Patent Publication Number: US-8983319-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-016517 filed Jan. 30, 2012. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to an image forming apparatus. 
     (ii) Related Art 
     Hitherto, as the aforementioned image forming apparatus, for example, the following type of image forming apparatus is available. This type of image forming apparatus has a structure that forms an image by driving a photoconductor drum while switching its speed to multiple speeds, and by developing an electrostatic latent image, formed on the surface of the photoconductor drum, using a developing device that is driven in accordance with a speed corresponding to the speed of the photoconductor drum. A developer supplying device supplies developer to the developing device when necessary. The developer supplying device may be driven at a constant speed regardless of the driving speed of the photoconductor drum. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including an image carrier that carries an electrostatic latent image; a developer supplying unit that supplies developer by being driven at a predetermined speed; a developing unit that develops the electrostatic latent image carried by the image carrier, while a transporting member transports the developer that is supplied from the developer supplying unit, a transport speed of the transporting member being switched to a plurality of speeds; a determining unit that determines whether or not an operation where a supply capacity of the developer supplying unit is greater than a transport capacity of the developing unit exceeds a predetermined threshold value and is continued; and a controller that performs control so that, when the determining unit determines that the operation exceeds the predetermined threshold value and is continued, an operation that was being executed immediately prior to the determination is stopped to forcefully drive the transporting member of the developing unit for a predetermined driving time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates the entire structure of an image forming apparatus according to a first exemplary embodiment of the present invention; 
         FIG. 2  illustrates the structures of developer supplying devices according to the first exemplary embodiment of the present invention; 
         FIG. 3  is a sectional view of the structure of each toner cartridge; 
         FIG. 4  is a sectional view of the structure of each toner cartridge that is mounted to a body of the image forming apparatus; 
         FIG. 5  illustrates the structure of a toner supply path for supplying toner to a developing device from the corresponding toner cartridge; 
         FIG. 6  is a perspective view of the structure of each developer supplying device; 
         FIG. 7  is a sectional view of the structure of each developer storing device; 
         FIG. 8  is a perspective view of the structure of each developing device; 
         FIG. 9  is a perspective view of the structure of each developing device; 
         FIG. 10  is a perspective view of the structure of each developing device; 
         FIG. 11  is a block diagram of a control circuit; 
         FIG. 12  is a flow chart of the operations of the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 13  illustrates a toner clog prevention operation; and 
         FIG. 14  is a flow chart of the operation of an image forming apparatus according to a second exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will hereunder be described with reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  shows a tandem full-color image forming apparatus serving as an image forming apparatus according to a first exemplary embodiment of the present invention. The tandem full-color image forming apparatus includes an image reading device, and also functions as a full-color copying machine. The image forming apparatus need not include an image reading device. The present invention is obviously not limited to a tandem image forming apparatus. Therefore, the present invention may be applied to, for example, a monochromatic image forming apparatus including only one photoconductor drum, or to what is called a four-cycle full-color image forming apparatus. 
     In  FIG. 1 , reference numeral  1  denotes the body of the image forming apparatus, with an image reading device  4  that reads an image on an original  2  being disposed at one end (left end in  FIG. 1 ) of an upper portion of the body  1  of the image forming apparatus. In the image reading device  4 , a light source  6  illuminates the original  2  placed on a platen glass  5  while the original  2  is pressed by an original holding member  3 , and an image formed by light reflected from the original  2  scans and exposes an image reading element  11  (including, for example, a charged coupled device (CCD)) through a reduction optical system (including a full-rate mirror  7 , half-rate mirrors  8  and  9 , and an imaging lens  10 ). The scanning and the exposure cause the image reading element  11  to read the image on the original  2  with a predetermined dot density. 
     The image on the original  2  read by the image reading device  4  is sent to an image processing device  12  as, for example, pieces of image data of three colors, red (R), green (G), and blue (B), each piece of image data being, for example, eight bits. The image processing device  12  performs predetermined image processing operations on the pieces of image data of the original  2 . The image processing operations include, for example, shading correction, positional displacement correction, brightness/color space conversion, gamma correction, frame erasure, and color/movement edition. The pieces of image data on which the predetermined image processing operations have been performed by the image processing device as mentioned above are converted into pieces of image data of four colors, cyan (C), magenta (M), yellow (Y), and black (K), by the image processing device  12 . The number of colors of the pieces of image data that are converted by the image processing device  12  is not limited to the four colors, cyan (C), magenta (M), yellow (Y), and black (K). Therefore, the colors of the pieces of image data may be converted into any number of colors, such as six colors including highly saturated cyan (HC) and highly saturated magenta (HM) in addition to the aforementioned four colors. The pieces of image data that are input to the controller  12  may obviously be sent from, for example, a personal computer through a communication line (not shown). 
     In the exemplary embodiment, the image forming apparatus includes image forming units that form images using toners of different colors. 
