Patent Publication Number: US-8995883-B2

Title: Developing device and image forming apparatus including same

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2013-90517, filed on Apr. 23, 2013 and Japanese Patent Application No. 2013-109237, filed on May 23, 2013, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a developing device incorporated in image forming apparatuses such as a copier, a facsimile, and a printer, and an image forming apparatus including the same, and in particular, the present invention relates to a method for reducing problems caused when toner is supplied, such as insufficient image density and fogging. 
     Known as conventional developing methods using a dry toner in an image forming apparatus employing an electro-photographic process are: a single-component developing method not using a carrier; and a two-component developing method of, by using a two-component developer for charging a non-magnetic toner with a magnetic carrier, developing an electrostatic latent image on an electrostatic latent image carrier (photosensitive body) with a magnetic brush of a toner and a carrier formed on a developing roller. 
     In a developing device, toner is consumed in a developing operation. Hence, there has been proposed a developing device in which, for the purpose of supplying toner to compensate for the amount consumed in a developing operation, a developer supply member is provided inside a developing container to supply developer to an agitating/conveying member. 
     For example, there has been known a developing device including first and second agitating/conveying members that agitate and convey developer, a developer supply member that rotates in the same direction as the first agitating/conveying member to supply the developer to the first agitating/conveying member, and a developing container that accommodates the first and second agitating/conveying members and the developer supply member. The developing container is provided with a supply section for supplying the developer from the developer supply member to the first agitating/conveying member. With this structure, by the rotation of the developer supply member, the developer is supplied from the developer supply member to the first agitating/conveying member in the supply section. 
     In such a developing device, toner is consumed through a developing operation. Hence, inside the developing device, there is provided a toner sensor that detects toner concentration (or toner amount), such that new toner is supplied to compensate for the amount consumed through the developing operation. 
     Here, when the developing device is operated continuously for a long time with a small amount of supply toner, as in a case where an image of a low printing rate is repeatedly printed and in a case where printing operations are repeated in an intermittent manner, mechanical stress on the toner circulating inside the developing device increases. As a result, electric charge on the toner in the developing device increases. 
     If, in this state, printing is performed at a high printing rate or continuously, initial toner charged to a comparatively low level is supplied into the developing device in a short period of time, and thus toner with a broad charge distribution is supplied to a developing roller, and this results in defective images such as those with insufficient image density and those with fogging. 
     To deal with this problem, there has been proposed a method for properly charging supply toner electrically, and there has been known a developing device in which, for example, a separation member and a pre-charging roller are provided in an agitating section, to thereby achieve stable charging of toner and reduce fogging occurring when toner is supplied anew. 
     There is also known a configuration where, in a developing device in which a supplying/conveying path, an agitating/conveying path, and collecting/conveying path are aligned substantially horizontally in this order from the vicinity of a developer regulating member, toner and carrier are supplied from a toner supplying position provided in the collecting/conveying path, and collected developer including the supplied toner and carrier is made to flow into the agitating/conveying path from a downstream-side end portion of the collecting/agitating path. 
     SUMMARY 
     According to one aspect of the present disclosure, a developing device includes: a developing container, a first agitating/conveying member, a second agitating/conveying member, a supply port, and a supply section. The developing container is separated into a developer supply passage, a first conveying chamber, and a second conveying chamber that are arranged substantially parallel to one another, and the developing container accommodates a developer including toner. The first agitating/conveying member agitates and conveys the developer in the first conveying chamber. The second agitating/conveying member agitates and conveys the developer in the second conveying chamber in a direction opposite to a direction in which the first agitating/conveying member agitates and conveys the developer. The developer supply member supplies the developer existing in the developer supply passage into the first conveying chamber. The supply port is provided in the developer supply passage to be connected to a developer supply mechanism. The supply section is provided between the developer supply passage and the first conveying chamber, and via the supply section, the developer is supplied from the developer supply passage into the first conveying chamber. The supply section is disposed upstream from a downstream-side end portion of the developer supply passage but downstream from the supply port with respect to a direction in which the developer is conveyed in the developer supply passage, and via the supply section, the developer supply member supplies the developer into the first conveying chamber in a direction opposite to a direction in which the developer is conveyed in the first conveying chamber. 
