Patent Publication Number: US-6341205-B1

Title: Image forming apparatus with cleaning bias feature

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus using an electrophotographic system, such as a copying machine or a laser printer, in which an image is formed by secondarily transferring a toner image to a recording medium from an intermediate transfer body on which the toner image has been transferred, and particularly to a cleaning technique of a transfer part in a secondary transfer portion. 
     2. Description of the Related Art 
     Conventionally, as an image forming apparatus using an electrophotographic system, such as the copying machine or laser printer, there is an image forming apparatus constructed such that toner images of yellow, magenta, cyan, black, etc. sequentially formed on a photoreceptor drum are transferred onto an intermediate transfer belt in a state where they overlap with each other, the multiple toner images of the respective colors transferred onto the intermediate transfer belt are transferred to a recording medium at the same time by pressing force and electrostatic attraction force of a backup roll and a secondary transfer roll, and then, the unfixed toner images of the respective colors are fixed onto the recording medium by a fixing device, whereby a color image is formed. 
     In the image forming apparatus, the secondary transfer roll is brought into press contact with the intermediate transfer belt on which the toner images of the respective colors have been transferred, and the multiple transferred toner images on the intermediate transfer belt are secondarily transferred onto the recording medium, so that the color image is formed. Thus, in the image forming apparatus, when the toner image transferred onto the intermediate transfer belt comes in contact with the secondary transfer roll, the toner is shifted to the secondary transfer roll by the pressing force, and the toner sticking to the secondary transfer roll adheres to the rear surface of the recording medium next conveyed to a secondary transfer position, which becomes a cause of rear surface stain of the recording medium. 
     For the purpose of preventing such rear surface stain of a recording medium caused by adhesion of toner to a secondary transfer roll, in an image forming apparatus using a transfer roll including a secondary transfer roll, a technique of cleaning the secondary transfer roll or the like has been already proposed in Japanese Patent Unexamined Publication No. Hei. 8-272235, No. Hei. 8-328401, and No. Hei. 9-6146. 
     An image forming apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 8-272235 includes an image carrier which electrostatically holds a toner image, a transfer member which comes in contact with the surface of the image carrier and applies a transfer bias, and a bias application part which sequentially applies bias currents having different polarities to the transfer member when a transfer material does not exist at a transfer position. In the image forming apparatus, the bias application part applies the same polarity current having the same polarity as the toner constituting the toner image, and then, applies the opposite polarity current having the polarity opposite to the toner and having a current value of an absolute value not lower than an absolute value of the same polarity current. 
     An image forming apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 8-328401 includes an image carrier which electrostatically holds a toner image, a transfer member which comes in contact with the surface of the image carrier and applies a transfer bias, and a bias application part which applies the transfer bias to the transfer member when a transfer material passes between the image carrier and the transfer member so as to transfer the toner image formed on the image carrier to the transfer material, and applies a current having a polarity opposite to the charged polarity of the toner to the transfer member after applying a current having the same polarity as the charged polarity of the toner at the time of cleaning when the transfer material does not exist at a transfer position. In the image forming apparatus, a bias having the same polarity as the transfer bias is applied to the transfer member at least in the period from the start of first image formation immediately after the cleaning to the application of the transfer bias. 
     An image forming apparatus disclosed in Japanese Patent Unexamined Publication No. Hei. 9-6146 includes a photoreceptor on which a toner image corresponding to an image to be formed is formed, a primary transfer member which transfers the toner image on the photoreceptor onto an intermediate transfer body, a secondary transfer member which is provided to be freely pressed on and separated from the intermediate transfer body and transfers the toner image on the intermediate transfer body onto a recording sheet, and an intermediate transfer body driving part which rotates and drives the intermediate transfer body. The image forming apparatus further includes a cleaner blade which is provided to be freely pressed on and separated from the intermediate transfer body and removes the toner image on the intermediate transfer body, a secondary transfer member holding part which holds the secondary transfer member while selectively changing a press contact state and a separating state to the intermediate transfer body, a cleaner blade holding part which holds the cleaner blade while selectively changing a press contact state and a separating state to the intermediate transfer body, a paper jam detection part which detects a paper jam in a recording sheet conveying passage, a paper jam release detection part which detects that a paper jam state is released, a voltage application part which applies a voltage to the secondary transfer member, and a control part which controls the operation of the respective holding parts, the driving part, and the voltage application part on the basis of detection signals by the respective detection parts. The control part performs such control that after the release of the paper jam state is detected, the cleaner blade is held in the press contact state to the intermediate transfer body, the secondary transfer member is held in the separating state to the intermediate transfer body, cleaning on the intermediate transfer body is performed while the intermediate transfer body is caused to make at least one rotation, the secondary transfer member is held while the press contact state to the intermediate transfer body is changed after the intermediate transfer body is cleaned, a voltage is applied to the secondary transfer member, and cleaning of the secondary transfer member is performed while the secondary transfer member is caused to make at least one rotation. 
     However, the foregoing prior art has the following problems. That is, in the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-272235 or No. Hei. 8-328401, at the time of cleaning when the recording medium does not exist at the transfer position, the voltage having the same polarity as the charged polarity of the toner is applied to the transfer part, and then, the voltage having the polarity opposite to the charged polarity of the toner is applied, so that the toner sticking to the transfer part is completely reversely transferred onto the image carrier, and the rear stain of the recording medium is prevented. In the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 9-6146, the contact and separation control of the cleaner blade and the secondary transfer member is performed, so that cleaning of the toner remaining on the intermediate transfer body and the secondary transfer member after the release of the paper jam can be effectively performed, and in the image forming operation carried out after the release of the paper jam, the stain such as fogging or stripes due to the remaining toner is not produced, a clear image can be formed, and the image quality of the formed image is improved. 
     However, there has been a problem that even if a constant cleaning voltage is applied to the transfer part, if consideration is not paid to the relation between the mechanical adhesion force of toner to the surface of the secondary transfer roll or the like as the transfer part and the adhesion force of toner to the surface of the image carrier with which the secondary transfer roll comes in contact, it is impossible to effectively remove the toner adhered to the surface of the secondary transfer roll or the like as the transfer part, and the surface of the secondary transfer roll can not be finely cleaned. 
     As a result, in the image forming apparatus such as a color copying machine, the cleaning part for applying the cleaning voltage to the secondary roll is made auxiliary, and the blade for cleaning the surface of the secondary transfer roll is used as a main cleaning part. Thus, there has been a problem that the surface of the secondary transfer roll is abraded by the press contact of the blade, and the life becomes short. Especially in the case where a color image is formed, the amount of toner to be cleaned is about  4  times as large as that of a black-and-white image, so that the cleaning part for applying the cleaning voltage to the secondary transfer roll is insufficient, and the load by the blade is large. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the problems of the prior art and provides an image forming apparatus which can finely clean a transfer roll at all times by regulating the relation between a mechanical adhesion force of toner to the surface of a transfer roll and a mechanical adhesion force of toner to the surface of an image carrier with which the transfer roll comes into press contact. 
     In order to solve the problems, according to an aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that at the time of cleaning the transfer roll, both an electrostatic adhesion force and a mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier are set larger than an electrostatic adhesion force and a mechanical adhesion force of the charged colorant to the surface of the transfer roll. 
     Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, a part which sets the electrostatic adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the electrostatic adhesion force of the charged colorant to the surface of the transfer roll includes a part which applies a cleaning bias voltage having a polarity opposite to the transfer bias voltage to at least one of the backup roll and the transfer roll. 
     Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, a part which sets the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the mechanical adhesion force of the charged colorant to the surface of the transfer roll includes a part which sets surface energy of the belt-like image carrier larger than surface energy of the transfer roll. 
     Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, a contact angle of water on the surface of the belt-like image carrier is set to 70° to 80°, and a contact angle of water on the surface of the transfer roll is set to 85° to 100°. 
     Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled so that a potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning. 
     Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled according to a system resistance between the backup roll and the transfer roll. 
     Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled according to an environmental variation. 
     Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled according to a use history of the image forming apparatus. 
     Besides, according to another aspect of the present invention, an image forming apparatus includes a belt-like image carrier which holds a visible image with a charged colorant and is circularly moved, a transfer roll which is disposed to be brought into press contact with a front surface of the image carrier through a recording medium and collectively transfers the visible image on the image carrier onto the recording medium, a backup roll which is disposed opposite to the transfer roll to be brought into press contact with a rear surface of the belt-like image carrier and forms a predetermined width transfer nip region to the transfer roll, and a transfer bias application part which applies a transfer bias voltage to at least one of the backup roll and the transfer roll, in which the image forming apparatus is characterized in that a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and a resistor of a predetermined value corresponding to a system resistance between the backup roll and the transfer roll is provided between a cleaning bias power supply and a transfer portion so that a potential difference between the belt-like image carrier and the transfer roll becomes a value suitable for cleaning. 
     Besides, according to another aspect of the present invention, in the foregoing image forming apparatus, the resistance value of the resistor is set so that an optimum cleaning bias can always be applied against a change of the system resistance of the transfer portion. 
     According to the present invention, at the time of cleaning the transfer roll, both the electrostatic adhesion force and the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier are set larger than the electrostatic adhesion force and the mechanical adhesion force of the charged colorant to the surface of the transfer roll. Thus, the charged colorant adhered to the surface of the transfer roll can be certainly shifted from the surface of the transfer roll to the surface of the belt-like image carrier by both the electrostatic adhesion force and the mechanical adhesion force, it becomes possible to finely clean the transfer roll at all times, and an excellent cleaning property of the transfer roll can be assured without using a specific cleaning part. 
