Patent Publication Number: US-7725054-B2

Title: Electrifying apparatus, a processing unit, and an image formation apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electrifying apparatus for electrifying (charging) at least one of the surface of a latent-image supporting object, such as a photoconductor, and toner adhered to the surface by a conductive member to which bias is supplied while the conductive member is contacting the surface of the latent-image supporting object. The present invention further relates to a processing unit and an image formation apparatus using the electrifying apparatus. 
   2. Description of the Related Art 
   Generally, image formation apparatuses that employ an electronic photography method form an image according to the following processes. Namely, a electrostatic latent image is formed by exposure scanning a latent-image supporting object, such as a photoconductor, that is uniformly electrified; then the electrostatic latent image is developed into a toner image by a development apparatus. Subsequently, the toner image is either directly transferred onto a recording object such as paper from the latent-image supporting object, or transferred to the recording object through a middle transfer object. 
   According to a conventional electrifying apparatus used by the image formation apparatuses, an electric bias is provided to an electrifying roller serving as a conductive member that touches the surface of the latent-image supporting object so that the surface of the latent-image supporting object may be uniformly electrified by the conductive member. 
   Further, Patent Reference 1 discloses an electrifying apparatus that includes an auxiliary electrifying apparatus, to which auxiliary electrifying apparatus an auxiliary electric bias is provided. The auxiliary electrifying apparatus is provided in addition to a main electrifying apparatus for principally electrifying the latent-image supporting object such as an electrifying roller and an electrifying charger of a scorotron method. The auxiliary electrifying apparatus contacts a part of the peripheral surface of the latent-image supporting object, which is endlessly moving, after the latent image supporting object passes a transfer process location and before advancing into a location of a main electrifying process to be performed by the main electrifying apparatus. In this way, auxiliary electrifying of the surface of the latent-image supporting object is carried out and residual toner adhered to the surface is charged to have a regular polarity by discharging or charge injection by the auxiliary electrifying apparatus before the uniform electrifying is performed by the main electrifying apparatus. Thereby, charge unevenness of the latent-image supporting object can be reduced, and background dirt due to residual toner with a low charge or a reverse charge being conveyed to a development area can be prevented. 
   With the conventional electrifying apparatuses, if residual toner is fixed to the electrifying apparatus and the auxiliary electrifying apparatus (the conductive member) that contact the latent-image supporting object, discharging and charge injection between the conductive member, the residual toner, and the latent-image supporting object are interfered with. Accordingly, the main electrifying and the auxiliary electrifying are degraded; therefore, charge unevenness is generated, and electrifying of the residual toner is degraded, causing the background dirt to be generated. Here, the background dirt is a phenomenon of the toner adhering to the background part (uniformly charged part) of the latent-image supporting object. 
   Then, an electrifying apparatus disclosed by Patent Reference 2 specifies that the electrifying apparatus have a pure-water contact angle of greater than 90 degrees so that the toner adhesion to the electrifying apparatus is reduced. 
   [Patent Reference 1] JPA 2005-62737 
   [Patent Reference 2] JPA 11-352752 
   However, the inventors of the present invention found out by experiments that the toner adhesion to the conductive member, such as the electrifying apparatus, could not be reduced over a long period of time by only satisfying the condition that the pure-water contact angle be comparatively great. For example, a conductive member having a pure-water contact angle of 100 degrees caused toner adhesion after printing out thousands of sheets. 
   SUMMARY OF THE INVENTION 
   The present invention is made in view of the above situation, and provides an electrifying apparatus, a processing unit, and an image formation apparatus using the electrifying apparatus that is capable of reducing charge unevenness and ground dirt due to toner adhesion to a conductive member for a long period of time. 
   That is, the present invention provides an electrifying apparatus, a processing unit, and an image formation apparatus using the electrifying apparatus that substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art. 
   Features of embodiments of the present invention are set forth in the description that follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Problem solutions provided by an embodiment of the present invention may be realized and attained by an electrifying apparatus, a processing unit, and an image formation apparatus therewith particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention. 
   To achieve these solutions and in accordance with an aspect of the invention, as embodied and broadly described herein, an embodiment of the invention provides an electrifying apparatus, a processing unit, and an image formation apparatus as follows. 
   Means for Solving a Subject 
   An aspect of the embodiment of the present invention provides an electrifying apparatus for electrifying at least one of the surface of a latent-image supporting object and toner adhered to the surface of the latent-image supporting object by a conductive member to which a bias voltage is provided while the electrifying apparatus contacts the surface of the latent-image supporting object that supports a latent image, wherein the pure-water contact angle of the surface of the conductive member is equal to or greater than 108 degrees, and the shore D hardness of the surface is equal to or less than 65. 