     Still other objects and specific advantages of the present disclosure will become apparent from the following descriptions of preferred embodiments given below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a schematic sectional view showing an overall configuration of an image forming apparatus  100  including a developing device  4  according to an embodiment of the present disclosure; 
         FIG. 2  is a side sectional view showing a structure of the developing device  4  of the present embodiment; 
         FIG. 3  is an external perspective view showing a state where a cover member  42  of the developing device  4  of the present embodiment is removed; 
         FIG. 4  is a plan sectional view showing a structure of an agitating section of the developing device  4  of the present embodiment; 
         FIG. 5  is a perspective view showing a structure of a first spiral  31  employed in the developing device  4  of the present embodiment; 
         FIG. 6  is an external perspective view of the developing device  4  of the present embodiment; and 
         FIG. 7  is a perspective view showing a structure of a developer supply member  35  employed in the developing device  4  of the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.  FIG. 1  is a schematic sectional view showing a configuration of an image forming apparatus  100  provided with a developing device  4  according to an embodiment of the present disclosure. In an image forming operation of the image forming apparatus  100  (for example, a monochrome printer), an electrostatic latent image is formed based on original image data sent from an unillustrated personal computer (PC) in an image forming section P inside a main body of the image forming apparatus  100 , and then, the developing device  4  makes toner adhere to the electrostatic latent image to form a toner image. The toner supplied to the developing device  4  comes from a toner container  5 . In the image forming apparatus  100 , an image forming process is performed with respect to a photosensitive drum  1  while the photosensitive drum  1  is rotating in the clockwise direction in  FIG. 1 . 
     In the image forming section P, along a rotation direction of the photosensitive drum  1  (in the clockwise direction), there are arranged a charging section  2 , an exposure unit  3 , the developing device  4 , a transfer roller  6 , a cleaning device  7 , and a charge neutralizing device (not shown). The photosensitive drum  1  has, for example, a photosensitive layer laid on an aluminum drum, and its surface is electrically charged by the charging section  2 . When the surface is irradiated with a laser beam from the exposure unit  3 , which will be described later, an electrostatic latent image is formed through attenuation of electric charge. Preferred as the photosensitive layer is, for example, but not limited to, amorphous silicon (a-Si), which is high in durability, or an organic photosensitive layer (OPC), which produces little ozone in charging and contributes to a high-resolution image. 
     The charging section  2  uniformly charges the surface of the photosensitive drum  1 . Employed as the charging section  2  is, for example, a corona discharge device which produces electric discharge by applying a high voltage to an electrode such as a piece of fine wire. Instead of a corona discharge device, there may be employed a contact-type charging device which achieves voltage application while a charging member, as exemplified by a charging roller, is in contact with the surface of a photosensitive drum. The exposure unit  3  irradiates the photosensitive drum  1  with a light beam (for example, a laser beam) according to image data, and thereby forms an electrostatic latent image on the surface of the photosensitive drum  1 . 
     The developing device  4  makes toner adhere to the electrostatic latent image formed on the photosensitive drum  1  to form a toner image. Here, a magnetic single-component developer (hereinafter may also be simply referred to as toner) composed of a magnetic toner component alone is accommodated in the developing device  4 . The details of the developing device  4  will be described later. The transfer roller  6  transfers, without disturbing, the toner image formed on the surface of the photosensitive drum  1  onto a sheet of paper which is conveyed along a sheet conveying passage  11 . The cleaning device  7  is provided with a cleaning roller, cleaning blade, and the like which make line contact with the photosensitive drum  1  in a longitudinal direction thereof, to remove residual toner remaining on the surface of the photosensitive drum  1  after the transfer of the toner image onto the sheet. 
     Based on the image data, which has been inputted beforehand, the exposure unit  3  irradiates the photosensitive drum  1  with a laser beam (light beam), and thereby, an electrostatic latent image based on the image data is formed on the surface of the photosensitive drum  1 . Then, the developing device  4  makes toner adhere to the electrostatic latent image to form a toner image. 
