Patent Publication Number: US-9897940-B2

Title: Developing device and conveying screw for developing an electrostatic latent image on an image bearing member

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a developing device that develops an electrostatic latent image formed on an image bearing member with a developer. 
     Description of the Related Art 
     In a well-known electrophotographic image forming apparatus, a two-component developer whose main components are a toner and a carrier is used. With such a configuration using the two-component developer, the toner is consumed by image formation, and the developer is supplied to replenish the toner. Accordingly, the toner is gradually replaced by a new one, but the carrier is not substantially consumed. A continuation of the image formation consequently causes a charging performance to be gradually impaired. For this reason, the following method (method of automatically replacing the developer) for maintaining the charging performance of the carrier has been known: while the developer containing the carrier mixed in the toner is supplied, an excess of the developer is discharged from a developer container and the carrier that becomes old is thereby discharged. 
     In a known example of such a configuration, the excess of the developer is discharged from a discharge passage formed on the downstream, side of a conveyance passage through which the developer is conveyed in the developer container (Japanese Patent Laid-Open No. 2002-072686) in the case of the configuration disclosed in Japanese Patent Laid-Open No. 2002-072686, a returning screw that conveys the developer in the direction opposite to the direction in which a conveying screw conveys the developer in the conveyance passage is disposed on the downstream side of the conveying screw. The bottom surface of the discharge passage (discharge outlet) formed on the downstream side of the conveyance passage is located at a position higher than the bottom surface of the conveyance passage. The developer passing through the returning screw is discharged via the discharge passage. A discharging conveying screw that conveys the developer toward the outside is disposed in the discharge passage. 
     In an example of the method of automatically replacing the developer, as illustrated in  FIG. 9 , a disk portion  212   a  is disposed at an upstream end portion of a returning screw  212 . The purpose is to suppress unstable discharging of the developer that is caused by the fact that the position of a blade of the upstream end portion of the returning screw is varied in accordance with a phase of the returning screw  212 , and accordingly, the developer falls on the side of a discharge passage  213 . 
     The above method of automatically replacing the developer has the following problem. 
     As illustrated in  FIG. 9 , the blade of the screw is not formed between the disk portion  212   a  on the upstream side of the returning screw  212  and a start position of the discharge passage  213  (area surrounded by a dotted line). The reason is as follows. There is a developer remaining in an immobile area between the disk portion  212   a  and the discharge passage  213 . In the case where a blade portion is located between the disk portion  212   a  and the discharge passage  213 , the developer remaining in this area is spattered. The spattered developer is discharged via the discharge passage. Consequently, the developer in the developer container is supplied between the disk portion  212   a  and the discharge passage  213 , and discharging of the developer is repeated. Consequently, the amount of the developer in a developing device becomes lower than the intentional amount, resulting in a failure of an image. 
     The temperature of an end portion of a first conveying screw  204  may be increased by friction against a bearing  216  due to its rotation. Accordingly, the increase in the temperature of the end portion of the first conveying screw  204  may increase the temperature of the developer remaining in the immobile area illustrated within the frame of the dotted line in  FIG. 9 , and an aggregation toner may be generated. Vibration of the developing device (for example, when the body of an image forming apparatus is moved, or a unit of the image forming apparatus is replaced by a new one) causes the aggregation toner to enter a developer circulation path in the inside of the developing device. Consequently, the aggregation toner may be developed and a toner stain image may be formed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a developing device using a method of replacing the developer that can suppress the generation of the aggregation toner in the immobile area around the returning screw, and a conveying screw. 