   According to another aspect of the embodiment, the shore D hardness of the surface of the conductive member of the electrifying apparatus is equal to or greater than 50. 
   According to another aspect of the embodiment, the surface resistivity of the conductive member of the electrifying apparatus is between 10 2  Ω/cm 2  and 10 8  Ω/cm 2 . 
   According to another aspect of the embodiment, the volume specific resistance of the conductive member of the electrifying apparatus is between 10 2  Ω-cm and 10 6  Ω-cm. 
   According to another aspect of the embodiment, the surface roughness Ra of the conductive member of the electrifying apparatus is between 0.1 μm and 0.6 μm. 
   The embodiment further provides a processing unit that is attachable to/detachable from the main body of an image formation apparatus, wherein the processing unit holds the electrifying apparatus and the latent-image supporting object in one body with a common supporting member. Here, the image formation apparatus includes 
   a latent-image supporting object for supporting a latent image, 
   a latent-image formation unit for forming the latent image onto the latent-image supporting object, 
   a developing unit for developing the latent image on the latent-image supporting object with toner, and 
   the electrifying apparatus for electrifying at least one of the latent-image supporting object and the toner adhered to the surface of the latent-image supporting object. 
   The embodiment further provides an image formation apparatus that includes 
   a latent-image supporting object for supporting a latent image, 
   a latent-image formation unit for forming the latent image onto the latent-image supporting object, 
   a developing unit for developing the latent image on the latent-image supporting object with toner, and 
   the electrifying apparatus as described above for electrifying at least one of the latent-image supporting object and the toner adhered to the surface of the latent-image supporting object. 
   According to another aspect of the embodiment, as for the image formation apparatus, the length of a section where the latent-image supporting object and the conductive member make contact in a direction of surface movement of the latent-image supporting object is between 2 mm and 7 mm. 
   According to another aspect of the embodiment, as for the image formation apparatus, contact pressure between the latent-image supporting object and the conductive member is between 2 kN/m 2  and 15 kN/m 2 . 
   According to another aspect of the embodiment, as for the image formation apparatus, the pure-water contact angle of the latent-image supporting object is greater than 90 degrees. 
   According to another aspect of the embodiment, as for the image formation apparatus, the toner contains an external additive at a density of 1 through 4 part(s) by weight of the toner particles. 
   According to another aspect of the embodiment, as for the image formation apparatus, toner particles having a diameter less than 5 μm are less than 15% of the total of the toner particles. 
   The image formation apparatus according to an aspect of the embodiment develops the latent image into a toner image using the developing unit with toner that is held on the surface of a developer supporting object, transfers the toner image formed on the surface of the latent-image supporting object by the developing unit to a transfer object using a transfer unit, and residual toner remaining on the latent-image supporting body is recouped to the surface of the developer supporting object after the transfer process is performed by the transfer unit. 
   EFFECTIVENESS OF INVENTION 
   Since, according to the embodiment of the invention, the surface pure-water contact angle of the conductive member is greater than 108 degrees, and the surface shore D hardness of the conductive member is 65 or less, toner adhesion to the conductive member is reduced over a long period of time as evidenced by experiments conducted by the inventors of the present invention as described below. Accordingly, generating the charge unevenness and ground dirt due to toner adhesion to the conductive member are reduced over a long period of time. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of the principal part of a printer according to an embodiment of the present invention; 
       FIG. 2  is a schematic diagram of a processing unit for K of the printer with a middle transfer belt; and 
       FIG. 3  is an expanded schematic diagram of the processing unit for K (black color) with a middle transfer belt according to a modification of the printer. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, embodiments of the present invention are described with reference to the accompanying drawings. 
   According to the embodiments, the present invention is applied to an image formation apparatus, and the image formation apparatus is a color laser printer (printer) of an electro photographic method. 
   First, the basic configuration of the printer according to the embodiment is described.  FIG. 1  shows an outline of the principal parts of the printer according to the embodiment. The printer includes four processing units, namely,  1 Y,  1 M,  1 C and  1 K for forming toner images in colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. The printer further includes an optical writing unit  50 , a resist roller pair  54 , and a transfer unit  60 . Where appropriate, suffixes Y, M, C, and K are used for indicating color specific units. 
   The optical writing unit  50  serves as a latent-image formation unit, and includes a luminous source consisting of four laser diodes for corresponding colors of Y, M, C, and K, a polygon mirror of a regular hexahedron, a motor for rotationally driving the polygon mirror, an fθ lens, a lens, and a reflective mirror. The luminous source emits laser lights L for corresponding colors. The laser lights L are reflected by one of the six faces of the polygon mirror, deflected with rotation of the polygon mirror, and reach one of four photoconductors described below. The surfaces of the four photoconductors are optically scanned by the corresponding laser lights L irradiated by the corresponding laser diodes. 
