Patent Publication Number: US-10788768-B2

Title: Charging device, image former, and image forming apparatus that electrically charge a surface of a charging target member

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2018-245408 filed on Dec. 27, 2018, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The technology relates to a charging device that electrically charges a surface of a charging target member, an image former provided with the charging device, and an image forming apparatus provided with the charging device. 
     An electrophotographic image forming apparatus is in widespread use. One reason for this is that the electrophotographic image forming apparatus allows a high-quality image to be obtained in a short time, as compared with an image forming apparatus of other method such as an inkjet method. 
     The electrophotographic image forming apparatus, hereinafter simply referred to as an “image forming apparatus,” includes an image former that performs a charging process and a developing process. The image former may form an electrostatic latent image on a surface of a photosensitive member by electrically charging the surface of the photosensitive member, and thereafter attach a toner to the electrostatic latent image. The image former therefore includes a charging device. The charging device includes a charging member that electrically charges the surface of the photosensitive member. 
     A configuration of a charging device influences a charging state of a surface of a photosensitive member, and accordingly influences quality of an image formed with use of an electrostatic latent image. Various considerations have been therefore given to the configuration of the charging device. For example, in order to electrically charge the surface of the photosensitive member uniformly, a charging member or a charging roller of a contact-charging type is used. For example, reference can be made to Japanese Unexamined Patent Application Publication No. 2015-090409. The charging roller rotates while being in contact with the photosensitive member and thereby electrically charges the surface of the photosensitive member. 
     SUMMARY 
     Although various proposals have been made regarding a configuration of a charging device to be mounted on an image forming apparatus or an image former, the configuration of the charging device is still insufficient from a viewpoint of ensuring quality of an image, leaving room for improvement. 
     It is desirable to provide a charging device, an image former, and an image forming apparatus that are capable of forming a high-quality image. 
     According to one embodiment of the technology, there is provided a charging device that includes a charging member and a cleaning member. The charging member electrically charges a surface of a charging target member and is rotatable while being in contact with the surface of the charging target member. The cleaning member is in contact with a surface of the charging member and removes a foreign object attached to the surface of the charging member. Surface free energy of the surface of the charging member is 5.00 dynes per centimeter or more. A dynamic friction coefficient of the surface of the charging member and a surface of the cleaning member falls within a range from 0.48 to 0.88 both inclusive. The surface free energy and the dynamic friction coefficient satisfy a relation represented by the following expression (1),
 
 E≤− 63μ+69  (1)
 
where “E” is the surface free energy in dynes per centimeter, and “μ” is the dynamic friction coefficient.
 
     According to one embodiment of the technology, there is provided an image former that includes a developing device and a charging device. The developing device includes a photosensitive member and performs a developing process with use of a toner. The charging device performs a charging process on a surface of the photosensitive member. The charging device includes a charging member and a cleaning member. The charging member electrically charges the surface of the photosensitive member and is rotatable while being in contact with the surface of the photosensitive member. The cleaning member is in contact with a surface of the charging member and removes a foreign object attached to the surface of the charging member. Surface free energy of the surface of the charging member is 5.00 dynes per centimeter or more. A dynamic friction coefficient of the surface of the charging member and a surface of the cleaning member falls within a range from 0.48 to 0.88 both inclusive. The surface free energy and the dynamic friction coefficient satisfy a relation represented by the following expression (1),
 
 E≤− 63μ+69  (1)
 
where “E” is the surface free energy in dynes per centimeter, and “μ” is the dynamic friction coefficient.
 
     According to one embodiment of the technology, there is provided an image forming apparatus that includes an image former, a transfer section, and a fixing section. The image former performs a charging process and a developing process. The transfer section performs a transfer process with use of a toner subjected to the developing process by the image former. The fixing section performs a fixing process with use of the toner subjected to the transfer process by the transfer section. The image former includes a developing device and a charging device. 
     The developing device includes a photosensitive member and performs a developing process with use of a toner. The charging device performs a charging process on a surface of the photosensitive member. The charging device includes a charging member and a cleaning member. The charging member electrically charges the surface of the photosensitive member and is rotatable while being in contact with the surface of the photosensitive member. The cleaning member is in contact with a surface of the charging member and removes a foreign object attached to the surface of the charging member. Surface free energy of the surface of the charging member is 5.00 dynes per centimeter or more. The dynamic friction coefficient of the surface of the charging member and a surface of the cleaning member falls within a range from 0.48 to 0.88 both inclusive. The surface free energy and the dynamic friction coefficient satisfy a relation represented by the following expression (1),
 
 E≤− 63μ+69  (1)
 
where “E” is the surface free energy in dynes per centimeter, and “μ” is the dynamic friction coefficient.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an example of a configuration of an image forming apparatus according to an embodiment of the technology. 
         FIG. 2  is a block diagram illustrating an example of the configuration of the image forming apparatus. 
         FIG. 3  is a plan view of an example of a configuration of a charging device. 
         FIG. 4  is a plan view for explaining an example of a method of measuring a dynamic friction coefficient. 
         FIG. 5  is a plan view of an example of a configuration of a print medium at a time of successive image formation. 
         FIG. 6  is a diagram illustrating an example of a correlation between surface free energy, the dynamic friction coefficient, and quality of an image (an image defect occurrence state). 
     
    
    
     DETAILED DESCRIPTION 
     Some example embodiments of the technology are described below in detail with reference to the drawings. Note that the following description is directed to illustrative examples of the technology and not to be construed as limiting to the technology. Factors including, without limitation, numerical values, shapes, materials, components, positions of the components, and how the components are coupled to each other are illustrative only and not to be construed as limiting to the technology. Further, elements in the following example embodiments which are not recited in a most-generic independent claim of the technology are optional and may be provided on an as-needed basis. The drawings are schematic and are not intended to be drawn to scale. Note that the like elements are denoted with the same reference numerals, and any redundant description thereof will not be described in detail. The description is given in the following order.
     1. Image Forming Apparatus (Image Former and Charging Device)
       1-1. Configuration
           1-1-1. Overall Configuration   1-1-2. Block Configuration   1-1-3. Detailed Configuration of Charging Device   
           1-2. Operation   1-3. Example Workings and Example Effects   
       2. Modification Examples   

