Patent Publication Number: US-8526863-B2

Title: Toner discharge mechanism, toner cartridge and image forming apparatus using the toner cartridge

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to Japanese patent application No. 2010-059731 filed on Mar. 16, 2010 whose priority is claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a toner discharge mechanism, a toner cartridge, and an image forming apparatus using the toner cartridge. 
     2. Description of the Related Art 
     A developing device using a two-component developer is structured such that, when a toner in the developing device has been consumed and reduced by image forming, a toner cartridge supplies the toner to the developing device so as to prevent a concentration of the toner of the developer from being less than or equal to a predetermined value. 
     For example, a toner cartridge is known which includes: a toner storage section for storing a toner to be supplied; a toner discharge outlet through which the toner is discharged toward a developing device; and a toner conveying screw (auger screw) for conveying the toner from the toner storage section to the toner discharge outlet (see, for example, Japanese Unexamined Patent Application No. 2006-235255). 
     Further, another toner discharge device is also known which includes: a toner conveying path formed in a cylindrical shape; an auger screw which is rotatably mounted in the toner conveying path, and a toner discharge outlet on a lowermost stream side in a toner conveying direction, wherein the auger screw includes a reversed helical blade (reversed spiral) on the lowermost stream side (see, for example, Japanese Unexamined Patent Application No. 2008-32769). 
     However, in a toner cartridge which allows a toner to be discharged through the toner discharge outlet by using the auger screw, a fluidity of the toner may be reduced due to, for example, the toner cartridge having been transported over a long distance or the toner cartridge having being left unused for a long period of time. Therefore, when driving of the auger screw is recommenced, the toner stored in the vicinity of the toner discharge outlet is not smoothly discharged through the toner discharge outlet. The toner which is accumulated on an inner wall surface (a wall surface for bearing the auger screw) in the lowermost stream portion of the toner cartridge, and the toner pushed to the downstream side in the toner conveying direction by the rotation of the auger screw cannot move anywhere, and are tightly packed, so that the auger screw stops rotating (by locking phenomenon). 
     Further, the use of the auger screw including the reversed helical blade on the lowermost stream side can prevent the toner from being accumulated on a surface (a wall surface for bearing the auger screw) in the lowermost stream portion of the toner cartridge. However, once the toner stored in the vicinity of the toner discharge outlet cannot be smoothly discharged through the toner discharge outlet, the toner pushed in the toner conveying downstream direction by the rotation of the auger screw, and the toner pushed back due to the opposite spiral by the rotation of the auger screw, hit against each other at one point. Therefore, a pressure rapidly rises, and the toner is tightly packed, resulting in occurrence of the locking phenomenon. That is, the problem that the locking phenomenon of the auger screw occurs is not solved. 
     SUMMARY OF THE INVENTION 
     The present invention is made in order to solve the aforementioned problems, and an object of the present invention is to provide a toner discharge mechanism and a toner cartridge which can prevent the tightly packed toner from causing occurrence of the locking of the auger screw, and an image forming apparatus using the toner cartridge. 
     The present invention provides a toner discharge mechanism which is attached to a toner cartridge comprising: 
     an auger screw for conveying and discharging a toner stored in the toner cartridge, the auger screw having an end portion downstream thereof, the end portion forming a taper portion having a predetermined length and an outer diameter diminishing toward a tip thereof; a cover member for covering the taper portion, the cover member having a conical inner wall configured to correspond to a taper shape of the taper portion; and a toner discharge outlet formed on the cover member. 
     According to the present invention, the tapered portion having an outer diameter reduced toward a tip thereof is formed in an end portion on a downstream side of the auger screw, and the end portion has a predetermined length. Further, the tapered portion is covered with a cover member having a conical inner wall which corresponds to a tapered shape of the tapered portion, and a toner discharge outlet is formed on the cover member. Therefore, in the cover member, an internal space in the vicinity of the toner discharge outlet is gradually reduced in the toner conveying direction, so that the conveyed toner is gradually pushed toward the toner discharge outlet, and is smoothly discharged. As a result, the toner conveyed toward the toner discharge outlet is slowly pushed to the toner discharge outlet, so that a tightly packed state of the toner which is caused by a rapid pressure change can be avoided, and occurrence of the locking phenomenon of the auger screw can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a structure of an image forming apparatus according to the present invention. 