   The processing units  1 Y,  1 M,  1 C, and  1 K include photoconductors  3 Y,  3 M,  3 C, and  3 K, respectively, developing units  40 Y,  40 M,  40 C and  40 K, respectively, and corresponding electrifying apparatuses. Each of the photoconductors  3 Y,  3 M,  3 C, and  3 K is shaped like a drum, serves as the latent-image supporting object, and is rotationally driven in the clockwise direction in the drawing at a predetermined linear speed by a driving unit that is not illustrated. The photoconductors  3 Y,  3 M,  3 C, and  3 K are optically scanned by the optical writing unit  50  that emits the laser lights L modulated by image information provided by an external source such as a personal computer that is not illustrated, and support corresponding electrostatic latent images that are generated for colors Y, M, C, and K, respectively. 
     FIG. 2  is an expanded schematic diagram showing the processing unit  1 K serving as a representative of the four processing units  1 Y,  1 M,  1 C and  1 K with a middle transfer belt  61  of a transfer unit  60  (refer to  FIG. 1 ). The processing unit  1 K includes a common unit casing (supporting body) containing the photoconductor  3 K, a discharging lamp that is not illustrated, and a developing unit  40 K. The processing unit  1 K is attachable to and detachable from the body of the printer. 
   The photoconductor  3 K, which is a charged body and serves as the latent image supporting object, is shaped like a drum the diameter of which is about 24 mm. The photoconductor  3 K is made of an aluminum tube. A photosensitive layer of an organic photoconductor (OPC) having a negative charging property is formed on the surface of the aluminum tube. The photoconductor  3 K is rotationally driven in the clockwise direction in the drawing by a driving unit that is not illustrated. In this way, the surface of the photoconductor  3 K passes through a primary transfer nip (contacting point with the middle transfer belt  61 ), an auxiliary electrifying (charging) nip, an electrifying (charging) nip, an optical writing position, and a development area. 
   The development apparatus  40 K for K includes a developing roller  42 K, a part of which is exposed through an opening in a casing  41 K. The developing roller  42 K serving as a developer supporting object is rotationally driven around a shaft supported by bearings (not illustrated). The casing  41 K contains toner in K color that is conveyed from the right-hand side to the left-hand side in the drawing by an agitator  43 K that is rotationally driven. A toner supply roller  44 K is provided on the left-hand side of the agitator  43 K, and the toner supply roller  44 K is rotationally driven in the counterclockwise direction by a driving unit that is not illustrated. A roller part of the toner supply roller  44 K is made of an elastic foam object, such as sponge, so that the K-color toner provided by the agitator  43 K can be efficiently received. The K-color toner is supplied to the developing roller  42 K at a position where the toner supply roller  44 K touches the developing roller  42 K. The K-color toner supported by the surface of the developing roller  42 K serving as the developer supporting object passes a position where a regulation blade  45 K makes contact. The thickness of the toner is regulated and friction charging is promoted with the rotational drive of the developing-roller  42 K in the counterclockwise direction. The toner is conveyed to the development area that counters the photoconductor  3 K. 
   In the development area, a development potential that causes the K toner having a negative polarity to move from the developing roller  42 K to the latent image is activated between the developing roller  42 K, to which a development bias of the negative polarity provided by a power source that is not illustrated is provided, and the electrostatic latent image on the photoconductor  3 K. Further, a non-developing potential that causes the K toner having the negative polarity to move from the background part to the developing roller  42 K is activated between the uniform charged part (background part) of the developing roller  42 K and the photoconductor  3 K. The K-color toner supported by the developing roller  42 K is transferred from the developing-roller  42 K to the electrostatic latent image on the photoconductor  3 K by the action of the development potential. In this way, the electrostatic latent image on the photoconductor  3 K is developed into a K-color toner image. 
   In addition, although a single component developer is used by the development apparatus  40 K of the printer, a 2-component developer consisting of K toner and a magnetic carrier may be used. 
   The K-color toner image developed in the development area is conveyed to the primary transfer nip where the photoconductor  3 K and the middle transfer belt  61  meet with the rotational movement of the photoconductor  3 K, and the toner image is transferred to the middle transfer belt  61 . At this point, toner that has not been transferred to the middle transfer belt  61  remains on the surface of the photoconductor  3 K after passing the primary transfer nip. Such toner is called residual toner. Handling of the residual toner is described below. 
   The electrifying apparatus includes an electrifying roller  7 K, and an auxiliary electrifying apparatus  10 K. The electrifying roller  7 K forms an electrifying nip at a position where the photoconductor  3 K is contacted as the electrifying roller  7 K is rotationally driven counterclockwise as illustrated. The auxiliary electrifying apparatus  10 K forms an auxiliary electrifying nip at a position where the photoconductor  3 K is contacted. The electrifying roller  7 K includes a metal rotational shaft, and a roller part made of a conductive and elastic material, such as conductive rubber. The roller part is provided such that it covers the metal rotational shaft. A bias voltage is provided to the metal rotational shaft by an electric bias supply unit that includes a power source that is not illustrated. By the bias voltage, electric discharge occurs between the electrifying roller  7 K and the photoconductor  3 K, and the surface of the photoconductor  3 K is charged in the same polarity as the toner. 
