Patent Document

CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from Japanese Patent Application No. 2005-190130, filed on Jun. 29, 2005, the entire subject matter of which is incorporated herein by reference.  
       TECHNICAL FIELD  
       [0002]     Aspects of the present invention relate to an image forming apparatus forming an image on a recording medium and, particularly, to an image forming apparatus that forms an electrostatic latent image on the surface of an image carrier, sticks toner on the electrostatic latent image and transfers the toner onto a recording medium to form an image, and to a process cartridge and developing cartridge used in the image forming apparatus.  
       BACKGROUND  
       [0003]     A conventional image forming apparatus includes an image carrier, such as a photosensitive drum, on the surface of which an electrostatic image is formed. The image forming apparatus further includes a developing roller developing the electrostatic latent image by sticking toner on the surface of the image carrier and forms an image by transferring the toner stuck on the surface of the image carrier by the developing roller onto a recording medium. In this type of image forming apparatus, the electrostatic latent image is developed by sticking toner on the surface of the image carrier bearing the electrostatic latent image using the developing roller. The image corresponding to the electrostatic latent image is formed on the recording medium by transferring the stuck toner onto the recording medium.  
         [0004]     In this type of image forming apparatus, various processing devices including a charger that uniformly charges the image carrier before the electrostatic latent image is formed on the surface of the image carrier by exposing and a transfer roller that transfers toner stuck on the surface of the image carrier onto the recording medium are provided around the image carrier. Voltage acting between the processing device or the above-mentioned developing roller and the image carrier may be applied (e.g. see JP-A-11-327288).  
         [0005]     In the image forming apparatus, the charger and the developing roller are accommodated in a cartridge and detachable from the image carrier. The components are replaceable according to each life span. JP-A-11-184195 discloses an image forming apparatus in which voltage is applied to a developing unit accommodating a developing roller through a charging unit accommodating a charger.  
       SUMMARY  
       [0006]     However, in the conventional image forming apparatus, an electric current may flow into the processing device although the developing cartridge accommodating the developing roller is separated. For example, in the image forming apparatus disclosed in JP-A-11-184195, it is structurally possible that an electric current may flow into a transfer charger and a transfer belt when the developing cartridge (developing unit) is detached. When the developing cartridge is detached, in general, an electric current flowing into a processing device is stopped by control of software. However, when the electric current is not normally controlled, voltage may be applied between an image carrier and a processing device regardless of separation of the developing cartridge.  
         [0007]     When the developing cartridge is separated, toner is not applied to the surface of the image carrier. In this situation, if voltage is applied between the image carrier and the processing device, the image carrier may be damaged due to the amount of charge excessively stored in the image carrier.  
         [0008]     Aspects of the invention provide an image forming apparatus preventing damage to an image carrier by stopping an electric current flowing into a processing device when a developing cartridge is separated from the image carrier, and a process cartridge and developing cartridge used in the image forming apparatus.  
         [0009]     According to an aspect of the present invention, there is provided an image forming apparatus including: an image carrier; a developing cartridge detachably mountable with respect to the image carrier; a processing device to which voltage acting between the image carrier and the processing device is applied; and a first electrode, wherein the developing cartridge further includes: a developing roller on which toner is capable of being held; and a second electrode capable of being electrically connected with the first electrode when the developing cartridge is mounted with respect to the image carrier, the processing device further includes: a third electrode capable of being electrically connected with the second electrode when the developing cartridge is mounted with respect to the image carrier, wherein the second electrode positions the developing cartridge in an axial direction of the developing roller. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a vertical cross-sectional view showing the inside configuration of a laser printer according to an aspect of the invention;  
         [0011]      FIGS. 2A and 2B  are a side view and a vertical cross-sectional view, respectively, of a developing cartridge and photosensitive cartridge of the laser printer;  
         [0012]      FIGS. 3A and 3B  are a side view and a rear view of the developing cartridge, respectively; and  
         [0013]      FIG. 4  is a partial top view of the photosensitive cartridge.  