     Toward the image forming section P in which the toner image is formed as described above, a sheet of paper is conveyed at predetermined timing from a paper accommodating section  10  via the paper conveying passage  11  and a registration roller pair  13  such that, in the image forming section P, the transfer roller  6  transfers the toner image on the surface of the photosensitive drum  1  onto the sheet. Then, the sheet carrying the transferred toner image is separated from the photosensitive drum  1 , and is conveyed to a fixing section  8 , where application of heat and pressure is performed on the sheet to thereby fix the toner image thereon. The sheet having passed through the fixing section  8  is ejected via an ejection roller pair  14  into a sheet ejection section  15 . 
       FIG. 2  is a side sectional view showing a structure of the developing device  4 . Detailed descriptions will be given of the structure of the developing device  4  with reference to  FIG. 2 . Note that  FIG. 2  shows a state as seen from a rear surface side of  FIG. 1 , and the arrangement of the members inside the developing device  4  is reversed left to right compared with the arrangement in  FIG. 1 . 
     As shown in  FIG. 2 , the developing device  4  includes a developing roller (developer carrier)  20 , a regulating blade  21 , an agitating/conveying member  30 , a developer supply member  35 , a developing container  40  in which these components are accommodated, and the like. 
     The developing container  40  constitutes an outer casing of the developing device  4 , and has a main body  41  and a cover member  42  that covers an upper portion of, for example, the developer supply member  35 . The developing container  40  is separated into a first conveying chamber  40   a  and a second conveying chamber  40   b  by a partition  41   a  provided in the main body  41 . A single-component developer composed of toner alone is accommodated in the first conveying chamber  40   a  and the second conveying chamber  40   b . The developing container  40  rotatably holds the agitating/conveying member  30 , the developer supply member  35 , and the developing roller  20 . The developing container  40  is further provided with an opening  40   c  through which the developing roller  20  is exposed toward the photosensitive drum  1  (see  FIG. 1 ). 
     The developing roller  20  is arranged to face the photosensitive drum  1  with a predetermined distance therebetween. The developing roller  20  supplies toner to the photosensitive drum  1  in a facing region where the developing roller  20  is close to the photosensitive drum  1 . The agitating/conveying member  30  is located obliquely downward to the left of the developing roller  20 . The regulating blade  21  is fixedly held by the developing container  40  on the left of the developing roller  20 . 
     The agitating/conveying member  30  includes two spirals, namely, a first spiral (first agitating/conveying member)  31  and a second spiral (second agitating/conveying member)  32 . The second spiral  32  is disposed inside the second conveying chamber  40   b  to be located obliquely downward to the left of the developing roller  20 , and the first spiral  31  is disposed inside the first conveying chamber  40   a  to be located adjacent to the left of the second spiral  32 . 
     The first and second spirals  31  and  32  convey the developer while agitating it. At both end portions in a longitudinal direction (a direction perpendicular to the plane of  FIG. 2 ) of the partition  41   a  that separates the first and second conveying chambers  40   a  and  40   b  from each other, communication sections (an upstream-side communication section  40   d  and a downstream-side communication section  40   e  which will be described later) are provided such that, when the first spiral  31  rotates, the developer is conveyed to the second spiral  32  via one of the communication sections provided in the partition  41   a , and thus the developer circulates in the first conveying chamber  40   a  and the second conveying chamber  40   b . Then, the developer is supplied from the second spiral  32  to the developing roller  20 . 
     The developing roller  20  includes a stationary shaft  20   a , a magnetic pole member  20   b , a developing sleeve  20   c  that is a cylindrical member formed of a nonmagnetic metal material. The developing roller  20  is made to rotate in the counterclockwise direction in  FIG. 2  by an unillustrated drive mechanism that includes a motor and a gear. 
     When the developing sleeve  20   c  having a developing bias applied thereto rotates, the toner carried on the developing sleeve  20   c  flies to the photosensitive drum  1  in a developing region (the facing region where the developing roller  20  and the photosensitive drum  1  face each other), due to a potential difference between the developing bias and the potential of an exposed part of photosensitive drum  1 . The flying toner sequentially adheres to the exposed part of the rotating photosensitive drum  1 , and thereby, an electrostatic latent image on the photosensitive drum  1  is developed. 
