Patent Publication Number: US-8538287-B2

Title: Cleaning member for image forming apparatus, charging device, unit for image forming apparatus, process cartridge, and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2010-151376 filed Jul. 1, 2010. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a cleaning member for an image forming apparatus, a charging device, a unit for an image forming apparatus, a process cartridge, and an image forming apparatus. 
     (ii) Related Art 
     According to an electrophotographic image forming apparatus, a surface of an image-carrying member including a photoconductor or the like is charged with a charging device to create charges and an electrostatic latent image is formed by, for example, a laser beam obtained by modulating an image signal. Then the electrostatic latent image is developed with charged toner to form a visible toner image. The toner image is electrostatically transferred onto a receiving member such as recording sheet either directly or via an intermediate transfer body and fixed onto a receiving member to obtain an image. 
     SUMMARY 
     According to an aspect of the invention, there is provided a cleaning member for an image forming apparatus, the cleaning member including a core and an elastic layer that is formed by a strip-shaped elastic member helically wound around an outer peripheral surface of the core. The cleaning member satisfies the following formula (1):
 
0.7 &lt;t/T&lt; 1.0  (1)
 
where t (mm) denotes a thickness of the elastic layer, the thickness being taken in a central portion in a helical width direction of the elastic layer, and T (mm) denotes a thickness of the central portion of the strip-shaped elastic member in a width direction before wound around the outer peripheral surface of the core.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic perspective view showing a cleaning member for an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  is a schematic side view of the cleaning member for an image forming apparatus according to the exemplary embodiment; 
         FIG. 3  is an enlarged cross-sectional view showing the thickness of an elastic layer of the cleaning member for an image forming apparatus according to the exemplary embodiment; 
         FIGS. 4A to 4C  are diagrams showing examples of steps of a method for manufacturing the cleaning member for an image forming apparatus according to the exemplary embodiment; 
         FIG. 5  is a schematic diagram showing an electrophotographic image forming apparatus according to an exemplary embodiment; 
         FIG. 6  is a schematic diagram showing a process cartridge according to an exemplary embodiment of the invention; and 
         FIG. 7  is an enlarged schematic diagram showing a vicinity of a charging member (charging device) shown in  FIGS. 5 and 6 . 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described. The components that have the same functions and effects are represented by the same reference symbols throughout the drawings and the descriptions therefore may be omitted to avoid redundancy. 
     (Cleaning Member) 
       FIG. 1  is a schematic perspective view showing a cleaning member for an image forming apparatus according to an exemplary embodiment.  FIG. 2  is a schematic side view of the cleaning member for an image forming apparatus according to the exemplary embodiment.  FIG. 3  is an enlarged cross-sectional view showing the thickness of an elastic layer of the cleaning member and is taken along line III-III in  FIG. 1 , i.e., in a direction orthogonal to the helical direction of the elastic layer. The elastic layer includes all layers formed on the core. 
     As shown in  FIGS. 1 to 3 , a cleaning member  100  of an image forming apparatus (simply referred to as “cleaning member  100 ” hereinafter) according to this exemplary embodiment is a roll-shaped member that includes a core  100 A and an elastic layer  100 B. The elastic layer  100 B is formed by helically winding a strip-shaped elastic member (referred to as “strip  100 C” hereinafter) on the surface of the core  100 A. In particular, the elastic layer  100 B is formed by helically winding a strip  100 C around the core  100 A serving as a helical axis from one end to the other end of the core  100 A at particular intervals. 
     The elastic layer  100 B satisfies conditional formula (A1) below where t (mm) represents the thickness of the elastic layer  100 B in the central portion in the helical width direction of the elastic layer formed on the outer peripheral surface of the core  100 A and T (mm) represents the thickness of the strip  100 C in the central portion in the strip width direction before wound around the outer peripheral surface of the core  100 A (refer to  FIG. 3 ):
 
0.7 &lt;t/T&lt; 1.0  Conditional formula A1
 
     According to the cleaning member  100  of this exemplary embodiment having such a structure, deformation of the elastic layer  100 B after storage (in particular, storage in a high-temperature, high-humidity environment, e.g., in a 40° C. 95% RH environment) is suppressed. Although the reason is not clear, it is presumed as follows. 
     When an elastic layer  100 B is disposed on the outer peripheral surface of the core  100 A by winding a strip  100 C around the core  100 A, the strip  100 C is wound around the outer peripheral surface of the core  100 A while being imparted predetermined tension in the longitudinal direction (winding direction). Application of tension is required in order to wind the strip  100 C around the core  100 A. Accordingly, the elastic layer  100 B wound around the core  100 A is elastically deformed (in other words, the thickness of the elastic layer  100 B is smaller than the thickness of the strip  100 C in the central portion in the width direction before winding). If the tension is excessively high, the degree of elastic deformation is increased and the elastic layer  100 B may be deformed after storage. This phenomenon is particularly frequent after storage in a high-temperature, high-humidity environment, e.g., a temperature of 40° C. and a humidity of 95% RH. 
     According to the cleaning member  100  of the exemplary embodiment, the elastic layer  100 B satisfies conditional formula (A1) above, in other words, the degree of elastic deformation (change in thickness in the central portion in the width direction) is minimized with respect to the strip  100 C before winding. This presumably suppresses deformation of the elastic layer  100 B after storage. 
     According to a charging device (unit for forming an image), process cartridge, and image-forming apparatus that includes the cleaning member  100  of the exemplary embodiment, image defects (such as banding) caused by the deformation of the elastic layer after storage is suppressed. 
     The elastic layer  100 B may satisfy conditional formula (A2) and preferably satisfy conditional formula (A3) below:
 
