Abstract:
A developing apparatus, comprises an endless belt shaped donor member; a developer supporting member to carry developer including toner and magnetic carrier and to form a toner layer on the donor member; a first supporting member arranged in the inside of the donor member with a predetermined gap against the developer supporting member and to strain the donor member; a second supporting member arranged in the inside of the donor member with a predetermined gap against the developer supporting member and to strain the donor member; a toner supply bias applying section to apply onto the first supporting member a toner supply bias for supplying toner from the developer supporting member to the donor member; and a toner recovery bias applying section to apply onto the second supporting member a toner recovery bias for recovering toner from the donor member to the developer supporting member.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application No. 2007-106886 filed on Apr. 16, 2007, in Japanese Patent Office, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a development technology in an electrophotography process, especially the development technology of a hybrid developing method. 
         [0003]    The hybrid developing method is a developing method which supplies toner to a donor member from developer which includes toner and carrier, forms a toner layer on the donor member, conveys toner to a developing area by the donor member, and develops a latent image with the toner on the donor member. As a result that the development is performed with toner having a uniform electrically charged amount, the hybrid developing method has the following excellent features that fogging caused by weakly electrically-charged toner or reversely electrically-charged toner can be refrained, there is no carrier adhesion, and so on. 
         [0004]    The hybrid developing method is disclosed in Patent documents 1 and 2 as explained below. 
         [0005]    Patent document 1 discloses a developing device in which a toner supplying function and a toner recovering function are separated by one magnetic brush made opposite and two electrodes provided inside of a donor belt. Patent document 2 discloses a developing device which comprises a drum-shaped donor member, a supply magnet roller to supply toner to the donor member, and a recovery magnet roller to recover toner from the donor member, and conveys developer from the supply magnet roller to the recovery magnet roller. 
         [0006]    Patent documents 1: Japanese Patent Unexamined Publication No. 6-102755 
         [0007]    Patent documents 2: Japanese Patent Unexamined Publication No. 2005-221938 
         [0008]    There are the following problems in the above technology. 
         [0009]    In Patent document 1, electrodes provided in a donor belt do not support the donor belt itself. Specifically, these electrodes are electrically conductive brush, and the gap between magnetic brush and the donor member at both of a toner feed section and a toner recovering section fluctuate easily. For this reason, the supply electric field and the recovery electric field formed between the donor member and the magnetic brush fluctuate, and the amount of toner on the donor member becomes uneven. As a result, uneven development may take place easily. Moreover, since toner may not be completely recovered from the donor member, a memory effect occurs easily. 
         [0010]    In Patent documents 2, the flow of developer is formed from a supply magnet roller to a recovery magnet roller. In the case where the flow of developer is formed in such the way, a path to return developer from a recovery magnet roller to a supply magnet roller is needed as a circulation path of the developer. Therefore, there are problems that the structure of the developing device becomes complicated, and a developing device becomes large size. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the present invention is to solve the above problems in prior art regarding the hybrid developing and to realize a developing apparatus which can form a high quality image with a uniform image density stably and is downsized. 
         [0012]    The above object can be attained by the following structure. 
         [0013]    A developing apparatus for forming a toner image on an image carrying member via a donor member, comprises: 
         [0014]    the donor member structured in an endless belt shape; 
         [0015]    a developer supporting member to carry developer including toner and magnetic carrier and to form a toner layer on the donor member; 
         [0016]    a first supporting member arranged in the inside of the donor member with a predetermined gap against the developer supporting member and to strain the donor member; 
         [0017]    a second supporting member arranged in the inside of the donor member with a predetermined gap against the developer supporting member and to strain the donor member; 
         [0018]    a toner supply bias applying section to apply onto the first supporting member a toner supply bias for supplying toner from the developer supporting member to the donor member; and a toner recovery bias applying section to apply onto the second supporting member a toner recovery bias for recovering toner from the donor member to the developer supporting member. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1(   a ) and  FIG. 1(   b ) are drawing showing a developing device according to Embodiment 1 of the present invention. 
           [0020]      FIG. 2  is a drawing showing a developing device according to Embodiment 2 of the present invention. 
           [0021]      FIG. 3  is a front view of a developing device according to Embodiment 3 of the present invention. 
           [0022]      FIG. 4  is a side elevation view of a developing device according to Embodiment 3 of the present invention. 
           [0023]      FIG. 5  is a drawing showing a developing device according to Embodiment 4 of the present invention. 
           [0024]      FIG. 6  is a drawing showing a developing device according to Embodiment 5 of the present invention. 
           [0025]      FIG. 7  is a drawing showing a developing device according to Embodiment 6 of the present invention. 
           [0026]      FIG. 8  is a drawing showing a developing device according to Embodiment of the present invention. 
           [0027]      FIG. 9  is a drawing showing a developing device used in Comparative example. 
           [0028]      FIG. 10(   a ) and  FIG. 10(   b ) are illustration showing images used for evaluation. 
           [0029]      FIG. 11  is a drawing showing change in electrically charged amount. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    While some preferred embodiments of the present invention are described below, the present invention shall not be limited to these embodiments. 
       Embodiment 1 
       [0031]      FIG. 1  is a drawing showing a developing device according to Embodiment 1. 
         [0032]    Reference number  1  is a photoreceptor as an image carrying member which carries an electrostatic latent image formed by processes of electrically-charging and imagewise exposing. As the image carrying member, a well-known one such as an OPC photoreceptor and so on can be arbitrarily employed. 
         [0033]    Reference number  2  is a donor member which is structured with a belt, and is semi-conductive. 
         [0034]    Reference numbers  3  to  5  are rollers which support the donor member  2 . 
         [0035]    Reference number  6  is a magnet roll as a developer carrying section. 
         [0036]    Reference number  3  is a roller as a third supporting member that makes the donor member  2  to oppose to the photoreceptor  1  at a development position P 1 , and a development gap (minimum distance) D 1  between the surface of the photoreceptor  1  and the surface of the donor member  2  is set up by the roller  3 . 
         [0037]    Contact type development or non-contact type development is set up by the size of the development gap D 1 . 