       FIG. 3  is an external perspective view showing a state where the cover member  42  of the developing device  4  is removed, and  FIG. 4  is a plan sectional view showing a structure of an agitating section of the developing device  4 . As shown in  FIG. 3  and  FIG. 4 , the partition  41   a , the first conveying chamber  40   a , the second conveying chamber  40   b , and the communication sections  40   d  and  40   e  are provided in the developing container  40 , and in addition, there is also provided a developer supply passage  40   f . The developer supply passage  40   f  is a passage for supplying a new developer (supply toner) from the toner container  5  to the first conveying chamber  40   a.    
     The first conveying chamber  40   a , the second conveying chamber  40   b , and the developer supply passage  40   f  are arranged parallel to one another. The partition  41   a  is provided to extend in the longitudinal direction of the developing container  40  so as to separate the first conveying chamber  40   a  and the second conveying chamber  40   b  from each other. A partition  41   b  is provided to extend in the longitudinal direction of the developing container  40  so as to separate the developer supply passage  40   f  and the first conveying chamber  40   a  from each other. In the first conveying chamber  40   a , the left side in  FIG. 3  is the upstream side while the right side in  FIG. 3  is the downstream side. In the second conveying chamber  40   b , the right side in  FIG. 3  is the upstream side while the left side in  FIG. 3  is the downstream side. 
     The communication sections  40   d  and  40   e  are provided at one and the other longitudinal end portions (direction-A1 end portion and direction-A2 end portion) of the partition  41   a , respectively. The communication section  40   d  allows the direction-A1 end portions of the first and second conveying chambers  40   a  and  40   b  to communicate with each other. The communication section  40   e  allows the direction-A2 end portions of the first and second conveying chambers  40   a  and  40   b  to communicate with each other. In this configuration, the developer is able to circulate in the first conveying chamber  40   a , the communication section  40   d , the second conveying chamber  40   b , and the communication section  40   e.    
       FIG. 5  is a perspective view showing a structure of the first spiral  31  employed in the developing device  4 . As shown in  FIGS. 3 to 5 , the first spiral  31  includes a rotation shaft  31   a  and a first spiral blade (blade)  31   b  that is integral with the rotation shaft  31   a  and that has a shape of a spiral at a predetermined pitch along the axial direction of the rotation shaft  31   a . The rotation shaft  31   a  is pivotally supported by the developing container  40 . The first spiral blade  31   b  conveys the developer inside the first conveying chamber  40   a  in direction A1 while agitating the developer. 
     The second spiral  32  has the same structure as the first spiral  31  shown in  FIG. 5 , except that orientations (phases) of the spiral blades are different. That is, the second spiral  32  includes a rotation shaft  32   a  and a second spiral blade  32   b . The second spiral blade  32   b  is integral with the rotation shaft  32   a  and has a shape of a spiral at the same pitch as the first spiral blade  31   b  along the rotation shaft  32   a  in its axial direction. The second spiral blade  32   b  is reversely oriented (reversely phased) with respect to the first spiral blade  31   b . The rotation shaft  32   a  is arranged parallel to the rotation shaft  31   a , and is pivotally supported by the developing container  40 . The second spiral blade  32   b  supplies the developer to the developing roller  20  by conveying the developer inside the second conveying chamber  40   b  in direction A2 (a direction opposite to direction A1) while agitating the developer. 
     As shown in  FIG. 6 , in a portion of the developer supply passage  40   f  on the direction-A1 side, there is provided a supply port  42   a  in the cover member  42  to supply a new developer therethrough into the developing container  40  from the toner container  5  that is disposed above the developing container  40 . On an inner wall of the first conveying chamber  40   a  or the second conveying chamber  40   b , there is provided a toner sensor (not shown) for detecting an amount of toner in the developing container  40 . According to detection results of the toner sensor, toner (developer) accommodated in the toner container  5  (see  FIG. 1 ) is supplied into the developing container  40  through the supply port  42   a.    
     The developer supply passage  40   f  is a passage for conveying in direction A2 the developer that is supplied to the portion thereof on the direction-A1 side, to thereby supply the developer to the upstream side of the first conveying chamber  40   a . In the developer supply passage  40   f , there is provided a supply section  40   g  for supplying the developer from the developer supply passage  40   f  to the first conveying chamber  40   a . With respect to a direction in which the developer is conveyed in the developer supply passage  40   f  (direction A2), the supply section  40   g  is located upstream from a downstream-side end portion (left end portion in  FIG. 4 ) of the developer supply passage  40   f  (the upstream is the right side in  FIG. 4 ) but downstream from the supply port  42   a  (the downstream side is the left side in  FIG. 4 ). 
