Patent Publication Number: US-2010111568-A1

Title: Charging device and image forming apparatus

Description:
This application is based on an application No. 2008-279521 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a charging device used in an electrographic image forming apparatus, and especially to a technique of suppressing production of ozone resulting from discharge in the charging device. The aim of suppressing the production of ozone is to prevent deterioration of the surrounding components caused by ozone and also to prevent an image defect resulting from degradation of the photoreceptor sensitivity caused by deteriorated substances. 
     (2) Description of the Related Art 
     Generally, an electrographic image forming apparatus is provided with a scorotron charging device. Regarding scorotron charging devices, there have been demands for further improvement in efficiency and reduction of ozone production. In order to meet the demand, it has become the mainstream to employ a discharge electrode having a serrated edge, rather than a conventional wire-type discharge electrode. 
     A discharge electrode with a serrated edge produces a discharge in limited directions as compared with that produced by a wire-type discharge electrode. By virtue of this property, the discharge electrode with a serrated edge is capable of producing a high energy discharge comparatively efficiently in view of the applied voltage and the supplied current. In addition, since the discharge is produced from the tips of the serrated edge, the amount of ozone produced is relatively small. 
     Yet, even with such a discharge electrode having a serrated edge, the following is still unavoidable. That is, ozone produced at the time of discharge eventually causes, by ozone oxidation and other actions, corrosion of the shielding member provided to encompass the discharge electrode. As a result, deteriorated substances such as rust are formed. The deteriorated substances tend to act as a source of a gas that affects the photoreceptor sensitivity and release the gas at the timing irrelevant to image forming process. As a result, the photoreceptor sensitivity is degraded. The degraded photoreceptor sensitivity in turn causes an undesirable phenomenon called image deletion in which an image is formed with voids appearing white. 
     In order to address the above, JP patent application publication No. 2007-316197 discloses a technique for suppressing the deterioration of a shielding member. 
     According to the publication, the charging device is provided with a member for suppressing corrosion and a cleaning mechanism for the corrosion suppressing member. The corrosion suppressing member is made of a material that is more prone to oxidation corrosion than materials of both the stabilizing case and of the control electrode. The oxidation corrosion caused by, for example, ozone produced as a result of discharge by discharge members advances more on the corrosion suppressing member than on the stabilizing case and the control electrode. The corroded corrosion suppressing member can be cleaned by the cleaning mechanism, so that it is maintained that the corrosion suppressing member is more apt to corrosion than the stabilizing case and the control electrode. As a consequence, the corrosion of the stabilizing case and the control electrode is suppressed. 
     Unfortunately however, the structure disclosed by the publication mentioned above is not desirable for the following reason. That is, the presence of the corrosion suppressing member made of metal inevitably changes the electric field distribution within the charging device and reduces the effective discharge current supplied to the photoreceptor. Thus, the application voltage needs to be increased to increase the amount of discharge current, which requires the provision of a power supply circuit capable of producing higher power output. 
     In addition, the increase in the amount of discharge current results in that a larger amount of ozone is produced to increase the ozone concentration. In addition, the provision of the corrosion suppressing member and the cleaning mechanism is not desirable also because it increases the complexity of the structure, size, and cost of the overall device. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide an image forming apparatus and a charging device used in the image forming apparatus that are capable of suppressing corrosion of the shielding member, without the need to increase the structural complexity, size, and cost of the device and apparatus. The image forming apparatus and the charging device according to the present invention are therefore capable of preventing occurrence of image deletion and capable of forming images with good quality over a long time. 
     In order to achieve the above aim, one aspect of the present invention provides a charging device for charging a surface of a rotating image carrier. The charging device includes: a shielding case having an opening at a location facing toward the image carrier; a corona electrode hung within the shielding case to longitudinally extend in a direction perpendicular to a direction of the rotation; and a control electrode disposed at a location of the opening of the shielding case. A pair of side plates of the shielding case are opposed to each other in the direction of the rotation, and at least one of the side plates has a portion that outwardly bulges at a location in a vicinity of the opening. 
     According to the structure above, with the shielding case having an opening of an adequate size and with a control electrode of an adequate size, the charging device is still capable of suppressing undesirable discharge affecting the shielding case. 
     For example, as compared with a conventional shielding case, the shielding case of the above structure is so configured that the electric field strength in the vicinity of the control electrode is kept lower. As a result, the amount of current flowing through the shielding case is reduced, which in turn reduces the amount of ozone produced in the vicinity of the control electrode. This achieves to decrease the corrosion rate of the shielding case and especially of the portion of the shielding case closer to the image carrier. The lower corrosion rate leads to reduction of the adverse influence of gas, which is one factor causing image deletion. 
