Patent Publication Number: US-9417561-B2

Title: Developing device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a developing device that is used in an image forming apparatus such as a copying machine or a laser printer that employs an electrostatic recording method or an electrophotographic method in which an electrostatic image that has been formed on an image-bearing member is developed by using a developer that includes toner and carrier. 
     2. Description of the Related Art 
     In the related art, in an image forming apparatus such as a copying machine, an electrostatic latent image that has been formed on a photoconductor drum, which is an image-bearing member, is developed by a developing device in such a manner as to convert the electrostatic latent image into a visible image. 
     Such a developing device is configured to cause a latent image to be visualized as a toner image by supplying toner to a photoconductor drum, and a developing device in which a two-component developer that is a mixture of non-magnetic toner and magnetic carrier is used as a developer is frequently used in addition to a developing device in which a mono-component developer that contains magnetic toner is used as a developer. In a developing device that uses a two-component developer, the charge amount of toner has good stability, and thus, a color image that has good color tone can be formed. Therefore, such a developing device may be used in color image forming apparatuses. 
     An example of the above-mentioned developing device is disclosed in Japanese Patent Laid-Open No. 11-143231 (Patent Document 1), and a representative figure thereof is illustrated in  FIG. 5 . 
     In a developing device  102 , a developing sleeve  120  that includes a cylindrical magnet  121 , which is non-rotatably arranged in an opening of a developer container  110  that contains a two-component developer, is disposed in such a manner as to face a photoconductor drum  1 . The photoconductor drum  1  rotates in the direction of a corresponding one of arrows in  FIG. 5 , and the developing sleeve  120  rotates in a direction opposite to the direction in which the photoconductor drum  1  rotates at a facing portion. 
     The developer, which is contained in the developer container  110 , is sucked in and held on the developing sleeve  120  at a position (pumping position) Q on a surface of the developing sleeve  120  that corresponds to the position of a pumping magnetic pole S 1  of the magnet  121  by the influence of the pumping magnetic pole S 1 . Then, the developer reaches a development portion after the layer thickness of the developer has been controlled by a developing blade  122 , forms a magnetic brush in the development portion by the influence of a developing magnetic pole N 1 , and develops a latent image on the photoconductor drum  1 . 
     The developer, whose toner density has been reduced through development, is separated from the developing sleeve  120  and deposited at a position (developer depositing position) P on the surface of the developing sleeve  120  between a releasing magnetic pole S 2  and the pumping magnetic pole S 1  where the magnetic flux density is low. As described above, the developer is sucked in and held on the developing sleeve  120 , from which the developer has been separated, again at the pumping position Q. 
     In the developer container  110 , a developing chamber  125  in which a first stirring conveying member  123  is disposed is formed below the developing sleeve  120 , and a stirring chamber  126  in which a second stirring conveying member  124  is disposed is formed in such a manner that a partitioning wall  140  is interposed between the developing chamber  125  and the stirring chamber  126 . Each of the first and second stirring conveying members  123  and  124  is a screw-type stirring conveying member, and such a screw-type stirring conveying member generally includes a helical screw blade. 
     Circulation of the developer in the developing device  102  will be described with reference to  FIG. 6 . The developing chamber  125  from which the developer is supplied to the developing sleeve  120  and the stirring chamber  126  that is arranged next to the developing chamber  125  in the horizontal direction are separated from each other by the partitioning wall  140  that has end portions  115  and  116  that are open. As indicated by arrows in  FIG. 6 , the developer is supplied to the developing sleeve  120  while the developer is made to circulate between the developing chamber  125  and the stirring chamber  126  by the first and second stirring conveying members  123  and  124 . The developer that has passed through a development region of the photoconductor drum  1  while being carried by the developing sleeve  120  is separated from the developing sleeve  120  and collected so as to be placed in the developing chamber  125  as indicated by dashed arrows in  FIG. 6 . 
     In this case, if the developer that has been separated from the developing sleeve  120  and that has a low toner density is not sufficiently stirred by the first stirring conveying member  123  and supplied to the developing sleeve  120  again, the developer develops a latent image on the photoconductor drum  1  while having a low toner density. This causes image density to be unstable, and undesirable effects such as inconsistencies in density are likely to occur. 