0.8 &lt;t/T&lt; 0.95  Conditional formula (A2)
 
0.8 &lt;t/T&lt; 0.9  Conditional formula (A3)
 
     The thickness of the elastic layer  100 B in the central portion in the helical width direction is measured as follows, for example. 
     The cleaning member is scanned with a laser analyzer (Laser Scan Micrometer, model LSM 6200 produced by Mitsutoyo Corporation) in a longitudinal direction (axis direction) of the cleaning member at a traverse speed of 1 mm/s while having the circumferential direction of the cleaning member fixed so as to determine the profile of the elastic layer thickness. Subsequently, the same measurement is conducted by shifting the position of the scanning in the circumferential direction (measurement is conducted at three positions 120° apart from each other). The thickness of the elastic layer  100 B in the central portion in the helical width direction is calculated on the basis of this profile. 
     The thickness T of the strip  100 C in the central portion in the width direction before winding may be measured in the same manner by using a laser analyzer (Laser Scan Micrometer, model LSM 6200 produced by Mitsutoyo Corporation) by attaching the strip  100 C to a zero curvature plate or the like. 
     Examples of the technique for rendering the elastic layer  100 B to satisfy the conditional formula described above in preparing the elastic layer  100 B by winding a strip  100 C around a core include techniques of adjusting the thickness of a strip, the angle at which the strip is wound, and the tension at which the strip is wound. 
     The elastic layer  100 B is helically disposed. In particular, the helical angle is 10° to 65° or about 10° to about 65° and preferably 20° to 50°. The helical width R 1  is 2 mm to 18 mm or about 2 mm to about 18 mm and preferably 3 mm to 10 mm. The helical pitch R 2  is 3 mm to 25 mm and preferably 15 mm to 22 mm. 
     In forming the elastic layer  100 B by winding the strip  100 C around the core  100 A, the helical angle and the helical width may be adjusted as above to render it easier for the elastic layer  100 B to satisfy the conditional formula described above. 
     The coverage by the elastic layer  100 B determined by (helical width R 1  of elastic layer  100 B/[helical width R 1  of elastic layer  100 B+helical pitch R 2  of elastic layer  100 B (R 1 +R 2 )])×100 is 20% to 70% and preferably 25% to 55%. 
     When the coverage is beyond this range, the length of time the elastic layer  100 B comes into contact with the member to be cleaned is increased and deposits on the surface of the cleaning member tend to re-contaminate the member to be cleaned. In contrast, when the coverage is below this range, the thickness of the elastic layer  100 B is not readily stabilized and the cleaning performance may be degraded. 
     The helical angle θ is the angle (acute angle) between the longitudinal direction P (helical direction) of the elastic layer  100 B and the axis direction Q (core axis direction) of the cleaning member  100 . 
     The helical width R 1  is the length of the elastic layer  100 B in the axis direction Q (core axis direction) of the cleaning member  100 . 
     The helical pitch R 2  is the length between adjacent parts of the elastic layer  100 B in the axis direction Q (core axis direction) of the cleaning member  100 . 
     The elastic layer  100 B refers to a layer composed of a material that returns to its original shape after being deformed by application of external force of 100 Pa. 
     The individual components will be described. 
     The core is described first. 
     Examples of the material for the core  100 A include metals (e.g., free-cutting steel, stainless steel, etc.) and resins (e.g., polyacetal (POM) resin, etc.). The material and the surface treatment method may be selected according to need. 
     When the core  100 A is composed of a metal, the core  100 A is preferably plated. When the core  100 A is composed of a material having no electrical conductivity, such as a resin, the material may be processed by a typical treatment such as plating to impart electrical conductivity or may be directly used as is. 
     The elastic layer is described next. 
     Examples of the material for the elastic layer  100 B include foaming resins such as polyurethane, polyethylene, polyamide, and polypropylene and rubber materials such as silicone rubber, fluorine rubber, urethane rubber, ethylene propylene diene rubber (EPDM), nitrile butadiene rubber (NBR), chloroprene rubber (CR), chlorinated polyisoprene, isoprene, acrylonitrile-butadiene rubber, styrene-butadiene rubber, hydrogenated polybutadiene, and butyl rubber, and any blends of two or more of these materials. Assistant agents such as such as a foaming aid, a foam stabilizer, a catalyst, a curing agent, a plasticizer, or a vulcanization accelerator may be added to these materials. 
     The material for the elastic layer  100 B may be a material having air bubbles, in other words, a foamed material. In particular, polyurethane foam highly resistant to stretching may be used in order not to scratch the surface of the member to be cleaned and in order to prevent shredding and breaking over a long term. 
     Examples of the polyurethane include reaction products between a polyol (e.g., polyester polyol, polyether polyester, or acryl polyol) and an isocyanate (such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4-diphenylmethane diisocyanate, tolidine diisocyanate, or 1,6-hexamethylene diisocyanate). The polyurethane may contain a chain extender such as 1,4-butanediol or trimethylol propane. Foaming of polyurethane is typically conducted by using a foaming agent such as water or an azo compound (e.g., azodicarbonamide, azobisisobutyronitrile, etc.). An assistant agent such as a foaming aid, a foam stabilizer, or a catalyst may be added to the polyurethane foam if needed. 
