Patent Publication Number: US-7917052-B2

Title: Charging device and an image forming apparatus provided with the charging device

Description:
This application is based on Japanese Patent Application No. 2009-156856 filed on Jul. 1, 2009, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Filed of the Invention 
     The present invention relates to a charging device and an image forming apparatus provided with the charging device, and more particularly to a charging device for charging an image bearing member and an image forming apparatus provided with the charging device. 
     2. Description of Related Art 
     An example of conventional charging devices is a corona charging device as disclosed by Japanese Patent Laid-Open Publication No. 2002-268342 (Reference 1). In the corona charging device, a corona wire is used. A high-voltage source is connected to the corona wire, and thereby, a discharge from the corona wire occurs. 
     Regarding such a corona charging device, as the discharge occurs again and again, silicon oxide and other substances adhere to the corona wire, and corona products, which are called as needles, are formed on the corona wire. Due to the corona products, the discharge from the corona wire becomes uneven, thereby causing a fault in charging. This is a cause of image noise. 
     In order to solve this problem, the corona charging device disclosed by Reference 1 has a cleaning assembly including a cylindrical grinding stone made of aluminum oxide. By a slide of the cylindrical grinding stone on the corona wire, the corona products deposited on the corona wire are removed. 
     In recent years, charging devices of a type that has a sheet electrode with aligned triangular pins are developed for practical use. This type of charging devices having a sheet electrode has the advantage over the charging device disclosed by Reference 1 of generating less ozone. However, the type of charging devices having a sheet electrode has the same problem as the charging device disclosed by Reference 1 in that corona products are generated. Therefore, also in this type of charging devices, it is necessary to clean the sheet electrode regularly. 
     The type of charging devices having a sheet electrode has also a problem that the pins are fragile. More specifically, an exemplary way of cleaning the sheet electrode is touching a grinding stone made of aluminum oxide as disclosed by Reference 1 to main surfaces of the sheet electrode; however, because the tips of the pins of the sheet electrode are sharp, the pins are relatively fragile. In carrying out this way of cleaning, therefore, the tips of the pins of the sheet electrode may be bent and/or cracked at a touch of the grinding stone. Then, the bent/cracked tips of the pins of the sheet electrode will cause a poor discharge, which results in degradation of picture quality. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a charging device wherein breaks of triangular pins of a sheet electrode can be prevented and an image forming apparatus provided with the charging device. 
     A charging device according to an embodiment of the present invention comprises: a stainless steel sheet electrode for charging an image bearing member, the stainless steel sheet electrode having a thickness within a range from 50 μm to 60 μm and comprising aligned triangular pins, each of the triangular pins having a vertex angle within a range from 10 degrees to 30 degrees; and a cleaner for cleaning the stainless steel sheet electrode, the cleaner having two grinding members comprising abrasive grains having an average diameter within a range from 2 μm to 9 μm, the two grinding members being in contact with, respectively, both main surfaces of the sheet electrode, wherein the cleaner and the sheet electrode are moved relative to each other at a constant speed by a force equal to or less than 2N. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a skeleton framework of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a skeleton framework of a charging device; 
         FIG. 3  is a configuration diagram of a sheet electrode provided in the charging device; 
         FIGS. 4   a ,  4   b  and  4   c  are configuration diagrams of the charging device; 
         FIG. 5  is a perspective view of a cleaner unit; 
         FIGS. 6   a  and  6   b  show a grinding sheet,  FIG. 6   a  being a sectional view and  FIG. 6   b  being a microgram; and 
         FIG. 7  is a graph showing the results of a second experiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A charging device according to an embodiment of the present invention and an image forming apparatus provided with the charging device are hereinafter described with reference to the drawings. 
     General Structure of the Image Forming Apparatus 
