JP2006084646A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of speedily bringing a nonuniform temperature distribution caused in a fixing belt into a uniform temperature distribution. <P>SOLUTION: The image forming apparatus includes a fixing device having the endless fixing belt trained about two rollers and also having a pressure roller for pressing one of the rollers via the fixing belt, and used to fix a toner image on a transfer material in a nip formed between the fixing belt heated by a prescribed fixing means and the pressure roller. The image forming apparatus is characterized by including a first heat pipe whose prescribed arcuate part is in firm contact with the surface of the fixing belt, and a second heat pipe whose prescribed arcuate part is in firm contact with the back of the fixing belt. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine having these functions, and more particularly to a fixing device that equalizes the temperature in the width direction of a fixing belt.

  2. Description of the Related Art In an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine having these functions, a latent image corresponding to an original is formed on a photosensitive member, and toner is applied to the latent image. Thus, the visualized toner image is transferred onto a recording paper (transfer material), and then the toner image transferred onto the recording paper is fixed.

  As a fixing device for fixing such a toner image, a heating roller incorporating a halogen heater or the like and a pressure roller that pressurizes the heating roller are used to heat and hold the recording paper onto which the toner image is transferred. There is a heat roller fixing type fixing device that pressurizes, and such a fixing device is widely used because it has a simple configuration and good fixability on a transfer material. Further, the heating roller and the pressure roller have a structure in which a rubber layer is provided on the surface of the cored bar.

  However, since the rubber layer has a low thermal conductivity, there is a problem that the time for heating the heating roller or the like becomes long and the warm-up time (WUT) becomes long.

  In order to solve this problem, a belt fixing device is used. The belt fixing device has a pressure roller that stretches an endless fixing belt between a heating roller and a fixing roller and presses the fixing roller via the fixing belt, and is heated by a heating unit provided inside the heating roller. The toner image on the transfer material is fixed at a nip formed between the fixing belt and the pressure roller. Since the fixing belt has a small heat capacity, the warm-up time is shortened and energy is saved.

  However, the fixing belt has a small heat capacity, a large temperature drop due to heat radiation, and a low thermal conductivity. For this reason, when recording paper having a width narrower than that of the fixing belt is passed, the temperature rises because heat is not taken away by the recording material at the end of the fixing belt where the recording paper does not pass. This temperature rise is promoted by continuous paper feeding.

  For example, when A4R size recording paper with a narrow paper width is continuously passed, the temperature of the end of the fixing belt through which the recording paper does not pass rises, and then when A4 size recording paper is passed, it is formed at the end. There are problems that uneven glossiness occurs in the toner image, the recording paper becomes wrinkled, and the toner at the end is offset to the fixing belt, so that a good fixed image cannot be obtained. Further, the fixing belt may be deformed due to a temperature difference.

  For this reason, conventionally, means for cooling the end portion of the fixing belt by blowing air from a fan has been used, but this is not preferable because the power consumption becomes large.

Since such a problem also occurs in a heat roller fixing type fixing device, it has been proposed to use a heat pipe to accelerate heat transfer and make the temperature distribution uniform. That is, there is known a heat pipe disposed inside a heating roller or a fixing film (see Patent Documents 1 and 2).
Japanese Patent Laid-Open No. 6-4006 JP 2001-147606 A

  The heat pipe has excellent heat distribution characteristics, and can make the temperature distribution of the object uniform. However, in the above-mentioned Patent Documents 1 and 2, since the heat pipe is in line contact with the heating roller and the fixing film in the rotation axis direction, the effect of the heat pipe is not fully exhibited, and the heating roller and the fixing film It takes time to relax the non-uniform temperature distribution and make it uniform. Therefore, as described above, for example, if A4 size recording paper is passed immediately after passing A4R size recording paper continuously, there is a risk of occurrence of uneven gloss or wrinkles.

