Patent Publication Number: US-6990309-B2

Title: Method and apparatus for image forming performing improved cleaning and discharging operation on image forming associated members

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/279,890, the entire contents of which are hereby incorporated by reference, and this application claims priority rights of and is based on Japanese patent applications respectively filed in the Japanese Patent Office as listed below, the entire contents of which are hereby incorporated by reference. 
   JPAP10-333074 filed on Nov. 24, 1998 
   JPAP10-346365 filed on Dec. 7, 1998 
   JPAP10-346334 filed on Dec. 7, 1998 
   JPAP10-346435 filed on Dec. 7, 1998 
   JPAPxx-xxxxxx filed on October xx, 1999 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming in which cleaning and discharging operations are efficiently performed relative to an image carrying member, an intermediate transfer member, and associated members. 
   2. Discussion of the Background 
   In image forming apparatuses such as copying machines, facsimile machines, printers, etc., a large number of techniques have-been introduced, relating to cleaning and discharging of members associated with an image forming operation involving usage of toner. In particular, cleaning and discharging are important in a full-color image forming apparatus which is provided with an intermediate transfer member in addition to a commonly-used image carrying member. In such a full-color image forming apparatus, primary and secondary transfer operations are in turn performed so as to transfer a plurality of mono-color-toner images separately formed on the image carrying member onto a transfer sheet at one time via the intermediate transfer member. 
   More specifically, the image carrying member and the intermediate transfer member are arranged to contact each other so as to perform a primary transfer operation for transferring each mono-color-toner image from the image carrying member to the intermediate transfer member. For this, the full-color image forming apparatus is provided with a charge applying member for applying a charge to the intermediate transfer member to generates an electric field which generates a force to help such primary transfer operation. After a number of times of the primary transfer operation, a plurality of mono-color-toner images are overlaid with precision as one full-color-toner image on the intermediate transfer member. Then, a secondary transfer operation is performed to transfer this full-color-toner image held on the intermediate transfer member onto a transfer sheet which is also in contact with the intermediate transfer member. 
   The above-described intermediate transfer member is often used in a belt shape or a drum shape. An intermediate transfer belt, for example, typically has a medium range of a volume resistivity from about 10 8  Ωcm to about 10 11  Ωcm, which normally does not require operations for discharging the surface of the intermediate transfer belt. This helps the cost reduction. 
   In using such an intermediate transfer member having a medium range of volume resistivity, the surface of the intermediate transfer member is applied with a bias to perform the primary transfer operation and thus has a charge thereon. However, this charge will leak through members in contact with the rear surface of the intermediate transfer member and no charge will therefore remain on the surface of the intermediate transfer member in a relatively short time period after the application of the charge. 
   As a result, the intermediate transfer member has the voltage which is 0 and greatly different from the voltage of the toner image transferred through the primary transfer operation. Due to this voltage difference, toner particles forming the toner image, particularly the topmost-laid mono-color-toner image, are attracted to the surface of the intermediate transfer member. This results in a toner dispersion in which the toner particles are dispersed on the surface of the intermediate transfer member. Such a toner dispersion may badly cause a dirty background of an image, a blur of an image such as letters, and so forth and therefore make an image deteriorated in quality. 
   To avoid this problem, the image forming apparatus has used the intermediate transfer member which has a high volume resistivity of about 10 13  Ωcm. In using the intermediate transfer member having the high volume resistivity, the surface of the intermediate transfer member charges during the primary transfer operation due to an occurrence of discharge from the image carrying member and thus increases the voltage on the surface. Because of the high volume resistivity, the charge on the surface of the intermediate transfer member will not leak through the members in contact with the rear surface of the intermediate transfer member. Thereby, the difference of voltages between the intermediate transfer member and the toner image held on the intermediate transfer member is made relatively smaller. This helps to prevent the above-described toner dispersion. 
   In this case using the intermediate transfer member having the high volume resistivity, or the volume resistivity of at least 10 11  Ωcm, the charge will remain on the surface of the intermediate transfer member till the time when the next primary transfer operation starts. This makes it difficult to generate the same electric field as made during the previous primary transfer operation. In this case, accordingly, the charge remaining on the surface of the intermediate transfer member need to be discharged before starting the next primary transfer operation. 
   In addition, when a transfer sheet is jammed during the image forming operation in the image forming apparatus, the toner image held on the intermediate transfer member may pass a region where the secondary transfer operation is conducted, without being actually transferred onto a transfer sheet. This toner image needs, of course, to be removed before the next toner image is formed on the intermediate transfer member. However, a common cleaning member such as a cleaning blade alone cannot sufficiently remove the toner because the full-color image forming apparatus uses a relatively large amount of toner during one time of the image forming process. 
   Conventionally, a corona charger is widely used as a non-contact-type discharging member for discharging the image carrying member and other members associated with the image forming process in an image forming apparatus. Such a non-contact type of discharging member typically generates ozone during discharging, which is undesired from the environmental aspect. In addition, the discharging member needs an application of discharging bias which is generated from an expensive high voltage AC (alternating current) power source. This increase a manufacturing cost. 
   In addition, the above-described intermediate transfer member having a relatively high volume resistivity changes its volume resistivity in accordance with various environmental factors such as temperature, humidity, and so forth. The intermediate transfer member also changes a charger level on the surface thereof in accordance with a number of layers of mono-color toner image. With these changes, if the discharging bias is not variable, the discharging operation may not sufficiently be performed, causing a reduction of efficiency of the primary transfer operation. 
   As for the cleaning in the full-color image forming apparatus, it is required a relatively high level of cleaning performance, as described above. Conventionally, this is achieved by pressing the cleaning member relative to the intermediate transfer member. However, since the intermediate transfer member is rotating, the adjustment of pressure by the cleaning member has a relatively narrow margin and therefore it cannot be adjusted in a satisfactory manner. 
   In addition, the above-described discharging operation is needed to be performed relative to a transfer sheet carrying member as well as the intermediate transfer member. The transfer sheet carrying member carries a transfer sheet having a toner image transferred from the intermediate transfer member through the secondary transfer operation. During the secondary transfer operation, the transfer sheet carrying member is commonly applied with a bias to help the performance of the secondary transfer operation. This bias may remain on the transfer sheet carrying member after the secondary transfer operation and interferes the generation of the electric field for the next secondary transfer operation, resulting in an inferior image quality. Such a charge problem on the transfer sheet carrying member is addressed by employing a non-contact-type discharging member which involves an ozone problem. 
   SUMMARY OF THE INVENTION 
   The present application relates to a novel image forming apparatus which includes an image carrying member, an intermediate transfer member, a charging member, a transfer mechanism, a discharging member, a direct current voltage source, and a direct current voltage controller. The image carrying member rotates and carries a toner image on a rotating surface thereof. The intermediate transfer member is deposited at a position facing and in contact with the image carrying member, rotates and receives the toner image from the image carrying member during a first transfer operation. The charging member applies a charge to the intermediate transfer member to cause an electric field around a region where the image carrying member and the intermediate transfer member contact with each other, where the electric field generates a force for initiating the first transfer operation. The transfer mechanism performs a second transfer operation for transferring the toner image from the intermediate transfer member to a transfer sheet. The discharging member performs a discharging operation for discharging the charge remaining on the intermediate transfer member with contacting the intermediate transfer member after a completion of the second transfer operation. The direct current voltage source applies a direct current voltage to the discharging member to cause the discharging member to perform the discharging operation. The direct current voltage controller controls the direct current voltage in accordance with a volume resistivity of the intermediate transfer member. 
   The above-mentioned volume resistivity of the intermediate transfer member may be in a range of about 10 11  Ωcm to about 10 14  Ωcm, or in a range of about 10 12  Ωcm to about 10 13  Ωcm. 
   The present application also relates to a novel method of image forming which includes the steps of providing, rotating, charge applying, performing, direct current voltage applying, and controlling. The providing step provides a toner image to an carrying member for rotating and carrying the toner image on a rotating surface thereof. The rotating step rotates an intermediate transfer member which is arranged at a position facing and in contact with the image carrying member. The charge applying step applies a charge to the intermediate transfer member to cause an electric field around a region where the image carrying member and the intermediate transfer member contact with each other so that the electric field generates a force for initiating a first transfer operation for transferring the toner image from the image carrying member to the intermediate transfer member. The performing step performs a second transfer operation for transferring the toner image from the intermediate transfer member to a transfer sheet. The direct current voltage applying step applies a direct current voltage to the discharging member to cause the discharging member which discharges the charge remaining on the intermediate transfer member with contacting the intermediate transfer member after a completion of the second transfer operation. The controlling step controls the direct current voltage in accordance with a volume resistivity of the intermediate transfer member. 
   Further, the present application relates to another novel image forming apparatus which includes an image carrying member, an intermediate transfer member, a charging member, a transfer mechanism, a discharging member, a direct current voltage source, a voltage detect sensor, and a direct current voltage controller. The image carrying member rotates and carries a toner image on a rotating surface thereof. The intermediate transfer member which is deposited at a position facing and in contact with the image carrying member, rotates and receives the toner image from the image carrying member during a first transfer operation. The charging member applies a charge to the intermediate transfer member to cause an electric field around a region where the image carrying member and the intermediate transfer member contact with each other, where the electric field generates a force for initiating the first transfer operation. The transfer mechanism performs a second transfer operation for transferring the toner image from the intermediate transfer member to a transfer sheet. The discharging member performs a discharging operation for discharging the charge remaining on the intermediate transfer member with contacting the intermediate transfer member after a completion of the second transfer operation. The direct current voltage source applies a direct current voltage V to the discharging member to cause the discharging member to perform the discharging operation. The voltage detect sensor detects a surface voltage Va of the intermediate transfer member. The direct current voltage controller controls the direct current voltage V in a way such that the direct current voltage V relative to the surface voltage Va satisfies a range of
 
