Patent Publication Number: US-8977168-B2

Title: Developer unit and image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 12/957,368, filed on Nov. 30, 2010, which claims priority from Japanese Patent Application No. 2009-271828, filed on Nov. 30, 2009, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     An aspect of the present invention relates to a developer unit for an image forming apparatus. 
     2. Related Art 
     An image forming apparatus for forming an image on a recording medium with a developer unit has been known. The developer unit is often provided with a developer device with a developer roller to carry a developer agent on a surface thereof, a supplier roller to supply the developer agent to the developer roller, and a spreader blade to spread the developer agent evenly on the surface of the developer roller. The developer unit may further be provided with a developer agent container to contain the developer agent to be supplied to the developer device. In order to supply the developer agent in homogenized condition so that consistent image-forming quality is maintained, the developer unit may be designed to have the developer agent to be circulated between the developer device and the developer agent container. In such a developer unit, the developer agent container may be arranged in a higher position with respect to the developer device. 
     SUMMARY 
     In the developer unit with the developer agent container arranged in the upper position with respect to the developer device, the developer agent in the developer agent container easily drop down in the developer device by effect of gravity; therefore, pressure of the developer agent in the developer agent container tends to increase easily. The increased pressure may cause troubles in the developer device. For example, the developer roller and the supplier roller may be damaged by the excessively increased pressure. For another example, the developer agent may leak from clearance between the developer roller and the spreader blade. Further, an obstacle may intervene in the clearance. 
     In view of the above deficiencies, the present invention is advantageous in that a developer unit, in which pressure increase of the developer agent in the developer device is prevented, is provided. 
     According to an aspect of the present invention, a developer unit for an image forming apparatus to form an image on a recording sheet is provided. The developer unit includes a developer device having a developer agent carrier, which carries a developer agent on a surface thereof, and a developer agent supplier, which supplies the developer agent to the developer agent carrier, a developer agent container, which contains the developer agent to be supplied to the developer device and is arranged in an upper position with respect to the developer device, and a curved wall, which separates the developer device from the developer agent container; is curved toward the developer agent container; and is formed to have a feeding opening, through which the developer agent from the developer agent container is supplied to the developer device, and a collecting opening, through which the developer agent in the developer device is retrieved in the developer agent container, and a conveyer, which is arranged along the developer agent supplier and rotated about a rotation axis to convey the developer agent supplied to the developer device through the feeding opening toward the collecting opening. The curved wall is arranged to be in proximity to the conveyer and to fit with outlines of the conveyer. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a multifunction peripheral device (MFP) having developer units according to an embodiment of the present invention. 
         FIG. 2  is a schematic view of the MFP and the developer units according to the embodiment of the present invention with a holder case removed out of a chassis. 
         FIG. 3  is a cross-sectional side view of the developer unit according to the embodiment of the present invention with first and second shutters in opening positions. 
         FIG. 4A  is a perspective view of a toner box of the developer unit according to the embodiment of the present invention with the first shutter in a closing position.  FIG. 4B  is a perspective view of the toner box of the developer unit according to the embodiment of the present invention with the first shutter in the opening position. 
         FIG. 5  illustrates a flow of the toner circulated in the developer unit according to the embodiment of the present invention. 
         FIG. 6  is a perspective view of the developer device according to the embodiment of the present invention. 
         FIG. 7  is a cross-sectional side view of the developer unit with the first and second shutters in the closing positions. 
         FIGS. 8A and 8B  are schematic views to illustrate opening and closing movements of the first and second shutters of the developer unit according to the embodiment of the present invention. 
         FIGS. 9A-9C  illustrate the flow of the toner circulation in the developer unit according to the embodiment of the present invention. 
         FIG. 10  is a cross-sectional side view of an integrally-formed developer unit according to an embodiment of the present invention with the toner box and the developer device undetachably fixed to each other. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. A color multifunction peripheral device (hereinafter, MFP)  1  represents an image processing device having a developer unit  61  according to the present invention. 
     Overall Configuration of the MFP 
     As shown in  FIG. 1 , the MFP  1  is equipped with a chassis  10  and a flatbed scanner  20 . In the chassis  10 , the MFP  1  is provided with a sheet-feed unit  30 , which feeds recording sheet P in a feeding path, an image forming unit  40 , which forms an image on the sheet P being fed, and a discharge unit  90 , which ejects the sheet P with the image formed thereon. 