     That is, as shown in  FIG. 1 , in the interior of the body  1  of the image forming apparatus  1  according to the exemplary embodiment, four image forming sections  13 Y,  13 M,  13 C, and  13 K corresponding to the colors, yellow (Y), magenta (M), cyan (C), and black (K), respectively, are disposed side by side horizontally so as to be spaced apart from each other by a certain interval. The image forming sections  13 Y,  13 M,  13 C, and  13 K serve as the image forming units. The order of disposition of the image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K), respectively, is not limited to that shown in  FIG. 1 . The image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K), respectively, are each formed into a unit, and are each replaceably mounted individually to the body  1  of the image forming apparatus. 
     As shown in  FIG. 1 , the four image forming sections  13 Y,  13 M,  13 C, and  13 K all have basically the same structure, and only differ in the type of toner that they use. Roughly speaking, each of the image forming sections  13 Y,  13 M,  13 C, and  13 K includes a photoconductor drum  15 , a scorotron  16 , an image exposing device  14 , a developing device  17 , and a cleaning device  18 . Each photoconductor drum  15  serving as an image carrier is driven along the direction of arrow A at predetermined rotational speeds. Each scorotron  16  serving as a first charging unit uniformly charges the surface of the corresponding photoconductor drum  15 . The image exposing devices  14  serving as latent image forming units form electrostatic latent images by exposing the surfaces of the photoconductor drums  15  to images corresponding to the respective colors. The developing devices  17  serving as developing units develop the electrostatic latent images formed on the corresponding photoconductor drums  15  with toners of the corresponding colors. The cleaning devices  18  clean residual toner remaining on the photoconductor drums  15  after transfer. 
     In the exemplary embodiment, the speed of an image forming operation that is determined by the rotational speed of each photoconductor drum  15 , that is, a process speed (peripheral speed) is switchable in four stages. These four stages are a full-color image forming mode corresponding to the highest speed of 308 mm/s, a high image quality mode corresponding to the second highest speed of 255 mm/s, a first thick-paper mode corresponding to the third highest speed of 200 mm/s for forming images on a recording medium that is thick paper having a relatively small paper weight, and a second thick-paper mode corresponding to the lowest speed of 103 mm/s for forming images on a recording medium that is thick paper having a relatively large paper weight. The process speed is not limited to a speed that is switched in four stages. Therefore, the process speed may obviously be switched in stages that is less than or greater than four stages. 
     The image forming apparatus is formed so that, for example, driving speeds of the developing devices  17  are switched in four stages in accordance with the process speeds determined by the rotational speeds of the corresponding photoconductor drums  15 . 
     As shown in  FIG. 1 , in each image exposing device  14 , a semiconductor laser  19  is modulated in accordance with image data, and a laser beam LB from the semiconductor laser  19  is emitted in accordance with the image data. The laser beam LB emitted from the semiconductor laser  19  is deflected by a rotating polygonal mirror  22  through reflecting mirrors  20  and  21  for scanning. With the focal length being adjusted in accordance with a scanning angle by a f-θ lens (not shown), each photoconductor drum  15  serving as an image carrier is scanned and exposed through reflecting mirrors  23  and  24 . The image exposing devices  14  are not limited to devices that perform image exposure by deflecting the laser beams LB and scanning with the laser beams LB. For example, they may be devices using LED arrays in which LED elements are disposed along an axial direction of the photoconductor drums  15 . Compared to the image exposing devices  14  that perform image exposure by deflecting the laser beams LB and scanning with the laser beams LB, the image exposing devices  14  using the LED arrays may be made considerably smaller, which is desirable from the viewpoint of reducing the size of the entire image forming apparatus. 
     The photoconductor drums  15 Y,  15 M,  15 C, and  15 K of the image forming sections  13 Y,  13 M,  13 C, and  13 K corresponding to yellow (Y), magenta (M), cyan (C), and black (K) are uniformly charged by scorotrons  16 Y,  16 M,  16 C, and  16 K to predetermined potentials. Thereafter, the image processing device  12  successively outputs the pieces of image data of the corresponding colors to the image exposing devices  14 Y,  14 M,  14 C, and  14 K of the image forming sections  13 Y,  13 M,  13 C, and  13 K for the corresponding colors, yellow (Y), magenta (M), cyan (C), and black (K). The light beams LB exiting from the corresponding image exposing devices  14 Y,  14 M,  14 C, and  14 K in accordance with the pieces of image data scan the surfaces of the corresponding photoconductor drums  15 Y,  15 M,  15 C, and  15 K along a main scanning direction (that is, an axial direction of the photoconductor drums  15 ) for exposing the surfaces to the light beams LB, to form electrostatic latent images. The electrostatic latent images formed on the corresponding photoconductor drums  15 Y,  15 M,  15 C, and  15 K are developed as toner images of the corresponding colors, yellow (Y), magenta (M), cyan (C), and black (K), by the corresponding developing devices  17 Y,  17 M,  17 C, and  17 K. 
     As shown in  FIG. 1 , the toner images of the corresponding colors, yellow (Y), magenta (M), cyan (C), and black (K), that are successively formed on the photoconductor drums  15 Y,  15 M,  15 C, and  15 K of the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K are first-transferred to an intermediate transfer belt  25  while the toner images are superposed upon the intermediate transfer belt  25  by first transfer rollers  26 Y,  26 M,  26 C, and  26 K. The intermediate transfer belt  25  serving as an intermediate transfer body is disposed below the image forming sections  13 Y,  13 M,  13 C, and  13 K. 