     An ether-based polyurethane foam is particularly preferred. This is because an ester-based polyurethane foam has a tendency to deteriorate under humidity and heat. A silicone oil foam stabilizer is typically used for the ether-based polyurethane. However, image defects caused by migration of silicone oil to the member to be cleaned (e.g., charging roller) may occur during storage (in particular, long-term storage at high temperature and high humidity). Accordingly, a foam stabilizer other than silicone oil is used to prevent image defects caused by the elastic layer  100 B. 
     Examples of the foam stabilizer other than silicone oil include Si-free organic surfactants (e.g., anionic surfactants such as dodecylbenzenesulfonic acid and sodium lauryl sulfate). A method disclosed in Japanese Unexamined Patent Application Publication No. 2005-301000 that does not use a silicone foam stabilizer may also be employed. 
     The elastic layer  100 B may have a single layer structure or a multilayer structure. In particular, the elastic layer  100 B may be constituted by a single layer of a foam body or may take a two-layer structure including a solid layer and a foam layer. 
     Next, a method for manufacturing the cleaning member  100  according to the exemplary embodiment is described. 
       FIGS. 4A to 4C  are diagrams showing examples of steps of a method for manufacturing the cleaning member  100  according to the exemplary embodiment. 
     Referring to  FIG. 4A , a sheet-shaped elastic layer component (polyurethane foam sheet or the like) being sliced to a target thickness is prepared. A double-sided adhesive tape (not shown) is attached on one surface of the sheet-shaped elastic layer component. The elastic layer component is blanked out using a punching die to obtain a strip  100 C (strip with a double-sided adhesive tape) having desired width and length. Meanwhile, the core  100 A is prepared. 
     Next, as shown in  FIG. 4B , the strip  100 C is placed with the surface on which the double-sided adhesive tape is attached facing upward. One end of the releasing paper of the double-sided adhesive tape is detached and one end of the core  100 A is placed on the portion of the double-sided adhesive tape from which the releasing paper is detached. 
     Then, as shown in  FIG. 4C , while detaching the releasing paper of the double-sided adhesive tape, the core  100 A is rotated at a target speed to helically wind the strip  100 C around the peripheral surface of the core  100 A to obtain a cleaning member  100  including a core  100 A and a elastic layer  100 B helically arranged on the peripheral surface of the core  100 A. 
     In winding the strip  100 C around the core  100 A to form the elastic layer  100 B, the position of the strip  100 C may be adjusted so that the angle (helical angle) formed between the longitudinal direction of the strip  100 C and the axial direction of the core  100 A is a desired angle. The outer diameter of the core  100 A is, for example, about 3 mm to 6 mm. 
     The tension applied during winding the strip  100 C around the core  100 A may be at a level that does not create a gap between the core  100 A and the double-sided adhesive tape of the strip  100 C. If excessive tension is applied, conditional formula (A1) is not easily satisfied. Moreover, the tension set is increased and the elastic force of the elastic layer  100 B tends to be lowered. In particular, the tension may be at a level that the length of the strip  100 C is stretched more than 0% but not more than 5% from the original length of the strip  100 C. 
     If the strip  100 C is wound around the core  100 A, the strip  100 C tends to be elongated. The elongation differs in the thickness direction of the strip  100 C. The outermost portion tends to show the largest elongation, which results in a decrease in elastic force. The elongation of the outermost portion after the strip  100 C is wound around the core  100 A is preferably about 5% with respect to the outermost portion of the original strip  100 C. 
     The elongation is controlled by the thickness of the strip  100 C and the radius of curvature at which the strip  100 C is wound around the core  100 A. The curvature at which the strip  100 C is wound around the core  100 A is controlled by the outer diameter of the core  100 A and the winding angle of the strip  100 C. 
     The curvature at which the strip  100 C is wound around the core  100 A is, for example, ((core outer diameter/2)+0.2 mm) or more and ((core outer diameter/2)+8.5 mm) or less and preferably ((core outer diameter/2)+0.5 mm) or more and ((core outer diameter/2)+7.0 mm) or less. 
     The thickness of the strip  100 C is, for example, about 1.5 to about 4 mm and preferably 1.5 to 3.0 mm. The width of the strip  100 C may be adjusted so that the coverage of the elastic layer  100 B is within the above-described range. The length of the strip  100 C is determined by, for example, the length (length in the axis direction) of the region on which the strip  100 C is wound around the core  100 A, the winding angle, and the tension applied during winding. 
     (Image-Forming Apparatus Etc.) 
     An image forming apparatus according to an exemplary embodiment of the present invention will be described with reference to the drawings. 
       FIG. 5  is a schematic diagram showing an image forming apparatus according to an exemplary embodiment. 
