Patent Publication Number: US-7720418-B2

Title: Toner change and flow stabilizing developing device, process cartridge, and image forming apparatus

Description:
TECHNICAL FIELD 
   The present invention relates to a developing device used for an image formation using an electrostatic copying process, and a process cartridge and an image forming apparatus employing the developing device. 
   BACKGROUND ART 
   Office automation is becoming increasingly prevalent, and use of color documents is growing. In the past, office equipment was mainly used for taking copies of documents consisting only of text. Now, documents including graphics such as graphs are prepared in personal computers, printed out from printers, and large amounts of copies are taken to produce presentation materials, for example. Images output from printers include solid images, line images, and halftone images. Thus, demands for image quality are changing, and high reliability is increasingly demanded. 
   Electrophotographic methods such as electrostatic recording and electrostatic printing include a developing process for developing an electrostatic image on an image carrier such as a photoconductor by applying a developer to the photoconductor, a transfer process for transferring the developed image from the photoconductor to a transfer medium such as paper, and a fixing process for fixing the image onto the paper. There are two types of developers for developing the electrostatic image formed on the photoconductor, i.e., a two-component developer including carriers and toner, and a single-component developer, which does not require carriers, including only magnetic toner or nonmagnetic toner. The two-component developer has the following disadvantages: the developer deteriorates as toner particles stick to the surfaces of the carriers; and a mixture of the toner and the carriers needs to be maintained at a certain ration, so that toner density in the developer does not decrease as the toner is consumed. Accordingly, a large-sized developing device is needed to realize such a configuration. On the other hand, the single-component developer is advantageous in that the developing device can be made compact, and that the developer can be used under any temperature or humidity conditions. Accordingly, the single-component developer is becoming a mainstream. 
   There are two types of single-component developers, i.e., a magnetic single-component developer including magnetic toner, and a nonmagnetic single-component developer including nonmagnetic toner. In a magnetic single-component developing method employing the magnetic single-component developer, a developing sleeve with a magnetic field generator such as a magnet provided inside holds the magnetic toner including magnetic substances such as magnetite, and a layer thickness restricting member reduces the thickness of the toner for the developing process. The magnetic single-component developer is widely used in compact printers. In a nonmagnetic single-component developing method employing the nonmagnetic single-component developer, the toner does not have a magnetic force, and therefore, a toner supplying roller is pressed against a developing sleeve to supply the toner to the developing sleeve, and the developing sleeve holds the toner by static electricity. A layer thickness restricting member reduces the thickness of the toner for the developing process. Because the toner does not include chromatic magnetic substances, the nonmagnetic single-component developer is useful for producing color images, and because the developing sleeve does not include a magnet, a light-weight, low-cost developing device can be realized. Accordingly, the nonmagnetic single-component developer is widely used in compact, full-color printers. 
   However, the single-component developing method has many problems to be solved. In the two-component developing method, the carriers are used to electrically charge and convey the toner. The toner and the carriers are sufficiently mixed and stirred together, and then conveyed to the developing sleeve for the developing process. Therefore, the two-component developer can be steadily charged and conveyed over a long time, and can be used in a high-speed developing device. On the other hand, the single-component developing method does not employ carriers that can steadily charge and convey the toner, and therefore, failures occur in the charging and conveying operations when the developing device is used over a long time or at a high speed. 
   Particularly in the nonmagnetic single-component developing method, the toner contacts friction-charged members such as the developing sleeve or the layer thickness restricting member only for a very short time. Therefore, there is a higher chance of creating low charged toner or reversely charged toner than in the two-component developing method. Furthermore, the layer thickness of toner on a toner conveying member, which conveys the toner to the image carrier, needs to be as thin as possible. Accordingly, the toner conveying member receives a force from the layer thickness restricting member, which pushes outer additives on the surface of the toner particles inside the toner particles. This significantly deteriorates the chargeability and the flowability of the toner. 
   To solve the above problems, technologies are disclosed in, for example, Japanese Patent Application Laid-Open No. H08-122559 and Japanese Patent Application Laid-Open No. 2005-062215. 
   DISCLOSURE OF INVENTION 
   Problem to be Solved by the Invention 
   However, with the conventional technologies, it is difficult to stabilize chargeability and flowability of toner, particularly a nonmagnetic single-component developer, in a developing device over a long time. 
   The present invention has been made in view of the above problem, and it is an object of the invention to provide a developing device that can prevent decreases in the chargeability and the flowability of toner in the developing device, particularly a nonmagnetic single-component developer, so that an image forming apparatus employing the developing device can produce high quality images over a long time. 
   Means for Solving Problem 
   To solve the above problems and to achieve the above objects, a developing device according to one aspect of the present invention includes a developing unit that develops a latent image on an image carrier with toner; a toner cartridge that is detachably arranged in parallel with the developing unit, and supplies the toner to the developing unit; and an opening that is disposed between the developing unit and the toner cartridge, through which the toner passes. The toner cartridge includes a space forming unit that forms a space in the toner stored in the toner cartridge. 
   An image forming apparatus according to another aspect of the present invention includes a charging unit that charges a surface of an image carrier that carries a latent image; an exposing device that forms an electrostatic latent image on the surface of the image carrier charged by the charging unit; a developing device that visualizes the electrostatic latent image formed on the surface of the image carrier, to form a visual image; a transferring device that transfers the visible image from the image carrier onto a recording medium directly or via intermediate transfer member; and a fixing device that fixes the visible image transferred onto the recording medium by using a heat or a pressure. The developing device includes a developing unit that develops the latent image on the image carrier with toner; a toner cartridge that is detachably arranged in parallel with the developing unit, and supplies the toner to the developing unit; and an opening that is disposed between the developing unit and the toner cartridge, through which the toner passes. The toner cartridge includes a space forming unit that forms a space in the toner stored in the toner cartridge. 
   A process cartridge according to still another aspect of the present invention integrally supports at least an image carrier and a developing device, and is detachably attached to an image forming apparatus. The developing device includes a developing unit that develops a latent image on the image carrier with toner; a toner cartridge that is detachably arranged in parallel with the developing unit, and supplies the toner to the developing unit; and an opening that is disposed between the developing unit and the toner cartridge, through which the toner passes. The toner cartridge includes a space forming unit that forms a space in the toner stored in the toner cartridge. 
   A developing device according to still another aspect of the present invention includes a developing unit that develops a latent image on an image carrier with toner; a toner cartridge that is detachably arranged in parallel with the developing unit, the toner cartridge supplying the toner to the developing unit; an opening that is disposed between the developing unit and the toner cartridge, through which the toner passes; and a control valve that controls an amount of the toner that passes through the opening. 
   An image forming apparatus according to still another aspect of the present invention includes a charging unit that charges a surface of an image carrier that carries a latent image; an exposing device that forms an electrostatic latent image on the surface of the image carrier charged by the charging unit; a developing device that visualizes the electrostatic latent image formed on the surface of the image carrier, to form a visual image; a transferring device that transfers the visible image from the image carrier onto a recording medium directly or via intermediate transfer member; and a fixing device that fixes the visible image transferred onto the recording medium by using a heat or a pressure. The developing device includes a developing unit that develops a latent image on an image carrier with toner; a toner cartridge that is detachably arranged in parallel with the developing unit, and supplies the toner to the developing unit; an opening that is disposed between the developing unit and the toner cartridge, through which the toner passes; and a control valve that controls an amount of the toner that passes through the opening. 
   A process cartridge according to still another aspect of the present invention integrally supports at least an image carrier and a developing device, and is detachably attached to an image forming apparatus. The developing device includes a developing unit that develops a latent image on an image carrier with toner; a toner cartridge that is detachably arranged in parallel with the developing unit, and supplies the toner to the developing unit; an opening that is disposed between the developing unit and the toner cartridge, through which the toner passes; and a control valve that controls an amount of the toner that passes through the opening. 
