Image forming device

Abstract

To solve the problem in which when a recording material with a length in a width direction equal to or more than a predetermined width is conveyed to a fixing nip part, steering control to move a fixing belt widely in the width direction is performed, thereby a difference in surface roughness of the fixing belt becomes conspicuous, and gloss unevenness may occur in an image on the recording material after fixing.SOLUTION: An image forming apparatus has a control unit that performs control to swing a steering roller. The control unit controls the steering roller in a first mode and a second mode. When an image area of a recording material conveyed to a fixing nip part in a width direction is smaller than a predetermined width, the control unit performs steering control in the first mode, and when the image area of the recording material conveyed to the fixing nip part is equal to or more than the predetermined width, the control unit performs the steering control in the second mode. The steering roller is controlled such that a fixing belt is moved to a middle position of the steering roller in the width direction in the second mode.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus capable of forming an image on a recording material. [Background technology]

[0002] The image forming apparatus has a fixing device that fixes the unfixed toner image on the recording material to the recording material.

[0003] The fixing device has a pair of rotors, including a fixing belt that applies heat to unfixed toner and is driven to rotate, and a pressure rotor that applies pressure to the fixing belt to form a nip between the fixing belt and the fixing belt and is driven to rotate. When a recording material with unfixed toner on it is conveyed through the nip, the heat of the fixing belt and the pressure of the pressure rotor are applied to the recording material, and the unfixed toner is fixed to the recording material.

[0004] Patent Document 1 discloses a steering control that moves a fixing belt back and forth in the width direction. By repeatedly moving the fixing belt back and forth within a predetermined area, it is possible to prevent the fixing belt from coming off the steering roller. It is also possible to prevent the edge of the recording material from repeatedly passing through the same area of the fixing belt. As a result, it is possible to prevent deterioration of the fixing belt surface.

[0005] Steering control can reduce scratches on the fixing belt surface caused by the edges of the recording material. However, repeated contact between the edges of the recording material and the fixing belt can cause dents on the fixing belt surface. The unevenness of the fixing belt surface is reflected in the gloss after fixing. Therefore, if fixing is performed while the fixing belt surface is dented, the gloss of the area where the dent is fixed will be different from that of the other areas, resulting in a line-like gloss unevenness. Therefore, a polishing roller is used to polish the fixing belt surface to make the gloss unevenness caused by the edges of the recording material less noticeable. Abrasive grains are densely adhered to the surface of the polishing roller, and the polishing roller polishes the surface of the fixing belt when it comes into contact with the surface. Patent Document 2 discloses a technology that uniforms the surface roughness of the fixing belt and reduces gloss unevenness in the fixed image. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2015-59964 [Patent Document 2] Patent Publication No. 2008-040363 Summary of the Invention [Problem to be solved by the invention]

[0007] In a configuration using a polishing member that is a polishing roller, a fixing belt, and a steering roller, the polishing member is shorter than the fixing belt in the width direction of the fixing belt, so there are areas on the fixing belt that can be polished by the polishing member and areas that cannot be polished by the polishing member.

[0008] Furthermore, steering control is performed while the abrasive member is abrading the fixing belt. Although steering control increases the area that the abrasive member can abrade, it also creates areas where the abrasion frequency is high and areas where the abrasion frequency is low.

[0009] On the other hand, when a recording material is passed through the fixing nip portion, steering control is performed to move the fixing belt over a wide range in the width direction in order to prevent damage to the edge.

[0010] Conventionally, even when a recording material with a large width is transported to the fixing nip, steering control is performed to move the fixing belt over a wide range in the width direction. As a result, the frequency of edge scratches increases in areas with low abrasion frequency, resulting in a significant difference in surface roughness between areas with high and low abrasion frequency. When a recording material with a large width is then transported to the fixing nip, fixing is performed by areas of the fixing belt with high and low abrasion frequency, which can cause gloss unevenness on the image surface.

[0011] Therefore, it is necessary to appropriately control the steering when a recording material having a large width is conveyed to the fixing nip portion. [Means for solving the problem]

[0012] In order to solve the above problems, the image forming apparatus of the present invention comprises a rotatable endless fixing belt, a heating roller that contacts the inner peripheral surface of the fixing belt and applies heat to the fixing belt, a steering roller that contacts the inner peripheral surface of the fixing belt together with the heating roller, a pressure rotating body that presses the fixing belt to form a fixing nip portion, and nip-conveys a recording material carrying unfixed toner into the fixing nip portion to fix the unfixed toner image onto the recording material, and a polishing member that contacts the surface of the fixing belt and polishes the fixing belt. the grinding member is movable between a position in contact with the fixing belt and a position separated from the fixing belt, and the grinding member is located at a position separated from the fixing belt when a recording material is transported to the fixing nip portion; the device is equipped with a belt position detection unit that detects the position of the fixing belt in a width direction of the fixing belt, and a control unit that controls to swing the steering roller so as to move the fixing belt to a predetermined position in the width direction based on the detection result of the belt position detection unit, the belt position detection unit can detect that the fixing belt is at a first predetermined position and a second predetermined position in the width direction, and in the width direction, a center position of the fixing belt at the first predetermined position and the second predetermined position is located on one end side of the steering roller with respect to the center of the steering roller, and the center position of the fixing belt at the first predetermined position is located on the one end side of the center position of the fixing belt at the second predetermined position, and the control unit controls the steering roller in a mode selected from a plurality of modes including a first mode and a second mode an area on the recording material where toner can be carried is defined as an image area; when the widthwise length of the image area of the recording material conveyed to the fixing nip portion is smaller than a predetermined width, the control unit controls the steering roller in the first mode; in the first mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit performs an operation to tilt the steering roller in a predetermined first direction; and when it is detected that the fixing belt is positioned at the second predetermined position, the control unit does not perform an operation to tilt the steering roller; when the widthwise length of the image area of the recording material conveyed to the fixing nip portion is equal to or greater than a predetermined width, the control unit controls the steering roller in the second mode; and in the second mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit performs an operation to tilt the steering roller in the first direction; and when it is detected that the fixing belt is positioned at the second predetermined position, the control unit performs an operation to tilt the steering roller in the first direction. [Effects of the Invention]

[0013] It is possible to suppress uneven gloss that occurs when fixing a recording material that is large in the width direction. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus. [Figure 2] FIG. [Figure 3] FIG. 2 is a schematic diagram showing a steering mechanism. [Figure 4] FIG. 2 is a schematic diagram showing a sensor unit that detects the position of a fixing belt. [Figure 5] FIG. 2 is a block diagram illustrating a control unit. [Figure 6] FIG. 2 is a schematic diagram showing a belt position detection unit for detecting the position of a fixing belt. [Figure 7] 4 is a schematic diagram showing the position of the fixing belt in the width direction at one end side of the fixing belt. FIG. [Figure 8] 10 is a flowchart showing steering control in a first mode. [Figure 9] FIG. 2 is a diagram showing the inclination angles (A and −A) of the steering roller. [Figure 10] 10 is a schematic diagram showing the other end of the fixing belt when the steering control is performed in the first mode when fixing the recording material S. FIG. [Figure 11] 10 is a flowchart showing steering control in a second mode. [Figure 12] FIG. 10 is a diagram showing the inclination angles (B and −B) of the steering roller. [Figure 13] FIG. 10 is a diagram showing the inclination angles (C and −C) of the steering roller. [Figure 14] 10 is a table showing the relationship between the fixing belt position and the substituted value in the first mode. [Figure 15]10 is a schematic diagram showing the other end of the fixing belt when the steering control is performed in the second mode when fixing the recording material S. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] <Image forming device> Fig. 1 is a schematic diagram showing the configuration of an image forming apparatus 100. As shown in Fig. 1, the image forming apparatus 100 has four image forming units for yellow, magenta, cyan, and black arranged along the direction of movement of an intermediate transfer belt 6. First, the process of forming a toner image on the intermediate transfer belt 6 will be explained using the yellow image forming unit PY as an example.

[0016] The surface of the photosensitive drum 3, which is rotated by the charger 2, is uniformly charged (charging). Then, the exposure device 5 irradiates the surface of the photosensitive drum 3 with a laser in accordance with input image data, forming an electrostatic latent image on the surface of the photosensitive drum 3 (exposure). Then, the development device 1 forms a yellow toner image on the photosensitive drum (development). The primary transfer roller 24 applies a voltage of the opposite polarity to the potential polarity of the yellow toner image to the intermediate transfer belt 6. This transfers the yellow toner on the photosensitive drum 3 to the intermediate transfer belt 6 (primary transfer). Any remaining yellow toner on the photosensitive drum surface is scraped off by the toner cleaner 4 and removed from the surface of the photosensitive drum 3. This series of processes is repeated for magenta PM, cyan PC, and black PK. As a result, a full-color toner image is formed on the intermediate transfer belt 6.

[0017] The toner image on the intermediate transfer belt 6 is transported to a secondary transfer portion n2 formed by a pair of secondary transfer rollers 11 and 14. In synchronization with the transport of the toner image, recording material A is taken out one by one from a recording material cassette 10 and fed to the secondary transfer portion n2. Then, the toner image on the intermediate transfer belt 6 is transferred onto the recording material A (secondary transfer).

[0018] The recording material A onto which the toner image has been transferred is transported to the fixing device 30, where it is fixed by heat and pressure (fixing). The recording material A onto which the toner image has been fixed is discharged onto the discharge tray 8.

[0019] Image forming apparatus 100 can also form monochrome images. When forming monochrome images, only the black image forming station PK is driven among the multiple image forming stations.

[0020] Now, we will explain double-sided printing, in which images are formed on both sides of a recording material. After recording material A with an image formed on one side is discharged from fixing device 30, it is guided to paper path 18 by flapper 7. When recording material A is transported from paper path 18 to reversing path 19, it is switched back and transported on reversing path 19. After that, recording material A passes through double-sided path 20 and is transported to paper path 21. At this time, recording material A is in an inverted state. After that, recording material A is transported again to secondary transfer section n2, where a toner image is transferred and fixed by fixing device 30. Then, recording material A with double-sided printing performed is discharged to output tray 8.

[0021] This process, which starts from charging and ends when the recording material A with the fixed toner image is discharged onto the discharge tray 8, is called an image forming process (print job). The period during which image formation is being performed is called an image forming process (print job).

[0022] <Fixing device> Next, the fixing device 30 of this embodiment will be described with reference to FIG.

[0023] This embodiment employs a fixing device that uses an endless fixing belt 310. In Fig. 2, the recording material is conveyed in the direction indicated by the arrow α. The fixing device 30 has a heating rotor 300 that has the fixing belt 310, and a pressure rotor 330 that contacts the fixing belt 310 and applies pressure to it, thereby forming a nip N with the fixing belt 310.

[0024] The heating rotator 300 has a fixing belt 310, a steering roller 350, a fixing pad 380 which is a pad member, and a heating roller 340. The fixing pad 380 and the heating roller 340 are in contact with the inner circumferential surface of the fixing belt. The fixing belt 310 is stretched between the fixing pad 380 and the heating roller 340.

