Patent Publication Number: US-7904001-B2

Title: Developing unit, process cartridge, and image forming apparatus having a plurality of conveyor members, a supply part, and a discharge part

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to image forming apparatuses using electrophotography, such as copiers, printers, facsimile machines, and multifunction machines having two or more of their functions, and developing units and process cartridges provided therein, and more particularly to a developing unit of the trickle development system, which suitably supplies new carrier into the developing unit, and a process cartridge and image forming apparatus including the same. 
     2. Description of the Related Art 
     There is a conventionally known technique that suitably supplies new carrier to a developing unit containing two-component developer formed of toner and carrier (in some cases, with an additive added thereto) in image forming apparatuses such as copiers and printers, which technique is referred to as the trickle development system. (See, for example, Japanese Laid-Open Patent Application No. 2001-183893.) 
     Toner is suitably supplied into the developing unit using a two-component developer through a toner supply opening provided in part of the developing unit in accordance with toner consumption in the developing unit. The supplied toner and the developer in the developing unit are agitated and mixed using a conveying member (an agitating member) such as a screw conveyor. Part of the agitated and mixed developer is supplied to a developing roller. The developer carried on the developing roller is restricted to an appropriate amount by a doctor blade. Thereafter, toner in the two-component developer adheres to a latent image on a photosensitive body drum at a position opposite the photosensitive body drum. 
     Thus, the carrier in the two-component developer contained in the developing unit remains in the developing unit without being consumed in a regular development process. Therefore, the carrier is degraded over time. In detail, the “film scraping phenomenon,” where the electrostatic charge capability of a carrier is reduced by the abrasion or separation of its coating layer due to lengthy agitation and mixing of the carrier in the developing unit, or the “spent phenomenon,” where the electrostatic charge capability of a carrier is reduced by adhesion of a toner component or additive to the surface of the carrier, occurs. 
     The trickle development system prevents degradation of the quality of an output image due to such carrier degradation over time. That is, this system maintains the amount and electrostatic charge capability of carrier contained in the developing unit by reducing a degraded portion of the carrier in the developing unit by suitably supplying new carrier (or new two-component developer) into the developing unit and suitably discharging part of the two-component developer contained in the developing unit from the developing unit. 
     Image forming apparatuses using this trickle development system achieve stabilization of the quality of an output image even over time compared with those requiring replacement of a developing unit or carrier with a new one every time there is degradation of the carrier with time. 
     Japanese Laid-Open Patent Application No. 2001-183893 describes a developing unit using the trickle development system, where overflow-type discharge means is employed for discharging a developer from the developing unit. In detail, a discharge opening (hole) is provided in the developing unit, and the developer (a portion made surplus by the supply of carrier) is discharged outside from the discharge opening when the surface of the developer conveyed to the position of the discharge opening exceeds a predetermined height. 
     According to the above-described developing unit of the trickle development system of Japanese Laid-Open Patent Application No. 2001-183893, when driving of the unit is started, the developer in the unit may be inclined to locally cause great undulations on its surface, so that there may be an unintended discharge of the developer. Repeated occurrence of such a phenomenon at every start and stop of the driving of the unit may cause an excessive discharge of the developer in the developing unit, thus causing a shortage of the amount of the developer. 
     This shortage of the amount of the developer in the developing unit causes the degraded condition of the developer to be unstable or the amount of electrostatic charge of toner to be reduced, thus causing a problem on an output image, such as a decrease in image density. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention may solve or reduce one or more of the above-described problems. 
     According to one or more embodiments of the present invention, there are provided a developing unit, a process cartridge, and an image forming apparatus in which one or more of the above-described problems may be solved or reduced. 
     According to one or more embodiments of the present invention, there are provided a developing unit of the trickle development system where even if developer in the developing unit is inclined in an undulatory manner, no variation is caused in the amount of the developer discharged outside and the quality of an output image is stabilized, and a process cartridge and an image forming apparatus including the same. 
     According to one embodiment of the present invention, there is provided a developing unit containing a developer including a carrier and a toner, and developing a latent image formed on an image carrier, the developing unit including a plurality of conveyor members configured to convey the contained developer in respective longitudinal directions so as to form a circulation channel; a supply part configured to supplement the carrier in the developing unit; a discharge part configured to discharge a first part of the contained developer outside the developing unit; and a bypass channel configured to cause a second part of the developer to return to an upstream side of the circulation channel without passing a position where the discharge part is provided. 
     According to one embodiment of the present invention, there is provided a developing unit containing a developer including a carrier and a toner, and developing a latent image formed on an image carrier, the developing unit including a plurality of conveyor members configured to convey the contained developer in respective longitudinal directions so as to form a circulation channel; a supply part configured to supplement the carrier in the developing unit; a discharge part configured to discharge a part of the contained developer outside the developing unit; and a projection part configured to control entrance of the developer into the discharge part, the projection part being provided at one of an upper end and a lower end of the discharge part. 