[−1.3Va−650#V#−1.3 va+550].
 
   Further, the present application also relates to a method of image forming which includes the steps of providing, rotating, charge applying, performing, detecting, direct current voltage applying, and controlling. The providing step provides a toner image to an carrying member for rotating and carrying the toner image on a rotating surface thereof. The rotating step rotates an intermediate transfer member which is arranged at a position facing and in contact with the image carrying member. The applying step applies a charge to the intermediate transfer member to cause an electric field around a region where the image carrying member and the intermediate transfer member contact with each other so that the electric field generates a force for initiating a first transfer operation for transferring the toner image from the image carrying member to the intermediate transfer member. The performing step performs a second transfer operation for transferring the toner image from the intermediate transfer member to a transfer sheet. The detecting step detects a surface voltage Va of the intermediate transfer member. The applying step applies a direct current voltage V to the discharging member to cause the discharging member which discharges the charge remaining on the intermediate transfer member with contacting the intermediate transfer member after a completion of the second transfer operation. The controlling step controls the direct current voltage V in a way such that the direct current voltage V relative to the surface voltage Va satisfies a range of
 
[−1.3Va−650#V#−1.3 va+550].
 
   Further, the present application also relates to a novel image forming apparatus which includes an image carrying member, an intermediate transfer member, a charging member, a transfer mechanisms a discharging member, a direct current voltage source, a judging mechanism, and a direct current voltage controller. The image carrying member rotates and carries a toner image on a rotating surface thereof. The intermediate transfer member which is deposited at a position facing and in contact with the image carrying member, rotates and receives the toner image from the image carrying member during a first transfer operation which is performed for one time in a mono color mode and is repeated for a plurality of times in a multiple color mode to overlay a plurality of mono color toner images in turn on the intermediate transfer member. The charging member applies a charge to the intermediate transfer member to cause an electric field around a region where the image carrying member and the intermediate transfer member contact with each other, where the electric field generates a force for initiating the first transfer operation. The transfer mechanism performs a second transfer operation for transferring the toner image from the intermediate transfer member to a transfer sheet. The discharging member performs a discharging operation for discharging the charge remaining on the intermediate transfer member with contacting the intermediate transfer member after a completion of the second transfer operation. The direct current voltage source applies a direct current voltage to the discharging member to cause the discharging member to perform the discharging operation. The judging mechanism judges as to whether the apparatus is in the mono color mode or in the multiple color mode. The direct current voltage controller controls the direct current voltage in accordance with a result of judgement by the judging mechanism. 
   Further, the present application also relates to a novel method of image forming which includes providing, rotating, charge applying, performing, judging, direct current voltage applying, and controlling. The providing step provides a toner image to an carrying member for rotating and carrying the toner image on a rotating surface thereof. The rotating step rotates an intermediate transfer member which is arranged at a position facing and in contact with the image carrying member. The charge applying step applies a charge to the intermediate transfer member to cause an electric field around a region where the image carrying member and the intermediate transfer member contact with each other so that the electric field generates a force for initiating a first transfer operation for transferring the toner image from the image carrying member to the intermediate transfer member. The above-mentioned first transfer operation is performed for one time in a mono color mode and is repeated for a plurality of times in a multiple color mode to overlay a plurality of mono color toner images in turn on the intermediate transfer member. The performing step performs a second transfer operation for transferring the toner image from the intermediate transfer member to a transfer sheet. The judging step judges as to whether the apparatus is in the mono color mode or in the multiple color mode. The direct current voltage applying step applies a direct current voltage to the discharging member to cause the discharging member to discharge the charge remaining on the intermediate transfer member with contacting the intermediate transfer member after a completion of the second transfer operation. The controlling step controls the direct current voltage in accordance with a result of judgement by the judging mechanism. 
   Further, the present application also relates to a novel lubricant applying apparatus for applying a lubricant to an intermediate transfer member in an image forming apparatus. The above-mentioned novel lubricant applying apparatus includes a lubricant applying member for applying a lubricant to the intermediate transfer member and discharging a charge remaining on the intermediate transfer member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  illustrates an exemplary configuration of an image forming apparatus according to a first embodiment of the present invention; 
       FIG. 2  illustrates an exemplary structure around a photosensitive drum of the image forming apparatus of  FIG. 1 ; 
       FIG. 3  illustrates an exemplary structure around a photosensitive drum of an image forming apparatus according to a second embodiment of the present invention; 
       FIG. 4  illustrates a block diagram of a specific example of a controller included in the image forming apparatus of  FIG. 2 ; 
       FIG. 5  is a graph for explaining a relationship between a volume resistivity of an intermediate transfer belt and a surface voltage of the intermediate transfer belt after a secondary transfer operation in the image forming apparatus of  FIG. 2 ; 
       FIGS. 6–8  illustrate block diagrams of other specific examples of the controller included in the image forming apparatus of  FIG. 2 ; 
       FIG. 9  illustrates a main portion of a printer of an image forming apparatus according to a third embodiment of the present invention; 
       FIG. 10  illustrates a block diagram of an exemplary controller of the image forming apparatus of  FIG. 9 ; 
       FIGS. 11A–11C  are graphs for explaining experimental results with variations of environmental conditions on an implementation version based on the image forming apparatus of  FIG. 9 ; 
       FIG. 12  is a time chart for explaining a timing of application of a discharging bias in the implementation version of the image forming apparatus of  FIG. 9 ; 
       FIG. 13  illustrates a main portion of a printer of an image forming apparatus according to a fourth embodiment of the present invention; 
       FIG. 14  illustrates an exemplary transfer unit of an image forming apparatus according to a fifth embodiment of the present invention; 
       FIG. 15  illustrates an exemplary configuration of an image forming apparatus according to a six embodiment of the present invention; 
       FIG. 16  illustrates an exemplary structure around a photosensitive drum of the image forming apparatus of  FIG. 15 ; 
       FIG. 17  illustrates an exemplary structure around a photosensitive drum of a modified version of the image forming apparatus of  FIG. 15 ; 
       FIG. 18  illustrates an exemplary structure around a photosensitive drum of an image forming apparatus according to a seventh embodiment of the present invention; 
       FIG. 19  illustrates a main portion of a printer of an image forming apparatus according to an eighth embodiment of the present invention; 
       FIG. 20  illustrates an enlarged cleaning blade of a belt cleaning unit of  FIG. 19 ; 
       FIG. 21  illustrates a main portion of a printer of an image forming apparatus according to a ninth embodiment of the present invention; 
       FIG. 22  is an enlarged view of a cleaning blade of a belt cleaning unit and a cleaning facing roller of an intermediate transfer belt included in the image forming apparatus of  FIG. 21 ; 
       FIG. 23  illustrates an exemplary transfer unit of an image forming apparatus according to a tenth embodiment of the present invention; 
       FIG. 24  illustrates an exemplary configuration of an image forming apparatus according to an eleventh embodiment of the present invention; 
       FIG. 25  illustrates an exemplary structure around a photosensitive drum of the image forming apparatus of  FIG. 24 ; 
       FIG. 26  illustrates an exemplary structure of a lubricant applying unit of the image forming apparatus of  FIG. 24 ; 
       FIG. 27  illustrates an exemplary structure of a brush roller of the lubricant applying unit of the image forming apparatus of  FIG. 24 ; 