   The auxiliary electrifying apparatus  10 K includes an elastic part  8 K made of an elastic material such as sponge, and a conductive sheet  9 K made of a conductive material. The surface of the elastic part  8 K is covered by the conductive sheet  9 K. The auxiliary electrifying apparatus  10 K is pressed toward the photoconductor  3 K by a holding member such that the conductive sheet  9 K contacts the photoconductor  3 K at a position after the primary transfer nip and before the electrifying nip. An auxiliary electric bias supply unit, which consists of a power source that is not illustrated, provides an auxiliary electric bias to the conductive sheet  9 K. The auxiliary electric bias is either a DC voltage of the same polarity as the toner, or an AC voltage onto which the DC voltage is superposed. 
   The residual toner adhered to the surface of the photoconductor  3 K after the primary transfer nip includes toner with a regular polarity, toner charged to an insufficient level of the regular polarity, and toner charged with the reverse polarity. The residual toner advances to an auxiliary electrifying nip with the rotation of the photoconductor  3 K. Then, the toner charged with the reverse polarity is fully charged to the regular polarity, i.e., negative polarity, by electric discharge between the auxiliary electrifying apparatus  10 K and the photoconductor  3 K, or by charge injection from the auxiliary electrifying apparatus  10 K. Further, the toner of low charge out of the residual toner is also fully charged by discharging or charge injection with the negative polarity. In this way, the ground dirt due to the toner with the reverse polarity and the toner that is insufficiently charged being conveyed to the development area is reduced. 
   When discharging is to take place between the conductive sheet  9 K of the auxiliary electrifying apparatus  10 K and the photoconductor  3 K, charge unevenness can be prevented by carrying out the auxiliary charging of the photoconductor  3 K by the electric discharge in advance of the main charging of the photoconductor  3 K by the electrifying roller  7 K. 
   The electrostatic latent image in the K color is formed on the surface of the photoconductor  3 K by the optical writing unit  50  scanning the uniformly charged surface, and the electrostatic latent image is developed by the development apparatus  40 K into the toner image in the K color. 
   Since the processing units  1 Y,  1 M, and  1 C for other corresponding colors have the same configuration as the processing unit  1 K, descriptions are not repeated. 
   The processing units  1 Y,  1 M,  1 C, and  1 K employ the so-called “cleanerless” method. According to the “cleanerless” method, an image formation process on the latent-image supporting object is performed without using a special unit for cleaning and recovery of the residual toner that is adhered to the latent-image supporting object such as the photoconductor  3 K. Here, the special unit is for separating the residual toner from the latent-image supporting object, conveying the separated residual toner to a disposed toner container without adhering the residual toner to the latent-image supporting object again, and/or recycling the separated residual toner in the development apparatus. A cleaning blade that scratches the residual toner from the latent-image supporting object is an example of the special unit described here. 
   The “cleanerless” method is described in more detail. The method includes a “scatter-pass” type, a “temporarily capture” type, and a combination of the two types. According to the “scatter-pass” type, adhesion of the residual toner to the latent-image supporting object is weakened by scratching the residual toner on the latent-image supporting object with a scattering unit such as a brush that makes sliding contact with the latent-image supporting object. Then, the residual toner on the latent-image supporting object is electrostatically transferred to the developing unit such as the developing roller in the development area or immediately before the development area. The development area is where the developing unit (such as the developing roller and a development sleeve) and the latent-image supporting object meet. In this way, the residual toner is recovered into the development apparatus. In advance of the recovery of the residual toner, the residual toner passes a position for optically writing the latent image. If the amount of the residual toner is comparatively small, there is no inadvertent influence on the latent image. However, if the residual toner includes toner that is charged in a polarity reverse to the regular polarity, such toner is not cleaned, is not recovered onto the developing unit, and causes the ground dirt to be generated. In order to reduce the amount of the ground dirt due to the toner that is charged in reverse, it is desirable to provide a toner electrifying apparatus for electrifying the residual toner on the latent-image supporting object in the regular polarity between a transfer location (for example, primary transfer nip) and a scattering location, or between the scattering location and the development area. The scattering unit may be one of 
   a fixed brush that has two or more brush-filling fibers that consist of conductive fibers stuck on a sheet metal, unit casing, and the like, 
   a brush roller that has two or more brush-filling fibers to a rotating metal shaft, and 
   a roller (for example, an electrifying roller) that is made of conductive sponge, and the like. The fixed brush has an advantage that it can be constituted with a small amount of the brush-filling fibers, and is therefore economical. However, if the fixed brush is to serve also as the electrifying apparatus for carrying out uniform charging of the latent-image supporting object, the fixed brush is not capable of providing sufficiently uniform charging. To the contrary, the brush roller is capable of charging with sufficient uniformity, and thus is more desirable. 
