Patent Publication Number: US-8532531-B2

Title: Image forming apparatus comprising a charging unit including plural conductive fibers

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image forming apparatuses, such as copying machines and laser printers, that adopt an intermediate transfer system of an electrophotographic system or an electrostatic recording system for transferring a toner image formed on an image bearing member onto an intermediate transfer member and thereafter transferring the toner image onto a transfer material. 
     2. Description of the Related Art 
     A known example of image forming apparatuses, such as copying machines and laser printers, uses an intermediate transfer member. 
     An image forming apparatus configured to use an intermediate transfer member transfers a toner image formed on the surface of a photosensitive drum serving as a first image bearing member onto an intermediate transfer member in a primary transfer process. Thereafter, by repeating this primary transfer process for a plurality of colors of toner images, the image forming apparatus forms the plurality of colors of toner images on the surface of the intermediate transfer member. Subsequently, as a secondary transfer process, the image forming apparatus transfers the plurality of colors of toner images formed on the surface of the intermediate transfer member onto a transfer material in a batch. The unfixed toner images transferred in a batch on the transfer material are thereafter fixed permanently by the fixing unit to form a full-color image on the transfer material. 
     At that time, part of the toner images are not sometimes transferred to the transfer material in the secondary transfer process and thus remains on the surface of the intermediate transfer member. By collecting the residual toner by a known cleaning unit, the next image formation can be started. 
     Japanese Patent Laid-Open No. 9-50167 discloses an image forming apparatus that collects residual toner on the intermediate transfer member after the secondary transfer process from the intermediate transfer member using a charging unit. This proposes a simultaneous transfer cleaning system in which an AC voltage is applied to a roller used as the charging unit to charge the residual toner to a polarity opposite to the charged state of the toner during development, and the residual toner charged to the opposite polarity is thereafter reversely transferred to a photosensitive drum in the next primary transfer process and is collected by a cleaning unit on the photosensitive drum. The above configuration allows the residual toner to be cleaned simultaneously with the primary transfer of the next page, thus allowing continuous image formation without slowing the printing speed. 
     Japanese Patent Laid-Open No. 2009-205012 discloses a method of using a roller member and a brush member as a charging unit. Specifically, this is configured to scatter residual toner on an intermediate transfer member substantially uniformly with the brush member and to charge the substantially uniformly scattered residual toner with the roller member. However, the use of the brush member as the charging unit may pose the following problem depending on the situation; that is, conductive fibers that constitute the brush member may cause electric discharge that causes a bad quality image. Specifically, an image forming apparatus in which toner is negatively charged during development will be described. 
     The brush member described above scatters residual toner substantially uniformly by coming into contact with the intermediate transfer member and charges the residual toner to a positive polarity opposite to the charged state of the toner during development when a DC voltage is applied. As shown in  FIG. 6A , the brush member  60  is provided with a predetermined amount of entry with respect to the intermediate transfer member  61 . Furthermore, the brush member  60  is connected to a voltage application unit (not shown) that applies a positive-polarity voltage. Therefore, conductive fibers  62  that constitute the brush member  60  are bent into contact with the surface of the intermediate transfer member  61  to form a minute gap L to or from the intermediate transfer member  61 . At that time, a large number of minute gaps L are generated between the surface of the intermediate transfer member  62  and the conductive fibers  62 , as shown in  FIG. 6B  that is an enlarge view of a contact portion S at which the intermediate transfer member  61  and the conductive fibers  62  contact in  FIG. 6A . 
       FIG. 7  illustrates a cross-sectional view of one of the conductive fibers  62  constituting the brush member  60  over which a conductive agent is dispersed. Since the whole outer circumferential surfaces of the conductive fibers  62  are covered with the scattered conductive agent, the electric conductive portions of the conductive fibers  62  and the intermediate transfer member  61  oppose each other to discharge electricity in all the minute gaps L. This provides discharging points corresponding to the number of the conductive fibers  62  (minute gaps L in which electric discharge occurs). 
     As a result, residual toner that passes through the charging portion that the brush member  60  forms is overcharged at a positive polarity (opposite polarity to the charged state of the toner during development) at the large number of charging points formed between the brush member  60  and the intermediate transfer member  61 , resulting in an excessive charge amount. When the overcharged residual toner is reversely transferred from the intermediate transfer member to the photosensitive drum at the primary transfer portion, the residual toner is reversely transferred to the photosensitive drum while drawing the negative-polarity toner developed on the photosensitive drum because of a large electric field generated in the surrounding, thus causing a bad quality image. 