[0025] The heating roller 340 is cylindrically formed from a metal such as aluminum or stainless steel. In this embodiment, it is formed from an aluminum pipe with an outer diameter of 80 mm. A halogen heater 341 is installed inside the heating roller 340 as a means for heating the fixing belt 310. The halogen heater 341 heats the heating roller 340 to a predetermined temperature. The heating roller 340, heated by the heat of the halogen heater 341, heats the fixing belt 310. The fixing belt 310 is controlled to a predetermined target temperature according to the basis weight of the recording material to be fixed, based on the temperature detection result by a fixing temperature detection sensor (not shown). Note that the heating means is not limited to a halogen heater, and may be configured to generate heat by electromagnetic induction heating (IH), for example. The heating roller 340 is driven to rotate in the direction of arrow R1 by being driven by a drive motor M1.

[0026] The fixing belt 310 has excellent thermal conductivity and heat resistance and is, for example, a thin, endless belt with an inner diameter of 120 mm. In this embodiment, the fixing belt 310 has a three-layer structure consisting of a base layer, an elastic layer on the outside of the base layer, and a release layer on the outside of the elastic layer. The base layer is 60 μm thick and made of polyimide resin (PI), the elastic layer is 300 μm thick and made of silicone rubber, and the release layer is 30 μm thick and made of PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) as a fluororesin. The fixing belt is rotated by the pressure rotor 330 (described later) pressing the fixing pad 380 via the fixing belt 310 and rotating it. Furthermore, since the heating roller 340 is rotated by receiving drive from the drive motor M1, the fixing belt 310 is also rotated by the rotation of the heating roller 340.

[0027] Fixing pad 380 is disposed on the inner circumferential surface of fixing belt 310 so as to face pressure rotor 330 with fixing belt 310 sandwiched therebetween. In this embodiment, a lubricant such as a lubricating sheet containing silicone oil or silicone oil is interposed between fixing pad 380 and fixing belt 310 to enable fixing belt 310 and fixing pad 380 to slide smoothly against each other. For this reason, oil such as silicone oil is applied to the inner circumferential surface of fixing belt 310.

[0028] The pressure rotor 330 has a cylindrical aluminum core, an elastic layer with a thickness of 1 mm on the outside of the core, and a release layer for enhancing separation from the toner on the outside of the elastic layer.

[0029] Furthermore, pressure rotating body 330 is rotated in the direction of arrow R2. Therefore, fixing belt 310 sandwiched between pressure rotating body 330 and fixing pad 380 is rotated in response to the rotation of pressure rotating body 330.

[0030] The pressure rotor 330 can be moved by a contact / separation mechanism that moves the pressure rotor 330 so that it can contact or separate from the fixing belt 310. The contact / separation mechanism includes a frame 385 and a drive motor (not shown). The frame 385 is supported by the image forming apparatus 100. The frame 385 supports the pressure rotor 330. The frame 385 is rotated by receiving drive force from a drive motor (not shown) around a rotation axis 332. When the drive motor (not shown) rotates the frame 385 around the rotation axis 332 clockwise on the paper, the pressure rotor 330 moves in the direction of arrow P. As a result, the pressure rotor 330 contacts the fixing pad 380 with the fixing belt 310 sandwiched between them (contact state) in a direction perpendicular to the recording material conveyance direction α. This forms a fixing nip N. In this embodiment, a total pressure of 2000 N is applied, and the width of the fixing nip N is 24 mm. When the frame 385 is rotated counterclockwise on the paper surface about the rotation axis 332, the pressure rotator 330 is separated from the fixing belt 310 (separated state).

[0031] As explained above, the recording material carrying the unfixed toner image is sandwiched and conveyed by the heating rotor 300 and the pressure rotor 330 in the fixing nip N, where heat and pressure are applied to fix the toner image.

[0032] <Steering roller> Next, the steering roller 350 in this embodiment will be described with reference to FIGS.

[0033] In this embodiment, a force of 2000 N is applied to the fixing belt 310 in the contact state. Therefore, the surface of the fixing belt 310 may be damaged by the edge of the recording material, which may cause uneven gloss. This will be described in detail below.

[0034] <Uneven gloss due to scratches on the paper edge> Paper edge scratches are scratches on the surface of the fixing belt caused by a scrap of recording material (edge) coming into contact with the surface of the fixing belt. When fixing unfixed toner to the recording material, the part of the fixing belt 310 that comes into contact with the edge (edge contact part) is subjected to greater stress than the part that does not come into contact with the edge (edge non-contact part). The area scratched by the edge of the recording material ends up with a more concave shape than the edge non-contact part. These indentations on the surface of the fixing belt 310 caused by the edge of the recording material are called paper edge scratches.

[0035] When fixing unfixed toner to the recording material, the fixing device 30 applies pressure and heat to the recording material. At this time, the surface condition of the fixing belt 310 is reflected in the gloss of the fixed image surface. If the surface of the fixing belt 310 is uneven, the uneven condition is reflected in the gloss of the image surface, resulting in gloss unevenness (uneven gloss) on the image surface. Therefore, if unfixed toner is fixed to the recording material with scratches on the fixing belt surface, gloss unevenness like a straight line will appear on the image surface.

[0036] In this embodiment, in order to prevent scratches on the paper edge of the surface of the fixing belt 310, steering control by a steering mechanism 400 is used, which moves the fixing belt 310 back and forth in the width direction of the fixing belt 310.

[0037] The steering control will be explained with reference to FIG.

[0038] 3, the steering mechanism 400 has a steering roller 350, a steering motor 401, a worm 402, a worm wheel 403, and a fork plate 404. The steering motor 401 is rotatable in both forward and reverse directions. When the steering motor 401 receives a signal from the control unit 600 and is driven to rotate, the worm 402 attached to the steering motor 401 is rotated.

[0039] The rotation of the worm 402 is converted into oscillation in the direction of the rotation axis of the steering motor 401 around the rotation shaft 405 as an oscillation center by a drive converting unit 410 in which the worm wheel 403 and the fork plate 404 are integrally formed. That is, the worm wheel 403 is meshed with the worm 402 and is provided so as to be able to move back and forth in the direction of the rotation axis of the steering motor 401 in accordance with the rotation of the worm 402. To achieve this, the meshing surface of the worm wheel 403 is formed in an arc shape so as to mesh with the worm 402 at the center in the direction of the rotation axis. In this way, the drive converting unit 410 can oscillate around the rotation shaft 405 as an oscillation center via the worm 402 and the worm wheel 403 in accordance with the rotation of the steering motor 401.

[0040] The steering mechanism 400 also has a steering operation shaft 406, a steering roller support arm 351, and a bearing unit 352. The steering operation shaft 406, the steering roller support arm 351, and the bearing unit 352 are integrally formed and attached to the steering roller 350. The bearing unit 352 rotatably supports the rotation shaft of the steering roller 350. The steering roller support arm 351 is rotatably provided, and rotatably supports the steering roller 350 by holding the bearing unit 352.

[0041] A steering operation shaft 406, which is fitted to the drive converter 410, is fixed to the steering roller support arm 351. The steering operation shaft 406 is fitted to the fork plate 404 of the drive converter 410 and can move together with the drive converter 410 while remaining fitted to the drive converter 410. In this way, the inclination of the steering roller 350 changes in conjunction with the swing of the drive converter 410. That is, by driving the steering motor 401, the angle of the steering roller 350 relative to the heating roller 340 (see FIG. 2) can be changed. When the steering angle of the steering roller 350 is adjusted in this way, the fixing belt 310, which is stretched between the steering roller 350 and the heating roller 340, rotates in the R1 direction, and the fixing belt 310 moves back and forth in a meandering pattern in the width direction while rotating in the R1 direction. This allows for steering control of the fixing belt 310, which moves the fixing belt 310 back and forth within a predetermined range in the width direction. The fixing belt 310 moves back and forth in opposite directions when the steering motor 401 is rotated forward to tilt the steering roller 350 and when the steering motor 401 is rotated backward to tilt the steering roller 350.

[0042] In this way, the steering mechanism 400 reciprocates the fixing belt 310 in the width direction to a predetermined position within the area of the steering roller 350. The reciprocating movement of the fixing belt 310 prevents the edge of the recording material from repeatedly passing over the same area on the surface of the fixing belt 310. This prevents scratches on the edge of the paper from occurring on the surface of the fixing belt 310.

[0043] <Polishing mechanism> As mentioned above, the edges of the recording material can cause unevenness on the surface of the fixing belt 310. Therefore, the fixing device 30 of this embodiment is equipped with a polishing mechanism to smooth out the uneven surface roughness of the fixing belt 310. The polishing mechanism will be described below.

[0044] The polishing mechanism includes polishing roller 420, which is a polishing member, and polishing member contact / separation mechanism 421. Polishing roller 420 has a metal core, such as stainless steel, with a diameter of 12 mm. Abrasive grains are densely adhered to the surface of the core, and each grain is 3 μm to 16 μm in size. The abrasive grains used here are made of alumina. Therefore, the abrasive grains are harder than the surface layer of fixing belt 310 and are suitable for polishing the surface of fixing belt 310. In this way, polishing roller 420 is formed by densely adhering alumina abrasive grains to the surface of the metal core. However, polishing roller 420 does not have to have abrasive grains. The abrasive grain portion may be made of metal with a roughened and uneven surface, thereby forming a polishing member in which the cylindrical roller portion and the portion that performs polishing are integrally formed.

[0045] The polishing roller 420 is disposed so that its center in the width direction coincides with the center position of the steering roller 350. However, since there is considerable variation between individual products, the center in the width direction of the polishing roller 420 and the center position of the steering roller 350 may be slightly misaligned. In this embodiment, the length of the polishing roller 420 in the width direction of the fixing belt 310 is shorter than that of the fixing belt 310. Even when the fixing belt 310 is moved in the width direction by steering control, the end of the fixing belt 310 in the width direction does not come into contact with the polishing roller 420. This is to prevent oil applied to the inner surface of the fixing belt 310 from adhering to the polishing roller 420 due to the end of the fixing belt 310 coming into contact with the polishing roller 420. If oil applied to the inner surface of the fixing belt 310 adheres to the polishing roller 420, the oil is transferred from the polishing roller 420 to the surface of the fixing belt 310. If an area of the surface of the fixing belt 310 with oil on it is used to fix a recording material, the oil may transfer to the recording material, causing image defects. For this reason, the length of the polishing roller 420 in the width direction is made shorter than the length of the fixing belt 310 minus the width of the deviation control. As a result, even if the fixing belt 310 is moved in the width direction by steering control, the end of the fixing belt 310 in the width direction does not come into contact with the polishing roller 420, and image defects caused by oil can be suppressed.

[0046] The polishing roller 420 presses against the heating roller 340 via the fixing belt 310. The polishing roller 420 can be moved by a polishing member contact / separation mechanism 421 between a contact state in which the polishing roller 420 contacts the fixing belt 310 and a separated state in which the polishing roller 420 is separated from the fixing belt 310. The polishing member contact / separation mechanism 421 is driven by a motor (not shown). When the polishing roller 420 contacts the fixing belt 310 to polish it, the polishing member contact / separation mechanism 421 rotates in the direction of arrow B in FIG. 2 with 421a as the rotation axis. As a result, the polishing roller 420 contacts the fixing belt 310 and is rotated by the motor (not shown). The polishing roller 420 rotates in the direction opposite to the rotation direction R2 of the fixing belt 310 in FIG. 2. As a result, the surface of the fixing belt 310 is polished. The term "polishing" as used herein means that the surface roughness Rz of the area of fixing belt 310 polished by polishing roller 420 is 0.5 μm or more and 2.0 μm or less. The time when polishing roller 420 is polishing the surface of fixing belt 310 is considered to be in the process of polishing.