   EFFECT OF THE INVENTION 
   The developing device and the image forming apparatus according to an embodiment of the present invention are able to maintain chargeability of toner, so that high-quality images can be produced over a long time. Moreover, flowability of toner is prevented from decreasing, so that images of high density can be steadily produced over a long time. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1A  is a schematic of a developing device according to a first embodiment of the present invention; 
       FIG. 1B  is a schematic of another example of the developing device according to the first embodiment; 
       FIG. 2  is a detailed schematic of a control valve in the developing device according to the first embodiment; 
       FIG. 3  is a diagram for explaining a space formed in a toner cartridge, and a flow of toner from a developing unit to the toner cartridge in the developing device according to the first embodiment; 
       FIG. 4  is another diagram for explaining a space formed in the toner cartridge, and a flow of toner from the developing unit to the toner cartridge the developing device according to the first embodiment; 
       FIG. 5  is still another diagram for explaining a space formed in the toner cartridge, and a flow of toner from the developing unit to the toner cartridge the developing device according to the first embodiment; 
       FIG. 6  is a schematic of an image forming apparatus according to the first embodiment; 
       FIG. 7  is a detailed schematic of a communicating opening in the developing device according to the first embodiment; 
       FIG. 8  is a schematic of a developing device according to a second embodiment of the present invention; 
       FIG. 9  is a diagram for explaining an operation of supplying toner from a toner cartridge to a developing unit in the developing device according to the second embodiment; 
       FIG. 10  is a diagram for explaining movement of toner between the toner cartridge and the developing unit in the developing device according to the second embodiment; 
       FIGS. 11A to 11P  are detailed schematics for explaining movement of toner between the developing unit and the toner cartridge in the developing device according to the second embodiment; 
       FIG. 12  is a perspective view of a first conveying paddle according to the second embodiment; 
       FIG. 13  is a schematic of a communicating opening in the developing device according to the second embodiment; 
       FIG. 14  is a schematic of a toner-charging-amount evaluating apparatus; 
       FIG. 15  is a graph of a result of evaluating a toner charging amount according to the second embodiment; 
       FIG. 16  is a graph of a result of evaluating a toner charging amount according to a conventional technology; 
       FIG. 17  is another graph of a result of evaluating a toner charging amount according to the second embodiment; 
       FIG. 18  is another graph of a result of evaluating a toner charging amount according to the conventional technology; 
       FIG. 19  is a graph of a percentage of a low inverse-charged toner when a toner cartridge is replaced; 
       FIG. 20  is a graph for showing a collected toner divided into each charging area; and 
       FIG. 21  is a graph of a result after 4-cycle execution. 
   

   EXPLANATIONS OF LETTERS OR NUMERALS 
   
       
         1  image forming apparatus 
         10  photoconductor unit 
         11  photoconductive belt 
         12  photoconductor cleaning device 
         13  charging roller 
         14  driving roller 
         15  primary transfer opposite roller 
         16  extension roller 
         20  writing optical device 
         21  semiconductor laser 
         22  polygon mirror 
         23   a ,  23   b ,  23   c  reflecting mirrors 
         30 ,  300  developing device 
         31  developing unit
         31   a  developing sleeve     31   b  supplying roller     31   c  restricting roller     31   d ,  310   d  first conveying paddle     311 ,  312  film     31   e  slide shutter     31   f  elastic member     31   g  windows     
         32  toner cartridge
         321  first storage space     322  second storage space     32   a  second conveying paddle     32   b  third conveying paddle     32   c  elastic member     32   d  slide shutter     32   e  fixing seal     
         33  communicating opening 
         34 ,  340  control valve
         34   a  support unit     34   b  elastic resin films     
         35  rib 
         40  intermediate transfer device 
         41  intermediate transfer belt 
         42  belt cleaning device 
         43  position detecting sensor 
         44  driving roller 
         45  primary transfer roller 
         46  secondary transfer opposite roller 
         47  cleaning opposite roller 
         48  tension roller 
         49  toner waste tank 
         50  secondary transfer device 
         51  secondary transfer roller 
         60  fixing device 
         61  fixing belt 
         62  pressurizing roller 
         65  duplex changeover claw 
         70  paper reversing device 
         71  reverse changeover claw 
         72  pair of reverse rollers 
         80  transfer paper cassette 
         81   a ,  81   b ,  81   c  paper feeding rollers 
         82  pair of registration rollers 
         83  manual feed tray 
         84  paper discharge tray 
         90  transfer sheet 
     
  
   BEST MODE(S) FOR CARRYING OUT THE INVENTION 
   Exemplary embodiments of the present invention will be described in detail below with reference to accompanying drawings. The present invention is not limited to the embodiments. 
     FIGS. 1A and 1B  are schematics of a developing device  30  according to a first embodiment of the present invention. The developing device  30  includes a developing unit  31  that develops a latent image on a photoconductive belt  11  (see  FIG. 6 ), which is an image carrier, and a toner cartridge  32  that supplies toner to the developing unit  31 . 
   The developing unit  31  faces the photoconductive belt  11 , and includes a developing sleeve  31   a  that is a developer carrier for conveying toner to a developing area formed between the photoconductive belt  11 , a supplying roller  31   b  that supplies toner onto the developing sleeve  31   a , a restricting roller  31   c  that is a layer thickness restricting member for restricting the amount of toner on the developing sleeve  31   a , and a first conveying paddle  31   d  that is a rotating body for conveying the toner. 
   The toner cartridge  32  includes a first storage space  321  and a second storage space  322  configured to store therein toner, a second conveying paddle  32   a  and a third conveying paddle  32   b  for conveying toner to the developing unit  31 , a rib  35  that is a plate member that protrudes from the bottom of the toner cartridge  32  beneath the second conveying paddle  32   a , and a control valve  34  that is a movable plate member for blocking a communicating opening  33 . 
   A single-component developer is used as the developer. When replacing deteriorated toner with fresh toner, in the case of a two-component developer, it is difficult to separate toner from carriers once they are mixed together. In the case of the single-component developer, the same kind of toner is stored in the toner cartridge  32  and the developing unit  31 , and can therefore be easily replaced. Thus, the developing device  30  can employ the single-component developer. It is particularly preferable to use a nonmagnetic single-component developer. When outer additives on the surface of toner particles of the nonmagnetic single-component developer decrease, chargeability and flowability of the toner decrease, thereby deteriorating developing properties. However, in the developing device  30 , a stable amount outer additives can be maintained on the surface of toner particles. 
   The developing unit  31  and the toner cartridge  32  are horizontally juxtaposed in the developing device  30 . Toner passes through the communicating opening  33  between the developing unit  31  and the toner cartridge  32 . 
   When toner is consumed in the developing unit  31 , fresh toner is supplied from the toner cartridge  32  to the developing unit  31  through the communicating opening  33 . Deteriorated toner is discharged from the developing unit  31  to the toner cartridge  32  through the communicating opening  33 . 
   The toner on the developing sleeve  31   a  receives suppress strength from the supplying roller  31   b  and the restricting roller  31   c . Accordingly, asperities on the surface of toner particles are crushed, and the surface becomes smooth. As a result, adherence of the toner increases so that the toner adheres more strongly to the photoconductive belt  11 , which makes it hard to clean the toner off. Although transferring properties improve when humidity in the environment decreases, cleaning failures occur, and a fog appears in a white background. Furthermore, the suppress strength pushes outer additives on the surface of toner particles inside the toner particles, because the outer additives are harder than the toner. A decrease in the amount of outer additives on the surface of a toner particle changes chargeability of the toner. For example, when silica is used as the outer additive, the toner is highly charged because silica has a large specific surface area. Therefore, chargeability of toner decreases significantly if the silica is pushed inside the toner particles. Moreover, flowability of the toner decreases when the outer additives are pushed inside the toner particles. Flowability affects the adherence of the toner, and therefore, when flowability is high, adherence between the toner and the photoconductive belt  11  decreases. High flowability also decreases the adherence between the toner and the developing sleeve  31   a , so that developing properties improve. As amounts of the outer additives on the surfaces of the toner particles decrease, the flowability decreases, thereby deteriorating developing properties. 
