Patent Publication Number: US-9429869-B1

Title: Developing device, and image forming apparatus including developing device

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2015-032052 filed on Feb. 20, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a developing device which develops an electrostatic latent image by electrophotography, and an image forming apparatus including the developing device. 
     Generally, in a developing device provided in an image forming apparatus such as a multifunction peripheral, for example, the layer thickness of developer formed on the surface of a developer carrying body is limited by a layer thickness limiting member. At this time, scattered toner is accumulated on the surface of the layer thickness limiting member. When the accumulated toner increases, the accumulated toner transfers from the layer thickness limiting member to a photosensitive body, whereby the image quality might be adversely affected. 
     In an image forming apparatus using a two-component developer, carrier contained in the two-component developer forms a magnetic brush on the surface of the developer carrying body. In the image forming apparatus, it is known that the developer carrying body is rotated in a direction opposite to a rotation direction in a developing process, to scrape the accumulated toner by the magnetic brush. 
     SUMMARY 
     A developing device according to one aspect of the present disclosure includes a developer reservoir, a developer carrying body, a magnetic pole member, a layer thickness limiting member, and a sheet member. In the developer reservoir, a two-component developer is stored. The developer carrying body is supported so as to be rotatable in a first rotation direction and a second rotation direction, and is configured to rotate in the first rotation direction, thereby carrying the two-component developer stored in the developer reservoir on an outer circumferential surface of the developer carrying body and feeding, in a first region on the outer circumferential surface, toner contained in the two-component developer to a toner carrying body at a next stage. The magnetic pole member is configured to generate a magnetic force in a direction to separate the two-component developer from the developer carrying body, in a second region on a downstream side in the first rotation direction relative to the first region on the outer circumferential surface of the developer carrying body. The layer thickness limiting member is provided via a gap at a first specific position on an upstream side in the first rotation direction relative to the first region on the outer circumferential surface of the developer carrying body, and is configured to limit a layer thickness of the two-component developer carried on the developer carrying body rotating in the first rotation direction. The sheet member is provided via a gap at a second specific position in a specific region between a position on a downstream side in the first rotation direction relative to the first specific position, and a high-magnetic-force position at which an intensity of the magnetic force is the highest in the second region, on the outer circumferential surface of the developer carrying body. The sheet member is configured to limit a layer thickness of the two-component developer carried on the developer carrying body rotating in the second rotation direction. The sheet member is able to elastically deform in accordance with application of a pressing force equal to or greater than a tolerable value in the first rotation direction or the second rotation direction. 
     An image forming apparatus according to another aspect of the present disclosure includes a photosensitive body and a developing device. The photosensitive body allows an electrostatic latent image to be formed on a surface thereof. The developing device is configured to feed the toner to the photosensitive body, thereby developing the electrostatic latent image into a toner image. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing the configuration of an image forming apparatus according to a first embodiment of the present disclosure. 
         FIG. 2  is a schematic diagram showing the configuration of a developing device. 
         FIG. 3  is a schematic diagram showing the magnetic flux distribution of a plurality of magnets enclosed in a sleeve portion. 
         FIG. 4A  and  FIG. 4B  are diagrams for explaining magnetization of carrier. 
         FIG. 5  is a flowchart showing a rotation process for the sleeve portion by a control portion. 
         FIG. 6A  and  FIG. 6B  are diagrams for explaining a layer thickness limiting function of a blade when the sleeve portion rotates forward. 
         FIG. 7  is a diagram showing the states of a two-component developer and a sheet member when the sleeve portion rotates forward. 
         FIG. 8A  and  FIG. 8B  are diagrams showing the states of the two-component developer and the sheet member when the sleeve portion rotates reversely. 
         FIG. 9A  and  FIG. 9B  are diagrams showing the states of the two-component developer and the sheet member when the sleeve portion rotates reversely. 
         FIG. 10A ,  FIG. 10B , and  FIG. 10C  are diagrams for explaining a removal process for accumulated toner on the blade when the sleeve portion rotates reversely. 
         FIG. 11  is a diagram showing the configuration of a sheet member in a second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. The following embodiments are examples in which the present disclosure is embodied, and are not intended to limit the technical scope of the present disclosure. 
     First, with reference to  FIGS. 1 to 3 , the configuration of an image forming apparatus  10  according to a first embodiment of the present disclosure will be described. In the following description, an up-down direction  501 , a right-left direction  502 , and a front-rear direction  503  defined in  FIG. 1  may be used. 
     The image forming apparatus  10  is an image forming apparatus of an electrophotography type. As shown in  FIG. 1 , the image forming apparatus  10  includes, in a housing  100 , a sheet feed portion  2 , a sheet conveyance portion  3 , an image forming portion  4 , a laser scanning portion  5 , a fixing portion  6 , a container installation portion  900 , and the like. 
     The image forming apparatus  10  shown in  FIG. 1  is a color printer, and is communicably connected to another communication device. The other communication device is, for example, a personal computer, and transmits job data indicating an image formation job to the image forming apparatus  10 , thereby requesting the image forming apparatus  10  to execute the image formation job. Based on the job data received from the other communication device, the image forming apparatus  10  executes the image formation job requested by the other communication device. The job data includes image data of an image to be formed on a recording sheet  9 , size information about the recording sheet  9 , or the like. 
