Fixing device and image forming apparatus

A fixing device includes a fixing belt that is rotatable and a pressure rotator that is pressed against the fixing belt to form a fixing nip therebetween. A polishing roller separably contacts the fixing belt and slides over an outer circumferential surface of the fixing belt while the polishing roller is pressed against the fixing belt. At least one support supports and stretches the fixing belt. A polishing aid, which is disposed opposite the polishing roller via the fixing belt, supports and stretches the fixing belt. The polishing roller is disposed opposite the polishing aid via the fixing belt to form a polishing nip between the polishing roller and the fixing belt while the polishing roller is pressed against the fixing belt. The polishing roller changes a rotation locus of the fixing belt along at least a part of a circumferential face of the polishing roller.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application Nos. 2016-165718, filed on Aug. 26, 2016, and 2016-202869, filed on Oct. 14, 2016, in the Japanese Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein.

BACKGROUND

Technical Field

Exemplary aspects of the present disclosure relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus incorporating the fixing device.

Description of the Background

Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a developing device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.

Such fixing device may include a fixing rotator, such as a fixing roller, a fixing belt, and a fixing film, heated by a heater and a pressure rotator, such as a pressure roller and a pressure belt, pressed against the fixing rotator to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. As the recording medium bearing the toner image is conveyed through the fixing nip, the fixing rotator and the pressure rotator apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.

SUMMARY

This specification describes below an improved fixing device. In one embodiment, the fixing device includes a fixing belt rotatable in a rotation direction and a pressure rotator pressed against the fixing belt to form a fixing nip between the fixing belt and the pressure rotator. A polishing roller separably contacts the fixing belt and slides over an outer circumferential surface of the fixing belt while the polishing roller is pressed against the fixing belt. At least one support supports and stretches the fixing belt. A polishing aid, which is disposed opposite the polishing roller via the fixing belt, supports and stretches the fixing belt. The polishing roller is disposed opposite the polishing aid via the fixing belt to form a polishing nip between the polishing roller and the fixing belt while the polishing roller is pressed against the fixing belt. The polishing roller changes a rotation locus of the fixing belt along at least a part of a circumferential face of the polishing roller.

This specification further describes an improved image forming apparatus. In one embodiment, the image forming apparatus includes an image forming device to form a toner image and a fixing device disposed downstream from the image forming device in a recording medium conveyance direction to fix the toner image on a recording medium. The fixing device includes a fixing belt rotatable in a rotation direction and a pressure rotator pressed against the fixing belt to form a fixing nip between the fixing belt and the pressure rotator. A polishing roller separably contacts the fixing belt and slides over an outer circumferential surface of the fixing belt while the polishing roller is pressed against the fixing belt. At least one support supports and stretches the fixing belt. A polishing aid, which is disposed opposite the polishing roller via the fixing belt, supports and stretches the fixing belt. The polishing roller is disposed opposite the polishing aid via the fixing belt to form a polishing nip between the polishing roller and the fixing belt while the polishing roller is pressed against the fixing belt. The polishing roller changes a rotation locus of the fixing belt along at least a part of a circumferential face of the polishing roller.

DETAILED DESCRIPTION OF THE DISCLOSURE

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly toFIG. 1, an image forming apparatus1000according to an embodiment is explained.

The image forming apparatus1000may be a copier, a facsimile machine, a printer, a multifunction peripheral or a multifunction printer (MFP) having at least one of copying, printing, scanning, facsimile, and plotter functions, or the like. According to this embodiment, the image forming apparatus1000is a color printer that forms a color toner image on a recording medium by electrophotography. Alternatively, the image forming apparatus1000may be a monochrome printer that forms a monochrome toner image on a recording medium.

Referring toFIG. 1, a description is provided of a construction of the image forming apparatus1000.

FIG. 1is a schematic vertical cross-sectional view of the image forming apparatus1000as one example. As illustrated inFIG. 1, the image forming apparatus1000is a printer employing a tandem intermediate transfer system. The image forming apparatus1000includes a body100and a sheet feed table200mounting the body100.

The body100accommodates an image forming portion20employing the tandem intermediate transfer system (hereinafter referred to as a tandem image forming portion). The image forming portion20includes a plurality of image forming devices18Y,18M,18C, and18K aligned horizontally. Suffixes Y, M, C, and K represent yellow, magenta, cyan, and black, respectively.

An intermediate transfer belt10serving as an intermediate transferor (e.g., an endless belt) is situated in a sheet conveyance region and a substantially center portion of the body100. The intermediate transfer belt10is looped over a plurality of rollers, that is, an intermediate transfer belt driving roller14, intermediate transfer belt support rollers15aand15b, a secondary transfer opposed roller16a, and the like. The intermediate transfer belt10is rotatable clockwise inFIG. 1in a rotation direction D10.

On the left of the secondary transfer opposed roller16ais an intermediate transfer belt cleaner17. The intermediate transfer belt cleaner17removes residual toner failed to be transferred onto a sheet S and therefore remaining on the intermediate transfer belt10therefrom.

Above an upper face of the intermediate transfer belt10stretched taut across the intermediate transfer belt driving roller14and the intermediate transfer belt support rollers15aand15bare the four image forming devices18Y,18M,18C, and18K aligned horizontally in the rotation direction D10of the intermediate transfer belt10to form yellow, magenta, cyan, and black toner images, respectively, thus constructing the tandem image forming portion20. The image forming devices18Y,18M,18C, and18K of the tandem image forming portion20include photoconductive drums40Y,40M,40C, and40K serving as image bearers that bear yellow, magenta, cyan, and black toner images, respectively.

Above the tandem image forming portion20are two exposure devices21. The left exposure device21is disposed opposite the two image forming devices18Y and18M. The right exposure device21is disposed opposite the two image forming devices18C and18K. For example, each of the exposure devices21employs an optical scanning method and includes two light sources (e.g., a semiconductor laser, a semiconductor laser array, or a multi-beam light source), a coupling optical system, a common optical deflector (e.g., a polygon mirror), and two scanning-image forming optical systems. The exposure devices21expose the photoconductive drums40Y,40M,40C, and40K according to yellow, magenta, cyan, and black image data, forming electrostatic latent images on the photoconductive drums40Y,40M,40C, and40K, respectively.

Each of the photoconductive drums40Y,40M,40C, and40K is surrounded by a developing device and a photoconductive drum cleaner. The developing device visualizes the electrostatic latent image that is formed by a charger and the exposure device21into a visible toner image, that is, yellow, magenta, cyan, and black toner images. Before the exposure devices21expose the photoconductive drums40Y,40M,40C, and40K, the chargers uniformly charge the photoconductive drums40Y,40M,40C, and40K, respectively. The photoconductive drum cleaners remove residual toner failed to be transferred onto the intermediate transfer belt10and therefore remaining on the photoconductive drums40Y,40M,40C, and40K therefrom, respectively.