     The above tendency is notable under a high-temperature, high-humidity environment in which the charge polarity of the residual toner before coming into contact with the brush member  60  tends to become opposite to the polarity during development. Since the toner itself absorbs moisture under the high-temperature, high-humidity environment, the resistance is low, so that the absolute value of the charge amount of the toner is small. The charge polarity of the residual toner is reversed due to the influence of the positive-polarity voltage received during the secondary transfer, which increases the proportion of positive-polarity toner, so that the foregoing phenomenon is prone to occur. 
     To reduce overcharging of the residual toner, the number of minute gaps L formed between the conductive fibers  62  constituting the brush member  60  and the intermediate transfer member  61  should be reduced. To reduce the number of minute gaps L, there is a method of reducing the number of points of contact between the residual toner and the conductive fibers  62  by decreasing the density of the conductive fibers  62  to reduce the number of the conductive fibers  62 . 
     However, this method reduces the points of contact between the conductive fibers  62  constituting the brush member  60  and the residual toner, thus resulting in a decrease in the effect of scattering the residual toner. In particular, if there is much residual toner, lumps of residual toner cannot be scattered by the brush member  60  in which the scattering effect is reduced. This excessively reduces the charge amount of the residual toner after it passes through the contact portion between the brush member  60  and the intermediate transfer member  61 . As a result, the insufficiently charged residual toner remains on the intermediate transfer member  61  when reversely transferred to the photosensitive drum from the intermediate transfer member  61  in the primary transfer portion, which tends to generate a bad quality image. 
     The above tendency is notable under a low-temperature, low-humidity environment in which the charge polarity of the residual toner hardly becomes positive. Since the electrical resistance of the toner itself is high, so that the absolute value of the charge amount of the toner during development is large during development under the low-temperature, low-humidity environment, which increases the proportion of negative-polarity residual toner, so that the foregoing phenomenon is prone to occur. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image forming apparatus in which bad quality images are reduced by using a brush member that assuredly scatters residual toner while suppressing overcharge or insufficient charge of residual toner. 
     According to an aspect of the present invention, there is provided an image forming apparatus, including an image bearing member configured to bear a toner image; a rotatable, endless intermediate transfer member; a primary transfer member configured to primarily transfer the toner image from the image bearing member to the intermediate transfer member at a primary transfer portion; a secondary transfer member configured to secondarily transfer the toner image from the intermediate transfer member to a transfer material at a secondary transfer portion; and a charging unit disposed upstream of the primary transfer portion and downstream of the secondary transfer portion in the rotating direction of the intermediate transfer member and configured to charge residual toner on the intermediate transfer member. The charging unit includes a brush member in which a plurality of conductive fibers including an electric insulating portion and an electric conductive portion are bundled. The brush member brushes the surface of the intermediate transfer member with the plurality of conductive fibers with the rotation of the intermediate transfer member. Part of the outer circumferential surface of the conductive fibers serves as the conductive portion, and the other part serves as the insulating portion. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an image forming apparatus according to a first embodiment. 
         FIG. 2  is a diagram illustrating a cleaning configuration of the first embodiment. 
         FIG. 3A  is a diagram illustrating a conductive fiber of the first embodiment. 
         FIG. 3B  is a diagram illustrating a charging brush of the first embodiment. 
         FIG. 4A  is a diagram illustrating the operation of the first embodiment. 
         FIG. 4B  is an enlarged view of the conductive fibers. 
         FIG. 5  is a diagram illustrating a conductive fiber used in a second embodiment. 
         FIG. 6A  is a diagram illustrating a brush member in related art. 
         FIG. 6B  is a diagram illustrating conductive fibers in the related art. 
         FIG. 7  is a cross-sectional view of one of the conductive fiber in the related art. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described in detail by way of example with reference to the drawings. The sizes, materials, forms, and relative configuration of components described in the following embodiments may be changed as appropriate depending on the configuration and conditions of an apparatus that incorporates the present invention. 