  The present invention has been made in view of such a problem, and heat pipes are efficiently pressure-bonded to the front and back surfaces of a fixing belt in a belt fixing device so that a non-uniform temperature distribution generated on the fixing belt can be quickly uniformized. An object of the present invention is to provide an image forming apparatus.

  The above object is achieved by the following means.

  [Claim 1] The fixing belt having an endless fixing belt stretched between two rollers and having a pressure roller that presses one of the rollers through the fixing belt, and heated by a predetermined heating means; In an image forming apparatus including a fixing device that fixes a toner image on a transfer material at a nip formed between the pressure roller and a first heat pipe that is pressure-bonded to the surface of the fixing belt with a predetermined arc. And an image forming apparatus comprising: a second heat pipe that is pressure-bonded to the back surface of the fixing belt with a predetermined arc.

  [Claim 2] Storage means for storing the width dimension of the transfer material based on information from the operation unit for setting the size of the transfer material or information from the transfer material size detection means for detecting the size of the transfer material, and the first A heat pipe position switching means for switching between a position for crimping at least one of the heat pipe and the second heat pipe to the fixing belt and a position for retracting from the fixing belt, and a transfer material stored in the storage means When the width is larger than a predetermined width, the heat pipe position switching means is driven to retract at least one of the first heat pipe or the second heat pipe from the fixing belt, When the width of the transfer material stored in the storage means is smaller than a predetermined width, the first heat pipe is driven by driving the heat pipe position switching means. The image forming apparatus according to fine the second heat pipe to claim 1, characterized in that a, and control means for controlling so as to crimp the fixing belt.

  [Claim 3] The temperature detecting means for detecting the temperature at the center in the width direction and at both ends in the width direction of the fixing belt, and at least one of the first heat pipe or the second heat pipe is fixed to the fixing belt. A heat pipe position switching means for switching between a position where the belt is crimped and a position where the belt is retracted from the fixing belt, and a temperature difference between the width direction central portion and the width direction both ends detected by the temperature detecting means is a predetermined value. If smaller, the heat pipe position switching means is driven to retract at least one of the first heat pipe or the second heat pipe from the fixing belt, and the temperature detection means detects When the temperature difference between the center portion in the width direction of the fixing belt and both end portions in the width direction is larger than a predetermined value, the heat pipe position switching unit is driven to The image forming apparatus according to claim 1, characterized in that one of the heat pipe and the second heat pipes are provided, and control means for controlling so as to crimp the fixing belt.

  [Claim 4] During the warm-up, after completion of a series of image formation under the set conditions, during idling, during low power mode, during recovery from idling or low power mode, and when a jam occurs, the control means 4. The heat pipe position switching unit is driven to retract at least one of the first heat pipe or the second heat pipe from the fixing belt. The image forming apparatus described.

  [Claim 5] The distance between the shafts of the two rollers on which the fixing belt is stretched is changed by the switching operation of the heat pipe position switching means. Image forming apparatus.

  According to the image forming apparatus of the present invention, when there is a possibility that the temperature in the width direction of the fixing belt may be non-uniform or when non-uniform, the heat pipe is pressure-bonded to the front and back surfaces of the fixing belt with a predetermined arc. Therefore, heat is efficiently and quickly transmitted by the heat pipe, and the temperature in the width direction of the fixing belt becomes uniform, resulting in uneven gloss on the recording paper, wrinkles on the recording paper, There is no problem that the toner on the portion is offset to the fixing belt and a good fixed image cannot be obtained.

  Embodiments of the image forming apparatus according to the present invention will be described below with reference to the drawings.

  First, an example of an image forming apparatus will be described based on the configuration diagram of FIG.

  The image forming apparatus includes an image forming apparatus main body GH and an image reading apparatus YS.

  The image forming apparatus main body GH is called a tandem type color image forming apparatus, and a plurality of sets of image forming units 10Y, 10M, 10C, and 10K, a belt-like intermediate transfer member 6, a sheet feeding and conveying unit, a fixing device 9, and the like. Consists of.