[0047] When polishing by polishing roller 420 is not being performed, polishing member contact / separation mechanism 421 separates polishing roller 420 from fixing belt 310. When separating, polishing member contact / separation mechanism 421 rotates around rotation axis 421a in the direction opposite to the arrow in FIG. 2. This separates polishing roller 420 from fixing belt 310.

[0048] In this embodiment, when a recording material is transported through the fixing nip N, the polishing roller 420 is separated from the fixing belt 310 and does not polish. This is to prevent the polishing roller 420 from being contaminated with offset toner. When a recording material is transported through the fixing nip N, toner on the recording material may not be fixed to the recording material and may adhere to the surface of the fixing belt 310. The toner adhering to the surface of the fixing belt 310 is called offset toner. The offset toner is transported downstream of the fixing nip N in the rotation direction of the fixing belt 310. If the polishing member is in contact with the fixing belt 310 when the recording material is transported through the fixing nip N, the offset toner will adhere to the polishing roller 420 and get into the gaps between the abrasive grains on the surface of the polishing roller 420. This reduces the polishing efficiency of the surface of the fixing belt 310. Therefore, when a recording material is transported through the fixing nip N, the polishing roller 420 is separated from the fixing belt 310.

[0049] Furthermore, when a recording material is being transported to the fixing nip portion N, the abrasive member contact / separation mechanism 421 does not cause the abrasive roller 420 to contact the fixing belt 310. If the abrasive roller 420 comes into contact with the fixing belt 310 while a recording material is being transported to the fixing nip portion N, the impact caused by the contact is transmitted to the fixing nip portion N. The impact caused by the contact may cause the toner on the recording material to shift and be fixed, resulting in a defective image. To prevent this, when a recording material is being transported to the fixing nip portion N, the abrasive member contact / separation mechanism 421 does not cause the abrasive roller 420 to contact the fixing belt 310.

[0050] The timing of polishing will be explained. First, when image formation is performed, the control unit 600 counts the number of recording materials of each width size that have passed through the fixing nip N. After that, the recording materials are no longer conveyed to the fixing nip N, and image formation ends. If the counted number of recording materials reaches a predetermined number (e.g., 500 sheets for A4 paper) or more at the end of image formation, polishing is performed after image formation ends. This polishing is performed for a period corresponding to the number of counted recording materials (approximately 10 seconds for 500 sheets of A4 paper). In other words, the more sheets of recording material are counted, the longer the polishing time. Once polishing is performed, the count of the number of recording materials is reset. During polishing, the fixing belt 310 is polished at least once. By polishing according to the count, polishing can be performed according to the surface roughness of the fixing belt 310, preventing excessive or insufficient polishing. If the number of counted sheets of recording material at the end of image formation does not reach a predetermined number (e.g., 500 sheets for A4 paper), polishing is not performed. In this case, the counted number of sheets of recording material is carried over to the next image formation. If multiple image formations for the same job are performed continuously for more than an hour without interruption (e.g., more than 6,000 sheets for A4 paper), one hour after image formation begins, the recording material is not transported to the fixing nip N, image formation is interrupted, and polishing is performed. Polishing is performed for approximately one minute. After polishing, the count of the number of sheets of recording material is reset. Edge scratches on the surface of the fixing belt 310 may progress each time the recording material passes through the fixing nip N. If image formation is performed for a long period of time, such as more than an hour, the edge scratches may progress too much by the time image formation is completed. If the edge scratches progress too much, polishing by the polishing roller 420 may not be able to uniform the surface roughness of the fixing belt 310. Therefore, when multiple image formations of the same job are performed for one hour or more, the surface roughness of the fixing belt 310 can be kept uniform by polishing the belt every hour.

[0051] The case where image formation is interrupted refers to the case where the rotation of the photosensitive drum 3 stops and no image formation is performed.

[0052] <Fixing belt position detection> 2, 3, and 4, a belt position detector for detecting the position of fixing belt 310 in the width direction will be described.

[0053] In this embodiment, a sensor unit 390 is provided to detect the position of the end of the fixing belt 310 in the width direction. The position of the end of the fixing belt 310 is detected based on the output signal of this sensor unit 390. The steering mechanism 400 described above is operated based on the detected end position of the fixing belt 310, thereby changing the tilt angle of the steering roller 350. The configuration of the sensor unit 390 will be described with reference to FIG. 4.

[0054] 4, the sensor unit 390 of this embodiment includes a contact member 391 that contacts the end of the fixing belt 310, an arm member 392 that supports the contact member 391, a belt position detection unit 393 that serves as a moving member, and three sensors 394, 395, and 396 that detect the positions of the end of the fixing belt 310. The sensors 394, 395, and 396 that serve as the belt position detection units may be optical sensors, for example. The contact member 391 is disposed on one end side of the arm member 392 so as to contact the end in the width direction.

[0055] Arm member 392 is biased by a coil spring (not shown) from the end portion of fixing belt 310 toward the center portion in the width direction. Arm member 392 is rotatably provided via a contact member 391 so as to follow the movement in the width direction. A belt position detection unit 393 serving as a moving member is provided on the other end side of arm member 392. Belt position detection unit 393 is, for example, a fan-shaped columnar member, and has a plurality of openings 393a and a plurality of detection target portions 393b formed on its arc-shaped outer peripheral surface. Three sensors 394, 395, and 396 are arranged at predetermined intervals along the rotational movement direction of belt position detection unit 393 so as to face the outer peripheral surface on which openings 393a and detection target portions 393b are formed relative to belt position detection unit 393.

[0056] In this embodiment, when fixing belt 310 moves from one end to the other end in the width direction, belt position detection unit 393 rotates in accordance with the movement of fixing belt 310. As belt position detection unit 393 rotates, the positional relationship between sensors 394, 395, and 396 and detection target portion 393b (or opening 393a) changes. Specifically, a detection state in which sensors 394, 395, and 396 detect detection target portion 393b is switched to a non-detection state in which sensors 394, 395, and 396 do not detect detection target portion 393b because they face opening 393a.

[0057] In this embodiment, optical sensors are used for the sensors 394, 395, and 396. The sensors 394, 395, and 396 each have a light-emitting unit that emits light and a light-receiving unit that receives reflected light emitted from the light-emitting unit. The sensors 394, 395, and 396 emit a predetermined amount of light toward the belt position detection unit 393 using the light-emitting unit. If the emitted light is blocked by the detection target portion 393b of the belt position detection unit 393, the light-receiving units of the sensors 394, 395, and 396 do not receive the light emitted from the light-emitting unit. On the other hand, if the emitted light is not blocked by the opening 393a of the belt position detection unit 393, the light-receiving units receive the light emitted from the light-emitting unit. In this way, whether or not each of the sensors 394, 395, and 396 receives light is determined according to the movement of the belt position detection unit 393.

[0058] <Control unit> As shown in Fig. 1, image forming apparatus 100 includes a control unit 600. The control unit 600 will be described using Fig. 5 with reference to Figs. 2 to 4. However, in addition to those shown in the figure, various devices such as motors and power supplies for operating image forming apparatus 100 are connected to control unit 600, but illustration and description of these devices will be omitted here as they are not the main focus of the invention.

[0059] The control unit 600 as a control means performs various controls such as image forming operations, and includes, for example, a CPU 601 (Central Processing Unit) and a memory 602. The memory 602 is configured with a ROM (Read Only Memory), a RAM (Random Access Memory), etc. The memory 602 stores various programs and various data for controlling the image forming apparatus 100. The CPU 601 can execute various programs stored in the memory 602, and can operate the image forming apparatus 100 by executing the various programs.

[0060] In this embodiment, the CPU 601 can execute "image formation job processing (program)" stored in the memory 602, "steering control" to be described later, and the like.

[0061] The memory 602 stores, for example, a "sensor value table" that is referenced to identify the end position of the fixing belt 310 that is reciprocated by steering control during the belt deviation control process, and to determine whether or not there is a malfunction in the sensor unit 390. The memory 602 can also temporarily store the results of calculations that accompany the execution of various programs.

[0062] The control unit 600 is connected to the operation unit 40 via an input / output interface. The operation unit 40 has, for example, a touch panel type liquid crystal screen (display unit) that enables the user to issue instructions to start various programs such as image formation job processing, and input various data such as the size of the recording material (A3, B4, etc.).

[0063] The LCD screen can display various screens including software keys, and various functions, such as instructions to start various pre-assigned programs, can be executed in response to a user's touch operation of the software keys. The LCD screen can also display various information, such as the operating status of the image forming apparatus and error information, to notify the user. That is, in this embodiment, the operation unit 40 can function as a notification means. Notification of various information, such as error information, is not limited to the above-described display notification method, but may be any other appropriate method, such as sound notification using a sound generating means such as a speaker.

[0064] It is also possible to input the widthwise length of the recording material from the operation unit 40. The information on the widthwise length of the recording material input from the operation unit 40 is sent to the control unit 600. Based on the sent information, steering control is changed depending on the widthwise length of the recording material, as will be described later. The means for sending the information on the widthwise length of the recording material to the control unit is not limited to information input from the operation unit 40. For example, the length in the widthwise direction of the recording material may be calculated by a means such as a recording material presence / absence detection sensor arranged in the widthwise direction (not shown).

[0065] The control unit 600 is further connected to the drive motor M1, steering motor 401, temperature sensor 370, halogen heater 341, sensor unit 390, position sensor 407, and a motor that drives the pressure rotor 330 via an input / output interface. When an instruction to start an image formation job is issued from the operation unit 40, the control unit 600 (more specifically, the CPU 601) executes an "image formation job" stored in memory 602. The control unit 600 controls the image forming apparatus 100 based on the execution of the "image formation job." Accordingly, the control unit 600 drives the drive motor M1 to rotate the heating roller 340, thereby rotating the fixing belt 310. Furthermore, the control unit 600 controls the halogen heater 341 based on the detection result of the temperature sensor 370 so that the surface temperature of the fixing belt 310 reaches a desired target temperature (180°C in this embodiment). Since the control unit 600 controls the motor that drives the pressure rotating body 330, it is also possible to determine whether the pressure rotating body 330 is in contact with or separated from the fixing belt 310.

[0066] In this embodiment, the control unit 600 controls the steering motor 401 based on the detection results of the sensor unit 390, specifically, based on a combination of output signals from three sensors 394, 395, and 396 (see FIG. 6(b) described later). That is, the control unit 600 detects the end position of the fixing belt 310 based on the detection results of the sensor unit 390, and rotates the steering motor 401 forward or reverse according to the rotation amount determined accordingly. In this way, the control unit 600 can operate the steering mechanism 400 described above using the steering motor 401 to steer the fixing belt 310.