     First, the general structure of an image forming apparatus according to an embodiment of the present invention is described.  FIG. 1  shows the general structure of an image forming apparatus  100  according to an embodiment of the present invention. The image forming apparatus  100  according to this embodiment is a monochromatic or a color copying machine, printer, facsimile or a machine having these functions. 
     The image forming apparatus  100  comprises a photosensitive drum  1 , a charging device  10 , an optical scanning device  31 , a developing device  32 , a transfer roller  33 , a cleaning device  34 , an eraser lamp  35  and a fixing device  36 . The photosensitive drum  1  is cylindrical and is driven by a motor (not shown) to rotate in a direction “A”. An electrostatic latent image is formed on the surface of the photosensitive drum  1 , and toner is applied to the surface thereof. Thus, the photosensitive drum  1  serves as an image bearing member for bearing a toner image in accordance with the electrostatic latent image. 
     The charging device  10  charges the surface of the photosensitive drum  1  evenly to a specified level. The optical scanning device  31  scans the surface of the photosensitive drum  1  with a beam modulated in accordance with image data and forms an electrostatic latent image on the surface of the photosensitive drum  1 . The developing device  32  supplies toner onto the surface of the photosensitive drum  1 , so that the electrostatic latent image is developed (visualized) into a toner image. The transfer roller  33  transfers the toner image formed on the surface of the photosensitive drum  1  to a sheet S traveling between the transfer roller  33  and the photosensitive drum  1 . The fixing device  36  performs a heat/pressure treatment toward the sheet S so as to fix the toner on the sheet S. 
     The cleaning device  34  collects residual toner from the surface of the photosensitive drum  1 . The eraser lamp  35  erases residual charge from the surface of the photosensitive drum  1 . 
     Structure of the Charging Device 
     Next, the structure of the charging device  10  is described.  FIG. 2  shows the general structure of the charging device  10 .  FIG. 3  shows the structure of a sheet electrode  13  provided in the charging device  10 .  FIGS. 4   a ,  4   b  and  4   c  show the structure of the charging device  10 .  FIG. 4   a  is a plan view,  FIG. 4   b  is a front view, and  FIG. 4   c  is a bottom view.  FIG. 5  is a perspective view of a cleaner unit  20 . In the following paragraphs, the lengthwise direction of the charging device  10  (that is, main-scanning direction) is referred to as x direction, and the rotating direction of the photosensitive drum  1  (that is, sub-scanning direction) is referred to as y direction. The direction perpendicular to the x direction and the y direction is defined to be z direction. 
     As shown by  FIGS. 2 and 4 , the charging device  10  comprises stabilizing plates  11   a  and  11   b , a mesh-type grid  12 , a sheet electrode  13 , holders  14   a  and  14   b , a shaft  15 , a support  17  and a cleaner unit  20 . 
     The stabilizing plates  11   a  and  11   b  have lengths in the x direction, each having an L-shape cross section. More specifically, as shown by  FIG. 2 , the stabilizing plate  11   a  also has a dimension in the z direction, and the end portion of the plate  11   a  at the negative side in the z direction is bent to the negative side in the y direction. Also, the stabilizing plate  11   b  has a dimension in the z direction, and the end portion of the plate  11   b  at the negative side in the z direction is bent to the positive side in the y direction. As seen in the cross sectional view of  FIG. 2 , the stabilizing plates  11   a  and  11   b  are U-shape in combination. The stabilizing plates  11   a  and  11   b  that are combined into U-shape have an opening that faces to the photosensitive drum  1 . The mesh-type grid  12  is disposed at the opening of the stabilizing plates  11   a  and  11   b . As shown in  FIGS. 4   a ,  4   b  and  4   c , the holders  14   a  and  14   b  for holding the stabilizing plates  11   a ,  11   b  and the mesh-type grid  12  are disposed at both ends in the lengthwise direction of the stabilizing plates  11   a  and  11   b  (in the x direction). 
     The sheet electrode  13  is disposed in a space enclosed by the stabilizing plates  11   a ,  11   b  and the mesh-type grid  12  with its both ends held by the holders  14   a  and  14   b . The sheet electrode  13  charges the surface of the photosensitive drum  1 . In the following, the structure of the sheet electrode  13  is described in detail. 
