Patent Publication Number: US-9405219-B2

Title: Developing device with reverse rotation control, image forming apparatus, and method for controlling developing device

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2014-235803 filed on Nov. 20, 2014, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a developing device which converts by electrophotography an electrostatic latent image into a manifest image, an image forming apparatus, and a method for controlling the developing device. 
     In general, in a developing device mounted in an image forming apparatus such as a multifunction peripheral, the thickness of the layer of a developer formed on the surface of a developer carrying member is regulated by a layer thickness regulating member. At this time, scattered toner accumulates on the surface of the layer thickness regulating member. If the accumulated toner is transferred from the layer thickness regulating member to a photosensitive member, the image quality of a printed matter may be adversely affected. 
     In order to address the above problem, there are cases where the following measures are taken. That is, in the image forming apparatus of this type, a magnetic brush is formed on the surface of the developer carrying member by the carrier in the developer including two components. By utilizing this, the image forming apparatus causes the developer carrying member to rotate in a direction reverse to the rotation direction during a developing process, thereby scraping the accumulated toner with the magnetic brush. 
     SUMMARY 
     A developing device according to one aspect of the present disclosure includes a developer carrying member, a first rotation control portion, a layer thickness regulating member, and a second rotation control portion. The developer carrying member is rotatably supported and is configured to rotate in a first rotation direction, thereby to carry a two-component developer on a surface thereof, to supply, at a first position, toner included in the two-component developer to a toner carrying member on a next stage, and further to cause the two-component developer carried on the surface to be detached therefrom at a second position on a downstream side in the first rotation direction relative to the first position. The first rotation control portion is configured to cause, when a developing process is performed, the developer carrying member to rotate in the first rotation direction at a first rotation speed previously determined. The layer thickness regulating member is provided, spaced from the surface of the developer carrying member, at a third position on an upstream side in the first rotation direction relative to the first position on an outer circumference of the developer carrying member, and is configured to regulate a thickness of a layer of the two-component developer carried by the developer carrying member rotating in the first rotation direction. The second rotation control portion is configured to, when the developing process is not performed, cause the developer carrying member to rotate at a second rotation speed faster than the first rotation speed in a second rotation direction reverse to the first rotation direction, and further cause the developer carrying member to rotate at a third rotation speed slower than the second rotation speed in the second rotation direction. 
     An image forming apparatus according to another aspect of the present disclosure includes a photosensitive member, a developing device, and a transfer portion. In the photosensitive member, an electrostatic latent image is formed on a surface thereof. The developing device is configured to supply the toner to the photosensitive member, to convert the electrostatic latent image into a toner image as a manifest image. The transfer portion is configured to transfer, to a recording sheet, the toner image formed on the photosensitive member. 
     A method for controlling a developing device according to another aspect of the present disclosure is a method for controlling a developing device including the developer carrying member and the regulation member, and includes two steps. In a first step, when a developing process is performed by the developer carrying member rotatably supported and configured to rotate in a first rotation direction, thereby to carry a two-component developer on a surface thereof, to supply, at a first position, toner included in the two-component developer to a toner carrying member on a next stage, and further to cause the two-component developer carried on the surface to be detached therefrom at a second position on a downstream side in the first rotation direction relative to the first position, the developer carrying member is caused to rotate in the first rotation direction at a first rotation speed previously determined. In a second step, when the developing process is not performed, the developer carrying member is caused to rotate at a second rotation speed faster than the first rotation speed in a second rotation direction reverse to the first rotation direction, and the developer carrying member is further caused to rotate at a third rotation speed slower than the second rotation speed in the second rotation direction. 
     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 configuration diagram of an image forming apparatus according to a first embodiment of the present disclosure. 
         FIG. 2  is a configuration diagram of a photosensitive drum and a developing device in an image forming portion of the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 3  shows a state in which toner is accumulated on the surface of a blade. 
         FIG. 4A  shows a state in which a magnetic roller is in reverse rotation. 
         FIG. 4B  shows a state in which, through reverse rotation of the magnetic roller, a magnetic brush scrapes toner accumulated on the surface of the blade. 
         FIG. 5  is a flow chart showing a process performed by a control portion. 
         FIG. 6  shows a result of verification of the effect of the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described in detail based on the drawings. It should be noted that the following embodiments are examples embodying the present disclosure, and, by nature, do not limit the technical scope of the present disclosure. 
