Patent Publication Number: US-9411271-B2

Title: Image forming apparatus to set target currents

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, such as a copying machine, a printer, and a facsimile apparatus, which uses an electrophotographic process or an electrostatic recording process. 
     2. Description of the Related Art 
     Recently, full color machines capable of outputting a plurality of colors have become the mainstream of electrophotographic and electrostatic recording image forming apparatuses. For example, many tandem type image forming apparatuses, in which a plurality of image bearing members of different developing colors is arranged along a rotation path of an intermediate transfer member to form a full color image, have been put to practical use. 
     Generally speaking, in electrophotographic and electrostatic recording image forming apparatuses, the electric resistances of an intermediate transfer member and transfer rollers and/or the thicknesses of surface layers of photosensitive members may change with a change in an atmospheric environment such as temperature and humidity, or with the use of the apparatuses. Thus, according to such changes, it is desirable to change a transfer voltage to be applied to a transfer member. To obtain a desired transfer voltage during an image forming operation, a voltage determination control for determining a control value (voltage value) for constant voltage control is performed before the image forming operation. For example, Japanese Patent Application Laid-Open No. 2-123385 discusses active transfer voltage control (ATVC) as a voltage determination method. In the ATVC, a desired constant current voltage is applied to a photosensitive member from a transfer roller during a non-image forming operation of the image forming apparatus, and the value of the voltage applied at that time is stored. The electric resistance of the transfer member is thereby detected, and a constant voltage according to the electric resistance value is applied to the transfer roller as a transfer voltage at the time of transfer in the image-forming operation. 
     Further, it is desirable that an image forming apparatus that performs continuous image formation change the transfer voltage applied to a transfer member, according to a change in the atmospheric environment inside the apparatus due to the continuous operation. Thus, sheet-to-sheet correction control may be performed to correct a primary transfer voltage value. The sheet-to-sheet correction control includes monitoring a primary transfer current at a timing corresponding to an interval between transfer materials during the continuous image formation (hereinafter also referred to as “an interval between sheets”), and increasing or decreasing the applied voltage by a certain value if a difference equal to or greater than a certain current amount occurs with respect to a target current. 
     Japanese Patent Application Laid-Open No. 2008-129471 discusses a method for enabling optimum constant voltage settings even under extreme conditions when the present electric resistance of a transfer member changes greatly due to a temporal change or temperature variations. More specifically, a target transfer current value is set based on a predetermined table according to the electric resistance of the transfer member obtained during the foregoing ATVC. The table contains values that are set in advance to uniquely reduce the target transfer current value as the electric resistance increases. 
     However, the foregoing conventional method by which the target transfer current value is determined according to the electric resistance obtained in the voltage determination control performed before the image formation may fail to apply an appropriate transfer voltage during the image formation. 
     In particular, if an intermediate transfer member of which the electric resistance changes greatly during the image formation is used, the optimum current varies during the image formation. As a result, the current value determined according to the electric resistance obtained before the image formation deviates gradually from the optimum current if a large amount of images are formed. 
     Furthermore, if the foregoing conventional control of uniquely reducing the target transfer current value with an increase in the electric current is used with an intermediate transfer member of which the electric resistance changes greatly during the image formation, the following problem may occur. If the electric resistance of the intermediate transfer member is low, the target transfer current value becomes high and an excessive current may flow through the photosensitive member, thereby causing a memory in the photosensitive member. On the other hand, if the electric resistance of the intermediate transfer member is high, the target transfer current value may be lowered too much, thereby causing a transfer defect. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an image forming apparatus includes an image bearing member configured to bear a toner image thereon, a movable intermediate transfer member configured to temporarily bear the toner image, the toner image being transferred from the image bearing member onto the intermediate transfer member in a transfer portion and then being transferred onto a recording material, a transfer member configured to electrostatically transfer the toner image formed on the image bearing member onto the intermediate transfer member in the transfer portion, a power supply configured to apply a voltage to the transfer member, a detection member configured to detect a current flowing through the transfer member during the application of the voltage to the transfer member, a control portion configured to control the voltage of the power supply in such a way that the current detected by the detection member has a predetermined target current value during continuous image formation, and a setting portion configured to, during the control of the voltage of the power supply by the control portion, (a) set the target current value to a first current value, in a case of a value related to an electric resistance obtained through the detection by the detection member being a first value or lower, (b) set the target current value to a correction current value based on the value related to the electric resistance, in a case of the value related to the electric resistance being greater than the first value and lower than a second value greater than the first value, and (c) set the target current value to a second current value lower than the first current value, in a case of the value related to the electric resistance being the second value or greater. 
     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 schematic sectional view of an image forming apparatus (in a full color mode). 
         FIG. 2  is a schematic sectional view of the image forming apparatus (in a black monochrome mode). 
         FIG. 3  is a schematic sectional view of a belt cleaning device. 
         FIG. 4  is a control block diagram of essential parts of the image forming apparatus. 
         FIG. 5  is a graph illustrating an example of a relationship between the number of images formed and a volume resistivity of an intermediate transfer belt. 
         FIG. 6  is a graph illustrating transfer efficiency and retransfer efficiency. 
         FIG. 7  is a graph illustrating an example of a relationship between the volume resistivity of the intermediate transfer belt and an optimum current value of a transfer current. 
         FIG. 8  is a graph illustrating an example of a transition of electrification of the intermediate transfer belt. 
         FIG. 9  is a graph illustrating another example of the transition of electrification of the intermediate transfer belt. 
         FIG. 10  is a graph illustrating an example of a relationship between the volume resistivity of the intermediate transfer belt and the degree of rise of a transfer voltage. 
         FIG. 11  is a flowchart illustrating an example of normal active transfer voltage control (ATVC). 
         FIG. 12  is a flowchart illustrating an example of sheet-to-sheet ATVC. 
         FIG. 13  is a schematic diagram illustrating a shift amount of a primary transfer roller. 
         FIG. 14  is a schematic sectional view illustrating a layer configuration of the intermediate transfer belt. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An image forming apparatus according to an exemplary embodiment of the present invention will be described in detail below with reference to the drawings. 
     1. Overall Configuration and Operation of Image Forming Apparatus 
       FIG. 1  is a schematic sectional view of an image forming apparatus according to a first exemplary embodiment of the present invention. The image forming apparatus  100  according to the present exemplary embodiment is a tandem type laser beam printer using an intermediate transfer system, which is capable of forming a full color image on a transfer material (such as a recording sheet, an overhead projector (OHP) sheet, and cloth) through an electrophotographic process. 
     The image forming apparatus  100  includes first, second, third, and fourth image forming units SY, SM, SC, and SK as a plurality of image forming units (stations). The image forming units SY, SM, SC, and SK form an image of yellow (Y), magenta (M), cyan (C), and black (K), respectively. In the present exemplary embodiment, the image forming units SY, SM, SC, and SK have a lot in common in terms of configuration and operation, except that each of the units uses toner of a different color. Therefore, in the following description, the suffixes “Y”, “M”, “C”, and “K” on image forming units S to indicate the elements provided for the respective colors will be omitted and the elements will be described in a generalized manner unless a distinction is particularly needed. 
     The image forming units S each include a photosensitive drum  1 , which is a drum-shaped (cylindrical) electrophotographic photosensitive member (photosensitive member) serving as an image bearing member arranged in a rotatable manner. The photosensitive drum  1  is driven to rotate by a drive motor (not illustrated) serving as a drive unit in a direction indicated by an arrow R 1  in  FIG. 1 . Around the photosensitive drum  1 , process devices are arranged, including a charging roller  2  serving as a charging unit, an exposure device  3  serving as an exposure unit, a developing device  4  serving as a developing unit, and a drum cleaning device  6  serving as a photosensitive member cleaning unit. The developing devices  4  of the image forming units SY, SM, SC, and SK store toner of yellow, magenta, cyan, and black, respectively. In the present exemplary embodiment, the photosensitive drum  1 K of the fourth image forming unit SK has a diameter greater than those of the other image forming units SY, SM, and SC. The fourth image forming unit SK also includes a sensor for detecting the density of a patch (described below). 
