Patent Publication Number: US-7212771-B2

Title: Grid electrode, scorotron charger, and image forming device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 USC 119 from Japanese Patent Application Nos. 2004-136776 and 2005-118757, the disclosures of which are incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a grid electrode at which an opening pattern is formed, a scorotron charger, and an image forming device. 
   2. Description of the Related Art 
   A device utilizing corona discharge is generally used as a charger of an image forming device. Corotron chargers and scorotron chargers are known as corona discharge devices. In a corotron charger, a wire electrode spans the interior of a shield case, substantially parallel to a photosensitive body. High voltage is applied to the wire electrode such that corona discharge is caused, and charges are applied to the surface of the photosensitive body. A corotron charger has the drawback that it is easy for fluctuations to arise in the potential of the surface of the photosensitive body due to the accuracy of the charger, errors in setting, and the like. Scorotron chargers, which are advantageous in that they can charge to a uniform potential, have been used recently. 
   The main portion of a scorotron charger is structured by a discharge electrode which supplies charges to a photosensitive body and charges the photosensitive body, a grid electrode disposed between the discharge electrode and the photosensitive body and controlling the potential of the photosensitive body, and a shield case which is supported so as to cover the discharge electrode. An opening pattern is formed at the grid electrode in order to carry out potential control well. The charge potential of the surface of the photosensitive body can be controlled so as to be uniform by applying a high voltage to the discharge electrode and simultaneously applying the appropriate voltage (i.e., the voltage desired to be charged to) to the grid electrode (see, for example, Japanese Patent Applications Laid-Open (JP-A) Nos. 2001-13765 and 8-36289). Note that, in order to prevent white deletion or the like (there are also cases in which black stripes are generated) from arising in the image due to discharge products such as O 3 , N OX , and the like which are generated at the time of discharging, there are cases in which the grid electrode is subjected to a surface treatment such as painting or the like in order to decompose the discharge products. 
   In order to control the potential of the photosensitive body well, it is preferable to make the opening ratio along a straight line, which runs along the moving direction of the photosensitive body (hereinafter called “opening ratio on the line of movement of the photosensitive body”), uniform at all of the regions at which the opening pattern is formed. Here, “opening ratio on the line of movement of the photosensitive body” means the ratio of the distance which a given portion of the photosensitive body moves over while opposing the opening portions, among the distance that that given portion of the photosensitive body moves over while opposing the grid electrode. If the opening ratio on the line of movement of the photosensitive body differs greatly in accordance with the position of the aforementioned portion of the photosensitive body, it is easy for the discharge distribution to become non-uniform, which is not preferable. Such a phenomenon is particularly marked in cases in which the discharge electrode has deteriorated or the discharge electrode and the grid electrode have become dirty. Further, there is also the drawback that the costs required for maintenance of the charger are high due to deterioration of the discharge distribution. Note that, in cases in which there is dispersion in the opening ratio on the line of movement of the photosensitive body, generally, the average value of the opening ratio on the line of movement of the photosensitive body is defined as the opening ratio on the line of movement of the photosensitive body at the entire grid electrode. 
   As an example of aiming to make uniform the opening ratio on the line of movement of the photosensitive body, for example, JP-A No. 2001-13765 discloses a grid electrode  78  (see  FIG. 9 ) in which this opening ratio is uniform and the ability to control the charge potential is excellent. Long, narrow openings  80 , at which the positions of the end portions thereof are uniform, are formed in a fixed direction in the grid electrode  78 . 
   However, in this grid electrode  78 , because the long side connecting portions, which connect the long sides of the openings, are extremely long, there is the problem that it is easy for these long side connecting portions to deform. In particular, in a scorotron charger having a mechanism for cleaning the grid electrode  78 , this problem arises markedly because, generally, the grid electrode is cleaned by a cleaning member being moved in a direction orthogonal to a moving direction X of the photosensitive body, in a state in which the cleaning member abuts the grid electrode  78 . Further, when an opening corner portion  82  which is acute-angled is formed at an opening edge  81  forming the opening  80 , there is the problem that the cleaning member (which is mainly bristles) is severed by the opening edge portion forming this acute-angled opening corner portion  82 , and image defects arise. There is also the problem that the grid opening ratio cannot be made that large, and the controlling ability of the grid electrode  78  is insufficient. 
   In order to improve the mechanical strength of a grid electrode, it has been thought to use a grid electrode  88  such as shown in  FIG. 10  and disclosed in Japanese Utility Model Application Publication (JP-Y) No. 3-042443. This grid electrode  88  is formed by a screen-like thin plate material having a larger number of minute openings  90  which are shaped as regular hexagons. Among the six sides of the openings  90 , two sides are disposed so as to be oriented parallel to the moving direction X of the photosensitive body. In this way, because the configurations of the openings  90  of the grid electrode  88  are regular hexagons, small openings can be arranged densely. 