         [0038]    The contact type development is a development performed on the condition where a toner layer on the donor member  2  comes in contact with the surface of the photoreceptor  1 , and the non-contact type development is a development performed on the condition where a toner layer on the donor member  2  does not come in contact with the surface of the photoreceptor  1 . 
         [0039]    A developing bias is applied to the roller  3  by a power source E 1 . The power source E 1  applies a bias voltage to form an electric field to move electrically-charged toner to the photoreceptor  1  from the donor member  2 , and applies as the bias voltage a direct current voltage or a voltage in which an alternating current voltage is superimposed on a direct current voltage. 
         [0040]    A roller  4  as a first supporting section supports the donor member  2  at a toner supply position P 2 , and forms a toner supply gap D 2  between the surface of the donor member  2  and a magnet roll  6 . A bias voltage as a toner supply bias is applied to the roller  4  by a power source E 2  (toner supply bias applying section). 
         [0041]    A roller  5  as a second supporting member supports the donor member  2  at a toner recovery position P 3 , and forms a toner recovery gap D 3  between the surface of the donor member  2  and the magnet roll  6 . A toner recovery bias voltage is applied to the roller  5  by a power source E 3  (toner recovery bias applying section). 
         [0042]    The development gap D 1  is the minimum distance (the shortest distance) between the surface of the photoreceptor  1  and the surface of the donor member  2 , and this distance corresponds to the shortest distance between the surface of the photoreceptor  1  and the surface of the roller  3 . Further, the toner supply gap D 2  and the toner recovery gap D 3  are the minimum distances between the surface of a magnet roller  6 A of the magnet roll  6  and the surface of the opposed donor member  2 . Namely, since the belt-shaped donor member  2  is supported by the surface of the roller  4  and the surface of the roller  5  at these positions, the toner supply gap D 2  and the toner recovery gap D 3  correspond to the minimum distance between the surface of the magnet roll  6  and respective surfaces of the roller  4  and roller  5 . 
         [0043]    In this embodiment, a reversal development which develops a latent image formed with negative electric charge by the use of negative electrically-charged toner is performed. 
         [0044]    The power source E 1  applies a bias voltage which is a negative direct current voltage or a negative direct current voltage superimposed with an alternating current voltage so as to form an electric field between the grounded photoreceptor  1  and the donor member  2  in order to shift negative electrically-charged toner to the photoreceptor  1 . 
         [0045]    The power source E 2  applies to the magnet roll  6  a relatively positive direct current voltage or a voltage in which an alternating current voltage is superimposed on a positive direct current voltage, and forms an electric field between the magnet roll  6  and the donor member  2  in order to shift negative electrically-charged toner to the donor member  2 . 
         [0046]    The power source E 3  applies a relatively negative voltage to the magnet roll  6 , and shifts negative electrically-charged toner from the donor member  2  to the magnet roll  6 . 
         [0047]    The power source E 4  is a power source which applies a voltage to the magnet roll  6 . 
         [0048]    The magnet roll  6  is a roll on the circumferential surface of which a plurality of magnetic poles are formed, and as shown in  FIG. 1(   b ), magnetic brush  6 A of developer including toner and carrier is formed on the circumferential surface by the magnetic poles. 
         [0049]    The photoreceptor  1 , the rollers  2  to  5 , and the magnet roll  6  rotate in directions like arrows respectively. 
         [0050]    At the toner supply position P 2 , toner in carrier moves to the donor member  2 , and a uniform toner layer is formed on the donor member  2 . 
         [0051]    At the development position P 1 , an electrostatic latent image on the photoreceptor  1  is developed, and a toner image is formed on the photoreceptor  1 . 
         [0052]    At the toner recovery position P 3 , toner on the donor member  5  is recovered to developer on a magnet roll  6 . 
         [0053]    At the toner supply position P 2 , the toner concentration of developer moving from the toner supply position P 2  to the toner recovery position P 3  decreases very much as a result of toner having shifted from the developer to the donor member  2 . 
         [0054]    At the toner recovery position P 3 , toner recovery is performed with high efficiency by an effect of the bias voltage by the power source E 3  and by an effect of the toner concentration decrease at the toner supply position P 1 . 
         [0055]    Therefore, toner remaining on the donor member  2  after development is fully recovered at the toner recovery position P 3 . 
         [0056]    With this, it becomes possible to prevent very well a memory phenomenon in which a history of a previous development process influences a next toner image. 
         [0057]    In this way, at the toner supply position P 2  and the toner recovery position P 3 , the donor member is supported by a surface of the roller  4  and a surface of the roller  5  respectively. Therefore, since a toner supply gap D 1  and a toner recovery gap D 2  are maintained at predetermined values respectively, constant electric fields are maintained at the toner supply position P 2  and the toner recovery position P 3 , stable toner supply and toner recovery are performed, and uniform development can be realized. 
       Embodiment 2 
       [0058]      FIG. 2  is a drawing showing a developing device according to Embodiment 2. 
         [0059]    This embodiment has a structure that a spacing roller  7  as a spacing device is provide between rollers  4  and  5  so that the donor member  2  is separated from the magnet roll  6 . 
         [0060]    Developer is separated by the spacing roller  7  from the donor member  2  between the toner supply position P 2  and the toner recovery position P 3  so that stress added to the developer is reduced. 
         [0061]    As a result, fatigue of developer, especially fatigue of carrier can be prevented. 
         [0062]    The spacing roller  7  has a length in an axial direction almost equal to the rollers  4  and  5  and the magnet roll  6 , and supports the donor member  7  overall its width. 
       Embodiment 3 
       [0063]      FIGS. 3 and 4  show a developing device according to Embodiment 3,  FIG. 3  is a front sectional view and  FIG. 4  is a side elevation view. 
         [0064]    This embodiment has the structure where two spacing rings  8  are provided between rollers  4  and  5  so as to separate the donor member  2  from a magnet roll  6 . The two spacing rings  8  support the donor member  2  at the ends of a shaft so as to separate the donor member  2  from a magnet roll  6 . 
         [0065]    Developer is separated from the donor member  2  between the toner supply position P 2  and the toner recovery position P 3  by the spacing ring  8 , whereby stress added to the developer is reduced. 
         [0066]    The spacing ring  8  has a large number of ribs similar to a gear, and supports the donor member  2  with the tip end of each rib. 
         [0067]    Stress applied to un-recovery remaining toner on the donor member  2  is reduced by the spacing ring  8  having the large number of ribs, whereby deterioration of toner can be prevented. 
       Embodiment 4 
       [0068]      FIG. 5  is a drawing showing a developing device according to Embodiment 4. 
         [0069]    This embodiment has the structure where a predetermined development gap D 1  (refer to  FIG. 1 ) is formed between the photoreceptor  1  and the donor member  2  at the development position P 1  by the roller  5 , and a predetermined toner recovery gap D 3  (refer to  FIG. 1 ) is formed between the donor member  2  and the magnet roll  6  at the toner recovery position P 3  by the same roller  5 . 
