Abstract:
An image forming apparatus, a process cartridge, and a developing unit includes a developer carrying member to carry developer, first and second rotary members arranged in parallel to each other and configured to rotate to agitate and convey the developer, and an enclosure having an inside space to contain the developer, the inside space being divided by a partition with communication openings formed therein at opposite ends thereof into a first chamber configured to maintain the developer above a first level and to hold the first rotary member therein which supplies the developer to the developer carrying member while agitating and conveying the developer, and a second chamber configured to communicate with the first chamber through the communication openings, to maintain the developer at a second level lower than the first level, and to hold the second rotary member therein which circulates the developer with the first chamber through the communication openings.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 12/506,764 filed Jul. 21, 2009 now U.S. Pat. No. 7,957,678, which is a division of U.S. application Ser. No. 11/287,305 filed Nov. 28, 2005 (now U.S. Pat. No. 7,650,101 issued on Jan. 19, 2010), and claims the benefit of priority to Japanese Patent Application No. 2004-341895 filed in the Japanese Patent Office on Nov. 26, 2004 and Japanese Patent Application No. 2005-250836 filed in the Japanese Patent Office on Aug. 31, 2005, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     This specification generally describes an apparatus for image forming, and more particularly describes an apparatus for image forming capable of effectively developing images. 
     2. Discussion of the Background 
     There is a widely known image forming apparatus having a developing unit including two conveyance screws and a development roller disposed above one of the two conveyance screws. 
       FIG. 1  is a cross-sectional view of a background developing unit  5  used in a background image forming apparatus. The developing unit  5  includes a first screw  55 , a second screw  56 , and a development roller  51  disposed above the first screw  55 .  FIG. 2  is a perspective view of the first screw  55  and the second screw  56 .  FIG. 3  illustrates another aspect of the background developing unit  5  of  FIG. 1 . 
     Referring to  FIG. 1 , the developing unit  5  further includes a developer-containing portion divided by a partition into a first chamber  53  and a second chamber  54 . The first chamber  53  is provided with the first screw  55 , and the second chamber  54  is provided with the second screw  56 . Developer dispensed into the second chamber  54  from above may be agitated and conveyed by the second screw  56  to the first chamber  53 . Part of the developer in the first chamber  53  is to be picked up by the development roller  51 . 
     Referring to  FIG. 2 , the first screw  55  and the second screw  56  have a substantially equal shape, volume, and conveying speed. That is, the first screw  55  and the second screw  56  convey a substantially equal amount of developer per time unit. The first chamber  53  and second chamber  54  have a substantially equal volume. Therefore, a surface of the developer in the first chamber  53  and a surface of the developer in the second chamber  54  may be at a substantially equal height. 
     As illustrated in  FIG. 1 , when the first chamber  55  is filled with the developer, the second chamber  54  is also filled with the developer. When the second screw  56  in the second chamber  54  is buried in the developer, an upper portion of the developer may not be sufficiently agitated by the second screw  56 . In other words, the developer may not be sufficiently charged. Those skilled in the art may appreciate that the use of insufficiently charged toner for development may cause drawbacks such as background contamination and a toner spatter. 
     On the other hand, as illustrated in  FIG. 3 , when the second chamber  54  has a relatively low level of developer, the first chamber  53  also has a relatively low level of developer. When the first chamber  53  has a relatively low developer, the height of the surface of the developer may vary before and after movement of a blade portion of the first screw  55 Y. As a result, an amount of the developer picked up by the development roller  51 Y may be unstable, and an abnormal image referred to as a conveyance-screw-pitch irregularity may be caused. When any rotary member for conveying developer by rotation is used instead of the screws, there is a possibility that a similar kind of abnormal image is caused. 