[0067] <Belt position detection unit> The belt position detection unit 393 will be described with reference to FIGS. 6(a) and 6(b). FIG. 6(a) is a top view illustrating the belt position detection unit 393, and FIG. 6(b) shows the combinations of output signals from sensors 394, 395, and 396 when the belt position detection unit 393 is used. The 27 regions in FIG. 6(a) are used when three sensors (394, 395, and 396) detect nine positions of the fixing belt 310 in the width direction. For example, when sensors 394, 395, and 396 are in a detection state detecting detection target portions 393b1 to 393b5, or in other words, when sensors 394, 395, and 396 are in a shielded state shielded by detection target portions 393b1 to 393b5, they output an output signal of "0." On the other hand, when each sensor 394, 395, 396 is in a non-detection state where it is not detecting the detection target portions 393b1 to 393b5, in other words, when it is in an open state (also called a non-shielded state) facing the openings 393a1 to 393a4, it outputs an output signal "1".

[0068] 6(b), the "first sensor" refers to sensor 394, the "second sensor" refers to sensor 395, and the "third sensor" refers to sensor 396, and the belt position is a value determined by a combination of the output signals of sensors 394, 395, and 396. In this embodiment, control unit 600 can detect the end position of fixing belt 310 at nine subdivided positions according to the belt position determined by a combination of the output signals (0 or 1) of sensors 394, 395, and 396.

[0069] The nine positions of the end of the fixing belt 310 are described below with reference to FIG. 7. FIG. 7 illustrates one end of the fixing belt 310, viewed from the conveyance direction α, with the pressure rotor 330 positioned below. The opposite side of the width from the one end is the other end. The detectable positions are a "first close-up position" where the fixing belt 310 moves to the one end as far as possible, a "second close-up position" where the fixing belt 310 moves to the other end as far as possible, and seven positions equally spaced apart between the "first close-up position" and the "second close-up position." The seven positions, in order from closest to the "first close-up position," are "front 3," "front 2," "front 1," "center," "rear 1," "rear 2," and "rear 3." The "front" and "rear" positions here refer to the side of the image forming device where the operation unit 40 is located, and the opposite side, the "rear" side.

[0070] In this embodiment, the detection target portions 393b1 to 393b5 are arranged so that two or more of the sensors 394, 395, and 396 are simultaneously in the detection state (0) or the non-detection state (1) depending on the movement position of the sensor flag 393. The detection target portions 393b1 to 393b5 are also arranged so that all of the sensors 394, 395, and 396 are in the detection state (or the non-detection state) when the fixing belt 310 is in the "first close-in" position or the "second close-in" position.

[0071] In this embodiment, the "front 3" position is the first predetermined position, the "front 1" position is the second predetermined position, and the "front 2" position is the third predetermined position. The "front 2" position is located closer to the "center" position than the "front 3" position. The "front 1" position is located closer to the "center" position than the "front 2" position.

[0072] When the fixing belt 310 is in the "middle" position, this indicates that the center position of the fixing belt 310 in the width direction is at the center position of the range of movement of the fixing belt or at the center position of the steering roller 350. When the fixing belt 310 is in the "front 1 to 3" position, this indicates that the center position of the fixing belt 310 in the width direction is located closer to one end than the center position of the steering roller 350. Conversely, when the fixing belt 310 is in the "rear 1 to 3" position, this indicates that the center position of the fixing belt 310 in the width direction is located closer to the other end than the center position of the steering roller 350. Therefore, when the belt position detection unit 393 detects that the fixing belt 310 is in the second predetermined position in the width direction, this indicates that the fixing belt 310 is located closer to the center position of the steering roller 350 than when the fixing belt 310 is in the first predetermined position.

[0073] Furthermore, the center position of the fixing belt 310 and the center position of the steering roller 350 may be slightly misaligned due to variations in assembly precision.

[0074] 7 shows nine positions from the first close-in position to the second close-in position. The nine positions are arranged at equal intervals, and in this embodiment, the intervals between each position are 3 mm (see FIG. 7). In this embodiment, the first close-in position is on one end side of the steering roller 350. The "middle" position is the center position of the nine equally spaced positions. Therefore, when the belt position detector 393 detects that the end of the fixing belt 310 is at the "middle" position, this means that the fixing belt is located at the center position of the steering roller 350 in the width direction.

[0075] As shown in FIG. 6(a), the sensor flag 393 is a sector-shaped columnar member, and five detection target portions 393b1 to 393b5 are formed on the outer peripheral surface where the sensors 394, 395, and 396 are arranged facing each other. In other words, four openings 393a1 to 393a4 are formed on the outer peripheral surface so that the five detection target portions 393b1 to 393b5 are formed. In this embodiment, the three sensors 394, 395, and 396 are arranged side by side at a predetermined interval along the movement direction (arrow X direction) of the sensor flag 393. Note that the number of detection target portions 393b1 to 393b5 may be four or more, which is more than the number of sensors.

[0076] Five detection target portions 393b1-393b5 are formed on sensor flag 393 so that one of sensors 394, 395, and 396 switches between a detection state and a non-detection state as sensor flag 393 moves. That is, detection target portions 393b1-393b5 are formed so that when fixing belt 310 moves in the width direction, as shown in FIG. 6B, only one of the output signals of sensors 394, 395, and 396 changes. Detection target portions 393b1-393b5 are formed with widths as shown in FIG. 6A, for example, when sensor flag 393 is divided into 27 regions at equal angles in the circumferential direction starting from rotation center O. Specifically, detection target portions 393b1 and 393b2 occupy two regions, detection target portions 393b3 and 393b5 occupy four regions, and detection target portion 393b4 occupies three regions.

[0077] As shown in FIG. 6B, when the sensor flag 393 shown in FIG. 6A is used, when the fixing belt 310 (specifically, the end position) is in the "second close-in" position, the output signals of the three sensors 394, 395, and 396 are all "0." That is, the three sensors 394, 395, and 396 are in a detection state, detecting the detection target portions 393b1, 393b3, and 393b4, respectively. When the fixing belt 310 moves from the "second close-in" position to the "rear 3" position, the output signal of the sensor 396 changes from "0" to "1," while the output signals of the other sensors 394 and 395 remain unchanged at "0." That is, only the output signal of the sensor 396 changes. At this time, the sensor 396 faces the opening 393a4.

[0078] When fixing belt 310 moves from the "rear 3" position to the "rear 2" position, only the output signal of sensor 394 changes from "0" to "1." At this time, sensor 394 faces opening 393a1. When fixing belt 310 moves from the "rear 2" position to the "rear 1" position, only the output signal of sensor 395 changes from "0" to "1." At this time, sensor 395 faces opening 393a3. In other words, all three sensors 394, 395, and 396 face openings 393a1, 393a3, and 393a4, respectively, and are in a non-detection state where they are not detecting any of detection targets 393b1 to 393b5. Therefore, the output signals of all three sensors 394, 395, and 396 are "1."

[0079] When the fixing belt 310 moves from the "rear 1" position to the "middle" position, only the output signal of the sensor 394 changes from "1" to "0." At this time, the sensor 394 detects the detection target 393b2. When the fixing belt 310 moves from the "middle" position to the "front 1" position, only the output signal of the sensor 396 changes from "1" to "0." At this time, the sensor 396 detects the detection target 393b5. When the fixing belt 310 moves from the "front 1" position to the "front 2" position, only the output signal of the sensor 394 changes from "0" to "1." At this time, the sensor 394 faces the opening 393a2. When the fixing belt 310 moves from the "front 2" position to the "front 3" position, only the output signal of the sensor 395 changes from "1" to "0." At this time, the sensor 395 detects the detection target 393b4. Furthermore, when the fixing belt 310 moves from the "near 3" position to the "first close-up" position, only the output signal of the sensor 394 changes from "1" to "0." At this time, the sensor 394 detects the detection target 393b3. When the fixing belt 310 is in the "first close-up" position, the output signals of all the sensors are "0." In other words, all three sensors 394, 395, and 396 are in a detection state, detecting the detection targets 393b3, 393b4, and 393b5, respectively. Note that when the fixing belt 310 moves from the "first close-up" position to the "second close-up" position, it is only necessary to reverse the change in the output signals of the sensors 394, 395, and 396, as described above, and therefore a description thereof will be omitted here.

[0080] To prevent the fixing belt 310 from coming off the steering roller 350, when the belt position detection unit 393 detects that the fixing belt 310 is in the "first close-up" position or the "second close-up" position, the control unit 600 determines via the sensor unit 390 that a close-up error has occurred. If the control unit 600 determines that a close-up error has occurred, it stops the print job and sets the pressure rotor 330 to a separated state. To notify the user of the close-up error, the operation unit 40 may display a close-up error. Setting the pressure rotor 330 to the separated state facilitates the work of a service technician to restore the image forming apparatus 100 from the close-up error to a state where image formation is possible, i.e., to recover. In this embodiment, this work involves moving the fixing belt 310 toward the "middle" position from the "first close-up" position or the "second close-up" position.

[0081] In this embodiment, a method for detecting the widthwise position of the belt using the belt position detection unit 393 and sensors 394, 395, and 396 has been described, but the means for detecting the widthwise position of the belt is not limited to this, and a line sensor, an eddy current sensor, or the like may also be used.

[0082] <Steering control in the first mode> When the length in the width direction of the recording material conveyed to the fixing nip portion N is smaller than a predetermined width, steering control is performed in the first mode. The steering control in the first mode will be described in detail below.

[0083] The predetermined width will now be described. The predetermined width is the length of the fixing belt in the width direction. The control unit 600 selects the steering control mode depending on whether the predetermined width is less than or equal to the predetermined width. The predetermined width is equal to or greater than the length obtained by subtracting the length by which the fixing belt 310 moves in the width direction due to steering control during grinding and the length by which the fixing belt 310 moves in the width direction due to steering control in the first mode when a recording material is transported to the fixing nip portion N from the length by which the grinding roller 420 grinds the fixing belt 310, and is equal to or less than the length by which the grinding roller grinds the fixing belt in the width direction.

[0084] The length over which polishing roller 420 polishes fixing belt 310 refers to the range over which the portion of polishing roller 420 to which abrasive grains are adhered contacts fixing belt 310 when polishing roller 420 polishes fixing belt 310. The length of this range in the width direction is defined as the length over which polishing roller 420 polishes fixing belt 310 (the length in the width direction between region 310a and the hatched region in FIG. 10e).

[0085] The length by which the fixing belt 310 moves in the width direction due to the steering control during polishing is the length in the width direction of the range in which the end 420a of the area where the abrasive grains are adhered of the polishing roller 420 moves during polishing. The distance Dd shown in Fig. 10(e) is the length by which the fixing belt 310 moves in the width direction due to the steering control during polishing (the shaded area in Fig. 10e).

[0086] The length over which the polishing roller 420 polishes the fixing belt 310 minus the length over which the fixing belt 310 moves in the width direction due to steering control during polishing is the length over which the polishing roller 420 is constantly in contact with the fixing belt during polishing. Here, this is the region where polishing is frequently performed (310a in FIG. 10).