  An image reading device YS including an automatic document feeder 201 and a document image scanning exposure device 202 is installed on the upper part of the image forming apparatus main body GH. The document d placed on the document table of the automatic document feeder 201 is transported by a transport unit, and an image on one or both sides of the document is scanned and exposed by the optical system of the document image scanning exposure device 202, and the line image sensor CCD is scanned. Is read.

  A signal formed by photoelectric conversion by the line image sensor CCD is subjected to analog processing, A / D conversion, shading correction, image compression processing, and the like in an image processing unit, and then exposure means 3Y, 3M, 3C, 3K. Sent to.

  In the image forming unit 10Y that forms a yellow (Y) image, a charging unit 2Y, an exposure unit 3Y, a developing device 4Y, and a cleaning unit 8Y are arranged around the photosensitive drum 1Y. In the image forming unit 10M that forms a magenta (M) color image, a charging unit 2M, an exposure unit 3M, a developing device 4M, and a cleaning unit 8M are arranged around the photosensitive drum 1M. In the image forming unit 10C that forms a cyan (C) color image, a charging unit 2C, an exposure unit 3C, a developing device 4C, and a cleaning unit 8C are arranged around the photosensitive drum 1C. In the image forming unit 10K that forms a black (K) image, a charging unit 2K, an exposure unit 3K, a developing device 4K, and a cleaning unit 8K are arranged around the photosensitive drum 1K. The charging unit 2Y and the exposure unit 3Y, the charging unit 2M and the exposure unit 3M, the charging unit 2C and the exposure device 3C, and the charging unit 2K and the exposure device 3K constitute a latent image forming unit.

  The developing devices 4Y, 4M, 4C, and 4K include a two-component developer including yellow (Y), magenta (M), cyan (C), and black (K) toner having a small particle diameter and a carrier.

  The intermediate transfer body 6 is wound around a plurality of rollers and is rotatably supported.

  The fixing device 9 includes a pressure roller 94 that stretches an endless fixing belt 91 between a heating roller 92 and a fixing roller 93 and presses the fixing roller 93 via the fixing belt 91. The toner image on the recording paper (transfer material) P is heated and pressed at a nip formed between the pressure roller 94 and fixed.

  Thus, each color image formed by the image forming units 10Y, 10M, 10C, and 10K is sequentially transferred onto the rotating intermediate transfer body 6 by the transfer means 7Y, 7M, 7C, and 7K (primary transfer). A toner image synthesized with a color image is formed. The recording paper P stored in the paper feeding cassette 20 is fed by the paper feeding means 21 and is conveyed to the transfer means 7A via the paper feeding rollers 22A, 22B, 22C, 22D, the registration rollers 23, etc. A color image is transferred onto P (secondary transfer). The recording paper P to which the color image has been transferred is heated and pressurized by the fixing device 9 and the color toner image on the recording paper P is fixed. Thereafter, the sheet is sandwiched between the sheet discharge rollers 24 and placed on the sheet discharge tray 25 outside the apparatus.

  On the other hand, after the color image is transferred to the recording paper P by the transfer means 7A, the residual toner is removed by the cleaning means 8A from the intermediate transfer body 6 from which the recording paper P is separated by curvature.

  Although the above is an image forming apparatus that forms a color image, an image forming apparatus that forms a monochrome image may be used.

  Next, the fixing device according to the present invention will be described with reference to FIGS.

  2 is a cross-sectional view of the fixing device when passing the recording paper of the maximum width, FIG. 3 is a basic configuration diagram of the heat pipe, and FIG. 4 is a cross-sectional view of the fixing device when passing the narrow recording paper, FIG. 5 is a diagram showing a temperature distribution in the width direction of the fixing belt, FIG. 6 is an enlarged view showing a state in which two heat pipes press the fixing belt, and FIG. 7 is a block diagram of a configuration for switching and controlling the position of the heat pipe. It is.

  First, each member will be described with reference to FIG.

