Patent Publication Number: US-8977166-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2010-244232 filed on Oct. 29, 2010, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to an image forming apparatus having a charger that charges a photosensitive drum. 
     BACKGROUND 
     There has been proposed a related-art image forming apparatus that includes a charger having a discharge wire (wire electrode) and a grid electrode and generating the corona discharge from the wire electrode in order to charge a surface of a photosensitive drum. In the related-art image forming apparatus, the charger is arranged such that the grid electrode (grid surface opposed to the photosensitive drum) is orthogonal to a plane including the wire electrode and a rotational center portion of the photosensitive drum. 
     SUMMARY 
     Illustrative aspects of the invention provide an image forming apparatus capable of reducing a size thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing an overall configuration of a color printer according to an exemplary embodiment of the invention; 
         FIG. 2  is an enlarged view showing a configuration around a photosensitive drum; and 
         FIGS. 3A-3B  illustrate effects of the invention, where  FIG. 3A  shows a configuration of a comparative example in which a grid surface is arranged in parallel with an orthogonal plane and  FIG. 3B  shows a configuration of an exemplary embodiment in which the grid surface is inclined to the orthogonal plane. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;General Overview&gt; 
     In recent years, as a size of the image forming apparatus is reduced, a diameter of the photosensitive drum is also made to be smaller. However, since the photosensitive drum has, on a circumferential surface thereof, an exposure position to be exposed by an exposure device and the charger, a developing roller, a transfer roller, a cleaning roller and the like are arranged around the photosensitive drum, gaps therebetween are reduced as the diameter of the photosensitive drum is made to be smaller. Specifically, since there is a limit to the miniaturization of the charger, there is also a limit to the miniaturization of the photosensitive drum (reducing a size of the image forming apparatus). 
     Therefore, illustrative aspects of the invention provide an image forming apparatus capable of reducing a size thereof. 
     According to illustrative aspects of the invention, there is provided an image forming apparatus comprising: a photosensitive drum configured to rotate about a rotational axis extending in an axial direction; and a charger configured to charge the photosensitive drum, wherein the charger comprises: a wire electrode, which extends in the axial direction, and to which a voltage for exposing the photosensitive drum is applied; a grid electrode having a grid surface that faces the photosensitive drum between the wire electrode and the photosensitive drum; and a pair of shield electrodes, which is arranged to face each other with the wire electrode being interposed therebetween, and which extends in a substantially orthogonal direction to the grid electrode, and wherein the grid surface is inclined to a second plane such that the grid electrode and one of the pair of shield electrodes come close to the photosensitive drum at a more downstream side in a rotating direction of the photosensitive drum than a first plane, wherein the first plane includes the wire electrode and the rotational axis of the photosensitive drum, and the second plane is an orthogonal plane to the first plane. 
     According to the image forming apparatus configured as described above, the grid surface is inclined to the orthogonal plane of the plane including the wire electrode and the rotational center portion of the photosensitive drum such that the grid electrode and the shield electrode come close to the photosensitive drum at the more downstream side in the rotating direction of the photosensitive drum than the plane including the wire electrode and the rotational center portion of the photosensitive drum. Accordingly, an end portion, which faces the photosensitive drum, of the shield electrode of the charger, which shield electrode is positioned at the downstream side in the rotating direction of the photosensitive drum, can be made to be distant from an optical axis of the exposing light. Thereby, it is possible to secure a space between the charger and the optical axis of the exposing light, and thus it is possible to closely arrange the charger to the optical axis so as to make a diameter of the photosensitive drum smaller. As a result, the size of the image forming apparatus can be further reduced. 
     In addition, the grid electrode is inclined such that the downstream side thereof in the rotating direction of the photosensitive drum is more close to the photosensitive drum than the plane including the wire electrode and the rotational center portion of the photosensitive drum. Accordingly, a distance between the surface of the photosensitive drum and the grid electrode (grid surface) is narrower at the downstream side in the rotating direction of the photosensitive drum and is wider at the upstream side in the rotating direction. Therefore, it is possible to reduce an amount of ions (ionized air by the corona discharge) flowing toward the downstream side (toward the exposure position) in the rotating direction of the photosensitive drum. According thereto, it is possible to suppress the surface of the photosensitive drum from being re-charged after the exposure. As a result, it is possible to form a favorable image. 
