Abstract:
There is disclosed a developing device including a toner hopper, a toner supply source, a toner table on which a toner is laid, an agitator for carrying the toner to the toner table from the toner hopper, a magnet roller disposed above the toner table, a developing sleeve which covers an outer periphery of the magnet roller, holds a magnetic carrier, allows the toner of the toner table to be adsorbed onto the magnetic carrier, and shifts and attaches the toner to an electrostatic latent image of a photosensitive drum, a doctor blade disposed close opposite to the developing sleeve, and a developer returning member which is disposed between the developing sleeve and the toner hopper and is positioned in a position higher than a position of the toner table to prevent the toner cut/removed by the doctor blade from dropping toward the toner hopper and to return the cut/removed toner to the toner table, and which has a tip end displaced toward the developing sleeve from the boundary of the toner table and the toner hopper to form a space for carrying the toner by the agitator between the tip end and the toner table.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to image forming apparatuses such as an electrophotographic copying machine and laser beam printer, and particularly to an improvement in a developing unit which supplies toner to an electrostatic image and develops the electrostatic image.  
           [0002]    For a developing device disposed in an image forming apparatus using an electrophotographic process, a two-component developer including a carrier and toner, and one-component developer including only the toner are known.  
           [0003]    The two-component developer developing device attaches the toner to particles of a carrier, carries the toner to an outer periphery of a developing sleeve, forms a developer layer on the outer periphery of the developing sleeve, brings the developer layer into contact with the surface of a photosensitive drum (developing roller), and uses a Coulomb force of an electrostatic latent image formed beforehand on the photosensitive drum to detach the toner from the carrier and attach the toner to the electrostatic latent image.  
           [0004]    The one-component developer developing device forms a thin layer only of the toner on the outer periphery of the developing sleeve, disposes the photosensitive drum (developing roller) opposite to the developing sleeve at a predetermined interval, selectively supplies the toner to the electrostatic latent image formed on the photosensitive drum, and attaches the toner to the electrostatic latent image.  
           [0005]    In recent years, a so-called “pseudotwo-component developing device” using a small amount of a magnetic carrier has been proposed in Jpn. Pat. Appln. KOKOKU Publication Nos. 7-40156 (1995-40156) and 7-43554 (1995-43554). The pseudotwo-component developing device is originally the one-component developing device, but is constituted to attach a start agent (toner including a small amount of the magnetic carrier) to the developing sleeve before starting developing the image and to supply only the toner to the developing sleeve after starting developing the image.  
           [0006]    An outline of the pseudotwo-component developing device will be described with reference to FIG. 1. A developing device  10  is used in an image forming apparatus (copying machine) of a vertical sheet conveyance system for conveying a sheet P upwards to the photosensitive drum (not shown). The device also includes a developing sleeve  13  including a magnet roller  12  in order to use the small amount of the magnetic carrier.  
           [0007]    A developer carrying mechanism includes a toner hopper  14 , agitator  15 , and toner table  17  (carrier container  11   a ). In the developing device  10  of the vertical sheet conveyance system, since the photosensitive drum (not shown) is disposed above the developing sleeve  13 , the developer carrying mechanism  11   a ,  14 ,  15 ,  17  cannot be disposed above the developing sleeve  13 . Therefore, a toner T is taken up (lifted up) from the toner hopper  14  by the agitator  15 , and carried to the developing sleeve  13 . A part of the toner T adheres to the developing sleeve  13 , and not-adhering remaining toner T returns (drops) to the toner table  17 .  
           [0008]    When an operation starts, the start agent obtained by mixing the carrier and toner beforehand is supplied to a housing  11 . Thereby, since the small amount of magnetic carrier is inserted into the vicinity of the developing sleeve  13 , the developing sleeve  13  is replenished only with the magnetic toner from this time on. In order to prevent the magnetic carrier dropping from the developing sleeve  13  from being mixed into the toner hopper  14  in this case, the toner table  17  (carrier container  11   a ) is disposed right under the developing sleeve  13 .  