     SUMMARY 
     An image forming apparatus, a process cartridge, and a developing unit includes a developer carrying member to carry developer, first and second rotary members arranged in parallel to each other and configured to rotate to agitate and convey the developer, and an enclosure having an inside space to contain the developer, the inside space being divided by a partition with communication openings at opposite ends thereof into a first chamber configured to maintain the developer above a first level and to hold the first rotary member therein which supplies the developer to the developer carrying member while agitating and conveying the developer, and a second chamber configured to communicate with the first chamber through the communication openings, to maintain the developer at a second level lower than the first level, and to hold the second rotary member therein which circulates the developer with the first chamber through the communication openings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional view of a developing unit of a background image forming apparatus in a state where both of a first chamber and a second chamber are filled with developer; 
         FIG. 2  is a perspective view of a first screw and a second screw used in the developing unit of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the developing unit of  FIG. 1  in a state where both of the first chamber and the second chamber have a relatively low level of developer; 
         FIG. 4  is a schematic diagram of an example image forming apparatus according to an example embodiment; 
         FIG. 5  is an enlarged sectional view of a process cartridge of the image forming apparatus of  FIG. 4 ; 
         FIG. 6  is a perspective view of a toner dispensing system for dispensing toner from a toner bottle to a developing unit of the process cartridge of  FIG. 5 ; 
         FIG. 7  is an illustration for explaining a configuration of the developing unit; 
         FIG. 8  is a perspective view of the developing unit with a top cover removed; 
         FIG. 9  is a perspective view of the developing unit of  FIG. 8  with a development roller and a frame member further removed; 
         FIG. 10  is a perspective view of a first screw and the second screw used in the developing unit; 
         FIG. 11  is an illustration of the developing unit in a state that developer in a first chamber has a higher surface than developer in a second chamber; 
         FIGS. 12 and 13  are perspective views of pairs of a first screw and the second screw used in the developing unit of  FIG. 11  according to different example embodiments; 
         FIG. 14  is a cross-sectional view of a developing unit according to another example embodiment; 
         FIGS. 15 ,  16 , and  17  are perspective views of pairs of a first screw and the second screws used in the developing unit  10  according to different example embodiment; and 
         FIG. 18  is a cross-sectional top view of a developing unit according to another example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 4 , an image forming apparatus  100  according to an example embodiment is described. 
     As illustrated in  FIG. 4 , the image forming apparatus  100  includes four process cartridges  6 Y (yellow),  6 M (magenta),  6 C (cyan), and  6 K (black), an exposure unit  7 , an intermediate image transfer unit  15 , a secondary transfer roller  19 , a fixing unit  20 , a sheet cassette  26 , a feed roller  27 , a registration roller pair  28 , an output roller pair  29 , a stacking portion  30 , and a bottle container  31 . The intermediate image transfer unit  15  includes an intermediate image transfer belt (simply belt hereinafter)  8  as an intermediate image transfer body being stretched and endlessly moved. 
     The four process cartridges  6 Y,  6 M,  6 C, and  6 K for forming toner images of yellow, magenta, cyan, and black, respectively, have substantially similar configurations except that toner of different colors are used as image forming materials. The process cartridges  6 Y,  6 M,  6 C, and  6 K can be replaced when reaching their end of life. In addition, the process cartridges  6 Y,  6 M,  6 C, and  6 K are detachable from the image forming apparatus  100  so that consumable parts may be replaced at a time. In this specification, the process cartridge  6 Y for forming a yellow toner image will be described as a representative example for purposes of explanation. 
     As illustrated in  FIG. 5 , the process cartridge  6 Y includes a photoconductive member  1 Y serving as a drum-type image carrier, a drum cleaner unit  2 Y, a charger unit  4 Y, and a developing unit  5 Y. 
     The charger unit  4 Y uniformly charges a surface of the photoconductive member  1 Y rotated clockwise as viewed in  FIG. 5  by a driving device. When the uniformly charged surface of the photoconductive member  1 Y is scanned by, that is, exposed to a laser beam L, the photoconductive member  1 Y carries a latent image. The developing unit  5 Y develops the latent image into a yellow toner image using yellow toner. Then, the yellow toner image is transferred onto the belt  8 , which operation is referred to an intermediate image transfer. The drum cleaner unit  2 Y cleans the surface of the photoconductive member  1 Y by removing toner remaining after the intermediate image transfer. Charges remaining on the surface of the photoconductive member  1 Y after the cleaning may be discharged by a discharger unit (not shown), so that the surface of the photoconductive member  1 Y is initialized so as to be ready for the next image formation. 
     In the other process cartridges  6 M,  6 C, and  6 K, toner images of magenta, cyan, and black are formed and are transferred onto the intermediate image transfer belt  8  in a similar manner. 
     Under the process cartridges  6 Y,  6 M,  6 C, and  6 K as viewed in  FIG. 4 , there is disposed the exposure unit  7 . The exposure unit  7  includes a light source (not shown) emitting the laser beam L to irradiate each photoconductive member of the process cartridges  6 Y,  6 M,  6 C, and  6 K according to image information of yellow, magenta, cyan, and black, respectively. In the exposure unit  7 , for each color, the laser beam L is generated by the light source thereof and is reflected and adjusted by a polygon shaped mirror, rotationally driven by a motor, and a plurality of optical lenses and mirrors to scan the corresponding photoconductive member. According to the exposure, latent images of yellow, magenta, cyan, and black are formed on the photoconductive members  1 Y,  1 M,  1 C, and  1 K, respectively. 
