Patent Publication Number: US-6711371-B2

Title: Developing device having means to remove developer from a developer supplying member

Description:
FIELD OF THE INVENTION 
     The present invention relates to a developing device, and more particularly to a developing device in an electrophotographic recording apparatus. 
     RELATED ART 
     Generally, in an electrophotographic recording apparatus, a photosensitive body, the surface of which is uniformly charged, is irradiated with light to form an electrostatic latent image, and toner is deposited on the electrostatic latent image portion by an electrostatic force to produce a visible image, which is transferred to a printing medium, and fixed by heating the toner on the printing medium. 
     FIG. 11 is a block diagram of a developing device of reversal development type in a conventional electrophotographic recording apparatus. The device in FIG. 11 comprises a photosensitive drum  11 , a charging roller  12 , a print head  13 , a developing device  14 , and a transfer roller  15 . 
     The photosensitive drum  11  is a cylindrical photosensitive body, which is a working part to have an electrostatic latent image formed by the charging roller  12  and the print head  13 , have toner from the developing device  14  deposited on the latent image, and get the toner transferred to a printing medium  16 . The charging roller  12  is a cylindrical roller set face to face with the photosensitive drum  11  to uniformly electrify the surface of the photosensitive drum  11 . The print head  13  is arranged downstream of the charging roller  12  in the rotating direction of the photosensitive drum  11  to form an electrostatic latent image by irradiating the photosensitive drum  11  with light for exposure. 
     The developing device  14  to develop the electrostatic image to produce a toner image is arranged downstream of the print head  13  in the rotating direction of the photosensitive drum  11 , and comprises a developing roller  21 , sponge roller  22 , toner  23 , an agitator rod  24 , a developer scraping blade  25 , and a case  26 . The case  26  contains and holds the toner  23 . 
     The developing roller  21  is a cylindrical roller, which rotates in the arrow direction  21   a , is arranged face to face with the photosensitive drum  11  to deposit toner on the electrostatic latent image formed on the photosensitive drum  11 . The sponge roller  22  is in contact with the developing roller  21  and rotates in the same direction (in the arrow direction  22   a ) as the developing roller  21 . After the toner is deposited on the photosensitive drum  11 , the sponge roller  22  removes surplus toner from the developing roller  21 , and, on the other hand, supplies toner afresh to the developing roller  21 . The sponge roller  22  has at its center a conductive rod  22   b  to which a voltage Vs is applied. 
     The toner  23  is developing powder which is deposited on the electrostatic latent image on the photosensitive drum  11  to develop the image. The agitator rod  24  agitates the toner  23  in the case  26  to homogenize it. The developer scraping blade  25  thins down the toner  23  deposited on the surface of the developing roller  21  to a thin film of a constant thickness. 
     The transfer roller  15  rotates in the direction of the arrow and is arranged downstream of the developing device  14  in the rotating direction of the photosensitive drum  11  and is pressed to the photosensitive drum  11 . The transfer roller  15  has a toner image transferred to the printing medium  16  fed between the photosensitive drum  11  and the transfer roller  15 . 
     In the electrophotographic recording apparatus configured as described, when a motor, not shown, rotates to feed the printing medium  16  in the direction of the arrow  16   a , the developing roller  21  and the sponge roller  22  in the developing device  14  rotate in the directions of arrows  21   a  and  22   a  respectively, and the toner  23  in the developing device  14  is agitated by the agitator rod  24  which rotates in the direction of the arrow  24   a . At this time, negative voltages are applied to the developing roller  21  and the sponge roller  22  simultaneously, but in such a way that the absolute value of the applied negative voltage of the developing roller  21  is smaller. In the case of reversal development, negatively-charged toner  23  that was agitated is deposited on the surface of the sponge roller  22 , and the deposited toner  23  is supplied to the surface of the developing roller  21  through contact of the sponge roller  22  with the surface of the developing roller  21  having a negative voltage the absolute value of which is smaller than that of the sponge roller  22 . The toner deposited on the surface of the developing roller  21  is made thinner by the developer scraping blade  25  to a thin toner layer. 