     A developing device according to an embodiment of the present invention includes a developer container that contains a developer, a conveying screw that conveys the developer and includes a rotating shaft rotatably disposed in the developer container, a spiral first blade portion that conveys the developer in a first direction of the rotating shaft, a spiral second blade portion that is formed on the rotating shaft at an end portion of the first blade portion and that conveys the developer in a direction opposite to the first direction, and a disk portion that is disposed on the rotating shaft at an end portion of the second blade portion distal from the first blade portion in an axial direction and that is disposed so as to protrude from the rotating shaft in a radial direction, a first conveyance passage in which the conveying screw is disposed and through which the developer is conveyed, a second conveyance passage that is formed on a downstream side of the disk portion in the first direction so as to be in communication with the first conveyance passage, that accommodates the rotating shaft in an inside thereof, and that is located at a position higher than a bottom surface of the first conveyance passage that the first blade portion faces, an outlet that is formed in the second conveyance passage and via which a toner is discharged, and a circular portion that is disposed on the rotating shaft at an end portion of the disk portion distal from the second blade portion in the axial direction and whose end portion distal from the disk portion has an outer diameter larger than an outer diameter of the rotating shaft and smaller than an outer diameter of the disk portion. 
     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 schematic configuration diagram of an image forming apparatus according to a first embodiment. 
         FIG. 2  is a schematic configuration diagram of a developing device according to the first embodiment. 
         FIG. 3  is a schematic configuration diagram of the developing device according to the first embodiment. 
         FIG. 4  is an enlarged view of the developing device according to the first embodiment around an outlet. 
         FIG. 5A  is a diagram illustrating a problem of a conventional developing device. 
         FIG. 5B  is a diagram illustrating the problem of the conventional developing device. 
         FIG. 5C  is a diagram illustrating the problem of the conventional developing device. 
         FIG. 6A  is a diagram illustrating the developing device according to the first embodiment. 
         FIG. 6B  is a diagram illustrating the developing device according to the first embodiment. 
         FIG. 7A  is a diagram illustrating the developing device according to the first embodiment. 
         FIG. 7B  is a diagram illustrating the developing device according to the first embodiment. 
         FIG. 8A  is a diagram illustrating a developing device according to a second embodiment. 
         FIG. 8B  is a diagram illustrating the developing device according to the second embodiment. 
         FIG. 9  is a diagram illustrating a problem of a conventional developing device. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will hereinafter be described in detail with respect to the drawings. 
     First Embodiment 
     Image Forming Apparatus 
       FIG. 1  is a schematic configuration diagram of an image forming unit of an image forming apparatus according to a first embodiment of the present invention. 
     As illustrated in  FIG. 1 , a developing device according to the first embodiment is used in a full-color image forming apparatus of a so-called tandem type. Drum cartridges that form four color toner images of yellow, magenta, cyan, and black are arranged in parallel. After four colors are superposed on an intermediate transfer belt  104 , the colors are collectively transferred to a transfer material. Then, a full color image is obtained by pressing and heating with a fixing unit  106 . In the following description, components represented simply by numerals, with the symbols of Y, M, C, and K being omitted, are common components of the drum cartridges for yellow, magenta, cyan, and black in  FIG. 1 . 
     An image forming action of the image forming apparatus thus configured will be described. 
     When the image forming action is started, photosensitive drums  100  as image bearing members rotate in the directions of arrows a. Surfaces of the photosensitive drums  100  are uniformly charged with primary chargers  101  as charging devices. Electrostatic latent images are subsequently formed on the surfaces of the photosensitive drums  100  that are exposed to light by a laser exposure device, not illustrated. 
     The electrostatic latent images thus formed are developed with developing devices  102  by using a two-component developer containing a magnetic carrier and a non-magnetic toner and are visualized. The toner images developed with the developing devices  102  are transferred to the intermediate transfer belt  104  with primary transfer rollers  103  as transfer devices in a multi-layer transfer manner. A toner image after the multi-layer transfer is transferred to a transfer material  110  conveyed to a secondary transfer unit  103   z . The toner image transferred to the transfer material  110  is subsequently fixed with the fixing unit  106  as a fixing device. After the toner image is transferred, residues of the transferred toner attached to surfaces of the photosensitive drums  100  and the intermediate transfer belt  104  are removed with cleaners  105 , and the photosensitive drums  100  and the intermediate transfer belt  104  are used in subsequent image formation. 