   In the developing device  30 , toner that is deteriorated after consumption in the developing unit  31  is discharged to the toner cartridge  32  through the communicating opening  33 . The deteriorated toner is mixed with fresh toner present in the toner cartridge  32  so that the ratio of deteriorated toner is decreased, and the mixture is then supplied to the developing unit  31  through the communicating opening  33 . 
   The developing device  30  includes a space forming unit that forms a space in the toner stored in the toner cartridge  32 . The toner gradually fills the space formed in the toner cartridge  32 , as the toner cartridge  32  is shaken by an image forming operation of an image forming apparatus, or by the gravity of the toner. To prevent this, the toner is periodically stirred to form a space inside the toner cartridge  32 . Accordingly, the flowability of the toner is prevented from decreasing. 
   The space forming unit causes toner to flow from the developing unit  31  into the space formed in the toner cartridge  32 . The space forming unit includes the second conveying paddle  32   a  and the control valve  34 . The control valve  34  is provided in the toner cartridge  32  at the communicating opening  33 . The control valve  34  shown in  FIG. 1A  opens at the top. The control valve  34  shown in  FIG. 1B  opens at the bottom.  FIG. 2  is a detailed schematic of the control valve  34  in the developing device  30  according to the first embodiment. The control valve  34  is pasted to a support unit  34   a  fixed in a casing of the toner cartridge  32 . The control valve  34  includes rectangular plates, which are elastic resin films  34   b , provided alternately corresponding to the communicating opening  33 , so that a plate is not provided where there is no opening. The support unit  34   a  is made of a rigid metal such as SUS, Cu, and Al. The elastic resin films  34   b  are made of polypropylene, polyethylene, polyester resin, fluorine resin, etc. 
   The toner moves between the toner cartridge  32  and the developing unit  31  through the communicating opening  33 . When the toner cartridge  32  is filled with a large amount of toner, it is difficult to discharge toner from the developing unit  31  to the toner cartridge  32 . Thus, the space forming unit forms a space in the toner cartridge  32  near the communicating opening  33 , so that toner can flow from the developing unit  31  into the toner cartridge  32 . 
     FIG. 3  is a diagram for explaining a space formed in the toner cartridge  32 , and a flow of toner from the developing unit  31  to the toner cartridge  32  in the developing device  30 . As shown in ( 1 ), the second conveying paddle  32   a  is long enough to contact the control valve  34 . The second conveying paddle  32   a  has a paddling film made of an elastic material such as fluorine resin and silicon resin. As shown in ( 2 ), the paddling film rotates and contacts the control valve  34 . As shown in ( 3 ), the second conveying paddle  32   a  holds down the control valve  34  and blocks the communicating opening  33 . As shown in ( 4 ), when the second conveying paddle  32   a  passes by and releases the control valve  34 , the control valve  34  quickly flips back open by its own elasticity, and forms a space. The control valve  34  moves between an open state and a closed state when contacted by the second conveying paddle  32   a , thereby forming a space between the control valve  34  and the communicating opening  33 . 
   As shown in ( 5 ), toner is drawn into the toner cartridge  32  from the developing unit  31  through the communicating opening  33 . 
   When the second conveying paddle  32   a  holds down the control valve  34  as shown in ( 2 ) and ( 3 ), toner on the control valve  34  is pushed into the developing unit  31  through the communicating opening  33 . 
   The control valve  34  shown in  FIG. 3  opens at the top; however, the control valve  34  can open at the bottom, and move the toner in the same manner. 
   The second conveying paddle  32   a  includes one bendable paddling film, and the rotation frequency is 0.04 hertz to 0.4 hertz. The paddling film is made of resin such as fluorine resin and polyester resin. Fluorine resin is particularly preferable because it has good slipping properties, and has a low friction coefficient. Moreover, the bendable, resin paddling film can be made longer than a paddling film made of a stiff material, because the length of the stiff film would have to be limited in order to rotate inside the toner cartridge  32 . Therefore, a space can be formed within the toner without leaving a dead space between the paddling film and the casing of the toner cartridge  32 . The second conveying paddle  32   a  includes one paddling film. If there are two paddling films, the toner in the toner cartridge  32  receives excessive pressure, and a space cannot be formed properly. The space needs to be formed for a sufficient time to let the toner from the developing unit  31  move into the space. However, if the toner in the toner cartridge  32  receives pressure, the space is quickly filled with the toner before the space reaches the vicinity of the communicating opening  33 . 
   The rotation frequency of the second conveying paddle  32   a  is 0.04 hertz to 0.4 hertz. If the rotation frequency is less than 0.04 hertz, the space moves too slowly, and by the time the space formed at the bottom of the toner cartridge  32  moves to the vicinity of the communicating opening  33 , the space is filled with toner. If the rotation frequency exceeds 0.4 hertz, the toner receives large pressure, and by the time the space formed at the bottom of the toner cartridge  32  moves to the vicinity of the communicating opening  33 , the space is filled with toner. 
   Accordingly, the developing device  30  can form a space in the toner with the second conveying paddle  32   a , which is a rotating body, in the toner cartridge  32 . 
     FIG. 4  is a diagram for explaining a space formed in the toner cartridge  32 , and a flow of toner from the developing unit  31  to the toner cartridge  32  in the developing device  30 . 
   The space forming unit described in  FIG. 4  includes the second conveying paddle  32   a  and the rib  35 , and does not include the control valve  34 . As shown in ( 1 ), the second conveying paddle  32   a  rotates in the toner cartridge  32 . As shown in ( 2 ), the second conveying paddle  32   a  contacts the rib  35 , and while the rib  35  is blocking toner, the second conveying paddle  32   a  continues rotating, so that a space is formed in the toner. As shown in ( 3 ), as toner crosses over the rib  35 , and the space reaches the communicating opening  33 , toner in the developing unit  31  flows into the toner cartridge  32  by pressure. 
   The second conveying paddle  32   a  includes one bendable paddling film. The paddling film rotates in contact with inner walls of the toner cartridge  32 , conveying toner from an upstream side of a rotational direction to the communicating opening  33 , without letting toner slipping into a space formed beneath the paddling film. At the communicating opening  33 , the pressure of the toner conveyed by the paddling film is stronger than toner from the developing unit  31 , so that the toner is sent into the developing unit  31 . Because the paddling film contacts the inner walls of the toner cartridge  32 , the toner above the paddling toner is prevented from slipping into the space, so that the space can be maintained for a sufficient time. A length L 1  of the paddling film corresponds to 110% to 170% of a length L from the center of the rotating body of the second conveying paddle  32   a  to the inner walls of the toner cartridge  32 . If the length L 1  is less than 110% of the length L, a dead space is left between the paddling film and the inner walls of the toner cartridge  32 . If the length L 1  exceeds 170% of the length L, the toner receives pressure for an excessive time. As a result, toner sticks to the inner walls of the toner cartridge  32 , and forms a toner film. The toner film becomes thick over time. 
