Patent Publication Number: US-8526847-B2

Title: Image forming apparatus and developing device

Description:
This application is based on application No. 2010-151323 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an image forming apparatus and a developing device for developing an electrostatic latent image on an image carrier using a developer, the image forming apparatus including the developing device. 
     (2) Description of the Related Art 
     An image forming apparatus, such as a copier or a printer, includes a developing device which contains a two-component developer including carriers and toner, and develops an electrostatic latent image formed on a photosensitive drum. 
     Examples of a developing method include a so-called trickle developing method. In this method, a developer including fresh carriers (i.e., fresh developer) is supplied into a housing of a developing device little by little via an inlet of the housing. At the same time, developer in the housing is agitated and circularly conveyed. While the developer is being conveyed, an excess amount of developer due to the supply of the fresh developer overflows and is discharged outside the housing via an outlet of the housing (Japanese examined patent application publication No. H02-21591). 
     According to the trickle developing method, the fresh developer is supplied into the housing from a cartridge, and whereby old carriers in the housing are replaced by fresh carriers and discharged outside the housing. Therefore, deteriorated carriers are less likely to remain in the housing, and deterioration in the developer is suppressed for a long period of time. This realizes high image quality in forming images. 
     Although realizing high image quality, the use of a developing device for the trickle developing method as described above is costly. This is because such a developing device requires a large amount of fresh developer for supply, and a two-component developer is discharged from the developing device. 
     Suppose here that a user sometimes needs to print out images having high image quality, such as pictures, but other times needs to print out images that do not require high image quality, such as images including only characters. In this case, the user may not wish to use the developing device for the trickle developing method all the time, since it is costly although providing high image quality. 
     Here, a normal developing method, which is not the trickle developing method, is less costly. In this method, a two-component developer in a housing is not discharged via an outlet, and a fresh developer including toner and not carriers is supplied into the housing from a cartridge. However, such a normal developing method cannot provide high image quality for a long period of time. 
     For example, it is possible to prepare two developing devices, i.e., one for the trickle developing method and the other for a normal non-trickle developing method. Then, the user may switch between the two developing devices as necessary. This structure, however, is costly as it requires two types of developing devices. Also, the user needs to keep one of the developing devices not mounted in the image forming apparatus somewhere different from the image forming apparatus, which is bothersome. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of the above problems, and an aim thereof is to provide an image forming apparatus and a developing device that realize high image quality and low cost as desired by a user. 
     The above aim is achieved by an image forming apparatus for developing an electrostatic latent image on an image carrier with use of a two-component developer including carriers and toner, the image forming apparatus comprising: a developing device having (i) a conveyance path in which the two-component developer is conveyed and (ii) an outlet from which the two-component developer is partially discharged while the two-component developer is conveyed; a hopper to which (i) a first cartridge containing a first developer including carriers and (ii) a second cartridge containing a second developer including toner and not carriers are attachable, and operable, when the first cartridge is attached thereto, to supply the first developer to the developing device, and when the second cartridge is attached thereto, to supply the second developer to the developing device; and a switching part operable, when the first developer is supplied, to switch the developing device to a first state in which the two-component developer is partially discharged from the outlet, and when the second developer is supplied, to switch the developing device to a second state in which the two-component developer is not discharged from the outlet. 
     Also, the above aim is achieved by a developing device mounted in an image forming apparatus, and for developing an electrostatic latent image on an image carrier with use of a two-component developer including carriers and toner, the developing device comprising: a conveyance path in which the two-component developer is conveyed, an outlet that is provided in the conveyance path and from which the two-component developer is partially discharged while the two-component developer is conveyed; an inlet that is provided in the conveyance path and from which (i) a first developer including carriers is supplied when a cartridge containing the first developer is attached to the image forming apparatus and (ii) a second developer including toner and not carrier is supplied when a cartridge containing the second developer is attached to the image forming apparatus; and a switching part operable, when the first developer is supplied, to switch the developing device to a first state in which the two-component developer is discharged from the outlet, and when the second developer is supplied, to switch the developing device to a second state in which the two-component developer is not discharged from the outlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and the other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  shows an overall structure of a copier  1  according to Embodiment 1. 
         FIG. 2  is a sectional view showing a structure of a developing device in the copier. 
         FIG. 3  schematically shows a hopper in the copier supplying toner to the developing device. 
         FIG. 4  shows the developing device seen from the front of the copier. 
         FIG. 5  is a sectional view of the developing device taken along the line A-A in  FIG. 2 . 
         FIG. 6  is a sectional view of the developing device taken along the line A-A in  FIG. 2 . 
         FIGS. 7A and 7B  show a structure of a shutter in the developing device. 
         FIG. 8  is a schematic perspective view showing a structure of the shutter. 
         FIG. 9A  shows a state where the shutter is located in a first position corresponding to a trickle developing method, and  FIG. 9B  shows a state where the shutter is located in a second position corresponding to a non-trickle developing method. 
         FIG. 10A  shows a structure example of a collection tank mounted in the copier;  FIG. 10B  shows a state where the shutter is switched to the first position by the collection tank; and  FIG. 10C  shows a state where the shutter is switched to the second position by the collection tank. 
         FIG. 11  is a block diagram showing a structure of a controller in the copier. 
         FIG. 12  is a flowchart showing the details of control for displaying a warning message, the control being performed by the controller. 
         FIGS. 13A ,  13 B, and  13 C show a structure of a shutter according to Embodiment 2. 
         FIG. 14  is a flowchart showing the details of control for switching the position of the shutter according to Embodiment 3, the control being performed by the controller. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     The following describes an embodiment of an image forming apparatus according to the present invention, by taking a tandem-type color digital copier (hereinafter, simply “copier”) as an example. 
       FIG. 1  shows an overall structure of a copier  10  according to Embodiment 1. 
     As shown in  FIG. 1 , the copier  10  includes an image reader  11 , an image former  12 , a paper feeder  13 , a fixer  14 , a controller  15 , an operation panel  16 , etc. The copier  10  forms a color image using yellow (Y), magenta (M), cyan (C) and black (K) or a monochrome image using black, for example, and reproduces the image on a recording sheet. Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, respectively. 
     The image reader  11  obtains image data by reading an image on a document set thereto, and outputs the image data. 
     The image former  12  includes imaging parts  20 Y to  20 K that respectively correspond to colors Y to K, an intermediate transfer belt  21 , hoppers  7 Y to  7 K, supply motors  8 Y to  8 K, etc. 
     The imaging parts  20 Y to  20 K respectively include photosensitive drums  1 Y to  1 K, charge rollers  2 Y to  2 K, exposure parts  3 Y to  3 K, developing devices  4 Y to  4 K, primary transfer rollers  5 Y to  5 K, cleaners  6 Y to  6 K for cleaning the photosensitive drums  1 Y to  1 K, etc, and form toner images of Y to K on the photosensitive drums  1 Y to  1 K. The charge rollers  2 Y to  2 K, the exposure parts  3 Y to  3 K, the developing devices  4 Y to  4 K, the primary transfer rollers  5 Y to  5 K, and the cleaners  6 Y to  6 K are all disposed around the photosensitive drums  1 Y to  1 K. 
     Each of the developing devices  4 Y to  4 K contains a two-component developer that includes carriers and toner (hereinafter “developer”), and is switchable between (i) a trickle developing method as a first developing method and (ii) a normal developing method as a second developing method (hereinafter “non-trickle developing method”). 
     The intermediate transfer belt  21  is endless and suspended by a driving roller  22  and a driven roller  23  in a tensioned manner. The intermediate transfer belt  21  is driven to rotate in the direction of arrow A in  FIG. 1 . 
     A cartridge  9 Y, which contains a Y-color developer for supply, is attachable to the hopper  7 Y. The hopper  7 Y supplies the Y-color developer to the developing device  4 Y by an instruction from the controller  15 . The cartridge  9 Y comes in two types, i.e., a first cartridge containing a first developer that includes carriers and toner, and a second cartridge containing a second developer that includes toner and not carriers. The cartridge  9 Y is exchangeable between two types of cartridges, i.e., the first and second cartridges, and either the first cartridge or the second cartridge is attached to the hopper  7 Y. 
     The cartridge  9 Y is exchangeable between two types and attachable to the hopper  7 Y, so that when the developing method is switched between the trickle developing method and the non-trickle developing method, a developer corresponding to the developing method that has been switched to is supplied. The cartridge  9 Y is attachable and detachable by a user (i.e., operator). 
     The same applies to the other hoppers  7 M to  7 K, and cartridges  9 M to  9 K are detachable from the hoppers  7 M to  7 K, respectively. The cartridges  9 M to  9 K come in two types, i.e., first and second types, and contain developers M to K, respectively. The hoppers  7 M to  7 K supply developers in the cartridges  9 M to  9 K attached thereto, to the developing devices  4 M to  4 K, by an instruction from the controller  15 . 