     An intermediate transfer belt  7  constituting an endless belt serving as an intermediate transfer member is arranged to be opposed to the photosensitive drums  1  of the image forming units S. The intermediate transfer belt  7  is held by support members (stretching rollers) including a driving roller  71 , a tension roller  72 , a secondary transfer counter roller  73 , and push-up rollers  74  and  75 . The driving roller  71  transmits drive to the intermediate transfer belt  7 . The tension roller  72  applies a predetermined tension to the intermediate transfer belt  7 . The secondary transfer counter roller  73  serves as a counter member (counter electrode) of a secondary transfer roller  8  to be described below. The push-up rollers  74  and  75  form a primary transfer plane  70  for transferring a toner image onto the intermediate transfer belt  7 . The four image forming units SY, SM, SC, and SK are arranged in line along a horizontal portion of the primary transfer plane  70 . The driving roller  71  is driven to rotate at a circumferential speed of 350 mm/sec by a drive motor (not illustrated) serving as a drive unit such as a pulse motor. Consequently, the intermediate transfer belt  7  rotates (circulates) in a direction indicated by an arrow R 2  in  FIG. 1  (hereinafter also referred to as “rotation direction” or “conveyance direction”). The stretching rollers other than the driving roller  71  are driven to rotate by the rotation of the intermediate transfer belt  7 . 
     On the inner peripheral (back surface) side of the intermediate transfer belt  7 , primary transfer rollers  5  are arranged in positions opposed to the photosensitive drums  1  of the respective image forming units S. The primary transfer rollers  5  each are a primary transfer member having a roller shape serving as a primary transfer unit. Each of the primary transfer rollers  5  is biased (pressed) toward the photosensitive drum  1  via the intermediate transfer belt  7  to form a primary transfer portion (primary transfer nip) T 1  where the intermediate transfer belt  7  and the photosensitive drum  1  make contact with each other. The secondary transfer roller  8  is arranged in a position opposed to the secondary transfer counter roller  73  on the outer peripheral (front surface) side of the intermediate transfer belt  7 . The secondary transfer roller  8  is a secondary transfer member having a roller shape serving as a secondary transfer unit. The secondary transfer roller  8  is biased (pressed) toward the secondary transfer counter roller  73  via the intermediate transfer belt  7  to form a secondary transfer portion (secondary transfer nip) T 2  where the intermediate transfer belt  7  and the secondary transfer roller  8  make contact with each other. A belt cleaning device  9  serving as an intermediate transfer member cleaning unit is also arranged in a position opposed to the driving roller  71  on the outer peripheral side of the intermediate transfer belt  7 . 
     Each of the rotating photosensitive drums  1  is uniformly charged by the charging roller  2 . The charged photosensitive drum  1  is exposed by the exposure device  3  according to image information, whereby an electrostatic latent image (electrostatic image) according to the image information is formed thereon. The developing device  4  supplies toner of a color corresponding to the image forming unit S, whereby the electrostatic latent image formed on the photosensitive drum  1  is developed as a toner image. The toner image formed on the photosensitive drum  1  is primarily transferred onto the rotating intermediate transfer belt  7  in the primary transfer portion T 1  by the action of the primary transfer roller  5 . At this time, a primary transfer power supply  51  serving as an application unit applies a primary transfer bias (primary transfer voltage) to the primary transfer roller  5 , whereby a primary transfer field is formed in the primary transfer portion T 1 . The primary transfer bias is a direct-current voltage of opposite polarity to the charging polarity (normal charging polarity) of the toner at the time of development. In the present exemplary embodiment, primary transfer power supplies  51 Y,  51 M,  51 C, and  51 K are connected to the primary transfer rollers  5 Y,  5 M,  5 C, and  5 K of the image forming units SY, SM, SC, and SK, respectively. For example, during formation of a full color image, toner images of the respective colors, yellow, magenta, cyan, and black that are formed by the respective image forming units S are successively transferred onto the intermediate transfer belt  7  in an overlapping manner in the respective primary transfer portions T 1 . 
     The toner images transferred to the intermediate transfer belt  7  are secondarily transferred onto a transfer material P in the secondary transfer portion T 2  by the action of the secondary transfer roller  8 . At this time, a secondary transfer power supply  81  serving as an application unit applies a secondary transfer bias (secondary transfer voltage) to the secondary transfer roller  8 , whereby a secondary transfer field is formed in the secondary transfer portion T 2 . The secondary transfer bias is a direct-current voltage of a polarity opposite to a normal charging polarity of a toner. By this time, the transfer material P is fed out of a sheet cassette  10 , temporarily stopped by a registration roller  12 , and then conveyed to the secondary transfer portion T 2  at a predetermined timing. The transfer material P to which the toner images have been transferred is conveyed to a fixing device  11 . In the fixing device  11 , the toner images are firmly fixed to the transfer material P by heat and pressure. The transfer material P is then discharged (output) to the outside of the main body of the image forming apparatus  100 . 
     Residual transfer toner on the photosensitive drum  1 , which was not transferred onto the intermediate transfer belt  7  during the primary transfer, is removed and collected from the photosensitive drum  1  by the drum cleaning device  6 . Residual transfer toner on the intermediate transfer belt  7 , which was not transferred onto the transfer material P during the secondary transfer, is removed and collected from the intermediate transfer belt  7  by the belt cleaning device  9 . 
     2. Configuration of Each Component 
     2-1. Photosensitive Drums 
     The photosensitive drums  1  are each formed by applying an organic photoconductive layer (OPC) to the outer peripheral surface of an aluminum cylinder. The photosensitive drum  1  is rotatably supported by flanges at both end portions in its longitudinal direction (rotational axis direction). A driving force is transmitted from a drive motor (not illustrated) to one of the end portions, whereby the photosensitive drum  1  is driven to rotate. In the present exemplary embodiment, the photosensitive drum  1  has a negative charging polarity. 
     In the present exemplary embodiment, the photosensitive drums  1 Y,  1 M, and  1 C of the first, second, and third image forming units SY, SM, and SC for yellow, magenta, and cyan, respectively have an outer diameter of φ30 mm. The photosensitive drum  1 K of the fourth image forming unit SK for black has an outer diameter of φ80 mm. In other words, only the photosensitive drum  1 K for black is larger than the photosensitive drums  1 Y,  1 M, and  1 C for the other colors. 
     2-2. Charging Rollers 
     The charging rollers  2  each are a contact charging member which makes contact with the surface of the corresponding photosensitive drum  1  to uniformly charge the circumferential surface of the photosensitive drum  1 . The charging roller  2  is a conductive roller including a core (core material) around which an elastic layer is formed. The charging roller  2  is rotatably held by bearing members at both end portions in its longitudinal direction (rotational axis direction), and is biased toward the photosensitive drum  1  by a pressing spring serving as a biasing unit. As a result, the charging roller  2  is pressed against the surface of the photosensitive drum  1  by a predetermined pressing force, and is driven to rotate by the rotation of the photosensitive drum  1 . A charging power supply  21  (see  FIG. 4 ) serving as an application unit applies a charging bias (charging voltage) having a predetermined condition to the core of the charging roller  2 . The circumferential surface of the rotating photosensitive drum  1  is thereby charged to a predetermined potential of a predetermined polarity (negative polarity in the present exemplary embodiment). In the present exemplary embodiment, the charging bias is an oscillation voltage obtained by superposing an alternating-current voltage (Vac) on a direct-current voltage (Vdc). More specifically, the oscillation voltage is obtained by superposing a sinusoidal alternating-current voltage (alternating-current component) having a frequency f of 1 kHz and a peak-to-peak voltage Vpp of 1.5 kV on a direct-current voltage (direct-current component) of −600 V. As a result, the circumferential surface of the photosensitive drum  1  is uniformly charged to −600 V (dark potential Vd). 
     2-3. Exposure Devices 
     The exposure devices  3  each are a laser scanner device which includes a laser light source and a polygonal mirror and is controlled on/off by a driving circuit according to an image signal. The exposure device  3  projects a laser beam according to the image signal of a color component in a document corresponding to the image forming unit S, upon the photosensitive drum  1  via the polygonal mirror. 
     2-4. Developing Devices 
     The developing devices  4  each use a two-component developer including nonmagnetic toner and a magnetic carrier as a developer. In the present exemplary embodiment, the toner has a negative charging characteristic. The developing device  4  includes a developing container storing the developer. The developing device  4  also includes a developing sleeve serving as a developer bearing member. The developing sleeve is arranged to be partly exposed from an opening of the developing container opposed to the photosensitive drum  1 . The developing sleeve is arranged next to the surface of the photosensitive drum  1 , and driven to rotate by a drive motor (not illustrated) serving as a drive unit. A developing power supply (not illustrated) serving as an application unit applies a predetermined developing bias (developing voltage) to the developing sleeve. Consequently, toner is supplied from the developer borne and conveyed to the position (developing portion) opposite to the photosensitive drum  1  by the developing sleeve, whereby the electrostatic latent image on the photosensitive drum  1  is developed as a toner image. In the present exemplary embodiment, the developing device  4  forms the toner image by a reversal phenomenon of causing toner having a polarity same as the charging polarity of the photosensitive drum  1  to attach to exposed portions of the photosensitive drum  1  where the absolute value of the potential is lowered after the uniform charging of the photosensitive drum  1 . To improve releasability of the toner, an external additive is added to the toner. 
     2-5. Primary Transfer Rollers 