     An image forming apparatus  10  according to the exemplary embodiment is a tandem system color image forming apparatus shown in  FIG. 5 , for example. Process cartridges (also refer to  FIG. 6 ) each including a photoconductor (image-carrying member)  12 , a charging member  14 , a developing device, and other associated components are disposed inside the image forming apparatus  10 . In this exemplary embodiment, four process cartridges  18 Y,  18 M,  18 C, and  18 K are respectively provided for four colors, i.e., yellow, magenta, cyan, and black. The process cartridges are detachably mounted to the image forming apparatus  10 . 
     The photoconductor  12  is, for example, a conductive cylindrical body having a diameter of 25 mm and coated with a photoconductor layer composed of an organic photosensitive material formed on the surface, and is rotated at a process speed of 150 mm/sec by a motor not shown in the drawing. 
     The surface of the photoconductor  12  is charged with the charging member  14  disposed on the surface of the photoconductor  12  and irradiated with a laser beam LB emitted from an exposure device  16  so as to form an electrostatic latent image, which corresponds to image information, on the downstream side of the charging member  14  in the rotation direction of the photoconductor  12 . 
     Electrostatic latent images formed on the photoconductors  12  are respectively developed with developing devices  19 Y,  19 M,  19 C, and  19 K for yellow (Y), magenta (M), cyan (C), and black (K) to form toner images of the four colors. 
     For example, when a color image is to be formed, the process of charging, exposing, and developing is conducted on the surface of each of the photoconductors  12  corresponding to yellow (Y), magenta (M), cyan (C), and black (K) so as to form a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image on the photoconductors  12 , respectively. 
     The yellow (Y), magenta (M), cyan (C), and black (K) toner images sequentially formed on the photoconductors  12  are transferred onto a recording sheet  24  at positions where the photoconductors  12  contact the transfer devices  22  while the recording sheet  24  is transported on an outer peripheral surface of a sheet transport belt  20  given tension by and supported by supporting rolls  40  and  42  from the inner peripheral side. The recording sheet  24  that has received the toner images from the photoconductors  12  is transported to a fixing device  64  and heated and pressurized by the fixing device  64  to fix the toner images on the recording sheet  24 . The recording sheet  24  with toner images fixed thereon is ejected with a discharging roll  66  onto a discharge unit  68  in the upper part of the image forming apparatus  10  when the printing is to be performed on only one side of the sheet. 
     The recording sheet  24  is supplied from a sheet container  28  by using a supply roller  30  and transported with feed rolls  32  and  34  to the sheet transport belt  20 . 
     In the case where double-sided printing is to be conducted, the recording sheet  24  with toner images fixed on a first surface (front surface) by the fixing device  64  is not ejected onto the ejecting unit  68  by the discharging roll  66 . Instead, the discharging roll  66  holding the rear end of the recording sheet  24  is reversed while the transport path of the recording sheet  24  is switched to a sheet transport path  70  for double-sided printing. The recording sheet  24  with its side reversed is transported again onto the sheet transport belt  20  by using a feed roll  72  installed on the sheet transport path  70  so as to transfer toner images onto a second surface (rear surface) of the recording sheet  24  from the photoconductors  12 . The toner images on the second surface (rear surface) of the recording sheet  24  are fixed with the fixing device  64  and the recording sheet (receiving member) is ejected onto the ejecting unit  68 . 
     The surface of the photoconductor  12  after the toner image transfer step is cleaned with a cleaning blade  80  disposed downstream of the position that has come into contact with the transfer device  22  in the rotation direction of the photoconductor  12 . This cleaning is conducted every time the photoconductor  12  is rotated to remove residual toner, paper dust, etc., and to prepare for the next image formation. 
     As shown in  FIG. 7 , the charging member  14  is, for example, a roll including a rotatably supported conductive core  14 A and an elastic layer  14 B surrounding the core  14 A. A cleaning member  100  for cleaning the charging member  14  is in contact with a side of the charging member  14  remote from the photoconductor  12 . The cleaning member  100  is part of a charging unit. The cleaning member  100  of the exemplary embodiment is used as the cleaning member. 
     The description below concerns the case in which the cleaning member  100  is always in contact with the charging member  14  and driven by the charging member  14 . Alternatively, the charging member may be brought into contact with and driven by the charging member only during cleaning. Yet alternatively, the cleaning member  100  may be brought into contact with the charging member  14  only during cleaning and driven separately so as to have a peripheral speed different from that of the charging member  14 . However, having the cleaning member  100  always in contact with the charging member  14  and creating a difference in peripheral speed may be avoided since contamination on the charging member  14  accumulates on the cleaning member  100  and may re-deposit on the charging roll. 