     The image forming apparatus  10  is a tandem-type image forming apparatus. The image forming portion  4  includes an intermediate transfer belt  48 , a cleaning device  480 , and a secondary transfer device  49 . The image forming portion  4  includes a plurality of single-color image forming portions  411  respectively corresponding to cyan, magenta, yellow, and black. 
     Each single-color image forming portion  411  includes a photosensitive drum  41  which carries a toner image, a charging device  42 , a developing device  43 , a primary transfer device  45 , a cleaning device  47 , and the like. In each single-color image forming portion  411 , the photosensitive drum  41  rotates at a peripheral velocity according to the peripheral velocity (movement velocity) of the intermediate transfer belt  48 , and the charging device  42  charges the surface of the photosensitive drum  41  uniformly. Further, the laser scanning portion  5  scans a laser beam to draw an electrostatic latent image on the charged surface of the photosensitive drum  41 . The photosensitive drum  41  is an example of a photosensitive body of the present disclosure. 
     The developing device  43  feeds toner to the photosensitive drum  41 , thereby developing the electrostatic latent image. The developing device  43  in the present embodiment agitates a two-component developer  90  containing toner made from resin and carrier made from a magnetic material, thereby charging the toner, and feeds the charged toner to the photosensitive drum  41 . The charging device  42  charges a part, of the photosensitive drum  41 , on which the electrostatic latent image has not been drawn yet. The developing device  43  is detachable from the housing  100 . 
     The intermediate transfer belt  48  is an endless belt-like member formed in a loop shape, and circulates being stretched between two rollers. In the image forming portion  4 , each single-color image forming portion  411  forms an image for the corresponding color on the surface of the circulating intermediate transfer belt  48 . Thus, a color image composed of overlaid images for the respective colors is formed on the intermediate transfer belt  48 . 
     The secondary transfer device  49  transfers a toner image formed on the intermediate transfer belt  48  to the recording sheet  9 . The cleaning device  480  removes the remaining toner on a part, of the intermediate transfer belt  48 , that has passed through the secondary transfer device  49 . 
     The sheet feed portion  2  includes a sheet reception portion  21  and a sheet sending portion  22 . The sheet sending portion  22  sends the recording sheet  9  from the sheet reception portion  21  to a conveyance path  30 . The sheet conveyance portion  3  includes a registration roller  31 , a conveyance roller  32 , a discharge roller  33 , and the like. The registration roller  31  and the conveyance roller  32  convey the recording sheet  9  fed from the sheet feed portion  2 , to the secondary transfer device  49  of the image forming portion  4 . Further, the discharge roller  33  discharges the recording sheet  9  on which an image has been formed, onto a discharge tray  101  through a discharge port of the conveyance path  30 . 
     The image forming apparatus  10  includes a manual feed tray  60 . A sheet placed on the manual feed tray  60  is taken into the image forming apparatus  10  by a take-in roller  77 , and conveyed through the conveyance path  30  to a transfer position of the secondary transfer device  49 . 
     At an upper part of the housing  100 , a top cover (not shown) which is openable and closable is provided. When the top cover is turned upward (open direction), the container installation portion  900  is exposed. The container installation portion  900  is provided above the image forming portion  4 , and accommodates the toner containers  40 . The toner containers  40  are provided at respective locations in the image forming portion  4 . The toner containers  40  contain toners having colors corresponding to the respective colors of the image forming portion  4 . 
     As shown in  FIG. 2 , the developing device  43  has a device body  431 . In the device body  431 , a magnetic roller  430 , a developing roller  432 , an agitation mechanism  437 , and a blade  438  are provided. The magnetic roller  430 , the developing roller  432 , and the agitation mechanism  437  are rotatably supported around respective rotation shafts that are parallel with each other. 
     A lower part of the device body  431  is a developer reservoir  450  which stores the two-component developer  90 . The toner fed from the toner container  40  is stored and accumulated in the developer reservoir  450 . 
     The agitation mechanism  437  includes a screw member  451 . The screw member  451  is a long member elongated along a direction perpendicular to the drawing plane of  FIG. 2 . The screw member  451  is made from resin. The screw member  451  is rotatably supported by side walls (not shown), of the developer reservoir  450 , that are present at both ends in a direction perpendicular to the drawing plane of  FIG. 2 . By the screw member  451  rotating, the two-component developer  90  in the developer reservoir  450  is moved and agitated. By the agitation, the toner and the carrier are rubbed with each other. Static electricity caused by their friction charges the toner at a predetermined polarity. The carrier is charged at a polarity opposite to the charge polarity of the toner. By the electrostatic force, the toner is adhered to the carrier. 
     The magnetic roller  430  is provided inside the device body  431 . The magnetic roller  430  attracts, by a magnetic force, the two-component developer  90  agitated by the agitation mechanism  437 , from the developer reservoir  450 , and carries the two-component developer  90 . The magnetic roller  430  has a sleeve portion  460  and a magnetic pole portion  440 . 