Primary transfer rollers62Y,62M,62C, and62K serving as primary transferors are disposed opposite the photoconductive drums40Y,40M,40C, and40K via the intermediate transfer belt10to form primary transfer nips between the photoconductive drums40Y,40M,40C, and40K and the intermediate transfer belt10, respectively, where the yellow magenta, cyan, and black toner images formed on the photoconductive drums40Y,40M,40C, and40K are primarily transferred onto the intermediate transfer belt10as a color toner image. The primary transfer rollers62Y,62M,62C, and62K are disposed opposite the photoconductive drums40Y,40M,40C, and40K with the intermediate transfer belt10sandwiched between the primary transfer rollers62Y,62M,62C, and62K and the photoconductive drums40Y,40M,40C, and40K, respectively.

The intermediate transfer belt driving roller14is a driving roller that drives and rotates the intermediate transfer belt10. The intermediate transfer belt driving roller14is coupled to a motor through a driving force transmitter (e.g., a gear, a pulley, and a belt). In a print job to form a black toner image on the intermediate transfer belt10, a mover moves the intermediate transfer belt support rollers15aand15bwithout moving the intermediate transfer belt driving roller14. Thus, the mover isolates the intermediate transfer belt10from the photoconductive drums40Y,40M, and40C used for forming yellow, magenta, and cyan toner images, respectively.

A secondary transfer device22is disposed opposite the tandem image forming portion20via the intermediate transfer belt10. The secondary transfer device22includes a secondary transfer roller16bpressed against the secondary transfer opposed roller16avia the intermediate transfer belt10. The secondary transfer roller16bgenerates a transfer electric field to secondarily transfer the color toner image formed on the intermediate transfer belt10onto a sheet S (e.g., a transfer sheet) serving as a transfer medium or a recording medium.

Downstream from the secondary transfer device22in a sheet conveyance direction DS is a fixing device25that fixes the color toner image transferred from the intermediate transfer belt10onto the sheet S thereon. A detailed description of a construction of the fixing device25is deferred. The fixing device25includes a fixing belt26(e.g., an endless belt) and a pressure roller27pressed against the fixing belt26. The fixing belt26is looped over a plurality of support rollers including a fixing roller29and a heating roller30. A heater (e.g., a lamp or an induction heater employing an electromagnetic induction heating method) is disposed inside at least one of the support rollers (e.g., the heating roller30).

A conveyance belt24supported by two conveyance belt support rollers23aand23bconveys the sheet S bearing the color toner image transferred from the intermediate transfer belt10by the secondary transfer device22to the fixing device25. Instead of the conveyance belt24, a stationary guide, a conveyance roller, or the like may be used.

Below the secondary transfer device22and the fixing device25is a sheet reverse device28disposed in parallelism with the tandem image forming portion20. The sheet reverse device28reverses and conveys the sheet S for duplex printing to print another toner image on a back side of the sheet S. The sheet S bearing the fixed color toner image is ejected by an output roller pair91onto an output tray92.

A description is provided of a construction of the fixing device25.

The fixing device25(e.g., a fuser or a fusing unit) includes the fixing belt26, the pressure roller27, the fixing roller29, the heating roller30, a polishing roller51, and a polishing aid roller54. The fixing belt26is stretched taut across a plurality of supports and rotatable. The pressure roller27serving as a pressure rotator or a pressure member is pressed against the fixing roller29via the fixing belt26to form a fixing nip FN between the pressure roller27and the fixing belt26. The polishing roller51separably contacts the fixing belt26. The polishing roller51slides over an outer circumferential surface of the fixing belt26while the polishing roller51is pressed against the fixing belt26. The polishing aid roller54is at least one of the plurality of supports. The polishing aid roller54serves as a polishing aid that is disposed opposite the polishing roller51via the fixing belt26. While the polishing roller51is pressed against the fixing belt26, the polishing roller51is disposed opposite the polishing aid roller54via the fixing belt26to form a polishing nip PN between the polishing roller51and the fixing belt26. The polishing roller51changes a rotation locus of the fixing belt26along at least a part of a circumferential face of the polishing roller51.

The plurality of supports across which the fixing belt26is stretched taut includes the fixing roller29and the heating roller30. The fixing roller29presses against the pressure roller27via the fixing belt26to form the fixing nip FN between the fixing belt26and the pressure roller27. The heating roller30heats the fixing belt26.

Instead of the polishing aid roller54, the fixing roller29or a support that is provided separately from the fixing roller29and the heating roller30may serve as a polishing aid.

A straight line connecting a center (e.g., an axis) of the polishing roller51and a center (e.g., an axis) of the polishing aid roller54and a straight line passing through the center of the polishing roller51and being perpendicular to an outer circumferential surface of the polishing aid roller54form an angle θ also called a wound angle. The angle θ is not smaller than 10 degrees, preferably not smaller than 20 degrees.

A surface layer of at least one of the polishing roller51and the polishing aid roller54includes an elastic body.

In order to bring the polishing roller51into contact with the fixing belt26, the polishing roller51may move toward the polishing aid roller54or the polishing aid roller54may move toward the polishing roller51. The polishing roller51may be removably attached to the fixing device25so that a user attaches the polishing roller51to the fixing device25to cause the polishing roller51to polish the fixing belt26and removes the polishing roller51from the fixing device25when the polishing roller51does not polish the fixing belt26.

The polishing aid roller54may contact an inner circumferential surface of the fixing belt26constantly. The polishing aid roller54may exert tension to the fixing belt26to stretch the fixing belt26. For example, the polishing aid roller54may be a tension roller. The polishing aid may be a rotator (e.g., a roller) or a pad. If the polishing aid is the pad, the pad increases a length of the polishing nip PN in a rotation direction D26of the fixing belt26. However, compared to a configuration in which the polishing aid is the rotator (e.g., the polishing aid roller54), the pad may be susceptible to abrasion or the pad may cause the inner circumferential surface of the fixing belt26that contacts the pad to be susceptible to abrasion.

A description is provided of a configuration of a first comparative fixing device.

An electrophotographic image forming apparatus forms toner images of various sizes on sheets of various sizes and thicknesses.

After a plurality of small sheets is conveyed through the first comparative fixing device, when a large sheet is conveyed through the first comparative fixing device, a fixing belt may generate a gloss streak on the toner image on the large sheet at a portion of the fixing belt over which a lateral edge of the small sheets has slid. While the small sheets are conveyed over the fixing belt, the lateral edge of the small sheets may damage the fixing belt with a streaked scratch. Since burrs on a machined edge of the small sheet produce the streaked scratch on the fixing belt, the streaked scratch on the fixing belt is conspicuous as the number of the small sheets conveyed over the fixing belt increases.

To address this circumstance, the first comparative fixing device includes a polishing roller that restores the fixing belt damaged with the streaked scratch to suppress formation of a faulty toner image that suffers from the gloss streak.

The fixing belt is looped over a fixing roller and a heating roller. An opposed roller is interposed between the fixing roller and the heating roller. In order to polish the fixing belt sufficiently to prevent the gloss streak and decrease a total load imposed on the polishing roller, pressure with which the polishing roller contacts a center of the fixing belt in an axial direction thereof is smaller than pressure with which the polishing roller contacts a lateral end of the fixing belt in the axial direction thereof.

A description is provided of a configuration of a second comparative fixing device.