     The charging member  14  is pressed against the photoconductor  12  by application of a load F to both ends of the core  14 A so that a nip portion is formed along the peripheral surface of the elastic layer  14 B by elastic deformation. The cleaning member  100  is pressed against the charging member  14  by application of a load F′ to both ends of the core  100 A so that a nip portion is formed along the peripheral surface of the charging member  14  by elastic deformation of the elastic layer  100 B. As a result, a nip portion is formed in the axis direction of the charging member  14  and the photoconductor  12  while suppressing the deflection of the charging member  14 . 
     The photoconductor  12  is rotated in the arrow X direction by a motor not shown in the drawing and the charging member  14  is driven in the arrow Y direction by the rotation of the photoconductor  12 . The cleaning member  100  is driven by the rotation of the charging member  14  and rotates in the arrow Z direction. 
     —Structure of Charging Member— 
     The description of the charging member is given below but the structure of the charging member is not limited by the description. 
     The structure of the charging member is not particularly limited. For example, the charging member may include a core and an elastic layer or a resin layer instead of the elastic layer. The elastic layer may have a single-layer structure or a multilayer structure including two or more layers having various functions. The elastic layer may be surface-treated. 
     The material of the core may be free-cutting steel or stainless steel. The material and the surface treatment method may be adequately selected according to the property such as slidability. The core may be plated. When a material having no electrical conductivity is used, the material may be processed by a typical treatment such as plating to impart electrical conductivity or may be directly used as is. 
     The elastic layer is a conductive elastic layer. For example, the conductive elastic layer may contain, an elastic material such as rubber, a conductive material such as carbon black and an ion conductive material for adjusting the resistance of the conductive elastic layer, and any additives commonly used as needed, such as a softener, a plasticizer, a curing agent, a vulcanizing agent, a vulcanization accelerator, an antioxidant, and a filler such as silica or calcium carbonate. The elastic layer is formed by coating the peripheral surface of the conductive core with a mixture of these materials. Examples of the conductive agent for adjusting the resistance include carbon black blended with a matrix material and a dispersion of a conductive material that uses at least one of electrons and ions as charge carriers, such as an ion conductive material. The elastic material may be foamed. 
     The elastic material constituting the conductive elastic layer is formed by dispersing a conductive agent in a rubber material. Examples of the rubber material include silicone rubber, ethylene propylene rubber, epichlorohydrin-ethylene oxide copolymer rubber, epichlorohydrin-ethylene oxide-allyl glycidyl ether copolymer rubber, acrylonitrile-butadiene copolymer rubber, and blend rubber of these. These rubber materials may be foamed or unfoamed. 
     Examples of the conductive agent include electronic conductive agents and ion conductive agents. Examples of the electronic conductive agents include fine particles composed of carbon black such as Ketjenblack and acetylene black; pyrocarbon and graphite; various conductive metals such as aluminum, copper, nickel, and stainless steel and alloys thereof; conductive metal oxides such as tin oxide, indium oxide, titanium oxide, tin oxide-antimony oxide solid solution, and tin oxide-indium oxide solid solution; and insulating materials having surfaces treated to exhibit conductivity. Examples of the ion conductive agent include perchloric acid salts and chlorates such as tetraethylammonium and lauryltrimethylammonium; and perchloric acid salts and chlorates of alkali metals and alkaline earth metals such as lithium and magnesium. 
     These conductive agents may be used alone or in combination of two or more. The amounts of these conductive agents added are not particularly limited. The amount of the electronic conductive agent may be 1 to 60 parts by mass relative to 100 parts by mass of rubber material. The amount of the ion conductive agent may be 0.1 to 5.0 parts by mass relative to 100 parts by mass of rubber material. 
     A surface layer may be formed in the surface of the charging member. The material for the surface layer may be resin, rubber, or any other suitable material and is thus not particularly limited. Examples of the material for the surface layer include polyvinylidene fluoride, ethylene tetrafluoride copolymers, polyester, polyimide, and copolymer nylon. 
     Examples of the copolymer nylon include those that contain at least one of nylon 6,10, nylon 11, and nylon 12 as a polymerization unit. Examples of other polymerization unit contained in the copolymer include nylon 6 and nylon 6,6. The ratio of a polymerization unit constituted by nylon 6,10, nylon 11, and/or nylon 12 in the copolymer may be 10% by mass or more in total. 
     The polymer materials may be used alone or in combination of two or more. The number-average molecular weight of the polymer material is preferably 1,000 to 100,000 and more preferably 10,000 to 50,000. 
     A conductive material may be added to the surface layer to control the resistance. A conductive material may have a particle size of 3 μm or less. 
     Examples of the conductive agent for adjusting the resistance include carbon black and conductive metal oxide particles blended with a matrix material, and a dispersion of a conductive material that uses at least one of electrons and ions as charge carriers, such as an ion conductive material. 