     As described above, the present invention achieves to suppress corrosion of the shielding case and thus suppress image deletion, without the need to complicate the device structure and increase the device size and cost. As a result, it is ensured that the charging device according to the present invention is capable of preventing occurrence of image deletion and capable of forming images with good quality over a long time 
     In order to achieve the above aim, another aspect of the present invention provides an image forming apparatus including: an image carrier that is a rotating body; and a charger operable to charge a surface of the image carrier. The charging device includes: a shielding case having an opening at a location facing toward the image carrier; a corona electrode hung within the shielding case to longitudinally extend in a direction perpendicular to a direction of the rotation; and a control electrode disposed at a location of the opening of the shielding case. A pair of side plates of the shielding case are opposed to each other in the direction of the rotation, and at least one of the side plates has a portion that outwardly bulges at a location in a vicinity of the opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and the other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  is a schematic sectional view of an electrographic image forming apparatus consistent with Embodiment 1; 
         FIG. 2  is a view showing a charging device consistent with Embodiment 1 in detail; 
         FIG. 3  is a view showing the major part of the charging device shown in  FIG. 2 , in a cross section taken along a plane perpendicular to the rotation axis x of an image carrier, and also showing a schematic representation of the electrical connection; 
         FIG. 4  is a view showing the cross section of the charging device shown in  FIG. 3 , with the distribution of electric field strengths calculated by running simulations; 
         FIG. 5  is a view showing a cross section of a charging device of a comparative example, with the distribution of electric field strengths calculated by running simulations; 
         FIG. 6  is a view showing a table of results of experiment to check for occurrence of image deletion in relation to different length of discharge time; 
         FIG. 7  is a graph showing the distribution of electric field strengths in the vicinity of the image carrier surface; 
         FIG. 8  is a view showing the major part of a charging device consistent with Embodiment 2, in a cross section taken along a plane perpendicular to the rotation axis x of an image carrier, and also showing a schematic representation of the electrical connection; 
         FIG. 9  is a view showing the cross section of the charging device shown in  FIG. 8 , with the distribution of electric field strengths calculated by running simulations; 
         FIG. 10  is a view showing a table of results of experiment to check for occurrence of image deletion in relation to different length of discharge time; 
         FIG. 11  is a view showing a cross section of a charging device according to Modification 1, with the distribution of electric field strengths calculated by running simulations; and 
         FIG. 12  is a view showing a cross section of a charging device according to Modification 2, with the distribution of electric field strengths calculated by running simulations. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiment 1  
     &lt;Structure&gt; 
       FIG. 1  is a schematic sectional view of an electrographic image forming apparatus consistent with Embodiment 1. 
     As shown in  FIG. 1 , the image forming apparatus consistent with Embodiment 1 includes: an image carrier  1 , which is an OPC photoreceptor; a charging device  2  for uniformly charging the surface of the image carrier  1  at a predetermined potential; an exposure device  3  for forming an electrostatic latent image by directing laser light to the image carrier  1  having been charged at the predetermined potential; a developer device  5  for developing the latent image by attaching toner particles  4   a  mainly by the action of electrostatic force; a transfer device  7  for transferring the toner particles  4   a  from the image carrier  1  to a recording sheet  6 , such as a sheet of copier paper, by electrostatic force or pressure; a fusing device  8  for fusing the transferred toner particles  4   a  on the recording sheet  6 , by applying heat and pressure; a clearing device  9  for cleaning the image carrier  1  by electrically or mechanically removing residual toner particles  4   b  that remain on the image carrier  1  without being transferred to the recording sheet  6 ; and an antistatic device  10  for equalizing variations in the potential across the image carrier  1  after the cleaning, by exposure to light or with a charged brush, for example. 
     It should be noted that the charging device  2  has functions performed at an initial phase of the image forming. In order to stably produce high-quality images, it is therefore important that the charging device  2  is ensured to charge the surface of the image carrier  1  at a uniform and stable potential. 
       FIG. 2  shows the charging device  2  consistent with Embodiment 1 in detail. 
     As shown in  FIG. 2 , the charging device  2  consistent with Embodiment 1 includes a discharge electrode  20 , a control electrode  21 , a shielding case  22 , an air duct  23 , and a fun  24 . 
     The discharge electrode  20  is, for example, a corona electrode made of stainless steel to have a serrated edge. The discharge electrode  20  is provided inside the shielding case  22  so as to hang across the shielding case  22  in a direction perpendicular to a rotating direction R of the image carrier  1 . The tips of the serrated edge are each located a predetermined discharge distance away from the image carrier  1  and act as a discharging point. When a voltage Vc (−5000V in this example) is applied, a discharge starts from the discharging points (i.e. the tips to charge the image carrier  1 ) to charge the image carrier  1 . Note that the discharge electrode  20  may be composed of an array of needles or pins instead of the one having a serrated edge. Alternatively, the discharge electrode  20  may be composed of wire, such as tungsten wire. In such a case, the discharging point (s) correspond to the portion (s) of the discharge electrode  20  closest to the image carrier  1 . 