     According to one embodiment of the present invention, there is provided a process cartridge removably provided in a main body of an image forming apparatus, the process cartridge including the developing unit and the image carrier as set forth in any of the above-described developing units, the developing unit and the image carrier being integrated as a unit. 
     According to one embodiment of the present invention, there is provided an image forming apparatus including the developing unit and the image carrier as set forth in any of the above-described developing units. 
     According to one or more embodiments of the present invention, since there is provided a bypass channel for causing part of the developer to return to the upstream side of a circulation channel without passing a position where a discharge part is provided, it is possible to provide a developing unit of the trickle development system according to which no variation is caused in the amount of the developer discharged outside so that the quality of an output image is stabilized even if there is an undulatory inclination in the developer in the developing unit  23 ; and a process cartridge and an image forming apparatus including the developing unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram showing an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 2  is an enlarged view of a process cartridge provided in the image forming apparatus according to the first embodiment of the present invention; 
         FIG. 3  is a longitudinal cut-away view of a circulation channel in a developing unit according to the first embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of the circulation channel of  FIG. 3 , taken along the line Y 1 -Y 1  according to the first embodiment of the present invention; 
         FIG. 5  is a cross-sectional view of the circulation channel of  FIG. 3 , taken along the line Y 2 -Y 2  according to the first embodiment of the present invention; 
         FIG. 6  is a diagram showing the circulation channel of  FIG. 3 , where there is an undulatory inclination in developer, according to the first embodiment of the present invention; 
         FIG. 7  is a cross-sectional view of a developing unit according to a second embodiment of the present invention; 
         FIG. 8  is a perspective view of part of the developing unit according to the second embodiment of the present invention; and 
         FIG. 9  is a cross-sectional view of a variation of the developing unit according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description is given below, with reference to the accompanying drawings, of embodiments of the present invention. In the drawings, the same or corresponding elements are referred to by the same reference numerals, and a redundant description thereof is suitably simplified or omitted. 
     First Embodiment 
     A description is given, with reference to  FIG. 1  through  FIG. 6 , of a first embodiment of the present invention. 
     First, a description is given, with reference to  FIG. 1 , of an overall configuration and operation of an image forming apparatus according to the first embodiment of the present invention. 
     Writing parts  2 A,  2 B,  2 C, and  2 D are devices that write electrostatic latent images onto corresponding photosensitive body drums  21  (image carriers) after a charging process based on image information. The writing parts  2 A through  2 D are optical scanners using polygon mirrors  3 A,  3 B,  3 C, and  3 D and optical elements  4 A,  4 B,  4 C, and  4 D, respectively. Alternatively, the writing parts  2 A through  2 D may be LED arrays instead of optical scanners. 
     A paper feed part  61  contains transfer materials P such as OHP sheets and feeds the transfer materials P one by one to a transfer belt  30  at the time of forming an image. 
     The transfer belt  30 , which is an endless belt for conveying the transfer material P by having the transfer material P electrostatically attracted and adhered to its surface so that toner images formed on the photosensitive body drums  21  are transferred onto the transfer material P, has an attraction and adhesion roller  64  and a belt cleaner  65  provided on its exterior surface. 
     Transfer rollers  24  opposite the corresponding photosensitive body drums  21  across the transfer belt  30  each have a cored bar coated with a conductive elastic layer. The conductive elastic layer of each transfer roller  24  is an elastic body whose electrical resistance (volume resistivity) is adjusted to medium resistance by mixing and dispersing a conductivity imparting agent such as carbon black, zinc oxide, or tin oxide in an elastic material such as polyurethane rubber or ethylene-propylene-diene polyethylene (EPDM). 
     A fixing part  66 , which includes a heating roller  68  and a pressure roller  67 , fixes a composite toner image on the transfer material P thereonto with pressure and heat. 
     Four process cartridges  20 Y,  20 C,  20 M, and  20 BK are provided longitudinally along the transfer belt  30  for forming yellow, cyan, magenta, and black toner images, respectively. 
     The process cartridges  20 Y,  20 C,  20 M, and  20 BK have respective agent cartridges  28 Y,  28 C,  28 M, and  28 BK provided thereon as supply parts that supply carriers (magnetic carriers) and color (yellow, cyan, magenta, and black) toners (toner particles) to corresponding developing units  23 . 