     The intermediate transfer belt  25  extends on rollers, such as a drive roller  27 , a driven roller  28 , a tension applying roller  29 , a driven roller  30 , a back support roller  31  of a second transfer section, and a driven roller  32 , by a predetermined tension. The drive roller  27  that is rotationally driven by a dedicated drive motor (not shown) that excels in achieving constant speed is driven so as to circulate at a speed that is substantially equal to the rotational speeds (peripheral speeds) of the photoconductor drums  15 Y,  15 M,  15 C, and  15 K in the direction of arrow B. As the intermediate transfer belt  15 , for example, a synthetic resin film, such as a polyimide resin film or a polyamide-imide resin film, having flexibility and formed into an endless belt may be used. 
     The toner images of the corresponding colors, yellow (Y), magenta (M), cyan (C), and black (K), that have been transferred to the intermediate transfer belt  25  in a superimposed state are second-transferred collectively to recording paper  34  (serving as a recording medium), by a second-transfer roller  33  that press-contacts the back support roller  31  with the intermediate transfer belt  25  being disposed therebetween. The recording paper  34  to which the toner images of the corresponding colors have been transferred is transported to a fixing device  37  (serving as a fixing unit) by a double belt including transfer belts  35  and  36 . The recording paper  34  to which the toner images of the corresponding colors have been transferred is subjected to a fixing operation using heat provided by a heating belt  38  of the fixing device  37  and pressure provided by a pressure roller  39  of the fixing device  37 . Thereafter, in the case of one-side printing, the recording paper  34  is discharged as it is to a discharge tray  40  provided at an outer portion of the body  1  of the image forming apparatus. 
     As shown in  FIG. 1 , pieces of recording paper  34  having a predetermined size or formed of a predetermined material are temporarily transported from either one of sheet-feed trays  41  and  42  to registration rollers  46  while the pieces of recording paper  34  are separated one at a time through a sheet transport path  45  including a sheet-feed roller  43  and a pair of sheet transport rollers  44 . The recording paper  34  supplied from either one of the sheet-feed trays  41  and  42  is sent out to a second transfer position of the intermediate transfer belt  25  by the registration rollers  46  that are rotationally driven at a predetermined timing. 
     When forming images on both sides of the recording paper  34  by the image forming apparatus, the recording paper  34  to whose one side the images have been fixed by the fixing device  37  is not discharged out of the image forming apparatus. Instead, a switching gate (not shown) causes the transport path of the recording paper  34  to be switched to a lower transport path, as a result of which the front and back of the recording paper  34  are reversed through a reversal sheet transport path  47 . Thereafter, the reversed recording paper  34  is transported again to the second transfer position of the intermediate transfer belt  25  through a duplex-printing sheet transport path  48  and the ordinary sheet transport path  45 , so that images are transferred to the back side of the recording paper  34 . Thereafter, the images are fixed by heat provided by the heating belt  38  of the fixing device  37  and pressure provided by the pressure roller  39  of the fixing device  37 . The recording paper  34  to whose back side the images have been fixed is discharged to the discharge tray  40  provided at the outer portion of the body  1  of the image forming apparatus. 
     The surfaces of the photoconductor drums  15  to which the toner images have been first-transferred are cleaned by cleaning devices  18 . A surface of the intermediate transfer belt  25  to which the toner images have been second-transferred is cleaned by a belt cleaning device  49  disposed at the drive roller  27 . 
     As shown in  FIGS. 1 and 2 , developer supplying devices  50 Y,  50 M,  50 C, and  50 K are provided at the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K). The developer supplying devices  50 Y,  50 M,  50 C, and  50 K supply developers including at least toners of colors corresponding to the respective developing devices  17 Y,  17 M,  17 C, and  17 K. Although, in the exemplary embodiment, the developer supplying devices  50 Y,  50 M,  50 C, and  50 K are formed so as to supply the developers including only toners, the developer supplying devices  50 Y,  50 M,  50 C, and  50 K may obviously be formed so as to supply developers including toners and carriers. 
     As shown in  FIGS. 1 and 2 , the developer supplying devices  50 Y,  50 M,  50 C, and  50 K include, respectively, a toner cartridge  51 Y, a toner cartridge  51 M, a toner cartridge  51 C, and toner cartridges  51 K, serving as developer containers that contain toners as developers of the corresponding colors, yellow (Y), magenta (M), cyan (C), and black (K). Since the amount of consumption of black (K) toner is relatively large compared to that of each of the other color toners, two black (K) toner cartridges  51 K are disposed. When one of the toner cartridges  51 K becomes empty, the other toner cartridge  51 K is used. 
