Patent Publication Number: US-2013236215-A1

Title: Developing device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-049306 filed Mar. 6, 2012. 
     BACKGROUND 
     Technical Field 
     The present invention relates to developing devices and image forming apparatuses. 
     SUMMARY 
     According to an aspect of the invention, there is provided a developing device including an accommodation housing, multiple developing members, a supply member, a layer regulating member, and an electrode member. The accommodation housing accommodates a two-component developer containing a toner and a magnetic carrier. The multiple developing members are disposed facing an image bearing member having a latent image formed thereon due to a difference in electrostatic potential and include a first developing member and a second developing member. The first developing member has a substantially-cylindrical first sleeve rotationally driven in a circumferential direction thereof and a first magnet roller fixedly supported within the first sleeve and provided with magnetic poles at multiple positions in the circumferential direction. The second developing member has a substantially-cylindrical second sleeve rotationally driven in a circumferential direction thereof and a second magnet roller fixedly supported within the second sleeve and provided with magnetic poles at multiple positions in the circumferential direction. The supply member supplies the two-component developer onto the first sleeve of the first developing member. The layer regulating member faces the first sleeve and regulates a layer of the two-component developer supported on a peripheral surface of the first sleeve by the first magnet roller provided within the first sleeve. The electrode member faces the second sleeve with a certain distance therebetween and is disposed upstream, in a rotational direction of the second sleeve included in the second developing member, of a position where the second sleeve receives the two-component developer from the first sleeve and downstream of a position where the second developing member having received the two-component developer regulated by the layer regulating member faces the image bearing member. An electric field that causes the toner adhered to a peripheral surface of the second sleeve to be removed therefrom or an electric field that causes the toner adhered to a surface of the magnetic carrier to adhere to the peripheral surface of the second sleeve is generated between the electrode member and the second sleeve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  schematically illustrates the configuration of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  schematically illustrates the configuration of a developing device included in the image forming apparatus shown in  FIG. 1 , according to a first exemplary embodiment of the present invention; 
         FIG. 3  schematically illustrates how toner particles, magnetic carrier particles, and external additive particles behave at a position where a developing roller and a photoconductor drum face each other; 
         FIGS. 4A and 4B  schematically illustrate other modes for applying voltage to an electrode member and a developing roller; 
         FIG. 5  schematically illustrates a state where a cleaning device is provided for the electrode member included in the developing device shown in  FIG. 2 ; 
         FIG. 6  schematically illustrates an example in which the developing device is equipped with two electrode members; 
         FIG. 7  schematically illustrates a developing device according to a second exemplary embodiment of the present invention; and 
         FIG. 8  schematically illustrates a developing device according to a third exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. 
       FIG. 1  schematically illustrates the configuration of an image forming apparatus according to an exemplary embodiment of the present invention. 
     The image forming apparatus forms a color image by using toners of four colors and includes electrophotographic image forming units  10 Y,  10 M,  10 C, and  10 K that respectively output yellow (Y), magenta (M), cyan (C), and black (K) images, and an intermediate transfer belt  11  that faces these units. 
     The intermediate transfer belt  11  is wrapped around a drive roller  15  that is rotationally driven, an adjustment roller  16  that adjusts deviation of the intermediate transfer belt  11  in the width direction thereof, and an opposing roller  17 . The intermediate transfer belt  11  is disposed facing the image forming units  10 Y,  10 M,  10 C, and  10 K and is rotationally driven in a direction indicated by an arrow A in  FIG. 1 . 
     The image forming unit  10 Y that forms a yellow toner image, the image forming unit  10 M that forms a magenta toner image, the image forming unit  10 C that forms a cyan toner image, and the image forming unit  10 K that forms a black toner image are arranged in that order from the upstream side in the rotational direction of the intermediate transfer belt  11 , and a second-transfer member  12  for performing a second-transfer process is disposed in contact with the intermediate transfer belt  11  at the downstream side of the image forming unit  10 K. 
     A recording medium in the form of a sheet is transported from a recording-medium accommodation section  8  to a second-transfer position  13 , at which the second-transfer member  12  faces the intermediate transfer belt  11 , via a transport path  9 . A transport device  14  that transports the recording medium having toner images transferred thereon and a fixing device  7  that fixes the toner images onto the recording medium by heating and pressing the toner images are provided downstream of the second-transfer position  13  in the transport path  9  for the recording medium. 
     An output-sheet supporter (not shown) that supports a stack of recording media having toner images fixed thereon is disposed further downstream. 
