Fixing device with mechanism capable of minimizing damage to toner image and recording medium and image forming apparatus incorporating same

A fixing device includes at least one separator including a pawl separatably contacting a fixing rotary body to separate a recording medium discharged from a fixing nip formed between the fixing rotary body and a pressing rotary body from the fixing rotary body; at least one first rotary body rotatably supported by the at least one separator to contact and guide the recording medium separated by the pawl in a recording medium conveyance direction; at least one second rotary body disposed downstream from the first rotary body in the recording medium conveyance direction to contact and guide the recording medium guided by the first rotary body in the recording medium conveyance direction; and at least one brush mounted on an outer circumferential surface of at least one of the first rotary body and the second rotary body.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2011-146127, filed on Jun. 30, 2011, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Exemplary aspects of the present invention 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.

2. Description of the Related Art

Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; 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.

The fixing device installed in such image forming apparatuses may include a fixing roller and an opposed pressing roller that apply heat and pressure to a recording medium bearing a toner image. For example, the pressing roller is pressed against the fixing roller heated by a heater to form a fixing nip therebetween through which the recording medium bearing the toner image is conveyed. As the fixing roller and the pressing roller rotate and convey the recording medium through the fixing nip, they apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.

Thereafter, the recording medium bearing the toner image is discharged from the fixing nip toward the outside of the fixing device. However, the recording medium may adhere to the fixing roller due to an adhesive force of the toner image heated by the fixing roller. To address this circumstance, a separation pawl may be located at an exit of the fixing nip to separate the recording medium from the fixing roller. Since the separation pawl is also designed to contact and guide the recording medium to the outside of the fixing device, the separation pawl has a side effect of producing scratches on the toner image on the recording medium.

To address this problem, a guide roller may be disposed in proximity to the separation pawl to guide the recording medium separated from the fixing roller by the separation pawl toward the outside of the fixing device.

However, immediately after the recording medium is discharged from the fixing nip, the recording medium still stores heat conducted from the fixing roller, softening the toner image thereon. While the recording medium moves from the separation pawl to the guide roller, it comes into contact with the separation pawl and the guide roller. Hence, as the recording medium slides over the separation pawl and the guide roller, the separation pawl and the guide roller may scratch the softened toner image on the recording medium, thus producing scratches and glossy streaks on the toner image.

Additionally, as the recording medium conveyed through the fixing nip is applied with heat and pressure, moisture contained in the recording medium may be evaporated and adhered to an interior wall of a conveyance path through which the recording medium discharged from the fixing nip is conveyed as water droplets. When the water droplets move to the recording medium, they may be left as water droplet marks on the recording medium.

SUMMARY OF THE INVENTION

This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes a fixing rotary body rotatable in a predetermined direction of rotation and a pressing rotary body rotatably contacting the fixing rotary body to form a fixing nip therebetween through which a recording medium bearing a toner image is conveyed. A heater is disposed opposite at least one of the fixing rotary body and the pressing rotary body to heat the at least one of the fixing rotary body and the pressing rotary body. At least one separator separatably contacts an outer circumferential surface of the fixing rotary body and includes a pawl to contact and separate the recording medium discharged from the fixing nip from the fixing rotary body. At least one first rotary body is rotatably supported by the at least one separator to contact and guide the recording medium separated by the pawl of the at least one separator in a recording medium conveyance direction. At least one second rotary body is disposed downstream from the first rotary body in the recording medium conveyance direction to contact and guide the recording medium guided by the first rotary body in the recording medium conveyance direction. At least one brush is mounted on an outer circumferential surface of at least one of the first rotary body and the second rotary body.

This specification further describes an improved image forming apparatus. In one exemplary embodiment of the present invention, the image forming apparatus includes the fixing device described above.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular toFIG. 1, an image forming apparatus100according to an exemplary embodiment of the present invention is explained.

FIG. 1is a schematic vertical sectional view of the image forming apparatus100. The image forming apparatus100may be a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this exemplary embodiment, the image forming apparatus100is a printer for forming color and monochrome toner images on a recording medium by electrophotography.

The image forming apparatus100includes four process units1Y,1C,1M, and1K serving as image forming units detachably attached to the image forming apparatus100, respectively. Although the process units1Y,1C,1M, and1K contain yellow, cyan, magenta, and black toilers that form yellow, cyan, magenta, and black toner images, respectively, resulting in a color toner image, they have an identical structure. Hence, the following describes the structure of one of them, that is, the process unit1Y that forms a yellow toner image.

For example, the process unit1Y includes a drum-shaped photoconductor2Y serving as an image carrier that carries an electrostatic latent image and a resultant yellow toner image; a charging roller3Y serving as a charger that charges an outer circumferential surface of the photoconductor2Y; a development device4Y serving as a development unit that supplies a developer (e.g., yellow toner) to the electrostatic latent image formed on the outer circumferential surface of the photoconductor2Y, thus visualizing the electrostatic latent image into a yellow toner image with the yellow toner; and a cleaning blade5Y serving as a cleaner that cleans the outer circumferential surface of the photoconductor2Y.

Above the process units1Y,1C,1M, and1K is an exposure device6serving as an exposure unit that emits a laser beam L onto the outer circumferential surface of the respective photoconductors2Y,2C,2M, and2K to form an electrostatic latent image thereon. For example, the exposure device6, constructed of a light source, a polygon mirror, an f-θ lens, reflection mirrors, and the like, emits a laser beam L onto the outer circumferential surface of the respective photoconductors2Y,2C,2M, and2K according to image data sent from an external device such as a client computer.

Below the process units1Y,1C,1M, and1K is a transfer unit7that accommodates an endless intermediate transfer belt8serving as a transferor, a driving roller9, a driven roller10, four primary transfer rollers11Y,11C,11M, and11K, a secondary transfer roller12, and a belt cleaner13. Specifically, the endless intermediate transfer belt8is stretched over the driving roller9and the driven roller10that support the intermediate transfer belt8. As the driving roller9rotates counterclockwise inFIG. 1, the intermediate transfer belt8rotates counterclockwise inFIG. 1in a rotation direction R4.

Inside a loop formed by the intermediate transfer belt8and opposite the four photoconductors2Y,2C,2M, and2K are the four primary transfer rollers11Y,11C,11M, and11K serving as primary transferors that transfer the yellow, cyan, magenta, and black toner images formed on the photoconductors2Y,2C,2M, and2K, respectively, onto an outer circumferential surface of the intermediate transfer belt8. The primary transfer rollers11Y,11C,11M, and11K contact an inner circumferential surface of the intermediate transfer belt8and press the intermediate transfer belt8against the photoconductors2Y,2C,2M, and2K at opposed positions where the primary transfer rollers11Y,11C,11M, and11K are disposed opposite the photoconductors2Y,2C,2M, and2K, respectively, via the intermediate transfer belt8, thus forming primary transfer nips between the photoconductors2Y,2C,2M, and2K and the intermediate transfer belt8where the yellow, cyan, magenta, and black toner images formed on the photoconductors2Y,2C,2M, and2K are primarily transferred onto the intermediate transfer belt8to form a color toner image thereon. The primary transfer rollers11Y,11C,11M, and11K are connected to a power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto.

Opposite the driving roller9is the secondary transfer roller12serving as a secondary transferor that transfers the color toner image formed on the intermediate transfer belt8onto a recording medium P. The secondary transfer roller12contacts the outer circumferential surface of the intermediate transfer belt8and presses the intermediate transfer belt8against the driving roller9, thus forming a secondary transfer nip between the secondary transfer roller12and the intermediate transfer belt8where the color toner image formed on the intermediate transfer belt8is transferred onto the recording medium P. Similar to the primary transfer rollers11Y,11C,11M, and11K, the secondary transfer roller12is connected to a power supply that applies a predetermined direct current voltage and/or alternating current voltage thereto.