     Examples of carbon black used as a conductive agent include Special Black 350, Special Black 100, Special Black 250, Special Black 5, Special Black 4, Special Black 4A, Special Black 550, Special Black 6, Color Black FW200, Color Black FW2, and Color Black FW2V produced by Degussa, and MONARCH 1000, MONARCH 1300, MONARCH 1400, MOGUL-L, and REGAL 400R produced by CABOT CORPORATION. 
     Carbon black may have a pH of 4.0 or less. 
     The conductive metal oxide particles used as conductive particles for adjusting resistance is not particularly limited and may be any conductive particles that use electrons as charge carriers. Examples thereof include tin oxide, antimony-doped tin oxide, zinc oxide, anatase-type titanium oxide, and indium tin oxide (ITO). These may be used alone or in combination of two or more. The particle size may be any. The conductive particles are preferably tin oxide, antimony-doped tin oxide, or anatase-type titanium oxide and more preferably tin oxide or antimony-doped tin oxide. 
     The surface layer may be composed of a fluorine-based or silicone-based resin. In particular, the surface layer may be composed of a fluorine-modified acrylate polymer. Particles may be added to the surface layer. Insulating particles such as alumina or silica may be added to impart irregularities on the surface of the charging member so that the frictional load imposed during contact with the photoconductor is decreased and the wear resistance between the charging member and the photoconductor is improved. 
     The outer diameter of the charging member may be 8 mm to 16 mm. The outer diameter is measured with a commercially available caliper or a laser-system outer diameter measuring device. 
     The microhardness of the charging member may be 45° to 60°. In order to decrease hardness, the amount of plasticizer added may be increased or a low-hardness material such as silicone rubber may be used. 
     The microhardness of the charging member is the value determined with MD-1 durometer produced by Kobunshi Keiki Co., Ltd. 
     The image forming apparatus of the exemplary embodiment includes a photoconductor (image-carrying member), a charging device (unit constituted by a charging member and a cleaning member), a developing device, and a cleaning blade (cleaning device) but the image forming apparatus is not limited to this. For example, a charging device (unit constituted by a charging member and a cleaning member) and, if needed, at least one selected from a photoconductor (image-carrying member), an exposing device, a transfer device, a developing device, and a cleaning blade (cleaning device) may be combined to form a process cartridge. It should be noted that these devices and members need not be formed into a cartridge and may be directly installed in the image forming apparatus. 
     The image forming apparatus of the exemplary embodiment described above includes a charging device which is a unit constituted by a charging member and a cleaning member, in other words, a structure in which the charging member is the member to be cleaned. However, the structure is not limited to this. The member to be cleaning may be a photoconductor (image-carrying member), a transfer device (transfer member or transfer roll), and/or an intermediate transfer body (intermediate transfer belt). The unit constituted by the member to be cleaned and the cleaning member in contact with the member to be cleaned may be installed directly on the image forming apparatus or may be formed into a cartridge as with the process cartridge described above and installed in the image forming apparatus. 
     The image forming apparatus of the exemplary embodiment is not limited to one having the above-described structure. Image forming apparatuses of an intermediate transfer type and other known types may be employed. 
     EXAMPLES 
     The present invention will be described by using Examples below which do not limit the present invention. 
     Example 1 
     (Preparation of Cleaning Roll  1 ) 
     A double-sided adhesive tape 0.15 mm in thickness is attached to urethane foam (EPM-70, product of INOAC CORPORATION) 3.25 mm in thickness and a strip 3.4 mm in thickness (in the central portion in the width direction), 6 mm in width, and 356 mm in length is cut out. The strip is wound around a stepped metal core (outer diameter: 6 mm, length: 337 mm, outer diameter and length of bearing portion: 4 mm and 6.0 mm) at a winding angle of 40° so that the sheet length is stretched by 0% to 5% to form a helically arranged elastic layer and a cleaning roll  1 . 
     (Preparation of Charging Roll) 
     —Formation of Elastic Layer— 
     A mixture described below is kneaded with an open roll, applied on a surface of a conductive support composed of SUS 416 stainless steel 6 mm in diameter so as to form a cylindrical body having a thickness of 3 mm, placed in a cylindrical die having an inner diameter of 18.0 mm, vulcanized for 30 minutes at 170° C., released from the die, and polished to obtain a cylindrical conductive elastic layer A.
     Rubber material: 100 parts by mass   (epichlorohydrin-ethylene oxide-allyl glycidyl ether copolymer rubber) Gechron 3106: product of ZEON CORPORATION   Conductive agent (carbon black Asahi Thermal, product of ASAHI CARBON CO., LTD.) 25 parts by mass   Conductive agent (Ketjenblack EC: product of Lion Corporation) 8 parts by mass   Ion conductive agent (lithium perchlorate) 1 part by mass   Vulcanizing agent (sulfur) 200 mesh: product of Tsurumi Chemical Co.) 1 part by mass   Vulcanization accelerator (Nocceler DM: product of OUCHI SHINKO CHEMICAL INDUSTRIAL CO., LTD) 2.0 parts by mass   Vulcanization accelerator (Nocceler TT: product of OUCHI SHINKO CHEMICAL INDUSTRIAL CO., LTD) 0.5 parts by mass
 
