Patent Publication Number: US-7899373-B2

Title: Developing device and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a developing device that forms a toner image by supplying a magnetic developing agent toward an image carrier on which an electrostatic latent image is formed by an electrophotographic method, and an image forming apparatus equipped with the developing device. 
     2. Description of the Related Art 
     A developing device applied to an image forming apparatus adopting an electrophotographic method forms a toner image by supplying toner particles as a developing agent toward the peripheral surface of the photoconductive drum on which an electrostatic latent image is formed according to image data. The image forming apparatus equipped with such a developing device transfers the toner image formed on the photoconductive drum onto a recording medium, such as a sheet. The transferred toner image is fixed onto the recording medium by heating and applying a pressure to the recording medium by a fixing device provided downstream of the photoconductive drum in the sheet transportation direction. By these operations, the image forming apparatus forms an image according to the image data on the recording medium. 
     The developing device is provided with a rotary sleeve to supply toner particles to the photoconductive drum, and a blade is provided at a position upstream of the position at which the rotary sleeve and the photoconductive drum oppose each other in the rotation direction of the rotary sleeve. By regulating the thickness of a toner layer that is formed thick on the rotary sleeve with the blade, it is possible to prevent an excessive supply of toner particles to the photoconductive drum. The developing device configured in this manner is therefore able to supply toner particles in a homogeneous state to the photoconductive drum via the rotary sleeve. 
     As a developing device using a magnetic mono-component developing agent (magnetic toner particles), there is known, for example, a magnetic mono-component developing device that regulates the thickness of a toner layer made up of magnetic toner particles on the rotary sleeve accommodating a stationary magnet using a non-contact magnetic blade. In such a magnetic mono-component developing device, when the toner layer formed thick on the rotary sleeve passes by the position at which the rotary sleeve and the magnetic blade oppose each other (hereinafter, referred to as the layer thickness regulating position), the toner layer is divided in two between the rotary sleeve and the magnetic blade. Toner particles closer to the magnetic blade adhere to the magnetic blade and toner particles closer to the rotary sleeve move away from the magnetic blade as they are attracted to the rotary sleeve by a magnetic attraction force of the stationary magnet accommodated in the rotary sleeve. In this manner, the magnetic mono-component developing device regulates the thickness of the toner layer without the magnetic blade contacting with the rotary sleeve instead of scarping off the toner layer using the blade. Accordingly, the magnetic mono-component developing device has minor damage on the magnetic toner particles, the blade, and the rotary sleeve even when high-speed printing is executed, and is therefore suitably applied to an image forming apparatus that requires high-speed printing and high durability. 
     However, because the magnetic mono-component developing device regulates the thickness of the toner layer with the magnetic attraction force of the stationary magnet accommodated in the rotary sleeve or the like, it is susceptible to the surrounding environment, such as the charging property of toner particles, free fine toner particles, an external additive to toner particles, and the surface nature of the rotary sleeve. Accordingly, disturbance may possibly occur in the toner layer whose layer thickness has been regulated, and the toner layer formed under such a condition may have irregularities in density. 
     As a developing device that reduces the occurrence of irregularities in density in the toner layer as above, there is known a developing device as described in JP-A-2003-167426 (Reference D1), in which a magnet is attached to the surface of the blade on the side upstream in the rotation direction of the rotary sleeve. 
     The developing device described in Reference D1 is able to regulate the thickness of the toner layer without disturbing the toner layer when a strong magnetic field is generated at the end surface of the blade on the rotary sleeve side by the magnet attached to the blade, and a homogeneous toner layer having little irregularities in density can be obtained. However, depending on the location of the magnet attached to the blade, there may be a case where a strong magnetic field is also generated on the end surface of the magnet on the rotary sleeve side. In such a case, a large amount of magnetic toner particles adhere to the end surface of the magnet on the rotary sleeve side. In a case where the developing device is used over a long period or at a high temperature in this state, toner particles adhering to the magnet aggregate to form clusters. When such aggregated toner particles enter at the layer thickness regulating position, a part mixed with the aggregate toner particles is in a condition that the thickness and the charging state of the toner layer are different from those in the other parts. This gives rise to a streak-like image defect. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a developing device not only capable of forming a toner layer having little irregularities in density but also capable of preventing the occurrence of an image defect. Also, another object is to provide an image forming apparatus equipped with such a developing device. 