     The sleeve portion  460  has a cylindrical shape, and encloses the magnetic pole portion  440 . The sleeve portion  460  is formed by a nonmagnetic member. The sleeve portion  460  is supported by the device body  431  so as to be rotatable in a forward direction and a reverse direction. In a development process, the sleeve portion  460  rotates in one direction. In the following description, the rotation direction of the sleeve portion  460  in the development process is referred to as a forward rotation direction X 1 . The forward rotation direction X 1  corresponds to a first rotation direction. In the present embodiment, the forward rotation direction X 1  is the counterclockwise direction in  FIG. 2 . 
     The magnetic pole portion  440  is provided inside the sleeve portion  460 . In the magnetic pole portion  440 , a plurality of magnets  441  to  445  are provided. The positions of the magnets  441  to  445  are fixed inside the sleeve portion  460 . The plurality of magnets  441  to  445  are arranged via predetermined intervals therebetween along the circumferential direction of the sleeve portion  460 . 
     The positional relationship among the magnets  441  to  445  in the present embodiment is set as follows: using the magnet  444  as a reference, the angle interval to the magnet  441  is 229.5 degrees, the angle interval to the magnet  442  is 273 degrees, the angle interval to the magnet  443  is 314.5 degrees, and the angle interval to the magnet  445  is 95.5 degrees. 
     The magnetic poles on the sleeve portion  460  side of the magnets  441  to  445  are alternately arranged along the circumferential direction of the sleeve portion  460 . In the present embodiment, the magnetic poles on the sleeve portion  460  side of the magnets  441  to  445  are arranged such that the magnets  441 ,  443 ,  445  are S poles, and the magnets  442 ,  444  are N poles (see  FIG. 3 ).  FIG. 3  shows the distribution of the magnetic flux density in the normal direction of the sleeve portion  460 , formed by the magnets  441  to  445 . 
     A curve W 1  depicted by a two-dot dashed line in  FIG. 3  indicates the magnitude of the magnetic flux density in the normal direction on the surface of the sleeve portion  460 . That is, in  FIG. 3 , the magnitude of the magnetic flux density in the normal direction on the surface of the sleeve portion  460  is represented by undulation of the curve W 1 . As shown in  FIG. 3 , the magnetic flux density is maximized at a position, on the surface of the sleeve portion  460 , that is close to the magnetic pole on the sleeve portion  460  side of each magnet  441  to  445 , and decreases with increase in the distance from the position in the circumferential direction of the sleeve portion  460 . 
     The magnet  441  is provided at a position that faces the two-component developer  90  in the developer reservoir  450 , and attracts the two-component developer  90  stored in the developer reservoir  450 . Thus, in a transfer region E 1  of the surface of the sleeve portion  460 , which faces the magnet  441 , the two-component developer  90  is transferred and adhered to the sleeve portion  460 . 
     The magnet  442  is provided at a position that faces the blade  438  described later, and magnetizes the blade  438 . Thus, a magnetic field is formed in a gap G 1  between the end of the blade  438  and the magnet  442 . When the two-component developer  90  adhered to the surface of the sleeve portion  460  by the magnet  441  passes through the gap G 1 , the two-component developer  90  is magnetically held in the gap G 1  by the magnetic field and the layer thickness of the two-component developer  90  is regulated by the blade  438 . Thus, a developer layer having a uniform layer thickness is formed on the surface of the sleeve portion  460 . 
     The magnet  443  is provided at a position that is on the downstream side relative to the magnet  442  in the forward rotation direction X 1  and is adjacent to the magnet  442 . The magnet  443  causes the sleeve portion  460  to carry the two-component developer  90  thereon. On the developer layer formed on the sleeve portion  460 , a magnetic brush is formed. The magnetic brush is a plurality of chain bodies formed by the carriers which are contained in the two-component developer  90  and which are linked in a chain form from the surface of the sleeve portion  460  by magnetic forces of the magnets  441  to  445 . 
     More specifically, the carriers are made from a magnetic material. Therefore, the carriers are magnetized by the magnetic fields of the magnets  441  to  445 , and S poles and N poles are generated in the magnets  441  to  445  themselves. The magnetic poles generated in the carriers are arranged along the magnetic field lines extending from or entering the magnetic poles of the magnets  441  to  445 . For example, in the case of approaching the S pole of the magnet, as shown in  FIG. 4A , in each carrier, an N pole is generated at a part close to the S pole of the magnet and an S pole is generated at a part far from the S pole of the magnet, so as to be arranged along the magnetic field lines extending from the S pole of the magnet. On the other hand, in the case of approaching the N pole of the magnet, as shown in  FIG. 4B , in each carrier, an S pole is generated at a part close to the N pole of the magnet and an N pole is generated at a part far from the N pole of the magnet, so as to be arranged along the magnetic field lines entering the N pole of the magnet. 
     Such magnetization occurs in each particle of the carriers, and the particles of the carriers are attracted to each other by attractive forces due to magnetic forces generated between the different poles. Thus, the carriers are linked to form the chain body. Then, a plurality of such chain bodies are formed, whereby the magnetic brush is formed. 