           [0009]    Furthermore, a doctor blade  32  (developer regulating member) is disposed opposite to an outer peripheral surface of the developing sleeve  13 , and is constituted to cut/remove surplus toner T from the developing sleeve  13 , so that a toner layer having a constant thickness (amount) is formed on the outer peripheral surface of the developing sleeve  13 . The surplus toner T cut by the doctor blade  32  is returned to the toner hopper  14 , again agitated, and again supplied to the developing sleeve  13 .  
           [0010]    However, in the conventional developing device  10 , when the surplus toner T is returned to the toner hopper  14 , toner table  17  (shelf-shaped portion), and developing sleeve  13 , particularly to the toner hopper  14 , the magnetic carrier is also returned. Therefore, an amount of the magnetic carrier which is originally to be present in the vicinity of the developing sleeve  13  is partially reduced, and a concentration of the toner on the developing sleeve  13  becomes unstable. As a result, image defects such as an image density unevenness, image density dispersion, image density reduction, and fog are possibly generated.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    An object of the present invention is to provide an image forming apparatus in which a concentration of toner on a developing sleeve is stabilized, and an excellent image quality can steadily be obtained over a long period without causing an image density unevenness, image density dispersion, image density reduction, or fog by an unstable toner concentration on the developing sleeve in developing an image with a pseudotwo-component developer.  
           [0012]    According to the present invention, there is provided an image forming apparatus including: image reading means for reading an image; a photosensitive drum on which an electrostatic latent image corresponding to the image is formed based on image data given from the image reading means; a vertical conveyance path along which a transfer sheet is conveyed upwards to the photosensitive drum; and a developing device which is disposed below the photosensitive drum and which uses a small amount of magnetic carrier to allow the toner to be adsorbed onto the electrostatic latent image on the photosensitive drum.  
           [0013]    The developing device includes:  
           [0014]    a toner hopper which contains the toner;  
           [0015]    a toner supply source which supplies the toner to the toner hopper;  
           [0016]    a toner table which is disposed in a position higher than a position of the toner hopper, which has a boundary adjacent to the toner hopper, and on which the toner is laid;  
           [0017]    an agitator which is disposed in the toner hopper, agitates/mixes the contained toner, and carries the toner to the toner table from the toner hopper;  
           [0018]    a magnet roller disposed above the toner table;  
           [0019]    a developing sleeve which covers an outer periphery of the magnet roller, holds the magnetic carrier, allows the toner of the toner table to be adsorbed onto the magnetic carrier, is rotated/driven in a direction reverse to a rotating/driving direction of the magnet roller, and shifts and attaches the toner to the electrostatic latent image of the photosensitive drum;  
           [0020]    a doctor blade which has a tip end disposed close opposite to the developing sleeve at a predetermined interval, and whose tip end cuts/removes a surplus toner from the developing sleeve; and  
           [0021]    a developer returning member which is disposed between the developing sleeve and the toner hopper and is positioned in a position higher than a position of the toner table to prevent the toner cut/removed by the doctor blade from dropping toward the toner hopper and to return the cut/removed toner to the toner table, and which has a tip end displaced toward the developing sleeve from the boundary of the toner table and the toner hopper to form a space for carrying the toner by the agitator between the tip end and the toner table.  
           [0022]    The toner cut/removed by the doctor blade is inhibited from dropping toward the toner hopper by the developer returning member, and guided and returned to the toner table by the developer returning member. Thereby, the magnetic carrier is prevented from being guided together with the toner and mixed in the toner hopper, and is returned to the developing sleeve via the toner table. Moreover, since the tip end of the developer returning member is displaced toward the developing sleeve from the boundary of the toner table and toner hopper, a space sufficient for carrying the toner from the toner hopper is secured between the tip end and the toner table.  
           [0023]    Additionally, the developer returning member is preferably made up of a material by which the carrier and toner T do not cause any frictional charging, an acrylonitrile butadiene styrene (ABS) resin, polycarbonate, or a composite material which contains glass fibers or glass particles in the resin.  