     Besides, according the present invention, in the foregoing image forming apparatus, the part which sets the electrostatic adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the electrostatic adhesion force of the charged colorant to the surface of the transfer roll includes the part which applies the cleaning bias voltage having the polarity opposite to the transfer bias voltage to at least one of the backup roll and the transfer roll. Thus, by setting the cleaning bias voltage applied to at least one of the backup roll and the transfer roll, the electrostatic adhesion force of the charged colorant to the surface of the belt-like image carrier can be easily and certainly set larger than the electrostatic adhesion force of the charged colorant to the surface of the transfer roll. 
     Besides, according to the present invention, in the foregoing image forming apparatus, the part which sets the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the mechanical adhesion force of the charged colorant to the surface of the transfer roll includes the part which sets the surface energy of the belt-like image carrier larger than the surface energy of the transfer roll. Thus, by suitably setting the surface energy of the belt-like image carrier and the transfer roll, the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier can be easily and certainly set larger than the mechanical adhesion force of the charged colorant to the surface of the transfer roll. 
     Besides, according to the present invention, the image forming apparatus includes the belt-like image carrier which holds the visible image with the charged colorant and is circularly moved, the transfer roll which is disposed at the front surface of the image carrier to be brought into press contact through the recording medium and collectively transfers the visible image on the image carrier onto the recording medium, the backup roll which is disposed opposite to the transfer roll to be brought into press contact with the rear surface of the belt-like image carrier and forms the predetermined width transfer nip region to the transfer roll, and the transfer bias application part which applies the transfer bias voltage to at least one of the backup roll and the transfer roll, in which the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning. Thus, even in the case where an environmental variation or a change with time occurs, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times. 
     Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled according to the system resistance between the backup roll and the transfer roll. Thus, by actually measuring the system resistance between the backup roll and the transfer roll and controlling the output value of the cleaning bias voltage according to the measurement value of the system resistance, even in the case where an environmental variation or a change with time occurs, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times. 
     Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled according to the environmental variation. Thus, even in the case where an environment such as temperature or humidity is changed, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times. 
     Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled according to the use history of the image forming apparatus. Thus, even in the case where the system resistance of the transfer part is changed with the passage of time, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times. 
     Besides, according to the present invention, the image forming apparatus is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the resistor of the predetermined value corresponding to the system resistance between the backup roll and the transfer roll is provided between the cleaning bias power supply and the transfer portion so that the potential difference between the belt-like image carrier and the transfer roll becomes the value suitable for cleaning. Thus, by the simple structure that the resistor of the predetermined value corresponding to the system resistance between the backup roll and the transfer roll is provided between the cleaning bias power supply and the transfer portion, the potential difference between the belt-like image carrier and the transfer roll can be made the value suitable for cleaning, and even in the case where an environmental variation or a change with time occurs, it becomes possible to finely clean the transfer part at all times. 
     Besides, according to the present invention, in the foregoing image forming apparatus, the resistance value of the resistor is set so that the optimum cleaning bias can always be applied against the change of the system resistance of the transfer portion. Thus, even in the case where the environmental variation or the change with time occurs, the output value of the cleaning bias voltage can be maintained so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail based on the following figures, wherein: 
     FIG. 1 is a structural view showing an image forming part of a color electrophotographic copying machine as an image forming apparatus according to embodiment 1 of the present invention; 
     FIG. 2 is a structural view showing the color electrophotographic copying machine as the image forming apparatus according to the embodiment 1 of the present invention; 
     FIG. 3 is an explanatory view showing an image area and a non-image area of an intermediate transfer belt; 
     FIG. 4 is an explanatory view showing a patch for process control and a patch for registration control which are transferred onto an intermediate transfer belt; 
     FIG. 5 is an explanatory view showing a patch for process control and a patch for registration control which are transferred onto an intermediate transfer belt; 
     FIG. 6 is a block diagram showing a control circuit of the color electrophotographic copying machine as the image forming apparatus according to the embodiment 1 of the present invention; 
     FIG. 7 is an explanatory view showing a contact angle of water on the surface of a secondary transfer roll; 
     FIG. 8 is an explanatory view showing a mechanical adhesion force of toner to an intermediate transfer belt and a secondary transfer roll; 
     FIG. 9 is a timing chart showing an image forming operation of the color electrophotographic copying machine as the image forming apparatus according to the embodiment 1 of the present invention; 
     FIG. 10 is a structural view showing a tandem type color electrophotographic copying machine as an image forming apparatus according to embodiment 2 of the present invention; 
     FIG. 11 is a structural view showing a measurement part of a system resistance value of a secondary transfer portion; 
     FIG. 12 is a graph showing the relation between the system resistance value of the secondary transfer portion and reverse bias output value; 
     FIG. 13 is a graph showing the relation between absolute humidity and reverse bias output value; 
     FIG. 14 is a graph showing the relation between the number of prints and reverse bias output value; 
     FIG. 15 is a structural view showing the main part of a color electrophotographic copying machine as an image forming apparatus according to embodiment 6 of the present invention; and 
     FIG. 16 is a graph showing the relation between the change of environmental condition and voltage applied to a transfer portion. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below with reference to the drawings. 
     Embodiment 1 
     FIG. 2 shows a color electrophotographic copying machine as an image forming apparatus according to embodiment 1 of the present invention. 
     In FIG. 2, reference numeral  1  designates a main body of a color electrophotographic copying machine. An automatic document conveying device  3  for automatically conveying an original document  2  in a state where the documents are separated from each other, and a document reading device  4  for reading an image of the original document  2  conveyed by the automatic document conveying device  3  are disposed on an upper portion of the color electrophotographic copying machine main body  1 . In this document reading device  4 , the original document  2  put on a platen glass  5  is illuminated by a light source  6 , a reflected light image from the original document  2  is scanned and exposed onto an image reading element  11  made of CCDs or the like through a reducing optical system constituted by a full rate mirror  7 , half rate mirrors  8 ,  9 , and an imaging lens  10 , and a colorant reflected light image of the original document  2  is read by this image reading element  11  at a predetermined dot density (for example, 16 dots/mm). 
     The colorant reflected light image of the original document  2  read by the foregoing document reading device  4  is sent as, for example, original document reflectivity data of three colors of red (R), green (G) and blue (B) (each has 8 bits), to an image processing device  12 . In this image processing device  12 , predetermined image processing, such as shading correction, position shift correction, brightness/color space conversion, gamma correction, frame erasure, or color/movement edition, is carried out to the reflectivity data of the original document  2 . 
     The image data subjected to the predetermined image processing by the image processing device  12  as described above are sent, as original document colorant gradation data of four colors of yellow (Y), magenta (M), cyan (C), and black (BK)(each has 8 bits), to a ROS  13  (Raster Output Scanner). In this ROS  13 , image exposure by laser light is carried out in accordance with the original document colorant gradation data. 
     An image forming part A capable of forming plural toner images with difference colors is provided in the inside of the color electrophotographic copying machine main body  1 . This image forming part A is mainly constructed by a photoreceptor drum  17  as an image carrier on which an electrostatic latent image is formed, and a rotary system developing device  19  as a developing part capable of forming plural toner images having different colors by developing the electrostatic latent images formed on the photoreceptor drum  17 . 
     As shown in FIG. 2, in the ROS  13 , a not-shown semiconductor laser is modulated in accordance with the document reproduction colorant gradation data, and a laser beam LB is emitted from the semiconductor laser in accordance with the gradation data. The laser beam LB emitted from the semiconductor laser is deflected and scanned by a rotating polygon mirror  14 , and is scanned and exposed onto the photoreceptor drum  17  as the image carrier through a f θ lens  15  and a reflecting mirror  16 . 
     The photoreceptor drum  17  onto which the laser beam LB is scanned and exposed by the ROS  13  is driven to rotate at a predetermined speed in the direction of an arrow by a not-shown driving part. After the surface of the photoreceptor drum  17  is charged to a predetermined polarity (for example, minus polarity) and potential by a screen corotron  18  for primary charging, the laser beam LB is scanned and exposed in accordance with the document reproduction colorant gradation data, so that an electrostatic latent image is formed. The electrostatic latent image formed on the photoreceptor drum  17  is reversal developed by the rotary system developing device  19  equipped with developing units  19 Y,  19 M,  19 C, and  19 BK of four colors of yellow (Y), magenta (M), cyan (C), and black (BK) and by, for example, toner (charged colorant) charged to the minus polarity of the same polarity as the charged polarity of the photoreceptor drum  17 , and becomes a toner image of predetermined color. Incidentally, the toner image formed on the photoreceptor drum  17  receives charging of the minus polarity by a pre-transfer charging unit  20  as needed, and the amount of electric charge is adjusted. 
     The toner images of the respective colors formed on the photoreceptor drum  17  are transferred so as to overlap with each other by a primary transfer roll  22  as a first transfer part onto an intermediate transfer belt  21  as an intermediate transfer body disposed at an under portion of the photoreceptor drum  17 . This intermediate transfer belt  21  is supported by a driving roll  23 , a follower roll  24   a , a tension roll  24   b , and a backup roll  25  as an opposite roll constituting part of a secondary transfer part, in such a manner that the intermediate transfer belt can be rotated in the arrow direction at the same moving speed as a peripheral speed of the photoreceptor drum  17 . 