     A sheet feeding system includes the sheet cassette  26 , the feed roller  27 , and the registration roller pair  28 . Here, a plurality of sheet cassettes  26  may be provided. The sheet cassette  26  is loaded with a stack of sheets P of transfer paper as a recording medium, and a top sheet of the sheets P (i.e. the sheet P) in the sheet cassette  26  is in contact with the feed roller  27 . The feed roller  27  is rotated counterclockwise as viewed in  FIG. 4  to transport the sheet P toward the registration roller pair  28 . Both rollers of the registration roller pair  28  are rotationally driven to hold the sheet P. Immediately after holding the sheet P, the registration roller pair  28  temporarily stops rotating. Then with suitable timing in synchronism with transfer of the image on the belt  8 , the registration roller pair  28  sends the sheet P toward the secondary transfer roller  19 . 
     Above the process cartridges  6 Y,  6 M,  6 C, and  6 K, the intermediate image transfer unit  15  is disposed. In addition to the belt  8 , the intermediate image transfer unit  15  includes four primary transfer bias rollers  9 Y,  9 M,  9 C, and  9 K and a cleaning unit  10 . 
     The intermediate image transfer unit  15  further includes a secondary transfer backup roller  12 , a cleaning backup roller  13 , and a tension roller  14 . The belt  8  is stretched across the three rollers forming a loop and is endlessly moved counterclockwise as viewed in  FIG. 4  by at least one of the rollers rotationally driven. The endlessly moved belt  8  is held between the primary transfer bias rollers  9 Y,  9 M,  9 C, and  9 K and the photoconductive members  1 Y,  1 M,  1 C, and  1 K, respectively, to form respective nips for a primary image transfer. Each of the primary transfer bias rollers  9 Y,  9 M,  9 C, and  9 K applies a transfer bias having an opposite polarity (e.g. a positive polarity) from the polarity of the toner to a back side (i.e. an inside of the loop) of the belt  8 . All rollers other than the primary transfer bias rollers  9 Y,  9 M,  9 C, and  9 K are electrically grounded. 
     While the belt  8  is endlessly moved, the belt  8  sequentially passes through the nips for the primary image transfer for yellow, magenta, cyan, and black so that toner images of yellow, magenta, cyan, and black on the photoconductive members  1 Y,  1 M,  1 C, and  1 K are sequentially transferred to the belt  8  in a superposed manner, which operation is referred to as a primary image transfer. Thus, a four- or multi-color toner image, hereinafter referred to as a multi-color toner image, is created. 
     The secondary transfer backup roller  12  and the secondary transfer roller  19  hold the belt  8  therebetween to form a nip for a secondary image transfer. At the nip for the secondary image transfer, the multi-color toner image formed on the belt  8  is transferred onto the sheet P, which operation is referred to as the secondary image transfer. Toner remaining on the belt  8  after passing through the nip for the secondary image transfer is cleaned by the cleaning unit  10 . 
     At the nip for the secondary image transfer, the sheet P is held between the belt  8  and the secondary transfer roller  19 , both of which surfaces are moved in a forward direction. As a result, the sheet P is transported by the nip for the secondary image transfer toward the fixing unit  20 . 
     When the sheet P passes between rollers of the fixing unit  20 , the transferred multi-color toner image on a surface of the sheet P is fixed with heat and pressure. Thereafter, the sheet P is sent out of the apparatus through rollers of the output roller pair  29  onto the stacking portion  30  formed on an external top of the image forming apparatus  100 . The sheet P is sequentially stacked on the stacking portion  30 . 
     The bottle container  31  is disposed between the intermediate image transfer unit  15  and the stacking portion  30 . The bottle container  31  houses toner bottles  32 Y,  32 M,  32 C, and  32 K as developer containers for containing toner of yellow, magenta, cyan, and black, respectively. Toner bottles  32 Y,  32 M,  32 C, and  32 K are placed into respective locations in the bottle container  31  from above. The yellow, magenta, cyan, and black toner contained in the toner bottles  32 Y,  32 M,  32 C, and  32 K, respectively, are appropriately dispensed into the respective developing units of the process cartridges  6 Y,  6 M,  6 C, and  6 K by a toner dispensing system described below. Each of the toner bottles  32 Y,  32 M,  32 C, and  32 K and the process cartridges  6 Y,  6 M,  6 C, and  6 K are independently detachable from the image forming apparatus  100 . 
     Referring to  FIG. 6 , a toner dispensing system  40 Y for dispensing toner from the toner bottle  32 Y to the developing unit  5 Y will be described.  FIG. 6  depicts a developing unit  5 Y portion of the process cartridge  6 Y. In the image forming apparatus  100 , the toner dispensing system  40 Y is disposed under a position where the toner bottle  32 Y is placed. 