     As shown by  FIG. 3 , on the sheet electrode  13 , a multiple of triangular pins  13   a  are aligned in the x direction. The sheet electrode  13  has a thickness within a range from 40 μm to 60 μm. Each of the pins  13   a  has a vertex angle θ within a range from 5 degrees to 30 degrees and has a height H within a range from 1 mm to 3 mm. The pins  13   a  are arranged at a pitch P within a range from 1 mm to 3 mm. These values are designed for an efficient discharge. The sheet electrode  13  is made of stainless steel. 
     A voltage within a range from −6 kV to −7 kV (900 μA) is applied to the sheet electrode  13 , and thereby, a corona discharge from the pins  13   a  to the photosensitive drum  1  occurs. Also, a voltage within a range from −300V to −900V is applied to the mesh-type grid  12 , and thereby, the charge potential applied to the photosensitive drum  1  can be adjusted to a desired value. 
     As shown by  FIG. 5 , the cleaner unit  20  comprises a frame  21 , resin plates  25   a  and  25   b , grinding sheets  26   a  and  26   b , and pressers  30   a  and  30   b . The frame  21  is a parallelepiped, and a rectangular through-hole is made to pierce through the frame  21  in the lengthwise direction of the sheet electrode  13  (in the x direction). In other words, the frame  21  does not have surfaces on the sides in the x direction. In the frame  21 , at the negative side in the z direction, grooves  23   a  and  23   b  are made to extend parallel to the x direction. As shown in  FIG. 2 , the end of the stabilizing plate  11   a  at the negative end in the y direction serves as a rail  11   c , and the end of the stabilizing plate  11   b  at the positive end in the y direction serves as a rail  11   d . The rails  11   c  and  11   d  engage with the grooves  23   a  and  23   b , respectively. In this way, the frame  21  is disposed to be capable of sliding relative to the stabilizing plates  11   a  and  11   b  in the lengthwise direction of the stabilizing plates  11   a  and  11   b  (in the x direction). 
     As shown in  FIG. 5 , the resin plates  25   a  and  25   b , the grinding sheets  26   a  and  26   b  and the pressers  30   a  and  30   b  are disposed in the through-hole of the frame  21 . More specifically, the presser  30   a  is stuck on the inner surface of the side of the frame  21  that is at the positive side in the y direction as shown in  FIG. 5 , and the presser  30   a  is made of an elastic material. The presser  30   b  is stuck on the inner surface of the side of the frame  21  that is at the negative side in the y direction as shown in  FIG. 5 , and the presser  30   b  is made of an elastic material. Various elastic materials can be used for the pressers  30   a  and  30   b . For example, urethane foam may be used as the material for the pressers  30   a  and  30   b . The use of urethane foam is advantageous in that urethane foam is a foamed material with a bare possibility of permanent deformation and in that urethane foam is unresolved by ozone. 
     The resin plate  25   a  is stuck on the side of the presser  30   a  that is at the negative side in the y direction as shown in  FIG. 5 , and the resin plate  25   a  is made of a material harder than urethane foam, for example, made of PET. The resin plate  25   b  is stuck on the side of the presser  30   b  that is at the positive side in the y direction as shown in  FIG. 5 , and the resin plate  25   b  is made of a material harder than urethane foam, for example, made of PET. The resin plates  25   a  and  25   b  have thicknesses within a range from 0.5 mm to 1.0 mm. 
     The grinding sheet  26   a  is stuck on the side of the resin plate  25   a  that is at the negative side in the y direction as shown in  FIG. 5 . The grinding sheet  26   b  is stuck on the side of the resin plate  25   b  that is at the positive side in the y direction as shown in  FIG. 5 . Accordingly, the grinding sheets  26   a  and  26   b  face to each other with a space in-between.  FIG. 6   a  is a sectional view of the grinding sheets  26   a  and  26   b , and  FIG. 6   b  is a micrograph of the grinding sheets  26   a  and  26   b . Each of the grinding sheets  26   a  and  26   b  comprises a PET film  50 , abrasive grains  52  and a binder  54 . The PET film  50  is a base sheet and has a thickness within a range from 5 μm to 75 μm. The average diameter of the abrasive grains  52  is within a range from 2 μm to 9 μm, and the abrasive grains  52  are scattered on the PET film  50 . For the abrasive grains  52 , for example, a metal oxide, such as aluminum oxide, chrome oxide and iron oxide, or silicone carbide is used. The binder  54  binds the abrasive grains  52  so that the abrasive grains  52  will not fall off the PET film  50 . As shown by  FIGS. 6   a  and  6   b , the abrasive grains  52  are bound by the binder  54  to stick together densely without spaces. Thus, the abrasive grains  52  and the binder  54  form an abrasive layer, and the abrasive layer has a thickness that is equal to or greater than 10 μm. As the grinding sheets  26   a  and  26   b , for example, wrapping film sheets made by 3M, namely, model A3-2SHT (average diameter: 2 μm), model A3-3SHT (average diameter: 3 μm), model A3-5SHT (average diameter: 5 μm) and model A3-9SHT (average diameter: 9 μm) may be used. The wrapping film sheets made by 3M comprise abrasive grains of aluminum oxide. 
