Abstract:
There is provided a charging device including: a discharge electrode extending in a first direction; and a cleaning member configured to move in the first direction to clean the discharge electrode. The cleaning member includes a rigid member in contact with the discharge electrode, and a cushion member supporting the rigid member. The cushion member is positioned opposite to the discharge electrode with respect to the rigid member.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2013-074708 filed Mar. 29, 2013. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a charging device disposed in an electrophotographic image forming apparatus. 
     BACKGROUND 
     Conventional electrophotographic image-forming devices produce a developer image on a photosensitive member by first charging the photosensitive member with a charging device, then forming an electrostatic latent image on the photosensitive member and supplying developer thereto. One type of charging device used in this image-forming device is a scorotron charging device provided with a discharge wire and grid wires. 
     However, foreign matter may become deposited on the discharge wire and grid wires when the photosensitive member is charged, for example. When deposits build up on these wires, the quality of charge applied to the photosensitive member can degrade. For this reason, some scorotron charging devices known in the art are equipped with a cleaning member for removing foreign matter deposited on the discharge wire and grid wires. 
     In one proposed scorotron charging device, the cleaning member is provided with a sliding member that contacts both the discharge wire and the grid wires (See Japanese Utility Model Application Publication No. H04-24759). The sliding member is configured of a flexible insulation sheet. When the cleaning member in this scorotron charging device is moved, the sliding member slides over the discharge wire and grid wires to clean the same. 
     SUMMARY 
     However, since the sliding member in the scorotron charging device described above is configured of a flexible insulation sheet, it is difficult to achieve uniform contact between the sliding member and the discharge wire and grid wires. Consequently, the cleaning member may not be capable of reliably rubbing against the discharge wire and grid wires and, hence, may not sufficiently remove deposits from the same. 
     In view of the foregoing, it is an object of the present invention to provide a charging device that is capable of reliably cleaning at least one of a discharge electrode and a grid electrode. 
     In order to attain the above and other objects, there is provided a charging device including a discharge electrode and a cleaning member. The discharge electrode extends in a first direction, and the cleaning member is configured to move in the first direction to clean the discharge electrode. The cleaning member includes: a rigid member in contact with the discharge electrode; and a cushion member supporting the rigid member. The cushion member is positioned opposite to the discharge electrode with respect to the rigid member. 
     According to another aspect of the present invention, there is also provided a charging device including a discharge electrode, a grid electrode and a cleaning member. The discharge electrode extends in a first direction. The grid electrode extends in the first direction and is disposed in separation from the discharge electrode. The cleaning member is configured to move in the first direction to clean the grid electrode. The cleaning member includes: a rigid member in contact with the grid electrode; and a cushion member supporting the rigid member and configured to surround the grid electrode via the rigid member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is an upper right view of a drum cartridge provided with a charging unit according to a first embodiment of the present invention; 
         FIG. 2  is a side cross-sectional view of the charging unit according to the first embodiment as viewed from its front and right side, the charging unit including a cleaner, a discharge wire, and a grid electrode; 
         FIG. 3  is an exploded perspective view of the cleaner and the discharge wire shown in  FIG. 2  when viewed from their lower-right side; 
         FIG. 4  is an exploded perspective view of the cleaner, the discharge wire and the grid electrode shown in  FIG. 2  when viewed from their upper and right side; 
         FIG. 5A  is a plan view of the charging unit according to the first embodiment; 
         FIG. 5B  is a cross-sectional view of the charging unit according to the first embodiment taken along a plane  5 A- 5 A shown in  FIG. 5A ; 
         FIG. 6A  is a cross-sectional view of the charging unit according to the first embodiment taken along a plane  5 B- 5 B shown in  FIG. 5A , wherein the cleaner is at a home position; 
         FIG. 6B  is a cross-sectional view of the charging unit according to the first embodiment taken along a plane  5 C- 5 C shown in  FIG. 5A , wherein the cleaner is at a cleaning position; 
         FIG. 7  is a cross-sectional view of a cleaning member of the cleaner taken along a plane  6 D- 6 D shown in  FIG. 6A ; 
         FIG. 8  is a central cross-sectional view of a printer provided with the drum cartridge shown in  FIG. 1 ; 
         FIG. 9A  is a vertical cross-sectional view of a cleaning member according to a second embodiment of the present invention; 
         FIG. 9B  is a vertical cross-sectional view of a cleaning member according to a third embodiment of the present invention 
         FIG. 9C  is a vertical cross-sectional view of a cleaning member according to a fourth embodiment of the present invention 
         FIG. 10A  is a plan view of a charging unit according to a fifth embodiment of the present invention; and 
         FIG. 10B  is a cross-sectional view of the charging unit according to the fifth embodiment taken along a plane  10 E- 10 E shown in  FIG. 10A . 
     
    
    
     DETAILED DESCRIPTION 
     First, a drum cartridge  1  will be described. The drum cartridge  1  is provided with a charging unit  2  according to a first embodiment of the present invention. 