     In the developer supply passage  40   f , the developer supply member  35  is arranged parallel to the first and second spirals  31  and  32 . As shown in  FIG. 3  and  FIG. 7 , the developer supply member  35  includes a rotation shaft  35   a  and third and fourth spiral blades  35   b  and  35   c  that are integral with the rotation shaft  35   a . The third spiral blade  35   b  is formed in a spiral shape along the rotation shaft  35   a  in its axial direction. The third spiral blade  35   b  is reversely oriented (reversely phased) with respect to the first spiral blade  31   b , and extends from the supply port  42   a  (see  FIG. 6 ) to the supply section  40   g . The fourth spiral blade  35   c  is formed in a spiral shape and reversely oriented (reversely phased) with respect to the third spiral blade  35   b , and extends from the supply section  40   g  to a direction A2 end portion. Further, on the rotation shaft  35   a , at a portion thereof facing the supply section  40   g , there is provided a paddle-shaped conveying blade  35   d.    
     The developer supply member  35  is configured to rotate in the same direction as the first spiral  31  (in the counterclockwise direction in  FIG. 2 ), such that the developer supplied to the supply port  42   a  is conveyed toward the supply section  40   g . Since the third spiral blade  35   b  and the fourth spiral blade  35   c  have phases opposite in direction to each other, the developer conveyed by the third spiral blade  35   b  and the developer conveyed by the fourth spiral blade  35   c  collide with each other in the supply section  40   g  to be conveyed to the first conveying chamber  40   a.    
     The developer supply member  35 , the first spiral  31 , and the second spiral  32  are each driven to rotate by an unillustrated drive mechanism including a motor and a gear. 
     As shown in  FIG. 2 , the developer supply member  35  supplies the developer from below upward to the first spiral  31  by rotating in the counterclockwise direction. In an upper portion of the supply section  40   g  (a portion of the supply section  40   g  downstream from the developer supply member  35  with respect to a rotation direction of the developer supply member  35 ), a rib  42   b  is provided to extend downward (toward the upstream side with respect to the rotation direction of the developer supply member  35 ). A side  42   c  of the rib  42   b  on the first spiral  31  side is a flat surface extending in a vertical direction. The rib  42   b  is provided to extend substantially all along a width direction (direction A1 and direction A2) of the supply section  40   g . Further, the rib  42   b  is made of resin and integral with the cover member  42 . 
     An edge  42   b   1  (lower edge) of the rib  42   b  is located on a side above line L that connects a center O 35  of the developer supply member  35  and a center O 31  of the first spiral  31  (that is, on a downstream side of line L with respect to the rotation direction of the developer supply member  35 ). Further, the edge  42   b   1  of the rib  42   b  is located above the center O 35  of the developer supply member  35 . The center O 35  of the developer supply member  35  is located at a position higher than the center O 31  of the first spiral  31 . A bottom  40   g   1  of the supply section  40   g  is inclined downward from the developer supply passage  40   f  toward the first conveying chamber  40   a.    
     According to the present embodiment, as described above, in the upper portion of the supply section  40   g  (a portion on the downstream side with respect to the rotation direction of the developer supply member  35 ), the rib  42   b  is provided to extend downward (upstream with respect to the rotation direction of the developer supply member  35 ). This makes it possible to reduce backflow of the developer into the developer supply passage  40   f  from the first conveying chamber  40   a  caused by the first spiral  31  in an upper portion of the supply section  40   g . This contributes to improvement of the efficiency of supplying the developer from the developer supply member  35  to the first spiral  31  (from the developer supply passage  40   f  to the first conveying chamber  40   a ). 
     Furthermore, as described above, the edge  42   b   1  of the rib  42   b  is located on a side above line L that connects the center O 35  of the developer supply member  35  and the center O 31  of the first spiral  31  (that is, on the downstream side of line L with respect to the rotation direction of the developer supply member  35 ). This makes it possible to reduce blockage caused by the rib  42   b  in the supply of the developer from the developer supply member  35  to the first spiral  31 , and thus to achieve more efficient supply of the developer. 
