Patent Publication Number: US-8538300-B2

Title: Developing device with improved developer transport efficiency

Description:
This application is based on an application No. 2010-062744 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a developing device for developing an electrostatic latent image on a photoreceptor and to a process unit and an image forming apparatus that include the developing device. 
     (2) Description of the Related Art 
     Electrophotographic image forming apparatuses, such as a printer, copying machine, etc. normally function as follows. A developing device forms a toner image corresponding to image data on a photoreceptor drum. The toner image is then transferred to a recording sheet, such as recording paper, an OHP sheet, etc., either directly or indirectly via a component such as an intermediate transfer belt. Finally, a fixing device fixes the toner image to the recording sheet. 
     The developing device may use either mono-component developer, which includes only toner, or two-component developer, which includes toner and magnetic carrier. A developing device that uses two-component developer is normally provided with a developing roller, a feed screw, and a mixing screw inside housing that contains the two-component developer. The developing roller develops the electrostatic latent image on the photoreceptor drum. The mixing screw supplies the feed screw with two-component developer, and the feed screw feeds the developing roller with the two-component developer. The photoreceptor drum, feed screw, and mixing screw are parallel to each other in an approximately horizontal position. 
     The two-component developer in the housing is stirred and mixed during transport by the mixing screw. As a result, the toner becomes statically charged. Statically charged toner is transported along with the carrier to the feed screw. The feed screw transports the two-component developer in the opposite direction as the mixing screw. Toner and carrier transported by the feed screw are provided to the developing roller. 
     Carrier provided to the developing roller forms a magnetic brush on the peripheral surface of the developing roller. Toner that adheres to the magnetic brush develops the electrostatic latent image on the photoreceptor drum. Toner and carrier that are not provided to the developing roller are transported from a downstream edge of the feed screw to an upstream edge of the mixing screw, the terms downstream and upstream referring to the direction of transport of each of the screws. The mixing screw and the feed screw thus circulate toner and carrier through the inside of the housing. 
     In order to provide the photoreceptor drum with a constant amount of developer, it is preferable for the photoreceptor drum, developing roller, feed screw, and mixing screw in the image forming apparatus to be arranged in a horizontal position. When the image forming apparatus is not placed horizontally, the developer in the housing tends to accumulate in one location, causing the amount of developer provided to the photoreceptor drum not to be constant. The quality of the toner image formed on the photoreceptor drum may therefore degrade. 
     For example, if the developing device is inclined so that the upstream edge of the mixing screw is positioned lower than a downstream edge of the mixing screw (respectively corresponding to the downstream edge and an upstream edge of the feed screw), the developer in the housing flows to and accumulates at the upstream edge of the mixing screw (i.e. the downstream edge of the feed screw). 
     The mixing screw is provided in a mixing chamber, and when the developing device is thus inclined, the upper level of the developer in the mixing chamber rises higher than the transport range of the mixing screw at the upstream edge. The mixing screw therefore cannot stably transport developer that accumulates in this area to the upstream edge of the feed screw. Accordingly, the amount of developer transported to the feed screw decreases, which results in a decrease in the amount of developer fed to the developing roller. Toner thus cannot be stably fed to the photoreceptor drum. 
     In recent years, demand has increased for compact, lower-cost image forming apparatuses. The size of image forming apparatuses has been reduced by making developing devices smaller through a decreased amount of developer in the housing of the developing device and a reduction in size of the mixing screw and other components. Even when the developing device is positioned horizontally, i.e. not inclined, the amount of developer transported by the feed screw in such a compact image forming apparatus is decreased. 
     Accordingly, as described above, if the upstream edge of the mixing screw is positioned lower than the downstream edge, then even a slight inclination angle has a higher probability of impeding transport by the feed screw of a predetermined amount of developer. If the predetermined amount of developer is not transported to the developing roller, excellent development of the electrostatic latent image on the photoreceptor drum may not be possible. 
     Patent Literature 1 (Japanese Patent Application Publication No. 2001-117337) discloses providing a rectifying fin on the peripheral surface of a developing sleeve to rectify developer that flows in the direction in which the developing device is inclined. The rectifying fin redirects the developer towards the central region of the direction of transport of the developing sleeve. 
     Furthermore, Patent Literature 2 (Japanese Patent Application Publication No. 2002-278270) discloses providing a transport screw, fins, and a balancer. The transport screw transports, in the direction of the axis of the developing roller, developer that is removed from the developing roller by a doctor blade. The fins are provided in a separator that causes the transport screw to transport developer downstream. The fins are at an incline and guide developer to an upstream edge of the transport screw. Depending on where the developer accumulates, the balance changes the inclination of the fins. 
     The structures disclosed in Patent Literature 1 and 2 make the amount of developer transported from the feed screw to the peripheral surface of the developing sleeve uniform when the developing sleeve is inclined in a predetermined position. However, neither of the structures in Patent Literature 1 and 2 overcomes the problem of decreased transport efficiency by the mixing screw when the upstream edge of the mixing screw is lower than the downstream edge. 
     SUMMARY OF THE INVENTION 
     The present invention has been conceived in light of the above problem, and it is an object thereof to provide a developing device that circulates developer via a first developer transport member and a second developer transport member that transport developer in opposite horizontal directions, as do a mixing screw and a feed screw, and that does not exhibit decreased transport efficiency of the developer even when the transport directions of the first and second developer transport members are inclined with respect to the horizon. Another object of the present invention is to provide a process unit and an image forming apparatus that include the developing device. 
     In order to achieve the above objects, according to the present invention, a developing device for developing an electrostatic image on a photoreceptor using toner comprises: a mixing chamber including a first developer transport member that transports developer, which includes the toner, laterally in a first direction; a feed chamber including a second developer transport member that transports the developer in a second direction opposite to the first direction; a housing that encloses the mixing chamber and the feed chamber; and a developing roller provided in the feed chamber and configured to receive the developer transported by the second developer transport member and to develop the electrostatic image, wherein the housing includes: a first communication passage configured to direct the developer transported by the first developer transport member to an upstream edge, along the second direction, of the second developer transport member; and a second communication passage configured to direct the developer transported by the second developer transport member to an upstream edge, along the first direction, of the first developer transport member, and the second communication passage is configured to progressively reduce the amount of developer that is directed through the second communication passage to the upstream edge of the first developer transport member as an inclination angle of the housing increases so that the second communication passage is lower than the first communication passage. 