     1. Structure of the Drum Cartridge 
     As shown in  FIG. 1 , the drum cartridge  1  includes a drum unit  3 , and the charging unit  2 . 
     Directions related to the drum cartridge  1  in the following description will be given under the assumption that the drum cartridge  1  is resting on a level surface, and more specifically will be based on the directional arrows shown in the drawings. Further, forward, rearward, leftward, rightward, upward, and downward directions related to the charging unit  2  will be defined based on the state of the charging unit  2  when the charging unit  2  is mounted in the drum cartridge  1  resting on a level surface. 
     (1-1) Drum Unit 
     The drum unit  3  includes a drum-unit frame  50 , and a photosensitive drum  51 . 
     The drum-unit frame  50  has a box-like shape that is open on the top. The drum-unit frame  50  includes a pair of frame side walls  53 . 
     The frame side walls  53  are arranged parallel to each other and are separated in a left-right direction. The frame side walls  53  have a plate shape that is generally rectangular in a side view and elongated in a front-rear direction. Each frame side wall  53  has a grooved part  52 . 
     The grooved parts  52  are respectively formed in the outer left-right surfaces of the corresponding frame side walls  53 , in the rear portions thereof. The grooved parts  52  have a general U-shape in a side view that is open on the top. The grooved parts  52  are recessed inward in the respective left-right outer surfaces of the frame side walls  53 . 
     While not shown in the drawings, each of the frame side walls  53  includes a through-hole formed in the approximate center region of the grooved part  52  when viewed from the side. The through-holes penetrate the respective frame side walls  53  in the left-right direction. 
     The photosensitive drum  51  includes a drum body  54 , and a drum shaft  55 . 
     The drum body  54  is disposed between rear portions of the frame side walls  53 . The drum body  54  has a general cylindrical shape that is elongated in the left-right direction. More specifically, the drum body  54  includes a base metal tube having a general cylindrical shape, and a photosensitive resin layer coating the surface of the metal tube. 
     The drum shaft  55  is formed of a metal in a general columnar shape that is elongated in the left-right direction. The drum shaft  55  has a left-right length greater than the left-right dimension of the drum body  54 . The drum shaft  55  penetrates the drum body  54  in the left-right direction and is aligned with the center axis of the same. Both left and right ends of the drum shaft  55  protrude outward in corresponding left and right directions from the drum body  54 . The drum shaft  55  and drum body  54  are capable of rotating relative to each other. 
     The left and right ends of the drum shaft  55  are received in the through-holes (not shown) formed in the frame side walls  53  of the drum-unit frame  50  and through-holes  16  formed in frame side walls  8  of a charging-unit frame  4  (described later) so as to be incapable of rotating relative to the same. With this configuration, the frame side walls  53  support the photosensitive drum  51 , with the drum body  54  capable of rotating relative to the drum-unit frame  50 . 
     (1-2) Charging Unit 
     The charging unit  2  includes the charging-unit frame  4 , a scorotron charger  5 , and a cleaner  6 . 
     The charging-unit frame  4  includes a pair of frame side walls  8 , a charger support part  26 , and a top wall  10 . 
     The frame side walls  8  are arranged parallel to each other and are separated in the left-right direction. As shown in  FIG. 2 , each of the frame side walls  8  includes a body part  11 , and a protruding part  12 . 
     The body parts  11  have a plate shape that is generally rectangular in a side view and elongated in the front-rear direction. The protruding parts  12  are provided in positions corresponding to the grooved parts  52  formed in the frame side walls  53 . The protruding parts  12  are generally rectangular in a side view with a shape similar to the grooved parts  52 . The protruding parts  12  protrude downward from bottom edges of the respective body parts  11  in an approximate front-rear center region thereof. Each protruding part  12  has a through-hole  16 . 
     The through-holes  16  are generally circular in a side view and penetrate the center portion of the respective protruding parts  12  in the left-right direction. 
     The charger support part  26  is disposed between the approximate front-rear center regions of the body parts  11  constituting the frame side walls  8 . The charger support part  26  is configured of a pair of grid support parts  17 , and a pair of cleaner support parts  9 . 
     The grid support parts  17  are disposed adjacent to the respective body parts  11  on the inner left-right sides thereof and are positioned to correspond to the left and right ends of a grid  18  described later. The grid support parts  17  have a general U-shape in a side view that is open on the top. The grid support parts  17  are positioned in the approximate front-rear center regions on the inner left-right surfaces of the corresponding body parts  11  and protrude inward relative to the left-right direction. 
     The cleaner support parts  9  are spaced apart from each other in the front-rear direction so as to sandwich the pair of grid support parts  17  from the front and rear sides thereof. The cleaner support parts  9  bridge the body parts  11  of the frame side walls  8 . Each of the cleaner support parts  9  includes a vertical wall  27 , and a guide part  13 . 
     The vertical wall  27  has a plate shape that is generally rectangular in a front view and elongated in the left-right direction. The left and right ends of the vertical wall  27  are connected to the inner left and right surfaces of the corresponding body parts  11  constituting the frame side walls  8 . 