           [0024]    Moreover, a member similar to the developer returning member is described in an apparatus of Jpn. Pat. Appln. KOKAI Publication No. 9-197833 (1997-197833) (hereinfafter prior art 1). However, in the apparatus of the prior art 1, a developing unit is disposed substantially at the same height as that of the photosensitive drum, and a sheet is conveyed to the photosensitive drum from the developing unit in a horizontal direction (horizontal sheet conveyance system). Moreover, the apparatus of the prior art  1  is not a pseudotwo-component developer developing device which uses a small amount of magnetic carrier, but is a one-component or genuine two-component developer developing device.  
           [0025]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0026]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
         [0027]    [0027]FIG. 1 is an internal sectional view showing a conventional developing device.  
         [0028]    [0028]FIG. 2A is a characteristic diagram showing a toner concentration distribution on a developing sleeve when the conventional developing device is used to develop an image, and FIG. 2B is a characteristic diagram showing an image density distribution on a hardcopy when the conventional developing device is used to develop the image.  
         [0029]    [0029]FIG. 3 is an internal sectional view schematically showing a whole image forming apparatus.  
         [0030]    [0030]FIG. 4 is an enlarged internal sectional view showing a developing unit of the image forming apparatus according to an embodiment of the present invention.  
         [0031]    [0031]FIG. 5 is a block diagram schematically showing a toner supply section of the developing unit.  
         [0032]    [0032]FIG. 6 is an enlarged internal sectional view showing a part of the developing unit.  
         [0033]    [0033]FIG. 7 is an enlarged internal sectional view of a part of another developing unit.  
         [0034]    [0034]FIG. 8A is a characteristic diagram showing a toner concentration distribution on the developing sleeve when the image forming apparatus of the embodiment of the present invention is used, and FIG. 8B is a characteristic diagram showing an image density distribution on the hardcopy when the image forming apparatus of the embodiment of the present invention is used. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    A preferred embodiment of the present invention will be described hereinafter with reference to the accompanying drawings.  
         [0036]    As shown in FIG. 3, a digital copying machine  101  includes a scanner  102  for reading image information of a draft as brightness/darkness of light and generating an image signal, and an image forming section  103  for forming an image corresponding to the image signal supplied from the scanner  102  or an external apparatus (not shown). The scanner  102  includes an automatic draft feeder (ADF)  104  for feeding a sheet-like copy object during an image reading operation by the scanner  102 .  
         [0037]    The image forming section  103  includes an exposure unit  105 , photosensitive drum  106 , developing unit  10 A, and fixing (heating) unit  107 . The exposure unit  105  irradiates the photosensitive drum  106  with a laser beam corresponding to image data supplied from the scanner  102  or the external apparatus (not shown). The photosensitive drum  106  holds the image corresponding to the laser beam from the exposure unit  105  as an electrostatic latent image.  
         [0038]    The developing unit  10 A includes developing rollers  12 ,  13  and toner hopper  14  for supplying and attaching a toner to the electrostatic latent image on the photosensitive drum  106 . A transfer unit transfers a toner image developed on the photosensitive drum  106  by the developing unit  10 A onto a sheet P. The fixing unit  107  heats and melts the transferred toner image, and fixes the image onto the sheet P.  
         [0039]    An outline of an operation in the image forming apparatus  101  will next be described.  
         [0040]    The sheet P is taken out of a cassette  109  by a pickup roller  108 , conveyed along a vertical sheet conveyance path  110 , successively passed through the developing unit  10 A, photosensitive drum  106 , and fixing unit  107 , and discharged onto a tray  113  via a discharge roller  112 . Additionally, a reference numeral  111  in FIG. 3 denotes an aligning roller whose operation is controlled by a controller (not shown). The aligning roller  111  feeds the sheet P into a transfer position between the photosensitive drum  106  and the transfer unit at a predetermined timing.  
