Patent Publication Number: US-7221892-B2

Title: Developing device used in image forming device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a developing device provided with a removing member that removes residual nonmagnetic single-component developer from a developer-carrying member, and an image-forming device including the developing device. 
   2. Related Art 
   One type of developing device well known in the art includes a developer-carrying member that develops an electrostatic latent image formed on an image-carrying member by supplying a charged nonmagnetic single-component developer to the surface of the image-carrying member, and a supplying member that supplies developer to the developer-carrying member. In this type of developing device, an image formation is performed in an electrophotographic method. That is, the supplying member supplies a nonmagnetic single-component developer, such as toner, accommodated in a toner-accommodating chamber or the like to the developer-carrying member while tribocharging the developer. A thickness-regulating member, such as a blade or the like, regulates the developer at a uniform thin layer. When the developer reaches a position across from the image-carrying member, the developer carried on the developer-carrying member is selectively deposited on an electrostatic latent image formed on the image-carrying member. Then, the nonmagnetic single-component developer deposited on the image-carrying member is transferred onto a recording medium, such as a recording paper. 
   However, a problem caused by what is called sleeve ghosting can occur when some of the developer is not supplied to the image-carrying member for developing the electrostatic latent image and remains on the surface of the developer-carrying member after passing by the position opposing the image-carrying member. Thereupon, Japanese unexamined patent application publication No. HEI-9-236979 proposes to provide a removing member on the upstream side of the supplying member in the rotational direction of the developer-carrying member to remove the developer that is not supplied for developing the latent image but remains on the surface of the developer-carrying member. The occurrence of sleeve ghosting can be satisfactorily prevented in this case since the developer is supplied from the supplying member to the developer-carrying member after first removing the residual developer from the developer-carrying member. 
   However, in the device disclosed in Japanese unexamined patent application publication No. HEI-9-236979, the developer-carrying member is rotated so that the peripheral surface of the developer-carrying member opposing the supplying member moves from bottom to top in relation to the direction of gravity, while the removing member is rotated such that the peripheral surface of the removing member opposing the developer-carrying member moves from top to bottom. Further, the removing member is disposed at the bottom section of a developing cartridge. Accordingly, toner removed by the removing member collects in a space between the removing member and a bottom surface of an outer case of the developing cartridge, which can have an adverse effect on toner circulation. A worsening in toner circulation can lead to such problems as a drastic reduction in image quality in parts of the formed image. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to overcome the above problems and also to provide a developing device including a removing member that removes nonmagnetic single-component developer remaining on the surface of a developer-carrying member that was not supplied for developing an electrostatic latent image. It is another object of the present invention to provide an image-forming device including the developing device that can effectively circulate developer removed by the developing device. 
   In order to achieve the above and other objects, the present invention provides a developing device including a developer-carrying member that conveys a charged nonmagnetic single-component developer to a surface of an image-carrying member, a supplying member that supplies a developer to the developer-carrying member, and a removing member that removes a charged nonmagnetic single-component developer remaining on a peripheral surface of the developer-carrying member that was not supplied to the image-carrying member. The developer-carrying member rotates in a rotational direction such that the peripheral surface of the developer-carrying member opposing the supplying member moves vertically downward. The removing member is positioned vertically above the supplying member and upstream of the supplying member in the rotational direction of the developer-carrying member. 
   There is also provided a developing device including a developer-carrying member that conveys a charged nonmagnetic single-component developer to a surface of an image-carrying member, a supplying member that supplies a developer to the developer-carrying member, and a removing member that removes a nonmagnetic single-component developer remaining on a peripheral surface of the developer-carrying member that was not supplied to the image-carrying member. The removing member is positioned upstream of the supplying member in the rotational direction of the developer-carrying member. The removing member rotates such that a peripheral surface of the removing member opposing the developer-carrying member moves in the same direction as the peripheral surface of the developer-carrying member opposing the removing member while in contact with the peripheral surface of the developer-carrying member. 