     In the present embodiment, directions concerning the MFP  1  will be referred to in accordance with orientation of the MFP  1  shown in  FIG. 1 . That is, a viewer&#39;s right-hand side appearing in  FIG. 1  is referred to as a front side of the MFP, and a left-hand side, which is opposite side from the front side, is referred to as rear. Further, a viewer&#39;s nearer side is referred to as left, and a further side is referred to as right. Furthermore, vertical (up-down) direction of the MFP  1  corresponds to an up-down direction appearing in  FIG. 1 . Directions of the drawings in  FIG. 2  are similarly based on the orientation of the MFP  1  as defined above and correspond to those with respect to the MFP  1  shown in  FIG. 1 . In  FIGS. 3-7 , directions of the drawings are as indicated by arrows. 
     The flat bed scanner  20  is a known document reader, which is arranged on top of the chassis  10 . The flatbed scanner  20  irradiates light onto a source document to read an image formed thereon and creates image data representing the read image. Thus, the image on the source document can be copied. 
     The sheet-feed unit  30  is arrange in a lower section of the chassis  10 . The sheet-feed unit  30  includes a sheet-feed tray  31 , in which the sheets P are stored, and a sheet-feeder  32 , which conveys the sheets P one-by-one from the sheet-feed tray  31  to the image forming unit  40 . 
     The image forming unit  40  includes an exposure section  50 , a processing section  60 , a transfer section  70 , and a fixing section  80 . 
     The exposure section  50  is arranged in an upper section in the chassis  10  and includes a laser-beam source, a polygon mirror, a lens, and a reflection mirror, which are not shown. A laser beam emitted from the laser-beam source is reflected on the polygon mirror and the reflection mirror and transmits through the lens to be casted to scan on surfaces of photosensitive drums  61 A. 
     The processing section  60  is arranged between the sheet-feed unit  30  and the exposure section  50  and includes four developer units  61 , which are aligned in line along a front-rear direction, and a holder case  62  to hold the developer units  61 . 
     Each of the developer units  61  includes a toner box  100  and a developer device  200 . The toner box contains toner being a developer agent therein. Each toner in the toner box  100  is in a different color, and in the present embodiment, a colored image is formed in the four colored toners. The developer device  200  includes a photosensitive drum  61 A, a charger  61 B, a developer roller  61 C to carry the toner, a supplier roller  61 D, and a spreader blade  61 E (see also  FIG. 3 ). The developer unit  61  including the toner box  100  and the developer device  200  will be described later in detail. 
     The holder case  62  can be installed in the chassis  10  through an opening, which can be covered with a front cover  11 . The holder case  62  has a handle  62 H, and when the front cover  11  is open (see  FIG. 2 ), the holder case  62  can be withdrawn out of the chassis  10  by the handle  62 H. When the holder case  62  is outside the chassis  10 , the toner boxes  100  can be removed from the chassis  10  and replaced with new toner boxes  100 . The developer devices  200  may be either detachable from the holder case  62  or fixed to the holder case  62 . 
     The transfer section  70  is arranged between the sheet-feed unit  30  and the processing section  60 . The transfer section  70  includes a driving roller  71 , a driven roller  72 , and an endless conveyer belt  73 , which is extended to roll around the driving roller  71  and the driven roller  72 , and four transfer rollers  74 . The conveyer belt  73  is arranged to have its upper and outer surface to be in contact with the photosensitive drums  61 A. The transfer rollers  74  are arranged in positions to be in contact with an upper-inner surface of the conveyer belt  73  to nip the conveyer belt  73  with the photosensitive drums  61 A. 
     The fixing section  81  is arranged in a position closer to the rear of the MFP  1  and includes a heat roller  81  and a pressure roller  82 . The pressure roller  82  is arranged in a position opposite from the heat roller  81  and presses the sheet P against the heat roller  81 . 
     In the image forming unit  40 , the charger  61 B charges the surface of the photosensitive drum  61 A evenly, and the surface of the photosensitive drum  61 A is exposed to the laser beam emitted based on the image data from the exposure section  50  in order to form an electrostatic latent image thereon. Meanwhile, the toner in the toner box  100  is supplied to the developer roller  61 C via the supplier roller  61 D and spread evenly in a layer of a predetermined thickness between the spreader blade  61 E and the developer roller  61 C Thus, the toner is carried in a layer by the developer roller  61 C. 