     First, with reference to  FIGS. 1 and 2 , a configuration of an image forming apparatus  10  according to a first embodiment of the present disclosure will be described. The image forming apparatus  10  is an electrophotographic image forming apparatus. As shown in  FIG. 1 , the image forming apparatus  10  includes a sheet feed portion  2 , a sheet conveying portion  3 , toner supply portions  40 , an image forming portion  4 , an optical scanning portion  5 , a fixing portion  6 , and the like in a housing  100 . 
     The image forming apparatus  10  shown in  FIG. 1  is a tandem type image forming apparatus, and is a color printer. Thus, the image forming portion  4  further includes an intermediate transfer belt  48 , a secondary cleaning device  480 , and a secondary transfer device  49 . 
     The image forming portion  4  includes a plurality of single color image forming portions  4   x  respectively corresponding to colors of cyan, magenta, yellow, and black. Further, the image forming apparatus  10  includes a plurality of toner supply portions  40  which supply toners of colors of cyan, magenta, yellow, and black to later-described developer chambers  4300  (see  FIG. 2 ) of later-described developing devices  43 , respectively. Each toner supply portion  40  is removably mounted at a previously-determined position in the image forming apparatus  10 . In the present embodiment, each toner supply portion  40  is mounted at an upper position in the image forming portion  4 . 
     The image forming apparatus  10  is, for example, a printer, a copy machine, a facsimile machine, a multifunction peripheral, or the like. The multifunction peripheral also has the function of the printer, the function of the copy machine, and the like. 
     The sheet feed portion  2  includes a sheet receiving portion  21  and a sheet sending-out portion  22 . The sheet receiving portion  21  is configured such that a plurality of recording sheets  9  can be placed thereon in a stacked manner. Each recording sheet  9  is a sheet-like medium on which an image is to be formed, such as paper, coated paper, a postcard, an envelope, an OHP sheet, or the like. 
     The sheet sending-out portion  22  rotates in contact with the recording sheet  9 , to send out the recording sheet  9  from the sheet receiving portion  21  toward a conveying path  30 . 
     The sheet conveying portion  3  includes registration rollers  31 , conveying rollers  32 , discharge rollers  33 , and the like. The registration rollers  31  and the conveying rollers  32  convey the recording sheet  9  fed from the sheet feed portion  2 , toward the secondary transfer device  49  of the image forming portion  4 . Further, the discharge rollers  33  discharge the recording sheet  9  on which an image has been formed, from the discharge outlet of the conveying path  30  onto a discharge tray  101 . 
     The intermediate transfer belt  48  is an endless belt-like member formed in a loop shape. The intermediate transfer belt  48  rotates in a state of being extended on and between two rollers. In the image forming portion  4 , each single color image forming portion  4   x  forms an image of the color thereof on the surface of the intermediate transfer belt  48  which is rotating. Accordingly, a color image on which images in the respective colors are superposed is formed on the intermediate transfer belt  48 . 
     The secondary transfer device  49  transfers, to the recording sheet  9 , the toner image formed on the intermediate transfer belt  48 . The secondary cleaning device  480  removes toner remaining on a portion, of the intermediate transfer belt  48 , that has passed through the secondary transfer device  49 . 
     Each single color image forming portion  4   x  includes a photosensitive drum  41  which carries a toner image, a charging device  42 , a developing device  43 , a primary transfer device  45 , a primary cleaning device  47 , and the like. Each photosensitive drum  41  is one example of a photosensitive member which carries a toner image while rotating. Each primary transfer device  45 , the intermediate transfer belt  48 , and the secondary transfer device  49  correspond to a transfer portion which transfers, to the recording sheet  9 , the toner image formed on photosensitive drum  41 . 
     Each photosensitive drum  41  rotates at a circumferential speed in accordance with the circumferential speed (movement speed) of the intermediate transfer belt  48 . For example, it is conceivable that the photosensitive drum  41  is an organic photosensitive member. It is also conceivable that the photosensitive drum  41  is an amorphous silicon photosensitive member. 
     In each single color image forming portion  4   x , the photosensitive drum  41  rotates, and the charging device  42  uniformly charges the surface of the photosensitive drum  41 . Further, the optical scanning portion  5  performs scanning with laser light, thereby writing an electrostatic latent image on the charged surface of the photosensitive drum  41 . 
     The developing device  43  develops the electrostatic latent image by supplying toner to the photosensitive drum  41 . The developing device  43  in the present embodiment causes toner to be charged by agitating a two-component developer  90  including toner and carrier, and supplies the charged toner to the photosensitive drum  41 . 