   However, the grid electrode  88  disclosed in JP-Y No. 3-042443 has the following problems. Two sides of the regular hexagonal opening  90  of the grid electrode  88  are disposed so as to be oriented parallel to the moving direction X of the photosensitive body, and strip-shaped portions  94 , which are electrode portions between adjacent openings along the moving direction X of the photosensitive body, are lined up in one row at uniform intervals. On the other hand, the regions of the surface of the photosensitive body, which regions do not oppose the strip-shaped portions  94  (e.g., the regions of the photosensitive body surface opposing the regions  96  in  FIG. 10A ), are inclined at 30° or 150° with respect to the direction orthogonal to the rotating direction of the photosensitive body. Accordingly, with such an arrangement and configuration of openings  104 , the dispersion in the opening ratio on the line of movement of the photosensitive body is large, and the controlling ability of the grid electrodes is poor (see  FIGS. 10B and 10C ). 
   In order to make the dispersion in the opening ratio on the line of movement of the photosensitive body smaller, there are methods such as making the widths of the strip-shaped portions  94  narrower so as to change the configurations and sizes of the openings  90 , and the like. However, if the widths of the strip-shaped portions  94  are made narrow, the strength of the grid electrode deteriorates, and other problems arise, such as a predetermined controlling ability cannot be maintained when the openings  90  are made large, and the like. 
   Further, when the opening ratio on the line of movement of the photosensitive body is the same, the smaller the grid pitch, the better the ability to control the charge potential. (The grid pitch is the distance between both ends, in the moving direction X of the photosensitive body, at each opening portion, and is hereinafter abbreviated as “GP”.) However, if the openings  90  are shaped as regular hexagons as described above, a problems arises in that, as compared with openings of the same opening ratio but other configurations, the GP is large, which is not very preferable in terms of the controlling ability (see  FIGS. 10B and 10C ). 
   In order to suppress the dispersion in the opening ratio on the line of movement of the photosensitive body, it has been thought to adjust the orientation of the regular hexagon shapes (see, for example, JP-Y No. 62-181954). However, as compared with openings of the same opening ratio but other shapes, an increase in the GP is unavoidable. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances, and provides a grid electrode, a scorotron charger, and an image forming device. 
   A first aspect of the present invention is a grid electrode which is disposed along a corotron wire with a predetermined interval between the grid electrode and the corotron wire, including a planar, plate-shaped member having plural openings in each of which a longitudinal direction and a short side direction are formed, the longitudinal directions of the respective plural openings being lined up so as to be in a same direction, the plate-shaped member also having connecting portions connecting openings which are adjacent to each other in the opening longitudinal direction and connecting portions connecting openings which are adjacent to each other in the opening short side direction. At each opening, a shortest distance from a center point to an opening edge is S and S is less than 1.5 mm, a grid opening ratio is greater than or equal to 82%, and a length of the longitudinal direction of the opening is at least 1.3 times a length of the short side direction of the opening. The connecting portions connecting openings which are adjacent to each other in the opening longitudinal direction are not disposed on a same line which is orthogonal to the corotron wire. 
   The predetermined interval is set within a range such that the controllability of the charge potential at the time when the grid electrode is attached does not deteriorate. 
   A connecting portion allotted surface area, which is obtained by allotting, per opening, the surface area of the connecting portion connecting adjacent openings (e.g., forming the opening edge) at the discharge region, is determined. The grid opening ratio is the value obtained by dividing the surface area of one opening by the sum of the opening surface area and this connecting portion allotted surface area. In order to express this value in terms of percent, the value obtained by division is multiplied by 100. 
   Openings having two or more types of configurations may exist together as the openings. 
   In accordance with the first aspect of the present invention, by determining the opening short side direction by taking the moving direction of the photosensitive body into consideration, it is possible to obtain a grid electrode in which the opening ratio on the line of movement of the photosensitive body is uniform, without making the GP (grid pitch) large. Moreover, at each of the openings, the shortest distance from the center point to the opening edge is S, S is less than 1.5 mm, and the grid opening ratio is greater than or equal to 82%. Therefore, it is easy to realize a structure in which discharge that leads to a deterioration in image quality, such as bright spot discharge or the like, does not occur. 
   Moreover, the lengths in the longitudinal direction of the openings are at least 1.3 times the lengths in the short side direction of the openings. Therefore, it is easy to increase the mechanical strength of the grid electrode. Accordingly, the mechanical strength of the grid electrode can be improved without deteriorating the controlling ability of the grid electrode. 
   The grid electrode may be a grid electrode which is disposed along a corotron wire with a predetermined interval between the grid electrode and the corotron wire, and has, at a planar, plate-shaped member, plural openings which are lined-up in the same direction and whose long side lengths (lengths in a longitudinal direction of the openings) are a and short side lengths (lengths in a short side direction of the openings) are b, wherein, at each one opening of the plural openings, a long side connecting portion, which connects a long side of the one opening and long sides of two other openings which are adjacent to the one opening, and a short side connecting portion, which connects short sides of the two other openings, are connected at the substantial center of the long side of the one opening  30 J, and the relationship a≧ 2   b  is established. In this case, the widths of the long side connecting portions and the short side connecting portions are set such that the needed opening ratio on the line of movement of the photosensitive body is obtained and the mechanical strength of the grid electrode does not become too weak. 