     Accordingly, as a configuration for suppressing the occurrence of the inconsistencies in density due to a developer that has been separated from a developing sleeve and that has a low toner density, a developing device that employs a function separation system such as that disclosed in Japanese Patent Laid-open No. 2012-42737 (Patent Document 2) has been proposed. An exemplary embodiment of the developing device disclosed in Patent Document 2, which employs the function separation system, is illustrated in  FIG. 7 . 
     A developing device that employs the function separation system is a developing device that employs a developer circulation system in which a developer is supplied from a developing chamber, in which a first conveying screw is disposed, to a developing sleeve, and in which the developer after completion of development is collected so as to be placed in a stirring chamber in which a second conveying screw is disposed. By employing the function separation system, a developer whose toner density has been reduced as a result of being used for development can be prevented from being supplied to a developing sleeve again while not being sufficiently stirred, and the occurrence of a problem, such as inconsistencies in density as mentioned above, can be suppressed. 
     However, in a developing device that employs the function separation system such as that disclosed in Patent Document 2, there has been a problem in that uneven distribution of the developer level occurs. This uneven distribution occurs as a result of the developer being collected from the developing sleeve and placed in the stirring chamber. 
     Circulation of a developer in such a developing device that employs the function separation system will be described with reference to  FIG. 8  while being compared with  FIG. 6 . As indicated by arrows in  FIG. 8 , the developer is supplied to a developing sleeve  120  while the developer is made to circulate between a developing chamber  125  and a stirring chamber  126  by conveying screws. This configuration is the same as that of the developing device of  FIG. 6 , which does not employ the function separation system. However, in the developing device of  FIG. 8 , which employs the function separation system, the developer that has passed through a development region of a photoconductor while being carried by the developing sleeve  120  is separated from the developing sleeve  120  and then collected so as to be placed, not in the developing chamber  125 , but in the stirring chamber  126 , as indicated by dashed arrows in  FIG. 8 . This is a characteristic of a developing device that employs function separation system. 
     In the developing device, which is illustrated in  FIG. 6  and which does not employs the function separation system, the developer that has been supplied to the developing sleeve  120  from the developing chamber  125  returns the developing chamber  125 , and thus, the distribution of developer is relatively uniform. However, in the developing device, which is illustrated in  FIG. 8  and which employs the function separation system, the developer that has been supplied to the developing sleeve  120  from the developing chamber  125  is collected so as to be placed in the stirring chamber  126 , and thus, in the developing chamber  125 , the surface angle of the developer is likely to become inclined downward toward a downstream side. As a result, the amount of the developer on the downstream side of the developing chamber  125  that is to be supplied to the developing sleeve  120  is reduced, which in turn may result in coating failure. 
     Regarding a decrease in the level of a developer in a developer container of a developing device that employs the function separation system, such as that described above, for example, a developing device that is disclosed in Japanese Patent Laid-Open No. 11-84874 (Patent Document 3) has been proposed. Patent Document 3 proposes that, in a developing device, particularly a vertical-stirring-type developing device that employs the function separation system and in which a developing chamber is formed above a stirring chamber, unevenness in developer distribution is corrected by changing the pitch or the diameter of the blade of a conveying screw, which is disposed in the developing chamber. 
     However, a method for making the unevenness uniform during developer distribution in a developing chamber by changing the pitch or the diameter of the blade of a conveying screw, which is disposed in the developing chamber, such as that described in Patent document 3, has the following problem. 
     For example, although reducing the pitch of the blade of a conveying screw, which is disposed in a developing chamber, can increase the developer level, as the pitch of the blade is reduced, the inclination angle of the blade with respect to a screw shaft increases. When the inclination angle of the blade of the conveying screw with respect to the screw shaft is small, a force that is applied to the developer by the blade as a result of rotation of the conveying screw increases not only in a direction parallel to the screw shaft, but also in a direction perpendicular to the screw shaft. This force affects the ability of the blade to supply the developer to a developing sleeve. However, when the inclination angle of the blade of the conveying screw with respect to the screw shaft is large, the force that is applied to the developer by the blade as a result of rotation of the conveying screw concentrates in the direction perpendicular to the screw shaft, and thus, the ability of the blade to supply the developer to the developing sleeve decreases. 