       FIG. 28  illustrates an exemplary structure around a photosensitive drum with respect to a modification made on the image forming apparatus of  FIG. 24 ; 
       FIG. 29  illustrates a main portion of a printer of an image forming apparatus according to a twelfth embodiment of the present invention; 
       FIG. 30  illustrates an exemplary structure of a moving mechanism for moving a lubricant applying brush roller and a cleaning blade in the image forming apparatus of  FIG. 29 ; 
       FIGS. 31A and 31B  illustrate enlarged structures of the moving mechanism of  FIG. 30 ; 
       FIG. 32  illustrates a main portion of a printer of an image forming apparatus according to a thirteenth embodiment of the present invention; 
       FIG. 33  illustrates an exemplary transfer unit of an image forming apparatus according to a fourteenth embodiment of the present invention; 
       FIG. 34  illustrates an exemplary configuration of an image forming apparatus according to a fifteen embodiment of the present invention; 
       FIG. 35  illustrates an exemplary structure around a photosensitive drum of the image forming apparatus of  FIG. 34 ; 
       FIG. 36  illustrates an exemplary structure around a photosensitive drum with respect to a first modification made on the image forming apparatus of  FIG. 34 ; 
       FIG. 37  illustrates an exemplary structure around a photosensitive drum with respect to a second modification made on the image forming apparatus of  FIG. 34 ; 
       FIG. 38  is a graph for explaining a relationship between a discharging bias of a discharging brush and a surface voltage on the intermediate transfer belt when a conductive plate is provided and when a conductive plate is not provided; 
       FIG. 39  illustrates an exemplary structure around a photosensitive drum with respect to a third modification made on the image forming apparatus of  FIG. 34 ; 
       FIG. 40  illustrates a discharging brush roller of the third modification made on the image forming apparatus of  FIG. 34 ; 
       FIG. 41  is a graph for explaining a relationship between a filling density of the discharging brush roller and the surface voltage of the intermediate transfer belt in the third modification made on the image forming apparatus of  FIG. 34 ; 
       FIG. 42  is a graph for explaining a relationship between a brush pile gap of the discharging brush roller and the surface voltage of the intermediate transfer belt in the third modification made on the image forming apparatus of  FIG. 34 ; 
       FIG. 43  illustrates a main portion of a printer of an image forming apparatus according to a sixteenth embodiment of the present invention; 
       FIG. 44  illustrates a main portion of a printer of an image forming apparatus according to a seventeenth embodiment of the present invention; and 
       FIG. 45  illustrates a main portion of a printer of an image forming apparatus according to an eighteenth embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the present invention is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner. 
   Various embodiment of the present invention will hereinafter be described with reference to the accompanying drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views. 
   [Embodiment 1] 
   To begin with, a first embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied. 
     FIG. 1  is a cross-sectional view schematically illustrating the configuration of the copier according to the first embodiment, and  FIG. 2  is an enlarged view schematically illustrating the structure around a photosensitive drum serving as an image carrier in the copier of  FIG. 1 . The illustrated copier is generally formed of a color image reader unit  1  (hereinafter referred to as the “scanner unit  1 ”) and a color image recording unit  2  (hereinafter referred to as the “printer unit  2 ”). 
   First, the scanner unit  1  in the copier will be described in terms of the structure and operation. In this scanner unit  1 , an image of an original  3  carried on a contact glass is focused on a color sensor  7  through an illumination lamp  4 , a group of mirrors ( 5   a ,  5   b ,  5   c ), and a lens  6 . The sensor  7  reads color image information of the original  3 , for example, for each separated color light components Blue (hereinafter abbreviated as “B”), Green (“G”), and Red (“R”), and transduces the color image information to electrical image signals. The color sensor  7 , which is composed of B, G, R color separating means and a photo-electric transducing element such as a CCD (charge coupled device), has the ability of simultaneously reading three colors. Respective image signals B, G, R produced in the scanner unit  1  are subjected to color conversion processing in an image processing unit based on their respective intensity levels. The color conversion processing results in color image data composed of Black (hereinafter abbreviated as “Bk”), Cyan (“C”), Magenta (“M”), and Yellow (“Y”). More specifically, an illumination/mirror optical system of the scanner unit  1  is responsive to a start signal associated with the printer unit  2  to scan an original in a direction indicated by an arrow A in  FIG. 1  to acquire color image data. In the first embodiment, image data for one color is acquired each time the illumination/mirror optical system scans an original, so that the illumination/mirror optical system must scan a total of four times in order to acquire color image data for the four colors Bk, C, M, Y. 
   Next, the printer unit  2  of the copier according to the first embodiment will be described in terms of the structure and operation. 
   The printer unit  2  includes an optical writing unit  8  as an exposing means, and a photosensitive drum  10  as an image carrier. The optical writing unit  8  transduces color image data from the above-mentioned scanner unit  1  to an optical signal, and forms a negative latent image corresponding to an original image on the photosensitive drum  10  which is uniformly charged in the negative polarity. The optical writing unit  8  may be composed of a semiconductor laser  8   a ; a light emission driving controller, not shown, for controlling emission and driving of the semiconductor laser  8   a ; a polygon mirror  8   b ; a rotation driving motor  8   c  for rotating the polygon mirror  8   b ; an fθ lens  8   d ; and a reflection mirror  8   e . The photosensitive drum  10  is driven to rotate in the counter-clockwise direction, i.e., in a direction indicated by an arrow B in  FIG. 1 . 
   The printer unit  2  further includes, around the photosensitive drum  10 , a photosensitive drum cleaning unit  11 ; a discharging lamp  12 ; a charger  13 ; a potential sensor  14 ; a set of a Bk developing device  15 , a C developing device  16 , an M developing device  17  and Y developing device  18 ; a developer concentration pattern detector  19 ; and an intermediate transfer unit  20 . 
   As can be seen in  FIG. 2 , the photosensitive drum cleaning unit  11  has a pre-cleaning discharger  11   a , and a fur brush  11   b  and a photosensitive drum cleaning blade  11   c  as cleaning members, and is provided for cleaning the surface of the photosensitive drum  10  after primary transfer (transfer from the photosensitive drum to an intermediate transfer belt). 
   Each of the developing devices  15 – 18  has a developing paddle ( 15   b ,  16   b ,  17   b ,  18   b ) as an agitating means for scooping up and agitating an associated developer; a toner concentration sensor ( 15   c ,  16   c ,  17   c ,  18   c ) for sensing the toner concentration of the developer; and a developing sleeve ( 15   a ,  16   a ,  17   a ,  18   b ) as a developer carrier for bringing a sleeve or ear of the developer into contact with the surface of the photosensitive drum  10 . For developers contained in the four, developing devices, two-component developers may be used. Toners mixed in the developers are negatively charged. When the copier proceeds to a standby state, the four developing devices remove ears on the respective developing sleeves, and proceeds to an inoperative state. 
   The intermediate transfer unit  20  includes an intermediate transfer belt  21 ; a primary transfer bias roller  22  as a charge supply means; a primary transfer power supply  28  connected to the primary transfer bias roller  22 ; a ground roller  23  as a pre-primary transfer discharging means; a driving roller  24  as a belt driving means; and a driven roller  25 . The intermediate transfer belt  21  is passed over the primary transfer bias roller  22 , the ground roller  23 , the driving roller  24 , and the driven roller  25 . The driving roller  24 , connected to a driving motor  24   a , controls the driving of the intermediate transfer belt  21 . 
   The intermediate transfer belt  21  is formed in a multi-layer structure composed of a surface layer, an intermediate layer and a base layer, and is placed such that the surface layer is positioned on the outer peripheral side which contacts the photosensitive drum  10 , and the base layer is positioned on the inner peripheral side. In addition, an adhesive layer is interposed between the intermediate layer and the base layer for adhering the two layers. The intermediate transfer belt  21  is formed to have the volume resistivity ρv, as measured by the method described in JISK6911, in a range of 10 7  Ωcm to 10 14  Ωcm, preferably in a range of 10 12  Ωcm to 10 14  Ωcm, and more preferably equal to approximately 10 13  Ωcm. It should be noted that while a material having the volume resistivity of 10 14  Ωcm or more might be utilized, it is not suitable for the intermediate transfer belt for the intended purpose in the present invention from a viewpoint of durability and so on. 