  The fixing belt 91 is formed in an endless shape, for example, a metal belt made of a nickel electroformed belt having an inner diameter of 70 to 120 mm and a thickness of 20 to 80 μm as a base, or a heat resistance made of polyimide or polyamide having a thickness of 40 to 150 μm. Using a resin belt, the outer peripheral surface of the substrate is coated with heat-resistant silicone rubber having a thickness of 100 to 300 μm, and further, the surface is coated with PFA (perfluoroalkoxy) with a thickness of 30 to 50 μm as a release layer. Or cover the tube.

  The heating roller 92 incorporates a halogen lamp 92A as a heating means for heating the fixing belt 91, and the outer peripheral surface of a cylindrical hollow rotating body 92B formed of a metal such as stainless steel or a resin such as polyimide or polyamide is heat-resistant. The PFA tube 92C is coated with a hard roller. The outer diameter is, for example, 25 to 70 mm.

  Note that any heating means may be used as a heating means for heating the fixing belt 91. For example, an induction heating heating element using an exciting coil may be used. Further, the heating unit is not necessarily arranged in the heating roller 92, and may be arranged anywhere.

  A tension spring 95 is hung on a support shaft (not shown) of the heating roller 92, and the heating roller 92 is biased upward.

  The fixing roller 93 is formed of, for example, a silicon rubber foam having a rubber hardness of 10 to 40 Hs (JIS, A rubber hardness), and is configured as a soft roller.

  The pressure roller 94 is provided on a cored bar 94A made of a solid round bar made of stainless steel or the like, and an outer peripheral surface of the cored bar 94A, and is formed of a foam of silicon rubber having a rubber hardness of, for example, 10 to 40 Hs. The rubber roller layer 94B and the release layer 94C in which the outer peripheral surface of the rubber roller layer 94B is coated with a PFA tube are configured as a hard roller. A stainless steel pipe may be used as the core metal 94A, and the rubber hardness of the rubber roller layer 94B may be 40 to 80 Hs.

  When the fixing roller 93 is rotated counterclockwise by a drive source (not shown) configured as described above, the fixing belt 91 and the heating roller 92 are also rotated counterclockwise, and the fixing belt 91 contacting the heating roller 92 is halogenated. Heated by the lamp 92A. Since the pressure roller 94 is urged in the direction of the fixing roller 93 by an urging means (not shown), the nip portion N between the fixing belt 91 wound around the fixing roller 93 and the pressure roller 94. Thus, the recording paper P is heated and pressurized, and the toner image on the recording paper P is fixed.

  A temperature sensor S1 that detects the temperature of the heating roller 92 via the fixing belt 91 and a temperature sensor S2 that detects the temperature of the pressure roller 94 are provided.

  A heat pipe 111 (first heat pipe) is disposed on the surface side of the fixing belt 91, and is rotatably attached to one end of the lever 112. The lever 112 is rotatable around a support shaft 113 provided at the other end, and is biased counterclockwise by a spring (not shown). The lever 112 is connected to a solenoid 115 by a wire 114. When the solenoid 115 is on, the lever 114 is attracted and the lever 112 is rotated clockwise against a spring. Accordingly, the heat pipe 111 is retracted to the right of the figure from the surface of the fixing belt 91.

  Similarly, a heat pipe 121 (second heat pipe) is disposed on the back side of the fixing belt 91, and is rotatably attached to one end of the lever 122. The lever 122 is rotatable around a support shaft 123 provided at the other end, and is urged clockwise by a spring (not shown). The lever 122 is connected to the solenoid 125 by a wire 124. When the solenoid 125 is on, the lever 124 is sucked and the lever 122 is rotated counterclockwise against the spring. Accordingly, the heat pipe 121 is retracted to the left in the drawing from the back surface of the fixing belt 91.

  As described above, the levers 112 and 122 for switching the positions of the heat pipes 111 and 121, the support shafts 113 and 123, the wires 114 and 124, and the solenoids 115 and 125 are referred to as heat pipe position switching means.