     As described above, there is a concern that, if the pitch of the blade in the developing chamber is reduced, or if the diameter of the blade is changed, the ability of the blade to supply the developer to the developing sleeve also changes in response to these changes. Thus, there is a concern that, even if the developer level can be increased by changing the pitch or the diameter of the blade, at the same time, the ability of the blade to supply the developer to the developing sleeve also changes, and as a result, the advantageous effect of improving coating of the developing sleeve is limited. 
     Accordingly, a configuration in which the pitch of the blade in the developing chamber is reduced or the diameter of the blade is changed in an area outside a developer carrying area of the developing sleeve in a direction parallel to the rotation axis of the developing sleeve may also be considered. However, although a decrease in the level of the developer in the area outside the developer carrying area of the developing sleeve can be suppressed with the above-described configuration, the configuration is not sufficiently effective for the level of the developer in the developer carrying area of the developing sleeve. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a horizontal-stirring-type developing device that employs the function separation system and that has a configuration that suppresses a decrease in developer level on a downstream side in a developing chamber in a direction in which a developer is conveyed while suppressing a decrease in an ability to supply the developer to a developer-carrying member. 
     The present invention provides a developing device including
     a developer-carrying member that is rotatably arranged, that carries a developer, which includes toner and carrier, and that supplies the developer to a development region, which faces an image-bearing member,   a developing chamber in which the developer is supplied to the developer-carrying member at a position facing a circumferential surface of the developer-carrying member,   a collecting chamber that is positioned upstream of the developing chamber in a rotation direction of the developer-carrying member in such a manner as to face the circumferential surface of the developer-carrying member, in which the developer that has been collected from the developer-carrying member is placed, and in which a circulation path that allows the developer to circulate through communicating portions, which allow communication between the collecting chamber and the developing chamber at opposite ends of the chambers, is formed,   a partitioning wall that separates the developing chamber and the collecting chamber from each other,   a first screw that is rotatably arranged in the developing chamber and that conveys the developer, and   a second screw that is rotatably arranged in the collecting chamber and that conveys the developer.   In an axial direction of the developer-carrying member, a developer carrying area of the developer-carrying member is superposed with one of the communicating portions, which is a downstream communicating portion through which the developer is conveyed from the developing chamber to the collecting chamber, and   a conveying ability of the second screw in a first area that includes an area, which is superposed with the developer carrying area of the developer-carrying member and the downstream communicating portion in the axial direction of the developer-carrying member, is lower than the conveying ability of the second screw in a second area, which is positioned further downstream than the first area in a direction in which the developer is conveyed and which is superposed with the developer carrying area of the developer-carrying member, or is zero, or a direction in which the second screw conveys the developer in the first area and a direction in which the second screw conveys the developer in the second area are opposite to each other.   

     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an example of an image forming apparatus that includes a developing device according to a first embodiment of the present invention. 
         FIG. 2  is a diagram illustrating the developing device according to the first embodiment. 
         FIG. 3  is a diagram illustrating the developing device according to the first embodiment. 
         FIG. 4  is a diagram illustrating a developing device according to a second embodiment. 
         FIG. 5  is a diagram illustrating a developing device of the related art. 
         FIG. 6  is a diagram illustrating circulation of a developer in the developing device of the related art. 
         FIG. 7  is a diagram illustrating another developing device of the related art that employs a function separation system. 
         FIG. 8  is a diagram illustrating circulation of a developer in the other developing device of the related art, which employs the function separation system. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described in detail below with reference to the drawings. 