   Around the intermediate transfer belt  21 , there are disposed a contact-type discharger  50 ; a belt cleaning unit  29 ; and a transfer unit  30 . The belt cleaning unit  29  has a brush roller  29   a  and a rubber blade  29   b  as cleaning members, and a belt contact/separation mechanism  29   c . This belt contact/separation mechanism  29   c  enables the intermediate cleaning unit  29  to move into and out of contact with the intermediate transfer belt  21 . The transfer unit  30  also has a secondary transfer bias roller  31  opposite to the driving roller  24  of the intermediate transfer unit  20 ; a transfer cleaning blade  32 ; and a transfer contact/separation mechanism  33 . This transfer contact/separation mechanism enables the transfer unit  30  to move into and out of contact with the intermediate transfer belt  21 . 
   The primary transfer bias roller  22  for tensioning the intermediate transfer belt  21  is positioned downstream of a primary transfer region defined by a nip formed-by a contact between the intermediate transfer belt and the photosensitive drum  10  in a direction in which the surface of the intermediate transfer belt runs, i.e., in a belt moving direction. The primary transfer bias roller  22  is applied with a predetermined primary transfer bias by the primary transfer power supply  28 . The ground roller  23  is disposed upstream of the nip in the belt moving direction. The intermediate transfer belt  21  is pressed against the photosensitive drum  10  by the primary transfer bias roller  22  and the ground roller  23 , whereby the nip is formed. 
   The printer unit  2  also has a paper feed roller  41  for feeding a transfer paper  100  as a transfer material to a secondary transfer region formed between the secondary transfer bias roller  31  of the transfer unit  30  and the driving roller  24  of the intermediate transfer unit  20 ; a resist roller  42 ; transfer paper cassettes  43   a ,  43   b ,  43   c  for accommodating transfer papers  100  of various sizes; a hand feed tray  40  for use in copying an image on an OHP (overhead projector) sheet, rather thick paper, or the like; a paper conveying unit  44 ; a fixing unit  45 ; and a copy tray  46 . 
   Next, the operation of the copier will be described in connection with an illustrative image forming mode in which the development is performed in the order of Bk, C, M, Y. It should be of course understood that image formation is not limited to this particular order. 
   Once a copy operation is initiated, a Bk step is first started, wherein color image information of an original is read in the scanner unit  1 , and a Bk latent image is formed on the photosensitive drum  10  by laser light generated from the optical writing unit  8  based on Bk image data derived from the image information in the printer unit  20 . The Bk latent image is applied with toner by the Bk developing device  15 , and developed by forming a Bk toner image. In this event, the developing sleeve  15   a  has been previously rotated before the leading edge of the Bk latent image arrives at a developing position of the Bk developing device  15  in order to ensure that the Bk latent image is completely developed In this way, since the developer has already formed a sleeve or ear when the leading edge of the Bk latent image arrives at the developing position of the Bk developing device  15 , it is ensured that the entire Bk latent image can be developed. Also, in the Bk developing device  15 , at the time the trailing edge of the Bk latent image has passed the developing position, the sleeve or ear of the developer formed on the developing sleeve  15   a  is immediately discontinued. This causes the Bk developing device  15  to proceed to an inoperative state. At this time, the Bk developing device  15  should be completely inoperative before the leading edge of a C latent image, to be next developed, arrives at the developing position of the Bk developing device  15 . The developer ear may be discontinued by switching the developing sleep  15   a  to the direction reverse to the rotating direction during the developing operation. 
   The Bk toner image thus formed on the photosensitive drum  10  by the Bk developing device  15  is transferred to the surface of the intermediate transfer belt  21  which is driven at the same speed as the photosensitive drum  10  (primary transfer), followed by termination of the Bk step. 
   In parallel with the primary transfer of the Bk toner image, the next C step is started on the photosensitive drum  10 . Specifically, color image information of the original is again read at a predetermined timing, a C latent image is formed on the photosensitive drum  10  by laser light based on C image data derived from the image information, and a C toner image is formed by the C developing device  16 . The rotation of the developing sleeve  16   a  in the C developing device  16  is started after the trailing edge of the Bk latent image has passed a developing position of the C developing device  16  and before the leading edge of the C latent image arrives at the developing position. Then, at the time the trailing edge of the C latent image has passed the developing position, a developer ear formed on the developing sleeve  16   a  is discontinued as is the case of the aforementioned Bk developing device  15 , and the C developing device  16  is made inoperative. Again, in this event, the C developing device  16  should be completely inoperative before the leading edge of the next M latent image arrives. The C toner image thus developed and formed on the photosensitive drum  10  is transferred to an image surface area of the intermediate transfer belt  21  in precise register with the Bk toner image which has been transferred to the image surface area. 
   Subsequently, in an M step and a Y step, the formation of latent image, development, and primary transfer are performed respectively based on their respective image data in a manner similar to the aforementioned C step. By transferring the respective Bk, C, M and Y toner images sequentially formed on the photosensitive drum  10  to the same image surface area on the intermediate transfer belt  21 , a complete toner image formed of the four color images in accurate register with one another is formed on the intermediate transfer belt  21 . 
   Now, the operation of the intermediate transfer belt  21  will be described referring again to  FIG. 2 . 
   While the aforementioned Bk, C, M and Y toner images are transferred to the photosensitive drum  10 , for example, from the termination of the primary transfer of the first color (Bk) toner image to the initiation of the primary transfer of the second color (C) toner image, the intermediate transfer belt  21  may be driven in accordance with a constant speed forward mode, a skip forward mode, reciprocation (quick return) mode, or the like. While any driving mode selected from these illustrative driving modes may be fixedly employed for the intermediate transfer belt  21 , a suitable driving mode may be selected from the three modes in accordance with a copy size for increasing the copy speed, or a plurality of driving modes may be efficiently used in combination. 
   In the following, the illustrative driving modes will be briefly described. The constant speed forward mode performs the primary transfer while driving the intermediate transfer belt in one direction at a low speed. The skip forward mode, which also drives the intermediate transfer belt in one direction similarly to the constant speed forward mode, moves the intermediate transfer belt away from the photosensitive drum after a toner image has been transferred thereto, skip forwards the intermediate transfer belt in the same direction at a higher speed, and then brings the intermediate transfer belt back to the start position of the primary transfer for performing the next primary transfer. This sequence of operations is repeated for the four color toner images. The reciprocation (quick return) mode, unlike the skip forward mode, returns the intermediate transfer belt to the start position of the primary transfer in the reverse direction at a higher speed in preparation for the next primary transfer, after the primary transfer is performed to the intermediate transfer belt and the intermediate transfer belt is moved away from the photosensitive drum. This sequence of operations are repeated for the four color toner images. 
   During a time period in which a complete toner image is formed on the intermediate transfer belt  21 , specifically, during a time period from the time the first color (Bk) toner image had been transferred to the intermediate transfer belt  21  to the time the fourth color (Y) toner image has been transferred to the same, the discharging brush  51 , the belt cleaning unit  29 , and the transfer unit  30  are separated away from the intermediate transfer belt  21  by the respective contact/separation mechanisms. 