     Each of the image forming units  10  has a photoconductor drum  1  that functions as an image bearing member by having an electrostatic latent image formed on a surface thereof. The photoconductor drum  1  is surrounded by a charging device  2  that electrostatically charges the surface of the photoconductor drum  1 , a developing device  20  that forms a toner image by selectively transferring toner to the latent image formed on the photoconductor drum  1 , a first-transfer roller  5  that first-transfers the toner image on the photoconductor drum  1  onto the intermediate transfer belt  11 , and a cleaning device  6  that removes residual toner from the photoconductor drum  1  after the transfer process. Furthermore, for each of the photoconductor drums  1 , an exposure device  3  that generates image light based on an image signal is provided. The exposure device  3  radiates the image light onto the corresponding photoconductor drum  1  so as to write an electrostatic latent image on the electrostatically-charged photoconductor drum  1 . In this exemplary embodiment, the photoconductor drum  1  is electrostatically charged to −800 V by the charging device  2 . With regard to the electric potential on the surface of the photoconductor drum  1  when the electrostatic latent image is formed thereon, an image area where the electric potential is attenuated due to the exposure process is −400 V, whereas a non-image area is maintained at −800 V. 
     The second-transfer member  12  facing the opposing roller  17  with the intermediate transfer belt  11  interposed therebetween has a second-transfer roller  12   a , an auxiliary roller  12   b , and a second-transfer belt  12   c  wrapped around these rollers. The second-transfer belt  12   c  is nipped between the opposing roller  17  and the second-transfer roller  12   a  in a state where the second-transfer belt  12   c  overlaps the intermediate transfer belt  11 , and is rotated as the intermediate transfer belt  11  is rotationally driven. When a recording medium is delivered between the intermediate transfer belt  11  and the second-transfer belt  12   c , the two belts transport the recording medium by nipping the recording medium therebetween. In order to generate a transfer electric field between the second-transfer roller  12   a  and the opposing roller  17 , a transfer voltage is applied to the opposing roller  17 . 
     The fixing device  7  includes a heating roller  7   a  having a built-in heating source and a pressure roller  7   b  that is in pressure contact with the heating roller  7   a , and forms a nip at a position where these rollers are in contact with each other. The recording medium having the toner images transferred thereon is delivered to the nip, where the recording medium is heated and pressed between the rotationally-driven heating roller  7   a  and pressure roller  7   b , whereby the toner images are fixed onto the recording medium. 
     Referring to  FIG. 2 , each developing device  20  includes an accommodation housing  22  that accommodates a two-component developer  21  containing a toner, a magnetic carrier, and an external additive. A first developing roller  23  and a second developing roller  24  functioning as developing members are provided in an area where the accommodation housing  22  opens toward the corresponding photoconductor drum  1 . In the accommodation housing  22 , a first developer accommodation chamber  25  and a second developer accommodation chamber  26  are provided behind the developing rollers  23  and  24 . The developer accommodation chambers  25  and  26  are respectively provided with a first stirrer transport member  27  and a second stirrer transport member  28  that stir and transport the two-component developer  21  and supply the two-component developer  21  to the first developing roller  23 . The first developing roller  23  and the second developing roller  24  are surrounded by a layer regulating member  29  that regulates the layer thickness of the two-component developer  21  magnetically attached to the outer peripheral surface of the first developing roller  23 , a distributing member  30  that distributes the two-component developer  21  on the first developing roller  23  to the second developing roller  24 , an electrode member  31  disposed facing the outer peripheral surface of the second developing roller  24  and to which DC voltage is applied, and a guide member  32  that guides the two-component developer  21  removed from the outer peripheral surface of the second developing roller  24  to an operational area of the first stirrer transport member  27 . 
     The two-component developer (referred to as “developer” hereinafter)  21  contains a resinous toner, a magnetic carrier, and an external additive. When the developer  21  used in this exemplary embodiment is stirred, the magnetic carrier and the external additive are charged to positive polarity, whereas the toner is charged to negative polarity. By magnetically attaching the positively-charged magnetic carrier to the outer peripheral surfaces of the developing rollers  23  and  24 , the negatively-charged toner adhered around the magnetic carrier is transported together with the positively-charged external additive adhered to the toner. 
     The accommodation housing  22  accommodates the developer  21  and supports the two developing rollers  23  and  24 , the stirrer transport members  27  and  28 , the layer regulating member  29 , the distributing member  30 , the electrode member  31 , and the guide member  32 . In an opening of the accommodation housing  22  that faces the photoconductor drum  1 , the first developing roller  23  and the second developing roller  24  are disposed facing the photoconductor drum  1  with a certain distance therebetween. 
     The two stirrer transport members  27  and  28  are arranged along the axes of the developing rollers  23  and  24  and are screw-shaped members having helical blades around the central axes thereof. The stirrer transport members  27  and  28  are arranged in parallel to each other with a partition  33  interposed therebetween. The partition  33  has openings (not shown) at opposite ends thereof in the axial direction. The stirrer transport members  27  and  28  transport the developer  21  in the axial direction and are rotationally driven so as to transport the developer  21  in opposite directions from each other. Thus, the developer  21  is delivered between the two stirring areas via the openings provided in the partition  33  so as to circulate between the first developer accommodation chamber  25  and the second developer accommodation chamber  26  partitioned by the partition  33 . Then, the developer  21  is supplied to the first developing roller  23  by the first stirrer transport member  27 , whereby the developer  21  becomes magnetically attached to the outer peripheral surface of the first developing roller  23 . 