     A developing device using a magnetic mono-component developing agent according to one aspect of the invention to achieve the above and other objects includes: a rotary sleeve that accommodates a stationary magnet; and a toner layer thickness regulating member that regulates a thickness of a toner layer formed on the rotary sleeve. The toner layer thickness regulating member includes a blade formed of a plate-like member made of a magnetic material, and a magnet attached to the blade on a side upstream in a rotation direction of the rotary sleeve. The magnet is disposed in such a manner that a direction of a magnetic field generated in a portion of the magnet on a side of the rotary sleeve is almost in parallel with the rotation direction of the rotary sleeve. 
     An image forming apparatus according to another aspect of the invention includes an image carrier on which an electrostatic latent image is to be formed, and a developing device that develops the electrostatic latent image formed on the image carrier by supplying a magnetic mono-component developing agent to the image carrier. The developing device has the configuration described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing the configuration of an image forming apparatus (copying machine) equipped with a developing device according to one embodiment of the invention; 
         FIG. 2  is a schematic view showing the periphery of an image forming portion in the image forming apparatus in  FIG. 1 ; 
         FIG. 3  is a cross section showing a developing unit (developing device) shown in  FIG. 2 ; 
         FIG. 4  is a partially enlarged cross section showing a developing roller and a toner layer thickness regulating member shown in  FIG. 3 ; 
         FIG. 5  is a view showing the result of the analysis on magnetic fields in a case where magnetic poles are formed in the positional relation as specified in  FIG. 4 ; 
         FIG. 6  is a partially enlarged cross section showing a developing roller and a toner layer thickness regulating member in a developing device according to a comparative embodiment; 
         FIG. 7  is a view showing the result of the analysis on magnetic fields in a case where magnetic poles are formed in the positional relation as specified in  FIG. 6 ; and 
         FIG. 8  is a partially enlarged cross section showing the developing roller and the toner layer thickness regulating member in the developing units according to one embodiment of the invention and the comparative embodiment when the magnetic flux density was measured. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An example of an image forming apparatus equipped with a developing device according to one embodiment of the invention will be described. A copying machine will be described as an example of the image forming apparatus. It should be appreciated, however, that the image forming apparatus is not limited to a copying machine, and it may be a facsimile machine, a printer, and so forth. Also, a photoconductive drum, which is a drum-shaped photoconductor, will be described as an example of an image carrier. It should be appreciated, however, that the image carrier is not limited to a photoconductive drum, either, and the invention is also applicable to a belt of photoconductor and a sheet of photoconductor. 
       FIG. 1  is a view schematically showing the configuration of an image forming apparatus (copying machine)  60  equipped with a developing device according to one embodiment of the invention. The copying machine  60  is a so-called body-inside sheet discharging type copying machine, including a sheet feeing portion  200  provided at the bottom of the copying machine main body, an image forming portion  300  provided above the sheet feeding portion  200 , a fixing portion  400  provided closer to the discharging side than the image forming portion  300 , an image reading portion  500  provided on the top portion of the copying machine main body, and a sheet discharging portion  600  disposed between the copying machine main body and the image reading portion  500 . The copying machine main body is provided with a sheet transportation portion  100  that interconnects the sheet feeding portion  200 , the image forming portion  300 , the fixing portion  400 , and the sheet discharging portion  600 . 
     The image forming portion  300  forms a specific toner image on a sheet by an electrophotographic process, and includes a photoconductive drum  301  axially supported to be rotatable, and a charging unit  302 , an exposing unit  303 , a developing unit (developing device)  10 , a transfer unit  305 , and a cleaner  306  disposed on the periphery of the photoconductive drum  301  along the rotation direction A. The developing unit  10  forms a toner image on the surface of the photoconductive drum  301  by developing an electrostatic latent image using toner particles. 
     The fixing portion  400  is disposed downstream of the image forming portion  300  in the sheet transportation direction. It nips a sheet on which is transferred a toner image in the image forming portion  300  and heats and applies a pressure to the sheet using a pair of rollers (a heating roller  401  and a pressure roller  402 ) and thereby fixes the toner image on the sheet. 
     The image reading portion  500  reads image information of an original by irradiating light from an exposing lamp to the original placed on the contact glass and introducing reflected light to a photo-electric converting portion via a reflection mirror. 
     The sheet feeding portion  200  includes plural sheet feeding cassettes  201 ,  202 , and  221 . Of these cassettes, the sheet feeding cassette  221  serves as a bypass tray used to replenish sheets from the side surface of the copying machine, and it can be closed with a cover portion  222 . 