The second comparative fixing device includes a fixing roller and a polishing roller separably contacting the fixing roller. The polishing roller rotates separately from the fixing roller or in accordance with rotation of the fixing roller.

Since printing is not available while the polishing roller polishes the fixing roller, the polishing roller is requested to polish the fixing roller for a shortened time. To address this request, the polishing roller is driven and rotated separately from the fixing roller at a rotation velocity different from a rotation velocity of the fixing roller.

However, the polishing roller, while being driven and rotated separately from the fixing roller at the rotation velocity different from the rotation velocity of the fixing roller, may suffer from an increased total load and an increased driving torque. For example, a spring is anchored to a shaft of the polishing roller at each lateral end of the polishing roller in an axial direction thereof. The spring presses the polishing roller against the fixing roller or the fixing belt evenly. A gear mounted on one lateral end of the polishing roller in the axial direction thereof is driven and rotated. With such configuration, as a driving torque of the polishing roller increases, a reaction force against the driving torque generates on the gear. Accordingly, the polishing roller may not contact and press against the fixing roller or the fixing belt evenly. Consequently, one lateral end of the polishing roller may press against the fixing roller or the fixing belt with insufficient pressure, degrading polishing performance and increasing a polishing time taken to polish the fixing roller or the fixing belt.

Further, if the polishing roller contacts the fixing roller or the fixing belt in a small area, the polishing roller may polish the fixing roller or the fixing belt in a decreased polishing area. Accordingly, regardless of a driving method of the polishing roller, the polishing roller may not restore the fixing roller or the fixing belt satisfactorily when the polishing time is short.

Referring to drawings, a description is provided of embodiments of the present disclosure.

Referring toFIGS. 2 and 3, a description is provided of a configuration of a fixing device25S according to a first embodiment.

FIG. 2is a vertical cross-sectional view of the fixing device25S, illustrating the polishing roller51being isolated from the fixing belt26.FIG. 3is a vertical cross-sectional view of the fixing device25S, illustrating the polishing roller51contacting the fixing belt26. The fixing belt26is looped over the fixing roller29and the heating roller30. The heating roller30accommodates a heater31serving as a heater or a heat source. The fixing roller29is rotatably mounted on or supported by a frame of the fixing device25S. The heating roller30is rotatably mounted on or supported by the frame of the fixing device25S such that the heating roller30is pulled by a spring in a direction32. Thus, the heating roller30stretches the fixing belt26.

The pressure roller27is pressed against the fixing roller29via the fixing belt26. A driver drives and rotates the fixing roller29in a rotation direction D29which in turn rotates the fixing belt26in the rotation direction D26. Alternatively, the driver may drive and rotate the pressure roller27, instead of the fixing roller29, in a rotation direction D27which in turn rotates the fixing belt26.

The fixing roller29includes a surface layer made of an elastic body. As the polishing roller51presses against the fixing roller29via the fixing belt26, the surface layer of the fixing roller29deforms and produces the fixing nip FN between the fixing belt26and the pressure roller27. According to this embodiment, the fixing nip FN has a length of 2.4 mm in the sheet conveyance direction DS, for example.

A polishing unit50includes the polishing roller51, a holder52that holds the polishing roller51, and a spring53that presses the polishing roller51against the fixing belt26. A mover brings the polishing unit50into contact with the fixing belt26and separates the polishing unit50from the fixing belt26. The mover is not restricted and employs general mechanisms. A driver may drive and rotate the polishing roller51.

According to this embodiment, each of the fixing roller29, the heating roller30, and the pressure roller27has a diameter of 50 mm, for example. The polishing roller51has a diameter of 25 mm, for example.

FIG. 2illustrates an isolation position of the polishing roller51where the polishing roller51is isolated from the fixing belt26.FIG. 3illustrates a contact position of the polishing roller51where the polishing roller51contacts the fixing belt26. According to this embodiment, the polishing roller51contacts the fixing belt26at a separation start position P1thereon where the fixing belt26rotating in the rotation direction D26starts separating from the fixing roller29. The separation start position P1is situated in proximity to an end of a contact span in the rotation direction D26of the fixing belt26where the fixing belt26contacts the fixing roller29. The spring53presses the polishing roller51against the fixing roller29via the fixing belt26.

As illustrated inFIG. 3, the spring53brings the polishing roller51into contact with the fixing belt26and presses the polishing roller51against the fixing belt26, forming the polishing nip PN between the polishing roller51and the fixing belt26. Simultaneously, the fixing belt26is wound around a part of the circumferential face of the polishing roller51, changing the rotation locus of the fixing belt26. Additionally, the fixing belt26is wound around the fixing roller29in an increased span in the rotation direction D26of the fixing belt26.

As the polishing roller51presses against the fixing belt26, the polishing roller51engages the fixing belt26with a length of about 3 mm, for example. Accordingly, the fixing belt26displaces the heating roller30in a direction opposite the direction32in which the heating roller30exerts tension to the fixing belt26. However, an amount of displacement of the heating roller30is slight, causing no disadvantage.

A straight line1connecting the center (e.g., the axis) of the polishing roller51and a center (e.g., an axis) of the fixing roller29serving as a polishing aid and a straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26form the angle θ (e.g., the wound angle). The fixing belt26is wound around the polishing roller51in a wound span defined by the angle θ. In an example illustrated inFIG. 3, the angle θ is 25 degrees.

The angle θ formed when the polishing roller51is situated at the contact position where the polishing roller51contacts the fixing belt26as illustrated inFIG. 3is greater than the angle θ formed when the polishing roller51is situated at the isolation position where the polishing roller51is isolated from the fixing belt26as illustrated inFIG. 2. A pressurization direction in which the spring53presses the polishing roller51against the fixing belt26is within the wound span defined by the angle θ. In the example illustrated inFIG. 3, the pressurization direction of the spring53is defined by the straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26.

The polishing nip PN formed between the fixing belt26and the polishing roller51has a length of 2.4 mm in the rotation direction126of the fixing belt26. The angle θ formed by the straight lines L1and L2is 25 degrees. The polishing roller51contacts and slides over the fixing belt26at a predetermined circumferential velocity difference. Thus, the polishing roller51polishes the fixing belt26. According to this embodiment, the polishing roller51slides over the fixing belt26in a forward direction at a linear velocity of the polishing roller51that is three times as fast as a surface linear velocity of the fixing belt26. However, the linear velocity of the polishing roller51is not limited to the above.

Referring toFIGS. 4 and 5, a description is provided of a configuration of a fixing device25SC according to a first comparative example.

FIG. 4is a vertical cross-sectional view of the fixing device25SC, illustrating the polishing roller51being isolated from the fixing belt26.FIG. 5is a vertical cross-sectional view of the fixing device25SC, illustrating the polishing roller51contacting the fixing belt26. The polishing roller51contacts the fixing belt26at a contact position P2thereon where the fixing belt26is in contact with or wound around the fixing roller29. The spring53presses the polishing roller51against the fixing roller29via the fixing belt26. Accordingly, as illustrated inFIG. 5, even if the polishing roller51contacts the fixing belt26, the fixing belt26is not wound around the polishing roller51.