The belt cleaner13, disposed opposite the outer circumferential surface of the intermediate transfer belt8and in proximity to the secondary transfer nip, cleans the outer circumferential surface of the intermediate transfer belt8. Below the intermediate transfer unit7is a waste toner container14that collects waste toner conveyed from the belt cleaner13through a waste toner conveyance tube extending from the belt cleaner13to an inlet of the waste toner container14.

Below the waste toner container14in a lower portion of the image forming apparatus100is a paper tray15that loads a plurality of recording media P (e.g., sheets and OHP (overhead projector) transparencies). The paper tray15is attached with a feed roller16that feeds a recording medium P from the paper tray15toward a registration roller pair19. In an upper portion of the image forming apparatus100are an output roller pair17that discharges the recording medium P onto an outside of the image forming apparatus100and an output tray18that receives and stocks the recording medium P discharged by the output roller pair17.

The recording medium P fed by the feed roller16is conveyed upward through a conveyance path R that extends from the paper tray15to the output roller pair17. The conveyance path R is provided with the registration roller pair19disposed below the secondary transfer nip formed between the secondary transfer roller12and the intermediate transfer belt8, that is, upstream from the secondary transfer nip in a recording medium conveyance direction D1. The conveyance path R is further provided with a fixing device20disposed above the secondary transfer nip, that is, downstream from the secondary transfer nip in the recording medium conveyance direction D1.

For example, the fixing device20includes a fixing roller21serving as a fixing rotary body heated by a heater24; a pressing roller22serving as a pressing rotary body that contacts the fixing roller21to form a fixing nip N therebetween; a separator23that separates the recording medium P from the fixing roller21; and a fixing exit guide41that guides the recording medium P toward the output roller pair17. According to this exemplary embodiment, a pressing mechanism presses the pressing roller22against the fixing roller21, thus forming the fixing nip N therebetween. However, alternative configurations are possible.

For example, at least one of the fixing rotary body and the pressing rotary body may be an endless belt pressed against another one of the fixing rotary body and the pressing rotary body by a roller or a pad. Further, the pressing rotary body may not press against the fixing rotary body but may merely contact the fixing rotary body. The heater24may be a halogen lamp, a resistance heater, or the like. According to this exemplary embodiment, the heater24is situated inside the fixing roller21. Alternatively, the heater24may be situated inside the pressing roller22or inside each of the fixing roller21and the pressing roller22.

Referring toFIG. 1, the following describes an operation of the image forming apparatus100having the structure described above to form a color toner image on a recording medium P.

As a print job starts, a driver drives and rotates the photoconductors2Y,2C,2M, and2K of the process units1Y,1C,1M, and1K, respectively, clockwise inFIG. 1in a rotation direction R5. The charging rollers3Y,3C,3M, and3K uniformly charge the outer circumferential surface of the respective photoconductors2Y,2C,2M, and2K at a predetermined polarity. The exposure device6emits laser beams L onto the charged outer circumferential surface of the respective photoconductors2Y,2C,2M, and2K according to yellow, cyan, magenta, and black image data contained in image data sent from the external device, respectively, thus forming electrostatic latent images thereon. The development devices4Y,4C,4M, and4K supply yellow, cyan, magenta, and black toners to the electrostatic latent images formed on the photoconductors2Y,2C,2M, and2K, visualizing the electrostatic latent images into yellow, cyan, magenta, and black toner images, respectively.

As the driving roller9is driven and rotated counterclockwise inFIG. 1, the driving roller9drives and rotates the intermediate transfer belt8counterclockwise inFIG. 1in the rotation direction R4. A power supply applies a constant voltage or a constant current control voltage having a polarity opposite a polarity of toner to the primary transfer rollers11Y,11C,11M, and11K. Thus, a transfer electric field is created at the primary transfer nips formed between the primary transfer rollers11Y,11C,11M, and11K and the photoconductors2Y,2C,2M, and2K, respectively. Accordingly, the yellow, cyan, magenta, and black toner images formed on the photoconductors2Y,2C,2M, and2K, respectively, are primarily transferred onto the intermediate transfer belt8successively by the transfer electric field created at the respective primary transfer nips, in such a manner that the yellow, cyan, magenta, and black toner images are superimposed on a same position on the intermediate transfer belt8. Consequently, a color toner image is formed on the intermediate transfer belt8. After the primary transfer of the yellow, cyan, magenta, and black toner images from the photoconductors2Y,2C,2M, and2K onto the intermediate transfer belt8, the cleaning blades5Y,5C,5M, and5K remove residual toner not transferred onto the intermediate transfer belt8and therefore remaining on the photoconductors2Y,2C,2M, and2K therefrom. Then, dischargers discharge the outer circumferential surface of the respective photoconductors2Y,2C,2M, and2K, initializing the potential thereof so that the respective photoconductors2Y,2C,2M, and2K are ready for the next print job.

On the other hand, as the print job starts, the feed roller16is driven and rotated to feed a recording medium P from the paper tray15toward the registration roller pair19through the conveyance path R. The registration roller pair19feeds the recording medium P to the secondary transfer nip formed between the secondary transfer roller12and the intermediate transfer belt8at a time when the color toner image formed on the intermediate transfer belt8reaches the secondary transfer nip. The secondary transfer roller12is applied with a transfer voltage having a polarity opposite a polarity of the charged yellow, cyan, magenta, and black toners of the yellow, cyan, magenta, and black toner images constituting the color toner image formed on the intermediate transfer belt8, thus creating a transfer electric field at the secondary transfer nip. Accordingly, the yellow, cyan, magenta, and black toner images constituting the color toner image are secondarily transferred from the intermediate transfer belt8collectively onto the recording medium P by the transfer electric field created at the secondary transfer nip. The recording medium P bearing the color toner image is conveyed to the fixing device20where the fixing roller21and the pressing roller22apply heat and pressure to the recording medium P, fixing the color toner image on the recording medium P. The separator23separates the recording medium P bearing the fixed color toner image from the fixing roller21. Thereafter, the output roller pair17discharges the recording medium P onto the output tray18. After the secondary transfer of the color toner image from the intermediate transfer belt8onto the recording medium P, the belt cleaner13removes residual toner not transferred onto the recording medium P and therefore remaining on the intermediate transfer belt8therefrom. The removed toner is conveyed and collected into the waste toner container14.

The above describes the image forming operation of the image forming apparatus100to form the color toner image on the recording medium P. Alternatively, the image forming apparatus100may form a monochrome toner image by using any one of the four process units1Y,1C,1M, and1K or may form a bicolor or tricolor toner image by using two or three of the process units1Y,1C,1M, and1K.

Referring toFIGS. 2 and 3, the following describes a construction of the fixing device20installed in the image forming apparatus100described above.

FIG. 2is a vertical sectional view of the fixing device20in a state in which no recording medium P is conveyed therethrough.FIG. 3is a vertical sectional view of the fixing device20in a state in which a recording medium P is conveyed therethrough. As shown inFIG. 2, the fixing device20(e.g., a fuser unit) includes the fixing roller21heated by the heater24situated inside the fixing roller21. The fixing roller21is rotatable in a rotation direction R1and the pressing roller22is rotatable in a rotation direction R2counter to the rotation direction R1of the fixing roller21. The pressing roller22is pressed against the fixing roller21to form the fixing nip N therebetween.

As shown inFIG. 3, after the fixing roller21is heated by the heater24to a predetermined target fixing temperature, as the fixing roller21rotating in the rotation direction R1and the pressing roller22rotating in the rotation direction R2nip and convey a recording medium P bearing a toner image T through the fixing nip N formed between the fixing roller21and the pressing roller22, the fixing roller21and the pressing roller22apply heat and pressure to the recording medium P, thus melting and fixing the toner image T on the recording medium P.