  Here, the basic structure of the heat pipe will be described with reference to FIG.

  The heat pipe is obtained by providing a porous material called wick on the inner wall surface of a pipe-shaped container, and filling the container with a small amount of highly volatile working fluid after evacuating the container. When one end of the container is heated, the internal working fluid evaporates, and the generated vapor passes through the central passage through the heat insulating portion and moves to the other end at a low temperature. And the vapor | steam which touched the inner wall surface of a condensation part condenses, and discharge | releases a latent heat. The condensed hydraulic fluid returns to the evaporation section by the capillary force of the wick and is heated again to evaporate. By repeating this process, a large amount of heat can be transferred. The explanation of this heat pipe was referred to the Nikkan Kogyo Shimbun, published by the Association for Practical Education and Training, and the Dictionary of Machine Terms.

  As described above, in the configuration shown in FIG. 2, the condition that the solenoids 115 and 125 are turned on and the heat pipes 111 and 121 are retracted from the fixing belt 91 is that the recording paper P to be fed has the maximum width. There is when using. At this time, since the temperature distribution in the width direction of the fixing belt 91 is uniform, the heat pipes 111 and 121 for making the heat of the fixing belt 91 uniform are unnecessary. If the heat pipes 111 and 121 are brought into contact with the fixing belt 91, the heat of the heated fixing belt 91 is transferred to the heat pipes 111 and 121 in a meaningless manner.

  It should be noted that even if the temperature distribution at the end in the width direction of the fixing belt 91 rises and the temperature distribution becomes slightly non-uniform, if the temperature distribution is such that there is no practical problem, a predetermined width narrower than the maximum width is obtained. Similarly, when the recording paper P is used, the heat pipes 111 and 121 are retracted.

  In addition, during warm-up, after a series of image formations under the set conditions is completed (after a predetermined number of originals are imaged on a predetermined number of recording sheets and discharged to a paper discharge tray), during idling, low power The solenoids 115 and 125 may be turned on and the heat pipes 111 and 121 may be retracted from the fixing belt 91 during the mode, during the return from the idling or low power mode, or when a jam occurs.

  Next, when the width of the recording paper P to be fed is smaller than the maximum width or a predetermined width, the solenoid 115 is turned off. Then, as shown in FIG. 4, the lever 112 is rotated counterclockwise by a spring (not shown), and the heat pipe 111 is pressed against the surface of the fixing belt 91 with a predetermined arc. Similarly, since the solenoid 125 is also turned off, the lever 122 is rotated clockwise by a spring (not shown), and the heat pipe 121 is pressed against the back surface of the fixing belt 91 with a predetermined arc.

  As a result, the temperature of the end portion in the width direction of the fixing belt 91 through which the recording paper P does not pass should rise as shown in FIG. 5A, but the heat in the width direction of the fixing belt 91 is increased by the action of the heat pipes 111 and 121. The temperature distribution becomes uniform as shown in FIG.

  In particular, since the heat pipes 111 and 121 are pressure-bonded to the front and back surfaces of the fixing belt 91 with a predetermined arc, the heat pipes 111 and 121 are in surface contact with the fixing belt 91 and the heat pipes 111 and 121 are in contact with the fixing belt 91, unlike the prior art. The action of moving the heat of 91 to make it uniform is much more efficient.

  When the heat pipes 111 and 121 are pressed against the front and back surfaces of the fixing belt 91 with a predetermined arc, the right side of the fixing belt 91 bends from the state of FIG. 2, so that the heating roller 92 resists the urging force of the tension spring 95. And move downward. That is, the distance between the axes of the heating roller 92 and the fixing roller 93 changes between the state of FIG. 2 and the state of FIG.