     The paper feeder  13  includes a paper feed cassette  31 , a feed roller  32 , a conveyance roller pair  33 , a timing roller pair  34 , a secondary transfer roller  35 , etc. The paper feed cassette  31  contains sheets S. The feed roller  32  feeds the sheets S in the paper feed cassette  31 , one at a time, onto a conveyance path  37 . The conveyance roller pair  33  conveys the sheet S that has been fed, and the timing roller pair  34  adjusts the timing for conveying the sheet S to a secondary transfer position  351 . 
     The fixer  14  includes a fixing roller and a pressure roller that are pressed against each other, and a heater for heating the fixing roller. 
     The operation panel  16  includes keys and a liquid crystal display  17 . The keys receive instructions (e.g., a copying instruction), and selections (e.g., color mode or monochrome mode). The liquid crystal display  17  displays information for giving a notification or a warning to a user. 
     Upon receiving an instruction for copying or the like via the operation panel  16  from the user, the controller  15  controls the image reader  11 , the image former  12 , etc. to start a copying operation based on the instruction. Specifically, the controller  15  causes the image reader  11  to read a document image, thereby obtaining image data. Then, the controller  15  drives laser diodes of the exposure parts  3 Y to  3 K, based on the image data. As a result, the laser diodes of the exposure parts  3 Y to  3 K emit laser beams L respectively corresponding to the colors Y to K, and the photosensitive drums  1 Y to  1 K are exposure-scanned on a line-by-line basis. 
     The photosensitive drums  1 Y to  1 K are uniformly charged by the charge roller  2 Y to  2 K before the exposure scanning, so that electrostatic latent images are formed on the photosensitive drums  1 Y to  1 K by the exposure scanning of the laser beams L. The electrostatic latent images on the photosensitive drums  1 Y to  1 K are developed (i.e., visualized) by developers contained in the developing devices  4 Y to  4 K. As a result, toner images are formed on the photosensitive drums  1 Y to  1 K. 
     The toner images on the photosensitive drums  1 Y to  1 K are sequentially primary-transferred onto the intermediate transfer belt  21 , by the electrostatic force acting between the primary transfer rollers  5 Y to  5 K and the photosensitive drums  1 Y to  1 K. At this time, the toner images are transferred to the same position on the intermediate transfer belt  21  at different timings, so that they are superimposed on each other at the position. After being superimposed on the intermediate transfer belt  21 , the toner images of the respective colors are conveyed to the secondary transfer position  351  by the rotation of the intermediate transfer belt  21 . 
     In the meanwhile, the paper feeder  13  feeds a sheet S via the timing roller pair  34  to the secondary transfer position  351  at the timing when the toner images are conveyed thereto. The sheet S, which has been fed by the paper feeder  13 , passes through between the intermediate transfer belt  21  and the secondary transfer roller  35 . When the sheet S passes through therebetween, the toner images on the intermediate transfer belt  21  are collectively secondary-transferred onto the sheet S, by the electrostatic force acting between the secondary transfer roller  35  and the driving roller  22 . 
     After passing the secondary transfer position  351 , the sheet S is conveyed to the fixer  14 . When the sheet S passes through the fixer  14 , the toner images on the sheet S are fixed onto the sheet S by heat and pressure. The sheet S passes through an ejection roller pair  36  and is ejected onto an ejection tray  38 . 
     The above describes an operation in the case of color mode for forming a color image. However, in the case of monochrome mode for forming only an image of the color K for example, only the imaging part  20 K for the color K is driven to form a K-color toner image. 
       FIG. 2  is a sectional view showing a structure of the developing device  4 Y.  FIG. 3  schematically shows the hopper  7 Y supplying toner to the developing device  4 Y.  FIG. 4  shows the developing device  4 Y seen from the front of the copier  10 . For the convenience of description,  FIG. 3  does not show the exposure part  3 Y and so on. Although using developers of different colors, the developing devices  4 Y to  4 K and the hoppers  7 Y to  7 K have the same structures. Therefore, the following describes the developing device  4 Y and the hopper  7 Y, and descriptions of the structures of the other developing devices  4 M to  4 K and hoppers  7 M to  7 K are omitted. 
     As shown in  FIG. 2 , the developing device  4 Y includes a housing  50 , a developing roller  51 , a supply screw  52 , an agitation screw  53 , and a developer detection sensor  54 . The components from the housing  50  to the agitation screw  53  are elongated along an axis direction of the photosensitive drum  1 Y (i.e., a direction perpendicular to a paper surface: hereinafter “axis direction”). 
     The housing  50  is partitioned by a partition (i.e., dividing wall)  57  into upper and lower portions, i.e., a supply chamber  58 , an agitation chamber  59 , etc. The developing roller  51  and the supply screw  52  are disposed in the supply chamber  58  and the agitation screw  53  is disposed in the agitation chamber  59 . The supply chamber  58  and the agitation chamber  59  contain a developer D. 
     The developing roller  51  is disposed at an opening of the supply chamber  58 , the opening facing the photosensitive drum  1 Y. The developing roller  51  includes a developing sleeve  511  and a magnet roller  512 . The developing sleeve  511  is tubular, and the magnet roller  512  is inserted in the developing sleeve  511  along the axis direction. 
     The magnet roller  512  includes a plurality of magnetic poles, i.e., N-poles and S-poles. For example, the N-poles and the S-poles are disposed alternately along the circumferential direction of the magnet roller  512 . An edge of the magnet roller  512  in the axis direction is fixed to the housing  50  so as to prevent the magnet roller  512  from rotating. Each of the magnetic poles extends along the axis direction. 
     The developing sleeve  511  is partially exposed from the opening of the housing  50  so as to face the photosensitive drum  1 Y. The developing sleeve  511  is held by the housing  50  in a manner that the developing sleeve  511  is rotatable in the direction of an arrow shown in  FIG. 2 . The developing sleeve  511  rotates around the magnet roller  512  that is stationary, while holding (i.e., carrying) the developer D on its surface by the magnetic force of the magnet roller  512 . 
     The supply screw  52  is disposed in the supply chamber  58 , and faces the photosensitive drum  1 Y with the developing roller  51  in between. The supply screw  52  is rotatably held by the housing  50  parallel to the developing roller  51  in the axis direction. The supply screw  52  rotates in the direction of an arrow shown in  FIG. 2 , thereby conveying the developer D in the supply chamber  58  along the axis direction and supplying the developer D to the developing roller  51 . 
     The agitation screw  53  is disposed in the agitation chamber  59 , and is rotatably held by the housing  50  parallel to the developing roller  51  in the axis direction. The agitation screw  53  rotates in the direction of an arrow shown in  FIG. 2 , thereby conveying and agitating the developer D in the agitation chamber  59  along a direction opposite from the direction of the supply screw  52  conveying the developer D. The developing sleeve  511 , the supply screw  52 , and the agitation screw  53  rotate by the drive force of a motor that is not shown in figures. 
     The developer detection sensor  54  is disposed at the bottom of the agitation chamber  59 , and measures the ratio of carriers to toner that are both included in the developer D conveyed inside the agitation chamber  59 . The developer detection sensor  54  transmits, to the controller  15 , a detection signal indicating the ratio of the carriers to the toner. The controller  15  controls the supply of the developer from the hopper  7 Y to the developing device  4 Y, such that the ratio of the carriers to the toner falls within an appropriate range. Specifically, the controller  15  performs this control by driving the supply motor  8 Y, based on the detection signal transmitted from the developer detection sensor  54 . Details of this control are described later. 
     As shown in  FIG. 3 , the hopper  7 Y includes a cartridge housing  70 Y, a spiral roller  71 Y, and a cartridge type detection sensor  72 Y. 
     The cartridge housing  70 Y is provided for housing the cartridge  9 Y that contains a developer Da. The cartridge  9 Y is detachable from the cartridge housing  70 Y. As described above, the cartridge  9 Y comes in two types, i.e., the first cartridge containing the first developer that includes carriers and toner, and the second cartridge containing the second developer that includes toner and not carriers. 
     In  FIG. 3 , “ 9 Y” denotes the first cartridge, “Da” denotes the first developer in the first cartridge  9 Y, “ 9 Yb” denotes the second cartridge, and “Db” denotes the second developer in the second cartridge  9 Yb. 
     The first cartridge  9 Y corresponds to the trickle developing method, and the second cartridge  9 Yb corresponds to the non-trickle developing method. Therefore, in the case of using the trickle developing method, the first cartridge  9 Y is attached so as to supply the first developer that corresponds to the trickle developing method. 
