Patent Publication Number: US-2010124442-A1

Title: Developing device, developing method, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the priority of U.S. Provisional Application No. 61/115,180, filed on Nov. 17, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus of an electrophotographic recording system that superimposes toners of plural colors one on top of another to obtain a color image, and, more particularly to an improvement of a developing device. 
     BACKGROUND 
     In general, in an image forming apparatus of an electrophotographic recording system, plural photoconductive drums are arranged in parallel and laser beams are irradiated on the respective photoconductive drums to form electrostatic latent images. The photoconductive drums have toner images of respective colors formed by developing devices and multiply transfer the toner images of the respective colors onto sheet paper to obtain a color image. 
     The developing devices are respectively provided for the photoconductive drums. Plural toner cartridges are arranged to supply toners to the developing devices. The toners stored in the toner cartridges are carried to the developing devices. The developing devices include developing rollers for shifting the toners to the photoconductive drums and mixers that agitate the toners and carriers. The developing rollers and the mixers are rotated by motors. 
     A developing device using a two-component developer including a toner and a carrier has advantages such as stability of an image quality and durability of the device. However, since the developer is deteriorated, necessary to supply the developer to the developing device. Also necessary to discharge an excess developer according to the supply of the developer. 
     JP-B-2-21591 discloses a developing device including agitating means for agitating a carrier and a toner. In JP-B-2-21591, during toner supply, a developer as a mixture of a new toner and a new carrier is supplied into the developing device, an excess developer is caused to overflow from a discharge port, and a deteriorated developer is replaced with the new toner and the new carrier. However, the developer that does not need to be discharged is splashed by a mixer and discharged from the discharge port. 
     JP-A-2000-112238 discloses a developing device in which a member for preventing scattering of a developer is provided to be opposed to a discharge port for discharging an excess developer. However, since the developer is unnecessarily heaped and the developer that does not need to be discharged is discharged, stable discharge cannot be performed. 
     SUMMARY 
     According to an aspect of the present invention, there is provided a developing device including: 
     a developing roller configured to shift a toner onto the surface of an image bearing member; 
     a container configured to store a developer and have an agitating passage for supplying the developer to the developing roller, the agitating passage having a first wall surface and a second wall surface opposed to each other; 
     a rotatable agitating member configured to carry the developer along the agitating passage while agitating the developer; 
     a discharge port configured to be provided in one of the first wall surface and the second wall surface and discharge an excess developer involved in the supply of the developer; and 
     a heaping member configured to heap the developer relatively to the discharge port according to the agitation of the agitating member, wherein 
     the discharge port is located in a direction in which the developer is scraped down according to the agitation of the agitating member. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of an image forming apparatus according to an embodiment; 
         FIG. 2  is an enlarged view of the internal structure of the image forming apparatus; 
         FIG. 3  is a front view of a developing device according to the embodiment; 
         FIG. 4A  is a plan view of the developing device shown in  FIG. 3 ; 
         FIG. 4B  is a partially enlarged view of the developing device shown in  FIG. 3 ; 
         FIG. 5  is a side view of the developing device shown in  FIG. 3 ; 
         FIG. 6  is an evaluation table of density unevenness and a spill of a developer with respect to a developer amount; 
         FIG. 7  is a diagram for explaining fluctuation height during discharge of the developer; 
         FIG. 8  is a characteristic chart of a rate of increase of the developer with respect to the fluctuation height of the developer; 
         FIG. 9  is a front view of a developing device having structure for scraping up the developer; 
         FIG. 10  is a plan view of the developing device shown in  FIG. 9 ; 
         FIG. 11  is a side view of the developing device shown in  FIG. 9 ; 
         FIG. 12  is a characteristic chart of the fluctuation height of the developer in a scraping-down structure and a scraping-up structure; 
         FIGS. 13A to 13E  are diagrams for explaining the movement of the developer in the scraping-down structure; 
         FIGS. 14A and 14B  are diagrams for explaining the movement of the developer in the scraping-up structure; 
         FIG. 15  is a diagram for explaining an angle of repose of the developer; 
         FIG. 16  is a characteristic chart of the fluctuation height of the developer according to a difference in the angle of repose; 
         FIGS. 17A to 17E  are diagrams for explaining the movement of a developer having a high angle of repose; 
         FIG. 18  is a plan view of a developing device in which a developing roller and a mixer rotate in “with” directions; 
         FIG. 19  is a front view of the developing device shown in  FIG. 18 ; 
         FIG. 20  is an evaluation table of evaluation of printing by rotation in “against” directions and the “with” directions; 
         FIG. 21  is a characteristic chart of the fluctuation height of the developer with respect to a change in a pitch of a screw blade of a heaping member; and 
         FIG. 22  is a characteristic chart of the fluctuation height of the developer with respect to a change in the height of the screw blade of the heaping member. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout this description, the embodiment and example shown should be considered exemplars, rather than limitations on the apparatus of the present invention. 