     In accordance with the color of an image to be formed, toner images of all of or part of four colors of yellow (Y), magenta (M), cyan(C), and black (BK) formed on the photoreceptor drum  17  are sequentially transferred onto the intermediate transfer belt  21  by the primary transfer roll  22  in the state where they overlap with each other. The toner images transferred onto the intermediate transfer belt  21  are transferred onto a recording sheet  26  as a recording medium conveyed to a secondary transfer position at predetermined timing by a pressing force and an electrostatic attracting force of the backup roll  25  for supporting the intermediate transfer belt  21  and a secondary transfer roll  27  which constitutes part of a secondary transfer part and comes in press contact with the backup roll  25 . As shown in FIG. 2, the recording sheet  26  of a predetermined size is fed by feed rolls  28   a ,  29   a ,  30   a , and  31   a  from one of paper feed cassettes  28 ,  29 ,  30  and  31  as plural recording medium accommodating members disposed at a lower portion in the color electrophotographic copying machine main body  1 . The fed recording sheet  26  is conveyed to the secondary transfer position of the intermediate transfer belt  21  at the predetermined timing by plural conveying rolls  32  and a registration roll  33 . Then, as described above, by the backup roll  25  and the secondary transfer roll  27  as the secondary transfer part, the toner images of the predetermined colors are collectively transferred onto the recording sheet  26  from the intermediate transfer belt  21 . 
     After the recording sheet  26  onto which the toner images of the predetermined colors were transferred from the intermediate transfer belt  21  is separated from the intermediate transfer belt  21 , it is conveyed to a fixing device  35  by a conveying belt  34 . The toner images are fixed onto the recording sheet  26  by this fixing device  35  and with the heat and pressure, and in the case of one-sided copying, the sheet is directly discharged onto a paper discharge tray  36 , and the copying step of a color image is ended. 
     On the other hand, in the case of two-sided copying, the recording sheet  26  on a first surface (front surface) of which the color image was formed is not directly discharged onto the paper discharge tray  36 , but the conveying direction is changed downward by a not-shown inverting gate, and the sheet is once conveyed to an inverting passage  39  by a tri-roll  37  in which three rolls are in press contact with each other and an inverting roll  38 . Then, the recording sheet  26  is conveyed to a passage  40  for two-sided copying by the inverting roll  38  which is reversely rotated, and is once conveyed to the registration roll  33  by a conveying roll  41  provided on the passage  40  for two-sided copying and is stopped. The conveyance of the recording sheet  26  is started again by the registration roll  33  in synchronization with the toner images on the intermediate transfer belt  21 . After transfer and fixing steps of the toner images are carried out to the second surface (rear surface) of the recording sheet  26 , the sheet is discharged onto the discharge tray  36 . 
     Incidentally, in FIG. 2, reference numeral  42  designates a cleaning device for removing a remaining toner, paper powder, or the like from the surface of the photoreceptor drum  17  after the transfer step is ended;  43 , an intermediate transfer belt cleaner for cleaning the intermediate transfer belt  21 ; and  44 , a manual paper feed tray. 
     FIG. 1 is a structural view showing an image forming part A of the color electrophotographic copying machine. 
     In this color electrophotographic copying machine, as described above, after the surface of the photoreceptor drum  17  is uniformly charged to a predetermined potential by the screen corotron  18  for primary charging, an image corresponding to a predetermined color is exposed onto the surface of the photoreceptor drum  17  by the ROS  13 , and an electrostatic latent image is formed. The electrostatic latent image formed on the surface of the photoreceptor drum  17  correspondingly to each color is developed by the developing unit  19 Y,  19 M,  19 C or l 9 BK of the corresponding color, and a toner image T of the predetermined color is formed on the surface of the photoreceptor drum  17 . 
     For example, if the electrostatic latent image formed on the photoreceptor drum  17  corresponds to yellow, this electrostatic latent image is developed by the developing unit  19 Y for yellow, and the yellow toner image T is formed on the photoreceptor drum  17 . Besides, with respect to the other colors of magenta, cyan, and black as well, the toner images T of the corresponding colors are sequentially formed on the photoreceptor drum  17  by a similar process. 
     The toner images T of the respective colors sequentially formed on the photoreceptor drum  17  are transferred onto the surface of the intermediate transfer belt  21  from the photoreceptor drum  17  at the primary transfer position where the photoreceptor drum  17  comes in contact with the intermediate transfer belt  21 . At this primary transfer position, the semi-conductive bias roll  22  for the primary transfer is disposed at the rear surface of the intermediate transfer belt  21 , and the intermediate transfer belt  21  is brought into contact with the surface of the photoreceptor drum  17  by the bias roll  22  for the primary transfer. A voltage having a polarity opposite to the charged polarity of the toner is applied to the bias roll  22  for the primary transfer, and the toner images T formed on the photoreceptor drum  17  are transferred onto the intermediate transfer belt  21  by the pressing force and the electrostatic attracting force. 
     In the case where an image of a single color is formed, the toner image T of the predetermined color which has been primarily transferred onto the intermediate transfer belt  21  is immediately secondarily transferred onto the recording sheet  26 . However, in the case where a color image obtained by overlapping the toner images T of plural colors is formed, the formation of the toner image T of a predetermined color onto the photoreceptor drum  17 , and the step of primary transfer of the toner image T onto the intermediate transfer belt  21  are repeated plural times of the number of predetermined colors. 
     For example, in the case of forming an image of a full color in which the toner images T of four colors of yellow (Y), magenta (M), cyan (C) and black (BK) overlap with each other, the toner images T of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are sequentially formed on the photoreceptor drum  17  every rotation thereof, and the toner images of these four colors are primarily transferred onto the intermediate transfer belt  21  in the state where they overlap with each other. 
     At that time, the intermediate transfer belt  21  rotates in a period synchronous with the photoreceptor drum  17  while holding the unfixed toner image T of yellow first primarily transferred. At a predetermined position determined by a position detection sensor  45 , every rotation thereof, the unfixed toner images T of magenta, cyan, and black are transferred onto the intermediate transfer belt  21  in the state where they sequentially overlap with the yellow unfixed toner image T. 
     The unfixed toner images T primarily transferred onto the intermediate transfer belt  21  in this way are conveyed to the secondary transfer position facing to the conveying passage of the recording sheet  26  with the rotation of the intermediate transfer belt  21 . 
     As described above, the recording sheet  26  is fed from the predetermined paper feed cassette  28 ,  29 ,  30  or  31  by the feed rolls  28   a ,  29   a ,  30   a  and  31   a , is conveyed to the registration roll  33  by the conveying roll  32 , and is fed to a nip portion between the secondary transfer roll  27  and the intermediate transfer belt  21  by the registration roll  33  at predetermined timing. 
     At the rear surface side of the intermediate transfer belt  21  at the secondary transfer position, the backup roll  25  which is a counter electrode of the secondary transfer roll  27  is disposed. At the secondary transfer position, the semi-conductive secondary transfer roll  27  comes in press contact with the intermediate transfer belt  21  at the predetermined timing, and by applying the voltage having the polarity opposite to the charged polarity of the toner to the backup roll  25 , the unfixed toner images T transferred onto the intermediate transfer belt  21  are electrostatically secondarily transferred onto the recording sheet  26  at the secondary transfer position. 
     This embodiment is structured such that, as shown in FIG. 1, the voltage having the same polarity as the charged polarity of the toner is not directly applied to the secondary transfer roll  27 , but the voltage having the same polarity as the charged polarity of the toner is applied to the backup roll  25 , which comes in press contact with the secondary transfer roll  27  through the intermediate transfer belt  21 , by a bias roll  46  from a high voltage power source  47  for transfer bias as a transfer bias voltage application part. However, such a structure may be naturally adopted that the voltage having the same polarity as the charged polarity of the toner is directly applied to the secondary transfer roll  27 . 
     The recording sheet  26  onto which the unfixed toner images were transferred is separated from the intermediate transfer belt  21 , is sent to the fixing device  35  by an electrode member  48 , a guide plate  49  and a conveying belt  34  disposed at the downstream side of the secondary transfer portion, and a fixing treatment of the unfixed toner images T is carried out. 
     On the other hand, with respect to the intermediate transfer belt  21  after the secondary transfer of the unfixed toner images T is ended, the remaining toner is removed by the cleaner  44  for the intermediate transfer belt. 
     The intermediate transfer belt  21  is made of a synthetic resin, such as polyimide, polycarbonate, polyester, or polypropylene, or various kinds of rubber, containing a suitable amount of antistatic agent such as carbon black, and is formed so that its volume resistivity becomes 10 6  to 10 14 Ω·cm. The thickness of the intermediate transfer belt  21  is set to, for example, 0.1 mm. The peripheral length of the intermediate transfer belt  21  is set to integer times (for example, 3 times) as long as the peripheral length of the photoreceptor drum  17 . 
     The secondary transfer roll  27  and the intermediate transfer belt cleaner  44  are disposed so that they can be brought into contact with and be separated from the intermediate transfer belt  21 . In the case where a color image is formed, until the unfixed toner image T of the final color is primarily transferred onto the intermediate transfer belt  21 , at least the intermediate transfer belt cleaner  44  is separated from the intermediate transfer belt  21 . 
     Moreover, the secondary transfer roll  27  includes a surface layer made of a tube of urethane rubber in which carbon is dispersed, and an inner layer made of foamed urethane rubber in which carbon is dispersed. The surface of the secondary transfer roll  27  is coated with fluorine. The secondary transfer roll  27  is set such that its volume resistivity is 10 3  to 10 10 Ω·cm, a roll diameter is 28 φmm, and hardness is, for example, 30° (Askar C). 
     On the other hand, the backup roll  25  includes a surface layer made of a tube of blend rubber of EPDM and NBR in which carbon is dispersed, and an inner layer made of rubber of EPDM. The backup roll is set such that its surface resistivity is 10 7  to 10 10 Ω/□, a roll diameter is 28 φmm, and hardness is 70° (Askar C). 
     As the electrode member  48  disposed at the downstream side of the nip portion of the secondary transfer position, a metal plate is preferable as a conductive plate-like member. In this embodiment, a stainless steel plate with a thickness of 0.5 mm is used, and a needle-like portion is formed at the side of the recording sheet  26 . Further, the tip of the electrode member  48  at the side of the secondary transfer region is disposed at the side of the secondary transfer roll  27  by 1 mm from the line of the nip portion between the backup roll  25  and the secondary transfer roll  27 , and is apart from the outlet of the nip portion by 7 mm. 