     Sheet transportation paths  110  are connected to the respective sheet feeding cassettes  201 ,  202 , and  221 . The sheet transportation paths  110  are headed for the image forming portion  300  and further to the sheet discharging portion  600  by way of the fixing portion  400 . These transportation paths  110  together form the sheet transportation portion  100 . A sheet after the completion of the copying operation is discharged onto a discharge tray  610  from a discharging roller pair  605  in the sheet discharging portion 
       FIG. 2  is a schematic view showing the periphery of the image forming portion  300  in the image forming apparatus, such as the copying machine  60  of  FIG. 1 . The image forming portion  300  is a portion where a specific toner image is formed on a recording sheet  115  by the electrophotographic process. It includes the charging unit  302 , the exposing unit  303 , the developing unit  10 , the transfer unit  305 , a static eliminating unit  307 , and the cleaner  306  provided sequentially on the periphery of the photoconductive drum  301  having photosensitivity along the rotation direction A of the photoconductive drum  301 . The locations of the static eliminating unit  307  and the cleaner  306  may be exchanged. 
     The charging unit  302  provides specific potential to the surface of the photoconductive drum  301  through generation of a corona discharge. The exposing unit  303  forms an electrostatic latent image by irradiating light corresponding to a desired image on the surface of the photoconductive drum  301  to selectively attenuate the surface potential. The developing unit  10  forms a toner image by developing the electrostatic latent image formed on the surface of the photoconductive drum  301  with toner particles. The transfer unit  305  transfers the toner image formed on the photoconductive drum  301  onto a recording sheet  115 . The static eliminating unit  307  eliminates surface charges on the photoconductive drum  301  using lamp light. The cleaner  306  is formed of a fur brush  316  and a rubber blade  326 , and removes toner particles and additives thereof remaining on the surface of the photoconductive drum  301 . The cleaner  306  shown in the drawing as an example has both the fur brush  316  and the rubber blade  326 . It should be noted, however, that there is a cleaner having only one of these components. 
     Heat and a pressure are applied to the recording sheet  115  on which is transferred the toner image in the image forming portion  300  by the fixing portion  400  (the heating roller  401  and the pressure roller  402 ), and the toner image is fixed thereon. Subsequently, the recording sheet  115  is discharged onto the sheet discharge tray  610  by the sheet discharging roller pair  605  shown in  FIG. 1 . 
     Hereinafter, the developing unit (developing device)  10  according to one embodiment of the invention used in the copying machine will be described. 
       FIG. 3  is a cross section showing the developing unit (developing device)  10  shown in  FIG. 2  together with the photoconductive drum  301 . An unillustrated magnetic mono-component developing agent (toner particles) is accommodated in toner storing portions  354  of the developing unit  10 , and two stirring rollers  314  and  324  that stir toner particles and a developing roller  14  that forces toner particles to migrate onto the surface of the photoconductive drum  301  are provided. 
     The developing roller  14  is formed of a cylindrical rotary sleeve  13  made of a non-magnetic material, such as aluminum, and a stationary magnet  15  accommodated in the rotary sleeve  13 . It is configured in such a manner that the rotary sleeve  13  rotates about the stationary magnet  15  while the position thereof is fixed. In addition, a toner layer thickness regulating member  16  is provided oppositely to the rotary sleeve  13  of the developing roller  14 . 
     The stationary magnet  15  is a magnet of the 6-pole structure with the magnetic poles disposed as follows. An S pole (developing pole S 1 ) is disposed at a position opposing the photoconductive drum  301  and another S pole (blade pole S 3 ) is disposed at a position opposing the toner layer thickness regulating member  16 . An N pole (N 1 ), an S pole (S 2 ), and an N pole (N 2 ) are sequentially disposed downstream of the developing pole S 1  in the rotation direction B of the rotary sleeve  13 . Further, an N pole (N 3 ) is disposed downstream of the blade pole S 3  in the rotation direction B of the rotary sleeve  13 . 
     The toner layer thickness regulating member  16  is formed of a blade  11  and a magnet  12 . The blade  11  is disposed in a direction perpendicular to the rotation direction B of the rotary sleeve  13 , and the magnet  12  is attached to the surface of the blade  11  on the side upstream in the rotation direction B of the rotary sleeve  13 . 