     On the other hand, simultaneously with the rotation of the above-mentioned motor, not shown, a negative voltage is applied to the charging roller  12 , and the photosensitive drum  11  is charged uniformly while rotating in the direction of the arrow  11   a . After this, when the photosensitive drum  11  is irradiated with light from the print head  13 , the regions exposed to light decrease in the level of charge to 0[V], so that an electrostatic latent image is formed on the surface of the photosensitive drum  11 . The toner  23  deposited on the developing roller  21  is negatively-charged, and when the developing roller  21  contacts the electrostatic latent image on the photosensitive drum  11  or reaches a maximum proximate point, the toner  23  is deposited on the exposed regions. Meanwhile, because the regions not exposed to light are more negatively charged than the negatively-charged toner  23  on the developing roller  21 , the toner  23  is not deposited on the unexposed regions. Subsequently, the toner image is transferred from the photosensitive drum  11  to a printing medium  16 , and the toner image is fixed to the printing medium  16  by a fixing unit, not shown. 
     However, the conventional developing device has problems as follows. 
     FIG. 12 is a diagram for explaining the conventional problem. 
     The residues of toner (residual toner)  101  on the developing roller  21 , which was not used by the photosensitive drum  11  and returned to the developing device  14 , are scraped off on the downstream side of the contact area by the sponge roller  22 . However, in a continuous printing process, for example, the residual toner  101  gets into a sponge portion  22   c  (indicated as toner  102 ) covering the periphery of the sponge roller  22 , and solidifies to give rise to clogging of the sponge, thus deteriorating the toner removing performance of the sponge roller  22 . Under this condition, the residual toner  101  is not detached completely on the downstream side of the contact area, and stays deposited on the developing roller  21  as indicated by the residual toner  101   a . Consequently, there arises a difference in toner thickness on the surface of the developing roller  21  or a difference in potential of the toner between its first rotation and the second rotation when the residual toner  101   a  remains deposited on the developing roller  21 , which is responsible for irregular print density. 
     In solid print, the image, which was used for printing in the first rotation, remains as a thin residual image in the second rotation. In other words, toner is deposited on the previous toner image which was not consumed on the photosensitive drum  11  and remains on the developing roller  21 , with the result that a reverse image of the image in the first rotation is printed. 
     Further, irregular print density occurs for the reason as follows. The residual toner, which was once detached by scraping off from the developing roller  21 , still stays deposited on the sponge roller  22 . As the sponge roller  22  rotates, the residual toner is supplied together with other toner to the surface of the developing roller  21 , making the toner layer not uniform and giving rise to irregular print density. 
     Irregular print density occurs for another reason. Immediately before starting printing, the image forming process starts, and to this end, the photosensitive drum  11  is subjected to preliminary charging by the charging roller  12 . Simultaneously with the charging of the photosensitive drum  11 , the surface of the developing roller  21  is also charged. Therefore, in a printing process with high density which consumes a relatively large amount of toner, such as solid print, a difference occurs in toner layer thickness between the first rotation of the developing roller  21  and in the second rotation when the surface is reset by detaching the toner once. 
     As mentioned above, the problem with the prior art is that irregular print density occurs which is attributable to the unevenness of the thickness of the toner layer. 
     SUMMARY OF THE INVENTION 
     A developing device for supplying, by a developer carrying body, a developer to an electrostatic latent image formed on a photosensitive body is proposed. The device includes a developer supplying member to supply a developer to the developer carrying body and to separate the developer remaining on the developer carrying body. Developer removing means is provided for contacting the developer supplying member and for removing the developer adhering to the developer supplying member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a first embodiment of the developing device according to the present invention; 
     FIG. 2 is a diagram for explaining the operation of the first embodiment; 
     FIG. 3 is a block diagram of a second embodiment of the developing device according to the present invention; 
     FIG. 4 is a diagram for explaining the operation of the second embodiment; 
     FIG. 5 is a block diagram of a third embodiment of the developing device according to the present invention; 
     FIG. 6 is a diagram for explaining the operation of the third embodiment; 
     FIG. 7 is a block diagram of a fourth embodiment of the developing device according to the present invention; 
     FIG. 8 is a diagram for explaining the operation of the fourth embodiment; 
     FIG. 9 is a block diagram of a fifth embodiment of the developing device according to the present invention; 
     FIG. 10 is a time chart for explaining the operation of the fifth embodiment; 
     FIG. 11 is a block diagram of a conventional developing device; 
     FIG. 12 is a diagram for explaining a conventional problem; and 
     FIG. 13 is a timing chart for explaining control of applied voltages of the developer removing means and the sponge roller according to the present invention. 
    
    
     EMBODIMENTS OF THE INVENTION 
     The modes of carrying out the present invention will be described in detail with reference to the following embodiments. 
     &lt;Embodiment 1&gt; 
     &lt;Structure&gt; 
     A printer mechanism shown in FIG. 1 comprises a photosensitive drum  11 , a charging roller  12 , a print head  13 , a developing device  14 , and a transfer roller  15 . 