         FIG. 2  is a perspective view of a toner cartridge according to the present invention. 
         FIGS. 3(   a )- 3 ( e ) are diagrams illustrating structures of the toner cartridge and a toner discharge mechanism according to the present invention. 
         FIG. 4  is a cross-sectional view of the toner cartridge and the toner discharge mechanism according to the present invention. 
         FIG. 5  is a cross-sectional view of the toner discharge mechanism according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The toner discharge mechanism according to the present invention has the features described below. 
     Specifically, the present invention is directed to a toner discharge mechanism which is mounted in a toner cartridge having an auger screw operable to convey through and discharge from the toner cartridge a toner stored therein, and the toner discharge mechanism includes a tapered portion having an outer diameter reduced toward a tip thereof, in an end portion on a downstream side in a direction in which the toner is conveyed by the auger screw, the end portion having a predetermined length. Further, in the toner discharge mechanism, the tapered portion is covered with a cover member having a conical inner wall which corresponds to a tapered shape of the tapered portion, and a toner discharge outlet is formed on the cover member. 
     Further, the toner cartridge according to the present invention has the features described below. 
     Specifically, the present invention is directed to a toner cartridge which has an auger screw operable to convey through and discharge from the toner cartridge a toner stored therein, and the toner cartridge includes a tapered portion having an outer diameter reduced toward a tip thereof, in an end portion on a downstream side in a direction in which the toner is conveyed by the auger screw, the end portion having a predetermined length. Further, in the toner cartridge, the tapered portion is covered with a cover member having a conical inner wall which corresponds to a tapered shape of the tapered portion, and a toner discharge outlet is formed on the cover member. 
     The auger screw has an elongated shaft (axis) around which a helical blade is wound, and rotates the shaft to convey the toner by using a contact pressure of the helical blade against the toner. The auger screw may be also referred to as a toner conveying screw. 
     In the toner discharge mechanism and the toner cartridge according to the present invention, a minimal value of an inner diameter of the conical inner wall portion is preferably greater than or equal to 1.1 times a diameter of a rotating shaft, and is not greater than 1.5 times the diameter of the rotating shaft. 
     When the minimal value of the inner diameter of the conical inner wall portion is excessively great, an effect of pushing out the toner through the toner discharge outlet is insufficient, whereas when the minimal value is excessively small, a friction and a shear force generated in the toner between the rotating shaft and the inner wall is increased, so that the toner having its temperature increased is likely to become agglomerated, resulting in the locking phenomenon of the auger screw being likely to occur. However, according to the present invention, the toner is smoothly discharged through the toner discharge outlet, and the temperature increase and agglomeration of the toner are prevented, thereby enhancing an effect of preventing occurrence of the locking of the auger screw. 
     Further, the minimal value of the inner diameter of the conical inner wall portion is preferably greater than or equal to 0.2 times a maximal value of the inner diameter of the conical inner wall portion, and is not greater than 0.5 times the maximal value of the inner diameter of the conical inner wall portion. 
     When the minimal value of the inner diameter of the conical inner wall portion is excessively small, the diameter of the rotating shaft needs to be reduced, so that the rigidity of the rotating shaft is reduced, and a friction heat is likely to be generated due to contact between the conical inner wall and the helical blade. On the other hand, when the minimal value is excessively great, an effect of pushing out the toner through the toner discharge outlet is insufficient. According to the present invention, the toner is smoothly discharged through the toner discharge outlet, and the temperature increase and agglomeration of the toner are prevented, thereby enhancing an effect of preventing occurrence of the locking of the auger screw. 
     Further, another aspect of the present invention is directed to an image forming apparatus which includes: a photoconductor drum; a charging device for charging a surface of the photoconductor drum; an exposure unit for forming an electrostatic latent image on the surface of the photoconductor drum; a developing device for supplying a toner to the electrostatic latent image on the surface of the photoconductor drum, to form a toner image; a toner cartridge for supplying the toner to the developing device; a transfer unit for transferring, to a recording medium, the toner image on the surface of the photoconductor drum; and a fuser unit for fusing, onto the recording medium, the toner image having been transferred, and the toner cartridge has the features described above. 