     As shown in  FIG. 3 , toners T of the corresponding colors are contained in the corresponding toner cartridges  51 Y,  51 M,  51 C, and  51 K. In addition, as shown in  FIG. 3 , agitators  53  are rotatably disposed in the corresponding toner cartridges  51 Y,  51 M,  51 C, and  51 K. The agitators  53  serve as toner transporting members for supplying the toners T from corresponding toner supply openings  52  while mixing the toners T, and are formed by spirally bending linear members formed of a metal or synthetic resin. Each toner supply opening  52  opens in a bottom portion at one end of the corresponding toner cartridge in a longitudinal direction. As shown in  FIG. 4 , by mounting the toner cartridges  51 Y,  51 M,  51 C, and  51 K to the body  1  of the image forming apparatus, the agitators  53  are connected to corresponding cartridge motors  54 Y,  54 M,  54 C, and  54 K, and are rotationally driven thereby. The cartridge motors  54 Y,  54 M,  54 C, and  54 K, serving as first driving units, are provided at the body  1  of the image forming apparatus. As the cartridge motors  54 Y,  54 M,  54 C, and  54 K, for example, DC motors are used. The reasons DC motors are used as the cartridge motors  54 Y,  54 M,  54 C, and  54 K are that DC motors themselves are relatively smaller than other types of motors, can be made small even if combined with a speed-reduction gear box, and can be disposed in the interior of the body  1  of the image forming apparatus with a high degree of freedom. Regardless of a process speed, the cartridge motors  54 Y,  54 M,  54 C, and  54 K are driven at a predetermined constant speed corresponding to, for example, the highest process speed. 
     As shown in  FIG. 5 , the developer supplying devices  50 Y,  50 M,  50 C, and  50 K include corresponding developer storing devices  55 Y,  55 M,  55 C, and  55 K that temporarily store the toners that are supplied from the corresponding toner cartridges  51 Y,  51 M,  51 C, and  51 K, and that supply the toners to the developing devices  17 Y,  17 M,  17 C, and  17 K while mixing the toners. The developer storing devices  55 Y,  55 M,  55 C, and  55 K transport the toners T while mixing the toners T with predetermined amounts of toners T that are supplied from the toner supply openings  52  of the corresponding toner cartridges  51 Y,  51 M,  51 C, and  51 K being temporarily stored in the developer storing devices  55 Y,  55 M,  55 C, and  55 K. Then, through drop paths  57 , the toners T are supplied and drop towards the corresponding developing devices  17 Y,  17 M,  17 C, and  17 K from toner replenishment openings  56 . Each toner replenishment opening opens in a bottom surface at one end of a corresponding one of the developer storing devices  55 Y,  55 M,  55 C, and  55 K. 
       FIG. 6  is a perspective view of a state in which the toner cartridges  51 Y,  51 M,  51 C, and  51 K are removed from the corresponding toner cartridges  50 Y,  50 M,  50 C, and  50 K, as viewed obliquely from thereabove in the direction of arrow C in  FIG. 5 . An area  58  (described later) that is adjacent to the corresponding one of the developer storing devices  55 Y,  55 M,  55 C, and  55 K to which the toner T is supplied from the toner supply opening  52  of the corresponding one of the toner cartridges  51 Y,  51 M,  51 C, and  51 K can be seen. 
     As shown in  FIG. 7 , in the interiors of the developer storing devices  55 Y,  55 M,  55 C, and  55 K, the toners T are supplied from the toner supply openings  52  of the toner cartridges  51 Y,  51 M,  51 C, and  51 K to the rectangular areas  58  shown by broken lines. Two spiral agitators  59  and  60  are disposed parallel to each other in each of the developer storing devices  55 Y,  55 M,  55 C, and  55 K. The agitators  59  and  60  transport the toner T supplied from the corresponding one of the toner cartridges  51 Y,  51 M,  51 C, and  51 K so as to circulate the toner T while mixing the toner T. An auger  61  having the form of a screw is disposed between the two agitators  59  and  60  in each of the developer storing devices  55 Y,  55 M,  55 C, and  55 K. The augers  61  transport a portion of the toners T that are transported so as to replenish the developing devices  17 Y,  17 M,  17 C, and  17 K with the toners T while being mixed so as to be circulated by the two agitators  59  and  60 . The augers  61  are formed so that the toners T from the corresponding toner replenishment openings  56  that open in the bottom surfaces of the corresponding developer storing devices  55 Y,  55 M,  55 C, and  55 K drop and are supplied to the corresponding developing devices  17 Y,  17 M,  17 C, and  17 K. As shown in  FIGS. 5 and 7 , the two agitators  59  and  60  and the auger  61  are rotationally driven at a predetermined constant speed through gears by a corresponding one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K serving as second drive motors. As the toner supply motors  62 Y,  62 M,  62 C, and  62 K, for example, DC motors may be used due to the same reasons that DC motors are used for the cartridge motors  54 Y,  54 M,  54 C, and  54 K. The toner supply motors  62 Y,  62 M,  62 C, and  62 K are also driven at a predetermined constant rotational speed regardless of the process speed. 
       FIG. 8  shows the structure of each developing device to which toner of a corresponding color is supplied from the corresponding one of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K. 
     As shown in  FIG. 8 , each developing device  17  includes a developing-device housing  64  having an opening  63  in an area opposing the corresponding photoconductor drum  15 . In an internal portion of each developing-device housing  64 , a developing roller  65  is rotatably disposed at a position that faces the opening  63 . A developer chamber  64  that contains two-component developer  66  including toner and a carrier is provided at a back side of each developing roller  65 . Each developer chamber  64  is partitioned in two by a partition wall  68 . A mixing/supplying auger  69  is rotatably disposed at a side of its corresponding developing roller  65 . Each auger  69  serves as a transporting member that supplies the developer  66  to its corresponding developing roller  65  by transporting the developer  66  contained in the developer chamber  67  while mixing the developer  66 . A mixing/transporting auger  70  is disposed at a back side of the auger  69 . Each auger  70  serves as a transporting member that transports the developer  66  contained in the corresponding developer chamber  67  while mixing the developer  66 . The direction of transport of the developer  66  by each mixing/supplying auger  69  and the direction of transport of the developer  66  by each auger  70  are set in opposite directions. The augers  69  and  70  allow the developer  66  to pass so as to transport the developer  66  through paths  71  and  72  that open at respective ends of the corresponding partition wall  68  in a longitudinal direction thereof, to circulate the developer  66  while mixing the developer  66 . 
     As shown in  FIG. 9 , a toner density sensor  73  is provided near a downstream end portion of each auger  70  along an axial direction thereof at a bottom portion of the developer chamber  67  in the corresponding developing-device housing  64 . Each toner density sensor  73  is, for example, a permeability sensor that detects the density of the toner of the developer  66  contained in the corresponding developer chamber  67 . 
     As shown in  FIG. 10 , an end portion  69   a  of each auger  60  in a longitudinal direction thereof and an end portion  70   a  of each auger  70  in a longitudinal direction thereof extend so as to protrude beyond the corresponding developing roller  65 . The toners T of the corresponding colors are such as to drop and to be supplied from the corresponding developer supplying devices  50 Y,  50 M,  50 C, and  50 K to the end portions of the extending portions  69   a  and  70   a  of the respective augers  69  and  70 . 
     As shown in  FIG. 9 , a cover for the extending portions  69   a  and  70   a  cover the extending portions  69   a  and  70   a  of the corresponding augers  69  and  70 . In addition, as shown in  FIG. 9 , a toner receiving opening  74  opens in an upper end surface of each cover. Each toner receiving opening  74  receives the toner T that has dropped and that has been supplied from the corresponding one of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K through the drop path  57 . The toner T that has been received from the corresponding toner receiving opening  74  is primarily transported into the corresponding developing-device housing  64  along an axial direction by the corresponding mixing/transporting auger  70 , is transported while being mixed with the developer  66  contained in the corresponding developer chamber  67 , and is supplied to the developing roller  65  by its corresponding auger  69  in order to be used for development. 
     Each developing roller  65 , each mixing/supplying auger  69 , and each mixing/transporting auger  70  are rotationally driven by a drive motor (not shown) at a speed corresponding to a process speed. This causes the developer  66  contained in the developer chamber  67  of the corresponding developing-device housing  64  to be transported while being mixed, so that the electrostatic latent image formed on the surface of the corresponding photoconductor drum  15  by its corresponding developing roller  65  is developed. 
     In the image forming apparatus having the above-described structure, as shown in  FIG. 1 , the toners in the corresponding developing devices  17 Y,  17 M,  17 C, and  17 K are gradually consumed as the electrostatic latent images formed on the surfaces of the photoconductor drums  15 Y,  15 M,  15 C, and  15 K of the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K) are developed with the toners of the corresponding colors by the corresponding developing devices  17 Y,  17 M,  17 C, and  17 K. 
     When the toner densities of the developers  66  contained in the corresponding developer chambers  67  are detected by the corresponding toner density sensors  73 , and the toner densities in the corresponding developing devices  17 Y,  17 M,  17 C, and  17 K become lower than a preset threshold value, the developer supplying devices  50 Y,  50 M,  50 C, and  50 K supply the toners T of the corresponding colors to the corresponding developing devices  17 Y,  17 M,  17 C, and  17 K at a predetermined timing, such as after completion of the series of image forming operations or directly after forming images on a predetermined number of pieces of recording paper  34 . Toner replenishment is performed when necessary when forming images. 
     The supplying operations of the toners T performed by the corresponding developer supplying devices  50 Y,  50 M,  50 C, and  50 K are executed by rotationally driving the agitators  53  in the toner cartridges  51 Y,  51 M,  51 C, and  51 K by the corresponding cartridge motors  54 Y,  54 M,  54 C, and  54 K as shown in  FIG. 4 , and by rotationally driving at a predetermined constant speed the two agitators  59  and  60  and the auger  61  of each of the developer storing devices  55 Y,  55 M,  55 C, and  55 K by the corresponding one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K as shown in  FIGS. 5 and 6 . 
     As shown in  FIGS. 8 to 10 , the developing devices  17 Y,  17 M,  17 C, and  17 K to which the toners T are supplied from the developer supplying devices  50 Y,  50 M,  50 C, and  50 K are driven at a speed corresponding to the speed of the image forming operation, and the toners T supplied from the developer supplying devices  50 Y,  50 M,  50 C, and  50 K are transported into the corresponding developer chambers  67  by the corresponding mixing/supplying augers  69  and the corresponding mixing/transporting augers  70 . In addition, as shown in  FIGS. 8 to 10 , the toners T are transported while being mixed by the corresponding mixing/supplying augers  69  and the corresponding mixing/transporting augers  70 , so that the supplied toners T are frictionally electrified by being mixed with the developers  66  in the corresponding developer chambers  67 . 
     Accordingly, in each of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K, the toner T is supplied by the two agitators  59  and  60  and the auger  61  that are rotationally driven at a constant speed regardless of the process speed of the image forming apparatus, whereas, in each of the developing devices  17 Y,  17 M,  17 C, and  17 K, the mixing/supplying augur  69  and the mixing/transporting auger  70  are rotationally driven at a driving speed that is switched to more than one speed in accordance with the process speed of the image forming apparatus, so that the mixing and transport of the developer  66  including the toner T are executed. 
     Therefore, in the image forming apparatus, in the case in which an image forming operation is executed at a process speed that is less than 308 mm/s (which is the highest process speed), such as 200 mm/s (which is approximately ⅔ of 308 mm/s or the third highest speed) or 103 mm/s (which is approximately ⅓ of 308 mm/s or the lowest speed), when the toner T is supplied to any one of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K, the following may occur. That is, as shown in  FIG. 5 , the toner T may accumulate at, for example, a lower end of the drop path  57 , to which the toner T drops and is supplied from the any one of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K to the corresponding one of the developing devices  17 Y,  17 M,  17 C, and  17 K, when the capacity of supplying the toner T by the two agitators  59  and  60  and the auger  61  of the any one of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K becomes greater than the capacity of transporting the developer by the mixing/supplying auger and the mixing/transporting auger of the corresponding developing device  17 . 
     When the toner T accumulates in the drop path  57  that allows the toner T to drop and to be supplied to the corresponding one of the developing devices  17 Y,  17 M,  17 C, and  17 K from the corresponding one of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K, for example, the load of accumulated toner T causes excess toner T and developer  66  to adhere to the mixing/supplying auger  69  and the mixing/transporting auger  70  of the corresponding one of the developing devices  17 Y,  17 M,  17 C, and  17 K, thereby causing mixing failure and improper transport of the developer  66  and toner T to clog the drop path  57 . Therefore, developer density may be reduced because toner is not supplied to the corresponding one of the developing devices  17 Y,  17 M,  17 C, and  17 K. 
     In the exemplary embodiment, the image forming apparatus includes a determining unit and a controller. The determining unit determines whether or not an operation where supplying capacities of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K are greater than developer transport capacities of the developing devices  17 Y,  17 M,  17 C, and  17 K exceeds a predetermined threshold value and is continued. The controller performs control so that, when the determining unit determines that the operation exceeds the predetermined threshold value and is continued, an operation that was being executed immediately prior to the determination is stopped to forcefully drive the mixing/supplying auger and the mixing/transporting auger of the corresponding one of the developing devices  17 Y,  17 M,  17 C, and  17 K for a predetermined driving time. 
       FIG. 11  is a block diagram of a control circuit of the image forming apparatus. 
     In  FIG. 11 , reference numeral  100  denotes a central processing unit (CPU) that controls the operation of the entire image forming apparatus and that functions as the determining unit and the controller. The CPU  100  functions as the determining unit and the controller and controls the operation of the entire image forming apparatus while reading, for example, parameters, stored in RAM  102  (such a nonvolatile random-access memory (NVRAM)), as appropriate, on the basis of a program previously stored in ROM  101 . 
     As shown in  FIG. 11 , output signals from the toner density sensors, provided at the developing devices  17 Y,  17 M,  17 C, and  17 K of the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K), are input to the CPU  100 . Driving signals for driving the cartridge motors  54 Y,  54 M,  54 C, and  54 K, provided at the toner cartridges  51 Y,  51 M,  51 C, and  51 K of the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K), are output from the CPU  100  through a drive circuit (not shown). In addition, as shown in  FIG. 11 , driving signals for driving the toner supply motors  62 Y,  62 M,  62 C, and  62 K, provided at the developer storing devices  55 Y,  55 M,  55 C, and  55 K of the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K, are output from the CPU  100  through the drive circuit. 
     In the above-described structure, by performing the following, the image forming apparatus according to the exemplary embodiment can suppress developer clogs caused by the continuation of the operation where the supply capacities of the developer supplying units exceed the transport capacities of the developing units. 
     That is, in the image forming apparatus, as shown in  FIG. 2 , toner images of corresponding colors are formed on the photoconductor drums  15 Y,  15 M,  15 C, and  15 K of the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K). After the toner images of the corresponding colors formed on the photoconductor drums  15  of the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K have been first-transferred in a superposed state to the intermediate transfer belt  25 , the toner images are second-transferred collectively to the recording paper  34  from the intermediate transfer belt  25  at the second transfer position. 