     A process unit according to the present invention includes the developing device. 
     An image forming apparatus according to the present invention includes the developing device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and the other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention. 
       In the drawings: 
         FIG. 1  is a schematic diagram showing an image forming apparatus, specifically a tandem-type color digital printer, provided with the developing device according to Embodiment 1 of the present invention; 
         FIG. 2  is a cross-section diagram of the developing device provided in a process unit of the printer shown in  FIG. 1 ; 
         FIG. 3  is a perpendicular cross-section diagram along the line E-E in  FIG. 2 ; 
         FIG. 4  is a perpendicular cross-section diagram along the line F-F in  FIG. 2 ; 
         FIG. 5  is an enlarged diagram of a principal part shown by the letter P in  FIG. 3 ; 
         FIG. 6  is a graph showing experimental results on the adequacy of the amount of developer fed to the developing sleeve when the developing device according to Embodiment 1 is inclined at predetermined angles; 
         FIG. 7  is a cross-section diagram of the principal part in a modification of the developing device according to Embodiment 1; 
         FIG. 8  is a cross-section diagram of the principal part in another modification of the developing device according to Embodiment 1; 
         FIG. 9  is a graph showing experimental results on the adequacy of the amount of developer fed to the developing sleeve in the modifications of the developing device; 
         FIG. 10  is a cross-section diagram of the principal part in a developing device according to Embodiment 2 of the present invention; and 
         FIG. 11  is a cross-section diagram of the principal part to illustrate the operation of the developing device shown in  FIG. 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a schematic diagram showing an image forming apparatus, specifically a tandem-type color digital printer (hereinafter simply referred to as “printer”), provided with the developing device according to Embodiment 1 of the present invention. Upon receiving a print job instruction, this printer uses known electrophotography to form a full-color or monochrome image on a recording sheet, such as recording paper or an OHP sheet, based on the print job instruction. 
     The printer is provided with an image forming unit A and a paper feed unit B located below the image forming unit A. The image forming unit A forms a toner image on a recording sheet via yellow (Y), magenta (M), cyan (C), and black (K) toners. The paper feed unit B is provided with a paper feed cassette  51  storing recording sheets S that are provided to the image forming unit A. 
     The image forming unit A is provided with a pair of belt conveyor rollers  42  and  43  and an intermediate transfer belt  41 . The intermediate transfer belt  41  is wound around the belt conveyor rollers  42  and  43 , so as to be positioned horizontally, and rotates around the rollers. The intermediate transfer belt  41  is rotated in the direction indicated by the arrow X by a motor not shown in the drawings. 
     Process units  10 Y,  10 M,  10 C, and  10 K are provided below the intermediate transfer belt  41  in this order, starting at the upstream end of the intermediate transfer belt  41 . The process units  10 Y,  10 M,  10 C, and  10 K each form a toner image on the intermediate transfer belt  41  with yellow (Y), magenta (M), cyan (C), and black (K) toners respectively. 
     The process units  10 Y,  10 M,  10 C, and  10 K are each removable from the image forming unit A and are mounted in the image forming unit A by being inserted therein from the front of the image forming unit A towards the back. Upon reaching the end of its life, each of the process units  10 Y,  10 M,  10 C, and  10 K can be exchanged for a new process unit. 
     Apart from using a different color toner, the process units  10 Y,  10 M,  10 C, and  10 K have the same structure. Accordingly, the following description mainly focuses on the process unit  10 Y. 
     The process unit  10 Y is provided with a photoreceptor drum  11  positioned horizontally with respect to the intermediate transfer belt  41 . The photoreceptor drum  11  extends lengthwise along the direction of width of the intermediate transfer belt  41 , in a straight line from the front side of the printer to the back side of the printer. Each of the photoreceptor drums  11  rotates in the direction indicated by the arrow E. 
     An exposure device  17  is provided below the process units  10 Y,  10 M,  10 C, and  10 K and shines laser light on the surface of the photoreceptor drum  11  in each of the process units  10 Y,  10 M,  10 C, and  10 K to form an electrostatic latent image. The exposure device  17  is attached to the image forming unit A. 
     A charging device  12  is provided in the process unit  10 Y to uniformly charge the facing surface of the photoreceptor drum  11 . The charging device  12  faces the photoreceptor drum  11  and is positioned near the upstream edge, in the direction of rotation, of the surface portion of the rotating photoreceptor drum  11  that receives laser light from the exposure device  17 . An electrostatic latent image is formed by the laser light from the exposure device  17  on the surface of the photoreceptor drum  11 , which has been charged by the charging device  12 . 
     A developing device  20  is located at the downstream edge, in the direction of rotation, of the surface portion of the rotating photoreceptor drum  11  that receives laser light. The developing device  20  uses toner to develop the electrostatic latent image formed on the surface of the photoreceptor drum  11 . 
     Downstream from the developing device  20  in the direction of rotation of the photoreceptor drum  11  (i.e. above the photoreceptor drum  11 ), a first transfer roller  15  is provided facing the photoreceptor drum  11  so as to sandwich the intermediate transfer belt  41 . The toner image formed by the developing device  20  on the photoreceptor drum  11  is transferred to the outer circumferential surface of the intermediate transfer belt  41  by the first transfer roller  15 . Note that the first transfer roller  15  is attached to the image forming unit A. 
     A first transfer roller  15  is provided above the photoreceptor drum  11  in each of the other process units  10 M,  10 C, and  10 K to transfer the image formed on the respective photoreceptor drum  11  to the intermediate transfer belt  41 . Each of the first transfer rollers  15  is attached to the image forming unit A. The toner image formed on each of photoreceptor drums  11  is transferred to the same region of the intermediate transfer belt  41  by the corresponding first transfer roller  15 . 
     A cleaning device  14  is provided in each of the process units  10 Y,  10 M,  10 C, and  10 K to clean the surface of the corresponding photoreceptor drum  11  after the toner image is transferred to the intermediate transfer belt  41 . The cleaning devices  14  are respectively provided in the process units  10 Y,  10 M,  10 C, and  10 K downstream, in the direction of rotation of the corresponding photoreceptor drum  11 , from the position at which the toner image is transferred from the surface of the photoreceptor drum  11  to the intermediate transfer belt  41 . 