     With the cleaner unit  20  of the structure above, as shown by  FIG. 5 , the sheet electrode  13  passes through the space between the grinding sheets  26   a  and  26   b . Meanwhile, the grinding sheets  26   a  and  26   b  are pressed by the elastic pressers  30   a  and  30   b , respectively, on the both sides of the sheet electrode  13 . 
     As shown in  FIG. 5 , the shaft  15  is disposed on the lower surface of the side of the frame  21  that is at the negative side in the z direction, and the shaft  15  extends parallel to the x direction toward the negative side. The support  17  is fitted on the side of the holder  14   b  that is at the negative side in the z direction, and a through-hole is made in the support  17 . The shaft  15  pierces through the through-hole of the shaft  17 . Thus, the support  17  supports the shaft  15 . In this structure, a user can reciprocate the cleaner unit  20  in the x direction by sliding the shaft  15  along the x direction. The force necessary to move the cleaner unit  20  relative to the sheet electrode  13  at a constant speed is preferably larger than 0N and smaller than 2.0N. In other words, it is preferred that a frictional force that is larger than 0N and smaller than 2.0N acts between the grinding sheet  26   a  and the sheet electrode  13  and between the grinding sheet  26   b  and the sheet electrode  13 . The space between the grinding sheets  26   a  and  26   b  is so designed as to generate the frictional force. In order to achieve the designed space between the grinding sheets  26   a  and  26   b , the thicknesses of the resin plates  25   a  and  25   b , the grinding sheets  26   a  and  26   b  and the pressers  30   a  and  30   b  are adjusted. 
     Additionally, in order to generate an appropriate frictional force, it is necessary that the grinding sheets  26   a  and  26   b  apply a pressure uniformly to the sheet electrode  13 . For this purpose, in the charging device  10 , it is preferred that each of the pressers  30   a  and  30   b  is composed of ten or more cells arranged in the area of 3 mm (depth A 1 ) by 6 mm (height A 2 ) shown in  FIG. 5 . In this case, the pressers  30   a  and  30   b  are preferably made of urethane foam having a density within a range from 15 kg/m 3  to 60 kg/m 3 . 
     With the cleaner unit  20  of the structure above, a user pushes the shaft  15  to the positive side in the x direction to move the cleaner unit  20  to the neighborhood of the holder  14   a . Thereafter, the user pulls the shaft  15  to the negative side in the x direction to move the cleaner unit  20  to the neighborhood of the holder  14   b . Thereby, the both sides of the sheet electrode  13  are ground by the grinding sheets  26   a  and  26   b , respectively. Consequently, corona products adhering to the sheet electrode  13  can be removed therefrom. 
     Advantages 
     In the charging device  10 , each of the pins  13   a  of the sheet electrode  13  has a vertex angle θ within a range from 5 degrees to 30 degrees, and the sheet electrode  13  has a thickness within a range from 40 μm to 60 μm. The abrasive grains  52  of the grinding sheets  26   a  and  26   b  have an average diameter within a range from 2 μm to 9 μm. Further, a force larger than 0N and smaller than 2.0N starts a uniform motion of the cleaner unit  20  and the sheet electrode  13  relative to each other. Due to this structure of the charging device  10 , breaks of the triangular pins  13   a  of the sheet electrode  13  can be prevented. In the following, the advantages of the charging device  10  will be described with reference to results of experiments. 
     A first experiment is described. In the first experiment, in order to find out the conditions for preventing bends and/or abrasions of the pins  13   a , the inventors fabricated various samples of the charging device  10 . Then, the inventors operated the cleaner unit  20  in each of the samples and thereafter examined the pins  13   a  whether there were any bends/cracks or abrasions. More specifically, the inventors fabricated the first to the twenty-sixth samples shown by Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Abrasive 
                 Average 
                 Frictional 
               