     An image forming apparatus according to an embodiment of the present invention is explained in detail below with reference to the accompanying drawings. In the drawings, the same components are denoted by the same reference numerals and signs. 
       FIG. 1  is a front view of the image forming apparatus according to the embodiment. In  FIG. 1 , reference numeral  100  denotes the image forming apparatus, which is, for example, a MFP (Multi-Function Peripheral) as a machine having multiple functions, a printer, or a copying machine. In the following explanation, the MFP is explained as an example. 
     A document table is provided in an upper part of a main body  11  of the MFP  100 . An auto document feeder (ADF)  12  is openably and closably provided on the document table. An operation panel  13  is provided in the upper part of the main body  11 . The operation panel  13  includes an operation unit  14  including various keys and a display unit  15  of a touch panel type. 
     A scanner unit  16  is provided below the ADF  12  in the main body  11 . The scanner unit  16  reads a document fed by the ADF  12  or a document placed on the document table and generates image data. A printer unit  17  is provided in the center in the main body  11 . Plural cassettes  18  that store sheets of various sizes are provided in a lower part of the main body  11 . 
     The printer unit  17  includes photoconductive drums, lasers, and the like. The printer unit  17  processes the image data read by the scanner unit  16  or image data created by a PC (Personal Computer) or the like to form an image on a sheet. 
     The sheet having the image formed by the printer unit  17  is discharged to a paper discharge unit  40 . The printer unit  17  is, for example, a tandem color laser printer. The printer unit  17  scans a photoconductive member with a laser beam from a laser exposing device  19  and generates an image on the photoconductive member. 
     The printer unit  17  includes image forming units  20 K,  20 Y,  20 M, and  20 C for respective colors of black (K), yellow (Y), magenta (M), and cyan (C). The image forming units  20 K,  20 Y,  20 M, and  20 C are arranged in parallel on the lower side of an intermediate transfer belt  21  from an upstream side to a downstream side. 
     Since the image forming units  20 K,  20 Y,  20 M, and  20 C have the same configuration, the image forming unit  20 K is explained as a representative image forming unit. The configuration of the image forming unit  20 K is shown in enlargement in  FIG. 2 . 
     In  FIG. 2 , the image forming unit  20 K includes a photoconductive drum  22 K as an image bearing member. An electrifying charger  23 K, a developing device  50 K including a developing roller  24 K, a primary transfer roller  25 K, a cleaner  26 K, a blade  27 K, and the like are arranged around the photoconductive drum  22 K along a rotating direction t. The laser exposing device  19  irradiates a black laser beam on an exposing position of the photoconductive drum  22 K to form an electrostatic latent image on the photoconductive drum  22 K. 
     The electrifying charger  23 K of the image forming unit  20 K uniformly charges the entire surface of the photoconductive drum  22 K. The developing device  50 K includes mixers (explained later) that agitate a developer and the developing roller  24 K to which developing bias is applied. The developing device  50 K supplies, with the developing roller  24 K, a two-component developer including a toner and a carrier to the photoconductive drum  22 K. The cleaner  26 K removes a residual toner on the surface of the photoconductive drum  22 K using the blade  27 K. 