     The primary transfer rollers  5  each are a conductive roller including a core (core material) around which an elastic layer is formed. The core is a cylindrical member made of conductive metal and having a diameter of 8 mm. The elastic layer is a conductive foam member having a resistance of 1.0×10 4  to 5.0×10 6 Ω and a thickness of 0.5 mm. The elastic layer covers the periphery of the core. The primary transfer roller  5  also has a weight of 300 g. In the present exemplary embodiment, the primary transfer rollers  5  of all the image forming units S have the same outer diameter. 
     The primary transfer roller  5  transfers the toner images from the photosensitive drum  1  onto the intermediate transfer belt  7  by an electrical action and a pressing force. For that purpose, the primary transfer roller  5  is supported by a pressing mechanism so as to be brought into contact with the photosensitive drums  1  from the back side of the intermediate transfer belt  7 . In the present exemplary embodiment, the primary transfer roller  5  is pressed vertically upward by a pressing spring serving as a biasing unit at both end portions in its longitudinal direction (rotational axis direction). 
     The primary transfer roller  5  is shifted downstream in the conveyance direction of the intermediate transfer belt  7  with respect to the vertical direction passing through the rotation center of the photosensitive drum  1 . In the present exemplary embodiment, the primary transfer rollers  5 Y,  5 M, and  5 C of the first, second, and third image forming units SY, SM, and SC are shifted by an amount of 2.5 mm. The primary transfer roller  5 K of the fourth image forming unit SK is shifted by an amount of 4.5 mm. As illustrated in  FIG. 13 , suppose that the straight line that passes through the rotation center of the photosensitive drum  1  and is orthogonal to the intermediate transfer belt  7  is X 1 . Suppose also that the straight line that passes through the rotation center of the primary transfer roller  5  and is parallel to the straight line X 1  is X 2 . In such a case, in the present exemplary embodiment, the shift amount Z of the primary transfer roller  5  with respect to the photosensitive drum  1  can be represented by the shift amount of the straight line X 2  with respect to the straight line X 1 . 
     The pressing force of the primary transfer roller  5  can be measured by using a pressure measuring jig. For example, a pseudo metal counter roller that has the same diameter as that of the photosensitive drum  1  and is split into five parts in the rotational axis direction is prepared. The pressing force of the primary transfer roller  5  is then measured by detecting pressure acting on the metal counter roller by using a load cell. Such a measurement system may be provided inside the main body of the image forming apparatus  100 . This enables measurement of the pressure actually acting on the photosensitive drum  1  from the primary transfer roller  5 . Using the five-way split metal counter roller also enables measurement of pressure distribution in the longitudinal direction of the primary transfer roller  5 . In the present exemplary embodiment, the primary transfer rollers  5 Y,  5 M, and  5 C of the first, second, and third image forming units SY, SM, and SC have a total pressing force of 600 to 800 gf. On the other hand, the primary transfer roller  5 K of the fourth image forming apparatus SK has a pressing force of 1300 to 1500 gf. Adjusting the shift amounts and pressures according to the diameters of the photosensitive drums  1  pressed by the primary transfer rollers  5  provides favorable transferability. 
     In the present exemplary embodiment, the distance between the primary transfer portions T 1  of the adjoining image forming units S is 120 mm in the conveyance direction of the intermediate transfer belt  7 . 
     In the present exemplary embodiment, the image forming apparatus  100  is capable of performing a full color mode (first image forming mode) and a black monochrome mode (second image forming mode or monochrome image forming mode) as a plurality of image forming modes in which different numbers of image forming units S are used to form a toner image(s). In the full color mode, the first, second, third, and fourth image forming units SY, SM, SC, and SK form toner images, whereby a full color image can be formed. In the black monochrome mode, only the fourth image forming unit SK forms a toner image as the predetermined image forming unit among the first, second, third, and fourth image forming units SY, SM, SC, and SK, whereby a black image can be formed. The image forming apparatus  100  includes a belt contact/separation mechanism  170  (see  FIG. 4 ) which can keep the photosensitive drums  1 Y,  1 M, and  1 C of the image forming units SY, SM, and SC, which are not used in the black monochrome mode, out of contact with the intermediate transfer belt  7 . 
     In the present exemplary embodiment, the primary transfer plane  70  moves when the push-up rollers  74  and  75  and the primary transfer rollers  5 Y,  5 M, and  5 C of the first, second, and third image forming units SY, SM, and SC move vertically as illustrated in  FIG. 2 . In the full color mode, the primary transfer plane  70  is formed by the push-up rollers  74  and  75  and the tension roller  72 . On the other hand, in the black monochrome mode, the primary transfer plane  70  is formed by the push-up roller  75  on the downstream side in the conveyance direction of the intermediate transfer belt  7  and the tension roller  72 . Consequently, in the full color mode, the photosensitive drums  1 Y,  1 M,  1 C, and  1 K of the first, second, third, and fourth image forming units SY, SM, SC, and SK are brought into contact with the intermediate transfer belt  7 . In the black monochrome mode, the photosensitive drums  1 Y,  1 M, and  1 C of the first, second, and third image forming units SY, SM, and SC are separated from the intermediate transfer belt  7 . In such a manner, the image forming apparatus  100  is configured to be able to selectively switch between the black monochrome mode and the full color mode. The belt contact/separation mechanism  170  (see  FIG. 4 ) includes support members for the push-up rollers  74  and  75  and the primary transfer rollers  5 Y,  5 M, and  5 C of the first, second, and third image forming units SY, SM, and SC, and a switching unit for moving such rollers via the support members. In the present exemplary embodiment, a solenoid is used as the switching unit. The switching unit moves the rollers  74 ,  75 ,  5 Y,  5 M, and  5 C vertically, i.e., selectively between a first position where the intermediate transfer belt  7  is located closer to the photosensitive drums  1  and a second position where the intermediate transfer belt  7  is located farther from the photosensitive drums  1 . In the present exemplary embodiment, the photosensitive drums  1 Y,  1 M, and  1 C of the first, second, and third image forming units SY, SM, and SC, which are not used in the black monochrome mode, are separable from the intermediate transfer belt  7 . This increases the life of the photosensitive drums  1 Y,  1 M, and  1 C. Further, the photosensitive drum  1 K of the fourth image forming unit SK for black, of which the use frequency is often high, is configured to have a large diameter. This increases the life of the photosensitive drum  1 K. The image forming unit using a photosensitive drum of large diameter need not necessarily be the image forming unit  1 K for black or be the most downstream one in the conveyance direction of the intermediate transfer belt  7 . Further, a photosensitive drum of large diameter is not necessarily used for only one image forming unit such as the image forming unit  1 K for black. A plurality of image forming units may be configured to use a photosensitive drum having an outer diameter larger than that of the other image forming unit(s) (such a plurality of image forming units may use photosensitive drums of the same or different outer diameters). Alternatively, the photosensitive drums  1  of all the image forming units S may have the same diameter if desired. 
     2-6. Intermediate Transfer Belt 
     In the present exemplary embodiment, a belt having a plurality of layers and also having an elastic layer is used as the intermediate transfer belt  7  (hereinafter also referred to as an “elastic intermediate transfer belt”).  FIG. 14  is a schematic sectional view illustrating an example of a layer configuration of the elastic intermediate transfer belt  7 . In the present exemplary embodiment, the elastic intermediate transfer belt  7  has a three-layer structure including a base layer (resin layer)  7   a , an elastic layer  7   b , and a surface layer  7   c . To maintain image properties, the three-layer elastic intermediate transfer belt  7  according to the present exemplary embodiment has a surface resistivity of 10 12 Ω/□ and a volume resistivity of 10 9  Ω·cm. The resistivities were measured by using a high resistivity meter Hiresta UPM, CP-HT450, UR probe manufactured by Mitsubishi Chemical Corporation, with a measurement condition including an applied voltage of 1000 V and an application time of 10 seconds. As for the thicknesses of the layers of the elastic intermediate transfer belt  7 , it is desirable that the base layer  7   a  has a thickness of approximately 50 to 100 μm, the elastic layer  7   b  has a thickness of approximately 200 to 300 μm, and the surface layer  7   c  has a thickness of approximately 2 to 20 μm. In the present exemplary embodiment, the base layer  7   a  is 85 μm thick, the elastic layer  7   b  is 260 μm thick, and the surface layer  7   c  is 2 μm thick. Further, it is desirable that the three-layer elastic intermediate transfer belt  7  have an International Rubber Hardness Degrees (IRHD) hardness of approximately 40 to 90 degrees. In the present exemplary embodiment, the elastic intermediate transfer belt  7  has an IRHD hardness of 73±3 degrees. 
     The base layer  7   a  and the elastic layer  7   b  can be made of any material as long as the foregoing characteristics are satisfied. Typical examples include the following. Examples of resin materials that can be used to constitute the base layer (resin layer)  7   a  include polycarbonates, fluorine-BASED resins (ethylene tetrafluoroethylene (ETFE) and polyvinylidene difluoride (PVDF)), polyamide resins, and polyimide resins having a Young&#39;s modulus (compliant with Japanese Industrial Standards (JIS) K 7127) of 5.0×10 2  to 5.0×10 3  MPa. Examples of elastic materials (elastic rubbers and elastomers) that can be used to constitute the elastic layer  7   b  include butyl rubber, fluorine-based rubber, chloroprene rubber (CR), ethylene propylene diene monomer (EPDM), and urethane rubber having a Young&#39;s modulus of 0.1 to 1.0×10 2  MPa. The surface layer  7   c  is not limited to a particular material. It is desirable that the surface layer  7   c  be made of a material that reduces the adhesion of toner to the surface of the intermediate transfer belt  7  for improved secondary transferability. Examples include resin materials such as fluorine resins and fluorine compounds having a Young&#39;s modulus of 1.0×10 2  to 5.0×10 3  MPa, urethane type resins in which fluorine type resin particles are dispersed, and elastic materials. None of the base layer  7   a , the elastic layer  7   b , and the surface layer  7   c  is limited to the foregoing materials. In the present exemplary embodiment, as described above, the intermediate transfer member includes at least a plurality of layers, and the layer on the side of the surface for bearing a toner image has a hardness lower than that of the bottommost layer on the side of the surface for not bearing a toner image. 