     When the toner on the developer roller  61 C comes in contact with the photosensitive drum  61 A, the toner is supplied to the surface of regions corresponding to the electrostatic latent image formed on the photosensitive drum  61 A. Accordingly, the electrostatic latent image is developed to be a toner image on the photosensitive drum  61 A. As the photosensitive drum  61 A is rotated further, and when the sheet P conveyed on the conveyer belt  73  comes to a position opposite from the photosensitive drum  61 A and the transfer roller  74 , the toner image on the photosensitive drum  61 A is transferred to the sheet P. Thus, an image in a color of the toner is formed on the sheet P. As the sheet P is continuously conveyed in the image forming unit  40 , images in the different colors corresponding to the other toners are sequentially formed on the sheet P so that the colored image is completed. The sheet P is further conveyed in the fixing section  80  between the heat roller  81  and the pressure roller  82 , and the colored image is thermally fixed on the sheet P. 
     The discharge unit  90  includes a discharge guide  91 , which is formed to direct the sheet P from the fixing unit  80  toward upward-front of the MFP  1 , and a discharge roller  92  to eject the sheet P out of the discharge unit  90 . The sheet P with the thermally-fixed toner image is carried along the discharge guide  91  and directed to a discharge tray  12 , which is formed in an upper section of the chassis  10 . 
     Configuration of the Developer Unit 
     The developer unit  61  including the toner box  100  and the developer device  200  will be described in detail hereinbelow. 
     Firstly, the toner box  100  will be described. The toner box  100  is installed in the developer unit  61  in an upper and adjoining position with respect to the developer device  200  (see also  FIG. 2 ) to be detachably attached to the developer device  200 . The toner box  100  is formed to have a shape of a partially-dented cylinder with left and right side walls  101 . In particular, a circumferential surface of the cylinder is formed to have a fitting wall  110  in a position to be adjacent to the developer device  200  when the toner box  100  is attached to the developer device  200 . The fitting wall  110  is curved inward in an arc to fit with an outer peripheral surface of the adjoining developer device  200 . The fitting wall  110  is dented, in a cross-sectional view (see  FIG. 3 ), to center around a reference line BL, which extends in parallel with a rotation shaft  241  of an auger  240  in the developer device  200 . In particular, whilst the rotation shaft  241  extends laterally in the right-left direction (i.e., in the depth direction in  FIG. 3 ) in the MFP  1 , the reference line BL is included inside a rotation trajectory of spirals  242 ,  243  of the auger  240 . More specifically, the reference line BL coincides with a rotation axis C of the auger  240 , as shown in  FIG. 3 , when the auger  240  is viewed from a side. Description of the auger  240  will be given later in detail. 
     In other words, the fitting wall  110  is formed to fit with a part of an outer surface of the developer device  200 . In particular, the fitting wall  110  partially surrounds the auger  240 , which has a cross-sectional shape of a circle, via a half-pipe wall  210  of the developer device  200 . The fitting wall  110  is arranged in a position adjacent to the auger  240  via the half-pipe wall  210  whilst a small amount of clearance, which allows the toner to flow therein, is reserved between the auger  240  and an inner surface of the half-pipe wall  210 . 
     The fitting wall  110  is formed to have a first feeding hole  111  and two first collecting holes  112 . The first feeding hole  111  is an opening, through which the toner stored in the toner box  100  is supplied to the developer device  200 . A flow of supplying the toner through the first feeding hole  111  is indicated by a thick solid downward arrow in  FIG. 3 . The first collecting holes  112  are openings, through which the toner in the developer device  200  is retrieved to be stored in the toner box  100 . A flow of collecting the toner through the first collecting holes  112  is indicated by a thick broken arrow in  FIG. 3 . 
     The first feeding hole  111  and the first collecting holes  112  are formed in laterally (in the right-left direction) displaced positions. The right-left direction in the present embodiment corresponds to the direction of the rotation shaft  240  of the auger  240 . As shown in  FIGS. 4A  and  4 B, the first feeding hole  111  is formed in the fitting wall  110  in a central area with respect to the right-left direction and in an upper position in the central area. Each first collecting hole  112  is formed in vicinity of either a left or a right side end of the fitting wall  110  and in a lower position. 