   In this way, it is possible to obtain a grid electrode in which the opening ratio on the line of movement of the photosensitive body is uniform, without making the GP (grid pitch) large. Further, because the plural openings are disposed in accordance with the long side connecting portions and the short side connecting portions, it is easy to increase the mechanical strength of the grid electrode. Accordingly, the mechanical strength of the grid electrode can be improved, without deteriorating the controlling ability of the grid electrode. 
   Because the present invention is structured as described above, it is possible to realize a grid electrode, a scorotron charger, and an image forming device in which the mechanical strength of the grid electrode is improved without a deterioration in the controlling ability of the grid electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIG. 1  is a side view showing the structure of an image forming device of a first embodiment; 
       FIG. 2  is a perspective view of a scorotron charger used in the image forming device of the first embodiment; 
       FIG. 3  is a perspective view of the scorotron charger used in the image forming device of the first embodiment; 
       FIG. 4A  is a partial plan view of a grid electrode attached to the scorotron charger used in the image forming device of the first embodiment; 
       FIG. 4B  is a sectional view taken along arrow  4 B— 4 B of  FIG. 4A ; 
       FIG. 4C  is a sectional view taken along arrow  4 C— 4 C of  FIG. 4A ; 
       FIG. 5A  is a partial enlarged view of  FIG. 4A ; 
       FIG. 5B  is a partial enlarged view of  FIG. 5A ; 
       FIG. 6  is a partial enlarged plan view showing a modified example of a grid electrode in the first embodiment; 
       FIG. 7  is a partial enlarged plan view of a grid electrode of a second embodiment; 
       FIG. 8  is a partial enlarged plan view of a grid electrode of a third embodiment; 
       FIG. 9A  is a partial plan view of a grid electrode attached to a conventional scorotron charger; 
       FIG. 9B  is a sectional view taken along arrow  9 B 1 — 9 B 1  of  FIG. 9A , and is the same as a sectional view taken along arrow  9 B 2 — 9 B 2  of  FIG. 9A ; 
       FIG. 10A  is a partial plan view of a grid electrode attached to a conventional scorotron charger; 
       FIG. 10B  is a sectional view taken along arrow  10 B— 10 B of  FIG. 10A ; and 
       FIG. 10C  is a sectional view taken along arrow  10 C— 10 C of  FIG. 10A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be described hereinafter. Note that, from the second embodiment on, portions which are similar to structural elements described thereinbefore are denoted by the same reference numerals, and description thereof is omitted. 
   [First Embodiment] 
   First, a first embodiment will be described. As shown in  FIG. 1 , an image forming device  10  relating to the present embodiment has, at the periphery of a drum-shaped photosensitive body  14 , a scorotron charger  12  which relates to the present embodiment and which is replaceable. 
   As shown in  FIGS. 2 and 3 , the scorotron charger  12  is a long, narrow device which is provided along the direction of the rotational axis of the photosensitive body  14 , and has two corotron wires  16 , a grid electrode  18  relating to the present invention, and a cleaning mechanism  20 . The grid electrode  18  is disposed so as to be positioned between the corotron wires  16  and the photosensitive body  14 , and so as to be able to be replaced. The cleaning mechanism  20  moves in a direction orthogonal to the moving direction of the photosensitive body  14 , and cleans the grid electrode  18 . An electrode short side direction X of the scorotron charger  12  is a direction orthogonal to the corotron wires  16  and is oriented the same as the moving direction (the rotating direction) of the photosensitive body  14 . 
   The cleaning mechanism  20  has a brush  22  and a moving mechanism  24 . The brush  22  press-contacts the grid electrode  18  from the side at which the corotron wires  16  are disposed. The moving mechanism  24  slides the brush  22  along the rotational axis direction of the photosensitive body  14  (i.e., along an electrode longitudinal direction Y of the grid electrode) in a state in which the brush  22  press-contacts the grid electrode  18 . The cleaning mechanism  20  cleans the grid electrode  18  due to the brush  22  sliding along the electrode longitudinal direction Y (the direction of the rotational axis of the photosensitive body  14 ) with respect to the grid electrode  18 . 
   The grid electrode  18  is shaped so as to be long in the longitudinal direction of the scorotron charger  12 . An opening pattern  26  is formed in the grid electrode  18  so that the grid electrode  18  is mesh-like. 
   As shown in  FIGS. 4 and 5A , each of respective openings  30  is shaped as a long, narrow hexagon, and is structured by three pairs of parallel sides. As shown in  FIG. 5A , the lengths of the parallel sides are p, q, and r, respectively, and p and q are substantially the same and both are longer than r. 
   The openings  30  are disposed at the same orientation, so as to be staggered with respect to an opening short side direction U. In other words, openings which are adjacent to one another in the opening short side direction are disposed, along the opening longitudinal direction, such that the positions thereof are offset by a length which is substantially half of the opening longitudinal direction length. Namely, as seen from the opening short side direction U, a longitudinal direction end portion  30 HE of an opening  30 H, which is adjacent to an opening  30 G, is positioned at a longitudinal direction substantially central portion  30 GM of the opening  30 G As seen from the opening short side direction U, the longitudinal direction end portions of the respective openings are positioned alternately. 
   Namely, along the corotron wires  16 , the grid electrode  18  has, at a planar, plate-shaped member, plural openings  30  whose long side lengths (lengths in a longitudinal direction of the openings, that is, lengths along an opening longitudinal direction V) are a and short side lengths (lengths in a short side direction of the openings, that is, lengths along the opening short side direction U) are b, and which are lined up along the same direction. Further, a long side connecting portion (i.e., a long side connecting portion which connects openings which are adjacent to one another in the opening short side direction)  28 , which connects a long side of one opening  30 J structuring the plural openings  30  and the long sides two other openings  30 K which are adjacent to the one opening  30 J, and a short side connecting portion (i.e., a short side connecting portion which connects openings which are adjacent to one another in the opening longitudinal direction)  29 , which connects the short sides of the two other openings  30 K, are connected at the substantial center of the long side of the one opening  30 J. 