   There is also provided an image forming apparatus including an image-carrying member, a developer-carrying member that conveys a charged nonmagnetic single-component developer to a surface of the image-carrying member, a supplying member, formed of a conductive material, that supplies a developer to the developer-carrying member, a removing member, formed of a conductive material, that removes a charged nonmagnetic single-component developer remaining on a peripheral surface of the developer-carrying member that was not supplied to the image-carrying member, and a power source. The developer-carrying member rotates in a rotational direction such that the peripheral surface of the developer-carrying member opposing the supplying member moves vertically downward. The removing member is positioned vertically above the supplying member and upstream of the supplying member in the rotational direction of the developer-carrying member. A bias is applied by the power source to between the removing member and the developer-carrying member so as to attract the charged nonmagnetic single-component developer from the peripheral surface of the developer-carrying member to the removing member. A bias is applied by the power source to between the supplying member and the developer-carrying member so as to attract the charged nonmagnetic single-component developer from the supplying member to the developer-carrying member. 
   There is also provided an image forming apparatus including an image-carrying member, a developer-carrying member that conveys a charged nonmagnetic single-component developer to a surface of the image-carrying member, a supplying member, formed of a conductive material, that supplies a developer to the developer-carrying member, a removing member, formed of a conductive material, that removes a nonmagnetic single-component developer remaining on a peripheral surface of the developer-carrying member that was not supplied to the image-carrying member, and a power source. The removing member is positioned upstream of the supplying member in the rotational direction of the developer-carrying member. The removing member rotates such that a peripheral surface of the removing member opposing the developer-carrying member moves in the same direction as the peripheral surface of the developer-carrying member opposing the removing member while in contact with the peripheral surface of the developer-carrying member. The power source applies a bias to between the removing member and the developer-carrying member so as to attract the electrically-charged nonmagnetic single-component developer from the developer-carrying member to the removing member. The power source applies a bias to between the supplying member and the developer-carrying member so as to attract the electrically-charged nonmagnetic single-component developer from the supplying member to the developer-carrying member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a side cross-sectional view showing the general construction of a color laser printer according to a first embodiment of the present invention; 
       FIG. 2  is an enlarged view showing the construction of a developing device in the color laser printer of  FIG. 1 ; and 
       FIG. 3  is an enlarged view showing the construction of a developing device according to a second embodiment of the present invention. 
   

   PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
   A developing device and an image-forming device according to preferred embodiments of the present invention will be described while referring to the accompanying drawings. A first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 .  FIG. 1  is a side cross-sectional view showing the general construction of a color laser printer  1 , which serves as the image-forming device to which the present invention is applied. The printer  1  shown in  FIG. 1  includes a visible-image forming unit  4 , a belt-shaped intermediate transfer member  5 , a fixing unit  9 , a paper supply unit  9 , and a discharge tray  10 . 
   For each step in forming visible images with toner of the colors magenta (M), cyan (C), yellow (Y), and black (Bk), the visible-image forming unit  4  includes developing units  51 M,  51 C,  51 Y, and  51 Bk (collectively referred to as “developing units  51 ”), photosensitive drums  3 M,  3 C,  3 Y, and  3 Bk (collectively referred to as “photosensitive drums  3 ”), cleaning rollers  70 M,  70 C,  70 Y, and  70 Bk (collectively referred to as “cleaning rollers  70 ”), “charging units  71 M,  71 C,  71 Y, and  71 Bk (collectively referred to as charging units  71 ”), and exposure devices  72 M,  72 C,  72 Y, and  72 Bk (collectively referred to as “exposure devices  72 ”). 
   The aforementioned components will be described in greater detail. While the developing unit  51 M is shown as an example in  FIG. 2 , the constructions of the developing units  51 C,  51 Y, and  51 Bk are identical. 
   First, the developing unit  51 M will be described. It should be noted that since the developing units  51 M,  51 C,  51 Y, and  51 Bk are identical, only the developing unit  51 M will be described, and description of developing units  51 C,  51 Y, and  51 Bk will be omitted to avoid duplication in explanation. 