     The magnetic force acting on the outer circumferential surface of the sleeve portion  460  due to each magnetic pole of the magnets  441  to  445  is maximized at a position that faces the center position of the magnetic pole surface on the sleeve portion  460  side of each magnet  441  to  445 . Therefore, at the position that faces the center position of the magnetic pole surface, more carriers congest to be linked in a chain form. That is, at the position that faces the center position of the magnetic pole surface, the length of the chain body is maximized. Therefore, the layer thickness of the magnetic brush is maximized at the position that faces the center position of the magnetic pole surface of each magnet  441  to  445 . 
     The magnets  441  to  445  are arranged such that the magnetic field line extending from or entering the center position of each magnetic pole surface is along the normal direction of the sleeve portion  460 . Therefore, at the position that faces the magnetic pole surface of each magnet  441  to  445 , the magnetic brush erects along the normal direction. In a region other than the center position, the magnetic field line forms an arc that curves outward. Therefore, at the periphery of the center position, the magnetic brush is tilted. 
     Thus, the magnetic brush gradually becomes close to the erecting state as approaching the center position of the magnetic pole surface on the sleeve portion  460  side of each magnet  441  to  445 , and becomes the erecting state at the position that faces the center position. The magnetic brush is gradually tilted as departing from the position that faces the center position. 
     Different voltages are respectively applied to the sleeve portion  460  and the developing roller  432 , and a predetermined potential difference is generated between the sleeve portion  460  and the developing roller  432 . Owing to the potential difference, the toner contained in the two-component developer  90  carried on the sleeve portion  460  is transferred and adhered to the developing roller  432 . The transfer of the toner mainly occurs in a certain region including the position closest to the developing roller  432 , of the sleeve portion  460 . Hereinafter, this region E 2  is referred to as a transfer region E 2 . The transfer region E 2  is an example of a first region of the present disclosure. 
     The magnet  444  is provided at a position that faces the developing roller  432 , and attracts, onto the surface of the sleeve portion  460 , the carrier left on the sleeve portion  460  after the toner has been transferred to the developing roller  432  in the transfer region E 2 . The carrier attracted onto the surface of the sleeve portion  460  by the magnet  444  keeps formation of the magnetic brush. 
     The magnet  445  generates a magnetic force in a direction to separate the two-component developer  90  from the sleeve portion  460 , in a separation region E 3  which is on the downstream side in the forward rotation direction X 1  relative to the transfer region E 2  on the outer circumferential surface of the sleeve portion  460 . Thus, after the toner has been transferred to the developing roller  432  in the transfer region E 2 , the magnet  445  separates the carrier left on the surface of the sleeve portion  460  from the surface by the magnetic force in the separation region E 3 , thereby dropping the carrier to the developer reservoir  450  below. The separation region E 3  is an example of a second region of the present disclosure. The magnet  445  is an example of a magnetic pole member of the present disclosure. 
     In the development process, the sleeve portion  460  receives and carries the two-component developer  90  from the developer reservoir  450  in the transfer region E 1  by the magnetic force of the magnet  441 , and conveys the two-component developer  90  by rotating in the forward rotation direction X 1 . When the two-component developer  90  is conveyed to the transfer region E 2  by the sleeve portion  460 , the toner contained in the two-component developer  90  is transferred to the developing roller  432  at the next stage by the potential difference between the sleeve portion  460  and the developing roller  432 . At this time, the two-component developer  90  in which the proportion of the carrier  555  is high is left on the surface of the sleeve portion  460 . The sleeve portion  460  is an example of a developer carrying body of the present disclosure. 
     The sleeve portion  460  further rotates in the forward rotation direction X 1 , to convey the two-component developer  90  in the forward rotation direction X 1 . Then, when the two-component developer  90  reaches the separation region E 3 , the two-component developer  90  is separated from the sleeve portion  460 . This is because the range of the magnetic field lines extending from the magnet  441  to the developer reservoir  450  side is very small due to the influence of the magnet  445  which has the same magnetic pole on the sleeve portion  460  side as that of the magnet  441 , and therefore the magnetic attractive force applied to the two-component developer  90  from the magnet  441  disappears. The two-component developer  90  separated from the sleeve portion  460  drops to the developer reservoir  450  below. 
     The blade  438  is provided via the gap G 1  from the surface of the sleeve portion  460 , at a position L 1  on the upstream side in the forward rotation direction X 1  relative to the transfer region E 2  on the outer circumference of the sleeve portion  460 . The blade  438  limits the layer thickness of the two-component developer  90  carried on the sleeve portion  460  rotating in the forward rotation direction X 1 . Hereinafter, the position L 1  is referred to as a first layer thickness limiting position L 1 . The first layer thickness limiting position L 1  is an example of a first specific position of the present disclosure. The blade  438  is an example of a layer thickness limiting member of the present disclosure. 
     The developing roller  432  is provided being opposed to the sleeve portion  460 . The developing roller  432  receives the toner from the sleeve portion  460  carrying the two-component developer  90 , and carries the received toner. 