   The toner image transferred to the intermediate transfer belt  21  in the manner described above is conveyed to the secondary transfer region for secondary transfer to a transfer paper  100 . In this event, the secondary transfer bias roller  31  of the transfer unit  30  is generally pressed against the intermediate transfer belt  21  by the transfer contact/separation mechanism  33  at the timing the toner image is transferred to the transfer paper  100 . Subsequently, the secondary transfer bias roller  31  is applied with a predetermined secondary transfer bias by a secondary transfer power supply, not shown, to form a secondary transfer electric field in the secondary transfer region. The secondary transfer electric field causes the toner image on the intermediate transfer belt  21  to be transferred to the transfer paper  100 . The transfer paper  100  is conveyed from a transfer paper cassette  43   a ,  43   b ,  43   c  of a size specified by an operator on an operation panel, not shown, in a direction toward the resist roller  42 , and fed into the secondary transfer region. More specifically, the transfer paper  100  is fed into the secondary transfer region at the timing coincident with the arrival of the leading edge of the toner image on the intermediate transfer belt  21  to the secondary transfer region. 
   The transfer paper  100 , on which the complete toner image formed of four color toner images in accurate register with one another has been collectively transferred from the intermediate transfer belt  21 , is subsequently conveyed to a fixing unit  45  by the paper conveying unit  44 . The unfixed toner image on the transfer paper  100  is melted between a pair of fixing rollers consisting of a fixing roller  45   a  controlled at a predetermined temperature and a press roller  45   b , and the unfixed toner image is fixed. Then, after the fixation, the transfer paper  100  is conveyed to and stacked on the copy tray  46 . 
   After the primary transfer, the surface of the photosensitive drum  10  is cleaned by the photosensitive drum cleaning unit  11 , and uniformly discharged by the discharging lamp  12 . Also, after the secondary transfer, the surface of the intermediate transfer belt  21  is cleaned by the belt cleaning unit  29  which is pressed against the intermediate transfer belt  21  by the belt cleaning contact/separation mechanism  29   c.    
   For repetitively copying the same original, in the scanner unit  1 , the first color (Bk) step is started for the second copy at a predetermined timing subsequent to the fourth color (Y) step on the first copy. In the printer unit  2 , in turn, a Bk latent image is formed on the photosensitive drum  10 . On the intermediate transfer belt  21 , on the other hand, the first color (Bk) toner image for the second copy is transferred to the region on the intermediate transfer belt  21 , which has been cleaned by the belt cleaning unit  29 , subsequent to the secondary transfer of the complete toner image  2  for the first copy. 
   While the operation of the copier has been described in connection with a copy mode for producing full-color or four-color copies, the same description is applicable to other copy modes, i.e, a three-color copy mode and a two-color copy mode, except that used colors and associated mechanisms are different. For a single-color copy mode, a developer in a developing device associated with a selected color is maintained to form a sleeve or ear, i.e., the developing device is maintained in operative state until a predetermined number of copies have been produced. Also, with the discharging brush  51 , the belt cleaning unit  29  and the transfer unit,  30  maintained in contact with the intermediate transfer belt  21  and with the intermediate transfer belt  21  maintained in contact with the photosensitive drum  10 , the intermediate transfer belt  21  is driven in the forward direction at a constant speed for producing copies. 
   In the following, description will be made on the configuration and operation of the contact-type discharger  50  which constitutes a characterizing portion of the first embodiment. 
   The contact-type discharger  50  of the first embodiment has the discharging brush  51  and the discharge power supply  59  for applying the discharging brush  51  with a discharging bias. As can be seen in  FIG. 2 , the contact-type discharger  50  is positioned downstream of the belt cleaning unit  29  and upstream of the ground roller  23  in the direction of the movement of the intermediate transfer belt  21 . Instead of the illustrated discharging brush  51 , a discharging blade, a discharge roller, and a discharging brush roller may be used by way of example. 
   The discharging brush  51  is grounded through the discharge power supply  59 . The discharging brush  51  is applied by the discharge power supply with a direct current or an alternate current discharging bias, or with a combination of direct current and alternate current discharging biases. In this event, when a direct current power supply for applying a direct current voltage is employed as the discharge power supply  59 , a reduction in cost is expected. The first embodiment employs a regulated direct current power supply as the discharge power supply  59 . In addition, since the residual potential on the intermediate transfer belt  21  is negative, the discharge power supply  59  applies the discharging brush  51  with a positive discharging bias. 
   The discharging bias thus applied to the discharging brush  51  forces a residual charge, which exists on the intermediate transfer belt  21  to form the residual potential, to efficiently flow into the discharging brush  51 , so that effective discharging can be accomplished. Thus, even when the surface moving speed of the intermediate transfer belt  21  is increased, for example, in order to perform the image formation at a higher speed, the intermediate transfer belt  21  can be stably discharged. 
   [Embodiment 2] 
   Next, a second embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”) that is, an image forming apparatus in which the present invention is applied. 
     FIG. 3  schematically illustrates the configuration of a main portion in a printer unit of the copier according to the second embodiment. The illustrated copier includes a scanner unit, not shown, which has the same configuration as that of the first embodiment. The second embodiment differs from the first embodiment in that a printer unit has a variable discharge power supply and a control unit for controlling the variable discharge power supply. Since the copier of the second embodiment performs image forming operations basically in the same manner as the first embodiment, description on those parts that are constructed and operated in a manner similar to the first embodiment is omitted. 
   A contact-type discharger  150  according to the second embodiment has a discharging brush  51 , and a variable discharge power supply  159  for applying the discharging brush  51  with a variable direct current voltage, similarly to that in the first embodiment. The variable discharge power supply  159  is connected to a control unit which controls a direct current voltage applied to the discharging brush  51 . 
   A specific example (hereinafter referred to as the “first example”) of the control unit for controlling the variable bias power supply  159  will be described below with reference to  FIG. 4 . 
     FIG. 4  is a block diagram illustrating the configuration of a controller  61  in the control unit  60  for controlling the variable discharge power supply  159  in accordance with the volume resistivity ρv of the intermediate transfer belt  21 . The controller  61  has a CPU  62 , a ROM  63 , a RAM  64 , and an I/O interface  65 . The I/O interface  65  is connected to the variable discharge power supply  159 ; a driving motor  24   a  coupled to a driving roller  24  for driving the intermediate transfer belt  21 ; a mark sensor  24   b  for detecting a mark attached on the inner peripheral surface of the intermediate transfer belt  21  for detecting a rotating position; and a calculator  66  for counting a total number of copies produced by the copier. The variable discharge power supply  159  for applying the discharging brush  51  with a direct current voltage is turned ON/OFF at a timing that is set based on an output signal of the mark sensor  24   b.    
   Now, explanation will be given of the relationship between the volume resistivity ρv of the intermediate transfer belt  21  and a surface potential on the intermediate transfer belt  21  after secondary transfer. 
     FIG. 5  shows a graph representing the relationship between the volume resistivity ρv of the intermediate transfer belt  21  and the surface potential on the intermediate transfer belt  21  after secondary transfer. Also, Table 1 below shows the relationship between the surface potential on the intermediate transfer belt  21  after secondary transfer and the evaluation for an image which is subsequently formed when the surface of the intermediate transfer belt  21  is charged at each potential as indicated. The image evaluation on Table 1 is made in the following manner: when an image produced in the next sequence of image formation with a surface potential equal to a value indicated in Table 1 exhibits a similar image quality to the preceding image, it is evaluated as ∘; and when such an image has a lower image quality than the preceding image, it is evaluated as Δ or X according to the degree of deterioration. 
   