   The developing unit  51 M includes a developing roller  52 M and a supply roller  53 M. The developing roller  52 M is formed in a cylindrical shape with a conductive silicon rubber as the base material, the surface of which is coated with a resin or a rubber material containing fluorine. However, the developing roller  52 M need not be configured of a conductive silicon rubber as the base material, but instead may be configured of a conductive urethane rubber. The average roughness (Rz) at ten points on the surface of the developing roller  52 M should be set to 3–5 μm in order to be smaller than the average particle size of toner, which is 9 μm. 
   The supply roller  53 M is formed of a conductive sponge roller and is configured to contact the developing roller  52 M with pressure applied by the elastic force of the sponge. The supply roller  53 M can be configured of an appropriate foam member formed of a conductive silicon rubber, EPDM, or urethane rubber. 
   The developing unit  51 M also includes a thickness-regulating blade  54 M and a removing roller  56 M. As shown in  FIG. 2 , the thickness-regulating blade  54 M includes a support part  54   a M and a contact part  54   b M. The support part  54   a M is formed of stainless steel with the base end fixed to a developing case  55 M. The contact part  54   b M is formed of an insulating silicon rubber or an insulating rubber or synthetic resin containing fluorine. The contact part  54   b M is provided on the other end of the support part  54   a M extending above the base end. The contact part  54   b M contacts the developing roller  52 M from the bottom side with pressure applied by the elastic force of the support part  54   a M. 
   The removing roller  56 M contacts the developing roller  52 M at a position above the supply roller  53 M in the gravitational direction. The removing capacity of the removing roller  56 M described later can be improved by configuring the removing roller  56 M of a solid conductive roller or a metal roller. However, conductive foam, such as silicon or urethane, can also be used. 
   Toner accommodated in the developing case  55 M is a positively charging nonmagnetic single-component developer. The toner includes base toner particles having an average size of 9 μm. The base toner particles are formed by adding an additive, such as carbon black, well known in the art and a charge-controlling agent or charge-controlling resin, such as nigrosine, triphenylmethane, or quaternary ammonium salt, to a styrene-acrylic resin formed in a spherical shape through suspension polymerization. The toner is configured by adding silica to the surface of the base toner particles. The silica additive undergoes hydrophobing according to a process known in the art using a silane coupling agent, silicon oil, or the like. The average particle size of the silica is 10 nm, with the additive accounting for a 0.6% of the base toner particle weight. Toner of the colors magenta, cyan, yellow, and black are accommodated in the developing cases  55 M,  55 C,  55 Y, and  55 Bk, respectively. 
   The toner is a suspension polymerized toner very nearly spherical in shape. Also, the hydrophobed silica having an average particle size of 10 nm has been added to the particles at 0.6% weight therefore, the toner has excellent fluidity, and a sufficient charge amount Can be obtained by tribocharging. Further, since the toner has no sharp edges like coarsely ground toner, the particles are less affected by mechanical forces and readily follow the electric field, thereby achieving efficient transfer. 
   As shown in  FIG. 2 , the photosensitive drum  3 M rotates in the counterclockwise direction in the drawing, while the developing roller  52 M rotates in the clockwise direction. Further, the removing roller  56 M and the supply roller  53 M both rotate in the counterclockwise direction in the drawing. Hence, in the present embodiment, the developing rollers  52  ( 52 M,  52 C,  52 Y,  52 Bk), the supply rollers  53  ( 53 M,  53 C,  53 Y,  53 Bk) and the removing rollers  56  ( 56 M,  56 C,  56 Y,  56 Bk) rotate such that their peripheral surfaces at the points of contact move in the same direction. The velocity ratios of corresponding peripheral surfaces at the points of contact are set between 0.7 and 1.3. 