     The charging device  42  includes a charging roller  420  which charges a portion, of the photosensitive drum  41 , on which the electrostatic latent image has not yet been written. 
     As shown in  FIG. 2 , each developing device  43  includes the developer chamber  4300 , a magnetic roller  430 , a developing roller  432 , an agitation mechanism  437 , and a blade  438 . The magnetic roller  430 , the developing roller  432 , and the agitation mechanism  437  are rotatably supported about the rotation axes thereof which are parallel to one another. 
     The developer chamber  4300  contains the two-component developer including toner and carrier. The toner is supplied from its corresponding toner supply portion  40  (see  FIG. 1 ). The toner is particles whose principal component is resin, and the carrier is particles including a magnetic material. The particle size of the toner is smaller than the particle size of the carrier. The toner is lighter in weight than the carrier. The magnetic material of the carrier is ferrite or the like, for example. As described later, the toner is agitated while being mixed with the carrier, thereby being charged with static electricity that occurs from friction between the toner and the carrier. With the presence of the carrier, in the two-component developer  90 , the toner can be more easily charged, than in the case of a one-component developer consisting of toner alone, and thus, high quality of the image can be realized. 
     The agitation mechanism  437  is rotatably provided inside the developer chamber  4300 . The agitation mechanism  437  agitates the two-component developer  90  in the developer chamber  4300 . 
     The agitation mechanism  437  includes a rotation shaft portion  4371  and an agitating member  4372 . 
     The rotation shaft portion  4371  is a shaft member formed in a shape that is long in a direction orthogonal to the drawing plane of  FIG. 2 . The rotation shaft portion  4371  is rotatably supported by side walls (not shown) at opposite ends in the direction orthogonal to the drawing plane of  FIG. 2  of the developer chamber  4300 . 
     The agitating member  4372  is a flexible member formed in a film shape. For example, the agitating member  4372  is a member including synthetic resin such as a PET (polyethylene terephthalate) resin. The principal material of the agitating member  4372  is not limited to a PET resin, and may be a synthetic resin such as vinyl chloride, polycarbonate, or the like. 
     The agitating member  4372  is attached to the rotation shaft portion  4371 . The agitating member  4372  is formed so as to extend along the longitudinal direction of the rotation shaft portion  4371  and so as to extend from the rotation shaft portion  4371  in a direction that crosses the longitudinal direction of the rotation shaft portion  4371 . In the present embodiment, the rotation shaft portion  4371  has a planar adhesion surface (not shown). An edge portion of the agitating member  4372  is bonded with an adhesive to the adhesion surface of the rotation shaft portion  4371 . 
     The agitating member  4372  rotates in conjunction with rotation of the rotation shaft portion  4371 , to move in the two-component developer  90  in the developer chamber  4300 . Accordingly, the two-component developer  90  in the developer chamber  4300  is agitated. This agitation causes friction between the toner and the carrier, and the toner is charged with static electricity caused by this friction, to a previously-determined polarity. The carrier is charged to a polarity opposite to the polarity to which the toner is charged. Then, the toner adheres to the carrier under static electricity. 
     The magnetic roller  430  is rotatably provided inside the developer chamber  4300 . The magnetic roller  430  attracts the two-component developer  90  agitated by the agitation mechanism  437 , from the developer chamber  4300  with magnetic force thereof, and carries the two-component developer  90  on the surface of the magnetic roller  430 . 
     The magnetic roller  430  includes a sleeve portion  430 S 1  and magnets  430 M. 
     The sleeve portion  430 S 1  has a cylindrical shape, and holds the magnets  430 M therein. The sleeve portion  430 S 1  is composed of a nonmagnetic member. The sleeve portion  430 S 1  is rotatable in forward/reverse directions. The sleeve portion  430 S 1  rotates in one direction during a developing process. In the description below, the rotation direction of the sleeve portion  430 S 1  during the developing process will be referred to as a development rotation direction X 1 . The development rotation direction X 1  corresponds to a first rotation direction. In the present embodiment, the development rotation direction X 1  is a counterclockwise direction on the drawing of  FIG. 2 . 
     A plurality of magnets  430 M are provided inside the sleeve portion  430 S 1 . The plurality of magnets  430 M are arranged with a predetermined interval therebetween along the circumferential direction. The positions of the magnets  430 M are fixed in the sleeve portion  430 S 1 . The plurality of magnets  430 M include a magnet  430 M- 1 , a magnet  430 M- 2 , a magnet  430 M- 3 , and a magnet  430 M- 4 . 