     This mechanical block has the photosensitive drum  11  of a cylindrical shape, on which an electrostatic latent image is formed, and toner  23  from the developing device  14  is deposited on the latent image, and then a toner image is transferred to a printing medium  16 . The charging roller  12  is a cylindrical roller arranged face to face with the photosensitive drum  11 , and uniformly electrifies the surface of the photosensitive drum  11 . The print head  13  is arranged downstream of the charging roller  12  in the rotating direction  11   a  of the photosensitive drum  11 , irradiates the photosensitive drum  11  with light for exposure to form an electrostatic latent image. 
     The developing device  14  is arranged downstream of the print head  13  in the rotating direction of the photosensitive drum  11 , and develops the electrostatic latent image to form a toner image on the photosensitive drum  11 . The developing device  14  comprises a developing roller  21 , a sponge roller  22 , toner  23 , an agitator rod  24 , a developer scraping blade  25 , a case  26 , and a plurality of cylindrical members  31 . The case  26  accommodates and holds the toner  23 . 
     The developing roller  21  is a cylindrical roller, which is arranged face to face with the photosensitive drum  11  and deposits toner  23  on the electrostatic latent image on the photosensitive drum  11 , and rotates in the direction of the arrow  21   a . The sponge roller  22  is a sponge-covered roller, which contacts the developing roller  21 , and rotates in the same direction as the developing roller  21  (the direction of the arrow  22   a ) to remove excess toner  23  on the developing roller  21  after the toner  23  was deposited on the photosensitive drum  11 , and supply toner  23  to the developing roller  21  anew. A voltage Vs (−600V) is applied to a conductive rod  22   b  provided in the center of the sponge roller  22 . 
     The toner  23  is a developer which is deposited on the electrostatic latent image on the photosensitive drum  11  to develop a toner image. The agitator rod  24  agitates the toner  23  in the case  26  to homogenize it. The developer scraping blade  25  is a plate member that scrapes off the toner  23  deposited on the developing roller  21  to a certain thickness. 
     The transfer roller  15  is arranged downstream of the developing device  14  in the rotating direction of the photosensitive drum  11  in a manner to be pressed against the photosensitive drum  11 , and transfers the toner image to a printing medium  16  fed between itself and the photosensitive drum  11 . 
     The plurality of cylindrical members  31  are conductive cylindrical members arranged at a lower position in the case  26  in a manner to contact the sponge portion  22   c  of the sponge roller  22 . The cylindrical members  31  are immovably fixed on a stationary base  32  and are non-rotatable with their side end faces fixed by a fixture, not shown. A passage  32   a  is provided in the stationary base  32 . This passage  32   a  interconnects a space  27   a  formed in the case  26  by the developing roller  21 , the sponge roller  22 , the cylindrical members  31 , and the stationary base  32 , and a space  27   b  filled with toner  23  in the case  26 . Therefore, through the passage  32   a , the toner  23  detached from the developing roller  21  by the sponge roller  22  and the cylindrical members  31  can move from the space  27   a  to the space  27   b.    
     The above-mentioned voltage Vs (−600V) is applied to the conductive rod  22   b  of metal or resin, for example, provided at the center of the sponge roller  22  covered with a semiconductor sponge, while a voltage Vr1 (−200V˜−300V) is applied to the cylindrical members  31 . 
     &lt;Operation&gt; 
     In FIG. 1, when a motor, not shown, rotates to feed a printing medium  16  in the direction of the arrow  16   a , the developing roller  21  and the sponge roller  22  in the developing device  14  rotate in the directions of the arrows  21   a ,  22   a , and the agitator rod  24  rotates in the direction of the arrow  24   a  to agitate the toner  23 . At the same time, a voltage of negative polarity, say, −300V is applied to the developing roller  21 . 
     Therefore, the agitated, negatively-charged toner  23  is first deposited on the surface of the sponge roller  22 , and when the surface of the sponge roller  22  contacts the surface of the developing roller  21  that has a smaller absolute value of applied voltage, owing to this voltage difference, the toner  23  is now deposited on the surface of the developing roller  21  and supplied. The toner  23  deposited on the surface of the developing roller  21  is made thinner by the developer scraping blade  25  to form a thin toner layer. 