         [0041]    The image information is supplied from the scanner  102  or the external apparatus, and the photosensitive drum  106  charged beforehand at a predetermined potential is then irradiated with the laser beam whose strength is modulated in accordance with the image information by the exposure unit  105 . Thereby, the electrostatic latent image corresponding to the image to be copied is formed on the photosensitive drum  106 .  
         [0042]    A toner T is supplied to the photosensitive drum  106  from the developing unit  10 A, and the electrostatic latent image is developed. The toner T of the toner image is transferred onto the sheet P by an electric field given from the transfer unit which is not described herein in detail. The transferred toner T on the sheet P is heated/molten in the fixing unit  107 , and fixed onto the sheet P.  
         [0043]    The developing unit of a first embodiment will next be described with reference to FIGS.  4  to  6 .  
         [0044]    As shown in FIG. 4, the developing unit  10 A includes a housing  11 , developing roller (magnet roller  12  and developing sleeve  13 ), toner hopper  14 , agitator  15 , toner table  17 , developer returning member  18 , toner cartridge holder  19 , sensor  21 , and doctor blade  32 . The developing rollers  12 ,  13  and toner cartridge holder  19  are disposed on opposite sides of the toner hopper  14 , and are positioned above the toner hopper  14 . The toner hopper  14  and toner table  17  are formed integrally with the housing  11 , and constitute a part  11   a  of the housing  11 .  
         [0045]    The toner table  17  is disposed in the close vicinity to an upper edge of the toner hopper  14 , and disposed to be flat and substantially horizontal. The toner table  17  is positioned right under the developing rollers  12 ,  13 , extends along an axial direction of the developing rollers  12 ,  13 , and is a little longer than the developing rollers  12 ,  13 . A position of the toner table  17  is higher than a position of the toner hopper  14 . The toner T is taken up from the toner hopper  14  by the agitator  15 , and passes a boundary  17   a  and rides onto the toner table  17 .  
         [0046]    The developing rollers  12 ,  13  are positioned right above the toner table  17 , and the photosensitive drum  106  (not shown) is further disposed right above the developing rollers  12 ,  13 . The toner T shifts to the developing sleeve  13  (outer peripheral portion of the developing roller) from the toner table  17 , and next shifts to the photosensitive drum  106 .  
         [0047]    A core of the developing roller is formed of the magnet roller  12 , and an outer peripheral portion of the developing roller is formed of the developing sleeve  13 . The magnet roller  12  and developing sleeve  13  are coaxially disposed, and the developing sleeve  13  covers the outer periphery of the magnet roller  12 . The magnet roller  12  and developing sleeve  13  are independently rotated/driven by a rotating/driving mechanism (not shown).  
         [0048]    Additionally, an interval between the outer peripheral surface of the developing sleeve  13  and that of the photosensitive drum  106  in the closest vicinity is set to be substantially 0.35 mm. Moreover, the developing sleeve  13  is rotated in the same direction in a position opposite to the outer peripheral surface of the photosensitive drum  106 , and at a movement speed of 254 mm/sec. The developing sleeve  13  has a diameter of 20 mm.  
         [0049]    As seen from a transverse section of the magnet roller  12  crossing at right angles to an axis, N and S poles are alternately disposed at a substantially uniform interval in a roller peripheral direction, and twelve poles in total are disposed in this manner. Additionally, a magnetic force of each magnetic pole of the magnet roller  12  is measured on the surface of the developing sleeve  13 , and substantially indicates  700  gausses. Moreover, the magnet roller  12  is rotated in a direction reverse to a direction of the developing sleeve  13  at a rotation speed, for example, of 2000 rpm.  
         [0050]    As shown in FIG. 5, a toner replenishment opening  14   a  is formed in an appropriate position of the toner hopper  14 , and a toner cartridge of the holder  19  is connected to the toner replenishment opening  14   a  via an auger conveyor  20 . The toner T is supplied into the toner hopper  14  from the toner cartridge of the holder  19  through the auger conveyor  20  and toner replenishment opening  14   a.    