     First Embodiment 
       FIG. 1  is a schematic diagram of an image forming apparatus. The configuration and operation of the image forming apparatus of this embodiment will be described with reference to  FIG. 1 . The image forming apparatus of this embodiment includes four image forming stations a, b, c, and d. A first image forming station corresponds to yellow (Y), a second image forming station corresponds to magenta (M), a third image forming station corresponds to cyan (C), and a fourth image forming station corresponds to black (Bk). The image forming operation will be described using the first station (Y). 
     Operation of Image Forming Apparatus 
     The image forming apparatus includes drum-like photosensitive members (hereinafter referred to as photosensitive drums)  1 . The photosensitive drums  1  are rotationally driven in the direction of the arrow at a predetermined circumferential speed (process speed). Here, the first image forming station will be described in detail. The photosensitive drum  1   a  of the first image forming station is an image bearing member that bears a toner image. The photosensitive drum  1   a  is uniformly charged to a predetermined polarity potential by a photosensitive-drum charging roller  2   a  during the rotation process and is then exposed to light by an image exposing unit  3   a . The photosensitive-drum charging roller  2   a  is for charging the photosensitive drum  1   a . Thus, an electrostatic latent image corresponding to a yellow component image of a target color image is formed on the photosensitive drum  1   a . Next, the electrostatic latent image is developed by a first developing unit (yellow developing unit)  4   a  at a developing position to be visualized as a yellow toner image. 
     A rotatable intermediate transfer member  10  is an endless intermediate transfer belt stretched by a driving roller  11 , a tension roller  12 , and a facing roller for secondary-transfer  13  (stretching members). The intermediate transfer member  10  rotates at substantially the same circumferential speed as that of the photosensitive drums  1 . The yellow toner image formed on the photosensitive drum  1   a  is transferred onto the intermediate transfer belt  10  (primary transfer) while passing through a contact portion (hereinafter referred to as a primary transfer portion) between the photosensitive drum  1   a  and the intermediate transfer belt  10 . At that time, a primary transfer voltage is applied to a primary transfer roller  14   a , which is a primary transfer member, from a primary transfer power supply  15   a . Residual toner T that remains on the photosensitive drum  1   a  is removed by a cleaning unit  5   a.    
     Likewise, a second-color magenta toner image, a third-color cyan toner image, and a fourth-color black toner image are formed by the respective image forming stations and are transferred onto the intermediate transfer belt  10  in sequence to form a combined color image corresponding to the target color image. 
     The four-color toner images on the intermediate transfer belt  10  are transferred collectively onto the surface of a transfer material P fed by a feeding member  50  during the process of passing through a secondary transfer portion formed between the intermediate transfer belt  10  and a secondary transfer roller  20  that is a secondary transfer member (secondary transfer) At that time, a secondary transfer voltage is applied to the secondary transfer roller  20  by a secondary transfer power supply  21 . Thereafter, the transfer material P that bears the four-color toner images are introduced to a fixing device  30 , where the transfer material P is heated and pressed, so that the four color toners are melted and mixed and are fixed onto the transfer material P. Thus, a full-color print image is formed. 
     The residual toner T remaining on the surface of the intermediate transfer belt  10  after the secondary transfer is uniformly scattered onto the intermediate transfer belt  10  (intermediate transfer member) and is uniformly charged by the charging unit. The charging unit is disposed downstream of a secondary transfer nip and upstream of a primary transfer nip in the rotating direction of the intermediate transfer belt  10 . 
     The charging unit of this embodiment includes a charging brush  16  which is a first charging member disposed upstream in the rotating direction of the intermediate transfer belt  10  and a charging roller  17  which is a second charging member disposed downstream. 
     The residual toner T remains scatteringly on the intermediate transfer belt  10  depending on the pattern of the toner image transferred to the transfer material P. To efficiently charge the residual toner T, it is desirable to charge the residual toner T by a charging member, with the residual toner T scattered into substantially one layer on the intermediate transfer belt  10 . 
     In this embodiment, the residual toner T is uniformly scattered onto the intermediate transfer belt  10  and is charged by the charging brush  16 . Thereafter, the residual toner T is charged by the charging roller  17  and is then reversely transferred to the photosensitive drum  1   a  during primary transfer of the next image. At that time, the residual toner T adherent to the photosensitive drum  1   a  is removed by the photosensitive-member cleaning unit  5   a.    