     The process cartridges  20 Y,  20 C,  20 M, and  20 BK and the agent cartridges  28 Y,  28 C,  28 M, and  28 BK can be attached to and detached from an apparatus main body  1  by rotating the transfer belt  30  around a rotational support shaft so that the transfer belt  30  (unit) is open with respect to the process cartridges  20 Y,  20 C,  20 M, and  20 BK and the agent cartridges  28 Y,  28 C,  28 M, and  28 BK. 
     The image forming apparatus of this first embodiment is a multifunction type serving as a copier and a printer. When the image forming apparatus serves as a copier, image information read from a scanner is subjected to various kinds of image processing, such as A/D conversion, MTF correction, and tone processing, and is converted into writing data. When the image forming apparatus serves as a printer, image information of a page-description language or bitmap format transmitted from a computer is subjected to image processing and converted into writing data. 
     At the time of forming an image, the writing parts  2 A through  2 D emit exposure lights corresponding to image information items of black, magenta, cyan, and yellow onto the process cartridges  20 BK,  20 M,  20 C, and  20 Y, respectively. That is, the exposure lights (laser lights) from respective light sources are emitted onto the corresponding photosensitive body drums  21  through the polygon mirrors  3 A through  3 D and the optical elements  4 A through  4 D, respectively. As a result, toner images corresponding to the exposure lights are formed on the photosensitive body drums  21  (image carriers) of the process cartridges  20 BK,  20 M,  20 C, and  20 Y. These toner images are transferred onto the transfer material P. 
     The transfer material P fed from the paper feeding part  61  is conveyed to the position of the transfer belt  30  after being timed for the conveyance at the position of a registration roller  63 . The attraction and adhesion roller  64  provided at the feed-in position of the transfer belt  30  causes the fed-in transfer material P to be attracted and adhered to the transfer belt  30  by applying voltage. The transfer material P, which moves as the transfer belt  30  runs in the direction indicated by an arrow in  FIG. 1 , successively passes the positions of the process cartridges  20 Y,  20 C,  20 M, and  20 BK, so that the toner images of respective colors are transferred onto the transfer material P in a superposed manner. 
     The transfer material P onto which the color toner images have been transferred is separated from the transfer belt  30  to reach the fixing part  66 . The toner images on the transfer material P are heated while being held between the heating roller  68  and the pressure roller  67 , so as to be fixed onto the transfer material P. On the other hand, the surface of the transfer belt  30  after separation of the transfer material P reaches the position of the belt cleaner  65 , so as to be cleaned of dirt such as toner adhered thereto. 
     Next, a description is given in detail of the process cartridges  20 Y,  20 C,  20 M, and  20 BK and the agent cartridges  28 Y,  28 C,  28 M, and  28 BK. 
     The process cartridges  20 Y,  20 C,  20 M, and  20 BK have substantially the same structure, and the agent cartridges  28 Y,  28 C,  28 M, and  28 BK also have substantially the same structure. Accordingly, in  FIG. 2 , the process cartridge and the agent cartridge are referred to by reference numerals  20  and  28 , respectively, without an alphabet letter (Y, C, M or BK). Likewise, the writing part is also referred to by reference numeral  2  without an alphabet letter (A, B, C or D). 
       FIG. 2  is an enlarged view of the process cartridge  20  and the corresponding agent cartridge  28  provided in the apparatus main body  1 .  FIG. 3  is a cut-away view from the direction of arrow X of a circulation channel in the corresponding developing unit  23 , taken along a plane perpendicular to the direction of arrow X.  FIG. 4  is a cross-sectional view of the circulation channel in the developing unit  23  of  FIG. 3 , taken along line Y 1 -Y 1 .  FIG. 5  is a cross-sectional view of the circulation channel in the developing unit  23  of  FIG. 3 , taken along line Y 2 -Y 2 . 
     Here, according to the present invention, the term “process cartridge” is defined as a unit into which an image carrier and at least one of a charging part that charges the image carrier, a developing part (developing unit) that develops a latent image formed on the image carrier, and a cleaning part that cleans the surface of the image carrier are integrated, and which is provided removably (detachably and reattachably) with respect to the main body of an image forming apparatus. 
     Referring to  FIG. 2 , the photosensitive body drum  21  serving as an image carrier, a charging part  22 , the developing unit  23  (developing part), and a cleaning part  25  are integrated into the process cartridge  20 , which adopts the trickle development system. 
     The photosensitive drum body  21  as an image carrier, which is a negatively-charged organic photosensitive body, is rotated counterclockwise by a rotating mechanism (not graphically illustrated). 