[0087] Next, the length by which the fixing belt 310 moves in the width direction due to the steering control in the first mode when a recording material is being transported to the fixing nip N will be described. The steering control in the first mode will be described later. When the fixing belt 310 is positioned at the "front 3" or "rear 3" position, the inclination angle of the steering roller 350 is changed. When the fixing belt 310 is positioned at the "front 3" position, the inclination angle of the steering roller 350 is inclined at angle A, and when the fixing belt 310 is positioned at the "rear 3" position, the inclination angle of the steering roller 350 is inclined at angle -A. The length between "front 3" and "rear 3" is defined as the length by which the fixing belt 310 moves in the width direction due to the steering control in the first mode when a recording material is being transported to the fixing nip N (the length between Dc in FIG. 10e).

[0088] The length obtained by subtracting the length of the high-abrasion-frequency region by which the fixing belt 310 moves in the width direction due to steering control in the first mode when the recording material is conveyed to the fixing nip N from the high-abrasion-frequency region is the length over which the high-abrasion-frequency region is always used in the fixing nip N. If the width of the image region, which is the region where toner can be carried on the recording material, exceeds the length over which the high-abrasion-frequency region is always used in the fixing nip N, there is a risk of gloss unevenness occurring in the image region. Therefore, in this embodiment, the predetermined width is set to the length over which the high-abrasion-frequency region is always used in the fixing nip N. By setting the predetermined width to the length over which the high-abrasion-frequency region is always used in the fixing nip N, when steering control is performed in the first mode, the image region of the recording material is fixed in the high-abrasion-frequency region. Therefore, the risk of gloss unevenness occurring in the image region is reduced.

[0089] However, gloss unevenness that occurs when the area with low grinding frequency is only slightly used for fixing is unlikely to significantly impair image quality. Therefore, the predetermined width is set to be equal to or greater than the length over which the area with high grinding frequency is always used in the fixing nip N, and equal to or less than the length over which the grinding roller 420 grinds the fixing belt 310.

[0090] In this embodiment, the length in the width direction of the area where the abrasive grains of the polishing roller 420 are adhered is 335 mm, and the length in the width direction of the range over which 420a moves during polishing is 6 mm, if it is defined as the distance between "front 1" and "rear 1." Also, the length over which the fixing belt 310 moves in the width direction due to the steering control in the first mode when the recording material is being transported to the fixing nip N is the distance between "front 3" and "rear 3," and therefore is 9 mm. Therefore, the predetermined width in this embodiment is 320 mm or more and 335 mm or less.

[0091] When fixing is performed by the fixing device 30 by applying heat and pressure to a recording material carrying an unfixed toner image, the pressure rotating body 330 comes into contact with the fixing belt 310 to form the fixing nip N. As mentioned in <Uneven Gloss Due to Paper Edge Scratches>, when the recording material is in contact with the pressure rotating body 330, the edge of the recording material causes scratches on the surface of the fixing belt 310. Therefore, in order to prevent deterioration of the surface of the fixing belt 310 due to scratches on the paper edge, the control unit 600 performs steering control to reciprocate the fixing belt 310 in the width direction.

[0092] The steering control by the control unit 600 will be described in detail later with reference to Figures 8 and 9. The explanation will be given along the flowchart of Figure 8.

[0093] First, when the control section 600 determines that the length in the width direction of the recording material conveyed to the fixing nip portion N is smaller than a predetermined width, the control section 600 performs steering control in the first mode.

[0094] S001 The belt position detection unit 393 performs a first detection of the position of the fixing belt 310. If the belt position detection unit 393 detects that the fixing belt 310 is at the first or second shift position (shift position), the control unit 600 issues a shift error. If it detects that the fixing belt 310 is not at the shift position, the process proceeds to S002.

[0095] S002 If the first detection by belt position detection unit 393 detects that fixing belt 310 is located at "front 1," "front 2," or "front 3," the process proceeds to S003. If the first detection by belt position detection unit 393 detects that fixing belt 310 is located at "middle," "rear 1," "rear 2," or "rear 3," the process proceeds to S006.

[0096] S003 The control unit 600 performs an operation to tilt the steering roller 350 to the first tilt angle.

[0097] The first tilt angle will be described using FIG. 9. FIG. 9 is a view from the direction of arrow α in FIG. 2. To explain the tilt angle of the steering roller 350, the fixing belt 310 is not shown. The pressure rotor 330 is located at the bottom of the drawing. 350a in FIG. 9 indicates the state where the steering roller is parallel to the heating roller 340. The first tilt angle is the angle at which the steering roller 350 is tilted relative to the steering roller 350a in order to move the fixing belt 310 toward the other end of the steering roller 350. In this embodiment, the control unit 600 tilts the steering roller 350a counterclockwise on the drawing in FIG. 9 to position 350b. The direction in which the steering roller 350a is tilted counterclockwise on the drawing is referred to as the first direction. As a result, the fixing belt 310 tends to move toward the other end of the steering roller 350. The tilt angle from the steering roller 350a to 350b at this time is referred to as the first tilt angle (angle A). In this embodiment, 350a is shown parallel to the heating roller 340, but the angle of the steering roller 350a may be slightly deviated due to variations in assembly precision.

[0098] As a result of the operation of tilting the steering roller 350a to the position 350b, the steering roller 350 is inclined at the first tilt angle.

[0099] It takes about 1.5 seconds to change the tilt angle of the steering roller 350. Therefore, there is a possibility that the fixing belt 310 may exceed the "near 3" position toward the first shift position (overshoot). When the fixing belt 310 reaches the first shift position and the belt position detection unit 393 detects that the fixing belt 310 is positioned at the first shift position, the control unit 600 outputs a shift error.

[0100] On the other hand, it is also possible that the fixing belt 310 has passed the "near 3" position toward the first close-up position but has not yet reached the first close-up position. In this case, the steering roller 350 is tilted at the first tilt angle, so that the fixing belt 310 is moved from between the "near 3" position and the first close-up position toward the other end of the steering roller 350. As a result, the belt position detection unit 393 detects that the fixing belt 310 has reached the "near 3" position, but the tilt angle of the steering roller 350 is tilted at the first tilt angle A, which is the first orientation.

[0101] Because the steering roller 350 is tilted at the first tilt angle, the fixing belt 310 is moved in the order of "Front 2," "Front 1" (second predetermined position), "Middle," "Rear 1," and "Rear 2." While the fixing belt 310 is being moved toward the other end, the belt position detection unit 393 detects the position of the fixing belt 310 at the "Front 2," "Front 1" (second predetermined position), "Middle," "Rear 1," and "Rear 2" positions, but no operation is performed to change the tilt angle of the steering roller 350 through steering control. The steering roller 350 is inclined at the first tilt angle. Note that a configuration is also possible in which the belt position detection unit 393 does not detect that the fixing belt 310 is located at the "Front 2," "Front 1" (second predetermined position), "Middle," "Rear 1," or "Rear 2" positions, and no operation is performed to tilt the steering roller 350 through steering control.

[0102] S004 If the belt position detection unit 393 detects that the fixing belt 310 has been moved to the other end side of the steering roller 350 and that the fixing belt 310 has reached the "rear 3" position, the process proceeds to S006. If the belt position detection unit 393 does not detect that the fixing belt 310 has reached the "rear 3" position, the process proceeds to S005.

[0103] S005 If it is detected that the fixing belt 310 is in the shift position, a shift error is issued. If it is not detected that the fixing belt 310 is in the shift position, the process returns to S003.

[0104] S006 The control unit 600 performs an operation to tilt the steering roller 350 at an angle −A in order to move the fixing belt 310 toward one end side.

[0105] In FIG. 9, 350a indicates the state when the steering roller 350 is parallel to the heating roller 340. The -A angle is the angle at which the steering roller 350 is tilted relative to the steering roller 350a in order to move the fixing belt 310 toward one end of the steering roller 350. In this embodiment, the steering roller 350a is tilted clockwise on the paper surface of FIG. 9 to position 350c. The direction in which the steering roller 350a is tilted clockwise on the paper surface is referred to as the second direction. In other words, the second direction is the direction in which the steering roller 350a is tilted clockwise, opposite to the first direction in which the steering roller 350a is tilted counterclockwise on the paper surface. This tends to move the fixing belt 310 toward one end of the steering roller 350. The tilt angle from the steering roller 350a to the steering roller 350c at this time is referred to as angle -A.

[0106] As a result of the operation of tilting the steering roller 350a to the position 350c, the steering roller 350 is inclined at the angle −A.

[0107] It takes about 1.5 seconds to change the tilt angle of the steering roller 350. Therefore, the fixing belt 310 may move beyond the "rear 3" position toward the second shift position. When the fixing belt 310 reaches the second shift position and the belt position detection unit 393 detects that the fixing belt 310 is positioned at the second shift position, the control unit 600 issues a shift error.

[0108] On the other hand, it is also possible that the fixing belt 310 has passed the "rear 3" position toward the second shift position but has not yet reached the second shift position. In this case, the steering roller 350 is tilted at an angle -A, so that the fixing belt 310 is moved from between the "rear 3" position and the second shift position toward one end of the steering roller 350. As a result, the belt position detection unit 393 detects that the fixing belt 310 has reached the "rear 3" position, but the steering roller 350 is tilted at an angle of -A.

[0109] Because the steering roller 350 is tilted at an angle of -A, the fixing belt 310 is moved in the order of "rear 2," "rear 1," "middle," "front 1" (second predetermined position), and "front 2." While the fixing belt 310 is being moved toward the one end side, the belt position detector 393 detects the position of the fixing belt 310 at the positions "front 2," "front 1" (second predetermined position), "middle," "rear 1," and "rear 2," but no operation to tilt the steering roller 350 by steering control is performed. The steering roller 350 is now tilted at the angle -A.

[0110] Furthermore, the belt position detection unit 393 may not detect that the fixing belt 310 is positioned at the "rear 2," "rear 1," "middle," "front 1," or "front 2" positions, and the control unit 600 may not perform the operation of tilting the steering roller 350.

[0111] S007 If the belt position detection unit 393 detects that the fixing belt 310 has been moved to one end side of the steering roller 350 and that the fixing belt 310 has reached the "Front 3" position, the process proceeds to S003. If the belt position detection unit 393 does not detect that the fixing belt 310 has reached the "Front 3" position, the process proceeds to S008.

[0112] S008 If it is detected that the fixing belt 310 is in the shift position, a shift error is issued. If it is not detected that the fixing belt 310 is in the shift position, the process returns to S006.

[0113] When the belt position detection unit 393 detects that the fixing belt 310 is positioned at any of the "Front 1," "Front 2," "Rear 1," and "Rear 2" positions, the control unit 600 does not tilt the steering roller 350. However, when the fixing belt 310 is positioned at the "Front 3" or "Rear 3" positions, the control unit 600 tilts the steering roller 350. Therefore, the fixing belt 310 is reciprocated between the "Front 3" (first predetermined position) position and the "Rear 3" position. In other words, the fixing belt 310 can be reciprocated over a wide range in the width direction without causing a close-to-edge error. Reciprocating the fixing belt over a wide range in the width direction expands the area of the fixing belt 310 through which the edge of the recording material can pass. This prevents the edge of the recording material from repeatedly passing over the same area of the fixing belt 310. This prevents deterioration of the surface of the fixing belt 310 due to scratches on the paper edge.