     The guide parts  13  are configured to guide movement of the cleaner  6 . Each guide part  13  includes a rail part  28 , and a protrusion  15 . 
     The rail part  28  has a general L-shape in a side view. That is, the rail part  28  extends inward relative to the front-rear direction from the inner front-rear surface of the corresponding vertical wall  27 , then bends and extends upward. The rail part  28  also extends across the entire left-right length of the vertical wall  27 . As shown in  FIG. 5B , the rail part  28  slopes slightly upward from the left end toward the right end. 
     As illustrated in  FIG. 2 , the rail part  28  of the front guide part  13  and the rail part  28  of the rear guide part  13  are aligned with each other in the front-rear direction and are spaced apart from each other in the front-rear direction. 
     The protrusion  15  has a general rectangular shape in a side view. The protrusion  15  protrudes downward from the approximate front-rear center region on the bottom surface of the corresponding rail part  28  and extends across the entire left-right dimension of the rail part  28 . 
     The top wall  10  has a plate shape that is generally rectangular in a plan view and elongated in the left-right direction. The top wall  10  protrudes rearward from the vertical wall  27  of the rear cleaner support part  9 . The left and right ends of the top wall  10  are coupled to the inner left-right surfaces of the body parts  11  constituting the corresponding frame side walls  8 . 
     As shown in  FIG. 1 , the charging-unit frame  4  is disposed on the rear portion of the drum-unit frame  50  such that the protruding parts  12  of the frame side walls  8  are fitted into the corresponding grooved parts  52  from above. In this state, the through-holes  16  formed in the frame side walls  8  of the charging-unit frame  4  communicate in the left-right direction with the through-holes (not shown) formed in the frame side walls  53 . 
     The scorotron charger  5  is supported on the charger support part  26  of the charging-unit frame  4 . The scorotron charger  5  includes a grid  18 , and a discharge wire  19 . 
     As shown in  FIG. 4 , the grid  18  includes a frame part  21 , and a ridged part  22 . 
     The frame part  21  has a general square U-shape in a side view, with the opening of the “U” facing upward, and is elongated in the left-right direction. The frame part  21  includes a pair of side plates  23 , a right bottom plate  24 , and a left bottom plate  25 . 
     The side plates  23  are arranged parallel to each other and are separated in the front-rear direction. The side plates  23  have a plate shape that is generally rectangular in a front view and elongated in the left-right direction. 
     The right bottom plate  24  has a plate shape that is generally rectangular in a plan view and elongated in the front-rear direction. The right bottom plate  24  couples the bottom edges of the side plates  23  in the front-rear direction at the right end thereof. 
     The left bottom plate  25  has a plate shape that is generally rectangular in a plan view and elongated in the front-rear direction. The left bottom plate  25  couples the bottom edges of the side plates  23  in the front-rear direction at the left end thereof. 
     The ridged part  22  is disposed between the right bottom plate  24  and left bottom plate  25 . The ridged part  22  is configured of a plurality of ridge members  39  extending linearly in the left-right direction and juxtaposed at intervals in the front-rear direction. Each of the ridge members  39  is connected to the right bottom plate  24  at the right end and to the left bottom plate  25  at the left end. 
     The left and right ends of the grid  18  are fitted into the corresponding grid support parts  17 . With this configuration, the charging-unit frame  4  supports the grid  18  between the pair of cleaner support parts  9  in the front-rear direction and between the pair of frame side walls  8  in the left-right direction. As shown in  FIG. 8 , the ridged part  22  of the grid  18  confronts but is separated from the top of the photosensitive drum  51 . 
     As shown in  FIG. 4 , the discharge wire  19  extends in the left-right direction and includes a wire  38 , and a pair of ring hooks  20 . 
     The wire  38  is formed of tungsten, for example. One of the ring hooks  20  is provided on each end of the wire  38 . The ring hooks  20  have a general annular shape. 
     The ring hooks  20  of the discharge wire  19  are engaged with engaging parts (not shown) provided in the grid support parts  17 . In this way, the charging-unit frame  4  supports the discharge wire  19  in a taut state. As shown in  FIG. 8 , the wire  38  of the discharge wire  19  is arranged at a position between and separated from the side plates  23  of the grid  18  with respect to the front-rear direction and at a position above and separated from the ridged part  22  of the grid  18 . The wire  38  of the discharge wire  19  is also positioned between a pair of sandwiching parts  33  described later. 
     As shown in  FIG. 4 , the cleaner  6  includes a retaining frame  30 , and a cleaning member  40 . 
     The retaining frame  30  includes an operating part  31 , a pair of coupling parts  32 , and the pair of sandwiching parts  33 . 
     The operating part  31  has a plate shape that is generally L-shaped in a front view and is elongated in the front-rear direction. 
     The coupling parts  32  are disposed to the lower right of the operating part  31 . The upper left ends of the coupling parts  32  are coupled to the operating part  31 . The coupling parts  32  are spaced apart in the front-rear direction. The coupling parts  32  have a plate shape that is generally L-shaped in a side view and elongated in the left-right direction. 