     The magnet  430 M- 1  is provided at a position opposed to the two-component developer  90  in the developer chamber  4300 . The magnet  430 M- 1  attracts the two-component developer  90  contained in the developer chamber  4300 . Accordingly, the two-component developer  90  adheres to the portion opposed to the magnet  430 M- 1  of the surface of the sleeve portion  430 S 1  in the magnetic roller  430 . A position L 1  shown in  FIG. 2  indicates a developer transfer position L 1  at which the two-component developer  90  contained in the developer chamber  4300  is transferred to the sleeve portion  430 S 1 . 
     The magnet  430 M- 2  is provided at a position adjacent to the magnet  430 M- 1  and on the downstream side in the development rotation direction X 1  relative to the magnet  430 M- 1 . The magnet  430 M- 2  causes the sleeve portion  430 S 1  to carry the two-component developer  90 . 
     Under the magnetic force of the magnet  430 M- 1  and the magnet  430 M- 2 , a developer layer is formed on the surface of the sleeve portion  430 S 1 . On this developer layer, a magnetic brush B 1  (see  FIG. 4 ) is formed. In the present embodiment, the sleeve portion  430 S 1  of the magnetic roller  430  is one example of the developer carrying member which carries the two-component developer  90 . 
     The magnetic brush B 1  is formed in the following manner: a plurality of carrier particles included in the two-component developer  90  are connected together to form a plurality of  : 10 
     chains on the surface of the magnetic roller  430  under the magnetic force of the magnets  430 M- 1  and  430 M- 2 , resulting in a stack of the chains of carrier particles. 
     Each of the magnetic roller  430  and the developing roller  432  has bias applied thereon, and a previously-determined potential difference is provided between the magnetic roller  430  and the developing roller  432 . Due to this potential difference, the toner included in the two-component developer  90  carried by the magnetic roller  430  is transferred to the developing roller  432 . A position L 2  shown in  FIG. 2  indicates a toner transfer position L 2  at which the toner included in the two-component developer  90  carried by the magnetic roller  430  is transferred to the developing roller  432 . The toner transfer position L 2  corresponds to a first position. 
     As described above, the magnetic roller  430  is rotatably supported in the developer chamber  4300  and rotates in the development rotation direction X 1 , thereby carrying the two-component developer  90  on the surface thereof and supplying, at the toner transfer position L 2 , the toner included in the two-component developer  90  to the developing roller  432  on the next stage. 
     The magnet  430 M- 3  is provided at a position opposed to the developing roller  432 , and attracts, to the sleeve portion  430 S 1 , the carrier remaining on the magnetic roller  430  as a result of the toner having been transferred to the developing roller  432  at the toner transfer position L 2 . The carrier attracted to the sleeve portion  430 S 1  by the magnet  430 M- 3  keeps the state where the magnetic brush B 1  is formed. 
     After the toner has been transferred to the developing roller  432  at the toner transfer position L 2 , the magnet  430 M- 4  separates, from the surface of the magnetic roller  430 , the carrier remaining on the surface with the magnetic force thereof, and causes the carrier to drop into the developer chamber  4300  located below. A position L 3  shown in  FIG. 2  indicates a separation position L 3  at which the carrier remaining on the surface of the magnetic roller  430  is separated from the surface with the magnetic force. The separation position L 3  corresponds to a second position. 
     During the developing process, the magnetic roller  430  receives the two-component developer  90  from the developer chamber  4300  at the developer transfer position L 1  with the magnetic force of the magnet  430 M- 1 , and conveys the two-component developer  90  through rotation of the sleeve portion  430 S 1  in the development rotation direction X 1 . When the two-component developer  90  has been conveyed to the toner transfer position L 2 , the toner included in the two-component developer  90  is transferred to the developing roller  432  on the next stage, due to the potential difference between the magnetic roller  430  and the developing roller  432 . At this time, the carrier remains on the surface of the magnetic roller  430 . 
     The magnetic roller  430  conveys the carrier to the separation position L 3  through further rotation of the sleeve portion  430 S 1  in the development rotation direction X 1 . When having conveyed the carrier to the separation position L 3 , the magnetic roller  430  causes the carrier to be detached from the magnetic roller  430 , with repulsive force acting between the carrier and the magnet  430 M- 4 . Accordingly, the separated carrier drops into the developer chamber  4300  located below. 