     Developing Device 
     The developing devices  102  will now be described in detail with reference to  FIG. 2  and  FIG. 3 . As illustrated in  FIG. 2  and  FIG. 3 , each developing device  102  includes a developer container  200  containing a two-component developer. The developing device  102  also includes a developing sleeve  201 , which is a developer bearing member, made of a non-magnetic material such as SUS or aluminum. The developing sleeve  201  is disposed on the developer container  200  so as to be rotatable in the direction of an arrow b. The developing sleeve  201  bears and conveys the developer to a developing area facing the corresponding photosensitive drum  100  and develops the electrostatic latent image formed on the corresponding photosensitive drum  100 . The diameter of the developing sleeve  201  is 20 mm in the first embodiment. A magroll  202  (illustrated in  FIG. 2  only), which is a means of generating a magnetic field, is secured to and disposed on the inside of the developing sleeve  201 . The surfaces of the developing sleeve  201  are rotated along the outer circumference of the magroll  202  at a speed of 500 rpm. A regulating blade  203  (illustrated in  FIG. 2  only), which is a means of developer regulation, is disposed so as to face the developing sleeve  201  at an interval and regulates the amount in which the developer borne by the developing sleeve  201  is coated. In the first embodiment, the interval between the developing sleeve  201  and the regulating blade  203  is 350 μm. 
     A first conveying screw  204  and a second conveying screw  205 , which are developer-agitating and -conveying members, are disposed inside the developing device  102 . The developer contained in the developer container  200  is conveyed from the upstream side in the direction of conveyance of the developer to the downstream side (front side direction in the figures) while being agitated by the first conveying screw  204  disposed in a first conveyance passage  206 . The developer contained in the developer container  200  is also conveyed from the upstream side in the direction of conveyance of the developer to the downstream side (back side direction in the figures) by the second conveying screw  205  in a second conveyance passage  207 . The first conveyance passage  206  and the second conveyance passage  207  are partitioned by a partition wall  209 . In this way, a circulation path through which the developer circulates is formed of the first conveyance passage  206  and the second conveyance passage  207  with the partition wall  209  interposed therebetween. Part of the developer circulating in the developer container  200  is supplied from the second conveyance passage  207  to the developing sleeve  201  by using the magnetic force of the magroll  202 . The developer supplied to the developing sleeve  201  is borne on a surface of the developing sleeve  201  by using the magnetic force of the magroll  202  and conveyed to the developing area facing the photosensitive drum  100  when the developing sleeve  201  is rotated. The first conveying screw  204  and the second conveying screw  205  are rotated at a speed of 550 rpm. Blades are spirally formed so as to be centered about screw axes in a period, of 20 mm, and the diameter of the outer circumference of each blade is 17 mm. The two-component developer that is attracted by the magnetic force at the developing area facing the photosensitive drum  100  comes into contact with the surface of the photosensitive drum  100 . A developing bias applied to the developing sleeve  201  causes only the toner to be transferred from the developing sleeve  201  to the photosensitive drum  100 . In this way, a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive drum  100 . The developing bias is applied such that an alternating current component is superimposed on a predetermined direct current component V dev  V. The alternating current component of the developing bias is a square wave having a frequency of 7 kHz and a peak-to-peak voltage of 1.3 kV. 
     The developer borne by the developing sleeve  201  after developing is returned to the inside of the developer container  200  when the developing sleeve  201  is rotated. The developer is subjected to magnetic repulsion, detached from the surface of the developing sleeve  201 , and returned to the second conveyance passage  207 . 