A thermistor61serving as a temperature detector that detects the temperature of the fixing roller21is disposed opposite an outer circumferential surface of the fixing roller21. Similarly, a thermostat62preventing overheating of the fixing roller21is disposed opposite the outer circumferential surface of the fixing roller21. A controller37, that is, a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, is operatively connected to the heater24, the thermistor61, and the thermostat62. The controller37controls the heater24based on a detection signal output from the thermistor61so as to adjust the temperature of the outer circumferential surface of the fixing roller21to a predetermined fixing temperature range.

A detailed description is now given of a construction of the fixing roller21.

The fixing roller21is a tube constructed of a thermal conductive base layer, an elastic layer coating the base layer, and an outer surface layer coasting the elastic layer. The thermal conductive base layer is made of a thermal conductive material having a predetermined mechanical strength, such as carbon steel, aluminum, or the like. The elastic layer is made of synthetic rubber such as silicone rubber, fluoro rubber, or the like. The outer surface layer is made of a heat-resistant, thermal conductive material that facilitates separation of the toner image T on the recording medium P from the fixing roller21and enhances the durability of the elastic layer. For example, the outer surface layer may be a fluoroplastic tube made of tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), a fluoroplastic coat made of PFA or polytetrafluoroethylene (PTFE), a silicone rubber layer, a fluoro rubber layer, or the like.

A detailed description is now given of a construction of the pressing roller22.

The pressing roller22is a tube constructed of a metal core, an elastic layer coating the metal core, and an outer surface layer coating the elastic layer. For example, the metal core is made of carbon steel tubes for machine structural purposes (STKM). The elastic layer is made of silicone rubber, fluoro rubber, silicone rubber foam, fluoro rubber foam, or the like. The outer surface layer is a heat-resistant fluoroplastic tube made of PFA, PTFE, or the like that facilitates separation of the toner image T on the recording medium P from the pressing roller22.

A detailed description is now given of a construction of the separator23.

As shown inFIG. 2, the separator23is disposed above the fixing roller21, that is, downstream from the fixing nip N in the recording medium conveyance direction D1and opposite the outer circumferential surface of the fixing roller21. A pawl23aconstituting a head of the separator23comes into contact with and separates from the outer circumferential surface of the fixing roller21. The pawl23ahas a round front edge that does not damage the fixing roller21as it slides over the fixing roller21. The pawl23acontacts the fixing roller21in a direction counter to the rotation direction R1of the fixing roller21. The separator23is rotatable about a shaft25, both axial ends of which are rotatably mounted on side plates rotatably mounting both axial ends of the respective fixing roller21and the pressing roller22. As the separator23rotates about the shaft25clockwise or counterclockwise inFIG. 2, the pawl23aof the separator23comes into contact with and separates from the fixing roller21independently from other separators23.FIG. 3illustrates the pawl23aof the separator23brought into contact with the fixing roller21.

A distance E in a range of from about 5 mm to about 6 mm is provided between an exit of the fixing nip N and a contact position where the pawl23aof the separator23contacts the fixing roller21in the recording medium conveyance direction D1. The distance E may be as long as possible within a range that does not obstruct separation of the recording medium P from the fixing roller21by the pawl23a, increasing time for cooling the recording medium P and minimizing damage inflicted on the toner image T on the recording medium P as the pawl23acontacts and separates the recording medium P from the fixing roller21.

The separator23is made of PFA, polyetherketone (PEK), polyether ether ketone (PEEK), or the like that facilitates separation from and sliding over the fixing roller21. Alternatively, a surface of the separator23may be coated with PFA or Teflon® that facilitates separation from and sliding over the fixing roller21.

A detailed description is now given of a configuration of a mechanism that moves the separator23with respect to the fixing roller21.

FIGS. 2 and 3illustrate one example of such mechanism, that is, a contact direction biasing member26, a separator presser27, and a separation direction biasing member29. It is to be noted that since the fixing device20includes a plurality of separators23, a plurality of contact direction biasing members26corresponds to the plurality of separators23. As shown inFIGS. 2 and 3, the contact direction biasing member26is anchored to a base23bof the separator23constituting one end of the separator23in a longitudinal direction thereof disposed opposite the pawl23aconstituting another end of the separator23in the longitudinal direction thereof. According to this exemplary embodiment, a tension coil spring is used as the contact direction biasing member26. Alternatively, other biasing members may be used as the contact direction biasing member26in view of location and manufacturing costs of the contact direction biasing member26. The contact direction biasing member26biases the separator23against the fixing roller21, thus bringing the separator23into contact with the outer circumferential surface of the fixing roller21.

On the other hand, the separator presser27separatably contacts the base23bof the separator23to separate the separator23from the fixing roller21. The separator presser27is supported by a shaft28in such a manner that the separator presser27is rotatable about the shaft28. As the separator presser27rotates about the shaft28clockwise and counterclockwise inFIG. 2, a free end27aof the separator presser27disposed opposite the base23bof the separator23comes into contact with and separates from the base23bof the separator23. The separator presser27extends in a direction parallel to an axial direction of the fixing roller21, thus separatably contacting all of the plurality of separators23aligned in the axial direction of the fixing roller21.

The separator presser27is made of lightweight resin having predetermined mechanical strength, heat resistance, and abrasion resistance, such as poly p-phenylene sulfide (PPS), polyphenylene sulfide (PPS), or polyetherketone (PEK). According to this exemplary embodiment, the shaft28made of SUS stainless steel is separately manufactured from the separator presser27to prevent bending of the separator presser27in a longitudinal direction thereof parallel to the axial direction of the fixing roller21. Alternatively, the separator presser27may be made of other materials according to the size of the fixing device20and a resilient bias exerted to the separator23by the contact direction biasing member26and the separation direction biasing member29.

The separation direction biasing member29is anchored to a linkage drivably connected to the separator presser27and exerts a resilient bias to the separator presser27that separates the separator23from the fixing roller21.FIGS. 2 and 3schematically illustrate the separation direction biasing member29anchored to a base27bof the separator presser27. As shown inFIG. 2, as the separation direction biasing member29pulls the base27bof the separator presser27in a direction B2, the free end27aof the separator presser27is brought into contact with the base23bof the separator23. Alternatively, other biasing members may be used as the separation direction biasing member29in view of location and manufacturing costs. The contact direction biasing member26constantly exerts a resilient bias to the separator23that brings the separator23into contact with the fixing roller21. Conversely, the separation direction biasing member29exerts a resilient bias to the separator23via the separator presser27as needed that separates the separator23from the fixing roller21.

A solenoid30is connected to the separator presser27and serves as a driver that drives the separator presser27. For example, the solenoid30is constructed of a body31incorporating a coil and a plunger32that protrudes from and retracts into the coil. The plunger32is connected to the linkage drivably connected to the separator presser27.FIGS. 2 and 3schematically illustrate the plunger32connected to the base27bof the separator presser27. As the coil incorporated in the body31is excited and the plunger32is refracted into the body31, the plunger32pulls down the base27bof the separator presser27, thus driving and rotating the separator presser27in a rotation direction R3as shown inFIG. 3.

Above the separator23inFIG. 2is a detent33that contacts and halts the separator23at a predetermined halting position where the separator23is isolated from the fixing roller21with a predetermined interval therebetween. Accordingly, even if the separation direction biasing member29constantly pulls the base27bof the separator presser27in the direction B2and therefore the separator presser27presses against the base23bof the separator23against the resilient bias exerted by the contact direction biasing member26, thus separating the separator23from the fixing roller21, the detent33halts the separator23at the predetermined halting position with an appropriate interval between the pawl23aof the separator23and the outer circumferential surface of the fixing roller21regardless of variation in dimension and assembly of parts constituting the separator23.