     The blade  438  is provided, spaced from the surface of the magnetic roller  430 , at a layer thickness regulation position L 4  on the upstream side in the development rotation direction X 1  relative to the toner transfer position L 2  on the outer circumference of the magnetic roller  430 . The blade  438  regulates the thickness of the layer of the two-component developer  90  carried by the magnetic roller  430  rotating in the development rotation direction X 1 . The blade  438  is one example of a layer thickness regulating member. The layer thickness regulation position L 4  corresponds to a third position. In the present embodiment, the separation position L 3  and the layer thickness regulation position L 4  are arranged at positions substantially opposite to each other relative to the rotation axis of the magnetic roller  430 . 
     The developing roller  432  receives, from the magnetic roller  430 , the toner included in the two-component developer  90  carried by the magnetic roller  430 . On the surface of the developing roller  432 , a toner layer is formed from the toner. 
     The developing roller  432  faces the photosensitive drum  41  in a non-contact state. Due to the bias applied to the developing roller  432 , the toner on the developing roller  432  is transferred to the portion of the electrostatic latent image formed on the outer circumferential surface of the photosensitive drum  41 . That is, the developing roller  432  converts the electrostatic latent image into a manifest image, by supplying the toner to the photosensitive drum  41  in which the electrostatic latent image is formed on the surface thereof. The developing roller  432  is one example of a toner carrying member. 
     The developing roller  432  rotates in the same direction as that of the magnetic roller  430  during the developing process. Accordingly, the portions, of the outer circumferential surfaces of the magnetic roller  430  and the developing roller  432 , that face each other move in reverse directions, respectively. 
     In addition, during the developing process, the developing roller  432  and the photosensitive drum  41  rotate in reverse directions with each other. Accordingly, the portions, of the outer circumferential surfaces of the developing roller  432  and the photosensitive drum  41 , that face each other move in the same direction. 
     In this manner, the toner included in the two-component developer  90  is consumed during the developing process. Thus, the toner is supplied from the toner supply portion  40  into the developer chamber  4300 , to supplement the consumed amount. On the other hand, the carrier included in the two-component developer  90  will remain in the developer chamber  4300  with scarcely being consumed, and provides flowability and the like to the toner supplied to the developer chamber  4300 . 
     The developing device  43  includes a drive motor  203 . The drive motor  203  drives the magnetic roller  430  so as to rotate. It is conceivable that the drive motor  203  is a DC brushless motor, a stepping motor, or the like. 
     The developing device  43  includes 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 kinds of calculation processes. The ROM is a nonvolatile storage portion in which information such as a control program for causing the CPU to execute various kinds of processes is previously stored. The RAM is a volatile storage portion to be used as a temporary storage memory (working area) for various kinds of processes executed by the CPU. The control portion  200  controls operation of the image forming apparatus  10 , by the CPU executing the program stored in the ROM. 
     In the ROM of the control portion  200 , a processing program is stored which causes the CPU of the control portion  200  to execute a process described later (see the flow chart in  FIG. 5 ). The processing program may be stored in the ROM at the time of shipment of the image forming apparatus  10 . Alternatively, the processing program may be stored in a computer-readable information storage medium such as a CD (Compact Disc), a DVD (Digital Versatile Disc), or a flash memory, and after the shipment, the processing program may be stored in the ROM of the control portion  200  from the information storage medium. It is also conceivable as another embodiment that part or a plurality of functions of the control portion  200  are implemented as electronic circuits. 
     Meanwhile, in the developing device  43 , toner scattered when the thickness of the layer of the developer on the magnetic roller  430  is regulated by the blade  438  accumulates on the surface of the blade  438  (see  FIG. 3 ). When the size of the accumulated toner Q 1  has become large, there is a risk that the accumulated toner is transferred from the blade  438 , via the developing roller  432 , to the photosensitive drum  41 , thereby adversely affecting the image quality. 
     In order to address the above problem, there are cases where, by utilizing the phenomenon that the magnetic brush B 1  is formed on the surface of the developer carrying member by the carrier included in the two-component developer  90 , the accumulated toner is scraped by the magnetic brush B 1  (see  FIG. 4B ) by rotating the magnetic roller  430  in a rotation direction X 2 , which is reverse (hereinafter, referred to as reverse rotation direction) to the rotation direction during the developing process (see  FIG. 4A ). 
     However, in the case where the control is performed in which the magnetic roller  430  is rotated in the reverse rotation direction X 2  to the rotation direction during the developing process, there is a demand for a technique that enhances the performance of removing the accumulated toner from the surface of the blade  438 . Thus, the present embodiment includes the following configuration. 
     In the present embodiment, the control portion  200  includes a first rotation control portion  201  and a second rotation control portion  202 . 