     Further, the color electrophotographic copying machine of this embodiment 1 is constructed such that in the case where a toner image transferred onto a non-image area of the intermediate transfer body is positioned on the intermediate transfer body other than the area corresponding to the recording medium, the second transfer part is provided with a transfer bias voltage application control part for making control to apply a reverse transfer bias voltage (cleaning bias voltage) having the polarity opposite to the transfer bias voltage to at least the area other than the recording medium. 
     That is, in the color electrophotographic copying machine of this embodiment 1, as shown in FIG. 3, the surface of the intermediate transfer belt  21  is previously divided into an image area  50  and a non-image area  51 . The image area  50  is set correspondingly to the recording sheet  26  of the maximum size (for example, A 3  size) which can be copied by the color electrophotographic copying machine. Two surfaces of the image areas  50  corresponding to the A 3  size recording sheet  26  are set on the surface of the intermediate transfer belt  21 , and a portion between these image areas  50  is the non-image area  51 . The intermediate transfer belt  21  is constructed such that, as shown in FIG. 1, a mark  52  provided at the reference position is detected by the position detection sensor  45 , so that the positions of the image area  50  and the nonimage area  51  are recognized, and the toner image T corresponding to the original document  2  is transferred onto the image area  50 . Besides, in the color electrophotographic copying machine, when an image is formed on the recording sheet  26  which has a size not larger than half of the A 3  size, for example, A 4  size, the image area  50  is divided into two areas  50   a  and  50   b  (for example, an area corresponding to A 4  size), and the toner image of the original document  2  can be transferred also onto the respective areas  50   a  and  50   b  of the image area  50 . 
     Besides, with the advance of picture quality, in order to assure the picture quality of the color image, the color electrophotographic copying machine is constructed such that a patch for process control and a patch for registration control are transferred onto the intermediate transfer body before the image forming operation or at the timing of a paper feed interval of the recording medium, the patch for the process control and the patch for the registration control are detected, and on the basis of the detection result, the image forming operation is controlled. 
     That is, in the color electrophotographic copying machine, when a power supply switch of the copying machine is turned on, when a predetermined number of copies are taken, or at the time of a setup operation after a copy button for starting a copying operation is pressed and before the copying operation is actually started, as shown in FIG. 4, in the image area  50  of the intermediate transfer belt  21 , plural patches  53 Y,  53 M,  53 C, and  53 BK for process control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) are formed at different densities, and sideways V-shaped patches  54 Y,  54 M,  54 C, and  54 BK for registration control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) are formed at a predetermined pitch. 
     Besides, in the color electrophotographic copying machine, as shown in FIG. 5, in the non-image area  50  corresponding to a paper feed interval of the recording sheet  26  on the intermediate transfer belt  21 , patches  55 Y,  55 M,  55 C, and  55 BK for process control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) are formed at two kinds of densities of 60% and 20%, and sideways V-shaped patches  56 Y,  56 M,  56 C, and  56 BK for registration control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) are formed at a predetermined pitch. 
     The patches  53 Y,  53 M,  53 C,  53 BK,  54 Y,  54 M,  54 C, and  54 BK for the process control formed on the intermediate transfer belt  21  and the patches  55 Y,  55 M,  55 C,  55 BK,  56 Y,  56 M,  56 C, and  56 BK for the registration control are detected, as shown in FIG. 1, by an optical sensor  57  disposed above the follower roll  24   a.    
     Moreover, the color electrophotographic copying machine is constructed such that in the case where at least part of the toner image transferred onto the non-image area  51  and the image area  50  of the intermediate transfer belt  21  is positioned on the intermediate transfer belt  21  other than an area corresponding to the recording sheet  26 , the secondary transfer part is provided with a CPU  60  as a transfer bias voltage application control part for making control to apply a reverse transfer bias voltage (cleaning bias voltage) having a polarity opposite to a transfer bias voltage to at least the area other than the recording sheet  26 . 
     Besides, in the color electrophotographic copying machine, as shown in FIG. 1, a sheet detection sensor  58  for detecting the recording sheet  26  is disposed at the upstream side of the secondary transfer position. This sheet detection sensor  58  detects the tip end, rear end, or the like of the recording sheet  26  conveyed to the secondary transfer position where it comes in contact with the intermediate transfer belt  21 . 
     Further, in the color electrophotographic copying machine, a humidity sensor  59  for detecting humidity and a temperature sensor  66  are provided in the inside of the intermediate transfer belt  21 . 
     FIG. 6 is a block diagram showing a control circuit of the color electrophotographic copying machine. 
     In FIG. 6, reference numeral  60  designates a CPU which controls an image forming operation of the color electrophotographic copying machine and functions also as a transfer bias voltage application control part;  61 , a user interface which specifies the number of copied sheets, copying magnification, size of the recording sheet  26 , and the like;  45 , a position detection sensor for detecting the mark  52  provided on the intermediate transfer belt  21 ;  57 , an optical sensor for detecting the patches  53 Y,  53 M,  53 C,  53 BK,  54 Y,  54 M,  54 C, and  54 BK for the process control transferred onto the intermediate transfer belt  21 , and the patches  55 Y,  55 M,  55 C,  55 BK,  56 Y,  56 M,  56 C, and  56 BK for the registration control;  58 , a sheet detection sensor disposed in front of the secondary transfer position;  62 , a cassette sensor for detecting the size of the recording sheet  26  accommodated in the sheet feed cassette  28 ,  29 ,  30  or  31  by a size detection portion provided at the sheet feed cassette  28 ,  29 ,  30  or  31 ;  59 , a humidity sensor for detecting the humidity of the inside of the color electrophotographic copying machine main body  1 ;  47 , a high voltage power supply for transfer bias voltage as a transfer bias voltage application part for applying a transfer bias voltage to the backup roll  27  as the secondary transfer part;  13 , a ROS which performs image exposure corresponding to an image of the original document  2  onto the photoreceptor drum  17  and performs image exposure to form the patches  53 Y,  53 M,  53 C,  53 BK,  54 Y,  54 M,  54 C, and  54 BK for the process control, and the patches  55 Y,  55 M,  55 C,  55 BK,  56 Y,  56 M,  56 C, and  56 BK for the registration control;  63 , a belt driving motor for rotating and driving the intermediate transfer belt  21 ;  64 , a ROM storing a program with which the CPU  60  performs the image forming operation of the color electrophotographic copying machine and the transfer bias voltage application control operation; and  65 , a RAM for storing data or the like with which the CPU  60  makes the control operation. 
     The color electrophotographic copying machine of the embodiment 1 is designed such that at the time of cleaning the transfer roll, both the electrostatic adhesion force and the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier become larger than the electrostatic adhesion force and the mechanical adhesion force of the charged colorant to the surface of the transfer roll. 
     Besides, this embodiment 1 is constructed such that the part which sets the electrostatic adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the electrostatic adhesion force of the charged colorant to the surface of the transfer roll includes the part which applies the cleaning bias voltage having the polarity opposite to the transfer bias voltage to at least one of the backup roll and the transfer roll. 
     Further, this embodiment 1 is constructed such that the part which sets the mechanical adhesion force of the charged colorant to the surface of the belt-like image carrier larger than the mechanical adhesion force of the charged colorant to the surface of the transfer roll includes the part which sets the surface energy of the belt-like image carrier larger than the surface energy of the transfer roll. 
     That is, in the color electrophotographic copying machine of the embodiment 1, in order to set the surface energy of the intermediate transfer belt  21  larger than the surface energy of the secondary transfer roll  27 , the contact angle of water on the surface of the secondary transfer roll  27  is set larger than the contact angle of water on the surface of the intermediate transfer belt  21 . 
     Specifically, the surface roughness Rz of the secondary transfer roll  27  to which fluorine coating is applied is set by surface polishing or the like such that Rz &lt;5 μm is established. At this time, as shown in FIG. 7, the contact angle of water on the surface of the secondary transfer roll  27  is controlled to be within the range of 85° to 100°. At this time, the contact angle of water on the surface of the intermediate transfer belt  21  is within the range of 70° to 80°, and the above relation is satisfied. Although the surface of the secondary transfer roll  27  is formed to be arc-shaped, since a waterdrop attached to the surface of the secondary transfer roll  27  is small, the surface of the secondary transfer roll  27  can be approximated to a plane. 
     By doing so, as shown in FIG. 8, the mechanical adhesion force F FR  of toner to the secondary transfer roll  27  is made lower than the mechanical adhesion force F FB  of toner to the intermediate transfer belt  21 , so that the toner becomes hard to shift from the intermediate transfer belt  21  to the secondary transfer roll  27 . 
     The CPU  60  makes control so that a predetermined voltage value of cleaning bias is applied to the backup roll  25  through the bias roll  46  by the high voltage power supply  47  for transfer bias. 
     The cleaning bias voltage applied to the backup roll  25  is set such that at the time of cleaning the secondary transfer roll  27 , the electrostatic adhesion force F CB  of toner to the surface of the intermediate transfer belt  21  becomes larger than the electrostatic adhesion force F CR  of toner to the surface of the secondary transfer roll  27 . 
     In the foregoing structure, in the case of the color electrophotographic copying machine of the embodiment 1, the relation between the mechanical adhesion force of the toner to the surface of the transfer roll and the mechanical adhesion force of the toner to the surface of the image carrier with which the transfer roll comes in press contact is regulated in the manner described below, so that the transfer roll can be finely cleaned at all times. 