   DC power sources  57   a ,  57   b , and  57   c  are provided to apply biases to the developing rollers  52 , the supply rollers  53 , and the removing rollers  56 , respectively. The DC power source  57   a  applies a voltage V 1  to the developing rollers  52 . The DC power source  57   b  supplies a voltage V 2  to the supply rollers  53 . The DC power source  57   c  supplies a voltage V 3  to the removing rollers  56 . When the toner is charged to a positive polarity, the voltages V 1 , V 2 , and V 3  have the relationship V 3 &lt;V 1 &lt;V 2 . Hence, the positively charged toner is supplied from the supply rollers  53  to the developing rollers  52 . After the toner is formed in a uniform thin layer on the developing rollers  52  by the thickness-regulating blades  54  ( 54 M,  54 C,  54 Y,  54 Bk), the toner is supplied from the developing rollers  52  to electrostatic latent images described later that are formed on the photosensitive drums  3 . Toner remaining on the developing rollers  52  that was not supplied for developing the electrostatic latent images is scraped off the developing rollers  52  by the removing rollers  56 . 
   The developing case  55 M is provided with a scraper  58 M for scraping off toner deposited on the surface of the removing roller  56 M. The scraper  58 M can achieve sufficient scraping capacity when formed of a synthetic resin film, such as PET, a urethane rubber, or a urethane sponge of approximately 200 μm thick. 
   The photosensitive drums  3  are formed, for example, of an aluminum base covered by a positively charged photosensitive layer. The photosensitive layer is formed at a thickness of 20 μm or greater. Further, the aluminum base is used as a grounding layer. In the present embodiment, the photosensitive drums  3  rotate at a different velocity from the intermediate transfer member  5 . 
   Returning to  FIG. 1 , the cleaning rollers  70  are formed of conductive materials, such as a conductive sponge, and are disposed below the corresponding photosensitive drums  3  in sliding contact with the same. A power source not shown in the drawings applies a voltage of negative polarity, the opposite polarity from the toner, to the cleaning rollers  70 . The cleaning rollers  70  remove residual toner on the photosensitive drums  3  by the frictional force on the photosensitive drums  3  and the effects of the electric field generated by the above voltages. Since the present embodiment employs a cleanerless developing method, residual toner removed from the cleaning rollers  70  is once again returned to the photosensitive drums  3  within a prescribed cycle after the developing process has been completed. 
   The charging units  71  are Scorotron-type charging devices and confront the surfaces of the photosensitive drums  3  from the bottoms thereof at a position downstream from the cleaning rollers  70  in the rotational direction of the photosensitive drums  3 . Roller-type charging devices that contact the photosensitive drums  3  may also be used as the charging units  71 . 
   The exposure devices  72  are each configured of a laser scanner unit well known in the art and are positioned downstream from the charging units  71  in the rotational direction of the photosensitive drums  3  in order to irradiate laser light onto the surfaces of the photosensitive drums  3 . The exposure devices  72  irradiate laser light beams onto the surfaces of the photosensitive drums  3  based on image data so as to form electrostatic latent images for each color on the surfaces of the photosensitive drums  3 . 
   This construction effectively develops positively charged latent images formed on the photosensitive drums  3  with the positively charged toner according to a reverse developing method in which the positively-charged toner is attracted to positively-charged areas of the photosensitive drums  3  at points of contact between the developing rollers  52  and the photosensitive drums  3 , thereby forming an image of very high quality. 
   The intermediate transfer member  5  is a conductive sheet formed of polycarbonate, polyimide, or the like that is configured in a belt shape. As shown in  FIG. 1 , the intermediate transfer member  5  is looped around two drive rollers  60  and  62 . Intermediate transfer rollers  61 M,  61 C,  61 Y, and  61 Bk are disposed near positions opposing the photosensitive drums  3 M,  3 C,  3 Y, and  3 Bk. The surface of the intermediate transfer member  5  on the side opposing the photosensitive drums  3  moves vertically downward, as shown in  FIG. 1 . 
   A prescribed voltage is applied to the intermediate transfer rollers  61  in order to transfer toner deposited on the photosensitive drums  3  to the intermediate transfer member  5 . A secondary transfer roller  63  is disposed at the position in which the toner image is transferred to a paper P, that is, opposite the drive roller  62  disposed at the lower end of the intermediate transfer member  5 . A prescribed potential is applied to the secondary transfer roller  63 , so that a four-color toner image carried on the intermediate transfer member  5  is transferred onto the paper P. 