     As shown in  FIG. 1 , a developer cartridge  28  that supplies developers to the developing devices  50 K,  50 Y,  50 M, and  50 C is provided above the image forming units  20 K,  20 Y,  20 M, and  20 C. In the developer cartridge  28 , developer cartridges  28 K,  28 Y,  28 M, and  28 C for the respective colors of black (K), yellow (Y), magenta (M), and cyan (C) are adjacent to one another. 
     Toner hoppers  54 K,  54 Y,  54 M, and  54 C that supply the developers are arranged between the developer cartridges  28 K,  28 Y,  28 M, and  28 C and the developing devices  50 K,  50 Y,  50 M, and  50 C. In  FIG. 1 , the specific configuration of the toner hoppers  54 K,  54 Y,  54 M, and  54 C is not shown. 
     The intermediate transfer belt  21  as a recording medium cyclically moves. For example, semi-conductive polyimide is used for the intermediate transfer belt  21  from the viewpoint of heat resistance and abrasion resistance. The intermediate transfer belt  21  is stretched and suspended around a driving roller  31  and driven rollers  32  and  33 . The intermediate transfer belt  21  is opposed to and set in contact with the photoconductive drums  22 K to  22 C. 
     The primary transfer roller  25 K applies primary transfer voltage to a position of the intermediate transfer belt  21  opposed to the photoconductive drum  22 K and primarily transfers a toner image on the photoconductive drum  22 K onto the intermediate transfer belt  21 . 
     A secondary transfer roller  34  is arranged to be opposed to the driving roller  31  that stretches and suspends the intermediate transfer belt  21 . When a sheet S passes between the driving roller  31  and the secondary transfer roller  34 , the secondary transfer roller  34  applies secondary transfer voltage to the intermediate transfer belt  21  to secondarily transfer the toner image on the intermediate transfer belt  21  onto the sheet S. A belt cleaner  35  is provided near the driven roller  33  of the intermediate transfer belt  21 . 
     The laser exposing device  19  includes a polygon mirror  19   a , a focusing lens system  19   b , and a mirror  19   c . The laser exposing device  19  scans a laser beam, which is emitted from a semiconductor laser element, in an axis direction of the photoconductive drums  22 K to  22 C. 
     A separation roller  36  that extracts the sheet S in the paper feeding cassettes  18 , conveying rollers  37 , and registration rollers  38  are provided along a path extending from the paper feeding cassettes  18  to the secondary transfer roller  34 . A fixing device  39  is provided downstream of the secondary transfer roller  34 . 
     The paper discharge unit  40  and a reversing conveying path  41  are provided downstream of the fixing device  39 . A sheet from the fixing device  39  is discharged to the paper discharge unit  40 . The reversing conveying path  41  reverses the sheet S and guides the sheet S in the direction of the secondary transfer roller  34 . The reversing conveying path  91  is used when duplex printing is performed. 
     The operation of the image forming apparatus  100  shown in  FIGS. 1 and 2  is explained below. When image data is input from the scanner unit  16 , the PC, or the like, the image forming units  20 K to  20 C sequentially form images. 
     The image forming unit  20 K is explained as an example. The laser exposing device  19  irradiates a laser beam corresponding to image data of black (K) on the photoconductive drum  22 K to form an electrostatic latent image. The developing device  50 K develops the electrostatic latent image on the photoconductive drum  22 K to form a black (K) toner image. 
     The photoconductive drum  22 K comes into contact with the rotating intermediate transfer belt  21  and primarily transfers, with the primary transfer roller  25 K, the black (K) toner image onto the intermediate transfer belt  21 . After the photoconductive drum  22 K primarily transfers the toner image onto the intermediate transfer belt  21 , a residual toner on the photoconductive drum  22 K is removed by the cleaner  26 K and the blade  27 K. And possible to perform the next image formation. 
     In the same manner as the toner image forming process for black (K), the image forming units  20 Y to  20 C form toner images of yellow (Y), magenta (N), and cyan (C), sequentially transfer the toner images to a position same as the position of the yellow (Y) toner image on the intermediate transfer belt  21  and multiply transfer the yellow (Y), magenta (N), and cyan (C) toner images onto the intermediate transfer belt  21  to obtain a full color toner image. 