   According to the “temporary capture” type of the “cleanerless” method, the residual toner is temporarily captured with a capturing unit such as a rotational brush that endlessly contacts the surface of the latent-image supporting object. Then, either after a printing job or between printing jobs, the residual toner captured by the capturing unit is “breathed out” to be transferred to the latent-image supporting object, is then transferred to the developing unit such as the developing roller by electrostatic force, and the residual toner is recovered into the development apparatus. According to the “scatter-pass” type, if the amount of the residual toner is great (such as when a solid image is formed and when a jam takes place) in excess of the recovering capacity of the developing unit, image formation may be degraded. In contrast, according to the “temporary capture” type, the residual toner captured by the capturing unit can be collected little by little into the developing unit, and the image formation degradation can be reduced. 
   The “cleanerless” method may be implemented by combining the “temporary capture” type and the “scatter-pass” type. Specifically, the rotational brush that contacts the latent-image supporting object serves as both scattering unit and capturing unit. The rotational brush functions as the scattering unit if a DC voltage is provided to the rotational brush, and functions as the capturing unit if a superposed voltage (DC+AC) is provided. 
   According to the embodiment, the printer employs a “scattering-penetration” type “cleanerless” method. Specifically, the photoconductor  3 K contacts the outer surface of the middle transfer belt  61 , and forms the primary transfer nip for K, while the photoconductor  3 K is rotationally driven at a predetermined linear speed in the clockwise direction as shown by an arrow in  FIG. 2 . Then, the adhesion of residual toner to the photoconductor  3 K is weakened by scratching the residual toner on the photoconductor  3 K with the electrifying roller  7 K and the auxiliary electrifying apparatus  10 K (serving as the scattering unit). Then, in the development area, the residual toner on the photoconductor  3 K is electrostatically collected by the developing roller  42 K of the development apparatus  40 K. At this time, if the amount of toner of low charge and reverse charge is great, such residual toner is not properly collected by the developing-roller  42 K, and the ground dirt is generated. 
   As shown in  FIG. 1 , the transfer unit  60  is provided under the processing units  1 Y,  1 M,  1 C, and  1 K. The transfer unit  60  includes the middle transfer belt  61  that is endlessly and rotationally driven in the counterclockwise direction. The middle transfer belt  61  is wound around two or more rollers. Specifically, the rollers include a follower roller  62 , a driving roller  63 , and four primary transfer bias rollers  66 Y,  66 M,  66 C, and  66 K. 
   The follower roller  62 , the primary transfer bias rollers  66 Y,  66 M,  66 C, and  66 K, and the driving roller  63  touch the inner surface (inner side of a loop formation) of the middle transfer belt  61 . The primary transfer bias rollers  66 Y,  66 M,  66 C, and  66 K are each made of a metal core that is covered with an elastic body, such as sponge, and are pressed to the photoconductors  3 Y,  3 M,  3 C, and  3 K, respectively. In this way, four primary transfer nips for Y, M, C, and K are formed, wherein the photoconductors  3 Y,  3 M,  3 C and  3 K contact the outer surface of the middle transfer belt  61  for a predetermined length in the belt movement direction. 
   A primary transfer bias voltage at a constant current is applied to the metal cores of the primary transfer bias rollers  66 Y,  66 M,  66 C, and  66 K by a transfer bias power source (not illustrated). Thereby, transfer charges are provided to the inner surface of the middle transfer belt  61  through the primary transfer bias rollers  66 Y,  66 M,  66 C, and  66 K, and a transfer electric field is formed in each of the primary transfer nips between the photoconductors  3 Y,  3 M,  3 C, and  3 K and the middle transfer belt  61 . In addition, although the printer includes the primary transfer bias rollers  66 Y,  66 M,  66 C, and  66 K, other members such as brushes and blades may be used instead. Further, transfer chargers may be used. 
   Toner images in Y, M, C, and K colors formed on the corresponding photoconductors  3 Y,  3 M,  3 C, and  3 K, respectively, are piled up on the middle transfer belt  61  at the corresponding primary transfer nips, which is called a primary transfer. In this way, on the middle transfer belt  61 , a 4-color superposed toner image (henceforth 4-color toner image) is formed. 
   A secondary transfer bias roller  67  is provided countering the driving roller  63  and sandwiching the middle transfer belt  61  such that the secondary transfer bias roller  67  touches the outer surface of the middle transfer belt  61 , and a secondary transfer nip is formed. A secondary transfer bias voltage is applied to the secondary transfer bias roller  67  by a voltage supplying unit (not illustrated) that includes a power source and wiring. In this way, a secondary transfer electric field is formed between the secondary transfer bias roller  67  and the driving roller  63  serving as a secondary transfer nip rear-side roller that is grounded. The 4-color toner image formed on the middle transfer belt  61  is conveyed into the secondary transfer nip as the middle transfer belt  61  is rotationally and endlessly driven. 
   The printer includes a feed cassette (not illustrated) for storing and feeding recording paper P. The topmost sheet of the recording paper P is sent out to a feed way at a predetermined timing. Then, the recording paper P is inserted into a resist nip formed by a resist roller pair  54  that is provided at the end of the feed way. 
   Both rollers of the resist roller pair  54  are rotationally driven in order to take the recording paper P into the resist nip, and stop the rotational drive shortly after the tip of the recording paper P is pinched by the resist nip. Then, the recording paper P is sent out to the secondary transfer nip at a timing that is in sync with the 4-color toner image on the middle transfer belt  61 . In the secondary transfer nip, the 4-color toner image on the middle transfer belt  61  is transferred to the recording paper P by the secondary transfer electric field and nip pressure (secondary transfer), and the full color image is formed on the recording paper P. 