               
                   
                 Sample No. 
                 Grains 
                 Diameter (μm) 
                 Force (N) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1 
                 Aluminum 
                 1 
                 1.5 
               
               
                   
                 2 
                 Oxide 
                   
                 2 
               
               
                   
                 3 
                   
                   
                 2.5 
               
               
                   
                 4 
                   
                 2 
                 0.5 
               
               
                   
                 5 
                   
                   
                 1 
               
               
                   
                 6 
                   
                   
                 1.5 
               
               
                   
                 7 
                   
                   
                 2 
               
               
                   
                 8 
                   
                   
                 2.5 
               
               
                   
                 9 
                   
                 3 
                 0.5 
               
               
                   
                 10 
                   
                   
                 1 
               
               
                   
                 11 
                   
                   
                 1.5 
               
               
                   
                 12 
                   
                   
                 2 
               
               
                   
                 13 
                   
                   
                 2.5 
               
               
                   
                 14 
                   
                 5 
                 0.5 
               
               
                   
                 15 
                   
                   
                 1 
               
               
                   
                 16 
                   
                   
                 1.5 
               
               
                   
                 17 
                   
                   
                 2 
               
               
                   
                 18 
                   
                   
                 2.5 
               
               
                   
                 19 
                   
                 8 
                 1.5 
               
               
                   
                 20 
                   
                   
                 2 
               
               
                   
                 21 
                   
                   
                 2.5 
               
               
                   
                 22 
                   
                 9 
                 0.5 
               
               
                   
                 23 
                   
                   
                 1 
               
               
                   
                 24 
                   
                   
                 1.5 
               
               
                   
                 25 
                   
                   
                 2 
               
               
                   
                 26 
                   
                   
                 2.5 
               
               
                   
                   
               
            
           
         
       
     
     The values listed as the frictional force in Table 1 were obtained by connecting the sheet electrode  13  to a push-pull gauge and by reading the scale of the push-pull gauge when the sheet electrode  13  was pulled while the cleaner unit  20  was fixed. The other conditions for the experiment were as follows. 
     The thickness of the sheet electrode  13  was 50 μm; the pitch P of the pins  13   a  was 1 mm; the height H of the pins  13   a  was 2 mm; the vertex angle θ of the pins  13   a  was 10 degrees; the thickness of the resin plates  25   a  and  25   b  was 75 μm; the thickness of the grinding layers of the grinding sheets  26   a  and  26   b  was 20 μm; and the thickness of the PET film  50  was 75 μm. 
     In each of the first to the twenty-sixth samples above, the cleaner unit  20  was reciprocated twenty times, and thereafter, the pins  13   a  were examined whether there were any bends/cracks or abrasions. Table 2 shows the results of the experiment. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Sample No. 
                 Bends/Cracks 
                 Abrasions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 X 
                 ◯ 
               