Conversely, according to the first embodiment illustrated inFIG. 3, the fixing belt26is wound around the polishing roller51in the wound span in the rotation direction D26of the fixing belt26that is defined by the angle θ formed by the straight lines L1and L2. Accordingly, the polishing roller51contacts the fixing belt26in a sufficient contact area, decreasing a polishing time taken to restore the outer circumferential surface of the fixing belt26. For example, a polishing time according to the first embodiment depicted inFIG. 3is shorter than a polishing time according to the first comparative example depicted inFIG. 4by 20 percent.

Referring toFIGS. 6 and 7, a description is provided of a configuration of the fixing device25according to a second embodiment.

FIG. 6is a vertical cross-sectional view of the fixing device25, illustrating the polishing roller51being isolated from the fixing belt26.FIG. 7is a vertical cross-sectional view of the fixing device25, illustrating the polishing roller51contacting the fixing belt26. The fixing belt26is looped over the fixing roller29and the heating roller30. The heating roller30accommodates the heater31serving as a heater or a heat source. The fixing roller29is rotatably mounted on or supported by a frame of the fixing device25. The heating roller30is rotatably mounted on or supported by the frame of the fixing device25such that the heating roller30is pulled by the spring in the direction32. Thus, the heating roller30stretches the fixing belt26. According to this embodiment, the polishing aid roller54serving as a polishing aid is disposed inside a loop formed by the fixing belt26. As illustrated inFIG. 6, the polishing aid roller54is rotatably supported by the frame of the fixing device25while the polishing aid roller54is isolated from the fixing belt26. In an example illustrated inFIG. 6, the polishing aid roller54is isolated from the fixing belt26with an isolation interval of 1 mm.

The pressure roller27is pressed against the fixing roller29via the fixing belt26. The driver drives and rotates the fixing roller29in the rotation direction D29which in turn rotates the fixing belt26in the rotation direction D26. Alternatively, the driver may drive and rotate the pressure roller27, instead of the fixing roller29, in the rotation direction D27which in turn rotates the fixing belt26.

The polishing aid roller54includes the surface layer made of the elastic body. As the polishing roller51presses against the polishing aid roller54via the fixing belt26, the surface layer of the polishing aid roller54deforms and produces the polishing nip PN between the fixing belt26and the polishing roller5disposed opposite the polishing aid roller54. According to this embodiment, the polishing nip PN has a length of 2.0 mm in the rotation direction D26of the fixing belt26, for example. While the polishing aid roller54is isolated from the fixing belt26as illustrated inFIG. 6, the polishing aid roller54does not rotate.

The polishing unit50includes the polishing roller51, the holder52that holds the polishing roller51, and the spring53that presses the polishing roller51against the fixing belt26. The mover brings the polishing unit50into contact with the fixing belt26and separates the polishing unit50from the fixing belt26. The mover is not restricted and employs general mechanisms. The driver may drive and rotate the polishing roller51.

According to this embodiment, each of the fixing roller29, the heating roller30, and the pressure roller27has the diameter of 50 mm, for example. Each of the polishing roller51and the polishing aid roller54has a diameter of 25 mm.

FIG. 6illustrates an isolation position of the polishing roller51where the polishing roller51is isolated from the fixing belt26.FIG. 7illustrates a contact position of the polishing roller51where the polishing roller51contacts the fixing belt26. According to this embodiment, the polishing roller51contacts the fixing belt26at a shift position shifted from a proximal position where the polishing aid roller54is in contact with or disposed in proximity to the fixing belt26. The shift position is shifted from the proximal position of the polishing aid roller54in the rotation direction D26of the fixing belt26or a direction opposite the rotation direction D26of the fixing belt26. InFIG. 7, the shift position is shifted from the proximal position of the polishing aid roller54leftward in the direction opposite the rotation direction D26of the fixing belt26. The spring53presses the polishing roller S1against the polishing aid roller54via the fixing belt26.

While the polishing aid roller54contacts the fixing belt26as illustrated inFIG. 7, the polishing aid roller54rotates in accordance with rotation of the fixing belt26while the polishing roller51polishes the fixing belt26. As illustrated inFIG. 7, the spring53brings the polishing roller51into contact with the fixing belt26and presses the polishing roller51against the fixing belt26, forming the polishing nip PN between the polishing roller51and the fixing belt26. Simultaneously, the fixing belt26is wound around a part of the circumferential face of the polishing roller51, changing the rotation locus of the fixing belt26. Similarly, the fixing belt26is wound around the polishing aid roller54in a wound span in the rotation direction D26of the fixing belt26.

As the polishing roller51presses against the fixing belt26, the polishing roller51engages the fixing belt26with a length of about 3 mm. Accordingly, the fixing belt26displaces the heating roller30in the direction opposite the direction32in which the heating roller30exerts tension to the fixing belt26. However, an amount of displacement of the heating roller30is slight, causing no disadvantage.

A straight line L3connecting the center (e.g., the axis) of the polishing roller51and the center (e.g., the axis) of the polishing aid roller54and the straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26form an angle θ (e.g., a wound angle). The fixing belt26is wound around the polishing roller51in the wound span defined by the angle θ. In an example illustrated inFIG. 7, the angle θ is 29 degrees.

The angle θ formed when the polishing roller51is situated at the contact position where the polishing roller51contacts the fixing belt26as illustrated inFIG. 7is greater than the angle θ formed when the polishing roller51is situated at the isolation position where the polishing roller51is isolated from the fixing belt26as illustrated inFIG. 6. The pressurization direction in which the spring53presses the polishing roller51against the fixing belt26is within the wound span defined by the angle θ. In the example illustrated inFIG. 7, the pressurization direction of the spring53is defined by the straight line L3passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26.

The polishing nip PN formed between the fixing belt26and the polishing roller51has a length of 2.0 mm in the rotation direction D26of the fixing belt26. The angle θ formed by the straight lines L2and L3is 29 degrees. The polishing roller51contacts and slides over the fixing belt26at a predetermined circumferential velocity difference. Thus, the polishing roller51polishes the fixing belt26. According to this embodiment, the polishing roller51slides over the fixing belt26in the forward direction at the linear velocity of the polishing roller51that is three times as fast as the surface linear velocity of the fixing belt26. However, the linear velocity of the polishing roller51is not limited to the above. Accordingly, the polishing roller51contacts the fixing belt26in the sufficient contact area, decreasing the polishing time taken to restore the outer circumferential surface of the fixing belt26.

According to the second embodiment illustrated inFIGS. 6 and 7, as the polishing unit50moves, the polishing roller51comes into contact with the fixing belt26and separates from the fixing belt26. Alternatively, the polishing unit50may be stationary. In this case, as the polishing aid roller54moves, the polishing roller51comes into contact with the fixing belt26and separates from the fixing belt26. Yet alternatively, the polishing aid roller54may be stationary and may not rotate in accordance with rotation of the fixing belt26while the fixing belt26slides over the polishing aid roller54.

Referring toFIGS. 8 and 9, a description is provided of a configuration of a fixing device25C according to a second comparative example.