[0114] As shown in FIG. 9, in the first mode, the steering roller 350 is inclined at angle A or angle -A. Angle A or angle -A is larger than angles B, -B, C, and -C shown in FIGS. 12 and 13, which will be described later. A larger inclination angle of the steering roller 350 allows the moving speed of the fixing belt 310 in the width direction to be increased. A higher moving speed of the fixing belt 310 tends to prevent the edge of the recording material from repeatedly passing the same area of the fixing belt 310. This makes it possible to prevent deterioration of the surface of the fixing belt 310 due to edge scratches.

[0115] Although the steering control is described as being performed between the "front 3" position and the "rear 3" position, the steering control may also be performed between the "front 2" and "rear 2" positions. In this case, when the belt position detection unit 393 detects that the fixing belt 310 is located at the "front 2" position, an operation to tilt the steering roller 350 is performed. At this time, the position of the steering roller 350 is tilted toward 350a with respect to 350b. Similarly, when the belt position detection unit 393 detects that the fixing belt 310 is located at the "rear 2" position, an operation to tilt the steering roller 350 is performed. At this time, the position of the steering roller 350 is tilted toward 350a with respect to 350c.

[0116] <Modification of the first mode> In this modification, the movement direction of the fixing belt 310 in the width direction is changed between "front 3" and "rear 3", and the inclination angle of the steering roller 350 is changed between "front 1, 2" and "rear 1, 2", but the movement direction of the fixing belt 310 is not changed. A detailed method for controlling the steering roller 350 will be described below. The steering control in this modification differs from the above example when the fixing belt 310 is located at the "front 1" or "rear 1" position. Therefore, the case where the fixing belt 310 is located at the "front 1" or "rear 1" position will be described.

[0117] In a modified example, the "Front 3" position is the first predetermined position and the "Front 1" position is the second predetermined position.

[0118] The belt position detector 393 detects the position of the fixing belt 310 .

[0119] When the fixing belt 310 is positioned at the "front 3" position, the control unit 600 tilts the steering roller 350 to a first tilt angle. When the fixing belt 310 is positioned at the "rear 3" position, the control unit 600 tilts the steering roller 350 to an angle -A.

[0120] When the control unit 600 determines that the fixing belt 310 is located at the "rear 1" position and the steering roller 350 is tilted at an angle A, the steering roller 350 is tilted at an angle B or C, which is an angle smaller than the angle A. As a result, the moving speed of the fixing belt 310 in the width direction becomes slower than when the steering roller 350 is tilted at an angle A.

[0121] When the control unit 600 determines that the fixing belt 310 is located at the "rear 1" position and the steering roller 350 is tilted at an angle -A, the steering roller 350 is tilted at an angle -B or -C, which is an angle smaller than the angle -A. As a result, the moving speed of the fixing belt 310 in the width direction becomes slower than when the steering roller 350 is tilted at an angle -A.

[0122] Similarly, when the control unit 600 determines that the fixing belt 310 is located at the "front 1" position and the steering roller 350 is tilted at an angle -A, the steering roller 350 is tilted at an angle -B or -C, which is an angle smaller than the angle -A. This reduces the moving speed of the fixing belt 310 in the width direction compared to when the steering roller 350 is tilted at an angle -A.

[0123] When the control unit 600 determines that the fixing belt 310 is located at the "front 1" position and the steering roller 350 is tilted at angle A, the steering roller 350 is tilted at angle B or C, which is an angle smaller than angle A. This reduces the moving speed of the fixing belt 310 in the width direction compared to when the steering roller 350 is tilted at angle A. The pressure rotator 330 transitions from a contact state to a separated state, which can prevent an increase in the moving speed of the fixing belt 310 in the width direction. This can prevent the occurrence of a bias error.

[0124] In this modification, the moving speed of the fixing belt 310 in the width direction may be reduced compared to the steering control in the first mode described above. However, it is possible to prevent the edge of the recording material from repeatedly passing the same point on the fixing belt 310. Therefore, it is possible to perform steering control sufficiently to prevent edge damage.

[0125] <Steering control in the second mode> In this embodiment, when the image area of the recording material conveyed to the fixing nip N in the width direction is equal to or greater than a predetermined width, the control unit 600 switches the steering control from the first mode to the second mode. Even when a recording material whose image area in the width direction is equal to or greater than a predetermined width is conveyed to the fixing nip N, if steering control is performed in the first mode, there is a risk of gloss unevenness occurring. The reason for this will be explained below. FIG. 10 will be used for explanation. FIG. 10 shows the fixing belt 310, the polishing roller 420, and the other end side of the recording material S. The recording material S in FIGS. 10 and 16 is a recording material whose image area is equal to or greater than a predetermined width, and in this embodiment, the recording material S having the maximum length in the width direction is illustrated (hereinafter abbreviated as recording material S).

[0126] In this embodiment, which includes the fixing belt 310, the steering roller 350, and the polishing roller 420, the length of the polishing roller 420 in the width direction is shorter than the length of the fixing belt 310. "Shorter" here means that even when the steering control is performed in the first mode when the polishing roller 420 comes into contact with the fixing belt 310, the end of the fixing belt 310 does not come into contact with the polishing roller 420. Because the length of the polishing roller 420 is shorter than that of the fixing belt 310, when the polishing roller 420 comes into contact with the surface of the fixing belt 310, there are regions on the surface of the fixing belt 310 where the polishable region of the polishing roller 420 comes into contact (FIG. 10(a) Da) and regions where the polishable region of the polishing roller 420 does not come into contact (FIG. 10(a) Db).

[0127] FIG. 10(b) shows the case where the recording material S is conveyed to the fixing nip N. FIG. 10(b) also shows the case where the fixing belt 310 is steered in the first mode, with the fixing belt 310 positioned at the "middle" position. FIG. 10(c) shows the case where the fixing belt 310 is positioned at the "rear 3" position. FIG. 10(d) shows the case where the fixing belt 310 is positioned at the "front 3" position. As can be seen from FIGS. 10(a) to 10(d), if the fixing belt 310 is steered in the first mode when the recording material S is conveyed to the fixing nip N, there is a risk that the edge of the recording material will come into contact with the arrow Dc in FIG. 10(e). Similarly, if the fixing of the recording material S is performed continuously, the surface roughness between the arrows Dc will increase with each fixing operation. Meanwhile, Dd in FIG. 10 indicates the range over which the end 420a of the polishing roller 420, to which the abrasive grains are attached, moves while the polishing roller 420 polishes the fixing belt 310. The steering roller 350 is also controlled during grinding. Assume that 420a moves between Dd. In this case, the area of the fixing belt 310 corresponding to the Dd interval (the shaded area in FIG. 10(e)) is not sufficiently ground compared to the area 310a closer to the one end of the Dd interval (the area where the grinding roller 420 is constantly in contact with the fixing belt 310 during grinding; this is an area where grinding is frequently performed). Therefore, when fixing of the recording material S is performed continuously, there is a risk of a difference in the surface roughness between Dd of the fixing belt 310 and the surface roughness of the area 310a. FIG. 10(f) shows a case where the fixing belt 310 is positioned at the "front 3" position and the recording material S is transported to the fixing nip N. At this time, there is a difference in the surface roughness between Dd and the area 310a of the fixing belt 310. When fixing of the recording material S is performed in this state, the Dd interval and the area 310a are used, which may result in gloss unevenness on the image surface.

[0128] Therefore, in the image forming apparatus of this embodiment, when fixing the recording material S, the control unit 600 performs steering control in the second mode. This makes it possible to suppress gloss unevenness that occurs when fixing the recording material S. The details and effects of the second mode are described below.

[0129] The occurrence of differences in the surface roughness of the fixing belt 310 can be suppressed by increasing the width of the polishing roller 420. By making the polishing roller 420 larger in the width direction than the fixing belt 310, it is possible to eliminate or reduce the area where the fixing belt 310 does not come into contact with the polishing roller 420 during polishing. However, if the polishing roller 420 is configured to be larger in the width direction than the fixing belt 310, when the fixing belt 310 abuts against the polishing roller 420, the edge of the fixing belt 310 in the width direction will come into contact with the polishing roller 420. Oil is applied to the inner circumferential surface of the fixing belt 310, and a considerable amount of oil also adheres to the edge of the fixing belt 310. Therefore, the oil on the inner circumferential surface of the fixing belt 310 trickles down the edge of the fixing belt 310 and adheres to the polishing roller 420. As a result, the oil that trickles down the surface of the polishing roller 420 due to the abutment between the fixing belt 310 and the polishing roller 420 spreads across the width of the fixing belt. There is a risk that the area of the fixing belt 310 with oil on it may come into contact with the recording material, which may cause image defects. For this reason, the polishing roller 420 is configured to be shorter than the fixing belt 310 in the width direction.

[0130] Furthermore, by increasing the width of the fixing belt 310 and the polishing roller 420 while maintaining a configuration in which the widthwise ends of the fixing belt 310 do not come into contact with the polishing roller 420, it is possible to suppress the occurrence of gloss unevenness due to polishing unevenness. However, if the polishing roller 420 and the fixing belt 310 are made larger than before, not only will the increased costs be incurred, but the fixing device 30 will also have to be made larger in the width direction. For this reason, it is difficult to use a configuration in which the polishing roller 420 is larger than the fixing belt 310.

[0131] Therefore, in this embodiment, when the recording material S is conveyed in the width direction to the fixing nip N, the control unit 600 controls the steering roller 350 in the second mode. In the second mode, a target position of the fixing belt 310 is set (the target position in this embodiment is the "middle" position), and the steering roller 350 is tilted so that the fixing belt 310 is moved to the target position.

[0132] First, the timing at which steering control switches to the second mode will be described. Before fixing the recording material S, the control unit 600 determines that the width of the image area on the recording material S passing through the fixing nip N is equal to or greater than a predetermined width. When the recording material S reaches the fixing nip N, steering control switches to the second mode. By performing steering control in the second mode when the recording material S reaches the fixing nip N, for example, if fixing was performed on a recording material smaller than a predetermined width before the recording material S was transported to the fixing nip N, the first mode is applied and steering control can be performed to prevent edge damage. This makes it possible to prevent deterioration of the fixing belt 310.

[0133] Furthermore, the timing at which the steering control is set to the second mode may be between after the recording material transported just before the recording material S passes through the fixing nip portion N and before the recording material S reaches the fixing nip portion N.

[0134] A specific method for determining the tilt angle after the steering control is switched to the second mode will be described with reference to the flowchart of FIG.

[0135] S30 The belt position detector 393 detects the position of the fixing belt 310 .

[0136] S31 If the fixing belt 310 is in the over-committed position, the process proceeds to S35, where an over-committed error is issued.

[0137] If the fixing belt 310 is not at the close-to-position, the process proceeds to S32.

[0138] S32 Based on the detection result (B.Pnow) of the belt position detection unit 393, the difference B.Pdif from the target position "middle" is calculated.