       FIG. 4  is a partially enlarged cross section showing the developing roller  14  and the toner layer thickness regulating member  16  shown in  FIG. 3 . The blade  11  is formed of a plate-like member made of a magnetic material. Examples of the plate-like member made of a magnetic material include but not limited to SUS340 and SUS430, and SUS430 is used preferably. The blade  11  has an end surface  11   a  opposing the rotary sleeve  13 . The end surface  11   a  is also referred to as the end face  11   a  of the blade  11  on the rotary sleeve side herein. The end surface  11   a  of the blade  11  on the rotary sleeve side is provided to extend in parallel with the axial direction of the rotary sleeve  13  while being spaced apart from the rotary sleeve  13  by a specific interval W 1 . The specific interval W 1  is preferably 0.2 to 0.4 mm, and for example, 0.3 mm. When W 1  is too narrow, there is a tendency that toner particles readily clog in a space between the blade  11  and the rotary sleeve  13 . Conversely, when W 1  is too wide, there is a tendency that irregularities in density readily occur in the toner layer. The thickness T 1  of the blade  11  can be any thickness as long as it is sufficient to firmly maintain the position of the end surface  11   a  on the rotary sleeve side without bending, and for example, 2 mm. In addition, the magnet  12  has an end surface  12   a  opposing the rotary sleeve  13 . The end surface  12   a  is also referred to as the end face  12   a  of the magnet  12  on the rotary sleeve side herein. It is preferable that the end surface  12   a  of the magnet  12  on the rotary sleeve side is at a position farther from the rotary sleeve  13  than the end surface  11   a  of the blade  11  on the rotary sleeve side. A distance between the end surface  11   a  of the blade  11  on the rotary sleeve side and the end surface  12   a  of the magnet  12  on the rotary sleeve side, that is, a protrusion length W 2  of the blade  11  with respect to the magnet  12  is preferably 0.3 to 0.8 mm, and for example, 0.5 mm. When W 2  is too narrow, there is a tendency that toner particles readily clog in a space between the blade  11  and the rotary sleeve  13  in a case where a small amount of toner particles adhere to the end surface  12   a  of the magnet  12  on the rotary sleeve side. Conversely, when W 2  is too wide, there is a tendency that the magnet  12  can no longer be disposed in the manner as described below (disposed so that the direction C of the magnetic field is almost in parallel with the rotation direction B of the rotary sleeve  13 ). 
     The magnet  12  is a plate-like magnet disposed in parallel with the plane direction of the blade  11 . It is sufficient for the magnet  12  to generate a specific magnetic field, and the thickness T 2  thereof is, for example, 4 mm, the length L thereof is, for example, 5 mm, and the length in parallel with the shaft center direction of the developing roller  14  is, for example, 218 mm. The magnet  12  has a magnetization direction perpendicular to the plane direction of the blade  11 . To be more concrete, the lamination surface side in contact with the blade  11  serves as an N pole, which has a polarity different from that of the magnetic pole (blade pole S 3 ) formed in a portion of the stationary magnet  15  included in the developing roller  14  so as to oppose the blade  11 , whereas the open surface side not in contact with the blade  11  serves as an S pole, which has the polarity same as that of the blade pole S 3 . In other words, in this embodiment, the stationary magnet  15  included in the developing roller  14  has a first magnetic pole (S pole) disposed at a position opposing the toner layer thickness regulating member  16 , whereas the magnet  12  has a second magnetic pole (N pole) formed on the lamination surface side in contact with the blade  11  to have a polarity different from that of the first magnetic pole and another magnetic pole (S pole) formed on the open surface side not in contact with the blade  11  to have the same polarity as the first magnetic pole. In other words, the magnet  12  is disposed in such a manner that the direction (a direction in which the magnetic field line is oriented) C of the magnetic field generated in a portion of the magnet  12  on the rotary sleeve side is almost in parallel with the rotation direction B of the rotary sleeve  13  (a direction in which the outer surface of the rotary sleeve  13  moves). 
       FIG. 5  is a view showing the result of the analysis on the magnetic fields when magnetic poles are formed in the positional relation as specified in  FIG. 4 . The magnetic fields were analyzed by making a distance between the magnet  12  and the stationary magnet  15  shorter than the actual distance in order to make a difference from a comparative embodiment described below easy to understand by indicating the directions of the magnetic fields clearly. As can be understood from  FIG. 5 , the magnet  12  disposed in the positional relation as specified in  FIG. 4  is disposed in such a manner that the direction C of the magnetic field generated in a portion of the magnet  12  on the rotary sleeve side is almost in parallel with the rotation direction B of the rotary sleeve  13 . 
     It is sufficient to dispose the magnet  12  so that the direction C of the magnetic field is almost in parallel with the rotation direction B of the rotary sleeve  13  as described above, and the magnet  12  does not necessarily have to be disposed according to the positional relation as specified in  FIG. 4 . 