     The intermediate transfer belt  21  collectively secondarily transfers the full color toner image onto the sheet S with transfer bias of the secondary transfer roller  34 . In synchronization with the full color toner image on the intermediate transfer belt  21  reaching the secondary transfer roller  34 , the sheet S is fed from the paper feeding cassette  18  to the secondary transfer roller  34 . 
     The sheet S having the toner image secondarily transferred reaches the fixing device  39 . The toner image is fixed on the sheet S. The sheet S having the toner image fixed is discharged to the discharge unit  40 . On the other hand, after the secondary transfer ends, the belt cleaner  35  cleans a residual toner on the intermediate transfer belt  21 . 
     A developing device  50  representing the developing devices  50 K,  50 Y,  50 M, and  50 C is explained in detail below with reference to  FIGS. 3 to 5 .  FIG. 3  is a front view of the developing device  50 .  FIG. 4A  is a plan view of  FIG. 3  and  FIG. 4B  is a partial enlarged view of  FIG. 3 .  FIG. 5  is a side view of the developing device  50  viewed from an arrow X 1  direction in  FIG. 4A . A part of the developing device  50  is shown in section. 
     The developing devices  50 K,  50 Y,  50 M, and  50 C are provided to correspond to the developing rollers  24 K,  24 Y,  24 M, and  24 C. However, since the developing devices  50 K,  50 Y,  50 M, and  50 C, the developing rollers  24 K,  24 Y,  24 M, and  24 C, and other components respectively have the same configurations, the signs K, Y, M, and C are omitted in the following explanation. 
     In  FIG. 3 , the developing device  50  includes a developer container  51 . The developer container  51  is arranged substantially in parallel to the axis direction of the photoconductive drum  22 . The developing roller  24  is rotatably provided in the developer container  51 . 
     The developing roller  24  has a magnet in the inside and is also called magnet roller. The developing roller  24  is opposed to the photoconductive drum  22 . A carrier and a toner are carried on the surface of the developing roller  24 . The developing roller  24  rotates to feed the toner onto the photoconductive drum  22 . 
     The developer container  51  is partitioned into two spaces  531  and  532  by a partition plate  52 . The toner hopper  54  supplies a developer to one space  531 . 
     A first mixer  55  is provided in one space  531  of the developer container  51 . A second mixer  56  is provided in the other space  532 . The mixers  55  and  56  configure an agitating member that agitates and carries the developer (the toner and the carrier) in the developer container  51  and supplies the developer to the developing roller  24 . 
     As shown in  FIG. 4A , the mixers  55  and  56  have first screw blades  552  and  562  having a spiral shape attached to rotating shafts  551  and  561 . The mixers  55  and  56  agitate and carry the developer according to the rotation of the first screw blades  552  and  562 . As indicated by a thick like shown in  FIG. 4 , the developer in the developer container  51  is circulated to be carried from the front to the depth of the space  531  and carried in the counterclockwise direction from the depth to the front of the space  532 . The spaces  531  and  532  configure an agitating passage. The agitating passage has a first wall surface and a second wall surface opposed to each other. 
     A toner density sensor  57  ( FIG. 3 ) is provided in the space  531 . The toner density sensor  57  detects toner density of the developer agitated and carried by the mixer  55 . When the toner density detected by the toner density sensor  57  falls to be equal to or lower than a value set in advance, the developer including the toner and the carrier is supplied from the toner hopper  54 . 
     A discharge port  58  is provided in the developer container  51 . In  FIG. 5 , the developer is agitated and carried from the left to right. A discharge port is provided in a position indicated by a broken line  58 . An excess developer is discharged from the discharge port  58  by an overflow. A heaping member (a second screw blade  563 ) that heaps the developer relatively to the discharge port  58  is provided in a section of the mixer  56  opposed to the discharge port  58 . As shown in  FIG. 4B  in enlargement, the second screw blade  563  is provided coaxially with the first screw blade  562  and has a diameter and a pitch smaller than a diameter and a pitch of the first screw blade  562 . 