     First Embodiment 
     As illustrated in  FIG. 1 , a developing unit according to a first embodiment is used in a full-color image forming apparatus that employs a so-called tandem system. In the developing device, drum cartridges each of which corresponds to one of four colors of yellow, magenta, cyan, and black, and each of which performs processing for forming a toner image of the corresponding color are arranged side-by-side. The toner images of the four colors are superposed with one another on an intermediate transfer belt  24  and collectively transferred onto a transfer sheet. Then, the transfer sheet, to which the toner images have been transferred, is applied with pressure and heated by a fixing unit  25 , so that a full-color image is obtained. In the following description, components that are common to the drum cartridges illustrated in  FIG. 1 , each of which corresponds to one of the four colors of yellow, magenta, cyan, and black, will be simply indicated by reference numerals, and reference letters Y, M, C, and K will be omitted. 
     [Image Forming Apparatus] 
     An image forming apparatus of the present embodiment will be described below.  FIG. 1  is a diagram illustrating a toner image forming operation of each of the drum cartridges. Primary chargers  21  that serve as charging devices charge surfaces of photoconductor drums  28  that serve as image-bearing members. Then, the surfaces of the photoconductor drums  28  are exposed to laser beams by lasers  22  that serve as exposure devices, and consequently, electrostatic latent images are formed on the photoconductor drums  28 . These electrostatic latent images are developed by developing units  1  that serve as developing devices, so that toner images are obtained. The toner images are transferred onto the intermediate transfer belt  24  in such a manner that the toner images are superposed with one another by primary transfer rollers  23  that serve as transfer units. Residual toner that remains on the photoconductor drums  28  after the toner images have been transferred is removed by cleaners  26 . 
     [Developing Device] 
     The developing units  1  of the present embodiment will be described in detail with reference to  FIG. 2  and  FIG. 3 . Each of the developing units  1  includes a developer container  2  that contains a two-component developer and a developing sleeve  3  that serves as a developer-carrying member and that is arranged in an opening of the developer container  2 . Regarding the developer, in the present embodiment, a two-component development system is employed as a development system, and a mixture of non-magnetic toner having a negative charge polarity and magnetic carrier is used as the developer. The non-magnetic toner is powder that is formed by grinding or polymerizing a resin such as a polyester resin, a styrene-acrylic resin, or the like that contains a colorant, a wax component, and the like. The magnetic carrier is formed by coating, with a resin, surface layers of cores that include resin particles, which are obtained by kneading ferrite particles and magnetic powder. In the present embodiment, the toner density of the developer in an initial state (the weight ratio of toner that is included in the developer in an initial state) is 8%. 
     In the opening of the developer container  2 , which is a portion that faces a corresponding one of the photoconductor drums  28  and that is open, the developing sleeve  3  is rotatably arranged in such a manner as to be partially exposed. The developing sleeve  3  is made of a non-magnetic material and includes a stationary magnet  4  that serves as a magnetic field generating unit therein. The developing sleeve  3  rotates in the direction of arrow B in  FIG. 2  and conveys the developer, which has been caused to be held on the developing sleeve  3  at a position corresponding to a pumping magnetic pole N 1  of the stationary magnet  4 , in a direction toward a blade  5 . The developer that has been napped by a regulating magnetic pole S 1  is subjected to a shearing force from the blade  5 , so that the amount of the developer is regulated, and after the developer has passed through a gap between the developing sleeve  3  and the blade  5 , a developer layer that has a predetermined layer thickness is formed on the developing sleeve  3 . The developer layer is carried to a development region that faces the photoconductor drum  28  and develops an electrostatic latent image that has been formed on a surface of the photoconductor drum  28  while being caused to form a magnetic brush by a developing magnetic pole N 2 . The developer that has been used for the development is separated from the developing sleeve  3  in a magnetic-force-free zone that is defined between a releasing magnetic pole N 3  and the pumping magnetic pole N 1  as a result of these poles being adjacent to each other. 
     The developer container  2  is divided by a partitioning wall  15  into a developing chamber  11  (a first developer containing chamber) and a stirring chamber  12  (a second developer containing chamber) that serves as a collecting chamber. Each of the developing units  1  of the present embodiment is a developing device that employs a so-called function separation system. In other words, in each of the developing units  1 , the developer is supplied to the developing sleeve  3  at a position in the developing chamber  11  that faces a circumferential surface of the developing sleeve  3 . The developer is collected so as to be placed in the stirring chamber  12  from the developing sleeve  3  at a position that faces the circumferential surface of the developing sleeve  3 . 