FIG. 8is a vertical cross-sectional view of the fixing device25C, illustrating the polishing roller51situated at an isolation position where the polishing roller51is isolated from the fixing belt26.FIG. 9is a vertical cross-sectional view of the fixing device25C, illustrating the polishing roller51situated at a contact position where the polishing roller51contacts the fixing belt26.

The fixing belt26is looped over the fixing roller29and the heating roller30. The heating roller30accommodates the heater31serving as a heater or a heat source. The fixing roller29is rotatably mounted on or supported by a frame of the fixing device25C. The heating roller30is rotatably mounted on or supported by the frame of the fixing device25C such that the spring exerts tension to the heating roller30in the direction32. Thus, the heating roller30stretches the fixing belt26.

The pressure roller27is pressed against the fixing roller29via the fixing belt26. The driver drives and rotates the fixing roller29in the rotation direction D29which in turn rotates the fixing belt26in the rotation direction D26. Alternatively, the driver may drive and rotate the pressure roller27, instead of the fixing roller29, in the rotation direction D27which in turn rotates the fixing belt26.

As illustrated inFIG. 8, the polishing roller51is disposed outside the loop formed by the fixing belt26and is isolated from the fixing belt26. The polishing aid roller54is disposed inside the loop formed by the fixing belt26. The polishing aid roller54is rotatably supported by the frame of the fixing device25C such that the polishing aid roller54is isolated from the fixing belt26slightly.

As illustrated inFIG. 9, the straight line L3connecting the center of the polishing roller51and the center of the polishing aid roller54and the rotation locus of the fixing belt26form a substantially right angle.

The polishing roller51contacts the fixing belt26at a proximal position where the polishing aid roller54is in contact with or disposed in proximity to the fixing belt26. The spring53presses the polishing roller51against the polishing aid roller54via the fixing belt26. While the polishing roller51is at the isolation position illustrated inFIG. 8, the polishing aid roller54is isolated from the fixing belt26with an isolation interval of 1 mm. Accordingly, as illustrated inFIG. 9, although the fixing belt26is wound around the polishing roller S1slightly, the fixing belt26is not wound around the polishing aid roller54.

For example, as illustrated inFIG. 9, the straight line L3connecting the center of the polishing roller51and the center of the polishing aid roller54and the straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26form the angle θ (e.g., the wound angle) of 3 degrees. The polishing nip PN formed between the fixing belt26and the polishing roller51has a length of 0.6 mm in the rotation direction D26of the fixing belt26that is calculated based on the angle θ of 3 degrees. The length of 0.6 mm of the polishing nip PN is within the length of 2.0 mm of the polishing nip PN described above by referring toFIG. 7, thus not increasing the wound span (e.g., a contact area) in the rotation direction D26of the fixing belt26where the fixing belt26is wound around the polishing roller51.

While the polishing aid roller54is isolated from the fixing belt26with the isolation interval not smaller than 1 mm as illustrated inFIG. 8, the polishing roller51engages the fixing belt26with an increased engagement amount, thus attaining the angle θ not smaller than 3 degrees with which the fixing belt26is wound around the polishing roller51. However, the rotation locus of the fixing belt26changes excessively, degrading conveyance of the sheet S by the fixing belt26.

A description is provided of disadvantages caused by driving of the polishing roller51.

The holder52supports each lateral end of the polishing roller51in an axial direction thereof through the spring53. The polishing roller51is driven and rotated by a gear mounted on a shaft situated at one lateral end of the polishing roller51in the axial direction thereof. As a cam or the like moves the polishing unit50toward the polishing aid roller54, the polishing roller51presses against the fixing belt26as illustrated inFIG. 9.

As illustrated inFIG. 9, as a substantial driving torque generates on the shaft that mounts the gear of the polishing roller51, a reaction force to the driving torque degrades a pressure balance between the springs53contacting both lateral ends of the polishing roller51in the axial direction thereof. Accordingly, the polishing roller51may not press against or contact the fixing belt26evenly throughout the entire span of the fixing belt26in an axial direction thereof. For example, the polishing roller51may press against one lateral end of the fixing belt26but may not press against another lateral end of the fixing belt26in the axial direction thereof. In order to cause the polishing roller51to polish the another lateral end of the fixing belt26that is not pressed by the polishing roller51sufficiently so as to restore the fixing belt26to a predetermined condition, the polishing time when the polishing roller51polishes the fixing belt26may increase.

The straight line L3connecting the center of the polishing roller51and the center of the polishing aid roller54and the rotation locus of the fixing belt26form the substantially right angle. Accordingly, the fixing belt26may be barely wound around the polishing roller51, resulting in an insufficient contact area where the polishing roller51contacts the fixing belt26and an increased polishing time taken for the polishing roller51to polish the fixing belt26.

Referring toFIGS. 10 and 11, a description is provided of a configuration of a fixing device25T according to a third embodiment.

FIG. 10is a vertical cross-sectional view of the fixing device25T, illustrating the polishing roller51being isolated from the fixing belt26.FIG. 11is a vertical cross-sectional view of the fixing device25T, illustrating the polishing roller51contacting the fixing belt26.FIG. 10illustrates an isolation position of the polishing roller51where the polishing roller51is isolated from the fixing belt26.FIG. 11illustrates a contact position of the polishing roller51where the polishing roller51contacts the fixing belt26.

The fixing belt26is looped over the fixing roller29and the heating roller30. The heating roller30accommodates the heater31serving as a heater or a heat source. The fixing roller29and the heating roller30are rotatably mounted on or supported by a frame of the fixing device25T. Thus, the fixing roller29and the heating roller30stretch the fixing belt26.

According to this embodiment, the polishing aid roller54and a tension roller55serving as polishing aids are disposed inside the loop formed by the fixing belt26. The polishing aid roller54also serves as a first polishing aid or a first polishing aid roller and the tension roller55also serves as a second polishing aid or a second polishing aid roller. The polishing aid roller54, together with the fixing roller29and the heating roller30, is rotatably mounted on or supported by the frame of the fixing device25T. The tension roller55is rotatably mounted on or supported by the frame of the fixing device25T such that a spring55acauses the tension roller55to exert tension to the inner circumferential surface of the fixing belt26outward. Thus, the tension roller55stretches the fixing belt26.

The pressure roller27is pressed against the fixing roller29via the fixing belt26. The driver drives and rotates the fixing roller29in the rotation direction D29which in turn rotates the fixing belt26in the rotation direction D26. Alternatively, the driver may drive and rotate the pressure roller27, instead of the fixing roller29, in the rotation direction D27which in turn rotates the fixing belt26.

The polishing aid roller54includes the surface layer made of the elastic body. As the polishing roller51presses against the polishing aid roller54via the fixing belt26, the surface layer of the polishing aid roller54deforms and produces the polishing nip PN between the fixing belt26and the polishing roller S1disposed opposite the polishing aid roller54. According to this embodiment, the polishing nip PN has a length of 1.8 mm in the rotation direction D26of the fixing belt26.