     On the other hand, simultaneously with the rotation of a motor, not shown, a voltage of negative polarity, say, −1200V is applied to the charging roller  12 , the photosensitive drum  11 , while rotating in the direction of the arrow  11   a , is uniformly charged. Consequently, the potential of the charge on the surface of the photosensitive drum  11  will be −700V, for example. After this, when the photosensitive drum  11  is irradiated with light from the print head  13 , the regions exposed to light decrease in the level of charge to −50[V], so that an electrostatic latent image is formed on the surface of the photosensitive drum  11 . Because the toner  23  deposited on the surface of the developing roller  21  is negatively charged, when the developing roller  21  contacts the electrostatic latent image on the photosensitive drum  11  or reaches a maximum approximate point, the toner  23  is deposited on the image. Meanwhile, the regions of the surface of the photosensitive drum  11  which were not exposed to light are more negatively charged (−700V) than the toner  23  on the developing roller  21 , so that the toner  23  is not deposited on the unexposed regions on the photosensitive drum  11 . 
     Next, description will be made of the detachment and removal of the residual toner in the developing device  14 . 
     FIG. 2 is a diagram for explaining the operation of the first embodiment. 
     As shown in FIG. 2, in the developing device  14 , residual toner  101  which was not deposited on the photosensitive drum  11  but remains on the surface of the developing roller  21  is scraped off by the sponge roller  22 . The toner is mechanically detached in the direction of the arrow  201 , for example. 
     As the sponge roller  22  rotates, the sponge portion  22   c  moves while it is deformed elastically by a plurality of cylindrical members  31 . Therefore, as the sponge portion  22   c  is deformed repeatedly (as if it were vibrating), so that the toner  101  solidifies in clots in the sponge portion  22   c , and is pushed out in the directions of the arrows  202 ,  203 . Therefore, the sponge roller  22  can maintain its toner removing performance. 
     To electrically detach the toner from the sponge roller  22  by means of the cylindrical members  31 , a negative voltage of Vr1 (−200V˜−300V) is applied to the cylindrical members  31 . Vr1 is set to meet a relation of the Vr1 to the voltage Vs (−600V) applied to the sponge roller  22  such that |Vr1|&lt;|Vs|. Therefore, the negatively-charged toner is deposited on the cylindrical members  31 . The removed residual toner  101  passes through the passage  32   a , circulates and is supplied to the sponge roller  22  again. 
     Subsequently, the toner image is transferred from the photosensitive drum  11  to a printing medium  16 , fixed to the printing medium  16  by a fixing unit, not shown, by which printing is finished. 
     &lt;Effect&gt; 
     As has been described, according to the first embodiment, the provision of a plurality of cylindrical members  31  contacting the sponge roller  22  makes it possible to mechanically and electrically separate the toner by the cylindrical members  31  when the sponge roller  22  removes the residual toner  101  by scraping off from the developing roller  21 . Therefore, according to this embodiment, it is possible to improve and maintain the toner removing performance of the sponge roller  22 . Consequently, irregular print density occurring at the periods of the developing roller  21  and the residual image phenomenon can be prevented, making it possible to provide an image forming device that produces high-quality and stable images. 
     In the first embodiment, the cylindrical members  31  are fixed not to prevent them from rotating, but the cylindrical members  31  may be rotatably supported. In this latter case, as the sponge roller  22  rotates, the cylindrical members  31  rotate as driven elements. According to this structure, the abrasion of the sponge portion  22 C can be reduced, and the life of the sponge roller  22  can be prolonged. 
     Further, in the first embodiment, the developer removing means is structured as a plurality of cylindrical members  31  arranged in parallel, but the developer removing means is not limited to this design. Parts having an undulated sectional profile can achieve the same effects so long as they contact the surface of the sponge roller  22  and have a plurality of ups and downs in the moving direction of the sponge roller  22 . 
     &lt;Embodiment 2&gt; 
     In a second embodiment of the present invention, a net-like member  41  is used for the developer removing means. 
     &lt;Structure&gt; 
     FIG. 3 is a block diagram of the second embodiment. 
     A printer mechanism in FIG. 3 comprises a photosensitive drum  11 , a charging roller  12 , a print head  13 , a developing device  14 , a transfer roller  15 , and a printing medium  16 , and a developing device  14  comprises a developing roller  21 , a sponge roller  22 , toner  23 , an agitator rod  24 , a developer scraping blade  25 , a case  26 , and a net-like member  41 . The components, from the photosensitive drum  11  up to case  26 , are identical with those in the first embodiment shown in FIG. 1, and they are designated by the same reference numerals and will not be explained here. 
     The net-like member  41  is a solid-structured conductive net installed upright with one side of it in contact with the sponge portion  22   c  of the sponge roller  22 , and fixed to the bottom of case  26 . A negative voltage Vr2 (−200V to −300V) is applied to the net-like member  41 . 