     The developing roller  432  is opposed to the photosensitive drum  41  in a contactless manner. As described above, voltage is applied to the developing roller  432 . Thus, a predetermined potential difference is generated between the developing roller  432  and the electrostatic latent image formed on the photosensitive drum  41 . Owing to the potential difference, in a transfer region E 4 , the toner carried on the developing roller  432  is transferred to a part corresponding to the electrostatic latent image formed on the outer circumferential surface of the photosensitive drum  41 . Thus, the electrostatic latent image is developed. The developing roller  432  conveys the toner to the transfer region E 4  for passing the toner to the photosensitive drum  41  having the electrostatic latent image formed on the surface thereof. The developing roller  432  is an example of a toner carrying body. 
     In the development process, the developing roller  432  rotates in the same direction as the sleeve portion  460 . Thus, the mutually opposed portions of the outer circumferential surfaces of the sleeve portion  460  and the developing roller  432  respectively move in the opposite directions. 
     In the development process, the developing roller  432  and the photosensitive drum  41  respectively rotate in the opposite directions. Thus, the mutually opposed portions of the outer circumferential surfaces of the developing roller  432  and the photosensitive drum  41  move in the same direction. 
     Thus, the toner  777  contained in the two-component developer  90  is consumed in the development process. Therefore, the toner  777  is supplied from the toner container  40  to the developer reservoir  450 , to compensate the consumption. Meanwhile, the carrier  555  contained in the two-component developer  90  is hardly consumed but is left in the developer reservoir  450 , and gives fluidity and the like to the toner  777  supplied to the developer reservoir  450 . 
     The developing device  43  has a drive motor  203 . The drive motor  203  rotationally drives the sleeve portion  460 . The drive motor  203  may be a DC brushless motor, a stepping motor, or the like. 
     The developing device  43  has a control portion  200 . The control portion  200  includes a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). 
     The CPU is a processor that executes various calculation processes. The ROM is a nonvolatile storage portion in which information such as a control program for causing the CPU to execute the various processes is stored in advance. The RAM is a volatile storage portion used as a temporary storage memory (working area) for the various processes executed by the CPU. The control portion  200  causes the CPU to execute the control program stored in the ROM, thereby controlling operation of the image forming apparatus  10 . 
     In the ROM of the control portion  200 , a processing program for causing the CPU of the control portion  200  to execute a rotation process described later (see a flowchart in  FIG. 5 ) is stored. The processing program may be stored in the ROM at a stage of shipment of the image forming apparatus  10 . Alternatively, the processing program may be stored in a computer-readable non-transitory information storage medium such as a CD, a DVD, or a flash memory, and then after the shipment, the processing program may be stored into the ROM of the control portion  200  from the information storage medium. In another embodiment, a part or a plurality of the functions of the control portion  200  may be provided as an electronic circuit. 
     The control portion  200  functions as a rotation control portion  201  by the CPU executing the processing program stored in the ROM. 
     During execution of the development process, the rotation control portion  201  performs forward rotation control to rotate the sleeve portion  460  in the forward rotation direction X 1  at a predetermined first rotation velocity V 1 . In addition, at a predetermined timing excluding a period during which the development process is executed, the rotation control portion  201  performs reverse rotation control to rotate the sleeve portion  460  in a rotation direction (hereinafter, referred to as a reverse rotation direction) X 2  opposite to the forward rotation direction X 1  at a rotation velocity V 2  which is the same as or different from the first rotation velocity V 1 . The reverse rotation direction X 2  corresponds to a second rotation direction of the present disclosure. 
     In the case where control for rotating the sleeve portion  460  in the reverse rotation direction X 2  is performed in the image forming apparatus  10 , technology for further enhancing the performance for removing the accumulated toner from the surface of the blade  438  is required. Considering this, in the developing device  43  and the image forming apparatus  10  according to the present embodiment, the performance for removing the accumulated toner from the blade  438  which limits the layer thickness of the two-component developer on the surface of the sleeve portion  460  is enhanced as described below. 
     As shown in  FIG. 2 , the developing device  43  includes a sheet member  666 . The sheet member  666  is attached at a predetermined position on the inner wall of the device body  431 . The sheet member  666  extends along the rotation axis of the sleeve portion  460 , and has a rectangular shape. The sheet member  666  is mainly composed of, for example, urethane, and is elastic or flexible. 
     The sheet member  666  in a natural state extends toward a part at a position L 2  between the transfer region E 2  and the separation region E 3  on the outer circumference of the sleeve portion  460 . That is, the position L 2  is on the upstream side in the forward rotation direction X 1  relative to the separation region E 3 , and on the downstream side in the forward rotation direction X 1  relative to the transfer region E 2 . The position L 2  is an example of a second specific position of the present disclosure. Hereinafter, the position L 2  is referred to as a second layer thickness regulating position L 2 . 
     When the sheet member  666  is in a natural state, the end of the sheet member  666  reaches a position separated from the surface of the sleeve portion  460  by a predetermined distance. In other words, a gap G 2  having a length corresponding to the predetermined distance is provided between the end of the sheet member  666  and the second layer thickness regulating position L 2  on the surface of the sleeve portion  460 . The gap G 2  will be described later. 
     The sheet member  666  enables further enhancement of the performance for removing the accumulated toner from the surface of the blade  438 . 