     In the present exemplary embodiment, the elastic intermediate transfer belt described above is used as the intermediate transfer belt  7 . Alternatively, a single-layer belt such as a resin belt may be used. 
     In the present exemplary embodiment, the photosensitive drums  1  and the intermediate transfer belt  7  are driven so that a difference in speed between the surfaces of the photosensitive drums  1  and the surface of the intermediate transfer belt  7  falls within the range of 0% to 0.5%. 
     2-7. Secondary Transfer Roller 
     The secondary transfer roller  8  is a conductive roller including a core (core material) around which an elastic layer of ion conductive foam rubber (nitrile-butadiene rubber (NBR)) is formed. The secondary transfer roller  8  has an outer diameter of 24 mm and a roller surface roughness Rz of 6.0 to 12.0 μm. The secondary transfer roller  8  also has a resistance of 1.0×10 5  to 1.0×10 8 Ω in measurement at normal temperature and normal humidity (N/N) (23° C., 50% in relative humidity (RH)) with an application of 2 kV. 
     In the present exemplary embodiment, the image forming apparatus  100  includes a secondary transfer roller contact/separation mechanism  180  (see  FIG. 4 ) for bringing the secondary transfer roller  8  into contact with the intermediate transfer belt  7  or separating the secondary transfer roller  8  from the intermediate transfer belt  7 . The secondary transfer roller  8  is thus configured to be able to selectively switch between an operating state and a non-operating state. In the operating state, the secondary transfer roller  8  is brought into contact with the intermediate transfer belt  7  and rotates with the rotation of the intermediate transfer belt  7 . In the non-operating state, the secondary transfer roller  8  is separated from the intermediate transfer belt  7 . The secondary transfer roller contact/separation mechanism  180  includes a support member for the secondary transfer roller  8  and a switching unit for moving the secondary transfer roller  8  via the support member. In the present exemplary embodiment, a solenoid is used as the switching unit. The switching unit moves the secondary transfer roller  8  vertically, i.e., selectively between a first position where the secondary transfer roller  8  is brought into contact with the intermediate transfer belt  7  and a second position where the secondary transfer roller  8  is separated from the intermediate transfer belt  7 . In the present exemplary embodiment, the secondary transfer roller  8  is separated from the intermediate transfer belt  7  when a patch passes through the secondary transfer portion T 2 . Further, in the present exemplary embodiment, in a case where the secondary transfer roller  8  has been in contact with the intermediate transfer belt  7  for two seconds or more during a period (e.g., a sheet-to-sheet interval) other than a period (sheet passing period) during which a transfer material P passes through the secondary transfer portion T 2 , the secondary transfer roller  8  immediately gets separated from the intermediate transfer belt  7 . This prevents the backside of the transfer material P from being stained by toner adhering to the secondary transfer roller  8 . 
     2-8. Belt Cleaning Device (Electrostatic Fur Cleaning) 
     In the present exemplary embodiment, the belt cleaning device  9  using an electrostatic cleaning method for removing toner in an electrostatic manner is used as the intermediate transfer member cleaning unit.  FIG. 3  is a schematic sectional view of the belt cleaning device  9  according to the present exemplary embodiment. The belt cleaning device  9  is arranged upstream of the primary transfer portions T 1  (more specifically, the most upstream primary transfer portion T 1 Y) and downstream of the secondary transfer portion T 2  in the conveyance direction of the intermediate transfer belt  7 . 
     The belt cleaning device  9  includes an upstream fur brush  91   a  and a downstream fur brush  91   b  in a housing  95 . The upstream fur brush  91   a  serves as a first collection member which is arranged on an upstream side in the conveyance direction of the intermediate transfer belt  7 . The downstream fur brush  91   b  serves as a second collection member which is arranged on a downstream side in the conveyance direction of the intermediate transfer belt  7 . The upstream and downstream fur brushes  91   a  and  91   b  make contact with the intermediate transfer belt  7  in respective positions opposed to the driving roller  71  via the intermediate transfer belt  7 , and form first and second electrostatic cleaning portions CL 1  and CL 2  for collecting toner from the intermediate transfer belt  7 . The belt cleaning device  9  further includes an upstream bias roller  92   a  and a downstream bias roller  92   b  in the housing  95 . The upstream bias roller  92   a  serves as a first voltage application member which makes contact with the upstream fur brush  91   a . The downstream bias roller  92   b  serves as a second voltage application member which makes contact with the downstream fur brush  91   b . The belt cleaning device  9  further includes an upstream blade  93   a  and a downstream blade  93   b  in the housing  95 . The upstream blade  93   a  serves as a first removal member which makes contact with the upstream bias roller  92   a . The downstream blade  93   b  serves as a second removal member which makes contact with the downstream bias roller  92   b.    
     The upstream and downstream fur brushes (cleaning brushes)  91   a  and  91   b  are electrically conductive fur brushes. In the present exemplary embodiment, the upstream and downstream fur brushes  91   a  and  91   b  have a diameter of 32 mm. The upstream and downstream bias rollers  92   a  and  92   b  are formed by metal rollers made of aluminum. In the present exemplary embodiment, the upstream and downstream bias rollers  92   a  and  92   b  have a diameter of 20 mm. The upstream and downstream blades  93   a  and  93   b  are formed by plate-like members made of urethane rubber. 
     The upstream and downstream fur brushes  91   a  and  91   b  are arranged to make sliding contact with the intermediate transfer belt  7  with an intrusion amount of approximately 1.0 mm with respect to the intermediate transfer belt  7 . The upstream and downstream fur brushes  91   a  and  91   b  are driven to rotate in a direction indicated by an arrow R 3  in  FIG. 3  at a speed (circumferential speed) of 50 mm/sec by a drive motor (not illustrated) serving as a drive unit. The moving direction indicated by the arrow R 3  is opposite to the moving direction of the intermediate transfer belt  7  in the first and second electrostatic cleaning portions CL 1  and CL 2 . The upstream and downstream bias rollers  92   a  and  92   b  are arranged with an intrusion amount of approximately 1.0 mm with respect to the upstream and downstream fur brushes  91   a  and  91   b . The upstream and downstream bias rollers  92   a  and  92   b  are driven to rotate in a direction indicated by an arrow R 4  in  FIG. 3  at a speed (circumferential speed) equivalent to that of the upstream and downstream fur brushes  91   a  and  91   b  by a drive motor (not illustrated) serving as a drive unit. The moving direction indicated by the arrow R 4  is opposite to the moving direction of the upstream and downstream fur brushes  91   a  and  91   b  in the contact portions with the upstream and downstream fur brushes  91   a  and  91   b.    
     A first cleaning power supply  94   a  serving as an application unit applies a direct-current voltage of negative polarity to the upstream bias roller  92   a  as a cleaning bias (cleaning voltage). A second cleaning power supply  94   b  serving as an application unit applies a direct-current voltage of positive polarity to the downstream bias roller  92   b  as a cleaning bias. 
     3. Patch Sensors 
     The image forming apparatus  100  according to the present exemplary embodiment includes an on-belt patch sensor  150  and an on-drum patch sensor  160  as detection units for detecting a patch. The patch refers to an adjustment toner image used in an adjustment operation of the image forming apparatus  100 . 
     4. Control Configuration 
       FIG. 4  illustrates a schematic control configuration of essential parts of the image forming apparatus  100  according to the present exemplary embodiment. The image forming apparatus  100  includes a central processing unit (CPU)  110  serving as a control unit for controlling the image forming apparatus  100  in a centralized manner, and a memory  111  serving as a storage unit. The memory  111  includes a read-only memory (ROM) and a random access memory (RAM). The RAM stores detection results of sensors and calculation results. The ROM stores a control program and a data table determined in advance. As far as the present exemplary embodiment is concerned, the CPU  110  controls an image formation control unit  112 , a charging bias control unit  113 , a primary transfer bias control unit  114 , a secondary transfer bias control unit  115 , and a cleaning bias control unit  116 . The CPU  110  also controls the on-belt patch sensor  150 , the on-drum patch sensor  160 , the belt contact/separation mechanism  170 , the secondary transfer roller contact/separation mechanism  180 , and a temperature and humidity sensor  190 . 
     The image formation control unit  112  controls exposure timing of the exposure devices  3 . The charging bias control unit  113  can output a constant voltage-controlled voltage from the charging power supply  21  to the charging rollers  2 . The primary transfer bias control unit  115  can output a constant current-controlled voltage and a constant voltage-controlled voltage from the primary transfer power supplies  51  to the primary transfer rollers  5 . The secondary transfer bias control unit  115  operates in a similar manner to the primary transfer bias control unit  114 . 
     5. Change in Volume Resistivity of Intermediate Transfer Belt 
     Next, a change in the volume resistivity of the intermediate transfer belt  7  used in the present exemplary embodiment will be described. 
     The rubber layer (elastic layer)  7   b  of the intermediate transfer belt  7  used in the present exemplary embodiment uses ion conductive CR. In other words, the intermediate transfer member according to the present exemplary embodiment contains an ion conductive agent. Ion conductive rubber materials are known to develop polarization and cause a gradual increase in the volume resistivity if a voltage continues to be applied thereto over a long period of time. In the present exemplary embodiment, the volume resistivity of the entire intermediate transfer belt  7  has also been confirmed to increase in a long period of use because of the increase in the volume resistivity of the rubber layer  7   b.    