—Formation of Surface Layer—
   

     A dispersion obtained by dispersing the mixture below with a bead mill is diluted with methanol, applied on a surface of the conductive elastic layer A by dip-coating, and thermally dried at 140° C. for 15 minutes to form a surface layer having a thickness of 4 μm to obtain a conductive roll. This conductive roll is used as a charging roll.
     Polymer material 100 parts by weight   (copolymer nylon) Amilan CM8000: product of Toray Industries. Inc.   Conductive agent: 30 parts by mass   (antimony-doped tin oxide) SN-100P: product of ISHIHARA SANGYO KAISHA LTD.   Solvent (methanol) 500 parts by mass   Solvent (butanol) 240 parts by mass   

     Example 2 
     (Preparation of Cleaning Roll  2 ) 
     A helically arranged elastic layer and a cleaning roll  2  are prepared as in Example 1 except that the thickness of the polyurethane foam sheet is 2.85 mm and a strip having a thickness (thickness in the central portion in the width direction) of 3 mm is used. 
     Preparation of Charging Roll 
     A charging roll is prepared as in Example 1. 
     Example 3 
     (Preparation of Cleaning Roll  3 ) 
     A helically arranged elastic layer and a cleaning roll  3  are prepared as in Example 1 except that the thickness of the polyurethane foam sheet is 2.6 mm and a strip having a thickness (thickness in the central portion in the width direction) of 2.75 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Example 4 
     (Preparation of Cleaning Roll  4 ) 
     A helically arranged elastic layer and a cleaning roll  4  are prepared as in Example 1 except that the thickness of the polyurethane foam sheet is 2.05 mm and a strip having a thickness (thickness in the central portion in the width direction) of 2.2 mm is used. 
     Preparation of Charging Roll 
     A charging roll is prepared as in Example 1. 
     Example 5 
     (Preparation of Cleaning Roll  5 ) 
     A helically arranged elastic layer and a cleaning roll  5  are prepared as in Example 1 except that the thickness of the polyurethane foam sheet is 1.65 mm and a strip having a thickness (thickness in the central portion in the width direction) of 1.8 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Example 6 
     (Preparation of Cleaning Roll  6 ) 
     A helically arranged elastic layer and a cleaning roll  6  are prepared as in Example 1 except that the thickness of the polyurethane foam sheet is 1.6 mm and a strip having a thickness (thickness in the central portion in the width direction) of 1.75 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Examples 7 to 12 
     (Preparation of Cleaning Rolls  7  to  12 ) 
     Cleaning rolls  7  to  12  are prepared as in Examples 1 to 6 except that the core is a cylindrical core having an outer diameter of 4 mm and a length of 337 mm and the helical angle is set to 26°. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Example 13 
     (Preparation of Cleaning Roll  13 ) 
     A cleaning roll  13  is prepared as in Example 1 except that the helical angle is 65°, the thickness of the polyurethane foam sheet is 1.85 mm, and a strip having a thickness (thickness in the central portion in the width direction) of 2.0 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Example 14 
     (Preparation of Cleaning Roll  14 ) 
     A cleaning roll  14  is prepared as in Example 1 except that the core is a cylindrical core having an outer diameter of 4 mm and a length of 337 mm, the helical angle is 10°, the thickness of the polyurethane foam sheet is 2.85 mm, and a strip having a thickness (thickness in the central portion in the width direction) of 3.0 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Comparative Example 1 
     (Preparation of Cleaning Roll  15 ) 
     A helically arranged elastic layer and a cleaning roll  15  are prepared as in Example 1 except that the thickness of the polyurethane foam sheet is 3.35 mm and a strip having a thickness (thickness in the central portion in the width direction) of 3.5 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Comparative Example 2 
     (Preparation of Cleaning Roll  16 ) 
     A helically arranged elastic layer and a cleaning roll  16  are prepared as in Example 1 except that the thickness of the polyurethane foam sheet is 1.45 mm and a strip having a thickness (thickness in the central portion in the width direction) of 1.6 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Comparative Example 3 
     (Preparation of Cleaning Roll  17 ) 
     A cleaning roll  17  is prepared as in Example 7 except that the thickness of the polyurethane foam sheet is 3.35 mm, and a strip having a thickness (thickness in the central portion in the width direction) of 3.5 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Comparative Example 4 
     (Preparation of Cleaning Roll  18 ) 
     A cleaning roll  18  is prepared as in Example 7 except that the thickness of the polyurethane foam sheet is 1 45 mm, and a strip having a thickness (thickness in the central portion in the width direction) of 1.6 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Comparative Example 5 
     (Preparation of Cleaning Roll  19 ) 
     A cleaning roll  19  is prepared as in Example 1 except that the helical angle is 70°, the thickness of the urethane sheet foam is 1.85 mm, and a strip having a thickness (thickness in the central portion in the width direction) of 2.0 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     Comparative Example 6 
     (Preparation of Cleaning Roll  20 ) 
     A cleaning roll  20  is prepared as in Example 7 except that the helical angle is 5°, the thickness of the urethane sheet foam is 2.85 mm, and a strip having a thickness (thickness in the central portion in the width direction) of 3.0 mm is used. 
     (Preparation of Charging Roll) 
     A charging roll is prepared as in Example 1. 
     [Evaluation] 
     (Evaluation of Property) 
     The thickness (thickness in the helical width direction) of the elastic layer of the cleaning rolls prepared in the examples is investigated. The results are shown in Table 1. 
     (Storage Evaluation) 
     A cleaning roll and a charging roll of each example are installed in a color copier DocuCentre-III C3300 produced by Fuji Xerox Co., Ltd., modified so that the charging power source is a DC power source only. The installation is performed after the process cartridge of the color copier including the cleaning roll and the charging roll preliminarily installed therein is stored at a temperature of 40° C. and 95% RH for one month. 
     Half-tone images are output from the color copier and the extent of banding (image defects immediately after storage) is evaluated. 
     (Evaluation of Cleaning Property and Color Spots) 
     The cleaning roll and the charging roll of each example are installed in a color copier DocuCentre-III C3300 produced by Fuji Xerox Co., Ltd., modified so that the charging power source is a DC power source only. Printing is conducted on 300,000 A4 sheets. Half-tone images are then output. Whether banding (cleaning property) caused by non-uniform cleaning of the charging roll occurs and whether there are color spots caused by cleaning roll segments are evaluated on the basis of the following standards. The results are shown in Table 1. 
     Evaluation Standard of Image Defects Immediately after Storage 
     