     1. Image Forming Apparatus (Image Former and Charging Device) 
     Description is given first of an image forming apparatus according to an example embodiment of the technology. Since each of an image former according to an example embodiment of the technology and a charging device according to an example embodiment of the technology is a portion, or one component, of the image forming apparatus, the image former and the charging device are described together below. 
     [1-1. Configuration] 
     For example, an image forming apparatus described below may form an image on a print medium M with use of toners of two or more colors, as will be described later. The image forming apparatus may be, for example, a so-called electrophotographic full-color printer (see  FIG. 1 .) The image forming apparatus may employ, for example, a direct transfer method that uses no intermediate transfer medium to form an image on the print medium M. 
     The print medium M is not particularly limited in its type; however, the print medium may be, for example, one or more of paper, a film, and any other printable medium. Specific but non-limiting examples of the print medium M as the paper may include plain paper, copy paper, special paper, and an envelope. Specific but non-limiting examples of the print medium M as the film may include an overhead projector (OHP) sheet. 
     [1-1-1. Overall Configuration] 
       FIG. 1  illustrates an example of a planar configuration of the image forming apparatus. As illustrated in  FIG. 1 , the image forming apparatus may include, for example, a cassette  10 , a hopping roller  20 , a developing section  30 , a charging section  40 , an exposure section  50 , a transfer section  60 , a fixing section  70 , and a conveying roller  80 . The image forming apparatus may convey the print medium M in a conveyance direction D 1  along a conveyance path P indicated by a dashed line. The developing section  30  and the charging section  40  may correspond to an “image former” in one specific but non-limiting embodiment of the technology. The developing section  30  may correspond to a “developing device” in one specific but non-limiting embodiment of the technology. The charging section  40  may correspond to a “charging device” in one specific but non-limiting embodiment of the technology. 
     A series of rollers described below, i.e., a series of components including a term “roller” in their names, may each be a cylindrical member that extends in a direction intersecting a paper plane of  FIG. 1 , i.e., an X-axis direction, and is rotatable about a rotational axis extending in the X-axis direction. 
     [Cassette and Hopping Roller] 
     The cassette  10  may be, for example, an accommodation member that accommodates the print medium M. The cassette  10  may be attachable and detachable. The cassette  10  may accommodate two or more print medium M stacked on each other, for example. The hopping roller  20  may be a feeding member, or a print medium feeding roller, that feeds the print medium M to the conveyance path P by picking up the print medium M from the cassette  10 . 
     [Developing Section] 
     The developing section  30  performs a developing process with use of a toner. In one specific but non-limiting example, the developing section  30  may use Coulomb force and thereby attach the toner to a surface of a photosensitive drum  311  that has been subjected to a charging process by the charging section  40 , for example. The photosensitive drum  311  will be described later. In a more specific but non-limiting example, the developing section  30  may thus attach the toner to an electrostatic latent image formed on the surface of the photosensitive drum  311 . The photosensitive drum  311  may correspond to a “charging target member” and a “photosensitive member” in one specific but non-limiting embodiment of the technology. 
     The developing section  30  may include a developing process unit  31  that performs the developing process. The developing process unit  31  may include, for example, the above-described photosensitive drum  311 , a developing roller  312 , a feeding roller  313 , and a cleaning blade  314 . 
     The photosensitive drum  311  may be a cylindrical member that extends in the X-axis direction, and may be rotatable about a rotational axis extending in the X-axis direction. The photosensitive drum  311  may be, for example, an organic photosensitive member that includes a cylindrical electrically-conductive shaft and a photoconductive layer. The electrically-conductive shaft may extend in the X-axis direction. The photoconductive layer may cover an outer peripheral surface of the electrically-conductive shaft. 
     The electrically-conductive shaft may be, for example, a metal pipe that includes one or more of metal materials such as aluminum or stainless steel. The photoconductive layer may include, for example, charge generation layers and charge transport layers that are alternately stacked on each other. The photoconductive layer may include any other layer, for example. The charge generation layer may include, for example, a charge generation substance and binder resin. The charge generation layer may include any other material, for example. The charge transport layer may include, for example, a charge transport substance and binder resin. The charge transport layer may also include various additives such as an antioxidant or a sensitizer on an as-needed basis. The charge transport layer may include any other material, for example. 
     The charge generation substance may include, for example, one or more of materials such as an organic pigment or an organic dye. Other than the above, non-limiting examples of the charge generation substance may include metal-free phthalocyanine, copper, indium chloride, gallium chloride, oxytitanium, metal, an oxide of the metal, and an azo pigment. Non-limiting examples of the metal may include tin, zinc, and vanadium. Non-limiting examples of the azo pigment may include monoazos, hisazos, trisazos, and polyazos. The binder resin may include, for example, one or more of polymer materials. Non-limiting examples of the polymer materials may include polyester, polyvinyl acetate, polyacrylate ester, polymethacrylate ester, polycarbonate, polyvinyl acetoacetal, polyvinylpropional, polyvinyl butyral, phenoxy resin, epoxy resin, urethane resin, cellulose ester, and cellulose ether. 
     The charge transport substance may include, for example, one or more of electron donating substances. Non-limiting examples of the electron donating substances may include a heterocyclic compound, an aniline derivative, a hydrazone compound, an aromatic amine derivative, and a stilbene derivative. Non-limiting examples of the heterocyclic compound may include carbazole, indole, imidazole, oxazole, pyrazole, oxadiazole, pyrazoline, and thiadiazole. It is to be noted that the charge transport substance may be, for example, a polymer having a group containing the above-described electron donating substance in a main chain or a side chain. The binder resin may include, for example, one or more of polymer materials. Non-limiting examples of the polymer materials may include polycarbonate, polymethyl methacrylate, polystyrene, a vinyl polymer, polyester, polyester carbonate, polysulfone, polyimide, phenoxy resin, epoxy resin, and silicone resin. Non-limiting examples of the vinyl polymer may include polyvinyl chloride. It is to be noted that the binder resin may be, for example, a polymer of any two or more of the above-described series of polymer materials or a partially-cross-linked cured product of any two or more of the above-described series of polymer materials. 
     The developing roller  312  may be in contact with the photosensitive drum  311 . The developing roller  312  may attach the toner to an electrostatic latent image formed on the photosensitive drum  311 . The developing roller  312  may include, for example, a metallic cylindrical shaft and an electrically-semiconductive urethane rubber layer. The electrically-semiconductive urethane rubber layer may cover an outer peripheral surface of the shaft. 
     The feeding roller  313  may be in contact with the developing roller  312 . The feeding roller  313  may feed the toner discharged from an unillustrated toner cartridge to the developing roller  312 . The cleaning blade  314  may be a plate-shaped member that is in contact with the photosensitive drum  311 . The cleaning blade  314  may scrape off a foreign object on the surface of the photosensitive drum  311 . The foreign object may be, for example but not limited to, an unnecessary remaining of the toner on the surface of the photosensitive drum  311 . The cleaning blade  314  may include one or more of flexible materials. Non-limiting examples of the flexible materials may include a rubber material and a polymer material. 
     In the example embodiment, the developing section  30  may include, for example, four developing process units  31  (i.e., developing process units  31 Y,  31 M,  31 C, and  31 K.) The developing process units  31 Y,  31 M,  31 C, and  31 K may be disposed in this order from downstream side to upstream side in the conveyance direction D 1 , for example. The developing process units  31 Y,  31 M,  31 C, and  31 K may have respective configurations similar to each other except that types, e.g., colors, of toners used in the developing process differ from each other, for example. In one specific but non-limiting example, the developing process unit  31 Y may be mounted with a yellow toner. The developing process unit  31 M may be mounted with a magenta toner, for example. The developing process unit  31 C may be mounted with a cyan toner, for example. The developing process unit  31 K may be mounted with a black toner, for example. 
     The toner may include, for example, a toner base particle and an external additive. The toner base particle may include a material such as a colorant, binder resin, a mold release agent, or a charge control agent. The external additive may be fixed to a surface of the toner base particle. The agent such as the colorant, the mold release agent, or the charge control agent is a so-called inside additive included inside the toner base particle. In contrast, the external additive is an additive to be added on outer side of the toner base particle. The colorant may include one or more of materials such as a pigment or a dye each having a color corresponding to the color of the toner. The binder resin may include, for example, one or more of polymer materials such as polyester. The external additive may be, for example, a plurality of inorganic particles that prevents the toner from aggregating. The inorganic particles may include, for example, one or more of silicon dioxide (silica) particles, titanium oxide particles, and any other suitable particles. The toner is not particularly limited in its degree of circularity; however, the degree of circularity of the toner may fall within a range from about 0.94 to about 0.98 both inclusive, for example. The toner is not particularly limited in its average particle size (median size D50); however, the average particle size of the toner may be, for example, about 7 μm. The external additive including a plurality of particles is not particularly limited in its average particle size (median size D50); however, the average particle size of the external additive may fall within a range from about 50 nm to about 200 nm both inclusive, for example. 
     [Charging Section] 
     The charging section  40  performs a charging process on the surface of the photosensitive drum  311 . In one specific but non-limiting example, the charging section  40  may electrically charge the surface of the photosensitive drum  311  uniformly in order to form an electrostatic latent image on the surface of the photosensitive drum  311 . The charging section  40  may be disposed, for example, for each of the developing process units  31 . The charging section  40  may include a charging roller  41  that electrically charges the surface of the photosensitive drum  311 . A detailed configuration of the charging section  40  will be described later with reference to  FIG. 3 . The charging roller  41  may correspond to a “charging member” in one specific but non-limiting embodiment of the technology. 