     
    
     DETAILED DESCRIPTION  
       [0014]     An aspect of the invention will be described hereinafter with reference to the accompanying drawings.  FIG. 1  is a vertical cross-sectional view showing the inside configuration of a laser printer  1 . As shown in  FIG. 1 , a sheet feed cassette  3  is mounted in a lower portion of a casing  2 . The casing  2  covers the main body of the laser printer  1  from the outer circumference. The sheet feed cassette  3  can be drawn out in a front direction of the laser printer  1  (i.e. the front when the laser printer  1  is set; the left side in  FIG. 1 ).  
         [0015]     A supporting plate  5  pushed up by a spring  6  is provided inside the sheet feed cassette  3 . A sheet feed roller  9  is disposed above a front end of the supporting plate  5 . The sheet feed roller  9  separates a pile of sheets P as recording media disposed on the supporting plate  5  and sends it one-by-one to an image forming portion  7 . A guide  11  that reverses a sheet P conveyed by the sheet feed roller  9 , conveying rollers  12  and  12  that subsequently convey the sheet P, and a pair of registration rollers  14  and  15  that stop a leading end of the sheet P and corrects the skew of the sheet P are sequentially disposed on a conveying path of the sheet P from the sheet feed roller  9  to the image forming portion  7 .  
         [0016]     The image forming portion  7  includes a photosensitive drum  21  that serves as an image carrier and disposed inside a photosensitive cartridge  20  that serves as an image carrier cartridge. The image forming portion  7  further includes a transfer roller  22  that serves as a processing device and transfer unit. The transfer roller  22  is disposed to oppose the photosensitive drum  21 . The photosensitive drum  21  is a well-known drum formed by applying an organic photo conductor (OPC) on the surface of a grounded metal body.  
         [0017]     A sheet P with an image formed by toner (described below) by passing between the photosensitive drum  21  and transfer roller  22  is sent to a fixing portion  31 . The toner image formed on the sheet P is nipped between a heating roller  33  and a pressing roller  35  and fixed by heat at the fixing portion  31 . The sheet P with the fixed image is then conveyed by a pair of conveying rollers  36  and  36 .  
         [0018]     The sheet P conveyed by the conveying rollers  36  is guided to an upper portion of the casing  2  by a guide  37  and then discharged though a pair of sheet discharge rollers  38  and  38  onto a sheet discharge tray  39  provided on the top of the casing  2 . A scanner unit  90 , which exposes the photosensitive drum  21  to laser light L, is disposed between the sheet discharge tray  39  and the photosensitive cartridge  20 . The scanner unit  90  forms an electrostatic latent image by exposing the surface of the photosensitive drum  21  to the laser light L. The scanner unit  90  includes a laser light source, a polygon mirror, an fθ lens, and a reflecting mirror, etc (all not shown).  
         [0019]     The configuration of the image forming portion  7  will be described hereafter in detail. The photosensitive cartridge  20  has the rotatable photosensitive drum  21 , the transfer roller  22  and a scorotron charger  23  that uniformly charges the surfaces of the photosensitive drum  21 . By the laser light L irradiated by the scanner unit  90 , an electrostatic latent image is formed on the surface of the photosensitive drum  21  charged by the scorotron charger  23 . A developing roller  41  (a developing unit), which is provided in a developing cartridge  40  (to be described below), applies toner onto the surface of the photosensitive drum  21  and the electrostatic latent image is subsequently developed. The toner stuck to the photosensitive drum  21  is transferred onto a sheet P passing between the photosensitive drum  21  and the transfer roller  22 . Thus, the image is formed on the sheet P through the above-described operations.  
         [0020]     The developing roller  41  is rotatably supported in the developing cartridge  40  and rotates while contacting with the photosensitive drum  21 . The developing cartridge  40  also includes a toner accommodating portion  42  accommodating toner, an agitator  43  agitating the toner in the toner accommodating portion  42 , a feed roller  44  applying toner discharged from the toner accommodating portion  42  by the agitator  43  to the developing roller  41 , a developing blade  45  frictionally charging the toner stuck on the surface of the developing roller  41  and forming a thin layer of the toner, etc.  