     A supplementary developer is supplied from a supply port  210  to replenish the toner consumed during such a developing process. The supplementary developer is contained in a hopper, not illustrated, connected to the supply port  210 . In the first embodiment, the amount of the toner to be supplied is controlled on the basis of a result of detection by a magnetic permeability sensor, not illustrated, disposed inside the developer container  200 . Specifically, the average magnetic permeability of the developer is detected with the magnetic permeability sensor, and the weight ratio of the toner to the developer is calculated from the detected value. When the calculated value is less than 8%, the supplementary developer is supplied. The supply is performed in a manner in which a screw disposed inside the hopper is rotated to convey the supplementary developer in the hopper to the supply port  210 . The supplementary developer supplied from the supply port  210  is conveyed while being agitated by using the first conveying screw  204  together with the other developer circulating through the developer container  200 . 
     The supplementary developer used at this time contains a small amount of the carrier mixed in the toner, and the weight ratio of the carrier to the supplementary developer is 10%. The toner is consumed during image formation but the carrier is not consumed. Accordingly, the amount of the developer in the developer container  200  continuously increases when the supply of the supplementary developer continues. For this reason, an outlet  211  is formed at the most downstream position in the direction of conveyance by the first conveying screw  204  in the first conveyance passage  206 . Details of the mechanism of the passage extending to the outlet  211 , which is a feature of the first embodiment, will be described later. A small amount of the developer is discharged from the outlet  211  such that the amount of the developer in the developer container  200  is maintained within a predetermined range. The above supply enables the toner and the carrier to be continuously replaced by a new toner and a new carrier. Accordingly, a long lifetime can be achieved. 
     Two-Component Developer 
     The two-component developer composed of the non-magnetic toner and the magnetic carrier that is used in the first embodiment will now be described. The toner contains a resin binder, a colorant, and, as needed, coloration resin particles containing another additive, and coloration particles containing an external additive such as colloidal silica fine powder. The toner is a negatively charged polyester resin and its volume average particle diameter is preferably no less than 5 μm and no more than 8 μm. In an experiment described later, a toner having a volume average particle diameter of 7.0 μm was used. 
     Preferable examples of the carrier include iron whose surface oxidizes or does not oxidize, nickel, cobalt, manganese, chromium, a metal such as a rare earth element, an alloy thereof, and oxide ferrite. A method of manufacturing the magnetic particles is not particularly limited. The carrier has a volume average particle diameter of 20 to 50 μm, preferably 30 to 40 μm, and a resistivity of 1.0×10 7  Ω·cm or more, preferably 1.0×10 8  Ω·cm or more. In the experiment described later, the carrier had a volume average particle diameter of 40 μm, a resistivity of 5.0×10 7  Ω·cm, and a magnetization of 260 emu/cc. 
     Configuration of Automatically Replacing Developer 
     A configuration of automatically replacing the developer, which is a feature of the first embodiment, will now be described. As illustrated in  FIG. 4 , a rotating shaft  214  is rotatably disposed in the first conveyance passage  206 . The first conveying screw  204  is disposed on the circumference of the rotating shaft  214 . The first conveying screw  204  is disposed as a main spiral portion (first blade portion) that includes a first blade portion  204  formed in a spiral shape and that conveys the developer contained in the first conveyance passage  206 . The returning screw  212  is disposed on the downstream side of the first conveying screw  204  in the direction of conveyance by the first conveying screw  204 . The returning screw  212  is disposed as a spiral sub-portion (second blade portion) including a second blade portion  212  wound around the rotating shaft  214  in the direction opposite co the direction in which the first blade portion  204  is wound. The disk portion  212   a  is disposed on the rotating shaft  214  on the downstream side of the returning screw  212  in the direction of conveyance by the first conveying screw  204 . The disk portion  212   a  is disposed so as to protrude in the radial direction of the rotating shaft  214 . The disk portion  212   a  prevents the absence of a blade at the upstream end of the returning screw  212  without being affected by the phase of the rotating shaft  214 . Accordingly, discharging of the developer can be stable. 