     &lt;Operation&gt; 
     In FIG. 3, the basic toner depositing operation is the same as in the first embodiment, and will not be described here. The removal process of the residual toner in the developing device  14  will be described in the following. 
     FIG. 4 is a diagram for explaining the operation of the second embodiment. 
     As shown in FIG. 4, in the developing device  14 , when the sponge roller  22  scrapes off the residual toner  101  on the surface of the developing roller  21 , the net-like member  41  prevents the scraped-off toner  101  from solidifying inside the sponge portion  22   c , and removes the toner  101  by sending it, for example, in a direction  201  or in a direction  204  in which the toner  101  passes through the net-like member  41 . Because the sponge portion  22   c  is deformed, the toner  102  that is solidified in the sponge portion  22   c  is pushed in a direction  202  or in a direction  205  after passing through the net-like member  41 , for example. Therefore, the sponge roller  22  can maintain its toner removing performance. 
     To electrically detach the toner from the sponge roller  22 , and for this purpose, to let the toner from the sponge roller  22  be deposited on the net-like member  41 , a negative voltage Vr2 of −200V˜−300V is applied to the net-like member  41 . The negative voltage Vr2 is set to meet a relation of the Vr2 to the voltage Vs of −600V applied to the sponge roller  22  such that |Vr2|&lt;|Vs|. Consequently, the negatively-charged toner moves toward the net-like member  41 . Because the scraped-off toner can pass through the net-like member  41 , the toner  23  is kept circulating even when the printing process is continued. 
     &lt;Effect&gt; 
     As has been described, according to the second embodiment, in which the net-like member  41  is so provided as to contact the sponge roller  22 , when the residual toner  101  on the developing roller  21  is removed by being scraped off by the sponge roller  22 , the net-like member  41  can mechanically and electrically detach the toner  101 . Therefore, the toner removal performance of the sponge roller  22  can be improved and prolonged. Because this toner removing means, being of a net type, permits the detached toner to pass through it, and can avoid uneven distribution of toner in the case  26 , and the toner can circulate smoothly even if printing is carried out continuously. As a result, irregular print density occurring with rotating periods of the developing roller  21  and the residual image phenomenon can be prevented and an image forming device can be realized which produces high-quality and stable images. 
     In the second embodiment, the net-like member  41  is mounted upright, but it can be mounted in any manner so long as one side of it is in contact with the surface of the sponge roller  22 . For example, the net-like member  41  may be installed at the bottom or at the top of the sponge roller  22  though this requires that a fixture be provided separately. 
     &lt;Embodiment 3&gt; 
     In a third embodiment of the present invention, as the developer removing means, there is provided a roller member  51  which contacts the sponge roller  22  and rotates in the same direction as the sponge roller  22  (the contact faces of the two rollers move in opposite directions). 
     &lt;Structure&gt; 
     FIG. 5 is a block diagram of the third embodiment. 
     A printer mechanism shown in FIG. 5 comprises a photosensitive drum  11 , a charging roller  12 , a print head  13 , a developing device  14 , a transfer roller  15 , and a printing medium  16 ; and the developing device  14  comprises a developing roller  21 , a sponge roller  22 , toner  23 , an agitator rod  24 , a developer scraping blade  25 , a case  26 , and a roller member  51 . The components, from the photosensitive drum  11  to case  26  are identical with those of the first and second embodiments, the common components are designated by the same reference numerals, and their descriptions are omitted. 
     The roller member  51  is a conductive and inelastic member of metal, for example, of circular cross section, so arranged as to interfere on the outer circumferential surface with the sponge portion  22   c  of the sponge roller  22  (contact the sponge portion  22   c  to such an extent to slightly deform it). The roller member  51  is so formed as to be rotatably driven in the same direction as the rotating direction of the sponge roller  22  by driving means, not shown. A voltage Vr3 (−200V˜−300V) of negative polarity is applied to the roller member  51 . 
     &lt;Operation&gt; 
     In FIG. 5, the basic toner depositing operation is the same as in the first and second embodiments, and its description is omitted here. Description will be made of the removing process of toner  23  in the developing device  14 . 
     FIG. 6 is a diagram for explaining the operation of the third embodiment of the present invention. 