     In the case of switching to the non-trickle developing method, the second cartridge  9 Yb is attached so as to supply the second developer that corresponds to the non-trickle developing method. The same applies to the case with the other colors. That is, first cartridges  9 M,  9 C, and  9 K correspond to the trickle developing method, and second cartridges  9 Mb,  9 Cb, and  9 Kb (not shown) correspond to the non-trickle developing method. 
     The spiral roller  71 Y rotates by the drive force of the supply motor  8 Y, and conveys, to a pipe  73 Y, the first developer Da (or the second developer Db) in the cartridge  9 Y (or the cartridge  9 Yb) that is attached to the hopper  7 Y. After being conveyed to the pipe  73 Y, the first developer Da (or the second developer Db) passes through the pipe  73 Y to be conveyed into the housing  50  of the developing device  4 Y, via a developer receiver  55 Y (see  FIG. 4 ) provided at an upper portion of the housing  50 . 
     The cartridge type detection sensor  72 Y is provided at the cartridge housing  70 Y, and outputs, to the controller  15 , a signal indicating whether the cartridge set in the cartridge housing  70 Y is the first cartridge  9 Y or the second cartridge  9 Yb. In the present embodiment, the first cartridge  9 Y and the second cartridge  9 Yb are provided with identification information pieces at predetermined positions thereof. Each identification information piece indicates whether the corresponding cartridge is the first cartridge  9 Y or the second cartridge  9 Yb. The cartridge type detection sensor  72 Y reads the identification information piece of the cartridge set to the cartridge housing  70 Y, and transmits the identification information piece to the controller  15 . For example, the identification information pieces may be marks that each have a different shape, and the cartridge type detection sensor  72 Y may detect a difference between the mark of the cartridge set to the cartridge housing  70 Y and the other mark. Alternatively, the identification information pieces may be barcodes, the cartridge type detection sensor  72 Y may read the barcode of the cartridge set to the cartridge housing  70 Y. 
     Upon receiving an identification information piece, the controller  15  judges whether the cartridge set to the cartridge housing  70 Y is the first cartridge  9 Y or the second cartridge  9 Yb, based on the identification information piece (judgment of the type of cartridge). Based on a result of the judgment, the controller  15  outputs, to the user, a warning that indicates switching of the position of a shutter (described later) of the developing device  4 Y. Details of the outputting of the warning are described later. 
     When using the trickle developing method, the user attaches the first cartridge  9 Y to the hopper  7 Y. When switching to the non-trickle developing method, the user detaches the first cartridge  9 Y from the hopper  7 Y and attaches the second cartridge  9 Yb to the hopper  7 Y. When switching from the non-trickle developing method to the trickle developing method, the user detaches the second cartridge  9 Yb from the hopper  7 Y, and attaches the first cartridge  9 Y to the hopper  7 Y. In this way, a developer corresponding to the developing method currently being used is supplied from the cartridge containing the developer to the developing device  4 Y. 
     In  FIG. 3 , a reference sign  80  shown by a dashed line denotes a collection tank (see  FIG. 10 ) for collecting the developer D discharged in the trickle developing method. The collection tank  80  is detachably attached to the copier  10 . In the present embodiment, an outer cover is provided on a front side of the copier  10  such that the outer cover can be freely opened and closed. The user can open the outer cover so as to attach/detach the collection tank  80  and to change the cartridge from the front of the copier  10 . Note that the collection tank  80  also collects residual toner discharged from the cleaners  6 Y to  6 K. The residual toner is removed from the photosensitive drums  1 Y to  1 K by the cleaners  6 Y to  6 K cleaning the photosensitive drums  1 Y to  1 K. 
       FIG. 5  is a sectional view of the developing device  4 Y taken along the line A-A in  FIG. 2 .  FIG. 6  is a sectional view of the developing device  4 Y taken along the line B-B in  FIG. 2 . In  FIGS. 5 and 6 , a crosswise direction corresponds to the axis direction. The right side along the axis direction indicates the front of the copier, and the left side along the axis direction indicates the back of the copier. Note that the developer D is not shown in  FIGS. 5 and 6 . 
     As shown in  FIGS. 5 and 6 , the supply chamber  58  and the agitation chamber  59  in the housing  50  are tubular and elongated along the axis direction. The supply chamber  58  and the agitation chamber  59  are separated from each other by the partition  57 , but in communication with each other via (i) opening  93  (see  FIG. 5 ) at one end (right side) of the partition  57  along the axis direction and (ii) opening  94  (see  FIG. 5 ) at the other end (left side) of the partition  57  along the axis direction. 
     The supply screw  52  is composed of a rotational shaft and a spiral blade provided along the outer circumferential surface of the rotational shaft. Both ends of the supply screw  52  in the axis direction are rotatably held by side walls of the supply chamber  58  via bearings. 
     The agitation screw  53  is composed of a rotational shaft and a spiral blade provided along the outer circumferential surface of the rotational shaft. Both ends of the agitation screw  53  in the axis direction are rotatably held by side walls of the agitation chamber  59  via bearings. 
     A gear is attached to each of the developing roller  51 , the supply screw  52 , and the agitation screw  53 , specifically to each of their shaft portions at the back side of the copier  10 . The gears are engaged with each other, so that an external rotational drive force is transmitted to the developing roller  51 , the supply screw  52 , and the agitation screw  53  via the gears, thus driving the developing roller  51 , the supply screw  52 , and the agitation screw  53  to rotate. 
     When the developing roller  51 , the supply screw  52 , and the agitation screw  53  are driven to rotate, the developer D in the supply chamber  58  is conveyed by the supply screw  52  in an X direction shown by the arrow X. While being conveyed by the supply screw  52 , the developer D is supplied to the developing roller  51 . 
     When the developer D arrives at the opening  93 , which is located at the front side of the copier  10  and at the end of the supply screw  52  in the downstream side in the developer conveyance direction of the supply screw  52 , the developer D is conveyed to the agitation chamber  59  via the opening  93 . 
     After being conveyed to the agitation chamber  59 , the developer D is further conveyed by the agitation screw  53  in an X′ direction shown by the arrow X′ (i.e., a direction opposite from the X direction). When the developer D arrives at the opening  94 , which is located at the back side of the copier  10  and at the end of the agitation screw  53  in the downstream side in the developer conveyance direction of the agitation screw  53 , the developer D is conveyed to the supply chamber  58  via the opening  94 . In this way, the developer D is circularly conveyed in the housing  50  in the direction shown by the arrows X and X′ in  FIG. 5 . 
     In the supply chamber  58 , a conveyance path in which the developer D is conveyed by the supply screw  52  in the X direction is referred to as a first conveyance path  92 . Also, in the agitation chamber  59 , a conveyance path in which the developer D is conveyed by the agitation screw  53  is referred to as a second conveyance path  92 . 
     The opening  93  allows the downstream side of the first conveyance path  91  in the developer conveyance direction to communicate with the upstream side of the second conveyance path  92  in the developer conveyance direction. The opening  93  also serves as a first communication path  93  in which the developer D is conveyed from the first conveyance path  91  to the second conveyance path  92 . The opening  94  allows the downstream side of the second conveyance path  92  in the developer conveyance direction to communicate with the upstream side of the first conveyance path  91  in the developer conveyance direction. The opening  94  also serves as a second communication path  94  in which the developer D is conveyed from the second conveyance path  92  to the first conveyance path  91 . 
     As shown in  FIG. 6 , the first developer Da (or the second developer Db) supplied from the hopper  7 Y is conveyed from the developer receiver  55 Y of the housing  50 , which is provided at the front side of the copier  10 , to the agitation chamber  59 , via an inlet  98  of the agitation chamber  59  at an end portion thereof located at the front side of the copier  10 . When conveyed to the agitation chamber  59 , the first developer Da (or the second developer Db) is conveyed and agitated by the agitation screw  53 , together with the developer D in the agitation chamber  59 . 
     An outlet  99  of the developer is provided at the bottom of the supply chamber  58  in an end portion thereof located at the front side of the copier  10 . Also, a shutter  56  for opening and closing the outlet  99  is provided in the vicinity of the outlet  99 . 
       FIGS. 7A and 7B  correspond respectively to  FIGS. 5 and 6 . In  FIGS. 7A and 7B , the supply screw  52  and the agitation screw  53  are not shown so that a structure of the shutter  56  is easily understood.  FIG. 8  is a schematic perspective view showing the structure of the shutter  56  and a part of the housing  50  at the front side of the copier  10 . 
     As shown in  FIGS. 7A ,  7 B and  8 , the shutter  56  includes a first shutter part  61 , a second shutter part  62 , and a holder  60 . The first shutter part  61  and the second shutter part  62  are formed integrally on the holder  60 . The shutter  56  is held by the housing  50  such that the shutter  56  is slidable (i.e., movable) by a predetermined distance along the axis direction (i.e., an E direction shown by an arrow E in  FIG. 8 ). 