     The control electrode  21  may be formed, for example, of mesh. One specific example of the control electrode  21  is a grid mesh made of stainless steel added with nickel. The control electrode  21  is disposed at an opening of the shielding case  22 . That means, the control electrode  21  is located between the discharge electrode  20  and the image carrier  1 . Upon an application of an intermediate voltage Vg (−500V in this example) between the potential of the discharge electrode  20  and the image carrier  1 , the control electrode  21  serves to stabilize the discharge of the discharge electrode. 
     The shielding case  22  may be a housing made of stainless steel and having an elongated shape. The shielding case  22  has the opening across its length on the surface facing toward the image carrier  1 , and the discharge electrode  20  is housed within the shielding case  22 . At a location coinciding with the opening of the shielding case  22 , the control electrode  21  is disposed. The shielding case  22  serves to contain the electric fields generated as a result of voltage application to the discharge electrode  20  and the control electrode  21 . 
     Note that the shape of the shielding case  22  is partly different from that of a conventional shielding case. That is, the shielding case  22  has a pair of side plates S 1  and S 2  opposing to each other in the rotating direction R of the image carrier  1 . Unlike a conventional shielding case having such side plates each having the shape of a flat plate, at least either of the side plates S 1  and S 2  of the shielding case  22  (only the one side located on a downstream side in Embodiment 1) has a bulged portion at a location in the vicinity of the opening. In other words, the portion of the side plate close to the image carrier  1  flares outwardly (i.e., in a direction away from the discharge electrode  20 ). The outline of the shielding case  22  will be described later in more detail. 
     The air duct  23  is used to exhaust ozone (O 3 ), nitrogen oxide (NOx), and the like that are produced in an unneglectable amount during the discharge. 
     The fun  24  is a blower that is driven by an electric motor to produce a flow current in the air duct  23  and the charging device  2 . 
       FIG. 3  shows the major part of the charging device  2  shown in  FIG. 2  in a cross section taken along a plane perpendicular to the rotation axis x of the image carrier  1 .  FIG. 3  also shows a schematic representation of the electrical connection. 
     Note that although the image carrier  1  described herein has a roller-type photoreceptor and thus has the rotation axis x, the present invention is not limited to such a roller-type photoreceptor and also applicable to any structural component of any shape as long as it needs to by charged. That is to say, the image carrier  1  may be formed of a belt-type photoreceptor. In such a case, the rotation of the image carrier  1  refers to the driving of the belt to run around. Yet, since the belt-type image carrier  1  does not have a rotation axis, the description given above does not fit. In view of this, the sectional view shown in  FIG. 3  may be defined as a plane taken along “a plane substantially perpendicular to the extending direction of the discharge electrode  20 ”. 
     As shown in  FIG. 3 , the charging device  2  consistent with the present embodiment has anti-ionic-wind sheets  25   a  and  25   b.  The anti-ionic-wind sheet  25   a  is attached to the side plate S 2  located at the upstream side of the rotation direction, whereas the anti-ionic-wind sheet  25   b  is attached to the side plate S 1  located at the downstream side of the rotation direction. The discharge electrode  20  is connected to a high voltage source  26   a  that includes a high-voltage transformer. The control electrode  21  and the shielding case  22  are electrically connected and at the same potential. The control electrode  21  and the shielding case  22  are also connected to a high voltage source  26   b  of which output voltage is lower than the output voltage of the high voltage source  26   a  connected to the discharge electrode  20 . 
     In response to output voltages of the predetermined levels from the respective high voltage source  26   a  and  26   b,  a discharge starts from the discharge electrode  20  toward the control electrode  21  and the image carrier  1 . As a result of the discharge, the surface of the image carrier  1  becomes charged. Thus, by rotating the image carrier  1  while the discharge is maintained, the surface of the image carrier  1  becomes uniformly charged. 
       FIG. 4  is a view showing the cross section of the charging device  2  shown in  FIG. 3 , with the distribution of electric field strengths calculated by running simulations. 