     With reference to  FIGS. 5 to 10 , the rotation process for the sleeve portion  460  by the control portion  200  will be described. In the flowchart in  FIG. 5 , steps S 501 , S 502 , . . . indicate the numbers of steps in the processing procedure. The process by the control portion  200  shown in  FIG. 5  is started when an image formation job accompanied by the development process is executed. 
     &lt;Step S 501 &gt; 
     In step S 501 , the control portion  200  determines whether or not the image formation job has been finished. If it is determined that the image formation job has not been finished (NO in step S 501 ), the control portion  200  executes the processing in step S 501  again. 
     During execution of the image formation job, the rotation control portion  201  performs the forward rotation control to rotate the sleeve portion  460  in the forward rotation direction X 1  (see  FIG. 6A ). In the forward rotation control, when the two-component developer  90  carried on the surface of the sleeve portion  460  enters the gap G 1  between the end of the blade  438  and the magnet  442 , the layer thickness is limited by the blade  438  (see  FIG. 6B ). 
     On the other hand, in the forward rotation control, the sheet member  666  is in a natural state so that the end of the sheet member  666  reaches the position separated from the surface of the sleeve portion  460  by a predetermined distance. 
     Here, as described above, the second layer thickness regulating position L 2  is a position between the transfer region E 2  and the separation region E 3 . In more detail, between the transfer region E 2  and the separation region E 3 , the second layer thickness regulating position L 2  is within a range from the position L 3  at which the magnetic flux density is minimized to a position L 4  at which the magnetic flux density is maximized by the magnet  445  (see arrow Q 1  in  FIG. 3 ). The position L 4  is a position within the separation region E 3 . The region between the position L 3  and the position L 4  is an example of a specific region of the present disclosure. 
     In the region from the position L 3  to the position L 4 , the magnetic flux density gradually increases in the forward rotation direction X 1 . Therefore, when the sleeve portion  460  rotates in the forward rotation direction X 1 , the second layer thickness regulating position L 2  is a position through which the height of the magnetic brush is increasing from a low state. On the other hand, when the sleeve portion  460  rotates in the reverse rotation direction X 2 , the second layer thickness regulating position L 2  is a position through which the height of the magnetic brush is decreasing from a high state. In other words, when the sleeve portion  460  rotates in the forward rotation direction X 1 , the second layer thickness regulating position L 2  is a position through which the layer thickness of the two-component developer  90  is increasing from a thin state, and when the sleeve portion  460  rotates in the reverse rotation direction X 2 , the second layer thickness regulating position L 2  is a position through which the layer thickness of the two-component developer  90  is decreasing from a thick state. 
     Therefore, the second layer thickness regulating position L 2  is a position at which, due to the magnetic force of the magnet  445 , the layer thickness of the two-component developer  90  when entering the gap G 2  in the case of rotation in the reverse rotation direction X 2  is greater than in the case of rotation in the forward rotation direction X 1 . 
     When the sleeve portion  460  rotates in the forward rotation direction X 1 , the distance from the end of the sheet member  666  to the surface of the sleeve portion  460  is longer than the layer thickness of the two-component developer that is approaching the second layer thickness regulating position L 2  from the transfer region E 2 . Therefore, in the forward rotation control for the sleeve portion  460 , the two-component developer  90  approaching the second layer thickness regulating position L 2  from the transfer region E 2  passes under the sheet member  666  without colliding therewith (see  FIG. 7 ). 
     In step S 501 , if it is determined that the image formation job has been finished (YES in step S 501 ), the control portion  200  advances the process to step S 502 . 
     &lt;Step S 502 &gt; 
     After determining that the image formation job has been finished, the control portion  200  determines whether or not a start condition for starting the reverse rotation control of the sleeve portion  460  has been satisfied. The start condition may be that, for example, the count value of a counter (not shown) described later exceeds a numerical value indicating a predetermined number of sheets. The numerical value is, for example, 10000. 
     If it is determined that the start condition has not been satisfied (NO in step S 502 ), the control portion  200  ends the process. On the other hand, if it is determined that the start condition has been satisfied (YES in step S 502 ), the control portion  200  advances the process to step S 503 . 
     &lt;Step S 503 &gt; 
     In step S 503 , the control portion  200  resets the count value of the counter. The counter counts the number of the recording sheets  9  on which images have been formed. For example, the counter may be realized by the CPU executing a program for counting up the number of times the image formation process has been executed, in the control portion  200 . After the processing in step S 503 , the control portion  200  advances the process to step S 504 . 
     &lt;Step S 504 &gt; 
     In step S 504 , the control portion  200  starts the reverse rotation control for the sleeve portion  460 . Under the reverse rotation control, the control portion  200  rotates the sleeve portion  460  in the reverse rotation direction X 2 . 
     Here, as described above, in the rotation in the reverse rotation direction X 2 , the second layer thickness regulating position L 2  is a position through which the layer thickness of the two-component developer  90  is decreasing from a thick state. In addition, in the case where the sleeve portion  460  rotates in the reverse rotation direction X 2 , the layer thickness of the two-component developer  90  when entering the gap G 2  between the end of the sheet member  666  and the surface of the sleeve portion  460  is greater than in the case where the sleeve portion  460  rotates in the forward rotation direction X 1 . 