     According to the present invention, occurrence of the locking of the toner discharge member can be prevented, and a stable image concentration can be obtained. 
     Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings. 
     (Image Forming Apparatus) 
       FIG. 1  is a diagram illustrating a structure of an image forming apparatus according to the present invention. 
     As shown in  FIG. 1 , the image forming apparatus  100  includes: photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  having surfaces on which electrostatic latent images are formed; developing devices  102   a ,  102   b ,  102   c , and  102   d  for supplying a toner to the electrostatic latent images on the surfaces of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , respectively, to form toner images; toner cartridges  200  for supplying the toner through toner supply pipes  105   a ,  105   b ,  150   c , and  150   d  to the developing devices  102   a ,  102   b ,  102   c , and  102   d , respectively; a secondary transfer roller (transfer unit)  14  for transferring, onto a paper, a toner image of each of the surfaces of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  through an intermediate transfer belt  11 ; and a fuser unit  15  for fusing the toner image onto the paper. Each of the toner cartridges  200  includes a toner discharge mechanism of which details will be described later. The image forming apparatus  100  forms an image by using a toner in an electrophotographic method. 
     The image forming apparatus  100  forms, as a visible image, a multicolored image or a monochromatic image on a predetermined sheet (recording paper), based on image data contained in an input command, such as image data transmitted from the outside via a communication network and/or the like. 
     The image forming apparatus  100  includes: an exposure unit E; the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  which act as image holders on which latent images are formed by the exposure unit E; the developing devices  102   a ,  102   b ,  102   c , and  102   d ; charging rollers  103   a ,  103   b ,  103   c , and  103   d ; cleaning units  104   a ,  104   b ,  104   c , and  104   d ; the intermediate transfer belt  11 ; primary transfer rollers  13   a ,  13   b ,  13   c , and  13   d ; the secondary transfer roller  14 ; the fuser unit  15 ; paper conveying paths P 1 , P 2 , and P 3 ; a paper feed cassette  16 ; a manual bypass tray  17 ; a paper output tray  18 ; a toner cartridge unit  20 ; and the like, as shown in  FIG. 1 . 
     Image data of color images processed by the image forming apparatus  100  are image data corresponding to four hues of black (K), cyan (C), magenta (M), and yellow (Y) colors, and the image forming apparatus  100  forms visible images by using the image data corresponding to the four hues by means of image forming units  55   a ,  55   b ,  55   e , and  55   d.    
     The image forming units  55   a ,  55   b ,  55   c , and  55   d  process color images by using the respective colors. Therefore, the number of the developing devices  102   a ,  102   b ,  102   e , and  102   d  provided, the number of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  provided, the number of the charging rollers  103   a ,  103   b ,  103   c , and  103   d  provided, and the number of the cleaning units  104   a ,  104   b ,  104   c , and  104   d  provided are each four so as to form four kinds of latent images corresponding to the respective colors. 
     The image forming units  55   a ,  55   b ,  55   e , and  55   d  have the same structure therebetween. For example, the black image forming unit  55   a  includes: the photoconductor drum  101   a ; the developing device  102   a ; the charging roller  103   a ; the transfer roller  13   a ; the cleaning unit  104   a , and the like. 
     The image forming units  55   a ,  55   b ,  55   c , and  55   d  are aligned along the moving direction (secondary scanning direction) in which the intermediate transfer belt  11  moves. Reference numerals a, b, c, and d as described above represent black color, cyan color, magenta color, and yellow color, respectively. The components described above are classified so as to be represented by a corresponding one of the reference numerals, and form a corresponding one of the four image forming units  55   a ,  55   b ,  55   c , and  55   d.    
     The exposure unit E, which is an exposure device, includes a semiconductor laser (not shown), a polygon mirror  4 , first reflective mirrors  7 , second reflective mirrors  8 , and the like. The exposure unit E is operable to apply, to the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , laser beams which have been modulated based on image data corresponding to the hues of black, cyan, magenta, and yellow colors, respectively. 