     The first shutter part  61  has the shape of a curved plate, and is positioned outside the housing  50 . The first shutter part  61  is bent in an arc so as to be in contact with an outer bottom surface of the supply chamber  58  in the housing  50 , and to coincide with the shape of the outer bottom surface thereof. When the shutter  56  is moved to the front side of the copier  10 , the first shutter part  61  moves to a closing position for closing the outlet  99  and closes the outlet  99 . When the shutter  56  is moved to the back side of the copier  10 , the first shutter part  61  moves away from the outlet  99  and to an opening position for opening the outlet  99  and opens the outlet  99 . 
     The second shutter part  62  has the shape of a flat plate elongated in the E direction, and extends inside the housing  50  via an elongated hole  503  (see  FIG. 8 ) that is a through-hole provided at the bottom of the housing  50 . The second shutter part  62  is held parallel to the partition  57  (see  FIG. 2 ) provided inside the housing  50 . 
     When the shutter  56  is moved to the back side of the copier  10 , an end portion  621  of the second shutter part  62  at the back side of the copier  10  makes contact with the partition  57 . As a result, the second shutter part  62  becomes a part of the partition  57  that partitions the housing  50  into the supply chamber  58  and the agitation chamber  59 . 
     When the shutter  56  is moved to the front side of the copier  10 , the end portion  621  (see  FIG. 8 ) of the second shutter part  62  moves away from the partition  57 . As a result, a part of the partition  57  disappears, and a communication path  95  (see  FIG. 9B ) for conveying the developer D is newly formed between the supply chamber  58  and the agitation chamber  59 . 
     The communication path  95  (hereinafter, “third communication path”) communicates a portion  911  (see  FIG. 5 ) of the first conveyance path  91  with a portion  921  (see  FIG. 5 ) of the second conveyance path  92 . The portion  911  is located more upstream in the developer conveyance direction in the first conveyance path  91  than the first communication path  93 , and the portion  921  is located more downstream in the developer conveyance direction in the second conveyance path  92  than the first communication path  93 . When the developing method is switched to the non-trickle developing method, the third communication path  95  is used as a part of a circulation conveyance path in which the developer D is circularly conveyed. Details of switching of the developing method are described later. The third communication path  95  is formed between the developing roller  51  and the first communication path  93  in the axis direction. 
     As shown in  FIG. 8 , the holder  60  is connected to the second shutter part  62  via a connecting part  622 . A back surface of the holder  60 , which is a surface closer to the back of the copier  10 , has a hole  65  (shown by a dashed line) that is elongated. The hole  65  extends from the back surface toward the front side of the copier  10  along the E direction. A pin  69  is disposed at a position facing the opening of the hole  65  of the holder  60 . The pin  69  is arranged immediately below the housing  50 , parallel to the E direction, pointing toward the front side of the copier  10 . The elongated hole  503 , which is a through-hole elongated in the E direction, is provided in a part of the housing  50 , the part being located at the front side of the copier  10  and between the first conveyance path  91  and the second conveyance path  92 . 
     The second shutter part  62  is inserted from the outside (bottom part) to the inside (upper part) of the housing  50  via the elongated hole  503 . In this way, the second shutter part  62  is arranged inside the housing  50 , and the connecting part  622  is engaged with the elongated hole  503 . 
     The pin  69  provided for the housing  50  is fit in and engaged with the hole  65  of the holder  60 . Also, the connecting part  622  of the shutter  56  is engaged with the elongated hole  503  of the housing  50 . In this state, the shutter  56  is held by the housing  50  such that the shutter  56  is slidable along the E direction. 
     The pin  69  is inserted in a compression coil spring  68 . The compression coil spring  68  constantly applies, to the holder  60 , a biasing force directed toward the front side of the copier  10 . The holder  60  has a head portion  601  at the front side of the copier  10 . The head portion  601  has two states, i.e., (i) a pressed state in which the head portion  601  is pressed toward the back side of the copier  10 , against the biasing force of the compression coil spring  68 , and (ii) a non-pressed state of not being pressed. The state of the head portion  601  is switched between the pressed state and the non-pressed state, thus causing the position of the shutter  56  to be switched between a first position corresponding to the trickle developing method and a second position corresponding to the non-trickle developing method. 
       FIG. 9A  shows the shutter  56  being placed at the first position (trickle corresponding position).  FIG. 9B  shows the shutter  56  being placed at the second position (non-trickle corresponding position). 
     As shown in  FIG. 9A , in the trickle corresponding position, the first shutter part  61  is positioned away from the outlet  99  so as to open the outlet  99 . Also, the second shutter part  62  is positioned at a blocking position for blocking the third communication path  95  by cutting across the third communication path  95 . As a result, the second shutter part  62  makes contact with the partition  57  to be a part of the partition  57  and closes the third communication path  95 . 
     When the third communication path  95  is blocked, the developer D conveyed in the X direction by the supply screw  52  in the supply chamber  58  cannot pass through the third communication path  95 . Accordingly, the developer D is conveyed toward the first communication path  93  at the front of the copier  10 , passes over the outlet  99 , and is thereafter conveyed to the agitation chamber  59  via the first communication path  93 . While the developer D passes over the outlet  99 , a part of the developer D is discharged outside the housing  50  from the outlet  99 . According to the trickle developing method described above, although the developer D in the developing device  4 Y gets deteriorated, the developer D is gradually replaced with fresh developer. This makes it possible to maintain high image quality over a long period of time, and to reduce the frequency of replacing the deteriorated developer with new one by a service man or the like. As a result, the life of the developer D is prolonged. 
     In the trickle developing method, the first conveyance path  91 , the first communication path  93 , the second conveyance path  92 , and the second communication path  94  constitute a circulation conveyance path  90  along which the developer D is circularly conveyed. 
     As shown in  FIG. 9B , at the non-trickle corresponding position, the first shutter part  61  moves to the closing position for closing the outlet  99  and closes the outlet  99 . Also, the second shutter part  62  moves away from the partition  57  to an opening position for opening the third communication path  95 . In this way, a part of the partition  57  disappears, and the third communication path  95  is opened. 
     Due to the third communication path  95  being open, the developer D in the supply chamber  58 , which is conveyed by the supply screw  52  in the X direction, is mostly conveyed to the agitation chamber  59  via the third communication path  95 . 
     In the present Embodiment, the agitation chamber  59  is located obliquely downward with respect to the supply chamber  58 , as shown in  FIG. 2 . Therefore, at the time of being conveyed from the supply chamber  58  to the agitation chamber  59 , the developer D mainly drops to the agitation chamber  59  via the third communication path  95  due to gravity. The remaining part of developer D that has not dropped via the third communication path  95  is conveyed to the front side of the copier  10  by the supply screw  52 . 
     The part of the developer D conveyed to the front side of the copier  10  by the supply screw  52  passes over the outlet  99 . Since the outlet  99  is closed, the developer D is not discharged outside the housing  50  and conveyed to the agitation chamber  59  via the first communication path  93 . Unlike the trickle developing method, the developer D in the developing device  4 Y is not discharged. This makes it possible to reduce the amount of developer required for developing, thus reducing cost. 
     In the non-trickle developing method, the first conveyance path  91 , third communication path  95 , the second conveyance path  92 , and the second communication path  94  mainly constitute a circulation conveyance path  901  along which the developer D is circularly conveyed. 
       FIG. 10A  is a sectional view showing a state before the collection tank  80  is mounted in the copier  10 , where a part of the collection tank  80  is shown by a side view.  FIGS. 10B and 10C  each show a state after the collection tank  80  is mounted in the copier  10 .  FIG. 10B  shows an example of the case of the trickle developing method.  FIG. 10C  shows an example of the case of the non-trickle developing method. 
     As shown in  FIG. 10A , a surface  85  of the collection tank  80  that faces the developing device  4 Y is provided with an open hole  81 . When being mounted in the copier  10 , the collection tank  80  is fixedly held by the copier  10  in a state where an end portion  501  (a portion including the outlet  99 ) of the developing device  4 Y at the front side of the copier  10  is inserted into the collection tank  80  via the open hole  81 , as shown in  FIGS. 10B and 10C . 
     The collection tank  80  has a projection  82 , which is slidable up and down, at a position on the surface  85  thereof, in particular, at a position lower than the open hole  81 . The projection  82  makes contact with the head portion  601  of the shutter  56  of the developing device  4 Y, so as to switch the position of the shutter  56  between the trickle corresponding position and the non-trickle corresponding position. The position of the projection  82  is manually switchable by the user. When the user applies a certain degree of pressure to the projection  82 , the projection  82  is switched between an uppermost position (shown by a solid line) and a lowermost position (shown by a dashed line) as shown in  FIG. 10A . The position of the projection  82  is not switched by a pressure produced by, for example, vibrations of the copier  10  when images are formed. 