   At each of the openings  30  forming the opening pattern  26 , a shortest distance from a center point to an opening edge is S, and S is less than 1.5 mm. In the present embodiment, because the opening  30  is hexagonal as described above, the distance between opening edge portions  31   p , which each form a side of length p and which oppose one another, is a value equal to 2×S. Note that the distance between opening edge portions  31   q , which each form a side of length q and which oppose one another, also is a value equal to 2×S. 
   Further, in the present embodiment, because the opening  30  is the above-described hexagonal shape, in the discharge region, the sum of the opening surface area and the connecting portion allotted surface area, which is obtained by allotting, per opening, the surface area of the connecting portions connecting adjacent openings (e.g., forming the opening edge), is the surface area of the region within the two-dot chain line of  FIG. 5B . A value, which is equal to the product obtained by multiplying by 100 the quotient obtained by dividing the opening surface area by the surface area of this region within the two-dot chain line, is the grid opening ratio as expressed in percent. In the present embodiment, the grid opening ratio is 82% or more. 
   An angle α, which is formed by the electrode short side direction (the moving direction of the photosensitive body  14 ) X and the opening short side direction U, is within the range of 0° to 20°, in consideration of the controllability of the charge potential. 
   Regarding the dimensions of the opening  30 , the long side length a (length in the longitudinal direction of the opening) is made to be at least 1.3 times the short side length (length in the short side direction of the opening). Further, a longest distance L from the center point to the opening edge is determined so as to satisfy the relationship L≧2×S. 
   The angles of opening corner portions  32 A through  32 F, which are formed by the opening edge  31 , are all obtuse angles, and no acute-angled opening corner portions are formed. 
   As described above, the openings  30  are formed to be long and narrow, and are disposed at the same orientation so as to be in a staggered form with respect to the opening short side direction. Namely, openings which are adjacent to one another in the opening short side direction are disposed, along the opening longitudinal direction, such that the positions thereof are offset by a length which is substantially half of the longitudinal direction length of the opening, and the short side connecting portions  29  connecting openings which are adjacent to each other in the opening longitudinal direction V are not disposed on a same line which is orthogonal to the corotron wire  16 . Further, the angle α, which is formed by the electrode short side direction (the moving direction of the photosensitive body  14 ) X and the opening short side direction U, is within the range of 0° to 20° in consideration of the controllability of the charge potential. Moreover, the grid opening ratio is 82% or more, the aforementioned shortest distance S is 1.5 mm or less, and a is made to be at least 1.3 times b. In addition, as shown in  FIG. 4 , the opening ratio on the line of movement of the photosensitive body in cross-section  4 B— 4 B, and the opening ratio on the line of movement of the photosensitive body in cross-section  4 C— 4 C are substantially the same. Still further, at the entire region where the opening pattern  26  is formed, the opening ratios on the line of movement of the photosensitive body at straight lines which run along the electrode short side direction (the moving direction of the photosensitive body  14 ) X are substantially the same, and there is markedly little dispersion in the opening ratio on the line of movement of the photosensitive body. Furthermore, the GP in cross-section  4 B— 4 B (see  FIG. 4B ) along the moving direction of the photosensitive body  14  (i.e., the electrode short side direction), and the GP in cross-section  4 C— 4 C (see  FIG. 4C ), are much smaller than the GP of the conventional grid electrode. 
   The controlling ability of the grid electrode  18  is therefore sufficiently high. 
   Because the openings  30  forming the opening pattern  26  are arranged in a staggered form, even if the width of the electrode member forming the openings  30  is not particularly small, the mechanical strength of the grid electrode  18  can be made to be sufficiently high. 
   The angles of the opening corner portions  32 A through  32 F formed by the opening edge  31  are all obtuse angles, and no opening corner portion which is an acute angle is formed. Therefore, when the brush  22  slides in order to clean, it is difficult for the bristles forming the brush  22  to be severed at the opening edge  31 . 
   Further, in the present embodiment, the widths of the long side connecting portion  28  and the short side connecting portion  29  are both 0.2 mm or less, whereby a structure is obtained in which the grid opening ratio is unlikely to be reduced. Further, the plate thickness of the plate-shaped member forming the grid electrode  18  is in a range of 0.1 to 0.2 mm, whereby strength and flexure are preferable and whereby, moreover, stable etching can be carried out at the time of forming the opening pattern. 
   EXAMPLES 
   Grid electrodes are fabricated which are the grid electrode  18  described in the first embodiment, and at which the thickness t of the electrode member is a constant 0.1 mm, and (2×S) (i.e., twice the aforementioned S) and (2×L) (i.e., twice the aforementioned L) are changed as parameters. The respective values of t, 2×S, 2×L of each grid electrode are as shown in Tables 1 through 5. Note that, in the present Example, a width W of the electrode members which partition adjacent openings  30  (i.e., the width W of the long side connecting portion  28  and the short side connecting portion  29 ) is 0.1 which is the same as t. 
   