The polishing unit50includes the polishing roller51, the holder52that holds the polishing roller51, and the spring53that presses the polishing roller51against the fixing belt26. The mover brings the polishing unit50into contact with the fixing belt26and separates the polishing unit50from the fixing belt26. The mover is not restricted and employs general mechanisms. The driver may drive and rotate the polishing roller51.

According to this embodiment, each of the fixing roller29, the heating roller30, and the pressure roller27has the diameter of 50 mm. Each of the polishing roller51, the polishing aid roller54, and the tension roller55has a diameter of 25 mm.

According to this embodiment, the polishing roller51contacts the fixing belt26in an interval between the polishing aid roller54and the tension roller55in the rotation direction D26of the fixing belt26. The spring53presses the polishing roller51against the polishing aids, that is, the polishing aid roller54and the tension roller55, via the fixing belt26.

As illustrated inFIG. 11, the spring53brings the polishing roller51into contact with the fixing belt26and presses the polishing roller51against the fixing belt26, forming the polishing nip PN between the polishing roller51and the fixing belt26. Simultaneously, the fixing belt26is wound around a part of the circumferential face of the polishing roller51, changing the rotation locus of the fixing belt26. Similarly, the fixing belt26is wound around the polishing aid roller54and the tension roller55in a wound span in the rotation direction D26of the fixing belt26.

As the polishing roller51presses against the fixing belt26, the polishing roller51engages the fixing belt26with a length of about 5.5 mm, thus displacing the tension roller55. However, an amount of displacement of the tension roller55is slight, causing no disadvantage.

The straight line L3connecting the center (e.g., the axis) of the polishing roller51and the center (e.g., the axis) of the polishing aid roller54and the straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26form an angle θ1 (e.g., a wound angle). A straight line L4connecting the center (e.g., the axis) of the polishing roller51and a center (e.g., an axis) of the tension roller55serving as a polishing aid and the straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26form an angle θ2 (e.g., a wound angle). The angles θ1 and θ2 are combined into an angle θ3 (e.g., a wound angle). The fixing belt26is wound around the polishing roller51in a wound span defined by the angle θ3. In an example illustrated inFIG. 11, the angle θ3 is 80 degrees.

The angle θ3 formed when the polishing roller51is situated at the contact position where the polishing roller51contacts the fixing belt26as illustrated inFIG. 11is greater than the angle θ3 formed when the polishing roller51is situated at the isolation position where the polishing roller51is isolated from the fixing belt26as illustrated inFIG. 10. The pressurization direction in which the spring53presses the polishing roller51against the fixing belt26is within the wound span defined by the angle θ3.

The polishing nip PN formed between the fixing belt26and the polishing roller51has a length of 1.8 mm in the rotation direction D26of the fixing belt26. The polishing roller51contacts the fixing belt26to create the angle θ3 of 80 degrees. The polishing roller51slides over the fixing belt26at the predetermined circumferential velocity difference. Thus, the polishing roller51polishes the fixing belt26. According to this embodiment, the polishing roller51slides over the fixing belt26in the forward direction at the linear velocity of the polishing roller51that is three times as fast as the surface linear velocity of the fixing belt26. However, the linear velocity of the polishing roller51is not limited to the above. Accordingly, the polishing roller51contacts the fixing belt26in the sufficient contact area, decreasing the polishing time taken to restore the outer circumferential surface of the fixing belt26.

Referring toFIGS. 12 and 13, a description is provided of a configuration of a fixing device25U according to a fourth embodiment.

FIG. 12is a vertical cross-sectional view of the fixing device25U, illustrating the polishing roller51being isolated from the fixing belt26.FIG. 13is a vertical cross-sectional view of the fixing device25U, illustrating the polishing roller51contacting the fixing belt26.FIG. 12illustrates an isolation position of the polishing roller51where the polishing roller51is isolated from the fixing belt26.FIG. 13illustrates a contact position of the polishing roller51where the polishing roller51contacts the fixing belt26.

The fixing device25U includes the fixing belt26looped over the fixing roller29, the heating roller30, and the tension roller55. The heating roller30accommodates the heater31serving as a heater or a heat source. The fixing roller29and the heating roller30are rotatably mounted on or supported by a frame of the fixing device25U. The fixing device25U further includes the polishing aid roller54and a pressurization assembly that presses the polishing aid roller54against the polishing roller51via the fixing belt26. The fixing device25U further includes the tension roller55. The tension roller55is rotatably mounted on or supported by the frame of the fixing device25U such that the spring55acauses the tension roller55to exert tension to the inner circumferential surface of the fixing belt26outward. Thus, the tension roller55stretches the fixing belt26.

The pressure roller27is pressed against the fixing roller29via the fixing belt26. The driver drives and rotates the fixing roller29in the rotation direction D29which in turn rotates the fixing belt26in the rotation direction D26. Alternatively, the driver may drive and rotate the pressure roller27, instead of the fixing roller29, in the rotation direction D27which in turn rotates the fixing belt26.

The polishing aid roller54is disposed inside the loop formed by the fixing belt26. Each lateral end of the polishing aid roller54in an axial direction thereof is supported by the frame of the fixing device25U such that each lateral end of the polishing aid roller54is slidable in an elongate hole59of a holder58. A spring57serving as a presser is anchored to the holder58and the polishing aid roller54. The spring57biases each lateral end of the polishing aid roller54in the axial direction thereof toward the fixing belt26. The polishing aid roller54, the holder58, and the spring57construct a polishing aid roller unit5. The polishing aid roller unit5is supported by the frame of the fixing device25U such that the polishing aid roller unit5is slidable in a longitudinal direction thereof. A cam60disposed opposite the polishing aid roller54via the holder58moves and slides the polishing aid roller unit5.

The polishing roller51is disposed outside the loop formed by the fixing belt26. The polishing roller51is rotatably supported by the frame of the fixing device25U and is disposed opposite the polishing aid roller54disposed inside the loop formed by the fixing belt26. A driver6coupled to the polishing roller51drives and rotates the polishing roller51through the gear mounted on the shaft situated at one lateral end of the polishing roller51in the axial direction thereof. Since the polishing roller51is supported by the frame of the fixing device25U against a reaction force that generates as the polishing roller51is driven and rotated, even if a force that drives and rotates the polishing roller51is great, the polishing roller51rotates stably.

The polishing aid roller54includes the surface layer made of the elastic body. As the polishing roller51presses against the polishing aid roller54via the fixing belt26, the surface layer of the polishing aid roller54deforms and produces the polishing nip PN between the fixing belt26and the polishing roller51disposed opposite the polishing aid roller54. According to this embodiment, the polishing nip PN has a length of 1.8 mm in the rotation direction D26of the fixing belt26.

According to this embodiment, each of the fixing roller29, the heating roller30, and the pressure roller27has the diameter of 50 mm. Each of the polishing roller51, the polishing aid roller54, and the tension roller55has the diameter of 25 mm.

Referring toFIG. 12, a description is provided of the isolation position of the polishing roller51where the polishing roller51is isolated from the fixing belt26.

The polishing roller51is isolated from the fixing belt26slightly. As the cam60rotates, the cam60moves and slides the polishing aid roller unit5to an isolation position where the polishing aid roller unit5isolates the polishing aid roller54from the inner circumferential surface of the fixing belt26. The spring57presses the polishing aid roller54against one end of the elongate hole59of the holder58.