     
       
         
             
             
             
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Surface Potential (V) 
               0 
               −100 
               −200 
               −300 
               −400 
               −500 
             
             
                 
             
           
          
             
               Image 
               ◯ 
               ◯ 
               Δ 
               X 
               X 
               X 
             
             
                 
             
          
         
       
     
   
   The graph of  FIG. 5  shows that when the volume resistivity ρv is 10 11  Ωcm or more, the residual potential due to a residual charge on the surface of the intermediate transfer belt after the secondary transfer is at −100 volts or less. Then, Table 1 shows that an image formed in the next sequence of image formation fails to be evaluated as ∘ when the residual potential is at −100 volts or less. Consequently, it is found that with the volume resistivity ρv equal to or higher than 10 11  Ωcm, the residual potential on the surface of the intermediate transfer belt adversely affects the next primary transfer, with the result that an image formed in such an environment suffers from a degraded quality. It is thought that the degraded image quality of a subsequently formed image as compared with that of the previously formed image is caused by an insufficient primary transfer bias due to-the residual potential. It is therefore effective to provide such the intermediate transfer belt  21  with a discharging means. In addition, the volume resistivity ρv of the intermediate transfer belt  21  set in a range of 10 13  Ωcm to 10 14  Ωcm or more is preferable because dusts can be prevented from remaining on the intermediate transfer belt  21  after the primary transfer. 
     FIG. 5  also shows that as the volume resistivity ρv is higher, the residual potential on the intermediate transfer belt  21  is also higher. For preferably performing the primary transfer in the next image formation process, the discharging bias must be selected such that the intermediate transfer belt  21  is not discharged insufficiently or excessively, that is, such that the surface potential on the intermediate transfer belt is at −100 volts or lower. For this purpose, the variable discharge power supply  159  is controlled to generate a direct current which provides an optimal discharging bias in accordance with the volume resistivity ρv of the employed intermediate transfer belt  21 . 
   Further, while the volume resistivity ρv of the intermediate transfer belt  21  is determined in a design stage of a copier, the intermediate transfer belt  21  is deteriorated as it is repetitively used over time. This deterioration appears as a lower volume resistivity ρv, so that if a direct current voltage applied by the variable discharge power supply  159  is kept unchanged from the initial setting, an actually applied discharging bias will deviate from an optimal value. To solve this problem, the control unit  60  in the second embodiment controls the direct current generated by the variable discharge power supply  159  in accordance with this decreasing volume resistivity ρv over time. 
   Specifically, when the total number of copies counted by the calculator  66  reaches a predetermined value, the controller  61  in the control unit  60  controls the variable discharge power supply  159  to generate a higher direct current voltage. As a result, it is possible to correct a deviation of the discharging bias from the optimal value due to the decreasing volume resistivity ρv associated with the deteriorated intermediate transfer belt  21 , and hence accomplish stable and exact discharging over a long term. 
   Another specific example (hereinafter referred to as the “second example”) of the control unit for controlling the variable bias power supply  159  will be described below with reference to  FIG. 6 . 
     FIG. 6  is a block diagram illustrating the configuration of a controller  61   a  in a control unit  60   a  for controlling the variable discharge power supply  159  in accordance with a surface potential of the intermediate transfer belt  21 . The control unit  60   a  of the second example has the same configuration as the control unit  60  in the foregoing first example except that an I/O interface  65   a  in the controller  61   a  is connected to a potential sensor  67  for sensing the potential on the surface of the intermediate transfer belt  21  instead of the calculator  66  in the first example. The potential sensor  67  is disposed upstream of the position at which the discharging brush  51  is disposed in the direction of the movement of the intermediate transfer belt  21 . 
   An optimal value for a direct current applied by the variable discharge power supply  159  varies depending on the potential on the surface of the intermediate transfer belt  21 , more specifically, the surface of the intermediate transfer belt  21  after secondary transfer. It is therefore desirable to control the variable discharge power supply  159  in accordance with the surface potential in order to achieve effective discharging. 
   In the second example, the potential sensor  67  senses the surface potential on the intermediate transfer belt  21  before it is discharged, and supplies the sensed surface potential data to a CPU  62   a  in the controller  61   a  to control the variable discharge power supply  159  in response to the surface potential data. 
   While an exact bias potential can be applied by virtue of the discharging bias control in response to the surface potential on the intermediate transfer belt  21  using the potential sensor  67 , the implementation of such control may result in a complicated configuration and an increased cost. Thus, when the discharging bias control is applied to a copier which has a single-color mode in which a single-color toner image formed on the photosensitive drum  10  is transferred to the intermediate transfer belt  21  and then transferred again from the intermediate transfer belt  21  to the transfer paper  100 , and a multi-color mode in which a plurality of toner images sequentially formed on the photosensitive drum are transferred to the intermediate transfer belt  21  one after the other in accurate register with one another, and the complete multi-color image is transferred to the transfer paper, the variable discharge power supply may be controlled to apply different discharging biases in accordance with the single-color mode or the multi-color mode. Specifically, when a copier has a single-color mode for producing copies using only a single developer for one color (hereinafter referred to as the “1C mode”) and a multi-color mode for producing copies using developers for four colors (hereinafter referred to as the “4C mode”), the controller  61   a  may control the variable discharge power supply  159  such that different discharging biases are applied corresponding to these copy modes. 
   Further, when the discharging bias control is applied to a copier which has a plurality of multi-color modes in accordance with the number of times toner images are transferred or superimposed, in which a plurality of toner images sequentially formed on the photosensitive drum  10  are transferred to the intermediate transfer belt  21  one after the other in accurate register with one another, and the complete multi-color image is transferred to the transfer paper, the control unit  61  may control the variable discharge power supply  159  to generate different direct current voltages in accordance with the number of toner images, in other words, the number of times toner images are transferred or superimposed one after the other to the intermediate transfer belt  21 . In this case, if a copier has an additional two-color mode (hereinafter referred to as the “2C mode”) for producing copies using developers for two colors in addition to the aforementioned 1C mode and 4C mode, the controller  61   a  may control the variable discharge power supply  159  to apply different discharging biases corresponding to the respective copy modes. 
   Next, a further specific example (hereinafter referred to as the “third example”) of the control unit for controlling the variable bias power supply  159  will be described below with reference to  FIG. 7 . 