   Thus, after the full color image is formed on the recording paper P, the recording paper P is discharged from the secondary transfer nip, and is conveyed to a fixing apparatus (not illustrated) so that the full color image is fixed onto the recording paper P. 
   Residual toner adhering to the surface of the middle transfer belt  61  after passing the secondary transfer nip is removed by a belt cleaning apparatus  68 . 
   Here, the toners for Y, M, C, and K colors of the printer according to the embodiment are of a negative polarity charge. Accordingly, the photoconductors  3 Y,  3 M,  3 C, and  3 K are first uniformly charged in the negative polarity by the corresponding electrifying apparatuses. Then, the charge in the negative polarity is reduced with reference to the background by optical scanning, and the toners in the negative polarity are adhered to the corresponding latent images, that is, a negative-positive development method is used. 
   Next, the printer according to the embodiment is described. 
   The inventors hereto prepared a testing machine having the same configuration as the printer of the embodiment as shown in  FIG. 1 . Further, various conductive sheets were prepared for the conductive sheet  9 K of the auxiliary electrifying apparatus  10 K of the processing unit  1 K for the K color (refer to  FIG. 2 ). The conductive sheets were different in respect to the pure-water contact angle and in respect to the shore D hardness. With each of the conductive sheets being used as the conductive sheet  9 K, a monochrome half chart (2×2 half-tone gradation images) was continuously printed onto 10,000 A4-size sheets at an image area rate of 5%. 
   The pure-water contact angle of the conductive sheet was measured by a droplet method using a contact angle meter model CA-DT-A manufactured by Kyowa Interface Science Co., Ltd., following the handling manual of the contact angle meter. 
   The shore D hardness of the conductive sheet  9 K was measured at 25° C. based on a method described in ASTM D-2240. 
   A DC voltage of −1100 V was used as the electric bias to be provided to the electrifying roller  7 K. Further, a DC voltage of −700 V was used as the auxiliary electric bias to be provided to the conductive sheet  9 K of the auxiliary electrifying apparatus  10 K. With this configuration, by the auxiliary charge provided by the conductive sheet  9 K, residual toner of low charge and reverse charge was charged with the regular polarity, i.e., negative polarity, such that generating of ground dirt was reduced. Further, the auxiliary charging of the photoconductor  3 K was carried out with the auxiliary electric bias voltage that was smaller than the main electric bias voltage in advance of the main charging by the electrifying roller  7 K, and generating of charge unevenness was reduced. If the toner was adhered to the conductive sheet  9 K, the auxiliary charge would be degraded and charge unevenness would take place. In this case, a vertical stripe would become conspicuous in the image. 
   The toner contained toner particles having an average diameter of 8.5 μm, and was manufactured by a grinding method. The toner contained an external additive. 
   At the time of printing the half chart, the photoconductors  3 Y,  3 M,  3 C, and  3 K were rotationally driven at a linear speed of 120 mm/s. 
   The electrifying roller (for example,  7 K) of each color includes a metal rotating shaft having a diameter of 6 mm covered by a conductive rubber layer making the diameter of the electrifying roller be 10 mm φ. 
   Although various kinds of the conductive sheets having different pure-water contact angles and shore D hardnesses were prepared for the conductive sheet  9 K of the auxiliary electrifying apparatus  10 K, the surface resistivity and the thickness of all of the conductive sheets were 10 5  Ω/cm 2 . and 0.1 mm, respectively. 
   The elastic part  8 K of the auxiliary electrifying apparatus  10 K was made of 5 mm thick sponge. When the auxiliary electrifying apparatus  10 K was pushed toward the photoconductor  3 K by the holding member, the elastic part  8 K was compressed to a thickness of 2 mm so that the conductive sheet  9 K of the auxiliary electrifying apparatus  10 K made pressing contact with the photoconductor  3 K. 
   Then, presence/absence of image dirt due to condensed toner lumping was investigated for all the 10,000 printed sheets for every test printing using each of the different kinds of the conductive sheets as the conductive sheet  9 K. The toner tended to be condensed at the position where the conductive sheet  9 K contacts the photoconductor  3 K (refer to  FIG. 2 ). When the toner was condensed and collected to some extent, the toner would fall by gravity from the contact position, would be conveyed to the primary transfer nip, and would cause the image dirt to be generated either indirectly via the middle transfer belt  61  or directly on the recording paper P. The image dirt was evaluated in three steps of “poor” (100 or more toner adhesion dirt spots greater than 0.5 mmφ/10 sheets), “fair” (100 or more toner adhesion dirt spots greater than 0.5 mmφ/100 sheets), and “excellent” (no toner adhesion dirt spots in 100 sheets). Here, the tolerance of the image dirt in general commercial printers is either “fair” or “excellent”. 
   Further, presence/absence of a vertical stripe was observed in a printed image of the 10,000th sheet in each test printing. The vertical stripe is generated when the toner is adhered to the conductive sheet  9 K and charge unevenness is generated on the photoconductor  3 K. The vertical stripe was evaluated in three steps of “poor” (two or more stripes were easily visible), “fair” (less than 10 small/thin stripes were visible if eyes were aided), and “excellent” (no stripes were visible). Here, the tolerance of the stripes in the general commercial printers is either “fair” or “excellent”. 
   Results of the experiments described above are shown in the following Table 1. 
   