     More specifically, each of the coupling parts  32  includes a vertical part  36 , and a horizontal part  37 . The vertical part  36  has a plate shape that is generally rectangular in a front view and elongated in the left-right direction. The horizontal part  37  has a plate shape that is generally rectangular in a plan view and elongated in the left-right direction. The horizontal part  37  is formed continuously with the bottom edge of the corresponding vertical part  36  and protrudes outward in the corresponding front or rear direction. 
     The left portions of the vertical parts  36  at the top edges thereof are coupled with the bottom right edge of the operating part  31 . In other words, the operating part  31  and coupling parts  32  are integrally formed. 
     As shown in  FIG. 3 , one of the sandwiching parts  33  is provided on the bottom of each coupling part  32 , so that a gap is formed between the sandwiching parts  33  in the front-rear direction. The sandwiching parts  33  have a rail-like shape that is elongated vertically. The sandwiching parts  33  extend downward from the bottom surfaces of the corresponding horizontal parts  37  near the left edge thereof. As shown in  FIG. 6A , the bottom ends of the sandwiching parts  33  slope outward in the front-rear direction toward the bottom thereof so that the gap between the sandwiching parts  33  grows larger toward its bottom. 
     As shown in  FIG. 2 , the retaining frame  30  is arranged between the pair of vertical walls  27  such that both the front and rear horizontal parts  37  are positioned below the corresponding rail parts  28 , and both the front and rear ends of the operating part  31  are positioned above the corresponding rail parts  28 . The horizontal parts  37  are also disposed adjacent to the corresponding protrusions  15  on the inner front-rear sides thereof. 
     With this configuration, the retaining frame  30  can move in the left-right direction along the rail parts  28 , as will be described later in greater detail. However, the retaining frame  30  is normally disposed in a prescribed position (hereinafter called the “home position”) wherein the operating part  31  is disposed above the left ends of the guide parts  13 . 
     As shown in  FIG. 6A , the cleaning member  40  is folded in half to form a general U-shape in a side view with the opening of the “U” facing downward and is disposed between the pair of sandwiching parts  33 . The cleaning member  40  includes a cushion member  42 , a first double-sided tape strip  41 , a second double-sided tape strip  44 , and a rigid member  43 . 
     The cushion member  42  is formed of an elastic foam, such as a urethane sponge. While not shown in the drawings, the cushion member  42  has a rail-like shape that extends in the front-rear direction prior to being mounted in the charging-unit frame  4 . Further, the 25% compression load value of the cushion member  42  is preferably between 0.04 and 1.37 MPa, and more preferably between 0.14 and 0.39 MPa. The 25% compression load can be measured using a common compression tester based on the JIS K 6767 standard (as defined by Japan Industrial Standard). 
     The first double-sided tape strip  41  is a strip of tape having adhesive applied to both surfaces and is adhered to the top surface of the cushion member  42 . As shown in  FIG. 7 , the left-right dimension of the first double-sided tape strip  41  is equivalent to that of the cushion member  42 . 
     The second double-sided tape strip  44  is a strip of tape having adhesive applied to both surfaces and is adhered to the bottom surface of the cushion member  42 . The left-right dimension of the second double-sided tape strip  44  is equivalent to that of the cushion member  42 . Consequently, a right edge  91  of the second double-sided tape strip  44  is aligned with a right edge  94  of the cushion member  42  in the left-right direction, and a left edge  93  of the second double-sided tape strip  44  is aligned with a left edge  95  of the cushion member  42  in the left-right direction. 
     The rigid member  43  is formed of a polymer film, such as polyethylene terephthalate, polyacetal, or polytetrafluoroethylene. Further, the compressive elasticity modulus of the rigid member  43  (at 25° C.) is between 1 GPa and 10 GPa, and preferably between 2 GPa and 6 GPa, for example. Thus, the rigid member  43  is harder than the cushion member  42 . Here, the compressive elasticity modulus can be measured using a common compression tester. 
     The left-right dimension of the rigid member  43  is greater than that of the cushion member  42 . 
     The rigid member  43  is affixed to the bottom surface of the second double-sided tape strip  44  such that a right edge  90  of the rigid member  43  is positioned further right than the right edge  91  of the second double-sided tape strip  44  and the right edge  94  of the cushion member  42 , and a left edge  92  of the rigid member  43  is positioned further left than the left edge  93  of the second double-sided tape strip  44  and the left edge  95  of the cushion member  42 . In other words, the right edge  90  of the rigid member  43  is positioned outside the right edge  94  of the cushion member  42  in the left-right direction, and the right edge  91  of the second double-sided tape strip  44  is positioned inside the right edge  90  of the rigid member  43  in the left-right direction. Further, the second double-sided tape strip  44  is fixed to the cushion member  42  and rigid member  43 , and the cushion member  42  supports the rigid member  43  through the second double-sided tape strip  44 . 