     The toner dispensing system  40 Y includes a conveyance pipe  43 Y for conveying toner to the developing unit  5 Y and a toner hopper portion  48 Y. The conveyance pipe  43 Y has an opening  45 Y. The developing unit  5 Y includes, for being dispensed with toner, a toner dispensing portion  58 Y and a toner-dispensing hole  61 Y. 
     By sliding the process cartridge  6 Y in the direction indicated by an arrow α, the process cartridge  6 Y is placed into the image forming apparatus  100 , and an edge of the conveyance pipe  43 Y is engaged with the toner dispensing portion  58 Y of the developing unit  5 Y. New toner filled in the toner bottle  32 Y is first dispensed into the toner hopper portion  48 Y of the toner dispensing system  40 Y. The toner stored in the toner hopper portion  48 Y is dispensed into the developing unit  5 Y when a toner density is determined to be low by a toner detection device (not shown) in the developing unit  5 Y. The toner in the toner hopper portion  48 Y is supplied into the toner-dispensing hole  61 Y of the developing unit  5 Y through the conveyance pipe  43 Y and from the opening  45 Y of the conveyance pipe  43 Y. 
     In this manner, the toner and the carriers which serve as the developer are contained in the developing unit  5 Y in advance. The toner consumed by development is replenished from the toner bottle  32 Y into the developing unit  5 Y. 
     It is to be noted that, although the developing unit  5 Y according to the example embodiment uses two-component developer including toner and carriers, a single component developer may also be used. 
     Referring now to  FIGS. 7 through 9 , the developing unit  5 Y in the process cartridge  6 Y is described in detail. As illustrated in  FIG. 7 , the developing unit  5 Y includes a development roller  51 Y serving as a developer carrying member, a partition  59 Y, a first chamber  53 Y, a second chamber  54 Y, a first screw  55 Y serving as a first rotary member, a second screw  56 Y serving as a second rotary member, magnets P 1 , P 2 , P 3 , P 4 , and P 5 , a doctor blade  52 Y, and a top cover  70 Y.  FIG. 8  is a perspective view of the developing unit  5 Y with the top cover  70 Y removed. As illustrated in  FIG. 8 , the developing unit  5 Y further includes a frame member  71 Y including a communicating hole  72 Y.  FIG. 9  is a perspective view of the developing unit  5 Y similar to  FIG. 8  with the development roller  51 Y and the frame member  71 Y further removed. 
     Referring to  FIG. 7 , the development roller  51 Y is partly exposed to the outside via an opening of a casing of the developing unit  5 Y. The development roller  51 Y includes a magnet roller, which is a magnetic field generation device having five magnets P 1 , P 2 , P 3 , P 4 , and P 5 , and a developer sleeve rotating coaxially around the magnetic roller. 
     The magnets P 3  and P 4  are disposed to generate magnetic fields having a common polarity. Each of the magnets P 1 , P 2 , and P 5  is disposed to generate a magnetic field having a polarity opposite to a polarity of the adjacent magnets. 
     The doctor blade  52 Y is disposed at a location opposed to a downstream side of the magnet P 5  in a surface movement direction of the developer sleeve. The doctor blade  52 Y regulates the film thickness of the developer. 
     Below the development roller  51 Y, there is provided an enclosure having an inside space configured to contain the developer, which is divided by the partition  59 Y into the first chamber  53 Y on a side of the development roller  51 Y and the second chamber  54 Y. The first chamber  53 Y holds a first screw  55 Y, and the second chamber  54 Y holds the second screw  56 Y. Each of the first screw  55 Y and the second screw  56 Y has a spiral screw shape having an axis portion and a blade portion. 
     Referring to  FIG. 8 , between the development roller  51 Y and the first screw  55 Y, there is provided the frame member  71 Y including the communicating hole  72 Y through which the developer is supplied to the development roller  51 Y. 
     Referring to  FIG. 9 , the partition  59 Y forms a first opening  59 Ya and a second opening  59 Yb through which the developer may pass between the first chamber  53 Y and the second chamber  54 Y. 
     The manner in which the developer circulates around the first and second chambers  53 Y and  54 Y is now described. Toner replenished from the toner bottle  32 Y is dispensed into the second chamber  54 Y of the developing unit  5 Y via the toner-dispensing hole  61 Y, which is marked in  FIGS. 8 and 9  by broken-line boxes. The toner dispensed into the second chamber  54 Y is agitated with carriers by the second screw  56 Y to form developer. The second screw  56 Y conveys and agitates the developer in a direction indicated by an arrow B until the developer reaches the end of the second chamber  54 Y. Then the developer passes through the first opening  59 Ya to enter the first chamber  53 Y. In the first chamber  53 Y, the first screw  55 Y conveys and agitates the developer in a direction indicated by an arrow A. When the developer reaches the end of the first chamber  53 Y, the developer passes through the second opening  59 Yb to enter the second chamber  54 Y. Thus, the developer is circulated in the developing unit  5 Y by the first screw  55 Y and the second screw  56 Y. 