A recording medium detector34is located upstream from the fixing nip N in the recording medium conveyance direction D1and detects a recording medium P conveyed toward the fixing nip N. The recording medium detector34is constructed of a shaft35and a feeler36swingably or rotatably supported by the shaft35. As shown inFIG. 2, before the recording medium P touches the feeler36, the feeler36intersects the conveyance path R. As the recording medium P conveyed through the conveyance path R comes into contact with and abuts the feeler36, the feeler36detects the recording medium P and swings or rotates counterclockwise inFIG. 2to a position shown inFIG. 3where the feeler36retracts from the conveyance path R, allowing the recording medium P to move to the fixing nip N. After the recording medium P passes through the recording medium detector34, the feeler36returns to a default position shown inFIG. 2by its own weight or a bias exerted by a biasing member (e.g., a torsion coil spring). Specifically, the feeler36comes into contact with and is halted by a detent at the default position shown inFIG. 2.

For example, the feeler36is located at a position in proximity to a center of the conveyance path R in a width direction thereof orthogonal to the recording medium conveyance direction D1, thus preventing the recording medium P from being skewed by contact with the feeler36. Such location of the feeler36facilitates smooth conveyance of the recording medium P that prevents creasing of the recording medium P and warping of a toner image T on the recording medium P, enhancing reliability in conveyance of the recording medium P.

According to this exemplary embodiment, the recording medium detector34is a contact detector that detects the recording medium P by contacting it. Alternatively, a non-contact detector, such as a reflection optical sensor or a transmission optical sensor, which detects the recording medium P without contacting it may be used. The non-contact detector provides an advantage of precluding skew of the recording medium P because it does not contact the recording medium P.

Further, a jam detector for detecting a jammed recording medium P may be located upstream from the fixing nip N in the recording medium conveyance direction D1. In this case, such jam detector may also serve as the recording medium detector34. Accordingly, a separate detector that detects the recording medium P is unnecessary, downsizing the fixing device20and reducing manufacturing costs.

The solenoid30is driven based on a detection signal output from the recording medium detector34. For example, the solenoid30is electrically connected to the recording medium detector34via a driving circuit38and the controller37. The controller37is the CPU incorporating an input-output (I/O) port. When the recording medium detector34detects the recording medium P conveyed toward the fixing nip N and generates a detection signal, the controller37drives the solenoid30via the driving circuit38based on the detection signal sent from the recording medium detector34.

A detailed description is now given of an operation of the above-described mechanism that moves the separator23with respect to the fixing roller21.

FIG. 2illustrates a non-contact state in which the separator23is isolated from the fixing roller21before the recording medium P reaches the recording medium detector34. Since the recording medium detector34does not yet detect the recording medium P, the controller37does not drive the solenoid30. Hence, the solenoid30does not drive the separator presser27. Conversely, the separation direction biasing member29exerts a resilient bias to the separator presser27that pulls the base27bof the separator presser27upward inFIG. 2toward the separation direction biasing member29, applying a rotation moment M3clockwise inFIG. 2to the separator presser27. Accordingly, the rotation moment M3causes the free end27aof the separator presser27to press down the base23bof the separator23.

The separator presser27pressing down the base23bof the separator23applies a rotation moment M2counterclockwise inFIG. 2to the separator23. Simultaneously, the contact direction biasing member26pulls the base23bof the separator23upward inFIG. 2in a direction B1, applying a rotation moment M1clockwise inFIG. 2to the separator23. That is, the separator23receives the two forces in the opposite directions: the clockwise rotation moment M1and the counterclockwise rotation moment M2counter to the rotation moment M1. However, the rotation moment M2applied by the separation direction biasing member29is greater than the rotation moment M1applied by the contact direction biasing member26. Accordingly, the pawl23aof the separator23separates from the fixing roller21. Before the recording medium P is conveyed through the fixing nip N, the pawl23aof the separator23is isolated from the fixing roller21, minimizing wear of the fixing roller21due to contact with the pawl23aand therefore allowing the fixing roller21to apply heat and pressure to the recording medium P bearing the toner image T for an extended period of time for formation of a high quality toner image. Further, the separator23moved by the separation direction biasing member29is halted by the detent33at the predetermined halting position where the separator23is isolated from the fixing roller21with the predetermined interval therebetween.

As shown inFIG. 3, when the recording medium P conveyed toward the fixing nip N comes into contact with the feeler36of the recording medium detector34, the controller37drives the solenoid30via the driving circuit38based on a detection signal sent from the recording medium detector34. Specifically, when a predetermined electric current is supplied to the solenoid30, the plunger32is retracted into the body31in a direction D2against a resilient bias exerted by the separation direction biasing member29. The plunger32pulls down the base27bof the separator presser27, rotating the free end27aof the separator presser27counterclockwise inFIG. 3in the rotation direction R3. Accordingly, the free end27aof the separator presser27separates from the base23bof the separator23and thus the separator presser27no longer presses down the separator23.

As the separator presser27is isolated from the separator23, the separator23receives only the rotation moment M1applied by the contact direction biasing member26. Accordingly, the separator23rotates clockwise inFIG. 3, bringing the pawl23ainto contact with the fixing roller21.

Referring toFIGS. 4 to 7, a detailed description is now given of a construction of the fixing exit guide41incorporated in the fixing device20.

FIG. 4is a perspective view of the fixing exit guide41, the fixing roller21, and the pressing roller22.FIG. 5is a horizontal front view of the fixing exit guide41and the fixing roller21.FIG. 6is a perspective view of guide roller pairs43-1,43-2, and43-3and a guide roller shaft42.FIG. 7is a vertical sectional view of the fixing device20illustrating the fixing exit guide41. As shown inFIG. 7, the fixing exit guide41is located above the fixing roller21, that is, downstream from the fixing nip N in the recording medium conveyance direction D1.

The lightweight fixing exit guide41has a heat resistance great enough to endure radiant heat from the fixing roller21and is made of a material readily molded into a complex shape such as polyethylene terephthalate (PET) containing glass fiber. As shown inFIG. 4, the fixing exit guide41is constructed of a substantially vertical guide face41aconstituting a front face of the fixing exit guide41that guides the recording medium P discharged from the fixing nip N. A plurality of slanted ribs41bis mounted on the guide face41athat guides the recording medium P. The slanted ribs41bfacilitate contact of the recording medium P therewith, shortening the time for which the recording medium P contacts the slanted ribs41b. A lower edge, that is, an upstream edge41cin the recording medium conveyance direction D1, of the guide face41aof the fixing exit guide41is located beside pawl roller pairs40-1,40-2,40-3, and40-4serving as a first rotary body and aligned with the pawl roller pairs40-1,40-2,40-3, and40-4in an axial direction thereof. A predetermined interval is provided between the upstream edge41cof the guide face41aand the outer circumferential surface of the fixing roller21, preventing the upstream edge41cof the guide face41afrom coming into contact with and damaging the fixing roller21and jamming the recording medium P between the upstream edge41cof the guide face41aand the fixing roller21.

The three guide roller pairs43-1,43-2, and43-3serving as a second rotary body mounting brushes described below rotatably protrude from the guide face41aof the fixing exit guide41. Each of the guide roller pairs43-1,43-2, and43-3is constructed of a base made of a heat-resistant material such as polybutylene terephthalate (PBT). As shown inFIGS. 5 and 6, the three guide roller pairs43-1,43-2, and43-3are evenly spaced on the guide roller shaft42with an interval D between the adjacent guide roller pairs43-1and43-2and the adjacent guide roller pairs43-2and43-3in a longitudinal direction of the guide roller shaft42. Hence, six rollers constitute the three guide roller pairs43-1,43-2, and43-3. However, the number of the rollers constituting the guide roller pairs may be changed as needed. The guide roller shaft42may be attached to the fixing exit guide41with a single motion.