     The intermediate transfer belt  41  transports the toner images transferred by the first transfer rollers  15  to the belt conveyor roller  42 , which is located near the process unit  10 K that uses color K toner. A second transfer roller  54  is provided facing the belt conveyor roller  42  with the intermediate transfer belt  41  therebetween. The second transfer roller  54  is pressed into contact with the intermediate transfer belt  41 , thus forming a transfer nip that transfers the toner image to a recording sheet. 
     A recording sheet S in the paper feed cassette  51  provided in the paper feed unit is transported to the transfer nip by a paper feed roller  52 . The recording sheet S is transported by a pair of timing rollers  53  at the same time that the toner image formed on the intermediate transfer belt  41  is transported to the transfer nip. The toner image formed on the intermediate transfer belt  41  is pressed into contact with the recording sheet S that passes through the transfer nip and is wholly transferred to the recording sheet S by the electrostatic force of the electrical field formed by the second transfer roller  54 . 
     Note that the toner that forms the toner image on the intermediate transfer belt  41  is not completely transferred to the recording sheet by the electrostatic force of the second transfer roller  54 . Rather, a portion of the toner may remain on the intermediate transfer belt  41 . In this case, the residual toner on the intermediate transfer belt  41  is electrically or mechanically removed by a residual toner removal device  57 . The residual toner removal device  57  is provided opposite the belt conveyor roller  43  that is located near the process unit  10 Y, with the intermediate transfer belt  41  therebetween. The removed toner is accumulated as waste toner in a waste toner bottle  58 . 
     The recording sheet S that has passed through the transfer nip is transported to a fixing unit  60  provided in the upper part of the image forming unit A. The fixing unit  60  is provided with a heat roller  61  and a pressure roller  62  that are pressed into contact with each other. A fixing nip is formed between the heat roller  61  and pressure roller  62  by the two rollers being pressed together. A heat lamp  63  is provided in the center of the shaft of the heat roller  61  in order to heat the heat roller  61 . 
     The recording sheet S passes through the transfer nip, is transported to the fixing unit  60 , and then is heated and pressed while passing through the fixing nip. This heating and pressing fixes the toner image on the recording sheet S. With the toner image fixed thereon by the fixing unit  60 , the recording sheet S is ejected into a copy receiving tray  56  provided at an upper part of the printer. The side of the recording sheet S bearing the toner image faces down. 
     The developing device  20  provided in each of the process units  10 Y,  10 M,  10 C, and  10 K has the same structure, and a two-component developer (carrier and toner) is located in each of the developing devices  20 . Each developing device  20  uses two-component developer to develop the electrostatic latent image formed on the corresponding photoreceptor drum  11  provided facing the developing device  20 . 
       FIG. 2  is a cross-section diagram of the developing device  20 .  FIG. 3  is a perpendicular cross-section diagram along the line E-E in  FIG. 2 , and  FIG. 4  is a perpendicular cross-section diagram along the line F-F in  FIG. 2 . 
     As shown in  FIG. 2 , the developing device  20  includes a housing  21  that extends along the length of the photoreceptor drum  11 . The housing  21  contains the two-component developer, which includes toner and carrier. A feed chamber  21   s  and a mixing chamber  21   p  are provided inside the housing  21 . The feed chamber  21   s  is provided near the photoreceptor drum  11 , and the mixing chamber  21   p  is provided at a lower part of the housing  21 , on the opposite side of the feed chamber  21   s  than the photoreceptor drum  11 . The feed chamber  21   s  and the mixing chamber  21   p  are separated by a partition  26 . 
     At an upper part of the feed chamber  21   s  in the housing  21 , an opening  21   a  is provided facing a lower part of the outer circumferential surface of the photoreceptor drum  11 . Also at the upper part of the feed chamber  21   s , a developing roller  25  is provided facing the lower part of the outer circumferential surface of the photoreceptor drum  11  through the opening  21   a . As the second developer transport member, a feed screw  23  is provided at a lower part of the feed chamber  21   s , facing the developing roller  25 . 
     Like a developing roller in a conventional developing device that uses two-component developer, the developing roller  25  includes a cylindrical developing sleeve  25   a  and a magnetic roller  25   b . The developing sleeve  25   a  is provided parallel to the photoreceptor drum  11 , and the magnetic roller  25   b  is provided in the developing sleeve  25   a . The position of the magnetic roller  25   b  is fixed with respect to the housing  21 , and the developing sleeve  25   a  is supported by the housing  21  so as to rotate about the magnetic roller  25   b . The developing sleeve  25   a  rotates in the direction indicated by the arrow Y in  FIG. 2 . 
     As shown in  FIG. 3 , the feed screw  23  includes a rotating shaft  23   a  and a spiral transport blade  23   b . The transport blade  23   b  is wound in a predetermined direction and is provided along the rotating shaft  23   a , except at either edge thereof. The rotating shaft  23   a  is parallel to the developing sleeve  25   a , which is in a horizontal position. The edges of the rotating shaft  23   a  are rotatably supported by faces  21   b  and  21   c  of the housing  21 . The face  21   b  is at the front of the printer (hereinafter “front face”), and the face  21   c  is at the back of the printer (hereinafter “back face”). 
     The rotating shaft  23   a  rotates in a predetermined direction due to a turning force. When the rotating shaft  23   a  rotates in the predetermined direction, the transport blade  23   b  rotates along with the rotating shaft  23   a , and developer is transported in the horizontal direction indicated by the arrow D 3  in  FIG. 3 . A portion of the developer transported by the transport blade  23   b  is provided to the outer circumferential surface of the developing sleeve  25   a.    
     As shown in  FIG. 2 , a mixing screw  22  is provided in the mixing chamber  21   p  as the first developer transport member. The mixing screw  22  is positioned close to and diagonally lower than the feed screw  23 , with the partition  26  therebetween. The mixing screw  22  is parallel to the feed screw  23  with respect to the horizon. The axis of the mixing screw  22  is located on a plane that forms approximately a 15° angle, for example, with respect to a horizontal plane that traverses the axis of the feed screw  23 . 
     As shown in  FIG. 3 , the partition  26  is in the form of a plate, extending in the direction of length of the mixing chamber  21   p  and the feed chamber  21   s . The direction of width of the partition  26  (perpendicular to the directions of length and thickness) is perpendicular to a plane that traverses the axes of both the mixing screw  22  and the feed screw  23 . Accordingly, the direction of width of the partition  26  is inclined at approximately a 15° angle with respect to a vertical plane. 