     Electrostatic latent images based on the image data of the hues of the black, cyan, magenta, and yellow colors are formed on the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , respectively. 
     The exposure unit E which uses a laser scanning unit (LSU) including a laser application section and a reflective mirror is used is some cases. However, for example, the exposure unit which uses an EL (electroluminescence) device having light emitting devices aligned in an array, or which uses an LED write head, may be used. 
     The photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  are provided above the exposure unit E. Each of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  is formed as an image holder having almost a cylindrical shape, and is controlled so as to rotate in a predetermined direction by driving means and control means which are not shown. The photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  each have a photoconductive layer formed on a base component. 
     For example, each of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  is structured such that a metal drum formed of aluminium and/or the like is used as a base component, and a photoconductive layer formed of an amorphous silicon (a-Si), selenium (Se), an organic photo semiconductor (OPC), and/or the like, is formed as a thin film on the outer diameter surface of the base component. The structure of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  is not limited to this structure. 
     Each of the charging rollers  103   a ,  103   b ,  103   c , and  103   d  is a contact type charger which is operable to uniformly charge the surface of a corresponding one of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  at a predetermined potential. In the present embodiment, as shown in  FIG. 1 , contact roller type charging rollers  103   a ,  103   b ,  103   c , and  103   d  are used as chargers. However, a charger-type charging device or a brush-type charger may be used. 
     The developing devices  102   a ,  102   b ,  102   c , and  102   d  supply the toner onto the surfaces of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , respectively, on which the electrostatic latent images are formed, to develop the electrostatic latent images into toner images. The developing devices  102   a ,  102   b ,  102   c , and  102   d  store toners of the hues of the black, cyan, magenta, and yellow colors, respectively, and develop the electrostatic latent images of the hues formed on the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , into toner images of the hues of the black, cyan, magenta, and yellow colors, respectively, to form visible images. 
     The cleaning units  104   a ,  104   b ,  104   c , and  104   d  remove and collect toners which are left on the surfaces of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , respectively, by using a lubricant and/or the like, after the development and image transfer. 
     The intermediate transfer belt  11  is provided above the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , and is extended on a driving roller  11   a  and a follower roller  11   b  to form a loop-shaped moving path. The outer diameter surface of the intermediate transfer belt  11  is opposed to the photoconductor drum  101   d , the photoconductor drum  101   c , the photoconductor drum  101   b , and the photoconductor drum  101   a  in order, respectively. 
     The primary transfer rollers  13   a ,  13   b ,  13   c , and  13   d  are provided at positions opposing the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , respectively, so as to sandwich the intermediate transfer belt  11 . Positions at which the intermediate transfer belt  11  is opposed to the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  correspond to primary transfer positions, respectively. The intermediate transfer belt  11  is formed as an endless belt by using a film having a thickness ranging from about 100 μm to about 150 μm. 
     Primary transfer bias for transferring, onto the intermediate transfer belt  11 , the toner images held on the surfaces of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , are applied to the primary transfer rollers  13   a ,  13   b ,  13   c , and  13   d , respectively, by constant voltage control, and the primary transfer bias represents a polarity opposite to a polarity of a charged toner. Thus, the toner images of the hues formed on the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , respectively, are sequentially transferred and superimposed on the outer diameter surface of the intermediate transfer belt  11 , so that a full color toner image is formed on the outer diameter surface of the intermediate transfer belt  11 . 
     However, when image data of only one or some of the hues of the yellow, magenta, cyan, and black colors is inputted, the electrostatic latent image and the toner image are formed on only the photoconductor drum(s), corresponding to the hue(s) of the inputted image data, among the four photoconductor drums  101   a ,  101   b ,  101   c , and  101   d.    
     For example, when a monochromatic image is formed, the electrostatic latent image and the toner image are formed on only the photoconductor drum  101   a  corresponding to the hue of the black color, and only the black toner image is transferred onto the outer diameter surface of the intermediate transfer belt  11 . 