     Further, as shown in the cross-sectional view in  FIG. 3 , the first feeding hole  111  is formed in the fitting wall  110  in an upper-stream position in a rotating direction of an agitator  140  with respect to a nearest point NP, which is in a shortest distance from a rotation axis  141  of the agitator  140 . Meanwhile, the first collecting holes  112  are formed in lower-stream positions in the rotating direction of the agitator  140  with respect to the nearest point NP. The agitator  140  will be described later in detail. 
     According to the present embodiment, an angle θ 2  between a direction D 2 , along which the first collecting holes  112  are oriented, and a vertical (up-down) direction DV, is larger than an angle θ 1  between a direction D 1 , along which the first feeding hole  111  is oriented, and the vertical direction DV. Therefore, inclination of the direction D 2 , which corresponds to the orientation of the first collecting holes  112 , is closer to a horizontal line DH than inclination of the direction D 1 , which corresponds to the orientation of the first feeding hole  111 . Further, the inclination of the direction D 1  is closer to the vertical direction DV than the inclination of the direction D 2 . 
     The toner box  100  includes a first shutter  120  (see  FIGS. 3 ,  4 A, and  4 B), which is slidable along the curvature of the fitting wall  110 , to cover and expose the first feeding hole  111  and the first collecting holes  112 . The first shutter  120  includes a metal plate  121 , which is formed to curve along the fitting wall  110 , and a pair of slider pieces  122 , which are attached to right and left side edges of the metal plate  121 . 
     The metal plate  121  is formed to have two openings  123 . Each opening  123  is formed in the vicinity of the right and left side edges of the first shutter  120 . When the first shutter  120  is in a closing position (i.e., a front position as shown in  FIG. 4A ), the first feeding hole  111  and the first collecting holes  112  are covered with the metal plate  121 . When the first shutter  120  is shifted in an opening position (i.e., a rear position as shown in  FIG. 4B ), the first feeding hole  111  is uncovered, and the openings  123  coincide with the first collecting holes  112 . Accordingly, the first collecting holes  112  and the first feeding hole  111  are exposed. 
     The slider pieces  122 , respectively arranged on the right and left side edges of the metal plate  121 , are supported by right and left side ends of the fitting wall  110  and slidable with respect to and along the curvature of the fitting wall  110 . Each slider piece  122  is formed to have a plurality of dents  124  on its outer circumferential edge. Further, the slider piece  122  is formed to have a plurality of teeth  125  on its inner circumferential edge (see also  FIGS. 8A ,  8 B). 
     The toner box  100  further includes a shutter handler  130 , which can manipulate the first shutter  120  and a second shutter  220 . The shutter handler  130  includes a pair of supporting parts  131  and a handle  132 . The supporting parts  131  are attached to the side walls  101  by a rotation shaft  130 A and rotatable about the rotation shaft  130 A. The handle  132  is a bar extending in parallel with the rotation shaft  130 A and connects the left and right supporting parts  131 . 
     The supporting part  131  is formed to have a plurality of teeth  134 , which partially surround the rotation shaft  130 A, on a side across the rotation shaft  130 A from the handle  132 . The teeth  134  can be interlocked with the dents  124  in the slider piece  122  so that the first shutter  120  is shifted along the circumference of the fitting wall  110  in cooperation with rotating movement of the supporting parts  131 , which are rotated according to a user&#39;s manipulation to the handle  132  (see  FIGS. 8A ,  8 B). 
     Inside the toner box  100 , an agitator  140  to stir the toner in the toner box  100  is provided (see  FIG. 3 ). The agitator  140  has a rotation shaft  141 , a support  142 , and a plurality of wings  143  (see also  FIG. 5 ). The rotation shaft  141  is rotatably supported by the left and right side walls  101 . The support  142  extends from the rotation shaft  141  radially, and the wings  143  being flexible sheets are fixed to the support  142 . 
     The agitator  140  is rotated by driving force from a motor (not shown) transmitted to the rotation shaft  141 . In the present embodiment, the agitator  140  is rotated in a counter clockwise direction as indicated by a curved arrow inside the toner box  100  in  FIG. 3 . As the agitator  140  rotates, free ends of the wings  143  sweep inner surfaces of the toner box  100 , including an inner surface of the fitting wall  110 , and the toner in the toner box  100  is stirred. 