     The charging part  22  is an elastic roller charging device having a roller-shaped medium-resistance urethane foam layer formed of polyurethane, carbon black as conductive particles, a sulfidizing agent, a foaming agent, etc., around a cored bar. Examples of the material of the medium resistance layer of the charging part  22  include a rubber material, which may be expanded, where a conductive material such as carbon black or metal oxide is dispersed for resistance adjustment in urethane, ethylene-propylene-diene polyethylene (EPDM), acrylonitrile-butadiene rubber (NBR), silicone rubber, or isoprene rubber. 
     The cleaning part  25 , in which a cleaning brush (or cleaning blade) that comes into sliding contact with the photosensitive body drum  21  is provided, mechanically removes and collects untransferred toner on the photosensitive drum body  21 . 
     The developing unit  23  has a developing roller  23   a  serving as a developer carrier placed in proximity to the photosensitive body drum  21 , so that a development area where the photosensitive body drum  21  and a magnetic brush come into contact is formed where the developing roller  23   a  and the photosensitive body drum  21  face each other. The developing unit  23  contains developer G (two-component developer) formed of toner T and carrier C. The developing unit  23  develops an electrostatic latent image formed on the photosensitive body drum  21  (forms a toner image). A detailed description is given below of the configuration and operation of the developing unit  23 . 
     Here, the developing unit  23  according to the first embodiment adopts the trickle development system, so that new carrier C (developer G) is suitably supplied into the developing unit  23  from the agent cartridge  28  and the degraded developer G is discharged to an agent reservoir  70  provided external to the developing unit  23 . 
     Referring to  FIG. 2 , the agent cartridge  28  contains the developer G (toner T and carrier C) to be supplied into the developing unit  23 . The agent cartridge  28  serves as a toner cartridge that supplies new toner T to the developing unit  23  and as a supply part that supplies new carrier C to the developing unit  23 . Specifically, the agent cartridge  28  performs the opening and closing operations of a shutter mechanism  80  based on toner density information (the proportion of the toner T in the developer G) detected by a magnetic sensor  26  ( FIG. 3 ) provided in the developing unit  23 , so as to suitably supply the developer G into the developing unit  23  from the agent cartridge  28  as a supply part. 
     In this first embodiment, the mixture ratio of the toner T to the carrier C in the developer G of the agent cartridge  28  (toner density) is relatively high. 
     A supply pipe  29  serving as a supply part ensures introduction of the developer G (toner T and carrier C) supplied from the agent cartridge  28  into the developing unit  23 . That is, the developer G discharged from the agent cartridge  28  is supplied into the developing unit  23  through the supply pipe  29 . 
     Next, a description is given of an image forming process performed on the photosensitive body drum  21 . 
     Referring to  FIG. 2 , when the photosensitive body drum  21  is rotated counterclockwise, first, the surface of the photosensitive body drum  21  is evenly charged at the position of the charging part  22 . Thereafter, the charged surface of the photosensitive body drum  21  reaches the position of exposure to exposure light L, where an exposure process is performed by the writing part  2 . That is, the surface of the photosensitive body drum  21  is selectively discharged in accordance with image information through exposure to the exposure light L, so as to generate a difference from the electric potential of a non-image part that has not been exposed (voltage contrast), thereby forming an electrostatic latent image. In this exposure process, a charge generation material receives light so as to generate an electric charge in the photosensitive layer of the photosensitive body drum  21 , and generated holes counteract the electric charge on the charged surface of the photosensitive body drum  21 . 
     Thereafter, the surface of the photosensitive body drum  21  on which the latent image is formed reaches a position opposite the developing unit  23 . The electrostatic latent image on the photosensitive body drum  21  comes into contact with a magnetic brush on the developing roller  23   a , so that the negatively charged toner T in the magnetic brush is adhered to the electrostatic latent image. As a result, the electrostatic latent image is visualized. 
     In detail, the developer G drawn up by the magnetic force of a magnetic pole of the developing roller  23   a  is adjusted to an appropriate amount by a doctor blade  23   c , and is thereafter conveyed to the development area, where the developing roller  23   a  faces the photosensitive body drum  21 . The carrier C comes into sliding contact with the photosensitive body drum  21  with chains or clusters of its particles in the development area. At this point, the toner T mixed in the carrier C is negatively charged through friction with the carrier C. On the other hand, the carrier C is positively charged. A predetermined development bias is applied to the developing roller  23   a  from a power supply part (not graphically illustrated). As a result, an electric field is formed between the developing roller  23   a  and the photosensitive drum  21  so as to cause the negatively charged toner T to be selectively adhered to only an image part on the photosensitive body drum  21 , so that a toner image is formed. 
     Thereafter, the surface of the photosensitive body drum  21  on which the toner image is formed reaches a position where the transfer belt  30  and the transfer roller  24  face each other. Then, the toner image on the photosensitive body drum  21  is transferred onto the transfer material P that has been timed to be conveyed to the facing position for the transfer of the toner image. At this point, a predetermined voltage is applied to the transfer roller  24 . 
