Patent Publication Number: US-7899372-B2

Title: Developing device and image forming apparatus

Description:
CROSS REFERENCE 
     This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2008-000637 filed in Japan on Jan. 7, 2008, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE TECHNOLOGY 
     The technology relates to a developing device for use in an electrophotographic image forming apparatus of the type using a two-component developer comprising a toner and a carrier. More particularly, the technology relates to a developing device employing a counter developing system in which a developer bearing member feeds a developer at a developing region in a developer feeding direction which is opposite to an electrostatic latent image feeding direction in which an electrostatic latent image bearing member feeds an electrostatic latent image, as well as an image forming apparatus including such a developing device. 
     An electrophotographic image forming apparatus includes a developing device having a developer bearing member, and an electrostatic latent image bearing member for carrying an electrostatic latent image thereon. The developer bearing member supplies a developer to the peripheral surface of the electrostatic latent image bearing member at a developing region in which the electrostatic latent image bearing member and the developer bearing member face each other, to visualize the electrostatic latent image. 
     One developing system for such a developing device is a forward developing system in which the developer bearing member feeds the developer at the developing region in the same direction as the direction in which the electrostatic latent image bearing member feeds the electrostatic latent image, as described in Japanese Patent Laid-Open Publication No. H05-289522 for example. 
     With the forward developing system, when a high density region is developed in succession to development of a halftone region, a pinhole  201  is likely to occur in the halftone region as shown in  FIG. 1 . The pinhole  201  is considered to occur for the reason that electric flux lines  207  in a boundary area  206  of a halftone region  204  adjacent to a high density region  205  is deflected toward the high density region  205  as shown in  FIG. 2  and, hence, an electric field is weakened in the boundary area  206 . 
     With the forward developing system, after passage of the developer through the high density region  205 , the amount of toner contained in the developer projecting like spikes is reduced, while an electric charge called “counter charge” appears at the tips of the spikes, whereby the developing ability of the developer is lowered. For this reason, the electrostatic latent image is more difficult to develop in the boundary area  206  in which the electric field is weakened, with the result that the pinhole  201  is likely to occur. 
     By contrast, with the counter developing system in which the developer feeding direction is opposite to the electrostatic latent image feeding direction of the electrostatic latent image bearing member, fresh spikes of the developer which have not passed through the developing region are applied to a downstream-side region of the electrostatic latent image bearing member  202  in the electrostatic latent image feeding direction. For this reason, the amount of toner contained in the developer is relatively large and the counter charge does not appear at the tips of the spikes. Therefore, the developing ability of the developer is relatively high in the boundary area  206 , so that the pinhole  201  is less likely to occur in the halftone region  204  developed prior to the high density region  205 . 
     With the counter developing system, however, a phenomenon so-called “sweeping together” that a trailing end portion of an image (i.e., an upstream end portion of the electrostatic latent image in the electrostatic latent image feeding direction) is developed excessively densely, is likely to occur. 
     The reason why such a phenomenon occurs is as follows. On the side upstream of a proximal position at which the electrostatic latent image bearing member and the developer bearing member are closest to each other in the direction of rotation of the developer bearing member, the developer projecting like spikes has not passed through the proximal position yet and, hence, spikes of the developer are relatively high. For this reason, the developer collides with the peripheral surface of the electrostatic latent image bearing member to cause toner particles to float. In the trailing end portion of the image located adjacent to an image-free portion, the density of electric flux lines directed from the peripheral surface of the developer bearing member toward the peripheral surface of the electrostatic latent image bearing member is likely to increase and, hence, floating toner particles collect easily. Further, since the side upstream of the proximal position at which the developer bearing member is closest to the electrostatic latent image in the developer feeding direction corresponds to the side downstream of the proximal position in the electrostatic latent image feeding direction, the quality of the image tends to be determined on the side downstream of the proximal position in the electrostatic latent image feeding direction. In a conventional image forming apparatus employing the counter developing system, floating toner particles are likely to collect on the trailing end portion of an image in a region in which the image quality tends to be determined. 
     The technology intends to provide a developing device which is capable of suppressing the occurrence of a pinhole in the halftone region developed prior to the high density region, as well as the occurrence of the phenomenon that the trailing end portion of an image becomes excessively dense. 
     SUMMARY OF THE TECHNOLOGY 
     According to the technology, there is provided a developing device for developing an electrostatic latent image carried on an electrostatic latent image bearing member rotating as to feed the electrostatic latent image at a developing region in a predetermined first direction. The developing device includes a developer bearing member, a magnetic field generating member, a layer thickness restricting member, and a spike-height restricting member. The developer bearing member has a cylindrical shape and is configured to feed a developer to a developing region in which the developer bearing member faces the electrostatic latent image bearing member by rotating in a second direction which is opposite to the first direction while carrying the developer on a peripheral surface thereof. The magnetic field generating member is unrotatably placed inside the developer bearing member and has a plurality of magnetic poles including a main magnetic pole positioned in the vicinity of the developing region. The layer thickness restricting member is configured to restrict a layer thickness of the developer born on the peripheral surface at a position upstream of the developing region in the second direction. The spike-height restricting member is placed at a predetermined position upstream of a proximal position at which the electrostatic latent image bearing member and the developer bearing member are closest to each other and downstream of the layer thickness restricting member in the second direction and is configured to restrict a spike height of the developer caused to project like spikes on the peripheral surface by a magnetic field generated by the main magnetic pole. 