     As shown in  FIG. 6A , the cleaning member  40  is folded in half and positioned between the pair of sandwiching parts  33  such that the rigid member  43  is pressed against the wire  38  of the discharge wire  19  from both sides. By fixing the first double-sided tape strip  41  to the inner front-rear surfaces of the sandwiching parts  33 , the first double-sided tape strip  41  is fixed to the sandwiching parts  33 . Thus, the cushion member  42  is folded in half with the rigid member  43  nipping the wire  38  of the discharge wire  19 . 
     With this configuration, the rigid member  43  surrounds the entire circumference of the wire  38 , making surface contact with the wire  38 . Further, the cushion member  42  is positioned opposite to the wire  38  (which the rigid member  43  contacts) with respect to the rigid member  43 . By elastically deforming, the cushion member  42  allows the rigid member  43  to deform to conform to the peripheral surface of the wire  38 . 
     2. Cleaning Operation 
     Next, an operation performed by the cleaner  6  to clean the discharge wire  19  will be described. 
     In order to clean the discharge wire  19 , an operator moves the operating part  31  of the retaining frame  30  rightward from its home position shown in  FIG. 1 . While holding the cleaning member  40 , the retaining frame  30  is guided along the rail parts  28  of the guide parts  13  so as to move slightly upward from left to right. That is, as illustrated in  FIG. 5B , the guide parts  13  guide the movement of the cleaner  6  so that a direction X in which the cleaner  6  and, hence, the retaining frame  30  moves is sloped relative to the left-right direction (the horizontal direction). Here, the moving direction X of the cleaner  6  and the left-right direction form an acute angle θ. 
     Accordingly, the retaining frame  30  is moved from its home position depicted in solid lines in  FIGS. 5A and 5B  to a cleaning position depicted in dashed lines in  FIGS. 5A and 5B . In the cleaning position, the operating part  31  of the retaining frame  30  is disposed above the right end of the guide parts  13 . 
     While the retaining frame  30  moves from its home position to the cleaning position, the cleaning member  40  held in the retaining frame  30  moves together with the retaining frame  30 . Therefore, the cleaning member  40  moves gradually upward relative to the discharge wire  19 , as illustrated in  FIGS. 6A and 6B . 
     By sliding over the wire  38  of the discharge wire  19 , the rigid member  43  of the cleaning member  40  scrapes deposits off the wire  38 , thereby cleaning the discharge wire  19 . Once the cleaning operation is complete, the operator returns the retaining frame  30  from the cleaning position to the home position. This ends the operation to clean the discharge wire  19  with the cleaner  6 . 
     3. Overall Structure of a Printer 
     Next, a printer  60  will be described. The printer  60  is equipped with the drum cartridge  1  described above, which has the charging unit  2  according to the first embodiment of the present invention. 
     As shown in  FIG. 8 , the printer  60  includes a generally box-shaped main casing  61 . An access opening  62  is formed in one side wall of the main casing  61 . A front cover  63  is provided over the access opening  62  on the same side wall. The front cover  63  can be pivoted about its bottom edge in order to expose or cover the access opening  62 . 
     In the following description, the side of the main casing  61  on which the front cover  63  is provided will be called the front side, and its opposite side will be called the rear side. Left and right sides of the main casing  61  will be defined based on the perspective of a user facing the front of the printer  60 . 
     The printer  60  is further provided with a process cartridge  68 . The process cartridge  68  includes the drum cartridge  1  described above, and a developing cartridge  69 . 
     The drum cartridge  1  is detachably mounted in the main casing  61 . In addition to the photosensitive drum  51  and the scorotron charger  5  described above, the drum cartridge  1  also includes a transfer roller  70 . 
     The developing cartridge  69  is detachably mounted on the drum cartridge  1  while the drum cartridge  1  is mounted in the main casing  61 . In this way, the developing cartridge  69  is detachably mounted in the main casing  61 . The developing cartridge  69  includes an enclosure  77 , an agitator  76 , a developing roller  71 , a supply roller  72 , and a thickness-regulating blade  73 . 
     The internal space of the enclosure  77  is configured to include a toner-accommodating chamber  74 , and a developing chamber  75 . 
     The toner-accommodating chamber  74  constitutes a front portion of the interior space in the enclosure  77  and serves to accommodate toner. The developing chamber  75  constitutes a rear portion of the interior space in the enclosure  77 . 
     The agitator  76  is disposed in an approximate front-rear and vertical center region of the toner-accommodating chamber  74 . The agitator  76  is capable of rotating relative to the enclosure  77 . 
     The developing roller  71  is disposed in the developing chamber  75  such that a rear surface of the developing roller  71  is exposed outside the enclosure  77 . The developing roller  71  can also rotate relative to the enclosure  77 . 
     The supply roller  72  is disposed in the developing chamber  75  and contacts a lower front surface of the developing roller  71  with pressure. The supply roller  72  can also rotate relative to the enclosure  77 . 
     The thickness-regulating blade  73  is disposed in the developing chamber  75  so as to contact an upper front surface of the developing roller  71 . 