     Moreover, as described above, the center O 35  of the developer supply member  35  is located at a position higher than the center O 31  of the first spiral  31 , and the edge  42   b   1  of the rib  42   b  is located above the center O 35  of the developer supply member  35 . This makes it possible to further reduce blockage caused by the rib  42   b  in the supply of the developer from the developer supply member  35  to the first spiral  31 . 
     Also, as has described above, the developer supply member  35  rotates so as to supply the developer from below upward to the first spiral  31 , and the rib  42   b  is disposed in the upper portion of the supply section  40   g . As a result, in comparison with a case where the developer supply member  35  is made to rotate so as to supply the developer from above downward to the first spiral  31  and the rib  42   b  is disposed in a lower portion of the supply section  42   g , it is possible to easily improve the efficiency of supplying the developer from the developer supply member  35  to the first spiral  31 . 
     Also, as described above, the rib  42   b  is integral with the cover member  42 . This facilitates production of the rib  42   b , reducing increase of components in number. 
     Furthermore, as described above, the side  42   c  of the rib  42   b  on the first spiral  31  side is a flat surface extending in the vertical direction. As a result, in comparison with a case where the side  42   c  of the rib  42   b  on the first spiral  31  side is, for example, an arc-like surface curved along an area where the blade of the first spiral  31  passes, it is possible to reduce backflow of the developer from the first spiral  31  to the developer supply member  35 . This makes it possible to further improve the efficiency of supplying the developer from the developer supply member  35  to the first spiral  31 . 
     Moreover, as described above, the bottom  40   g   1  of the supply section  40   g  is inclined downward from the developer supply passage  40   f  toward the first conveying chamber  40   a . This makes it possible to easily improve the efficiency of supplying the developer from the developer supply member  35  to the first spiral  31 . 
     As shown in  FIG. 2  and  FIG. 4 , the supply section  40   g  is an opening having a predetermined opening width in a horizontal direction (right-left direction in  FIG. 4 ), and a predetermined opening width in a perpendicular direction (up-down direction in  FIG. 2 ). If the opening width in the perpendicular direction is smaller than an outer diameter of the developer supplying member  35  (a diameter of the third and fourth spiral blade  35   b  and  35   c ), it is impossible to achieve sufficient toner supplying performance. On the other hand, if the opening width in the perpendicular direction is larger than the outer diameter of the developer supply member  35 , it causes backflow of the toner from the first conveying chamber  40   a  to the developer supply passage  40   f . To prevent these inconveniences, the supply section  40   g  is formed such that the opening width in the perpendicular direction is substantially equal to the outer diameter of the developer supply member  35 . 
     In the present embodiment, as has been discussed above, toner (developer) supplied into the developer supply passage  40   f  from the supply port  42   a  is conveyed through the developer supply passage  40   f  in direction A2 (as indicated by a broken line arrow in  FIG. 4 ). On the other hand, the toner inside the first conveying chamber  40   a  is conveyed in direction A1 (as indicated by a solid line arrow in  FIG. 4 ). Then, the toner conveyed in the developer supply passage  40   f  in direction A2 joins, in the supply section  40   g , circulation toner conveyed in direction A1 while circulating in the first conveying chamber  40   a.    
     With this feature, in feeding supply toner from the developer supply passage  40   f  into the first conveying chamber  40   a , the supply toner conveyed through the developer supply passage  40   f  in direction A2 and the circulation toner conveyed in the first conveying chamber  40   a  in direction A1 collide with each other near the supply section  40   g , such that part of the circulation toner in the first conveying chamber  40   a  is taken from the supply section  40   g  into the developer supply passage  40   f , to be agitated with the supply toner. As a result, the supply toner in the developer supply passage  40   f  and the circulation toner in the first conveying chamber  40   a  are fully agitated and mixed together in the supply section  40   g , and this makes it possible to quickly achieve uniform distribution of toner electric charge in the developing container  40 . Thus, it is possible to reduce occurrence of problems such as deficient image density caused by excessively charged toner and image fogging caused by insufficiently charged toner. 
