Fixing device and image forming apparatus

Abstract

To prevent various troubles that may occur due to a temperature difference between a first area and a sheet non-contact area on outer peripheral surfaces of fixing rotating bodies.SOLUTION: A fixing device 40 brings outer peripheral surfaces of heated fixing rotating bodies 41, 42, 43 into contact with a sheet to fix an image to the sheet, and comprises: first temperature detection means 49A that detects the temperature of a first area of the fixing rotating bodies as a first temperature T1; second temperature detection means 49B that detects the temperature of a second area of the fixing rotating bodies as a second temperature T2; a contact rotating body 46 that is in contact with both the first area and the sheet non-contact area of the fixing rotating bodies and is rotatable with a relative speed difference to the fixing rotating bodies; and switching means that switches the rotation operation of the contact rotating body based on detection results T1, T2 from the first temperature detection means and the second temperature detection means.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a fixing device and an image forming apparatus. [Background technology]

[0002] 2. Description of the Related Art Conventionally, there has been known a fixing device that fixes an image onto a sheet by bringing the outer peripheral surface of a heated fixing rotor into contact with the sheet.

[0003] For example, Patent Document 1 discloses a fixing device equipped with an external heating roller (contact rotator) that contacts the outer circumferential surface of a fixing roller (fixing rotator). This fixing device has a heat treatment mode in which the external heating roller is rotated in a driven manner relative to the fixing roller to heat the fixing roller, and a roughening mode in which the external heating roller is rotated so as to have a peripheral speed difference relative to the fixing roller to change the surface roughness of the fixing roller. Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional fixing devices, when using a sheet with a narrow sheet width, if a large temperature difference occurs between the sheet contact area and the sheet non-contact area of ​​the fixing rotor in the sheet width direction, this temperature difference can cause various problems. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides a fixing device that fixes an image on a sheet by bringing an outer peripheral surface of a heated fixing rotor into contact with the sheet, the fixing device comprising: a first temperature detection means that detects, as a first temperature, a temperature of a first region of the fixing rotor that contacts both a first sheet having a first sheet width and a second sheet having a second sheet width narrower than the first sheet width; a second temperature detection means that detects, as a second temperature, a temperature of a second region of the fixing rotor that contacts the first sheet but not the second sheet; a contact rotor that contacts both the first region and the second region of the fixing rotor and is rotatable at a relative speed difference with respect to the fixing rotor; and a switching means that switches the rotational operation of the contact rotor based on detection results of the first temperature detection means and the second temperature detection means. The switching means performs the switching so that, when a difference between the first temperature and the second temperature is less than a predetermined value, the contact rotor is rotated so that the relative speed of the contact rotor with respect to the fixing rotor becomes a first relative speed, and, when the difference is equal to or greater than a predetermined value, the contact rotor is rotated so that the relative speed becomes a second relative speed which is smaller than the first relative speed. It is characterized by the following. [Effects of the Invention]

[0006] According to the present invention, even if a large temperature difference occurs between the sheet contact area and the sheet non-contact area of ​​the fixing rotor in the sheet width direction when using a sheet with a narrow sheet width, this temperature difference can be reduced to suppress various problems. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of a printer according to an embodiment. [Figure 2] 2 is a cross-sectional view showing a schematic configuration of a fixing device in the printer, taken along a direction perpendicular to the roller axes of a fixing roller and a heating roller; FIG. [Figure 3] 2 is a cross-sectional view showing a schematic configuration of the fixing device, taken along a direction perpendicular to the roller axes of the fixing roller and the heating roller; FIG. [Figure 4] 2 is a cross-sectional view of a fixing roller and a heating roller taken along the roller axis, showing a schematic configuration of the fixing device; FIG. [Figure 5] FIG. 10 is an explanatory diagram showing an example of a state in which glossy streaks occur when a solid image is formed on a wide sheet of paper after a certain number of narrow sheets have been passed through. [Figure 6]10A and 10B are explanatory diagrams showing the temperature distribution of the fixing belt in the paper width direction after continuous paper feed of paper narrower than the maximum paper feed width in the embodiment. [Figure 7] 10A and 10B are explanatory diagrams showing the outer diameter profile of the fixing roller in the paper width direction and the contact pressure distribution between the refreshing roller and the fixing belt after continuous paper passing through the fixing roller with a paper width narrower than the maximum paper passing width in an embodiment. [Figure 8] 4 is a flowchart relating to control in a polishing mode and a temperature difference suppression mode in the embodiment. [Figure 9] 10 is an explanatory diagram showing the temperature distribution of the fixing belt in the paper width direction after continuous paper feed of paper narrower than the maximum paper feed width in a modified example. [Figure 10] 10A and 10B are explanatory diagrams showing the outer diameter profile of the fixing roller in the paper width direction and the contact pressure distribution between the refreshing roller and the fixing belt after continuous paper passing through the fixing roller with a paper width narrower than the maximum paper passing width in a modified example. [Figure 11] 10 is a flowchart relating to control of a polishing mode and a temperature difference suppression mode in a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0008] An embodiment in which the present invention is applied to a fixing device of an image forming apparatus will be described below. FIG. 1 is a schematic diagram showing an example of an electrophotographic printer as an image forming apparatus according to an embodiment. 1, the printer 100 of this embodiment includes four imaging units 2Y, 2M, 2C, and 2K for forming toner images of yellow (Y), magenta (M), cyan (C), and black (K). The printer 100 has a so-called tandem configuration in which these four imaging units 2 are arranged side by side along the endless movement direction of an intermediate transfer belt 61 serving as an image carrier.

[0009] The printer 100 includes a paper feed path 30, a pre-transfer transport path 31, a manual paper feed path 32, a manual feed tray 33, a pair of registration rollers 34, a transport belt unit 35, a fixing device 40, a transport switching device 50, a paper discharge path 51, a pair of paper discharge rollers 52, and a paper discharge tray 53. The printer 100 also includes two optical writing units 1YM and 1CK, a primary transfer unit 60, a secondary transfer unit 78, a first paper feed cassette 101, a second paper feed cassette 102, and the like.

[0010] The four imaging units 2Y, 2M, 2C, and 2K each have a drum-shaped photosensitive element 3Y, 3M, 3C, and 3K that serves as a latent image carrier. The first paper feed cassette 101 and the second paper feed cassette 102 each contain a stack of sheets P. The topmost sheet P in the stack of sheets is sent toward the paper feed path 30 by the rotation of the first feed roller 101a or the second feed roller 102a.

[0011] 1, a manual feed tray 33 is disposed so as to be able to open and close relative to the housing, and a stack of sheets is manually fed onto the upper surface of the tray when it is open relative to the housing. The topmost sheet P of the manually fed stack of sheets is sent out by a feed roller of the manual feed tray 33 towards the paper feed path 30 via the manual feed path 32.

[0012] Each of the two optical writing units 1YM and 1CK has a laser diode, a polygon mirror, various lenses, etc. The laser diode is driven based on image information read by a scanner, which is an external device of the printer 100, or image information sent from a personal computer, to optically scan the photoconductors 3Y, 3M, 3C, and 3K of the imaging units 2Y, 2M, 2C, and 2K. Specifically, the photoconductors 3Y, 3M, 3C, and 3K of the imaging units 2Y, 2M, 2C, and 2K are each driven to rotate counterclockwise in FIG. 1 by a driving means.