     [Exposure Section] 
     The exposure section  50  may perform an exposure process by means of exposure light. In one specific but non-limiting example, the exposure section  50  may form an electrostatic latent image on the surface of the photosensitive drum  311 , for example, by applying the exposure light to the surface, of the photosensitive drum  311 , that has been electrically charged by the charging section  40 . 
     The exposure section  50  may be disposed, for example, for each of the developing process units  31 . The exposure section  50  may include a light source  51 . Non-limiting examples of the light source  51  may include a light emitting diode (LED) and a laser device. Other than the above, the exposure section  50  may also include, for example, a component such as a lens array that forms an image of the exposure light on the surface of the photosensitive drum  311 . In the example embodiment, the image forming apparatus may include, for example, four exposure sections  50  corresponding to the four developing process units  31 , i.e., the developing process units  31 Y,  31 M,  31 C, and  31 K. 
     [Transfer Section] 
     The transfer section  60  performs a transfer process with use of the toner that has been subjected to the developing process by the developing section  30 . In one specific but non-limiting example, the transfer section  60  may transfer, onto the print medium M, the toner attached to the electrostatic latent image. 
     The transfer section  60  may include, for example, a driving roller  61 , an idler roller  62 , a transfer belt  63 , a transfer roller  64 , and a cleaning blade  65 . 
     The driving roller  61  may be rotatable by means of a drive source such as a motor, for example. The idler roller  62  may be rotatable in accordance with the rotation of the driving roller  61 , for example. The transfer belt  63  may be, for example, an endless belt. The transfer belt  63  may travel in a travel direction D 2  in accordance with the rotation of the driving roller  61  while lying on the driving roller  61  and the idler roller  62  and being stretched by the driving roller  61  and the idler roller  62 , for example. 
     The transfer roller  64  may be in contact with the photosensitive drum  311  with the transfer belt  63  in between. The transfer roller  64  may transfer, onto the print medium M, the toner attached to the electrostatic latent image. In the example embodiment, the transfer section  60  may include, for example, four transfer rollers  64 , i.e., transfer rollers  64 Y,  64 M,  64 C, and  64 K, corresponding to the four developing process units  31 , i.e., the developing process units  31 Y,  31 M,  31 C, and  31 K. The cleaning blade  65  may be, for example, a plate-shaped member that is in contact with a surface of the transfer belt  63 . The cleaning blade  65  may scrape off a foreign object on the surface of the transfer belt. The foreign object may be, for example, an unnecessary remaining of the toner attached to the surface of the transfer belt  63 . 
     [Fixing Section] 
     The fixing section  70  performs a fixing process with use of the toner that has been subjected to the transfer process by the transfer section  60 . In one specific but non-limiting example, the fixing section  70  may fix the toner to the print medium M by, for example, applying pressure to the print medium M provided with the transferred toner while heating the print medium M. 
     The fixing section  70  may include, for example, a heating roller  71  and a pressure-applying roller  72 . The heating roller  71  may include a heat source built in the heating roller  71 . Non-limiting examples of the heat source may include a halogen lamp. The heating roller  71  may heat the print medium M on which the toner has been transferred. The pressure-applying roller  72  may be in contact with the heating roller  71 . The pressure-applying roller  72  may apply pressure to the print medium M on which the toner has been transferred. 
     [Conveying Roller] 
     The conveying roller  80  may convey the print medium M along the conveyance path P. The conveying roller  80  may include, for example, a pair of rollers opposed to each other with the conveyance path P in between. In one example embodiment, the image forming apparatus may include, for example, two conveying rollers  80 , i.e., conveying rollers  81  and  82 . Since the conveying roller  80  is, however, not limited in its number, any number of conveying rollers  80  may be provided. 
     [1-1-2. Block Configuration] 
       FIG. 2  illustrates an example of a block configuration of the image forming apparatus.  FIG. 2  also illustrates some of the components of the image forming apparatus already described above. 
     For example, as illustrated in  FIG. 2 , the image forming apparatus may include a controller  100 , a reception memory  111 , an image data editing memory  112 , an operation panel  113 , a sensor group  114 , and a power supply circuit  120 . 
     The controller  100  may control the image forming apparatus as a whole. The controller  100  may include, for example, one or more of electronic components such as a control circuit, a memory, an input-output port, or a timer. The control circuit may include, for example but not limited to, a central processing unit (CPU.) The memory may include one or more of memory devices such as a read-only memory (ROM) or a random-access memory (RAM.) 
     The controller  100  may include, for example, a main controller  101 , an interface controller  102 , an exposure controller  103 , a fixing controller  104 , a conveyance controller  105 , and a drive controller  106 . The main controller  101  may generally control operation of the image forming apparatus as a whole. The interface controller  102  may receive information supplied to the image forming apparatus from an external device. Non-limiting examples of the information may include image data. Non-limiting example of the external device may include a personal computer. The exposure controller  103  may control operation of the exposure section  50 , e.g., the light source  51 . The fixing controller  104  may control operation of the fixing section  70 , e.g., the heating roller  71  and the pressure-applying roller  72 . The conveyance controller  105  may control operation of the conveying roller  80 . The drive controller  106  may control operation of the transfer section  60 , e.g., the transfer belt  63 . 
     The reception memory  111  may hold the information, such as the image data, supplied from the external device to the image forming apparatus. The image data editing memory  112  may hold, for example, the image data subjected to an editing process. The operation panel  113  may serve as a display device that displays information necessary for a user to operate the image forming apparatus. The operation panel  113  may also serve as an input device to be used by the user to operate the image forming apparatus. The operation panel  113  may include, for example, a component such as a display lamp, a display panel, or an operation button. Non-limiting examples of the display lamp may include an LED lamp. Non-limiting examples of the display panel may include a touch panel. The sensor group  114  may include, for example, one or more of sensors such as a temperature sensor, a humidity sensor, an image density sensor, a print medium position detection sensor, a toner remaining amount detection sensor, or a human presence sensor. 
     The power supply circuit  120  may include, for example, a charging-roller power supply  121 , a developing-roller power supply  122 , a feeding-roller power supply  123 , and a transfer-roller power supply  124 . 
     The charging-roller power supply  121  may apply a voltage to the charging roller  41 . The developing-roller power supply  122  may apply a voltage to the developing roller  312 . The feeding-roller power supply  123  may apply a voltage to the feeding roller  313 . The transfer-roller power supply  124  may apply a voltage to the transfer roller  64 . 
     [1-1-3. Detailed Configuration of Charging Device] 
       FIG. 3  illustrates an example of a planar configuration of the charging section  40 .  FIG. 3  also illustrates a portion of the photosensitive drum  311  and the charging-roller power supply  121  together. 
     The charging section  40  may include the charging roller  41  and a cleaning roller  42 , as illustrated in  FIG. 3 . The photosensitive drum  311  may include a surface  311 M to be electrically charged by the charging section  40 . The cleaning roller  42  may correspond to a “cleaning member” in one specific but non-limiting embodiment of the technology. 
     [Charging Roller] 
     The charging roller  41  may electrically charge the surface  311 M of the photosensitive drum  311 , as described above. The charging roller  41  may be in contact with the photosensitive drum  311 . The charging roller  41  may include, for example, an electrically-conductive shaft  411  and an electrically-conductive elastic layer  412 . The shaft  411  may be, for example, a metal core. The elastic layer  412  may correspond to a “first surface layer” in one specific but non-limiting embodiment of the technology. 
     As illustrated in  FIG. 3 , the shaft  411  may be a cylindrical member that extends in the X-axis direction. The shaft  411  may include one or more of metal materials. The shaft  411  may include any other material, for example. The metal material is not particularly limited in its type. Non-limiting examples of the metal material may include free-cutting steel (SUM) and stainless steel (SUS.) A surface of the shaft  411  may be plated with a metal material such as nickel by a plating method such as an electroless plating method, for example. The shaft  411  may be coupled to the charging-roller power supply  121 . This may allow the charging-roller power supply  121  to apply a voltage to the shaft  411 . 
     As illustrated in  FIG. 3 , the elastic layer  412  may cover an outer peripheral surface of the shaft  411 . The elastic layer  412  may include a surface  41 M that is in contact with the surface  311 M of the photosensitive drum  311 . The charging roller  41  or the elastic layer  412  may be rotatable about the shaft  411 , serving as a rotational axis, in a rotation direction R 2  while being in contact with the surface  311 M of the photosensitive drum  311 . The rotation direction R 2  of the charging roller  41  may be opposite to a rotation direction R 1  of the photosensitive drum  311 . In one specific but non-limiting example, the rotation direction R 2  may be a counterclockwise direction and the rotation direction R 1  may be a clockwise direction. The elastic layer  412  may be, for example, a single layer or a multi-layer. 
     The elastic layer  412  may include, for example, one or more of polymer materials such as rubber or thermoplastic elastomer. Specific but non-limiting examples of the polymer material may include epichlorohydrin rubber (CO, ECO, GECO), ethylene propylene rubber (EPM, EPDM), acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), chloroprene rubber (CR), urethane rubber, and silicone rubber. In one example embodiment, the polymer material may be a mixture of epichlorohydrin rubber (ECO) and acrylonitrile-butadiene rubber (NBR.) One reason is that this makes it easier for surface free energy E of the surface  41 M of the charging roller  41  to be decreased appropriately, as will be described later. 