         [0021]      FIG. 2A  is a side view of the developing cartridge  40  and  FIG. 2B  is a vertical cross-sectional view of the photosensitive cartridge  20 . As shown in  FIG. 2B , the developing cartridge  40  and photosensitive cartridge  20  that compose a process cartridge indicated by a solid line are detachable from the laser printer  1 . The developing cartridge  40  is detachably mounted in the photosensitive cartridge  20  by fitting a rotational shaft  41   a  of the developing roller  41  into guide grooves  20   a  formed at both sides of the photosensitive cartridge  20 .  
         [0022]     The developing cartridge  40  is provided with an electrode  51  having one end  51   a  protruding downward from the developing roller  41  in a direction perpendicular to the rotational shaft  41   a  and the other end  51   b  protruding from the right side (this side in a direction perpendicular to the sheet of  FIGS. 1, 2A  and  2 B) of the toner accommodating portion  42  in the direction of the rotational shaft  41   a . When the process cartridge is mounted in the main body of the laser printer  1 , the end  51   b  of the electrode  51  contacts with a leaf spring-shaped electrode  53  provided in the main body of the laser printer  1 , as shown in  FIG. 3A . As shown in a side view and rear view of  FIGS. 3A and 3B , the end  51   a  of the electrode  51  is formed in a plate shape that is perpendicular to the rotational shaft  41   a  of the developing roller  41 .  
         [0023]     The lower surface of the developing cartridge  40  functions as a guiding surface that guides a sheet P between the photosensitive drum  21  and the transfer roller  22 . A plurality of ribs  40   a  for guiding is provided along the conveying direction of the sheet P. As shown in  FIG. 3B , the largest width Wa for sticking toner on the photosensitive drum  21  using the developing roller  41  and the largest sheet width Wb that is available to the laser printer  1  are shown for reference. As shown in  FIG. 3B , the end  51   a  of the electrode  51  is disposed outside the largest available sheet width Wb and protrudes downward from the guiding surface.  
         [0024]      FIG. 4  is a partial view of a part of the photosensitive cartridge  20  lower than the photosensitive drum  21 . As shown in  FIG. 4 , the upper surface of the photosensitive cartridge  20  is also used as a guiding surface guiding a sheet P between the photosensitive drum  21  and the transfer roller  22  and has guiding ribs  20   b  that face the above-mentioned ribs  40   a . An end  61   a  of an electrode  61  is disposed to face the end  51   a  of the electrode  51 . The end  61   a  branches into two parts to interpose the end  51   a  of the electrode  51  between them and they hold the end  51   a  tight in an axial direction of the rotational shaft  41   a  using resin elasticity. On the other hand, the other end  61   b  of the electrode  61  is in contact with a metallic rotational shaft  22   a  of the transfer roller  22 . The end  61   b  is formed into a leaf spring, and in contact with the rotational shaft  22   a , by pressing the end of the rotational shaft  22   a  in the axial direction.  
         [0025]     Accordingly, when the developing cartridge  40  is mounted in the photosensitive cartridge  20  as shown in  FIG. 2B , the end  51   a  of the electrode  51  is interposed between the two parts of the end  61   a  of the electrode  61  and they are electrically connected. Since the end  51   a  of the electrode  51  is interposed between the two parts of the end  61   a  of the electrode  61 , the developing cartridge  40  is positioned in the axial direction of the developing roller  41  with respect to the photosensitive cartridge  20 .  
         [0026]     When the photosensitive cartridge  20  and the developing cartridge  40  that are combined into a unit (a process cartridge) are mounted in the main body of the laser printer  1 , the end  51   b  of the electrode  51  is brought into contact with the electrode  53  and bias voltage is applied from the main body to the transfer roller  22  through the path of the electrode  53 →the electrode  51 →the electrode  61 →the rotational shaft  22   a . The bias voltage is constant-current-controlled by a control circuit (not shown). As described above, toner stuck on the photosensitive drum  21  is transferred onto a sheet passing between the photosensitive drum  21  and the transfer roller  22  by electrostatic attractive force. As the developing cartridge  40  is positioned, the photosensitive drum  21  and developing roller  41  are appropriately positioned with respect to each other. As a result, the electrostatic latent image formed on the photosensitive drum  21  is developed by toner excellently.  