Referring toFIG. 13, a description is provided of the contact position of the polishing roller51where the polishing roller51contacts the fixing belt26.

As the cam60rotates, the cam60moves and slides the polishing aid roller unit5so that the polishing aid roller54moves toward the polishing roller51. While the cam60rotates, the polishing aid roller54comes into contact with the fixing belt26. As the cam60rotates further, the polishing aid roller54presses against the polishing roller51via the fixing belt26. As the cam60finishes rotation, the polishing aid roller54separates from the elongate hole59of the holder58. Thus, the spring57presses the polishing aid roller54against the polishing roller51via the fixing belt26.

As illustrated inFIG. 13, the spring57presses the polishing aid roller54against the polishing roller51via the fixing belt26, forming the polishing nip PN between the polishing roller51and the fixing belt26. Simultaneously, the fixing belt26is wound around a part of the circumferential face of the polishing roller51, changing the rotation locus of the fixing belt26. Since the fixing device25U includes the tension roller55, even when the rotation locus of the fixing belt26changes, the tension roller55exerts an appropriate tension to the fixing belt26constantly.

The straight line L3connecting the center (e.g., the axis) of the polishing roller51and the center (e.g., the axis) of the polishing aid roller54and the straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26form the angle θ (e.g., a wound angle). The fixing belt26is wound around the polishing roller51in the wound span defined by the angle θ. In an example illustrated inFIG. 13, the angle θ is 19 degrees.

The spring57presses the polishing aid roller54against the polishing roller51via the fixing belt26. Accordingly, the polishing nip PN formed between the fixing belt26and the polishing roller51has a length of 1.8 mm in the rotation direction D26of the fixing belt26with the angle θ of 19 degrees.

The driver6coupled to at least one lateral end of the polishing roller51in the axial direction thereof drives and rotates the polishing roller51such that the polishing roller51slides over the fixing belt26rotating in the rotation direction D26with the predetermined circumferential velocity difference. Thus, the polishing roller51polishes the fixing belt26. According to this embodiment, the polishing roller51slides over the fixing belt26in the forward direction at the linear velocity of the polishing roller51that is three times as fast as the surface linear velocity of the fixing belt26. However, the linear velocity of the polishing roller51is not limited to the above.

With the construction of the fixing device25U depicted inFIGS. 12 and 13, the fixing belt26contacts and presses against the polishing roller51evenly. Additionally, the polishing roller51contacts the fixing belt26in a sufficient contact area, decreasing a polishing time taken to restore the outer circumferential surface of the fixing belt26.

Referring toFIGS. 14 and 15, a description is provided of a configuration of a fixing device25V according to a fifth embodiment.

FIG. 14is a vertical cross-sectional view of the fixing device25V, illustrating the polishing roller51being isolated from the fixing belt26.FIG. 15is a vertical cross-sectional view of the fixing device25V, illustrating the polishing roller51contacting the fixing belt26.FIG. 14illustrates an isolation position of the polishing roller51where the polishing roller51is isolated from the fixing belt26.FIG. 15illustrates a contact position of the polishing roller51where the polishing roller51contacts the fixing belt26.

The fixing belt26is looped over the fixing roller29, the heating roller30, and the polishing aid roller54also serving as a tension roller. The heating roller30accommodates the heater31serving as a heater or a heat source. The fixing roller29and the heating roller30are rotatably mounted on or supported by a frame of the fixing device25V. The polishing aid roller54is rotatably mounted on or supported by the frame of the fixing device25V such that the spring57causes the polishing aid roller54to exert tension to the inner circumferential surface of the fixing belt26outward. Thus, the polishing aid roller54exerts tension to the fixing belt26and stretches the fixing belt26.

The fixing device25V according to the fifth embodiment incorporates the polishing aid roller54that attains the functions of the polishing aid roller54and the tension roller55separately provided from the polishing aid roller54, which are incorporated in the fixing device25U according to the fourth embodiment depicted inFIGS. 12 and 13. Accordingly, the fixing device25V reduces the number of parts incorporated therein and shortens the circumferential length of the fixing belt26, downsizing the fixing device25V.

The pressure roller27is pressed against the fixing roller29via the fixing belt26. The driver drives and rotates the fixing roller29in the rotation direction D29which in turn rotates the fixing belt26in the rotation direction D26. Alternatively, the driver may drive and rotate the pressure roller27, instead of the fixing roller29, in the rotation direction D27which in turn rotates the fixing belt26.

Each lateral end of the polishing aid roller54in the axial direction thereof is supported by the frame of the fixing device25V such that each lateral end of the polishing aid roller54is slidable in the elongate hole59of the holder58. The spring57biases each lateral end of the polishing aid roller54in the axial direction thereof toward the fixing belt26. The fixing device25V includes a polishing aid roller unit5V that includes the polishing aid roller54, the holder58provided with the elongate hole59, the spring57, and a shaft56. The shaft56(e.g., a fulcrum) is disposed at substantially a center of the holder58or the polishing aid roller unit5V in a longitudinal direction thereof. As the holder58pivots about the shaft56, the polishing aid roller54supported by the holder58moves. The cam60is disposed opposite the polishing aid roller54via the holder58. The cam60and a spring61rotate the polishing aid roller54and press the polishing aid roller54against the fixing belt26.

The spring57biases the polishing aid roller54in a separation direction in which the polishing aid roller54separates from the shaft56. The spring61biases the polishing aid roller54to pivot about the shaft56. A biasing direction in which the spring57biases the polishing aid roller54is substantially perpendicular to a biasing direction in which the spring61biases the polishing aid roller54.

The polishing roller51is disposed outside the loop formed by the fixing belt26. The polishing roller51is rotatably supported by the frame of the fixing device25V and is disposed counter to a rotation direction D54of the polishing aid roller54disposed inside the loop formed by the fixing belt26. The driver6coupled to the polishing roller51drives and rotates the polishing roller51through the gear mounted on the shaft situated at one lateral end of the polishing roller51in the axial direction thereof. Since the polishing roller51is supported by the frame of the fixing device25V against a reaction force that generates as the polishing roller51is driven and rotated, even if a force that drives and rotates the polishing roller51is great, the polishing roller51rotates stably.

The polishing aid roller54includes the surface layer made of the elastic body. As the polishing roller51presses against the polishing aid roller54via the fixing belt26, the surface layer of the polishing aid roller54deforms and produces the polishing nip PN between the fixing belt26and the polishing roller51disposed opposite the polishing aid roller54. According to this embodiment, the polishing nip PN has a length of 1.8 mm in the rotation direction D26of the fixing belt26.

According to this embodiment, each of the fixing roller29, the heating roller30, and the pressure roller27has the diameter of 50 mm. Each of the polishing roller51and the polishing aid roller54has the diameter of 25 mm.

As illustrated inFIG. 14, when the polishing roller51is isolated from the fixing belt26slightly, the cam60moves the polishing aid roller unit5V and the polishing aid roller54moves leftward from the polishing roller51. The spring57causes the polishing aid roller54to exert tension to the inner circumferential surface of the fixing belt26outward.