     In one example embodiment, electrical conductivity of the elastic layer  412  may be set in relation to appropriate electrical resistance. One reason for this is as follows. If the electrical resistance is excessively large, there is a possibility the surface  311 M of the photosensitive drum  311  is charged non-uniformly or insufficiently. If the electrical resistance is excessively small, there is a possibility that a leakage current is generated due to a reason such as a scratch on the surface  311 M of the photosensitive drum  311 . Accordingly, in one example embodiment, the elastic layer  412  may include one or more of materials such as an ion conductive material, an ion conductive agent, carbon black, or a metal oxide to achieve a desired electrical conductivity. The electrical conductivity of the elastic layer  412  may be electronic conductivity or ionic conductivity. In one example embodiment, the elastic layer  412  may have ionic conductivity. One reason is that this suppresses variation in electrical resistance. 
     The elastic layer  412  is not particularly limited in its volume resistance; however, the volume resistance of the elastic layer  412  may fall within a range from 10 6 Ω to 10 9 Ω. A value of the volume resistance varies depending on conditions such as temperature, humidity, or measurement voltage in a case where the elastic layer  142  has ionic conductivity; however, the value of the volume resistance described in the example embodiment is a value measured under environmental conditions with a temperature of 20° C. and humidity of 50% RH. 
     The elastic layer  412  is not particularly limited in its hardness; however, the hardness of the elastic layer  412  may fall within a range from 35° degrees to 80° both inclusive, for example. One reason is that this provides a fine gap between the surface  41 M of the charging roller  41  and the surface  311 M of the photosensitive drum  311 , which ensures a region contributing to discharge based on Paschen&#39;s law, in other words, ensures an appropriate nip state. The hardness of the elastic layer  412  described above may be measured by peaks with use of a micro durometer MD-1capa (Type A) available from KOBUNSHI KEIKI CO., LTD. located in Kyoto, Japan. It is to be noted that the hardness of the elastic layer  412  may also serve to absorb a factor such as variation in a cylindrical shape of each of the charging roller  41  and the photosensitive drum  311 . Therefore, the hardness of the elastic layer  412  may be settable to any value as long as the above-described appropriate nip state is obtained. 
     The elastic layer  412  may be formed by, for example, a process such as a cutting process, a polishing process, or a molding process. Therefore, a surface shape of the elastic layer  412  may be so adjusted as to have a desired polishing mark and desired surface roughness. The surface roughness, e.g., maximum height Ry according to Japanese Industrial Standards (JIS) B 0601: 1994, of the charging roller  41  or the elastic layer  412  may vary to some extent depending on conditions such as an applied voltage or a use environment. In one specific but non-limiting example, however, the surface roughness of the charging roller  41  or the elastic layer  412  may fall within a range from about 1 μm to about 40 μm both inclusive based on Paschen&#39;s law. 
     The elastic layer  412  may be subjected to a surface treatment. One reason is that this prevents the photosensitive drum  311  from being contaminated by the components in the elastic layer  412 , and allows for adjustment of the surface resistance of the elastic layer  412 . Another reason is that this prevents a material such as a toner or an external additive attached to the surface  311 M of the photosensitive drum  311  from being attached to the surface  41 M of the charging roller  41 . 
     In one non-limiting example, the surface treatment may be an irradiation process such as an ultraviolet irradiation process or an electron beam irradiation process. In another non-limiting example, the surface treatment may be a coating process in which a coating solution is supplied to the surface  41 M of the elastic layer  412  by means of a dipping process, spraying, a coater, or any other suitable method. The coating solution may include, for example, one or more of materials such as an isocyanate compound or polyol. Non-limiting examples of the isocyanate compound may include toluene diisocyanate (TDI), methylene diisocyanate (MDI), xylylene diisocyanate (XDI), naphthalene diisocyanate (NDI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI.) Non-limiting examples of the polyol may include polyester-based polyol, polycarbonate-based polyol, silicone-based polyol, acrylic-fluorine-based polyol, acrylic-silicone-based polyol, and fluorine-based polyol. The polyol may be, for example but not limited to, a multimer or a denaturant. 
     The coating solution may include an electrically-conductive material such as carbon black or an ionic conductive agent on an as-needed basis. The coating solution may include a plurality of particles on an as-needed basis. The particles may include, for example, one or more of polymer materials such as acrylic resin, urethane resin, fluororesin, polyamide resin, polycarbonate resin, polyester resin, or isocyanate resin. 
     In one example embodiment, since the coating solution includes fluororesin, the surface treatment may be performed on the elastic layer  412  with a coating solution including the fluororesin. In other words, in one example embodiment, the elastic layer  412  may include fluororesin. One reason is that this makes it easier for the surface free energy E of the surface  41 M of the charging roller  41  to be decreased appropriately, as will be described later. The fluororesin is a generic term for resin, or a polymer material, that contains fluorine (F) as a constituent element. The fluororesin may include, for example, one or more of materials such as polytetrafluoroethylene (PTFE). 
     [Cleaning Roller] 
     The cleaning roller  42  may remove a foreign object attached to the surface  41 M of the charging roller  41 . The cleaning roller  42  may be in contact with the surface  41 M of the charging roller  41 . The cleaning roller  42  may remove the foreign object attached to the surface  41 M of the charging roller  41  by winding up the foreign object while rotating. In one specific but non-limiting example, the cleaning roller  42  may include, for example, a shaft  421  and an elastic layer  422 . The shaft  421  may be, for example, a core body. The elastic layer  422  may correspond to a “second surface layer” in one specific but non-limiting embodiment of the technology. 
     As illustrated in  FIG. 3 , the shaft  421  may be a cylindrical member that extends in the X-axis direction. The shaft  421  may include one or more of a metal material, a polymer material, and any other suitable material. The metal material is not particularly limited in its type. Non-limiting examples of the metal material may include free-cutting steel (SUM) and stainless steel (SUS.) The surface of the shaft  411  may be plated with a metal material such as nickel, for example, by an electroless plating method or any other suitable method. The polymer material is not particularly limited in its type. Non-limiting examples of the polymer material may include polyacetal (POM.) 
     For example, as illustrated in  FIG. 3 , the elastic layer  422  may cover an outer peripheral surface of the shaft  421 . The elastic layer  422  may include a surface  42 M that is in contact with the surface  41 M of the charging roller  41 . It is to be noted that, in one example, the elastic layer  422  may cover only a middle portion of the outer peripheral surface of the shaft  421 , i.e., a portion excluding both ends of the outer peripheral surface of the shaft  421  in a longitudinal direction. In another example, the elastic layer  422  may spirally cover the outer peripheral surface of the shaft  421 . The cleaning roller  42  or the elastic layer  422  may be rotatable about the shaft  421 , serving as a rotational axis, in a rotation direction R 3  while being in contact with the surface  41 M of the charging roller  41 , for example. The rotation direction R 3  of the cleaning roller  42  may be opposite to the rotation direction R 2  of the charging roller  41 . In one specific but non-limiting example, the rotation direction R 3  may be a clockwise direction and the rotation direction R 4  may be a counterclockwise direction. The elastic layer  422  may be, for example, a single layer or a multi-layer. The elastic layer  422  may have, for example, a foamed structure, or may have a structure in which a solid layer and a foamed layer are stacked on each other. 
     The elastic layer  422  may include, for example, one or more of polymer materials such as foamable resin or a rubber material. The foamable resin is not particularly limited in its type. Non-limiting examples of the foamable resin may include polyurethane, polyethylene, polyamide, and polypropylene. The rubber material is not particularly limited in its type. Non-limiting examples of the rubber material may include silicone rubber, fluororubber, urethane rubber, ethylene propylene rubber (EPM, EDPM), acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), styrene-butadiene rubber (SBR), butadiene rubber (BR), isoprene rubber (IR), and chloroprene rubber (CR.) The elastic layer  422  may include one or more of aids such as a foaming aid, a foam stabilizer, a catalyst, a curing agent, a plasticizer, or a vulcanization accelerator, on an as-needed basis. 
     In one example embodiment, the elastic layer  422  may include foamable resin having bubbles. One reason is that this makes it easier for a dynamic friction coefficient μ of the surface  41 M of the charging roller  41  and the surface  42 M of the cleaning roller  42  to be decreased appropriately, as will be described later. Accordingly, it is easier for the cleaning roller  42  or the elastic layer  422  to remove the foreign object attached to the surface  41 M of the charging roller  41 . 
     In one example embodiment, the elastic layer  422  may include foamable polyurethane or foamed polyurethane. One reason for this is as follows. The foamed polyurethane has resistance to tearing, pulling, etc. This makes it more difficult for the surface  41 M of the charging roller  41  to be damaged, and also makes it more difficult for the charging roller  41  be damaged, for example, be teared. 
     The elastic layer  422  is not particularly limited in its density, e.g., density according to JIS K7222. For example, the density of the elastic layer  422  may fall within a range from 20 kg/m 3  to 80 kg/m 3  both inclusive. The elastic layer  422  is not particularly limited in its hardness, 25% compressive hardness according to JIS K6400-2. For example, the hardness of the elastic layer  422  may fall within a range from 100 N to 410 N both inclusive, for example. The elastic layer  422  is not particularly limited in its tensile strength, e.g., tensile strength according to JIS K6400-5. For example, the tensile strength of the elastic layer  422  may fall within a range from 60 kPa to 300 kPa both inclusive. The elastic layer  422  is not particularly limited in its elongation, e.g., elongation according to JIS K6400-5. For example, the elongation of the elastic layer  422  may fall within a range from 100% to 220% both inclusive. 
     [Physical Property] 
     In the charging section  40 , a physical property of each of the charging roller  41  and the cleaning roller  42  may be so optimized as to make it more difficult for the external additive to be fixed to the surface  41 M of the charging roller  41  and as to make it easier for the external additive adsorbed to the surface  41 M of the charging roller  41  to be removed by the cleaning roller  42 , when the external additive falls off from the toner. 
     Firstly, the surface free energy E (dyn/cm) of the surface  41 M of the charging roller  41  or the elastic layer  412  is 5.00 dyn/cm or more. Secondly, the dynamic friction coefficient μ of the surface  41 M of the charging roller  41  or the elastic layer  412  and the surface  42 M of the cleaning roller  42  or the elastic layer  422  falls within a range from 0.48 to 0.88 both inclusive. Thirdly, the surface free energy E and the dynamic friction coefficient μ described above satisfy a relation represented by the following expression (1).
 
 E≤− 63μ+69  (1)
 
In the above-described expression (1), “E” is the surface free energy in dynes per centimeter, and “μ” is the dynamic friction coefficient.
 
     One reason why the surface free energy E falls within the above-described range is that it allows the surface free energy E to be decreased appropriately, which in turn allows a polarity of the surface  41 M of the charging roller  41  to be decreased appropriately. Accordingly, adsorption force of the external additive to the surface  41 M is decreased. This makes it more difficult for the external additive to be fixed to the surface  41 M. As a result, it is easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 . 
     One reason why the dynamic friction coefficient μ falls within the above-described range is that it allows the dynamic friction coefficient μ to be decreased appropriately, which in turn allows the force by which the surface  41 M of the charging roller  41  and the surface  42 M of the cleaning roller  42  are in sliding contact with each other to be decreased appropriately. This makes it more difficult for the cleaning roller  42  to rub the external additive on the surface  41 M of the charging roller  41 , and thereby makes it more difficult for the external additive to be deposited on the surface  41 M. As a result, it is easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 . 
     One reason why the surface free energy E and the dynamic friction coefficient μ satisfy the relation represented by the expression (1) is that the relation between the surface free energy E and the dynamic friction coefficient μ is optimized. This makes it more difficult for the cleaning roller  42  to rub the external additive against the surface  41 M of the charging roller  41  while the adsorption force of the external additive to the surface  41 M is decreased. Accordingly, it is more difficult for the external additive to be fixed to the surface  41 M. As a result, it is further easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 . 
     It is to be noted that the surface free energy E is not particularly limited as long as the surface free energy E is 5.00 dyn/cm or more. In one example embodiment, the surface free energy E may be 13.49 dyn/cm or less. One reason is that this allows polarity of the surface  41 M of the charging roller  41  to be decreased sufficiently. This sufficiently decreases the adsorption force of the external additive to the surface  41 M, which makes the external additive hardly fixed to the surface  41 M. Accordingly, it is more difficult for the surface  41 M of the charging roller  41  to be damaged due to contact or friction with the external additive. As a result, a life of the charging roller  41  is made longer. 
     In the example embodiment, a method of measuring the surface free energy E may be as follows. In a case of measuring the surface free energy E, a contact angle of the surface  41 M of the charging roller  41  may be measured with use of three types of liquid samples described in Table 1. The three liquid samples may be water (H 2 O), diiodomethane (CH 2 I 2 ), and dodecane (C 12 H 26 .) Surface free energy components yd (dyn/cm), γp (dyn/cm), γh (dyn/cm), and γtotal (dyn/cm) related to the three liquid samples are as described in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 γd 
                 γP 
                 γh 
                 γtotal 
               
               
                 Liquid sample 
                 (dyn/cm) 
                 (dyn/cm) 
                 (dyn/cm) 
                 (dyn/cm) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Water 
                 29.1 
                 1.3 
                 42.4 
                 72.8 
               
               
                 Diiodomethane 
                 46.8 
                 4.0 
                 0 
                 50.8 
               
               
                 Dodecane 
                 25.4 
                 0 
                 0 
                 25.4 
               
               
                   
               
            
           
         
       
     
     In a case of measuring the contact angle, the contact angle may be measured under ambient temperature and ambient humidity environmental conditions with use of a contact angle meter by a drop method with a liquid volume in a range from 0.22×10 −3  ml to 0.27×10 −3  ml, i.e., a range from 0.22 mm 3  to 0.27 mm 3 . The ambient temperature and ambient humidity environmental conditions may be, for example, at a temperature of 23±3° C. and at humidity of 55±10% RH. As the contact angle meter, for example, a contact angle meter of CA-X type available from Kyowa Interface Science Co., LTD. located in Saitama, Japan may be used. Thereafter, the surface free energy E may be calculated with use of an equation proposed by Kitazaki and Hata and a Young-Dupre equation on the basis of a result of the measurement of the contact angle. 
     An example of a method of measuring the dynamic friction coefficient μ may be as follows.  FIG. 4  illustrates an example of a planar configuration of components including the charging roller  41  in order to explain the method of measuring the dynamic friction coefficient μ. In the case of measuring the dynamic friction coefficient μ, Euler&#39;s belt formula may be used. 
     In one specific but non-limiting example, as illustrated in  FIG. 4 , after an inner surface of a belt  201  is brought into contact with the surface  41 M of the charging roller  41  or the elastic layer  412 , a first end of the belt  201  may be coupled to a weight  202 , and a second end of the belt  201  may be coupled to a load meter  203 . In this case, a winding angle, i.e., an angle at which the inner surface of the belt  201  is wound around the surface  41 M of the charging roller  41  may be 0 (rad). 
     When the charging roller  41  is rotated at a predetermined rotational speed in a rotation direction R 4  while causing the charging roller  41  to be in contact with the belt  201  in this state, the following relational expression (2) may be established between the winding angle θ (rad), a load W (gf) of the weight  202 , force F (gf) measured by the load meter  203 , and the dynamic friction coefficient μ of the surface  41 M of the charging roller  41  and the inner surface of the belt  201 .
 
μ=(1/θ)ln( F/W )  (2)
 
In a case of measuring the dynamic friction coefficient μ of the surface  41 M of the charging roller  41  and the surface  42 M of the cleaning roller  42 , the dynamic friction coefficient μ may be calculated with use of the belt  201 . The belt  201  may include the same material as that of the elastic layer  422 . In this case, the winding angle θ may be π/2 rad, a width of the belt  201  may be 7 mm, a thickness of the belt  201  may be 4 mm, the load W may be 50 gf, and the rotational speed of the charging roller  41 , i.e., a speed at which the charging roller  41  slides against the belt  201  while being in contact with the belt  201 , may be 1.0 mm/sec. After the force F is thereby measured with use of the load meter  203 , the dynamic friction coefficient μ may be calculated with use of the relational expression (2).
 