     A shape and a number of the wings  143  are arbitrarily decided in consideration of efficiency to move the toner in the toner box  100  from the right and left end areas, in which the first collecting holes  112  are formed, toward the central area, in which the first feeding hole  111  is formed (see also  FIG. 5 ). A configuration of such an agitator is known; therefore detailed explanation of that is herein omitted. 
     Next, the developer device  200  will be described. The developer device  200  includes a developer case  201  being a frame, and the developer roller  61 C, the supplier roller  61 D, and the spreader blade  61 E inside the developer case  201 . The developer device  200  further includes an auger  240 . 
     The developer roller  61 C carries the toner on a surface thereof and supplies the toner to an electrostatic latent image formed on the surface of the photosensitive drum  61 A. The supplier roller  61 D supplies the toner to the developer roller  61 C and is arranged in a position closer to the front with respect to the developer roller  61 C. The spreader blade  61 E restricts thickness of the toner being carried on the surface of the developer roller  61 C. 
     The spreader blade  61 E is arranged in an upper position with respect to the developer roller  61 C to be in contact with the developer roller  61 C. 
     The developer case  201  includes a half-pipe wall  210 , which is curved outward (toward the toner box  100 ) in an arc in cross-section to fit with the fitting wall  110  of the toner box  100  when the toner box  100  is installed in the developer unit  61 . The half-pipe wall  210  is formed to surround a part of the auger  240  in a cross-sectional view and arranged in a position in proximity to the auger  240 , which has a cross-sectional shape of a circle. More specifically, the half-pipe wall  210  surrounds a part of the auger  240 , which includes at least a top section of the auger  240 , Therefore, a plane P 1 , which extends in parallel with an upper edge (i.e., an edge closer to the toner box  100 ) of the half-pipe wall  210  in the cross-section, is lower than a plane P 2 , which extends in parallel with a top level of the auger  240  (see  FIG. 7 ). In other words, the half-pipe wall  210  is arranged in a position in proximity to the auger  240  with a small amount of clearance, which allows the toner to flow therein, being reserved between the auger  240  and the inner surface thereof. Thus, the auger  240  is substantially surrounded by the half-pipe wall  210  to effectively convey the toner. The auger  240  is not surrounded by the half-pipe wall  210  at a part, which faces the developer roller  61 D. Whilst the half-pipe wall  210  is formed in an arc to fit with the curvature of the fitting wall  110  by an outer surface thereof, the half-pipe wall  210  is curved to also center around the reference line BL, which coincides with the rotation axis C of the auger  240 . 
     The half-pipe wall  210  is formed to have a second feeding hole  211  and second collecting holes  212 . The second feeding hole is formed in a position to coincide with the first feeding hole  111  of the toner box  212 , and the second collecting holes  212  are formed in positions to respectively coincide with the first collecting holes  112  of the toner box  100 , when the toner box  100  is attached to the developer device  200 . Further, a direction, in which the second feeding hole  211  is oriented, corresponds to the orientation of the first feeding hole  111  (i.e., the direction D 1 ), and a direction, in which the second collecting holes  212  are oriented, corresponds to the orientation of the first collecting holes  112  (i.e., the direction D 2 ). In other words, the first feeding hole  112  and the second feeding hole  212  are oriented in the same direction D 1  to be in communication with each other whilst the first collecting holes  112  and the second collecting holes  212  are oriented in the same direction D 2  to be in communication with each other. 
     The developer device  200  has a second shutter  220  (see  FIG. 6 ), which is movable along curvature of an outer peripheral surface of the half-pipe wall  210  to cover and expose the second feeding hole  211  and the second collecting holes  212 . The second shutter  220  includes a metal plate  221 , which is formed to curve along the half-pipe wall  210 , and a pair of rotary discs  222 , which are fixed to right and left side edges of the metal plate  221 . 
     The metal plate  221  is arranged in a position to vertically overlap the metal plate  121  of the first shutter  120  when the toner box  100  is attached to the developer device  200 . In the metal plate  221 , two openings  223  are formed in positions to correspond to the openings  123  of the first shutter  120 . 