[0139] A number between 1 and 7 is assigned to B.Pnow. The relationship between the position of the fixing belt 310 and the number assigned to B.Pnow is as shown in Fig. 14. For example, when the fixing belt 310 is at the front position 3 (first predetermined position), 1 is assigned, and when the fixing belt 310 is at the rear position 3, 7 is assigned. B.Pdif = 4 - B.Pnоw Equation 1 S33 The integral gain I and the cumulative integral value Itotal of the previous step are added to the difference BPdif. The initial value of Itotal is 0. Itotal(n) = I×BPdif + Itotal(n-1)...Equation 2 S34 The sum of the difference BPdif multiplied by the proportional gain P and the cumulative integral value Itotal(n) is set to the steering angle. Steering angle = P×BPdif + Itotal(n) Equation 3 In this embodiment, the proportional gain P is 100, the integral gain I is 1, and the calculation is performed every 0.2 seconds. For example, if the detection result of the belt position detection unit 393 is 1 at the rear, 5 is substituted into B.Pnow. In this case, the steering angle is as follows:

[0140] Rudder angle = 100×(4-5)+ 1×(4-5) = -101 The tilt angle of the steering roller 350 is determined by the steering angle value obtained from the above calculation.

[0141] The tilt angle can be positive or negative with respect to the steering roller 350a. When the value calculated from Equations 1 to 3 is positive, the steering roller 350 is tilted in order to move the fixing belt 310 toward the other end of the steering roller 350. When the steering angle is positive, the steering roller 350 is tilted counterclockwise in the plane of FIGS. 9, 12, and 13 (first direction). Similarly, when the steering angle is negative, the steering roller 350 is tilted in order to move the fixing belt 310 toward one end of the steering roller 350. When the steering angle is negative, the steering roller 350 is tilted clockwise in the plane of FIGS. 9, 12, and 13 (second direction).

[0142] In this embodiment, in the first mode steering control, when the fixing belt 310 is in the "near 1" (second predetermined position) position, no operation is performed to tilt the steering roller 350. In the second mode steering control, when the fixing belt 310 is in the "near 1" (second predetermined position) position, an operation is performed to tilt the steering roller 350 to an angle B.

[0143] Angle B will now be described. Angle B refers to the second tilt angle. In FIG. 12, angle B is the angle at which steering roller 350 is tilted relative to steering roller 350a in order to move fixing belt 310 toward the other end of steering roller 350. When steering roller 350 is tilted counterclockwise in the first direction, fixing belt 310 tends to move toward the other end of steering roller 350. When belt position detector 393 detects that fixing belt 310 is positioned nearer to 1 (second predetermined position), steering roller 350a is tilted to position 350d, which is closer to 350a than position 350b. The tilt angle at this time is designated angle B (second tilt angle). In other words, the first tilt angle is greater than angle B (second tilt angle).

[0144] Similarly, when belt position detector 393 detects that fixing belt 310 is at the "rear 1" position, steering roller 350 is tilted clockwise in the drawing, which is the second direction. In this case, compared to when steering roller 350 is tilted at angle -A, steering roller 350 is tilted to position 350e (angle -B) toward 350a.

[0145] Furthermore, when the fixing belt 310 is positioned at the "front 2" (third predetermined position) position in the second mode, the control unit 600 performs an operation to tilt the tilt angle of the steering roller 350 to angle C (third tilt angle).

[0146] In FIG. 13 , angle C is the angle at which the steering roller 350 is tilted relative to the steering roller 350a in order to move the fixing belt 310 toward the other end of the steering roller 350. When the steering roller 350 is tilted counterclockwise in the drawing, the fixing belt 310 tends to move toward the other end of the steering roller 350. When the belt position detector 393 detects that the fixing belt 310 is positioned at the "Front 2" position, the steering roller 350 is tilted toward 350b, i.e., to position 350f, compared to when the steering roller 350 is tilted at angle B. This tilt angle is designated angle C (third tilt angle). In other words, the relationship is: first tilt angle > third tilt angle > second tilt angle.

[0147] Similarly, when the belt position detector 393 detects that the fixing belt 310 is at the "rear 2" position, the steering roller 350 is tilted clockwise in the second direction. In this case, compared to when the steering roller 350 is tilted at the angle -B, the steering roller 350 is tilted to the 350c side, that is, the position 350g (angle -C).

[0148] From Equations 1, 2, and 3, when the fixing belt 310 is positioned at the target "middle" position, the steering angle is 0 in this embodiment. At this time, the steering roller 350 is tilted so that the fixing belt 310 is maintained at the "middle" position. As a result, when the fixing belt 310 is positioned at the target "middle" position, the fixing belt 310 is prevented from moving from the target position toward the other end or one end of the steering roller 350. In FIGS. 9, 12, and 13, the steering angle 0 indicates when the steering roller 350 is at 350a, which indicates when the steering roller 350a is parallel to the heating roller 340. However, due to variations in assembly precision, the steering angle 0 of the steering roller 350 may not be parallel to the heating roller 340. Therefore, it is acceptable for the steering angle 0 of the steering roller 350 to deviate slightly from the state in which the steering roller 350 is parallel to the heating roller 340.

[0149] The larger the absolute value of the steering angle obtained, the larger the amount of clockwise or counterclockwise movement of 350a shown in Figures 9, 12, and 13. In other words, the farther the position of fixing belt 310 is from the target "middle" position in the width direction, the larger the inclination angle of steering roller 350.

[0150] When steering control is performed in the second mode, steering control is performed at the "front 1" and "rear 1" positions, which are positions closer to the center position of the steering roller 350 than the "front 3" and "rear 3" positions. The steering control at this time aims to move the fixing belt 310 to the "middle" position, and the steering roller 350 is tilted. As a result, the fixing belt 310 tends to move between the "front 1" and "rear 1" positions in the width direction.

[0151] Using Figure 15, we will explain the effect of performing steering control in the second mode when the recording material S is transported to the fixing nip N, and performing steering control so that the fixing belt 310 is positioned at the ``middle'' position.

[0152] First, FIG. 15 will be described. The widthwise length of the recording material S in FIG. 15 is the maximum width that the image forming apparatus of this embodiment can print. All of FIG. 15 (abcd) show the case where the recording material S is conveyed to the fixing nip portion and steering control is performed in the second mode. FIG. 15 (a, b, c) shows the cases where the fixing belt 310 is located in the "middle" position, the "front 1" position, and the "rear 1" position, respectively. FIG. 15 (d) is a diagram illustrating the area Df where the area De where edge scratches are caused by the recording material S overlaps with the area Dd where edge scratches are less frequently polished. FIG. 15 (e) shows the case where edge scratches in the area less frequently polished are used for fixing the recording material S. The area Dd shown in FIG. 15, like the area Dd shown in FIG. 10, indicates the range of movement of 420a during polishing.

[0153] When steering control is performed in the second mode, the steering roller 350 is controlled to move the fixing belt 310 to the "center" position. In other words, the fixing belt 310 tends to move back and forth between "front 1" and "rear 1." Therefore, when continuous recording material S is fed, there is a risk of edge damage occurring mainly between De shown in (abcd) of FIG. 15 . When continuous fixing of recording material S is performed, the surface roughness between De increases. However, even when steering control is performed in the second mode, depending on the moving speed of the fixing belt 310 in the width direction, the fixing belt 310 may move back and forth between "front 3" and "rear 3." Therefore, although there is a risk of edge damage occurring in the area between Dc in FIG. 10 , when steering control is performed in the second mode, the fixing belt 310 tends to be maintained in the "center" position and tends to move back and forth between "front 1" and "rear 1," and therefore it is described as a risk of edge damage occurring mainly between De. In this embodiment, the width of the De interval corresponds to the width between "Front 1" and "Rear 1," and is therefore approximately 6 mm. Also, as in FIG. 10, the region of the fixing belt 310 corresponding to the Dd interval shown in FIG. 15(d) is polished less frequently than the region 310a. Therefore, the overlapping portion Df (the shaded area in FIG. 15(d)) between the Dd interval and the De interval is not polished sufficiently. Therefore, the region Df is an area where there is a risk of a difference in surface roughness from 310a when steering control is performed in the second mode while the recording material S is being continuously fed. In this embodiment, when the fixing belt 310 is positioned at the "middle" position, one end of the Dd interval coincides with the center of the De interval in the width direction. Therefore, the Df interval is approximately 3 mm.

[0154] The following explains how steering control in the second mode reduces the frequency of edge scratches caused by the recording material S in areas with low abrasion frequency. When steering control is performed in the second mode, the fixing belt 310 is more likely to be positioned in the "middle" position, while when steering control is performed in the first mode, the fixing belt 310 moves back and forth between "front 3" and "rear 3." Therefore, when steering control is performed in the second mode, the edge of the recording material passes through the same area of the fixing belt 310 more frequently than in the first mode, but edge scratches are less likely to occur in the area Dd, which is an area with low abrasion frequency. As a result, when a recording material with an image area equal to or greater than a predetermined width is transported through the fixing nip, steering control in the second mode reduces the difference in surface roughness between areas with high and low abrasion frequency on the fixing belt 310 compared to steering control in the first mode. This reduces gloss unevenness on the image surface.

[0155] This section explains how the second-mode steering control during fixing of the recording material S reduces the use of the area between Df for fixing and suppresses the occurrence of gloss unevenness. FIG. 15(e) illustrates a case in which the recording material S is passed through with a difference in surface roughness between the area between Df and area 310a. The second-mode steering control is performed during the passing of the recording material S, and the fixing belt 310 is positioned at the "Front 1" position. FIG. 15(e) illustrates a case in which the other end of the area between Df when the fixing belt 310 is positioned at the "Front 1" position coincides with the other end of the recording material S in the width direction. When the fixing belt 310 is positioned at the "Front 1" position as shown in FIG. 15(e), there is a risk that the area between Df will be used for fixing. However, the second-mode steering control during fixing of the recording material S prevents the fixing belt 310 from moving to the "Front 2, 3" position, which is closer to the one end than the "Front 1" position. This prevents the area between Df from coming into contact with the image area, which is the area where the recording material carries toner, and allows for fixing. Although there is a risk of contact with the "margins" of the recording material, which are areas where toner is not carried, since these are areas where toner is not carried, there is no problem with the occurrence of gloss unevenness. This makes it possible to prevent gloss unevenness caused by edge scratches that occur when fixing the recording material S.

[0156] Even when the image area of the recording material exceeds a predetermined width, performing fixing in the first mode can reduce the frequency of edge scratches occurring in the same area of the fixing belt 310. However, this increases the frequency of edge scratches occurring in areas that are less frequently polished. If these areas are used for fixing, there is a risk of significant degradation of image quality. Therefore, by performing steering control in the second mode when the image area of the recording material exceeds a predetermined width, the fixing belt 310 is more likely to be kept in the "middle" position, reducing the frequency of edge scratches occurring in areas that are less frequently polished. This also reduces the frequency with which areas that are less frequently polished are used for fixing, thereby suppressing the risk of image quality degradation. Therefore, in this embodiment, when the image area of the recording material exceeds a predetermined width, steering control is performed in the second mode.