     In the developing unit  10 , a strong magnetic field is generated at the layer thickness regulating position by the S 3  pole in the stationary magnet  15  and the S pole on the end surface  12   c  of the magnet  12  on the open surface side. In addition, as can be understood from the fact that the direction C of the magnetic field as described above is formed on the end surface  11   a  of the blade  11  on the rotary sleeve side, a strong N pole is induced. Further, because a magnetic field is generated in a portion of the magnet  12  on the rotary sleeve side in such a manner that the direction C of the magnetic field is almost in parallel with the rotation direction B of the rotary sleeve  13 , a strong magnetic pole is not formed on the end surface  12   a  of the magnet  12  on the rotary sleeve side. 
     Hereinafter, operations of the developing unit  10  will be described. 
     Toner particles inside the toner storing portions  354  in the developing unit  10  are attracted by magnetic forces of the N 2  pole and the N 3  pole in the stationary magnet  15  and transported to the layer thickness regulating position while being moved upward by rotations of the rotary sleeve  13 . In this instance, because a strong magnetic field has been generated between the blade  11  and the rotary sleeve  13  by the N pole on the end surface  11   a  of the blade  11  on the rotary sleeve side and the S 3  pole in the stationary magnet  15 , the toner layer passes by the layer thickness regulating position in an almost homogeneous state where irregularities in density are reduced to an extremely low extent. When the toner layer moves away from the layer thickness regulating position, toner particles closer to the blade  11  adhere to the blade  11  owing to the N pole on the end surface  11   a  of the blade  11  on the rotary sleeve side. Meanwhile, toner particles closer to the rotary sleeve  13  are attracted toward the rotary sleeve  13  by the S 3  pole in the stationary magnet  15  and move away from the blade  11 . In this manner, the toner layer on the rotary sleeve  13  is divided in two in a homogeneous state so that the layer thickness is regulated. Thereafter, the toner thin layer whose layer thickness is regulated and made thin is headed for the photoconductive drum  301 . 
     As has been described, in this embodiment, because a strong N pole is induced on the end surface  11   a  of the blade  11  on the rotary sleeve side by the magnet  12  in the toner layer thickness regulating member  16 , a strong magnetic force acts between the blade  11  and the rotary sleeve  13 . This makes the toner layer at the layer thickness regulating position least susceptible to the environment other than this strong magnetic force. Consequently, disturbance in the toner layer at the layer thickness regulating position is eliminated. It is thus possible to form an almost homogeneous toner layer having little irregularities in density even when the layer thickness is regulated. 
     Further, because a strong magnetic pole is not formed on the end surface  12   a  of the magnet  12  on the rotary sleeve side, toner particles hardly adhere to the end surface  12   a  of the magnet  12  on the rotary sleeve side. It is thus possible to prevent the occurrence of an image defect resulting from aggregated toner particles. 
     As has been described, the developing device  10  configured as above is not only capable of forming the toner layer having little irregularities in density, but is also capable of preventing the occurrence of an image defect. 
     A developing device  20  in which the magnet  12  is located differently from the invention will now be described as a comparative embodiment for the purpose of comparison with the invention. Because the developing device  20  is the same as the developing device  10  of the invention except for the location of the magnet  12 , like components are labeled with like reference numerals with respect to  FIG. 4  and descriptions will not be repeated herein. 
       FIG. 6  is a partially enlarged cross section showing the developing roller  14  and the toner layer thickness regulating member  16  in the developing device  20 . A magnet  17  attached to the blade  11  has an S pole, which has a polarity same as that of the magnetic pole (blade pole S 3 ) formed in a portion of the stationary magnet  15  included in the developing roller  14  so as to oppose the blade  11 , on the end surface  17   a  on the rotary sleeve side and an N pole on the end surface  17   b  on the opposite side (the side farther from the rotary sleeve  13 ). The magnet  17  is therefore located in such a manner that the direction D of the magnetic field generated in a portion of the magnet  17  on the rotary sleeve side is almost perpendicular to the rotation direction B of the rotary sleeve  13 . 
       FIG. 7  is a view showing the result of the analysis on the magnetic fields when the magnetic poles are formed in the positional relation specified in  FIG. 6 . As with  FIG. 5 , the magnetic fields were analyzed by making a distance between the magnet  17  and the stationary magnet  15  shorter than the actual distance. As can be understood from  FIG. 7 , the magnet  17  disposed in the positional relation as specified in  FIG. 6  is disposed in such a manner that the direction D of the magnetic field generated in a portion of the magnet  17  on the rotary sleeve side is almost perpendicular to the rotation direction B of the rotary sleeve  13 . 
