Patent Publication Number: US-7912407-B2

Title: Developing unit, developing device and image forming apparatus for peeling off magnetic developer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2007-341418 filed Dec. 28, 2007. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a developing unit, a developing device and an image forming apparatus. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a developing unit including: a developer carrying member that visualizes an electrostatic latent image held on an image carrying member using magnetic developer; a peeling member that peels off the magnetic developer held on the developer carrying member by a magnetic force; and a movement controller that controls movement of the peeling member, when the peeling member peels off the magnetic developer, the movement controller moving the peeling member to a position close to the developer carrying member, and when the peeling member has peeled off the magnetic developer, the movement controller moving the peeling member to a position away from the developer carrying member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a cross-sectional view schematically showing the structure of an image forming apparatus according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a cross-sectional view showing a first example of a developing device; 
         FIGS. 3A to 3C  are expanded views showing a state where developer held on a developing roller is peeled off with a magnet; 
         FIG. 4  is an expanded view showing a modification of the magnet in  FIG. 2  and its peripheral portion; 
         FIGS. 5A to 5C  are cross-sectional views showing the structure of a second example of the developing device; 
         FIGS. 6A to 6C  are cross-sectional views showing the structure of a third example of the developing device; 
         FIG. 7  is a cross-sectional view showing a state where developer is supplied from a developer container to the developing roller in a fourth example of the developing device; 
         FIG. 8  is a cross-sectional view showing a state where the peeling member starts peeling of the developer from the developing roller in the fourth example of the developing device; 
         FIG. 9  is a cross-sectional view showing a state where the developer container has moved into a developing device main body in a lower position in the fourth example of the developing device; 
         FIGS. 10A to 10C  are cross-sectional views showing the structure of a fifth example of the developing device; 
         FIGS. 11A to 11C  are cross-sectional views showing the structure of a sixth example of the developing device; 
         FIGS. 12A and 12B  are cross-sectional views showing the structure of a seventh example of the developing device; and 
         FIG. 13  is a cross-sectional view schematically showing the structure of the image forming apparatus according to a second exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Next, a first exemplary embodiment of the present invention will be described based on the drawings. 
       FIG. 1  schematically shows the structure of an image forming apparatus  10  according to the first exemplary embodiment of the present invention. The image forming apparatus  10  has an image forming apparatus main body  12 . A paper feed unit  14  which is e.g. a single-stage unit is provided in the rear side (right side in  FIG. 1 ) of the image forming apparatus main body  12 . 
     The paper feed unit  14  has a paper feed cassette  16  in which print sheets are contained. A pickup roller  18  to pick up a print sheet from the paper cassette  16  is provided above the paper feed cassette  16 , and a feed roller  20  and a retard roller  22  are provided ahead of the pickup roller  18 . 
     The feed roller  20  is driven by a drive unit (not shown), and is rotated to feed a print sheet toward a conveyance passage  24  to be described later. The retard roller  22  is in press-contact with the feed roller  20  and is rotatable in correspondence with the rotation of the feed roller  20 . The retard roller  22  is rotated along with the feed roller  20  to feed the print sheets one by one toward the conveyance passage  24  to be described later. 
     The conveyance passage  24  is a print sheet passage from the feed roller  20  to a discharge outlet  26 . The conveyance passage  24 , provided in an upper part of the image forming apparatus main body  12 , is formed approximately horizontally from the paper feed unit  14  to the discharge outlet  26 . In the conveyance passage  24 , a second transfer roller  60  and a second transfer backup roller  54  to be described later are provided on the upstream side of a fixing device  28 , and a registration roller  30  is provided on the upstream side of the second transfer roller  60  and the second transfer backup roller  54 . Further, in the conveyance passage  24 , a discharge roller  32  is provided in the vicinity of the discharge outlet  26 . 
     Accordingly, the print sheets picked up with the pickup roller  18  from the paper feed cassette  16  of the paper feed unit  14  are retarded with the feed roller  20  and the retard roller  22 , and only the top sheet is guided onto the conveyance passage  24 . Then the print sheet is temporarily stopped with the registration roller  30 , and at predetermined timing, a developer image is transferred onto the sheet while the sheet is passed between the second transfer roller  60  and the second transfer backup roller  54  to be described later. The transferred developer image is fixed to the print sheet by the fixing device  28 . The print sheet is discharged with the discharge roller  32  from the discharge outlet  26 . 
     The image forming apparatus main body  12  is provided with a developing device  34  in e.g. its approximate central portion. The developing device  34  visualizes an electrostatic latent image held on an image holder  36  to be described later with yellow, magenta, cyan and black color developers. 