         [0070]    Since the development position P 1  and the toner recovery position P 3  are arranged at a near position to each other, this embodiment has the advantage that the developing device can be miniaturized. 
         [0071]    By the setting of the applied voltage of the power source E 1  such that the electric potential of the donor member  2  is higher than that of an exposure section of the photoreceptor  1  and also higher than that of magnet roll  6  (higher in absolute value), the development of an electrostatic latent image and the recovery of toner are performed very well. 
       Embodiment 5 
       [0072]      FIG. 6  is a drawing showing a developing device according to Embodiment 5. 
         [0073]    This embodiment is a modification of Embodiment 4 corresponding to Embodiment 2, and has a spacing roller  7  which separates the donor member  2  from the magnet roll  6  between the toner supply position P 2  and the toner recovery position P 3 . 
       Embodiment 6 
       [0074]      FIG. 7  is a drawing showing a developing device according to Embodiment 6. 
         [0075]    This embodiment is a modification of Embodiment 4 corresponding to Embodiment 3, and has a spacing ring  8  which separates the donor member  2  from the magnet roll  6  between the toner supply position P 2  and the toner recovery position P 3 . 
         [0076]    In the embodiments of the present invention explained above, at the toner supply position, since the first supporting member supports the donor member so as to maintain the toner supply gap uniformly, a toner layer is formed with a uniform thickness on the donor member, whereby uniform development is performed. 
         [0077]    Moreover, when developer passes through the toner supply position, the toner concentration of the developer decrease. However, the toner is conveyed to a toner recovery position, and at the toner recovery position, since the second supporting member supports the donor member so as to maintain the toner recovery gap uniformly, the toner remaining after development is fully removed from the donor member, whereby a memory effect can be prevented. 
         [0078]      FIG. 8  shows a process cartridge which has a developing device according to the embodiments of the present invention and an image forming apparatus which has the process cartridge. 
         [0079]    In  FIG. 8 , reference number  50  is a photoreceptor drum being an image carrying member, and the photoreceptor drum is a photoreceptor in which an organic photosensitive layer is coated on a drum, is grounded and rotated in the clockwise direction. Reference number  52  is a Scorotoron electrically-charging device which electrically-charges the photoreceptor drum uniformly by the corona discharging. In advance to the electrically-charging with this electrically-charging device  52 , in order to eliminate a history of the photoreceptor in the previous image formation, exposure may be performed for the photoreceptor by an exposing section  51  which employs light emitting diode and the like. As a result, electric charge on the circumference surface of the photoreceptor is eliminated. 
         [0080]    After the photoreceptor  50  is charged uniformly, imagewise exposure is performed for the photoreceptor  50  based on image signals by an image exposure device  53 . The image exposure device  53  in this figure uses as an exposure light source a laser diode which is not illustrated. The photoreceptor  50  is scanned with a laser beam which enters into the photoreceptor drum  50  through a rotating polygon mirror, a fθ lens, and so on, whereby an electrostatic latent image is formed on the photoreceptor  50 . 
         [0081]    Subsequently, the electrostatic latent image is developed with a developing device  54 . Then, a toner image is formed in a peripheral surface of the photoreceptor drum  50 . 
         [0082]    The developing device  54  conducts developing by hybrid development explained above. 
         [0083]    After image formation, a recording sheet P is fed out to a transfer region by the rotational operation of the feed roller  57 , when the transfer timing is ready. 
         [0084]    In the transfer region, a transfer roller (transfer device)  58  is brought in pressure contact with the circumferential surface of the photoreceptor drum  50  in synchronization with the transfer timing, the fed-out recording sheet P is pressed between the photoreceptor drum  50  and the transfer roller  57 , whereby a toner image is transferred onto the recording sheet P. 
         [0085]    Subsequently, a separating brush (eliminator)  59  which is brought into pressure contact with the photoreceptor drum almost simultaneously with the transfer roller  57  eliminates charge on the recording sheet P, and separates the recording sheet P from the circumferential surface of the photoreceptor drum  50 , and then the recording sheet P is conveyed to a fixing device  60 . 
         [0086]    In the fixing device  60 , a toner is melted with heat and pressure, whereby a toner image is fixed on the recording sheet P. 
         [0087]    The recording sheet P on which the toner image is fixed is discharged by a delivery roller  61 . 
         [0088]    On the other hand, after the recording sheet P is separated from the photoreceptor drum  50 , the photoreceptor drum  50  passes through a cleaning device  62 , whereby remaining toner is removed from the photoreceptor  50 . 
         [0089]    Reference number  70  is a process cartridge in which the photoreceptor  50 , the electrically-charging device  52 , the developing device  54 , the transfer device  48 , the eliminator  59 , and the cleaning device are united into one body which can be detached and attached. 
         [0090]    When the process cartridge  70  is pulled out in the direction toward the front side of the sheet of the drawing, the process cartridge  70  is taken out from the image forming apparatus. 
         [0091]    Reference number  80  is a reading section to read a document. constituted by the different body with the photoreceptor Generally, the process cartridge includes an integral-type cartridge and a discrete-type cartridge. In the integral-type cartridge, at least one of an electrically-charging device, an image exposure device, a developing device, a transfer device, or an eliminator, and a cleaning device is constituted in one body with a photoreceptor so as to be detachably attached the apparatus body. In the discrete-type cartridge, at least one of an electrically-charging device, an image exposing deice, a developing device, a transfer device or an eliminator and a cleaning devices which are structured as a separate body from a photoreceptor is formed in one body. These process cartridges can be attached detachably to an apparatus main body, and when these process cartridges are attached in the apparatus main body, they are united with a photoreceptor. The process cartridge in the present invention includes the both type cartridges stated above. 
         [0092]    With the developing device and the image forming apparatus according to the embodiments of the present invention explained above, uniform development can be performed, whereby high quality image can be formed stably. In addition, the miniaturization of these device and apparatus becomes possible. 
       EXAMPLE 
       [0093]    Hereafter, an example and a comparative example are explained. 
       (1) Example 
     &lt;Structural Functional Conditions&gt; 
       [0094]    With regard to Examples 1 to 3 using the developing device of Embodiment 1 to 3 shown in  FIGS. 1-4 , the developing device was operated on the following structural functional conditions so as to conduct developing. 
         [0095]    &lt;Developer&gt;
       Average toner particle size: 6.5 micrometers   Average carrier particle size: 33 micrometers   Toner concentration: 7.5 mass W       
 