     In the developing unit  20 , a strong magnetic field is generated at the layer thickness regulating position by the S 3  pole in the stationary magnet  15  and the S pole on the end surface  17   a  of the magnet  17  on the rotary sleeve side. In addition, as can be understood from the fact that the direction D of the magnetic field as described above is formed on the end surface  11   a  of the blade  11  on the rotary sleeve side, a strong N pole is induced. Because a magnetic field is generated in a portion of the magnet  17  on the rotary sleeve side in such a manner that the direction D of the magnetic field is almost perpendicular to the rotation direction B of the rotary sleeve  13 , a strong magnetic pole is formed on the end surface  17   a  of the magnet  17  on the rotary sleeve side, which is a significant difference from the developing unit  10  described above. 
     In the developing unit  20 , because a strong N pole is induced on the end surface  11   a  of the blade  11  on the rotary sleeve side, as with the developing unit  10 , it is possible to form a toner layer having little irregularities in density. However, because a strong magnetic pole is formed on the end surface  17   a  of the magnet  17  on the rotary sleeve side, a large amount of toner particles adhere to the end surface  17   a  of the magnet  17  on the rotary sleeve side. Accordingly, in a case where the developing device  20  is used over a long period or at a high temperature, toner particles adhering to the magnet  17  may possibly aggregate to form clusters. When aggregated toner particles enter at the layer thickness regulating position, a part mixed with aggregated toner particles is in a condition that the layer thickness and the charging state of the toner layer are different from those in the other parts, which gives rise to the occurrence of a streak-like image defect. 
     Hereinafter, the magnetic flux density actually measured in the developing unit  10  according to one embodiment of the invention and in the developing unit  20  according to the comparative embodiment for the purpose of comparison with the invention will be described. 
       FIG. 8  is a partially enlarged cross section showing the developing roller  14  and the toner layer thickness regulating member  16  in the developing units  10  and  20  in which the magnetic flux density was measured. The dimensions and the positional relations of the respective members in the developing units  10  and  20  in which the magnetic flux density was actually measured were as follows. The interval W 1  between the end surface  11   a  of the blade  11  on the rotary sleeve side and the rotary sleeve  13  was 0.3 mm. The thickness T 1  of the blade  11  was 2 mm. The protrusion length W 2  of the blade  11  with respect to the magnet  12  or  17  was 0.5 mm. The magnets  12  and  17  had the thickness T 2  of 4 mm and the length L of 5 mm, and the length in a direction in parallel with the shaft center direction of the developing roller  14  was 218 mm. The diameter R of the rotary sleeve  13  was 16 mm. The angle α between the plane direction of the blade  11  and the longitudinal direction of the stationary magnet  15  opposing the blade  11  was 4 degrees. The developing unit  10  and  20  employed a magnet of a 70 mT as the magnet  12 , a magnet of a 70 mT as the magnet  17  and a magnet of 90 mT as the stationary magnet  15 , and the magnetic flux density was measured under the measurement conditions as specified above (shapes and the positional relations). The magnetic flux density was measured using a Tesla meter. 
     In the developing unit  10  according to one embodiment of the invention, an N pole of 40 mT was formed on the edge  11   c  of the end surface  11   a  of the blade  11  on the rotary sleeve side upstream in the rotation direction of the rotary sleeve  13 . An S pole of 70 mT was formed in the center portion on the end surface  12   c  of the magnet  12  on the open surface side. An S pole of 30 mT was formed in the center portion on the S pole side on the end surface  12   a  of the magnet  12  on the rotary sleeve side. Accordingly, it is understood that a strong magnetic pole was formed on the end surface  11   a  of the blade  11  on the rotary sleeve side but a strong magnetic pole was not formed on the end surface  12   a  of the magnet  12  on the rotary sleeve side in the developing unit  10 . 
     In the developing unit  20  according to the comparative embodiment, an N pole of 40 mT was formed on the edge  11   c  of the end surface  11   a  of the blade  11  on the rotary sleeve side upstream in the rotation direction of the rotary sleeve  13 . An N pole of 20 mT was formed in the center portion on the N pole side on the end surface  17   d  of the magnet  17  on the open surface side, and an S pole of 20 mT was formed in the center portion on the S pole side on the end surface  17   c  on the open surface side. An S pole of 70 mT was formed in the center portion on the end surface  17   a  of the magnet  17  on the rotary sleeve side. Accordingly, it is understood that a strong magnetic pole was formed on the end surface  11   a  of the blade  11  on the rotary sleeve side, and at the same time, a strong magnetic pole was also formed on the end surface  17   a  of the magnet  17  on the rotary sleeve side in the developing unit  20 . 