     The foregoing and other features and attendant advantages of the technology will become more apparent from the reading of the following detailed description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating one exemplary pinhole; 
         FIG. 2  is a view showing part of a conventional developing device including a developer bearing member facing an electrostatic latent image bearing member; 
         FIG. 3  is a sectional front elevational view schematically showing an image forming apparatus including a developing device; 
         FIG. 4  is a sectional front elevational view of the developing device; 
         FIG. 5  is a view illustrating the position of a spike-height restricting blade included in the developing device; 
         FIG. 6(A)  is a photograph, taken from above a developing sleeve, of a developer projecting like spikes on a peripheral surface of the developing sleeve of the developing device provided with the spike-height restricting blade; 
         FIG. 6(B)  is a photograph, taken transversely of the developing sleeve, of the developer projecting like spikes on the peripheral surface of the developing sleeve of the developing device provided with the spike-height restricting blade; 
         FIG. 6(C)  is a photograph, taken from above a developing sleeve, of a developer projecting like spikes on a peripheral surface of the developing sleeve of a developing device not provided with the spike-height restricting blade; 
         FIG. 6(D)  is a photograph, taken transversely of the developing sleeve, of the developer projecting like spikes on the peripheral surface of the developing sleeve of the developing device not provided with the spike-height restricting blade; 
         FIG. 7  is a graph plotting the relationship between the value of A/(B+C) and the average height of projecting spikes of the developer with varying angle A; 
         FIG. 8  is a table showing the relationship between the circumferential velocity ratio obtained by dividing the linear velocity of the peripheral surface of the developing sleeve by the linear velocity of the peripheral surface of a photosensitive drum and the condition of the developer during feeding; 
         FIG. 9  is a front elevational view showing a developing region in an image forming apparatus including a developing device employing the forward developing system as a comparative example; 
         FIG. 10  is a front elevational view showing a developing region in an image forming apparatus including a developing device employing the counter developing system; 
         FIG. 11  is a front elevational view showing a developing region in an image forming apparatus including a developing device employing the counter developing system not provided with the spike-height restricting blade as a comparative example; 
         FIG. 12  is a front elevational view showing a developing region in an image forming apparatus including a developing device employing the counter developing system provided with the spike-height restricting blade; 
         FIG. 13(A)  is a view showing an image comprising plural lines which is formed by the image forming apparatus including the developing device employing the counter developing system not provided with the spike-height restricting blade as a comparative example; and 
         FIG. 13(B)  is a view showing an image comprising plural lines which is formed by the image forming apparatus including the developing device employing the counter developing system provided with the spike-height restricting blade. 
     
    
    
     DETAILED DESCRIPTION OF THE TECHNOLOGY 
     Hereinafter, the best mode for carrying out the technology will be described with reference to the drawings. 
       FIG. 3  is a sectional front elevational view schematically showing an image forming apparatus  100  including a developing device. The image forming apparatus  100  is a tandem-type color image forming apparatus configured to form an image using four color developers for yellow, magenta, cyan and black. The color developers used in the image forming apparatus  100  are each a two-component developer comprising a toner and a carrier. The image forming apparatus  100  forms a color image or a monochrome image on a recording sheet as a recording medium according to image data read by a document reader or transmitted from terminal equipment, such as a PC (personal computer), connected thereto via a non-illustrated network. 
     The image forming apparatus  100  includes a sheet feeding tray  110 , an intermediate transfer unit  120 , an image forming unit  130 , a secondary transfer roller  140 , and a fixing device  150 . 
     The sheet feeding tray  110  accommodates therein a multiplicity of recording sheets on each of which an image is to be formed. 
     The intermediate transfer unit  120  includes an intermediate transfer belt  121 , a driving roller  122 , and a driven roller  123 . The intermediate transfer belt  121  is an endless belt entrained about the driving roller  122  and the driven roller  123  and rotates clockwise in  FIG. 3 . 
     The image forming unit  130  includes photosensitive drums  31 A to  31 D, electrostatic charger devices  32 A to  32 D, exposure units  33 A to  33 D, developing devices  10 A to  10 D, primary transfer rollers  35 A to  35 D, and cleaning units  36 A to  36 D. Each of the photosensitive drums  31 A to  31 B is equivalent to an electrostatic latent image bearing member. The developing devices  10 A to  10 D have developing rollers  11 A to  11 D, respectively. 
     The image forming unit  130  comprises four image forming sections  30 A to  30 D for forming an image using image data items corresponding to respective of four colors including the three subtractive primary colors: cyan, magenta and yellow, obtained by color separation of a color image, and black. 
     The image forming section  30 A for black, image forming section  30 B for cyan, image forming section  30 C for magenta and image forming section  30 D for yellow are arranged in a row in this order along the intermediate transfer belt  121 . 
     Description will be made mainly of the image forming section  30 A for black. Each of the image forming sections  30 B to  30 D for other colors has the same arrangement as the image forming section  30 A. 
     The image forming section  30 A includes the photosensitive drum  31 A which is rotatable counterclockwise in  FIG. 3 . Around the photosensitive drum  31 A, there are disposed the electrostatic charger device  32 A, exposure unit  33 A, developing device  10 A, primary transfer roller  35 A and cleaning unit  36 A in this order in the direction of rotation of the photosensitive drum  31 A. The primary transfer roller  35 A is placed to face the photosensitive drum  31 A across the intermediate transfer belt  121 . 
     The electrostatic charger device  32 A electrostatically charges a peripheral surface of the photosensitive drum  31 A to a predetermined potential uniformly. Though the present embodiment uses a contact-type electrostatic charger device using a roller, an electrostatic charger device of the type using a charger or a brush may be used. 
     The exposure unit  33 A irradiates the photosensitive drum  31 A with a laser beam modulated according to the image data item for black. A portion of the peripheral surface of the photosensitive drum  31 A that is irradiated with the laser beam loses its electric potential by the photoconductive action of a photosensitive layer, to form an electrostatic latent image corresponding to the image data item for black. The photosensitive drums  31 B to  31 D each form an electrostatic latent image corresponding to a respective one of the image data items for cyan, magenta and yellow. The exposure unit  33 A may comprise a laser scanning unit (LSU) or a writing device having an array of light-emitting devices such as ELs or LEDs. 