     During the developing process, the first rotation control portion  201  performs forward rotation control of causing the magnetic roller  430  to rotate in the development rotation direction X 1  at a first rotation speed V 1  previously determined. 
     When the developing process is not performed, the second rotation control portion  202  performs reverse rotation control of causing the magnetic roller  430  to rotate in the reverse rotation direction X 2  to the development rotation direction X 1 . Here, in the reverse rotation control, the second rotation control portion  202  causes the magnetic roller  430  to rotate at a second rotation speed V 2  which is faster than the first rotation speed V 1 , and then, causes the magnetic roller  430  to rotate at a third rotation speed V 3  which is slower than the second rotation speed V 2  in the reverse rotation direction X 2 . The reverse rotation direction X 2  corresponds to a second rotation direction. 
     The rotation angle by which the magnetic roller  430  is rotated at the second rotation speed V 2  is greater than or equal to the rotation angle from the separation position L 3  to the layer thickness regulation position L 4  in the reverse rotation direction X 2 , and less than the angle corresponding to one rotation. 
     The toner removing performance obtained through reverse rotation of the magnetic roller  430  is higher in the case where the third rotation speed V 3  is less than the first rotation speed V 1 , than in the case where the third rotation speed V 3  is greater than or equal to the first rotation speed V 1 . Thus, it is conceivable that the third rotation speed V 3  is slower than the first rotation speed V 1 . 
     In the description below, the control in which the second rotation control portion  202  causes the magnetic roller  430  to rotate at the second rotation speed V 2  in the reverse rotation direction X 2  will be referred to as fast reverse rotation control, and the control in which the second rotation control portion  202  causes the magnetic roller  430  to rotate at the third rotation speed V 3  in the reverse rotation direction X 2  will be referred to as slow reverse rotation control. 
     Next, with reference to  FIG. 5 , the process performed by the control portion  200  will be described. In the flow chart shown in  FIG. 5 , steps S 501 , S 502 , . . . represent the numbers of steps in the processing procedure. The process by the control portion  200  shown in  FIG. 5  is started at the time when an image forming job involving the developing process is executed. 
     &lt;Step S 501 &gt; 
     In step S 501 , the control portion  200  determines whether an image forming job has ended. When determining that the image forming job has not ended (NO in step S 501 ), the control portion  200  executes the process of step S 501  again. On the other hand, when determining that the image forming job has ended (YES in step S 501 ), the control portion  200  advances the process to step S 502 . 
     &lt;Step S 502 &gt; 
     Upon determining that the image forming job has ended, the control portion  200  determines whether a start condition on which to start the reverse rotation control of the magnetic roller  430  has been satisfied. As the start condition, for example, a condition can be employed that the count value of a counter (not shown) described later has exceeded a value indicating the number of sheets previously determined. The value is 10000, for example. 
     When determining that the start condition has not been satisfied (NO in step S 502 ), the control portion  200  ends the process. On the other hand, when determining 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 recording sheets  9  on which image forming has been performed. It is conceivable that the counter is provided in the control portion  200 . After executing the process of 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 fast reverse rotation control on the magnetic roller  430 . Under this fast reverse rotation control, the two-component developer  90  attracted on the magnetic roller  430  under the magnetic force of the magnet  430 M slips on the surface of the magnetic roller  430 . In other words, the control portion  200  causes the magnetic roller  430  to rotate at the second rotation speed V 2  that causes the two-component developer  90  to slip on the surface of the magnetic roller  430 . 
     As a result of the slipping, the magnetic brush B 1  is gathered, forming a lump of the magnetic brush B 1 . After executing the process of 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 the rotation time period under the fast reverse rotation control on the magnetic roller  430  has reached a rotation time period Tth 1  previously determined. The rotation time period for the magnetic roller  430  is set so as to attain an intended rotation angle. It is conceivable that the rotation time period Tth 1  is, for example, a time period required for the magnetic roller  430  to rotate by an angle from the separation position L 3  to the layer thickness regulation position L 4  in the reverse rotation direction X 2 . 
     When determining that the rotation time period has not reached the rotation time period Tth 1  (NO in step S 505 ), the control portion  200  executes the process of step S 505  again. On the other hand, when determining that the rotation time period has reached the rotation time period Tth 1  (YES in step S 505 ), the control portion  200  advances the process to step S 506 . 
     Here, the fast reverse rotation control is ended when the rotation time period has reached the rotation time period Tth 1 , but a mode is also conceivable in which: a sensor is provided which detects that the reverse rotation angle has reached the intended angle; and the fast reverse rotation control is ended depending on the detection result from the sensor. 