   After the paddling film passes by the communicating opening  33 , the paddling film continues rotating in contact with the inner walls of the toner cartridge  32 , so that toner does not flow from the toner cartridge  32  to the developing unit  31 , and toner is discharged from the developing unit  31  to the space formed in the toner cartridge  32 . Because the toner above the paddling film is prevented from slipping into the space, the size of the space is maintained, so that toner can surely flow into the toner cartridge  32  from the developing unit  31 . 
   Because the space is surely formed for a sufficient time, toner that is well stirred in the developing unit  31  flows into the toner cartridge  32 . 
   The rib  35  is arranged in a range of −30% to +30% of the length L from a position vertically below the second conveying paddle  32   a.    
     FIG. 5  is another diagram for explaining a space formed in the toner cartridge  32 , and a flow of toner from the developing unit  31  to the toner cartridge  32  in the developing device  30 . An angle at which the paddling film contacts the rib  35  changes according to the position of the rib  35 . If the rib  35  is positioned upstream, which is −30% to 0% from the position vertically below the second conveying paddle  32   a , it is difficult to block the toner and form a space. This decreases the amount of toner that flows from the developing unit  31  to the toner cartridge  32 . On the other hand, if the rib  35  is positioned downstream, which is 0% to +30% from the position vertically below the second conveying paddle  32   a , the space becomes small. This also decreases the amount of toner that flows from the developing unit  31  to the toner cartridge  32 . 
   Accordingly, by arranging the rib  35  within a range of −30% to +30% from the position vertically below the second conveying paddle  32   a , a large space can be formed, thereby increasing the amount of toner that flows from the developing unit  31  to the toner cartridge  32 . 
   The length of the paddling film of the second conveying paddle  32   a  is longer than a distance from the center of the rotating body of the second conveying paddle  32   a  to the furthest edge of the communicating opening  33  by 10 millimeters or more. An average diameter of toner particles is 10 micrometers or less, and therefore, it is difficult to completely block the toner, regardless of the flowability. In reality, toner slips into the space formed by the paddling film. However, if the paddling film is longer by 10 millimeters or more than the distance from the center of the rotating body to the furthest edge of the communicating opening  33 , there is enough time for toner to be discharged from the developing unit  31  into the space formed in the toner cartridge  32  at the communicating opening  33 . 
   More than one communicating opening  33  can provided. By providing plural communicating openings  33 , the amount of toner flowing between the developing unit  31  and the toner cartridge  32  can be increased, and the amount of toner can be easily controlled. The communicating openings  33  are arranged in an axial direction, and the shape of each opening is not limited. The shape of each of the communicating openings  33  can be an oval, a rectangle, a diamond, or a parallelogram. Moreover, a mesh can be provided at the communicating opening  33 . 
     FIG. 6  is a schematic of an image forming apparatus  1  according to the first embodiment. The image forming apparatus  1  includes a photoconductor unit  10 , a writing optical device  20 , the developing device  30 , an intermediate transfer device  40 , a secondary transfer device  50 , a fixing device  60 , and a paper reversing device  70  used for duplex printing. Color images of black (hereinafter, “Bk”), cyan (hereinafter “C”), magenta (hereinafter, “M”), and yellow (hereinafter, “Y”) are sequentially formed on the photoconductive belt  11  of the photoconductor unit  10 , and are superposed to form a full-color image. Around the photoconductive belt  11 , there are arranged a photoconductor cleaning device  12 , a charging roller  13 , a plurality of developing devices  30 , an intermediate transfer belt  41  of the intermediate transfer device  40 . The photoconductive belt  11  is extended around a driving roller  14 , a primary transfer opposite roller  15 , and an extension roller  16 , and is rotated in a direction indicated by an arrow A by a driving motor. The writing optical device  20  converts color image data into optical signals, performs optical writing corresponding to each color image, and forms electrostatic latent images on the photoconductive belt  11 . The writing optical device  20  includes a semiconductor laser  21  as a light source, a polygon mirror  22 , and three reflecting mirrors  23   a ,  23   b , and  23   c.    
   The developing device  30  includes a Bk developing unit  30 K storing black toner, a C developing unit  30 C storing cyan toner, an M developing unit  30 M storing magenta toner, and a Y developing unit  30 Y storing yellow toner, in this order from the lower side of the image forming apparatus  1 . These are herein referred to as the developing device  30  when a particular color is not specified. A contact-separation mechanism is provided for moving each developing device to the left and the right as viewed in  FIG. 6  so as to contact and separate from the photoconductive belt  11 . 
   Toner inside the developing device  30  is charged to a predetermined polarity, a developing bias is applied to the developing sleeve  31   a  (see  FIG. 1A ) by a developing bias power source, and the developing sleeve  31   a  is biased to a predetermined potential with respect to the photoconductive belt  11 . The contact-separation mechanism includes an electromagnetic clutch, not shown, for communicating a driving force from a motor to each of the developing devices  30 . When the electromagnetic clutch is switched on, a driving force moves the developing device  30  toward the photoconductive belt  11 . At the developing process, a selected developing device  30  moves to contact the photoconductive belt  11 . When the electromagnetic clutch is turned off and the driving force is not communicated, the developing device  30  separates from the photoconductive belt  11 . 
   When the image forming apparatus  1  is in a standby state, the developing device  30  is set at a position separated from the photoconductive belt  11 . When the image forming operation starts, optical writing is performed with laser beams based on color image data, and electrostatic latent images are formed (hereinafter, an electrostatic latent image of Bk image data is referred to as a Bk electrostatic latent image; similarly for C, M, and Y). Just before the leading edge of the Bk electrostatic latent image reaches a Bk developing position, a Bk developing sleeve  31   a  starts to rotate, so that the Bk electrostatic latent image is developed with Bk toner. When the trailing edge of the Bk electrostatic latent image passes by the Bk developing position, the Bk developing unit  30 K separates from the photoconductive belt  11 , and the developing device  30  of the next color contacts the photoconductive belt  11 . These operations are completed at least before the leading edge of an electrostatic latent image of the next color reaches the corresponding developing position. 
   The intermediate transfer device  40  includes the intermediate transfer belt  41 , a belt cleaning device  42 , and a position detecting sensor  43 . The intermediate transfer belt  41  is extended around a driving roller  44 , a primary transfer roller  45 , a secondary transfer opposite roller  46 , a cleaning opposite roller  47 , and a tension roller  48 , and is driven by a not shown driving motor. A plurality of position detection marks is provided along the rim of the intermediate transfer belt  41 , outside an image forming area on the intermediate transfer belt  41 . Image formation starts from a time point when the position detecting sensor  43  detects any one of these position detection marks. The belt cleaning device  42  includes a cleaning brush  42   a , a contact-separation mechanism. While a Bk image, which is the first color image, and the second, third, and fourth color images are being transferred to the intermediate transfer belt  41 , the contact-separation mechanism separates the cleaning brush  42   a  from the intermediate transfer belt  41 . 
   The secondary transfer device  50  includes a secondary transfer roller  51  and a contact-separation mechanism that causes, with a clutch etc., the secondary transfer roller  51  to contact and separate from the intermediate transfer belt  41 . In synchronization with a timing when a transfer sheet  90  reaches a transfer position, the contact-separation mechanism causes the secondary transfer roller  51  move by pivoting on a rotational axis of the contact-separation mechanism. Accordingly, the transfer sheet  90  is pressed against the intermediate transfer belt  41  by the secondary transfer roller  51  and the secondary transfer opposite roller  46  at a predetermined pressure. Precision of the parallel position of the secondary transfer roller  51  to the secondary transfer opposite roller  46  is controlled by a positioning member, not shown, provided in the intermediate transfer device  40 . A positioning roller (not shown) provided in the secondary transfer roller  51  stabilizes the contact pressure of the secondary transfer roller  51  on the intermediate transfer belt  41 . When the secondary transfer roller  51  contacts the intermediate transfer belt  41 , a transfer bias of a polarity opposite to that of toner is applied to the secondary transfer roller  51 , so that toner images superposed on the intermediate transfer belt  41  are transferred to the transfer sheet  90  at once. 