     The partition  26  has a constant thickness throughout (10 mm in Embodiment 1), and the surface facing the inside of the feed chamber  21   s  (feed chamber inner surface  26   b ) and the surface facing the inside of the mixing chamber  21   p  (mixing chamber inner surface  26   c ) are parallel. Accordingly, the feed chamber inner surface  26   b  and the mixing chamber inner surface  26   c  are both inclined at approximately a 15° angle with respect to the vertical plane. 
     As shown in  FIGS. 3 and 4 , the mixing screw  22  provided in the mixing chamber  21   p  is longer than the feed screw  23  and includes a rotating shaft  22   a  and a spiral transport blade  22   b . One edge (front edge) of the rotating shaft  22   a  is rotatably supported by the front face  21   b  of the housing  21 . The transport blade  22   b  is wound along the rotating shaft  22   a , except at either edge thereof. The transport blade  22   b  is wound in a spiral in the same direction as the transport blade  23   b  of the feed screw  23 . 
     The back edge of the rotating shaft  22   a  of the mixing screw  22  is located nearer the back of the printer than the back face  21   c  of the housing  21  and is covered by a cylindrical casing  21   d . The casing  21   d  is integrated into the back face  21   c  that supports the rotating shaft  23   a  of the feed screw  23 . A back edge of the rotating shaft  22   a  of the mixing screw  22  is rotatably supported by a back face  21   h  of the casing  21   d . A feed port  21   e  is provided at the top of the casing  21   d  for toner fed by a toner transport tube (not shown in the figures). 
     The rotating shaft  22   a  of the mixing screw  22  rotates in the opposite direction as the rotating shaft  23   a  of the feed screw  23 . The transport blade  22   b  of the mixing screw  22  rotates along with the rotating shaft  22   a , thereby transporting developer in the mixing chamber  21   p  in the opposite direction as the direction in which the feed screw  23  transports developer, i.e. in the approximately horizontal direction indicated by the arrow D 1  in  FIG. 3 . 
     As shown in  FIG. 2 , a bottom  21   f  of the mixing chamber  21   p  is semi-cylindrical, in accordance with the region in which the transport blade  22   b  of the mixing screw  22  rotates. A bottom  21   g  of the feed chamber  21   s  is also semi-cylindrical, in accordance with the region in which the transport blade  23   b  of the feed screw  23  rotates. The bottom  21   f  of the mixing chamber  21   p  and the bottom  21   g  of the feed chamber  21   s  both connect with the partition  26 . 
     The bottom  21   f  of the mixing chamber  21   p  is continuous with a lower part of the cylindrical casing  21   d , i.e. a semi-cylindrical circumferential portion of the casing  21   d . The inside of the casing  21   d  forms part of the mixing chamber  21   p . Through the feed port  21   e  provided at the top of the casing  21   d , toner flows into the casing  21   d  at the upstream edge of the mixing screw  22 , flowing into the region in which the transport blade  22   b  of the mixing screw  22  rotates. 
     Toner that is fed at the upstream edge of the mixing screw  22  is transported by rotation of the mixing screw  22 . Toner is transported together with developer in the mixing chamber  21   p  horizontally along the rotating shaft  22   a  towards the front face  21   b  of the housing  21 , as indicated by the arrow D 1  in  FIG. 3 . The developer is thus transported to the downstream edge of the mixing screw  22 . 
     As shown in  FIG. 3 , a first communication passage  28  and a second communication passage  29  are provided in the housing  21  at both sides of the partition  26  in a direction of length, in order to connect the inside of the mixing chamber  21   p  to the inside of the feed chamber  21   s . The first communication passage  28  faces the downstream edge of the mixing screw  22  (first developer transport member) and the upstream edge of the feed screw  23  (second developer transport member). Conversely, the second communication passage  29  faces the upstream edge of the mixing screw  22  and the downstream edge of the feed screw  23 . 
     At the downstream edge of the mixing screw  22  in the mixing chamber  21   p , developer transported by the mixing screw  22  traverses the first communication passage  28 , as indicated by the arrow D 2  in  FIG. 3 , and is fed to the feed chamber  21   s . Developer fed to the feed chamber  21   s  is transported by the feed screw  23  in the direction indicated by the arrow D 3  in  FIG. 3 . During transport, a portion of the developer is provided to the outer circumferential surface of the developing sleeve  25   a  in the developing roller  25 . 
     Developer not fed to the developing sleeve  25   a  by the feed screw  23  is transported to the downstream edge of the feed screw  23 . At the downstream edge, the developer flows in the direction indicated by the arrow D 4  in  FIG. 3  into and through the second communication passage  29 , thus being fed to the mixing chamber  21   p . Developer fed to the mixing chamber  21   p  is transported by the mixing screw  22  towards the first communication passage  28 . Developer thus circulates through the housing  21 . 
     Due to magnetic properties of the magnetic roller  25   b , developer that is provided to the developing roller  25  by the feed screw  23  forms a magnetic brush on the outer circumferential surface of the developing sleeve  25   a , which rotates around the fixed magnetic roller  25   b . The toner attached to the magnetic brush attaches to the electrostatic latent image on the photoreceptor drum  11 . The electrostatic latent image is thereby developed with toner. 
     Note that during such development, the concentration of toner in the developer stored in the housing  21  is detected by, for example, a magnetic sensor (not shown in the figures). If the concentration is below a predetermined level, toner is fed via a toner transport tube (not shown in the figures) from a corresponding toner cartridge (not shown in the figures) to the feed port  21   e  provided in the casing  21   d  of the housing  21 . 
     The first communication passage  28  is provided between (i) one edge of the partition  26  in the direction of length thereof near the upstream edge of the feed screw  23  and (ii) a guide member  24  provided at the front face  21   b  of the housing  21 . The central axis of the first communication passage  28  is perpendicular to the directions of length of the mixing screw  22  and the feed screw  23 . The width of the first communication passage  28  is constant along the edge of the partition  26 . 
       FIG. 5  is an enlarged diagram of a principal part shown by the letter P in  FIG. 3 . As shown in  FIG. 5 , the second communication passage  29  is formed between (i) an edge of the partition  26  in the direction of length thereof near the downstream edge of the feed screw  23  and (ii) a guide member  27  provided at the back face  21   c  of the housing  21  facing this edge of the partition  26 . The dimensions of the guide member  27  in the directions of thickness and width of the partition  26  are equal to the corresponding dimensions of the partition  26 , and the guide member  27  is provided across from the partition  26 . 