         [0099]    &lt;Structure and Function of a Developing Device&gt;
       Toner conveyances amount of a donor member: 5 g/m 2      Toner conveyances amount of a magnet roll: 300 g/m 2      Photoreceptor: a drum-shaped OPC photoreceptor with a radius of 60 mm   Gap between a photoreceptor and a donor member: 0.15 mm   Gap between a donor member and a magnet-roll at a toner supply position: 0.31 mm   Gap between a donor member and a magnet roll at a toner recovery position: 0.31 mm   Photoreceptor rotational speed (linear velocity): 400 mm/s   Donor member shifting speed: 600 mm/s   Magnet roll rotational speed (linear velocity): 900 mm/s       
 
         [0109]    (The photoreceptor and the donor member moved in the same direction at the development position, and the donor member and the magnet roll moved in counter directions at the toner supply position and the toner recovery position.) 
         [0110]    Examples 1-3 were conducted on the following electric potential conditions. 
         [0111]    &lt;Electric Potential Conditions&gt; 
         [0112]    Photoreceptor:
       Electrically-charged potential V 0 : −450 V   Exposure electric potential Vi: −50 V   Bias voltage Vdc: −50 V at the toner supply position   (Power supply E 2 )
           Vpp: 1.2 kV   Frequency: 2 kHz   Bias voltage Vdc: −320V at the development position   
           (Power supply E 1 )
           Vpp: 1.2 kV   Frequency: 2 kHz   Bias voltage Vdc: −620V at the photoreceptor toner recovery position   
           (Power supply E 3 )
           Vpp: 1.2 kV   Frequency: 2 kHz   Magnet-roll Potential Vdc: −500 V   
           (Power supply E 4 )       
 