     A part of the developer being agitated and conveyed by the first screw  55 Y in the first chamber  53 Y is attracted by the development roller  51 Y so as to be carried on the development roller  51 Y. That is, the carriers in the developer is attracted to the development roller  51 Y by magnetic force generated by the magnet P 4  or P 5  of the magnet roller to be carried on the development roller  51 Y. Meanwhile, the toner in the developer has been charged with an opposite polarity to the polarity of the carriers by being agitated, generating electrostatic force between the toner and the carriers. Therefore, the toner is carried on the development roller  51 Y with the carriers. 
     Passing through a gap (i.e. doctor gap) between the doctor blade  52 Y and the surface of the development roller  51 Y regulates thickness of the developer carried on the development roller  51 Y. When the developer of which thickness has been regulated is conveyed to a development area opposed to the photoconductive member  1 , magnetic force generated by the magnet P 1  of the magnet roller causes the developer to erect in a brush-like form. Here, in the development area, the surface of the development roller  51 Y has a higher linear velocity than a surface of the photoconductive member  1 Y while moving in a same direction. While sliding over the surface of the photoconductive member  1 Y, the carriers erecting in a brush-like form on the development roller  51 Y supplies the toner adhering to the carriers to the surface of the photoconductive member  1 Y. 
     At this time, a development bias is applied to the development roller  51 Y by a power source (not shown) thereby forming a development field in the development area. Electrostatic force is generated between the latent image on the photoconductive member  1 Y and the development roller  51 Y, which attracts the toner on the development roller  51 Y to the latent image. Thereby, the toner on the development roller  51 Y adheres to the latent image on the photoconductive member  1 Y. 
     In this manner, the latent images on the respective photoconductive members  1 Y,  1 M,  1 C, and  1 K are developed into toner images having respective colors. In addition, the development roller  51 Y according to an example embodiment is connected to a driving unit via a clutch (not shown), so that rotation of the development roller  51 Y may be temporarily stopped by the clutch. 
     Referring now to  FIG. 10 , a specific description is given of a first screw  55 Ya used in the developing unit  5 Y according to the example embodiment. As illustrated in  FIG. 10 , the first screw  55 Ya has a bigger axis diameter than the second screw  56 Y. With the exception that the first screw  55 Ya is bigger in axis diameter, the first screw  55 Ya and the second screw  56 Y are the same in such respects as a screw pitch of the blade portion and a rotation speed of the screws. 
     Specifically, the first screw  55 Ya preferably has a screw member of 7.0-mm axis diameter, where the second screw  56 Y employs a screw member of 5.0-mm axis diameter. The inventors compared four types of axis diameter, 5.0 mm, 6.0 mm, 7.0 mm, and 8.0 mm for the first screw  55 Ya. Among the axis diameters, 7.0 mm was most preferable. 
     It should be noted that the diameter of the second screw  56 Y is not limited to 5.0 mm. Generally, the axis diameter ranging from approximately 3 mm to 12 mm is used depending on an amount of the developer to be conveyed. Thus, a preferable range of the axis diameter of the first screw  55 Ya, which is bigger than the second screw  56 Y, may change accordingly. 
       FIG. 11  illustrates a developing unit  5 Ya using the first screw  55 Ya. When the first screw  55 Ya has a bigger axis diameter than the second screw  56 Y, the first screw  55 Ya has a larger volume than the second screw  56 Y. Thus, the first chamber  53 Y has a smaller developer-containing space than the second chamber  54 Y. Therefore, as illustrated in FIG.  11 , the height surface of the developer in the first chamber  53 Y becomes higher than a surface of the developer in the second chamber  54 Y. 
     It should be noted that, at a point when the developing unit  5 Ya is placed into the image forming apparatus  100 , height of the surfaces of the developer in the first chamber  53 Y and second chamber  54 Y are at substantially same level. 
     Here, the first chamber  53 Y has a smaller amount of the developer than the second chamber  54 Y. Since the first screw  55 Ya has a bigger axis diameter than the second screw  56 Y (i.e. the first screw  55 Ya has a larger volume than the second screw  56 Y), the first chamber  53 Y has a smaller developer-containing space than the second chamber  54 Y. 
     While the height of the surfaces of the developer in the first chamber  53 Y and second chamber  54 Y are at substantially the same level, the amount of developer passing through the first opening  59 Ya is larger than an amount of the developer passing through the second opening  59 Yb, thus increasing the amount of the developer in the first chamber  53 Y. Therefore, the surface of the developer in the first chamber  53 Y becomes higher, thus increasing the amount of the developer passing through the second opening  59 Yb. 