     
       
         
             
             
             
             
             
           
             
                 
                 
             
             
                 
               Pure-water 
                 
                 
                 
             
             
                 
               contact 
                 
                 
                 
             
             
                 
               angle 
               Shore D 
               Toner 
                 
             
             
                 
               [degrees] 
               hardness 
               adhesion 
               Image dirt 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               114 
               50 
               Excellent 
               Excellent 
             
             
                 
               115 
               65 
               Excellent 
               Excellent 
             
             
                 
               108 
               64 
               Excellent 
               Excellent 
             
             
                 
               108 
               50 
               Excellent 
               Excellent 
             
             
                 
               100 
               64 
               Poor 
               Excellent 
             
             
                 
               108 
               80 
               Poor 
               Excellent 
             
             
                 
               110 
               48 
               Excellent 
               Fair 
             
             
                 
                 
             
          
         
       
     
   
   As shown in Table 1, when the shore D hardness of the conductive sheet was between 50 and 65 and the pure-water contact angle was equal to or greater than 108 degrees, both image dirt and stripes were made within the tolerance for 10,000 sheet printing. On the other hand, when the shore D hardness was less than 50, image dirt in excess of the tolerance was generated in 10,000 sheet printing. This was considered to be because of the following reason. That is, if the shore D hardness was less than 50, the “softness” of the conductive sheet excessively increased the adhesiveness between the conductive sheet and the photoconductor, which made it difficult for the toner to pass the position where the conductive sheet and the photoconductor were in contact. 
   Further as shown in Table 1, if the shore D hardness was greater than 65, vertical stripes occurred in excess of the tolerance in the middle of 10,000 sheet printing even if the pure-water contact angle was equal to or greater than 108 degrees. That is, it was difficult to suppress the toner adhesion to the conductive sheet over a long period of time. This was considered to be because of the following reason. That is, if the shore D hardness was greater than 65, the conductive sheet tended not to follow the rotation of the photoconductor, but just rubbed the surface of the photoconductor due to the “hardness” of the conductive sheet even if minute projections in the magnitude of nm to μm on the surface of the conductive sheet were in contact with the photoconductor. Accordingly, fine vibration that would otherwise be generated by sliding contact with the photoconductor by the surface of the conductive sheet was not properly generated. On the other hand, when the shore D hardness of the conductive sheet  9 K was 65 or less, the “softness” contributed to the fine vibration. The fine vibration caused the toner adhered to the surface of the conductive sheet to be separated, and the toner easily passed through the contacting position of the conductive sheet and the photoconductor in a short time. 
   In view of the experiment results described above, the conductive sheet, such as  9 K, of the auxiliary electrifying apparatus, such as  10 K, of the processing units  1 Y,  1 M,  1 C, and  1 K of the printer according to the embodiment is specified as follows: 
   the shore D hardness is equal to or greater than 50, and equal to or less than 65; and 
   the surface pure-water contact angle is equal to or greater than 108 degrees. 
   In addition, although the conductive sheet was pushed toward the photoconductor by the elastic part of the auxiliary electrifying apparatus in the experiment described above, a conductive blade of the same quality as the conductive sheet may be employed to contact the photoconductor, one end of which conductive blade is supported, to obtain the same results as shown in Table 1. 
   Further, although the embodiment employed DC voltage as the electric bias, an AC voltage superimposed on a DC voltage may be used. Furthermore, although the embodiment employed DC voltage as the auxiliary electric bias, an AC voltage superimposed on a DC voltage may be used. 
   It is desirable that the conductive sheet of the auxiliary electrifying apparatus have a thickness between 50 μm and 2 mm. If the thickness becomes less than 50 μm, durability will be degraded. If the thickness is greater than 2 mm, the flexibility, and therefore the adhesiveness of the conductive sheet are degraded. 
   It is desirable that a torque generated by the conductive sheet in sliding contact with the photoconductor be between 0.2 and 1.2 N·m in the case that the sliding contact length in an axial direction of the photoconductor is 240 mm. As the torque exceeds 1.2 N·m, the conductive sheet is rapidly worn. As the torque becomes less than 0.2 N·m, the toner rapidly tends to be stuck to the conductive sheet due to weakened sliding contact force between the conductive sheet and the photoconductor. 
   It is desirable that the surface resistivity of the conductive sheet of the auxiliary electrifying apparatus be between 10 2  ω/cm 2  and 10 8  Ω/cm 2 . If the surface resistivity is less than 10 2  Ω/cm 2 , a current flowing between the conductive sheet and the photoconductor degrades electric discharging between the conductive sheet and the photoconductor, which causes charge unevenness and ground dirt to be generated. As the surface resistivity exceeds 10 8  Ω/cm 2 , the uniformity of the electric discharge is remarkably degraded, and the charge unevenness is generated. 