     In the simulations, the output voltage of the high voltage source  26   a  was −5000V, whereas the output voltage of the high voltage source  26   b  was −500V. In  FIG. 4 , the electric field strength in the vicinity of the discharging point A on the discharge electrode  20  during the discharge is taken as “1” (i.e., as “reference electric field strength”). The electric field strength distribution within the shielding case  22  is shown by classifying into the following five regions: the first region where the electric field strength falls within a range from &lt;90% to 100% of the reference electric field strength; the second region where the electric field strength falls within a range from &lt;80% to 90%; the third region where the electric field strength falls within a range from &lt;40% to 80%; the fourth region where the electric field strength falls within a range from &lt;10% to 40%; and the fifth region where the electric field strength falls within a range from 0% to 10%. 
     In the cross section shown in  FIG. 4 , the side plate S 1  having the bulged portion shows such an outline that outwardly tapered (i.e., the amount of bulge gradually increases) up to a position that is a predetermined distance away from the opening of the shielding case  22  and then inwardly tapered (i.e., the amount of budge gradually decreases) as the distance from the position increases. That is, the outline of the bulged portion of the side plate S 1  according to the present embodiment has a cranked shape. 
     With reference to the cross section shown in  FIG. 4 , the following are defined for purposes of description. First, let “B” denote a point that is closest to the discharging point A of the discharge electrode  20  and on a line coinciding with the control electrode  21 . In addition, let “AB” denotes a straight line segment connecting the discharging point A and the point B. Let “B 1 ” denote a point of intersection between (i) a straight line that is substantially perpendicular to the line segment AB and passing through the point B and (ii) the inner surface of the side plate S 1 , which is located at the downstream side of the rotation direction. Let “C” denote the mid-point of the line segment AB, and “D” denote an arbitrary point located between the point B and the mid-point C. Let “D 1 ” a point of intersection between (i) a straight line that is substantially perpendicular to the line segment AB and passing through the point D and (ii) the inner surface of the side plate S 1 . Then, the shielding case  22  has such a shape and dimensions that satisfy the following condition. That is, an arbitrary straight line connecting every possible point D and the point D 1  is longer than the straight line connecting the points B and B 1 . As long as this condition is satisfied, the side plate having a bulged portion having the outline of any shape other than a cranked shape serves to produce the advantageous effect of suppressing undesirable discharge to the shielding case  22 . The advantageous effect will be described later in more detail. 
     With reference to the cross section shown in  FIG. 4 , the following are further defined for purposes of description. Let “A 1 ” denotes a point of intersection between (i) a straight line that is substantially perpendicular to the line segment AB and passing through the discharging point A and (ii) the inner surface of the side plate S 1 . Then, the shielding case  22  further satisfies the following condition. That is, the straight line connecting the discharging point A and the point Al is shorter than the straight line connecting every possible point D and the point D 1 . As long as this condition is further satisfied, it is ensured that the shielding case on the whole is kept compact in size, while sufficiently suppressing undesirable discharge to the shielding case  22 . 
     &lt;Verification&gt; 
     Firstly, the following verifies whether or not occurrences of image deletion is effectively suppressed. 
       FIG. 5  shows a cross section of a charging device  102  of a comparative example, with the distribution of electric field strengths calculated by running simulations in the same manner to  FIG. 4  relating to the charging device  2 . 
     Note that the charging device  102  of the comparative example shown in  FIG. 5  corresponds to a conventional charging device and basically identical in structure to the charging device  2  shown in  FIG. 4 , except for the shape of the shielding case. Thus, components shown in  FIG. 4  that correspond to those shown in  FIG. 5  are identified with the same reference numerals. 
       FIG. 6  shows a table of results of experiment made on the following four charging devices to check for occurrence of image deletion (or “image blurring”) in relation to different length of discharge time. Note that four charging devices includes the charging device  2  shown in  FIG. 4  and the charging device  102  shown in  FIG. 5 . 
     With reference to  FIGS. 4 and 5 , the followings are defined for purposes of purposes of description. Let “B 2 ” denote a point of intersection between (i) the straight line substantially perpendicular to the line segment AB and passing through the point B and (ii) the inner surface of the side plate S 2 , which is located at the upstream side of the rotation direction. Let “C 1 ” and “C 2 ” denote points of intersection between (i) the straight line substantially perpendicular to the line segment AB and passing through the mid-point C and (ii) the inner surface of each of the side plates S 1  and S 2 . Let “a 1 ” denote the region located inwardly along the part of the inner surface of the side plate S 1  delimited between the points B 1  and C 1 . Let “a 2 ” denote the region located inwardly along the part of the inner surface of the side plate S 2  delimited between the points B 2  and C 2 . 
     As shown in the table in  FIG. 6 , the experiments were conducted on the four charging devices having the regions where the electric field strength is 10% or less of the reference electric field strength (i.e. the region classified as the “fifth region”) occupy about 40% of the total of the “a 1 +a 2  regions”, about 50%, about 70%, and 100%. Note that the device having the fifth regions occupying about 70% corresponds to the charging device  2  shown in  FIG. 4 , whereas the device having the fifth regions occupying about 40% corresponds to the charging device  102  shown in  FIG. 5 . 