     The developing device  10 A, which contains a black toner therein, has the developing roller  11 A. The developing roller  11 A feeds the developer to a developing region in which the peripheral surface of the photosensitive drum  31 A and the peripheral surface of the developing roller  11 A face each other and which allows toner particles to migrate to the peripheral surface of the photosensitive drum  31 A. The developing device  10 A supplies the toner to the electrostatic latent image formed on the peripheral surface of the photosensitive drum  31 A to visualize the electrostatic latent image into a toner image. 
     The developing devices  10 B to  10 D, each of which contains a respective one of cyan, magenta and yellow toners, visualize the electrostatic latent images for the respective colors formed on the respective photosensitive drums  31 B to  31 D into cyan, magenta and yellow toner images. 
     In the present embodiment, the toner is electrostatically charged to have the same polarity as the surface potential of the photosensitive drum  31 A. The polarity of the surface potential of the photosensitive drum  31 A and the polarity of the toner charged are both negative. 
     The primary transfer roller  35 A is applied with a primary transfer bias voltage having a polarity opposite to the polarity of the toner charged in order to transfer the toner image carried on the peripheral surface of the photosensitive drum  31 A to the intermediate transfer belt  121 . (In the present embodiment, the polarity of the primary transfer bias voltage is positive.) Thus, the black toner image formed on the photosensitive drum  31 A is transferred to the intermediate transfer belt  121  so as to be superimposed upon the toner images of other colors on the intermediate transfer belt  121 . The toner images of the respective colors are transferred to the intermediate transfer belt  121  by the respective image forming sections  30 A to  30 D so as to be superimposed one upon another, thereby forming a full-color toner image on the intermediate transfer belt  121 . 
     When image data items for some of yellow, magenta, cyan and black are inputted, only those photosensitive drums for the colors associated with the image data items thus inputted form electrostatic latent images and then their respective toner images. In a monochrome printing mode, for example, only the photosensitive drum  31 A for black forms an electrostatic latent image and then its toner image, while the intermediate transfer belt  121  receives only the black toner image transferred thereto. 
     The cleaning unit  36 A collects residual toner remaining on the peripheral surface of the photosensitive drum  31 A after the primary transfer operation following the developing operation. 
     The secondary transfer roller  140  is placed so as to face the driven roller  123  across the intermediate transfer belt  121 . A recording sheet fed from the sheet feeding tray  110  passes between the secondary transfer roller  140  and the intermediate transfer belt  121 . The secondary transfer roller  140  is applied with a secondary transfer bias voltage having a polarity opposite to the polarity of the toner charged. (In the present embodiment, the polarity of the secondary transfer bias voltage is positive.) Thus, the full-color toner image formed on the intermediate transfer belt  121  is transferred to the recording sheet passing between the intermediate transfer belt  121  and the secondary transfer roller  140 . 
     The fixing device  150  has a heating roller  151  and a pressurizing roller  152 . The recording sheet bearing the toner image transferred thereto is guided to the fixing device  150  and then heated and pressurized when the recording sheet passes between the heating roller  151  and the pressurizing roller  152 . Thus, the toner image is firmly fixed to the surface of the recording sheet. The recording sheet bearing the toner image fixed thereto is ejected onto a non-illustrated catch tray. 
       FIG. 4  is a sectional front elevational view of the developing device  10 A. The other developing devices  10 B to  10 D each have the same arrangement as the developing device  10 A. 
     The developing device  10 A includes, in addition to the developing roller  11 A, a layer thickness restricting blade  12 A, two stirring and feeding screws  13 A and  14 A, a developing tank  15 A, and a spike-height restricting blade  20 A. The layer thickness restricting blade  12 A is equivalent to a layer thickness restricting member. The spike-height restricting blade  20 A is equivalent to a spike-height restricting member. 
     The developing tank  15 A stores therein the two-component developer comprising the toner and the carrier. The stirring and feeding screws  13 A and  14 A are placed within the developing tank  15 A. A partition wall  16 A intervenes between the stirring and feeding screws  13 A and  14 A. The partition wall  16 A separates a region around the stirring and feeding screw  13 A and a region around the stirring and feeding screw  14 A from each other except spaces in the vicinity of opposite ends of each of the stirring and feeding screws  13 A and  14 A along the axis of rotation. 
     The toner contained in the developer stored in the developing tank  15 A is stirred together with the carrier by the stirring actions of the stirring and feeding screws  13 A and  14 A, thereby being frictionally electrified. 
     The developing tank  15 A has an opening  17 A in a portion facing the photosensitive drum  31 A. The developing roller  11 A is positioned within the developing tank  15 A so as to be partially exposed through the opening  17 A of the developing tank  15 A and to define a predetermined developing gap with the peripheral surface of the photosensitive drum  31 A. The developing gap is a spacing set as desired within a range from about 0.3 mm to about 1.0 mm for example. Usually, the developing gap is preferably as small as possible, for example, within a range from 0.3 mm to 0.5 mm. 
     The developing roller  11 A has a magnet roller  18 A and a non-magnetic developing sleeve  19 A. The magnetic roller  18 A is equivalent to a magnetic field generating member. The developing sleeve  19 A is equivalent to a developer bearing member. The magnet roller  18 A has a plurality of magnetic poles including a main magnetic pole positioned in the vicinity of the developing region. The plurality of magnetic poles are arranged circumferentially of the magnet roller  18 A. The magnet roller  18 A is fixed to the developing tank  15 A. The developing sleeve  19 A is shaped substantially cylindrical and formed from an aluminum alloy, brass or the like. The developing sleeve  19 A is fitted over the magnet roller  18 A so as to be rotatable in a predetermined direction. The developing sleeve  19 A is rotated by a non-illustrated driving source in such a direction as to feed the developer at the developing region in a predetermined feeding direction  92 . The feeding direction  92  is equivalent to the second direction defined by the technology. 