     Thereafter, the transfer material P having the superposed toner images transferred thereonto passes through the fixing part  66  to be ejected outside the apparatus by an ejecting roller  69  ( FIG. 1 ). 
     On the other hand, residual toner T on the photosensitive body drum  21 , which has not been transferred onto the transfer material P at the time of the transfer process, or untransferred toner, remains adhered onto the photosensitive body drum  21  and reaches a part opposite the cleaning part  25 . Then, the untransferred toner on the photosensitive body drum  21  is removed and collected in the cleaning part  25 . 
     Thereafter, the surface of the photosensitive body drum  21  passes a discharge part (not graphically illustrated). Thereby, the image forming process on the photosensitive body drum  21  is completed. 
     A detailed description is given below of a configuration and operation of the developing unit  23  in the image forming apparatus according to this embodiment. 
     Referring to  FIG. 2  through  FIG. 5 , the developing unit  23  includes the developing roller  23   a  as a developer carrier, first, second, and third conveyor screws  23   b   1 ,  23   b   2 , and  23   b   3  (auger screws) as conveying members, and the doctor blade  23   c.    
     The developing roller  23   a  is configured so that a cylinder-shaped sleeve of a non-magnetic material such as aluminum, brass, stainless steel, or conductive resin is rotated clockwise by a rotating mechanism (not graphically illustrated). In the sleeve of the developing roller  23   a , a magnet is fixed that forms a magnetic field so as to cause clustering of the developer G on the surface of the sleeve. Chains (clusters) of particles of the carrier C in the developer G are formed to stand on the sleeve along magnetic lines of force in a normal direction emanated from the magnet. Particles of the charged toner T are adhered to these standing chains of particles of the carrier C so as to form a magnetic brush. The magnetic brush is conveyed in the same direction as the sleeve (clockwise) by the rotation of the sleeve. 
     The doctor blade  23   c  is provided on the upstream side of the development area so as to restrict the developer G on the developing roller  23   a  to an appropriate amount. 
     The three conveyor screws  23   b   1  through  23   b   3  agitate and mix the developer G contained in the developing unit  23  while circulating the developer G in a longitudinal direction (a direction perpendicular to the plane of the paper of  FIG. 2 ). 
     The first conveyor screw  23   b   1  (first conveying member), which is provided at a position opposite the developing roller  23   a , conveys the developer G in a horizontal direction (a leftward direction indicated by a white arrow in  FIG. 3 ) and supplies the developer G onto the developing roller  23   a.    
     The second conveyor screw  23   b   2  (second conveying member) is provided at a position below the first conveyor screw  23   b   1  and opposite the developing roller  23   a . The second conveyor screw  23   b   2  conveys the developer G separated from the developing roller  23   a  (the developer G forcibly separated from the developing roller  23   a  by an agent separation pole after a development process) in a horizontal direction (a leftward direction indicated by a white arrow in  FIG. 3 ). The first conveyor screw  23   b   1  and the second conveyor screw  23   b   2  are provided so as to have their axes of rotation substantially horizontal the same as the developing roller  23   a  and the photosensitive body drum  21 . 
     The third conveyor screw  23   b   3  (third conveying member) is provided at an angle to a horizontal direction so as to linearly connect the downstream side of the channel of conveyance (conveyance channel) by the second conveyor screw  23   b   2  and the upstream side of the conveyance channel by the first conveyor screw  23   b   1 . (See  FIG. 3 .) The third conveyor screw  23   b   3  conveys the developer G conveyed by the second conveyor screw  23   b   2  to the upstream side of the conveyance channel by the first conveyor screw  23   b   1 , and conveys the developer G circulated from the downstream side of the conveyance channel by the first conveyor screw  23   b   1  through a falling channel  23   f  to the upstream side of the conveyance channel by the first conveyor screw  23   b   1  (diagonal conveyance to the upper right indicated by a white arrow in  FIG. 3 ). 
     The conveyance channel by the first conveyor screw  23   b   1 , the conveyance channel by the second conveyor screw  23   b   2 , and the conveyance channel by the third conveyor screw  23   b   3  are separated from one another by wall parts. 
     Referring to  FIG. 3 , the downstream side of the conveyance channel by the second conveyor screw  23   b   2  and the upstream side of the conveyance channel by the third conveyor screw  23   b   3  communicate with each other through a first link part  23   g . Further, the downstream side of the conveyance channel by the third conveyor screw  23   b   3  and the upstream side of the conveyance channel by the first conveyor screw  23   b   1  communicate with each other through a second link part  23   h . Further, the downstream side of the conveyance channel by the first conveyor screw  23   b   1  and the upstream side of the conveyance channel by the third conveyor screw  23   b   3  communicate with each other through the falling channel  23   f.    