     By rotating, the agitator  76  supplies toner from the toner-accommodating chamber  74  onto the supply roller  72  in the developing chamber  75 . Next, the rotating supply roller  72  supplies toner received from the agitator  76  onto the developing roller  71 . At this time, the toner is positively tribocharged between the supply roller  72  and developing roller  71 . As the developing roller  71  rotates, the thickness-regulating blade  73  regulates the thickness of toner carried on the developing roller  71 . As a result, the circumferential surface of the developing roller  71  carries a thin toner layer of uniform thickness. 
     In the meantime, the discharge wire  19  and grid  18  of the scorotron charger  5  are powered by a power supply (not shown) provided in the main casing  61  to apply a uniform charge of positive polarity to the surface of the photosensitive drum  51  as the photosensitive drum  51  rotates. Next, a scanning unit  64  disposed in an upper section of the main casing  61  selectively irradiates a laser beam (indicated by a dashed line in  FIG. 8 ) toward the positively charged surface of the photosensitive drum  51 , forming an electrostatic latent image on the surface of the photosensitive drum  51  based on image data. 
     Next, the developing roller  71  supplies the positively charged toner carried on its surface to the latent image formed on the surface of the photosensitive drum  51 , producing a toner image on the surface of the photosensitive drum  51 . 
     A paper tray  65  detachably mounted in a bottom section of the main casing  61  serves to accommodate sheets S of paper. A pickup roller  66  disposed above the front end of the paper tray  65  rotates and conveys the sheets S from the paper tray  65  along a U-shaped paper-conveying path  67 . The sheets S are subsequently conveyed between the photosensitive drum  51  and transfer roller  70  one sheet at a time and at a prescribed timing. At this time, the photosensitive drum  51  and transfer roller  70  work in conjunction to transfer the toner image from the photosensitive drum  51  onto the sheet S, forming an image on the sheet S. 
     Next, the sheet S is conveyed between a heating roller  79 , and a pressure roller  80  disposed in a rear section of the main casing  61 . As the sheet S passes between the heating roller  79  and pressure roller  80 , the heating roller  79  and pressure roller  80  apply heat and pressure to the sheet S, thermally fixing the image to the sheet S. Subsequently, the sheet S reaches discharge rollers  81  disposed in an upper rear section of the main casing  61 . The discharge rollers  81  discharge the sheet S onto a discharge tray  82  formed on a top surface of the main casing  61 . 
     4. Operational and Technical Advantages 
     (1) As shown in  FIGS. 6A and 6B , the cushion member  42  holds the rigid member  43 , which contacts the wire  38  of the discharge wire  19 . Hence, the rigid member  43  can be made to elastically contact the wire  38 . Through this elasticity, the rigid member  43  can contact the wire  38  uniformly around its circumference. In other words, the rigid member  43  can be made to contact the wire  38  as if being wrapped around the wire  38  circumferentially. 
     With this construction, the rigid member  43  can be made to reliably and uniformly scrape the wire  38  of the discharge wire  19  in order to reliably clean the discharge wire  19 . 
     (2) As shown in  FIGS. 6A and 6B , the rigid member  43  makes surface contact with the wire  38  of the discharge wire  19  to surround the entire circumference of the wire  38 . Hence, this configuration increases the area of contact between the rigid member  43  and wire  38  and can therefore more reliably clean the discharge wire  19 . 
     (3) As shown in  FIGS. 6A and 6B , the cushion member  42  is folded in half with the rigid member  43  sandwiching the wire  38  of the discharge wire  19 . Accordingly, this simple construction can reliably increase the area of contact between the rigid member  43  and wire  38 . 
     Further, by folding the cushion member  42  in half, the cleaning member  40  can be mounted in the retaining frame  30 , thereby facilitating the operation for mounting the cleaning member  40 . 
     (4) As shown in  FIG. 7 , the right edge  90  of the rigid member  43  is positioned outside (to the right of) the right edge  94  of the cushion member  42  in the left-right direction. Hence, this simple construction can even more reliably increase the area of contact between the rigid member  43  and wire  38 . 
     (5) As shown in  FIG. 7 , the cleaning member  40  of the cleaner  6  has the second double-sided tape strip  44  for adhering to the rigid member  43  and cushion member  42 . Accordingly, the rigid member  43  and cushion member  42  can be reliably fixed together by the second double-sided tape strip  44 . 
     Further, the right edge  91  of the second double-sided tape strip  44  is positioned inside (to the left of) the right edge  90  of the rigid member  43  in the left-right direction. Hence, the rigid member  43  covers the second double-sided tape strip  44  on the side confronting the discharge wire  19 , thereby restraining the second double-sided tape strip  44  from contacting the discharge wire  19 . 
     If the second double-sided tape strip  44  were to come into contact with the discharge wire  19 , the adhesive of the second double-sided tape strip  44  could become deposited on the discharge wire  19 . However, since the rigid member  43  of the first embodiment covers the second double-sided tape strip  44  from the side facing the discharge wire  19 , the discharge wire  19  is protected from being contacted by the second double-sided tape strip  44 , reducing the likelihood of adhesive becoming deposited on the discharge wire  19 . Hence, the above construction can restrain the second double-sided tape strip  44  from coming into contact with the discharge wire  19 , while ensuring that the rigid member  43  is reliably fixed to the cushion member  42 . 