     The photosensitive drum  31 A rotates in such a direction as to feed the electrostatic latent image at the developing region in a predetermined direction  91 . The feeding direction  91  is equivalent to a first direction. The image forming apparatus  100  employs the counter developing system wherein the feeding direction  92  in which the developing sleeve  19 A feeds the developer at the developing region is opposite to the feeding direction  91  in which the photosensitive drum  31 A feeds the electrostatic latent image at the developing region. 
     The carrier contained in the developer comprises a magnetic material. Toner particles adhere to the surface of the carrier by the Coulomb force resulting from frictional electrification. The developer is attracted onto the peripheral surface of the developing sleeve  19 A by magnetic fields generated by the magnetic poles of the magnet roller  18 A, to form a magnetic brush. The developer is fed into the developing region by rotation of the developing sleeve  19 A. 
     The layer thickness restricting blade  12 A is attached to the developing tank  15 A at a predetermined position upstream of the opening  17 A in the developer feeding direction  92  in such a manner that the tip thereof faces the peripheral surface of the developing sleeve  19 A. The layer thickness restricting blade  12 A restricts the layer thickness of the developer attracted on the peripheral surface of the developing sleeve  19 A. 
     The spike-height restricting blade  20 A is supported on the developing tank  15 A at a predetermined position upstream of a proximal position at which the peripheral surface of the photosensitive drum  31 A and the peripheral surface of the developing sleeve  19 A are closest to each other and downstream of the layer thickness restricting blade  12 A in the developer feeding direction  92  in such a manner that the tip thereof faces the peripheral surface of the developing sleeve  19 A. The spike-height restricting blade  20 A restricts the spike height of the developer caused to project like spikes on the peripheral surface of the developing sleeve  19 A under a magnetic field generated by the main magnetic pole of the magnet roller  18 A. The spike height of the developer projecting like spikes is about 1.2 mm before restriction by the spike-height restricting blade  20 A. Such spikes of the developer are cut to the same height, for example, 0.6 mm by restriction by the spike-height restricting blade  20 A. The spike-height restricting blade  20 A will be described in detail later. 
     The image forming apparatus  100  further includes a developing bias voltage applying section  161 . The developing bias voltage applying section  161  applies a developing bias voltage to the developing sleeve  19 A so that the potential difference between the developing sleeve  19 A and the photosensitive drum  31 A varies continuously and periodically. 
     The developing bias voltage is a voltage comprising a direct current component and an alternating current component which are superposed upon each other, that is, an oscillating bias voltage such that a developing potential and a reverse developing potential alternate with each other. The developing potential exerts a force on the electrostatically charged toner in a direction from the developing roller  11 A toward the photosensitive drum  31 A. The reverse developing potential exerts a force on the electrostatically charged toner in a direction from the photosensitive drum  31 A toward the developing roller  11 A. Toner particles fed into the developing region are caused to fly between the developing sleeve  19 A and the photosensitive drum  31 A by the developing bias voltage. For example, the developing bias voltage applying section  161  applies the developing sleeve  19 A with a rectangular wave oscillating bias voltage having a frequency of 9 kHz and an amplitude of 0.8 kV. 
     The developing device  10 A feeds the developer into the developing region at a predetermined feed rate per unit time. The toner contained in the developer fed into the developing region is attracted to the electrostatic latent image carried on the peripheral surface of the photosensitive drum  31 A by electrostatic force. Thus, the electrostatic latent image is developed into the toner image. 
     The developing device  10 A feeds the carrier and residual toner, which has not been used for development, of the developer fed to the developing region back into the developing tank  15 A by rotation of the developing sleeve  19 A. 
     Detailed description will be made of the spike-height restricting blade  20 A. The spike-height restricting blade  20 A is formed from a non-magnetic resin having elasticity such as urethane, PET (polyethylene terephthalate), or the like. 
     Since the spike-height restricting blade  20 A is formed from the non-magnetic material, the magnetic field in the developing region can be prevented from being disturbed by the spike-height restricting blade  20 A, unlike in the case where the spike-height restricting blade is formed from a magnetic material. 
     Also, since the spike-height restricting blade  20 A is formed from the resin having elasticity, the spike-height restricting blade  20 A can be placed in such a position that the tip thereof contacts the peripheral surface of the developing sleeve  19 A. For this reason, the spike-height restricting blade  20 A can be attached easily and positioned stably. Further, since the spike-height restricting blade  20 A has elasticity, the spike-height restricting blade  20 A is caused to float up from the peripheral surface of the developing sleeve  19 A by the developer fed thereto and, hence, the developer is fed in such a manner as to slip through the gap between the peripheral surface of the developing sleeve  19 A and the spike-height restricting blade  20 A. 
     The spike-height restricting blade  20 A may be formed from a metal. In cases where the spike-height restricting blade  20 A is formed from a hard material such as a metal, the spike-height restricting blade  20 A needs to be fixed firmly at a position adjacent the peripheral surfaces of respective of the developing sleeve  19 A and the photosensitive drum  31 A so as not to contact both of the peripheral surfaces. This is because the spike-height restricting blade  20 A of metal, when contacting the peripheral surface of the developing sleeve  19 A, prevents the developer from being fed into the developing region and because the spike-height restricting blade  20 A of metal, when contacting the peripheral surface of the photosensitive drum  31 A, might disorder the electrostatic latent image. In the present embodiment, the spike-height restricting blade  20 A comprises a PET film having a thickness of 0.1 mm and has a tip brought into contact with the peripheral surface of the developing sleeve  19 A. 