     The first developing roller  23  and the second developing roller  24  respectively include magnet rollers  23   a  and  24   a  fixedly supported by the accommodation housing  22  and substantially-cylindrical sleeves  23   b  and  24   b  supported in a rotatable manner along the outer peripheral surfaces of the magnet rollers  23   a  and  24   a.    
     The magnet rollers  23   a  and  24   a  have multiple magnetic poles in the circumferential direction thereof, and are capable of magnetically attaching or removing the developer  21  to or from the outer peripheral surfaces of the sleeves  23   b  and  24   b  by utilizing the effect of a magnetic force. Each of these magnetic poles is provided substantially uniformly in the axial direction of the corresponding magnet roller  23   a  or  24   a , such that substantially the same magnetic field is generated in the surrounding area thereof at any position in the axial direction. 
     The first sleeve  23   b  included in the first developing roller  23  is rotationally driven in a direction indicated by an arrow C in  FIG. 2 . Specifically, the first sleeve  23   b  is rotationally driven such that the peripheral surface thereof moves in the same direction as the photoconductor drum  1  at a position where the peripheral surface faces the photoconductor drum  1  driven in a direction indicated by an arrow B. The second sleeve  24   b  included in the second developing roller  24  is driven in a direction indicated by an arrow D. Thus, the opposing peripheral surfaces of the first sleeve  23   b  and the second sleeve  24   b  move in the same direction at a position where they face each other, that is, the delivery position of the developer  21 , whereas the opposing peripheral surfaces of the second sleeve  24   b  and the photoconductor drum  1  move in opposite directions at a position where they face each other. 
     As shown in  FIG. 2 , for example, in the following order in the rotational direction of the first sleeve  23   b  from a position to which the developer  21  supplied from the stirrer transport member  27  attaches, the magnetic poles provided in the first magnet roller  23   a  included in the first developing roller  23  include an attachment pole S 1  to which the supplied developer  21  attaches, a delivery pole N 2  that is magnetized at a position facing the second developing roller  24  and delivers the developer  21  supported by the first sleeve  23   b  to the second sleeve  24   b , a development pole S 3  magnetized at a position facing the photoconductor drum  1 , a transport pole N 4  that transports the developer  21  by attaching it to the outer peripheral surface of the first sleeve  23   b , and a removal pole S 5  provided adjacent to the attachment pole S 1  and having the same polarity as the attachment pole S 1 . 
     The second magnet roller  24   a  included in the second developing roller  24  similarly has five magnetic poles in the circumferential direction thereof. Specifically, in the following order in the rotational direction of the second sleeve  24   b  from a position facing the first developing roller  23 , the five magnetic poles include a reception pole S 6  magnetized at a position facing the first developing roller  23  and receiving the developer  21  from the first magnet roller  23   a , a development pole N 7  that orients the developer  21  toward the photoconductor drum  1  at a position facing the photoconductor drum  1 , a transport pole S 8  that transports the developer  21  by attaching it to the outer peripheral surface of the second sleeve  24   b , and two removal poles N 9  and N 10  magnetized to the same polarity and spaced apart from each other in the circumferential direction so as to remove the developer  21  therefrom by utilizing repulsive magnetic fields. 
     The aforementioned magnetic poles S 1  to S 8  are S-poles, whereas the aforementioned magnetic poles N 2  to N 10  are N-poles. 
     An AC superimposed on DC voltage is applied as a development bias voltage to each of the developing rollers  23  and  24 . In this exemplary embodiment, a DC voltage of −650 V and an AC voltage of 1000 V (i.e., a peak-to-peak voltage of 2000 V) are applied in a superimposed manner respectively from a DC power source  35  and an AC power source  36  to each of the magnet rollers  23   a  and  24   a  (the voltage applied to the first developing roller  23  is not shown in the drawings). 
     In this exemplary embodiment, the above voltages are applied to the magnet rollers  23   a  and  24   a . Alternatively, the sleeves  23   b  and  24   b  may be provided with electrically-conductive layers, and the above voltages may be applied to the electrically-conductive layers. 
     The layer regulating member  29  is a tabular member disposed such that an edge thereof faces the outer peripheral surface of the first sleeve  23   b , and regulates the amount of developer  21  that is moved while being attached to the first sleeve  23   b . The layer regulating member  29  is disposed downstream of a position where the developer  21  is supplied to the first developing roller  23  from the first stirrer transport member  27 , as viewed in the moving direction of the outer peripheral surface of the first sleeve  23   b.    
     The distributing member  30  protrudes from the downstream side in the rotational direction of the developing rollers  23  and  24  into a gap formed where the first developing roller  23  and the second developing roller  24  face each other, and extends continuously along the axes of the developing rollers  23  and  24 . An edge  30   a  of the distributing member  30  that protrudes into an opposed area  34  between the first developing roller  23  and the second developing roller  24  distributes the developer  21  linked between the first sleeve  23   b  and the second sleeve  24   b  toward the first developing roller  23  and the second developing roller  24 . 