     FIG. 7  is a block diagram illustrating the configuration of a controller  61   b  in a control unit  60   b  for controlling the variable discharge power supply  159  in accordance with an environmental condition around the intermediate transfer belt  21 . The control unit  60   b  of the third example has the same configuration as the control unit  60  in the foregoing first example except that an I/O interface  65   b  in the controller  61   b  is connected to a temperature and humidity sensor  68  for sensing an environmental condition around the intermediate transfer belt  21  instead of the calculator  66  in the first example. While the third example employs the combined temperature and humidity sensor  68  as an environmental condition sensing means, separate sensors may be provided for individually sensing a temperature and a humidity. Alternatively, the control unit  60   b  may be provided with another environmental condition sensing means such as that for sensing the volume resistivity ρv of the intermediate transfer belt  21 , or any other environmental condition, other than temperature and humidity, which may affect a contact resistance between the intermediate transfer belt  21  and the discharging brush  51 . 
   As mentioned above, an optimal value for a direct current voltage applied by the variable discharge power supply  159  varies depending on the volume resistivity ρv of the intermediate transfer belt  21 . The volume resistivity ρv in turn varies depending on environmental conditions, particularly, on temperature and humidity. Also, since the third example employs a discharging brush  51  which is a contact-type discharge member, a contact resistance between the discharging brush  51  and the intermediate transfer belt  21  is also included in factors which vary the optimal value for the direct current voltage. The contact resistance likewise varies depending on environmental conditions, particularly on temperature and humidity. It is therefore desirable to control the variable discharge power supply  159  in accordance with such environmental conditions as mentioned above which cause variations in the optimal value for the direct current voltage, in order to achieve effective discharging. 
   To meet the foregoing requirements, in the third example, the temperature and humidity sensor  68  senses the temperature and humidity around the intermediate transfer belt  21  and supplies sensed temperature data and humidity data to a CPU  62   b  in the controller  61   b  which controls the variable discharge power supply  159  in response to the supplied data. 
   Next, a further specific example (hereinafter referred to as the “fourth example”) of the control unit for controlling the variable bias power supply  159  will be described below with reference to  FIG. 8 . 
     FIG. 8  is a block diagram illustrating the configuration of a controller  61   c  in a control unit  60   c  for controlling the variable discharge power supply  159  in accordance with a surface moving speed the intermediate transfer belt  21 . The control unit  60   c  of the fourth example has the same configuration as the control unit  60  in the foregoing first example except that a CPU  62   c  in the controller  61   c  is supplied with a rotating speed of the driving motor  24   a  which drives the driving roller  24  for driving the intermediate transfer belt  21 . 
   An optimal value for a direct current voltage applied to the variable discharge power supply  159  varies depending on the surface moving speed of the intermediate transfer belt  21 . This is because a change in the surface moving speed causes variations in time period for which the discharging brush  51  remains in contact with the intermediate transfer belt  21 . More specifically, when the direct current voltage is fixed, an increase in the surface moving speed of the intermediate transfer belt  21  may result in insufficient discharging, while a decrease in the surface moving speed may result in excessive discharging. It is therefore beneficial to control the variable discharge power supply  159  in accordance with the surface moving speed of the intermediate transfer belt  21  which causes variations in the optimal value for the direct current voltage, in order to achieve effective discharging. 
   To meet the above requirements, in the fourth example, data on the rotating speed of the driving motor  24   a  is supplied to the CPU  62   c  in the controller  61   c  which calculates the current surface moving speed of the intermediate transfer belt  21  corresponding to the rotating speed of the driving motor  24   a , and controls the variable discharge power supply  159  so as to apply the discharging brush  51  with an optimal discharging bias for the calculated surface moving speed. 
   It should be noted that the control unit according to the second embodiment may advantageously utilize a combination of the foregoing examples as appropriate to more exactly discharge the intermediate transfer belt  21 . 
   [Embodiment 3] 
   Next, a third embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied. 
     FIG. 9  schematically illustrates the configuration of a main portion in a printer unit of the copier according to the third embodiment. The illustrated copier includes a scanner unit, not shown, which has the same configuration as that of the first embodiment, and performs basically the same image forming operations as the copier of  FIG. 1 . The third embodiment differs from the first embodiment mainly in the structure and operation of the printer unit. 
   As illustrated in  FIG. 9 , the printer unit of the third embodiment includes a photosensitive drum  10  as an image carrier, and, around the photosensitive drum  10 , an optical writing unit, not shown, as an exposing unit; a photosensitive drum cleaning unit  111 ; a charger  13 ; a revolver developing unit  110 ; and an intermediate transfer unit  120 . The printer unit also includes a transfer unit  130 ; an fixing unit  145 ; and a paper feed unit, a controller and so on, not shown, similar to those in the first embodiment. 
   The photosensitive drum cleaning unit  111  has a fur brush  111   b  and a photosensitive drum cleaning blade  111   c , and is provided for cleaning the surface of the photosensitive drum  10  after primary transfer. The fixing unit  145  has a pair of fixing rollers  145   a , and a pair of delivery rollers, not shown. 
   The revolver developing unit  110  has a Bk developing device  115 ; a C developing device  116 ; an M developing device  117 ; and a Y developing device  118 . A developing position in the developing device of each color opposite to the photosensitive drum  10  can be determined by the revolution of the revolver developing unit  110 . 
   The intermediate transfer unit  120  includes an intermediate transfer belt  121 ; a primary transfer bias roller  122  as a charge supply means; a primary transfer power supply  128  connected to the primary transfer bias roller  122 ; a ground roller  123  as a pre-primary transfer discharging means; a driving roller  124 ; a driven roller  125 ; a secondary transfer opposed roller  126 ; and a cleaning opposed roller  127 . The intermediate transfer belt  121  is passed over the primary transfer bias roller  122 , the ground roller  123 , the driving roller  124 , the driven roller  125 , the secondary transfer opposed roller  126 ; and the cleaning opposed roller  127 . The driving roller  124  is connected to a driving motor  124   a . All the rollers, over which the intermediate transfer roller  121  is passed, are made of an electrically conductive material, and all the rollers except for the primary transfer bias roller  122  are respectively grounded. The primary transfer bias roller  122  is applied by the primary transfer power supply  128  with a predetermined primary transfer bias which is controlled to be a constant voltage or a constant current. 
   Around the intermediate transfer belt  121 , there are disposed a discharging brush roller  151 ; a belt cleaning blade  129 ; and a transfer unit  130 . These components are moved into and out of contact with the intermediate transfer belt  121  by respective contact/separation mechanisms, not shown, associated therewith. 
   Like the aforementioned first embodiment, the intermediate transfer belt  121  is formed in a multi-layer structure composed of a surface layer, an intermediate layer and a base layer. In addition, an adhesive layer is interposed between the intermediate layer and the base layer for adhering the two layers. The intermediate transfer belt  121  is formed to have a volume resistivity ρv in a range of 10 12  Ωcm to 10 14  Ωcm, and preferably equal to approximately 10 13  Ωcm. Advantageously, with the intermediate transfer belt  21  having the volume resistivity ρv equal to or larger than 10 12  Ωcm, dusts can be prevented from remaining on the intermediate transfer belt  121  after primary transfer. It should be noted that while in the third embodiment, the intermediate transfer belt  121  has high resistance surface layer and intermediate layer, and a middle resistance base layer with the volume resistivity ρv in a range of 10 8  Ωcm to 10 11  Ωcm, the intermediate transfer belt  121  is not limited to this particular structure. Also, the intermediate transfer belt  121  is made such that the surface resistivity on the surface layer side thereof is in a range of 10 7  Ωcm to 10 14  Ωcm. 