       FIG. 5  is a view illustrating the position of the spike-height restricting blade  20 A included in the developing device  10 A. The spike height of the developer projecting like spikes on the peripheral surface of the developing sleeve  19 A can be adjusted by adjusting the position of the tip of the spike-height restricting blade  20 A. 
     Reference line  93  shown in  FIG. 5  is a line linking a rotation center Q 1  of the developing sleeve  19 A and a rotation center Q 2  of the photosensitive drum  31 A. First line  94  is a line liking the rotation center Q 1  and the tip of the spike-height restricting blade  20 A. Angle A is an angle defined between the reference line  93  and the first line  94  circumferentially of peripheral surface  191 A of the developing sleeve  19 A. 
     Second line  95  is a line linking the rotation center Q 1  and a circumferential position on the peripheral surface  191 A at which the intensity of a magnetic field  81  generated at the peripheral surface  191 A by the main magnetic pole assumes a maximum value in a radial direction of the developing sleeve  19 A. Angle B is an angle defined between the reference line  93  and the second line  95  circumferentially of the peripheral surface  191 A. 
     Third line  96  is a line linking the rotation center Q 1  and a circumferential position on the peripheral surface  191 A which is located upstream of the second line  95  in the developer feeding direction  92  and at which the intensity of the magnetic field  81  generated at the peripheral surface  191 A by the main magnetic pole is a half of the maximum value in the radial direction of the developing sleeve  19 A. Angle C is an angle defined between the second line  95  and the third line  96  circumferentially of the peripheral surface  191 A. Under this condition, the following formula holds:
 
 A≦B+C.  
 
     In brief, the tip of the spike-height restricting blade  20 A is positioned within a region between proximal position  82  at which the photosensitive drum  31 A and the developing sleeve  19 A are closest to each other and the circumferential position on the peripheral surface  191 A at which the intensity of the magnetic field  81  generated at the peripheral surface  191 A by the main magnetic pole is a half of the maximum value in the radial direction of the developing sleeve  19 A and which is located upstream of the circumferential position on the peripheral surface  191 A in the developer feeding direction  92  at which the intensity of the magnetic field  81  generated at the peripheral surface  191 A by the main magnetic pole assumes the maximum value in the radial direction of the developing sleeve  19 A. 
     In most cases, the developer is caused to project like spikes at a position downstream of the third line  96  in the developer feeding direction  92 . For this reason, the developer is not caused to project like spikes in a region in which the formula: A&gt;B+C holds. Therefore, if the tip of the spike-height restricting blade  20 A is positioned within the region in which the formula: A&gt;B+C holds, the spike height of the developer changes little. 
     Thus, by positioning the tip of the spike-height restricting blade  20 A within the region in which the formula: A≦B+C holds, the spike height of the developer can be adjusted to a predetermined value. 
       FIG. 6(A)  is a photograph, taken from above the developing sleeve  19 A, of the developer projecting like spikes on the peripheral surface  191 A of the developing sleeve  19 A of the developing device  10 A provided with the spike-height restricting blade  20 A.  FIG. 6(B)  is a photograph, taken transversely of the developing sleeve  19 A, of the developer projecting like spikes on the peripheral surface  191 A of the developing sleeve  19 A of the developing device  10 A provided with the spike-height restricting blade  20 A. In  FIGS. 6(A) and 6(B) , the angles A and B are set to 7° and 0°, respectively. 
       FIGS. 6(C) and 6(D)  shows a comparative example.  FIG. 6(C)  is a photograph, taken from above a developing sleeve, of the developer projecting like spikes on a peripheral surface of the developing sleeve of a developing device not provided with the spike-height restricting blade.  FIG. 6(D)  is a photograph, taken transversely of the developing sleeve, of the developer projecting like spikes on the peripheral surface of the developing sleeve of the developing device not provided with the spike-height restricting blade. 
       FIGS. 6(A) and 6(B)  are the photographs each showing the state of the developer at a predetermined position downstream of the tip of the spike-height restricting blade  20 A in the developer feeding direction  92 .  FIGS. 6(C) and 6(D)  are the photographs each showing the state of the developer at a position equivalent to the predetermined position described above. 
     The carrier used in the present embodiment is formed from ferrite core and has an average particle diameter of 40 μm. The magnet roller  18 A has a diameter of 18 mm and is configured such that: its main magnetic pole generates a magnetic field having a maximum intensity of 1100 mT in a radial direction of the developing sleeve  19 A; and the angle C is set to 14°. 
     As can be seen from comparison between  FIGS. 6(A) and 6(C)  by observation, the developer shown in  FIG. 6(A)  had a higher density of projecting spikes than that shown in  FIG. 6(C) . This proves that by restricting the spike height of the developer with the spike-height restricting blade  20 A, the density of projecting spikes of the developer was increased. As can be seen from comparison between  FIGS. 6(B) and 6(D)  by observation, the developer shown in  FIG. 6(B)  had a lower spike height than that shown in  FIG. 6(D)  and had spikes cut to a substantially even height. This proves that the spike-height restricting blade  20 A lowered the spike height of the developer and cut the spikes to a substantially even height. 
       FIG. 7  is a graph plotting the relationship between the value of A/(B+C) and the average height of projecting spikes of the developer with varying angle A. In  FIG. 7 , the average height of projecting spikes of the developer represented by the vertical axis was a value obtained by averaging the heights of 100 spikes selected at random from a photograph taken transversely of the developing sleeve  19 A as shown in  FIG. 6(B) . 