     (6) As shown in  FIG. 5B , the charging unit  2  is provided with the guide parts  13  for moving the cleaner  6 . This construction facilitates the operation for cleaning the discharge wire  19  with the cleaner  6 . 
     Further, the guide parts  13  guide movement of the cleaner  6  so that the moving direction X of the cleaner  6  is sloped relative to the left-right direction. Accordingly, the area of the rigid member  43  that contacts the wire  38  of the discharge wire  19  changes as the cleaner  6  moves, as illustrated in  FIGS. 6A and 6B . This arrangement reduces abrasion to the rigid member  43  caused by sliding friction between the rigid member  43  and wire  38 . 
     5. Second Through Fourth Embodiments 
     (1) Second Embodiment 
     Next, a cleaning member  240  according to a second embodiment of the present invention will be described with reference to  FIG. 9A . In the following description, like parts and components are designated by the same reference numerals with those of the first embodiment to avoid duplicating description. 
     In the first embodiment described above, the left-right dimension of the rigid member  43  is larger than the same dimension of the cushion member  42 , as shown in  FIG. 7 . Thus, the right edge  90  of the rigid member  43  is positioned further right than the right edge  94  of the cushion member  42 , and the left edge  92  of the rigid member  43  is positioned further left than the left edge  95  of the cushion member  42 . 
     The cleaning member  240  of the second embodiment includes a rigid member  243 , a second double-sided tape strip  244  and the cushion member  42 . Specifically, the left-right dimension of the rigid member  243  is smaller than the same dimension of the cushion member  42 , as illustrated in  FIG. 9A . In this case, a right edge  290  of the rigid member  243  is positioned further leftward than the right edge  94  of the cushion member  42 , and a left edge  292  of the rigid member  243  is positioned further rightward than the left edge  95  of the cushion member  42 . 
     In addition, the left-right dimension of the second double-sided tape strip  244  is equivalent to that of the rigid member  243 . Accordingly, the right edge  290  of the rigid member  243  is aligned with a right edge  291  of the second double-sided tape strip  244 , and the left edge  292  of the rigid member  243  is aligned with a left edge  293  of the second double-sided tape strip  244  with respect to the left-right direction. 
     With the configuration according to the second embodiment, the right edge  290  of the rigid member  243  is positioned further leftward (i.e., inward in the left-right direction) than the right edge  94  of the cushion member  42 , and the left edge  292  of the rigid member  243  is positioned further rightward (i.e., inward in the left-right direction) than the left edge  95  of the cushion member  42 . Therefore, even when filings from the rigid member  243  are produced through abrasion between the rigid member  243  and the wire  38  of the discharge wire  19  as the number of cleaning operations accumulates, the right edge  94  and left edge  95  of the cushion member  42  can remove these filings during the cleaning operation. Hence, this configuration restrains filings produced from the rigid member  243  from becoming deposited on the discharge wire  19 . 
     The configuration according to the second embodiment described above can obtain the same operational advantages described in the first embodiment. 
     (2) Third Embodiment 
     Next, a cleaning member  340  according to a third embodiment of the present invention will be described with reference to  FIG. 9B . In the following description, like parts and components are designated by the same reference numerals with those of the first embodiment to avoid duplicating description. 
     The cleaning member  340  of the third embodiment includes a rigid member  343 , a second double-sided tape strip  344  and the cushion member  42 . Specifically, the left-right dimension of the rigid member  343  is shorter than the same dimension of the cushion member  42 , and the left-right dimension of the second double-sided tape strip  344  is shorter than the same dimension of the rigid member  343 . Thus, a right edge  390  of the rigid member  343  is positioned rightward from a right edge  391  of the second double-sided tape strip  344 , and a left edge  392  of the rigid member  343  is positioned leftward from a left edge  393  of the second double-sided tape strip  344 . 
     With the configuration according to the third embodiment described above, the rigid member  343  covers the second double-sided tape strip  344  from the side confronting the discharge wire  19 , thereby restraining the second double-sided tape strip  344  from coming into contact with the discharge wire  19 . 
     Further, the configuration according to the third embodiment described above can obtain the same operational advantages described in the first embodiment. 
     (3) Fourth Embodiment 
     Next, a cleaning member  440  according to a fourth embodiment of the present invention will be described with reference to  FIG. 9C . In the following description, like parts and components are designated by the same reference numerals with those of the first embodiment to avoid duplicating description. 
     The cleaning member  440  of the fourth embodiment includes a rigid member  443 , a second double-sided tape strip  444 , and a cushion member  442 . Specifically, in the fourth embodiment, the left-right dimension of the rigid member  443  is equivalent to the same dimension of the cushion member  442  and the second double-sided tape strip  444  (and that of a first double-sided tape strip  441 ). Thus, a right edge  490  of the rigid member  443  is aligned with both a right edge  494  of the cushion member  442  and a right edge  491  of the second double-sided tape strip  444 , and a left edge  492  of the rigid member  443  is aligned with both a left edge  495  of the cushion member  442  and a left edge  493  of the second double-sided tape strip  444  with respect to the left-right direction. 