     According to the invention, the grid electrode is inclined to the orthogonal plane to the plane including the wire electrode and the rotational center portion of the photosensitive drum so that the grid electrode and the shield electrode come close to the photosensitive drum at the downstream side in the rotating direction of the photosensitive drum. Accordingly, it is possible to make the diameter of the photosensitive drum smaller, so that it is possible to further reduce the size of the image forming apparatus. 
     &lt;Exemplary Embodiments&gt; 
     Hereinafter, exemplary embodiments of the invention will now be described with reference to the drawings. In the following descriptions, a configuration of an image forming apparatus  1  will be briefly described and then the characteristics of the invention will be described. Incidentally, a color printer is one example of the image forming apparatus  1 . 
     Also, in the following descriptions, the directions are described on the basis of a user who uses the image forming apparatus  1 . That is, the left side of  FIG. 1  is referred to as the ‘front side’, the right side is referred to as the ‘rear side’, the front side is referred to as the ‘right side’ and the inner side is referred to as the ‘left side.’ Also, the upper-lower direction of  FIG. 1  are referred to as the ‘upper-lower’ direction. 
     (Overall Configuration of Image Forming Apparatus) 
     As shown in  FIG. 1 , the image forming apparatus  1  includes, in a body casing  10 , a feeder unit  20  that feeds sheets S, an image forming unit  30  that forms an image on the fed sheet S and a sheet discharge unit  90  that discharges the sheet S having an image formed thereon. 
     An upper cover  12  is provided to an upper side of the body casing  10 . The upper cover  12  is configured to be rotatable (opened and closed) about the rear side serving as a support point. A sheet discharge tray  13 , on which the sheet S discharged from the body casing  10  is put, is provided on an upper surface of the upper cover  12 . Four holding members  40  that hold LED units  40  are provided on a lower surface of the upper cover. 
     The feeder unit  20  is provided at the lower part in the body casing  20 . The feeder unit  20  includes a feeder tray  21 , which receives therein the sheets S, and a sheet feeding mechanism  22 , which feeds the sheet S from the feeder tray  21  to the image forming unit  30 . The sheets S in the feeder tray  21  are separated one at a time by the sheet feeding mechanism  22  and are then supplied to the image forming unit  30 . 
     The image forming unit  30  includes four LED units  40  that are an example of an exposure device, four process units  50 , a transfer unit  70  and a fixing unit  80 . 
     The LED unit  40  is opposed to the upper part of a photosensitive drum  52  at a closed state of the upper cover  12 . The LED unit  40  is configured to expose a surface of the photosensitive drum  52 . The LED unit  40  is held at the upper cover  12  by the holding member  14 , so that the LED unit  40  is spaced from the photosensitive drum  52  as the upper cover  12  is opened. The LED unit  40  will be specifically described later. 
     The process units  50  are arranged in parallel with each other in the front-rear direction between the upper cover  12  and the feeder unit  20 . The process units  50  are detachably mounted to the body casing  10  at an opened state of the upper cover  12 . The process unit  50  includes a process frame  51 , the photosensitive drum  52 , a charger  100 , a developing device  54  and a cleaning roller  55 . 
     The process frame  51  rotatably supports the photosensitive drum  52  and the cleaning roller  55 . Further, the process frame  51  supports a wire electrode  110 , a grid electrode  120  and shield electrodes  131 ,  132  of the charger  100 , which will be described later. 
     The photosensitive drum  52  has a photosensitive layer formed on a surface (outer peripheral surface) of a cylindrical drum body having conductivity. The photosensitive drum  52  is configured to rotate in a counterclockwise direction (arrow direction) of  FIG. 1 . 
     The charger  100  is a scorotron-type charger that charges the surface of the photosensitive drum  52 . The charger  100  is arranged obliquely upper-rear side of the photosensitive drum  52  and faces the photosensitive drum  52 . 
     The developing device  54  is detachably mounted to the process frame  51 . The developing device  54  is arranged obliquely upwards to the front of the photosensitive drum  52  at the mounted state to the process frame  51 . The developing device  54  includes a developing roller  54 A that is an example of a developer carrier to supply toner (developer) to the photosensitive drum  52 , a supply roller  54 B, a layer thickness regulation blade  54 C and a toner accommodating part  54 D that accommodates the toner therein. 