Below the guide roller pairs43-1,43-2, and43-3are the rotatable four pawl roller pairs40-1,40-2,40-3, and40-4, serving as a first rotary body mounting brushes51and53depicted inFIG. 7, protruding from the fixing exit guide41. Each of the pawl roller pairs40-1,40-2,40-3, and40-4is constructed of a base made of a heat-resistant material such as PBT. As shown inFIG. 5, each of the pawl roller pairs40-1,40-2,40-3, and40-4is constructed of two rollers sandwiching the pawl23adepicted inFIG. 2of respective separators23-1,23-2,23-3, and23-4collectively illustrated inFIG. 2as the separator23that separates the recording medium P from the fixing roller21. As shown inFIG. 4, according to this exemplary embodiment, eight rollers constitute the four pawl roller pairs40-1,40-2,40-3, and40-4. However, the number of the rollers used as the pawl roller pairs may be changed as needed.

As shown inFIG. 5, the four separators23-1,23-2,23-3, and23-4collectively illustrated inFIG. 2as the separator23are aligned in the axial direction of the fixing roller21. For example, the four separators23-1,23-2,23-3, and23-4are evenly or substantially evenly spaced in such a manner that the two separators23-1and23-2located in the left half inFIG. 5and the two separators23-3and23-4located in the right half inFIG. 5are symmetric with respect to a center line CL of a recording medium conveyance region in the axial direction of the fixing roller21. Specifically, an interval A is provided between the center line CL and the respective inboard separators23-2and23-3in the axial direction of the fixing roller21; an interval B greater than the interval A is provided between the center line CL and the respective outboard separators23-1and23-4in the axial direction of the fixing roller21. A substantially identical interval C is provided between the separators23-1and23-2, between the separators23-2and23-3, and between the separators23-3and23-4.

As shown inFIG. 5, the two separators23-1and23-2located in the left half inFIG. 5and the two separators23-3and23-4located in the right half inFIG. 5are symmetric with respect to the center line CL, maintaining the symmetrical shape of the recording medium P discharged from the fixing nip N and thereby preventing dog-ear or folded corner and jamming of the recording medium P for smooth conveyance of the recording medium P. The substantially evenly spaced, four separators23-1,23-2,23-3, and23-4cause the pawls23ato exert a uniform force to the recording medium P throughout a width direction thereof parallel to the axial direction of the fixing roller21to separate the recording medium P from the fixing roller21. For example, a particular one of the pawls23aof the separators23-1,23-2,23-3, and23-4may not damage the fixing roller21by exerting a greater force to the fixing roller21while the pawls23acontact the fixing roller21to separate the recording medium P from the fixing roller21as shown inFIG. 3.

As shown inFIG. 5, the three guide roller pairs43-1,43-2, and43-3are located downstream from the separators23-1,23-2,23-3, and23-4in the recording medium conveyance direction D1. The three guide roller pairs43-1,43-2, and43-3are aligned in the axial direction of the fixing roller21in the recording medium conveyance region in such a manner that the interval D is provided between the center guide roller pair43-2and each of the ambilateral guide roller pairs43-1and43-3. According to this exemplary embodiment, the interval D is about 58 mm Accordingly, the upstream, four separators23-1,23-2,23-3, and23-4and the downstream, three guide roller pairs43-1,43-2, and43-3create two staggered rows, downsizing the fixing exit guide41. The four separators23-1,23-2,23-3, and23-4and the three guide roller pairs43-1,43-2, and43-3aligned in the two staggered rows also exert uniform pressure to the recording medium P throughout the width direction thereof, preventing the pawls23aof the separators23-1,23-2,23-3, and23-4, the pawl roller pairs40-1,40-2,40-3, and40-4, and the guide roller pairs43-1,43-2, and43-3from producing scratches and glossy streaks on the toner image T on the recording medium P.

If the guide roller pairs43-1,43-2, and43-3are located downstream from the pawls23aof the separators23-1,23-2,23-3, and23-4in the recording medium conveyance direction D1in such a manner that the guide roller pairs43-1,43-2, and43-3and the separators23-1,23-2,23-3, and23-4do not create the two staggered rows, it is necessary to prevent the separators23-1,23-2,23-3, and23-4and the pawl roller pairs40-1,40-2,40-3, and40-4from touching or striking the guide roller pairs43-1,43-2, and43-3. For example, if the guide roller pairs43-1,43-2, and43-3and the pawl roller pairs40-1,40-2,40-3, and40-4have a greater diameter, a greater interval is needed between the guide roller pairs43-1,43-2, and43-3and the pawl roller pairs40-1,40-2,40-3, and40-4in the recording medium conveyance direction D1, obstructing downsizing of the fixing exit guide41. To address this problem, the guide roller pairs43-1,43-2, and43-3and the pawl roller pairs40-1,40-2,40-3, and40-4create the two staggered rows.

Referring toFIG. 8, a detailed description is now given of a construction of a pawl roller pair40representing the pawl roller pairs40-1,40-2,40-3, and40-4depicted inFIG. 5.

FIG. 8is a partial vertical sectional view of the pawl roller pair40, the separator23, and the fixing roller21. The pawl roller pair40is disposed in proximity to the pawl23aof the separator23. An axis O40of the pawl roller pair40extends in parallel with an axis O21of the fixing roller21. As shown inFIG. 8, the axis O40of the pawl roller pair40is inboard from or on the left of an extension line L3extending from a lift face23cof the pawl23aopposite a contact face thereof contacting the fixing roller21. According to this exemplary embodiment, the extension line L3extends substantially vertically inFIG. 8. The size or diameter of the pawl roller pair40is defined by a projection K of about 2 mm, that is, a gap between the extension line L3and a tangential line L4extending substantially vertically from an outer circumferential surface of the brushes51and53mounted on the pawl roller pair40in parallel with the extension line L3.

The greater projection K facilitates separation of the recording medium P from the pawl23aeven if the recording medium P is plain paper, thin paper having paper weight not greater than about 70 g/m2, or soft paper, thus preventing the pawl23afrom producing scratches or glossy streaks on the toner image T on the recording medium P. By contrast, if the recording medium P is thick paper having paper weight not smaller than about 150 g/m2or hard paper, the greater projection K presses the pawl roller pair40against the recording medium P with greater pressure, thus causing the pawl roller pair40to produce scratches or glossy streaks on the toner image T on the recording medium P. To address this problem, based on a result of a simulation measuring scratches and glossy streaks produced by the separator23and the pawl roller pair40, the projection K is set to about 2 mm.

The separator23includes a junction23dsituated at an intersection of the extension line L3and the outer circumferential surface of the brushes51and53mounted on the pawl roller pair40. Although the junction23dis hatched inFIG. 8, the junction23dmay be molded with the pawl23aor other part of the separator23. For example, the junction23dis substantially triangular in cross-section taken along a direction orthogonal to the axial direction of the fixing roller21with a first side23d1disposed opposite the outer circumferential surface of the brushes51and53mounted on the pawl roller pair40and a second side23d2contiguous to the lift face23cof the pawl23a. An extension line L2extending from the second side23d2of the triangular junction23dand the extension line L3extending from the lift face23cof the pawl23aform an angle α of about 23 degrees. An extension line L1extending from the first side23d1of the triangular junction23d, that is, a tangential line on the outer circumferential surface of the brushes51and53of the pawl roller pair40at a right edge of the first side23d1of the triangular junction23dinFIG. 8, and the extension line L2form an angle β of about 29 degrees. The angles α and β may be determined based on results of a simulation to facilitate movement of the recording medium P over the pawl23a, the junction23d, and the pawl roller pair40. It is to be noted that the angle α of about 23 degrees and the angle β of about 29 degrees constitute one non-limiting exemplary embodiment.