         [0051]    When the developer (carrier/toner) starts moving, the start agent obtained by mixing the carrier and toner beforehand is supplied to the housing  11 , and only the toner is subsequently supplied at a time of toner replenishment. The sensor  21  is attached to an appropriate position of the toner hopper  14 , and senses an amount of the toner T present in the toner hopper  14 . Additionally, as the toner, a magnetic toner having an average particle diameter of 9 μm and having magnetic properties imparted by mixing a magnetic material is used. Moreover, as the carrier, an Mn—Mg (ferrite) based magnetic carrier having an average particle diameter of 65 μm is used.  
         [0052]    The toner hopper  14  has a semispherical bowl shape. The agitator  15  is disposed in the toner hopper  14 . The agitator  15  extends in parallel to an axial line (Y axis) of a rotation center of the developing sleeve  13  and in a radial direction crossing at right angles to the axial line, and includes an arm  15   b  slightly shorter than a radius (inner diameter) of the toner hopper  14 . Moreover, the agitator  15  is disposed on a rotating/driving shaft  15   a , and has a film portion  15   d  slightly longer (by about 2 mm) than the radius (inner diameter) of the toner hopper  14 .  
         [0053]    The agitator arm  15   b  is connected to and supported by the rotating/driving shaft  15   a , and has a toner carrying portion  15   c  on a tip end thereof. The arm  15   b  is formed of an acrylonitrile butadiene styrene resin (ABS resin), polycarbonate, or a composite material which contains glass fibers or glass particles in the resin.  
         [0054]    Additionally, the rotating/driving shaft  15   a  of the agitator is positioned below a rotating/driving shaft  13   a  of the developing sleeve as seen on a Z-axis coordinate. Moreover, in the present embodiment, the agitator  15  is rotated in the same direction as a rotation direction of the magnet roller  12  (a counterclockwise direction in FIG. 4). Furthermore, a flexible sheet is attached to the tip end of the toner carrying portion  15   c , so that the toner T may be carried by the flexible sheet.  
         [0055]    The toner table  17  is used to increase an attached amount of the toner carried by the agitator  15  onto the developing sleeve  13  when the toner is attached to the peripheral surface of the developing sleeve  13  below the developing sleeve  13 .  
         [0056]    That is, a housing step  11   a  which connects a portion of the housing  11  covering the developing sleeve  13  (positioned below the developing sleeve  13 ) to the toner hopper  14  functions as the toner table  17  which temporarily holds the toner T carried by the agitator  15 .  
         [0057]    Elevation (height level) of the toner table  17  is lower than a center of the rotation shaft  15   a  of the agitator  15  disposed in the toner hopper  14 . Moreover, a horizontal length of the toner table  17  is equal to or shorter than a diameter of the developing sleeve  13 . In the present embodiment, the elevation of the toner table  17  is set to −5 mm. That is, the toner table  17  is positioned 5 mm below a lowermost portion of the developing sleeve  13 . In other words, the toner table  17  is positioned 2 mm below the center of the agitator rotation shaft  15   a . Moreover, the horizontal length of the toner table  17  was set to 17 mm in the present embodiment.  
         [0058]    A developer movement control assembly  30  of the first embodiment will next be described with reference to FIG. 6.  
         [0059]    The developing rollers  12 ,  13  and developer movement control assembly  30  are disposed in an upper opening of the housing  11 . The developer movement control assembly  30  controls movement of the developer (carrier and toner) in the developing unit. The assembly is attached to a frame of the housing  11  so as to be in close vicinity to the developing rollers  12 ,  13  in a position in which interference with a rotation track of the agitator tip end  15   c  does not occur.  
         [0060]    The developer movement control assembly  30  includes the developer returning member  18 , doctor bracket  31 , doctor blade  32 , and bolt  33 . The bolt  33  is inserted through an elongated hole of the doctor blade  32 , and the doctor bracket  31  having a screw hole is fixed to the housing  11 .  