     
       
         
             
             
             
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
           
          
             
               Dimension (t) 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
             
             
               Dimension (2 × S) 
               0.5 
               0.5 
               0.5 
               0.5 
               0.5 
               0.5 
             
             
               Dimension (2 × L) 
               1 
               1.5 
               2 
               2.5 
               3 
               3.5 
             
             
               Opening ratio (%) of 
               75.1 
               77.2 
               78.4 
               79.1 
               79.5 
               79.9 
             
             
               grid 
             
             
               Controlling ability of 
               135.0 
               110.0 
               100.0 
               90.0 
               88.0 
               85.0 
             
             
               grid electrode 
             
             
               (difference between Vg 
             
             
               and Vh) 
             
             
               Evaluation of controlling 
               X 
               X 
               X 
               X 
               X 
               X 
             
             
               ability of grid electrode 
             
             
               (magnitude of effect of 
             
             
               opening ratio) 
             
             
               Evaluation of non- 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
               actualization of wire 
             
             
               bright spot discharge 
             
             
               (magnitude of effect of 
             
             
               S) 
             
             
               Overall evaluation 
               X 
               X 
               X 
               X 
               X 
               X 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
             
             
           
             
               TABLE 2 
             
             
                 
             
           
          
             
               Dimension (t) 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
             
             
               Dimension (2 × S) 
               1 
               1 
               1 
               1 
               1 
               1 
             
             
               Dimension (2 × L) 
               1.5 
               2 
               2.5 
               3 
               3.5 
               4 
             
             
               Opening ratio (%) of 
               83.4 
               84.6 
               85.2 
               85.7 
               86.0 
               86.2 
             
             
               grid 
             
             
               Controlling ability of 
               58.0 
               50.0 
               44.0 
               42.0 
               38.0 
               35.0 
             