     In this embodiment, the developer is heaped and discharged from the discharge port  58 . A thick solid line  59  shown in  FIG. 5  indicates a developer surface. The developer surface  59  rises in the section of the discharge port  58  because the outer diameter of the second screw blade  563  of the mixer  56  is set small. Since an action for circulating and agitating the developer is reduced only in the section of the second screw blade  563 , the developer can be heaped. As shown in  FIGS. 3 and 5 , the discharge port  58  is arranged above the mixer  56 . 
     As shown in  FIG. 3 , the mixer  56  rotates in a direction (an arrow b 1 ) in which the mixer  56  scrapes down the developer from up to down near the discharge port  58 . Opposed surfaces of the developing roller  24  and the mixer  56  rotate in opposite directions (“against” directions) as indicated by arrows a 1  and b 1 . 
     As the shape of the second screw blade  563  near the discharge port  58  of the mixer  56 , a diameter and a pitch of the second screw blade  563  are changed from the diameter and the pitch of the first screw blade  562 . As shown in  FIG. 4A , when the radius (the height) of the first screw blade  562  is represented as H, the radius (height h 1 ) of the second screw blade  563  is smaller than H. As shown in  FIG. 5 , when the pitch of spirals of the first screw blade  562  is represented as P, a pitch p 1  of the second screw blade  563  is set smaller than the pitch P. 
     To obtain a satisfactory image necessary to stabilize the discharge of an excess developer and reduce fluctuation in a discharge amount. A method for stably discharging the excess developer is explained below. 
       FIG. 6  is a table of an evaluation result obtained by measuring an image state and a state of a spill of the developer when the developer is put in the developer container  51  with the discharge port  58  closed and a developer amount is gradually increased in order to check a satisfactory developer amount in the developer container  51 . 
     When a specified developer amount of the developer container  51  was set to 400 g and an image state and a state of a spill of the developer after taking one hundred copies of a photograph image at a printing ratio of 30% were observed, a result shown in  FIG. 6  was obtained. In  FIG. 6 , A indicates good, B indicates poor, and C indicates fair between good and poor. 
     As seen from  FIG. 6 , when the developer amount in the developer container  51  is equal to or smaller than 350 g, density unevenness occurs. When the developer amount is equal to or larger than 470 g, density unevenness due to spiral traces of the mixers  55  and  56  occurs. When the developer amount is equal to or larger than 450 g, a spill of the developer occurs. Therefore, the image state and the state of the spill are good when the developer amount is 360 g to 440 g, i.e., in a range of ±10% with respect to the specified developer amount (400 g). 
     Fluctuation height of the heap of the developer near the discharge port  58  is explained. The fluctuation height is calculated from a fluctuation state of the heap of the developer shown in  FIG. 7 . Specifically, when the heap height of the developer fluctuates from a minimum characteristic A 1  to a maximum characteristic A 2 , a difference of fluctuation (A 2 −A 1 ) is represented as fluctuation height HW. A characteristic A 0  is an intermediate characteristic between the characteristic A 1  and the characteristic A 2 . 
     A heap state of the developer was photographed by a video camera, photographed images were captured as still images, and the fluctuation height HW was calculated from a still image of a minimum heap state and a still image of a maximum heap state. 
       FIG. 8  is a graph of a result obtained by measuring a rate of increase of the developer (the ordinate) on the basis of the weight of the developer discharged from the discharge port  58  of the mixer  56  and verifying a relation between the rate of increase of the developer and the fluctuation height HW (the abscissa). The rate of increase of the developer fell as the fluctuation height HW decreased. At the fluctuation height HW equal to or smaller then 2 mm, the rate of increase of the developer fell to be equal to or lower than 10%. Stable discharge can be performed by reducing the fluctuation height HW. At the fluctuation height HW equal to or smaller than 2 mm, the rate of increase of the developer is equal to or lower than 10%. Therefore, a high-quality image without density unevenness and a developer spill can be stably output. The fluctuation height HW equal to or larger than 2 mm is undesirable because the rate of increase of the developer rises. 