     The developing chamber  11  and the stirring chamber  12  extend in a direction parallel to the rotation axis of the developing sleeve  3 . Both the ends of the partitioning wall  15  do not reach inner side walls of the developer container  2  that are positioned at the ends in the longitudinal direction of the developer container  2 , so that an upstream communicating portion  17  and a downstream communicating portion  18  that allow the developer to pass through between the developing chamber  11  and the stirring chamber  12  are formed. In the developing chamber  11  and the stirring chamber  12 , a circulation path that allows the developer to circulate between the developing chamber  11  and the stirring chamber  12  is formed. A first screw  13  and a second screw  14  that serve as conveying members are disposed in the developing chamber  11  and the stirring chamber  12 , respectively. The developing sleeve  3 , the first screw  13 , and the second screw  14  are configured to be driven in conjunction with one another by a gear train (not illustrated) and rotate by receiving a driving force from a developing-unit-driving gear (not illustrated). Rotations of the first and second screws  13  and  14  cause the developer to circulate and to be mixed and stirred within the developer container  2 . 
     In the present embodiment, the first screw  13  and the second screw  14  are disposed at positions where the first screw  13  and the second screw  14  are superposed with each other when viewed from the horizontal direction. In addition, in the present embodiment, the first screw  13  and the second screw  14  are disposed at positions where the center of rotation of at least one of the screws is superposed with the blade of the other one of the screws when viewed from the horizontal direction. As described above, the first screw  13  and the second screw  14  are substantially horizontally arranged. Compared with the case where the first screw  13  and the second screw  14  are arranged side-by-side in the vertical direction, the influence of conveying the developer against gravity can be reduced, and developer conveyance performance can be improved. 
     A guiding member  16 , which is an upper portion of the partitioning wall  15 , is positioned adjacent to the developing sleeve  3  in the vicinity of the magnetic-force-free zone of the developing sleeve  3  and is configured to cause the developer on the developing sleeve  3  not to return to the developing chamber  11  but to be contained in the stirring chamber  12  after the developer has been separated from the developing sleeve  3  by the releasing magnetic pole N 3 . 
     In other words, an end portion of the partitioning wall  15  is positioned above the second screw  14  in the vertical direction in such a manner as to face the developing sleeve  3 . In addition, the partitioning wall  15  guides the developer that has been collected from the developing sleeve  3  to the stirring chamber  12 . 
     As stated above, in such a developing device that employs the function separation system, there has been a problem in that uneven distribution of the developer level occurs. 
     In particular, in the present embodiment, an area in which the developing sleeve  3  is to be coated with the developer (hereinafter referred to as a developer coating area) is superposed with the upstream and downstream communicating portions  17  and  18 , which allow the developer to pass through between the developing chamber  11  and the stirring chamber  12 . Thus, as stated above, in the case of changing the pitch or the diameter of the blade of the first screw  13  in the developing chamber  11 , the influence in terms of supplying the developer to the developing sleeve  3  cannot be ignored. 
     [Configuration of Screw in Downstream Communicating Portion on Downstream Side in Developing Chamber] 
     A characteristic configuration of each of the developing units  1  according to the present embodiment will now be described. In the present embodiment, each of the developing units  1  has the following configuration in order to increase the developer level on the downstream side in the developing chamber  11 , particularly on the downstream side in the developer coating area. In other words, in the configuration of each of the developing units  1  of the present embodiment, which is a horizontal-stirring-type developing device in which the developing chamber  11  and the stirring chamber  12  are substantially horizontally arranged, the downstream communicating portion  18  that is used for conveying the developer from the developing chamber  11  to the stirring chamber  12  is positioned so as to be superposed with the developer coating area, which serves as a developer carrying area, in the direction parallel to the rotation axis of the developing sleeve  3 . 