     Each of the primary transfer rollers  13   a ,  13   b ,  13   c , and  13   d  has a shaft made of a metal (for example, a stainless steel) having a diameter ranging from 8 mm to 10 mm, and the surface of the shaft is covered with a conductive elastic material (for example, an EPDM). The conductive elastic material enables a high voltage to be uniformly applied to the intermediate transfer belt  11 . Although in the present embodiment the primary transfer rollers  13   a ,  13   b ,  13   c , and  13   d  are used as transfer electrodes, brushes may be also used as the transfer electrodes. 
     The toner images having been transferred onto the outer diameter surface of the intermediate transfer belt  11  at the respective primary transfer positions are conveyed to a secondary transfer position opposing the secondary transfer roller  14 , by the rotation of the intermediate transfer belt  11 . When an image is formed, the secondary transfer roller  14  presses and contacts, at a predetermined nipping pressure, the outer diameter surface of the intermediate transfer belt  11  which has the inner diameter surface contacting the outer surface of the driving roller  11   a.    
     In order to constantly obtain the nipping pressure, one of the secondary transfer roller  14  or the intermediate transfer belt driving roller  11   a  is formed of a hard material such as a metal, and the other thereof is formed of a flexible material as an elastic roller or the like (an elastic rubber roller, a foamable resin roller, or the like). 
     When a paper fed from the paper feed cassette  16  or the manual bypass tray  17  passes between the secondary transfer roller  14  and the intermediate transfer belt  11 , a high voltage having a polarity (+) opposite to a polarity (−) of a charged toner is applied to the secondary transfer roller  14 . In the manners described above, the electrostatic latent images on the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  are developed into visible images by using toners corresponding to the hues, to form toner images, respectively. The toner images are transferred and superimposed on the intermediate transfer belt  11 . 
     The superimposed toner image is moved to a contact position at which a paper having been conveyed contacts the intermediate transfer belt  11 , by the rotation of the intermediate transfer belt  11 , and the toner image is transferred onto the paper from the outer diameter surface of the intermediate transfer belt  11  by means of the secondary transfer roller  14  located at the contact position. 
     The Toner which has not been transferred during the transfer of the toner image from the intermediate transfer belt  11  onto the paper, and has been left on the intermediate transfer belt  11 , is removed and collected by the intermediate transfer belt cleaning unit  12  because the toner may cause mixture of the toner colors in subsequent process steps. 
     For example, a cleaning blade is provided, as a cleaning member of the intermediate transfer belt cleaning unit  12 , so as to contact the intermediate transfer belt  11 . A portion of the intermediate transfer belt  11  which contacts the cleaning blade is supported by the intermediate transfer belt follower roller  11   b  from the reverse side of the intermediate transfer belt  11 . 
     A paper on which the toner image has been transferred as a visible image is guided by the fuser unit  15  including a heating roller  15   a  and a pressure-applying roller  15   b , so as to pass between the heating roller  15   a  and the pressure-applying roller  15   b , thereby subjecting the paper to heating and pressure-applying processes. Thus, the toner image is fixedly fused onto the surface of the paper as a visible image. The paper onto which the toner image has been fused is discharged onto the paper output tray  18  by means of the paper discharge roller  18   a.    
     The image forming apparatus  100  includes the paper conveying path P 1 , and the paper conveying path P 1  almost vertically extends so as to convey a paper stored in the paper feed cassette  16  to the paper output tray  18  by passing the paper between the secondary transfer roller  14  and the intermediate transfer belt  11 , and through the fuser unit  15 . 
     In the paper conveying path P 1 , a pickup roller  16   a  for feeding a paper stored in the paper feed cassette  16 , one by one, into the paper conveying path P 1 , a conveying roller r 10  for conveying the fed paper in the upstream direction, a resist roller  19  for guiding the conveyed paper so as to pass the paper between the secondary transfer roller  14  and the intermediate transfer belt  11  at a predetermined time, and the paper discharge roller  18   a  for discharging the paper onto the paper output tray  18 , are provided. 