     In the case of switching to the trickle developing method, the user can mount the collection tank  80  to the copier  10  in the following manner. First, the user mounts the first cartridge  9 Y to the hopper  7 Y, and switches the position of the projection  82  to the uppermost position. Then, the user holds and inserts the collection tank  80  into the copier  10 , from the front side of the copier  10  in a direction shown by the arrow F (i.e., direction toward the back side of the copier  10 ). 
     As shown in  FIG. 10B , when the collection tank  80  is mounted in the copier  10  with the projection  82  being at the uppermost position, (i) the projection  82  makes contact with the head portion  601  of the shutter  56  of the developing device  4 Y, and (ii) the head portion  601  is pressed toward the back side of the copier  10  (i.e., pressed against the biasing force of the compression coil spring  68 ), whereby the shutter  56  moves to the trickle corresponding position. 
     At the trickle corresponding position, the outlet  99  of the developing device  4 Y is open. Therefore, while the developer D is circularly conveyed in the housing  50 , a part of the developer D is discharged from the outlet  99  and collected into the collection tank  80 . 
     In the case of switching to the non-trickle developing method, the user mounts the second cartridge  9 Yb to the hopper  7 Y instead of the first cartridge  9 Y, and switches the position of the projection  82  from the uppermost position to the lowermost position, and thereafter mounts the collection tank  80  to the copier  10 . 
     As shown in  FIG. 10C , when the collection tank  80  is mounted in the copier  10  with the projection  82  being at the lowermost position, the projection  82  does not make contact with the head portion  601  of the shutter  56  of the developing device  4 Y. As a result, the head portion  601  moves toward the front side of the copier  10  by the biasing force of the compression coil spring  68 , whereby the shutter  56  moves to the non-trickle corresponding position. 
     At the non-trickle corresponding position, the outlet  99  of the developing device  4 Y is closed. Therefore, although being circularly conveyed, the developer D is not discharged from the outlet  99 . 
     Note that the collection tank  80  corresponds to (i.e., is commonly used by) each of the developing devices  4 Y to  4 K. Therefore, a pair of the open hole  81  and the projection  82  is provided for each of the developing devices  4 Y to  4 K, so that the position of the projection  82  is individually switchable for each developing device. 
     As described above, changing both the type of cartridge and the position of the shutter for each color enables the user to switch between the trickle developing method and the non-trickle developing method as desired at any time. Accordingly, it is not necessary to prepare both a developing device dedicated to the trickle developing method and a developing device dedicated to the non-trickle developing method, allowing the user to easily switch between the developing methods at low cost. 
       FIG. 11  is a block diagram showing a structure of the controller  15 . 
     As shown in  FIG. 11 , the controller  15  includes a CPU  151 , a ROM  152 , a RAM  153 , a cartridge type judgment part  154 , and a display controller  155 . These components  151  to  155  exchange data with each other. 
     The CPU  151  controls operations of components such as the image reader  11 , the image former  12 , and the paper feeder  13 , based on a control program stored in the ROM  152 , and thereby realizes smooth operations for forming color and monochrome images. Also, the CPU  151  receives information input by the user using the operation panel  16 . Furthermore, based on the detection signals from the developer detection sensors  54 Y to  54 K, the CPU  151  judges whether the ratio of carriers to toner in the developer D currently contained in each of the developing devices  4 Y to  4 K falls within an appropriate range that is predetermined as a standard. When judging that the ratio of carriers and toner of any of the colors does not fall within the appropriate range, the CPU  151  performs control to cause one of the supply motors  8 Y to  8 K that corresponds to the color to be driven, so that the developer is supplied to the corresponding one of the developing devices  4 Y to  4 K and the ratio of carriers to toner of the color falls within the appropriate range. 
     This supply control is applied to both the trickle developing method and the non-trickle developing method. In this case, it is possible to employ a method in which the appropriate range is different for each color or each developing method. Alternatively, it is possible to employ a method of detecting the amount of developer contained in a developing device. Then, if the amount of developer in the developing device is detected to be smaller than an appropriate value that is a standard, an operation for supplying the corresponding developer may be performed until the amount of the developer reaches the appropriate value. 
     Also, the following method is possible. That is, the amount of toner consumed by developing is estimated based on the cumulative value of the number of pixels (i.e., dot count) when the laser beam emitted from an exposure part exposure-scans the corresponding photosensitive drum on a pixel-by-pixel basis. Then, the amount of developer corresponding to the estimated consumption amount is supplied to the corresponding developing device. By corresponding in advance one dot with the amount of toner supposedly consumed for forming a one-pixel image, the amount of consumed toner is estimated from the cumulative value of the dot count. Furthermore, it is possible to employ a method of detecting, with use of a reflective sensor or the like, the amount (i.e., density) of toner actually used to form a toner image on a photosensitive drum or on an intermediate transfer belt. According to this method, the detected amount of toner is considered as the amount of toner consumed for developing the toner image, and the amount of toner corresponding to the detected amount is supplied to the corresponding developing device. Also, it is possible to employ a supply method in which the aforementioned processes are combined. A supply method is selected based on the structure of a developing device, so that it is suitable for the developing device. 
     The RAM  153  is a work area for the CPU  151 . 
     The cartridge type judgment part  154  receives detection signals (identification information pieces each indicating the type of cartridge) from the cartridge type detection sensors  72 Y to  72 K, judges whether the cartridges set in the respective hoppers  7 Y to  7 K are the first cartridge or the second cartridge (determines the type of cartridge), and transmits results of the judgment to the display controller  155 . 
     The display controller  155  displays various pieces of information on the liquid crystal display  17  of the operation panel  16 . Also, based on the results of the judgment from the cartridge type judgment part  154 , the display controller  155  causes the liquid crystal display  17  to display a warning message which instructs the user to move the shutters  56  of the developing devices  4 Y to  4 K to the respective positions that correspond to the cartridges currently set to the developing devices  4 Y to  4 K. 
       FIG. 12  is a flowchart showing the details of control for displaying the warning message. 
     As shown in  FIG. 12 , the display controller  155  acquires the type of each of the cartridges set to the hoppers  7 Y to  7 K (step S 1 ). The acquisition is performed by receiving results of judgment from the cartridge type judgment part  154 . 
     When the type of cartridge is the first cartridge for the trickle developing method (“YES” in step S 2 ), the display controller  155  causes the liquid crystal display  17  to display a warning message instructing the user to switch the position of the shutter  56  of the corresponding developing device to the trickle corresponding position (step S 3 ), and ends the processing. 
     When the type of cartridge is the second cartridge for the non-trickle developing method (“NO” in step S 2 ), the display controller  155  causes the liquid crystal display  17  to display a warning message instructing the user to switch the position of the shutter  56  of the corresponding developing device to the non-trickle corresponding position (step S 4 ), and ends the processing. 
     The above processing is performed for each of the imaging parts  20 Y to  20 K. 
     Specifically, suppose that the first cartridges  9 Y,  9 M, and  9 C for the trickle developing method are used for the colors Y, M, and C, and the second cartridge  9 Kb for the non-trickle developing method is used for the color K. In this case, for example, the warning message indicates “Please set the projections  84  of the collection tank for the colors Y, M, and C to the uppermost positions, and set the projection  84  of the collection tank for the color K to the lowermost position”. 
     The user can look at the warning message on the liquid crystal display  17 , and switch, for each color, the position of the projection  84  on the collection tank  80  to either the trickle corresponding position (i.e., uppermost position) or the non-trickle corresponding position (i.e., lowermost position), or check whether the projections  84  are set to the respective positions indicated by the warning message. Note that the warning display described above is not limited to the message, and may be graphics showing the positions of the projections for the respective colors. Also, the warning described above does not always need to be displayed. For example, the warning may be provided by means of audio output or the like, as long as it can provide the user with a notification for indicating or warning about the positions of the shutters, in the same manner as the above warning message. 
     As for the timing for the warning, the user is preferably notified when the cartridge currently set is replaced with a new one. This is because of the following reason. If the new cartridge is of the same type as the previous one, the position of the shutter  56  does not need to be changed; however, if the new cartridge is of a different type from the previous one, the position of the shutter  56  needs to be changed to a position corresponding to the new cartridge. In the sense of encouraging the user to check the positions of the shutters  56 , a notification such as a warning display may be provided for the user all the time. 
     With the stated structure, the following advantages are obtained. During a period in which the user often needs to print out images having high image quality (e.g., pictures), the user can set the first cartridges  9 Y to  9 K for the respective colors to the hoppers  7 Y to  7 K, and switch the position of each shutter  56  to the trickle corresponding position (i.e., switch each projection  84  to the uppermost position), so as to switch the developing method to the trickle developing method that gives priority to image quality. This makes it possible to form images having high image quality. 