     As shown in FIG. 6, in the developing device  14 , when the sponge roller  22  scrapes off the residual toner  101  from the surface of the developing roller  21 , the roller member  51  is rotated in the direction of the arrow  51   a  by a motor, not shown, and ejects the scraped-off toner in the direction of the arrow  206 , for example, without letting it solidify and stay inside the sponge portion  22   c . As the sponge portion  22   c  is deformed, the toner that solidified in the sponge portion  22   c  is pushed away in the direction of the arrow  202 , for example. In this manner, the sponge roller  22  can maintain the toner removing performance. 
     To electrically separate the toner from the sponge roller  22 , and accordingly, to let the toner from the sponge roller  22  be attracted to the roller member  51 , a negative voltage Vr3 of −200V˜−300V is applied to the roller member  51 . The negative voltage Vr3 is set to meet a relation of the Vr3 to the voltage Vs of −600V applied to the sponge roller  22  such that |Vr3|&lt;|Vs|. Hence, the negatively-charged toner moves toward the roller member  51 . 
     &lt;Effect&gt; 
     As has been described, according to the third embodiment, in which the roller member  51  is provided so as to contact the sponge roller  22  and rotate in the opposite direction to the moving surface of the sponge roller  22 , when the residual toner  101 , which was not consumed and remains on the developing roller  21 , is removed by being scraped off by the sponge roller  22 , the roller member  51  can mechanically and electrically detach the toner. Therefore, the toner removing performance of the sponge roller  22  can be improved and prolonged. Therefore, irregular print density occurring at rotating periods of the developing roller  21  and the residual image phenomenon can be prevented, and an image forming device can be realized which produces high-quality and stable images. 
     &lt;Embodiment 4&gt; 
     In a fourth embodiment of the present invention, the roller member  51  in the third embodiment and a contact member  52  contacting the surface of the roller member  51  are provided as the developer removing means. 
     &lt;Structure&gt; 
     FIG. 7 is a block diagram of a fourth embodiment of the present invention. 
     A printer mechanism in FIG. 7 comprises a photosensitive drum  11 , a charging roller  12 , a print head  13 , a developing device  14 , a transfer roller  15 , and a printing medium  16 , and the developing device  14  comprises a developing roller  21 , a sponge roller  22 , toner  23 , an agitator rod  24 , a developer scraping blade  25 , a case  26 , a roller member  51 , and a contact member  52 . The components, from the photosensitive drum  11  up to the case  26 , are the same as in the first to third embodiments, and the common components are designated by the same reference numerals. The roller member  51  is the same as in the third embodiment, and its description is omitted here. 
     The contact member  52 , made of an elastic plate having electric conductivity, is at one end of it contacting the bottom portion of the roller member  51  and is fixed so as to be substantially horizontal. The contact member  52  has a plurality of slits  52   a  (See FIG. 8) formed at certain intervals along the longitudinal direction thereof (in a direction parallel with the rotating axis of the roller member  51 ) in an area away from the end portion thereof that contacts the roller member  51 , those slits  52   a  being openings through which the toner passes and circulates. 
     A negative voltage Vr4 of −50V˜−100V is applied to the contact member  52 . 
     &lt;Operation&gt; 
     In FIG. 7, the basic toner depositing operation is the same as in the first to third embodiments, and its description is omitted here. The residual toner removing process in the developing device  14  is described. 
     FIG. 8 is a diagram for explaining the operation of the fourth embodiment of the present invention. 
     As shown in FIG. 8, in the developing device  14 , when the sponge roller  22  scrapes off the residual toner  101  from the surface of the developing roller  21 , the roller member  51  is rotated in the direction  51   a  by a motor, not shown, mechanically ejects the scraped-off toner in the direction of the arrow, for example, without letting it solidify and stay inside the sponge portion  22   c . As the sponge portion  22   c  is deformed, the toner that solidified in the sponge portion  22   c  is pushed away in the direction of the arrow, for example. In this manner, the sponge roller  22  can preserve the toner removing performance. 
     To electrically separate the toner from the sponge roller  22 , and accordingly, to let the toner from the sponge roller  22  be attracted to the roller member  51 , a negative voltage Vr3 of −200V˜−300V is applied to the roller member  51 . The negative voltage Vr3 is set to meet a relation of the Vr3 to the voltage Vs of −600V applied to the sponge roller  22  such that |Vr3I&lt;|Vs|. Therefore, the negatively-charged toner moves toward the roller member  51 . The operations mentioned so far are the same as in the third embodiment. 