     At the downstream edge of the feed screw  23 , the guide member  27  has a flat guiding surface (inner surface)  27   a  to guide developer to the mixing chamber  21   p  after the developer flows into the second communication passage  29 . The guiding surface  27   a  is one of the two inner surfaces of the second communication passage  29 . The guide member  27  also has a lateral surface  27   b  that is located along the same plane as the mixing chamber inner surface  26   c  of the partition  26 . With respect to the lateral surface  27   b , the guiding surface  27   a  is inclined at a predetermined acute angle α. Note that the guiding surface  27   a  abuts the back face  21   c  of the housing  21  on the side of the feed chamber  21   s.    
     The partition  26  has a flat opposing surface (inner surface)  26   a  that faces the guiding surface  27   a  with the second communication passage  29  therebetween. With respect to the feed chamber inner surface  26   b  of the partition  26 , the opposing surface  26   a  is inclined at the predetermined acute angle α. The guiding surface  27   a  and the opposing surface  26   a  are parallel to each other, and accordingly, the opposing surface  26   a  is included at an obtuse angle (180°−α) with respect to the mixing chamber inner surface  26   c  of the partition  26 . 
     Developer that is transported by the feed screw  23  in the feed chamber  21   s  flows into the second communication passage  29  and then flows through the second communication passage  29  towards the mixing chamber  21   p . The central axis of the second communication passage  29  is parallel to the guiding surface  27   a  and the opposing surface  26   a  and is therefore inclined at the predetermined angle α with respect to the feed chamber inner surface  26   b  of the partition  26 . Accordingly, developer that flows into the second communication passage  29  is guided in the direction of the central axis of the second communication passage  29 , and upon reaching the mixing chamber  21   p , the developer then flows towards the first communication passage  28 . 
     The second communication passage  29  has an inlet, into which developer flows, facing the feed chamber  21   s . The inlet is defined by a first inflow edge  26   d  and a second inflow edge  27   d . The first inflow edge  26   d  is the edge formed where the feed chamber inner surface  26   b  of the partition  26  and the opposing surface  26   a  meet at the predetermined angle α, and the second inflow edge  27   d  is the edge formed where the guiding surface  27   a  of the guide member  27  and the back face  21   c  of the housing  21  meet. 
     The second communication passage  29  has an outlet, out of which developer flows, facing the mixing chamber  21   p . The output opening is defined by a first outflow edge  26   e  and a second outflow edge  27   c . The first outflow edge  26   e  is the edge formed where the mixing chamber inner surface  26   c  of the partition  26  and the opposing surface  26   a  meet, and the second outflow edge  27   c  is the edge formed where the guiding surface  27   a  of the guide member  27  and the lateral surface  27   b  of the guiding surface  27   a  meet. 
     In the second communication passage  29  in Embodiment 1, the first inflow edge  26   d  is located further downstream in the transportation direction of the feed screw  23  (upstream in the transportation direction of the mixing screw  22 ) than the second outflow edge  27   c . Therefore, the second outflow edge  27   c  is closer to the first communication passage  28  than the first inflow edge  26   d.    
     A printer with developing devices  20  having the above structure is usually placed in a room within an office or similar location so that the axes of the photoreceptor drums  11  (the main scanning direction) in the process units  10 Y,  10 M,  10 C, and  10 K are horizontal. However, rather than being placed horizontally, the printer may be positioned at an incline, with the front side higher than the back side. In this case, each of the developing devices  20  is therefore positioned at the same incline. 
     The central axis of the second communication passage  29  is inclined so that the central axis gradually approaches the first communication passage  28  from the inlet at the feed chamber  21   s  towards the outlet at the mixing chamber  21   p . Therefore, when the housing  21  is inclined so that the first communication passage  28  is higher than the second communication passage  29  as in the above case, the inclination of the central axis of the second communication passage  29  decreases with respect to the horizon. 
     The flow of developer from the feed chamber  21   s  through the second communication passage  29  into the mixing chamber  21   p  per unit time thus decreases. As a result, the problem of developer accumulating near the second communication passage  29  in the mixing chamber  21   p  does not occur, unlike in a conventional configuration. Accordingly, developer is smoothly transported by the mixing screw  22  without the problem, which occurs in a conventional developing device, of difficulty in transporting developer located at the back of the mixing screw  22 . 
     As a result, there is no risk of a decrease in the amount of developer flowing from the mixing chamber  21   p  through the first communication passage  28  to the feed chamber  21   s . Developer is thus stably fed by the feed screw  23  in the feed chamber  21   s  to the developing sleeve  25   a  in the developing roller  25 . Excellent development of the electrostatic latent image on the photoreceptor drum  11  is thus achieved by the developing sleeve  25   a  with the toner fed thereto. 
     Note that in the developing device  20  in Embodiment 1, the mixing screw  22  is positioned diagonally lower than, and parallel to, the feed screw  23  (see  FIG. 2 ). However, the developing device  20  is not limited to this structure. The mixing screw  22  may be positioned vertically below the feed screw  23 , or the mixing screw  22  and the feed screw  23  may be positioned parallel to each other in a horizontal plane. 
     If the mixing screw  22  is positioned vertically below the feed screw  23 , a cross-section diagram of a plane perpendicular to the horizon traversing the axis of both the mixing screw  22  and the feed screw  23  is the same as the diagram in  FIG. 5 . 
     In this case as well, when the housing  21  in the developing device  20  is inclined so that the first communication passage  28  is higher than the second communication passage  29 , the inclination of the central axis of the second communication passage  29  decreases with respect to the horizon. The amount of developer flowing from the feed chamber  21   s  through the second communication passage  29  to the mixing chamber  21   p  thus decreases. As a result, the amount of developer in the feed chamber  21   s  does not decrease, and developer is stably fed to the developing sleeve  25   a  in the developing roller  25 . 
     If the developing device  20  is structured so that the mixing screw  22  and the feed screw  23  are positioned parallel to each other in a horizontal plane, then in this case as well, when the housing  21  is inclined so that the first communication passage  28  is higher than the second communication passage  29 , the outlet of the second communication passage  29  located at the side of the mixing chamber  21   p  rises a greater amount than the inlet located at the side of the feed chamber  21   s , since the outlet is closer to the first communication passage  28 . 
     As a result, developer does not unnecessarily accumulate at the back of the mixing screw  22  due to a greater flow of developer than the transport ability of the mixing screw  22 , a situation which would cause a reduction in the total amount of developer in circulation. Rather, the amount of developer flowing from the feed chamber  21   s  to the mixing chamber  21   p  decreases. Accordingly, the amount of developer in the feed chamber  21   s  does not decrease, and developer is stably fed to the developing sleeve  25   a  in the developing roller  25 . 