     Although the developer  21  transported to the opposed area  34  by the first developing roller  23  is distributed to the first developing roller  23  and the second developing roller  24  in this exemplary embodiment, the distribution ratio may be changed where necessary. 
     The guide member  32  is a tabular member whose edge  32   a  is disposed facing the outer peripheral surface of the second sleeve  24   b  in an area where the repulsive magnetic fields generated by the two removal poles N 9  and N 10  provided in the second magnet roller  24   a  are effective. The guide member  32  guides the developer  21  removed from the second sleeve  24   b  along the planar surface thereof and introduces the developer  21  to an area within the accommodation housing  22  where the developer  21  is stirred by the first stirrer transport member  27 . 
     The electrode member  31  is a roller member composed of an electrically-conductive material and is rotatably supported about an axis thereof. The electrode member  31  extends in the axial direction of the second sleeve  24   b  at a position downstream of the development pole N 7  and upstream of the reception pole S 6  in the rotational direction of the second sleeve  24   b , and faces the second sleeve  24   b  with a certain distance therebetween. 
     In this exemplary embodiment, the electrode member  31  is disposed so as to face the transport pole S 8  provided downstream of the development pole N 7  and upstream of a position where the guide member  32  is disposed. 
     The distance between the second sleeve  24   b  and the electrode member  31  may range between 200 μm and 2000 μm, so that the distance may be substantially equal to the distance between the second sleeve  24   b  and the photoconductor drum  1  at a position where they face each other, that is, the development position. 
     A DC power source  37  applies voltage to the electrode member  31 , and the electric potential is set between, for example, −100 V and −800V. The electric potential set for the electrode member  31  may be changeable by using a variable resistor, a switch, or the like. In this exemplary embodiment, a switch  38  is used to switch between −300 V and −800 V so as to apply the voltage to the electrode member  31 . By changing the electric potential of the electrode member  31 , an electric field acting in a different direction with a different intensity is generated between the electrode member  31  and the second developing roller  24 . 
     The following description relates to how the toner, the magnetic carrier, and the external additive supplied onto the first sleeve  23   b  and the second sleeve  24   b  behave in each of the developing devices  20  having the above-described configuration. 
     The developer  21  stirred by the stirrer transport members  27  and  28  becomes attached onto the first sleeve  23   b  due to the effect of the attachment pole S 1  of the first magnet roller  23   a , and is transported as the first sleeve  23   b  rotates in the direction of the arrow C shown in  FIG. 2 . The layer regulating member  29  is disposed downstream of the attachment pole S 1  and regulates the layer thickness of the developer  21  supported on the first sleeve  23   b . Specifically, the amount of developer  21  transported on the first sleeve  23   b  is adjusted. 
     The developer  21 , whose layer thickness has been regulated, on the first sleeve  23   b  reaches the opposed area  34  where the first sleeve  23   b  and the second sleeve  24   b  face each other as the first sleeve  23   b  rotates. In the opposed area  34 , the delivery pole N 2  of the first magnet roller  23   a  and the reception pole S 6  of the second magnet roller  24   a  face each other, such that the magnetic carrier is linked between these magnetic poles having different polarities, whereby the developer  21  is supported and bridged between the two poles. 
     The distributing member  30  is disposed in the opposed area  34 . The edge  30   a  of the distributing member  30  abuts on the developer  21  with the magnetic carrier linked between the first sleeve  23   b  and the second sleeve  24   b  so as to distribute the developer  21  toward the first sleeve  23   b  and the second sleeve  24   b.    
     The developer  21  delivered to the second sleeve  24   b  from the first sleeve  23   b  in this manner is transported as the second sleeve  24   b  rotates, thereby reaching a position facing the photoconductor drum  1 . 
     In a development region facing the photoconductor drum  1 , the magnetic field of the development pole N 7  magnetized by the second magnet roller  24   a  causes magnetic carrier particles  41  to form chains, as shown in  FIG. 3 , and the development bias voltage applied between the photoconductor drum  1  and the second magnet roller  24   a  causes toner particles  42  adhered to the chained magnetic carrier particles  41  on the second sleeve  24   b  to transfer to the image area, that is, a latent image, on the photoconductor drum  1 . Specifically, an electric field generated between the image area (i.e., an area to which the toner particles  42  are to be adhered) in which the electric potential is attenuated to −400 V due to the photoconductor drum  1  being exposed to light and the second sleeve  24   b  receiving the AC voltage superimposed on the DC voltage of −650 V causes the negatively-charged toner particles  42  to transfer to the latent image on the photoconductor drum  1 . Therefore, the toner concentration in the developer  21  on the second sleeve  24   b  facing this image area decreases. In this case, the magnetic carrier particles  41  are constrained by the magnetic field of the second magnet roller  24   a  so as to be retained on the second sleeve  24   b . Furthermore, the external additive particles  43  are positively charged so as to be pulled toward the second sleeve  24   b , and some of them adhere to the surface of the second sleeve  24   b.    