     The cleaning roller  55  collects foreign substances such as paper dusts and dusts adhered on the surface of the photosensitive drum  52 , the toner remained on the surface of the photosensitive drum  52  and the like. The cleaning roller  55  is arranged to contact the rear part of the photosensitive drum  52 . 
     The transfer unit  70  is provided between the feeder unit  20  and the process unit  50 . The transfer unit  70  includes a driving roller  71 , a driven roller  72  and an endless conveyance belt  73  extending between the driving roller  71  and the driven roller  72 . The conveyance belt  73  contacts an outer surface of the respective photosensitive drums. At the inner side of the conveyance belt, the respective transfer rollers  74  are arranged to interpose the conveyance belt  73  between the respective transfer rollers and the respective photosensitive drums  52 . The fixing unit  80  is provided at the rear part of the process unit  50  and the transfer unit  70 . The fixing unit  80  includes a heating roller  81  and a pressing roller  82 , which is opposed to the heating roller  81  and presses the heating roller  81 . 
     In the image forming unit  30 , the surface of the photosensitive drum  52  is uniformly charged by the charger  100  and is then exposed by the LED unit  40 , so that an electrostatic latent image is formed on the photosensitive drum  52  based on image data. In addition, the toner in the toner accommodating part  54 D is supplied to the developing roller  54 A through the supply roller  54 B, is introduced between the developing roller  54 A and the layer thickness regulation blade  54 C and is then carried on the developing roller  54 A as a thin layer having a predetermined thickness. 
     The toner carried on the developing roller  54 A is supplied to the photosensitive drum  52 , so that the electrostatic latent image is visualized and a toner image is thus formed on the photosensitive drum  52 . After that, as the sheet S fed from the feeder unit  20  is conveyed between the photosensitive drums  52  and the conveyance belt  73  (transfer rollers  74 ), the toner images formed on the respective photosensitive drums  52  are sequentially overlapped and transferred on the sheet S. The sheet S, to which the toner images are transferred, is conveyed between the heating roller  81  and the pressing roller  82 , so that the toner images are heated and fixed. 
     The sheet discharge unit  90  includes a sheet discharge path  91 , which guides the sheet S conveyed from the fixing unit  80 , and a plurality of conveyance rollers  92  that conveys the sheet S. The sheet S, to which the toner images are heated and fixed, is conveyed through the sheet discharge path  91  by the conveyance rollers  92  and is discharged to the outside of the body casing  10  so as to be put on the sheet discharge tray  13 . 
     (Configuration Around Photosensitive Drum) 
     In the followings, detailed configuration around the photosensitive drum  52  will be described, based on the configuration and arrangement of the LED unit  40  and the charger  100 . 
     As shown in  FIG. 2 , the LED unit  40  includes an LED head  41 , which is an example of an exposure head, and a support frame  42 . The LED unit  40 , more specifically, the LED head  41  is arranged to face the surface of the photosensitive drum  52  between the developing roller  54 A (developing device  54 ) and the charger  100 . 
     The LED head  41  extends in the left-right direction (axial direction of the photosensitive drum  52 ). A lower end (leading end) of the LED head  41  is arranged adjacent to the surface of the photosensitive drum  52  at the closed state of the upper cover  12 . A plurality of light emitting parts (LEDs) (not shown) is arranged at the leading end of the LED head  41  in the left-right direction. In the invention, the light emitting parts may be linearly arranged or zigzag-arranged in the left-right direction. In addition, the row of the light emitting parts may be one or two or more rows of the light emitting parts may be provided side by side in the front-rear direction. The support frame  42  supports the LED head  41  to the upper cover  12 . An upper part of the support frame  42  is mounted to the upper cover  12  through the holding member  14  while a lower part thereof supports the LED unit  41 . 
     The LED unit  40  turns on and off the light emitting parts on the basis of the image data so as to expose the surface of the charged photosensitive drum  52 . In the following descriptions, a part of the surface of the photosensitive drum  52 , which is exposed by the LED unit  40 , is referred to as an exposure position PE. In this exemplary embodiment, an intersection point (intersection line) of the surface of the photosensitive drum  52  and an optical axis of the exposing light emitted from the LED head  41 , when seen from the left-right direction, is the exposure position PE. 