[0013] The first optical writing unit 1YM performs an optical scanning process by irradiating the driven yellow photoconductor 3Y and magenta photoconductor 3M with laser light while deflecting it in the direction of the rotation axis, thereby forming electrostatic latent images based on yellow image information and magenta image information on the yellow photoconductor 3Y and magenta photoconductor 3M, respectively. The second optical writing unit 1CK performs an optical scanning process by irradiating the driven cyan photoconductor 3C and black photoconductor 3K with laser light while deflecting it in the direction of their rotation axes, thereby forming electrostatic latent images based on cyan image information and black image information on the cyan photoconductor 3C and black photoconductor 3K, respectively.

[0014] The imaging units 2Y, 2M, 2C, and 2K are each a single unit, with the photoreceptors 3Y, 3M, 3C, and 3K, and various devices disposed around them, supported on a common support, and are detachably attached to the housing of the printer 100. The imaging units 2Y, 2M, 2C, and 2K have the same configuration except that they use different colors of toner. Therefore, in the following explanation, the letters Y, M, C, and K indicating the colors of toner used will be omitted as appropriate.

[0015] In addition to the photoconductor 3, the imaging unit 2 has a developing device 4 for developing the electrostatic latent image formed on the surface of the photoconductor 3 into a toner image. The imaging unit 2 also has a charging device 5 for uniformly charging the surface of the rotationally driven photoconductor 3. The imaging unit 2 also has a drum cleaning device 6 for cleaning residual toner adhering to the surface of the photoconductor 3 after it has passed through the primary transfer nips corresponding to each color, which will be described later.

[0016] The photoreceptor 3 is a drum-shaped body made of a base tube such as aluminum, coated with a photosensitive organic photosensitive material to form a photosensitive layer. Instead of a drum-shaped body, an endless belt-shaped body may also be used.

[0017] The developing device 4 is equipped with a rotatable developing sleeve made of a non-magnetic pipe and a magnet roller disposed within the hollow of the developing sleeve so as not to rotate with the sleeve. The electrostatic latent image on the photoreceptor 3 is developed with a two-component developer (hereinafter simply referred to as developer) containing magnetic carrier and non-magnetic toner of each color carried on the surface of the developing sleeve by the magnetic force generated by the magnet roller.

[0018] The developing devices 4Y, 4M, 4C, and 4K are supplied with toner of each color from toner bottles 103Y, 103M, 103C, and 103K by toner supply devices for each color as appropriate. A toner concentration sensor is provided in the developing device 4 as a toner concentration detection means. The toner concentration sensor detects the magnetic permeability of the developer due to the carrier, which is a magnetic material. The control unit controls the operation of the toner supply device for each color based on a comparison between the output value from the toner concentration sensor for each color and the target output value from the sensor, which is the target toner concentration value. This keeps the toner concentration of the developer within a certain range (for example, 4 [wt%] to 9 [wt%]).

[0019] The drum cleaning device 6 is a type that uses a polyurethane rubber cleaning blade that is in contact with the photoreceptor 3 to scrape off residual toner from the surface of the photoreceptor 3. Other types may be used instead of this type. In order to improve cleaning performance, the drum cleaning device 6 also has a rotatable fur brush that is in contact with the photoreceptor 3 in addition to the cleaning blade. This fur brush also serves to scrape the lubricant from the solid lubricant, turning it into fine powder and applying it to the surface of the photoreceptor 3.

[0020] A discharge lamp is disposed above the photoconductor 3, and this discharge lamp is also part of the imaging unit 2. The discharge lamp discharges the surface of the photoconductor 3 by irradiating it with light after it has passed the area facing the drum cleaning device 6. The discharged surface of the photoconductor 3 is then uniformly charged by the charging device 5 and optically scanned by the optical writing unit 1 described above. The charging device 5 is driven to rotate while receiving a charging bias from a power source. Instead of this type, a scorotron charger type that charges the photoconductor 3 without contact may be used.

[0021] A primary transfer unit 60 is disposed below the four imaging units 2Y, 2M, 2C, and 2K. This primary transfer unit 60 includes an intermediate transfer belt 61 that is stretched by a plurality of rollers 63, 67, 68, 69, and 70. The primary transfer unit 60 moves the intermediate transfer belt 61 endlessly in the clockwise direction (the direction of arrow A) in FIG. 1 by rotating one of the rollers while bringing the intermediate transfer belt 61 into contact with the photoconductors 3Y, 3M, 3C, and 3K. This forms primary transfer nips for each color where the intermediate transfer belt 61 contacts the photoconductors 3Y, 3M, 3C, and 3K.

[0022] Near the primary transfer nips for each color, primary transfer rollers 62Y, 62M, 62C, and 62K disposed inside the belt loop press the intermediate transfer belt 61 toward the photoconductors 3Y, 3M, 3C, and 3K. A primary transfer bias is applied to each of the four primary transfer rollers 62Y, 62M, 62C, and 62K by a primary transfer power supply. As a result, a primary transfer electric field is formed in the primary transfer nips for each color, which electrostatically moves the toner images on the photoconductors 3Y, 3M, 3C, and 3K toward the intermediate transfer belt 61.

[0023] 1, the Y toner image, the M toner image, the C toner image, and the K toner image are sequentially superimposed and primarily transferred onto the outer peripheral surface of the intermediate transfer belt 61, which passes through the primary transfer nips for each color in sequence. As a result of this superimposed primary transfer, a four-color superimposed toner image is formed on the outer peripheral surface of the intermediate transfer belt 61.

[0024] A secondary transfer unit 78 is disposed below the intermediate transfer belt 61 in Fig. 1. This secondary transfer unit 78 includes an endless secondary transfer belt 77, a ground driven roller 72, a secondary transfer drive roller 71, a secondary belt cleaning device 76, and a toner adhesion amount detection sensor 64. The secondary transfer belt 77 is stretched under tension by the ground driven roller 72 and the secondary transfer drive roller 71, both of which are disposed on its inner circumferential surface, and moves endlessly in the counterclockwise direction in Fig. 1 (the direction of arrow B) as the secondary transfer drive roller 71 is rotated.

[0025] The secondary transfer belt 77 of the secondary transfer unit 78 forms a secondary transfer nip by bringing the portion where it is wound around the grounded driven roller 72 into contact with the portion where it is wound around the secondary transfer bias roller 68 of the intermediate transfer belt 61 of the primary transfer unit 60. A secondary transfer bias output from a secondary transfer power supply is applied to the secondary transfer bias roller 68 on the inner circumferential surface side of the intermediate transfer belt 61, while the grounded driven roller 72 on the inner circumferential surface side of the secondary transfer belt 77 is grounded. This forms a secondary transfer electric field in the secondary transfer nip.

[0026] A pair of registration rollers 34 is disposed to the right of the secondary transfer nip in Figure 1, and the paper P sandwiched between the rollers is sent to the secondary transfer nip at a timing that allows it to be synchronized with the four-color superimposed toner image on the intermediate transfer belt 61. Within the secondary transfer nip, the four-color superimposed toner image on the intermediate transfer belt 61 is secondarily transferred all at once onto the paper P due to the influence of the secondary transfer electric field and nip pressure, and combined with the white color of the paper P, it becomes a full-color image.