     As can be seen from  FIG. 7 , the spike height of the developer obtained when A/(B+C)=1 is slightly lower than that obtained in the case where the spike-height restricting blade  20 A is not provided and, hence, the spike height is not restricted. As can be also seen from  FIG. 7 , the spike height of the developer obtained when A/(B+C)=1.2 is substantially equal to that obtained when A/(B+C)=1. These facts prove that the spike height of the developer obtained when A/(B+C)&gt;1 changes little whether or not the spike-height restricting blade  20 A is provided. On the other hand, the spike height of the developer is lowered by the spike-height restricting blade  20 A when A/(B+C)≦1, i.e., 
     A≦B+C. This proves that by positioning the tip of the spike-height restricting blade  20 A within the region in which the formula: A≦B+C holds, the spike height of the developer can be restricted to a predetermined height. 
     If it is possible to increase the angle A defined circumferentially of peripheral surface  191 A of the developing sleeve  19 A between the reference line  93  linking the rotation center Q 1  of the developing sleeve  19 A and the rotation center Q 2  of the photosensitive drum  31 A and the first line  94  linking the rotation center Q 1  and the tip of the spike-height restricting blade  20 A, the spike-height restricting blade  20 A can be positioned apart from the proximal position  82  at which the photosensitive drum  31 A and the developing sleeve  19 A are closest to each other (see  FIG. 5 ). By so doing, the space within which the spike-height restricting blade  20 A can be positioned is expanded for easy positioning of the spike-height restricting blade  20 A. 
     An example of means for increasing the angle A is setting the angle B to B&gt;0. Specifically, the main magnetic pole of the magnet roller  18 A is positioned upstream of the proximal position  82  at which the photosensitive drum  31 A and the developing sleeve  19 A are closest to each other in the developer feeding direction  92 . In the case where the hard material is used for the spike-height restricting blade  20 A, the space within which the spike-height restricting blade  20 A can be positioned is narrower than in the case where the resin material having elasticity is used for the spike-height restricting blade  20 A. Setting the angle B to B&gt;0 is particularly effective in such a case because the space within which the spike-height restricting blade  20 A can be positioned is expanded by such a setting. 
     Detailed description will be made of the developing sleeve  19 A. As described above, the developing sleeve  19 A is applied with the developing bias voltage comprising a direct current component and an alternating current component which are superposed upon each other. If the developing bias voltage does not comprise the alternating current component and consists only of the direct current component, the amount of developer to contact the photosensitive drum  31 A is reduced by an amount corresponding to a decrease in the spike height of the developer resulting from restriction with the spike-height restricting blade  20 A, so that the resulting image has a lowered density. Further, since the feed rate of the developer per unit area of the peripheral surface  19 A of the developing sleeve  19 A is lowered due to the lowered spike-height of the developer, the developing bias voltage consisting only of the direct current component sometimes causes the developer to be fed non-uniformly. In the image forming apparatus  100 , by contrast, the developing sleeve  19 A is applied with the developing bias voltage comprising the direct current component and the alternating current component which are superposed upon each other and, hence, it is easy to supply the toner to the electrostatic latent image uniformly and sufficiently. Therefore, it is possible to provide the image with a proper density and prevent the image quality from lowering due to non-uniform feeding of the developer. 
     The peripheral surface of the developing sleeve  19 A is sandblasted. The sandblasted peripheral surface of the developing sleeve  19 A allows the developer to form a high density of spikes uniformly on the peripheral surface. This results in a high-quality image. Further, the sandblasted peripheral surface of the developing sleeve  19 A feeds the developer at a lower feed rate per unit area than does a non-sandblasted smooth peripheral surface. For this reason, the ability of the spike-height restricting blade  20 A to restrict the spike height of the developer can be prevented from lowering. 
     In the present embodiment, the peripheral surface of the developing sleeve  19 A feeds the developer at a feed rate of 40 mg/cm 2 . As described above, the spike-height restricting blade  20 A is formed from the resin material having elasticity and has its tip brought into contact with the peripheral surface of the developing sleeve  19 A. When the developer reaches the spike-height restricting blade  20 A, the spike-height restricting blade  20 A deflects in such a manner that the tip thereof floats up from the peripheral surface of the developing sleeve  19 A. When the developer feed rate per unit area of the peripheral surface of the developing sleeve  19 A is not more than 60 mg/cm 2 , the gap between the floating tip of the spike-height restricting blade  20 A and the peripheral surface of the developing sleeve  19 A does not expand very much, with the result that the ability of the spike-height restricting blade  20 A to restrict the spike height of the developer can be prevented from lowering. Therefore, the developer feed rate per unit area of the peripheral surface of the developing sleeve  19 A is desirably not more than 60 mg/cm 2 , more desirably not more than 40 mg/cm 2 . 
       FIG. 8  is a table showing the relationship between the circumferential velocity ratio obtained by dividing the linear velocity of the peripheral surface of the developing sleeve  19 A by the linear velocity of the peripheral surface of the photosensitive drum  31 A and the condition of the developer during feeding. In  FIG. 8 , symbol “x” represents a bad condition, symbol “∘” represents a good condition, and symbol “ ” represents a particularly good condition. 
     The counter developing system can provide the image with a higher density than the forward developing system in which the developing sleeve  19 A feeds the developer at the developing region in the developer feeding direction  92  which is the same direction as the electrostatic latent image feeding direction  91  in which the photosensitive drum  31 A feeds the electrostatic latent image at the developing region, for the reason that the counter developing system allows a significantly larger amount of developer to contact the electrostatic latent image at the developing region, and a like reason. Accordingly, even when the circumferential velocity ratio is lowered by reducing the number of rotations of the developing sleeve  19 A, an image having an insufficient density is not likely to be formed. Therefore, by reducing the number of rotations of the developing sleeve  19 A, the stress exerted on the developer by the layer thickness restricting blade  12 A and the spike-height restricting blade  20 A can be reduced, whereby deterioration of the developer can be suppressed. 