     Portions for applying a voltage to the intermediate transfer belt  7  of the image forming apparatus  100  according to the present exemplary embodiment include the primary transfer portions T 1 , the secondary transfer portion T 2 , and the electrostatic cleaning portions CL 1  and CL 2 . In the primary transfer portions T 1 , a primary transfer current is applied to the intermediate transfer belt  7  from the photosensitive drums  1 , whereby toner images are primarily transferred onto the intermediate transfer belt  7 . In the secondary transfer portion T 2 , a current having an opposite polarity to that at the time of the primary transfer is applied to the toner images on the intermediate transfer belt  7 , whereby the toner images are transferred onto the transfer material P. In the electrostatic cleaning portions CL 1  and CL 2 , a current of the same polarity as and a current of opposite polarity to that of the primary transfer portions T 1  are applied in succession, the currents being intended to collect toner images remaining on the intermediate transfer belt  7  in electrostatic cleaning members  96   a  and  96   b  as residual transfer toner. In the case of a full color image, the four primary transfer portions T 1  and one of the electrostatic cleaning portions CL 1  and CL 2  have the polarity of increasing the volume resistivity of the intermediate transfer belt  7 . The secondary transfer portion T 2  and the other of the electrostatic cleaning portions CL 1  and CL 2  have the polarity of suppressing an increase in the volume resistivity of the intermediate transfer belt  7 . For a full color image, the voltage application in the primary transfer portions T 1  is performed four times in succession. Thus, the increase in the volume resistivity of the intermediate transfer belt  7  becomes the largest. 
       FIG. 5  illustrates a relationship between the volume resistivity of the intermediate transfer belt  7  and the number of full color images formed according to the present exemplary embodiment. The volume resistivity was measured at measurement timing which is before and after a day&#39;s use. The volume resistivity was measured by using a high resistivity meter Hiresta UPM, CP-HT450, UR probe manufactured by Mitsubishi Chemical Corporation, with a measurement condition including an applied voltage of 1000 V and an application time of 10 seconds. The initial volume resistivity was 5.0×10 9  Ω·cm. The volume resistivity is shown to increase gradually as the number of formed images increases. The volume resistivity increases with repeated ups and downs, which indicates daily variations. The volume resistivity is low before a day&#39;s use, and high after the day&#39;s use. The volume resistivity falls during the unused period before the next day&#39;s use, but is still higher than that on the previous day. This indicates that the distribution of the conductive agent polarized by the previous day&#39;s use restores during the unused period, but not completely, and that the volume resistivity is on the increase. 
     The intermediate transfer member used in the present exemplary embodiment exceeds a volume resistivity of 1.0×10 11  Ω·cm due to cumulative energization. The intermediate transfer member used in the present exemplary embodiment was found to change in volume resistivity by one digit or more if energized so that a value obtained by multiplying the amount of energization per unit area of the intermediate transfer member by the cumulative time is 30.0 A/m 2  or more. Such an intermediate transfer member can be said to be an intermediate transfer member of which resistance changes relatively largely due to image formation. 
     To examine primary transfer efficiencies at different volume resistivities, the intermediate transfer belt  7  was repeatedly used up to a volume resistivity of 1×10 12  Ω·cm, and the transfer efficiency and retransfer efficiency in primary transfer of a monochromatic solid image at a volume resistivity of 1.0×10 10  Ω·cm and 1×10 12  Ω·cm were determined. The transfer efficiency refers to a transfer rate in the primary transfer portion T 1  with the toner developed on the photosensitive drum  1  as 100%. The transfer efficiency is determined by dividing the amount of post-transfer toner by the amount of pre-transfer toner. The retransfer efficiency is determined by dividing the amount of retransferred toner on the photosensitive drum  1  by the amount of toner on the intermediate transfer belt  7  before the photosensitive drum  1  passes. The environment was 23° C. and 50% in RH. The density of the solid image was 0.5 mg/cm 2 . The electric resistance of the primary transfer roller  5  used was 2.0×10 6 Ω. 
       FIG. 6  illustrates the primary transfer efficiency of the intermediate transfer belt  7  at a volume resistivity of 1.0×10 10  Ω·cm and 1×10 12  Ω·cm. As illustrated in  FIG. 6 , an optimum transfer current (required current) for both the transfer efficiency and the retransfer efficiency decreases as the volume resistivity of the intermediate transfer belt  7  increases. The optimum transfer current refers to a current value that is well balanced so that the transfer efficiency is high and the retransfer efficiency is low. For example, the graph illustrated in  FIG. 6  shows that if the intermediate transfer belt  7  has a volume resistivity of 1.0×10 10  Ω·cm, the optimum transfer current is 50 μA. The graph illustrated in  FIG. 6  also shows that if the intermediate transfer belt  7  has a volume resistivity of 1.0×10 12  Ω·cm, the optimum transfer current is 33 μA. 
       FIG. 7  illustrates plots of the optimum transfer current obtained by similarly examining the transfer and retransfer efficiencies of the intermediate transfer belt  7  at other volume resistivities.  FIG. 7  shows that the lower the volume resistivity, the higher the optimum current value, and that the higher the volume resistivity, the lower the optimum current value. A possible reason for this is as follows: If the resistance of the primary transfer portion T 1  is low, areas with toner and without toner have a difference in resistance which corresponds to the toner in the primary transfer. Due to the difference in resistance, a current is less likely to flow through the area with toner, and more likely to flow through the area without toner. Consequently, the transfer field in the area where there is actually toner becomes relatively low, and the required current becomes high. In contrast, if the resistance of the primary transfer portion T 1  is high, the transfer field is uniform regardless of the presence or absence of toner, and the required current becomes low.  FIG. 7  shows that the optimum current value changes in the range of the volume resistivity of the intermediate transfer belt  7  from 5×10 9  Ω·cm to 1×10 12  Ω·cm, and remains almost the same in the range of the volume resistivity of the intermediate transfer belt  7  from 1×10 12  Ω·cm to 1×10 14  Ω·cm. The possible reason is that, with the volume resistivity of the intermediate transfer belt  7  less than 1×10 12  Ω·cm, a current difference occurs between the areas with toner and without toner, and with the volume resistivity of 1×10 12  Ω·cm or higher, such a current difference disappears. 
     Suppose, for example, that the optimum current according to the initial volume resistivity of the intermediate transfer belt  7  continues to be used with an increase in the amount of use of the intermediate transfer belt  7 . In such a case, the transfer efficiency decreases and the retransfer efficiency increases with an increase in the amount of use. This not only causes a loss of the transfer efficiency but also causes a void. More specifically, if there are toner layers arranged with a gap therebetween like a halftone image, an excessive current causes a discharge in the gap. This reverses the triboelectricity (electrification charge) of the toner, and the toner fails to be primarily transferred and returns to the photosensitive drum  1 . Such a phenomenon is referred to as a void. In addition, passing an excessive current through the primary transfer portion T 1  accelerates the degree of resistance increase of the members of which a resistance increase can occur due to energization, such as the intermediate transfer belt  7  and the primary transfer roller  5 . If the members such as the intermediate transfer belt  7  and the primary transfer roller  5  reach a certain resistance value (or the amount of use with which the members are predicted to usually reach the certain resistance value), the members are typically replaced, while being considered to have expired their parts life. The acceleration of the degree of resistance increase shortens the parts life. Adjusting the primary transfer current to an optimum current value according to the electric resistance of the primary transfer portion T 1  can keep the transfer efficiency high and the retransfer efficiency low regardless of an increase in the amount of use of the intermediate transfer belt  7 , and can also suppress a resistance increase due to unnecessary energization. 
     However, if the primary transfer current is uniformly set to the optimum current according to the electric resistance of the primary transfer portion T 1 , the following problem can occur. 
     A high primary transfer current needs to be set in a region where the volume resistivity is low, whereas the high primary transfer current may cause a memory in the photosensitive drum  1 . If an excessive primary transfer current is applied to the photosensitive drum  1 , a memory occurs in the photosensitive drum  1 , making it difficult for the charging roller  2  serving as a charging unit to charge the photosensitive drum  1  to a desired potential. The area in the photosensitive drum  1  which undergoes the excessive transfer bias may only be able to be charged to a value lower than a desired charging potential. If such a photosensitive drum  1  is exposed by the exposure device  3 , the exposure potential may become uneven, resulting in the occurrence of uneven development. In the present exemplary embodiment, if the initial volume resistivity of the intermediate transfer belt  7  is 5×10 9  Ω·cm, the optimum current value determined based on the foregoing transfer and retransfer efficiencies is 60 μA. It was found that the passing of a current of 60 μA or higher caused an excessive primary transfer current, resulting in a memory in the photosensitive drum  1 . 
     The graph illustrated in  FIG. 7  indicates that the optimum current value varies little in the region where the volume resistivity is high, more specifically, the volume resistivity is 1×10 12  Ω·cm or higher, as described above. Despite this, if the primary transfer current is set in a substantially linear manner according to the volume resistivity as is the case with 1×10 12  Ω·cm or lower, the primary transfer current value is reduced more than necessary. If the resulting primary transfer current is 25 μA or lower, a transfer defect image occurs due to an insufficient transfer current. 