     Moreover, in the image forming apparatus  100 , the paper conveying path P 2  is formed so as to extend from the manual bypass tray  17  to the resist roller  19 . In the paper conveying path P 2 , the pickup roller  17   a  and the conveying rollers r 10  are provided. Furthermore, the paper conveying path P 3  is formed so as to extend from the paper discharge roller  18   a  to the upstream portion of the paper conveying path P 1  above the resist roller  19 . 
     The paper discharge roller  18   a  is rotatable in both the forward direction and the opposite direction. In a one-surface image formation in which an image is formed on one surface of a paper, and in a second surface image formation of a both surfaces image formation in which images are formed on both surfaces of a paper, the paper discharge roller  18   a  is driven to rotate in the forward direction to discharge the paper onto the paper output tray  18 . 
     On the other hand, in a first surface image formation of the both surfaces image formation, the paper discharge roller  18   a  is driven to rotate in the forward direction until the back end edge of a paper has passed the fuser unit  15 , and thereafter the paper discharge roller  18   a  is driven to rotate in the opposite direction in a state where the back end portion of the paper is being held, to guide the paper into the paper conveying path P 3 . Thus, the paper on which an image has been formed only on one surface in the both surfaces image formation is guided into the paper conveying path P 1  with the front and the back surfaces being reversed and the head and the bottom being opposite. 
     The resist roller  19  guides the paper having been conveyed from the paper feed cassette  16  or the manual bypass tray  17 , or the paper having been conveyed through the paper conveying path P 3 , so as to pass the paper between the secondary transfer roller  14  and the intermediate transfer belt  11  in synchronization with the rotation of the intermediate transfer belt  11 . 
     Therefore, the resist roller  19  does not rotate when operations of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d , and the intermediate transfer belt  11  start. A paper which has been fed or conveyed prior to the rotation of the intermediate transfer belt  11  stops in the paper conveying path P 1  in a state where the head of the paper is in contact with the resist roller  19 . Thereafter, the resist roller  19  starts to rotate, at a time when the head portion of the paper is caused to oppose the front edge portion of the toner image formed on the intermediate transfer belt  11 , at a position at which the secondary transfer roller  14  and the intermediate transfer belt  11  press and contact each other. 
     When a full color image formation is performed in which all of the image forming units  55   a ,  55   b ,  55   c , and  55   d  form images, respectively, the primary transfer rollers  13   a ,  13   b ,  13   c , and  13   d  are operable to cause each of the photoconductor drums  101   a ,  101   b ,  101   c , and  101   d  and the intermediate transfer belt  11  to press and contact each other. On the other hand, when a monochromatic image formation is performed in which only the image forming unit  55   a  forms an image, only the primary transfer roller  13   a  is operable to cause the photoconductor drum  101   a  and the intermediate transfer belt  11  to press and contact each other. 
     (Toner Cartridge and Toner Discharge Mechanism) 
     Next, structures of the toner cartridge and the toner discharge mechanism according to the present embodiment will be described in detail with reference to the drawings. 
       FIG. 2  is a perspective view illustrating a structure of the toner cartridge unit  20  which includes the toner cartridges  200  and which is mounted to the image forming apparatus  100 .  FIG. 3(   a ) is a cross-sectional view of the toner cartridge  200 .  FIG. 3(   b ) is a cross-sectional view of the toner cartridge  200  taken along A-A′ of  FIG. 3(   a ).  FIG. 3(   c ) a cross-sectional view of the toner cartridge  200  taken along B-B′ of  FIG. 3(   a ).  FIG. 3(   d ) is a cross-sectional view of the toner cartridge  200  taken along C-C of  FIG. 3(   a ).  FIG. 3(   e ) is a cross-sectional view of the toner cartridge  200  taken along D-D′ of  FIG. 3(   a ).  FIG. 4  is a side view of an auger screw  202 .  FIG. 5  is a diagram illustrating a structure of the toner discharge mechanism. 
     As shown in  FIG. 2 , in the present embodiment, the number of the toner cartridges  200  is four, and the four toner cartridges  200  are aligned on the toner cartridge unit  20 . Each toner cartridge  200  is held such that the toner cartridge  200  is moved in the direction indicated by an arrow F, and is pressed against a stopper plate  20   b  by moving a locking lever  20   a  upward, as shown in  FIG. 2 . 