     When the second shutter  220  is in a closing position (see  FIG. 7 ), the second feeding hole  211  and the second collecting holes  212  are covered with the metal plate  221 . When the second shutter  220  is moved along the outer periphery of the half-pipe wall  210  to an opening position (see  FIG. 3 ), the second feeding hole  211  is uncovered, and the openings  223  coincide with the second collecting holes  212 . In this regard, when the first shutter  120  is also in the opening position, the second feeding hole  211  becomes in communication with the first feeding hole  111 , and the second collecting holes  212  become in communication with the first collecting holes  112  through the openings  223  and the openings  123 . 
     The rotary discs  222  (see  FIG. 6 ) are arranged on the right and left sides of the developer case  201 . The rotary discs  222  are supported by the developer case  201  to be rotatable about a rotation shaft  220 A. The rotary disc  222  is formed to have dents  225 , which are interlocked with the teeth  125  of the first shutter  120  when the toner box  100  is attached to the developer device  200  (see  FIG. 8 ). 
     When the toner box  100  is installed in the developer unit  61  to be attached to the developer device  200 , the teeth  125  provided to the first shutter  120  in the toner box  100  are interlocked with the dents  225  provided to the second shutter  220  in the developer device  200  (see  FIG. 8A ). In this regard, when the shutter handler  130  is manipulated to rotate about the rotation shaft  130 A, for example, in the counterclockwise direction see  FIG. 8B , the teeth  134  of the shutter handler  130  move the interlocking dents  124  of the first shutter  120  in the clockwise direction. Accordingly, the first shutter  120  is shifted to slide along the curvature of the fitting wall  110 . 
     In this regard, the teeth  125  of the first shutter  120  moves the interlocking dents  225  of the second shutter  220  in the same direction (i.e., the clockwise direction in  FIG. 8B ), and the second shutter  120  is shifted to slide along the curvature of the half-pipe wall  210 . Thus, the first shutter  120  and the second shutter  220  are moved collectively in cooperation with each other from the closing position to the opening position according to the movement of the shutter handler  130 . 
     The auger  240  is a roller with a rotation shaft  241  and spirals  242 ,  243  to convey the toner fed through the second feeding hole  211  (and the first feeding hole  111 ) toward the second collecting holes  212  (and the first collecting holes  112 ). The auger  240  is arranged in an upper front position with respect to the supplier roller  61 D (see  FIG. 3 ). The rotation shaft  241  is rotatably supported by right and left side walls of the developer case  201 , and the spirals  242 ,  243  twine around the rotation shaft  241 . 
     The spirals  242 ,  243  are respectively arranged on a right side and a left side of the rotation shaft  241 , which are divided at a lengthwise center of the rotation shaft  241  (see  FIG. 5 ). The spirals  242 ,  243  twine in different directions from each other. Accordingly, the toner in the right side area in the developer device  200  is conveyed leftward by the spiral  242 , and the toner in the left side area is conveyed rightward by the spiral  243 . 
     In the present embodiment, as the cross-sectional side view thereof is shown in  FIG. 3 , the half-pipe wall  210 , which separates the developer device  200  from the toner box  100  and faces the auger  240 , is arranged in the vicinity of the auger  240  to partially surround the auger  240 . In particular, the half-pipe wall  210  is arranged to be in proximity to the auger  240  whilst a small amount of clearance is maintained between outlines of the spirals  242 ,  243  of the auger  240  and the inner surface of the half-pipe wall  210  so that the auger  240  is allowed to rotate without being interfered with the inner surface of the half-pipe wall  210 . With this arrangement, the auger  240  can convey the toner sideward efficiently in cooperation with the inner surface of the half-pipe wall  210 . 
     According to the present embodiment, rotation axes of the developer roller  61 C, the supplier roller  61 D, and the auger  240  are aligned, in a side view (see  FIG. 3 ), on a line perpendicular to the rotation shaft  241  of the auger  240 . In particular, the supplier roller  61 D is arranged in a position to have a rotation center  61 G thereof to be on a line, which connects a rotation center  61 F of the developer roller  61 C and a rotation center C of the auger  240 . 
     Next, circulation of the toner within the developer unit  61  will be described. The toner in the toner box  100  is supplied to the developer device  200  through the first feeding hole  111  and the second feeding hole  211 , and a part of the toner in the developer device  200  is carried by the developer roller  61  and used in image forming. 