The junction23ddisposed in proximity to the pawl23afacilitates conveyance of the recording medium P from the pawl23ato the pawl roller pair40. Without the junction23d, an angle formed by the extension lines L3and L1, that is, the combined angle of the angles α and β is too great to prevent a leading edge of the recording medium P from striking the outer circumferential surface of the brushes51and53mounted on the pawl roller pair40substantially vertically, resulting in jamming of the recording medium P. Further, if each of the angles α and β is greater than about 30 degrees, as the recording medium P moves from the pawl23ato the junction23dand from the junction23dto the pawl roller pair40, the toner image T on the recording medium P may contact the pawl23a, the junction23d, and the pawl roller pair40with greater pressure, resulting in damage to the toner image T on the recording medium P and jamming of the recording medium P. To address these problems, each of the angles α and β is set to not greater than about 30 degrees.

Referring toFIGS. 5 and 7, a detailed description is now given of a construction of a guide roller pair43.

As shown inFIG. 7, the guide roller pair43representing the guide roller pairs43-1,43-2, and43-3protruding from the fixing exit guide41depicted inFIG. 5has an axis O43situated substantially directly above the axis O40of the pawl roller pair40in vertical cross-section in such a manner that a distance F from the exit of the fixing nip N to the axis O43of the guide roller pair43is about 20 mm. Conversely, in a horizontal front view illustrated inFIG. 5, the guide roller pairs43-1,43-2, and43-3and the pawl roller pairs40-1,40-2,40-3, and40-4create the two staggered rows. As shown inFIG. 7, the diameter of the guide roller pair43is greater than the diameter of the pawl roller pair40. A projection S, that is, a gap between a vertical tangential line L5extending substantially vertically from the outer circumferential surface of the brushes51and53mounted on the pawl roller pair40and a vertical tangential line L6extending substantially vertically from an outer circumferential surface of brushes52and54mounted on the guide roller pair43is about 2 mm.

The greater the projection S is, more effectively formation of a faulty toner image T on the recording medium P is prevented, such as scratches or glossy streaks produced on the toner image T on the recording medium P by the pawl23aand the pawl roller pair40contacting the recording medium P. However, the greater projection S adheres moisture evaporated from the recording medium P to a projected portion of the guide roller pair43corresponding to the projection S that is situated above the fixing nip N. Specifically, as the fixing roller21and the pressing roller22apply heat and pressure to the recording medium P conveyed through the fixing nip N, moisture contained in the recording medium P is released into the atmosphere as steam after the recording medium P is discharged from the fixing nip N. As the steam moves upward and strikes a part of the outer circumferential surface of the guide roller pair43corresponding to the projection S, the steam adheres to an outer circumferential surface of the guide roller pair43as water droplets. Then, the water droplets are transferred from the guide roller pair43onto the recording medium P. As the recording medium P is discharged from the fixing device20, the water droplets on the recording medium P are dried, leaving water droplet marks on the recording medium P. During duplex printing, a toner image T may not be transferred onto a back side of the recording medium P deteriorated by the water droplets adhered thereto, resulting in formation of a faulty toner image T on the recording medium P.

The shorter the distance F between the exit of the fixing nip N and the axis O43of the guide roller pair43, the smaller the pressure between the recording medium P and the pawl23aand the pawl roller pair40, preventing formation of a faulty toner image Ton the recording medium P with scratches and glossy streaks on the toner image T produced by the pawl23aand the pawl roller pair40. Further, the shorter distance F decreases a horizontal interval between the pawl roller pair40and the recording medium P, decreasing the projection S. However, even with the decreased projection S, moisture evaporated from the recording medium P may adhere to the guide roller pair43as water droplets. As described above, since the guide roller pairs43-1,43-2, and43-3and the pawl roller pairs40-1,40-2,40-3, and40-4create the two staggered rows, even with the shorter projection S, the guide roller pairs43-1,43-2, and43-3are exposed to moisture generated from the recording medium P heated at the fixing nip N. To address this circumstance, the fixing device20employs the brushes51to54described below.

Referring toFIGS. 9 to 10, a detailed description is now given of a construction of the brushes51to54.

FIG. 9is a perspective view of the pawl roller pair40and the brushes51and53.FIG. 10is a perspective view of the guide roller pair43and the brushes52and54.

As shown inFIG. 9, two types of brushes, that is, the brushes51and53, are mounted on an outer circumferential surface of the pawl roller pair40. Similarly, as shown inFIG. 10, two types of brushes, that is, the brushes52and54, are mounted on the outer circumferential surface of the guide roller pair43. The brushes51and52are different from the brushes53and54in that the brushes51and52have straight cut ambilateral edges in a longitudinal direction thereof and the brushes53and54have diagonally cut ambilateral edges in a longitudinal direction thereof. Other than that, the brushes51to54have an identical structure. Since the diameter of the pawl roller pair40is smaller than that of the guide roller pair43, the length of the brushes51and53in the longitudinal direction thereof is smaller than the length of the brushes52and54in the longitudinal direction thereof.

The brushes51to54are constructed of a heat-resistant fiber sheet made of nylon, aramid, polyurethane, or the like. According to this exemplary embodiment, a sheet implanted with fibers made of durable, heat-resistant nylon is cut into bands having lower adhesive faces51ato54a, respectively. For example, as shown inFIG. 9, the brush51is constructed of a base sheet51bmounting fibers51cat one face thereof and the adhesive face51aat another face thereof. Similarly, the brush53is constructed of a base sheet53bmounting fibers53cand the adhesive face53aas shown inFIG. 9; the brush52is constructed of a base sheet52bmounting fibers52cand the adhesive face52aas shown inFIG. 10; and the brush54is constructed of a base sheet54bmounting fibers54cand the adhesive face54aas shown inFIG. 10. The adhesive faces51aand53aare attached to the outer circumferential surface of the pawl roller pair40as shown inFIG. 9; the adhesive faces52aand54aare attached to the outer circumferential surface of the guide roller pair43as shown inFIG. 10. The fibers51cto54chave a length not smaller than about 0.8 mm, a density not smaller than about 10,000 pieces/cm2, and a diameter not greater than about 1.5 denier, thus contacting the recording medium P more softly and thereby minimizing scratches and glossy streaks produced on the toner image T on the recording medium P compared to fibers not configured to have the length, the density, and the diameter of the above-described range. Even if moisture generated from the recording medium P adheres to the pawl roller pair40and the guide roller pair43as water droplets, the brushes51to54absorb the water droplets, minimizing water droplets transferred onto the toner image T on the recording medium P and resultant water droplet marks thereon. The water droplets absorbed by the brushes51to54are evaporated by the entirely heated fixing device20, repeating absorption of the water droplets by the brushes51to54.

Each of the brushes51to54is a band having a length in the longitudinal direction thereof wound around the entire outer circumferential surface of the respective pawl roller pair40and the guide roller pair43once. Each of the brushes51and52has square ambilateral ends in the longitudinal direction thereof. By contrast, each of the brushes53and54has triangular ambilateral ends in the longitudinal direction thereof that have an angle γ.

The square ambilateral ends of the brushes51and52are manufactured at reduced costs with decreased variation in part size. However, if the leading edge of the recording medium P strikes a seam J1of the brushes51and52, the seam J1may damage, fold, or bend the recording medium P, thus jamming the recording medium P. Further, after the recording medium P strikes the seam J1of the brushes51and52repeatedly, it may gradually degrade an adhesive force of the adhesive faces51aand52aof the brushes51and52, and finally peel the brushes51and52off the pawl roller pair40and the guide roller pair43.

By contrast, the triangular ambilateral ends of the brushes53and54with the angle γ allow the leading edge of the recording medium P to strike a seam J2of the brushes53and54gradually, minimizing damage to the recording medium P and thereby improving durability of the brushes53and54compared to the square ambilateral ends of the brushes51and52having the seam J1. For example, the angle γ may be in a range of from about 30 degrees to about 60 degrees with respect to the recording medium conveyance direction D1. The angle γ smaller than about 30 degrees narrows a head of the triangular ambilateral ends of the brushes53and54, peeling the brushes53and54off the pawl roller pair40and the guide roller pair43easily. Conversely, the angle γ greater than about 60 degrees may raise the problems of the seam J1of the square ambilateral ends of the brushes51and52described above.

