and (2). Example 2) is indicated by reference numerals (3) and (4), respectively.

  As a result of the experiment, in (Example 1: (1)), (Example 2: (2)) in this example and (Comparative Example 2: (4)), 500 A4-size transfer sheets were used. It can be seen that the temperature rise on both sides of the non-sheet passing portion with respect to the sheet passing portion (width direction center portion) immediately after the continuous sheet passing is moderated as compared with the case of (Comparative Example 1: (3)).

  In the configuration described in (Example 1: (1), (Example 2: (2)) in this example, when A3 size transfer paper larger than A4 size was passed after continuous feeding, non-sheet passing No hot offset occurred in the range corresponding to the part.

The tension roller 73 in this embodiment can obtain the following effects by using graphite as a material having anisotropy in thermal conductivity.
The thermal conductivity of graphite has anisotropy of 450 (W / m / K) in the fiber surface direction and 20 (W / m / K) in the thickness direction. In contrast, since the thermal conductivity of aluminum is 230 (W / m / K), graphite is 1.5 times as much as aluminum in the axial direction and 11.5 minutes in the direction perpendicular to the axis with respect to the thermal conductivity. Therefore, the heat of the non-sheet passing portion in the axial direction is smoothly propagated to the sheet passing portion where the temperature is lowered, and the temperature gradient in the axial direction is eliminated and balanced. can do.

  On the other hand, in Comparative Example 1, the temperature of the non-sheet passing portion rises immediately after passing 500 sheets, and when the A3 size transfer paper is passed immediately after passing, the portion that was the non-sheet passing portion until then A hot offset occurred.

Regarding the temperature rise, as shown in FIG. 4, (Example 2), (Example 1), (Comparative Example 3), and (Comparative Example 4) rankings according to this example were obtained.
(Example 1) and (Example 1) according to the present embodiment are anisotropic thermal conductivity, and in particular, the thermal conductivity in the thickness direction is lowered with respect to the axial direction, so heat outflow is suppressed. This makes it possible to get up early. Further, in the case of (Example 2), unlike the case of (Example 1), the cored bar is a hollow member, so that the heat capacity is reduced, which also contributes to the improvement of the rise characteristics.

  On the other hand, in (Comparative Example 1) and (Comparative Example 2), since the thermal conductivity in the thickness direction is higher than that of graphite having anisotropic thermal conductivity, Heat is easily taken away, resulting in poor start-up characteristics.

  In the fixing device according to this embodiment, the fixing belt 70 is pressed to form a fixing nip portion, and the fixing belt 70 is pressed toward the fixing roller 72 using a pressure roller 74 as shown in FIG. By doing so, a part of the peripheral surface of the fixing roller 72 is deformed into a concave shape to enlarge the heat propagation area, or the opposing rollers are pressed against the fixing belt 70 to apply tension, instead of being targeted. A configuration in which a member such as a tension roller is used as a heating member, and a heat propagation region is expanded by using a fitting angle of the fixing belt 70 to the tension roller can be used.

  According to the present embodiment, a special heat insulating structure is required with a simple configuration in which a material having anisotropy in thermal conductivity is used on the outer peripheral surface of the core metal, in other words, the surface that contacts the fixing belt 70. The temperature distribution in the axial direction can be made uniform, and the heat loss from the fixing belt 70 due to the suppression of heat conduction in the thickness direction can be reduced to simultaneously prevent both the occurrence of offset and the temperature rise characteristic. It can be secured.

1 is a schematic diagram illustrating an example of an image forming apparatus in which a fixing device according to an embodiment of the present invention is used. FIG. 4 is a cross-sectional view for explaining a configuration of a tension roller used in a fixing device according to an embodiment of the present invention. FIG. 3 is a diagram for explaining a comparison result with a conventional structure regarding a temperature distribution in the width direction of the fixing belt when the tension roller having the configuration shown in FIG. 2 is used. FIG. 3 is a diagram for explaining a comparison result with a conventional structure regarding a temperature rise characteristic in a fixing belt when the tension roller having the configuration shown in FIG. 2 is used. It is a figure which shows the prior art example of a fixing device.

Explanation of symbols

2000 Color printer 7 Fixing device 70 Fixing belt 71 Heating roller 71A Halogen heater 72 Fixing roller 73 Tension roller 74 Pressure roller P Transfer paper

Claims (4)

An endless fixing belt that can be brought into contact with a recording medium to be fixed by being wound around a pair of opposed rollers, a means for heating the fixing belt, and a tension roller that contacts the fixing belt and applies tension. A pressing member that forms a fixing nip portion by pressing the fixing belt, and heat and pressure are applied to the unfixed image on the recording medium when the recording medium passes through the fixing nip portion. In a possible fixing device,
The tension roller has anisotropy in thermal conductivity, and the thermal conductivity in the axial direction is higher than the thermal conductivity in the thickness direction. The fixing device according to claim 1.
The fixing device is characterized in that the material having anisotropy in thermal conductivity is fibrous or fiber reinforced resin, and is arranged in parallel with the direction of the fiber and the axial direction of the tension roller. The fixing device according to claim 1 or 2,
A fixing device in which graphite is used as the material having anisotropy in thermal conductivity.   An image forming apparatus using the fixing device according to claim 1.

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