[0027] After passing through the secondary transfer nip, residual toner remains on the outer peripheral surface of the intermediate transfer belt 61. This residual toner is removed from the surface of the intermediate transfer belt 61 by the intermediate transfer belt cleaning device 75 of the primary transfer unit 60.

[0028] The paper P that has passed through the secondary transfer nip is separated from the intermediate transfer belt 61 and the secondary transfer belt 77 and is delivered to the conveyor belt unit 35. The conveyor belt unit 35 has an endless conveyor belt 36 stretched between a conveyor drive roller 37 and a conveyor driven roller 38, and is moved endlessly in the counterclockwise direction (the direction of arrow C) in FIG. 1 by the rotational drive of the conveyor drive roller 37. The paper P delivered from the secondary transfer nip is then held on the upper stretching surface of the conveyor belt 36 and conveyed in accordance with the endless movement of the conveyor belt 36, and the paper P is delivered to the fixing device 40.

[0029] The paper P sent into the fixing device 40 is sandwiched in a fixing nip formed by the contact between the endless fixing belt 41 and the pressure roller 44. Then, the toner image is fixed onto the surface of the paper P by the action of pressure, heat, etc.

[0030] The paper P, on whose first surface a toner image has been transferred at the secondary transfer nip and on whose first surface the toner image has been fixed by the fixing device 40, is sent toward the conveyance switching device 50. In the printer 100, the conveyance switching device 50, the re-feeding path 54, the switchback path 55, the post-switchback conveyance path 56, etc. constitute a re-feeding means. The conveyance switching device 50 switches the subsequent conveyance destination of the paper P received from the fixing device 40 between the paper discharge path 51 and the re-feeding path 54.

[0031] Specifically, when a print job in a single-sided mode in which an image is formed only on the first side of the paper P is executed, the conveyance destination is set to the paper discharge path 51. As a result, the paper P with an image formed only on the first side is sent to the paper discharge roller pair 52 via the paper discharge path 51 and discharged onto the paper discharge tray 53 outside the machine. Also, when a print job in a double-sided mode in which images are formed on both sides of the paper P is executed, the conveyance destination is also set to the paper discharge path 51 when the paper P with images fixed on both sides is received from the fixing device 40. As a result, the paper P with images formed on both sides is discharged onto the paper discharge tray 53 outside the machine.

[0032] On the other hand, when a print job in double-sided mode is executed and a sheet of paper P with an image fixed only on its first side is received from the fixing device 40, the transport destination is set to the re-sending path 54. A switchback path 55 is connected to the re-sending path 54, and the sheet of paper P sent to the re-sending path 54 enters this switchback path 55. Then, when the entire area of ​​the sheet of paper P in the transport direction enters the switchback path 55, the transport direction of the sheet of paper P is reversed and the sheet of paper P switches back.

[0033] In addition to the re-feed path 54, a post-switchback transport path 56 is connected to the switchback path 55, and the switched-back paper P enters this post-switchback transport path 56. At this time, the paper P is turned upside down. The turned-upside-down paper P then passes through the post-switchback transport path 56 and the above-mentioned paper feed path 30 and is sent again to the secondary transfer nip. The paper P, which has a toner image transferred to its second side at the secondary transfer nip, passes through the fixing device 40, where the toner image is fixed to the second side, and then passes through the transport switching device 50, paper discharge path 51, and paper discharge roller pair 52, and is discharged onto paper discharge tray 53.

[0034] 2 and 3 are cross-sectional views showing a schematic configuration of the fixing device 40 in this embodiment, taken along a direction perpendicular to the roller axes of the fixing roller 42 and the heating roller 43. FIG. FIG. 4 is a cross-sectional view of the fixing roller 42 and the heating roller 43 in the roller axis direction, showing a schematic configuration of the fixing device 40 in this embodiment.

[0035] 2 and 3 includes a fixing belt 41 that contacts the surface of paper P on which an unfixed toner image has been formed. The fixing belt 41 is stretched over a fixing roller 42, which is a fixing belt drive roller, and a heating roller 43, which is a fixing belt driven roller, and together with the fixing roller 42 and the heating roller 43, constitutes a fixing rotor. The fixing roller 42 is driven to rotate in the clockwise direction (the direction of arrow D) in FIGS. 2 and 3, causing the fixing belt 41 to rotate and run in the clockwise direction in FIGS. 2 and 3.

[0036] Heating roller 43 is a heating member made of a rotating body that incorporates fixing heater 45, which is a heating body, and fixing roller 42 is also made of a rotating body and is a fixing nip forming member that stretches fixing belt 41 together with heating roller 43. Heating roller 43 is heated by fixing heater 45, and fixing belt 41 is heated by heating roller 43.

[0037] The fixing device 40 includes a pressure roller 44 below the fixing roller 42. The pressure roller 44 is disposed opposite the fixing roller 42 with the fixing belt 41 sandwiched therebetween, and is pressed against the fixing roller 42 by a pressure mechanism via the fixing belt 41. The fixing roller 42 is driven to rotate by a drive mechanism, which rotates the fixing belt 41, and the pressure roller 44 rotates counterclockwise (in the direction of arrow E) in FIGS. 2 and 3 together with the fixing belt 41.

[0038] The surface temperature of fixing belt 41 is detected by two temperature sensors 49A and 49B (described later) serving as first and second temperature detection means. Controller 400 functions as temperature control means and controls fixing heater 45 based on the output value of temperature sensor 49A so that the surface temperature of fixing belt 41 reaches a target temperature. Fixing heater 45 is controlled using ON / OFF control, PID control, or the like.

[0039] The paper sheet P carrying an unfixed toner image is transported into the fixing device 40 along the entrance guide plate 49a as shown by the arrow α in Figure 2, and passes through the fixing nip formed by the fixing belt 41 and the pressure roller 44 via the fixing belt 41. In the fixing nip formed by the fixing belt 41 and the pressure roller 44, which are controlled to a target temperature, the toner image on the paper sheet P is melted and fixed to the paper sheet P. After passing through the fixing nip, the paper sheet P is separated from the fixing belt 41 and the pressure roller 44 by the fixing separation plate 49b and the pressure separation claw 49c at the exit of the fixing nip, and is sent out of the printer 100 body via the conveyance path and the paper discharge path 51 in the conveyance switching device 50.

[0040] 2 and 3, the fixing device 40 also includes a pressure roller cleaning device 110 that cleans the surface of the pressure roller 44. The pressure roller cleaning device 110 includes a cleaning web 11, which is a strip-shaped cleaning member, a web holding shaft 11b, a web take-up shaft 11a, and a web contact roller 11c. One end of the cleaning web 11 is fixed to the web take-up shaft 11a, and the other end is fixed to the web holding shaft 11b, and the portion of the cleaning web 11 between the web take-up shaft 11a and the web holding shaft 11b is pressed against the surface of the pressure roller 44 by the web contact roller 11c.