     Transfer Configuration 
     The primary transfer rollers  14   a  to  14   d  have an outside diameter of 12 mm and are formed by covering a nickel-plated steel rod having an outside diameter of 6 mm with foam sponge that is adjusted to a volume resistivity of 10 7  Ω·cm and a thickness of 3 mm and that is mainly composed of nitrile butadiene rubber (NBR) and epichlorohydrin rubber. The primary transfer rollers  14   a  to  14   d  are brought into contact with the photosensitive drums  1   a  to  1   d , respectively, via the intermediate transfer belt  10  under a pressure of 9.8 N and are driven with the rotation of the intermediate transfer belt  10 . The primary transfer rollers  14   a  to  14   d  are supplied with a voltage of 1,500 V as a primary transfer voltage from the primary transfer power supplies  15   a  to  15   d  to primarily transfer the toner on the photosensitive drums  1   a  to  1   d , respectively. 
     The intermediate transfer belt  10  has a thickness of 100 μm and is made from polyvinylidene fluoride (PVDF) whose volume resistivity is adjusted to 10 1  Ω·cm by mixing with carbon black as a conductive agent. The intermediate transfer belt  10  is stretched across three members, that is, the driving roller  11 , the tension roller  12 , and the facing roller for secondary-transfer  13 , and is stretched by the tension roller at a total tension of 60 N. 
     The secondary transfer roller  20  is a roller formed by covering a nickel-plated steel rod having an outside diameter of 8 mm with foam sponge that is adjusted to a volume resistivity of 10 8  Ω·cm and a thickness of 5 mm and that is mainly composed of NBR and epichlorohydrin rubber. The secondary transfer roller  20  is in contact with the intermediate transfer belt  10  under a pressure of 50 N. The secondary transfer roller  20  is driven with the rotation of the intermediate transfer belt  10 . When the toner on the intermediate transfer belt  10  is secondarily transferred onto the transfer material P, such as paper, a voltage of 2,500 V is applied as a secondary transfer voltage to the secondary transfer roller  20  from the secondary transfer power supply  21 . 
     This embodiment uses the charging brush  16  and the charging roller  17  as a residual toner T charging unit. The charging brush  16  is configured as an aggregate of a plurality of fibers having electrical conductivity (conductive fibers). The charging brush  16  is supplied with a voltage of 1,000 V from a high-voltage power supply  80  to charge the residual toner T. The configuration of the charging brush  16 , which is a feature of this embodiment, will be described later. 
     An elastic roller that is mainly composed of urethane rubber with a volume resistivity of 10 9  Ω·cm is used as the charging roller  17  (conductive roller). The conductive roller  17  is pushed against the facing roller for secondary-transfer  13  by a spring (not shown) via the intermediate transfer belt  10  under a total pressure of 9.8 N and is rotated with the rotation of the intermediate transfer belt  10  in the same direction. The conductive roller  17  is supplied with a voltage of 1,500V from a high-voltage power supply  70  to charge the residual toner T. Although this embodiment uses urethane rubber for the conductive roller  17 , it is not particularly limited; for example, ethylene propylene rubber or epichlorohydrin rubber may be used. 
     Method for Cleaning Intermediate Transfer Belt 
     With the configuration described above, a method for cleaning the intermediate transfer belt  10  will be described with reference to  FIG. 2 . 
     In this embodiment, as described above, the toner is negatively charged by the developing units  4   a  to  4   d  and is thereafter developed on the photosensitive drums  1   a  to  1   d . The toner developed on the photosensitive drums  1   a  to  1   d  is primarily transferred to the intermediate transfer belt  10  by the primary transfer rollers  14   a  to  14   d  that are supplied with a positive voltage by the primary transfer power supplies  15   a  to  15   d . The toner is transferred to the transfer material P, such as paper, from the intermediate transfer belt  10  by the secondary transfer roller  20  that is supplied with a positive voltage from the secondary transfer power supply  21 . 
     As shown in  FIG. 2 , the residual toner T remaining on the intermediate transfer belt  10  after the secondary transfer contains both positive-polarity and negative-polarity toners due to the influence of the positive-polarity voltage applied to the secondary transfer roller  20 . Furthermore, the residual toner T locally remains in a plurality of layers on the intermediate transfer belt  10  due to the influence of the irregularities of the surface of the transfer material P (portion A in  FIG. 2 ). The multilayered residual toner T is hardly charged as compared with single-layer residual toner. Thus, this embodiment is provided with the charging brush  16 . 
     For the residual toner T remaining on the intermediate transfer member  10 , the charging brush  16  located upstream in the rotating direction of the intermediate transfer belt  10  is fixed to the rotating intermediate transfer belt  10  and is disposed at a predetermined amount of entry with respect to the intermediate transfer member  10 . The charging brush  16  brushes the surface of the intermediate transfer belt  10  with the rotation of the intermediate transfer belt  10 . Therefore, the residual toner T deposited in multiple layers on the intermediate transfer belt  10  is scattered to substantially one layer owing to a difference in speed between the charging brush  16  and the rotating intermediate transfer member  10  (portion B in  FIG. 2 ). 
     The charging brush  16  is supplied with a positive-polarity voltage (in this embodiment, 1,000 V) from the high-voltage power supply  80 , so that the residual toner T is charged to a positive polarity opposite to the toner polarity during development while passing through a charging portion that the charging brush  16  forms. Thereafter, the residual toner T that has passed the charging portion formed by the charging brush  16  moves in the rotating direction of the intermediate transfer belt  10  to reach the conductive roller  17 . The conductive roller  17  is supplied with a positive-polarity voltage (in this embodiment, +1,500 V) from the high-voltage power supply  70 . The residual toner T that has passed through the charging portion formed by the charging brush  16 , where it is charged to a positive polarity, is further charged while passing through a charging portion that the conductive roller  17  forms to be given a positive charge best suited to cleaning (portion C in  FIG. 2 ). 
     The residual toner T that has given the optimum charge is reversely transferred to the photosensitive drum  1   a  due to the positive-polarity voltage applied to the primary transfer roller  14   a  at the primary transfer portion and is collected to the cleaning unit  5   a  disposed on the photosensitive drum  1   a.    
     In this embodiment, the conductive roller  17  is disposed downstream of the charging brush  16  in the rotating direction of the intermediate transfer belt  10 . This is for the purpose of making the charge amount of the residual toner T after secondary transfer that has passed through the charging brush  16  more uniform. Accordingly, if the charge amount of the residual toner T is within a predetermined range, the residual toner T can be charged only by the charging brush  16  without the conductive roller  17 . The charge amount of the residual toner T often depends on the environment, such as a temperature and humidity during secondary transfer, the charge amount of toner on the intermediate transfer belt  10 , and the kind of transfer material; thus, the use of the conductive roller  17  allows variations in the charge amount of the residual toner T described above to be coped with. 
     Next, the configuration of the charging brush  16  will be described with reference to  FIGS. 3A and 3B . The charging brush  16  that charges the residual toner T on the intermediate transfer belt  10  is a bundle of conductive fibers  16   a  including an electric insulating portion  16   b  and an electric conductive portion  16   c . Here, the insulating portion  16   b  and the conductive portion  16   c  of the conductive fiber  16   a  are different members, not all over which the conductive agent is scattered unlike that described with reference to  FIG. 7 . 
     The conductive fibers  16   a  of this embodiment are characterized in that part of the outer circumferential surface thereof is the conductive portion  16   c , as shown in  FIG. 3A . 
     Specifically, the conductive fibers  16   a  will be described with reference to  FIG. 3A  that is a cross-sectional view of one of the conductive fibers  16   a  constituting the charging brush  16 . The insulating portion  16   b  and the conductive portion  16   c  of the conductive fiber  16   a  are mainly composed of nylon and are configured such that the insulating portion  16   b  sandwiches the conductive portion  16   c  and that the conductive portion  16   c  is exposed at two portions of the outer circumferential surface of the conductive fiber  16   a . The proportion of the exposed portions when the whole outer circumferential surface is 100% is about 10% in total. 
     Furthermore, the resistance of one conductive fiber  16   a  per unit length is 10 8  Ω/cm. The length of the composite conductive fiber  16   a  is 5 mm.  FIG. 3B  is a diagram illustrating the charging brush  16  configured as an aggregate of the conductive fibers  16   a . As shown in  FIG. 3B , the charging brush  16  is configured such that the conductive fibers  16   a  are fixed to a foundation fabric  16   d  made of electric insulating polyester by being woven therein. Furthermore, the foundation fabric  16   d  is bonded onto a stainless used steel (SUS) plate  16   e  having a thickness of 1 mm with a conductive adhesive. By supporting the plate  16   e  in the apparatus main body, the charging brush  16  is fixed with respect to the intermediate transfer belt  10 . 
     The conductive fibers  16   a  used in this embodiment have a singe-yarn fineness of 5 dtex and a density of 100 kF/inch 2 . In this embodiment, although the charging brush  16  is configured by the conductive fibers  16   a  that are mainly composed of nylon, it is not particularly limited and may be made of polyester or acryl. 
     To charge the secondary-transfer residual toner T, the exposure amount of the conductive portion  16   c  of the composite conductive fiber  16   a  is preferably about 5 to 30% in total. To scatter lumps of the residual toner T into substantially one layer, the density of the conductive fibers  16   a  is preferably 20 kF/inch 2  to 300 kF/inch 2 . The end position of the charging brush  16  is fixed at an entry amount of about 1.0 mm with respect to the surface of the intermediate transfer belt  10 . 
     Next, the operation of this embodiment will be described. Since the charging brush  16  described above has the function of breaking down the deposited state of the residual toner T by coming into contact therewith, the charging brush  16  is provides with a predetermined amount of entry with respect to the intermediate transfer member  10 . As shown in  FIG. 4A , the conductive fibers  16   a  are in contact with the intermediate transfer member  10  while bending to the rotating direction of the intermediate transfer belt  10 . Therefore, a plurality of minute gaps L are formed between the conductive fibers  16   a  and the intermediate transfer belt  10 . In general, electric discharge occurs when the potential difference between objects and the size of the gaps therebetween satisfy predetermined relationship. When a predetermined potential difference or more is generated in one gap, electric discharge occurs. 
     In contrast, this embodiment is configured such that only part of the outer circumferential surface of each conductive fiber  16   a  is the conductive portion  16   c . The portion of the outer circumferential surface other than the conductive portion  16   c  is the insulating portion. Therefore, the conductive portions  16   c  of all the conductive fibers  16   a  do not always face the intermediate transfer belt  10 ; therefore, electric discharge do not occur in some minute gaps L formed between the conductive fibers  16   a  and the intermediate transfer belt  10 . 
       FIG. 4B  is a schematic enlarged view of the state of contact between the composite conductive fibers  16   a  and the intermediate transfer belt  10  shown in  FIG. 4A . 
     Referring to  FIG. 4B , electric discharge occurs in a minute gap L 1  in which the conductive portion  16   c  and the intermediate transfer belt  10  face; however, no electric discharge occurs in a minute gap L 2  in which the insulating portion  16   b  and the intermediate transfer belt  10  face. Therefore, electric discharge does not occur in all the minute gaps L formed between the conductive fibers  16   a  and the intermediate transfer belt  10 . 
     Accordingly, the charging brush  16  of this embodiment can reduce the number of minute gaps in which electric discharge occurs without decreasing the density of the conductive fibers  16   a . Furthermore, since there is no need to decrease the density, sufficient contact points between the conductive fibers  16   a  and the secondary-transfer residual toner T can be provided, thus allowing the charging brush  16  to sufficiently scatter the residual toner T by coming into contact therewith. 
     The exposure amount of each of the conductive portions  16   c  of the conductive fibers  16   a  constituting the charging brush  16  of this embodiment is about 10% of the outer circumferential surface, as described above. Therefore, the number of conductive fibers  16   a  whose conductive portions  16   c  come into contact with the intermediate transfer belt  10  and form discharging points is about 10% of the whole. That is, of the conductive fibers  16   a  with a density of 100 kF/inch 2 , the conductive portions  16   c  with a density of 10 kF/inch 2 , which is 10% of the charging brush  16  of this embodiment when expressed as the density of the charging brush  16 , is in contact with the intermediate transfer belt  10 . The study conducted by the applicant and the associated person showed that the density of the charging brush  16  of this embodiment at which the secondary-transfer residual toner T can be scattered is 20 kF/inch 2  or more. The use of the conductive fibers  16   a  with the configuration of this embodiment can efficiently reduce discharge points and can offer the effect of scattering a sufficient amount of residual toner T. 