     In the stirring chamber  12 , the ability of the second screw  14  to convey the developer in a first area that is at least part of a facing area in which the second screw  14  faces the downstream communicating portion  18  is set to be lower than that in a second area that is adjacent to the first area on the downstream side in a conveyance direction in which the developer is conveyed. In addition, the conveying ability of the second screw  14  in the facing area, in which the second screw  14  faces the downstream communicating portion  18 , is set to be lower than that of the second screw  13 . In the present embodiment, some of the blade of the second screw  14  is omitted in the first area in such a manner as to make the second screw  14  lose its conveying ability in the first area. 
     With this configuration, a portion of the first screw  13  that faces the downstream communicating portion  18 , which allows communication between the developing chamber  11  and the stirring chamber  12 , can convey a larger amount of the developer than a portion of the second screw  14  that faces the downstream communicating portion  18  in the direction in which the developer is conveyed. Therefore, the developer is likely to stay in an area including the upstream side and the downstream side of the downstream communicating portion  18 , and the level of the developer in the developing chamber  11  can be increased without changing the pitch or the diameter of the blade of the first screw in the developing chamber  11 . In addition, a configuration in which some of the blade of the second screw  14  at a position facing the developer coating area is omitted can be employed, and a decrease in the developer level in the developer coating area can be effectively suppressed. 
     In the present embodiment, the downstream communicating portion  18  through which the developer is conveyed from the developing chamber  11  to the stirring chamber  12  and the developer coating area are superposed with each other. In addition, the portion of the second screw  14 , which faces the downstream communicating portion  18 , is set to be capable of conveying a smaller amount of the developer than the amount of the developer that is conveyed in upstream and downstream of the portion in a direction in which the developer circulates. In other words, the conveying ability of the portion of the second screw  14 , which faces the downstream communicating portion  18 , is set to be lower than that of the portion of the first screw  13 , which faces the downstream communicating portion  18 . In addition, the conveying ability of the portion of the second screw  14 , which faces the downstream communicating portion  18 , is set to be lower than that of a portion of the second screw  14  that is positioned downstream of the portion. Therefore, in the conveyance direction, the developer level on the downstream side in the developing chamber  11  in the developer coating area can be effectively increased. In this case, such an improvement of the second screw  14  in the stirring chamber  12  will not affect the ability of the developing unit  1  to supply the developer to the developing sleeve  3 . 
     In the present embodiment, in order to make the second screw  14 , which is disposed in the stirring chamber  12 , lose its ability to convey the developer in an area (hereinafter referred to as A area), which is A in  FIG. 3  and which is in the vicinity of the downstream communicating portion  18  formed between the developing chamber  11  and the stirring chamber  12 , some of the blade of the second screw  14  is omitted. In the case where the blade of the second screw  14  that is positioned in the vicinity of the downstream communicating portion  18  is omitted, the developer in the A area cannot receive a force that causes the developer to be conveyed from the blade of the second screw  14 , and thus, the developer is likely to stay where it is without being conveyed. In addition, like a traffic jam, the developer that is present upstream of the A area is less likely to be conveyed due to the influence of the developer that has stayed in front of the developer, and thus, an increase in the developer level on the downstream side in the developing chamber  11 , which is positioned upstream of the A area, can be expected. However, in the case where the momentum of the developer that is conveyed from upstream of the A area is large, accumulation of the developer is less likely to occur, and thus, the level of the developer surface upstream of the A area cannot be increased very much. In the configuration of the present embodiment, the A area in which the blade of the second screw  14  is omitted is present not in the developing chamber  11 , but the stirring chamber  12  in the vicinity of the downstream communicating portion  18 , and thus, the A area is positioned along a direction perpendicular to the direction in which the developer is conveyed in the developing chamber  11 . Therefore, the momentum of the developer, which is conveyed, is less likely to be transmitted to the A area, in which the blade of the second screw  14  is omitted, in the stirring chamber  12 , and as a result, accumulation of the developer is likely to occur. 