[0041] The web take-up shaft 11a rotates in the direction of arrow G1 in FIG. 2 to wind up the cleaning web 11. The portion of the cleaning web 11 that contacts the pressure roller 44 and has been used for cleaning moves in the direction of arrow G2 in FIG. 2, and the unused portion comes into contact with the pressure roller 44. The unused portion of the cleaning web 11 is wound around the web holding shaft 11b. The cleaning web 11 is pulled as it moves in the direction of arrow G2 in FIG. 2, and the web holding shaft 11b rotates in the direction of arrow G3 in FIG. 2, unwinding the unused portion of the cleaning web 11 toward the contact point with the pressure roller 44. By cleaning the surface of the pressure roller 44 with a cleaning means that uses the cleaning web 11 as a cleaning member, it is possible to continue cleaning the pressure roller 44 using the portion of the cleaning member that is free of deposits. This makes it possible to maintain the surface of the pressure roller 44 free of deposits.

[0042] When a certain number of sheets of paper P narrower than the maximum paper passing width are passed through and then an image with a large amount of toner attached is output on the surface of a wider sheet of paper P, the following degradation in image quality may occur: In other words, in the paper width direction perpendicular to the paper passing direction (arrow α direction) of the paper P, at positions where both ends of the narrower sheets of paper that have passed through a certain number of times have passed, the fixation properties of the image on the wider paper thereafter will be different from other positions, resulting in degradation in image quality such as glossy streaks.

[0043] FIG. 5 is an explanatory diagram showing an example of a state in which glossy streaks 12 occur when a solid image is formed on a wide sheet of paper P after a certain number of narrow sheets of paper P have been passed through. In the example shown in Figure 5, gloss streaks 12, which are stripe-like gloss unevenness extending in the paper transport direction, have occurred in two places in the paper width direction (the direction of arrow β in Figure 5). This is for the following reason: When paper P narrower than the maximum width is fed for a certain period of time, burrs on both ends of the paper P repeatedly leave friction marks (paper edge streaks) at the same position in the width direction on the outer circumferential surface of the fixing belt 41. Repeated friction marks at the same position become edge scratches, and the image fixed at the edge scratched part on the outer circumferential surface of the fixing belt 41 appears in the output image as gloss streaks 12, causing this.

[0044] Therefore, the fixing device 40 of this embodiment is provided with a refreshing roller 46 as a contact rotating body that rubs against the surface of the fixing belt 41. The refreshing roller 46 is a polishing roller having a polishing layer as a surface layer around a metal core. The refreshing roller 46 is rotatably supported by a roller bracket 47, and can be rotated by a motor 46a in the co-rotating direction of the fixing belt 41 (the direction of arrow F in FIG. 3).

[0045] When the control unit 400 executes the polishing mode, the refreshing roller 46 is driven to rotate at a predetermined relative speed (first relative speed) with respect to the fixing belt 41 while in contact with the outer peripheral surface of the fixing belt 41 as shown in Fig. 3. As a result, the outer peripheral surface of the fixing belt 41 is rubbed and polished by the refreshing roller 46. This polishing can repair the rubbing marks (paper edge streaks) caused by burrs on both ends of the width direction of the paper P on the outer peripheral surface of the fixing belt 41, and can prevent the occurrence of gloss streaks 12 shown in Fig. 5, or can make gloss streaks 12 less visible even if they occur.

[0046] The rotation speed of the refreshing roller 46 in the fixing device 40 of this embodiment is preferably set so that its surface movement speed (linear speed) is within a range of three to six times the surface movement speed (linear speed) of the fixing belt 41. If it is less than three times, sufficient polishing will not be possible, and if it exceeds six times, excessive polishing may occur, reducing the life of the fixing belt 41. By setting the surface movement speed of the refreshing roller 46 within a range of three to six times the surface movement speed of the fixing belt 41, it is possible to recover the friction marks on the fixing belt 41 by polishing while suppressing a reduction in the life of the fixing belt 41.

[0047] The refreshing roller 46 of this embodiment is provided so as to contact the portion of the fixing belt 41 wound around the fixing roller 42. This allows the fixing roller 42 to receive the pressure applied by the refreshing roller 46 to press the fixing belt 41, and allows the refreshing roller 46 to contact the fixing belt 41 with high contact pressure. This allows the refreshing roller 46 to rub against the fixing belt 41 with high rubbing force.

[0048] In this embodiment, the roller bracket 47 that supports the refreshing roller 46 is supported so as to be rotatable about a rotation shaft 47a. The roller bracket 47 is biased by a biasing means in the clockwise direction in FIG. 2 about the rotation shaft 47a, and this biasing force causes the roller bracket 47 to come into contact with the cam surface of the eccentric cam 48, thereby positioning it.

[0049] The eccentric cam 48 is driven to rotate by a motor 48a controlled by the control unit 400, and the rotation position of the eccentric cam 48 is controlled by the control unit 400. During an image forming operation (at least while the paper P is passing through the fixing nip), the eccentric cam 48 is rotated to the rotation position shown in FIG. 2, the roller bracket 47 takes the rotation position shown in FIG. 2, and the refreshing roller 46 is positioned in a state separated from the fixing belt 41.

[0050] On the other hand, after the image forming operation is completed (for example, after the job ends), the control unit rotates the eccentric cam 48 to the rotation position shown in Fig. 3, the roller bracket 47 takes the rotation position shown in Fig. 3, and the refreshing roller 46 is positioned in contact with the fixing belt 41. Then, the control unit 400 executes a polishing mode for polishing the outer peripheral surface of the fixing belt 41, thereby recovering the friction marks (paper edge streaks) on the fixing belt 41.

[0051] Next, the temperature difference suppression mode, which is a characteristic feature of the present invention, will be described. FIG. 6 is an explanatory diagram showing the temperature distribution of fixing belt 41 in the paper width direction after paper P having a paper width narrower than the maximum paper passing width is continuously passed. FIG. 7 is an explanatory diagram showing the outer diameter profile of the fixing roller 42 in the paper width direction and the contact pressure distribution between the refreshing roller 46 and the fixing belt 41 after continuous paper P having a paper width narrower than the maximum paper passing width is passed.

[0052] Generally, in the paper width direction (the direction of the rotation axis of the fixing belt 41), the paper contact area of ​​the outer surface of the heated fixing belt 41 that comes into contact with the paper P becomes colder because heat is absorbed by the paper P, and the paper non-contact area that does not come into contact with the paper P becomes hotter because heat is not absorbed by the paper P. Therefore, when paper P whose width is narrower than the maximum paper passing width is continuously passed, as shown in FIG. 6, the temperature of both end portions of the fixing belt 41 that do not come into contact with the paper P (paper non-contact areas) becomes higher than the temperature of the center portion of the fixing belt 41 that comes into contact with the paper P (paper contact area).

[0053] If the temperature difference between the paper contact area and the paper non-contact area becomes large in this way, for example, when a wider paper sheet is subsequently fixed, the fixing temperature for this paper will be uneven across the paper width. More specifically, within the paper contact area of ​​fixing belt 41 that comes into contact with the wider paper sheet, there will be a paper contact area with a low temperature when the narrower paper sheet was passed through, and a paper non-contact area with a high temperature. As a result, the temperature when fixing the wider paper sheet will be low in the center of the paper width and high on both ends of the paper width, causing uneven fixing temperature across the paper width and uneven fixing performance, which can result in an abnormal image with a mixture of image portions with different image quality (glossiness, etc.).