     On the other hand, the non-image area on the photoconductor drum  1  is not exposed to the light so that the electric potential thereof is maintained at −800 V, whereby an electric field in the opposite direction from that in the image area is generated between the non-image area and the second sleeve  24   b . Therefore, the negatively-charged toner particles  42  are pulled toward the second sleeve  24   b , and some of the toner particles  42  adhered to the magnetic carrier particles  41  become detached from the magnetic carrier particles  41  so as to adhere to the surface of the second sleeve  24   b . Furthermore, the positively-charged external additive particles  43  and magnetic carrier particles  41  receive a pulling force toward the photoconductor drum  1 , so that some of the external additive particles  43  transfer to the photoconductor drum  1 . On the other hand, the magnetic carrier particles  41  are constrained by the magnetic field of the second magnet roller  24   a  so as to be retained on the second sleeve  24   b.    
     Accordingly, with regard to the developer  21  on the second sleeve  24   b , the amount of toner decreases in the area that faces the image area at the development region. In the area that faces the non-image area, the amount of external additive decreases, and the number of toner particles  42  directly adhered to the second sleeve  24   b  increases. In this state, the developer  21  on the second sleeve  24   b  moves to the position facing the electrode member  31 . Since DC voltage is applied to the electrode member  31 , an electric field is generated between the electrode member  31  and the second sleeve  24   b.    
     The DC voltage applied to the electrode member  31  is changeable by switching. For example, when the electrode member  31  receives −300 V, the electric field generated between the electrode member  31  and the second sleeve  24   b  receiving the AC voltage superimposed on the DC voltage of −650 V acts in the same direction as that when the second sleeve  24   b  faces the image area on the photoconductor drum  1  at the development region, so that the toner particles  42  supported on the second sleeve  24   b  are pulled toward the electrode member  31 . Specifically, some of the toner particles  42  supported by the magnetic carrier particles  41  on the second sleeve  24   b  fly toward the electrode member  31  so as to adhere to the surface of the electrode member  31 . At the same time, some of the toner particles  42  adhered to the surface of the second sleeve  24   b  move away from the surface of the second sleeve  24   b  so as to transfer to the electrode member  31 , whereas some of the toner particles  42  are supported by the magnetic carrier particles  41 . The external additive particles  43  receive a force that pulls them toward the surface of the second sleeve  24   b . Thus, in the area that faces the non-image area on the photoconductor drum  1  at the development region and where a large number of toner particles  42  are adhered to the surface of the second sleeve  24   b , the number of toner particles  42  directly adhered to the surface of the second sleeve  24   b  decreases, whereas the number of external additive particles  43  adhered to the surface of the second sleeve  24   b  increases. On the other hand, in the area that faces the image area on the photoconductor drum  1  at the development region and where a large number of toner particles  42  are not adhered to the surface of the second sleeve  24   b , the toner particles  42  adhered to and remaining on the surface of the second sleeve  24   b  or the magnetic carrier particles  41  transfer to the electrode member  31 , so that the toner particles  42  adhered on the second sleeve  24   b  are maintained at a small amount. Consequently, a difference in the amount of toner and external additive adhered to the surface of the second sleeve  24   b  between the areas on the surface of the second sleeve  24   b  that face the image area and the non-image area on the photoconductor drum  1  may be reduced. 
     On the other hand, when the electrode member  31  receives a DC voltage of −800 V, the electric field generated between the electrode member  31  and the second sleeve  24   b  receiving the AC voltage superimposed on the DC voltage of −650 V acts similarly to the area facing the non-image area at the development region, so that the toner particles  42  supported on the second sleeve  24   b  are pulled toward the second sleeve  24   b . The external additive particles  43  receive a force that pulls them away from the second sleeve  24   b . Specifically, in the area that faces the image area on the photoconductor drum  1  at the development region and where a large number of toner particles  42  are not adhered to the surface of the second sleeve  24   b , some of the toner particles  42  supported by the magnetic carrier particles  41  above the second sleeve  24   b  are pulled toward the surface of the second sleeve  24   b , so that the number of toner particles  42  directly adhered to the surface of the second sleeve  24   b  increases. Moreover, in the area that faces the non-image area on the photoconductor drum  1  at the development region and where a large number of toner particles  42  are adhered to the surface of the second sleeve  24   b , the number of toner particles  42  directly adhered to the surface of the second sleeve  24   b  also increases. However, since the toner particles  42  near the surface of the second sleeve  24   b  are already adhered to the surface of the second sleeve  24   b , an increase in the number of toner particles  42  adhered to the surface of the second sleeve  24   b  is smaller than that in the area that faces the image area on the photoconductor drum  1  at the development region. Therefore, a difference in the number of toner particles  42  adhered to the surface of the second sleeve  24   b  between the area facing the non-image area and the area facing the image area on the photoconductor drum  1  at the development region may be reduced. Furthermore, in the area facing the image area on the photoconductor drum  1  at the development region, the number of external additive particles  43  adhered to the surface of the second sleeve  24   b  decreases, so that a difference in the number of external additive particles  43  adhered to the surface of the second sleeve  24   b  between the area facing the non-image area and the area facing the image area on the photoconductor drum  1  at the development region is similarly reduced. 