     The carrier carried on the surface of the magnetic roller  430  is detached from the surface at the separation position L 3 . Accordingly, the carrier is scarcely present on the surface of the magnetic roller  430  from the separation position L 3  to the developer transfer position L 1  in the development rotation direction X 1 . Thus, even if the magnetic roller  430  is caused to reversely rotate by an angle greater than or equal to the rotation angle from the separation position L 3  to the layer thickness regulation position L 4 , the size of the lump of the magnetic brush B 1  formed as a result of the slipping hardly changes. 
     In order to avoid delay of the start of execution of an image forming job that could be newly generated, the time period required for the reverse rotation control is preferably as short as possible. 
     From the above, in the present embodiment, when the magnetic roller  430  is caused to reversely rotate at the fast speed by an angle from the separation position L 3  to the layer thickness regulation position L 4  in the reverse rotation direction X 2 , the toner accumulated on the surface of the blade  438  can be efficiently removed by the magnetic brush B 1  in a short time. However, the rotation angle when causing the magnetic roller  430  to reversely rotate at the fast speed is not limited to the rotation angle from the separation position L 3  to the layer thickness regulation position L 4  in the reverse rotation direction X 2 . 
     &lt;Step S 506 &gt; 
     When determining that the rotation angle has reached the previously determined rotation angle, the control portion  200  starts the slow reverse rotation control on the magnetic roller  430 . Under this slow reverse rotation control, the two-component developer  90  carried on the surface of the magnetic roller  430  does not slip on the surface of the magnetic roller  430 , and the lump of the magnetic brush B 1  formed under the fast reverse rotation control advances toward the surface of the blade  438 . 
     Then, the lump of the magnetic brush B 1  hits the toner accumulated on the surface of the blade  438 , thereby scraping the toner from the surface of the blade  438 . The scraped toner passes through the gap between the surface of the magnetic roller  430  and the tip of the blade  438 , and drops toward the developer chamber  4300 . 
     By the slow reverse rotation control being performed after the fast reverse rotation control, the lump of the magnetic brush B 1  hits the accumulated toner on the surface of the blade  438 . In this case, the performance of removing the toner accumulated on the surface of the blade  438  is improved compared with a case where the magnetic brush B 1  not having gathered hits the accumulated toner. After executing the process of step S 506 , the control portion  200  advances the process to step S 507 . 
     &lt;Step S 507 &gt; 
     The control portion  200  determines whether the rotation angle under the slow reverse rotation control on the magnetic roller  430  has reached a rotation time period Tth 2  previously determined. When determining that the rotation angle has not reached the rotation time period Tth 2  (NO in step S 507 ), the control portion  200  executes the process of step S 507  again. 
     On the other hand, when determining that the rotation angle has reached the rotation time period Tth 2  (YES in step S 507 ), the control portion  200  ends the reverse rotation control. When the sum of the rotation amount corresponding to the rotation time period Tth 1  and the rotation amount corresponding to the rotation time period Tth 2  is greater than or equal to an amount corresponding to one rotation at least, particularly excellent toner removing performance can be obtained. 
       FIG. 6  shows a result of a test verifying the effect of the present embodiment.  FIG. 6  shows a result of a verification test in which the amounts of toner accumulated on the blade  438  are compared among the present embodiment and two patterns of Comparative examples 1 and 2. 
     In the verification test, measured is the amount of toner accumulated on the surface of the blade  438  when an image has been formed on 10000 recording sheets  9  at the image coverage rate of 5%. The diameter of the magnetic roller  430  is 16 mm, the diameter of the developing roller is 16 mm, and the value of the rotation speed of the developing roller in terms of circumferential speed is 208 mm/sec. 
     Comparative example 1 is an example case in which the reverse rotation control on the magnetic roller  430  is not performed. Comparative example 2 is an example case in which: the magnetic roller  430  is caused to reversely rotate when image formation is not performed; but the rotation speed at that time is the same as the rotation speed during the developing process. The rotation speed (the first rotation speed V 1 ) of the magnetic roller  430  during the developing process in terms of circumferential speed is 235 mm/sec. 
     In the present embodiment, the value of the second rotation speed V 2  under the fast reverse rotation control in terms of circumferential speed and the rotation amount corresponding to the rotation time period Tth 1  are 470 mm/sec and 180 degrees, respectively. Further, in the present embodiment, the value of the third rotation speed V 3  under the slow reverse rotation control in terms of circumferential speed and the rotation amount corresponding to the rotation time period Tth 2  are 118 mm/sec and 360 degrees, respectively. At the time point when the magnetic roller  430  has been driven to rotate by one round after the slow reverse rotation control had been started on the magnetic roller  430 , the drive is stopped. 