     Incidentally, when the light emitting parts of the LED head  41  are zigzag-arranged or two or more rows of the light emitting parts are arranged side by side, a plurality of optical axes is formed when seen from the left-right direction. In this case, regarding the circumferential direction of the photosensitive drum  52 , a middle position between an intersection of the surface of the photosensitive drum  52  and the optical axis at the most upstream side in the rotating direction of the photosensitive drum  52  and an intersection of the surface of the photosensitive drum  52  and the optical axis at the most downstream side in the rotating direction is the exposure position. 
     The charger  100  includes the wire electrode  110 , the grid electrode  120  and a pair of shield electrodes  131 ,  132 . The charger  100  is arranged obliquely upper-rear side of the photosensitive drum  52  and faces the photosensitive drum  52  at a predetermined distance from the surface of the photosensitive drum  52 . 
     The wire electrode  110  is a metal wire that generates the corona discharge as a voltage for exposing the photosensitive drum  52  is applied. The wire electrode  110  is installed in parallel with the rotational axis (rotational center portion C) of the photosensitive drum  52  and extends in the left-right direction between the pair of shield electrodes  131 ,  132 . 
     The grid electrode  120  is a metal plate member that is set with a potential (including zero potential) different from that of the wire electrode  110  so as to control an amount of ions reaching the surface of the photosensitive drum  52 . The grid electrode  120  has a planar grid surface  121  that is opposed to the photosensitive drum  52  between the wire electrode  110  and the photosensitive drum  52 . The grid surface  121  extends in the left-right direction. The grid surface  121  includes a plurality of grid holes  122  for enabling the ions generated by the corona discharge of the wire electrode  110  to pass therethrough. 
     The shield electrodes  131 ,  132  are metal plate members that are arranged to face each other with the wire electrode  110  being interposed therebetween, when seen from the left-right direction, and extend in a substantially orthogonal direction to the grid electrode  120  (grid surface  121 ). The shield electrodes  131 ,  132  have the substantially long plate shapes extending along the rotational center portion C (the left-right direction) of the photosensitive drum  52  and are arranged in parallel with each other. Incidentally, in this exemplary embodiment, the shield electrodes  131 ,  132  are integrated with the grid electrode  120 . However, the invention is not limited thereto. For example, the shield electrodes may be parts different from the grid electrode  120 . 
     In the following descriptions, one of the shield electrodes  131 ,  132 , which is arranged at the downstream side (front side) of the photosensitive drum  52  in the rotating direction, is referred to as a front-side shield electrode  131 , and another one of the shield electrodes  131 ,  132 , which is arranged at the upstream side (back side) in the rotating direction, is referred to as a back-side shield electrode  132 . 
     In this exemplary embodiment, the charger  100 , more specifically, the grid surface  121  is inclined to an orthogonal plane PL 2  of a plane PL 1  including the wire electrode  110  and the rotational center portion C of the photosensitive drum  52  so that the grid electrode  120  and the front-side shield electrode  131  are close to the photosensitive drum  52  at a front side than the plane PL 1 . 
     The charger  100  is arranged as described above. Thereby, the configuration of this exemplary embodiment shown in  FIG. 3B  enables a lower end portion  133  of the front-side shield electrode  131  to be more distant from the LED unit  40  (optical axis of the exposing light (refer to a plane PL 3 )), compared to a configuration shown in  FIG. 3A  in which a grid surface  121 ′ of a charger  100 ′ is arranged along the orthogonal plane PL 2 . According to this configuration, it is possible to secure a space between the charger  100  and the LED unit  40 , and thus it is possible to closely arrange the charger  100  and the LED unit  40 , as shown in  FIG. 3B . Therefore, it is possible to make a diameter of the photosensitive drum  52  smaller, so that it is possible to further reduce the size of the image forming apparatus  1 . 
     Incidentally, as shown in  FIG. 2 , an angle θ 1  between the grid surface  121  and the orthogonal plane PL 2  is preferably equal to or more than 5 degrees and equal to or less than 15 degrees. According to this configuration, it is possible to favorably charge the surface of the photosensitive drum  52  while inclining the charger  100  (grid surface  121 ). 
     In addition, an angle θ 2  between the plane PL 1  and the plane PL 3  including the rotational center portion C of the photosensitive drum  52  and the exposure position PE is preferably equal to or less than 30 degrees. According to this configuration, since the charger  100  and the LED unit  40  come closer to each other, it is possible to make the diameter of the photosensitive drum  52  much smaller. As a result, it is possible to further reduce the size of the image forming apparatus  1 . 