     Subsequently, when the user needs to print out K-color images that do not require high image quality, such as images including only characters, the user can switch the developing method to the non-trickle developing method with respect to only the color K so as to give priority to cost-effectiveness. Specifically, the user can replace the first cartridge  9 K with the second cartridge  9 Kb, and switch only the position of the shutter  56  of the developing device  4 K to the non-trickle corresponding position (i.e., switch the projection  84  to the lowermost position). This makes it possible to form character images of the color K at low cost. As described above, the developing method can be switched for each color between the trickle developing method and the non-trickle developing method. 
     Although it is described above that the shutter  56  is manually switched by the user as described above, the present invention is not limited to such. The present invention may take another structure as long as it enables switching between a first state and a second state, the first state being a state in which the two-component developer is discharged from the outlet  99  when the first developer Da is supplied, the second state being a state in which the two-component developer is not discharged from the outlet  99  when the second developer Db is supplied. 
     For example, it is possible, as described below, to dispose an actuator (e.g., solenoid) that has a function of switching the position of the shutter  56  between the trickle corresponding position and the non-trickle corresponding position by applying a pressure to the shutter  56  in the axis direction. Also, keys for receiving a user input specifying the position of the shutter  56  may be provided for the operation panel  16 . Then, the actuator may be driven such that, when a key operation using the keys is performed by the user, the position of the shutter  56  is switched to the position specified by the key operation. This improves operability since the user does not need to directly hold the projection  84  and move the projection  84  up and down in order to switch the position of the shutter  56 . 
     As described above, the shutter  56  is formed by integrating the first shutter part  61  for opening and closing the outlet  99  with the second shutter part  62  for opening and closing the third communication path  95 , so that the outlet  99  and the third communication path  95  are opened and closed at the same time in conjunction with each other. However, the first shutter part  61  and the second shutter part  62  may be separately provided so that they can move separately from each other. In this way, the opening and closing of the outlet  99  may be performed separately from the opening and closing of the third communication path  95 . 
     Embodiment 2 
     In Embodiment 1, the shutter  56  has a function of opening and closing both the outlet  99  and the third communication path  95 . Embodiment 2 is different from Embodiment 1 in that the shutter opens and closes only the outlet  99  and an actuator opens and closes the third communication path  95 . Hereinafter, descriptions that are the same as in Embodiment 1 are omitted to avoid repetition, and the same reference signs are given to components that are the same as in Embodiment 1. 
       FIG. 13A  shows a structure of a shutter  201  in the present embodiment, and  FIG. 13B  shows a structure of an actuator  202  (hereinafter also referred to as “magnetic field generators”) for opening and closing the third communication path  95 . 
     As shown in  FIG. 13A , the shutter  201  corresponds to the first shutter part  61  in Embodiment 1, and does not include a part corresponding to the second shutter part  62  that serves as a part of the partition  57  between the supply chamber  58  and the agitation chamber  59 . The part corresponding to the second shutter part  62  is provided for the housing  50  as a partition  203  that is stationary. 
     Accordingly, when the shutter  201  is slid in the axis direction, only the outlet  99  is opened and closed. In  FIG. 13A , the shutter  201  is in a closed state which corresponds to the non-trickle developing method. Note that the shutter  201  includes the holder  60  in the same manner as the shutter  56  in Embodiment 1. Therefore, the outlet  99  is opened and closed by switching the position of the projection  84  of the collection tank  80  between the uppermost position and the lowermost position. 
     The third communication path  95  is not provided with a member that mechanically closes the third communication path  95 . Instead, a pair of magnetic field generators  202  is provided outside the housing  50 , as shown in  FIG. 13B . Specifically, one of the magnetic field generators  202  is provided at a first position and the other of the magnetic field generators  201  is provided at a second position, the first and second positions sandwiching the third communication path  95  therebetween and being located along a direction perpendicular to the conveyance direction of the developer D. The magnetic field generators  202  generate a magnetic field, and thereby form a shield that is made of the developer D to close the third communication path  95 . 
       FIG. 13C  schematically shows a state in which a shield  204  made of the developer D is formed in the third communication path  95 . 
     Each of the magnetic field generators  202  is made of an electromagnet, and generates a magnetic field when an electric current is applied thereto. The carriers included in the developer D are magnetic. Therefore, when a magnetic field is generated by the magnetic field generators  202 , the carriers in the developer D existing in the third communication path  95  and the toner attached to the carriers are attracted to the magnetic field generators  202  by a magnetic force, and stay in the third communication path  95 . 
     The developer D staying in the third communication path  95  forms a shield  204  that separates the supply chamber  58  from the agitation chamber  59 , and thereby blocks the third communication path  95  that is a path along which the developer D is conveyed. When the application of the electric current to the magnetic field generators  202  is stopped, the magnetic force that attracts the developer D to the magnetic field generators  202  disappears. As a result, the shield formed by the developer D is cleared, and the third communication path  95  in which the developer D is conveyed is opened. 
     Switching between the application and block of the electric current with respect to the magnetic field generators  202  is performed based on a result of judgment pertaining to the type of cartridge currently being set. That is, when the cartridge type judgment part  154  judges that the type of cartridge is the first cartridge for the trickle developing method, the controller  15  applies the electric current to the magnetic field generators  202  since the third communication path  95  needs to be closed. On the other hand, when the cartridge type judgment part  154  judges that the type of cartridge is the second cartridge for the non-trickle developing method, the controller  15  prohibits the application of the electric current to the magnetic field generators  202  since the third communication path  95  needs to be opened. 
     Note that the position of the shutter  201  is switched manually by the user in the same manner as in Embodiment 1. Therefore, a warning message is displayed that instructs the user which position the shutter  201  needs to be switched. The above structure is applied to each of the developing devices  4 Y to  4 K. 
     As described above, the third communication path  95  can be opened and closed by the magnetic field generated by the magnetic field generators  202 , instead of the shutter. Note that the generation and suspension of the magnetic field, which are performed by the magnetic field generators  202 , may be switched therebetween by the user using a switch or the like. 
     Embodiment 3 
     In the above embodiments, switching of the position of the shutter is manually performed by the user. Embodiment 3 is different from Embodiments 1 and 2 in that the user does not need to manually perform the switching operation. 
     Specifically, the actuator  301  (see  FIG. 11 ), such as a solenoid, is disposed on the developing device or the copier  10  itself. The actuator  301  can switch the position of the shutter  56  in Embodiment 1 between the trickle corresponding position and the non-trickle corresponding position by applying a pressure to the shutter  56  in the axis direction. As shown by the flowchart of  FIG. 14  pertaining to control of the position of the shutter, the cartridge type judgment part  154  in the controller  15  judges the type of cartridge currently being set (step S 21 ). When the cartridge type judgment part  154  judges that the type of cartridge is the first cartridge for the trickle developing method (“YES” in step S 22 ), the controller  15  controls the drive of the actuator  301  so as to switch the shutter  56  to the trickle corresponding position (step S 23 ), and ends the processing. 
     When the cartridge type judgment part  154  judges that the type of cartridge is the second cartridge for the non-trickle developing method (“NO” in step S 22 ), the controller  15  controls the drive of the actuator  301  so as to switch the shutter  56  to the non-trickle corresponding position (step S 24 ), and ends the processing. The above structure is applied to each of the developing devices  4 Y to  4 K. 
     With the stated structure, it is not necessary to display a warning for the user so as to encourage switching of the shutter position. Also, the user can switch between the developing methods by only setting a cartridge that corresponds to a desired developing method, to the corresponding hopper, thus improving convenience for the user. 
     The actuator  301  is not limited to a solenoid as long as it can move the shutter  56  by applying a pressure to the shutter  56  in the axis direction. For example, the actuator  301  may be a linear motor or a feed screw mechanism. 
     The present invention is not limited to an image forming apparatus and a developing device, and may be a method for switching between the aforementioned developing methods. Furthermore, the present invention may be a program that causes a computer to execute the method. Also, the program according to the present invention may be recorded on a computer-readable recording medium. Examples of such a computer-readable medium include (i) a magnetic disk such as a magnetic tape or a flexible disk, (ii) an optical recording medium such as a DVD, a CD-ROM, a CD-R, an MO, and a PD, and (iii) a flash memory-type recording medium including such as Smart Media™. The program may be manufactured and provided in the form of any of the aforementioned recording media. The program may also be transmitted and provided via a wired or wireless network including the Internet, broadcast, an electric communication line, satellite communication, etc. 
     &lt;Modification&gt; 
     Although the present invention has been described based on the embodiment, it is obvious that the present invention is not limited to the above-mentioned embodiment, and various modifications may be implemented. 
     (1) According to the above embodiments, one developing device is used for both the trickle developing method and the non-trickle developing method. In addition to this structure, it is possible to notify the user of the timing for switching between the developing methods. 