     Rotating directions of the mixers  55  and  56  were verified.  FIGS. 9 to 11  are diagrams of the developing device  50  in which the arrangement positions of the mixers  55  and  56  are horizontally reversed and the position of the discharge port  58  is also changed to a position of the mixer  56  opposed to the second screw blade  563 . The position of the developing roller  24  is also changed to the right side as shown in  FIGS. 10 and 11 .  FIG. 11  is a side view of the developing device  50  viewed from an arrow X 2  direction in  FIG. 10 . A part of the developing device  50  is shown in section. 
     In the developing device  50  shown in  FIG. 9 , the rotating directions of the mixers  55  and  56  and the developing roller  24  are the same as shown in  FIG. 3 . Therefore, as shown in  FIG. 10 , the developer circulates in the clockwise direction from the mixer  55  to the mixer  56  as indicated by a thick arrow. The mixer  56  rotates in a direction (an arrow b 1 ) for scraping up the developer toward the discharge port  58  as shown in  FIG. 9 . The opposed surfaces of the developing roller  24  and the mixer  56  rotate in the “against” directions (arrows a 1  and b 1 ). 
     A result obtained by checking a relation of the rate of increase of the developer and the fluctuation height HW with respect to the rotating direction of the mixer  56  is shown in  FIG. 12 . 
     B 1  in  FIG. 12  indicates the rate of increase of the developer (the ordinate) and the fluctuation height HW (the abscissa) at the time when the mixer  56  is rotated in a scraping-down direction ( FIG. 3 ). B 2  indicates at the time when the mixer  56  is rotated in a scraping-up direction ( FIG. 9 ). 
     As seen from  FIG. 12 , the rate of increase of the developer with respect the fluctuation height HW is lower when the mixer  56  is rotated in the scraping-down direction. When the mixer  56  is rotated in the scraping-down direction, at the fluctuation height HW equal to or smaller than 2 mm, the rate of increase of the developer is equal to or lower than 10%. When the flow of the developer was observed, the following was found. 
       FIGS. 13A to 13E  are diagrams in which the movement of the developer surface  59  in the developer container  51  is simulatively shown plainly. The mixer  56  rotates in the counterclockwise direction and, near the discharge port  58 , rotates in the direction for scraping down the developer. The discharge port  58  is provided in the wall surface of the developer container  51  on a side on which the developer is scraped down. 
     In  FIG. 13A , an initial state of the developer surface  59  is shown, a state in which the developer does not start to be carried yet. If the developer surface  59  is in a substantially flat state, when the mixer  56  rotates in the counterclockwise direction, in  FIG. 13B , the developer surface  59  moves to the left while rising on the right side (the first wall surface side) of the developer container  51 . In  FIG. 13C , the developer surface  59  rises in the center and falls on the right side. In  FIG. 13D , the developer surface  59  moves to the left while rising on the left side (the second wall surface side). In  FIG. 13E , the developer surface  59  returns to the original state ( FIG. 13A ). The developer surface  59  repeats the states shown in  FIGS. 13A to 13D  and moves as if waves sweep toward the discharge port  58 . Therefore, the developer is discharged from the discharge port  58  by an overflow not only in a state in which the developer surface  59  rises but also in a state in which the developer is pushed out. 
     The movement of the developer surface  59  that occurs when the discharge port  58  is provided on a side on which the developer is scraped up as shown in  FIG. 9  is explained with reference to  FIGS. 14A and 14B . In  FIG. 9 , the developer near the discharge port  58  moves in the scraping-up direction. When the mixer  56  starts rotating in a state shown in  FIG. 14A , as shown in  FIG. 14B , the developer surface  59  moves in a direction opposite to the discharge port  58  (the left direction) while rising near the discharge port  58 . 
     The developer is discharged by an overflow from the discharge port  58 . However, since a rising portion moves to the opposite side of the discharge port  58 , unlike the state shown in  FIG. 13D , the developer is not pushed out. 
     Therefore, seen that the discharge of the developer is more stable when the discharge port  58  is provided on the side on which the developer is scraped down (the second wall surface) as shown in  FIG. 3 . 