     An experiment was performed on the developing device  20  according to Embodiment 1, the developing device  20  having been structured so that the mixing screw  22  was located vertically below the feed screw  23  ( FIG. 5  showing a perpendicular cross-section diagram that traverses the mixing screw  22  and the feed screw  23 ). The experiment was performed under the following conditions to test whether feeding developer to the developing sleeve  25   a  in the developing roller  25  becomes problematic when the housing  21  is inclined so that the first communication passage  28  is higher than the second communication passage  29 . 
     An aluminum (Al) cylinder with an outside diameter of 16 mm was used as the developing sleeve  25   a  in the developing roller  25 . Sandblasting was performed on the outer circumferential surface of the developing sleeve  25   a . The rotation frequency of the developing sleeve  25   a  was set to 360 rpm. 
     The feed screw  23  was formed by a rotating shaft  23   a  with a diameter of 6 mm and a spiral transport blade  23   b  with a diameter (screw diameter) of 14 mm and a pitch of 25 mm. The rotation frequency of the feed screw  23   a  was set to 340 rpm. 
     The mixing screw  22  was formed with the same structure as the feed screw  23 . However, so that developer would be transported in the opposite direction, the spiral transport blade  22   b , which faced the same direction as the transport blade  23   b  in the feed screw  23 , was rotated in the opposite direction as the transport blade  23   b.    
     The developer was composed of ferrite core carrier, having a mean particle diameter of 33 μm and to which acrylic Micro Engineering Coating (MEC) was applied, and toner having a mean particle diameter of 6.5 μm manufactured by emulsion polymerization of a styrene acrylic resin. The developer had a toner concentration of 7%, with 200 g of toner stored in the housing  21 . 
     The openings of the first communication passage  28  were 37 mm×16 mm rectangles, and the openings of the second communication passage  29  were 10 mm×10 mm squares. The second communication passage  29  was formed at an inclination of 30° with respect to the axis of the feed screw  23 . 
     Under the above conditions, the housing  21  in the developing device  20  was inclined so that the first communication passage  28  was positioned both higher and lower than the second communication passage  29 , and the amount of developer fed to the developing sleeve  25   a  was tested as the inclination was varied. Note that determination of the amount of developer fed to the developing sleeve  25   a  was based on the amount of developer that attached to the outer circumferential surface of the developing sleeve  25   a . The graph in  FIG. 6  shows the results of the experiment. 
     The graph in  FIG. 6  shows both the case when the first communication passage  28  was positioned higher than the second communication passage  29 , as well as the opposite case when the second communication passage  29  was positioned higher than the first communication passage  28 , respectively indicated by a negative and a positive inclination angle. Note that when the amount of toner transported to the outer circumferential surface of the developing sleeve  25   a  is equal to or less than 200 g/m 2 , the feed of developer to the developing sleeve  25   a  was considered insufficient. 
     As the graph in  FIG. 6  shows, even at an inclination angle of the developing device  20  of −10°, the problem of insufficient feed of developer to the developing sleeve  25   a  did not occur. However, at an inclination angle of less than −10°, developer that accumulated near the second communication passage  29  on the side of the feed chamber  21   s  flowed out through the opening  21   a  in the housing  21 , i.e. the opening into which the developing roller  25  fits. 
     For comparison, an experiment was performed under the same conditions as the above experiment using a developing device structured in the same way as the developing device  20  used in the above experiment, with the exception that the guiding surface  27   a  facing the second communication passage  29  was formed to be perpendicular to the transport directions of the mixing screw  22  and the feed screw  23 . The dashed line (comparative example) in the graph in  FIG. 6  indicates the experimental results. In this case, the feed of developer to the developing sleeve  25   a  was insufficient when the inclination angle of the housing was below −2°. 
     Note that when the mixing screw  22  is positioned vertically below the feed screw  23  in the developing device  20 , it is preferable that the second outflow edge  27   c  (which is along the guiding surface  27   a  at the side of the mixing chamber  21   p , i.e. located at the upstream edge of the mixing screw  22 ) be positioned closer to the first communication passage  28  than the first inflow edge  26   d  (which is along the opposing surface  26   a  that faces the second communication passage  29 , i.e. located at the downstream edge of the feed screw  23 ), as in Embodiment 1. 
     With this structure, if the housing  21  in the developing device  20  is inclined so that the second communication passage  29  is positioned higher than the first communication passage  28 , then developer flows through the second communication passage  29  at a reduced speed, since the inclination angle of the guiding surface  27   a , which the developer hits and flows along, is smaller with respect to the horizon. The amount of developer fed from the feed chamber  21   s  to the mixing chamber  21   p  per unit time thus decreases. 
     Note that Embodiment 1 is not limited to this structure. The second outflow edge  27   c  along the guiding surface  27   a  may be positioned closer to the downstream edge of the feed screw  23  (the upstream edge of the mixing screw  22 ) than the first inflow edge  26   d  along the opposing surface  26   a  that faces the second communication passage  29 . 
       FIGS. 7 and 8  show examples of a second communication passage  29  with this structure.  FIG. 7  shows a developing device  20  in which the mixing screw  22  is positioned vertically below the feed screw  23 , and the inclination angle of the guiding surface  27   a  with respect to the axis of the feed screw  23  is 180°-60° (accordingly, the inclination angle of the opposing surface  26   a  with respect to the axis of the feed screw  23  is 60°). Furthermore, the opposing surface  26   a  facing the second communication passage  29  does not vertically overlap with the guiding surface  27   a.    
       FIG. 8  shows a developing device  20  in which the mixing screw  22  is positioned vertically below the feed screw  23 , the thickness of the partition  26  is half (i.e. 5 mm) the thickness of the partition  26  in Embodiment 1, and the opposing surface  26   a  facing the second communication passage  29  does not vertically overlap with the guiding surface  27   a.    
     In the developing device  20  structured as in  FIGS. 7 and 8 , the amount of developer that passes through the second communication passage  29  increases slightly over the developing device  20  in  FIG. 5 , yet the problem of insufficient toner fed to the developing sleeve  25   a  due to an inclination of the housing  21  is still controlled. 