     Once the amounts of the developer passing through the first opening  59 Ya and the second opening  59 Yb per time unit become substantially equal, the amount of the developer is stabilized in a state that the first chamber  53 Y has a higher surface than the second chamber  54 Y as illustrated in  FIG. 11 . 
     Compared to the background examples described with reference to  FIGS. 1 and 3 , even when the developer is filled to the top portion of the first chamber  53 Y, the developer is not to be filled to the top portion of the second chamber  54 Y. 
     When there is a big difference between the amounts of the developer to be conveyed by the first screw  55 Y and by the second screw  56 Y, the developer may sometimes accumulate in the vicinity of where the first screw  55 Y starts to convey the developer, that is, the first opening  59 Ya. 
     As a result, the surface of the developer may become temporarily high at an edge portion of the developing unit  5 Ya, causing the developer to slip into a gap between the edge portion of the development roller  51 Y and a casing member of the developing unit  5 Ya, so that the developer may stop the rotation of the development roller  51 Y or leak to the outside. 
     However, the developing unit  5 Ya according to the embodiment is provided with the frame member  71 Y including the communicating hole  72 Y, thus covering an upper portion of the first opening  59 Ya. Therefore, even when the amount or height of the developer is temporarily increased at the edge portion of the developing unit  5 Ya, the developer may be prevented from reaching the edge portion of the development roller  51 Y. 
     Referring now to  FIGS. 12 through 14 , the developing unit  5 Ya according to different example embodiments are described. 
     In the following example embodiments, some components of the developing unit  5 Ya in the image forming apparatus  100  such as a shape of the first screw  55 Y may be different; however, the basic configuration of the image forming apparatus  100  may be substantially equal. Following descriptions are focused on matters different from the above example embodiments, and matters in common with the above example embodiments are to be omitted. 
     As illustrated in  FIG. 12 , a first screw  55 Yb used in the image forming apparatus  100  according to another example embodiment has a blade portion thicker than the second screw  56 Y. Except for the thickness of the blade portion, the first screw  55 Yb and the second screw  56 Y have substantially similar shapes in such respects as axis diameter, and pitch of the blade portion. 
     When the first screw  55 Yb has a thicker blade portion than the second screw  56 Y, the first screw  55 Yb has a larger volume than the second screw  56 Y. As a result, the first chamber  53 Y has a smaller developer-containing space than the second chamber  54 Y. Therefore, in the image forming apparatus  100  having the first screw  55 Yb, the developer in the first chamber  53 Y has a higher surface than the second chamber  54 Y as illustrated in  FIG. 11 . 
     As illustrated in  FIG. 13 , a blade portion of a first screw  55 Yc used in the image forming apparatus  100  according to another example embodiment has more blades than a blade portion of the second screw  56 Y. 
     When the first screw  55 Yc has more blades in the blade portion than the second screw  56 Y, the first screw  55 Yc has a larger volume than the second screw  56 Y. As a result, the first chamber  53 Y has a smaller developer-containing space than the second chamber  54 Y. Therefore, in the image forming apparatus  100  having the first screw  55 Yc, the developer in the first chamber  53 Y has a higher surface than the second chamber  54 Y as illustrated in  FIG. 11 . 
     Specifically, the first screw  55 Yc preferably has two spiral blades where the second screw  56 Y has one spiral blade around each axis portion. Except for the number of blades, the first screw  55 Yc and the second screw  56 Y have substantially similar shapes in such respects as an axis diameter, a pitch of the blade portion, and a thickness of the blade portion. 
     Each of the first screws  55 Ya,  55 Yb, and  55 Yc described with reference to  FIGS. 10 ,  12 , and  13  may have a higher strength due to its larger volume. Further, each of the first screw  55 Ya,  55 Yb, and  55 Yc has a larger cross-sectional area than a conventional first screw. Generally, a screw having a large cross-sectional area is less subject to a run-out caused by rotation. 
       FIG. 14  illustrates a developing unit  5 Yb of the image forming apparatus  100  according to another example embodiment. As illustrated in  FIG. 14 , the first chamber  53 Ya has a smaller volume, that is, a smaller developer-containing space, than the second chamber  54 Y. Therefore, in the image forming apparatus  100  having the first chamber  53 Ya, the developer in the first chamber  53 Y has a higher surface than the second chamber  54 Y, in a similar manner to the developing unit  5 Ya having the first screws  55 Ya,  55 Yb, and  55 Yc. 
     Referring now to  FIGS. 15 to 18 , descriptions are given on the image forming apparatus  100  according to different example embodiments. In the following example embodiments, the developer in the first chamber  53 Y moves slower than the developer in the second chamber  54 Y so that the developer in the first chamber  53 Y has a higher surface than the second chamber  54 Y. 