     In addition, the distance from the end of the sheet member  666  to the surface of the sleeve portion  460  is shorter than the layer thickness of the two-component developer  90  that is approaching the second layer thickness regulating position L 2  from the separation region E 3  in the reverse rotation control for the sleeve portion  460 . Therefore, in the reverse rotation control for the sleeve portion  460 , a part of the two-component developer  90  approaching the second layer thickness regulating position L 2  from the separation region E 3  collides with the sheet member  666  (see  FIG. 8A ). Therefore, a part of the two-component developer  90  is dammed by the sheet member  666 . That is, the layer thickness of the two-component developer  90  carried on the sleeve portion  460  rotating in the reverse rotation direction X 2  is limited. 
     The two-component developer  90  dammed by the sheet member  666  receives a conveyance force in the reverse rotation direction X 2  from the sleeve portion  460  rotating in the reverse rotation direction X 2 . Thus, the sheet member  666  is pressed with a pressing force F 1  in the reverse rotation direction X 2  from the two-component developer  90  dammed by the sheet member  666  (see  FIG. 8A ). 
     As the sleeve portion  460  rotates in the reverse rotation direction X 2 , the two-component developer  90  dammed by the sheet member  666  increases (see  FIG. 8B ). Thus, a mass of the two-component developer  90  is generated and the mass gradually enlarges. Then, as the mass of the two-component developer  90  gradually enlarges, the pressing force F 1  in the reverse rotation direction X 2  applied to the sheet member  666  from the mass of the two-component developer  90  increases. 
     When the sheet member  666  becomes unable to keep the natural state due to the pressing force F 1  equal to or greater than a tolerable value being applied in the reverse rotation direction X 2 , the sheet member  666  starts to elastically deform (see  FIG. 9A ), and then is bent toward the downstream side in the reverse rotation direction X 2  (see  FIG. 9B ). Thus, the mass of the two-component developer  90  that has been dammed by the sheet member  666  passes under the bent sheet member  666  and passes through the gap G 2 . 
     After passing under the sheet member  666 , the mass of the two-component developer  90  approaches the surface of the blade  438  (see  FIG. 10A ). A part of the mass of the two-component developer  90  is caught on the upper surface of the blade  438 . The two-component developer  90  caught on the upper surface of the blade  438  runs onto the upper surface of the blade  438  while dispersing, to come into contact with the toner accumulated on the upper surface (see  FIG. 10B ). 
     The carrier  555  contained in the two-component developer  90  that has run onto the upper surface of the blade  438  has a polarity electrically opposite to that of the toner  777  accumulated on the upper surface of the blade  438 . Therefore, the toner  777  accumulated on the upper surface of the blade  438  is adhered to the carrier  555  on the upper surface of the blade  438  by an electrostatic force. 
     The carrier  555  to which the toner  777  has been adhered, i.e., the two-component developer  90  is attracted to the sleeve portion  460  by an electrostatic force, and then drops through the gap G 1  toward the developer reservoir  450  (see  FIG. 10C ). Thus, the toner  777  accumulated on the upper surface of the blade  438  is scraped by the magnetic brush K 1 . 
     After the processing in step S 504 , the control portion  200  advances the process to step S 505 . 
     &lt;Step S 505 &gt; 
     The control portion  200  determines whether or not a rotation time under the reverse rotation control for the sleeve portion  460  has reached a predetermined rotation time Tth 1 . The rotation time Tth 1  is at least a time needed for the mass of the two-component developer  90  to be formed by the sheet member  666  damming the two-component developer  90  and then to reach the position of the blade  438  by pushing away the sheet member  666 . 
     If it is determined that the rotation time has not reached the rotation time Tth 1  (NO in step S 505 ), the control portion  200  executes the processing in step S 505  again. On the other hand, if it is determined that the rotation time has reached the rotation time Tth 1  (YES in step S 505 ), the control portion  200  ends the process. 
     Here, the reverse rotation control is finished when the rotation time reaches the rotation time Tth 1 . However, a sensor for detecting that the reverse rotation angle reaches a desired angle may be provided, and the reverse rotation control may be finished as a result of the detection by the sensor. 
     Thus, the image forming apparatus  10  has the sheet member  666  for damming a part of the two-component developer  90  carried on the surface of the sleeve portion  460  and generating a mass of the two-component developer  90  when the sleeve portion  460  rotates in the reverse rotation direction X 2 . When the sheet member  666  receives a pressing force equal to or greater than a tolerable value in the reverse rotation direction X 2  from the mass of the two-component developer  90 , the sheet member  666  elastically deforms to release the damming of the two-component developer  90 . Then, by the release of the damming, the image forming apparatus  10  causes the mass of the two-component developer  90  conveyed to the blade  438  to come into contact with the toner  777  accumulated on the upper surface of the blade  438 , thereby removing the toner  777  from the upper surface of the blade  438 . 
     Thus, as compared to the conventional configuration having no sheet member  666 , the amount of the carrier  555  to which the toner  777  accumulated on the upper surface of the blade  438  is adhered increases, whereby the performance for removing toner can be improved as compared to the conventional configuration. 
     While preferred embodiments of the present disclosure have been described above, the present disclosure is not limited to the above content, but various modifications may be applied. 