     Furthermore, the diameter D 1  of the photosensitive drum  52  is preferably equal to or less than 18 mm. At this time, a distance D 2  between the shield electrodes  131 ,  132  is preferably equal to or more than 7 mm. According to this configuration, since the spark can be prevented from being occurred, it is possible to stabilize the corona discharge. 
     In this exemplary embodiment, the grid electrode  120  is inclined so that the front side thereof than the plane PL 1  is close to the photosensitive drum  52 . Accordingly, a distance between the surface of the photosensitive drum  52  and the grid surface  121  is narrower at the front side than the plane PL 1  and is wider at the back side than the plane PL 1 . Therefore, the configuration of this exemplary embodiment shown in  FIG. 3B  can reduce the amount of ions flowing toward the LED head  41  (refer to the arrows), compared to the configuration shown in  FIG. 3A . Thereby, it is possible to suppress the surface of the photosensitive drum  52  from being re-charged after the exposure. Hence, it is possible to make a difference of potentials of the exposed part and the non-exposed part greater, so that it is possible to form a favorable image. 
     The invention configured as described above is particularly useful in the configuration in which the exposure device is arranged to face the photosensitive drum  52  between the developing roller  54 A and the charger  100 , as the image forming apparatus  1 . That is, according to such configuration, it is not easy to make the diameter of the photosensitive drum  52  smaller, compared to a configuration in which the laser light is illuminated from an exposure device arranged above a plurality of process units to expose the photosensitive drums. Accordingly, by applying the invention to the image forming apparatus having the configuration such as the image forming apparatus  1 , it is possible to make the diameter of the photosensitive drum smaller and to thus further reduce the size of the image forming apparatus  1 . 
     Although the exemplary embodiment of the invention has been described, the invention is not limited thereto. The specific configuration can be appropriately changed without departing from the scope of the invention. 
     In the above-described exemplary embodiment, the front-side shield electrode  131  faces the LED unit  40 . However, the invention is not limited thereto. For example, a frame wall of the charger  100  supporting the shield electrodes  131 ,  132  and the grid electrode  120  may be provided between the front-side shield electrode  131  and the LED unit  40 . Incidentally, when the invention is applied to such a configuration, the frame wall is also inclined to the orthogonal plane PL 2  together with the shield electrodes  131 ,  132  and the grid electrode  120 . 
     In the above-described exemplary embodiment, in the process unit  50 , the developing device  54  (developing roller  54 A) is detachably mounted to the photosensitive drum  52 . However, the invention is not limited thereto. For example, the process unit may have a configuration in which the photosensitive drum and the developing roller (developer carrier) are integrated (i.e., the detachable mounting is impossible). 
     In the above-described exemplary embodiment, the LED head  41  having the plurality of LEDs (light emitting parts) arranged thereto has been exemplified as the exposure head that is arranged adjacent to the surface of the photosensitive drum  52 . However, the invention is not limited thereto. For example, an EL device, a fluorescent member and the like may be adopted as the light emitting part. In addition, the exposure head may be configured so that an optical shutter such as liquid crystal device, PLZT device and the like is arranged at an output side of a backlight such as fluorescent lamp, LED and the like. 
     In the above-described exemplary embodiment, the LED unit  40  has been exemplified as the exposure device that is arranged to face the photosensitive drum  52  between the developing roller  54 A (developer carrier) and the charger  100 . However, the invention is not limited thereto. For example, the exposure device that is arranged to face the photosensitive drum between the developer carrier and the charger may be a laser scanner that is provided in correspondence to each photosensitive drum and scans the laser light on the surface of the photosensitive drum at high speed and thus exposes the surface of the photosensitive drum after the charge. 
     In addition, the exposure device is not limited to the configuration in which it is arranged to face the photosensitive drum between the developer carrier and the charger. For example, one exposure device (laser scanner and the like) may be arranged above the plurality of processing units (photosensitive drums). 
     The image forming apparatus  1  may not be limited to the color printer. For example, the image forming apparatus  1  may be a printer that forms a black-white image. Alternatively, the image forming apparatus  1  may be a copier or multi-function device.