     That is, as shown in FIG. 2, in the color electrophotographic copying machine of the embodiment 1, when the original document  2  is set at a predetermined position, the user interface  61  is operated to specify the number of sheets to be copied, copying magnification, size of the recording sheet  26 , or the like, and the copy button is pressed, by the control of the CPU  60 , the photoreceptor drum  17  is driven to rotate, and as shown in FIG. 9, the belt driving motor  63  for rotation driving the intermediate transfer belt  21  is turned ON, and the intermediate transfer belt  21  is driven to rotate. Besides, by the CPU  60  as the transfer bias voltage application control part, at the same time as the rotation of the intermediate transfer belt  21 , the reverse transfer bias voltage (cleaning bias voltage) of, for example, +600 to 700 V is applied through the high voltage power supply  47  for the transfer bias voltage to the backup roll  25  of the secondary transfer part. Incidentally, the embodiment 1 is constructed such that while the belt driving motor  63  rotates, the secondary transfer roll  27  is put in the state where it remains being in press contact with the intermediate transfer belt  21 . 
     Onto the image area  50  of the intermediate transfer belt  21 , as shown in FIG.  4  and FIG. 9, at the setup operation after the copy button for starting the copying operation is pressed and before the copying operation is actually started, the plural patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) formed on the photoreceptor drum  17  are transferred at different densities, and the sideways V-shaped patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) are transferred at a predetermined pitch. The patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the sideways V-shaped patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors are detected by the optical sensor  57  as shown in FIG.  1 . The data of density and position of the patches for the process control and the patches for the registration control are sent to the CPU  60  as shown in FIG.  6 . The CPU  60  judges whether the data of density and position of the patches for the process control and the patches for the registration control are within a predetermined range, and controls various parameters for image formation so that the data of density and position of the patches are placed within the predetermined range. 
     Besides, also when the patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors pass through the secondary transfer position, the CPU  60  maintains the state, as shown in FIG. 9, where the reverse transfer bias voltage of, for example, +600 V, remains applied to the backup roll  25  through the high voltage power supply  47  for the transfer bias voltage. Thus, when the patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors pass through the secondary transfer position, they are placed in the state where they remain transferred on the intermediate transfer belt  21  by the reverse transfer bias voltage applied to the backup roll  25 , and are not transferred onto the secondary transfer roll  27  which is in press contact with the intermediate transfer belt  21 . Incidentally, the patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors are thereafter removed from the intermediate transfer belt  21  by the intermediate transfer belt cleaner  43 . 
     Next, onto the image area  50  of the intermediate transfer belt  21  and the area corresponding to the paper feed interval of the recording sheet  26 , as shown in FIG.  5  and FIG. 9, a toner image  70  corresponding to the image of the original document  2  formed on the photoreceptor drum  17  and the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at two kinds of densities, and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at the predetermined pitch. 
     At that time, in the case where a full color image is formed, the toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are sequentially transferred onto the image area  50  of the intermediate transfer belt  21  every rotation of the photoreceptor drum  17 . Besides, onto the area corresponding to the paper feed interval of the recording sheet  26 , the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at two kinds of densities every rotation of the photoreceptor drum  17  as shown in FIG. 5, and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at the predetermined pitch. 
     Besides, until the final toner image  70 , that is, the toner image  70  in which toner images of four colors of yellow (Y), magenta (M), cyan (C) and black (BK) have been transferred so as to overlap with each other in the case where a full color image is formed, or the toner image  70  of one color to three colors in the case where an image of one color to three colors among yellow (Y), magenta (M), cyan (C) and black (BK) is formed, is transferred onto the image area  50  of the intermediate transfer belt  21 , the backup roll  25  is in the state where the reverse transfer bias voltage of, for example, +600 V, remains applied. 
     When the final toner image  70  transferred onto the image area  50  of the intermediate transfer belt  21  in the manner as described above passes through the secondary transfer position, as shown in FIG. 9, the CPU  60  causes the high voltage power supply  47  for the transfer bias voltage to apply the transfer bias voltage of, for example, −2.2 KV to the backup roll  25 . Thus, when the final toner image  70  transferred onto the image area  50  of the intermediate transfer belt  21  passes through the secondary transfer position, by the transfer bias voltage applied to the backup roll  25 , the final toner image is transferred from the intermediate transfer belt  21  onto the recording sheet  26  conveyed to the secondary transfer position in synchronization with the toner image. 
     At that time, the transfer bias voltage applied to the backup roll  25  of the secondary transfer part is not limited to −2.2 KV, but is set to an optimum value to transfer the final toner image  70 , which has been transferred onto the intermediate transfer belt  21 , onto the recording sheet  26  when it passes through the secondary transfer position. Thus, in the color electrophotographic copying machine of the embodiment, as shown in FIG. 1, the humidity in the copying machine main body  1  is detected by the humidity sensor  59 , and on the basis of the detection result of the humidity sensor  59 , the CPU  60  controls the transfer bias voltage applied to the backup roll  25  within the range of −1.5 KV to −3.0 KV so that the optimum transfer property of the toner image  70  can be obtained. 
     On the other hand, when the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors transferred onto the area of the intermediate transfer belt  21  corresponding to the paper feed interval of the recording sheet  26 , and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors pass through the secondary transfer position as shown in FIG. 9, the CPU  60  causes the state where the reverse transfer bias voltage of, for example, +600 V remains applied to the backup roll  25  all over the non-image area  51  according to the size of the selected recording sheet  26  through the high voltage power supply  47  for the transfer bias voltage. Thus, when the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors transferred onto the non-image area  51  of the intermediate transfer belt  21  corresponding to the paper feed interval of the recording sheet  26 , and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors pass through the secondary transfer position, by the reverse transfer bias voltage applied to the backup roll  25 , they are placed in the state where they remain transferred on the intermediate transfer belt  21 , and they are not transferred onto the secondary transfer roll  27  which is in press contact with the intermediate transfer belt  21 . 
     Thereafter, when the belt driving motor  63  for rotation driving the intermediate transfer belt  21  is stopped, application of the bias voltage to the backup roll  25  is turned OFF. 
     Incidentally, in the embodiment 1, as shown in FIG. 7, the contact angle of water on the surface of the secondary transfer roll  27  is controlled so that it becomes in the range of 85° to 100°. At this time, the contact angle of water on the surface of the intermediate transfer belt  21  is in the range of 70° to 80°, and the above relation is satisfied. 
     By doing so, as shown in FIG. 8, the mechanical adhesion force of toner to the secondary transfer roll  27  is made lower than the mechanical adhesion force of toner to the intermediate transfer belt  21 , so that the toner becomes hard to shift from the intermediate transfer belt  21  to the secondary transfer roll  27 . 
     Besides, the cleaning bias voltage applied to the backup roll  25  is set such that at the time of cleaning the secondary transfer roll  27 , the electrostatic adhesion force of toner to the surface of the intermediate transfer belt  21  becomes larger than the electrostatic adhesion force of toner to the surface of the secondary transfer roll  27 . 
     Like this, at the time of cleaning the secondary transfer roll  27 , both the electrostatic adhesion force and the mechanical adhesion force of the toner to the surface of the intermediate transfer belt  21  are set larger than the electrostatic adhesion force and the mechanical adhesion force of the toner to the surface of the secondary transfer roll  27 . Thus, while the toner is prevented from shifting to the surface of the secondary transfer roll  27 , the toner adhered to the surface of the secondary transfer roll  27  can be certainly sifted from the surface of the secondary transfer roll  27  to the surface of the intermediate transfer belt  21  by both the electrostatic adhesion force and the mechanical adhesion force, and it becomes possible to finely clean the secondary transfer roll  27  at all times. Thus, without using a specific cleaning part, such as a blade, for cleaning the surface of the secondary transfer roll  27 , the excellent cleaning property of the transfer roll can be assured. Since the surface of the secondary transfer roll  27  is not abraded by the cleaning part such as the blade, the reliability can be improved and the life of the secondary transfer roll  27  can be extended to about twice the original life. Besides, since it is not necessary to use the specific cleaning part, such as the blade, for cleaning the surface of the secondary transfer roll  27 , the cleaning part for the secondary transfer roll  27  becomes unnecessary, and the number of parts can be decreased. 
     Embodiment 2 
     FIG. 10 shows embodiment 2 of the present invention. The embodiment 2 is different from the embodiment 1 in the structure of an image forming part. The image forming part includes plural image forming units each of which includes an image carrier on which an electrostatic latent image is formed and a developing part for developing the electrostatic latent image formed on the image carrier with a toner of a predetermined color. Plural toner images of different colors are sequentially formed by the plural image forming units. 
     FIG. 10 shows a tandem type color electrophotographic copying machine as an image forming apparatus of the embodiment 2 of the present invention. 
     In FIG. 10, reference numeral  101  designates a main body of the tandem type digital color copying machine. A platen cover  103  for pressing an original document  102  onto a platen glass  105 , and an original document reading device  104  for reading an image of the original document  102  put on the platen glass  105  are disposed on the upper portion of the digital color copying machine main body  101  at one end side. In this original document reading device  104 , the original document  102  put on the platen glass  105  is illuminated by a light source  106 , a reflected light image from the original document  102  is scanned and exposed onto an image reading element  111  made of CCDs and the like through a reducing optical system constituted by a full rate mirror  107 , half rate mirrors  108 ,  109 , and an imaging lens  110 , and a colorant reflected light image of the original document  102  is read by this image reading element  111  at a predetermined dot density (for example, 16 dots/mm). 
     The colorant reflected light image of the original document  102  read by the original document reading device  104  is sent as, for example, original document reflectivity data of three colors of red (R), green (G) and blue (B) (each has 8 bits), to an IPS  112  (Image Processing System). In this IPS  112 , predetermined image processing, such as shading correction, position shift correction, brightness/color space conversion, gamma correction, frame erasure, or color/movement edition, is carried out to the reflectivity data of the original document  102 . 