     The image holder  36  having e.g. a photoreceptor is provided so as to be in press-contact with the developing device  34  from a front upper position in the image forming apparatus  10 . A charging device  38  having e.g. a charging roller to uniformly charge the image holder  36  is provided on the rear side of the image holder  36 . That is, a developing bias is applied to the image holder  36 . Further, an image holder cleaner  40  is provided in contact with the image holder  36  on the upstream side of the charging device  38  in a rotational direction of the image holder  36 . The image holder cleaner  40  scrapes off developer remaining on the image holder  36  after e.g. first transfer. 
     An exposure device  42  to write a latent image using light such as laser beam on the image holder  36  charged by the charging device  38  is provided above the developing device  34 . Further, an intermediate transfer device  44  on which a developer image visualized by the developing device  34  is first-transferred in a first transfer position and which conveys the developer image to a second transfer position to be described later, is provided on the front side of the image holder  36 . 
     The intermediate transfer device  44  has an intermediate transfer body  46  such as an intermediate transfer belt, a first transfer roller  48 , support rollers  50  and  52 , the second transfer backup roller  54  opposite to the second transfer roller  60  in the second transfer position to support the intermediate transfer body  46 , and tension rollers  56  and  58 . The intermediate transfer body  46  has a first transfer portion in wrap-contact with the image holder  36  between the support roller  50  provided upstream of the first transfer roller  48  and the support roller  52  provided downstream of the first transfer roller  48 . The intermediate transfer body  46  winds around the image holder  36  only in a predetermined range, and rotates in accordance with the rotation of the image holder  36 . The tension rollers  56  and  58  provide a predetermined tension force to the intermediate transfer body  46 . Developer images on the image holder  36 , i.e., yellow, magenta, cyan and black developer images, in e.g. this order, are first-transferred while they are overlaid onto the intermediate transfer body  46  with the first transfer roller  48 . The intermediate transfer body  46  conveys the first-transferred developer image toward the second transfer roller  60  to be described later. 
     In the intermediate transfer device  44 , the second transfer roller  60  is provided to be opposite to the second transfer backup roller  54  with the conveyance passage  24  therebetween. That is, the second transfer position is set between the second transfer roller  60  and the second transfer backup roller  54 . The second transfer roller  60  second-transfers the developer image first-transferred on the intermediate transfer body  46  onto a print sheet in the second transfer position by assistance of the second transfer backup roller  54 . Note that the second transfer roller  60  is away from the intermediate transfer body  46  while the intermediate transfer body  46  rotates three times, i.e., while the yellow, magenta and cyan developer images are conveyed. When the black developer image has been transferred, the second transfer roller  60  becomes in contact with the intermediate transfer body  46 . Note that a predetermined potential difference is caused between the second transfer roller  60  and the second transfer backup roller  54 . For example, when a high voltage is applied to the second transfer roller  60 , the second transfer backup roller  54  is connected to the ground (GND) or the like. 
     The fixing device  28  is provided downstream of the second transfer position in the conveyance passage  24 . The fixing device  28  has a heating roller and a pressure roller. The fixing device  28  fixes the developer image, second-transferred on the print sheet with the second transfer roller  60  and the second transfer backup roller  54 , to the print sheet, and conveys the sheet toward the discharger roller  32 . 
     Further, a controller (including a movement controller)  62  to control the respective constituent elements of the image forming apparatus  10  is provided in the image forming apparatus  10 . 
     Next, the developing device  34  will be described in detail. 
       FIG. 2  shows a first example of the developing device  34 . The developing device  34  has a developing device main body  66 , a developing roller  68 , a layer thickness regulating member  70 , a blocking member  72 , a developing roller accommodation wall  74  and a peeling member  76 . 
     The developing device main body (rotating body)  66  is provided with detachable developer containers  80   a  to  80   d  respectively containing yellow, magenta, cyan and black developers. The developing device main body  66  is rotatably provided e.g. counterclockwise (counterclockwise in  FIG. 2 ) about a rotation shaft  82 , such that the developer containers  80   a  to  80   d  are sequentially moved to a position opposite to the developing roller  68 . The developer containers  80   a  to  80   d  have container main bodies  84   a  to  84   d , first augers  86   a  to  86   d  and second augers  88   a  to  88   d . Developer cartridges (not shown) containing the above-described four color developers are attached to the container main bodies  84   a  and  84   d . Further, the developer containers  80   a  to  80   d  contain developer supplied to the developing roller  68 , and contain developer peeled off (collected) from the developing roller  68  after development. 
     Note that the developer contained in the developer containers  80   a  to  80   d  is two component developer including e.g. non-magnetic toner and magnetic carrier. The developer is stirred with the first stir-conveyance member  86  and the second stir-conveyance member  88 , thereby the non-magnetic toner is attached around the magnetic carrier. 
     Hereinbelow, when any one of plural constituent elements such as developer containers  80   a  to  80   d  is described without particularly specifying it, it may be simply referred to as e.g. “developer container  80 ”. 
     The first stir-conveyance member  86  conveys developer supplied from the developer cartridge (not shown) to the second stir-conveyance member  88 . The second stir-conveyance member  88  stirs the developer, and when an opening of the container main body  84  is moved to a position opposite to the developing roller  68  by rotation of the developing device main body  66 , the second stir-conveyance member  88  supplies the developer to the developing roller  68 . 
     The developing roller  68  has a fixed magnet member  90  extending in its axial direction, with plural magnetic poles to attract developer (e.g., N poles) and magnetic poles to repel the developer (e.g., two continuous S poles) on its outer surface, and a cylindrical member  92  such as a sleeve, including the magnet member  90 , which rotates and conveys developer in its circumferential direction in correspondence with a magnetic force of the magnet member  90 . In the magnet member  90 , the N poles to attract the developer from the developer container  80  are provided in e.g. upper positions on the developer container  80  side, and the S poles to repel the developer are provided in e.g. lower positions on the image holder  36  side and on the developer container  80  side (peel-off position). Further, the developing roller  68  is accommodated in a developing roller accommodation wall  74  opened such that a portion opposite to the image holder  36  is exposed. The developing roller  68  visualizes an electrostatic latent image on the image holder  36  with respective color developers held with the magnetic force while forming a predetermined gap between the roller and the image holder  36 . 
     The layer thickness regulating member  70  regulates the layer thickness of developer attached to the surface of the developing roller  68  (held on the developing roller  68 ). The blocking member  72  moves in correspondence with control by the controller  62 , and blocks supply of the developer from the developer container  80  to the developing roller  68 . Note that the supply of the developer from the developer container  80  to the developing roller  68  can also be blocked by stopping the rotation of the second stir-conveyance member  88 . 
     The peeling member  76  has a rotation shaft  94  provided approximately parallel to the developing roller  68  and a magnet  96  of e.g. rare earth which rotates clockwise (clockwise in  FIG. 2 ) about the rotation shaft  94  under the control of the controller  62 . The peeling member  76  peels off the developer held on the developing roller  68  with its magnetic force (by e.g. attraction with the N poles) and guides the developer to the developer container  80 . 
     In the developing device, for various purposes, it is necessary to peel off developer on the developing roller  68  as much as possible. For example, when developer is changed, it is necessary to remove as much of the developer on the developing roller  68  and in the developer container  80  as possible. Further, in a case where development is performed on a single developing roller  68  using plural color toners, it is necessary to remove current color developer from the developing roller  68  and then supply the next color developer to the developing roller  68 . 
     In  FIG. 2 , when the peeling member provided in the example of the present invention is not provided, the developing roller having magnetic poles inside peels off developer by using the above-described magnetic poles to repel the developer (peeling poles: e.g., two continuous S poles in  FIG. 2 ). In this case, when the amount of the developer on the developing roller is reduced, as the flow of the developer for peeling is suppressed, developer remaining on the peeling poles occurs. 
       FIGS. 3A to 3C  are expanded view showing a state where developer (including developer around the peel-off position) remaining on the developing roller  68  is peeled off by the magnet  96 . As shown in  FIG. 3A , when the peeling member  76  provided below the developing roller  68  via the developing roller accommodation wall  74  rotates and the magnet  96  approaches the developing roller  68 , the magnet  96  attracts the developer to the lower side of the developing roller accommodation wall  74 . Further, as shown in  FIG. 3B , when the peeling member  76  rotates and the magnet  96  begins to move away from the developing roller  68 , the developer is peeled off from the developing roller  68  with the magnetic force of the magnet  96 . Further, as shown in  FIG. 3C , when the peeling member  76  further rotates and the magnet  96  further moves away from the developing roller  68 , the developer is moved downward along the developing roller accommodation wall  74  while the developer is attracted with the magnetic force of the magnet  96 , and further, drops downward in a region not influenced by the above-described magnetic force. 
     Note that the developing roller accommodation wall  74  is a partition wall partitioning the peeling member  76  from the developer. The developing roller accommodation wall  74  is formed to have an area in which distances to the developing roller  68  and the magnet  96  are increased as the magnet  96  moves away from the position close to the developing roller  68 . 
     In this manner, when the peeling member  76  peels off developer, the magnet  96  approaches the developing roller  68  side such that the magnetic force of the magnet  96  acts on the developer held on the developing roller  68 . When the peeling member  76  has peeled off the developer, the magnet  96  rotate-moves (clockwise in  FIG. 3B ) away from the developing roller  68  such that the peeled developer is isolated from the developing roller  68 . 
     Note that when the developing roller  68  visualizes an electrostatic latent image, the controller  62  stops the magnet  96  in a position where the action of the magnetic force of the magnet  96  on the developer held on the developing roller  68  is lowered (on the side opposite to the developing roller  68 ). Then, when the blocking member  72  has blocked supply of developer, the controller  62  rotate-moves the magnet  96  such that the magnetic force of the magnet  96  acts on the developer held on the developing roller  68 . 
     Next, the operation of the first exemplary embodiment of the present invention will be described. 
     When an image forming signal is sent, the image holder  36  is uniformly charged by the charging device  38 , and light is emitted from the exposure device  42  based on the image signal to the charged image holder  36 . The light from the exposure device  42  exposes the surface of the image holder  36 , and an electrostatic latent image is formed. 
     The electrostatic latent image held on the image holder  36  is developed by the developing device  34  by developer color, and first-transferred onto the intermediate transfer body  46 . Then the respective color developer images are overlaid on the intermediate transfer body  46 , and a color image is formed. 
     The yellow, magenta, cyan and black developers are supplied from the developer containers  80   a  to  80   d  to the developing roller  68 . Then, developer excessively supplied to the developer  68  is collected into the same color developer container  80  before supply of the next color developer to the developing roller  68 . Upon first transfer, waste developer remaining on the image holder  36  is scraped off with the image holder cleaner  40 , and is collected. 
     On the other hand, in accordance with a paper feed signal or the like, print sheets contained in the paper feed cassette  16  are picked up with the pickup roller  18 , retarded with the feed roller  20  and the retard roller  22 , and only the top print sheet is guided to the conveyance passage  24 . The print sheet is temporarily stopped with the registration roller  30 , and at predetermined timing, guided between the second transfer roller  60  and the second transfer backup roller  54 . When the print sheet has been guided between the second transfer roller  60  and the second transfer backup roller  54 , the developer image first-transferred on the intermediate transfer body  46  is second-transferred onto the print sheet with the second transfer roller  60  and the second transfer backup roller  54 . 
     The print sheet on which the developer image has been transferred is guided to the fixing device  28 , and the developer image is fixed to the print sheet with the heating roller and the pressure roller. The print sheet on which the developer image has been fixed is discharged with the discharge roller  32  from the discharge outlet  26 . 
     Note that the image forming apparatus  10  may form only a monochrome image. For example, the developing device  34  may be arranged to have a developer container  80 , the developing roller  68 , the layer thickness regulating member  70 , the blocking member  72 , the developing roller accommodation wall  74  and the peeling member  76 . In this case, the developing device  34  acts as a developing unit to visualize an electrostatic latent image with one developer. 
     Next, a modification of the magnet  96  will be described. 
       FIG. 4  is an expanded view showing a modification of the magnet  96  and its peripheral portion. As shown in FIG.  4 , the magnet  96  may have two pairs of a magnetic pole (S pole) to repel developer with a magnetic field formed between the magnetic pole and the S pole of the developing roller  68  and a magnetic pole (N pole) to attract the developer, arrayed sequentially from e.g. the downstream side in its moving direction. In this manner, in a case where the magnet  96  has two pairs of a magnetic pole to repel developer and a magnetic pole to attract the developer from the downstream side in the moving direction, when the peeling member  76  rotates clockwise (clockwise in  FIG. 4 ) in correspondence with control by the controller  62 , the magnet  96  repels developer positioned between the peeling member  76  and the developing roller  68  to the developer container  80  side, then attracts developer held on the developing roller  68  and remaining on the upstream side of the rotational direction of the peeling member  76  thereby peel off the developer from the developing roller  68 , and guides the developer to the developer container  80  side. The magnet  96  performs this operation twice. 
     Next, a second example of the developing device will be described. 
       FIGS. 5A to 5C  are cross-sectional views showing the structure of the second example of the developing device (developing device  100 ). Note that it may be arranged such that in the developing device  100  shown in  FIGS. 5A to 5C , having four developer containers, supplies developer from one developer container to the developing roller, and visualizes an electrostatic latent image with one color developer. 
     Further, in the developing device  100 , constituent elements substantially the same as those of the developing device  34  shown in  FIG. 2  have the same reference numerals. 
     The developing device  100  has the developer container  80 , the developing roller  68 , the layer thickness regulating member  70 , the blocking member  72 , the developing roller accommodation wall  74  and a magnet  102 . The magnet  102 , fixed in the developer container  80  below the developing roller  68 , attracts developer with its magnetic force. In the developing device  100 , the developer container  80  is moved with respect to the developing roller accommodation wall  74  under the control of the controller  62  such that the magnet  102  moves close to or away from the developing roller  68 . 
     Note that the magnet  102  may form, with another member, the peeling member to peel off developer from the developing roller  68 . 
     As shown in  FIG. 5A , in the developing device  100 , when developer is supplied from the developer container  80  to the developing roller  68 , the developer container  80  is positioned in a lower position (lower side in  FIG. 5A ) such that the magnet  102  is away from the developing roller  68 . Further, as shown in  FIG. 5B , in the developing device  100 , when the magnet  102  starts peeling of the developer from the developing roller  68 , the developer container  80  is moved upward (upper side in  FIG. 5B ) such that the magnet  102  approaches the developing roller  68  after blocking the supply of the developer from the developer container  80  to the developing roller  68  by the blocking member  72 . Then, as shown in  FIG. 5C , in the developing device  100 , the developer container  80  is moved downward in a state where the blocking member  72  blocks the supply of the developer from the developer container  80  to the developing roller  68 , thereby the magnet  102  peels off the developer held on the developing roller  68 . 
     Note that the developing device  100  may be arranged such that, when, e.g., developer is supplied to the developing roller from one of four developer containers of the rotating developing device, some of the developer attracted by the magnet  102  is peeled off from the magnet  102  by a centrifugal force by rotation of the developing device, free fall or the like. 
     Further, the developing device  100  may be arranged such that the second stir-conveyance member  88  is a magnetized member thereby the developer attracted by the magnet  102  is peeled off from the magnet  102 . 
     Next, a third example of the developing device will be described. 
       FIGS. 6A to 6C  are cross-sectional views showing the structure of the third example of the developing device (developing device  104 ). Note that the developing device  104  shown in  FIGS. 6A to 6C  may be arranged such that developer is supplied from one of four developer container to the developing roller and an electrostatic latent image is visualized with one color developer. 
     Further, in the developing device  104 , constituent elements substantially the same as those of the developing device  100  shown in  FIGS. 5A to 5C  have the same reference numerals. 
     The developing device  104  has the developer container  80 , the developing roller  68 , the layer thickness regulating member  70 , the blocking member  72 , the developing roller accommodation wall  74 , the magnet  102  and a repeeling member  106 . The repeeling member  106  is movable upward/downward together with the magnet  102  and is movable leftward/rightward along an upper surface of the magnet  102 . The repeeling member  106  scrapes off developer attracted to the upper surface of the magnet  102 . 
     As shown in  FIG. 6A , in the developing device  104 , when developer is supplied from the developer container  80  to the developing roller  68 , the developer container  80  is positioned in a lower position (lower side in  FIG. 6A ) such that the magnet  102  is away from the developing roller  68  together with the repeeling member  106 . Further, as shown in  FIG. 6B , in the developing device  104 , when the magnet  102  starts peeling of the developer from the developing roller  68 , the developer container  80  is moved upward (upper side in  FIG. 6B ) such that the magnet  102  approaches the developing roller  68  together with the repeeling member  106  after blocking the supply of the developer from the developer container  80  to the developing roller  68  by the blocking member  72 . Then, as shown in  FIG. 6C , in the developing device  104 , the developer container  80  is moved downward in a state where the blocking member  72  blocks the supply of the developer from the developer container  80  to the developing roller  68 , thereby the magnet  102  peels off the developer held on the developing roller  68  and the repeeling member  106  scrapes off the developer attracted to the magnet  102 , and the developer is collected in the developer container  80 . 
     Next, a fourth example of the developing device will be described. 
       FIGS. 7 to 9  show the fourth example of the developing device (developing device  110 ). Note that in the developing device  110  shown in  FIGS. 7 to 9 , constituent elements substantially the same as those of the developing device  34  shown in  FIG. 2  and the developing device  100  shown in  FIGS. 5A to 5C  have the same reference numerals. 
     The developing device  110  has the developing device main body  66 , the developing roller  68 , the layer thickness regulating member  70 , the blocking member  72  and the developing roller accommodation wall  74 . The developer containers  80   a  to  80   d  provided in the developing device main body  66  are respectively provided with the magnet  102 . Further, the developing device main body  66  is provided opening/closing members  112   a  to  112   d  to close the openings of the developer containers  80   a  to  80   d  opened in the position opposite to the developing roller  68  when the developer containers  80   a  to  80   d  move to another position. The opening/closing member  112  closes the opening of the developer container  80  when the developer container  80  moves from the position opposite to the developing roller  68 , and scrapes off developer attracted to the upper surface of the magnet  102  and collects the developer in the developer container  80 . 
     Note that the opening/closing members  112   a  to  112   d  may be respectively provided in the developer containers  80   a  to  80   d.    
     As shown in  FIG. 7 , in the developing device  110 , when developer is supplied from the developer container  80  to the developing roller  68 , the opening/closing member  112  is opened and the developer container  80  is moved upward from the developing device main body  66  such that the developing roller  68  approaches the image holder  36  in a state where the magnet  102  is away from the developing roller  68 . Further, as shown in  FIG. 8 , in the developing device  110 , when the magnet  102  starts peeling of the developer from the developing roller  68 , the developer container  80  is moved upward (upper side in  FIG. 8 ) such that the magnet  102  further approaches the developing roller  68  after blocking the supply of the developer from the developer container  80  to the developing roller  68  by the blocking member  72 . Then, as shown in  FIG. 9 , in the developing device  110 , the developer container  80  is moved in the developing device main body  66  in a lower position in a state where the blocking member  72  blocks the supply of the developer from the developer container  80  to the developing roller  68 , thereby the magnet  102  peels off the developer held on the developing roller  68  and the opening/closing member  112  scrapes off the developer attracted to the magnet  102 , and collects the developer in the developer container  80 . 
     Next, a fifth example of the developing device will be described. 
       FIGS. 10A to 10C  are cross-sectional views showing the structure of the fifth example of the developing device (developing device  114 ). Note that the developing device  114  shown in  FIGS. 10A to 10C  may be arranged such that developer is supplied from one of four developer containers to the developing roller and an electrostatic latent image is visualized with single-color developer. 
     Further, in the developing device  114 , constituent elements substantially the same as those of the developing device  100  shown in  FIGS. 5A to 5C  have the same reference numerals. 
     The developing device  114  has the developer container  80 , the developing roller  68 , the layer thickness regulating member  70 , the blocking member  72 , the developing roller accommodation wall  74  and a magnet  116 . The magnet  116  is movable close to or away from the developing roller  68  within the developer container  80  under the control of the controller  62 . The magnet  116  attracts developer with its magnetic force. In the developing device  114 , the developer container  80  is fixed with respect to the developing roller accommodation wall  74 . 
     Note that the magnet  116  may form, with another member, the peeling member to peel off developer from the developing roller  68 . 
     As shown in  FIG. 10A , in the developing device  114 , when developer is supplied from the developer container  80  to the developing roller  68 , the magnet  116  is positioned in a lower position in the developer container  80  (lower side in  FIG. 10A ) such that the magnet  116  is away from the developing roller  68 . Further, as shown in  FIG. 10B , in the developing device  114 , when the magnet  116  starts peeling of the developer from the developing roller  68 , the magnet  116  is moved upward in the developer container  80  (upper side in  FIG. 10B ) such that the magnet  116  approaches the developing roller  68  after blocking the supply of the developer from the developer container  80  to the developing roller  68  by the blocking member  72 . Then, as shown in  FIG. 10C , in the developing device  114 , the magnet  116  is moved downward in a state where the blocking member  72  blocks the supply of the developer from the developer container  80  to the developing roller  68 , thereby the magnet  116  peels off the developer held on the developing roller  68 . 
     Next, a sixth example of the developing device will be described. 
       FIGS. 11A to 11C  are cross-sectional views showing the structure of the sixth example of the developing device (developing device  118 ). Note that the developing device  118  shown in  FIGS. 11A to 11C  may be arranged such that developer is supplied from one of four developer containers to the developing roller and an electrostatic latent image is visualized with one color developer. 
     Further, in the developing device  118 , constituent elements substantially the same as those of the developing device  114  shown in  FIGS. 10A to 10C  have the same reference numerals. 
     The developing device  118  has the developer container  80 , the developing roller  68 , the layer thickness regulating member  70 , the blocking member  72 , the developing roller accommodation wall  74 , the magnet  116  and a repeeling member  120 . The repeeling member  120  is movable upward/downward together with the magnet  116  and is movable leftward/rightward along an upper surface of the magnet  116 . The repeeling member  120  scrapes off developer attracted to the upper surface of the magnet  116 . 
     As shown in  FIG. 11A , in the developing device  118 , when developer is supplied from the developer container  80  to the developing roller  68 , the magnet  116  is positioned in a lower position in the developer container  80  (lower side in  FIG. 11A ) such that the magnet  116  is away from the developing roller  68  together with the repeeling member  120 . Further, as shown in  FIG. 11B , in the developing device  118 , when the magnet  116  starts peeling of the developer from the developing roller  68 , the magnet  116  is moved upward in the developer container  80  (upper side in  FIG. 11B ) such that the magnet  116  approaches the developing roller  68  together with the repeeling member  120  after blocking the supply of the developer from the developer container  80  to the developing roller  68  by the blocking member  72 . Then, as shown in  FIG. 11C , in the developing device  118 , the magnet  116  is moved downward in a state where the blocking member  72  blocks the supply of the developer from the developer container  80  to the developing roller  68 , thereby the magnet  116  peels off the developer held on the developing roller  68 , and the repeeling member  120  scrapes off the developer attracted to the magnet  116 , and collects the developer in the developer container  80 . 
     Next, a seventh example of the developing device will be described. 
       FIGS. 12A and 12B  are cross-sectional views showing the structure of the seventh example of the developing device (developing device  122 ). Note that the developing device  122  shown in  FIGS. 12A and 12B  is applied to a developing unit to visualize an electrostatic latent image with e.g. one color developer. 
     Further, in the developing device  122 , constituent elements substantially the same as those of the developing device  114  shown in  FIGS. 10A to 10C  have the same reference numerals. 
     The developing device  122  has the developer container  80 , the developing roller  68 , the layer thickness regulating member  70 , the developing roller accommodation wall  74  and a magnet  124 . The magnet  124  attracts developer held on the developing roller  68  with its magnetic force, and slides in contact with an inner upper portion of the container main body  84  thereby repeels off the attracted developer. Further, in the developing device  122 , the developer container  80  is movable in a direction where the second stir-conveyance member  88  approaches or moves away from the developing roller  68 . 
     Note that the magnet  124  may form, with another member, the peeling member to peel off developer from the developing roller  68 . 
     As shown in  FIG. 12A , in the developing device  122 , when developer is supplied from the developer container  80  to the developing roller  68 , the magnet  124  is positioned below an inner upper portion of the container main body  84 . Further, the developer container  80  is stopped in a state where the second stir-conveyance member  88  is close to the developing roller  68 . 
     Further, as shown in  FIG. 12B , in the developing device  122 , when the magnet  124  peels off the developer from the developing roller  68 , the magnet  124  approaches the developing roller  68  and attracts the developer held on the developing roller  68 , thereafter, slides in contact with the inner upper portion of the container main body  84  after the attraction of the developer, thereby repeels off the attracted developer. That is, the developer attracted by the magnet  124  is collected in the developer container  80 . Note that the developer container  80  is stopped in a state where the second stir-conveyance member  88  has moved away from the developing roller  68 . 
     Next, a second exemplary embodiment of the present invention will be described based on the drawings. 
       FIG. 13  is a cross-sectional view schematically showing the structure of an image forming apparatus  210  according to the second exemplary embodiment of the present invention. The image forming apparatus  210  has an image forming apparatus main body  212 . A paper feed unit  214  which is e.g. a single-stage unit is provided in a lower part of the image forming apparatus main body  212 . 
     The paper feed unit  214  has a paper feed cassette  216  in which print sheets are contained. A pickup roller  218  to pick up a print sheet from the paper cassette  216  is provided above the paper feed cassette  216 , and a feed roller  220  and a retard roller  222  are provided ahead of the pickup roller  218 . 
     The feed roller  220  is driven by a drive unit (not shown), and is rotated to feed a print sheet toward a conveyance passage  224  to be described later. The retard roller  222  is in press-contact with the feed roller  220  and is rotatable in correspondence with the rotation of the feed roller  220 . The retard roller  222  is rotated in correspondence with rotation of the feed roller  220  to feed the print sheets one by one toward the conveyance passage  224  to be described later. 
     The conveyance passage  224  is a print sheet passage from the feed roller  220  to a discharge outlet  226 . The conveyance passage  224 , provided on the rear side of the image forming apparatus main body  212  (left side in  FIG. 13 ), has an approximately vertical portion extending from the paper feed unit  214  to the discharge outlet  226 . In the conveyance passage  224 , an image holder  236  and a transfer roller  260  to be described later are provided on the upstream side of a fixing device  228 , and a registration roller  230  is provided on the upstream side of the image holder  236  and the transfer roller  260 . Further, in the conveyance passage  224 , a discharge roller  232  is provided in the vicinity of the discharge outlet  226 . 
     Accordingly, the print sheets picked up with the pickup roller  218  from the paper feed cassette  216  of the paper feed unit  214  are retarded with the feed roller  220  and the retard roller  222 , and only the top sheet is guided onto the conveyance passage  224 . Then the print sheet is temporarily stopped with the registration roller  230 , and at predetermined timing, a developer image in e.g. black is transferred onto the sheet while the sheet is passed between the image holder  236  and the transfer roller  260  to be described later. The transferred black developer image is fixed to the print sheet by the fixing device  228 . The print sheet is discharged with the discharge roller  232  from the discharge outlet  226 . 
     In the case of color printing, the print sheet is returned to a repeat passage three times. That is, in the conveyance passage  224 , a part ahead of the discharge roller  232  is branched into two passages, and a switch device  246  is provided in the branch portion and a repeat passage  248  to return from the branch portion to the registration roller  230  is formed. Conveyance rollers  250 ,  252 ,  254  and  256  are provided on the repeat passage  248 . In the case of color printing, the switch device  246  is switched to the side to open the repeat passage  248 , then the print sheet is guided to the repeat passage  248 , then passed through the registration roller  230 , between the image holder  236  and the transfer roller  260  and through the fixing device  228  four times, and is discharged from the discharge outlet  226 . 
     The image forming apparatus main body  212  is provided with a developing device  234  in e.g. its approximate central portion. The developing device  234  visualizes an electrostatic latent image held on the image holder  236  to be described later with yellow, magenta, cyan and black color developers. Note that the developing device  234 , having a developing device main body  266 , a developing roller  268 , a layer thickness regulating member  270 , a blocking member  272 , a developing roller accommodation wall  274 , a peeling member  276  and developer containers  280   a  to  280   d  and the like, has substantially the same configuration as that of the developing device  34  shown in  FIG. 2 . 
     The image holder  236  having e.g. a photoreceptor is in contact with the developing device  234  from an upper position on the rear side of the image forming apparatus  210 . A charging device  238  having e.g. a charging roller to uniformly charge the image holder  236  is provided on the front side of the image holder  236 . That is, a developing bias is applied to the image holder  236 . Further, an image holder cleaner  240  is provided in contact with the image holder  236  on the upstream side of the charging device  238  in a rotational direction of the image holder  236 . The image holder cleaner  240  removes developer remaining on the image holder  236  after transfer. 
     An exposure device  242  to write an electrostatic latent image using light such as laser beam on the image holder  236  charged by the charger  238  is provided above the developing device  234 . Further, the transfer roller  260  to transfer a developer image visualized by the developing device  234  onto a print sheet in a transfer position is provided on the rear side of the image holder  236 . 
     The fixing device  228  is provided downstream of the transfer position in the conveyance passage  224 . The fixing device  228  has a heating roller and a pressure roller. The fixing device  228  fixes the developer image, transferred on the print sheet with the image holder  236  and the transfer roller  260 , to the print sheet, and conveys the sheet. 
     Further, a controller (including a movement controller)  262  to control the respective constituent elements of the image forming apparatus  210  is provided in the image forming apparatus  210 . 
     Next, the operation (color printing) of the second exemplary embodiment of the present invention will be described. 
     When an image forming signal is sent, the image holder  236  is uniformly charged by the charger  238 , and light corresponding to an yellow image is emitted from the exposure device  242  based on the image signal to the charged image holder  236 . The light from the exposure device  242  exposes the surface of the image holder  236 , and an electrostatic latent image is formed. 
     The electrostatic latent image held on the image holder  236  is developed with yellow developer supplied to the developing roller  268  of the developing device  234 , and transferred onto a print sheet fed from the paper feed unit  214 . Then the print sheet on which the yellow developer image has been transferred is guided to the fixing device  228 , and the developer image is fixed to the print sheet with the heating roller and the pressure roller. The print sheet on which the yellow developer image is fixed is returned by the switch device  246  to the repeat passage  248 . 
     The developer held on the developing roller  268  (remaining on the developing roller  268 ) is peeled off by the peeling member  276  and is collected in the developer container  280   a . Then, the developing device main body  266  of the developing device  234  rotates, thereby magenta developer is supplied to the developing roller  268 . The image holder  236  is uniformly charged by the charger  238 , and light corresponding to a magenta image is emitted from the exposure device  242  based on the image signal to the charged image holder  236 . The light from the exposure device  242  exposes the surface of the image holder  236 , and an electrostatic latent image is formed. 
     The electrostatic latent image held on the image holder  236  is developed with magenta developer supplied to the developing roller  268  of the developing device  234 , and transferred onto the print sheet. Then the print sheet on which the magenta developer image has been transferred is guided to the fixing device  228 , and the developer image is fixed to the print sheet with the heating roller and the pressure roller. The print sheet on which the magenta developer image is fixed is returned by the switch device  246  to the repeat passage  248 . 
     As the print sheet is returned to the repeat passage  248  three times, as in the case of the yellow and magenta developer images, developer images developed with cyan and black developers are fixed to the print sheet by the fixing device  228 , then a color image in which the developer images are overlaid is formed on the print sheet. The print sheet on which the color image is fixed is guided to the discharge roller  232  by the switch device  246  and is discharged. 
     Note that in the above exemplary embodiment, an electrostatic latent image is visualized using two component developer, however, the developer is not limited to this type of developer. For example, the developing device  34  may visualize an electrostatic latent image using single component developer. 
     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 exemplary 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.