     Another part of the toner remaining in the developer device  200  is carried leftward and rightward by the auger  240  to be retrieved through the second collecting holes  212  and the first collecting holes  112  in the toner box  100 . The retrieved toner in the toner box  100  is stirred by the agitator  140  and collected in the central area of the toner box  100  to be supplied to the developer device  200  again through the first feeding hole  111  and the second feeding hole  211 . 
     In this regard, due to the communication between the first feeding hole  111  and the second feeding hole  211  along the direction D 1  (see  FIG. 3 ), which is inclined to be closer to the vertical direction DV than the inclination of the direction D 2 , the toner in the toner box  100  drops down effectively and smoothly in the developer device  200  by use of gravity. 
     As shown in  FIG. 9A , the first feeding hole  111  is formed in the fitting wall  110  in the upper-stream position with respect to the nearest point NP in the rotating direction of the agitator  140 . As the wings  143  rotate in the counterclockwise direction in  FIG. 9A , the wings  143  moving closer to the first feeding hole  111  press the toner T staying in space between the first feeding hole  111  and the wings  143  downward so that the toner T drops down in the developer device  200 . Thus, the toner T is supplied from the toner box  100  to the developer device  200  smoothly. 
     Further, as shown in  FIG. 9B , the first collecting holes  112  are formed in the fitting wall  110  in the lower-stream position with respect to the nearest point NP in the rotating direction of the agitator  140 . As the wings  143  rotate in the counterclockwise direction in  FIG. 11B , the wings  143  moving further away from the first collecting holes  112  sweep away the toner T staying in areas in vicinities of the first collecting holes  112 . Thus, the areas in the vicinities of the first collecting holes  112  are cleared so that following toner T from the developer device  200  can be moved in the cleared areas. 
     The toner supplied to the developer device  200  is carried leftward and rightward by the auger  240  (see  FIG. 5 ) and accumulate in vicinities of left and right side ends of the auger  240  by pressure of the auger  240 . The densely accumulated toner T (see  FIG. 9C ) is pushed out of the developer device  200  through the second collecting holes  212  by the following toner T, which is carried by the auger  240  to the vicinities of the left and right side ends of the auger  240 . Thus, the toner T is retrieved in the toner box  100 . 
     In this regard, due to the communication between the first collecting holes  112  and the second collecting holes  212  nearly along the horizontal direction DH, the toner T in the developer device  200  can be moved smoothly to be retrieved in the toner box  100 . When the toner T is pressed through the first collecting holes  112 , because the areas in the vicinities of the first collecting holes  112  are cleared by the rotation of the wings  143 , the toner T can be smoothly accepted to be retrieved in the toner box  100 . 
     According to the above configuration of the developer unit  61 , in which the toner box  100  is arranged in the upper and adjoining position with respect to the developer device  200 , the toner can be smoothly circulated and agitated to be homogenized. 
     As has been described above, according to the present embodiment, the fitting wall  110  and the half-pipe wall  210  are formed to surround the auger  240  in adjoining positions to have a small amount of clearance between the half-pipe wall  210  and the auger  240 . Therefore, a large part of toner supplied to the developer device  200  can be efficiently conveyed by the auger  240  sideward, and a smaller but substantial amount of the toner is supplied to the supplier roller  61 D, which is arranged on a rear side with respect to the auger  240 . Thus, an amount of the toner to be supplied to the supplier roller  16 D can be effectively restricted. Accordingly, pressure of the toner in the developer device  200  can be prevented from being excessively increased. 
     Specifically, in the present embodiment, the half-pipe wall  210  is curved in an arc to center around the reference line BL, which extends in parallel with the axial direction of the rotation shaft  241  of the auger  240 . In other words, the half-pipe wall  210  is curved to fit with the outline of the auger  240  whilst the small amount of clearance is reserved between the inner surface of the half-pipe wall  210  and the auger  240 . Accordingly, the auger  240  can convey the toner efficiently, and excessive increase of pressure of the toner remaining in the developer device  200 , specifically in an area surrounding the spreader blade  61 E, can be prevented. 