     For example, it is possible to cumulatively calculate, for each color, the operation time of the corresponding developing device in the non-trickle developing method. When the cumulative time exceeds a predetermined time length, the image forming apparatus may notify the user that the developing device needs to be switched to the trickle developing method, because the developer (especially carriers) in the developing device is deteriorated to a degree that lowers image quality by progression of abrasion of carrier particles by agitation or the like. Here, the notification may be provided for the user by, for example, display or audio as described above. 
     Suppose that the user is notified of switching from the non-trickle developing method to the trickle developing method, with respect to the developer device corresponding to the color Y. In this case, the user may perform the operations as described above. That is, the user may open the outer cover on the front side of the image forming apparatus, and set the first cartridge  9 Y to the hopper  7 Y, instead of the second cartridge  9 Yb. After detaching the collection tank  80  and switching the position of the projection  84  for the color Y from the lowermost position to the uppermost position, the user may attach the collection tank  80  back to the image forming apparatus. 
     The above describes a structure example of notifying of switching from the non-trickle developing method to the trickle developing method. However, it is possible to employ a structure for notifying of switching from the trickle developing method to the non-trickle developing method instead. In other words, it is possible to cumulatively calculate, for each color, the operation time of the corresponding developing device in the trickle developing method. When the cumulative time exceeds a predetermined time length, the image forming apparatus may notify the user that the developing device is recommended to be switched to the non-trickle developing method, because the deterioration of the developer is resolved by the fresh developer replacing the deteriorated developer to a sufficient extent. In a case where the user switches to the non-trickle developing method according to the notification, the user may perform operations opposite from the aforementioned operations for replacing the cartridge and switching the position of the projection  84  of the collection tank  80 . 
     The condition of notifying the user is not limited to the operation time, and may be information that indicates the degree of deterioration of the developer. For example, the condition may be the number of times images are formed (hereinafter “image formation count”), the remaining amount of the developer in the cartridge, the ratio of the number of times color images are formed (hereinafter “color image formation count”) to the number of times monochrome images are formed (hereinafter “monochrome image formation count”), or the like. 
     The larger the image formation count, the longer the operation time. This means that deterioration in the developer has progressed. Therefore, when the image formation count exceeds a predetermined count, the image forming apparatus may notify the user that the non-trickle developing method needs to be switched to the trickle developing method. 
     Similarly, the smaller the remaining amount of the developer in the cartridge, the longer the operation time. Therefore, when the remaining amount is smaller than a predetermined amount, the image forming apparatus may notify the user that the non-trickle developing method can be switched to the trickle developing method. 
     Also, the larger the ratio of the color image formation count to the monochrome image formation count, the greater the frequency of use of the developing devices  4 Y,  4 M, and  4 C for forming color images. This means that the deterioration of developers contained in the developing devices  4 Y,  4 M, and  4 C has progressed. Therefore, when the ratio exceeds a predetermined value, the image forming apparatus may notify the user that the developing methods of only the developing devices for forming color images need to be switched from the non-trickle developing method to the trickle developing method. Note that the developing method of the developing device  4 K may also be switched to the trickle developing method, together with the developing devices  4 Y,  4 M, and  4 C. The user can arbitrarily switch between the developing methods as desired. 
     In the case where the position of the shutter  56  is automatically switched as described in Embodiment 3, the following structure is possible. That is, after the user switches a cartridge according to the aforementioned notification, the position of the shutter  56  may be automatically switched to the position corresponding to the type of the cartridge set after the switching. Regarding the notification of switching between the developing methods as described above, it is possible to notify either one or both of (i) the switching from the non-trickle developing method to the trickle developing method and (ii) the switching from the trickle developing method to the non-trickle developing method. 
     (2) In Embodiment 1, the projection  84  is provided for the collection tank  80  which is a different member from the shutter  56 . The projection  84  is capable to be switched between a state of being engaged with the shutter  56  and a state of being disengaged with the shutter  56 , so as to switch the position of the shutter  56  between the trickle corresponding position and the non-trickle corresponding position. However, the present invention is not limited to such. The present invention may employ a different member as long as the member can switch the position of the shutter  56 . For example, the shutter  56  may be fixed to the housing  50  by a fixing screw at either the trickle corresponding position or the non-trickle corresponding position. When switching the position of the shutter  56 , the user may remove the fixing screw and, after switching the position, tighten the fixing screw again. The present invention may employ another structure or method different from those described above. 
     (3) In the above embodiments, the first communication path  93  and the third communication path  95  are provided as parts of the circulation conveyance path of the developer D. Also, the outlet  99  is arranged in the first conveyance path  91 , at a portion from (i) a connected portion in which the first conveyance path  91  is connected to the third communication path  95  to (ii) a connected portion in which the first conveyance path  91  is connected to the first communication path  93  located in the downstream side in the developer conveyance direction (i.e., X direction), inclusive of the connected portions. However, the position of the outlet  99  is not limited to such. 
     For example, the outlet  99  may be arranged in the first communication path  93 . Alternatively, the outlet  99  may be arranged in the second conveyance path  92 , at a portion from (i) a connected portion in which the second conveyance path  92  is connected to the first communication path  93  to (ii) the inlet  98  (i.e., supply position) for receiving the developer located in the downstream side in the developer conveyance direction (i.e., X′ direction), inclusive of the connected portion and exclusive of the inlet  98 . 
     Also, depending on the positional relationship with the inlet  98 , the outlet  99  may be arranged in a portion different from the aforementioned portions. For example, suppose that the inlet  98  is arranged in a portion of the second conveyance path  92 , at either the connected portion in which the second conveyance path  92  is connected to the third communication path  95  or a portion of the second conveyance path  92  that is located more downstream than the connected portion in the developer conveyance direction. In this case, the outlet  99  may be arranged in any of the following portions: (i) a portion of the first conveyance path  91  that is located more downstream in the developer conveyance direction than the connected portion in which the first conveyance path  91  is connected to the third communication path  95 ; (ii) the first communication path  93 ; and (iii) a portion of the second conveyance path  92  that is located more upstream in the developer conveyance direction than the connected portion in which the second conveyance path  92  is connected to the third communication path  95 . Note that the outlet  99  does not always need to be arranged at the bottom of a chamber of the housing  50 , such as the supply chamber  58 . The outlet  99  may be arranged at a side of a chamber of the housing  50  as long as the developer D currently being conveyed can be discharged outside the housing  50 . 
     Furthermore, according to the above embodiment, the shutter  56  opens the outlet  99  and closes the third communication path  95  in the trickle developing method. Also, the shutter  56  closes the outlet  99  and opens the third communication path  95  in the non-trickle developing method. However, it is not limited to such. The present invention may employ a different structure as long as the structure enables switching between a state in which the developer is discharged from the outlet and a state in which the developer is not discharged from the outlet. 
     For example, the following structure is possible. That is, the first conveyance path  91  may be split into two conveyance paths, i.e., a conveyance path A and a conveyance path B (each corresponding to a communication path), at the end of the first conveyance path  91  on the downstream side in the developer conveyance direction. Also, the conveyance paths A and B may be joined together at the end portion of the second conveyance path  92  in the upstream side in the developer conveyance direction. Further, the conveyance path A may be provided with an inlet whereas the conveyance path B may not be provided with an inlet. In this state, the image forming apparatus may be capable of switching the conveyance path of the developer D from the conveyance path A to the conveyance path B or from the conveyance path B to the conveyance path A. 
     In this case, the conveyance path is switched to the conveyance path A in the trickle developing method, and to the conveyance path B in the non-trickle developing method. According to the stated structure, the outlet is not opened or closed. Instead, the conveyance path for the developer D is switched between the conveyance path A having the outlet and the conveyance path B having no outlet, whereby the state of the image forming apparatus is switched between the state in which the developer is discharged from the outlet and the state in which the developer is not discharged from the outlet. 
     Also, the circulation conveyance path is not limited to the above embodiments. For example, the circulation conveyance path of the housing  50  may not include the third communication path  95 , and may only include the first conveyance path, the first communication path, the second conveyance path, and the second communication path. Then, the outlet may be arranged in any of the following portions: a portion of the first conveyance path that is more downstream than a portion of the first conveyance path facing the developing roller  51  in the developer conveyance direction; the first communication path; and a portion of the second conveyance path that is more upstream than the inlet  98  in the developer conveyance direction. The outlet may be opened or closed depending on whether the developing method is the trickle developing method or the non-trickle developing method. Regarding the opening and closing of the outlet, it is not limited to the structure for switching the shutter between the position for closing the outlet and the position away from the outlet to open the outlet. For example, it is possible to employ a structure where the outlet is provided with a diaphragm mechanism including a plurality of overlapping plates. In this case, the outlet is opened and closed by moving the overlapping plates. 