     The same experiment as described above was also performed on the developing devices  20  structured as in  FIG. 7  and as in  FIG. 8  to test whether the problem of insufficient developer fed to the developing sleeve  25   a  occurred. The graph in  FIG. 9  shows the results of the experiments. The solid line in the graph in  FIG. 9  indicates the experimental results for the developing device  20  structured as in  FIG. 7 , and the alternating long and short dashed line indicates the experimental results for the developing device  20  structured as in  FIG. 8 . Note that the dashed line in the graph in  FIG. 9  indicates the experimental results of the comparative example shown by the dashed line in the graph in  FIG. 6 . 
     In the case of the developing device  20  structured as in  FIG. 7 , when the inclination angle of the developing device  20  was below −4° as compared to the horizon, the problem of insufficient developer fed to the developing sleeve  25   a  occurred. This inclination angle, however, is still larger than the inclination angle of the developing device  20  in the comparative example at which the problem of insufficient developer fed to the developing sleeve  25   a  occurred (−2°). 
     In the case of the developing device  20  structured as in  FIG. 8 , when the inclination angle of the developing device  20  was below −5° as compared to the horizon, the problem of insufficient developer fed to the developing sleeve  25   a  occurred. In this case as well, however, this inclination angle is still larger than the inclination angle of the developing device  20  in the comparative example at which the problem of insufficient developer fed to the developing sleeve  25   a  occurred (−2°). 
     Note that in Embodiment 1, factors such as the amount of developer transported by the mixing screw  22  and the feed screw  23  in the developing device  20  vary depending on the structure and location of the mixing screw  22  and the feed screw  23 , the position of the screws, the composition of the toner and the carrier, etc. Therefore, the inclination angle of the developing device  20  at which the problem of insufficient developer fed to the developing sleeve  25   a  occurs also varies. Therefore, the inclination angle, axis length, cross-sectional area, cross-sectional shape, etc. of the second communication passage  29  should be appropriately changed in accordance with factors such as the configuration of the mixing screw  22  and the feed screw  23 . 
     For example, if the guiding surface  27   a  in the second communication passage  29  is perpendicular to the transport direction of the feed screw  23 , then based on the inclination angle of the developing device  20  at which the problem of insufficient developer fed to the developing sleeve  25   a  occurs, the inclination angle of the second communication passage  29  with respect to the feed screw  23 , the axis length and the cross-sectional area of the second communication passage  29 , etc. may be determined. 
     Embodiment 2 
       FIG. 10  is a cross-section diagram of a second communication passage  29  in a developing device  20  according to Embodiment 2 of the present invention. In Embodiment 2, the mixing screw  22  is positioned vertically below the feed screw  23 . Therefore, when the developing device  20  is positioned along the main scanning direction, the partition  26  is horizontal. The second communication passage  29  is provided between the back face  21   c  of the housing  21  and the opposing surface  26   a  of the partition  26 . The opposing surface  26   a  of the partition  26  is parallel to the back face  21   c  of the housing  21 . 
     A compartment  26   m  is formed inside the partition  26  at the back edge thereof. The compartment  26   m  is formed along almost the entire width of the partition  26 . At the center, in the direction of width, of the side wall of the partition  26  constituting the opposing surface  26   a , a communication aperture  26   n  is provided to connect the inside of the compartment  26   m  with the second communication passage  29 . Inside the compartment  26   m , an opening space reconfiguration unit  30  is provided. By changing the width of the opening of the second communication passage  29  in the direction of length of the partition  26 , the opening space reconfiguration unit  30  changes the space through which the developer passes. 
     The opening space reconfiguration unit  30  includes a closure plate  31  that extends across almost the entire width of the partition  26 . The closure plate  31  is disposed in the compartment  26   m  so as to be capable of parallel displacement in the direction of length of the partition  26 . The closure plate  31  undergoes parallel displacement in the compartment  26   m  due to its own weight when the partition  26  is inclined so that the edge of the partition  26  containing the compartment  26   m  (by the second communication passage  29 ) is lowered. The closure plate  31  thus protrudes into the second communication passage  29  through the communication aperture  26   n  provided in the partition  26 . 
     When the partition  26  is inclined so that the edge of the partition  26  containing the compartment  26   m  lowers, and the closure plate  31  protrudes through the communication aperture  26   n , then the closure plate  31  partially closes off the second communication passage  29  on the side of the partition  26 . The opening of the second communication passage  29  in the direction of length of the partition  26  (inclination direction) thus becomes smaller, decreasing the opening space in the second communication passage  29  through which the developer flows. The amount of developer flowing through the second communication passage  29  thus decreases. 
     A stopper  31   a  is provided on the closure plate  31  at the end of the closure plate  31  towards the back of the compartment  26   m . The stopper  31   a  prevents the closure plate  31  from leaving the compartment  26   m  when a predetermined length of the closure plate  31  protrudes into the second communication passage  29  through the communication aperture  26   n.    
     Note that it is preferable to set the dimensions of the stopper  31   a  so that the closure plate  31  does not completely cut off the second communication passage  29  even if the developing device  20  is inclined to a greater degree than assumed. If the second communication passage  29  is completely cut off by the stopper  31   a , no developer will be transported to the mixing chamber  21   p , causing developer to accumulate in the feed chamber  21   s  and likely causing the developing device  20  to break down. 
     An extension spring  32  is provided inside the compartment  26   m  as a biasing means to provide a bias force to the closure plate  31  in the compartment  26   m . The extension spring  32  is provided, for example, at the center of the direction of width of the partition  26 . One edge of the extension spring  32  is attached to the back surface of the compartment  26   m , and the other edge of the extension spring  32  is attached to the closure plate  31 . The spring constant of the extension spring  32  is set to a predetermined value so that, when the partition  26  is inclined so that the edge of the partition  26  containing the compartment  26   m  (by the second communication passage  29 ) lowers, the length of the closure plate  31  protruding into the second communication passage  29  increases by 1 mm each time the inclination angle increases by one degree. 
     In a developing device  20  with the above structure, when the housing  21  in the developing device  20  is inclined so that the first communication passage  28  is higher than the second communication passage  29 , a gravitational force proportional to the inclination angle is exerted on the closure plate  31 . As shown in  FIG. 11 , the gravitational force causes the closure plate  31  to protrude into the second communication passage  29  through the communication aperture  26   n , shortening the width of the second communication passage  29  along the partition  26 . 