     To check the influence of fluidity of the developer, fluidity was verified by using developers having different angles of repose. The angle of repose is explained with reference to  FIG. 15 . When the developer is dropped from a funnel  60 , the developer gradually piles up in an isosceles triangular mountain shape with an angle of repose α. When the angle of repose α reaches a certain inclination angle, the mountain collapses. The inclination angle at the limit of collapse is set as the angle of repose. The fluidity of the developer is poorer as the angle of repose is larger. 
     A result obtained by putting developers having different angles of repose in the mixer  56  shown in  FIGS. 3 to 5  and checking a relation between rates of increase of the developers and the fluctuation height HW is shown in  FIG. 16 . Developers having angles of repose of 35 degrees to 55 degrees were used. The developer having a larger angle of repose a has a higher rate of increase of the developer with respect to the fluctuation height HW. And also seen that the developer having the angle of repose of 55 degrees has a rate of increase of the developer exceeding 10%. 
     When the flow of the developer was observed, the following was found. That is confirmed that the developer surface  59  of the developer having the angle of repose of 40 degrees moved as shown in  FIGS. 13A to 13E . On the other hand, the developer surface  59  of the developer having the angle of repose of 55 degrees moved as shown in  FIGS. 17A to 17E . 
     The mixer  56  rotates counterclockwise as indicated by an arrow. The discharge port  58  is formed on the left wall surface such that the developer is scraped down. When the mixer  56  rotates, as shown in  FIG. 17B , the developer surface  59  moves to the left while rising on the right side. And, as shown in  FIG. 17C , the developer surface  59  rises in the center, falls on the right side, and moves to the left while rising on the left side. 
     The developer is discharged from the discharge port  58  by an overflow in a state in which the developer surface  59  rises and the developer is pushed out. However, when the flow of the developer was closely observed, confirmed that the developer flowed only in the periphery of the mixer  56  and hardly flowed near the wall surfaces of the developer agitating passage. 
     When the fluidity of the developer is deteriorated, the flow of the developer near the agitating passage worsens. Even in a state in which the rotation of the mixer  56  is stopped, as shown in  FIG. 17A  or  17 E, the developer surface  59  slightly rises near the agitating passage (near the wall surfaces) compared with the center of the mixer  56 . Specifically, although the developer surface  59  moves while rising on the left side as shown in  FIGS. 17B to 17D , the flow of pushing out the developer to the discharge port  58  is hindered compared with  FIGS. 13A to 13E . Therefore, the fluidity of the developer is deteriorated, the discharge of an excess developer worsens, and the developer increases. 
     The developer having the angle of repose of 45 degrees moved in the same manner as the developer having the angle of repose of 40 degrees. When the angle of repose was 50 degrees, the developer moved in an intermediate manner between the manners shown in  FIGS. 13 and 17 . 
     In order to check the rotating directions of the developing roller  24  and the mixer  56 , a situation of occurrence of image unevenness was verified by using the developing device  50  shown in  FIGS. 18 and 19 . In the developing device  50  shown in  FIGS. 18 and 19 , the mixers  55  and  56  arranged to be horizontally reversed from shown in  FIG. 4  and having a spiral direction different from that shown in  FIG. 4  are used. The rotating direction of the mixers  55  and  56  is different from that shown in  FIG. 4 . 
     In  FIG. 18 , the developer circulates in the clockwise direction as indicated by a thick arrow. As shown in  FIG. 19 , the mixer  56  rotates in a developer scraping-down direction (an arrow b 2 ). The opposed surfaces of the developing roller  24  and the mixer  56  rotate in circumferential directions coinciding with each other (“with” directions) as indicated by arrows a 1  and b 2 . 
     As verification, a degree of unevenness that occurs in images output by the developing device  50  shown in  FIGS. 3 to 5  and the developing device  50  shown in  FIGS. 18 and 19  was evaluated in terms of the number of sheets as a limit of occurrence of the unevenness. A verification result is shown in  FIG. 20 . 