     The configuration according to the fourth embodiment described above can obtain the same operational advantages described in the first embodiment. 
     6. Fifth Embodiment 
     Next, a cleaner  506  according to a fifth embodiment of the present invention will be described with reference to  FIGS. 10A and 10B . In the following description, like parts and components are designated by the same reference numerals with those of the first embodiment to avoid duplicating description. 
     In the first embodiment, the cleaner  6  is configured to clean the wire  38  of the discharge wire  19 . However, the cleaner  506  according to the fifth embodiment is configured to clean the ridge members  39  of the grid  18 , as well as the wire  38  of the discharge wire  19 . 
     Specifically, the cleaner  506  of the fifth embodiment includes a retaining frame  530  having a pair of sandwiching parts  100 . The sandwiching parts  100  are disposed below the pair of coupling parts  32  and are spaced apart from each other in the front-rear direction. The sandwiching parts  100  have a general L-shape in a side view. Each sandwiching part  100  includes a wire-side fixing part  101 , and a grid-side fixing part  102 . 
     The wire-side fixing parts  101  have a rail-like shape that is elongated vertically. The wire-side fixing parts  101  extend downward from the bottom surface of the horizontal part  37  of the corresponding coupling part  32 . The grid-side fixing parts  102  have a rail-like shape that is elongated in the front-rear direction. The grid-side fixing parts  102  are formed continuously with the bottom ends of the corresponding wire-side fixing parts  101  and extend outward therefrom relative to the front-rear direction. 
     The pair of sandwiching parts  100  supports a cleaning member  540  therebetween. 
     The cleaning member  540  is configured of a cushion member  542 , a second double-sided tape strip  544  and a rigid member  543 , as in the first embodiment. The cleaning member  540  of this construction is folded in half to form a general U-shape in a side view, with the opening of the “U” facing downward to provide a wire cleaning part  103  and grid cleaning parts  104 . 
     The wire cleaning part  103  is disposed between the wire-side fixing parts  101  of the sandwiching parts  100 . The wire cleaning part  103  has a general U-shape in a side view, with the opening of the “U” facing downward, such that the rigid member  543  is pressed against both left and right sides of the wire  38 . The wire cleaning part  103  is adhesively fixed to the wire-side fixing parts  101  through a first double-sided tape strip  541 . 
     The grid cleaning parts  104  are formed continuously with the bottom ends of the wire cleaning part  103 , extending outward in respective front and rear directions. The grid cleaning parts  104  are disposed beneath the grid-side fixing parts  102  of the sandwiching parts  100 . The grid cleaning parts  104  are adhesively fixed to the bottom surfaces of the grid-side fixing parts  102  through the first double-sided tape strip  541 . 
     The grid cleaning parts  104  contact the ridge members  39  of the grid  18  from above with pressure. At this time, the cushion member  542  elastically deforms due to the pressure so that the rigid member  543  surrounds and contacts each ridge members  39  around its entire circumference, as shown in  FIG. 10B . In other words, a portion of the rigid member  543  is positioned opposite to the cushion member  542  with respect to each ridge member  39 . Thus, the rigid member  543  is made to wrap around the ridge members  39  in the circumferential direction thereof so as to contact the ridge members  39  uniformly around their circumference. 
     As in the cleaning operation described in the earlier embodiments, the retaining frame  530  of the cleaner  506  is moved rightward from its home position toward the cleaning position. Through this operation, the cleaning member  540  can clean the wire  38  as well as the ridge members  39 . 
     According to the fifth embodiment described above, the cushion member  542  supports the rigid member  543  so as to contact the wire  38  of the discharge wire  19  and the ridge members  39  of the grid  18 , as illustrated in  FIG. 10B . Accordingly, the rigid member  543  can be made to elastically contact the wire  38  and ridge members  39 . 
     This construction enables the rigid member  543  to contact the wire  38  and ridge members  39  uniformly, ensuring that the rigid member  543  scrapes the wire  38  and ridge members  39  during an operation to clean the grid  18  and discharge wire  19 . Thus, the cleaner  506  according to the fifth embodiment can reliably clean the discharge wire  19  and grid  18  in a single operation. 
     Further, the configuration according to the fifth embodiment described above can obtain the same operational advantages described in the first embodiment 
     7. Variations of the Embodiments 
     In the first through fifth embodiments described above, the charging unit  2  is provided in the drum cartridge  1  that is detachably mounted in the main casing  61 , as shown in  FIG. 1 , but the present invention is not limited to this arrangement. For example, the photosensitive drum  51  and charging unit  2  may be provided in the main casing  61 , while only the developing cartridge  69  may be detachably mounted in the main casing  61 . 
     With this construction as well, the same operational advantages described in the first embodiment can be obtained. 
     Incidentally, the configurations of the above-described first through fifth embodiments can be combined appropriately. 
     While the invention has been described in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention.