Abstract:
The developing apparatus has a housing, a developer bearing member rotatably provided at an opening of the housing, a developer containing room, a carrying member for carrying the developer in the developer containing room toward the developer bearing member, and a developer accumulating portion arranged near the developer bearing member and adapted to accumulate the developer carried by the carrying member. In the developing apparatus, the developer accumulating portion is provided above a rotational center of the carrying member.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image forming apparatus, such as a copying machine or a printer, using the electrophotographic or electrostatic recording system and to a developing apparatus to be used in the image forming apparatus.  
           [0003]    2. Related Art  
           [0004]    Nowadays, an increase in speed, a reduction in size, an improvement in image quality, and an increase in service life are required of image forming apparatuses. In addition, with the development of the information technology, there is an increasing demand for color image output.  
           [0005]    An example of a technique for meeting the demand for an increase in speed is a tandem type image forming apparatus in which developing apparatuses for four colors are arranged laterally or longitudinally. Despite the fact that a plurality of developing apparatuses are arranged, no increase in apparatus size is involved since each developing unit is thin. FIG. 13 shows an example of such an image forming apparatus.  
           [0006]    [0006]FIG. 13 is a side sectional view showing the inner construction of a tandem type printer. As shown in the drawing, a sheet conveying belt  111  is arranged substantially at the center of the interior of the apparatus. The belt  111 , whose end portions are held by a driving roller  112  and a driven roller  113 , circulates counterclockwise as indicated by an arrow A. On top of the belt  111 , there are arranged four photosensitive drums  114 ( 1144   a ,  114   b ,  114   c , and  114   d ) in the sheet conveying direction (from the right to the left in the drawing).  
           [0007]    On the upstream side of the belt  111  with respect to the sheet conveying direction, there is provided a registration roller pair  124  for correcting skew feeding of a sheet being fed. On the upstream side thereof, a sheet guiding path  125  is provided, and on the upstream side thereof, a sheet feeding roller  126  is provided. A sheet cassette  104  is arranged on the upstream side of the sheet feeding roller  126 .  
         Description of a Process Cartridge in an Image Forming Apparatus  
         [0008]    [0008]FIG. 14 is an enlarged sectional view of a process cartridge  134  to be mounted in the printer of FIG. 13. The process cartridge  134  is obtained by forming the two sub units: a drum unit  135  and a developing unit  136  into an integral unit. The drum unit  135  is equipped with the photosensitive drum  114  mentioned above, a cleaner  115  arranged on the left-hand side thereof, and a charging roller  116  arranged above and on the left-hand side of the photosensitive drum. Light in accordance with image information emitted from a scanner  117  mounted in the printer main body impinges upon the photosensitive drum  114  at a position indicated by reference numeral  137 .  
           [0009]    The photosensitive drum  114  consists of a conductive metal roller whose surface is uniformly coated with a photoconductive material; the metal roller portion is grounded, and rotates clockwise as indicated by an arrow c.  
           [0010]    The charging roller  116  is connected to a high-voltage power source and applies a blank-state charged potential (initialization potential applied uniformly; in a system it is applied in positive polarity, in another in negative polarity) to the photosensitive drum  114 , whereby the photosensitive layer on the peripheral surface of the photosensitive drum  114  is uniformly charged, for example, in negative high potential and initialized.  
           [0011]    The scanner  117  is equipped with a laser light source or an LED light source, and performs selective exposure in accordance with image information on the peripheral surface of the photosensitive drum  114  charged in negative high potential. As a result of this exposure, there is formed on the surface of the photosensitive drum  114  a negative low potential portion with attenuated potential, and there is formed an electrostatic latent image consisting of the low potential portion and the high potential portion obtained through the initialization.  
           [0012]    In the developing unit  136 , a non-magnetic toner is stored so as to substantially fill a developing container  118 , and a toner agitating member  144  is arranged so as to be buried under the toner; in the lowermost portion, a toner supply roller  145  formed of a sponge material is held in press contact with a developing roller  143 . A developing blade  146  in the form of a plate spring is held in press contact with the upper right portion of the developing roller  143 .  
         Description of the Operation of the Image Forming Apparatus  
         [0013]    The operation of the above-described image forming apparatus will be briefly described with reference to FIGS. 13 and 14. First, when the power source is turned on and instructions on the property and number of the sheets to be used, printing mode, etc. are input through keys or as signals from a host apparatus connected to the image forming apparatus, one of the sheets accommodated in a sheet cassette  104  is supplied to the registration roller pair  124  by way of the sheet feeding roller  126 . The rotation of the registration roller pair  124  is temporarily stopped, and the conveyance of the sheet is on standby with the leading end of the sheet abutting the nip portion defined by the pair of rollers.  
           [0014]    Subsequently, the driving roller  112  rotates counterclockwise, and the driven roller  113  also rotates counterclockwise, whereby the entire belt  111  circulates counterclockwise, with its upper portion in contact with the four photosensitive drums  114 .  
           [0015]    At the same time, the developing containers  118  and the photosensitive drums  114  are successively driven in synchronism with printing. The photosensitive drums  114  rotate clockwise, and the charging rollers  116  impart a uniform high negative charge to the peripheral surfaces of the photosensitive drums  114 ; the scanner  117  performs exposure on the peripheral surfaces of the photosensitive drums  114  in accordance with an image signal to form low potential portions, whereby electrostatic latent images are formed which consist of high negative potential portions due to initialization and low negative potential portions due to exposure. The developing rollers  143  of the developing containers  118  transfers toner to the low potential portions of the electrostatic latent images to thereby form toner images on the peripheral surfaces of the photosensitive drums  114  (reversal development).  
           [0016]    As the forward end of the toner image on the peripheral surface of the upstream-end photosensitive drum  114   a  approaches an opposite portion of the belt  111 , the registration roller pair  124  starts rotation such that the printing start position of the sheet is matched with the opposite portion, and the sheet is fed to the sheet inlet portion.  
           [0017]    The driven roller  113  and the attraction roller convey the sheet while holding it with the belt  111 . The sheet is attracted by the belt  111  and conveyed to the first transfer portion formed by the photosensitive drum  114   a  and a transfer roller  1194   a.    
           [0018]    The transfer rollers  1194   a ,  119   b ,  119   c , and  119   d  apply transfer current output from a transfer bias power source (not shown) to the sheet through the belt  111 . By the transfer current applied from the transfer rollers  119 , the toner images on the photosensitive drums  114  are transferred to the sheet.  
           [0019]    The sheet to which the toner images in four colors have been transferred is separated from the belt  111  and is conveyed to a fixing apparatus  128 . The fixing apparatus  128  fixes the toner images to the sheet by heat and pressure. After the image fixation, the sheet is discharged onto a discharge tray  106  by a discharge roller pair  132  with the toner image facing downwards.  
           [0020]    However, as shown in FIG. 14, in the process cartridge  134 , the toner supply roller  145  is provided in the lowermost portion of the developing apparatus, so that the toner is carried and supplied downward from above, and the stress on the developing portion is rather large, which makes the toner subject to deterioration.  
           [0021]    In this developing unit  136 , there is provided a developing roller  143  arranged opposite to the photosensitive drum  114 , the developing roller  143  carrying and conveying toner on its surface.  
           [0022]    The developing roller  143  is opposed to the developing blade  146 . The toner supplied onto and carried by the developing roller  143  hits the developing blade  146 , whereby the thickness of the toner layer is regulated.  
           [0023]    Thus, the toner in the vicinity of the developing blade  146  is under the pressure due to the formation of the toner layer. In addition, a large pressure due to compression of the toner in the gravitational direction is applied thereto. As a result, the toner in the vicinity of the developing blade  146  is not circulated, and suffers stress due to the friction with the blade  146  and the toner supply roller  145 , which makes the toner subject to deterioration.  
           [0024]    In view of this, it might be possible to arrange the developing roller above the toner storage chamber. In such a construction, the toner in the vicinity of the developing blade would not undergo compression due to the gravitational action, and the toner would not easily deteriorate.  
           [0025]    However, since it is a construction in which toner is supplied upwards from below, it is rather difficult to secure stability in the toner supply to the developing roller.  
         SUMMARY OF THE INVENTION  
         [0026]    The present invention has been made in view of the problem described above in the related art. It is an object of the present invention to provide a developing apparatus in which toner does not easily deteriorate and an image forming apparatus equipped with such a developing apparatus.  
           [0027]    Another object of the present invention is to provide a developing apparatus in which toner does not easily deteriorate and in which toner is supplied to the developing roller in a stable manner and an image forming apparatus equipped with such a developing apparatus.  
           [0028]    Still another object of the present invention is to provide a developing apparatus including:  
           [0029]    a housing;  
           [0030]    a developer bearing member rotatably provided at an opening of the housing;  
           [0031]    a developer containing room;  
           [0032]    a carrying member for carrying the developer in the developer containing room toward the developer bearing member; and  
           [0033]    a developer accumulating portion arranged near the developer bearing member and adapted to accumulate the developer carried by the carrying member,  
           [0034]    wherein the developer accumulating portion is provided above a rotational center of the carrying member.  
           [0035]    Still another object of the present invention is to provide an image forming apparatus including:  
           [0036]    an image bearing member;  
           [0037]    developing means for developing an electrostatic latent image formed on the image bearing member; and  
           [0038]    transfer means for transferring a developer image on the image bearing member to a recording material, the transfer means being arranged above the image bearing member,  
           [0039]    wherein the developing means includes:  
           [0040]    a housing;  
           [0041]    a developer bearing member rotatably provided at an opening of the housing;  
           [0042]    a developer containing room;  
           [0043]    a carrying member for carrying the developer in the developer containing room toward the developer bearing member; and  
           [0044]    a developer accumulating portion arranged near the developer bearing member and adapted to accumulate the developer carried by the carrying member,  
           [0045]    wherein the developer accumulating portion is provided above a rotational center of the carrying member.  
           [0046]    Further objects of the present invention will become apparent from the following detailed description with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]    [0047]FIG. 1 is a schematic sectional view of an image forming apparatus to which the present invention is applied;  
         [0048]    [0048]FIG. 2 is a schematic sectional view of a developing apparatus according to a first embodiment;  
         [0049]    [0049]FIG. 3 is a graph showing variation in image density when printing is performed on a sheet;  
         [0050]    [0050]FIG. 4 is a schematic sectional view of a developing apparatus according to second and third embodiments;  
         [0051]    [0051]FIG. 5 is a graph showing the relationship between toner remaining amount and image density;  
         [0052]    [0052]FIG. 6 is a graph showing the relationship between the number of sheets undergoing continuous printing and fog;  
         [0053]    [0053]FIG. 7 is a schematic sectional view of a developing apparatus according to a fourth embodiment;  
         [0054]    [0054]FIG. 8 is a graph showing the relationship between environment and fog;  
         [0055]    [0055]FIG. 9 is a schematic sectional view of a developing apparatus according to a fifth embodiment;  
         [0056]    [0056]FIG. 10 is a table showing the relationship between the number of sheets undergoing continuous printing and occurrence of streaks;  
         [0057]    [0057]FIG. 11 is a schematic sectional view of a developing apparatus according to a sixth embodiment;  
         [0058]    [0058]FIG. 12 is a graph showing the relationship between toner cohesion degree and image density;  
         [0059]    [0059]FIG. 13 is a schematic sectional view of a printer serving as a help to understand the present invention; and  
         [0060]    [0060]FIG. 14 is a schematic sectional view of a developing apparatus serving as a help to understand the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0061]    A first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic explanatory diagram showing an image forming apparatus, FIG. 2 is an explanatory diagram illustrating a developing unit according to the first embodiment, and FIG. 3 is a graph showing variation in image density when printing is performed on a sheet.  
       Image Forming Apparatus  
       [0062]    First, an example of an image forming apparatus consisting of a laser beam printer utilizing the electrophotographic process and containing a developing apparatus will be described with reference to FIG. 1.  
         [0063]    As shown in FIG. 1, a sheet conveying belt  11  is arranged substantially at the center of the interior of the image forming apparatus. The belt  11 , whose end portions are held by a driving roller  12  and a driven roller  13 , circulates clockwise as indicated by an arrow A. Under the belt  11 , four photosensitive drums  14  ( 144   a ,  14   b ,  14   c , and  14   d ) are arranged side by side in the sheet conveying direction (from the right to the left).  
         [0064]    On the upstream side of the belt  11  with respect to the conveying direction, there is arranged a registration roller pair  24 , and, below it, there is arranged a sheet guiding path  25 , at the lower end portion of which a sheet feeding roller  26  is arranged. Below the sheet feeding roller  26 , a sheet cassette  4  is arranged.  
       Description of Image Forming Process Cartridge  
       [0065]    In the image forming apparatus of FIG. 1, four process cartridges  34  for image formation are arranged side by side, and each process cartridge is detachable with respect to the image forming apparatus main body. FIG. 2 is an enlarged side sectional view of one of the image formation process cartridges  34 . The process cartridge  34  shown in the drawing is formed as an integral unit consisting of two sub units: a drum unit  35  and a developing unit  36  serving as a process means acting on the photosensitive drum  14  of the drum unit  35 .  
         [0066]    The drum unit  35  is equipped with the photosensitive drum  14 , a cleaner  15  arranged to the left thereof, and a charging roller  16  arranged below and to the left thereof.  
         [0067]    The photosensitive drum  14  consists of a conductive metal roller whose surface is uniformly coated with a photoconductive material; the metal roller portion is grounded and rotates counterclockwise as indicated by an arrow C. As shown in FIG. 1, a scanner  17  for performing exposure on the photosensitive drum  14  is arranged below the process cartridge  34 , and light in accordance with image information emitted from the scanner  17  impinges upon the photosensitive drum  14  at the position indicated by reference numeral  37 .  
         [0068]    The charging roller  16  is connected to a high-voltage power source and applies a blank-state charged potential (initialization potential applied uniformly; in a system it is applied in positive polarity, in another in negative polarity) to the photosensitive drum  14 , whereby the photosensitive layer on the peripheral surface of the photosensitive drum  14  is uniformly charged, for example, in negative high potential, and initialized.  
         [0069]    The scanner  17  is equipped with a laser light source or an LED light source, and performs selective exposure in accordance with image information on the peripheral surface of the photosensitive drum  14  charged in negative high potential. As a result of this exposure, there is formed on the surface of the photosensitive drum  14  a negative low potential portion with attenuated potential, and there is formed an electrostatic latent image consisting of the low potential portion and the high potential portion obtained through the initialization.  
         [0070]    In the developing unit  36 , a non-magnetic toner is contained so as to substantially fill a developing container  18 , and a toner agitating member  53  serving as a toner carrying means is arranged so as to be buried under the toner; below and to the right of the developing roller  43  serving as a toner carrying member, a developing blade  46  (layer thickness regulating member) in the form of a plate spring is held in press contact with the developing roller  43 .  
       Description of the Operation of the Image Forming Apparatus  
       [0071]    The operation of the above-described image forming apparatus will be described with reference to FIGS. 1 and 2. First, when the power source is turned on and instructions on the property and number of the sheets to be used, printing mode, etc. are input through keys or as signals from a host apparatus connected to the image forming apparatus, one of the sheets accommodated in the sheet cassette  4  is supplied to the registration roller pair  24  by way of the sheet feeding roller  26 . The rotation of the registration roller pair  24  is temporarily stopped, and the conveyance of the sheet is on standby with the leading end of the sheet abutting the nip portion defined by the pair of rollers.  
         [0072]    Subsequently, the driving roller  12  rotates clockwise, and the driven roller  13  also rotates clockwise, whereby the entire belt  11  circulates clockwise, with its lower portion in contact with the four photosensitive drums  14 .  
         [0073]    At the same time, the developing containers  18  and the photosensitive drums  14  are successively driven in synchronism with printing. The photosensitive drums  14  rotate counterclockwise, and the charging rollers  16  impart a uniform high negative charge to the peripheral surfaces of the photosensitive drums  14 ; the scanner  17  performs exposure on the peripheral surfaces of the photosensitive drums  14  in accordance with an image signal to form low potential portions, whereby latent images are formed which consist of high negative potential portions due to initialization and low negative potential portions due to exposure. The developing rollers  43  of the developing containers  18  transfer toner to the low potential portions of the electrostatic latent images to thereby form toner images on the peripheral surfaces of the photosensitive drums  14  (reversal developing).  
         [0074]    As the forward end of the toner image on the peripheral surface of the upstream-end photosensitive drum  14   a  approaches an opposite portion of the belt  11 , the registration roller pair  24  starts rotation such that the printing start position of the sheet is matched with the opposite portion, and the sheet is fed to the sheet inlet portion. The sheet fed is attracted by the belt  11 , and conveyed to the first transfer portion formed by the photosensitive drum  14   a  and the transfer roller  194   a.    
         [0075]    The transfer rollers  194   a ,  19   b ,  19   c , and  19   d  apply transfer current output from a transfer bias power source (not shown) to the sheet through the belt  11 . By the transfer current applied from the transfer rollers  19 , the toner images on the photosensitive drums  14  are transferred to the sheet.  
         [0076]    The sheet to which the toner images in four colors have been transferred is separated from the belt  11  and is conveyed to the fixing apparatus  28 . The fixing apparatus  28  fixes the toner images to the sheet by heat and pressure. After the image fixation, the sheet is discharged onto a discharge tray  6  by a discharge roller pair  32  with the toner images facing upward (face-up). It goes without saying that it is possible to provide a discharge path (not shown), enabling the sheet to be discharged with the toner images facing downwards (face-down)  
       Description of the Developing Unit  
       [0077]    Next, the developing unit  36  according to the first embodiment will be described with reference to FIGS. 2 and 3. While in this embodiment the developing unit  36  is formed as a sub unit of the process cartridge  34 , this should not be construed restrictively. The developing unit  36  may be formed as an independent developing cartridge. The constructions of the developing unit  36  and the drum unit  35  of this embodiment are suitable for a printer of the type in which the sheet passes above the cartridge.  
         [0078]    In FIG. 2, in the lower portion of the developing container  18 , there is provided a toner vessel (chamber)  54 , which contains a non-magnetic mono-component toner  57 . Further, a paddle  53   a  serving as a toner carrying means is provided therein so as to be rotatable about a rotational center  53   c . A thin-plate elastic member  53   b  consisting of Mylar or the like is attached to the forward end of the paddle  534   a . Further, the developing roller  43  is equipped with a toner accumulating plate member  52  serving as a toner accumulating member and situated under the developing blade  46 .  
         [0079]    When supplying the toner  57 , the paddle  53   a  is rotated, whereby the thin-plate elastic member  53   b  is rotated while being bent to scrape up the toner  57  in the toner vessel  54 . In its upper position, the thin-plate elastic member  53   b  is released, and, together with the synergetic effect of its torque, throws the toner  57  onto the toner accumulating portion  58 . The toner accumulating plate member  52  is substantially positioned as shown in FIG. 2, and is situated below the position where the developing roller  43  is held in contact with the developing blade  46  and above the rotational center of the agitating paddle  53 . Due to this positioning, the toner in the vicinity of the developing roller  43  can be supplied to the developing roller  43  after being carried to the toner accumulating portion  58  between the toner accumulating plate member  52  and the developing blade  46  by the agitating paddle  534   a.    
         [0080]    Due to this construction, in which some toner always exists at the toner supply position from which toner is supplied to the developing roller  43 , it is possible to stabilize the toner supply to the developing roller  43  while preventing excessive pressure from being applied to the toner in the vicinity of the developing blade  46 . The amount of toner supplied onto the developing roller  43  is larger than the requisite amount of toner for development, with substantially no charge being imparted to the toner. This surplus toner is regulated by the developing blade  46  and charged, forming a layer on the developing roller  43  in an amount as required. The charging of the toner is also effected through friction by the rotation of the paddle  53 . The toner thus carried onto the developing roller  43  is sent to the development region formed by the developing roller  43  and the photosensitive drum  14 , and development is effected in the development electric field.  
         [0081]    Next, the effect of the first embodiment will be specifically illustrated.  
         [0082]    [0082]FIG. 3 shows how density varies on a single sheet in the first embodiment and in a construction with no toner accumulating plate member  52  (comparative example 1) when an A4 size sheet (paper) is passed longitudinally (in the longer-side direction of the A4 size sheet) to effect solid black printing on the entire surface of the sheet.  
         [0083]    The developing roller  43  was rotated at a speed that is 150% of the speed of the photosensitive drum  14 . In the apparatus used, the photosensitive drum  14  was φ 30 , and the developing roller  43  was φ 16 . The sheet conveying speed of the apparatus was 120 (mm/sec). Thus, when the developing roller makes approximately nine rotations, the printing of an A4 size sheet (length: 297 mm) is completed.  
         [0084]    As shown in the graph, in comparative example 1, from the second cycle onwards of the developing roller, the density in solid black printing is reduced to an extreme degree. This extreme reduction in density is due to the fact that when solid black printing is effected, the toner on the developing roller is lost as a result of the development on the photosensitive drum  14  using the toner on the developing roller  43 , and that there exits no toner on the developing roller  43  until toner is carried to the vicinity of the developing blade  46  through agitation by the paddle  534   a.    
         [0085]    In the first embodiment, in contrast, toner is accumulated on the toner accumulating plate member  52 , so that if the toner on the developing roller  43  is consumed, it is always possible to maintain the state in which there is toner in the vicinity of the blade. Thus, even immediately after the consumption of the toner on the developing roller as a result of solid black printing, it is possible to supply toner onto the developing roller  43 . Thus, it is possible to carry toner onto the developing roller  43  over the range from the tip end to the rear end of the sheet, so that there is no reduction in density within the sheet, making it possible to obtain a satisfactory solid image.  
         [0086]    Thus, even in the case of a developing apparatus of the type in which the carrying and supplying of toner onto the developing roller is effected upwardly from below, it is possible to effect development free from a deterioration in toner for along period of time.  
       Second Embodiment  
       [0087]    Next, a second embodiment of the present invention will be described in detail. The components that are the same as those of the first embodiment are indicated by the same reference numerals, and a description of such components will be omitted. FIG. 4 is an explanatory drawing showing a developing apparatus according to the second embodiment and a third embodiment (described below). FIG. 5 is a chart showing the relationship between toner remaining amount and image density.  
       Description of the Developing Unit  
       [0088]    A developing unit  36   b  according to the second embodiment will be described with reference to FIG. 4. While in this embodiment the developing unit  36   b  is formed as a subunit of the process cartridge  34 , this should not be construed restrictively as in the first embodiment. This also applies to the other embodiments described below.  
         [0089]    The second embodiment is characterized in that there is provided a member (referred to as a toner supply member  55 ) for supplying the toner accumulated on the toner accumulating plate member  52  onto the developing roller  43 . The toner supply member  55  is in the form of a roller made of urethane sponge, which rotates in a non-contact state with respect to the developing roller  43 . The toner carried onto the toner accumulating portion  52  by the paddle  53  is raised by the rotation of the supply roller  55  and adheres to the surface of the developing roller  43 , whereby it is possible to efficiently supply the toner accumulated on the toner accumulating plate member  52  onto the developing roller  43 . FIG. 4 is a schematic sectional view showing this construction.  
         [0090]    [0090]FIG. 5 shows the results of an experiment in which the second embodiment is compared with the first embodiment.  
         [0091]    The toner supply member  55  for supplying toner onto the developing roller  43  is formed of an elastic sponge roller of φ 12 . It is not in contact with the developing roller  43 , and the gap between the developing roller  43  and the supply roller  55 , serving as the toner supply member  55 , is 0.5 mm.  
         [0092]    In accordance with the second embodiment, due to the toner supply member  55 , it is possible to carry toner onto the developing roller  43  in a satisfactory manner, so that it is possible to obtain a satisfactory image in more stable manner than in the first embodiment. When it is said that a satisfactory image can be obtained, it means that it is possible to maintain a satisfactory image density from the early stage to the end of the service life of the developing unit  36 .  
         [0093]    As shown in FIG. 5, the initial toner amount is 500 g. As can be seen from the chart, the first embodiment involves a reduction in image density when the remaining toner amount becomes approximately 100 g. In contrast, in the second embodiment, it is possible to achieve a satisfactory image density even when the remaining toner amount has been reduced to a level lower than that of the first embodiment. This is due to the fact that, in the second embodiment, the toner supply member  55  positively supplies the toner accumulated on the accumulating plate member  52  onto the developing roller  43 .  
       Third Embodiment  
       [0094]    Next, a third embodiment of the present invention will be described. The components that are the same as those of the above-described embodiments are indicated by the same reference numerals and a description of such components will be omitted. Since the developing unit of the third embodiment exhibits the same sectional configuration as that of the second embodiment, FIG. 4 will be referred to in the following description. FIG. 6 is a chart showing the relationship between endurance number of sheets and fog in the third and second embodiments.  
       Description of the Developing Unit  
       [0095]    A developing unit  36   c  according to the third embodiment will be described with reference to FIGS. 4 and 6.  
         [0096]    The third embodiment is characterized in that the toner supply member  55  in the second embodiment is in contact with the developing roller  43 . Thus, the toner supply member  55  in the third embodiment not only supplies toner onto the developing roller  43 , but also functions so as to take away the toner on the developing roller  43  remaining thereon without being used in the development on the photosensitive drum  14  after passing the development region opposed to the photosensitive member; by taking away the toner on the developing roller  43 , it refreshes the developing roller  43 .  
         [0097]    As in the case in which the image forming apparatus forms a lot of images at a low printing ratio, if, of the toner on the developing roller  43  having passed the developing blade  46 , the amount of toner adhering to the photosensitive drum  14  is small, it is possible in this embodiment to take away a lot of toner on the developing roller  43  which has not been used for development to thereby refresh the developing roller. Further, it is possible, over a long period of time, to reduce the stress on the toner due to the contact and friction with the developing blade and the photosensitive member.  
         [0098]    [0098]FIG. 6 shows the results of an experiment in which the developing unit  36   c  of the third embodiment and the developing unit  36   b  of the second embodiment were compared with each other. In the experiment, the fog on the photosensitive drum  14  when a text image of a low printing ratio of 2% was continuously printed was measured.  
         [0099]    To measure the fog on the photosensitive drum  14 , the drum rotation was forcibly stopped during the image formation of low printing ratio and taping was effected on the drum  14  at that time. And the difference in reflectance before and after the taping was obtained. For the taping, a mending tape manufactured by Sumitomo Three M, Co. was used. For the measurement of reflectance, a Densitometer manufactured by Tokyo Denshoku, Co. was used.  
         [0100]    In the second embodiment, the fog on the photosensitive drum  14  was increased throughout the endurance test, whereas, in the third embodiment, there was little generation of fog throughout the endurance test. When the fog on the photosensitive drum  14  is not more than 2%, the image on the sheet is not contaminated and no problem is involved. According to the third embodiment, even in a severe condition of use for the developing unit  36   c , that is, even when image formation is repeated at low printing ratio, it is possible to obtain a satisfactory image free from fog.  
         [0101]    This is due to the fact that the toner supply member  55  in contact with the developing roller  43  takes away the remaining toner on the developing roller  43  while supplying toner onto the developing roller  43 . Thus, if image formation is repeated at low printing ratio, it is possible to prevent the toner on the developing roller  43  being subjected to stress as a result of being continuously rubbed by the developing blade  46  and the photosensitive drum  14 .  
         [0102]    In the second embodiment, the reason for the increase in fog in the latter half of the endurance test is that when the toner is subjected to stress for a long period of time, the extraneous additive on the toner surface is separated or buried under the toner, with the result that the charging performance and fluidity of the toner change.  
       Fourth Embodiment  
       [0103]    Next, a fourth embodiment of the present invention will be described in detail. The components, which are the same as those of the above-described embodiments are indicated by the same reference numerals, and a description of such components will be omitted. FIG. 7 is an explanatory drawing showing a developing unit according to the fourth embodiment. FIG. 8 is a graph showing the relationship between environment and fog in the fourth and third embodiments.  
       Description of the Developing Unit  
       [0104]    A developing unit  36   d  according to the fourth embodiment will be described with reference to FIGS. 7 and 8.  
         [0105]    As shown in FIG. 7, the fourth embodiment is characterized in that an opening  52   a  is provided in the toner accumulating plate member  52 . Otherwise, it is the same as the third embodiment. Due to this construction, some of the toner accumulated on the toner accumulating plate member  52  passes through the opening  52   a , and it is possible to prevent excessive toner from being accumulated on the accumulating plate member  52 , so that, when the toner has reached the requisite amount, the surplus toner can be returned to the toner vessel  54  through the opening  52   a , whereby it is possible to control to an appropriate level the amount of toner accumulating in the accumulating portion  58  surrounded by the developing blade  46 , the developing roller  43 , and the accumulating plate member  52 . Further, since it is always possible to refresh the toner existing in the toner accumulating portion  58 , it is possible to perform a satisfactory development.  
         [0106]    Further, when toner is accumulated on the accumulating plate member  52 , the toner pressure in the accumulating portion  58  gradually increases. Due to this toner pressure, the pressure with which the developing blade portion  46  is held in contact with the developing roller can vary. The toner undergoes friction charging as it passes between the developing blade  46  and the developing roller  43  while being rubbed thereby. Thus, when the toner pressure with respect to the developing blade  46  is unstable, the toner charging is also likely to become unstable.  
         [0107]    In particular, in a high-temperature/high-humidity environment (32.5° C. and 80% RH; hereinafter referred to as HH) and in a low-temperature/low-humidity environment (10° C. and 20% RH; hereinafter referred to as LL), the toner charging is more likely to become unstable as compared with a normal-temperature/normal-humidity environment (23° C. and 50% RH). When, in such an environment, the toner pressure with respect to the developing blade  46  is unstable, the toner charging becomes further unstable.  
         [0108]    [0108]FIG. 8 shows the results of comparison of the third embodiment and the fourth embodiment in terms of the fog on the photosensitive drum  14  in NN environment, HH environment, and LL environment. The fog measurement was performed in the same manner as described in relation to the third embodiment.  
         [0109]    As shown in FIG. 8, the developing unit  36   d  of the fourth embodiment involves less fog than the developing unit  36   c  of the third embodiment in any of NN environment, HH environment, and LL environment, which means the developing unit  36   d  of the fourth embodiment is capable of performing image formation in a more stable manner.  
       Fifth Embodiment  
       [0110]    Next, a fifth embodiment of the present invention will be described in detail. The components that are the same as those of the above-described embodiments are indicated by the same reference numerals and a description of such components will be omitted. FIG. 9 is an explanatory drawing showing a developing unit  36   e  of the fifth embodiment, and FIG. 10 is a table showing the relationship between print number of sheets and occurrence of streak when an image of low printing ratio is continuously printed by using the fifth embodiment and the third embodiment.  
       Description of the Developing Unit  
       [0111]    A developing unit  36   e  according to the fifth embodiment will be described with reference to FIGS. 9 and 10.  
         [0112]    As shown in FIG. 9, the fifth embodiment is characterized in that the toner accumulating plate member  52  is flexible. By making the accumulating plate member  52  flexible, it is possible to prevent blocking of the toner accumulated in the accumulating portion  58 .  
         [0113]    The accumulating plate member  52  vibrates when the developing roller  43  and the photosensitive drum  14  rotate and vibrate and when the agitating paddle  53   a  comes into direct contact with the accumulating plate member  52 . As a result, the surplus toner accumulating in the accumulating portion  58  is released. Thus, if the fluidity of the toner is reduced in HH environment, it is possible to prevent blocking of the toner accumulated in the accumulating portion  58 .  
         [0114]    When blocking occurs in the accumulating portion  58 , the toner is allowed to fuse starting from the forward end of the developing blade  46 . When toner is fused to the developing blade  46 , a satisfactory layer formation cannot be effected, and there is the danger of streak occurring in a halftone image.  
         [0115]    In the portion where toner is fused to the developing blade  46 , it is impossible for the toner layer to be thin and uniform. Thus, unevenness in density is generated in a halftone image, and appears as a streak. In the case of a character image, this streak is generated especially when severe fusion occurs.  
         [0116]    [0116]FIG. 10 shows the results of evaluation of streak occurrence in halftone and character images in HH environment in the fifth embodiment and the third embodiment. In the evaluation, continuous printing of low printing ratio was first performed in HH environment in order to create a state in which blocking is likely to occur in the accumulating portion  58 , and then halftone and character images were formed for visual evaluation of streak level. In FIG. 10, symbol O indicates no streak generation; symbol Δ indicates an acceptable level of streak; and symbol × indicates a bad streak level.  
         [0117]    It can be seen from FIG. 10 that, in the third embodiment, when the print number of sheets at low printing ratio in HH environment increases, a streak is likely to be generated in a halftone image, and that, in the fifth embodiment, no streak is generated if continuous printing is conducted at low printing ratio. For example, a mechanism is adopted in which the accumulating plate member  52  consists of a Mylar member and in which when a pressure not less than a fixed level is applied to the accumulating portion  58 , the Mylar is deflected, causing toner to fall.  
         [0118]    Thus, when the amount of toner accumulated in the vicinity of the accumulating member  52  has reached the requisite level, the partition member is deflected to return toner to the toner vessel  54 , whereby it is possible to control the amount of toner accumulated in the accumulating portion  58  surrounded by the developing blade  46 , the developing roller  43 , and the accumulating plate member  52 .  
       Sixth Embodiment  
       [0119]    Next, a sixth embodiment of the present invention will be described in detail. The components that are the same as those of the above-described embodiments are indicated by the same reference numerals, and a description of such components will be omitted. FIG. 11 is an explanatory drawing showing a developing unit  36   f  according to the sixth embodiment. FIG. 12 is a graph showing the relationship between toner cohesion degree and image density in the sixth and third embodiments.  
       Description of the Developing Unit  
       [0120]    A developing unit  36   f  according to the sixth embodiment will be described with reference to FIGS.  11  to  13 .  
         [0121]    As shown in FIG. 11, the sixth embodiment is characterized in that the toner accumulating plate member  52  has an opening  52   b  equipped with an opening/closing valve, whereby, even when a toner of good fluidity is used, it is possible to accumulate toner on the toner accumulating plate member  52 , and to maintain the toner pressure in the accumulating portion  58  at an appropriate level.  
         [0122]    [0122]FIG. 12 shows the results of comparison in image density when toners differing in fluidity are used in the constructions of the sixth embodiment and the third embodiment. Here, cohesion degree is used as the index of toner fluidity. The smaller the value of cohesion degree is, the better the toner fluidity becomes. The cohesion degree measurement was performed as follows.  
         [0123]    First, 2 g of toner passed through a sieve of 200 meshes was obtained. Then, sieves of 60 mesh, 100 mesh, and 200 mesh were stacked together in that order from above in a power tester (manufactured by Hosokawa Micron, Co.), and the 2 g of specimen obtained was gently put on the set of sieves and a vibration of an amplitude of 1 mm was imparted thereto for 65 seconds. And the weight of the magnetic iron oxide remaining on each sieve was measured to calculate the cohesion degree by the following equation:  
         [0124]    Cohesion degree=(the weight of the specimen remaining on the 60 mesh sieve)/(the weight of the specimen put on the sieve set)×100+ (the weight of the specimen remaining on the 100 mesh sieve×⅗)/(the weight of the specimen put on the sieve set)×100+ (the weight of the specimen remaining on the 200 mesh sieve×⅕)/(the weight of the specimen put on the sieve set)×100.  
         [0125]    It can be seen from FIG. 12 that, in the third embodiment, a reduction in image density is to be observed with toners of small cohesion degree value, i.e., of good fluidity. This is due to the fact that a toner of good fluidity is caused to fall by the gravitational force onto the toner vessel  54  of the developing container  18  through the opening, so that toner is not easily accumulated on the toner accumulating plate member  52 .  
         [0126]    In the sixth embodiment, in contrast, it is possible to obtain a satisfactory image density even with a toner of a small cohesion degree value, i.e., of good fluidity. This is due to the fact that the opening of the accumulating plate remains closed until the requisite amount of toner is accumulated and that when excessive toner is going to be accumulated, the Mylar (valve) is automatically deflected downwards due to the toner pressure in the accumulating portion  58 , and toner is allowed to fall. Thus, even with a toner of good fluidity, it is always possible to achieve a satisfactory image density.  
       Other Embodiments  
       [0127]    While in the above-described embodiments the image forming apparatus is a four-color printer, this should not be construed restrictively. The image forming apparatus may also be a facsimile apparatus or a copying machine. Further, the image forming apparatus is not restricted to a colored one. It may also be a monochrome one or of a plurality of colors other than four colors.  
         [0128]    The present invention is not restricted to the above-described embodiments, and modifications are possible without departing from the technical scope of the invention.