     In  FIG. 20 , unevenness of images in the “against” directions and the “with” directions at a printing ratio of 5% and a printing ratio of 100% is shown. As seen from  FIG. 20 , at the printing ratio of 5%, no unevenness of images is seen in both the “against” directions and the “with” directions. However, at the printing ratio of 100%, although no unevenness of images occurred in the “against” directions until twenty sheets were printed, unevenness of images occurred in the “with” directions when the number of sheets exceeded ten. 
     From verification result, seen that unevenness of images less easily occurs when the developing roller  24  and the mixer  56  rotate in the “against” directions. According to the rotation in the “against” directions, the developer is easily accumulated on the developing roller  24  and the supply of the toner onto the developing roller  24  is stabilized. At the printing ratio of 100% with large toner consumption, when the developing roller  24  and the mixer  56  rotated in the “against” directions, the images were satisfactorily formed until sheets twice as many as printed when the developing roller  24  and the mixer  56  rotated in the “with” directions were printed. 
     In other words, in a copying machine mounted with the developing device  50 , when images having a high printing ratio are continuously output, toner supply is immediately required and convenience falls in a configuration in which the developing roller  24  and the mixer  56  rotate in the “with” directions. 
     In the developing device  50  shown in  FIGS. 4 and 5 , the fluctuation height HW was measured with the shape (the width, the diameter, and the pitch) of the second screw blade  563  changed. 
     First, the shape (the pitch, the diameter, and the width) of the second screw blade  563  is explained. When the pitch of the first screw blade  562  of the mixer  56  is represented as P as shown in  FIG. 5 , the fluctuation height HW was measured with the pitch p 1  of the second screw blade  563  set to ¼, 2/4, and ¾ with respect to the pitch P. 
     The diameter is the outer diameter (the blade height) of the second screw blade  563 . The fluctuation height HW was measured with the height h 1  of the second screw blade  563  set to ¼, 2/4, and ¾ with respect to the height H of the first screw blade  562 . 
     As shown in  FIG. 4B , the width is area width L 1  in the axis direction of the second screw blade  563 . When the width of the pitch of the first screw blade  562  is represented as L, the fluctuation height HW was measured with the area width L 1  of the second screw blade  563  set to ¼ to two times with respect to the width L. 
       FIG. 21  is a characteristic chart of the fluctuation height HW at the time when the area width L 1  of the second screw blade  563  (the abscissa) and the pitch p 1  of the second screw blade  563  are changed. The fluctuation height HW decreases as the width L 1  increases. And found that, when the width L 1  was larger than L/2 (=0.5 times) and the pitch p 1  was P* 2/4 and P*¼, the fluctuation height HW was smaller than 2 mm. Therefore seen that, if the width L 1  is larger than L/2 (=0.5) and the pitch p 1  is equal to or smaller than P*½, the fluctuation height HW can be held down to be equal to or smaller than 2 mm. 
       FIG. 22  is a characteristic chart of the fluctuation height HW at the time when the area width L 1  of the second screw blade  563  (the abscissa) and the height h 1  of the second screw blade  563  are changed. The fluctuation height HW decreases as the width L 1  increases. And found that, when the width L 1  was larger than L/2 (=0.5) and the blade height h 1  was H* 2/4 and H*¼, the fluctuation height HW was smaller than 2 mm. Therefore seen that, if the width L 1  is larger than L/2 (=0.5) and the blade height h 1  is equal to or smaller than H/2, the fluctuation height HW can be held down to be equal to or smaller than 2 mm. 
     Consequently, to reduce the fluctuation height HW to be equal to or smaller than 2 mm, desirable to set the area width L 1  of the second screw blade  563  to be equal to or larger than L/2 and set the pitch p 1  to be equal to or smaller than P/2 or set the width L 1  to be equal to or larger than L/2 and set the blade height h 1  to be equal to or smaller than H/2. 
     According to the embodiment explained above, possible to stably discharge the excess developer by reducing fluctuation in the heap of the developer near the discharge port  58 . 
     The present invention is not limited to the embodiment and various modifications of the embodiment are possible. For example, although the system employing the intermediate transfer belt  21  is explained above, a system not employing the intermediate transfer belt  21  may be adopted.