     As can be seen from  FIG. 8 , when the image forming apparatus  100  was subjected to a predetermined long-term continuous operation test at a circumferential velocity ratio of 1.9, particles of the developer agglomerated due to the stress exerted by the layer thickness restricting blade  12 A and the like, thereby causing the developer to be fed in a bad condition. By contrast, when the test was conducted at a circumferential velocity ratio of 1.8, the developer was fed in a good condition. The developer was fed in a particularly good condition when the circumferential velocity ratio was 1.7. 
     When the circumferential velocity ratio is less than 1.0, the linear velocity of the peripheral surface of the developing sleeve  19 A is lower than the linear velocity of the peripheral surface of the photosensitive drum  31 A. As a result, unevenness in the spike height of the developer projecting like spikes on the peripheral surface of the developing sleeve  19 A is likely to occur, which affects the image quality seriously. Therefore, the circumferential velocity ratio is desirably not less than 1.0. More desirably, the circumferential velocity ratio is not less than 1.3 because unevenness in the spike height of the developer is difficult to occur. 
     In view of the above, the circumferential velocity ratio is desirably not less than 1.0 and not more than 1.8, more desirably not less than 1.3 and not more than 1.8, further more desirably not less than 1.3 and not more than 1.7. 
     With reference to  FIGS. 9 and 10 , description will be made of the difference between the case where the spike-height restricting blade  20 A is used in the forward developing system and the case where the spike-height restricting blade  20 A is used in the counter developing system. 
       FIG. 9  is a front elevational view showing a developing region in an image forming apparatus including a developing device employing the forward developing system as a comparative example. In  FIG. 9 , like reference characters are used to designate like members common to the image forming apparatus  100  and the comparative example for convenience. As described above, in the forward developing system, the developing sleeve  19 A feeds the developer at the developing region in developer feeding direction  97  which is the same direction as the electrostatic latent image feeding direction  91  in which the photosensitive drum  31 A feeds the electrostatic latent image at the developing region. 
     In the forward developing system, in general, the linear velocity of the peripheral surface of the developing sleeve  19 A is higher than that of the peripheral surface of the photosensitive drum  31 A and, hence, the developer  83  projecting like spikes on the peripheral surface of the developing sleeve  19 A passes the electrostatic latent image on the photosensitive drum  31 A. 
     In region E in which the developer  83  is in a state before contacting the peripheral surface of the photosensitive drum  31 A, the spike height of the developer is restricted by the spike-height restricting blade  20 A to a value that is slightly larger than the gap defined between the peripheral surface of the photosensitive drum  31 A and that of the developing sleeve  19 A at the proximal position  82 . For this reason, the developer  83  fails to contact the peripheral surface of the photosensitive drum  31 A in the region E and, hence, the developer is less susceptible to the potential of the peripheral surface of the photosensitive drum  31 A. Thus, the potential of the developer exhibits high uniformity. 
     In region F which the developer  83  reaches after having contacted the peripheral surface of the photosensitive drum  31 A, the spike height of the developer  83  becomes equal to the gap defined between the peripheral surface of the photosensitive drum  31 A and that of the developing sleeve  19 A at the proximal position  82 . Since the tips of spikes of the developer  83  reaching the region F have contacted or passed close by the peripheral surface of the photosensitive drum  31 A, the potential at the tips of the spikes of the developer  83  has been disturbed by the effect of the potential of the peripheral surface of the photosensitive drum  31 A. 
     For example, when the tips of spikes of the developer  83  contact an image-free portion of the peripheral surface of the photosensitive drum  31 A in which the electrostatic latent image is not formed, toner particles move back away from carrier particles at the tips of the spikes toward the spike root side, with the result that the tips of the spikes carry an electrical charge called “counter charge” having a polarity opposite to the normal polarity of the toner. When the developer  83  carrying the counter charge is fed into the region F, the toner image is disordered in the region F. 
     Even without the provision of the spike-height restricting blade  20 A, the tips of spikes of the developer  83  pass through the proximal position  82  while contacting the peripheral surface of the photosensitive drum  31 A. For this reason, the developer  83  often carries the counter charge in the region F. 
     The quality of the toner image tends to be determined in the region F, particularly on the downstream side of the proximal position  82  in the electrostatic latent image feeding direction  91 . With the forward developing system, however, even in the case where the spike-height restricting blade  20 A is provided, the developer  83  in the region F carries the counter charge as in the case where the spike-height restricting blade  20 A is not provided. For this reason, the toner image is likely to be disordered in the region F. Therefore, with the forward developing system, a difference is not likely to occur in toner image quality between the case where the spike-height restricting blade  20 A is provided and the case where the spike-height restricting blade  20 A is not provided. 
       FIG. 10  is a front elevational view showing the developing region in the image forming apparatus  100  including the developing device  10 A employing the counter developing system. As described above, in the counter developing system, the developing sleeve  19 A feeds the developer at the developing region in the developer feeding direction  92  which is opposite to the electrostatic latent image feeding direction  91  in which the photosensitive drum  31 A feeds the electrostatic latent image at the developing region. 
     In the case where the counter developing system is provided with the spike-height restricting blade  20 A, the region F in which the developer  83  often carries the counter charge is located upstream of the proximal position  82  in the electrostatic latent image feeding direction  91 . In the region E which is located downstream of the proximal position  82  in the electrostatic latent image feeding direction  91  and in which the quality of the toner image tends to be determined, the developer  83  does not carry the counter charge. Since the spike height of the developer  83  is restricted to an appropriate height by the spike-height restricting blade  20 A in the region E, an excessive supply of toner to the electrostatic latent image is suppressed. For this reason, the toner image is rather restored than disordered in the region E. Therefore, the toner image quality is improved. 