     The image data subjected to the predetermined image processing by the IPS  112  as described above are converted into original document colorant gradation data of four colors of yellow (Y), magenta (M), cyan (C), and black (BK) (each has 8 bits), and are sent to ROSs  114 Y,  114 M,  114 C, and  114 BK (Raster Output Scanner) of image forming units  113 Y,  113 M,  113 C and  113 BK of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK). In these ROSs  114 Y,  114 M,  114 C and  114 BK, image exposure by a laser beam is carried out in accordance with the original document colorant gradation data of a predetermined color. 
     In the inside of the tandem type digital color copying machine main body  101 , the four image forming units  113 Y,  113 M,  113 C and  113 BK of yellow (Y), magenta (M), cyan (C) and black (BK) are disposed in the horizontal direction at a constant interval and in parallel. 
     All of these four image forming units  113 Y,  113 M,  113 C and  113 BK are structured in the same way, and each unit is roughly constructed by a photoreceptor drum  115  rotating in the direction of an arrow at a predetermined rotation speed, a primary charging screen corotron  116  which uniformly charges the surface of the photoreceptor drum  115 , a ROS  114  for forming an electrostatic latent image by exposing an image corresponding to each color onto the surface of the photoreceptor drum  115 , a developing unit  117  for developing the electrostatic latent image formed on the photoreceptor drum  115 , and a cleaning device  118 . 
     As shown in FIG. 10, in the ROS  114 , a semiconductor laser  119  is modulated in accordance with the original document colorant gradation data, and this semiconductor laser  119  emits a laser beam LB in accordance with the gradation data. The laser beam LB emitted from the semiconductor laser  119  is deflected and scanned by a rotary polygon mirror  122  through reflecting mirrors  120  and  121 , and is again scanned and exposed onto the photoreceptor drum  115  as an image carrier through the reflecting mirrors  120  and  121  and plural reflecting mirrors  123  and  124 . 
     Image data of the respective colors are sequentially outputted from the EPS  12  to the ROSs  114 Y,  114 M,  114 C and  114 BK of the image forming units  113 Y,  113 M,  113 C and  113 BK of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK). The laser beam LB emitted from the respective ROSs  114 Y,  114 M,  114 C and  114 BK in accordance with the image data is scanned and exposed onto the surface of the respective photoreceptor drums  115 Y,  115 M,  115 C and  115 BK and electrostatic latent images are formed. The electrostatic latent images formed on the respective photoreceptor drums  115 Y,  115 M,  115 C and  115 BK are developed as toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) by the developing units  117 Y,  117 M,  117 C and  117 BK. 
     The toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) sequentially formed on the photoreceptor drums  115 Y,  115 M,  115 C and  115 BK of the respective image forming units  113 Y,  113 M,  113 C and  113 BK are transferred so as to overlap with each other by primary transfer rolls  126 Y,  126 M,  126 C and  126 BK onto an intermediate transfer belt  125  disposed below the respective image forming units  113 Y,  113 M,  113 C and  113 BK. This transfer belt  125  is put with a constant tension around a drive roll  127 , a stripping roll  128 , a steering roll  129 , an idle roll  130 , a backup roll  131 , and an idle roll  132 , and is driven to circulate at a predetermined speed in the direction of an arrow by the drive roll  127  which is driven to rotate by a not-shown dedicated driving motor having an excellent constant speed property. As the transfer belt  125 , for example, a synthetic resin film of PET or the like having flexibility is formed into a belt shape, and both ends of the beltshaped synthetic resin film are connected by a method such as welding to form an endless belt shape one, which is used. 
     The toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) transferred onto the transfer belt  125  so as to overlap with each other are secondarily transferred onto a recording sheet  134  through a pressing force and an electrostatic force by a secondary transfer roll  133  which comes in press contact with the backup roll  131 . The recording sheet  134  onto which the toner images of the respective colors have been transferred is conveyed to a fixing unit  137  by two conveying belts  135  and  136 . Then the recording sheet  134  on which the toner images of the respective colors have been transferred is subjected to a fixing process through heat and pressure by the fixing unit  137 , and is discharged onto a discharge tray  138  provided at the outside of the copying machine main body  101 . 
     As shown in FIG. 10, the recording sheet  134  of a predetermined size is once conveyed from either one of plural sheet feed cassettes  139 ,  140  and  141  to a registration roll  147  through a sheet conveying passage  146  made of a sheet feed roll  142  and sheet conveying roll pairs  143 ,  144  and  145 . The transfer sheet  134  supplied from either one of the sheet feed cassettes  139 ,  140  and  141  is sent onto the intermediate transfer belt  125  by the registration roll  147  which is driven to rotate at predetermined timing. 
     In the four image forming units  113 Y,  113 M,  113 C and  113 BK of yellow, magenta, cyan and black, as described above, the toner images of yellow, magenta, cyan and black are sequentially formed at the predetermined timing. 
     Incidentally, with respect the photoreceptor drums  115 Y,  115 M,  115 C and  115 BK, after the transfer step of the toner images is ended, the remaining toner, paper powder, and the like are removed by the cleaning devices  118 Y,  118 M,  118 C and  118 BK, and they are prepared for a next image forming process. Besides, with respect to the intermediate transfer belt  125 , the remaining toner is removed by a cleaner  148  for a belt. 
     Similarly to the embodiment 1, the tandem type color electrophotographic copying machine is also constructed such that at the time of cleaning the secondary transfer roll  133 , both the electrostatic adhesion force and the mechanical adhesion force of toner to the surface of the intermediate transfer belt  125  becomes larger than the electrostatic adhesion force and the mechanical adhesion force of toner to the surface of the secondary transfer roll  133 . 
     Since the other structures and functions are the same as the embodiment 1, their description will be omitted. 
     A color electrophotographic copying machine according to embodiment 3 is constructed such that a cleaning bias voltage having a polarity opposite to a transfer bias voltage is applied to at least one of a backup roll and a transfer roll, and an output value of the cleaning bias voltage is controlled so that a potential difference between a belt-like image carrier and the transfer roll becomes optimum for cleaning. 
     More specifically, the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and the output value of the cleaning bias voltage is controlled according to a system resistance between the backup roll and the transfer roll. 
     That is, the color electrophotographic copying machine of the embodiment 3 is constructed such that as shown in FIG. 11, a constant bias current (for example, 60 μA) is made to flow to a backup roll  25  through a bias roll  46 , and a voltage applied to a secondary transfer roll  27  is measured by a resistance measurement part  80 , so that the system resistance of a secondary transfer portion is measured. Here, the system resistance of the secondary transfer portion means a resistance between the backup roll  25  and the secondary transfer roll  27  which are in press contact with each other through an intermediate transfer belt  21 . 
     As shown in FIG. 12, a CPU  60  is designed to operate so that an optimum voltage value of cleaning bias is determined according to a resistance value of the system resistance from a table of resistance values of the system resistance and the cleaning bias previously stored in a RAM  65  or the like, and the optimum voltage value of the cleaning bias is applied to the backup roll  25  through the bias roll  46  by a high voltage power supply  47  for transfer bias. 
     In the above structure, according to the color electrophotographic copying machine of the embodiment  3 , even in the case where an environment variation or a change with time occurs, it is possible to finely clean the transfer part at all times through the following manner. 
     That is, as shown in FIG. 2, in the color electrophotographic copying machine of the embodiment  3 , when the original document  2  is set at a predetermined position, the user interface  61  is operated to specify the number of sheets to be copied, copying magnification, size of the recording sheet  26 , or the like, and the copy button is pressed, by the control of the CPU  60 , the photoreceptor drum  17  is driven to rotate, and as shown in FIG. 9, the belt driving motor  63  for rotation driving the intermediate transfer belt  21  is turned ON, and the intermediate transfer belt  21  is driven to rotate. Besides, by the CPU  60  as the transfer bias voltage application control part, at the same time as the rotation of the intermediate transfer belt  21 , the reverse bias voltage (cleaning bias voltage) of, for example, +600 V is applied through the high voltage power supply  47  for the transfer bias voltage to the backup roll  25  of the secondary transfer part. Incidentally, the embodiment 3 is constructed such that while the belt driving motor  63  rotates, the secondary transfer roll  27  is put in the state where it remains being in press contact with the intermediate transfer belt  21 . 
     Onto the image area  50  of the intermediate transfer belt  21 , as shown in FIG.  4  and FIG. 9, at the setup operation after the copy button for starting the copying operation is pressed and before the copying operation is actually started, the plural patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) formed on the photoreceptor drum  17  are transferred at different densities, and the sideways V-shaped patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK) are transferred at a predetermined pitch. The patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the sideways V-shaped patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors are detected by the optical sensor  57  as shown in FIG.  1 . The data of density and position of the patches for the process control and the patches for the registration control are sent to the CPU  60  as shown in FIG.  6 . The CPU  60  judges whether the data of density and position of the patches for the process control and the patches for the registration control are within a predetermined range, and controls various parameters for image formation so that the data of density and position of the patches are placed within the predetermined range. 
     Besides, also when the patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors pass through the secondary transfer position, the CPU  60  maintains the state, as shown in FIG. 9, where the reverse transfer bias voltage of, for example, +600 V, remains applied to the backup roll  25  through the high voltage power supply  47  for the transfer bias voltage. Thus, when the patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors pass through the secondary transfer position, they are placed in the state where they remain transferred on the intermediate transfer belt  21  by the reverse transfer bias voltage applied to the backup roll  25 , and are not transferred onto the secondary transfer roll  27  which is in press contact with the intermediate transfer belt  21 . Incidentally, the patches  53 Y,  53 M,  53 C and  53 BK for the process control of the respective colors transferred onto the image area  50  of the intermediate transfer belt  21 , and the patches  54 Y,  54 M,  54 C and  54 BK for the registration control of the respective colors are thereafter removed from the intermediate transfer belt  21  by the intermediate transfer belt cleaner  43 . 
     Next, onto the image area  50  of the intermediate transfer belt  21  and the area corresponding to the paper feed interval of the recording sheet  26 , as shown in FIG.  5  and FIG. 9, a toner image  70  corresponding to the image of the original document  2  formed on the photoreceptor drum  17  and the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at two kinds of densities, and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at the predetermined pitch. 
     At that time, in the case where a full color image is formed, the toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are sequentially transferred onto the image area  50  of the intermediate transfer belt  21  every rotation of the photoreceptor drum  17 . Besides, onto the area corresponding to the paper feed interval of the recording sheet  26 , the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at two kinds of densities every rotation of the photoreceptor drum  17  as shown in FIG. 5, and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) are transferred at the predetermined pitch. 
     Besides, until the final toner image  70 , that is, the toner image  70  in which toner images of four colors of yellow (Y), magenta (M), cyan (C) and black (BK) have been transferred so as to overlap with each other in the case where a full color image is formed, or the toner image  70  of one color to three colors in the case where an image of one color to three colors among yellow (Y), magenta (M), cyan (C) and black (BK) is formed, is transferred onto the image area  50  of the intermediate transfer belt  21 , the backup roll  25  is in the state where the reverse transfer bias voltage of, for example, +600 V, remains applied. 
     When the final toner image  70  transferred onto the image area  50  of the intermediate transfer belt  21  in the manner as described above passes through the secondary transfer position, as shown in FIG. 9, the CPU  60  causes the high voltage power supply  47  for the transfer bias voltage to apply the transfer bias voltage of, for example, −2.2 KV to the backup roll  25 . Thus, when the final toner image  70  transferred onto the image area  50  of the intermediate transfer belt  21  passes through the secondary transfer position, by the transfer bias voltage applied to the backup roll  25 , the final toner image is transferred from the intermediate transfer belt  21  onto the recording sheet  26  conveyed to the secondary transfer position in synchronization with the toner image. 
     At that time, the transfer bias voltage applied to the backup roll  25  of the secondary transfer part is not limited to −2.2 KV, but is set to an optimum value to transfer the final toner image  70 , which has been transferred onto the intermediate transfer belt  21 , onto the recording sheet  26  when it passes through the secondary transfer position. Thus, in the color electrophotographic copying machine of the embodiment, as shown in FIG. 1, the humidity in the copying machine main body  1  is detected by the humidity sensor  59 , and on the basis of the detection result of the humidity sensor  59 , the CPU  60  controls the transfer bias voltage applied to the backup roll  25  within the range of −1.5 KV to −3.0 KV so that the optimum transfer property of the toner image  70  can be obtained. 
     On the other hand, when the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors transferred onto the area of the intermediate transfer belt  21  corresponding to the paper feed interval of the recording sheet  26 , and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors pass through the secondary transfer position, as shown in FIG. 9, the CPU  60  causes the state where the reverse transfer bias voltage of, for example, +600 V remains applied to the backup roll  25  all over the non-image area  51  according to the size of the selected recording sheet  26  through the high voltage power supply  47  for the transfer bias voltage. Thus, when the patches  55 Y,  55 M,  55 C and  55 BK for the process control of the respective colors transferred onto the non-image area  51  of the in intermediate transfer belt  21  corresponding to the paper feed interval of the recording sheet  26 , and the sideways V-shaped patches  56 Y,  56 M,  56 C and  56 BK for the registration control of the respective colors pass through the secondary transfer position, by the reverse transfer bias voltage applied to the backup roll  25 , they are placed in the state where they remain transferred on the intermediate transfer belt  21 , and they are not transferred onto the secondary transfer roll  27  which is in press contact with the intermediate transfer belt  21 . 
     Thereafter, when the belt driving motor  63  for rotation driving the intermediate transfer belt  21  is stopped, application of the bias voltage to the backup roll  25  is turned OFF. 
     In the embodiment  3 , as shown in FIG. 11, a constant bias current (for example, 60 μA) is made to flow to the backup roll  25  through the bias roll  46 , and a voltage applied to the secondary transfer roll  27  is measured by the resistance measurement part  80 , so that the system resistance of the secondary transfer portion is measured. Then, as shown in FIG. 12, the CPU  60  operates so that the optimum voltage value of cleaning bias is determined according to the resistance value of the system resistance from the table of resistance values of the system resistance and the cleaning bias previously stored in the RAM  65  or the like, and the optimum voltage value of the cleaning bias is applied to the backup roll  25  through the bias roll  46  by the high voltage power supply  47  for transfer bias. 
     Like this, since the output value of the cleaning bias voltage is controlled by actually measuring the system resistance between the backup roll  25  and the transfer roll  27  and control is made according to the measurement value of the system resistance, even in the case where an environmental variation or a change with time occurs, the output value of the cleaning bias voltage can be maintained such that the potential difference between the intermediate transfer belt  21  and the transfer roll  27  becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times. 
     Since the other structures and functions are the same as the embodiment 1, their description is omitted. 
     Embodiment 4 
     FIG. 13 shows embodiment 4 of the present invention, and the same portions as the embodiment 1 are designated by the same symbols. This embodiment 4 is constructed such that a cleaning bias voltage having a polarity opposite to a transfer bias voltage is applied to at least one of a backup roll and a transfer roll, and an output value of the cleaning bias voltage is controlled according to environmental variation. 
     That is, in the embodiment 4, as shown in FIG. 1, the humidity in the inside of the copying machine main body  1  is detected by the humidity sensor  59  provided in the inside of the copying machine main body  1 . Then, the CPU  60  obtains the absolute humidity from the following equation on the basis of the relative humidity detected by the humidity sensor  59 . 
     
       
         absolute humidity×10 −3 =15.375−0.077×(humidity)+0.027×(temperature) 2  ×(relative humidity)/100 
       
     
     Incidentally, as shown in FIG. 1, the temperature of the copying machine main body  1  is detected by the temperature sensor  66  provided together with the humidity sensor  59 . 
     As shown in FIG. 13, the CPU  60  determines an optimum voltage value of cleaning bias according to the absolute humidity from a table of absolute humidity and cleaning bias previously stored in the RAM  65  or the like, and the optimum voltage value of the cleaning bias is applied to the backup roll  25  through the bias roll  46  by the high voltage power supply  47  for transfer bias. 
     Like this, this embodiment is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to the backup roll  25 , and the output value of the cleaning bias voltage is controlled according to the environmental variation of humidity or the like. Thus, even in the case where the environment of temperature or humidity is changed, the output value of cleaning bias voltage can be maintained so that the potential difference between the intermediate transfer belt  21  and the secondary transfer roll  27  becomes optimum for cleaning, and it becomes possible to finely clean the transfer part at all times. 
     Since the other structures and functions are the same as the embodiment 1, their description is omitted. 
     Embodiment 5 
     FIG. 14 shows embodiment 5 of the present invention, and the same portions as the embodiment 1 are designated by the same symbols. This embodiment 5 is constructed such that a cleaning bias voltage having a polarity opposite to a transfer bias voltage is applied to at least one of a backup roll and a transfer roll, and an output value of the cleaning bias voltage is controlled according to a use history of the image forming apparatus. 
     That is, in this embodiment 5, the number of prints indicating the use history of the apparatus is counted with the CPU  60 , and as shown in FIG. 14, the CPU  60  determines an optimum voltage value of cleaning bias according to the number of prints from a table of the number of prints and cleaning bias previously stored in RAM  65  or the like, and the optimum voltage value of cleaning bias is applied to the backup roll  25  through bias roll  46 . 
     Like this, this embodiment is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to the backup roll  25  and the output value of the cleaning bias voltage is controlled according to the use history of the copying machine. Thus, even in the case where the system resistance of the secondary transfer portion is changed with the passage of time, the output value of the cleaning bias voltage can be maintained so that the potential difference between the intermediate transfer belt  21  and the transfer roll  27  becomes optimum for cleaning, and it becomes possible to finely clean the secondary transfer roll  27  at all times. 
     Since the other structures and functions are the same as the embodiment 1, their description is omitted. 
     Embodiment 6 
     FIG. 15 shows embodiment 6 of the present invention, and the same portions as the embodiment 1 are designated by the same symbols. This embodiment 6 is constructed such that a cleaning bias voltage having a polarity opposite to a transfer bias voltage is applied to at least one of a backup roll and a transfer roll, and a resistor of a predetermined value corresponding to a system resistance between the backup roll and the transfer roll is provided between a cleaning bias power supply and a transfer portion so that a potential difference between a belt-like image carrier and the transfer roll becomes a value suitable for cleaning. The resistance value of the resistor is set so that the optimum cleaning bias can always be applied against the change of the system resistance of the transfer portion. 
     That is, it has been clarified by the study of the present inventor et al. that the system resistance of the secondary transfer portion is changed within a range by the environment of temperature, humidity or the like or with the passage of time. Then, the embodiment 6 is constructed such that a resistor  90  is provided between the secondary transfer portion and the transfer bias power supply  47  so that an optimum cleaning bias can be obtained against the change of the system resistance of the secondary transfer portion. With respect to the resistance value of the resistor  90 , for example, in the case where the resistance of the secondary transfer portion is changed within the range of 30 MΩ to 300 MΩ, when the resistor of 10 MΩ is provided, even if a specific control such as a constant voltage control is not carried out, as shown in FIG. 16, it becomes possible to apply a low value corresponding to a case where the resistance value of system resistance is low, and a high value corresponding to a case where the resistance value of the system resistance is high. Incidentally, in FIG. 16, environment A indicates a high temperature high humidity environment, environment B indicates an environment at general room temperature, and environment C indicates a low temperature low humidity environment. 
     Since the control such as the constant voltage control is not carried out, it does not take a time required for the control and a responsibility also becomes excellent. 
     In the case where the resistance value of the resistor  90  provided between the secondary transfer portion and the transfer bias power supply  47  is changed, the relation between the actually applied voltage and environment (resistance value) is shown in FIG.  16 . 
     Like this, this embodiment is constructed such that the cleaning bias voltage having the polarity opposite to the transfer bias voltage is applied to the backup roll  25 , and the resistor of the predetermined value corresponding to the system resistance between the backup roll  25  and the secondary transfer roll  27  is provided between the cleaning bias power supply  47  and the transfer portion so that the potential difference between the intermediate transfer belt  21  and the secondary transfer roll  27  becomes the value suitable for cleaning. Thus, by the simple structure that the resistor  90  of the predetermined value corresponding to the system resistance between the backup roll  25  and the secondary transfer roll  27  is provided between the cleaning bias power supply  47  and the transfer portion, the potential difference between the intermediate transfer belt  21  and the secondary transfer roll  27  can be made the value suitable for cleaning, and even in the case where the environmental variation or change with time occurs, it becomes possible to finely clean the secondary transfer roll  27  at all times. 
     Since the other structures and functions are the same as the embodiment 1, their description is omitted. 
     Embodiment 7 
     FIG. 10 shows embodiment 7 of the present invention. The embodiment 7 is different from the embodiment 1 in the structure of an image forming part. The image forming part includes plural image forming units each of which includes an image carrier on which an electrostatic latent image is formed and a developing part for developing the electrostatic latent image formed on the image carrier with a toner of a predetermined color. Plural toner images of different colors are sequentially formed by the plural image forming units. 
     FIG. 10 shows a tandem type color electrophotographic copying machine as an image forming apparatus of the embodiment 7 of the present invention. 
     In FIG. 10, reference numeral  101  designates a main body of the tandem type digital color copying machine. A platen cover  103  for pressing an original document  102  onto a platen glass  105 , and an original document reading device  104  for reading an image of the original document  102  put on the platen glass  105  are disposed on the upper portion of the digital color copying machine main body  101  at one end side. In this original document reading device  104 , the original document  102  put on the platen glass  105  is illuminated by a light source  106 , a reflected light image from the original document  102  is scanned and exposed onto an image reading element  111  made of CCDs and the like through a reducing optical system constituted by a full rate mirror  107 , half rate mirrors  108 ,  109 , and an imaging lens  110 , and a colorant reflected light image of the original document  102  is read by this image reading element  111  at a predetermined dot density (for example, 16 dots/mm). 
     The colorant reflected light image of the original document  102  read by the original document reading device  104  is sent as, for example, original document reflectivity data of three colors of red (R), green (G) and blue (B) (each has 8 bits), to an IPS  112  (Image Processing System). In this IPS  112 , predetermined image processing, such as shading correction, position shift correction, brightness/color space conversion, gamma correction, frame erasure, or color/movement edition, is carried out to the reflectivity data of the original document  102 . 
     The image data subjected to the predetermined image processing by the IPS  112  as described above are converted into original document colorant gradation data of four colors of yellow (Y), magenta (M), cyan (C), and black (BK) (each has 8 bits), and are sent to ROSs  114 Y,  114 M,  114 C, and  114 BK (Raster Output Scanner) of image forming units  113 Y,  113 M,  113 C and  113 BK of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK). In these ROSs  114 Y,  114 M,  114 C and  114 BK, image exposure by a laser beam is carried out in accordance with the original document colorant gradation data of a predetermined color. 
     In the inside of the tandem type digital color copying machine main body  101 , the four image forming units  113 Y,  113 M,  113 C and  113 BK of yellow (Y), magenta (M), cyan (C) and black (BK) are disposed in the horizontal direction at a constant interval and in parallel. 
     All of these four image forming units  113 Y,  113 M,  113 C and  113 BK are structured in the same way, and each unit is roughly constructed by a photoreceptor drum  115  rotating in the direction of an arrow at a predetermined rotation speed, a primary charging screen corotron  116  which uniformly charges the surface of the photoreceptor drum  115 , a ROS  114  for forming an electrostatic latent image by exposing an image corresponding to each color onto the surface of the photoreceptor drum  115 , a developing unit  117  for developing the electrostatic latent image formed on the photoreceptor drum  115 , and a cleaning device  118 . 
     As shown in FIG. 10, in the ROS  114 , a semiconductor laser  119  is modulated in accordance with the original document colorant gradation data, and this semiconductor laser  119  emits a laser beam LB in accordance with the gradation data. The laser beam LB emitted from the semiconductor laser  119  is deflected and scanned by a rotary polygon mirror  122  through reflecting mirrors  120  and  121 , and is again scanned and exposed onto the photoreceptor drum  115  as an image carrier through the reflecting mirrors  120  and  121  and plural reflecting mirrors  123  and  124 . 
     Image data of the respective colors are sequentially outputted from the IPS  12  to the ROSs  114 Y,  114 M,  114 C and  114 BK of the image forming units  113 Y,  113 M,  113 C and  113 BK of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK). The laser beam LB emitted from the respective ROSs  114 Y,  114 M,  114 C and  114 BK in accordance with the image data is scanned and exposed onto the surface of the respective photoreceptor drums  115 Y,  115 M,  115 C and  115 BK and electrostatic latent images are formed. The electrostatic latent images formed on the respective photoreceptor drums  115 Y,  115 M,  115 C and  115 BK are developed as toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) by the developing units  117 Y,  117 M,  117 C and  117 BK. 
     The toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) sequentially formed on the photoreceptor drums  115 Y,  115 M,  115 C and  115 BK of the respective image forming units  113 Y,  113 M,  113 C and  1113 BK are transferred so as to overlap with each other by primary transfer rolls  126 Y,  126 M,  126 C and  126 BK onto an intermediate transfer belt  125  disposed below the respective image forming units  113 Y,  113 M,  113 C and  113 BK. This transfer belt  125  is put with a constant tension around a drive roll  127 , a stripping roll  128 , a steering roll  129 , an idle roll  130 , a backup roll  131 , and an idle roll  132 , and is driven to circulate at a predetermined speed in the direction of an arrow by the drive roll  127  which is driven to rotate by a not-shown dedicated driving motor having an excellent constant speed property. As the transfer belt  125 , for example, a synthetic resin film of PET or the like having flexibility is formed into a belt shape, and both ends of the beltshaped synthetic resin film are connected by a method such as welding to form an endless belt shape one, which is used. 
     The toner images of the respective colors of yellow (Y), magenta (M), cyan (C) and black (BK) transferred onto the transfer belt  125  so as to overlap with each other are secondarily transferred onto a recording sheet  134  through a pressing force and an electrostatic force by a secondary transfer roll  133  which comes in press contact with the backup roll  131 . The recording sheet  134  onto which the toner images of the respective colors have been transferred is conveyed to a fixing unit  137  by two conveying belts  135  and  136 . Then the recording sheet  134  on which the toner images of the respective colors have been transferred is subjected to a fixing process through heat and pressure by the fixing unit  137 , and is discharged onto a discharge tray  138  provided at the outside of the copying machine main body  101 . 
     As shown in FIG. 10, the recording sheet  134  of a predetermined size is once conveyed from either one of plural sheet feed cassettes  139 ,  140  and  141  to a registration roll  147  through a sheet conveying passage  146  made of a sheet feed roll  142  and sheet conveying roll pairs  143 ,  144  and  145 . The transfer sheet  134  supplied from either one of the sheet feed cassettes  139 ,  140  and  141  is sent onto the intermediate transfer belt  125  by the registration roll  147  which is driven to rotate at predetermined timing. 
     In the four image forming units  113 Y,  113 M,  113 C and  113 BK of yellow, magenta, cyan and black, as described above, the toner images of yellow, magenta, cyan and black are sequentially formed at the predetermined timing. 
     Incidentally, with respect the photoreceptor drums  115 Y,  115 M,  115 C and  115 BK, after the transfer step of the toner images is ended, the remaining toner, paper powder, and the like are removed by the cleaning devices  118 Y,  118 M,  118 C and  118 BK, and they are prepared for a next image forming process. Besides, with respect to the intermediate transfer belt  125 , the remaining toner is removed by a cleaner  148  for a belt. 
     Similarly to the embodiment 3, the tandem type color electrophotographic copying machine is also constructed such that there is provided a CPU as a transfer bias voltage application control part which makes such control that in the case where at least a part of the toner image transferred onto the non-image area  51  and the image area  50  on the intermediate transfer belt  125  is positioned on an area of the intermediate transfer belt  125  other than an area corresponding to the recording medium  134 , with respect to the backup roll  131  and the secondary transfer roll  133  as the second transfer part, a reverse transfer bias voltage having a polarity opposite to a transfer bias voltage is applied to at least the area other than the recording medium  134 . In this embodiment 7, similarly to the embodiments 3 to 6, for example, a cleaning bias voltage having a polarity opposite to the transfer bias voltage is applied to at least one of the backup roll and the transfer roll, and an output value of the cleaning bias voltage is controlled so that the potential difference between the belt-like image carrier and the transfer roll becomes optimum for cleaning. 
     Since the other structures and functions are the same as the embodiments 3 to 6, their description is omitted. 
     As described above, the present invention can provide the image forming apparatus capable of finely cleaning the transfer part at all times even in the case where the environmental variation or change with time occurs. 
     Moreover, as described above, the present invention can provide the image forming apparatus capable of finely cleaning the transfer roll at all times by regulating the relation between the mechanical adhesion force of toner to the surface of the transfer roll and the mechanical adhesion force of toner to the surface of the image carrier with which the transfer roll comes in press contact.