   As shown in  FIG. 1 , a cleaning unit  6  is disposed on the opposite side of the intermediate transfer member  5  from the photosensitive drums  3 . The cleaning unit  6  includes a scraping device  65  and a case  66 . Toner remaining on the intermediate transfer member  5  is scraped off by the scraping device  65  and accumulates in the case  66 . 
   The fixing unit  8  includes first and second heating rollers  81  and  82 . A paper P carrying a four-color toner image is heated and compressed by the first and second heating rollers  81  and  82  while being conveyed therebetween, thereby fixing the toner image to the paper P. 
   The paper supply unit  9  is disposed on the bottom of the printer  1  and includes a loading tray  91  for accommodating the stacked paper P and a pickup roller  92  for feeding the paper P. The paper supply unit  9  feeds the paper P at a prescribed timing in relation to the image forming process performed by the exposure devices  72 , the visible-image forming unit  4 , the photosensitive drums  3 , and the intermediate transfer member  5 . A pair of conveying rollers  100  conveys the paper P fed by the paper supply unit  9  to the nip point between the intermediate transfer member  5  and the secondary transfer roller  63 . 
   The discharge tray  10  is disposed at the top of the printer  1  and at the discharge end of the fixing unit  8 . The discharge tray  10  accommodates paper P discharged from the fixing unit  8  and conveyed by pairs of conveying rollers  101 ,  102 , and  103 . 
   In the present embodiment, a front cover  2  is configured to swing open about a shaft  20  in the direction indicated by an arrow in  FIG. 1 . By opening the front cover  2 , the developing units  51  can be easily replaced. 
   Next, the operations of the printer  1  according to the present embodiment will be described. First, the charging units  71  apply a uniform charge to the photosensitive layers on the photosensitive drums  3 . Next, these photosensitive layers are exposed to the exposure devices  72  based on the image data for the colors magenta, cyan, yellow, and black. The developing units  51 M,  51 C,  51 Y, and  51 Bk deposit magenta toner, cyan toner, yellow toner, and black toner on the electrostatic latent images formed on the photosensitive layers of the corresponding photosensitive drums  3 M,  3 C,  3 Y, and  3 Bk to develop the magenta, cyan, yellow, and black colors of the image. The toner images in magenta, cyan, yellow, and black that formed in this way are temporarily transferred onto the surface of the intermediate transfer member  5 . 
   Next, toner remaining on the photosensitive drums  3  following the transfer is temporarily retained by the cleaning rollers  70 . The toner image for each color is formed at slightly different times with consideration for the velocity of the intermediate transfer member  5  and the positions of the photosensitive drums  3  in order to superimpose the toner images of each color on the intermediate transfer member  5  during the transfer process. 
   The four-color toner image formed on the intermediate transfer member  5  as described above is transferred to the paper P fed from the paper supply unit  9  at the nip point between the secondary transfer roller  63  and the intermediate transfer member  5 . After the toner image is fixed to the paper P in the fixing unit  8 , the paper P is discharged onto the discharge tray  10 . Hence, a four-color image is formed on the paper P according to this process. 
   Further, in the developing units  51  described above, positively charged toner is supplied from the supply rollers  53  to the developing rollers  52  and, after the thickness-regulating blades  54  regulate the toner at a uniform thin layer, is supplied to the photosensitive drums  3  for developing electrostatic latent images formed thereon. Toner remaining on the developing rollers  52  that was not supplied for developing the latent images is subsequently stripped from the developing rollers  52  by the removing rollers  56 . 
   The removing rollers  56  are disposed upstream of the supply rollers  53  in the rotational direction of the developing rollers  52  and are disposed higher than the supply rollers  53  while overlapping the same vertically (see  FIG. 2 ). A portion of the toner stripped by the removing rollers  56  falls down toward the supply rollers  53  and is again supplied to the developing rollers  52  by the supply rollers  53 . Accordingly, the printer  1  of the present embodiment can effectively circulate toner. 