     In view of the foregoing, in comparison with the developing unit  20  of the comparison embodiment, the developing device (developing unit)  10  of this embodiment is capable of maintaining almost the same magnetic force on the end surface  11   a  of the blade  11  on the rotary sleeve side while suppressing the magnetic force on the end surface  12   a  of the magnet  12  on the rotary sleeve side. Accordingly, by the operations as described above, the developing device (developing unit)  10  of this embodiment is not only capable of forming a toner layer having little irregularities in density, but is also capable of preventing the occurrence of an image defect, in particular, a streak-like image defect appearing in a half-tone image by reducing an amount of toner particles adhering to the end surface  12   a  of the magnet  12  on the rotary sleeve side. 
     The embodiment above described a magnetic mono-component developing device using a magnetic mono-component magnetic agent (magnetic toner particles). It should be appreciated, however, that the invention is not limited to this configuration, and similar advantages can be achieved as long as a developing device uses a magnetic developing agent. Examples of a magnetic developing agent include but not limited to a two-component developing agent containing magnetic toner particles and a non-magnetic carrier, a two-component developing agent containing non-magnetic toner particles and a magnetic carrier, and a two-component developing agent containing magnetic toner particles and a magnetic carrier. 
     The specific embodiment described above chiefly includes inventions having the following configurations. 
     A developing device using a magnetic mono-component developing agent according to one aspect of the invention includes a rotary sleeve that accommodates a stationary magnet, and a toner layer thickness regulating member that regulates a thickness of a toner layer formed on the rotary sleeve. The toner layer thickness regulating member includes a blade formed of a plate-like member made of a magnetic material, and a magnet attached to the blade on a side upstream in a rotation direction of the rotary sleeve. The magnet is disposed in such a manner that a direction of a magnetic field generated in a portion of the magnet on a side of the rotary sleeve is almost in parallel with the rotation direction of the rotary sleeve. 
     According to this configuration, it is possible to regulate the thickness of the toner layer formed thick on the rotary sleeve using the toner layer thickness regulating member, which allows the toner particles to be supplied to the image carrier from the toner layer whose layer thickness has been regulated. 
     In addition, because the magnet is attached to the blade formed of a plate-shaped member made of a magnetic material on the side upstream in the rotation direction of the rotary sleeve, the toner layer thickness regulating member is able to generate a strong magnetic field on the end surface of the blade on the rotary sleeve side. Accordingly, a strong magnetic force acts between the blade and the rotary sleeve, which makes the toner layer at the layer thickness regulating position least susceptible to the environment other than this magnetic force. Consequently, disturbance in the toner layer hardly occurs at the layer thickness regulating position. It is thus possible to form a toner layer having little irregularities in density even when the layer thickness is regulated. 
     In addition, the magnet is disposed in such manner that the direction of the magnetic field generated in the portion on the rotary sleeve side is almost in parallel with the rotation direction of the rotary sleeve. Hence, in comparison with a case where the magnet is disposed in such a manner that the direction of the magnetic field is perpendicular to the rotation direction of the rotary sleeve, even when the magnetic field generated on the end surface of the blade on the rotary sleeve side has about the same strength, the magnetic field generated on the end surface of the magnet on the rotary sleeve side is weak. Consequently, toner particles hardly adhere to the end surface of the magnet on the rotary sleeve side. It is thus possible to prevent the occurrence of an image defect resulting from aggregated toner particles. 
     As has been described, the developing device of the invention is not only capable of forming the toner layer having little irregularities in density, but is also capable of preventing the occurrence of an image defect. 
     In the configuration described above, to be more concrete, it is preferable that: the blade is disposed in a direction perpendicular to the rotation direction of the rotary sleeve; the magnet is a plate-like magnet to be disposed in parallel with a plane direction of the blade; and the plate-like magnet has a magnetization direction perpendicular to the plane direction of the blade. 
     According to this configuration, it is possible to dispose the magnet with ease in such a manner that the direction of the magnetic field generated in the portion of the magnet on the rotary sleeve side is almost in parallel with the rotary direction of the rotary sleeve. Hence, not only is it possible to form a toner layer whose layer thickness is regulated so as to have little irregularities in density, but it is also possible to prevent the occurrence of an image defect resulting from aggregated toner particles. 
     In the configuration described above, it is preferable that: the magnet has an end surface opposing the rotary sleeve; the blade has an end surface opposing the rotary sleeve; and the end surface of the magnet is at position farther from the rotary sleeve than the end surface of the blade. 