             
               grid electrode 
             
             
               (difference between Vg 
             
             
               and Vh) 
             
             
               Evaluation of controlling 
               Δ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
               ability of grid electrode 
             
             
               (magnitude of effect of 
             
             
               opening ratio) 
             
             
               Evaluation of non- 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
               actualization of wire 
             
             
               bright spot discharge 
             
             
               (magnitude of effect of 
             
             
               S) 
             
             
               Overall evaluation 
               Δ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
             
             
           
             
               TABLE 3 
             
             
                 
             
           
          
             
               Dimension (t) 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
             
             
               Dimension (2 × S) 
               1.5 
               1.5 
               1.5 
               1.5 
               1.5 
               1.5 
             
             
               Dimension (2 × L) 
               2 
               2.5 
               3 
               3.5 
               4 
               4.5 
             
             
               Opening ratio (%) of 
               87.3 
               88.1 
               88.6 
               88.9 
               89.2 
               89.3 
             
             
               grid 
             
             
               Controlling ability of 
               30.0 
               25.0 
               23.0 
               22.0 
               21.0 
               20.0 
             
             
               grid electrode 
             
             
               (difference between Vg 
             
             
               and Vh) 
             
             
               Evaluation of controlling 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
               ability of grid electrode 
             
             
               (magnitude of effect of 
             
             
               opening ratio) 
             
             
               Evaluation of non- 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
               actualization of wire 
             
             
               bright spot discharge 
             
             
               (magnitude of effect of 
             
             
               S) 
             
             
               Overall evaluation 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
             
             
           
             
               TABLE 4 
             
             
                 
             
           
          
             
               Dimension (t) 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
             
             
               Dimension (2 × S) 
               2 
               2 
               2 
               2 
               2 
               2 
             
             
               Dimension (2 × L) 
               2.5 
               3 
               3.5 
               4 
               4.5 
               5 
             
             
               Opening ratio (%) of 
               89.7 
               90.3 
               90.7 
               90.9 
               91.1 
               91.3 
             
             
               grid 
             
             
               Controlling ability of 
               20.0 
               18.0 
               16.0 
               15.0 
               14.0 
               13.0 
             
             
               grid electrode 
             
             
               (difference between Vg 
             
             
               and Vh) 
             
             
               Evaluation of controlling 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
               ability of grid electrode 
             
             
               (magnitude of effect of 
             
             
               opening ratio) 
             
             
               Evaluation of non- 
               Δ 
               Δ 
               Δ 
               Δ 
               Δ 
               Δ 
             
             
               actualization of wire 
             
             
               bright spot discharge 
             
             
               (magnitude of effect of 
             
             
               S) 
             
             
               Overall evaluation 
               Δ 
               Δ 
               Δ 
               Δ 
               Δ 
               Δ 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
             
             
           
             
               TABLE 5 
             
             
                 
             
           
          
             
               Dimension (t) 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
               0.1 
             
             
               Dimension (2 × S) 
               3 
               3 
               3 
               3 
               3 
               3 
             
             
               Dimension (2 × L) 
               3.5 
               4 
               4.5 
               5 
               5.5 
               6 
             
             
               Opening ratio (%) of 
               92.4 
               92.7 
               93.0 
               93.1 
               93.2 
               93.3 
             
             
               grid 
             
             
               Controlling ability of 
               12.0 
               11.0 
               10.0 
               9.0 
               8.0 
               7.0 
             
             
               grid electrode 
             
             
               (difference between Vg 
             
             
               and Vh) 
             
             
               Evaluation of controlling 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
               ◯ 
             
             
               ability of grid electrode 
             
             
               (magnitude of effect of 
             
             
               opening ratio) 
             
             
               Evaluation of non- 
               X 
               X 
               X 
               X 
               X 
               X 
             
             
               actualization of wire 
             
             
               bright spot discharge 
             
             
               (magnitude of effect of 
             
             
               S) 
             
             
               Overall evaluation 
               X 
               X 
               X 
               X 
               X 
               X 
             
             
                 
             
          
         
       
     
   