   That is, when the stripped toner collects in portions of the developing cases  55  ( 55 M,  55 C,  55 Y,  55 Bk), the quality in parts of the formed images may decline radically. However, by circulating the toner as described above in the present embodiment, it is possible to form high-quality images. Moreover, since the Scrapers  58  ( 58 M,  58 C,  58 Y,  58 Bk) scrape off toner adhering to the surfaces of the removing rollers  56  in the present embodiment, removal of toner by the removing rollers  56  can be performed more effectively and stably, enabling the removing rollers  56  to maintain a stable toner removing capability to further suppress the generation of sleeve ghosting. 
   The scrapers  58  are disposed such that the point of contact between the scrapers  58  and the removing rollers  56  is above and vertically overlapping the supply rollers  53 . Accordingly, toner removed from the removing rollers  56  by the scrapers  58  falls down toward the supply rollers  53  and is again supplied to the developing rollers  52 . Accordingly, the printer  1  of the present embodiment can more effectively circulate toner. 
   In the present embodiment, the removing rollers  56  and developing rollers  52  rotate such that their peripheral surfaces at the point of confrontation move in the same direction while contacting one another. Further, velocity ratios of peripheral surfaces on each of the developing rollers  52 , the supply rollers  53 , and the removing rollers  56  opposing one another are set at 0.7–1.3. Accordingly, the frictional force applied to the toner is not large, effectively suppressing the degradation of toner, such as degradation caused by the additives becoming embedded in the base toner particles. (The frictional force applied to the toner increases as the velocity ratio of the contacting surfaces on the removing rollers  56  and the developing rollers  52  becomes farther from 1.) Moreover, the removing rollers  56  are configured of a conducting material, and biases are applied to all of the rollers  52 ,  53 , and  56  to attract toner from the developing rollers  52  onto the removing rollers  56 . Accordingly, toner on the developing rollers  52  can be stripped by the removing rollers  56  using an electrostatic force. Since opposing peripheral surfaces  20  of the removing rollers  56  and the developing rollers  52  move in the same direction, as described above, residual toner can be effectively removed, further suppressing the generation of sleeve ghosting, further decreasing the frictional force and the like that must be applied to remove residual toner, and thereby suppressing the degradation of toner. Further, toner can be smoothly and effectively supplied from the supply rollers  53  to the developing rollers  52 , effectively preventing the blurring or fading of images and the like. 
   Further, according to the present embodiment, the surface of the intermediate transfer member  5  opposing the photosensitive drums  3  moves vertically downward, while the secondary transfer roller  63  transfers the toner image to the paper P at the bottom end of the intermediate transfer member  5 . Accordingly, the paper supply unit  9  can be disposed in the bottom section of the printer  1 , reducing the footprint of the printer  1 . Moreover, when positioning the paper supply unit  9  in the bottom section of the printer  1 , the printer  1  has advantages over devices that provide cassette-type units in the top of the device, for example, as in the ease of loading the paper P. Further, as shown in  FIG. 1 , the conveying path of the paper P to the secondary transfer roller  63  can be shortened, thereby shortening the time required to complete the printing operation. 
   Further, since toner in the present embodiment is formed through a polymerization method, as described above, the toner has excellent fluidity, thereby more effectively preventing the accumulation of toner. Moreover, since the polymer toner has excellent fluidity, there is little decline in image quality, even when performing two transfer processes using the intermediate transfer member  5  of the present embodiment. Since there is less residual toner after a transfer using polymer toner, such residual toner can be reliably removed when using a cleanerless developing system. Use of a cleanerless developing system also eliminates the space required for a waste toner receptacle, thereby reducing the overall size of the printer  1 . 
   Further, in the present embodiments the paper supply unit  9 ; the processing unit including the intermediate transfer member  5 , the photosensitive drums  3 , and the developing units  51 ; and the discharge tray  10  can be disposed in a vertically overlapping configuration in the order of the paper supply unit  9 , the processing unit, and the discharge tray  10  from bottom to top. Accordingly, parts protruding from the main body of the printer  1  can be eliminated thereby reducing the footprint of the printer  1 . 