     According to this configuration, the three conveyor screws  23   b   1  through  23   b   3  form a circulation channel that circulates the developer G in a longitudinal direction in the developing unit  23 . Here, when the developing unit  23  is put into operation, the developer G contained in the developing unit  23  flows as indicated by oblique lines (hatching) in  FIG. 3 . 
     Referring to  FIG. 3 , the surface of the developer G is lower on the downstream side than on the upstream side in the conveyance channel by the first conveyor screw  23   b   1 . This is because part of the developer G in conveyance is supplied to the developing roller  23   a . That is, the developer G that is not supplied to the developing roller  23   a  moves to the upstream side of the third conveyor screw  23   b   3  through the falling channel  23   f.    
     The magnetic sensor  26  serving as a toner density sensor is provided in the conveyance channel by the third conveyor screw  23   b   3 . The developer G of a predetermined toner density is supplied from the agent cartridge  28  serving as a supply part into the developing unit  23  based on the information of toner density detected by the magnetic sensor  26 . 
     Here, referring to  FIG. 3  and  FIG. 4 , a discharge opening  23   d  serving as a discharge part through which part of the developer G contained in the developing unit  23  is discharged outside (to the agent reservoir  70 ) is provided in the conveyance channel by the first conveyor screw  23   b   1 . In detail, the discharge opening  23   d  is for discharging a surplus of the developer G to the agent reservoir  70  when the (upper) surface of the developer G conveyed to the position of the discharge opening  23   d  exceeds a predetermined height because of an increase in the amount of the developer G in the developing unit  23  due to supply of the developer G from the agent cartridge  28  into the developing unit  23  through the supply pipe  29 . That is, the surplus of the developer G exceeds the height (vertical dimension) of the lower part of the discharge opening  23   d  to be discharged from the discharge opening  23   d , and gravitates toward the agent reservoir  70  via a discharge channel  71  ( FIG. 4 ). Thus, the carrier C contaminated by the base resin or external additive of the toner T and degraded is automatically discharged outside the developing unit  23 . Therefore, it is possible to suppress the degradation of image quality even over time. 
     Further, according to this first embodiment, in the circulation channel of the developer G in the developing unit  23 , a bypass channel for causing part of the developer G to return to the upstream side of the circulation channel without passing the position where the above-described discharge opening  23   d  (discharge part) is provided. Specifically, referring to  FIG. 3  and  FIG. 5 , an opening  23   e  is provided on the upstream side of the discharge opening  23   d  (at a position relatively close to the discharge opening  23   d ) in the conveyance channel by the first conveyor screw  23   b   1 . This opening  23   e  serves as the entrance to the bypass channel, and the exit of the bypass channel is provided in (the vicinity of the center of) the conveyance channel by the second conveyor screw  23   b   3 . 
     Thus, by providing a bypass channel in the circulation channel of the developer G in the developing unit  23 , it is possible to prevent the problem of discharging a greater amount of the developer G than necessary from the developing unit  23  because of variations in the amount of the developer G discharged from the discharge opening  23   d  even when there is an undulatory inclination in the development G in the developing unit  23 . 
       FIG. 6  is a diagram showing the circulation channel of the developer G in the developing unit  23 , where there is an undulatory inclination in the developer G. 
     As shown in  FIG. 6 , there may be an undulatory inclination with large vertical variations in the circulation channel of the developer G. Such an undulatory inclination is most obvious immediately after the operation of the developing unit  23  is started (immediately after its restart). If such an undulatory inclination is caused, conventionally, all the developer G positioned higher than the lower part of the discharge opening  23   d  (part of the developer G corresponding to height H 2  in  FIG. 6 ) is discharged from the discharge opening  23   d . Originally, it is not intended (planned) to discharge the developer G thus discharged. Therefore, repeated occurrence of such a phenomenon may cause a shortage of the developer G in the developing unit  23  so as to destabilize the degraded condition of the developer G or reduce the amount of electric charge of the toner T. As a result, a problem such as a decrease in image density may be caused in an output image. 
     On the other hand, according to this first embodiment, the opening  23   e  that communicates with a bypass channel is provided on the upstream side of the discharge opening  23   d . Therefore, part of the developer G positioned higher than the lower part of the discharge opening  23   d  is returned to the conveyance channel in the third conveyor screw  23   b   3  through the opening  23   e  without being discharged from the discharge opening  23   d . As a result, it is possible to prevent the problem of an excessive discharge of the developer G from the discharge opening  23   d.    
     Here, according to this first embodiment, the lower part of the opening  23   e  in the bypass channel is positioned higher than the lower part of the discharge opening  23   d  by height H 1 . 