     In the fourth embodiment, the toner deposited on the roller member  51  is separated by the contact member  52 , and moves, for example, in the direction of the arrow  207  and in the direction of the arrow  208  to pass through the slit  52   a  of the contact member  52 . As a negative voltage Vr4 of −50V˜−100V is applied to the contact member  52  to meet a relation of the Vr4 to the voltage Vr3 of −200V˜−300V applied to the roller member  51  such that Vr4&lt;Vr3, and therefore the negatively-charged toner is attracted to the contact member  52 . 
     &lt;Effect&gt; 
     As described above, according to the fourth embodiment, the structure of the third embodiment is added with the contact member  52  which contacts the roller member  51 , and therefore the effects of the third embodiment are further added with an effect that the toner removing performance of the roller member  51  can be made more durable. 
     The contact member  52  may be formed of a conductive elastic resin or may be structured as a brush which is conductive. In short, the contact member  52  may be any type so long as it is conductive and structured so as to scrape toner off. 
     The contact member  52  is not limited to the fixed position mentioned above, but may be in any fixed condition. For example, the contact member  52  may be installed on the side of the roller member  51  opposite the side where the roller member  51  contacts the sponge roller  22 , or at the top of the roller member  51 . 
     &lt;Embodiment 5&gt; 
     In a fifth embodiment of the present invention, the voltage applied to the developing roller  21  in preliminary charging is controlled so that a difference between this voltage and a voltage, at which the photosensitive drum  11  is charged, is small. 
     &lt;Structure&gt; 
     FIG. 9 is a block diagram of the fifth embodiment. 
     A printer mechanism shown in FIG. 9 comprises a photosensitive drum  11 , a charging roller  12 , a print head  13 , a developing device  14 , a transfer roller  15 , and a printing medium  16 , and the developing device  14  comprises a developing roller  21 , a sponge roller  22 , toner  23 , an agitator rod  24 , a developer scraping blade  25 , a case  26 , and a controller  61 . The components, from the photosensitive drum  11  to the case  26 , are the same as in the first to fourth embodiments, and their descriptions are omitted here. 
     The controller  61  controls the voltage of the developing roller  21  so that, during preliminary charging of the photosensitive drum  11 , a potential difference between the preliminary charging voltage to the photosensitive drum  11  and the applied voltage of the developing roller  21  is smaller than a potential difference during print charging. More specifically, the controller  61  controls the voltage of the developing roller  21  so that if a voltage Vd applied to the developing roller  21  during print charging is designated as Vd1 and the voltage Vd during preliminary charging is designated as Vd2, the relation between Vd1 and Vd2 is |Vd1|&lt;|Vd2|, and that a voltage difference (|Vo−Vd|) between the charging voltage Vo to the photosensitive drum  11  (in other words, the voltage applied to the charging roller  12 ) and the applied voltage of the developing roller  21  during preliminary charging is decreased. 
     The developing roller  21  comprises a resin, such as rubber, and has a metal rod disposed at its center, and the voltage Vd is applied to this conductive metal rod. 
     &lt;Operation&gt; 
     In FIG. 9, the basic toner depositing operation is the same as in the other embodiments except that the voltage applied to the developing roller  21  changes. Therefore, description of the toner depositing operation is omitted here, and the operation different from the other embodiments will be described in the following. 
     FIG. 10 is a time chart showing changes in the voltage applied to the developing roller  21 . 
     A print charging period is an actual printing operation period when toner is supplied from the developing roller  21  to the photosensitive drum  11 , and a preliminary charging period is a process in which the photosensitive drum  11  is subjected to preliminary charging before the printing operation is started or before a printing step after a paper space time. 
     The controller  61  sets the applied voltage Vd2 of the developing roller  21  at −300V during preliminary charging (˜time T1), and sets the applied voltage Vd1 at a normal voltage of −200V during print charging (time T1˜time T2). These settings apply to the subsequent preliminary charging (time T2˜time T3) and print charging (time T3˜). The charging voltage Vo to the photosensitive drum  11  (namely, the applied voltage of the charging roller  12 ) Vo is −700V, and the applied voltage Vs of sponge roller  22  is −350V. 
     In the fifth embodiment, the applied voltage of the developing roller  21  is set to −300V during a preliminary charging period, a voltage difference |Vo−Vd2| is reduced to 400V, which is less than the voltage difference of 500V in cases where the applied voltage of the developing roller  21  is fixed at −200V. Therefore, the developing roller  21  is made less likely to be affected by charge increase caused by the photosensitive drum  11 . 
     In other words, if there is a potential difference of about 500V or more between the photosensitive drum  11  and something that comes into contact with the photosensitive drum  11 , it is well known that charge-up occurs between them. For this reason, in the fifth embodiment, the potential difference between the photosensitive drum  11  and the developing roller  21  during preliminary charging is limited to not more than 500V to prevent charge-up on the developing roller  21 . 