     The rotating shaft  214  is received by the bearing  216 . The bearing  216  in the first embodiment is a resin bearing made of POM (polyoxymethylene). The present invention, however, is not limited thereto. 
     The discharge passage  213 , as a discharge passage (second conveyance passage) through which the two-component developer is continuously discharged to the first conveyance passage  206 , is formed on the downstream side in the direction in which the two-component developer is conveyed by the first conveying screw  204 . The discharge passage  213  is formed so as to be in communication with the first conveyance passage  206 . The discharge passage  213  accommodates the rotating shaft  214  in the inside thereof and is located around the rotating shaft  214  so as to face and be a predetermined clearance away from the rotating shaft  214  of the returning screw  212 . The height of a bottom surface  213   a  or one discharge passage  213  is higher than the height of a bottom surface  206   a  of the first conveyance passage  206 . An excess of the developer can be discharged to the outside via the clearance between the discharge passage  213  and the rotating shaft  214 . 
     The supplementary developer is a developer in which the carrier is contained in the toner at a predetermined ratio (a weight ratio of about 10%). The ratio is not limited thereto. The toner consumed by the image formation is replenished with a supplier, not illustrated. The supplier contains the supplementary developer containing the carrier at a predetermined ratio as described above and supplies the supplementary developer by using rotation of a supply screw, not illustrated. The supplementary developer is supplied from the upstream side of the developer container  200  in the direction in which the developer is conveyed by the first conveying screw  204 . 
     The supply is controlled such that the concentration of the toner in the developer in the developer container  200  is kept constant. When the supply is thus controlled, the amount of the developer in the developer container  200  increases as the image formation is repeated. The supplementary developer contains 90% of the toner and 10% of the carrier. Accordingly, the toner is consumed by the image formation, but the carrier is not consumed and remains in the developer container. For this reason, the amount of the developer increases as the supply is repeated. When the amount of the developer increases, the surface D of the developer ascends and the developer is conveyed to the outlet  211  beyond the returning screw  212 . The developer conveyed to the outlet  211  is discharged from the outlet  211 , conveyed to a collection container (not illustrated), collected and stored. 
     The consumed toner is replenished by the supplementary developer, as described above. The amount of the carrier supplied at the same time becomes excessive, and accordingly, the two-component developer is gradually replaced by a new one automatically such that the amount of the developer in the developer container  200  is kept constant. In this way, a function of automatically discharging the developer is achieved. 
     A problem occurred in the case where the function of automatically discharging the developer is used will be described with reference to  FIG. 5A ,  FIG. 5B , and  FIG. 5C . 
     As illustrated in  FIG. 5A , in the case where the returning screw  212  is near to an entrance of the discharge passage  213 , the developer swirled up by the returning screw  212  easily enters the entrance of the discharge passage  213 . In fact, the swirled developer was consequently discharged from the outlet  211 , and the developer was excessively discharged. The amount of the developer in the developing device  102  was accordingly decreased, and a faint image and an image having a variation in contrast were formed due to a decrease in the amount in which the developing sleeve  201  was coated with the developer. In view of this, as illustrated in  FIG. 5B , the distance between the returning screw  212  and the entrance of the discharge passage  213  was increased, and the excessive discharge of the developer was thereby suppressed. The distance k between the upstream end portion of the returning screw  212  and the entrance of the discharge passage  213  is preferably 1.5 mm or more in order to suppress the excessive discharge of the developer and is 2.5 mill in the first embodiment. 
     As illustrated in  FIG. 5B , in the case where the distance k between the returning screw  212  and the entrance of the discharge passage  213  is a predetermined distance or more, there is the immobile area of the developer as illustrated in  FIG. 9 . The temperature of the end portion of the first conveying screw  204  may be increased by friction against the bearing  216  due to its rotation. Accordingly, the increase in the temperature of the end portion of the first conveying screw  204  may increase the temperature of the developer remaining in the immobile area illustrated within the frame of the dotted line in  FIG. 9 , and the aggregation toner may be generated. The aggregation toner enters the developer circulation path using developer-agitating and -conveying screws due to vibration of the developing device (for example, when the body of the image forming apparatus is transported or when a unit in the image forming apparatus is replaced by a new one). The aggregation toner may consequently be developed, and accordingly, a toner stain image may be formed. 