[1-2. Operation]
 
     The image forming apparatus may form an image on the print medium M by the following procedure, for example. In the following, reference is made where appropriate to  FIGS. 1 to 3  which have already been described. 
     In a case of forming an image on the print medium M, when image data is transmitted from the external device to the image forming apparatus, the interface controller  102  may receive the image data. Thereafter, the image data may be stored in the reception memory  111 , and image data subjected to an editing process may be stored in the image data editing memory  112 . 
     In this case, the conveyance controller  105  may drive the conveying roller  80  and the hopping roller  20  may be thereby caused to pick up the print medium M from the cassette  10 . Thereafter, the print medium M may be conveyed along the conveyance path P in the conveyance direction D 1 . Thereafter, the image forming apparatus may perform, for example, the charging process, the exposure process, the developing process, the transfer process, and the fixing process in this order as described below. The series of processes may be controlled by the controller  100  or the main controller  101 . 
     [Charging Process] 
     First, in the charging section  40 , when the charging-roller power supply  121  applies a voltage to the charging roller  41  or the shaft  411 , the charging roller  41  may rotate while being in contact with the photosensitive drum  311 . The charging roller  41  may thereby electrically charge the surface  311 M of the photosensitive drum  311  uniformly. In this case, since the cleaning roller  42  rotates while being in contact with the charging roller  41 , the foreign object attached to the surface  41 M of the charging roller  41  may be removed by the cleaning roller  42 . 
     [Exposure Process] 
     Thereafter, in the exposure section  50 , the exposure controller  103  may drive the light source  51  and thereby cause the light source  51  to apply the exposure light to the surface  311 M of the photosensitive drum  311  on the basis of the image data subjected to the editing process. As a result, an electrostatic latent image may be formed on the surface  311 M of the photosensitive drum  311 . 
     [Developing Process] 
     Thereafter, in the developing section  30  or the developing process unit  31 , when the feeding-roller power supply  123  applies a voltage to the feeding roller  313  and the developing-roller power supply  122  applies a voltage to the developing roller  312 , the feeding roller  313  may feed the toner to the surface of the developing roller  312 , and the developing roller  312  may attach the toner to the electrostatic latent image. In this case, the foreign object such as an unnecessary remaining of the toner attached to the surface  311 M of the photosensitive drum  311  may be scraped off by the cleaning blade  314 . 
     [Transfer Process] 
     Thereafter, in the transfer section  60 , when the drive controller  106  drives the transfer belt  63  and the transfer-roller power supply  124  applies a voltage to the transfer roller  64 , the transfer belt  63  may travel in a travel direction D 2  and the transfer roller  64  may be pressed against the photosensitive drum  311  with the transfer belt  63  in between. As a result, the toner attached to the electrostatic latent image may be transferred onto the print medium M. In this case, the foreign object such as the unnecessary remaining of the toner attached to the surface of the transfer belt  63  may be scraped off by the cleaning blade  65 . 
     A combination of which of the four developing process units  31 , i.e., the developing process units  31 Y,  31 M,  31 C, and  31 K and the four transfer rollers  64 , i.e., the transfer rollers  64 Y,  64 M,  64 C, and  64 K to perform the developing process and transfer process may be determined on the basis of a combination of colors of the toners required to form the image. 
     [Fixing Process] 
     Thereafter, in the fixing section  70 , the toner transferred onto the print medium M may be heated by the heating roller  71  while being applied with pressure by the pressure-applying roller  72 . The toner may be thereby fixed to the print medium M. As a result, the image may be formed on the print medium M, and the image forming operation may be completed. 
     [1-3. Example Workings and Example Effects] 
     In the image forming apparatus according to the example embodiment of the technology, the surface free energy E of the surface  41 M of the charging roller  41  or the elastic layer  412  in the charging section  40  is 5.00 dyn/cm or more, the dynamic friction coefficient μ of the surface  41 M of the charging roller  41  and the surface  42 M of the cleaning roller  42  or the elastic layer  422  in the charging section  40  falls within the range from 0.48 to 0.88 both inclusive, and the surface free energy E and the dynamic friction coefficient μ satisfy the relation represented by the expression (1). Accordingly, it is possible to form a high-quality image for the following reasons. 
     In a case where a toner in which an external additive, e.g. a plurality of inorganic particles, is fixed to a surface of a toner base particle, the external additive can fall off from the surface of the toner base particle. Examples of a possible cause of the external additive falling off from the surface of the toner base particle may include contact between the toners and contact between the toner and another object. 
     In this case, since the charging roller  41  is in contact with the photosensitive drum  311 , when the external additive fallen off from the toner base particle is attached to the surface  311 M of the photosensitive drum  311 , it is easier for the external additive to be transferred or adsorbed from the surface  311 M of the photosensitive drum  311  to the surface  41 M of the charging roller  41 . When the external additive is adsorbed to the surface  41 M of the charging roller  41 , charging may be insufficient, for example, a potential may drop in a region, of the surface  311 M of the photosensitive drum  311 , corresponding to the adsorption region of the external additive at a time of the charging process. This makes it easier for the toner to be attached unintentionally to the region where the charging is insufficient at the time of the developing process. One reason for this is that low electrical conductivity of the external additive makes it more difficult for the charging roller  41  to electrically charge the surface  311 M of the photosensitive drum  311 . 
     For such a reason, when the external additive is deposited on the surface  311 M of the photosensitive drum  311 , it is easier for an image defect to occur at a position corresponding to the adsorption region of the external additive in the image formed on the print medium M by the image forming apparatus. Non-limiting examples of the image defect may include a vertical streak and blurring each having a color corresponding to the type of toner attached to the region where the charging is insufficient. Such an image defect may be an image quality defect resulting independently of the image data, e.g., a formation pattern of the electrostatic latent image. 
     Therefore, in order to suppress the occurrence of the image defect in the image, it may be necessary to improve performance of the cleaning roller  42  cleaning the charging roller  41 . That is, it may be necessary to make it more difficult for the external additive to be fixed to the surface  41 M of the charging roller  41  or to remain on the surface  41 M of the charging roller  41 , when the external additive is attached to the surface  41 M of the charging roller  41 . It may be also necessary to make it easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 , when the external additive is attached to the surface  41 M of the charging roller  41 . 
     In this regard, in the image forming apparatus of the example embodiment, the surface free energy E is optimized to fall within the predetermined range, the dynamic friction coefficient μ is also optimized to fall within the predetermined range, and the surface free energy E and the dynamic friction coefficient μ are optimized to satisfy the predetermined relation, as described above. 
     In this case, since the surface free energy E is decreased appropriately as described above, the adsorption force of the external additive to the surface  41 M of the charging roller  41  is decreased sufficiently, as compared with a case where the surface free energy E is not decreased appropriately. This makes it more difficult for the external additive to be fixed to the surface  41 M. Accordingly, it is easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 . 
     Further, since the dynamic friction coefficient μ is decreased appropriately, it is more difficult for the cleaning roller  42  to rub the external additive against the surface  41 M of the charging roller  41 , even when the cleaning roller  42  rotates while being in contact with the charging roller  41 , as compared with the case where the dynamic friction coefficient μ is not decreased appropriately. This makes it more difficult for the external additive to be fixed to the surface  41 M. Accordingly, it is further easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 . 
     Further, since the surface free energy E and the dynamic friction coefficient μ satisfy the appropriate relation, one of the surface free energy E and the dynamic friction coefficient μ is optimized in a relation with the other. In this case, it is further easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 , as compared with the case where the surface free energy E and the dynamic friction coefficient μ do not satisfy the appropriate relation. 
     As described above, optimizing of the physical properties of the charging roller  41  and the cleaning roller  42 , i.e., the surface free energy E, the dynamic friction coefficient μ, and the relation between the surface free energy E and the dynamic friction coefficient μ, makes it sufficiently easier for the cleaning roller  42  to remove the external additive from the surface  41 M of the charging roller  41 . Accordingly, it is more difficult for the image defect to occur in the image, which makes it possible to form a high-quality image. 
     Further, in the image forming apparatus according to one example embodiment, where the surface free energy E is 13.49 dyn/cm or less, the external additive is hardly fixed to the surface  41 M of the charging roller  41 . This makes it more difficult for the charging roller  41  to be damaged due to the contact between the charging roller  41  and the external additive. Accordingly, the life of the charging roller  41  is made longer and a high-quality image is formed more stably, which makes it possible to obtain a higher effect. 
     In one example embodiment where the elastic layer  412  of the charging roller  41  includes the fluororesin, it is easier for the surface free energy E to be decreased appropriately, which makes it possible to obtain a higher effect. In this case, in one example embodiment where the elastic layer  412  includes the mixture of epichlorohydrin rubber and acrylonitrile-butadiene rubber, it is further easier for the surface free energy E to be decreased, which makes it possible to obtain a further higher effect. 
     In one example embodiment where the elastic layer  422  of the cleaning roller  42  include foamed polyurethane, it is easier for the dynamic friction coefficient μ to be decreased appropriately, which makes it possible to obtain a higher effect. 
     The example workings and the example effects related to the image forming apparatus described above are similarly obtained in the charging section  40  corresponding to the charging device and in the developing section  30  and the charging section  40  corresponding to the image former. 
     2. Modification Examples 
     The configuration of the image forming apparatus described above are modifiable as appropriate. For example, as long as the surface free energy E falls within the above-described appropriate range, the elastic layer  412  does not necessarily include the mixture of epichlorohydrin rubber and acrylonitrile-butadiene rubber described above, and may include any other material. For example, as long as the dynamic friction coefficient μ falls within the above-described appropriate range, the elastic layer  422  does not necessarily include the foamed polyurethane described above, and may include any other material. It is also possible to obtain a similar effect in the above-described cases as long as the above-described appropriate condition is satisfied related to each of the surface free energy E, the dynamic friction coefficient μ, and the relation between the surface free energy E and the dynamic friction coefficient μ. 
     Although a rotatable member, e.g., the cleaning roller  42 , may be used as the cleaning member in the above-described example embodiment, a non-rotatable member may be used as the cleaning member. Non-limiting examples of the non-rotatable member may include a sponge. It is also possible to obtain a similar effect in the above-described case as long as the above-described appropriate condition is satisfied related to each of the surface free energy E, the dynamic friction coefficient μ, and the relation between the surface free energy E and the dynamic friction coefficient μ. 
     WORKING EXAMPLES 
     Some working examples of an example embodiment of the technology are described in detail. 
     Experimental Examples 1 to 16 
     An image was formed on the print medium M with use of the image forming apparatus and quality of the image was evaluated thereafter by the following procedures. 
     [Preparation of Image Forming Apparatus, etc.] 
     The image forming apparatus provided with the charging section  40 , the print medium M, and a toner, i.e., a cyan toner, were prepared. The charging section  40  included the charging roller  41  and the cleaning roller  42 . 
     As the image forming apparatus, an electrophotographic full-color printer (a printer C542dnw available from Oki Data Corporation located in Tokyo, Japan) was used. As the print medium M, plain paper of A4 size was used. 
     The cyan toner included a cyan colorant (phthalocyanine blue), binder resin (amorphous polyester), a mold release agent (paraffin wax), a charge control agent, and an external additive (composite oxide particles, colloidal silica, and silica powder.) 
     As the cleaning roller  42 , a roller was used in which the elastic layer  422  (foamed polyurethane, urethane foam moltopren SM-55 available from INOAC CORPORATION located in Aichi, Japan, having an outer diameter of 6 mm) was provided on an outer peripheral surface of the shaft  421  (free-cutting steel (SUM) plated with nickel by an electroless plating method and having an outer diameter of 4 mm). 
     As the charging roller  41 , a roller fabricated by the following procedure was used. In a case of fabricating the charging roller  41 , first, prepared was a roller precursor in which the elastic layer  412  (a rubber material whose main component was a mixture of epichlorohydrin-hydrolin rubber (ECO) and acrylonitrile-butadiene rubber (NBR), having an outer diameter of 9.5 mm) was prepared on an outer peripheral surface of the shaft  411  (free-cutting steel (SUM) plated with nickel by an electroless plating method and having an outer diameter of 6 mm). Thereafter, the surface  41 M of the elastic layer  412  was dry-polished by a cylindrical polishing method by means of a grinder, while rotating the roller precursor. Thereafter, the surface  41 M of the elastic layer  412  was wet-polished by a tape-polishing method. 
     Thereafter, 100 parts by mass of an organic solvent (ethyl acetate), 15 parts by mass of an isocyanate compound (hexamethylene diisocyanate (HDI)), and 0.3 parts by mass to 3 parts by mass of fluororesin (polytetrafluoroethylene (PTFE)) were mixed, and the organic solvent was stirred. A coating solution was thereby prepared. 
     Thereafter, the coating solution was applied to the surface  41 M of the elastic layer  412  by an immersion method. Thereafter, the coating solution was dried. This caused the coating solution to penetrate into the elastic layer  412 , and the coating solution was cured while the organic solvent was volatilized. The elastic layer  412  was thereby subjected to the surface treatment. As a result, the charging roller  41  was completed. 
     In this case, each of the surface free energy E and the dynamic friction coefficient μ was varied as described in Table 2 by changing the fabrication conditions of the charging roller  41 . Specifically, firstly, a particle size of the tape used in the tape polishing method was varied, and a polishing rate at a time of polishing using the tape was varied. Secondly, surface roughness (maximum height Ry according to JIS B 0601: 1994) of the surface  41 M of the elastic layer  412  was varied within a range from 1 μm to 25 μm. Thirdly, a concentration of the fluororesin in the coating solution was varied within a range from 0.3 parts by mass to 3 parts by mass. The method of measuring each of the surface free energy E and the dynamic friction coefficient μ was as described above. 
     Table 2 describes a value (=−63μ+69) of the surface free energy calculated by the expression (1), as a threshold defining the relation between the surface free energy E and the dynamic friction coefficient μ. 
                                         TABLE 2                   Surface   Dynamic                       free energy   friction                   Experimental   E   coefficient   Threshold       Smudge       example   (dyn/cm)   μ   (−63μ + 69)   Quality   level                                                        1   13.49   0.79   19.23   A   5       2   12.02   0.48   38.76   A   5       3   12.49   0.68   26.16   A   5       4   8.32   0.87   14.19   A   5       5   13.49   0.88   13.56   A   5       6   27.34   0.65   28.05   A   4       7   38.03   0.49   38.13   A   4       8   24.68   0.49   38.13   A   3       9   5.00   0.82   17.34   A   5       10   32.09   0.65   28.05   B   —       11   27.31   0.82   17.34   B   —       12   53.55   1.73   −39.99   B   —       13   45.05   1.63   −33.69   B   —       14   19.19   1.32   −14.16   B   —       15   51.55   1.75   −41.25   B   —       16   60.51   1.39   −18.57   B   —                    
[Formation and Evaluation of Image]
 