   When the image forming operation starts, the transfer sheet  90  is conveyed by paper feeding rollers  81   a ,  81   b , and  81   c  from a transfer paper cassette  80  or a manual feed tray  83 , and is pressed against a nip between a pair of registration rollers  82 . When the leading edge of the superposed toner image on the intermediate transfer belt  41  reaches the secondary transfer roller  51 , the pair of registration rollers  82  is rotated such that the leading edge of the transfer sheet  90  is aligned with the leading edge of the toner images. The toner images on the intermediate transfer belt  41  contact the transfer sheet  90  as the transfer sheet  90  passes a secondary transfer position. The transfer sheet  90  is charged by the transfer bias of the secondary transfer roller  51 , so that the toner images are transferred onto the transfer sheet  90 . The transfer sheet  90  is conveyed to the fixing device  60 , and the toner images are melt-fixed onto the transfer sheet  90  at a nip between a fixing belt  61  and a pressurizing roller  62 . The transfer sheet  90  is sent out of the image forming apparatus  1  in a direction indicated by an arrow C, and stacked face-down on a paper discharge tray  84 . The operation of producing a full-color image is thus completed. 
   Duplex printing is performed as follows. After passing through the fixing device  60 , a duplex changeover claw  65  guides the transfer sheet  90  to the paper reversing device  70 . In the paper reversing device  70 , a reverse changeover claw  71  guides the transfer sheet  90  in a direction indicated by an arrow D. As the trailing edge of the transfer sheet  90  passes the reverse changeover claw  71 , a pair of reverse rollers  72  stops rotating, and the transfer sheet  90  stops. After a predetermined time, the pair of reverse rollers  72  rotates in a reverse direction, so that the transfer sheet  90  switches back. This time, the reverse changeover claw  71  is switched to guide the transfer sheet  90  in a direction indicated by an arrow E, to the pair of registration rollers  82 . The transfer sheet  90  is pressed against the nip between the pair of registration rollers  82 , and stops for a predetermined time. The pair of registration rollers  82  rotates to send the transfer sheet  90  to the secondary transfer position, superposed toner images on the intermediate transfer belt  41  are transferred onto the transfer sheet  90 , the toner images are melt-fixed onto the transfer sheet  90  at the fixing device  60 , and the transfer sheet  90  is sent out of the image forming apparatus  1 . 
   After primary transfer, the surface of the photoconductive belt  11  is cleaned by the photoconductor cleaning device  12 , and can be uniformly discharged by a discharging lamp etc. to facilitate the cleaning operation. After toner images are transferred to the transfer sheet  90 , the surface of the intermediate transfer belt  41  is cleaned by pressing the cleaning brush  42   a  against the intermediate transfer belt  41 . The toner cleaned off from the intermediate transfer belt  41  is accumulated in a toner waste tank  49 . 
   The developing device  30  includes the developing unit  31  (see  FIG. 1A ) including the developing sleeve  31   a  that rotates and carries toner on the surface thereof for developing an electrostatic latent image on the photoconductive belt  11 , and the first conveying paddle  31   d  (see  FIG. 1A ) that rotates to scoop and stir toner, and the toner cartridge  32  configured to store toner. The developing device  30  is divided into two units because the developing unit  31  has the durability to last while the toner cartridge  32  is replaced many times. 
     FIG. 7  is a detailed schematic of the communicating opening  33 . A slide shutter  31   e  is provided on the outside of the casing of the developing unit  31 , and an elastic member  31   f  adheres to the slide shutter  31   e . By opening and closing the slide shutter  31   e , the communicating opening  33  of the developing unit  31  is opened and closed. The toner cartridge  32  is provided with an elastic member  32   c  that has openings corresponding to the communicating opening  33 , a slide shutter  32   d  that prevents toner from spilling from the communicating opening  33  when closed and allows toner to pass through when open, and a fixing seal  32   e  that fixes the elastic member  32   c  and the slide shutter  32   d  to the casing of the toner cartridge  32 . The elastic member  32   c  is preferably made of foamed material such as urethane resin and silicon resin. 
   When the toner cartridge  32  is arranged in the developing device  30 , and the slide shutter  31   e  and the slide shutter  32   d  are open, the communicating opening  33  is open to let toner pass through. 
   When the toner cartridge  32  is not arranged in the developing device  30  or the image forming apparatus  1 , the slide shutter  31   e  is closed so that toner is prevented from spilling out of the developing unit  31  through the communicating opening  33 . 
   When the developing unit  31  is not arranged in the developing device  30  or the image forming apparatus  1 , the slide shutter  32   d  is closed so that toner is prevented from spilling out of the toner cartridge  32  through the communicating opening  33 . 
   Windows  31   g  are provided on the slide shutter  31   e . The windows  31   g  match the communicating opening  33 . When the communicating opening  33  is closed, the communicating opening  33  is blocked by the slide shutter  31   e  where there are no windows  31   g . When the communicating opening  33  is open, the slide shutter  31   e  is slid so that the windows  31   g  match with the communicating opening  33 . 
   In the developing device  30 , the first conveying paddle  31   d  stirs toner and conveys the toner to the supplying roller  31   b , the supplying roller  31   b  is rubbed against the developing sleeve  31   a , so that the toner charged by friction-charging. The charged toner is adheres to the developing sleeve  31   a  by image force, and is conveyed on the developing sleeve  31   a . The restricting roller  31   c  restricts the amount of toner conveyed by the developing sleeve  31   a  to the developing area. A thin toner layer formed on the developing sleeve  31   a  is developed onto the photoconductive belt  11  by a developing bias in the developing area. 
   When the toner on the supplying roller  31   b  is rubbed by the developing sleeve  31   a , the toner receives large pressure so that asperities on the surface of toner particles are crushed, which increases adherence of the toner. The large pressure pushes outer additives on the surface of toner particles inside. As a result, flowability of the toner decreases, and a charging amount of the toner decreases, because the charging amount cannot be adjusted by the outer additives. Thus, developing properties, transferring properties, and cleaning properties of the toner deteriorate. 
   When toner is consumed, the percentage of deteriorated toner increases in the developing unit  31 . Therefore, fresh toner is supplied into the developing unit  31  from the toner cartridge  32  through the communicating opening  33 . The second conveying paddle  32   a  and the third conveying paddle  32   b  provided in the first storage space  321  and the second storage space  322 , respectively, rotate in contact with the inner walls of the toner cartridge  32 . By rotation of the second conveying paddle  32   a  and the third conveying paddle  32   b , the toner is pushed into the developing unit  31  through the communicating opening  33 . 
   Toner in the developing unit  31  is discharged to the toner cartridge  32  through the communicating opening  33 , and the toner is mixed with toner in the toner cartridge  32 . The toner cartridge  32  stores a large amount of unused toner, which is mixed with the deteriorated toner from the developing unit  31 . Accordingly, outer additives adhering on surfaces of the unused toner particles are distributed to the deteriorated toner, so that chargeability and flowability of the deteriorated toner becomes close to that of the unused toner. The toner discharged from the developing unit  31  to the first storage space  321  is conveyed to the second storage space  322  by the second conveying paddle  32   a , and then returned to the first storage space  321  by the third conveying paddle  32   b . During this operation, the outer additives are distributed to the deteriorated toner. 
   The toner that is brought back to an unused state is returned to the developing unit  31  from the first storage space  321 . The toner brought back to an unused state and unused toner in the developing unit  31  form a thin layer on the developing sleeve  31   a  for developing a toner image. Accordingly, high quality images can be obtained over a long time. 