     As described above, according to this embodiment, a charging member that charges the residual toner T on the intermediate transfer belt  10  is the charging brush  16  constituted by the conductive fibers  16   a  including the insulating portion  16   b  and the conductive portion  16   c . Since only part of the surface of the conductive fiber  16   a  serves as the conductive portion  16   c , the residual toner T on the intermediate transfer belt  10  can be scattered without forming lumps, thereby preventing overcharging of the secondary-transfer residual toner T. This allows the residual toner T to be charged to a proper charge amount. 
     In this embodiment, although a bar-type fixed member is used as a cleaning brush, a fur brush type roller that uses the foregoing conductive fibers  16   a  can also offer the same advantages when rotated at a peripheral speed different from that of the intermediate transfer belt  10 . 
     The charging brush  16  of this embodiment can be used more effectively if the intermediate transfer member  10  has an ion conductive resistance characteristic obtained by dispersing hydrophilic macromolecules in polyvinylidene fluoride (PVDF). Since the intermediate transfer belt  10  that exhibits an ion conductive resistance characteristic performs electric conduction via ions, the resistance is more uniform in the surface of the intermediate transfer member  10  than that of the electron conducting intermediate transfer member  10  in which carbon is dispersed. This may be because the intermediate transfer belt  10  that uses hydrophilic macromolecules as a conducting agent conducts electricity by the movement of water ions, so that the resistance of the intermediate transfer member  10  is stable irrespective of the location although the resistance changes depending on the absolute moisture amount. On the other hand, since electronic conductivity is caused when electrons move between conductive fillers, such as carbon, while hopping due to a tunnel effect, the resistance depends on the dispersion state of the conductive fillers. 
     Therefore, the resistance of the intermediate transfer belt  10  is stable irrespective of the position of contact with the charging brush  16 , thus preventing concentration of electric discharge on a specific portion of the intermediate transfer belt  10 . This therefore stabilizes electric discharge that occurs between the composite conductive fibers  16   a  and the intermediate transfer belt  10 , allowing the residual toner T to be charged more uniformly. 
     In other words, since the ion conductive intermediate transfer belt  10  has high resistance uniformity in the surface, electric discharge generated between the conductive fibers  16   a  and the intermediate transfer member  10  can easily be stabilized. 
     Second Embodiment 
     In the configuration of an image forming apparatus of this embodiment, the same components as those of the first embodiment are given the same reference signs and descriptions thereof will be omitted. The sizes and arrangements of the charging brush  16  and the charging roller  17  used as a residual toner T charging unit are the same as those of the first embodiment. 
     In this embodiment, conductive fibers  16   f  differ from the composite conductive fibers  16   a  of the first embodiment. The conductive fibers  16   f  are mainly composed of polyester and have a cross-sectional form in which the conductive portion  16   c  and the insulating portion  16   b  are arranged alternately, as shown in  FIG. 5 . The conductive portion  16   c  of the conductive fiber  16   f  is exposed at three portions on the outer circumferential surface, and the proportion of the exposed portions is 15% in total when the whole outer circumferential surface is 100%. The resistance of one conductive fiber  16   f  per unit length is 10 8  Ω/cm. 
     The charging brush  16  configured as an aggregate of conductive fibers  16   f  can be made into a brush form by weaving the composite conductive fibers  16   f  into a fundamental fabric  16   d  formed of electric insulating nylon. The foundation fabric  16   d  is bonded on a SUS plate  16   e  having a thickness of 1 mm with a conductive adhesive. The conductive fibers  16   f  of the charging brush  16  have a singe-yarn fineness of 3 dtex and a density of 70 kF/inch 2 . 
     In this embodiment, although the charging brush  16  is configured by the conductive fibers  16   f  that are mainly composed of polyester, it is not particularly limited and may be made of nylon or acryl. The end position of the charging brush  16  is fixed at an amount of entry of about 1.0 mm with respect to the surface of the intermediate transfer belt  10 , thus causing a difference in peripheral speed between the charging brush  16  and the intermediate transfer belt  10 . 
     Since the conductive fiber  16   f  having three exposed conductive portions  16   c  have more discharging points at which the intermediate transfer belt  10  and the conductive portion  16   c  face, as in this embodiment, as compared with that having two exposed conductive portions, the residual toner T charging performance is enhanced. In particular, in the case where there is much negatively charged residual toner T, the residual toner T can be charged to a proper charge amount by using the conductive fibers  16   f.    
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2009-286886 filed Dec. 17, 2009, which is hereby incorporated by reference herein in its entirety.