     Referring to  FIG. 15 , a first screw  55 Yd used in the image forming apparatus  100  according to another example embodiment has a smaller screw pitch in a blade portion than the second screw  56 Y. A smaller screw pitch may reduce a distance for which the developer is conveyed by one rotation of the first screw  55 Yd. 
     Assuming that the first screw  55 Yd and the second screw  56 Y have the same rotational speed, the first screw  55 Yd has a lower conveying speed than the second screw  56 Y. That is, the developer in the first chamber  53 Y moves slower than the developer in the second chamber  54 Y. 
     At a point when the developing unit  5 Ya having the first screw  55 Yd is placed into the image forming apparatus  100 , height of the surfaces and amounts of the developers in the first chamber  53 Y and second chamber  54 Y are at substantially same level. When the developing unit  5 Ya is driven at this state, since the conveying speed of the first screw  55 Yd is lower than the conveying speed of the second screw  56 Y, the amount of the developer passing through the first opening  59 Ya becomes larger than an amount of the developer passing through the second opening  59 Yb, thus increasing the amount of the developer in the first chamber  53 Y. When the amount of the developer in the first chamber  53 Y increases, the surface of the developer in the first chamber  53 Y becomes higher, thus increasing the amount of the developer passing through the second opening  59 Yb per time unit. 
     When more developer passes through the first opening  59 Ya than the second opening  59 Yb, the developer in the second chamber  54 Y decreases, so that the amount of the developer passing through the first opening  59 Ya per time unit is decreased. 
     Once the amount of developer passing through the first opening  59 Ya and the second opening  59 Yb per time unit become substantially equal, the amount of the developer is stabilized in a state that the first chamber  53 Y has a higher surface than the second chamber  54 Y as illustrated in  FIG. 11 . 
     As described, since the conveying speed of the first screw  55 Yd is lower than the conveying speed of the second screw  56 Y, the developer in the first chamber  53 Y may have a higher surface than the developer in the second chamber  54 Y as illustrated in  FIG. 11 . Even when the developer is filled to the top portion of the first chamber  53 Y, the developer is not to be filled to the top portion of the second chamber  54 Y. 
       FIG. 16  illustrates a first screw  55 Ye and the second screw  56 Y used in the image forming apparatus  100  according to another example embodiment. As illustrated in  FIG. 16 , the first screw  55 Ye has a notch  91  in a blade portion. Provision of the notch on the first screw  55 Ye causes a part of the developer, which is to be pressed by the blade portion and conveyed to an axial direction as the first screw  55 Ye rotates, to escape through the notch  91 , thus reducing a conveyance efficiency. 
     Assuming that the first screw  55 Ye and the second screw  56 Y have a same rotational speed, the first screw  55 Ye has a lower conveying speed than the second screw  56 Y. That is, the developer in the first chamber  53 Y moves slower than the developer in the second chamber  54 Y. Therefore, in the image forming apparatus  100  having the first screw  55 Ye, the developer in the first chamber  53 Y may have a higher surface than the developer in the second chamber  54 Y as illustrated in  FIG. 11 . 
       FIG. 17  illustrates a first screw  55 Yf and a second screw  56 Y used in the image forming apparatus  100  according to another example embodiment. As illustrated in  FIG. 17 , the first screw  55 Yf has a rib  92  as a plate member parallel in axial direction to the axis member of the first screw  55 Yf. By providing the rib  92  on the first screw  55 Yf, the developer, which is to be pressed by the blade portion and conveyed to an axial direction as the first screw  55 Yf rotates, is subjected to a force moving in a rotation direction, thereby reducing conveyance efficiency in the axial direction. 
     Assuming that the first screw  55 Yf and the second screw  56 Y have a same rotational speed, the first screw  55 Yf has a lower conveying speed than the second screw  56 Y. That is, the developer in the first chamber  53 Y moves slower than the developer in the second chamber  54 Y. Therefore, in the image forming apparatus  100  having the first screw  55 Yf, the developer in the first chamber  53 Y may have a higher surface than the developer in the second chamber  54 Y as illustrated in  FIG. 11 . 
     Since each of the first screws  55 Yd,  55 Ye, and  55 Yf used in the image forming apparatus  100  as described with reference to  FIGS. 15 to 17  has a relatively low conveying speed, load applied thereon may be reduced even when the first chamber  53 Y is filled with the developer to the top portion. 
     Further, the first screws  55 Yd and  55 Yf in  FIG. 15  and  FIG. 17  not only have a lower conveying speed but also may have a larger volume than the respective second screws  56 Y. Therefore, the image forming apparatus  100  including the first screw  55 Yd or  55 Yf may have an effect similar to the image forming apparatus  100  including any one of the first screws  55 Ya,  55 Yb, and  55 Yc described with reference to  FIGS. 10 ,  12 , and  13 . 