         [0129]    Example 4 using Embodiment 4 shown in  FIG. 5  was conducted on the following electric potential conditions. 
         [0130]    The structure and function of a developing device were as same as in Examples 1 to 3. 
         [0131]    &lt;Electric Potential Conditions&gt; 
         [0132]    Photoreceptor:
       Electrically-charged potential V 0 : −650 V   Exposure electric potential Vi: −50 V   Bias voltage Vdc: −50 V at the toner supply position   (Power supply E 2 )
           Vpp: 1.2 kV   Frequency: 2 kHz   Bias voltage Vdc: −520 V in development and recovery   
           (Power supply E 1 )
           Vpp: 1.2 kV   Frequency: 2 kHz   Magnet-roll Potential Vdc: −400 V   
           (Power supply E 4 )
           Vdc: direct-current potential   Vpp: peak to peak voltage of alternating voltage   Wave of alternating voltage: Square wave   
               
 
       (2) Comparative Example 
       [0148]    In Comparative example, an image was formed by the use of the developing device having the structure shown in  FIG. 9 . 
         [0149]    In  FIG. 9 , a donor member  2  locating opposite to a magnet roll  6  is backed up with electrodes  12  and  13 . 
         [0150]    The position of the electrode  12  forms the toner supply position to supplies toner from a magnet roll  6  to the donor member  2 , and the electrode  13  forms the toner recovery position to recover toner from the donor member  2  to a brush structural member  6 . 
         [0151]    Developer is the same as that of Example 1. 
       &lt;Electric Potential Conditions&gt; 
       [0152]    Photoreceptor
       Electrically-charged potential V 0 : −450 V   Exposure electric potential Vi: −50 V   Bias voltage at the toner supply position (voltage applied to the electrode  12 )
           Vdc: −50 V   Vpp: 1.2 kV   Frequency: 2 kHz (square wave)   
           Bias voltage at the development position
           Vdc: −320 V   Vpp: 1.2 kV   Frequency: 2 kHz (square wave)   
           Bias voltage at the toner recovery position (voltage applied to the electrode  13 ):
           Vdc: −620 V   Vpp: 1.2 kV   Frequency: 2 kHz (square wave)   
           Electric potential of the magnet roll  6 
           Vdc: −500 V   
               