     Subsequently, the second sleeve  24   b  reaches the position where the removal pole N 9  is provided. The removal pole N 10  having the same polarity as the removal pole N 9  is provided downstream thereof such that repulsive magnetic fields are generated therebetween. Thus, the magnetic carrier particles  41  are released and removed from the second sleeve  24   b  together with the toner particles  42  and the external additive particles  43  adhered to the magnetic carrier particles  41 . The guide member  32  is disposed such that the edge  32   a  thereof protrudes to this position. Thus, the removed developer  21 , that is, the magnetic carrier particles  41  having the toner particles  42  and the external additive particles  43  adhered thereto, moves along the guide member  32  so as to be returned to the area where the first stirrer transport member  27  is driven. Then, the surface of the second sleeve  24   b  from which the magnetic carrier particles  41 , having the toner particles  42  and the external additive particles  43  adhered thereto, are removed moves again to the opposed area  34  between the second sleeve  24   b  and the first sleeve  23   b . In the opposed area  34 , the developer  21  on the first sleeve  23   b  is distributed so as to be used for forming a toner image at the development region where each sleeve faces the photoconductor drum  1 . 
     Although the magnetic carrier particles  41  having the toner particles  42  and the external additive particles  43  adhered thereto are removed at the position facing the guide member  32 , as described above, many of the toner particles  42  and the external additive particles  43  directly adhered to the surface of the second sleeve  24   b  remain on the second sleeve  24   b . With regard to the residual toner particles  42  and external additive particles  43 , the differences in the amounts thereof adhered to the surface of the second sleeve  24   b  between the area facing the non-image area and the area facing the image area on the photoconductor drum  1  when previously passing through the development region are reduced, so that unevenness in density of an image to be developed when subsequently passing through the development region may be reduced. 
     If the aforementioned electrode member  31  is not provided, the toner particles  42  and the external additive particles  43  unevenly adhered to the surface of the second sleeve  24   b  by passing through the development region would be transported to the position provided with the removal pole N 9 . When the magnetic carrier particles  41  are subsequently removed due to the repulsive magnetic fields, the toner particles  42  and the external additive particles  43  directly adhered to the surface of the second sleeve  24   b  would remain thereon without being removed therefrom. With regard to the distribution of residual toner particles  42  and external additive particles  43 , the unevenness occurring based on the image on the photoconductor drum  1  facing the second sleeve  24   b  at the development region may possibly be maintained. If the developer  21  is supplied again to the opposed position between the first sleeve  23   b  and the second sleeve  24   b  and is transported to the development region while such unevenness remains, unevenness in density based on the opposing image in the previous rotation may occur in a subsequent image to be developed. 
     In contrast, in the developing device  20  described above, unevenness in the amount of toner and external additive directly adhered to the surface of the second sleeve  24   b  may be reduced at the position facing the electrode member  31  having received the DC voltage of −300 V or −800 V, thereby reducing unevenness in density of a subsequent image to be developed. 
     With regard to the first sleeve  23   b  after delivering a portion of the developer  21  to the second sleeve  24   b  at the opposed area  34  between the first sleeve  23   b  and the second sleeve  24   b , the outer peripheral surface thereof rotates so as to transport the developer  21  to the position facing the photoconductor drum  1 . Then, the first sleeve  23   b  transfers the toner to the latent image on the photoconductor drum  1 , so that the latent image is developed. The first sleeve  23   b  supporting the developer  21  containing the residual toner and external additive after the developing process continues to rotate so that the developer  21  remaining on the first sleeve  23   b  is removed therefrom at the removal pole S 5 . The removed developer  21  is returned to the operational area of the first stirrer transport member  27  where the developer  21  and the other developer therein are stirred together. Subsequently, the developer  21  is supplied again onto the first sleeve  23   b  at the position where the attachment pole S 1  is provided. 
     When the developer  21  supported on the first sleeve  23   b  passes through the development region where the photoconductor drum  1  and the first sleeve  23   b  face each other, the toner particles transfer to the image area, and the toner particles and the external additive particles adhere to the surface of the first sleeve  23   b , similarly to when the developer  21  passes through the region where the photoconductor drum  1  and the second sleeve  24   b  face each other. However, a portion of the developer  21  attached to the first sleeve  23   b  is retained at the upstream side of the position facing the layer regulating member  29  and is rubbed against the outer peripheral surface of the first sleeve  23   b , as well as being stirred. Therefore, unevenness in toner particles and external additive particles occurring when passing through the development region may be eliminated, thereby reducing the occurrence of the image history appearing in the subsequent image. 