         [0027]     When the developing cartridge  40  is separated from the photosensitive cartridge  20 , the electrodes  51  and  61  are electrically disconnected. Accordingly, when only the photosensitive cartridge  20  is mounted in the main body of the laser printer  1 , bias voltage is not applied to the transfer roller  22 . Therefore, bias voltage is prevented from being applied to the transfer roller  22  when the developing cartridge  40  is not mounted in the printer, thus effectively preventing damage to the photosensitive drum  21 .  
         [0028]     The both ends  51   a  and  61   a  are disposed outside the sheet conveying path formed by the guiding surfaces where ribs  40   a  and  20   b  are formed. Accordingly, when the ribs  20   b  and  40   a  are not normally positioned and a sheet P is not guided between the photosensitive drum  21  and transfer roller  22 , the ends  51   a  and  61   a  are disconnected, and bias voltage is not applied to the transfer roller  22 . Therefore, bias voltage is prevented from being applied to the transfer roller  22  when a sheet P cannot be guided between the photosensitive drum  21  and transfer roller  22 . As a result, damage to the photosensitive drum  21  is surely prevented.  
         [0029]     Although the aspect of the present invention has been described in connection with the detailed aspects of the present invention, it will be apparent that various modifications and changes may be made thereto without departing from the scope and spirit of the invention. For example, the image carrier may not be formed in a drum shape, but may be a belt shape of a photosensitive belt, and may be detachably provided in the main body of the laser printer  1 . The transfer unit may not be limited to the roller, but may be a transfer belt or a transfer charger. The processing device may not be limited to the transfer unit, but may be other processing devices such as a conventional charging device that uniformly charges the surface of the photosensitive drum, or a conventional cleaning device that removes the toner or dusts from the surface of the photosensitive drum.  
         [0030]     As was described, according to the above configuration, when the developing cartridge is mounted with respect to the image carrier, the electrodes provided at the processing device and the developing cartridge, respectively, are connected with each other and an electric current flows into the processing device. Accordingly, voltage is applied between the processing device and image carrier and an image is formed on a recording medium.  
         [0031]     Further, when the developing cartridge is mounted with respect to the image carrier, the developing cartridge is positioned in the axial direction of the developing roller by connection of the pair of electrodes. Therefore, an image is excellently formed on a recording medium. Because the above-mentioned axial arrangement does not require severe precision, the electrodes are enough for the arrangement and other control members may not be required.  
         [0032]     When the image carrier is separated from the developing cartridge, electric current cannot flow into the processing device in view of the structure because the electrodes are separated. Accordingly, when toner cannot be applied onto the image cartridge due to the separation of the developing cartridge, voltage is not applied between the image carrier and processing device. Therefore, it is possible to prevent damage to the image carrier.  
         [0033]     Although the configuration of the electrodes is not limited, the developing cartridge may be configured so as to be detachable in a direction perpendicular to the axial direction of the developing roller and may be positioned by pinching one electrode of the developing cartridge or processing device by the other electrode in the axial direction.  
         [0034]     A variety of processing device are considered, but the processing device may be a transfer unit to which bias voltage acting between the image carrier and the transfer unit is applied and that transfers the toner stuck on the surface of the image carrier onto the recording medium. In general, a transfer unit is constant-current-controlled for maintaining predetermined charged amount of a recording medium. In this configuration, when the developing cartridge is separated and the transfer unit is constant-current-controlled, excessive voltage may be applied to the image carrier. However, when the processing device is the transfer unit, voltage is not excessively applied to the image carrier. Accordingly, damage to the image carrier is effectively prevented.

Technology Category: 3