     (1) In the above embodiment, the sheet member  666  has a rectangular shape. However, in this case, in the reverse rotation of the sleeve portion  460 , when the sheet member  666  contacts with the two-component developer  90  on the surface of the sleeve portion  460 , the toner is scattered by, mainly, corner portions at both ends of the sheet member  666 . 
     Considering this, in the present embodiment, as shown in  FIG. 11 , the sheet member  666  having a long shape along the rotational axis of the sleeve portion  460  has cutouts  667  and  668  at both ends in the rotation axis direction. That is, the sheet member  666  is configured such that a length R 1  of the gap G 2  between the sheet member  666  and the sleeve portion  460  in end regions  662  and  663  in predetermined ranges from both ends in the rotation axis direction is greater than a length R 2  of the gap G 2  between the sheet member  666  and the sleeve portion  460  in a central region  661  which is a region other than the end regions  662  and  663 . The cutouts  667  and  668  are an example of a limitation moderating portion of the present disclosure. 
     The sleeve portion  460  has a carrying region  731  which allows the two-component developer  90  to be carried thereon, and non-carrying regions  732  and  733  which are provided on both sides in the axial direction of the carrying region  731  and on which the two-component developer  90  is not carried. In the present embodiment, the length of the sheet member  666  is the same as the axial length of the carrying region  731  of the sleeve portion  460 . 
     Thus, it becomes possible to prevent the toner from being scattered by corner portions at both ends of the sheet member  666  when the sheet member  666  dams a part of the magnetic brush on the surface of the sleeve portion  460  during the reverse rotation of the sleeve portion  460 . 
     (2) In the above embodiment, the second layer thickness regulating position L 2  is a position between the transfer region E 2  and the separation region E 3  on the outer circumference of the sleeve portion  460 , or a position between the position L 3  at which the magnetic flux density is minimized and the position L 4  at which the magnetic flux density is maximized due to the magnet  445 . However, the second layer thickness regulating position L 2  is not limited to a position between the position L 3  and the position L 4 , as long as the second layer thickness regulating position L 2  is a position at which, due to the magnetic force of the magnet  445 , the layer thickness of the two-component developer  90  when entering the gap G 2  in the case of rotation in the reverse rotation direction X 2  is greater than in the case of rotation in the forward rotation direction X 1 . 
     For example, the second layer thickness regulating position L 2  may be a position between a position L 5  at which the magnetic flux density is minimized between the magnet  442  and the magnet  443 , and a position L 6  at which the magnetic flux density is maximized due to the magnet  443 . Alternatively, the second layer thickness regulating position L 2  may be a position between a position L 7  at which the magnetic flux density is minimized between the magnet  443  and the magnet  444 , and a position L 8  at which the magnetic flux density is maximized due to the magnet  444 . The region between the position L 5  and the position L 6 , and the region between the position L 7  and the position L 8  are examples of a specific region of the present disclosure. 
     However, when the sheet member  666  elastically returns to the natural state after the mass of the two-component developer has passed, the two-component developer adhered to the surface of the sheet member  666  might be scattered. If the scattered two-component developer is adhered to the developing roller  432 , the image quality is adversely affected. Therefore, it is preferable that the second layer thickness regulating position L 2  is a position between the position L 3  and the second layer thickness regulating position L 2 . 
     In the above embodiment, the mass of the two-component developer  90  is generated on the surface on the downstream side in the forward rotation direction X 1 , of the sheet member  666 . However, without limitation thereto, in some embodiments, the mass of the two-component developer  90  may be generated on the surface on the upstream side in the forward rotation direction X 1 , of the sheet member  666 . 
     In this case, the second layer thickness regulating position L 2  may be a position between the first layer thickness limiting position L 1  and the position L 5 , a position between the position L 6  and the position L 7 , or a position between the position L 8  and the position L 3 . The region between the position L 1  and the position L 5 , the region between the position L 6  and the position L 7 , and the region between the position L 8  and the position L 3  are examples of a specific region of the present disclosure. In this case, the sheet member  666  elastically deforms when the sheet member  666  becomes unable to keep the natural state due to the pressing force F 1  equal to or greater than a tolerable value being applied in the forward rotation direction X 1 . 
     (3) The developing device  43  according to the above embodiment is a device that develops an electrostatic latent image on the surface of the photosensitive drum  41  by a so-called interactive touchdown method. However, the developing device provided in the image forming apparatus  10  is not limited thereto. That is, the developing device provided in the image forming apparatus  10  may be a developing device of a type in which the magnetic roller  430  is not provided and the developing roller  432  receives the two-component developer  90  stored in the developer reservoir  450  and supplies the toner to the photosensitive drum  41 . In this case, the developing roller  432  corresponds to a developer carrying body that carries the agitated two-component developer  90 , and the photosensitive drum  41  corresponds to a toner carrying body. 
     (4) When the reverse rotation control by the rotation control portion  201  has been finished, the rotation control portion  201  may rotate the sleeve portion  460  in the forward rotation direction X 1  in advance in preparation for generation of an image formation job. Thus, when an image formation job is generated, the developer layer has been already formed on the surface of the sleeve portion  460 . Therefore, the generated image formation job can be swiftly executed. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.