     In the present embodiment, the direction of rotation of the first screw  13  in the developing chamber  11  is the direction of arrow C in  FIG. 2 . This is because, in an area below the first screw  13  in which the developer is present, the momentum of the developer, which is conveyed in the developing chamber  11 , is further less likely to be transmitted to the stirring chamber  12  as a result of the blade of the first screw  13  rotating in a direction away from the stirring chamber  12 . This configuration further increases the probability that the developer will accumulate. 
     The developer level on the downstream side in the developing chamber  11 , particularly in the developer coating area was effectively increased by employing the configuration of the present embodiment. 
     In order to obtain the advantageous effects of the present invention, at least part of the A area in the stirring chamber  12 , in which the blade of the second screw  14  is omitted, may face part of the downstream communicating portion  18  between the developing chamber  11  and the stirring chamber  12  in the developer coating area. The advantageous effects of the present invention can be obtained as long as at least the part of the A area in the stirring chamber  12 , in which the blade of the second screw  14  is omitted, faces the downstream communicating portion  18 . However, the blade of the second screw  14  may be omitted in the entire area in which the second screw  14  faces the downstream communicating portion  18 . In other words, the entire first area, in which the blade of the second screw  14  is omitted, may be included in the downstream communicating portion  18 . 
     In the present embodiment, in the stirring chamber  12 , a blade that conveys the developer in the same direction as that in which other portions of the second screw  14  conveys the developer (the same direction as the developer conveyance direction in the stirring chamber  12 ) is disposed in an area (hereinafter referred to as B area), which is B in  FIG. 3  and which is positioned further upstream than the A area in which the blade of the second screw  14  is omitted in the developer conveyance direction in the stirring chamber  12 . This blade can suppress flowing of the developer that has stayed in the A area, in which the blade of the second screw  14  is omitted, into the B area, which is positioned upstream of the A area. As a result, a larger amount of the developer can stay in the A area, in which the blade of the second screw  14  is omitted and where accumulation of the developer occurs, and accumulation of the developer in the developing chamber  11  is likely to be induced. 
     In addition, this blade is also configured to, by rotating, supply a force to the developer in the A area, in which the blade of the second screw  14  is omitted, in such a manner that the developer slowly flows toward the downstream side in the stirring chamber  12 . As a result, the developer in the A area, in which the blade of the second screw  14  is omitted, receives a force in a direction parallel to a shaft of the second screw  14 , and thus, occurrence of flowing back of the developer into a portion of the developing chamber  11  that is positioned along a direction perpendicular to the shaft of the second screw  14  can be suppressed. However, in the case where the force that is supplied to the developer by the blade is too large, the developer is less likely to stay in the A area, in which the blade of the second screw  14  is omitted, and as a result, the level of the developer surface in the developing chamber  11  may not be increased very much. Regarding this matter, the blade, which is positioned upstream of the A area, may be disposed so as not to be superposed with the downstream communicating portion  18 . Since an area upstream of the downstream communicating portion  18  in the stirring chamber  12  is positioned outside the circulation path, the amount of the toner that is present in the area is small. Therefore, the developer in the A area, in which the blade of the second screw  14  is omitted, receives the force from the blade, which is positioned at the area outside the circulation path, via a force that causes the developer that leaks from the downstream communicating portion  18  to flow back, and thus, the developer can slowly flow. 
     In addition, in the present embodiment, a supplementary toner is supplied to the B area in which the blade is disposed and which is positioned upstream of the A area in the stirring chamber  12 , in which the blade of the second screw  14  is omitted. 
     As mentioned above, the advantageous effects of the present invention can be obtained as long as the developing chamber  11  and the stirring chamber  12  are substantially horizontally arranged, and in the case where the developing chamber  11  and the stirring chamber  12  are arranged in such a manner as to be superposed with each other in the top-bottom direction, the level of the developer surface in the downstream communicating portion  18  may decrease due to the influence of gravity, resulting in a decrease in the level of the developer surface in the developer coating area. Here, the term “substantially horizontally arranged” refers to setting the inclination angle of the bottom surface of the downstream communicating portion  18  between the developing chamber  11  and the stirring chamber  12  with respect to the horizontal direction to be about 30 degrees or smaller. When the inclination angle is larger than about 30 degrees, it becomes difficult to cause the developer to stay in the vicinity of the downstream communicating portion  18 , or it becomes difficult to contribute to an increase in the level of the developer surface in the developing chamber  11 , and thus, the advantageous effects of the present invention is less likely to be obtained. The inclination angle is preferably 15 degrees or smaller. This holds true both in the case where the developing chamber  11  is positioned above the stirring chamber  12  and in the case where the developing chamber  11  is positioned below the stirring chamber  12 . 