     For this reason, at print charging, the potential on the developing roller  21  is quickly switched to a normal level of −200V, and this makes it possible to reduce a difference in toner layer thickness between in the first rotation and the second and following rotations in high-density printing, such as solid print, thereby reducing irregularity in print density. 
     In a preliminary charging period, a potential difference between the developing roller  21  and the sponge roller  22  is small, but this does not affect the printing operation because the printing operation is not performed during the preliminary charging period, and toner need not be supplied. 
     In the fifth embodiment, the applied voltage to the developing roller  21  in a preliminary charging period is set at −300V, but this should not be construed to show a limited value and may be varied if it is necessary according to the charging voltage to the photosensitive drum  11 , the applied voltage to the developing roller  21  or a kind of material of the developing roller  21  so long as it is possible to decrease the voltage difference between the photosensitive drum  11  and the developing roller  21 . 
     &lt;Effect&gt; 
     As has been described, according to the fifth embodiment, the applied voltage to the developing roller  21  is controlled so that a difference between this applied voltage and the surface potential of the photosensitive drum  11  during a preliminary charging period, and therefore the developing roller  21  is not likely to be affected by charge-up caused by the photosensitive drum  11  and irregular print density can be eliminated even in a high-density printing process, such as solid print. 
     &lt;Mode of Use&gt; 
     In the third and fourth embodiments, the roller member  51  as the developer removing means is rotatably driven in the same direction as the sponge roller  22 , but it may be arranged that the roller member  51  is rotatably driven in the opposite direction of the rotation of the sponge roller  22  (such that the surface of the roller member  51  moves in the same direction as the moving direction of the surface of the sponge roller  22 ). Note that in this case, it is necessary to arrange that the moving speed of the surface of the roller member  51  differs from the moving speed of the surface of the sponge roller  22 . In other words, if it is arranged that the surface of the sponge roller  22  moves relative to the surface of the roller member  51 , the same effects can be obtained as in a case where the surface of the sponge roller  22  moves in the opposite direction of the moving direction of the surface of the roller member  51  as described in the third and fourth embodiments. 
     In the first to fourth embodiments, the relation of the applied voltage (Vr) of the cylindrical members  31 , the net-like member  41 , or the roller member  51  with respect to the applied voltage of the sponge roller  22  is set such that |Vr|&lt;|Vs|, but moreover it is possible to perform control as follows. 
     For example, as shown in FIG. 13, voltages are applied such that |Vr3(−300V)|&lt;|Vs(−600V)| in normal printing. Further, it is possible to apply voltages such that |Vr(−700V)|&gt;|Vs(−600V)| at timing not concerned with printing, such as preliminary charging. This leaves a possibility that if voltages are applied such that |Vr3|&lt;|Vs| at all times, toner removed from the sponge roller  22  will be left deposited on the surface of the roller member  51 . However, by applying voltages such that |Vr3|&gt;|Vs| at timing not concerned with printing, at this time in point, the toner moves from the roller member  51  to the sponge roller  22 , by which the surface of the roller member  51  is refreshed. Consequently, the roller member&#39;s action of removing the toner from the sponge roller  22  can be preserved for a long period of time, making it possible to maintain stable, high-quality of images. Above all else, even when the contact member  52  is provided as in the fourth embodiment, the toner not completely scraped off by the contact member  52  can be allowed to move to the side of the sponge roller  22 . This further improves the toner removing action of the roller member  51 . 
     The structure of the fifth embodiment may be combined with any of the first to fourth embodiment or modifications of the third and fourth embodiments. In other words, it may be arranged that means to control an applied voltages in preliminary charging to the developing roller  21  should be added to the developing device  14  in which a developer removing means is provided for the sponge roller  22 . 
     The surfaces of the cylindrical members  31  and the roller member  51  should preferably be made smooth to make it possible to remove the toner deposited on the sponge roller  22  without damaging the sponge roller  22 . 
     Further, in each embodiment, a photosensitive body has been described as the photosensitive drum  11 , a charging means as the charging roller  12 , an exposure means as the print head, a developer carrying body as the developing roller  21 , and a developer supplying member as the sponge roller  22 . Any form of component may be used so long as it achieves the same function. For example, the photosensitive body may be a belt running in an elliptical track. The present invention may be applied to various types of mechanisms. Each embodiment has been described referring to a reversal development type developing device, but the present invention may be applied to normal development type developing devices.