     In the experiment, first, each of the subject devices were operated to cause accelerative discharge and sequentially brought into the six different states in terms of the lengths of discharge time. In each state, the respective devices were operated to print out a test pattern image on 1,000 sheets of A4-size paper in succession. After being allowed to stand for 24 hours, the printed sheets were compared against limit samples to make judgments on the image quality. In  FIG. 6 , the mark “◯” indicates the judgment that the image quality is acceptable without any problem at all. The mark “Δ” indicates the judgment that the image quality is acceptable, although slight image deletion is noted. More specifically, the image deletion is of minor nature as compared with those of the limit samples and an acceptable level of legibleness is still ensured. The mark “×” indicates the judgment that the image quality is not acceptable since the image deletion is observed and the image quality is judged to be inferior to the limit samples. 
       FIG. 6  shows that image deletion occurred less and less with the increase in the percentage of the fifth region in the total of “a 1 +a 2  regions”, even if the discharge time became longer. 
     Regarding the charging device having the fifth region occupying about 40% of the total of “a 1 +a 2  regions” (i.e., the charging device  102  of the comparative example), the following is noted. While the discharge time was 0 hours (initial state), no problem was observed at all. However, when the discharge time reached about 25 hours, slight image deletion was observed. When the discharge time reached about 50 hours or more, the image quality of the printed output was no longer acceptable. This experimental result shows that the life of this charging device is shorter than the estimated life of typical compact printers. It is thus concluded that this device is not applicable to printers in general. 
     Regarding the charging device having the fifth region occupying about 50% of the total of “a 1 +a 2  regions”, the following is noted. Up until the discharge time reached 25 hours, the device exhibited no problem at all. However, when the discharge time reached about 50 hours and about 75 hours, slight image deletion was observed. After the discharge time reached about 100 hours, the image quality of the printed output was no longer acceptable. This experimental result shows that this charging device is configured to withstand the discharge time of 25 hours or more, which is longer than the estimated life of typical compact printers. Thus, this charging device is applicable to compact printers. 
     Regarding the charging device having the fifth region occupying about 70% of the total of “a 1 +a 2  regions” (i.e., the charging device  2  shown in  FIG. 4 ), the following is noted. Up until the discharge time reached 50 hours, the charging device exhibited no problem at all. After the discharge time reached about 75 hours and about 100 hours, slight image deletion was observed. After the discharge time reached about 120 hours or more, the image quality of the printed output was no longer acceptable. This experimental result shows that the charging device is configured to withstand the discharge time of 50 hours or more, which is longer than the estimated life of general-purpose printers. Thus, this type of charging device is applicable to general-purpose printers. 
     Regarding the charging device having the fifth region occupying about 100% of the total of “a 1 +a 2  regions”, the following is noted. Up until the discharge time reached 75 hours, this charging device exhibited no problem at all. After the discharge time reached about 100 hours, slight image deletion was observed. After the discharge time reached about 120 hours or more, the image quality of the printed output was no longer acceptable. This experimental result shows that the charging device is configured to withstand the discharge time of 75 hours or more, which is longer than the estimated life of typical large printers. Thus, this type of charging device is applicable to typical large printers. 
     As shown by the experimental results, the size and shape of the bulged portion of the shielding case  22  maybe adjusted so that the percentage of the fifth region occupying the total of “a 1 +a 2  regions” fall in a desired one of the ranges suitable for image forming apparatuses of different specifications (such as large and small). With such an adjustment, the charging device suitable for a specific image forming apparatus is provided. 
     Next, the following now verifies the original performance of the charging device. 
       FIG. 7  is a graph showing the distribution of electric field strengths in the vicinity of the surface of the image carrier  1 , regarding both the charging device  2  shown in  FIG. 4  and the charging device  102  shown in  FIG. 5 . In  FIG. 7 , the horizontal axis of the graph indicates the measuring locations [mm] on the surface of the image carrier  1 . Each measuring location is indicated by the distance from the ZERO location toward the upstream direction. The ZERO location is set at a location that corresponds, in the vertical direction, to a downstream-side edge of the control electrode  21 . The vertical axis of the graph indicates the electric field strength [V/m] measured on the respective measuring locations on the surface of the image carrier  1 . The electric field strength is a substitutional characteristic for the charged amount. 
     As apparent from  FIG. 7 , the charging device  2  shown in  FIG. 4  and the charging device  102  shown in  FIG. 5  both exhibited substantially identical distributions of the electric field strengths in the vicinity of the surfaces of the respective image carriers  1 . This means that the two devices are without any notable difference regarding the balance of the discharge current and identical in terms of the performance. 