Referring toFIG. 11, the following describes a configuration of electrostatic implantation of fibers on the pawl roller pair40and the guide roller pair43.

FIG. 11is a vertical sectional view of the pawl roller pair40and the guide roller pair43electrostatically implanted with fibers. As shown inFIG. 11, an adhesive57is applied to an outer circumferential surface of the base, made of PBT or the like, of the pawl roller pair40and the guide roller pair43. Then, lots of fibers58serving as a brush are implanted to the adhesive57and the base of the pawl roller pair40and the guide roller pair43is electrostatically charged. For example, the fibers58are nylon fibers having a length not smaller than about 0.8 mm, a density not smaller than about 10,000 pieces/cm2, and a diameter not greater than about 1.5 denier. As the base of the pawl roller pair40and the guide roller pair43is electrostatically charged in a state in which the fibers58are implanted to the adhesive57, electrostatic repulsion causes the fibers58to stand upright on the outer circumferential surface of the base of the pawl roller pair40and the guide roller pair43until the adhesive57is solidified. Since every fiber58is adhered to the pawl roller pair40and the guide roller pair43with the adhesive57, there are no seams J1and J2of the brushes51to54depicted inFIGS. 9 and 10that may peel the fibers58off the pawl roller pair40and the guide roller pair43. Additionally, the fibers58adhered to the pawl roller pair40and the guide roller pair43attain improved durability compared to the brushes53and54having the triangular ambilateral ends.

Further, since the sheet brushes51to54have variation in size and error in adhesion to the pawl roller pair40and the guide roller pair43, it is difficult to adhere the brushes51to54to the entire outer circumferential surface of the pawl roller pair40and the guide roller pair43. To address this circumstance, it is necessary to adhere the brushes51to54to the reduced outer circumferential surface of the pawl roller pair40and the guide roller pair43in view of variation in size and error in adhesion to the pawl roller pair40and the guide roller pair43. Conversely, the fibers58are adhered to the entire outer circumferential surface of the pawl roller pair40and the guide roller pair43under electrostatic implantation, increasing the contact area where the fibers58contact the recording medium P compared to the sheet brushes51to54adhered to the pawl roller pair40and the guide roller pair43of the identical size. Accordingly, the fibers58contact the recording medium P more softly, minimizing scratches and glossy streaks produced on the toner image T on the recording medium P more precisely.

Referring toFIGS. 12 and 13, the following describes an operation of the pawl23aof the separator23, the pawl roller pair40, and the guide roller pair43.

A detailed description is now given of an operation of the pawl23aof the separator23.

FIG. 12is a vertical sectional view of the fixing device20illustrating movement of the recording medium P. As shown inFIG. 12, as the pawl23acontacts the fixing roller21, even if the recording medium P discharged from the fixing nip N still adheres to the fixing roller21, the pawl23aseparates the recording medium P from the fixing roller21. For example, when the leading edge of the recording medium P mounts the lift face23cof the pawl23a, the recording medium P slides over the lift face23cof the pawl23atoward the outer circumference surface of the pawl roller pair40. Immediately before the leading edge of the recording medium P mounts the pawl roller pair40, it moves from the lift face23cof the pawl23ato the junction23dthat forms the angle α depicted inFIG. 8with the lift face23cof the pawl23a. Since the angle α is smaller than about 30 degrees, for example, about 23 degrees, the lift face23cof the pawl23aand the junction23dpress against the recording medium P with moderate pressure. Accordingly, even if the recording medium P stores heat and therefore bears the soft toner image T softened by the heat, the lift face23cof the pawl23aand the junction23ddo not produce scratches and glossy streaks on the soft toner image T on the recording medium P.

As the leading edge of the recording medium P mounts the pawl roller pair40from the junction23d, it comes into contact with the brushes51and53mounted on the outer circumferential surface of the pawl roller pair40softly, thus rotating the pawl roller pair40. Simultaneously, the recording medium P moves upward while it is supported by a head of lots of fibers constituting the outer circumferential surface of the brushes51and53. Since the angle β depicted inFIG. 8is smaller than about 30 degrees, for example, about 29 degrees, the junction23dand the pawl roller pair40press against the recording medium P with moderate pressure. Accordingly, even if the recording medium P stores heat and therefore bears the soft toner image T softened by the heat, the junction23dand the pawl roller pair40do not produce scratches and glossy streaks on the soft toner image T on the recording medium P.

Moisture evaporated from the recording medium P by heat and pressure applied at the fixing nip N moves upward along the recording medium conveyance direction D1in accordance with movement of the recording medium P. When the moisture comes into contact with the separator23, it is cooled by the separator23into water droplets. However, the brushes51and53mounted on the outer circumferential surface of the pawl roller pair40absorb the water droplets, preventing adhesion of the water droplets to the recording medium P and resultant water droplet marks on the recording medium P that may damage the toner image T during duplex printing.

Referring toFIG. 13, a detailed description is now given of a configuration of a comparative fixing device20C without the brushes51and53.

FIG. 13is a vertical sectional view of the comparative fixing device20C illustrating movement of the recording medium P. As shown inFIG. 13, the comparative fixing device20C does not include the brushes51and53depicted inFIG. 12and therefore the pawl roller pair40exposes its PFA tube. Since the outer circumferential surface of the pawl roller pair40is harder than the brushes51and53, the leading edge of the recording medium P strikes the outer circumferential surface of the pawl roller pair40with a greater impact. As the greater impact bends the leading edge of the recording medium P, the bent leading edge of the recording medium P may trigger jamming of the recording medium P. Further, as the toner image T on the recording medium P comes into contact with the outer circumferential surface of the pawl roller pair40, the pawl roller pair40may produce scratches and glossy streaks on the toner image T on the recording medium P. Moreover, as moisture evaporated from the recording medium P adheres to the outer circumferential surface of the pawl roller pair40as water droplets, the water droplets may be transferred to the toner image T on the recording medium P, producing water droplet marks on the toner image T on the recording medium P. Thus, the pawl roller pair40without the brushes51and53may raise failures that may degrade the quality of the toner image T formed on the recording medium P.

As shown inFIG. 7, after the leading edge of the recording medium P passes over the pawl roller pair40, it moves upward to the guide roller pair43located above the pawl roller pair40. While the recording medium P moves upward to the guide roller pair43, the recording medium P is cooled by the atmosphere and the toner image T on the recording medium P is solidified. The axis O43of the guide roller pair43is situated at the distance F from the exit of the fixing nip N, which is determined in view of cooling of the recording medium P while the recording medium P is conveyed from the exit of the fixing nip N to the guide roller pair43. As the leading edge of the recording medium P mounts the guide roller pair43, it comes into contact with the brushes52and54mounted on the outer circumferential surface of the guide roller pair43softly, thus rotating the guide roller pair43. Simultaneously, the recording medium P moves upward further in a state in which it is supported by the head of lots of fibers constituting the outer circumferential surface of the brushes52and54.

The angle at which the leading edge of the recording medium P contacts the outer circumferential surface of the guide roller pair43is changed according to the projection S of the guide roller pair43. However, since the projection S is set to about 2 mm according to this exemplary embodiment, the leading edge of the recording medium P contacts the guide roller pair43at a moderate angle. Thus, the protrusion S of the guide roller pair43, cooling of the recording medium P, the angle at which the recording medium P contacts the guide roller pair43, and the advantages of the brushes52and54mounted on the guide roller pair43prevent production of scratches and glossy streaks on the toner image T on the recording medium P and formation of water droplet marks on the toner image T on the recording medium P.