     Each toner cartridge  200  includes a toner container  201  which contains a toner; an auger screw  202 , and an agitating paddle  207 , as shown in  FIG. 3(   a ) and  FIG. 3(   b ). The toner container  201  includes a toner storage section  206  having an agitating paddle  207  mounted therein. 
     Further, the toner container  201  is a container which has a substantially prismatic shape, and stores the toner. The toner container  201  includes a toner discharge mechanism  204  ( FIG. 3(   a )) in one end portion having a predetermined length, and the toner container  201  rotatably supports the auger screw  202  such that the auger screw  202  extends through the toner storage section  206  and the toner discharge mechanism  204 . 
     The toner discharge mechanism  204  includes a cover member  204   b  having a cylindrical inner wall portion and a conical inner wall portion. In the conical inner wall portion of the cover member  204   b , the auger screw  202  has a tapered portion which diminishes toward the tip thereof in the toner conveying direction. The conical inner wall portion has an inner diameter gradually reduced toward the tip of the auger screw  202  so as to correspond to a tapered shape of the tapered portion. 
     In the present embodiment, a minimal value of the inner diameter of the conical inner wall portion is 0.3 times a maximal value of the inner diameter of the conical inner wall portion. A toner discharge outlet  204   a  is formed on a curved surface forming the conical inner wall of the cover member  204   b.    
     The toner discharge mechanism  204  will be described in more detail. 
     The auger screw  202  includes a rotating shaft  202   a , a helical blade  202   b , and a drive gear  202   c , as shown in  FIGS. 3 and 4 . The auger screw  202  rotates coaxially with a central axis of the cylindrical inner wall portion and the conical inner wall portion of the cover member  204   b , to convey the toner in the toner storage section  206 , toward the toner discharge outlet  204   a , that is, to convey the toner in the rightward direction (in the direction indicated by the arrow F) in  FIG. 3(   a ). 
     The helical blade  202   b  has its outer diameter gradually reduced toward the right edge of the auger screw  202 , in a position facing the toner discharge outlet  204   a , that is, in a position above the toner discharge outlet  204   a  as shown in  FIG. 3(   a ) (see  FIG. 3(   c ),  FIG. 3(   d ), and  FIG. 3(   e )). 
     In the present embodiment, the minimal value of the inner diameter of the conical inner wall portion is greater than or equal to 1.1 times the diameter of the rotating shaft  202   a  of the auger screw  202 , and is not greater than 1.5 times the diameter of the rotating shaft  202   a  of the auger screw  202 . Further, the minimal value of the inner diameter of the conical inner wall portion is greater than or equal to 0.2 times the maximal value of the inner diameter of the conical inner wall portion, and is not greater than 0.5 times the maximal value of the inner diameter of the conical inner wall portion. 
     A minimal value of the outer diameter of the helical blade  202   b  of the auger screw  202  is set to be 0.25 times the maximal value of the inner diameter of the conical inner wall portion. A maximal value of the outer diameter of the helical blade  202   b  of the auger screw  202  is set to be 0.9 times the maximal value of the inner diameter of the conical inner wall portion. 
     The helical blade  202   b  has a helix structure adapted to convey the toner toward the toner discharge outlet  204   a  by the rotation of the auger screw  202 , and the outer diameter of the helical blade  202   b  is reduced toward the downstream side. Therefore, an amount of the toner conveyed is reduced toward the downstream side. 
     The agitating paddle  207  ( FIG. 3(   b )) is an agitator having four agitating blades formed around a rotating shaft. The agitating paddle  207  rotates to loosen the toner in the toner storage section  206 . 
     The toner discharge outlet  204   a  is a quadrate opening formed on the bottom of the toner discharge mechanism  204  in the toner container  201 . The toner conveyed by the auger screw  202  is discharged through the toner discharge outlet  204   a  toward a corresponding one of the developing devices  102   a ,  102   b ,  102   c , and  102   d  ( FIG. 1 ).