     Note that the image carrier  1  according to the present embodiment is an OPC photoreceptor. However, an AL photoreceptor may be employed instead of the OPC photoreceptor in the structures shown in  FIGS. 4 and 5 . In the case where an AL photoreceptor is employed in each structure, the percentage of the amount of electric current flowing through the AL photoreceptor, the control electrode, the shielding case is as follows. Regarding the structure shown in  FIG. 4 , the amount electric current flowing through the AL photoreceptor is 51%, the control electrode is 34.5%, and the shielding case is 14.5% of the entire electric current. With the structure shown in  FIG. 5 , the amount electric current flowing through the AL photoreceptor is 51.5%, the control electrode is 36%, and the shielding case is 12.5% of the entire electric current. The comparison between them show that the charging device  2  shown in  FIG. 4  and the charging device  102  shown in  FIG. 5  exhibit no notable difference regarding the balance of the discharge current, even if the image carrier  1  is an AL photoreceptor instead of an OPC photoreceptor. Thus, the two devices are identical in terms of the performance. 
     &lt;Recapitulation&gt; 
     As has been described above, in the charging device for use in an electrographic image forming apparatus according to the present embodiment, at least either of the pair of side plates of the shielding case has an outline that outwardly bulges at a portion closer to the opening. This structure produces an excellent advantageous effect of suppressing corrosion of the shielding case and suppressing occurrences of image deletion. In addition, these effects are achieved without complicating the device structure, inhibiting the size reduction, and increasing the cost. 
     Embodiment 2  
     &lt;Structure&gt; 
     According to Embodiment 1 described above, one of the side plates has a cranked shape. Embodiment 2 differs from Embodiment 1 in that both the side plates have a cranked shape. 
       FIG. 8  is a cross sectional view showing the major part of a charging device  202  consistent with Embodiment 2. The cross section is taken along a plane perpendicular to the rotation axis x of an image carrier  1 .  FIG. 8  also shows a schematic representation of the electrical connection. The charging device  202  of Embodiment 2 is basically identical to the charging device  2  of Embodiment 1, except for the shape of the shielding case. Thus, the identical components are identified with the same reference numerals. 
     As shown in  FIG. 8 , the charging device  202  consistent with the present embodiment has a shielding case  222  and anti-ionic-wind sheets  25   a  and  25   b  each attached to the outer surface of either side plate of the shielding case  222 . A discharge electrode  20  is connected to the high voltage source  26   a  that includes a high-voltage transformer. A control electrode  21  and the shielding case  222  are electrically connected and at the same potential. The control electrode  21  and the shielding case  222  are also connected to a high voltage source  26   b  of which output voltage is lower than the output voltage of the of a high voltage source  26   a  connected to the discharge electrode  20 . Note that the anti-ionic-wind sheets  25   a  according to Embodiments 1 and 2 are different in shape. Yet, there is no difference in their functionality, so that both the sheets are identified with the same reference numeral. 
       FIG. 9  is a view showing the cross section of the charging device  202  shown in  FIG. 8 , with the distribution of electric field strengths calculated by running simulations. 
     Similarly to Embodiment 1, in  FIG. 9 , the output voltage the high voltage source  26   a  is −5000V, whereas the output voltage of the high voltage source  26   b  is −500V. In addition, the electric field strength in the vicinity of the discharging point A on the discharge electrode  20  during the discharge is taken as “1” (i.e., as “reference electric field strength”). The electric field strength distribution within the shielding case  222  is shown by classifying into the following five regions: the first region where the electric field strength falls within a range from &lt;90% to 100% of the reference electric field strength; the second region where the electric field strength falls within a range from &lt;80% to 90%; the third region where the electric field strength falls within a range from &lt;40% to 80%; the fourth region where the electric field strength falls within a range from &lt;10% to 40%; and the fifth region where the electric field strength falls within a range from 0% to 10%. 
     In the cross section shown in  FIG. 9 , each of the side plates S 1  and S 2  has a bulged portion. Each bulged portion shows such an outline that outwardly tapered (i.e., the amount of bulge gradually increases) up to a position that is a predetermined distance away from the opening of the shielding case  222  and then inwardly tapered (i.e., the amount of budge gradually decreases) as the distance from the position increases. That is, the outline of each bulged portion according to the present embodiment has a cranked shape. 