         [0061]    The doctor bracket  31  has an L-shaped section, and the developer returning member  18  is supported by a bent end  31   b  of the bracket. When the bolt  33  is loosened, and the doctor blade  32  is slid backwards and forwards, the doctor blade  32  is attached to or detached from the developing sleeve  13  and the position of the blade can thus be changed.  
         [0062]    A tip end  32   a  of the doctor blade  32  is positioned in close vicinity to the outer peripheral surface of the developing sleeve  13  so that an interval between the tip end  32   a  and the developing sleeve  13  is about 0.25 mm at minimum. The doctor blade  32  removes the surplus toner T from the developing sleeve  13 , so that a developer layer formed on the outer peripheral surface of the developing sleeve  13  has a desired thickness.  
         [0063]    The developer returning member  18  is disposed on a downstream side from the doctor blade  32  in a developer movement direction of the sleeve  13 . The developer returning member  18  prevents the toner T cut/removed from the sleeve  13  by the doctor blade  32  from dropping into the toner hopper  14 , and has a function of returning the cut/removed toner T to the toner table  17 .  
         [0064]    The developer returning member  18  has an L-shaped section, and extends from end to end of the developing rollers  12 ,  13  along the developing rollers  12 ,  13 . A tip end  18   a  (point  1 ) of the developer returning member  18  is positioned above the toner table  17 , and the tip end  18   a  (point  1 ) is positioned toward the developing rollers  12 ,  13  from the boundary  17   a  (point  2 ) of the toner table  17  and toner hopper  14 . That is, the tip end  18   a  (point  1 ) of the developer returning member deviates onto the right side in FIG. 6 by a distance S 1  in an X-axis direction from the boundary  17   a  (point  2 ). The shift distance S 1  is preferably set to a range of 0 to 10 mm, most preferably 3 to 6 mm. In the present embodiment the shift distance S 1  was set to 4 mm.  
         [0065]    Moreover, the tip end  18   a  (point  1 ) of the developer returning member is positioned to be higher than the toner table  17  by an interval H 1 . The interval Hi is preferably in a range of 3 to 10 mm, most preferably 4 to 8 mm. In the present embodiment, the interval Hi was set to 5 mm. The toner T is supplied to the toner table  17  and developing sleeve  13  from the toner hopper  14  through the interval Hi by the agitator  15 .  
         [0066]    Furthermore, a bent portion of the developer returning member  18  is inclined toward the toner table  17  by an angle θ 1  with respect to Z-axis (vertical axis). An inclination angle θ 1  is preferably in a range of 0° to 90°, most preferably 30° to 60°. The inclination angle θ 1  was set to 50° in the present embodiment.  
         [0067]    Furthermore, the developer returning member  18  is preferably formed of a material by which the carrier and toner T do not cause any frictional charging, an acrylonitrile butadiene styrene (ABS) resin, polycarbonate, or a composite material which contains glass fibers or glass particles in the resin. In the present embodiment the developer returning member  18  was formed of a glass fibers containing ABS resin.  
         [0068]    The developer movement control assembly  30  of a second embodiment will next be described with reference to FIG. 7. Additionally, description of a redundant part of the present embodiment with respect to the aforementioned embodiment will be omitted.  
         [0069]    The developer movement control assembly  30  of the present embodiment has a developer returning member  28  whose sectional shape is curved in a J shape. A tip end  28   a  (point  1 ) of the developer returning member  28  is positioned above the toner table  17 , and the tip end  28   a  (point  1 ) deviates toward the developing rollers  12 ,  13  from the boundary  17   a  (point  2 ) of the toner table  17  and toner hopper  14 . That is, the tip end  28   a  (point  1 ) of the developer returning member deviates on the right side from the boundary  17   a  (point  2 ) by a distance S 2  in an X-axis direction in FIG. 7. The shift distance S 2  is preferably in a range of 0 to 10 mm, most preferably 3 to 6 mm. The shift distance S 2  was set to 5 mm in the present embodiment.  