By the time the recording medium P reaches the guide roller pair43, the toner image T on the recording medium P may have already been cooled and solidified. Further, addition of the pawl roller pair40mounting the brushes51and53may attain moisture proof sufficiently. In this case, even if the recording medium P contacts the guide roller pair43, the guide roller pair43neither produces scratches and glossy streaks on the toner image T on the recording medium P nor creates water droplet marks on the recording medium P. Accordingly, the pawl roller pair40may mount the brushes51and53but the guide roller pair43may not mount the brushes52and54at all or at least one guide roller pair43may mount the brushes52and54.

As shown inFIG. 7, the recording medium P is guided by the pawl23aof the separator23, the junction23d, the pawl roller pair40, and the guide roller pair43in this order in the recording medium conveyance direction D1. When a trailing edge of the recording medium P is discharged from the fixing nip N and passes over the pawl23aof the separator23, the pawl23aseparates from the fixing roller21as shown inFIG. 2. For example, after the tailing edge of the recording medium P passes through the fixing nip N, the controller37interrupts electric current supply to the solenoid30and therefore the plunger32no longer retracts into the body31. Accordingly, the separator presser27presses down the base23bof the separator23by a resilient bias exerted by the separation direction biasing member29that generates the rotation moment M3. Consequently, the separator23rotates counterclockwise inFIG. 2by the rotation moment M2.

As described above, the rotation moment M2exerted by the separation direction biasing member29to the separator23to rotate the separator23counterclockwise inFIG. 2is greater than the rotation moment M1exerted by the contact direction biasing member26to the separator23to rotate the separator23clockwise inFIG. 2. Accordingly, the separator23rotates counterclockwise inFIG. 2and therefore the pawl23aseparates from the fixing roller21. Whenever the recording medium P is conveyed through the fixing nip N, the separator23is brought into contact with and separated from the fixing roller21.

Referring toFIG. 14, the following describes results of an experiment to verify advantages attained by the exemplary embodiments described above.

The experiment verifies three advantages, that is, elimination of scratches and glossy streaks; elimination of water droplet marks; and improvement in durability, for the brushes51to54shown inFIGS. 9 and 10, the fibers58shown inFIG. 11, and the pawl roller pair40without fibers shown inFIG. 13.FIG. 14is a lookup table showing the results of the experiment. In the table, “with fibers” denotes electric fibers such as the fibers58depicted inFIG. 11and a sheet brush such as the brushes51to54depicted inFIGS. 9 and 10; “without fibers” denotes the PFA roller pair constructed of the base coated with the fluoroplastic (e.g., PFA) tube, such as the pawl roller pair40of the comparative fixing device20C shown in FIG.13. The elimination of scratches and glossy streaks is ranked on a scale of 1 to 5 where rank5defines the best and rank1defines the worst.

Rank5defines a state in which no scratches and glossy streaks are identified on the toner image T on the recording medium P. Rank4defines a state in which no scratches and glossy streaks are identified on the toner image T on the recording medium P when viewed from the front but some scratches and glossy streaks are identified when viewed from an angle other than the front. Rank3defines a state in which slight scratches and glossy streaks are identified on the toner image T on the recording medium P when viewed from the front. Rank2defines a state in which scratches and glossy streaks are identified clearly on the toner image T on the recording medium P when viewed from the front. Rank1defines a state in which scratches and glossy streaks are identified clearly on the toner image T on the recording medium P when viewed from any angle, the toner image T is chipped in parts, and white streaks appear on the toner image T.

The elimination of water droplet marks is ranked on a scale of “good”, “fair”, and “poor”. “Good” defines a state in which no water droplet marks are identified on the toner image T on the recording medium P. “Fair” defines a state in which water droplet marks are identified slightly on the toner image T on the recording medium P under a limited condition, such as on initial several recording media P. “Poor” defines a state in which water droplet marks are identified frequently on the toner image Ton the recording medium P.

The improvement in durability is ranked on a scale of “good”, “fair”, and “poor”. “Good” defines superior durability. “Fair” defines inferior durability compared to “good” and decreased durability under some condition. “Poor” defines poor durability.

As shown inFIG. 14, the PFA roller pair without fibers is ranked as 3 for elimination of scratches and glossy streaks and “fair” for elimination of water droplet marks. By contrast, electrostatic fibers and sheet brush with fibers are ranked as 4 for elimination of scratches and glossy streaks and “good” for elimination of water droplet marks. Especially, electrostatic fibers also attain “good” for improvement in durability.

Referring toFIG. 7, the following describes advantages of the fixing device20according to the exemplary embodiments described above.

As shown inFIG. 7, the fixing device20includes the fixing roller21serving as a fixing rotary body rotatable in the rotation direction R1; the pressing roller22serving as a pressing rotary body rotatable in the rotation direction R2counter to the rotation direction R1of the fixing roller21and contacting the fixing roller21to form the fixing nip N therebetween through which a recording medium P bearing a toner image T is conveyed; the heater24to heat at least one of the fixing roller21and the pressing roller22; and the plurality of separators23aligned in the axial direction of the fixing roller21and separatably contacting the fixing roller21. Each separator23includes the pawl23ato separatably contact the fixing roller21to separate the recording medium P discharged from the fixing nip N from the fixing roller21. The fixing device20further includes the plurality of pawl roller pairs40serving as a first rotary body. Each pawl roller pair40is rotatably supported by each separator23and contacts and guides the recording medium P separated from the fixing roller21by the pawl23ain the recording medium conveyance direction D1. The fixing device20further includes the fixing exit guide41to guide the recording medium P passing over the pawl roller pair40in the recording medium conveyance direction D1; the guide roller pair43serving as a second rotary body rotatably supported by the fixing exit guide41and guiding the recording medium P in the recording medium conveyance direction D1; and at least one of the brushes51to54and the fibers58, depicted inFIG. 11, serving as a brush mounted on the outer circumferential surface of the pawl roller pair40and the guide roller pair43.

The recording medium P discharged from the fixing nip N moves over the pawl23aof the separators23, the pawl roller pair40, and the guide roller pair43in this order in the recording medium conveyance direction D1, distributing pressure exerted by the recording medium P among the pawl23a, the pawl roller pair40, and the guide roller pair43. Additionally, even if the recording medium P bears the soft toner image T softened by heat from the fixing roller21as it is conveyed through the fixing nip N, the brushes51and53mounted on the pawl roller pair40absorb impact given to the recording medium P as the recording medium P comes into contact with the pawl roller pair40, thus preventing scratches and glossy streaks produced on the toner image T on the recording medium P by the pawl roller pair40. When the recording medium P reaches the guide roller pair43, the toner image T has been cooled and solidified, preventing formation of a faulty toner image T such as scratches and glossy streaks produced on the toner image T on the recording medium P due to contact with the guide roller pair43. If the guide roller pair43also mounts the brushes52and54similarly to the pawl roller pair40mounting the brushes51and53, the brushes52and54prevent scratches and glossy streaks produced on the toner image T on the recording medium P more precisely.

The moisture-proof brushes51to54absorb moisture evaporated from the recording medium P by heat and pressure applied to the recording medium P at the fixing nip N. Accordingly, moisture evaporated from the recording medium P does not adhere to an interior wall of a conveyance path through which the recording medium P passes and does not accumulate on the interior wall of the conveyance path as water droplets that may be transferred onto the recording medium P and left on the recording medium P as water droplet marks.

The present invention is not limited to the details of the exemplary embodiments described above, and various modifications and improvements are possible. For example, according to the exemplary embodiments described above, the fixing roller21serves as a fixing rotary body. Alternatively, the fixing rotary body may be an endless belt, an endless film, or the like. Further, according to the exemplary embodiments described above, the pressing roller22serves as a pressing rotary body. Alternatively, the pressing rotary body may be an endless belt, a pad, a plate, or the like.

The present invention has been described above with reference to specific exemplary embodiments. Note that the present invention is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.