     With reference to the cross section shown in  FIG. 9 , the following are defined for purposes of description. First, let “B” denotes a point that is closest to the discharging point A of the discharge electrode  20  and on a line coinciding with the control electrode  21 . Let “AB” denote a straight line segment connecting the discharging point A and the point B. Let “B 1 ” denote a point of intersection between (i) a straight line that is substantially perpendicular to the line segment AB and passing through the point B and (ii) the inner surface of the side plate S 1 , which is located at the downstream side of the rotation direction. Let “B 2 ” denote a point of intersection between (i) the straight line that is substantially perpendicular to the line segment AB and passing through the point B and (ii) the inner surface of the side plate S 2 , which is located at the upstream side of the rotation direction. Let “C” denote the mid-point of the line segment AB, and “D” denote an arbitrary point located between the point B and the mid-point C. Let “D 1 ” denote a point which of intersection between (i) a straight line that is substantially perpendicular to the line segment AB and passing through the point D and (ii) the inner surface of the side plate S 1 . Let “D 2 ” denote a point intersection between (i) the straight line that is substantially perpendicular to the line segment AB and passing through the point D and (ii) the inner surface of the side plate S 2 . Then, the shielding case  222  has such a shape and dimensions that satisfy the following condition. That is, an arbitrary straight line connecting every possible point D and the point D 1  is longer than the straight line connecting the points B and B 1 . In addition, an arbitrary straight line connecting every possible point D and the point D 2  is longer than the straight line connecting the point B and the point B 2 . As long as this condition is satisfied, the pair of side plates each having a bulged portion having the outline of any shape other than a cranked shape serves to produce the advantageous effect of suppressing undesirable discharge to the shielding case  222 . The advantageous effect will be described later in more detail. 
     With reference to the cross section shown in  FIG. 9 , the following are further defined for purposes of description. Let “A 1 ” denote a point of intersection between (i) a straight line that is substantially perpendicular to the line segment AB and passing through the discharging point A and (ii) the inner surface of the side plate S 1 . Let “A 2 ” denote a point of intersection between (i) the straight line that is substantially perpendicular to the line segment AB and passing through the discharging point A and (ii) the inner surface of the side plate S 2 . Then, the shielding case  222  further satisfies the following condition. That is, the straight line connecting discharging point A and the point Al is shorter than the straight line connecting every possible point D and the point D 1 . Further, the straight line connecting discharging point A and the point A 2  is shorter than the straight line connecting every possible point D and the point D 2 . As long as this condition is further satisfied, it is ensured that the shielding case  222  on the whole is kept compact in size, while sufficiently suppressing undesirable discharge to the shielding case  222 . 
     &lt;Verification&gt; 
       FIG. 10  shows a table of results of experiment made on the following four charging devices, including the charging device  202  shown in  FIG. 9  to check for occurrence of image deletion (or “image blurring”) in relation to different length of discharge time. 
     With reference to  FIG. 9 , the followings are defined for purposes of purposes of description. Let “A 1 ” denote a point of intersection between (i) the straight line substantially perpendicular to the line segment AB and passing through the discharging point A and (ii) the inner surface of the side plate S 1 . Let “A 2 ” denote a point of intersection between (i) the straight line substantially perpendicular to the line segment AB and passing through the discharging point A and (ii) the inner surface of the side plate S 2 . Let “B 1 ” denote a point of intersection between (i) the straight line substantially perpendicular to the line segment AB and passing through the point B and (ii) the inner surface of the side plate S 1 . Let “B 2 ” denote a point of intersection between (i) the straight line substantially perpendicular to the line segment AB and passing through the point B and (ii) the inner surface of the side plate S 2 . Let “β 1 ” denote the region located inwardly along the part of the inner surface of the side plate S 1  delimited between the points B 1  and Al. Let “β 2 ” denote the region located inwardly along the part of the inner surface of the side plate S 2  delimited between the points B 2  and A 2 . 
     As shown in the table in  FIG. 10 , the experiments were conducted on the four charging devices having the regions where the electric field strength is 10% or less of the reference electric field strength (i.e. the region classified as the “fifth region”) occupy about 40% of the total of the “β 1 +β 2  regions”, about 50%, about 70% (the charging device  202  shown in  FIG. 9 ), and 100%. 
     Similarly to Embodiment 1, in the experiment, each of the subject devices were operated to cause accelerative discharge and sequentially brought into the six different states in terms of the lengths of discharge time. In each state, the respective devices were operated to print out a test pattern image on 1,000 sheets of A4-size paper in succession. After being allowed to stand for 24 hours, the printed sheets were compared against limit samples to make judgments on the image quality. In  FIG. 10 , the mark “◯” indicates the judgment that the image quality is acceptable without any problem at all. The mark “Δ” indicates the judgment that the image quality is acceptable, although slight image deletion is noted. More specifically, the image deletion is of minor nature as compared with those of the limit samples and an acceptable level of legibleness is still ensured. The mark “×” indicates the judgment that the image quality is not acceptable since the image deletion is observed and the image quality is judged to be inferior to the limit samples. 