     In the case where the counter developing system is not provided with the spike-height restricting blade  20 A, the spikes of the developer  83  are relatively high in the region E and, hence, the developer  83  may contact the peripheral surface of the photosensitive drum  31 A. For this reason, the toner image on the peripheral surface of the photosensitive drum  31 A tends to be disordered. Accordingly, the toner image quality is lowered. 
     Thus, the spike-height restricting blade  20 A exhibits little effect in the forward developing system. It is in the counter developing system that the spike-height restricting blade  20 A exhibits the effect of improving the image quality. 
     With reference to  FIGS. 11 and 12 , description will be made of the difference between the case where the counter developing system is provided with the spike-height restricting blade  20 A and the case where the counter developing system is not provided with the spike-height restricting blade  20 A. 
       FIG. 11  is a front elevational view showing a developing region in an image forming apparatus including a developing device employing the counter developing system not provided with the spike-height restricting blade  20 A as a comparative example.  FIG. 12  is a front elevational view showing the developing region in the image forming apparatus  100  including the developing device  10 A employing the counter developing system provided with the spike-height restricting blade  20 A. In  FIGS. 11 and 12 , arrows extending from the peripheral surface of the developing sleeve  19 A toward the peripheral surface of the photosensitive drum  31 A indicate electric flux lines. 
     In the case where the developing device is not provided with the spike-height restricting blade  20 A, in general, the spike height of the developer  83  is larger than the gap defined between the peripheral surface of the photosensitive drum  31 A and the peripheral surface of the developing sleeve  19 A at the proximal position  82  because the spike height of the developer  83  is not restricted on the upstream side of the proximal position  82  in the developer feeding direction  92 , as shown in  FIG. 11 . For this reason, the tips of spikes of the developer  83  collide with the peripheral surface of the photosensitive drum  31 A on the upstream side of the proximal position  82  in the developer feeding direction  92  to cause toner particles adhering to the carrier to float in a region G adjacent to the peripheral surface of the photosensitive drum  31 A. 
     The developer  83  having collided with the peripheral surface of the photosensitive drum  31 A is further fed to pass through the proximal position  82 . The spike height of the developer  83  is restricted to a lower height when the developer  83  passes through the proximal position  82 . The developer  83  is fed further downstream in the feeding direction  92 . 
     As described above, the quality of the toner image tends to be determined on the downstream side of the proximal position  82  in the electrostatic latent image feeding direction  91 . In the counter developing system, the region located downstream of the proximal position  82  in the electrostatic latent image feeding direction  91  is the region located upstream of the proximal position  82  in the developer feeding direction  92  in which the tips of spikes of the developer  83  collide with the peripheral surface of the photosensitive drum  31 A. 
     Since trailing end portion  84 A of electrostatic latent image  84  is located adjacent to image-free portion  85 , the density of electric flux lines directed from the peripheral surface of the developing sleeve  19 A toward the peripheral surface of the photosensitive drum  31 A tends to increase in the trailing end portion  84 A of the electrostatic latent image  84  in the electrostatic latent image feeding direction  91 . Therefore, floating toner particles are likely to collect in the trailing end portion  84 A of the electrostatic latent image  84 . For this reason, in the case where the developing device is not provided with the spike-height restricting blade  20 A, the phenomenon so-called “sweeping together” that the trailing end portion  84 A of the electrostatic latent image  84  in the electrostatic latent image feeding direction  91  is developed excessively densely, is likely to occur. 
       FIG. 13(A)  is a view showing an image comprising plural lines which is formed by the image forming apparatus including the developing device employing the counter developing system not provided with the spike-height restricting blade  20 A as a comparative example. Lines in the trailing end portion of the image are excessively dense and each widened to such an extent as to contact an adjacent line. 
     As shown in  FIG. 12 , the image forming apparatus  100  including the developing device employing the counter developing system provided with the spike-height restricting blade  20 A restricts, at a position upstream of the proximal position  82  in the developer feeding direction  92 , the spike height of the developer  83  to a value slightly larger than the gap defined between the peripheral surface of the photosensitive drum  31 A and the peripheral surface of the developing sleeve  19 A at the proximal position  82 . Therefore, the spikes of the developer  83  can hardly collide with the peripheral surface of the photosensitive drum  31 A at the position upstream of the proximal position  82  in the developer feeding direction  92  and, hence, toner particles can hardly float. 
     Thus, the image forming apparatus  100  including the developing device  10 A employing the counter developing system provided with the spike-height restricting blade  20 A is capable of suppressing the occurrence of the phenomenon that the trailing end portion  84 A of the electrostatic latent image  84  is developed excessively densely, at the position upstream of the proximal position  82  in the developer feeding direction  92 , i.e., at the position downstream of the proximal position  82  in the electrostatic latent image feeding direction  91 . 
       FIG. 13(B)  is a view showing an image comprising plural lines which is formed by the image forming apparatus including the developing device  10 A employing the counter developing system provided with the spike-height restricting blade  20 A. In the trailing end portion of the image, the density and width of each line are not increased and the spacing between adjacent lines is not narrowed. Any portion having a pinhole, in particular, is not observed. 
     Thus, in the image forming apparatus including the developing device employing the counter developing system, the spike-height restricting blade  20 A restricts the spike height of the developer  83  caused to project like spikes under the magnetic field generated by the main magnetic pole at the position upstream of the proximal position  82  in the developer feeding direction  92 , thereby making it possible to suppress the occurrence of a pinhole in the halftone region developed prior to the high density region as well as the occurrence of the phenomenon that the trailing end portion of the image becomes excessively dense. 
     The foregoing embodiment is illustrative in all points and should not be construed to limit the technology. The scope of the technology is defined not by the foregoing embodiment but by the following claims. Further, the scope is intended to include all modifications within the scopes of the claims and within the meanings and scopes of equivalents.