  Further, as shown in FIG. 4, it is desirable to arrange the members so that the heat pipes 111 and 121 press each other with the fixing belt 91 interposed therebetween. This is because, in the enlarged view of the heat pipes 111 and 121 in FIG. 6, even if the heat pipes 111 and 121 are separated from each other with the fixing belt 91 interposed therebetween, as shown in FIG. The front and back surfaces of the fixing belt 91 are pressed and contacted with a predetermined circular arc. As shown in FIG. 6B, if the heat pipes 111 and 121 are pressed against each other with the fixing belt 91 interposed therebetween, the heat pipe The arcs when the front and rear surfaces 111 and 121 are pressed against the front and back surfaces of the fixing belt 91 become larger, the contact area increases, and the heat pipes 111 and 121 heat the fixing belt 91 more efficiently.

  Of course, even in the case of FIG. 6A, if the heat pipe 111 is moved to the left and the heat pipe 121 is moved to the right, the arc to be pressure-bonded to the fixing belt 91 becomes large. The movement amount of the heat pipes 111 and 121 is increased, and the reaction force of the fixing belt 91 is increased, which is not preferable, and driving by the solenoids 115 and 125 becomes difficult.

  In the above configuration, in some cases, either the heat pipe 111 or the heat pipe 121 is fixed in a state where the fixing belt 91 is pressed with an arc, and only the other heat pipe 121 or the heat pipe 111 is based on the above-described conditions. You may make it move.

  Further, the solenoids 115 and 125 are turned on in FIG. 2 and turned off in FIG. 4, but the biasing directions of the springs applied to the levers 112 and 122 are reversed so that the solenoids 115 and 125 are turned on / off. The off condition may be reversed.

  Further, the solenoids 115 and 125 are not necessarily used as the heat pipe position switching means, and any mechanism may be adopted depending on the situation. For example, a mechanism for rotating the levers 112 and 122 by rotating the support shafts 113 and 123 by a stepping motor may be used.

  Although the heat pipes 111 and 121 are moved according to the width of the recording paper P as described above, the heat pipes 111 and 121 may be moved by detecting the temperature distribution of the fixing belt. That is, a temperature sensor for detecting the temperature at the center and both ends of the fixing belt 91 is arranged, and a large temperature difference occurs between the center and both ends in the width direction of the fixing belt 91 as shown in FIG. For example, when it is detected that a temperature difference of 30 ° C. or more has occurred, it is necessary to make the heat of the fixing belt 91 uniform. Therefore, as shown in FIG. , 121 is pressed and when the temperature difference is less than 30 ° C., at least one of the heat pipes 111, 121 is retracted from the fixing belt 91 as shown in FIG.

Next, a configuration for switching and controlling the positions of the heat pipes 111 and 121 as shown in FIG. 2 to FIG. 4 will be described based on the block diagram of FIG. Control.

  The operation unit 202 is arranged on the upper part of the image forming apparatus main body GH shown in FIG. 1 (not shown in FIG. 1), and the size of the recording paper P used by the user's operation (for example, A3, B4, A4, A4R, B5 etc.) is selected and set.

  The paper size detecting means (transfer material size detecting means) 203 automatically detects the size of the recording paper P fed from the paper feed cassette 20. For example, the size of the original conveyed to the image reading device YS is detected, and the corresponding recording paper P is selected.

  The operation state detection unit 204 is in a state where the image forming apparatus is operable, in an operation state, during warming up, after a series of image formations under a set condition, during idling, or in a low power mode. Detecting whether the vehicle is returning from idling or low power mode, jamming, and the like.

  The temperature sensor 205 detects the temperatures at the center and both ends of the fixing belt 91.

  A storage unit 206 is a memory for storing the width dimension of the recording paper P and the state of the image forming apparatus.

  The heat pipe position switching unit 207 corresponds to the above-described solenoid or stepping motor that switches the positions of the heat pipes 111 and 121.