     Although the case in which some of the blade of the second screw  14  in the stirring chamber  12  is omitted has been described in the present embodiment, the present invention is not limited to this. For example, a configuration in which the diameter of the blade of the second screw  14  in an area in the stirring chamber  12 , in which the blade of the second screw  14  is omitted in the first embodiment, is set to be smaller than that of the blade of the second screw  14  in other areas in the stirring chamber  12  in such a manner as to reduce the conveying ability of the second screw  14  in a direction in which the developer circulates, so that the developer is caused to stay in the area may be employed. Instead of changing the diameter of the blade of the second screw  14 , the pitch of the blade may be changed. In both the cases, the conveying ability of the second screw  14  is set to be lower than that of the second screw  13  at a position in the downstream communicating portion  18  that faces the developer coating area. In addition, in this case, the conveying ability of the second screw  14  in an area in which the second screw  14  faces the downstream communicating portion  18  is set to be lower than that in an area that is positioned downstream of the area. 
     Although in the configuration of the present embodiment, a partitioning wall is disposed between the downstream communicating portion  18  through which the developer is conveyed from the developing chamber  11  to the stirring chamber  12  and the corresponding inner side wall of the developer container  2 , a configuration in which such a partition wall is not disposed may be employed. 
     Second Embodiment 
     Overviews of an image forming apparatus and developing devices of a second embodiment are the same as those of the first embodiment, and descriptions thereof will be omitted. Configurations of the developing devices of the second embodiment different from those of the developing devices of the first embodiment will now be described. 
       FIG. 4  is an enlarged view of an area in the vicinity of the downstream communicating portion  18  between the developing chamber  11  and the stirring chamber  12 , which is a characteristic portion of one of the developing devices according to the second embodiment. A difference between the second embodiment and the first embodiment is that a helical blade that is wound in a direction opposite to the direction in which the blade of the second screw  14  is wound is formed in the area, in which some of the blade of the second screw  14  is omitted in the first embodiment, in such a manner as to form the flow of the developer in a direction opposite to the developer circulation direction. The rest of the configuration of the second embodiment is the same as that of the first embodiment. 
     Such a configuration can more effectively cause the developer that is present in the vicinity of the downstream communicating portion  18  to stay where it is than that of the first embodiment. However, if an excessive amount of the developer is conveyed in the opposite direction, the developer in the stirring chamber  12  may flow back into the developing chamber  11 . Accordingly, in the present embodiment, the diameter of the blade, which is wound in the direction opposite to the direction in which the other blades are wounded, is smaller than that of the other blades. With this configuration, accumulation of the developer can slowly occur. 
     In the present embodiment, the direction of rotation of the second screw  14  in the stirring chamber  12  is the direction of arrow D in  FIG. 2 . 
     This is because, in an area below the second screw  14  in which the developer is present, the momentum of the developer, the blade of the second screw  14  rotates in a direction away from the developing chamber  11 , so that the developer can be prevented from flowing back into the developing chamber  11 . 
     Also with this configuration, uneven distribution of the level of developer in the developing chamber  11  can be suppressed, and the probability of coating failure can be reduced. 
     According to the present invention, a horizontal-stirring-type developing device that employs the function separation system and that has a configuration that suppresses a decrease in developer level on a downstream side of a developing chamber in a direction in which a developer is conveyed while suppressing a decrease in an ability to supply the developer to a developer-carrying member can be provided. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2013-257155, filed Dec. 12, 2013, which is hereby incorporated by reference herein in its entirety.