   In each of Tables 1 through 5, (2×S) (i.e., twice the aforementioned S) is constant, and (2×L) (i.e., twice the aforementioned L) is changed as a parameter. 
   The grid opening ratio is determined by the values 2×S and 2×L. The grid opening ratios are also shown in Tables 1 through 5. 
   Moreover, the potential difference (Vg−Vh) between the potential (Vg) of the grid electrode upon discharging and the potential (Vh) of the drum-shaped photosensitive body  14  is determined in accordance with the grid opening ratio. If this potential difference is too large due to the grid opening ratio being too small, the controlling ability of the grid electrode is poor. In the present Example, the controlling ability of the grid electrode, which is determined by the potential difference, is evaluated, and the results of evaluation also are shown in Tables 1 through 5. 
   If the value of S is too large, bright spot discharge at the corotron wires  16  (wire bright spot discharge) occurs, and the image quality deteriorates. In the present Example, to what extent the wire bright spot discharge is suppressed (i.e., to what extent it does not actualize) is evaluated visually. These results of evaluation are also shown in Tables 1 through 5. 
   Moreover, an overall evaluation of (1) the controlling ability of the grid electrode based on the above-described potential difference, and (2) non-actualization of wire bright spot discharge are conducted. The results of the lower (inferior) one of above (1) and (2) are used as the results for the overall evaluation. These evaluation results also are shown in Tables 1 through 5. 
   As can be understood from Table 1, at a grid opening ratio of 79.9% or less, the potential difference (Vg−Vh) is greater than or equal to 85.0 V, and the controlling ability of the grid electrode is poor (an evaluation of “BAD” is given, the “BAD” is denoted by a cross in the Tables). As can be understood from Tables 2 through 5, when the grid opening ratio is greater than 79.9%, the potential difference is less than 85.0 V, and the deterioration in the controlling ability of the grid electrode is suppressed (evaluations of “NOT BAD”, “GOOD”, the “NOT BAD” and the “GOOD” are denoted, respectively, by a triangle and a circle in the Tables). Further, as can be understood from Table 2, when L is greater than or equal to twice the value of S (L≧2S) and the grid opening ratio is 84.6% or more, the potential difference is less than or equal to 50.0 V, and the controlling ability is markedly good (an evaluation of “GOOD”). 
   As can be understood from Tables 3 through 5, even when L is not greater than or equal to twice the value of S, if the grid opening ratio is greater than or equal to 84.6%, the controlling ability of the grid electrode is markedly good. However, L being greater than S (e.g., 2S or more, and even 3S or more) made it easy to suppress the dispersion in the opening ratio on the line of movement of the photosensitive body along the moving direction X of the photosensitive body  14 , and easy to make the grid opening ratio larger. 
   Further, as can be understood from Table 5, when 2×S is greater than or equal to 3 mm (i.e., when S is greater than or equal to 1.5 mm), wire bright spot discharge clearly is clearly actualized (an evaluation of “BAD”). As can be understood from Tables 1 through 4, when 2×S is smaller than 3 mm (i.e., when S is less than 1.5 mm), the actualization of wire bright spot discharge is suppressed (evaluations of “NOT BAD”, “GOOD”). Further, when 2×S is 1.5 mm or less, wire bright spot discharge is sufficiently made to not actualize. 
   As described above, in the grid electrodes prescribed in Tables 1 through 5, the thickness t is 0.1 mm and the width W of the electrode members partitioning the adjacent openings  30  (i.e., the long side connecting portion  28  and the short side connecting portion  29 ) is 0.1 mm. Therefore, the strength of the grid electrode  18  is sufficient, and even if a cleaning member such as a brush or the like are to move in a state of abutting the grid electrode  18 , the grid electrode  18  would not deform. 
   [Second Embodiment] 
   A second embodiment of the present invention will be described next. 
   As shown in  FIGS. 4 and 5 , in the present embodiment, each of the respective openings  30  forming the opening pattern  26  is shaped as a long, narrow hexagon, and is structured by three pairs of parallel sides. As shown in  FIG. 5 , the lengths of the parallel sides are p, q, and r, and p and q are substantially equal, which is the same as in the first embodiment. However, in the present embodiment, p, q, and r satisfy the relationships p≧3r, q≧3r. 
   Further, in the same way as in the first embodiment, the openings  30  are disposed at the same orientation, so as to be staggered with respect to the opening short side direction U. In other words, openings which are adjacent to one another in the opening short side direction are disposed, along the opening longitudinal direction, such that the positions thereof are offset by a length which is substantially half of the longitudinal direction length of the opening. Namely, as seen from the opening short side direction U, the longitudinal direction end portion  30 HE of the opening  30 H, which is adjacent to the opening  30 G, is positioned at the longitudinal direction substantially central portion  30 GM of the opening  30 G. As seen from the opening short side direction U, the longitudinal direction end portions of the respective openings are positioned alternately. 
   In the present embodiment, a width b of the opening  30  along the opening short side direction U which is orthogonal to the opening longitudinal direction V, is less than or equal to 3 mm. Further, as shown in  FIG. 4 , the opening ratio on the line of movement of the photosensitive body in cross-section  4 B— 4 B, and the opening ratio on the line of movement of the photosensitive body in cross-section  4 C— 4 C, are substantially the same. Still further, at the entire region where the opening pattern  26  is formed, the opening ratios on the line of movement of the photosensitive body at straight lines which run along the electrode short side direction (the moving direction of the photosensitive body  14 ) X are substantially the same, and there is markedly little dispersion in the opening ratio on the line of movement of the photosensitive body. Furthermore, the GP in cross-section  4 B— 4 B (see  FIG. 4B ) along the moving direction of the photosensitive body  14  (i.e., the electrode short side direction) X, and the GP in cross-section  4 C— 4 C (see  FIG. 4C ), are much smaller than the GP of the conventional grid electrodes. 
   The controlling ability of the grid electrode  18  is thereby sufficiently high. 
   In the present embodiment, the grid opening ratio of the openings  30  can be calculated as follows by using a, b, and r. 
   A surface area N 1  of the region within the opening  30  is calculated by the following formula.
 
 N 1= a ×( b+r )/2
 
   N 2 , which is the sum (the surface area within the broken line shown in  FIG. 5-2 ) of the opening surface area N 1  and the connecting portion allotted surface area, which is obtained by allotting, per opening, the surface area of the connecting portion connecting adjacent openings (i.e., forming the opening edge), is similarly calculated. 
   Then, by determining (N 1 /N 2 )×100, the grid opening ratio as expressed in percent can be calculated. 
   Further, it suffices to set the angle α, which is formed by the opening short side direction U and the electrode short side direction (the moving direction of the photosensitive body  14 ) X, such that, at all of the regions where the opening pattern  26  is formed, a straight line running along the electrode short side direction X successively passes over openings which are adjacent in the opening short side direction U, i.e., such that this straight line is not positioned on the both ends of a strip-shaped electrode portion (e.g., an electrode portion  19  shown in  FIG. 5A ) which forms the opening side of length r. In this way, the dispersion in the opening ratio on the line of movement of the photosensitive body can be reduced more. Further, the dispersion in the opening ratio on the line of movement of the photosensitive body can be suppressed even more by adjusting the difference in the lengths p and q. Moreover, as shown in  FIG. 6 , the openings may be made even longer and narrower, and a longitudinal direction length a′ and a short side direction width b′ of an opening  40  may satisfy the relationship a′≧3b′. The controllability can thereby be further improved. 
   &lt;Example of Second Embodiment&gt; 
   In the present Example, the longitudinal direction length a of the opening  30  is 3.5 mm, the short side direction width b is 1.0 mm, α is 10°, the width W of the electrode members which partition the adjacent openings  30  (i.e., the long side connecting portion  28  and the short side connecting portion  29 ) is 0.1 mm, and the thickness of the electrode members is 0.1 mm. Further, in the present Example, r is 0.41 mm. 
   In accordance with the present Example, it is possible to easily realize a grid electrode which has a good controlling ability and sufficiently high mechanical strength. 
   [Third Embodiment] 
   Next, a third embodiment will be described. In the third embodiment, the configurations of the openings formed at the grid electrode are different. Namely, as shown in  FIG. 7 , openings  50  which are formed at the grid electrode are formed as rectangles which are long and narrow in the electrode longitudinal direction. 
   In the present embodiment, in the same way as in the first embodiment, along the corotron wires, the grid electrode has, at a planar, plate-shaped member, the plural openings  50  whose long side lengths (lengths in the longitudinal direction of the openings) are a and short side lengths (lengths in the short side direction of the openings) are b, and which are lined up in the same direction. Further, a long side connecting portion  48 , which connects a long side of one opening  50 J structuring the plural openings  50  and the long sides of two other openings  50 K which are adjacent to the one opening  50 J, and a short side connecting portion  49 , which connects the short sides of the two other openings  50 K, are connected at the substantial center of the long side of the one opening  50 J. 
   In this way, a grid electrode, in which the configurations of the openings are simpler than in the first embodiment, is realized. 
   [Fourth Embodiment] 
   A fourth embodiment will be described next. In the fourth embodiment, as shown in  FIG. 8 , four opening corner portions  62 A through  62 D of an opening  60  are formed to be arc-shaped, as compared with the third embodiment. In this way, marked effects can be seen in preventing the bristles of the brush  22  (see  FIG. 3 ) from being severed by opening edges  61  of the openings  60  at the time of cleaning. 
   Although embodiments of the present invention have been described above, these embodiments are examples, and various modifications are possible within a scope which does not deviate from the gist of the present invention. Further, the scope of the right of the present invention is, of course, not limited to the above-described embodiments.