               
                 2 
                 X 
                 ◯ 
               
               
                 3 
                 X 
                 ◯ 
               
               
                 4 
                 — 
                 — 
               
               
                 5 
                 — 
                 — 
               
               
                 6 
                 Δ 
                 ◯ 
               
               
                 7 
                 Δ 
                 ◯ 
               
               
                 8 
                 X 
                 ◯ 
               
               
                 9 
                 ◯ 
                 ◯ 
               
               
                 10 
                 ◯ 
                 ◯ 
               
               
                 11 
                 ◯ 
                 ◯ 
               
               
                 12 
                 ◯ 
                 ◯ 
               
               
                 13 
                 Δ 
                 ◯ 
               
               
                 14 
                 ◯ 
                 ◯ 
               
               
                 15 
                 ◯ 
                 ◯ 
               
               
                 16 
                 ◯ 
                 ◯ 
               
               
                 17 
                 ◯ 
                 ◯ 
               
               
                 18 
                 Δ 
                 ◯ 
               
               
                 19 
                 ◯ 
                 ◯ 
               
               
                 20 
                 ◯ 
                 ◯ 
               
               
                 21 
                 Δ 
                 Δ 
               
               
                 22 
                 — 
                 — 
               
               
                 23 
                 — 
                 — 
               
               
                 24 
                 ◯ 
                 ◯ 
               
               
                 25 
                 ◯ 
                 Δ 
               
               
                 26 
                 Δ 
                 X 
               
               
                   
               
            
           
         
       
     