     Thus, in the present exemplary embodiment, if the volume resistivity is 1×10 10  Ω·cm or lower, the primary transfer current value is controlled to be constant. If the volume resistivity is higher than 1×10 10  Ω·cm and lower than 1×10 12  Ω·cm, the primary transfer current is variably controlled. If the volume resistivity is 1×10 12  Ω·cm or higher, the primary transfer current value is controlled to be constant. Such control can provide favorable images. The volume resistivity of 1×10 10  Ω·cm corresponds to a threshold value Vc 1  to be described below. The volume resistivity of 1×10 12  Ω·cm corresponds to a threshold value Vc 2  to be described below. In actual control, as will be described in detail below, a transfer contrast voltage value is regarded as the electric resistance of the primary transfer portion T 1 . According to this transfer contrast voltage value, the regions where the primary transfer current value is to be variably controlled and where the primary transfer current value is to be controlled to be constant are determined. 
     6. Charge Amount of Intermediate Transfer Belt During Image Formation 
     Next, a change in the charge amount of the intermediate transfer belt  7  used in the present exemplary embodiment during image formation will be described. 
     As described above, in the image forming apparatus  100  according to the present exemplary embodiment, primary transfer currents are applied in the primary transfer portions T 1  to transfer toner images onto the intermediate transfer belt  7 . In the secondary transfer portion T 2 , a current of opposite polarity to that of the primary transfer portions T 1  is applied to the toner images on the intermediate transfer belt  7 , whereby the toner images are transferred onto the transfer material P. Toner images remaining on the intermediate transfer belt  7  are conveyed as transfer residual toner to the electrostatic cleaning portions CL 1  and CL 2 , where a current of the same polarity as and a current of opposite polarity to that of the primary transfer portions T 1  are successively applied to the transfer residual toner. In the case of a full color image, the electric current application in the primary transfer portions T 1  is performed four times in succession. 
     The charge amount of the intermediate transfer belt  7  is associated with the amounts of currents applied in the four primary transfer portions T 1 , the secondary transfer portion T 2 , and the electrostatic cleaning portions CL 1  and CL 2 , and the amount of charge eliminated by the grounded stretching rollers for the intermediate transfer belt  7 . Moreover, the higher the process speed is and the smaller the distance between the primary transfer portions T 1  of the adjoining image forming units S in the conveyance direction of the intermediate transfer belt  7  is, the greater the charge amount of the intermediate transfer belt  7  becomes as continuous image formation continues. The reason is that the intermediate transfer belt  7  charged in a preceding step (for example, in the primary transfer portion T 1 Y of the first image forming unit SY) proceeds to the next step (for example, the primary transfer portion T 1 C of the second image forming unit SC) without being electrically discharged, whereby the charge amounts are superposed. Further, the higher the volume resistivity of the intermediate transfer belt  7 , the greater the charge amount of the intermediate transfer belt  7  during image formation. 
     As the charge amount of the intermediate transfer belt  7  increases, the voltages required to be applied in the primary transfer portions T 1  increase by as much as the charge held by the intermediate transfer belt  7 . Therefore, it is desirable that the voltages to be applied in the primary transfer portions T 1  be adjusted to an optimum voltage value according to the charge amount of the intermediate transfer belt  7 . 
     7. Voltage-Current Characteristic in Primary Transfer Portions 
     A voltage-current characteristic in the primary transfer portions T 1  according to the present exemplary embodiment will be described. In the following description, the first, second, third, and fourth image forming units SY, SM, SC, and SK may be referred to as “Y station,” “M station,” “C station,” and “K station,” respectively. 
       FIG. 8  illustrates changes in the primary transfer voltages at the Y, M, and C stations when the intermediate transfer belt  7  has a volume resistivity of 5.0×10 10  Ω·cm. None of the Y, M, and C stations shows a change in the primary transfer voltage even during continuous image formation. 
       FIG. 9  illustrates changes in the primary transfer voltages at the Y, M, and C stations when the intermediate transfer belt  7  has a volume resistivity of 1.0×10 12  Ω·cm. A first characteristic observed in this case is that a difference occurs between the primary transfer voltage at the Y station where the primary transfer is performed first and the primary transfer voltage at the C station where the primary transfer is performed third. A second characteristic is that the primary transfer voltages rise gradually while continuous image formation is performed. 
       FIG. 10  illustrates a relationship between the actually obtained volume resistivity of the intermediate transfer belt  7  and the amount of rise (degree of rise) in voltage at the C station.  FIG. 10  indicates that the primary transfer voltage during continuous sheet passing rises sharply if the volume resistivity of the intermediate transfer belt  7  exceeds 1.0×10 11  Ω·cm. An actual attenuation of the electrification charge of the intermediate transfer belt  7  between the primary transfer portions T 1  of the adjoining image forming units S was calculated to determine a time constant. Assume that the intermediate transfer belt  7  has a constant permittivity s. In the present exemplary embodiment, the distance between the primary transfer portions T 1  of the adjoining image forming units S is 120 mm, and the process speed is 350 mm/s (which corresponds to the circumferential speed of the intermediate transfer belt  7 ). If the voltage accumulated in the intermediate transfer belt  7  immediately after primary transfer is 100 V and the volume resistivity ρv of the intermediate transfer belt  7  is 9.0×10 10  Ω·cm or lower, the accumulated voltage attenuates to substantially 0 V in the primary transfer portion T 1  of the next image forming unit S. However, if the volume resistivity ρv of the intermediate transfer belt  7  is 1.0×10 11  Ω·cm, the accumulated voltage attenuates to approximately 20 V. If the volume resistivity ρv is 1.0×10 12  Ω·cm, the accumulated voltage attenuates to approximately 50 V. If the volume resistivity ρv is 1.0×10 13  Ω·cm, the accumulated voltage attenuates to 92 V. This indicates that the attenuation amount of the electrification charge of the intermediate transfer belt  7  decreases sharply as the volume resistivity of the intermediate transfer belt  7  increases. In particular, if the volume resistivity ρv of the intermediate transfer belt  7  exceeds 1.0×10 11  Ω·cm, self attenuation becomes impossible. For example, if continuous image formation is performed in the full color mode, the electric resistances of the primary transfer portions T 1  vary while several hundreds of images are continuously formed. For example, the optimum currents change in several minutes if the process speed is 80 sheets/min. 
     8. Control of Primary Transfer Voltages 
     8-1. Overview 
     In the present exemplary embodiment, the image forming apparatus  100  is capable of performing two types of control (described below) as a method for determining the primary transfer voltages. 
     One control includes applying a voltage to each of the primary transfer portions T 1  and detecting a current value, detecting a voltage value when passing a certain value of current, and determining a required primary transfer voltage based on the voltage-current characteristic before starting image formation (hereinafter, such control is referred to as “normal ATVC”). In the present exemplary embodiment, the normal ATVC is performed during a pre-rotation operation which is a preparatory operation before the image formation. 
     The other control includes performing similar detection and feedback to those in the normal ATVC during an interval between sheets and correcting the primary transfer voltage to maintain an optimum primary transfer voltage while continuous image formation is performed on a plurality of transfer materials P (hereinafter, such control is referred to as “sheet-to-sheet ATVC”). The sheet-to-sheet ATVC can be performed in any region (timing) other than a region (timing) where a toner image is formed on the surface of a transfer material P. 
     Assume that the voltage applied to the primary transfer portion T 1  is V 1 , the potential of the photosensitive drum  1  is Vd, and the current flowing through the primary transfer portion T 1  is I 1 . The primary transfer current I 1  flows due to a potential difference Vc (=V 1 −Vd) (hereinafter referred to as a “transfer contrast voltage”) between the primary transfer portion T 1  and the photosensitive drum  1 . The electric resistance of the primary transfer portion T 1  can thus be expressed as (V 1 −Vd)/I 1 . The transfer contrast voltage Vc is employed here because all the electric resistances of the potentials of the intermediate transfer belt  7 , the primary transfer roller  5 , and the photosensitive drum  1  that are applied to the primary transfer portion T 1  contribute to the transfer of toner. 
     In the present exemplary embodiment, both the normal ATVC and the sheet-to-sheet ATVC are performed by the CPU  110 . 
     8-2. Normal ATVC 
     First, the normal ATVC will be described with reference to the block diagram illustrated in  FIG. 4 , the flowchart illustrated in  FIG. 11 , and the table illustrated in Table 1. Table 1 illustrates a table for determining optimum primary transfer current values according to transfer contrast voltages Vc in each environment examined in advance. In the present exemplary embodiment, the temperature and humidity sensor (environment sensor)  190  serving as an environment detection unit obtains a relative humidity (hereinafter referred to as an “ambient humidity” or simply as a “humidity”) based on the amount of moisture on the developing device  4  as the amount of moisture in the apparatus main body and a temperature outside the apparatus main body. In the table illustrated in Table 1, the optimum primary transfer current values are determined and set for each ambient humidity. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                   
                 Environmental category 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Humidity 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 5% 
                 10% 
                 25% 
                 37% 
                 47% 
                 57% 
                 67% 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Target current Ic1 [μA]  
                 Full  
                 Y 
                 50.0 
                 50.0 
                 50.0 
                 50.0 
                 45.0 
                 40.0 
                 40.0 
               
               
                 (Target current  
                 color  
                 M 
                 50.0 
                 50.0 
                 50.0 
                 50.0 
                 45.0 
                 40.0 
                 40.0 
               
               
                 corresponding to 
                 mode 
                 C 
                 50.0 
                 50.0 
                 50.0 
                 50.0 
                 45.0 
                 40.0 
                 40.0 
               
               
                 target current change 
                   
                 K 
                 55.0 
                 55.0 
                 55.0 
                 55.0 
                 50.0 
                 45.0 
                 45.0 
               
               
                 transfer contrast  
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 voltage value Vc1 and lower) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Target current Ic2 [μA] 
                 Full  
                 Y 
                 37.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
               
               
                 (Target current  
                 color  
                 M 
                 37.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
               
               
                 corresponding to target  
                 mode 
                 C 
                 37.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
                 33.0 
               
               
                 current change transfer 
                   
                 K 
                 42.0 
                 42.0 
                 42.0 
                 42.0 
                 41.0 
                 40.0 
                 40.0 
               
               
                 contrast voltage value Vc2  
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 and higher)  
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Target current change transfer  
                 Full  
                 Y 
                 2400 
                 2359 
                 2323 
                 2285 
                 2278 
                 2188 
                 2150 
               
               
                 contrast voltage value [V]  
                 color  
                 M 
                 2400 
                 2359 
                 2323 
                 2285 
                 2278 
                 2188 
                 2150 
               
               
                 (Vc1 = V1 − Vd) 
                 mode 
                 C 
                 2400 
                 2359 
                 2323 
                 2285 
                 2278 
                 2188 
                 2150 
               
               
                   
                   
                 K 
                 2300 
                 2259 
                 2223 
                 2185 
                 2178 
                 2088 
                 2050 
               
               
                 Target current change transfer  
                 Full  
                 Y 
                 3800 
                 3759 
                 3723 
                 3685 
                 3678 
                 3588 
                 3550 
               
               
                 contrast voltage value [V]  
                 color  
                 M 
                 3800 
                 3759 
                 3723 
                 3685 
                 3678 
                 3588 
                 3550 
               
               
                 (Vc2 = V2 − Vd) 
                 mode 
                 C 
                 3800 
                 3759 
                 3723 
                 3685 
                 3678 
                 3588 
                 3550 
               
               
                   
                   
                 K 
                 3400 
                 3359 
                 3323 
                 3285 
                 3278 
                 3188 
                 3150 
               
               
                   
               
            
           
         
       
     
     In step S 101 , the CPU  110  starts the normal ATVC. In step S 102 , the CPU  110  charges the photosensitive drums  1  by using the charging rollers  2  so that the photosensitive drums  1  have a predetermined potential. In step S 103 , the CPU  110  determines target currents I 0  from the table illustrated in Table 1 based on the transfer contrast voltages applied just before the control and the ambient humidity during execution of the control. 
     As illustrated in Table 1, the target currents I 0  in a region where the transfer contrast voltage value is from the initial value up to Vc 1  (that is, a region where the transfer contrast voltage value is Vc 1  or lower) are denoted by Ic 1 . The target currents I 0  in a region where the transfer contrast voltage value is Vc 2  and over (that is, a region where the transfer contrast voltage value is Vc 2  or higher) are denoted by Ic 2 . If the transfer contrast voltage value falls between Vc 1  and Vc 2  (that is, the transfer contrast voltage value is higher than Vc 1  and lower than Vc 2 ), the target currents I 0  are determined by linear interpolation of the voltage-current characteristic of Vc 1  and Ic 1  and that of Vc 2  and Ic 2 . For the purpose of the subsequent description, the region where the transfer contrast voltage value is from the initial value up to Vc 1  is referred to as a “region  1 ”, the region where the value is over Vc 1  up to (Vc 1 +Vc 2 )/2 as a “region  2 ”, the region where the value is over (Vc 1 +Vc 2 )/2 and under Vc 2  as a “region  3 ,” and the region where the value is Vc 2  and over as a “region  4 ”. 
     In step S 104 , a constant current control high-voltage substrate of the primary transfer bias control unit  114  outputs constant current voltages so that the constant target currents I 0  flow. In step S 105 , a voltage detection circuit of the primary transfer bias control unit  114  detects the values of the applied voltages for a single turn of the primary transfer rollers  5 , and determines and stores the averages of the values (initial voltages V 0 ) into the memory  111 . In step S 106 , the CPU  110  determines difference voltages ΔVx to be used in the next step from the detected initial voltages V 0 . The suffix “x” of the difference voltages ΔVx indicates the number of the corresponding region. In the case of the region  1 , the difference voltages are denoted by ΔV 1 . In the case of the region  2 , the difference voltages are denoted by ΔV 2 . The difference voltages ΔVx are determined so as to be ΔV 1 =ΔV in the region  1 , ΔV 2 =ΔV×3 in the region  2 , ΔV 3 =ΔV×4 in the region  3 , and ΔV 4 =ΔV×6 in the region  4 . 
     In step S 107 , voltages V 1  (=V 0 −ΔVx) obtained by subtracting the difference voltages ΔVx from the initial voltages V 0  are applied to the respective primary transfer rollers  5  for a single turn. At this time, a current value detection circuit of the primary transfer bias control unit  114  detects the values of the currents flowing through the primary transfer rollers  5 , and determines and stores the averages of the values into the memory  111 . In step S 108 , voltages V 2  (=V 0 +ΔVx) obtained by adding the difference voltages ΔVx to the initial voltages V 0  are applied to the respective primary transfer rollers  5  for a single turn. The current value detection circuit of the primary transfer bias control unit  114  detects the values of the currents flowing through the primary transfer rollers  5 , and determines and stores the averages of the values into the memory  111 . The average current values with the application of V 1  are denoted by I 1 . The average current values with the application of V 2  are denoted by I 2 . 
     In step S 109 , the CPU  110  determines final target currents It in the current normal ATVC control based on the initial voltages V 0  determined in step S 105  and the table illustrated in Table 1. In step S 110 , the CPU  110  calculates voltage values Vt for the final target currents It determined by the current normal ATVC control, based on the obtained linear expressions of the relationship of V 1 , V 2 , I 1 , and I 2  (voltage-current characteristic). In such a manner, the primary transfer voltages to be applied in constant voltage control during the subsequent image formation are determined. The target currents It serve as target current values during correction at an interval between sheets to be described below. 
     The CPU  110  stores the determined target currents It and target voltage values Vt of the image forming units SY, SM, SC, and SK as backup values It 1 , It 2 , It 3 , and It 4 , and Vt 1 , Vt 2 , Vt 3 , and Vt 4 , respectively. The CPU  110  applies the target voltages Vt 1 , Vt 2 , Vt 3 , and Vt 4  as the primary transfer voltages when the image formation is started. 
     As described above, ΔVx is changed region by region, or more specifically, changed by multiplication of a coefficient according to the electric resistance of the primary transfer portion T 1 . The reason is that it is considered that the gradient of the voltage-current (V-I) curve tends to decrease as the electric resistance increases. As a result, the linear interpolation of V 1 , V 2 , I 1 , and I 2  can be calculated without a reduction in accuracy. 
     8-3. Sheet-to-Sheet ATVC 
     Next, the sheet-to-sheet ATVC will be described. When image formation is continuously performed, the primary transfer voltages may gradually change. For example, if the outside air temperature is low, the ion-conductive intermediate transfer belt  7  has a high volume resistivity. However, if the main body of the image forming apparatus  100  is powered on and the temperature of the fixing device  11  increases and/or motors are actuated, the temperature inside the apparatus main body gradually rises and the volume resistivity of the intermediate transfer belt  7  decreases accordingly. Therefore, the application voltages Vt required to pass optimum current values It become lower than the application voltages Vt initially determined. Meanwhile, the ion-conductive intermediate transfer belt  7  increases in electric resistance if continuously energized for a long period of time. Further, in the present exemplary embodiment, as described above, a volume resistivity exceeding 1.0×10 12  Ω·cm makes self attenuation impossible. For example, if continuous sheet passing is performed in the full color mode, the electric resistances of the primary transfer portions T 1  vary in the course of forming several hundreds of images. If the image formation is continuously performed after the stabilization of the temperature inside the apparatus main body, the application voltages Vt required to pass optimum current values It increase with the increasing number of formed images. In such a case, the optimum current values It are not able to be obtained unless the application voltages Vt are increased. As described above, the charge amounts of the primary transfer portions T 1  fail to attenuate and increase between the primary transfer portions T 1  of the adjoining image forming units S. The application voltages Vt therefore need to be corrected during the image formation by the image forming units S. 
     In the present exemplary embodiment, the CPU  110  applies a voltage to each of the primary transfer portions T 1  and detects a current value at a timing (interval between sheets in the present exemplary embodiment) when none of images to be formed on transfer materials P is formed. If the current value deviates from an optimum current value by a predetermined value or more, the CPU  110  performs control to add or subtract a predetermined correction voltage (correction amount) ΔVt. 
       FIG. 12  illustrates the flowchart of the sheet-to-sheet ATVC. In step S 201 , the CPU  110  starts the sheet-to-sheet ATVC. In step S 202 , the CPU  110  stores, into the memory  111 , a current value I N  detected at an interval of a predetermined number N of sheets. In steps S 203  and S 204 , the CPU  110  determines whether the detected current value I N  is higher than or lower than the target current It currently backed up. In steps S 205  and S 206 , the CPU  110  corrects Vt into V N  by adding or subtracting the correction amount ΔVt. For example, if I N  is smaller than It (YES in step S 203 ), then in step S 205 , the CPU  110  adds the correction amount ΔVt to Vt to determine the corrected voltage value V N . If I N  is greater than It (YES in step S 204 ), then in step S 206 , the CPU  110  subtracts the correction amount ΔVt from Vt to determine the corrected voltage value V N . 
     The value of X in steps S 203  and S 204  may be zero. Alternatively, a predetermined numerical value may be set to X so that the correction is not performed within the range of the target current It±X pA. 
     In step S 207 , the CPU  110  determines whether the value obtained by dividing the corrected transfer contrast voltage by the detected current value I N  falls within a range where variable control needs to be performed on the target current It determined based on the table illustrated in Table 1. If the value is determined to fall within the region where the variable control needs to be performed (YES in step S 207 ), then in step S 208 , the CPU  110  calculates, changes, and determines the target current It based on the transfer contrast voltage V N −Vd and the linear interpolation of (Vc 1 ,Ic 1 ) and (Vc 2 ,Ic 2 ) on the table illustrated in Table 1. If the value is determined to not fall within the range where the variable control needs to be performed in step S 207  (NO in step S 207 ), then in step S 209 , the CPU  110  determines the target current It to be Ic 1  or Ic 2  according to the value of the transfer contrast voltage V N −Vd. 
     The CPU  110  switches to the primary transfer voltage value changed and determined by the sheet-to-sheet ATVC between the Nth and (N+1)th sheets. The CPU  110  similarly performs the next sheet-to-sheet ATVC by using detected current values I N  at the (N+2)th to (2N+2)th sheets. The CPU  110  continues the correction until the continuous image formation ends. The interval of the number of sheets N has only to be such that a value equivalent to an average value of the detected current values I N  during normal image formation can be monitored. In the present exemplary embodiment, N is set to five. The current value I N  is detected at eight points between sheets. An average value of a total of 40 points is used. 
     The CPU  110  may perform the normal ATVC by interrupt control during an interval between sheets after a predetermined number of sheets, e.g., after 400 A4-equivalent sheets are passed. However, in the present exemplary embodiment, the CPU  110  performs the normal ATVC during an interval between sheets in an interrupt manner if the detected current value I N  of the sheet-to-sheet ATVC exceeds the range of the target current value It±5 μA. This can immediately restore the target current value It if the actually-applied current value deviates from the target current value It. This can also preclude unnecessary normal ATVC if the actually-applied current value does not deviate from the target current value It. As a result, a desired target current value It can be obtained without a reduction in productivity. The interruption timing of the normal ATVC and other types of control (for example, image density adjustment control and color shift correction control) may be performed in a synchronized manner. The result in the sheet-to-sheet ATVC can thus be reflected in the determination of the execution timing of the normal ATVC. 
     As described above, the transfer contrast voltage applied just before the normal ATVC in the processing of step S 103  in the flow of the normal ATVC illustrated in  FIG. 11  is the transfer contrast voltage corrected by the sheet-to-sheet ATVC in a case where image formation is performed just before the normal ATVC. Thus, using the transfer contrast voltage applied just before the normal ATVC enables interpolation calculation from a V-I curve that is closer to the target current value It in the processing of step S 110  in the flow of the normal ATVC illustrated in FIG.  11 , whereby the primary transfer voltage can be accurately determined. 
     As described above, according to the present exemplary embodiment, in the normal ATVC, the target current value It and the application voltage value are determined regardless of the region of the transfer contrast voltage value. 
     In contrast, in the sheet-to-sheet ATVC, the target current value It is variably controlled and the application voltage is corrected to the target current value It only in the region where the transfer contrast voltage value is higher than Vc 1  and lower than Vc 2 . This region corresponds to the range where the volume resistivity of the intermediate transfer belt  7  is higher than 1×10 10  Ω·cm and lower than 1×10 12  Ω·cm. In the region, the intermediate transfer belt  7  cannot attenuate by itself and a charge-up accelerates. The target current value It thus needs to be variably controlled by the sheet-to-sheet ATVC during continuous image formation so as to be controlled to an optimum value when needed. 
     In the sheet-to-sheet ATVC, in the region where the transfer contrast voltage value is Vc 1  or lower, the application voltage is corrected to the target current value It determined by the normal ATVC but the target current value It is not changed. This region corresponds to the range where the volume resistivity of the intermediate transfer belt  7  is 1×10 10  Ω·cm or lower. In the region, a memory is prevented from occurring in the photosensitive drum  1  due to the target current value It being set too high by variably controlling the target current value It according to the electric resistance of the primary transfer portion T 1  during continuous image formation. 
     In the sheet-to-sheet ATVC, in the region where the transfer contrast voltage value is Vc 2  or higher, the application voltage is corrected to the target current value It determined by the normal ATVC but the target current value It is not changed. This region corresponds to the range where the volume resistivity of the intermediate transfer belt  7  is 1×10 12  Ω·cm or higher. In the region, a defect due to an insufficient transfer current such as deteriorating graininess is prevented from occurring due to the target current value IT being set too low by variably controlling the target current value IT according to the electric resistance of the primary transfer portion T 1  during continuous image formation. 
     As described above, in the present exemplary embodiment, the image forming apparatus  100  includes the detection unit (the primary transfer bias control unit in the present exemplary embodiment)  114  that detects the values (transfer contrast voltages) correlated with the electric resistances of the primary transfer portions T 1 . The image forming apparatus  100  also includes the control unit (the CPU in the present exemplary embodiment)  110  that controls the voltages (primary transfer voltages) to be applied to the primary transfer members  5  for primary transfer in continuous image formation on a plurality of transfer materials P, before performing the continuous image formation. The image forming apparatus  100  further includes the correction unit (the CPU in the present exemplary embodiment)  110  that corrects the primary transfer voltages while performing the continuous image formation on the plurality of transfer materials P. The control unit  110  determines, based on the detection results by the detection unit  114 , the target values of the currents (target current values) to be supplied to the primary transfer portions  5  for the primary transfer in the continuous image formation, and then controls the primary transfer voltages according to the target values. The correction unit  110  is also capable of performing the following first and second modes. In the first mode, the correction unit  110  controls the primary transfer voltages according to the target values determined by the control unit  110 . In the second mode, the correction unit  110  changes the target values determined by the control unit  110  based on the detection results by the detection unit  114 , and controls the primary transfer voltages according to the changed target values. 
     The correction unit  110  selectively performs the first mode and the second mode in the following manner. If the electric resistances indicated by the detection results of the detection unit  114  are a first value or lower, or a second value, which is higher than the first value, or higher, the correction unit  110  performs the first mode. On the other hand, if the electric resistances indicated by the detection results of the detection unit  114  are higher than the first value and lower than the second value, the correction unit  110  performs the second mode. In particular, in the present exemplary embodiment, the correction unit  110  performs voltage correction if a difference between a target value and the value of the current being supplied to the primary transfer portion T 1  exceeds a predetermined range. The control unit  110  can determine the execution timing of the voltage control based on the result of a comparison by the correction unit  110  between the target value and the value of the current being supplied to the primary transfer portion T 1 . In such a case, if the correction unit  110  detects that the difference between the target value and the value of the current being supplied to the primary transfer portion T 1  exceeds the predetermined range, the correction unit  110  can perform the voltage control by interrupt control. 
     As described above, according to the present exemplary embodiment, even if the electric resistance of the intermediate transfer belt  7  changes during continuous image formation, appropriate primary transfer current values can be accordingly supplied to maintain favorable transferability. 
     Up to this point, a specific exemplary embodiment of the present invention has been described. However, the present invention is not limited to the foregoing exemplary embodiment. 
     For example, in the foregoing exemplary embodiment, the normal ATVC is performed during a pre-rotation operation, and the sheet-to-sheet ATVC is performed at an interval between sheets. However, the normal ATVC may be performed in any other timing during a non-image forming operation other than the image forming operation during which an output image to be transferred and output to a transfer material P is being formed. The image forming operation refers to a period in which formation of an electromagnetic latent image, development, primary transfer, and secondary transfer are performed for an output image. The non-image forming operation refers to any other period. Examples of the non-image forming operation include a pre-multi-rotation operation, a pre-rotation operation, a sheet-to-sheet operation, and a post-rotation operation. The pre-multi-rotation operation is a preparatory operation performed upon power-on of the image forming apparatus  100 . The pre-rotation operation is a preparatory operation between when an image formation start instruction is input and when the image formation is actually started. The sheet-to-sheet operation corresponds to an interval between one transfer material P and another when forming images on a plurality of transfer materials P. The post-rotation operation is an arrangement operation (preparatory operation) after the end of the image formation. For example, if a plurality of jobs (a series of image forming operations on one or a plurality of transfer materials P by a single image formation start instruction) is on standby, the sheet-to-sheet ATVC can be performed during the post-rotation operation after a job and before the next job. Furthermore, performing the normal ATVC before continuous image formation on a plurality of transfer materials P refers not only to performing the normal ATVC before the job of the continuous image formation. For example, in the case of interrupting a job to perform the normal ATVC by interrupt control, the normal ATVC is performed before the continuous image formation of the job that is resumed after the end of the normal ATVC. 
     The primary transfer members and the secondary transfer member are not limited to roller-shaped ones. Transfer members of any configuration may be used. Examples include plate-like (blade-like), sheet-like, brush-like, and block-like ones that are arranged to make contact with and frictionally slide over the moving intermediate transfer member. 
     In the foregoing exemplary embodiment, the intermediate transfer member has been described as an intermediate transfer belt  7  constituting an endless belt. However, the intermediate transfer member is not limited thereto. For example, the intermediate transfer member may be an intermediate transfer drum having a drum shape formed by stretching a sheet, which is made of similar materials to those of the intermediate transfer belt  7  according to the foregoing exemplary embodiment, over a frame body. 
     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. 2014-107613, filed May 23, 2014, which is hereby incorporated by reference herein in its entirety.