     Since the spring constant of the extension spring  32  is set so that the length of the closure plate  31  protruding into the second communication passage  29  increases by 1 mm each time the inclination angle of the developing device  20  increases by one degree with respect to the horizon, the length of the closure plate  31  protruding into the second communication passage  29  increases proportionally to an increase in the inclination angle of the housing  21 . Accordingly, as the inclination angle of the developing device  20  increases with respect to the horizon, the area of the second communication passage  29  that is cut off by the closure plate  31  increases. 
     Therefore, when the closure plate  31  protrudes into the second communication passage  29 , a portion of the second communication passage  29  near the opposing surface  26   a  of the partition  26  is cut off, and the width of the second communication passage  29  along the direction of length of the partition  26  decreases. Since the length of the closure plate  31  protruding into the second communication passage  29  increases proportionally to an increase in the inclination angle of the housing  21  with respect to the horizon, the width of the second communication passage  29  also decreases proportionally to an increase in the inclination angle of the housing  21  with respect to the horizon. 
     As the width of the second communication passage  29  decreases, the amount of developer transported per unit of time from the feed chamber  21   s  through the second communication passage  29  to the mixing chamber  21   p  decreases. The amount of developer flowing through the second communication passage  29  into the mixing chamber  21   p  thus decreases. As a result, the problem of developer accumulating in the mixing chamber  21   p  by the second communication passage  29  does not exist, and the amount of developer transported by the mixing screw  22  to the feed chamber  21   s  does not decrease, as is also the case in the developing device  20  in Embodiment 1. Developer can thus be stably fed to the developing sleeve  25   a  in the developing roller  25 . 
     The same experiment as performed on the developing device  20  in Embodiment 1 was also performed under the following conditions on the developing device  20  in Embodiment 2 to test whether the problem of insufficient toner fed to the developing sleeve  25   a  occurred. 
     The thickness (height in a vertical direction) of the partition  26  in the developing device  20  of Embodiment 2 was 10 mm. The length of the closure plate  31  in the direction of length of the partition  26  was 12 mm including the stopper  31   a  (10 mm excluding the stopper  31   a ). The thickness of the closure plate  31  (height in the vertical direction of the partition  26 ) excluding the stopper  31   a  was 4 mm. The length of the stopper  31   a  in the direction of length of the partition  26  was 2 mm, and the thickness of the stopper  31   a  (height in the vertical direction of the partition  26 ) was 6 mm. The closure plate  31  was made entirely of steel use stainless (SUS) and had a mass of 40 g. 
     The spring constant of the extension spring  32  was set to 0.0068 N/mm so that, when housing  21  in the developing device  20  was inclined so that the first communication passage  28  was higher than the second communication passage  29 , the length of the closure plate  31  protruding into the second communication passage  29  increased by 1 mm for each degree of inclination angle (in this context, the inclination angle is negative). 
     In the developing device  20  in Embodiment 2 structured as above, even at an inclination angle of the housing  21  of −10°, the problem of insufficient feed of developer to the developing sleeve  25   a  did not occur. However, at an inclination angle of the housing  21  of less than −10°, developer that accumulated near the second communication passage  29  in the feed chamber  21   s  ran the risk of flowing out through the opening  21   a  in the housing  21 , i.e. the opening into which the developing roller  25  fits. 
     Note that in Embodiment 2 as well, factors such as the amount of developer transported by the mixing screw  22  and the feed screw  23  in the developing device  20  vary depending on the structure and location of the mixing screw  22  and the feed screw  23 , the position of the screws, the composition of the toner and the carrier, etc. Therefore, the inclination angle of the developing device  20  at which the problem of insufficient toner fed to the developing sleeve  25   a  occurs also varies. Therefore, the mass of the closure plate  31 , the spring constant of the extension spring  32 , etc. should be appropriately changed in accordance with factors such as the configuration of the mixing screw  22  and the feed screw  23 . 
     Modifications 
     The present invention is not limited to the above Embodiments. For example, the present invention is not limited to a developing device that uses two-component developer, but may also be adapted for use in a developing device that uses mono-component developer. 
     The present invention may also be adapted for use when only the developing device  20  is removable from the image forming unit A. Furthermore, the image forming apparatus according to the present invention is not limited to a tandem-type color digital printer, but may also be a printer that forms a monochrome image. Instead of a printer, the image forming apparatus may also be a copy machine, FAX, Multiple Function Peripheral (MFP), etc. that can form a color or monochrome image. 
     Summary of Embodiments 
     In the developing device of the present invention with the above structure, when the housing is inclined so that the second communication passage, which is located at the upstream edge of the first developer transport member, is lower than the first communication passage, the amount of developer that is directed through the second communication passage and into the mixing chamber is reduced. Therefore, there is no risk of developer accumulating in the mixing chamber near the second communication passage. Accordingly, transport efficiency of the developer by the first developer transport member does not decrease due to developer accumulating near the second communication passage, and the amount of developer in the feed chamber does not decrease, thus allowing developer to be fed stably to the developing roller in the feed chamber. 
     Preferably, the second communication passage has an outlet located at the mixing chamber and an inlet located at the feed chamber, the outlet being closer to the first communication passage than the inlet. 
     Preferably, the second communication passage has a first inner surface and a second inner surface that oppose each other, the first inner surface being a guiding surface located further downstream in the second direction than the second inner surface and being inclined so as to continuously approach the first communication passage from the inlet to the outlet, and an edge of the guiding surface located at the outlet is closer to the first communication passage than an edge of the second inner surface located at the inlet. 
     Preferably, the first inner surface and the second inner surface are parallel. 
     Preferably, an opening space reconfiguration unit configured to progressively reduce an open space in the second communication passage along a direction of inclination of the housing as the inclination angle of the housing increases, the housing being inclined so that the second communication passage is lower than the first communication passage. 
     Preferably, the opening space reconfiguration unit includes: a closure plate protruding into the second communication passage due to self weight when the housing is inclined so that the second communication passage is lower than the first communication passage and partially closing off the second communication passage; and a biasing unit to provide a bias force to the closure plate so that a length of the closure plate protruding into the second communication passage is approximately proportional to the inclination angle. 
     Preferably, carrier is stored along with toner in the housing. 
     In the context of a developing device in which a first developer transport member and a second developer transport member circulate developer, the first developer transport member transporting developer horizontally while mixing the developer, and the second developer transport member feeding developer, which has been transported by the first developer transport member, to a developing roller while transporting the developer in the opposite direction as the first developer transport member, the present invention is useful when the developing device is inclined with respect to the horizon. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.