     As shown in  FIG. 2 , the recording paper  34  to which the toner images of the corresponding colors, yellow (Y), magenta (M), cyan (C), and black (K), have been second-transferred collectively are heated and pressed by the fixing device  37  to fix the unfixed toner images, after which the recording paper  34  is discharged onto the discharge tray  40 , provided at the outer portion of the body  1  of the image forming apparatus. 
     In the image forming apparatus, the following control is performed when the above-described image forming operations are performed. 
     First, as shown in  FIG. 12 , the CPU  100  determines whether or not the setting is that for executing a toner clog prevention mode in Step S 101 . When the setting is that for not executing the toner clog prevention mode, the process immediately ends, whereas, when the setting is for executing the toner clog prevention mode, the CPU  100  determines whether or not the process speed that is set in an image forming operation that is being executed is a speed at which a toner clog occurs (which is a process speed stored in RAM  102 ) in Step S 102 . As the speed at which a toner clog occurs, for example, a speed other than 308 mm/s (which is the highest process speed), that is, 255 mm/s, 200 mm/s, or 103 mm/s is set. However, the speed is not limited thereto. For example, a speed other than 308 mm/s (which is the highest process speed) and 255 mm/s (which is the next highest process speed), that is, 200 mm/s or 103 mm/s may be set. 
     As shown in  FIG. 11 , when the CPU  100  determines that the process speed that is set is a speed at which a toner clog occurs, that is, a process speed other than 308 mm/s (which is the highest process speed), that is, any one of 255 mm/s, 200 mm/s, and 103 mm/s, the CPU  100  cumulatively counts the rotation times of the toner supply motors  62 Y,  62 M,  62 C, and  62 K in Step S 103 , and determines whether or not the accumulated rotation time of any one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K is greater than or equal to a threshold value, stored in RAM  102 , in Step S 104 . When the accumulated rotation time of any one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K is less than the threshold value stored in RAM  102 , the process returns to Step S 103 . Alternatively, when the accumulated rotation time of any one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K is less than the threshold value stored in RAM  102 , the process may return to Step S 101 . 
     In contrast, in the CPU  100 , as shown in  FIG. 12 , when the accumulated rotation time of any one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K is greater than or equal to the threshold value stored in RAM  102 , a printing operation is stopped and a cycle-down operation is executed in Step S 105 , after which the process speed is switched to 308 mm/s (which is the highest process speed), to execute a toner clog prevention operation. As the toner clog prevention operation, for example, as shown in  FIG. 13 , a cycle-up operation is executed, and a toner breakage operation and a density adjustment operation in a process control operation are executed. 
     As an operation of reducing toner density in the process control operation, for example, as shown in  FIG. 13 , in the image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K), uniform halftone images (having a density of, for example, 10%) are formed on, for example, a predetermined number of pieces of A4-size recording paper  34  (such as approximately 20 pieces of recording paper  34 ), to forcefully consume the toner T that has been supplied to the developing device  17  up to this time. Here, the supply of toner to the developing device  17  is prohibited. The operation of forcefully consuming the toner T may only be performed on the developing device  17  where the accumulated rotation time of the corresponding one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K is determined as being greater than or equal to a set value that is stored in RAM  102 , or on more than one of the developing devices  17  where the accumulated rotation times of the corresponding toner supply motors are determined as being greater than or equal to the set value that is stored in RAM  102 . 
     In the toner clog prevention operation, when necessary, it is determined whether or not the operation of reducing the toner density of the process control operation has been executed for a set number of times that is stored in RAM  102 . When the operation has not been performed for the set number of times, the image forming apparatus waits until the operation is performed for the set number of times, after which idle rotation is executed. Idle rotation is performed for maintaining the toner densities in the developing devices  17  at proper values. The idle rotation is performed while forming uniform halftone images (having a density of, for example, 10%) on, for example, a predetermined number of pieces of A4-size recording paper  34  (such as approximately 20 pieces of recording paper  34 ) while supplying toner under ordinary conditions to the developing devices  17  in the corresponding image forming sections  13 Y,  13 M,  13 C, and  13 K for yellow (Y), magenta (M), cyan (C), and black (K) as shown in  FIG. 13 . 
     Thereafter, in the toner clog prevention operation, it is determined whether or not the idle rotation has been executed for the set number of times that is stored in RAM  102 . When the idle rotation has not been executed for the set number of times, the image forming apparatus waits until the idle rotation is performed for the set number of times, and cumulatively counts how many times these operations for the corresponding colors have been executed. 
     Next, as shown in  FIG. 13 , in the toner clog prevention operation, a mixing operation is executed in a corresponding one of the developing devices  17 . 
     Thereafter, as shown in  FIG. 12 , the CPU  100  causes count values of the accumulated rotation times of the corresponding toner supply motors  62 Y,  62 M,  62 C, and  62 K to be reset in Step S 107 , and causes a cycle-down operation to be executed in Step S 108 . As shown in  FIG. 13 , in Step S 109 , printing is continued after a cycle-up operation. 