   It is desirable that the rate of volume specific resistance of the conductive sheet of the auxiliary electrifying apparatus be between 10 2  Ω-cm and 10 6  Ω-cm. It is desirable that the rate of volume specific resistance be slightly lower than the surface resistivity for obtaining uniform electric discharge. As the rate of volume specific resistance becomes less than 10 2  Ω-cm, a current flowing between the conductive sheet and the photoconductor will cause the electric discharge between the conductive sheet and the photoconductor to be rapidly degraded. 
   It is desirable that the surface roughness Ra of the conductive sheet of the auxiliary electrifying apparatus be between 0.6 μm and 0.1 μm. If the surface roughness Ra is greater than 0.6 μm, the toner tends to stay, adhere, and be fixed to a concave part of fine unevenness of the surface of the conductive sheet. As the surface roughness Ra becomes less than 0.1 μm, poor adhesiveness to the photoconductor tends to rapidly generate the image background dirt. 
   It is desirable that the width of an auxiliary electrifying nip (a length in a direction of surface movement of the photoconductor at the position where the conductive sheet of the auxiliary electrifying apparatus contacts the photoconductor) be between 2 mm and 7 mm. As the width of the auxiliary electrifying nip becomes less than 2 mm, the charge of the residual toner and the photoconductor is rapidly degraded. If the width of the auxiliary electrifying nip is greater than 7 mm, production cost will rise due to a greater size of the apparatus. 
   It is desirable that the contact pressure between the photoconductor and the conductive sheet of the auxiliary electrifying apparatus be between 2 kN/m 2  and 15 kN/m 2 . As the contact pressure becomes less than 2 kN/m 2 , the contact becomes unstable. If the contract pressure is greater than 15 kN/m 2 , the photoconductor rapidly tends to be damaged. 
   It is desirable that the pure-water contact angle of the photoconductor be greater than 90 degrees. As the pure-water contact angle becomes less than 90 degrees, the toner fixation to the photoconductor will rapidly take place. 
   It is desirable that the toner is added with an external additive in an amount of 1-4 parts by weight of the toner particles. The external additive helps reduce fixation of the toner to the conductive sheet, where the external additive intervenes between the toner particles and the conductive sheet even if the toner advances into the contact position of the conductive sheet of the auxiliary electrifying apparatus and the photoconductor. In addition, as the contents of the external additive exceed 4 parts by weight, an inadvertent influence is rapidly generated by the external additive transiting to various components. 
   It is desirable that the toner particles having a diameter less than 5 μm be less than 15% of all the toner particles. As the ratio exceeds 15%, toner fixation to the conductive sheet will rapidly take place. 
     FIG. 3  shows the outline of a second printer that is a variation of the printer described in detail above. The modified printer (the second printer) includes an electrifying brush roller  4 K instead of the electrifying roller as the electrifying apparatus. The electrifying brush roller  4 K includes a metal revolving shaft  5 K that is supported by bearings that are not illustrated, and two or more conductive brush-filling fibers  6 K that are implanted on the surface of the metal revolving shaft  5 K. The electrifying brush roller  4 K is rotationally driven by a driving unit that is not illustrated in the counterclockwise direction as shown in  FIG. 3 , where the metal revolving shaft  5 K serves as an axle, such that tips of the conductive brush-filling fibers  6 K make sliding contact with the photoconductor  3 K. An electric bias supply unit including a power source and wiring (not illustrated) is connected to the metal revolving shaft  5 K, and an electric bias, e.g., an AC voltage to which a DC voltage is superimposed is provided to the metal revolving shaft  5 K. At an electrifying nip where the conductive brush-filling fibers  6 K contact the photoconductor  3 K, and in its neighborhood, electric discharge occurs between the conductive brush-filling fibers  6 K and the photoconductor  3 K so that uniform charging of the surface of the photoconductor  3 K is carried out, for example, with a negative polarity. 
   The modified printer employs the “temporary capture” type “cleanerless” method. Specifically, at the electrifying nip, electric discharge is carried out between the conductive brush-filling fibers  6 K and the photoconductor  3 K, and the negative polarity uniform charging of the surface of the photoconductor  3 K is carried out. Simultaneously, by action of the electric bias, the residual toner adhered to the photoconductor  3 K is transferred to the conductive brush-filling fibers  6 K, i.e., the residual toner is temporarily captured. Then, the residual toner captured by the conductive brush-filling fibers  6 K is transferred to the photoconductor  3 K by switching the electric bias to a DC voltage from the superposed AC+DC voltage. Then, the residual toner is recovered by the development apparatus  40 K from the photoconductor  3 K via the developing roller  42 K after finishing the print job, or between printing jobs. 
   Although the printer according to the embodiment and the variation thereof has been described as employing the “cleanerless” type processing unit, the present invention is applicable to a printer with a cleaning unit wherein residual toner is removed from the photoconductor surface after passing the primary transfer nip and before advancing into the electrifying nip. 
   Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
   The present application is based on Japanese Priority Application No. 2006-345331 filed on Dec. 22, 2006 with the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.