     In contrast, as illustrated in  FIG. 5C , in the case where the entire shaft  214  is thickened, for example, the immobile area of the developer is reduced. However, the height of the bottom surface  213   a  of the discharge passage becomes low, and a step between the bottom surface  213   a  and the bottom surface  206   a  of the first conveyance passage becomes small. Accordingly, the developer in the first conveyance passage easily exits via the outlet  211 , and the amount of the developer in the developer container  200  is excessively decreased. 
     In view of this, in the first embodiment, as illustrated in  FIG. 6A , a circular portion J that increases the diameter of the rotating shaft  214  is formed between the disk portion  212   a  and the discharge passage  213  so as to fill a clearance (gap) between the rotating shaft  214  and the container. More specifically, the diameter of the rotating shaft  214  at a position downstream of the disk portion  212   a  in the direction of conveyance by the first conveying screw  204  and upstream of the discharge passage  213  in the direction of conveyance by the first conveying screw  204  is smaller than the outer diameter of the disk portion  212   a . The diameter of the rotating shaft  214  at the position downstream of the disk portion  212   a  in the direction of conveyance by the first conveying screw  204  and upstream of the discharge passage  213  in the direction of conveyance by the first conveying screw  204  is larger than the diameter of the rotating shaft  214  at a portion facing the discharge passage  213 . In this way, the clearance between the rotating shaft  214  and the container is filled between the disk portion  212   a  and the discharge passage  213 . The circular portion J is disposed so as to be concentric with the rotating shaft  214  and formed into a cylindrical shape. The diameter of the circular portion J is smaller than the outer diameter of the disk portion  212   a  and is larger than the diameter of the portion of the rotating shaft  214  that faces the discharge passage  213 . This causes the immobile area of the developer to be unlikely to occur. In this way, the formation of the toner stain image due to the aggregation toner can be suppressed and stable image formation can be performed over a long period of time. 
     The range in which the immobile area is filled will be described with reference to  FIG. 6 k   . A height H 1  of the lowest portion of the circular portion J with respect to the bottom surface  206   a  of the first conveyance passage preferably satisfies H 1 ≦H 2  where H 2  is the height of the highest portion of the bottom surface  213   a  of the discharge passage. The length of the circular portion J, which is calculated by k−L, is preferably 1 mm or more. The reason is that, in the case where an area with which the immobile area is filled is smaller than these, the effect of suppressing the generation of the aggregation toner is reduced. When the distance between the lowest portion of the returning screw  212  and H 2  in the vertical direction is divided into three equal distances, a dashed line m in  FIG. 6B  is a line connecting a position the divided distance away from the height position of the lowest portion to the lowest portion of the returning screw  212  at the most upstream position. The circular portion J preferably does not protrude to an area below the line m. The reason is that, in the case where the circular portion J becomes larger than this, there is a provability that the circular portion J itself spatters the developer, resulting in the excessive discharge of the developer. The symbol L represents a horizontal distance between the highest portion of the bottom surface  213   a  of the discharge passage and the circular portion J and is preferably at least 1 mm or more. The reason is that, when L is less than 1 mm, there is a probability that the developer is packed and is unlikely to be discharged to the discharge passage  213  or a new aggregate is generated at a narrow area due to friction against the circular portion J. 
     In the first embodiment, H 1 =4.5 mm, H 2 =6.5 mm, and L=1.5 mm hold. 