     Here, the supply section  40   g  is disposed upstream from the downstream-side (direction A2-side) end portion of the developer supply passage  40   f  with respect to the direction in which the developer is conveyed in the developer supply passage  40   f , and the developer supply passage  40   f  extends downstream past the supply section  40   g  with respect to the direction in which the developer is conveyed in the developer supply passage  40   f . Between the supply section  40   g  and the direction A2-side end portion, the fourth spiral blade  35   c  is provided to be reversely oriented (reversely phased) with respect to the third spiral blade  35   b . With this feature, near the supply section  40   g , a toner current is caused to occur by the fourth spiral blade  35   c . Thus, it is possible to agitate the toner existing near the supply section  40   g  and including both the supply toner and the circulation toner more efficiently. 
     Moreover, the supply section  40   g  is disposed downstream from the upstream-side communication section  40   e  that is located on the upstream side with respect to the direction in which the developer is conveyed in the first conveying chamber  40   a  (direction A1). This feature allows the circulation toner inside the first conveying chamber  40   a  and the supply toner inside the developer supply passage  40   f  to collide with each other from opposite directions to be fully agitated and mixed together. 
     Here, if the supply section  40   g  is disposed in the vicinity of the supply port  42   a , toner that is just supplied from the supply port  42   a  and thus is not electrically charged yet might be immediately fed into the first conveying chamber  40   a , and fogging might be caused by the insufficient charge of the toner. To prevent this by separating the supply port  42   a  from the supply section  40   g  as much as possible, it is preferable, as in the present embodiment, to dispose the supply port  42   a  at the upstream-side end portion (right side in  FIG. 6 ) with respect to the direction in which the developer is conveyed in the developer supply passage  40   f , and dispose the supply section  40   g  closer to the downstream-side end portion (left side in  FIG. 6 ) than the center portion of the developer supply passage  40   f.    
     It should be understood that the present disclosure is not limited to the above embodiments, and various modifications are possible within the scope of the present disclosure. For example, the above embodiment employs the first and second spirals  31  and  32  where the spiral blades  31   b  and  32   b  are continuously provided along the rotation shafts  31   a  and  32   a , respectively, but this is not meant as a limitation. Instead, there may be employed an agitating/conveying member such that, for example, a plurality of semilunar plates (each obtained by dividing a circular plate in half) are alternately arranged at a predetermined inclination angle along the rotation shafts  31   a  and  32   a.    
     Furthermore, the above-discussed embodiments have dealt with examples where the developer supply member rotates so as to supply the developer from below upward to the first agitating/conveying member, but this is not meant as a limitation, and the developer supply member may supply the developer from above downward to the first agitating/conveying member. In this case, a rib should be disposed in the lower portion (a portion on the downstream side with respect to the rotation direction of the developer supply member) of the supply section. Moreover, the center O 35  of the developer supply member  35  may be located at a position higher than the center O 31  of the first spiral  31 , and the edge of the rib  42   b  may be located below the center O 31  of the first spiral  31 . 
     Also, the above-discussed embodiments have dealt with examples where the rib  42   b  is integrally formed with the cover member  42 , but this is not meant as a limitation. For example, a rib may be integrally formed with the partition  41   b  that separates the developer supply passage  40   f  and the first conveying chamber  40   a  from each other. 
     In addition, the above-discussed embodiments have dealt with examples where a single-component developer composed of toner alone is used as the developer, but this is not meant as a limitation, and a two-component developer composed of carrier and toner may be used as the developer. 
     The application of the present disclosure is not limited to monochrome printers as illustrated in  FIG. 1 , but the present disclosure is applicable to various image forming apparatuses such as digital or analog monochrome copiers, color printers, color copiers, facsimile machines, and the like that incorporate a developing device including the first agitating/conveying member, the second agitating/conveying member, and the developer supply member. 
     The present disclosure is adaptable for use in developing devices that include the developer supply passage, the first conveying chamber, and the second conveying chamber which are arranged substantially parallel to one another. Adoption of the present disclosure makes it possible to quickly achieve uniform distribution of toner charge, and thus to obtain an image forming apparatus that is capable of reducing occurrence of defective images such as those with insufficient image density and those with fogging.