     In contrast, as shown in  FIG. 12 , when, in Step S 102 , the CPU  100  determines that the process speed that is set is not the speed at which a toner clog occurs, the CPU  100  determines whether or not the process speed that is set is the speed at which the toner clog prevention operation is executed in Step S 110 . When the CPU  100  determines that the process speed that is set is not the speed at which the toner clog prevention operation is executed, the process returns to Step S 101 . 
     When the CPU  100  determines that the process speed that is set is the speed at which the toner clog prevention operation is executed, the CPU  100  cumulatively counts the number of pieces of recording paper  34  on which images have been printed, after conversion to the number of pieces of A4 LEF recording paper at 308 mm/s (which is the process speed that is set) in Step S 111 . 
     The CPU  100  determines whether or not a value obtained by cumulatively counting the number of pieces of recording paper  34  on which the images have been printed is greater than or equal to a previously stored threshold value in Step S 112 . When the CPU  100  determines that the value is not greater than or equal to the previously stored threshold value, the process returns to Step S 101 . In contrast, when the CPU  100  determines that the value is greater than or equal to the previously stored threshold value, the count values of the accumulated use of the number of pieces of recording paper  34  as the operation is performed are reset in Step S 113 . 
     In the exemplary embodiment, as shown in  FIG. 12 , the CPU  100  determines whether or not the process speed is, for example, other than 308 mm/s (which is the highest speed). When the CPU  100  determines that the process speed is, for example, other than 308 mm/s (which is the highest speed), and that the accumulated rotation time of any one of the toner supply motors  62 Y,  62 M,  62 C, and  62 K is greater than or equal to the threshold value stored in RAM  102 , the CPU  100  causes the printing to be stopped and to execute the operation of forcefully consuming the toner in the corresponding developing device  17 . This makes it possible to suppress or prevent, for example, a reduction in image density caused by a toner clog, or a mixing failure or an improper transport of the developers  66  when the capacities of supplying the toners T by the developer supplying devices  50 Y,  50 M,  50 C, and  50 K become greater than the capacities of transporting the developers by the developing devices  17 . 
     Second Exemplary Embodiment 
       FIG. 14  illustrates a second exemplary embodiment of the present invention. Portions corresponding to those of the previous exemplary embodiment will be given the same reference numerals. In the second exemplary embodiment, the determining unit is formed so that, when the developer density in the developing unit immediately after supplying the developer to the corresponding one of the developing units from the corresponding one of the developer supplying units is less than a predetermined density, the determining unit determines that an operation where the supplying capacity of the developer supplying unit is greater than the developer transport capacity of the developing unit exceeds a predetermined threshold value and is continued. 
     That is, in the second exemplary embodiment, as shown in  FIG. 14 , the CPU  100  determines whether or not the setting is that for executing a toner clog prevention mode in Step S 101 . When the setting is that for not executing the toner clog prevention mode, the process immediately ends, whereas, when the setting is for executing the toner clog prevention mode, the CPU  100  determines whether or not the process speed that is set in an image forming operation that is being executed is a speed at which a toner clog occurs (which is a process speed stored in RAM  102 ) in Step S 102 . Here, as the speed at which a toner clog occurs, for example, a speed other than 308 mm/s (which is the highest process speed), that is, 255 mm/s, 200 mm/s, or 103 mm/s is set. However, the speed is not limited thereto. For example, a speed other than 308 mm/s (which is the highest process speed) and 255 mm/s (which is the next highest process speed), that is, 200 mm/s or 103 mm/s may be set. 
     As shown in  FIG. 14 , when the CPU  100  determines that the process speed that is set is a speed at which a toner clog occurs, that is, a process speed other than 308 mm/s (which is the highest process speed), that is, any one of 255 mm/s, 200 mm/s, and 103 mm/s, the CPU  100  determines whether or not a timing is a toner replenishment timing in Step S 120 . When the CPU  100  determines that the timing is the toner replenishment timing, a toner replenishment operation is executed in Step S 121 . 
     As shown in  FIG. 14 , the CPU  100  determines whether or not the toner density in the developing device  17  at which the toner replenishment operation is executed is less than a specified value that is previously stored in RAM  102  in Step S 122 . When the CPU  100  determines that the toner density is greater than or equal to the specified value that is previously stored in RAM  102 , the process immediately ends. 
     In contrast, when the CPU  100  determines that the toner density is less than the specified value that is previously stored in RAM  102 , as in the first exemplary embodiment, a printing operation is stopped and a cycle-down operation is executed in Step S 105 . Subsequently to this, the operations from Steps S 106  to S 109  excluding Step S 107  are executed. 
     In the second exemplary embodiment, when, after executing the toner replenishment operation, the CPU  100  determines that the toner density is less that the specified value that is previously stored in RAM  102 , the CPU  100  determines that, for example, toner is clogging the toner supply path, and causes an operation that does not forcefully consume the toner to be executed. This makes it possible to suppress or prevent, for example, a reduction in image density caused by a toner clog, or a mixing failure or an improper transport of the developers  66  when the capacity of supplying the toner T by any one of the developer supplying devices  50 Y,  50 M,  50 C, and  50 K becomes greater than the capacity of transporting the developer  66  by the corresponding one of the developing devices  17 . 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.