     In Table 2, a circle in the column of “Bends/Cracks” means that neither bends nor cracks occurred to the pins  13   a . A triangle in the column of “Bends/Cracks” means that although some bends and/or cracks occurred to the pins  13   a , the bends/cracks were in a small degree not to cause a problem. A cross in the column of “Bends/Cracks” means that some bends and/or cracks in such a degree to cause a problem occurred to the pins  13   a . Here, to “cause a problem” means to cause image noise in forming an image. A dash in the column of “Bends/Cracks” means that the sample was not subjected to the experiment. 
     In Table 2, also, a circle in the column of “Abrasions” means that no abrasions occurred to the pins  13   a . A triangle in the column of “Abrasions” means that although some abrasions occurred to the pins  13   a , the abrasions were in a small degree not to cause a problem. A cross in the column of “Abrasions” means that some abrasions in such a degree to cause a problem occurred to the pins  13   a . Here, to “cause a problem” means to cause image noise in forming an image. A dash in the column of “Abrasions” means that the sample was not subjected to the experiment. 
     Referring to Table 1 and Table 2, in the samples wherein the frictional force was equal to or less than 2N and the average diameter of the abrasive grains  52  was within a range from 2 μm to 9 μm (in the sixth and the seventh samples, the ninth to the twelfth samples, the fourteenth to the seventeenth samples, the nineteenth and the twentieth samples, and the twenty-fourth and the twenty-fifth samples), neither bends/cracks nor abrasions in such a degree to cause a problem occurred to the pins  13   a . As a result of the first experiment, it was found out that when the sheet electrode  13  has the following specifications: the thickness of the electrode  13  is 50 μm the vertex angle θ of the pins  13   a  is 10 degrees; the pitch P of the pins  13   a  is 1 mm; and the height H of the pins  13   a  is 2 mm, it is possible to prevent bends/cracks and abrasions of the pins  13   a  by setting the frictional force to or less than 2N and by using abrasive grains with an average diameter within a range from 2 μm to 9 μm. 
     In the first experiment, the sheet electrode  13  was made to have a thickness of 50 μm. If the sheet electrode  13  is thicker, the sheet electrode  13  will be less liable to bend and/or crack. Therefore, the thickness of the sheet electrode  13  shall be equal to or greater than 50 μm. Further, for the sake of an efficient corona discharge, as mentioned above, the thickness of the sheet electrode  13  is preferably within a range from 40 μm to 60 μm. In the charging device  10 , therefore, the thickness of the sheet electrode  13  is preferably within a range from 50 μm to 60 μm. 
     In the first experiment, the vertex angle θ of the pins  13   a  was ten degrees. If the vertex angle θ of the pins  13   a  is larger, the pins  13   a  will be stronger. Accordingly, if the vertex angle θ of the pins  13   a  is larger, the pins  13   a  will be less liable to bend and/or crack. Therefore, the vertex angle θ of the pins  13   a  of the sheet electrode  13  shall be equal to or greater than 10 degrees. Further, for the sake of an efficient corona discharge, as mentioned above, the vertex angle θ of the pins  13   a  of the sheet electrode  13  is preferably within a range from 5 degrees to 30 degrees. In the charging device  10 , therefore, the vertex angle θ of the pins  13   a  of the sheet electrode  13  is preferably within a range from 10 degrees to 30 degrees. 
     The possibility that bends/cracks will occur to the pins  13   a  is hardly influenced by the height H of the pins  13   a . In the first experiment, the bends/cracks of the pins  13   a  occurred in areas within 30 μm from the respective tips of the pins  13   a . When the height H of the pins  13   a  is equal to or greater than 30 μm, the possibility that bends/cracks will occur to the pins  13   a  does not depend on the height H and depends on other conditions. 
     The pins  13   a  are bent/cracked and/or are abraded by contact with the grinding sheets  26   a  and  26   b . The possibility that bends/cracks and/or abrasions will occur to the pins  13   a  does not depend on the pitch of the pins  13   a  and depends on other conditions. 
     Referring to Table 1 and Table 2, in the samples wherein the frictional force was equal to or less than 2N and the average diameter of the abrasive grains  52  was within a range from 3 μm to 8 μm (in the ninth to the twelfth samples, the fourteenth to the seventeenth samples, and the nineteenth and the twentieth samples), neither bends/cracks nor abrasions occurred to the pins  13   a . Therefore, the frictional force is preferably equal to or less than 2N, and the average diameter of the abrasive grains  52  is preferably within a range from 3 μm to 8 μm. 
     Next, a second experiment is described. The second experiment was conducted to certify that the grinding sheets  26   a  and  26   b  are highly effective in cleaning the sheet electrode  13 . In the second experiment, the sixteenth sample was used as a sample of the charging device  10 , and a twenty-seventh sample was fabricated as a comparative example. The twenty-seventh sample was different from the sixteenth sample in that two pieces of pile fabric were used instead of the grinding sheets  26   a  and  26   b . In each of the sixteenth sample and the twenty-seventh sample, a discharge was continued for 100 hours, and thereafter, the sheet electrode  13  was cleaned by the cleaner unit  20 . 
       FIG. 7  is a graph showing the results of the second experiment. The y axis shows the rank of image, and the x axis shows the number of reciprocations of the cleaner unit  20 . The rank of image is determined by the width of a black stripe (image noise) that occurred on an image. More specifically, the rank  5  means that the width of a black stripe on an image was 0 mm. The rank  4  means that the width of a black stripe on an image was 1 mm. The rank  3  means that the width of a black stripe on an image was 1.5 mm. The rank  2  means that the width of a black stripe on an image was 2.5 mm. The rank  1  means that the width of a black stripe on an image was 4 mm. Images of rank  3  or more are good. 
     As shown by  FIG. 7 , in the twenty-seventh sample, one reciprocation of the cleaner unit  20  did not improve the rank of image, but in the sixteenth sample, one reciprocation of the cleaner unit  20  greatly improved the rank of image. Also, in the twenty-seventh sample, repetitious reciprocations of the cleaner unit  20  resulted in only rank  3 , but in the sixteenth sample, repetitious reciprocations of the cleaner unit  20  resulted in rank  5 . Thus, the use of the grinding sheets  26   a  and  26   b  is more effective in cleaning the sheet electrode  13 , compared with the case of using pile fabric. The reason is that while the pile fabric merely wipes corona products, the grinding sheets  26   a  and  26   b  remove corona products from the sheet electrode  13  by grinding the sheet electrode  13  with abrasive grains. 
     Thus, in the charging device according to this embodiment, the cleaner unit  20  cleans the sheet electrode  13  effectively without breaking the triangular pins  13   a  of the sheet electrode  13 . 
     Although the present invention has been described in connection with the embodiment above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the invention.