         
         AA: No banding occurs in the image and the cleaning roll does not undergo deformation. 
         A: No banding occurs in the image but slight deformation occurs in the cleaning roll. 
         B: Slight banding occurs in the image. 
         C: Banding occurs in the image.
 
Evaluation Standard of Cleaning Property
 
         AA: No banding occurs in the image and the cleaning roll does not undergo deformation. 
         A: No banding occurs in the image but slight deformation occurs in the cleaning roll. 
         B: Slight banding occurs in the image. 
         C: Banding occurs in the image.
 
Evaluation Standard for Color Spots
 
         A: Color spots are not found in the image. 
         C: Color spots are found in the image. 
       
    
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Core outer 
                 Helical 
                 Elastic layer 
                 Thickness of strip in a 
                   
                 Image defect 
                   
                   
               
               
                   
                 diameter 
                 angle 
                 thickness in helical 
                 central portion in the width 
                   
                 immediately 
                 Cleaning 
                 Color 
               
               
                   
                 (mm) 
                 (°) 
                 width direction 
                 direction before winding 
                 t/T 
                 after storage 
                 property 
                 spot 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 6 
                 40 
                 2.4 
                 3.4 
                 0.71 
                 B 
                 A 
                 A 
               
               
                 Example 2 
                 6 
                 40 
                 2.35 
                 3 
                 0.78 
                 A 
                 A 
                 A 
               
               
                 Example 3 
                 6 
                 40 
                 2.2 
                 2.75 
                 0.80 
                 AA 
                 A 
                 A 
               
               
                 Example 4 
                 6 
                 40 
                 1.87 
                 2.2 
                 0.85 
                 AA 
                 A 
                 A 
               
               
                 Example 5 
                 6 
                 40 
                 1.62 
                 1.8 
                 0.90 
                 AA 
                 A 
                 A 
               
               
                 Example 6 
                 6 
                 40 
                 1.6 
                 1.75 
                 0.91 
                 B 
                 A 
                 A 
               
               
                 Example 7 
                 4 
                 26 
                 2.4 
                 3.4 
                 0.71 
                 B 
                 A 
                 A 
               
               
                 Example 8 
                 4 
                 26 
                 2.35 
                 3 
                 0.78 
                 A 
                 A 
                 A 
               
               
                 Example 9 
                 4 
                 26 
                 2.2 
                 2.75 
                 0.80 
                 AA 
                 A 
                 A 
               
               
                 Example 10 
                 4 
                 26 
                 1.87 
                 2.2 
                 0.85 
                 AA 
                 A 
                 A 
               
               
                 Example 11 
                 4 
                 26 
                 1.62 
                 1.8 
                 0.90 
                 AA 
                 A 
                 A 
               
               
                 Example 12 
                 4 
                 26 
                 1.6 
                 1.75 
                 0.91 
                 B 
                 A 
                 A 
               
               
                 Example 13 
                 6 
                 65 
                 1.42 
                 2.0 
                 0.71 
                 A 
                 A 
                 A 
               
               
                 Example 14 
                 4 
                 10 
                 2.72 
                 3.0 
                 0.91 
                 A 
                 A 
                 A 
               
               
                 Comparative 
                 6 
                 40 
                 2.45 
                 3.5 
                 0.70 
                 C 
                 A 
                 A 
               
               
                 Example 1 
               
               
                 Comparative 
                 6 
                 40 
                 1.52 
                 1.6 
                 0.95 
                 C 
                 A 
                 A 
               
               
                 Example 2 
               
               
                 Comparative 
                 4 
                 26 
                 2.45 
                 3.5 
                 0.70 
                 C 
                 A 
                 A 
               
               
                 Example 3 
               
               
                 Comparative 
                 4 
                 26 
                 1.52 
                 1.6 
                 0.95 
                 C 
                 A 
                 A 
               
               
                 Example 4 
               
               
                 Comparative 
                 6 
                 70 
                 1.4 
                 2.0 
                 0.70 
                 C 
                 A 
                 A 
               
               
                 Example 5 
               
               
                 Comparative 
                 4 
                 5 
                 2.85 
                 3.0 
                 0.95 
                 C 
                 A 
                 A 
               
               
                 Example 6 
               
               
                   
               
            
           
         
       
     
     The results show that, compared to Comparative Example, Examples had less image defects immediately after storage. 
     The results also show that the Examples exhibited higher cleaning property and less color spots caused by polishing powder and the like. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments are chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.