 
       (3) Evaluation 
       [0169]    Memory generating situations were evaluated by the following procedures. 
         [0170]    Recording sheet P of A 3  size was conveyed in the direction of Y, and images shown in  FIG. 10  was formed on it. 
         [0171]    In  FIG. 10 , character images A to H and a square image (black) which were solid images were formed on a region R 1  and a halftone image of a uniform image density was formed on a region R 2 . 
         [0172]    When there was no memory effect, the image shown in  FIG. 10(   a ) was formed, but when there was a memory effect, as shown in  FIG. 10(   b ), the reverse images of the images of A-H and the black square on the region R 1 , in other words, whitened images of the characters of A-H and the square appeared on the region R 2  by the memory effect. 
         [0173]    In the image area RA in  FIG. 10(   b ), the density difference between the middle image density of the background and the whitened images was large, and in the image area RB, the density difference was small. 
         [0174]    This phenomenon was due to the reason that after the donor member has circled one time, a memory effect decrease. 
         [0175]    Evaluation results for images by visual observation are shown in Table 1. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                 Comp. 
               
               
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 
               
               
                   
               
             
             
               
                 A 
                 A 
                 A 
                 A 
                 C 
               
               
                   
               
               
                 A: Image was excellent. 
               
               
                 C: Image failure occurred due to memory phenomena. 
               
             
          
         
       
     
         [0176]    As shown in Table 1, uniform halftone images were formed in each of Examples 1 to 4, but in Comparative uneven density images were formed due to memory effect. 
       &lt;Developer Deterioration Test&gt; 
       [0177]    Under the high temperature and high humidity environment where the deterioration of developer occurs rapidly, an image formation was conducted and the electrically charged amount of toner was measured. 
         [0178]    Test results are shown in  FIG. 11 . 
         [0179]    As recognized clearly from  FIG. 11 , in Embodiments 2 and 3 in which the donor member  2  was separated from the magnet roll  6  between the toner supply position P 2  and the toner recovery position P 3 , the grade of an electrically charged amount reduction of toner is small, and the progress of the deterioration of developer was refrained. 
         [0180]    In Embodiment 1 in which the donor member  2  was not separated, it was observed that the grade of an electrically charged amount reduction of toner is large, and the progress of the deterioration of developer was slightly rapid.