[0157] When the recording material S is conveyed to the fixing nip N, steering control is performed in the second mode. The effect of performing steering control even when the fixing belt 310 is positioned at the "Front 2" or "Rear 2" position will now be described. Steering control may not be possible when the fixing belt 310 is positioned at the "Front 1" or "Rear 1" position due to factors such as the fast movement speed of the fixing belt 310 in the width direction. In this case, the fixing belt 310 reaches the "Front 2" or "Rear 2" position (overshoot). However, even if the fixing belt 310 passes the "Front 1" or "Rear 1" position toward one end or the other, steering control is performed at the "Front 2" or "Rear 2" position with the aim of moving the fixing belt 310 to the "middle" position. When the fixing belt 310 is positioned at the "Front 2" position, the steering roller 350 is inclined at angle B. When the fixing belt 310 is positioned at the "Rear 2" position, the steering roller 350 is inclined at angle -B. Even if the steering control is not fast enough when the fixing belt 310 is positioned at the "Front 1" or "Rear 1" position, the fixing belt 310 tends to move between the "Front 2" and "Rear 2" positions in the width direction. In this way, even if an overshoot occurs when steering control is performed in the second mode, steering control is performed at the "Front 2" or "Rear 2" position. Therefore, when the recording material S is transported to the fixing nip N, edge scratches are less likely to occur in areas that are less frequently polished when steering control is performed in the second mode than when steering control is performed in the first mode. This makes it possible to suppress the occurrence of gloss unevenness when fixing the recording material S.

[0158] In the second mode of this embodiment, the inclination angle of the steering roller 350 is changed depending on the position of the fixing belt 310. The closer the fixing belt 310 is to the "center" position, the gentler the inclination angle of the steering roller 350. The steering roller 350 is inclined to move the fixing belt 310 to the "center" position. Specifically, the steering roller 350 is inclined at angle A, which is the first inclination angle, at the "front 3" position, and angle B, which is the second inclination angle, at the "front 1" position. This reduces the widthwise movement speed of the fixing belt 310 as it approaches the "center" position. This makes it easier to maintain the fixing belt 310 in the "center" position. Similarly, when a recording material S is passed through, the fixing belt 310 is more likely to be maintained in the "center" position, making it easier for edge scratches to occur on the recording material S in the region between Df. This reduces gloss unevenness that occurs when the recording material S is used for fixing.

[0159] The distance between the center position of the movement range of the fixing belt 310 and the center position of the fixing belt 310 in the width direction increases as the fixing belt 310 moves from the "rear 1" position to the "rear 3" position. Similarly, the distance increases as the fixing belt 310 moves from the "front 1" position to the "rear 3" position. After the fixing belt 310 moves from the "middle" position, the first operation to tilt the steering roller is performed when the fixing belt is at the "front 1" position or the "rear 1" position when steering control is performed in the second mode. On the other hand, when steering control is performed in the first mode, the first operation to tilt the steering roller is performed when steering control is performed in the "rear 3" position or the "rear 3" position. Therefore, it can be said that the distance between the center position of the fixing belt where the first operation to tilt the steering roller is performed after the center position of the movement range of the fixing belt 310 moves away from the center of the movement range of the fixing belt in the width direction and the center position of the movement range of the fixing belt is smaller when steering control is performed in the second mode than when steering control is performed in the first mode. As a result, the range of edge scratches on the recording material S tends to be smaller in the area De caused by steering control in the second mode than in the area Dc caused by steering control in the first mode. Therefore, when fixing the recording material S, gloss unevenness is less likely to occur on the image surface by performing steering control in the second mode.

[0160] In this embodiment, steering control while the polishing roller 420 is polishing the surface of the fixing belt 310 is performed in the second mode. If steering control is performed in the first mode during polishing, the area Dd in FIG. 15(d) corresponds to the distance the fixing belt 310 moves in the width direction from "front 3" to "rear 3". The polishing roller 420 does not always contact the area Dd of the fixing belt 310 during polishing. Depending on the position of the fixing belt 310, some areas of the area Dd of the fixing belt 310 are polished and some are not. Therefore, the area Dd of the fixing belt 310 is not sufficiently polished compared to 310a.

[0161] Therefore, when steering control is performed in the second mode during polishing, the region between Dd corresponds to the distance the fixing belt 310 moves in the width direction from "Front 1" to "Rear 1." The distance the fixing belt 310 moves from "Front 1" to "Rear 1" is shorter than the distance the fixing belt 310 moves from "Front 3" to "Rear 3." This makes it possible to increase the amount of polishing in the region where the polishing frequency was reduced in the first mode. Therefore, even when fixing is performed in a region where the polishing frequency is reduced due to steering control and a region where polishing is always performed during polishing, gloss unevenness on the image surface can be suppressed.

[0162] In this embodiment, steering control is performed in the first mode and the second mode. However, three or more steering control modes may be provided. For example, the present invention describes the steering control modes when the polishing roller 420 is provided. Therefore, in the case of an image forming apparatus that does not have the polishing roller 420, a third mode may be provided. [Explanation of symbols]

[0163] 30 Fixing device 100 Image forming device 310 Fixing belt 330 Pressurized Rotating Body 340 Heating Roller 350 steering roller 420 Polishing Roller 421 Polishing member contact / separation mechanism N Fixing nip

Claims

1. a rotatable endless fixing belt; a heating roller that contacts the inner circumferential surface of the fixing belt and applies heat to the fixing belt; a steering roller that contacts the inner circumferential surface of the fixing belt together with the heating roller; a pressure rotating body that forms a fixing nip portion by pressing the fixing belt, and nip-conveys a recording material carrying unfixed toner into the fixing nip portion, thereby fixing the unfixed toner image onto the recording material; a polishing member that contacts the surface of the fixing belt and polishes the fixing belt; the polishing member is movable between a position in contact with the fixing belt and a position away from the fixing belt, the abrasive member is located at a position separated from the fixing belt when the recording material is conveyed to the fixing nip portion, a belt position detection unit that detects a position of the fixing belt in a width direction of the fixing belt; a control unit that controls the steering roller to swing so as to move the fixing belt to a predetermined position in the width direction based on a detection result of the belt position detection unit, the belt position detection unit can detect whether the fixing belt is at a first predetermined position and whether the fixing belt is at a second predetermined position in the width direction; In the width direction, a center position of the fixing belt at the first predetermined position and the second predetermined position is located on one end side of the steering roller with respect to the center of the steering roller, and a center position of the fixing belt at the first predetermined position is located on the one end side of the center position of the fixing belt at the second predetermined position, the control unit controls the steering roller in a mode selected from a plurality of modes including a first mode and a second mode; The area on the recording material where the toner can be carried is defined as an image area, when the length in the width direction of the image area of the recording material conveyed to the fixing nip portion is smaller than a predetermined width, the control portion controls the steering roller in the first mode; In the first mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit performs an operation to tilt the steering roller in a predetermined first direction, and when it is detected that the fixing belt is positioned at the second predetermined position, the control unit does not perform an operation to tilt the steering roller, when the length in the width direction of the image area of the recording material conveyed to the fixing nip portion is equal to or greater than a predetermined width, the control portion controls the steering roller in the second mode; In the second mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit performs an operation to tilt the steering roller in the first direction, and when it is detected that the fixing belt is positioned at the second predetermined position, the control unit performs an operation to tilt the steering roller in the first direction. An image forming apparatus characterized by:

2. a rotatable endless fixing belt; a heating roller that contacts the inner circumferential surface of the fixing belt and applies heat to the fixing belt; a steering roller that contacts the inner circumferential surface of the fixing belt together with the heating roller; a pressure rotating body that forms a fixing nip portion by pressing the fixing belt, and nip-conveys a recording material carrying unfixed toner into the fixing nip portion, thereby fixing the unfixed toner image onto the recording material; a polishing member that contacts the surface of the fixing belt and polishes the fixing belt; the polishing member is movable between a position in contact with the fixing belt and a position away from the fixing belt, the abrasive member is located at a position separated from the fixing belt when the recording material is conveyed to the fixing nip portion, a belt position detection unit that detects a position of the fixing belt in a width direction of the fixing belt; a control unit that controls the steering roller to swing so as to move the fixing belt to a predetermined position in the width direction based on a detection result of the belt position detection unit, the belt position detection unit can detect whether the fixing belt is at a first predetermined position and whether the fixing belt is at a second predetermined position in the width direction; In the width direction, a center position of the fixing belt at the first predetermined position and the second predetermined position is located on one end side of the steering roller with respect to the center of the steering roller, and a center position of the fixing belt at the first predetermined position is located on the one end side of the center position of the fixing belt at the second predetermined position, the control unit controls the steering roller in a first mode and a second mode; The area on the recording material where the toner can be carried is defined as an image area, When the length in the width direction of the image area of the recording material conveyed to the fixing nip portion is smaller than a predetermined width, the control portion controls the steering roller in the first mode; In the first mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit tilts the steering roller in a predetermined first direction, when the fixing belt moves from a position where the center of the fixing belt is at the center of the steering roller in the width direction to the second predetermined position, the steering roller is inclined in a second direction opposite to the first direction, when the length in the width direction of the image area of the recording material conveyed to the fixing nip portion is equal to or greater than a predetermined width, the control portion controls the steering roller in the second mode; In the second mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit tilts the steering roller in the first direction, When the fixing belt is moved from a position where the center of the fixing belt is at the center of the steering roller in the width direction to a second predetermined position, the steering roller is inclined in the first direction. An image forming apparatus characterized by:

3. the steering roller is movable between a position inclined at a first tilt angle and a position inclined at a second tilt angle smaller than the first tilt angle relative to a position in which the steering roller is parallel to the heating roller; In the first mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit performs an operation of tilting the steering roller in the first direction at the first tilt angle, In the second mode, when it is detected that the fixing belt is positioned at the first predetermined position, the control unit performs an operation of tilting the steering roller in the first direction at the first tilt angle, and when it is detected that the fixing belt is positioned at the second predetermined position, the control unit performs an operation of tilting the steering roller in the first direction at the second tilt angle.

3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

4. In the width direction, after the center position of the fixing belt moves away from the center of the moving range of the fixing belt, the distance between the center position of the fixing belt where the steering roller is first tilted and the center position of the moving range of the fixing belt is smaller when the control unit controls the steering roller in the second mode than when the control unit controls the steering roller in the first mode.

4. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

5. the belt position detection unit is capable of detecting that the fixing belt is at a third predetermined position in the width direction, and a center position of the fixing belt at the third predetermined position is located on the other end side in the width direction opposite to the one end side with respect to a center position of the fixing belt at the first predetermined position, and is located on the one end side with respect to a center position of the fixing belt at the second predetermined position, When the control unit controls the steering roller in the second mode, if the belt position detection unit detects that the fixing belt is positioned at the third predetermined position, The control unit performs an operation to tilt the steering roller in the first direction, and the steering roller is tilted at a third tilt angle that is greater than the second tilt angle and smaller than the first tilt angle with respect to a position where the steering roller is parallel to the heating roller.

4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium.

6. the predetermined width is equal to or greater than the length obtained by subtracting from the length of the grinding member grinding the fixing belt in the width direction the length of the fixing belt moving in the width direction by the steering control during grinding and the length of the fixing belt moving in the width direction by the steering control in the first mode when a recording material is transported to the fixing nip portion, The length of the polishing member in the width direction is equal to or less than the length of the polishing member that polishes the fixing belt.

6. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

7. When the steering control is performed in the first mode while the recording material is being conveyed to the fixing nip portion, a length obtained by subtracting, from the length of time that the abrasive member abrades the fixing belt in the width direction, a length by which the fixing belt moves in the width direction due to steering control during abrasion and a length by which the fixing belt moves in the width direction due to steering control in the first mode when a recording material is transported to the fixing nip portion, is equal to or greater than the length of the image area in the width direction; 7. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

Images