     The aforementioned DC voltage applied to the electrode member  31  may be changed by switching the switch  38 , and this switching operation may be performed on the basis of predetermined conditions. For example, after the latent image formed on the photoconductor drum  1  passes through a region that faces the second developing roller  24 , that is, the development region, the switching may be performed before a subsequent latent image reaches the development region. When developing the latent image on the photoconductor drum  1  by transferring toner thereto, a voltage of −300 V is applied to the electrode member  31 , and the voltage is switched to −800 V after this latent image has passed the development region. Then, the voltage is switched back to −300 V before the subsequent latent image arrives. In other words, the voltage of −300 V is used for image formation, whereas the voltage of −800 V is used when not forming an image. 
     By periodically switching the voltage to be applied to the electrode member  31  in the order: −300 V, −800 V, −300 V, and −800 V, the direction of the electric field generated between the electrode member  31  and the second sleeve  24   b  is repeatedly inverted so that the toner-pulling direction is changed. Thus, a continuous increase in the amount of toner retained on the surface of the electrode member  31  may be prevented. Furthermore, since the toner particles  42  may be prevented from being retained on the second sleeve  24   b  or the electrode member  31  over a long period of time, toner fixation may be suppressed. 
     The voltage applied to the electrode member  31  may be the same as the DC component of the voltage applied to the second developing roller  24  as a development bias voltage. Specifically, as shown in  FIG. 4A , a DC voltage of −650 V may be applied to the electrode member  31  from the DC power source  35  used for applying the development bias voltage. When the voltage is applied in this manner, the toner particles  42  supported on the second sleeve  24   b  does not receive a pulling force toward the electrode member  31  or the second sleeve  24   b . However, the AC component of the development bias voltage applied to the second developing roller  24 , that is, an AC voltage of 1000 V, causes the toner particles  42  to vibrate on the second sleeve  24   b  toward and away from the surface of the second sleeve  24   b . Thus, some of the toner particles  42  adhered to the surface of the second sleeve  24   b  adhere to the magnetic carrier particles  41 . Consequently, the toner particles  42  are removed from the second sleeve  24   b  together with the magnetic carrier particles  41  at the position where the removal pole N 9  is provided, whereby the amount of toner directly adhered to the surface of the second sleeve  24   b  decreases. Therefore, unevenness in the amount of toner remaining on the second sleeve  24   b  after the magnetic carrier particles  41  are removed therefrom may be reduced, whereby unevenness in density of a subsequent image to be developed may be reduced. 
     When a voltage that is the same as the DC component of the development bias voltage applied to the second developing roller  24  is applied to the electrode member  31  in this manner, the toner may be prevented from being retained on the electrode member  31  or fixed on the electrode member  31  and the second sleeve  24   b.    
     Furthermore, referring to  FIG. 4B , the voltage that is the same as the DC component of the development bias voltage may be switched to a voltage lower than the aforementioned voltage or a voltage higher than the aforementioned voltage by using a switch  39  before being applied to the electrode member  31 . 
     Referring to  FIG. 5 , a cleaning member, such as a cleaning brush  51 , may be disposed in contact with the peripheral surface of the electrode member  31 . With such a cleaning member, the toner or the external additive adhered to the surface of the electrode member  31  can be scraped off so as to be returned to the developer layer formed on the second sleeve  24   b . Therefore, the toner and the external additive may be prevented from being retained on the electrode member  31 . 
     As an alternative to the above exemplary embodiment in which a single electrode member  31  is provided, multiple electrode members  52  and  53  that receive different DC voltages may be provided, as shown in  FIG. 6 . For example, when a voltage of −300 V is applied to the first electrode member  52  disposed at the upstream side in the rotational direction of the second sleeve  24   b  and a voltage of −800 V is applied to the second electrode member  53  disposed at the downstream side, the toner on the second sleeve  24   b  is pulled toward the first electrode member  52 , and the external additive on the second sleeve  24   b  is pulled toward the second electrode member  53 . Thus, unevenness in the toner and the external additive supported on the second sleeve  24   b  may be reduced. 
     The voltages applied to the first electrode member  52  and the second electrode member  53  may be switched by using switches  54  and  55  in accordance with predetermined conditions. For example, the voltage for the first electrode member  52  previously receiving −300 V may be switched to −800 V, and the voltage for the second electrode member  53  previously receiving −800 V may be switched to −300 V, thereby inverting the directions of the electric fields. Such switching of the voltages may be performed, for example, every time an image is to be formed on a single sheet. 
       FIG. 7  schematically illustrates a developing device according to a second exemplary embodiment of the present invention. 
     A developing device  60  uses a guide member  61  in place of the guide member  32  in the first exemplary embodiment. Specifically, the guide member  61  has the same shape as the guide member  32  but has an additional function of an electrode member by receiving DC voltage. Because the configuration of the developing device  60  is similar to the developing device  20  according to the first exemplary embodiment, the following description will be directed to the guide member  61 . The remaining components of the developing device  60  will be given the same reference numerals as in the first exemplary embodiment, and descriptions of such components will be omitted. 