     As a result, of the developer G positioned higher than the lower part of the discharge opening  23   d , a portion corresponding to the difference between H 1  and H 2  (H 2 −H 1 ) is returned to the conveyance channel in the third conveyor screw  23   b   3  through the opening  23   e  without being discharged from the discharge opening  23   d . As a result, it is possible to ensure prevention of the problem of an excessive discharge of the developer G from the discharge opening  23   d  while maintaining the original function of the discharge part. 
     Preferably, the longitudinal distance W between the discharge opening  23   d  and the opening  23   e  is as short as possible. 
     As described above, according to this first embodiment, there is provided a bypass channel (opening  23   e ) for causing part of the developer G to return to the upstream side of the circulation channel without passing the position where the discharge opening  23   d  (discharge part) is provided. Therefore, it is possible to provide the developing unit  23  of the trickle development system according to which no variation is caused in the amount of the developer G discharged to the agent reservoir  70  so that the quality of an output image is stabilized even if there is an undulatory inclination in the developer G in the developing unit  23 . 
     According to this first embodiment, the present invention is applied to the developing unit  23  in which the three conveyor screws  23   b   1  through  23   b   3  serving as conveying members are provided. Alternatively, the present invention may also be applied to a developing unit in which two or more than three conveyor screws are provided. In this case also, the same effects as those of this first embodiment can be produced by providing a bypass channel for causing part of the developer to return to the upstream side of a circulation channel without passing the position where a discharge part is provided. 
     Further, according to this first embodiment, the third conveyor screw  23   b   3  is provided at an angle to a horizontal direction. Alternatively, the third conveyor screw  23   b   3  may also be provided horizontally. 
     Further, according to this first embodiment, the developer G (toner T and carrier C) is supplied from the agent cartridge  28  as a supply part to the developing unit  23 . Alternatively, it is also possible to supply only the carrier C from the supply part to the developing unit  23 . In this case, a toner cartridge that contains only toner is provided separately from the agent cartridge (carrier cartridge), and the toner contained in the toner cartridge is suitably supplied to the developing unit  23  based on the result of detection by the magnetic sensor  26 . Even in such a case, the same effects as those of this first embodiment can be produced. 
     Further, according to this first embodiment, the present invention is applied to the image forming apparatus where the process cartridge  20  forms part of the image forming part. However, the application of the present invention is not limited to this, and the present invention may also be applied to an image forming apparatus where the image forming part is not formed of a process cartridge. Specifically, the present invention may be applied to the case where the developing unit  23  is formed as a unit that can be independently attached to and detached from the image forming apparatus. 
     Second Embodiment 
     A description is given, with reference to  FIG. 7  through  FIG. 9 , of a second embodiment of the present invention. 
       FIG. 7  is a cross-sectional view of a developing unit  23 A according to the second embodiment.  FIG. 7  corresponds to  FIG. 4  in the above-described first embodiment. One of the differences between the developing unit  23  of the first embodiment and the developing unit  23 A of the second embodiment lies in that a projection part  23   k  is provided at the upper end of the discharge opening  23   d  in the developing unit  23 A. 
     Like the developing unit  23  of the first embodiment, the developing unit  23 A of this second embodiment also includes the discharge opening  23   d  as a discharge part and the opening  23   e  for returning part of the developer G to the conveyance channel by the third conveyor screw  23   b   3 . 
     Here, according to the second embodiment, referring to  FIG. 7 , the projection part  23   k  (eaves) is provided at the upper end of the discharge opening  23   d . This projection part  23   k  serves as a prevention member that prevents a developer G 1  churned up by the first conveyor screw  23   b   1  rotating in the direction indicated by an arrow in  FIG. 7  from entering the discharge opening  23   d . That is, by providing the projection part  23   k  as a prevention member, it is possible to prevent the developer G 1  thrown up by the first conveyor screw  23   b   1  from entering the discharge opening  23   d  and being collected and stored in the agent reservoir  70 , so that it is possible to prevent a shortage of the developer G in the developing unit  23 . 
     Thus, according to this second embodiment, the developer G 1  churned up by the first conveyor screw  23   b   1  is returned to the conveyance channel by the first conveyor screw  23   b   1  after colliding with the projection part  23   k  without entering the discharge opening  23   d . This allows the discharge opening  23   d  to satisfactorily fulfill its function as a discharge part without any side-effects. 