[0054] 7, for example, the outer diameter of the fixing roller 42 corresponding to the high-temperature non-contact areas (end portions) becomes larger than the outer diameter of the low-temperature contact area (center portion) of the fixing roller 42 due to thermal expansion. This causes a step at the boundary between the center portion and both end portions of the fixing roller 42, and also causes a step on the outer circumferential surface of the fixing belt 41 wound around the fixing roller 42.

[0055] As a specific example, when narrow paper P is continuously fed, the temperature of both end portions (non-paper contact areas) of fixing belt 41 may reach 220°C, and the temperature of the center portion (paper contact area) of fixing belt 41 may reach 170°C (temperature difference = 50°C), as shown in Fig. 6. In this case, the step on the roller surface of fixing roller 42 is approximately 0.5 mm, as shown in Fig. 7.

[0056] If such a step occurs, for example, when the refreshing roller 46 rubs and polishes the outer peripheral surface of the fixing belt 41 in the polishing mode, the contact pressure of the refreshing roller 46 becomes higher at both end portions corresponding to the high-temperature non-paper-contact areas, as shown in Figure 7. Also, the contact pressure of the refreshing roller 46 becomes lower at the center portion corresponding to the low-temperature paper-contact area. As a result, appropriate polishing cannot be performed at the locations where the contact pressure of the refreshing roller 46 is low, and the rubbing marks (paper edge streaks) that have occurred on the outer peripheral surface of the fixing belt 41 cannot be restored, making it impossible to eliminate uneven gloss.

[0057] FIG. 8 is a flowchart relating to the control of the polishing mode and the temperature difference suppression mode in this embodiment. In this embodiment, the control unit 400 determines whether the conditions for executing the polishing mode are met at a predetermined timing, such as after the image forming operation is completed (after the job ends) (S1). The conditions for executing the polishing mode include a condition that may cause friction marks (paper edge streaks) on the outer peripheral surface of the fixing belt 41, such as a condition that a predetermined number of sheets of paper narrower than the maximum paper passing width have been passed continuously.

[0058] If it is determined that the conditions for executing the polishing mode are met (Yes in S1), the control unit 400 controls the motor 48a to rotate the eccentric cam 48 to the rotation position shown in Fig. 3, and brings the refreshing roller 46 into contact with the fixing belt 41 (S2). Thereafter, the control unit 400 of this embodiment determines whether to execute the temperature difference suppression mode before executing the polishing mode.

[0059] Specifically, the control unit 400 acquires temperature detection results T1 and T2 from a center temperature sensor 49A that detects the temperature of the center portion of the fixing belt 41 in the paper width direction, and an end temperature sensor 49B that detects the temperatures of both end portions of the fixing belt 41 in the paper width direction (S3). The temperature detection result T1 indicates the temperature of the outer surface portion (paper contact area) of the fixing belt 41 that has been in contact with the paper P that has passed through the fixing nip up to that point. The temperature detection result T2 indicates the temperature of the outer surface portion (paper non-contact area) of the fixing belt 41 that has not been in contact with the paper P that has passed through the fixing nip up to that point.

[0060] Then, the control unit 400 determines whether the temperature difference (T2-T1) obtained by subtracting the temperature detection result T1 (temperature of the paper contact area) of the central temperature sensor 49A from the temperature detection result T2 (temperature of the paper non-contact area) of the end temperature sensor 49B is greater than or equal to a predetermined value (S4).

[0061] On the other hand, if it is determined that the temperature difference (T2-T1) is equal to or greater than the predetermined value (Yes in S4), there is a large step at the boundary between the paper contact area and the paper non-contact area on the fixing belt 41. Therefore, if the polishing mode is executed as is and the fixing belt 41 is polished by rubbing with the refresh roller 46, polishing failure occurs in which the area near the step cannot be polished properly, and it becomes difficult to fully recover the rubbing marks (paper edge streaks).

[0062] Therefore, when the control unit 400 determines that the temperature difference (T2-T1) is equal to or greater than a predetermined value (Yes in S4), it executes a temperature difference suppression mode to reduce this temperature difference before executing the polishing mode (S5). Specifically, the control unit 400 functions as a switching unit, and executes an operation of rotating the refreshing roller 46, which is in contact with the fixing belt 41, in accordance with the rotation of the fixing belt 41 for a predetermined time t.

[0063] Generally, the smaller the relative speed difference between the refreshing roller 46 and the fixing belt 41, the more easily heat from the fixing belt 41 is transferred to the refreshing roller 46. Normally, in areas of the fixing belt 41 where the temperature is high (i.e., paper non-contact areas), the rate of heat transfer to the refreshing roller 46 is faster than in areas of the fixing belt 41 where the temperature is low (i.e., paper contact areas), and the temperature is more likely to drop. Therefore, by controlling the rotational operation of the refreshing roller 46 so that the relative speed difference between the fixing belt 41 and the refreshing roller 46 in contact therewith is smaller than the relative speed difference in the polishing mode, the temperature difference between the sheet contact area and the sheet non-contact area can be reduced.

[0064] In particular, in this embodiment, the outer peripheral surface portion of the fixing belt 41 that contacts the refreshing roller 46 is the portion wound around the fixing roller 42, whose outer diameter changes due to thermal expansion. Therefore, the outer peripheral surface portion of the fixing belt 41 that contacts the refreshing roller 46 deforms to conform to the roller surface shape of the fixing roller 42, whose outer diameter changes due to thermal expansion. That is, a step is created on this outer peripheral surface portion of the fixing belt 41, where the low-temperature paper-contact area is recessed and the high-temperature paper-non-contact area is protruding. As a result, the refreshing roller 46 is in close contact with the protruding paper-contact area but not with the recessed paper-contact area. Due to this difference in contact, in this embodiment, the non-paper-contact area of ​​the fixing belt 41 transfers heat to the refreshing roller 46 faster than the paper-contact area, and the temperature is more likely to drop.

[0065] In this embodiment, in the temperature difference suppression mode, in order to efficiently reduce the temperature difference between the sheet contact area and the sheet non-contact area, the refreshing roller 46 is rotated in a driven manner to reduce the relative speed difference between the refreshing roller 46 and the fixing belt 41 to zero. However, the relative speed difference between the refreshing roller 46 and the fixing belt 41 in the temperature difference suppression mode does not necessarily have to be reduced to zero, as long as it is smaller than the relative speed difference in the polishing mode.

[0066] The control unit 400 executes the temperature difference suppression mode, and rotates the refreshing roller 46 for a predetermined time t, and then executes the polishing mode (S6). In the polishing mode, the control unit 400 controls the motor 46a to rotate the refreshing roller 46 in the co-rotating direction of the fixing belt 41. As described above, it is preferable that the rotation speed of the refreshing roller 46 in the polishing mode is set so that its surface movement speed (linear speed) is within a range of three to six times the surface movement speed (linear speed) of the fixing belt 41.