     The guide member  61  in the developing device  60  according to this exemplary embodiment is similar to that in the first exemplary embodiment in that the guide member  61  is disposed in an area where the repulsive magnetic fields generated by the two removal poles N 9  and N 10  provided in the second magnet roller  24   a  are effective. The DC power source  37  applies DC voltage to the guide member  61 , and the switch  38  is used to switch between −300 V and −800 V so as to apply the voltage to the guide member  61 . 
     When a DC voltage of, for example, −300 V is applied to the guide member  61 , the toner directly adhered to the surface of the second sleeve  24   b  is pulled toward the guide member  61  so as to transfer toward the guide member  61  together with the magnetic carrier removed from the second sleeve  24   b  due to the repulsive magnetic fields generated by the removal poles N 9  and N 10 . Thus, the amount of toner directly adhered to the surface of the second sleeve  24   b  decreases, whereby unevenness in the amount of toner adhered to the surface of the second sleeve  24   b  may be reduced. 
     On the other hand, when −800 V is applied to the guide member  61 , the toner is pulled toward the second sleeve  24   b  and becomes detached from the magnetic carrier removed therefrom due to the effect of the removal poles N 9  and N 10 , thereby adhering to the surface of the second sleeve  24   b . Thus, in the area previously facing the image area on the photoconductor drum  1  at the development region, the amount of toner adhered to the surface of the second sleeve  24   b  increases. Since the toner is already adhered to the surface of the second sleeve  24   b  in the area previously facing the non-image area on the photoconductor drum  1  at the development region, the amount of toner that is to be additionally adhered onto the second sleeve  24   b  is smaller than that in the area facing the image area. Therefore, a difference in the amount of toner adhered to the surface of the second sleeve  24   b  between the area facing the non-image area and the area facing the image area on the photoconductor drum  1  at the development region may be reduced. 
     Similar to the first exemplary embodiment, a DC voltage of −650 V may be applied to the guide member  61  functioning as an electrode member. 
       FIG. 8  schematically illustrates a developing device according to a third exemplary embodiment of the present invention. 
     A developing device  70  differs from that in the first exemplary embodiment in terms of the position of an electrode member  71 , but is similar to the first exemplary embodiment in terms of the remaining components excluding the electrode member  71 . Therefore, the following description will be directed to the electrode member  71 . The remaining components will be given the same reference numerals as in the first exemplary embodiment, and descriptions of such components will be omitted. 
     In this exemplary embodiment, the electrode member  71  is disposed downstream of the guide member  32  in the rotational direction of the second sleeve  24   b  and upstream of the reception pole S 6  magnetized by the second magnet roller  24   a . The shape of the electrode member  71  may be the same as that in the first exemplary embodiment. Furthermore, the DC voltage to be applied is set such that an electric field that causes the toner on the second sleeve  24   b  to be pulled toward the electrode member  71  or an electric field that causes the toner on the electrode member  71  to be pulled toward the second sleeve  24   b  is generated. For example, the DC voltage to be applied may be switched between −300 V and −800 V. 
     In this developing device  70 , the distribution of the toner or the external additive adhered to the surface of the second sleeve  24   b  after passing through the development region is uneven, as shown in  FIG. 3 . When the developer in this state reaches the position where the removal pole N 9  is provided, a large amount of toner and external additive is removed together with the magnetic carrier. However, a large amount of toner directly adhered to the surface of the second sleeve  24   b  remains thereon in an uneven state without being removed therefrom. As the second sleeve  24   b  further rotates so as to face the electrode member  71  receiving −300 V in a state where the magnetic carrier is removed from the second sleeve  24   b  while the toner remains thereon, the toner adhered thereto is pulled toward the electrode member  71  so as to adhere to the electrode member  71 . Consequently, the amount of toner adhered on the surface of the second sleeve  24   b  decreases, whereby unevenness in the amount of toner adhered thereto may be reduced. 
     With regard to the toner transferred to the electrode member  71 , for example, the DC voltage applied to the electrode member  71  is switched to −800 V when the latent image on the photoconductor drum  1  is not being developed, so that the toner can be returned onto the second sleeve  24   b . Alternatively, a cleaning member may be provided for scraping off the toner. 
     The above exemplary embodiments of the present invention are not limited thereto and may be implemented as other exemplary embodiments so long as they are within the scope thereof. 
     For example, the number and the arrangement pattern of magnetic poles provided in the first magnet roller and the second magnet roller are not limited to those in the above exemplary embodiments. Furthermore, the number of developing rollers is not limited to two, and may be three or more. Moreover, the rotational direction of the developing rollers may be changed. 
     Furthermore, the development bias voltage applied to each developing roller may be set to various values depending on the characteristics of the developing device. Moreover, the DC voltage or voltages applied to the electrode member or members may be set in correspondence with the aforementioned development bias voltage or voltages. Furthermore, as an alternative to inter-switching the DC voltage between a voltage higher than the DC component of the development bias voltage applied to the second developing roller and a voltage lower than the DC component, the DC voltage may be switched between multiple voltages including substantially the same voltage as the DC component of the development bias voltage. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.