     FIG. 3  is an enlarged view showing the relative parts of a developing unit  181 M according to a second embodiment of the present invention. The developing unit  151 M can be applied to devices in which the photosensitive drum  3 M rotates in the clockwise direction of the drawing. An image-forming device in which the photosensitive drum  3 M rotates in this way can be easily understood by imagining the pickup roller  92  of the printer  1  in  FIG. 1  being disposed on the left side of the drawing. 
   The developing unit  151 M is provided with a developing roller  152 M and a removing roller  156 M disposed below the developing roller  152 M. A supplying roller  153 M and a thickness-regulating blade  154 M are disposed in sequence downstream from the removing roller  156 M in the rotational direction of the developing roller  152 M. The constructions of the developing roller  152 M, the supplying roller  153 M, the thickness-regulating blade  154 M, and the removing roller  156 M are identical to the developing roller  52 M, the supply roller  53 M, the thickness-regulating blade  54 M, and the removing roller  56 H described above, and the same biases are applied to each. Further, the developing roller  152 M, supplying roller  153 M, and the removing roller  156 M of the second embodiment rotate such that the peripheral surfaces of each at points of confrontation move in the same direction at a velocity ratio of 0.7–1.3. Hence, the developing roller  152 M rotates in the counterclockwise direction of the drawing, while the supplying roller  153 M and the removing roller  156 M rotate in the clockwise direction. A scraper  158 M is also provided on a developing case. The scraper  158 M contacts the top surface of the removing roller  156 M. 
   In the present embodiment having the above construction, a portion of the toner scraped off by the removing roller  156 M is conveyed downstream in the rotational direction of the developing roller  152 M and resupplied to the developing roller  152 M by the supplying roller  153 M disposed at a position vertically overlapping the removing roller  156 M. Remaining portion of the toner scraped off by the scraper  158 M accumulates at the position and reaches the supplying roller  153 M when the accumulated toner reaches a certain amount, so that the toner is resupplied to the developing roller  152 M by the supplying roller  153 M. Hence, the present embodiment can effectively circulate toner that has been removed. Further, in the present embodiment, the velocity ratios of the peripheral surfaces on the rollers and the biases applied thereto are set to the same, values described in the first embodiment. Hence, this construction can achieve the same effects as the first embodiment. That is, the second embodiment can effectively suppress the degradation of toner and moreover can more effectively suppress the generation of sleeve ghosting and can effectively prevent blurred images and the like. While  FIG. 3  shows the developing unit  151 M, it is obvious that the cyan developing unit, the yellow developing unit, and the black developing unit can be similarly configured. 
   In the embodiments described above, the lengths (widths) of the rollers  52 ,  53 ,  56 ,  152 M,  153 M, and  156 M with respect to their axial direction should be set to achieve the relationship: (image forming range of the photosensitive drum  3 )&lt;(width of the removing roller  56 ,  156 M)=(width of a developing chamber inside the developing unit  51 ,  151 M)&lt;(width of the developing roller  52 ,  152 M) With this construction, image formation can be performed more effectively. 
   In the present invention, the removing roller is disposed upstream of the supplying roller in the rotational direction of the developing roller. Moreover, the removing roller and the developing roller rotate such that their peripheral surfaces at the point of confrontation move in the same direction while in contact with each other. Accordingly, toner removed by the removing roller is conveyed to the downstream side in the rotational direction of the developing roller and supplied again to the developing roller by the supplying roller. Accordingly, the present invention can effectively circulate the nonmagnetic single-component developer that has been removed. 
   While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
   For example, in the embodiments described above, the developing member, the supplying members, and the removing members are each configured of a roller, but can also be configured of a sleeve or belt. Further, the removing roller can be a fixed member rather than one that rotates, such as a plate-shaped scraper. Further, the developing device and image-forming device of the present invention can be applied to black-ink or single-color image-forming devices. Further, the nonmagnetic single-component developer may also be a negatively charged type, in which case the voltages V 1 –V 3  should be set such that V 3 &gt;V 1 ≧V 2 .