[0067] Thus, in this embodiment, when the temperature difference (T2-T1) is equal to or greater than a predetermined value (e.g., 30°C), the temperature difference suppression mode is executed, and the polishing mode is executed after the temperature difference between the paper contact area and the paper non-contact area has decreased. As a result, even if a large temperature difference (T2-T1) causes a large step at the boundary between the paper contact area and the paper non-contact area on the fixing belt 41, the temperature difference can be reduced to reduce the step before polishing is performed. Therefore, even if the temperature difference (T2-T1) is equal to or greater than a predetermined value, polishing defects can be suppressed, and the friction marks (paper edge streaks) can be sufficiently restored.

[0068] On the other hand, if the control unit 400 determines that the temperature difference (T2-T1) is less than a predetermined value (e.g., 30°C) (No in S4), it does not execute the temperature difference suppression mode (S5) but executes the polishing mode (S6). In this case, since the step that occurs at the boundary between the paper contact area and the paper non-contact area on the fixing belt 41 is small, polishing defects do not occur and the friction marks (paper edge streaks) can be sufficiently restored.

[0069] Next, an effect confirmation test in this embodiment will be described. In this test, we compared this embodiment, in which the temperature difference suppression mode is executed before the polishing mode is executed if the temperature difference (T2-T1) is greater than or equal to a predetermined value (30°C), with a comparative example, in which the polishing mode is executed as is (the temperature difference suppression mode is not executed) even if the temperature difference is greater than or equal to the predetermined value.

[0070] Table 1 below summarizes the results of this effectiveness confirmation test, which shows the relationship between the temperature difference between both end portions (non-paper contact areas) and the center portion (paper contact area) of the fixing belt 41 and the evaluation rank of paper edge streaks after performing a predetermined polishing mode. Note that the higher the evaluation rank of paper edge streaks, the less paper edge streaks there are.

[0071] [Table 1]

[0072] As shown in Table 1 above, when the temperature difference (T2 - T1) between the paper non-contact area and the paper contact area was 20°C, the difference in outer diameter between the paper non-contact area and the paper contact area on the roller surface of the fixing roller 42 was 0.2 mm. In this case, even in the comparative example, the evaluation rank of the paper edge streaks after execution of the polishing mode was an acceptable rank 4. Furthermore, when the temperature difference (T2 - T1) was 20°C, the temperature difference suppression mode was not executed in the embodiment, so the operation was the same as in the comparative example, and the evaluation rank of the paper edge streaks after execution of the polishing mode was rank 4, the same as in the comparative example.

[0073] On the other hand, when the temperature difference (T2 - T1) between the paper non-contact area and the paper contact area was 30°C, the outer diameter difference between the paper non-contact area and the paper contact area on the roller surface of the fixing roller 42 was 0.5 mm, as shown in Table 1 above. In this case, in the comparative example, the evaluation rank for paper edge streaks after execution of the polishing mode was unacceptable, Rank 3. In contrast, in the embodiment, because the temperature difference (T2 - T1) was equal to or greater than the predetermined value (30°C), the temperature difference suppression mode was executed, and then the polishing mode was executed. As a result, execution of the temperature difference suppression mode reduced the outer diameter difference between the paper non-contact area and the paper contact area to 0.2 mm. As a result, the evaluation rank for paper edge streaks after execution of the polishing mode was maintained at acceptable Rank 4.

[0074] In this embodiment, the temperature difference suppression mode is executed to suppress poor polishing by the refresh roller 46, but the temperature difference suppression mode may also be executed for the purpose of suppressing other problems caused by, for example, a large temperature difference between the paper contact area and the paper non-contact area.

[0075] For example, if a certain number of sheets of paper narrower than the maximum paper feed width are fed and the temperature difference between the paper contact area and the paper non-contact area for the narrow paper becomes large, when a wider sheet of paper is subsequently fed and fixed, uneven fixing temperature occurs in the paper width direction. As a result, uneven fixing occurs in the paper width direction, and an abnormal image with a mixture of image portions with different image quality (glossiness, etc.) may occur. In this case, if the temperature difference suppression mode is executed after a certain number of narrow sheets of paper have been fed and before a wider sheet of paper is fed, uneven fixing temperature for the wide sheet of paper is suppressed, and an abnormal image with a mixture of image portions with different image quality (glossiness, etc.) is suppressed.

[0076] In addition, in this embodiment, an example has been described in which the refresh roller 46 is used as the contact rotating body, but the contact rotating body may be a rotating body other than the refresh roller 46 as long as it rotates in contact with both the paper contact area and the paper non-contact area of ​​the fixing belt 41.

[0077] [Modification] A modification of the temperature difference suppression mode in this embodiment will be described. FIG. 9 is an explanatory diagram showing the temperature distribution of fixing belt 41 in the paper width direction after continuous passage of paper P whose paper width is narrower than the maximum paper passing width in this modified example. Figure 10 is an explanatory diagram showing the outer diameter profile of the fixing roller 42 in the paper width direction and the contact pressure distribution between the refresh roller 46 and the fixing belt 41 after continuous paper P having a paper width narrower than the maximum paper passing width is passed through in this modified example.

[0078] This modified example differs from the above-described embodiment in that the target temperature of the fixing belt 41 in the temperature difference suppression mode is set to a temperature higher than the target temperature during image formation operation, and in that a heat pipe is used for the core of the refreshing roller 46.

[0079] FIG. 11 is a flowchart relating to the control of the polishing mode and the temperature difference suppression mode in this modified example. In this modified example, when the control unit 400 determines that the conditions for executing the polishing mode are met at a predetermined determination timing (Yes in S1), it brings the refreshing roller 46 into contact with the fixing belt 41 (S2). Then, the control unit 400 acquires the temperature detection results T1 and T2 of the paper contact area and the paper non-contact area of ​​the fixing belt 41 (S3), and when it determines that the temperature difference (T2-T1) is equal to or greater than a predetermined value (Yes in S4), it executes the temperature difference suppression mode (S5).

[0080] In this modified example, when the temperature difference suppression mode is executed, the target temperature of the fixing belt 41 is set to a temperature higher than the target temperature during the image formation operation (S10). As a result, the temperature of the outer peripheral surface of the fixing belt 41 becomes higher than the temperature during the image formation operation. As a result, in this modified example, as shown by the two-dot chain line in FIG. 9, the temperature of the paper contact area becomes higher than when it remains the same as during the image formation operation (the solid line in FIG. 9, which is the same as FIG. 6 of the above-described embodiment). As a result, according to this modified example, by executing the temperature difference suppression mode, the temperature of the paper contact area shifts toward the temperature of the paper non-contact area, which is higher.

[0081] On the other hand, in this modified example, a heat pipe is used for the core of the refreshing roller 46. Therefore, the refreshing roller 46 has a higher cooling effect on the fixing belt 41 (faster heat transfer rate) than the above-described embodiment in which the core of the refreshing roller 46 is made of metal. In particular, the temperature of the paper non-contact area in close contact with the refreshing roller 46 drops significantly lower than the temperature of the paper contact area not in close contact with the refreshing roller 46. As a result, even if the target temperature of the fixing belt 41 is set higher than that during image formation, the temperature of the paper non-contact area in this modified example is lower than that in the above-described embodiment (solid line in FIG. 9, which is the same as FIG. 6 for the above-described embodiment), as shown by the two-dot chain line in FIG. 9 . As a result, according to this modified example, by executing the temperature difference suppression mode, the temperature of the paper non-contact area shifts toward the temperature of the paper contact area, which is lower.