       FIG. 18  is a cross-sectional top view of the developing unit  5 Ya of the image forming apparatus  100  according to another example embodiment. As illustrated in  FIG. 18 , the first chamber  53 Y includes a conveyance inhibiting member  53 Yb for inhibiting conveyance of the developer. 
     The conveyance inhibiting member  53 Yb is a soft film member formed of resin such as polyethylene. Since the conveyance inhibiting member  53 Yb is soft and easily deformed, even when contacting the first screw  55 Y, the conveyance inhibiting member  53 Yb becomes easily deformed and does not inhibit rotation of the first screw  55 Y. The conveyance inhibiting member  53 Yb may inhibit movement of the developer particles to some extent, thereby reducing the conveyance efficiency of the developer. 
     Assuming that the first screw  55 Y and the second screw  56 Y have the same rotational speed, the first screw  55 Y has a lower conveying speed than the second screw  56 Y. That is, the developer in the first chamber  53 Y moves slower than the developer in the second chamber  54 Y. Therefore, in the image forming apparatus  100  having the conveyance inhibiting member  53 Yb, the developer in the first chamber  53 Y may have a higher surface than the developer in the second chamber  54 Y as illustrated in  FIG. 11 . 
     In the image forming apparatus  100  including the first screw  55 Yd,  55 Ye, and  55 Yf and conveyance inhibiting member  53 Yb described with reference to  FIGS. 15 through 18 , the developer in the first chamber  53 Y moves slower than the developer in the second chamber  54 Y. Therefore, the surface of the developer in the first chamber  53 Y, which supplies the developer to the development roller  51 Y, becomes higher than the surface of the developer in the second chamber  54 Y, into which toner is dispensed from an upper portion. 
     A configuration in which the developer in the first chamber  53 Y moves slower than the developer in the second chamber  54 Y may also be achieved by lowering a rotation speed of the first screw  55 Y than the second screw  56 Y. 
     Lowering the rotational speed of the first screw  55 Y may be achieved differently depending on a configuration of a driving portion (not shown) for the first screw  55 Y and the second screw  56 Y: when the first screw  55 Y and the second screw  56 Y are driven by one motor, gear ratios of the first and second screws  55 Y and  56 Y are to be changed; and when the first screw  55 Y and second screw  56 Y are driven by individual motors, number of revolutions of a drive source for the first screw  55 Y is to be reduced. In both cases, the first screw  55 Y and the second screw  56 Y are substantially same except that the rotation speed of the first screw  55 Y is slower than the second screw  56 Y. 
     When the first screw  55 Y has a lower rotation speed than the second screw  56 Y, the first screw  55 Y may have a lower conveying speed than the second screw  56 Y. Accordingly, the image forming apparatus  100  in which the first screw  55 Y has a lower rotation speed than the second screw  56 Y may have effects similar to that of the image forming apparatus  100  including the first screw  55 Yd,  55 Ye, and  55 Yf and the conveyance inhibiting member  53 Yb described with reference to  FIGS. 15 through 18 . 
     It should be noted that, although the developing units  5 Ya and  5 Yb using yellow toner have been described above, configurations of the developing unit  5 Ya and  5 Yb discussed above may also be applied to the developing unit  5 M,  5 C, and  5 K. 
     In principle, the difference between the surfaces of the developer may be caused by a difference in a developer-containing space or a difference in a developer-conveying speed between the first chamber  53 Y and the second chamber  54 Y. 
     Numerous additional modifications and variations are possible in light of the above teachings. For example, the image forming apparatus  100  may employ as a rotary member a conveyance coil formed of resin, metal, and other materials instead of the conveyance screws. 
     When the image forming apparatus  100  uses conveyance coils, a conveyance coil, which corresponds to the first screw, having configurations similar to any one of the first screw  55 Ya,  55 Yb,  55 Yc,  55 Yd,  55 Ye, and  55 Yf may be used. For example, the conveyance coil may have a thick wire diameter, a relatively low rotation speed leading to a low conveying speed, a smaller coil pitch, and a thick core if the conveyance coil has a core formed by solder brazing, etc. 
     When the image forming apparatus  100  includes such a conveyance coil, developer in the first chamber  53 Y may have a higher surface than developer in the second chamber  54 Y in a similar manner to the image forming apparatus  100  having configurations described with reference to  FIGS. 10 through 18 . 
     It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
     This patent specification is based on Japanese patent applications, No. JPAP 2005-250836 filed on Aug. 31, 2005 and No. 2004-341895 filed on Nov. 26, 2004, in the Japanese Patent Office, the entire contents of which are incorporated by reference herein.