         [0070]    Moreover, the tip end  28   a  (point  1 ) of the developer returning member is positioned to be higher than the toner table  17  by an interval H 2 . The interval H 2  is preferably in a range of 3 to 10 mm, most preferably 4 to 8 mm. In the present embodiment the interval H 2  was set to 7 mm. The toner T is supplied to the toner table  17  and developing sleeve  13  from the toner hopper  14  through the interval H 2  by the agitator  15 .  
         [0071]    Furthermore, a bent portion of the developer returning member  28  is inclined toward the toner table  17  by an angle θ 2  with respect to the Z-axis (vertical axis). The inclination angle θ 2  is preferably in a range of 0° to 90°, most preferably 30° to 60°. In the present embodiment the inclination angle θ 2  was set to 45°.  
         [0072]    Additionally, in the aforementioned embodiment, the developer returning member has an L or J-shaped transverse section, but the present invention is not limited to this, and the developer returning member may have a U-shaped or semicircular transverse section.  
         [0073]    [0073]FIG. 8A is a characteristic diagram showing a toner concentration distribution with the number of copy sheets on the abscissa and a toner concentration (wt %) on the developing sleeve on the ordinate, when the apparatus of the present embodiment was used to conduct a sheet passing test. FIG. 8B is a characteristic diagram showing an image density distribution with the number of copy sheets on the abscissa and an image density (by no unit) on a hardcopy on the ordinate, when the apparatus (apparatus having the developer returning member  18 ) of the present embodiment was used to conduct the sheet passing test. Moreover, the toner concentration on the developing sleeve  13  was measured using a carbon amount analysis apparatus. Furthermore, the image density on the hardcopy was measured using a Macbeth densitometer.  
         [0074]    In the diagrams a characteristic line F shows a concentration/density measurement result of a front portion (portion in the vicinity of one end of the developing sleeve  13 ) of the hardcopy, a characteristic line C shows a concentration/density measurement result of a center portion (middle portion of the developing sleeve  13 ) of the hardcopy, and a characteristic line R shows a concentration/density measurement result of a rear portion (portion in the vicinity of the other end of the developing sleeve  13 ) of the hardcopy. As apparent from FIG. 8A, it has been confirmed that the toner concentration on the developing sleeve  13  has only a small dispersion in each portion and with the number of passed sheets. Moreover, as apparent from FIG. 8B, it has also been confirmed that the image density on the hardcopy has only a small dispersion in each portion and with the number of passed sheets. It has been confirmed that both the toner concentration on the developing sleeve and the image density on the hardcopy are stable in this manner, and that drawbacks such as image density unevenness, image density dispersion, image density reduction, and fog are not generated in the apparatus of the present embodiment.  
         [0075]    On the other hand, FIG. 2A shows the measurement result of the toner concentration on the developing sleeve, when the conventional apparatus shown in FIG. 1 (apparatus which does not have the developer returning member  18 ) was used to conduct the sheet passing test. FIG. 2B shows the measurement result of the image density on the hardcopy, when a similar sheet passing test was conducted. As apparent from FIGS. 2A, 2B, both the toner concentration on the developing sleeve and the image density on the hardcopy had a large dispersion, and the drawbacks such as the image density unevenness, image density dispersion, image density reduction, and fog were generated in the conventional apparatus. Additionally, it has been seen that a mixture ratio (T/C) of the toner to the magnetic carrier largely fluctuated in a range of 30 to 80% in the conventional apparatus, but was stable at about 50% on average in the apparatus of the present invention.  
         [0076]    As described above, according to the present invention, the surplus developer (carrier, toner) cut by the doctor blade can securely be guided to the developing sleeve and toner table by the developer returning member, and the toner concentration unevenness (dispersion) of the developer on the developing sleeve does not easily occur. According to the image forming apparatus of the present invention, a stable image having no drawbacks such as the image density unevenness, image density dispersion, image density reduction, and fog can be obtained over a long period.  
         [0077]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general invention concept as defined by the appended claims and their equivalents.