       FIG. 10  shows that image deletion occurred less and less with the decrease in the percentage of the fifth region in the total of “β 1 +β 2  regions”, even if the discharge time became longer. 
     Regarding the charging device having the fifth region occupying about 40% of the total of “β 1 +β 2  regions”, the following is noted. Up until the discharge time reached 50 hours, the device exhibited no problem at all. However, when the discharge time reached about 75 hours and about 100 hours, slight image deletion was observed. After the discharge time reached about 120 hours, the image quality of the printed output was no longer acceptable. This experimental result shows that this charging device is configured to withstand the discharge time of 50 hours or more, which is longer than the estimated life of general-purpose printers. Thus, this type of charging device is applicable to general-purpose printers. 
     Regarding the charging device having the fifth region occupying about 50% of the total of “β 1 +β 2  regions”, the following is noted. Up until the discharge time reached 75 hours, the charging device exhibited no problem at all. After the discharge time reached about 100 hours, slight image deletion was observed. After the discharge time reached about 120 hours or more, the image quality of the printed output was no longer acceptable. This experimental result shows that the charging device is configured to withstand the discharge time of 75 hours or more, which is longer than the estimated life of most of general-purpose printers. Thus, this type of charging device is applicable to almost all types of printers. 
     Regarding the charging device having the fifth region occupying about 70% of the total of “β 1 +β 2  regions”, the following is noted. Up until the discharge time reached 100 hours, the charging device exhibited no problem at all. After the discharge time reached about 120 hours, slight image deletion was observed. 
     Regarding the charging device having the fifth region occupying about 100% of the total of “β 1 +β 2  regions”, the following is noted. In the state where the discharge time reached  120  hours, the charging device exhibited no problem at all. 
     &lt;Recapitulation&gt; 
     As has been described above, the charging device for use in an electrographic image forming apparatus according to the present embodiment, each of the pair of side plates of the shielding case has an outline that outwardly bulges at a portion closer to the opening. Although being a little more complex and expensive as compared with the structure of Embodiment 1, the structure of Embodiment 2 produces an excellent advantageous effect of suppressing corrosion of the shielding case and suppressing occurrences of image deletion. In addition, these effects are achieved without complicating the device structure, inhibiting the size reduction, and increasing the cost. 
     [Modification 1] 
     According to Embodiment 1, one of the side plates of the shielding case has a bulged portion with a cranked outline, at a portion in the vicinity of the opening. According to Modification 1, the budged portion has a circularly curved outline instead of a cranked outline. 
     Similarly to  FIG. 4  relating to Embodiment 1,  FIG. 11  shows the section of a charging device  302  according to Modification 1, with the distribution of electric field strengths calculated by running simulations. 
     As apparent from  FIG. 11 , the charging device  302  consistent with Modification 1 is basically identical to the charging device  2  according to Embodiment 1, except for that the structure of a shielding case  322  is partly different from that of the shielding case  22 . That is, the charging device  302  exhibits a similar electric field strength distribution and achieves similar advantageous effects as those of Embodiment 1. 
     [Modification 2] 
     According to Embodiment 2, each of the side plates of the shielding case has a budged portion with a cranked outline, at a portion in the vicinity of the opening. According to Modification 2, each budged portion has a circularly curved outline instead of a cranked outline. 
     Similarly to  FIG. 9  relating to Embodiment 2,  FIG. 12  shows the section of a charging device  402  according to Modification 2, with the distribution of electric field strengths calculated by running simulations. 
     As apparent from  FIG. 12 , the charging device  402  consistent with Modification 2 is basically identical to the charging device  202  according to Embodiment 2, except for that the structure of a shielding case  422  is partly different from that of the shielding case  222 . That is, the charging device  402  exhibits a similar electric field strength distribution and achieves similar advantageous effects as those of Embodiment 2. 
     Note in addition, the bulged portion according to Modifications 1 and 2 has the circularly curved outline. Alternatively, however, the outline of the budged portion may include either a curve defining a portion of a circular or elliptical shape or a curve defining a combination of portions of circular and elliptical shapes. In either case, the bulged portion has a continuously-curved outline, which allows manufacturing with ease and invariant properties. 
     Note in addition, although Embodiments 1 and 2 and Modifications 1 and 2 employ the image carrier  1  having a roller-type photoreceptor, the present invention is applicable to any structural component of any shape as long as it needs to by charged. For example, the image carrier  1  may be an intermediate transfer medium. In addition, the image carrier  1  may be a belt-type image carrier or a drum-type image carrier. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.