     By the following procedures, an image was formed on the print medium M, and thereafter, quality of the image was evaluated and durability of the charging roller  41  was evaluated. Table 2 describes results of the evaluations. 
       FIG. 5  illustrates a planar configuration of the print medium M at a time of successive formation of images in order to explain an image pattern at the time of the successive formation of the images.  FIG. 6  illustrates a correlation between the surface free energy E, the dynamic friction coefficient μ, and quality of the image (image defect occurrence state.) 
     [Evaluation Procedure of Quality of Image] 
     In a case of evaluating quality of images, the images were first successively formed on the print medium M under ambient temperature and ambient humidity environmental conditions (at a temperature of 23±3° C. and humidity of 55±10% RH.) In this case, as illustrated in  FIG. 5 , images each including a four-stripe pattern of a printing rate of 5% was formed with use of the cyan toner while successively conveying the print media M with a longitudinal direction of the print media M of A4 size corresponding to the conveyance direction D 1 . Number of images to be formed per day was set within a range from 3500 to 4000, and in such a condition, the images were successively formed until total number of the formed images reached 50000. 
     When the above-described successive formation of images was performed, an image for evaluation was formed on the print medium M every day before the successive formation of images was performed, and an image for evaluation was formed on the print medium M every day also after the successive formation of images was performed. The image for evaluation had a halftone image pattern (a 2×2 pattern and a 1×1 pattern.) 
     Lastly, after the successive formation of the images was completed, the quality of the images was evaluated by visually observing the series of images for evaluation formed on the print media M. Specifically, a case was evaluated as “A” where no image defect occurred in any of the images for evaluation. A case was evaluated as “B” where an image defect occurred in any of the images for evaluation. In  FIG. 6 , data evaluated as “A” in Table 2 is represented by “∘ (circle)”, and data evaluated as “B” in Table 2 is represented by “× (cross)”. 
     [Evaluation Procedure of Durability of Charging Roller] 
     In a case of examining the durability of the charging roller  41 , after the above-described successive formation of the images was completed, the cleaning roller  42  was collected from the image forming apparatus and the surface  42 M of the cleaning roller  42  was visually observed. A state, i.e., a smudge level, of the surface  42 M was evaluated in five levels. 
     Specifically, in a case where little amount of the external additive was attached to the surface  42 M, little amount of the external additive was attached to the surface  41 M of the charging roller  41 . It was therefore determined that the life of the charging roller  41  would be sufficiently longer. This case was evaluated as having the smudge level of “5.” In a case where an extremely-small amount of the external additive was attached to the surface  42 M, only an extremely-small amount of external additive was attached to the surface  41 M. It was therefore determined that the life of the charging roller  41  would be longer. This case was evaluated as having the smudge level of “4.” 
     In a case where a small amount of the external additive was attached to the surface  42 M, only a small amount of the external additive was attached to the surface  41 M. It was therefore determined that the life of the charging roller  41  would be slightly longer. This case was evaluated as having the smudge level of “3.” In a case where the external additive was attached to the surface  42 M, some amount of the external additive was attached to the surface  41 M. It was therefore determined that the life of the charging roller  41  would be shorter. This case was evaluated as having the smudge level of “2.” In a case where a great amount of the external additive was attached to the surface  42 M, a great amount of the external additive was attached to the surface  41 M. It was therefore determined that the life of the charging roller  41  would be remarkably shorter. This case was evaluated as having the smudge level of “1.” 
     That is, in the above-described five-level evaluation result based on the smudge level, the greater value of the smudge level indicates that the external additive is hardly adsorbed to the surface  41 M of the charging roller  41 , or even if the external additive is adsorbed to the surface  41 M of the charging roller  41 , the cleaning roller  42  sufficiently removes the external additive, and the life of the charging roller  41  is therefore made longer. 
     [Discussion] 
     As described in Table 2 and illustrated in  FIG. 6 , the quality of the image (the image defect occurrence state) varied in accordance with the physical properties of the charging roller  41  and the cleaning roller  42 , i.e., the surface free energy E, the dynamic friction coefficient μ, and the relation between the surface free energy E and the dynamic friction coefficient μ.) Specifically, as described in Table 2, no image defect occurred in a case where the three conditions were satisfied at the same time, i.e., the condition that the surface free energy E was 5.00 dyn/cm or more, the condition that the dynamic friction coefficient μ fell within the range from 0.48 to 0.88 both inclusive, and the condition that the surface free energy E and the dynamic friction coefficient μ satisfied the relation (E≤−63μ+69) represented by the expression (1) were satisfied at the same time (Experimental examples 1 to 9), unlike in a case where the three conditions were not satisfied at the same time (Experimental examples 10 to 16). 
     In  FIG. 6 , a range surrounded by dashed lines L 1  to L 4 , i.e., a hatched region represents a range in which no image defect occurred. The dashed line L 1  is a straight line representing the surface free energy E of 5.00 dyn/cm. The dashed line L 2  is a straight line representing the dynamic friction coefficient μ of 0.48. The dashed line L 3  is a straight line representing the dynamic friction coefficient μ of 0.88. The dashed line L 4  is a straight line representing y=−63μ+69 where “y” is the surface free energy E and “x” is the dynamic friction coefficient μ. 
     In particular, in the case where the above-described three conditions were satisfied at the same time, when the surface free energy E fell within a range from 5.00 dyn/cm to 13.49 dyn/cm both inclusive (Experimental examples 1 to 5 and 9), the smudge level was 5. Therefore, the external additive was hardly attached to the surface  42 M of the cleaning roller  42 . Accordingly, the external additive was hardly attached to the surface  41 M of the charging roller  41 , which made the life of the charging roller  41  sufficiently longer. 
     According to the results described in Table 2 and illustrated in  FIG. 6 , when the above-described three conditions related to the physical properties of the charging roller  41  and the cleaning roller  42 , i.e., the surface free energy E, the dynamic friction coefficient μ, and the relation between the surface free energy E and the dynamic friction coefficient μ, were satisfied at the same time, the quality of the image was improved. As a result, a high-quality image was formed. 
     Although one embodiment of the technology has been described above with reference to some example embodiments, the embodiment of the technology is not limited to the above-described example embodiments. 
     For example, the image forming apparatus according to one embodiment of the technology is not limited to a color image forming apparatus. In one specific but non-limiting example, the image forming apparatus according to one embodiment of the technology may be a monochrome image forming apparatus. For example, the image forming apparatus according to one embodiment of the technology is not limited to a printer. In one specific but non-limiting example, the image forming apparatus according to one embodiment of the technology may be any other apparatus that forms an image such as a copier, a facsimile machine, or a multifunction peripheral. For example, the image forming apparatus according to one embodiment of the technology is not limited to that of a direct transfer method which uses no intermediate transfer medium. In one specific but non-limiting example, the image forming apparatus according to one embodiment of the technology may be that of an intermediate transfer method using the intermediate transfer print medium. Other than the above, for example, the charging device according to one embodiment of the technology is applicable to any application other than the image forming apparatus such as the printer. 
     Furthermore, the technology encompasses any possible combination of some or all of the various embodiments and the modifications described herein and incorporated herein. It is possible to achieve at least the following configurations from the above-described example embodiments of the technology. 
     [1] 
     A charging device including: 
     a charging member that electrically charges a surface of a charging target member and is rotatable while being in contact with the surface of the charging target member; and 
     a cleaning member that is in contact with a surface of the charging member and removes a foreign object attached to the surface of the charging member, in which 
     surface free energy of the surface of the charging member is 5.00 dynes per centimeter or more, 
     a dynamic friction coefficient of the surface of the charging member and a surface of the cleaning member falls within a range from 0.48 to 0.88 both inclusive, and 
     the surface free energy and the dynamic friction coefficient satisfy a relation represented by the following expression (1),
 