     According to this configuration, even when a small amount of toner particles adhere to the end surface of the magnet on the rotary sleeve side, toner particles hardly clog in a space between the blade and the rotary sleeve. It is thus possible to prevent the occurrence of an image defect resulting from the clogging of toner particles. 
     In the configuration described above, it is preferable that: the stationary magnet has a first magnetic pole disposed at a position opposing the toner layer thickness regulating member; and that the magnet has a second magnetic pole having a polarity different from a polarity of the first magnetic pole on a lamination surface side in contact with the blade and a magnetic pole having a polarity same as the polarity of the first magnetic pole on an open surface side not in contact with the blade. 
     According to this configuration, it is possible to dispose the magnet with more ease in such a manner that the direction of the magnetic field generated in the portion of the magnet on the rotary sleeve side is almost in parallel with the rotation direction of the rotary sleeve. 
     In the configuration described above, it is preferable that a distance between the rotary sleeve and the blade is 0.2 to 0.4 mm. According to this configuration, toner particles hardly clog in a space between the blade and the rotary sleeve, which makes it possible to form a suitable toner layer. 
     In the configuration described above, it is preferable that a length of the blade protruding with respect to the magnet is 0.3 to 0.8 mm. According to this configuration, even when a small amount of toner particles adhere to the end surface of the magnet on the rotary sleeve side, toner particles hardly clog in a space between the blade and the rotary sleeve. It is thus possible to prevent the occurrence of an image defect resulting from the clogging of toner particles. 
     Also, a developing device using a magnetic developing agent according to another aspect of the invention includes a rotary sleeve that accommodates a stationary magnet, and a developing agent layer thickness regulating member that regulates a thickness of a developing agent layer formed on the rotary sleeve. The developing agent layer thickness regulating member includes a blade formed of a plate-like member made of a magnetic material, and a magnet attached to the blade on a side upstream in a rotation direction of the rotary sleeve. The magnet is disposed in such a manner that a direction of a magnetic field generated in a portion of the magnet on a side of the rotary sleeve is almost in parallel with the rotation direction of the rotary sleeve. 
     In the configuration described above, to be more concrete, it is preferable that the blade is disposed in a direction perpendicular to the rotation direction of the rotary sleeve; the magnet is a plate-like magnet to be disposed in parallel with a plane direction of the blade; and the plate-like magnet has a magnetization direction perpendicular to the plane direction of the blade. 
     In the configuration described above, it is preferable that: the magnet has an end surface opposing the rotary sleeve; the blade has end surface opposing the rotary sleeve; and the end surface of the magnet is at a position farther from the rotary sleeve than an end surface of the blade. 
     In the configuration described above, it is preferable that: the stationary magnet has a first magnetic pole disposed at a position opposing the developing agent layer thickness regulating member; and that the magnet has a second magnetic pole having a polarity different from a polarity of the first magnetic pole on a lamination surface side in contact with the blade and a magnetic pole having a polarity same as the polarity of the first magnetic pole on an open surface side not in contact with the blade. 
     In the configuration described above, it is preferable that a distance between the rotary sleeve and the blade is 0.2 to 0.4 mm. 
     In the configuration described above, it is preferable that a length of the blade protruding with respect to the magnet is 0.3 to 0.8 mm. 
     Further, an image forming apparatus according to still another aspect of the invention includes an image carrier on which an electrostatic latent image is to be formed, and a developing device that develops the electrostatic latent image formed on the image carrier by supplying a magnetic mono-component developing agent to the image carrier. The developing device includes a rotary sleeve that accommodates a stationary magnet, and a toner layer thickness regulating member that regulates a thickness of a toner layer formed on the rotary sleeve. The toner layer thickness regulating member includes a blade formed of a plate-like member made of a magnetic material, and a magnet attached to the blade on a side upstream in a rotation direction of the rotary sleeve. The magnet is disposed in such a manner that a direction of a magnetic field generated in a portion of the magnet on a side of the rotary sleeve is almost in parallel with the rotation direction of the rotary sleeve. 
     According to this configuration, because the developing device as described above is incorporated into the image forming apparatus, the developing device is not only capable of forming a toner layer having little irregularities in density, but is also capable of preventing the occurrence of an image defect. the image forming apparatus is thereby able to form a high-quality image on a recording medium. 
     This application is based on patent application No. 2007-021011 filed in Japan, the contents of which are hereby incorporated by references. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to embraced by the claims.