     According to this second embodiment, a slope (inclined plane)  23   k   1  is formed on the upper part of the projection part  23   k  as a prevention member so as to prevent a developer from being deposited thereon. This configuration causes the developer G 1  colliding with the projection part  23   k  after being churned up by the first conveyor screw  23   b   1  to slide down along the slope  23   k   1  and return to the conveyance channel by the first conveyor screw  23   b   1  without being deposited on the projection part  23   k . Accordingly, it is possible to prevent poor circulation of the developer G that may be caused by deposition of the developer G (G 1 ) on the projection part  23   k.    
       FIG. 8  is a perspective view of the vicinity of the discharge opening  23   d  and the opening  23   e , taken from the side of the conveyance channel by the first conveyor screw  23   b   1 . 
     Referring to  FIG. 8 , according to this second embodiment, the discharge opening  23   d  and the opening  23   e , each of which may have a slit shape, are formed in a plate-shaped member  23   n  removably provided in the developing unit  23 . In detail, the conveyance channel by the first conveyor screw  23   b   1  and the conveyance channel by the third conveyor screw  23   b   3  are separated by a wall part  23   m . The plate-shaped member  23   n  is formed so as to be detachable from and attachable (reattachable) to this wall part  23  in the directions indicated by a double-headed white arrow in  FIG. 8 . 
     By thus forming or providing the plate-shaped member  23   n  having the discharge opening  23   d  and the opening  23   e  provided therein removably with respect to the developing unit  23 , it is possible to change the height (vertical position) of each of the discharge opening  23   d  and the opening  23   e  with ease. That is, it is possible to change the height of each of the discharge opening  23   d  and the opening  23   e  merely by changing (replacing) the plate-shaped member  23   n  without changing (replacing) the entire developing unit  23 . 
     This configuration is particularly useful in the case where it is desired to make the developing unit  23  common to two types of image forming apparatuses different in speed of conveying the transfer material P (process linear velocity). The rotational speeds of the conveyor screws  23   b   1  through  23   b   3  differ, and accordingly the form of the undulatory inclination of a developer differs, between developing units having different process linear velocities. Accordingly, even if the same developing unit is used, the appropriate positions of the discharge opening  23   d  and the opening  23   e  differ depending on the rotational speeds of the conveyor screws  23   b   1  through  23   b   3 . According to this second embodiment, the plate-shaped members  23   n  that are different from each other in the heights (vertical positions) of the discharge opening  23   d  and the opening  23   e  are interchangeable with each other. This makes it possible to increase the commonality (compatibility) of the developing unit  23  between two types of image forming apparatuses having different process linear velocities. 
     As shown in  FIG. 8 , the wall face that separates a bypass channel F 1  and a discharge channel F 2  does not have to cover the entire boundary therebetween. Specifically, the wall face is not provided in the area where a developer discharged from the discharge opening  23   d  and a developer discharged from the opening  23   e  do not mix with each other because of their free fall. 
     Here, according to this second embodiment, the projection part  23   k  (eaves) as a prevention member is provided at the upper end of the discharge opening  23   d . Alternatively, if the first conveyor screw  23   b   1  (conveying member) rotates in the direction indicated by an arrow (clockwise) as shown in  FIG. 9 , it is preferable to provide the projection part  23   k  (eaves) at the lower end of the discharge opening  23   d . That is, it is preferable to provide the projection part  23   k  on the upstream side of the discharge opening  23   d  in the rotational direction of the first conveyor screw  23   b   1  facing the discharge opening  23   d . This is because the developer G churned up by the first conveyor screw  23   b   1  moves so as to enter the discharge opening  23   d  from its lower side when the first conveyor screw  23   b   1  rotates in the direction shown in  FIG. 9 . Accordingly, by providing the projection part  23   k  at the lower end of the discharge opening  23   d , it is possible to efficiently prevent the developer G churned up by the first conveyor screw  23   b   1  from entering the discharge opening  23   d . Further, in this case also, it is possible to prevent the developer G (G 1 ) from being deposited on the projection part  23   k  by forming the slope  23   k   1  on the upper part of the projection part  23   k.    
     As described above, in this second embodiment as well as in the above-described first embodiment, there is provided a bypass channel (opening  23   e ) for causing part of the developer G to return to the upstream side of the circulation channel without passing the position where the discharge opening  23   d  (discharge part) is provided. Therefore, it is possible to provide the developing unit  23  of the trickle development system according to which no variation is caused in the amount of the developer G discharged to the agent reservoir  70  so that the quality of an output image is stabilized even if there is an undulatory inclination in the developer G in the developing unit  23 . 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. The number, positions, or shapes of the above-described components (elements) are not limited to those of the above-described embodiments, and may be determined so as to be suitable for implementing the present invention. 
     The present application is based on Japanese Patent Applications No. 2006-063645, filed on Mar. 9, 2006, and No. 2007-036928, filed on Feb. 16, 2007, the entire contents of which are hereby incorporated by reference.