[0082] As a result of the above, in this modified example, by executing the temperature difference suppression mode, the temperature difference (T2 - T1) between the paper contact area and the paper non-contact area can be reduced more quickly than in the above-described embodiment, as shown by the two-dot chain line in Figure 9. Therefore, as shown by the two-dot chain line in Figure 10, the time required to reduce the step on the roller surface of the fixing roller 42 to the allowable range of 0.2 mm is shortened. Therefore, in the temperature difference suppression mode of processing step S5' in this modified example, the predetermined time t' for which the refreshing roller 46 is driven to rotate can be made shorter than the predetermined time t in the above-described embodiment, thereby shortening the processing time.

[0083] In this modified example, the target temperature of the fixing belt 41 in the temperature difference suppression mode is set higher than that during image formation operation, and a heat pipe is used for the core of the refresh roller 46. However, the effect of shortening the processing time can be obtained with either one of these.

[0084] The above description is merely an example, and each of the following aspects provides unique effects. [First aspect] The first aspect is a fixing device 40 that fixes an image on a sheet (e.g., paper P) by bringing the outer peripheral surface of a heated fixing rotor (e.g., fixing belt 41, fixing roller 42, and heating roller 43) into contact with the sheet, the fixing device 40 including a first temperature detection means (e.g., central temperature sensor 49A) that detects, as a first temperature T1, the temperature of a first region (e.g., paper contact region) of the fixing rotor that contacts both a first sheet (e.g., a wide sheet) having a first sheet width (e.g., a maximum sheet width) and a second sheet (e.g., a narrow sheet) having a second sheet width narrower than the first sheet width; The fixing rotor is characterized by comprising a second temperature detection means (e.g., end temperature sensor 49B) that detects the temperature of a second region (e.g., a paper non-contact region) of the fixing rotor that contacts the first region but not the second sheet as a second temperature T2, a contact rotor (e.g., a refresh roller 46) that contacts both the first region and the second region of the fixing rotor and is rotatable with a relative speed difference with respect to the fixing rotor, and a switching means (e.g., control unit 400) that switches the rotation operation of the contact rotor based on the detection results T1, T2 of the first temperature detection means and the second temperature detection means. Generally, in the direction of the rotation axis of the fixing rotor, the sheet contact area of ​​the outer peripheral surface of the heated fixing rotor is low in temperature because the sheet absorbs heat, while the sheet non-contact area is high in temperature because the sheet does not absorb heat. For example, when a second sheet having a narrow width is continuously fixed, if the temperature difference between the sheet contact area and the sheet non-contact area becomes large, various problems will occur. For example, if a narrow second sheet is fixed in succession, and the temperature difference between the sheet contact area (first area) and the sheet non-contact area (second area) at that time becomes large, and then a wider first sheet is fixed, the sheet contact area of ​​the fixing rotor that comes into contact with the first sheet contains both the sheet contact area and the sheet non-contact area at the time of fixing the second sheet, causing temperature unevenness, which can lead to image quality degradation due to differences in image quality within the sheet. Furthermore, when a second sheet having a narrower width is continuously fixed, the temperature difference between the sheet contact area (first area) and the sheet non-contact area (second area) becomes large. The outer diameter of the high-temperature sheet non-contact area becomes larger than the outer diameter of the low-temperature sheet contact area due to thermal expansion. This causes a step to appear on the outer surface of the fixing rotor between the sheet contact area and the sheet non-contact area. As a result, for example, in a fixing device equipped with a polishing roller (contact rotor) that rubs against the outer surface of the fixing rotor to polish scratches on the outer surface of the fixing rotor, the polishing roller cannot contact the outer surface near the step, preventing proper polishing. This can result in uneven polishing on the outer surface of the fixing rotor. Conventionally, there are many fixing devices equipped with a contacting rotator that contacts both the first and second regions of the fixing rotator and rotates at a relative speed difference with respect to the fixing rotator. Examples of such a contacting rotator include a polishing roller that polishes the fixing rotator, an external heating roller that heats the fixing rotator, and a tension roller that applies tension to a belt-shaped fixing rotator. In this embodiment, such a contacting rotator is used to reduce the temperature difference that occurs between the first and second regions. Specifically, generally, the smaller the relative speed difference between the contact rotor and the fixing rotor, the easier it is for heat from the fixing rotor to be transferred to the contact rotor. Therefore, by switching the rotational movement of the contact rotor, which is driven to rotate at a relative speed difference with respect to the fixing rotor, so as to change the relative speed difference, it is possible to change the rate at which heat is transferred from the fixing rotor to the contact rotor (heat transfer rate). For example, typically, the higher the temperature of a portion of the fixing rotor, the faster the heat transfer rate to the contact rotor, and the easier it is for the temperature to drop. Therefore, by switching the rotational movement of the contact rotor so as to reduce the relative speed difference with respect to the fixing rotor, the temperature difference between the low-temperature sheet contact area and the high-temperature sheet non-contact area is reduced, making it possible to reduce that temperature difference. In this aspect, the rotational operation of the contact rotor, which can rotate at a relative speed difference with respect to the fixing rotor, is switched based on the detection results of the first temperature detection means that detects the temperature of the first region (first temperature) and the second temperature detection means that detects the temperature of the second region (second temperature). As a result, when using a contact rotor such as a polishing roller that rotates at a large relative speed difference with respect to the fixing rotor, if a temperature difference between the sheet contact region (first region) and the sheet non-contact region (second region) of the fixing rotor is large when a narrow sheet is used, the rotational speed of the contact rotor can be switched to reduce the relative speed difference. This allows a contact rotor such as a polishing roller, which rubs against the fixing rotor, to also reduce the temperature difference between the regions when a narrow sheet is used, thereby preventing various problems that may arise due to the temperature difference between these regions.

[0085] [Second mode] The second aspect is characterized in that, in the first aspect, the point at which the contacting rotor abuts in the direction of movement of the outer surface of the fixing rotor is a point at which the outer surface displaces in the direction normal to the outer surface due to thermal expansion. In this aspect, thermal expansion causes a step on the outer peripheral surface of the fixing rotor with which the contact rotor contacts, where the low-temperature sheet contact area is recessed and the high-temperature sheet non-contact area is protruding. As a result, the contact rotor adheres closely to the protruding sheet non-contact area but not to the recessed sheet contact area. In this aspect, due to this difference in adhesion, the sheet non-contact area of ​​the fixing rotor transfers heat to the contact rotor faster than the sheet contact area, and the temperature is more likely to drop. Therefore, the temperature difference between the low-temperature sheet contact area and the high-temperature sheet non-contact area can be reduced.