     When the pressure of the remaining toner is controlled, excessive pressure to the developer roller  61 C and the supplier roller  61 D can be prevented, and leakage of the toner from the clearance between the developer roller  61 C and the spreader blade  61 C can be prevented. Further, intervention of an obstacle being caught between the developer roller  61 C and the spreader blade  61 E can be prevented. 
     Additionally to the above configuration, the auger  240  (specifically, the rotation shaft  241 ) may be provided with guiding wings in positions opposite from the first and second collecting holes  112 ,  212  to guide the toner to the toner box  101  more smoothly. 
     It is to be noted, in the developer unit  61 , that the first feeding hole  111  and the first collecting holes  112  are formed in laterally (in the right-left direction) displaced positions. Meanwhile, the developer device  200  is equipped with the auger  240 , which moves the toner supplied through the first and second feeding holes  111 ,  211  leftward and rightward to be retrieved back in the toner box  100  through the first and second collecting holes  112 ,  212 . Therefore, fluidity of the toner between the toner box  100  and the developer device  200  is improved to be better than fluidity of toner in a toner box and a developer device with the first feeding hole  111  and the first collecting holes  112  being formed in laterally coinciding positions. 
     In the above embodiment, the toner box  100  is equipped with the first shutter  120  to cover and uncover the first feeding hole  111  and the first collecting holes  112  so that leakage of the toner out of the toner box  100  is prevented when the toner box  100  is not attached to the developer device  200 . Further, the first feeding hole  111  and the first collecting holes  112  are collectively covered or uncovered by the single opening/closing movement of the first shutter  120 . 
     In the above embodiment, the first shutter  120  and the second shutter  220  are formed in arcs; therefore, rigidity of the metal plates  121 ,  221  can be maintained even when the metal plates  121 ,  221  are formed in thin plates. Further, when the arc-formed shutters  120 ,  220  are rotated, smaller amounts of twist-deformation can be expected in the arc-formed shutters  120 ,  220  compared to an amount of deformation which can be caused in slidable plane shutters. In other words, smooth and stable movement of the first shutter  120  and the second shutter  200  can be maintained. 
     Although an example of carrying out the invention has been described, those skilled in the art will appreciate that there are numerous variations and permutations of the developer unit that fall within the spirit and scope of the invention as set forth in the appended claims. It is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or act described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. 
     For example, the shape of the fitting wall  110  and the half-pipe wall  210  are not limited to an arc in cross-section, but may be in a different shape as long as the walls are formed and arranged to fit around the auger  240 . 
     For another example, the first feeding hole  111  and the first collecting holes  112  may not necessarily be formed in laterally (in the right-left direction, which is the axial direction of the rotation shaft  241  of the agitator  240 ), but may be formed in same positions in the right-left direction. 
     For another example, the directions D 1  and D 2 , in which the first and second feeding holes  111 ,  211  and the first and second collecting holes  112 ,  212  are respectively oriented, may not necessarily be limited as described in the above embodiment and described in the drawings. The first and second feeding holes  111 ,  211  may be oriented in the vertical direction, and/or the first and second collecting holes  112 ,  212  may be oriented in the horizontal direction. 
     Further, a number, sizes, and shapes of the first feeding holes  111  and the first collecting holes  112  are not limited to those described in the above embodiment. For example, a single first feeding hole may be formed in a position corresponding to one axial end of the auger  240 , and a single collecting hole may be formed in a position corresponding to the other axial end of the auger  240 . 
     Furthermore, the auger  240  to carry the toner sideward may be replaced with, for example, a coil spring. According to the present invention, a trajectory of the coil spring includes a region inside the coils. 
     In the above embodiment, the developer unit  61  with the toner box  100  detachable from the developer device  200  is described. However, a developer unit  61  having a toner container undetachably fixed to the developer device may be provided (see  FIG. 10 ). When a toner container is undetachably fixed to the developer device, one of the fitting wall  110  (and the first shutter  120 ) and the half-pipe wall  210  (and the second shutter  210 ) can be omitted. 
     Furthermore, the sheet P to have an image formed thereon may be, for example, an OHP sheet. 
     In the above embodiment, the MFP  1  being an image forming apparatus having the developer unit according to the present invention is described. However, the image forming apparatus may be, for example, a copier and a printer. Furthermore, the number of the developer unit  61  is not limited to four, but may be, for example, one.