As illustrated inFIG. 15, when the polishing roller51contacts the fixing belt26, as the cam60rotates, the polishing aid roller unit5V pivots and the polishing aid roller54starts moving rightward toward the polishing roller51. While the cam60rotates, the holder58separates from the cam60. The spring61biases the polishing aid roller54rightward. The polishing aid roller54is brought into contact with the fixing belt26and pressed against the polishing roller51via the fixing belt26by the spring61. The spring57causes the polishing aid roller54to exert tension to the inner circumferential surface of the fixing belt26outward.

As illustrated inFIG. 15, the spring57presses the polishing aid roller54against the polishing roller51via the fixing belt26, forming the polishing nip PN between the polishing roller51and the fixing belt26. Simultaneously, the fixing belt26is wound around a part of the circumferential face of the polishing roller51, changing the rotation locus of the fixing belt26.

The straight line L3connecting the center (e.g., the axis) of the polishing roller51and the center (e.g., the axis) of the polishing aid roller54and the straight line L2passing through the center of the polishing roller51and being perpendicular to the outer circumferential surface of the fixing belt26form the angle θ (e.g., a wound angle). The fixing belt26is wound around the polishing roller51in the wound span defined by the angle θ. In an example illustrated inFIG. 15, the angle θ is 30 degrees.

The spring61presses the polishing aid roller54against the polishing roller51via the fixing belt26. Accordingly, the polishing nip PN formed between the fixing belt26and the polishing roller51has a length of 1.8 mm in the rotation direction D26of the fixing belt26with the angle θ of 30 degrees.

The driver6drives and rotates the polishing roller51through the gear mounted on at least one lateral end of the polishing roller51in the axial direction thereof such that the polishing roller51slides over the fixing belt26rotating in the rotation direction D26with the predetermined circumferential velocity difference. Thus, the polishing roller51polishes the fixing belt26. According to this embodiment, the polishing roller51slides over the fixing belt26in the forward direction at the linear velocity of the polishing roller51that is three times as fast as the surface linear velocity of the fixing belt26. However, the linear velocity of the polishing roller51is not limited to the above.

With the construction of the fixing device25V depicted inFIGS. 14 and 15, the fixing belt26contacts and presses against the polishing roller51evenly. Additionally, the polishing roller51contacts the fixing belt26in a sufficient contact area, decreasing a polishing time taken to restore the outer circumferential surface of the fixing belt26.

The following describes examples of the specification of the components incorporated in the fixing devices25,25S,25T,25U, and25V according to the embodiments described above.FIGS. 2, 3, 6, 7, and 10 to 15illustrate the fixing devices25S,25,25T,25U, and25V each of which includes the fixing belt26that is looped over two rollers, that is, the fixing roller29and the heating roller30, and heated by the heater31disposed inside one of the two rollers (e.g., the heating roller30). Alternatively, for example, instead of the fixing roller29, a presser33(e.g., a pressure pad) that is stationary and is not rotatable may be employed as illustrated inFIG. 16.FIG. 16is a vertical cross-sectional view of a fixing device25W incorporating the presser33. As illustrated inFIG. 16, the fixing belt26is stretched taut across the presser33and the heating roller30. The heater31may be a halogen heater or an induction heater (IH) using electromagnetic induction.

For example, the fixing belt26is a multi-layer endless belt constructed of a base layer, an elastic layer coating the base layer, and a release layer coating the elastic layer. The base layer, having a layer thickness of about 90 micrometers, is made of polyimide (Pt) resin. The elastic layer is made of silicone rubber or the like.

The elastic layer, having a layer thickness of about 200 micrometers, for example, is made of an elastic material such as silicone rubber, fluoro rubber, and silicone rubber foam.

The release layer, having a layer thickness of about 20 micrometers, for example, is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polyimide (PI), polyether imide (PEI), polyether sulfide (PES), or the like. The release layer serving as a surface layer of the fixing belt26facilitates separation or peeling-off of toner of a toner image on a sheet serving as a recording medium from the fixing belt26. The release layer fixes the toner image on the sheet properly and separates the sheet from the fixing belt26precisely.

FIG. 17Ais a vertical cross-sectional view of the polishing roller51serving as a slider that slides over the fixing belt26. Since the polishing roller51is pressed against the fixing belt26with predetermined pressure, the polishing roller51rotates forward and backward to the rotation direction D26of the fixing belt26with a linear velocity difference.

FIG. 17Bis a partially enlarged horizontal cross-sectional view of the polishing roller51. As illustrated inFIG. 17B, the polishing roller51includes a core bar51aand a slide layer51bdisposed on the core bar51a. The slide layer51bserves as an abrasive grain layer or a polishing layer, for example. The slide layer51bas a surface layer includes a binder resin and abrasive grains51cdispersed in the binder resin. The abrasive grains51cproject beyond an outer circumferential surface of the slide layer51bto define slight surface asperities. The abrasive grains51care alumina abrasive grains as general abrasive particles, for example, white fused alumina #1500. Alternatively, the abrasive grains51cmay be made of other materials. The grain size of the abrasive grains51cmay be identical throughout the entire slide span of the polishing roller51that slides over the fixing belt26.

A description is provided of advantages of the fixing devices25,25S,25T,25U,25V, and25W.

As illustrated inFIGS. 2, 3, 6, 7, and 10 to 16, a fixing device (e.g., the fixing devices25,25S,25T,25U,25V, and25W) includes a fixing belt (e.g., the fixing belt26), a pressure rotator (e.g., the pressure roller27), a polishing roller (e.g., the polishing roller51), and a polishing aid (e.g., the fixing roller29and the polishing aid roller54).

The fixing belt is stretched taut across a plurality of supports (e.g., the fixing roller29, the heating roller30, and the polishing aid roller54) and rotatable in a rotation direction (e.g., the rotation direction D26). The pressure rotator is pressed against the fixing belt to form a fixing nip (e.g., the fixing nip FN) therebetween. The polishing roller separably contacts the fixing belt. The polishing roller slides over an outer circumferential surface of the fixing belt while the polishing roller is pressed against the fixing belt. The polishing aid is at least one of the plurality of supports. The polishing aid is disposed opposite the polishing roller via the fixing belt. While the polishing roller is pressed against the fixing belt, the polishing roller is disposed opposite the polishing aid via the fixing belt to form a polishing nip (e.g., the polishing nip PN) between the polishing roller and the fixing belt. The polishing roller changes a rotation locus of the fixing belt along at least a part of a circumferential face (e.g., the slide layer51b) of the polishing roller.

Accordingly, the polishing roller contacts and presses against the fixing belt evenly to polish the fixing belt in a sufficient contact area, shortening a polishing time when the polishing roller polishes the fixing belt.

According to the embodiments described above, the fixing belt26serves as a fixing belt. Alternatively, a fixing film, a fixing sleeve, or the like may be used as a fixing belt. Further, the pressure roller27serves as a pressure rotator. Alternatively, a pressure belt or the like may be used as a pressure rotator.