[0086] [Third aspect] A third aspect is characterized in that, in the first or second aspect, the switching means performs the switching so that, when the difference (T2-T1) between the first temperature T1 and the second temperature T2 is less than a predetermined value, the contacting rotor rotates so that the relative speed of the contacting rotor with respect to the fixing rotor becomes a first relative speed (e.g., the relative speed in a polishing mode), and, when the difference is equal to or greater than a predetermined value, the contacting rotor rotates so that the relative speed becomes a second relative speed (e.g., the relative speed in a temperature difference suppression mode) that is smaller than the first relative speed. According to this aspect, when the temperature difference between the sheet contact area and the sheet non-contact area exceeds a predetermined value, the relative speed of the contacting rotator with respect to the fixing rotator is reduced to the second relative speed, thereby reducing the temperature difference, thereby suppressing problems that may occur due to the large temperature difference.

[0087] [Fourth aspect] A fourth aspect is the third aspect, characterized in that when the difference is equal to or greater than a predetermined value, the switching means causes the contact rotor to rotate in a driven manner relative to the fixing rotor so that the second relative speed becomes zero. This allows the temperature difference between the sheet contact area and the sheet non-contact area to be reduced more quickly.

[0088] [Fifth mode] A fifth aspect is characterized in that, in the third or fourth aspect, a temperature control means (e.g., control unit 400) is provided for controlling the temperature of the fixing rotor, and the temperature control means makes the temperature of the fixing rotor higher when the switching means switches the rotational operation of the contact rotor so that the relative speed becomes the second relative speed (e.g., in the temperature difference suppression mode) than when the switching means rotates the contact rotor so that the relative speed becomes the first relative speed (e.g., in the polishing mode). This allows the temperature difference between the sheet contact area and the sheet non-contact area to be reduced more quickly, as explained in the modified example above.

[0089] [Sixth aspect] A sixth aspect is the one according to any one of the first to fifth aspects, characterized in that the contact rotating body includes a heat pipe extending along the rotation axis direction. This allows the temperature difference between the sheet contact area and the sheet non-contact area to be reduced more quickly, as explained in the modified example above.

[0090] [Seventh aspect] A seventh aspect is characterized in that in any one of the first to sixth aspects, the contact rotor has an abrasive layer as a surface layer. In this way, the contact rotor rotates with a relative speed difference relative to the fixing rotor, so that the outer circumferential surface of the fixing rotor can be polished by the polishing layer of the contact rotor, thereby repairing scratches such as paper edge streaks that may occur on the outer circumferential surface of the fixing rotor. In particular, when the outer peripheral surface of the fixing rotor with which the contact rotor abuts is thermally expanded, the low-temperature sheet contact area is recessed and the high-temperature sheet non-contact area is protruded, and if the temperature difference between the sheet contact area and the sheet non-contact area is large, this step becomes large. In this case, polishing defects may occur, where the contact rotor cannot properly polish the area around this step. In this aspect, as described above, the temperature difference between the sheet contact area and the sheet non-contact area can be reduced, thereby suppressing polishing defects.

[0091] [Eighth aspect] The eighth aspect is the seventh aspect, characterized in that, when the difference is less than a predetermined value, the switching means drives the contact rotor to rotate in a direction parallel to the fixing rotor at a surface movement speed within a range of three to six times the surface movement speed of the fixing rotor. This makes it possible to suppress a decrease in the life of the fixing rotor while sufficiently polishing the outer peripheral surface of the fixing rotor with the contact rotor.

[0092] [Ninth aspect] A ninth aspect is an image forming apparatus characterized by comprising any one of the first to eighth fixing devices. This makes it possible to provide an image forming apparatus that can reduce the temperature difference between the sheet contact area and the sheet non-contact area on the outer surface of the fixing rotor, thereby suppressing various problems that may arise due to this temperature difference. [Explanation of symbols]

[0093] 1: Optical writing unit 2: Imaging unit 3: Photoreceptor 4: Developing device 5: Charging device 6: Drum cleaning device 11: Cleaning Web 12: Glossy streaks 34: Registration roller pair 35: Conveyor belt unit 40: Fixing device 41: Fixing belt 42: Fuser roller 43: Heating roller 44: Pressure roller 45: Fixing heater 46: Refresh roller 46a: Motor 47: Roller bracket 47a: Rotating shaft 48: Eccentric cam 48a: Motor 49A: Central temperature sensor 49B: End temperature sensor 49a: Entrance guide plate 49b: Fixing separation plate 49c: Pressure separation claw 60: Primary transcription unit 61: Intermediate transfer belt 62: Primary transfer roller 68: Secondary transfer bias roller 71: Secondary transfer drive roller 75: Intermediate transfer belt cleaning device 76: Secondary belt cleaning device 77: Secondary transfer belt 78: Secondary transfer unit 100: Printer 110: Pressure roller cleaning device 400: Control section P:Paper [Prior art documents] [Patent documents]

[0094] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-96728

Claims

1. A fixing device that fixes an image on a sheet by bringing an outer peripheral surface of a heated fixing rotor into contact with the sheet, a first temperature detecting means for detecting, as a first temperature, a temperature of a first region of the fixing rotating body that contacts both a first sheet having a first sheet width and a second sheet having a second sheet width narrower than the first sheet width; a second temperature detecting means for detecting a temperature of a second region of the fixing rotating body that is in contact with the first sheet but not in contact with the second sheet as a second temperature; a contact rotor that is in contact with both the first region and the second region of the fixing rotor and is rotatable at a relative speed difference with respect to the fixing rotor; a switching means for switching the rotational operation of the contact rotor based on the detection results of the first temperature detection means and the second temperature detection means, the switching means performs the switching so that, when a difference between the first temperature and the second temperature is less than a predetermined value, the contact rotor is rotated so that the relative speed of the contact rotor with respect to the fixing rotor becomes a first relative speed, and, when the difference is equal to or greater than a predetermined value, the switching means rotates the contact rotor so that the relative speed becomes a second relative speed that is smaller than the first relative speed.

2. 2. The fixing device according to claim 1, The fixing device is characterized in that the portion of the fixing rotor in the direction of movement of the outer circumferential surface thereof with which the contact rotor abuts is a portion where the outer circumferential surface is displaced in the direction normal to the outer circumferential surface due to thermal expansion.

3. In the fixing device according to claim 1 or 2, The fixing device is characterized in that, when the difference is equal to or greater than a predetermined value, the switching means rotates the contact rotor in a driven manner relative to the fixing rotor so that the second relative speed becomes zero.

4. 4. The fixing device according to claim 1, a temperature control means for controlling the temperature of the fixing rotor; The fixing device is characterized in that, when the switching means switches the rotational operation of the contact rotor so that the relative speed becomes the second relative speed, the temperature control means increases the temperature of the fixing rotor compared to when the switching means switches the rotational operation of the contact rotor so that the relative speed becomes the first relative speed.

5. 5. The fixing device according to claim 1, The fixing device, wherein the contact rotary body includes a heat pipe extending along the rotation axis direction.

6. 6. The fixing device according to claim 1, The fixing device is characterized in that the contact rotary body has an abrasive layer as a surface layer.

7. 7. The fixing device according to claim 6, The fixing device is characterized in that, when the difference is less than a predetermined value, the switching means drives the contact rotor to rotate in the direction of rotation relative to the fixing rotor at a surface movement speed within a range of 3 to 6 times the surface movement speed of the fixing rotor.

8. An image forming apparatus comprising the fixing device according to claim 1 .

Images