     In the first embodiment, as illustrated in, for example,  FIG. 7A  and  FIG. 7B , in the case where a plurality of the circular portions are disposed on the shaft  214 , it goes without saying that the same effects are achieved. The height H 1  and the distance L in the case where the circular portions are disposed are the same as illustrated in  FIG. 7B . 
     The following description gives a difference in the amount of the aggregation toner generated by continuous image formation between the case where the circular portion was formed on the shaft  214  in the immobile area of the developer that was located on the upstream side of the returning screw as illustrated in  FIG. 6A  and  FIG. 6B  and the case where the immobile area was not filled as illustrated in  FIG. 5B . 
     In the case of the configuration illustrated in  FIG. 5B , when images were formed at 30° C., the toner stain image due to the aggregation toner was generated when 10000 sheets of paper were fed. In contrast, in the case of the configuration illustrated in  FIG. 6A  and  FIG. 6B , when images were formed at 30° C., no toner stain image due to the aggregation toner was generated after 10000 sheets of paper were fed. 
     Thus, in the developing device including a mechanism of automatically replacing the developer, the circular portion is formed on the shaft  214  in the immobile area of the developer between the disk portion  212   a  and the discharge passage  213  in order to fill the immobile area. This causes the immobile area of the developer to be unlikely to occur and accordingly suppresses the formation of the toner stain image due to the aggregation toner, enabling stable image formation over a long period of time. 
     Second Embodiment 
     A second embodiment will now be described. An image forming process in the second embodiment is substantially the same as in the first embodiment, and accordingly, a duplicative description is omitted. 
     In the second embodiment, as illustrated in  FIG. 8A  and  FIG. 8B , an inclined portion S as the circular portion that increases the diameter of the rotating shaft  214  is disposed between the disk portion  212   a  and the discharge passage  213 . The inclined portion S is inclined with respect to the axial direction of the rotating shaft  214 . In this way, the clearance (gap) between the rotating shaft  214  and the container is filled between the disk portion  212   a  and the discharge passage  213 . This causes the immobile area of the developer to be unlikely to occur and accordingly suppresses the formation of the toner stain image due to the aggregation toner, enabling stable image formation over a long period of time. 
     The range in which the immobile area is filled will be described with reference to  FIG. 8B . A height H 1  at which the inclination of the inclined portion begins preferably satisfies H 1 ≦H 2 . In the case where an area with which the immobile area is filled is smaller than this, the effect of suppressing the generation of the aggregation toner is reduced. The inclined portion S preferably does not protrude to an area below the line m. The reason is that, in the case where the inclined portion S becomes larger than this, there is a provability that the inclined portion S itself spatters the developer, resulting in the excessive discharge of the developer. The value of L is preferably at least 1 mm or more. The reason is that, when L is less than 1 mm, there is a probability that the developer is packed and is unlikely to be discharged to the discharging path  213  or a new aggregate is generated at a narrow area due to friction against the inclined portion S. In the second embodiment, H 1 =1 mm, H 2 =6.5 mm, and L=1 mm hold. The inclination of the inclined portion S is constant. The inclination of the inclined portion S, however, may be freely determined within the above conditions. The inclination of the inclined portion S may be formed of a combination of plural inclinations. Although the inclined portion S extends just in front of the discharge passage  213  in the second embodiment, it goes without saying that the effects of the present invention are not affected also in the case where the inclined portion S enters the discharge passage  213  in the horizontal direction, provided that the above conditions are satisfied. 
     Thus, in the developing device including a mechanism of automatically replacing the developer, the inclined portion is formed on the shaft  214  in the immobile area of the developer between the disk portion  212   a  and the discharge passage  213 . In this way, the immobile area is filled. This causes the immobile area of the developer to be unlikely to occur and accordingly suppresses the formation of the toner stain image due to the aggregation toner, enabling stable image formation over a long period of time. 
     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. 2015-170589, filed Aug. 31, 2015, which is hereby incorporated by reference herein in its entirety.