  With the above configuration, the paper size of the recording paper P is set by the user in the operation unit 202 or automatically detected by the paper size detection unit 203 and is input to the control unit 201 to determine the width of the recording paper P. The dimensions are stored in the storage means 206. Further, the status of the image forming apparatus detected by the operating state detection unit 204 is also input to the control unit 201 and stored in the storage unit 206. The control unit 201 drives the heat pipe position switching unit 207 according to the paper size stored in the storage unit 206 and the state of the image forming apparatus and the temperature difference of the fixing belt 91 detected by the temperature sensor 105. The positions of 111 and 121 are switched to one of the two states described above.

1 is a configuration diagram of an image forming apparatus. FIG. 3 is a cross-sectional view of a fixing device when a maximum width recording sheet is passed. It is a basic lineblock diagram of a heat pipe. FIG. 3 is a cross-sectional view of a fixing device when a narrow recording sheet is passed. FIG. 6 is a diagram showing a temperature distribution in the width direction of the fixing belt. It is an enlarged view showing a state where two heat pipes press the fixing belt. It is a block diagram of the structure which switches and controls the position of a heat pipe.

Explanation of symbols

DESCRIPTION OF SYMBOLS 9 Fixing apparatus 91 Fixing belt 92 Heating roller 93 Fixing roller 94 Pressure roller 95 Tension spring 111,121 Heat pipe 112,122 Lever 115,125 Solenoid 201 Control means 202 Operation part 203 Paper size detection means 204 Operating state detection means 205 Temperature Sensor 206 Storage means 207 Heat pipe position switching means N Nip part P Recording paper

Claims (5)

An endless fixing belt is stretched between two rollers, and includes a pressure roller that presses one of the rollers through the fixing belt, and the fixing belt heated by a predetermined heating unit, the pressure roller, In an image forming apparatus including a fixing device that fixes a toner image on a transfer material at a nip portion formed between
An image forming apparatus, comprising: a first heat pipe that is crimped to the surface of the fixing belt with a predetermined arc; and a second heat pipe that is crimped to the back surface of the fixing belt with a predetermined arc. Storage means for storing the width dimension of the transfer material according to information from the operation unit for setting the size of the transfer material or information from the transfer material size detection means for detecting the size of the transfer material;
Heat pipe position switching means for switching between a position where one of the first heat pipe and the second heat pipe is pressed against the fixing belt and a position where the first heat pipe is retracted from the fixing belt;
When the width of the transfer material stored in the storage means is larger than a predetermined width, the heat pipe position switching means is driven to at least one of the first heat pipe or the second heat pipe. Is retracted from the fixing belt, and when the width of the transfer material stored in the storage means is smaller than a predetermined width, the heat pipe position switching means is driven to drive the first heat pipe and the second heat pipe. Control means for controlling the heat pipe to be pressure-bonded to the fixing belt;
The image forming apparatus according to claim 1, further comprising: Temperature detecting means for detecting the temperatures of the center portion in the width direction and both ends in the width direction of the fixing belt;
Heat pipe position switching means for switching between a position where one of the first heat pipe and the second heat pipe is pressed against the fixing belt and a position where the first heat pipe is retracted from the fixing belt;
When the temperature difference detected by the temperature detection means between the widthwise central part and the widthwise both ends of the fixing belt is smaller than a predetermined value, the heat pipe position switching means is driven to drive the first heat pipe. Alternatively, at least one of the second heat pipes is withdrawn from the fixing belt, and the temperature difference between the center portion in the width direction and the both ends in the width direction detected by the temperature detecting unit is larger than a predetermined value. When there is, a control means for driving the heat pipe position switching means to control the first heat pipe and the second heat pipe to be pressure-bonded to the fixing belt;
The image forming apparatus according to claim 1, further comprising: During warm-up, after completion of a series of image formation under a set condition, during idling, during low power mode, during recovery from idling or low power mode, and when a jam occurs, the control means is the heat pipe position switching means 4. The image forming apparatus according to claim 1, wherein at least one of the first heat pipe and the second heat pipe is withdrawn from the fixing belt. . 5. The image forming apparatus according to claim 2, wherein a distance between the axes of the two rollers on which the fixing belt is stretched is changed by the switching operation of the heat pipe position switching unit.

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