 E≤− 63μ+69  (1)
 
     where “E” is the surface free energy in dynes per centimeter, and “μ” is the dynamic friction coefficient. 
     [2] 
     The charging device according to [1], in which the surface free energy is 13.49 dynes per centimeter or less. 
     [3] 
     The charging device according to [1] or [2], in which the charging member includes a first surface layer having a surface that includes fluororesin. 
     [4] 
     The charging device according to [3], in which the first surface layer includes a mixture of epichlorohydrin rubber and acrylonitrile-butadiene rubber. 
     [5] 
     The charging device according to any one of [1] to [4], in which the cleaning member includes a second surface layer having a surface that includes foamed polyurethane. 
     [6] 
     An image former including: 
     a developing device that includes a photosensitive member serving as a charging target member and performs a developing process with use of a toner, and 
     the charging device according to any one of [1] to [5], the charging device performing a charging process on a surface of the photosensitive member. 
     [7] 
     An image forming apparatus including: 
     the image former according to [6], the image former performing a charging process and a developing process, 
     a transfer section that performs a transfer process with use of a toner subjected to the developing process by the image former; and 
     a fixing section that performs a fixing process with use of the toner subjected to the transfer process by the transfer section. 
     According to any of the charging device, the image former, and the image forming apparatus of one embodiment of the technology, the surface free energy of the surface of the charging member in the charging device is 5.00 dyn/cm or more, the dynamic friction coefficient of the surface of the charging member and the surface of the cleaning member in the charging device falls within the range from 0.48 to 0.88 both inclusive, and the surface free energy and the dynamic friction coefficient satisfy the relation represented by the expression (1). Therefore, it is possible to form a high-quality image. 
     Although the technology has been described in terms of exemplary embodiments, it is not limited thereto. It should be appreciated that variations may be made in the described embodiments by persons skilled in the art without departing from the scope of the invention as defined by the following claims. The limitations in the claims are to be interpreted broadly based on the language employed in the claims and not limited to examples described in this specification or during the prosecution of the application, and the examples are to be construed as non-exclusive. For example, in this disclosure, the term “preferably”, “preferred” or the like is non-exclusive and means “preferably”, but not limited to. The use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. The term “substantially” and its variations are defined as being largely but not necessarily wholly what is specified as understood by one of ordinary skill in the art. The term “about” or “approximately” as used herein can allow for a degree of variability in a value or range. Moreover, no element or component in this disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.