   In the developing device  30  according to the first embodiment, the control valve  34  is provided in the toner cartridge  32  for blocking the communicating opening  33 , so that the developing unit  31  discharges toner to the toner cartridge  32 , the toner is mixed with toner in the toner cartridge  32 , and the toner is supplied once again to the developing unit  31 . In a developing device  300  according to a second embodiment of the present invention, a control valve  340  is provided in the developing unit  31  at the communicating opening  33 , so that the developing unit  31  discharges toner to the toner cartridge  32 , the toner is mixed with toner in the toner cartridge  32 , and the toner is supplied once again to the developing unit  31 . Toner can be steadily supplied to and discharged from the developing device  300 . Moreover, the developing device  300  includes a first conveying paddle  310   d  that uniformly stirs toner in the developing unit  31 . 
     FIG. 8  is a schematic of the developing device  300 . 
   The components in the second embodiment that perform same or similar function or that have same or similar configuration as those in the first embodiment are denoted by the same reference numerals as the first embodiment, and overlapping descriptions are omitted. The developing device  300  includes the developing unit  31  and the toner cartridge  32 . The developing unit  31  includes the developing sleeve  31   a , the supplying roller  31   b , the restricting roller  31   c , and the first conveying paddle  310   d . The toner cartridge  32  includes the first storage space  321 , the second storage space  322 , the second conveying paddle  32   a , the third conveying paddle  32   b , and the rib  35 . Toner passes through the communicating opening  33  between the developing unit  31  and the toner cartridge  32 . The control valve  340  is provided in the developing unit  31  at the communicating opening  33 . 
   The control valve  340  has a similar structure to that of the control valve  34  according to the first embodiment, and is therefore described with reference to  FIG. 2 . The control valve  340  is provided corresponding to the communicating opening  33 , and is pasted to the support unit  34   a  fixed in the casing of the toner cartridge  32 . The control valve  340  includes rectangular plates, which are the elastic resin films  34   b , provided alternately corresponding to the communicating opening  33 , so that a plate is not provided where there is no opening. The support unit  34   a  is made of a rigid metal such as SUS, Cu, and Al. The elastic resin films  34   b  are made of polypropylene, polyethylene, polyester resin, fluorine resin, etc. 
   The first conveying paddle  310   d  has a paddling film that rotates and conveys toner supplied from the toner cartridge  32  to the developing sleeve  31   a . The paddling film of the first conveying paddle  310   d  can be single or plural. Specifically, the paddling film can be a single, long film, or a plurality of rectangular films that contact the rectangular films of the control valve  340  arranged in a comb-like form. A combination a long film and rectangular films can be employed as the paddling film of the first conveying paddle  310   d.    
     FIG. 9  is a diagram for explaining the operation of supplying toner from the toner cartridge  32  to the developing unit  31  in the developing device  300 . The first conveying paddle  310   d  rotates and contacts the control valve  340 , and holds down the control valve  340 . When the first conveying paddle  310   d  passes by and releases the control valve  340 , the control valve  340  quickly flips back open by its own elasticity, so that toner pushed towards the developing unit  31  from the toner cartridge  32  is drawn into the developing unit  31  through the communicating opening  33 . 
     FIG. 10  is a diagram for explaining movement of toner between the toner cartridge  32  and the developing unit  31  in the developing device  300 . In the toner cartridge  32 , toner in the second storage space  322  is conveyed to the first storage space  321  by the third conveying paddle  32   b , and is then conveyed to the developing unit  31  by the second conveying paddle  32   a . The second conveying paddle  32   a  includes a single paddling film, which is rotated to convey toner to the developing unit  31 . The first storage space  321  includes the rib  35 , so that toner is stopped at the rib  35 , and a space is formed between the rib  35  and the paddling film. Although the space is gradually filled with toner having high flowability, the space is maintained for a predetermined time. When the paddling film is further rotated, toner moves into the space from above, until the space is filled up. 
   Accordingly, when toner is pushed into the developing unit  31  at a time coinciding with when the control valve  340  is not held down by the paddling film of the first conveying paddle  310   d , i.e., when the control valve  340  is in an open state, toner moves from the toner cartridge  32  to the developing unit  31 . 
   The toner enters the developing unit  31  towards the control valve  340 . Subsequently, when the toner is pushed from the developing unit  31  to the toner cartridge  32  by rotation of the paddling film of the first conveying paddle  310   d  at a time coinciding with when a space formed by the second conveying paddle  32   a  reaches the communicating opening  33 , the toner is discharged from the developing unit  31  to the toner cartridge  32 . 
     FIGS. 11A to 11P  are detailed schematics for explaining movement of toner between the developing unit  31  and the toner cartridge  32 . The developing sleeve  31   a  etc. in the developing unit  31  are omitted herein. 
   In  FIG. 11A , the control valve  340  is arranged at an angle θ with respect to an inner wall of the developing unit  31 . The first conveying paddle  310   d  includes a plurality of paddling films, which are rotated. The second conveying paddle  32   a  and the third conveying paddle  32   b  each includes a single paddling film. As shown in  FIG. 11B , the first conveying paddle  310   d  rotates the plurality of paddling films that holds down the control valve  340 . If the control valve  340  is pressed to block the communicating opening  33  when toner in the toner cartridge  32  is contacting the communicating opening  33 , toner between the control valve  340  and the communicating opening  33  cannot enter the communicating opening  33 . Therefore, the toner in the developing unit  31  is pushed out from the sides of the control valve  340 , returning into the developing unit  31 . As shown in  FIG. 11C , the paddling films of the first conveying paddle  310   d  further presses the control valve  340 , so that there is substantially no space between the control valve  340  and the communicating opening  33 . As shown in  FIG. 11D  and  FIG. 11E , as each paddling film moves away from the control valve  340 , the control valve  340  returns to the position at the angle θ. A this position, a large space is formed between the control valve  340  and the communicating opening  33 , so that toner moves from the toner cartridge  32  to the developing unit  31  through the communicating opening  33 . 
   As shown in  FIG. 11F , the control valve  340  is pressed once again by another paddling film of the first conveying paddle  310   d . At this time point, the paddling film of the second conveying paddle  32   a  is contacting the rib  35  in the second storage space  322  of the toner cartridge  32 . As shown in  FIG. 11G , as the paddling film of the first conveying paddle  310   d  is further rotated, the control valve  340  is further pressed down, so that there is no space between the control valve  340  and the communicating opening  33 . At this time point, the paddling film of the second conveying paddle  32   a  rotates past the rib  35 , and toner is blocked by the rib  35 , so that a space is formed in the toner of the toner cartridge  32 . As shown in  FIG. 11H  and  FIG. 11I , the paddling film of the first conveying paddle  310   d  releases the control valve  340 , so that the control valve  340  returns to the angle θ, thereby forming a large space between the control valve  340  and the communicating opening  33 . Thus, toner that is lifted up by the paddling film of the second conveying paddle  32   a  moves from the toner cartridge  32  into the developing unit  31  through the communicating opening  33 . 
   The control valve  340  is pressed yet once again by the other paddling film of the first conveying paddle  310   d . At the previous time the control valve  340  was pressed down, there was toner in the toner cartridge  32  near the communicating opening  33 . Therefore, the toner in the developing unit  31  was pushed out from the sides of the control valve  340 , returning into the developing unit  31 . This time, however, because there is a space in the toner cartridge  32  near the communicating opening  33 , the toner is discharged from the developing unit  31  into the toner cartridge  32  through the communicating opening  33  as the paddling film of the first conveying paddle  310   d  presses the control valve  340 , as shown in  FIGS. 11J to 11L . 
   By rotating the first conveying paddle  310   d  at a higher speed than the second conveying paddle  32   a , toner is discharged from the developing unit  31  to the toner cartridge  32 , as shown in  FIGS. 11M to 11P . 
   These operations are repeated so that toner moves between the developing unit  31  and the toner cartridge  32  through the communicating opening  33 . 
   Rotational frequencies of the first conveying paddle  310   d  in the developing unit  31  and the second conveying paddle  32   a  in the toner cartridge  32  can be controlled to adjust the amount of toner moving between the developing unit  31  and the toner cartridge  32 . For example, by rotating the first conveying paddle  310   d  in the developing unit  31  at a higher rotational frequency than the second conveying paddle  32   a  in the toner cartridge  32 , the number of times that a space formed in the toner cartridge  32  contacts the communicating opening  33  is decreased, while the number of times that the first conveying paddle  310   d  presses the control valve  340  is increased. Accordingly, the number of times that toner is supplied into the developing unit  31  is increased. 
   The amount of toner moving between the developing unit  31  and the toner cartridge  32  can be adjusted by the number of the communicating openings  33 . More than one communicating opening  33  can provided. The number of communicating openings  33  is determined based on the speed of the image forming operation. 
   The control valve  340  provided corresponding to the communicating opening  33  can be a comb-like form, including a plurality of valves. The control valves  340  adjacent to each other can be operated alternately. Specifically, each of the two paddling films of the first conveying paddle  310   d  can be a comb-like form with films provided alternately, so that half of the control valves  340  are pressed by one paddling film, and the other half of the control valves  340  are pressed by the other paddling film. Accordingly, toner in the developing unit  31  can be evenly discharged, without leaving a dead space. 
     FIG. 12  is a perspective view of the first conveying paddle  310   d.    
   Films  311  and a rectangular film  312  are attached to a quadrangle axis. The films  311  include two films, each extending in opposite directions from opposite surfaces. The films  311  are in a comb-like form with alternate concavities and convexities, and the convexities of the two films  311  are shifted from each other. Thus, adjacent control valves  340  are alternately pressed by the two films  311 . The convexities are provided corresponding to the communicating opening  33 , and are long enough to press down the control valve  340 . The concavities are arranged so as not to contact the control valve  340 . 
   Each convexity is tapered so that the base of the convexity is wider than the tip. Accordingly, the convexities also move toner in a longitudinal direction of the developing unit  31  as the first conveying paddle  310   d  rotates, so that the toner is stirred in the longitudinal direction. 
   As the first conveying paddle  310   d  rotates, the convexities press the control valve  340 , so that toner under the control valve  340  moves into the toner cartridge  32 . 
   The first conveying paddle  310   d  is rotated at a higher speed than the second conveying paddle  32   a , so that the first conveying paddle  310   d  presses the control valve  340  several times while a space is formed in the toner cartridge  32 , thereby efficiently conveying toner to the toner cartridge  32 . 
   When the convexities of the films  311  pass by the control valve  340 , the control valve  340  is released from suppress strength, and flips back open by elasticity. Accordingly, toner on the control valve  340  is returned into the developing unit  31 , and a space for letting toner in from the toner cartridge  32  is formed under the control valve  340 . 
   The convexities of the two films  311  are shifted from each other, so that the control valves  340  adjacent to each other are alternately pressed and released. Accordingly, toner is efficiently circulated between the developing unit  31  and the toner cartridge  32 . 
   The first conveying paddle  310   d  includes the two comb-like films  311 , and therefore, toner is efficiently circulated, and mixed in the longitudinal direction. However, with only the two comb-like films  311 , toner accumulates near the communicating opening  33  in the developing unit  31 . As a result, the toner surface in the developing unit  31  ripples in a vertical direction, forming mountain parts and valley parts. If a mountain part is formed at the communicating opening  33 , toner from the toner cartridge  32  is inhibited from moving in, and the amount of toner supply in the developing unit  31  decreases. Moreover, a toner flow from the toner cartridge  32  is created at the foot of the mountain part. As a result, the toner is somewhat inhibited from being evenly stirred. To improve these problems, the rectangular film  312  that has no convexities or concavities, which is shorter than the convexity of the films  311 , is provided between the two films  311  at an angle of 90 degrees with respect to the films  311 . Accordingly, the mountain part of the toner is eliminated, so that the surface of toner in the developing unit  31  is substantially flat. 
   The rectangular film  312  is provided between the two films  311  to steadily move toner between the developing unit  31  and the toner cartridge  32 , so that the toner is sufficiently circulated. The rectangular film  312  also prevents a toner flow from being created at a local position, so that the toner is evenly stirred in the developing unit  31 . 
     FIG. 13  is a schematic of the communicating opening  33  in the developing device  300 . Each of the communicating openings  33  is diamond-shaped in the second embodiment; however, the communicating opening  33  can be rectangular, oval, etc. A diamond shape is more advantageous than a rectangle, because the width of each of the communicating openings  33  is wider, and toner can be spread further in a longitudinal direction. Thus, with diamond-shaped communicating openings  33 , toner can be circulated more efficiently between the toner cartridge  32  and the developing unit  31 , and the number of films configuring the control valve  340  can be reduced. 
   The width of the control valve  340  is equal to or wider than the width of the communicating opening  33  by less than 20 millimeters. If the control valve  340  is narrower than the communicating opening  33 , toner in the developing unit  31  blocks the communicating opening  33 , thereby inhibiting toner supply from the toner cartridge  32 . Toner in between the control valve  340  and the communicating opening  33  is discharged from the developing unit  31  to the toner cartridge  32 , so that a large amount of toner is not discharged at once. However, if the control valve  340  is narrower than the communicating opening  33 , a large amount of toner is discharged, and the amount of toner in the developing unit  31  becomes insufficient. 
   On the other hand, if the control valve  340  is excessively wider than the communicating opening  33 , the amount of toner that enters from the sides of the control valve  340  in between the control valve  340  and the communicating opening  33  is reduced. Accordingly, the amount of toner discharged from the developing unit  31  to the toner cartridge  32  is reduced, so that deteriorated toner is not sufficiently replaced with fresh toner. The toner supplied from the toner cartridge  32  moves to the bottom of the communicating opening  33 , and is mixed with toner at the bottom of the control valve  340 . Thus, if the control valve  340  is excessively wide, the amount of toner supplied is reduced, and therefore, the toner is not mixed sufficiently evenly. 
   The width of the control valve  340  is preferably equal to or wider by less than 20 millimeters than the width of the communicating opening  33 . Accordingly, the amount of toner supplied and discharged is controlled, and toner supplied in the developing unit  31  is mixed evenly. 
   Intervals between the control valves  340  are 2 millimeters to 20 millimeters. If the intervals are less than 2 millimeters, the amount of toner that enters in between the control valve  340  and the communicating opening  33  decreases, so that the amount of toner discharged from the developing unit  31  decreases. If the intervals exceed 20 millimeters, the number of communicating openings  33  decreases, so that the amount of toner discharged from and supplied to the developing unit  31  decreases. 
   The length of the control valve  340  is 10 millimeters to 25 millimeters. The length of the control valve  340  determines the size of the space formed between the control valve  340  and the communicating opening  33 . Thus, if the length of the control valve  340  is less than 10 millimeters, the amount of toner discharged from the developing unit  31  decreases, so that deteriorated toner is not sufficiently replaced with fresh toner. If the length of the control valve  340  exceeds 25 millimeters, the amount of toner discharged from the developing unit  31  increases, so that the amount of toner in the toner hopper in the developing unit  31  is insufficient. 
   The angle of the control valve  340  is 20 degrees to 45 degrees when open, and 0 degrees to 15 degrees when closed, against the communicating opening  33 . The control valve  340  bends because it is elastic. Therefore, the angle of the control valve  340  is defined assuming that a line connecting the point where the control valve  340  contacts the wall of the developing unit  31  and the tip of the control valve  340  is straight. The angle of the control valve  340  determines the size of the space formed between the control valve  340  and the communicating opening  33 . Thus, if the angle of the control valve  340  is less than 20 degrees when open, the amount of toner discharged from the developing unit  31  decreases, so that replacement of deteriorated toner with fresh toner is not performed sufficiently. If the angle of the control valve  340  exceeds 45 degrees when open, an excessive amount of toner is discharged from the developing unit  31 , so that the amount of toner in the toner hopper of the developing unit  31  is insufficient. 
   The developing device  300  can be used in the same image forming apparatus as described in the first embodiment with reference to  FIG. 6 , and therefore, overlapping explanations are omitted. Details of the communicating opening  33  are the same as described in the first embodiment with reference to  FIG. 7 , and therefore, overlapping explanations are omitted. 
   The developing device according to the first and the second embodiments can be integrated with at least a photoconductor, and employed in a process cartridge that is detachably attached to an image forming apparatus. In the developing device, outer additives are reapplied to deteriorated toner so that high-quality images can be obtained over a long time. 
   The developing device  300  according to the second embodiment is compared with a conventional developing device. The conventional developing device is configured in such a manner that the control valve  340  and the rib  35  from the developing unit  30 K, and the films  311  and  312  of the first conveying paddle  310   d  is replaced by a single paddling film, similarly as the second conveying paddle  32   a . With this configuration, in the conventional developing device, the toner passing through the communicating opening  33  flows in substantially one way from the toner cartridge  32  to the developing unit  31 , and unlike in the developing device  300  according to the second embodiment, is not virtually discharged to the toner cartridge  32  in the developing unit  31  through the communicating opening  33 . 
   A series of experiments have been performed to compare toner charging characteristics between the developing device  300  according to the second embodiment and the conventional developing device. The developing unit  30 K is set in the image forming apparatus  1 . After outputting 10,000 sheets with an image area ratio of 3%, each of the toner in the developing unit  31  and the toner in the toner cartridge  32  is collected to evaluate a charging amount of the toner.  FIG. 14  is a schematic of a toner-charging-amount evaluating apparatus. The evaluation of the toner charging amount was performed using an E-SPART analyzer (evaluating apparatus) manufactured by HOSOKAWA MICRON COORPORATION, shown in  FIG. 14 . 
   The E-SPART analyzer can obtain a particle diameter and a charging amount at the same time by measuring a phase lag and a deflection amount of a charged particle that moves in response to a vibration by an acoustic wave and an influence by an electric field, using a laser Doppler method. As shown in  FIG. 14 , the E-SPART analyzer includes a measurement area  100 , a particle input port  101 , a particle output port  102 , an acoustic vibration plate  103 , an electrode  104 , a focusing lens  105 , and a photo multiplier tube  106 . 
     FIG. 15  is a graph of a result of evaluating a toner charging amount according to the second embodiment.  FIG. 16  is a graph of a result of evaluating a toner charging amount in the conventional developing device. 
   As shown in  FIG. 16 , in the conventional developing device, a peak of the toner charging amount in the developing unit  31  is deviated from a peak of the toner charging amount in the toner cartridge  32 . When there are mixed toners having different charging amounts, a background contamination occurs, and when the mixed amount is not even, a density fluctuation occurs in a vertical strip. 
   On the other hand, as shown in  FIG. 15 , the peak of the toner charging amount in the developing unit  31  almost matches with the peak of the toner charging amount in the toner cartridge  32  in the developing device  300  according to the second embodiment. In the case of the second embodiment, there is no difference in the charging amounts, and even at a time of outputting 1,000 sheets, a good image with no background contamination or density fluctuation could be obtained. It is because the film presses the control valve  340  by a rotation of the first conveying paddle  310   d , so that the toner under the control valve  340  returns to the toner cartridge  32 , resulting in an enough mixing of the toner in the toner cartridge  32  and the toner in the developing unit  31 . 
   Although the above experiments were performed using the black toner, even for the yellow toner, the magenta toner, and the cyan toner, it was confirmed that the toner in the toner cartridge  32  and the toner in the developing unit  31  were sufficiently mixed. 
   To evaluate a stability of time-dependent toner charging, following experiment was performed using the developing unit  30 K of the developing device  300  according to the second embodiment and the conventional developing device. 
   The developing unit  30 K in a state in which the toners are filled enough in the developing unit  31  and the toner cartridge  32  (a state of a product to be shipped) is set in the image forming apparatus  1 , and an image formation was performed until the toner end is displayed with the image area ratio of 2%. After that, the toner cartridge  32  was replaced with a new one, and the image formation was continued. 
   To evaluate the stability of the time-dependent toner charging for the developing device  300  according to the second embodiment and the conventional developing device, the toner on the developing sleeve  31   a  was collected at (a) the time when the developing unit  30 K was set in the image forming apparatus  1 , (b) the time when the toner end was displayed, and (c) the time when the toner cartridge  32  was replaced with a new one (however, the evaluation was performed after operating the developing device for about 90 seconds after replacing the toner cartridge, to figure out mixing state of the toner), to evaluate the toner charging amount. The evaluation of the toner charging amount was performed using the E-SPART analyzer manufactured by HOSOKAWA MICRON COORPORATION. 
     FIG. 17  is a graph of a result of evaluating the toner charging amount of the developing device  300  according to the second embodiment.  FIG. 18  is a graph of a result of evaluating the toner charging amount of the conventional developing device. 
   The peak of the toner charging amount tends to shift to a side of a low inverse-charging area with time in both cases. However, compared with the conventional developing device, an increase of a low inverse-charged toner is suppressed in the developing device  300  according to the second embodiment. 
   As for the toner charging amount, a high charging area is set to equal to or less than −2 (fc/μm: femto-coulomb/micrometer), the low inverse-charging area is set to equal to or more than −0.2, and an intermediate area between the high charging area and the low inverse-charging area is set to a medium charging area. If a percentage of the toner in the low inverse-charging area increases, the background contamination or the density fluctuation occurs. 
     FIG. 19  is a graph of a percentage of a low inverse-charged toner when the toner cartridge  32  is replaced. As shown in  FIG. 19 , compared with the conventional developing device, the developing device  300  according to the second embodiment shows the percentage of the low inverse-charged toner decreased by about 35%, which means that the toner in the toner cartridge  32  and the toner in the developing unit  31  were sufficiently mixed, indicating a good circulation of the toner. 
     FIG. 20  is a graph for showing the collected toner at (a) to (c) divided into each of the charging areas. Compared with the conventional developing device, the developing device  300  according to the second embodiment can reduce changes of percentages of the toners in the high charging area and the low inverse-charging area, making it possible to stabilize the toner charging amount on the developing sleeve  31   a  from the time when the developing unit  30 K is set to the time when the toner cartridge  32  is replaced with a new one. 
     FIG. 21  is a graph of a result after 4-cycle execution, taking (a) to (c) as one cycle. In the second to the fourth cycles, the evaluation of the toner charging amount was performed also at an intermediate time between (a) and (b), and a result of the evaluation was added. 
   For all of the four cycles, compared with the conventional developing device, the developing device  300  according to the second embodiment can reduce changes of percentages of the toners in the high charging area and the low inverse-charging area, making it possible to stabilize the toner charging amount on the developing sleeve  31   a  over a long period of time. 
   INDUSTRIAL APPLICABILITY 
   The developing device, the image forming apparatus, and the process cartridge according to the present invention is useful in forming images by an electrostatic copying process, and particularly useful in preventing a decrease in chargeability and flowability of toner.