     (4) The above embodiments are described with an example of the tandem-type color digital copier. However, the present invention is not limited to such, and is applicable to a color or monochrome image forming apparatus in general, such as a printer, a FAX (facsimile), or an MFP (Multiple Function Peripheral) as long as the image forming apparatus includes at least one developing device and a hopper for supplying developer to the developing device. 
     Also, according to the above embodiments, the first developer Da includes carriers and toner. However, the first developer Da may include carriers and not toner. In this case, the image forming apparatus further includes another hopper for supplying toner, and performs supply control separately on each of the hoppers, i.e., (i) the hopper for supplying toner and (ii) the hopper for supplying the first developer Da including carriers and not toner. Details of a structure for supplying carriers and toner separately are disclosed in the aforementioned Japanese examined patent application publication, for example. 
     Furthermore, the present invention may include a structure where the developing device in the image forming apparatus is detachable by the user or the like. 
     Also, according to the above embodiments, the photosensitive drums are used as image carriers for forming an electrostatic latent image. However, it is not limited to such. For example, it is possible to employ belt-like image carriers. Furthermore, it is described above that the developing roller having the sleeve is used as a developer carrier for carrying the developer D. However, it is not limited to such. It is possible to employ a different member as a developer carrier, as long as it carries the developer D. 
     Also, the present invention may be any combination of the embodiments and the modifications described above. 
     CONCLUSION 
     Each of the embodiment and modifications shown above is an aspect of the present invention to solve the problems discussed in the section of “BACKGROUND OF THE INVENTION”. The embodiment and modifications shown above are summarized as follows. 
     (1) A first aspect of the present invention is an image forming apparatus for developing an electrostatic latent image on an image carrier with use of a two-component developer including carriers and toner, the image forming apparatus comprising: a developing device having (i) a conveyance path in which the two-component developer is conveyed and (ii) an outlet from which the two-component developer is partially discharged while the two-component developer is conveyed; a hopper to which (i) a first cartridge containing a first developer including carriers and (ii) a second cartridge containing a second developer including toner and not carriers are attachable, and operable, when the first cartridge is attached thereto, to supply the first developer to the developing device, and when the second cartridge is attached thereto, to supply the second developer to the developing device; and a switching part operable, when the first developer is supplied, to switch the developing device to a first state in which the two-component developer is partially discharged from the outlet, and when the second developer is supplied, to switch the developing device to a second state in which the two-component developer is not discharged from the outlet. 
     (2) A second aspect of the present invention is the image forming apparatus of (1), wherein the switching part switches the developing device to the first state by opening the outlet, and switches the developing device to the second state by closing the outlet. 
     (3) A third aspect of the present invention is the image forming apparatus of (2), wherein the conveyance path is a circulation conveyance path in which the two-component developer is circularly conveyed, and includes: a first conveyance path that is linear and in which the two-component developer is conveyed in a first conveyance direction; a second conveyance path that is arranged parallel to the first conveyance path, and in which the two-component developer is conveyed in a second conveyance direction opposite the first conveyance direction; a first communication path for communicating a first portion of the first conveyance path located downstream in the first conveyance direction, with a first portion of the second conveyance path located upstream in the second conveyance direction; a second communication path for communicating a second portion of the second conveyance path located downstream in the second conveyance direction, with a second portion of the first conveyance path located upstream in the first developer conveyance path; and a third communication path for communicating a third portion of the first conveyance path with a third portion of the second conveyance path, the third portion of the first conveyance path located more upstream in the first conveyance direction than the first portion thereof, the third portion of the second conveyance path located more downstream in the second conveyance direction than the first portion thereof, the outlet is arranged in any of the following portions: (i) a portion of the first conveyance path located more downstream in the first conveyance direction than the third portion of the first conveyance path; (ii) the first communication path; and (iii) a portion of the second conveyance path located more upstream in the second conveyance direction than the third portion of the second conveyance path, and the switching part closes the third communication path when opening the outlet, and opens the third communication path when closing the outlet. 
     (4) A fourth aspect of the present invention is the image forming apparatus of (3), wherein the switching part includes: a first shutter operable to move between a first opening position and a first closing position, the first opening position being for opening the outlet by the first shutter separating from the outlet, the first closing position being for closing the outlet by the first shutter covering the outlet; and a second shutter operable to move between a second opening position and a second closing position, the second opening position being for opening the third communication path by the second shutter separating from the third communication path, the second closing position being for closing the third communication path by the second shutter cutting across the third communication path. 
     (5) A fifth aspect of the present invention is the image forming apparatus of (4), wherein the first shutter is integrated with the second shutter to constitute an integrated shutter, and the integrated shutter is operable to move between a first position and a second position, the first position being a position at which the first shutter is located at the first opening position for opening the outlet and the second shutter is located at the second closing position for closing the third communication path, the second position being a position at which the first shutter is located at the first closing position for closing the outlet and the second shutter is located at the second opening position for opening the third communication path. 
     (6) A sixth aspect of the present invention is the image forming apparatus of (5), wherein the integrated shutter is manually movable by a user. 
     (7) A seventh aspect of the present invention is the image forming apparatus of (6) further comprising: a detector operable to detect a type of cartridge mounted in the hopper; and a notifier operable to notify the user that the integrated shutter needs to be located at the first position when the type of cartridge is detected to be the first cartridge, and that the integrated shutter needs to be located at the second position when the type of cartridge is detected to be the second cartridge. 
     (8) An eighth aspect of the present invention is the image forming apparatus of (5) further comprising: a detector operable to detect a type of cartridge mounted in the hopper; an actuator operable to move the integrated shutter between the first position and the second position; and a controller operable to control the actuator, wherein the controller operable to drive the actuator to cause the integrated shutter to move to the first position when the type of cartridge is detected to be the first cartridge, and to cause the integrated shutter to move to the second position when the type of cartridge is detected to be the second cartridge. 
     (9) A ninth aspect of the present invention is the image forming apparatus of (3), wherein the switching part includes: a shutter operable to open and close the outlet; and a magnetic field generator operable to switch between generation and suspension of a magnetic field, the magnetic field being for forming a developer shield by causing the two-component developer existing in the third communication path to stay therein. 
     (10) A tenth aspect of the present invention is the image forming apparatus of (9), wherein the shutter is manually movable by a user between (i) an opening position for opening the outlet by the shutter separating from the outlet and (ii) a closing position for closing the outlet by the shutter covering the outlet. 
     (11) An eleventh aspect of the present invention is the image forming apparatus of (10) further comprising: a detector operable to detect a type of cartridge mounted in the hopper; a notifier operable to notify the user that the shutter needs to be located at the opening position when the type of cartridge is detected to be the first cartridge, and that the shutter needs to be located at the closing position when the type of cartridge is detected to be the second cartridge; and a controller operable to control the magnetic field generator to generate the magnetic field when the type of cartridge is detected to be the first cartridge, and to suspend the generation of the magnetic field when the type of cartridge is detected to be the second cartridge. 
     (12) A twelfth aspect of the present invention is the image forming apparatus of (1) further comprising: an acquisition part operable to acquire information indicating a degree of deterioration of the two-component developer contained in the developing device; and a switching notifier operable to notify a user of a timing of switching between a first developing method and a second developing method based on the information acquired by the acquisition part, the first developing method being a method for developing the electrostatic latent image with a supply of the first developer, the second developing method being a method for developing the electrostatic latent image with a supply of the second developer. 
     (13) A thirteenth aspect of the present invention is a developing device mounted in an image forming apparatus, and for developing an electrostatic latent image on an image carrier with use of a two-component developer including carriers and toner, the developing device comprising: a conveyance path in which the two-component developer is conveyed, an outlet that is provided in the conveyance path and from which the two-component developer is partially discharged while the two-component developer is conveyed; an inlet that is provided in the conveyance path and from which (i) a first developer including carriers is supplied when a cartridge containing the first developer is attached to the image forming apparatus and (ii) a second developer including toner and not carrier is supplied when a cartridge containing the second developer is attached to the image forming apparatus; and a switching part operable, when the first developer is supplied, to switch the developing device to a first state in which the two-component developer is discharged from the outlet, and when the second developer is supplied, to switch the developing device to a second state in which the two-component developer is not discharged from the outlet. 
     According to the stated structure, the user can switch between the first developing method and the second developing method as desired. Specifically, when the user desires high image quality, images are developed by means of the first developing method in which, while the first developer is supplied from the cartridge containing the first developer, the two-component developer in the developing device is discharged from the outlet. Also, when the user desires low cost, images are developed by means of the second developing method in which, although the second developer is supplied from the cartridge containing the second developer, the two-component developer in the developing device is not discharged from the outlet. As a result, the user does not need to prepare two types of developing devices, i.e., one for the first developing method and the other for the second developing method. This eliminates the need for the user to manage two types of developing devices, and thus improves convenience for the user. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.