     As shown in  FIG. 6 , when Comparative example 1 is compared with Comparative example 2, the amount of accumulated toner is 0.2 g in Comparative example 1, whereas, in Comparative example 2, the amount of accumulated toner is 0.07 g, reducing the amount of accumulated toner to about ⅓. 
     When the present embodiment is compared with Comparative example 2, an experimental result has been obtained in which, in the present embodiment, the amount of accumulated toner has been reduced to 0.025 g, which is about ⅓ of the amount of accumulated toner in Comparative example 2. 
     As described above, in the present embodiment, compared with a case where the magnetic roller  430  is caused to reversely rotate at a constant speed, the amount of toner accumulated on the surface of the blade  438  can be further reduced. 
     A preferred embodiment of the present disclosure has been described above. However, the present disclosure is not limited to the above embodiment, and various modifications can be made. 
     (1) The developing device  43  according to the embodiment is a device that develops an electrostatic latent image on the surface of the photosensitive drum  41  by a so-called interactive touch-down system. However, the developing device mounted in the image forming apparatus  10  is not limited thereto. 
     That is, the developing device mounted in the image forming apparatus  10  may be a developing device of a type that does not include the magnetic roller  430 , and in which the developing roller  432  receives the two-component developer  90  contained in the developer chamber  4300  and supplies the toner to the photosensitive drum  41 . In this case, the developing roller  432  corresponds to the developer carrying member which carries the agitated two-component developer  90 , and the photosensitive drum  41  corresponds to the toner carrying member. 
     (2) The toner is charged through friction with the carrier caused by agitation by the magnetic roller  430 . As the charged toner is left to stand, the electric charge is discharged from the toner with the elapse of time. This phenomenon is referred to as charge decay, electrostatic diffusion, or the like. The charge decay characteristic of the toner, that is, the discharging speed of electric charge, is different depending on the kind of the toner. The developing device  43  can be loaded with any of various kinds of toner having different charge decay speeds. 
     Toner having a fast charge decay speed is more likely to scatter to the surroundings than toner having a slow charge decay speed. Thus, when the developing device  43  is loaded with the toner having a fast charge decay speed, a large lump of accumulated toner is more likely to be formed on the surface of the blade  438 . In this case, unless the reverse rotation amount of the magnetic roller  430  is increased in accordance with increase in the size of the accumulated toner, the accumulated toner cannot be sufficiently removed from the blade  438 . 
     Thus, the rotation angle (rotation amount) in at least one of the fast reverse rotation and the slow reverse rotation may be set in accordance with the charge decay speed of the toner loaded into the developing device  43 . 
     That is, the developing device  43  includes an obtaining portion and a reverse rotation amount setting portion described below. 
     The obtaining portion obtains toner information regarding the charge decay characteristic of the toner contained in the toner supply portion  40 . The toner supply portion  40  is provided with an information storage medium retaining the toner information therein. The obtaining portion can read the toner information from the information storage medium. 
     The reverse rotation amount setting portion sets the reverse rotation amount of the magnetic roller  430  such that, in the case where the charge decay speed of the toner is high, the reverse rotation amount of the magnetic roller  430  is increased compared with that in the case where the charge decay speed is low. Specifically, the reverse rotation amount setting portion sets the reverse rotation amount of the magnetic roller  430  based on the charge decay characteristic of the toner indicated by the toner information. The reverse rotation amount is a rotation amount in at least one of the fast reverse rotation and the slow reverse rotation. 
     When the developing process is not performed, the second rotation control portion  202  causes the magnetic roller  430  to reversely rotate by the reverse rotation amount set by the reverse rotation amount setting portion. 
     Accordingly, the operation of removing the toner accumulated on the blade  438  can be performed, by a proper amount, in accordance with the charge decay speed of the toner. The configuration described above is effective in particular for a developing device in which the separation position L 3  is not set in the magnetic roller  430 . 
     (3) When the reverse rotation control by the second rotation control portion  202  has ended, the first rotation control portion  201  may cause the magnetic roller  430  to previously rotate in the development rotation direction X 1 , so as to be ready for generation of another image forming job. Accordingly, when another image forming job has been generated, a developer layer has been formed on the surface of the magnetic roller  430 , and thus, the image forming job that has been generated can be quickly 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.