   The discharging brush  151  is connected to a variable discharge power supply  159  for applying the discharging brush  151  with a direct current voltage. The variable discharge power supply  159  in turn is connected to a control unit  160  which controls the direct current voltage applied to the discharging brush  151 . 
   The transfer unit  130  has a paper transfer belt  134 ; a transfer cleaning blade  132  for cleaning the surface of the paper transfer belt  134 ; a secondary transfer bias roller  131  opposing a secondary transfer opposed roller  126  of the intermediate transfer unit  120 ; a secondary transfer power supply  139  connected to the secondary transfer bias roller  131 ; a first supporting roller  135   a  positioned at one end of the paper feed unit; a third supporting roller  135   c  opposing the transfer cleaning blade  132 ; a transfer paper discharger  136 ; and a transfer belt discharger  137 . The paper transfer belt  134  is made of PVDF (polyvinylidene fluoride) to have a high volume resistance of 10 13  Ωcm or higher. It should be understood that the transfer unit  130  is not limited to the foregoing structure, and that in an alternative, the transfer unit  130  may employ, for example, a member of a different shape such as a drum instead of the paper transfer belt  134 . 
   Next, the operation of the copier according to the third embodiment will be described in connection with an illustrative image forming mode in which the development is performed in the order of Bk, C, M, Y. Before starting an image forming cycle, the photosensitive drum  10  is driven to rotate in a direction indicated by an arrow C in  FIG. 9 , i.e., in the counter-clockwise direction, causing the charger  113  to initiate corona discharge. In this event, in the third embodiment, the photosensitive drum  10  is uniformly charged at a predetermined potential with a negative charge. Also, the intermediate transfer belt  121  of the intermediate transfer unit  120  is driven at the same speed as the photosensitive drum  10  to rotate in a direction indicated by an arrow D, i.e., in the clockwise direction. 
   Like the aforementioned first embodiment, in the scanner unit, color image information of an original is read at a predetermined timing, and Bk image data derived from the image information is optically written onto the photosensitive drum  10  using laser light produced by the optical writing unit (for example, raster exposure). As a result, a Bk latent image is formed on the photosensitive drum  10  corresponding to the Bk image data. Subsequently, the Bk latent image formed on the photosensitive drum  10  is reversely developed with a negatively charged toner by the Bk developing device  115  in the revolver developing unit  110 . In this way, a Bk toner image is formed on the photosensitive drum  10 . 
   The Bk toner image thus formed on the photosensitive drum  10  is transferred to the surface of the intermediate transfer belt  121  by the action of a transfer electric field existing in a primary transfer region. The transfer electric field is formed by a charge given to the intermediate transfer belt  121  by the primary transfer bias roller  122 . In this event, the primary transfer bias roller  122  is applied by the primary transfer power supply  128  with a primary transfer bias of a suitable magnitude. For example, the primary transfer bias may be at 1.5 kV for the first color (Bk) toner image; in a range of 1.6 to 1.8 kV for a second color (C) toner image; in a range of 1.8 to 2.0 kV for a third color (M) toner image; and in a range of 2.0 to 2.2 kV for a fourth color (Y) toner image. The toner used in the primary transfer and remaining on the photosensitive drum  10  after the development is removed by the photosensitive drum cleaning unit  111 . 
   The image forming surface on the intermediate transfer belt  121 , on which the Bk toner image has been transferred, is again returned to the primary transfer region. In this event, the discharging brush roller  151  and the belt cleaning blade  129  are moved away from the intermediate transfer belt  121  by respective contact/separation mechanisms associated therewith so as not to disturb the toner image. Further, the first supporting roller  135   a  and the secondary transfer bias roller  131  in the transfer unit  130  are moved by associated transfer contact/separation mechanisms, not shown, such that the secondary transfer bias roller  131  is moved away from the intermediate transfer belt  121 . In this event, the secondary transfer power supply  139  connected to the secondary transfer bias roller  131  is inhibited from applying a voltage. 
   The above-mentioned state is held until the toner image transferred to the intermediate transfer belt  121  is transferred to a transfer paper  100 . 
   After the Bk step is terminated, a C step is started on the photosensitive drum  10 . Specifically, color image information of the original is again read at a predetermined timing, a C latent image is formed on the photosensitive drum  10  by laser light based on C image data derived from the image information, and a C toner image is formed by the C developing device  116 . 
   In the third embodiment, after the trailing edge of the Bk latent image has passed, the revolver developing unit  110  is immediately rotated. The rotation of the revolver developing unit  110  is completed before the leading edge of the C latent image formed on the photosensitive drum  10  arrives at a developing position of the C developing device  116 . In this way, the C developing device  116  is aligned to the developing position, so that it develops the C latent image coming up to the developing position with a C toner. 
   Subsequently, in either of an M step and a Y step, the formation of a latent image, development, and primary transfer are performed respectively based on their respective image data in a manner similar to the aforementioned C step. By transferring the respective Bk, C, M and Y toner images sequentially formed on the photosensitive drum  10  to the same image surface area on the intermediate transfer belt  21 , a complete toner image formed of the four color images in accurate register with one another is formed on the intermediate transfer belt  21 . 
   The complete toner image thus transferred to the intermediate transfer belt  121  is conveyed to the secondary transfer region for secondary transfer of the toner image to a transfer paper  100 . In this event, the secondary transfer bias roller  131  of the transfer unit  130  is pressed against the intermediate transfer belt  121  by a transfer contact/separation mechanism, not shown. Subsequently, the secondary transfer bias roller  131  is applied with a predetermined secondary transfer bias to form a secondary transfer electric field in the secondary transfer region. The secondary transfer electric field causes the toner image on the intermediate transfer belt  121  to be transferred to the transfer paper  100 . The transfer paper  100  is fed into the secondary transfer region at the timing coincident with the arrival of the leading edge of the toner image on the intermediate transfer belt  121  to the secondary transfer region. 
   The transfer paper  100 , on which the complete toner image formed of four color toner images in accurate register with one another has been collectively transferred from the intermediate transfer belt  121 , is subsequently conveyed to an area opposite to the transfer paper discharger  136  in the transfer unit  130 . When the transfer paper  100  passes this opposing area, the transfer paper  100  is discharged by the transfer paper discharger charger  136  now in operative state, and separated from the paper transfer belt  134 . Then, the separated transfer paper  100  is conveyed to pass between the pair of fixing rollers  145   a  of the fixing unit  145 . The unfixed toner image on the transfer paper  100  is melted in a fixing region formed of a nip between the fixing rollers  145   a , and the unfixed toner image is fixed. Then, after the fixation, the transfer paper  100  is conveyed to and stacked on the copy tray  46 . 
   After the secondary transfer, the belt cleaning blade  129  is pressed against the intermediate transfer belt  121  by a contact/separation mechanism, not shown, to remove the toner used in the secondary transfer and remaining on the surface of the intermediate transfer belt  121 . In addition, the charge remaining on the surface of the paper transfer belt  134 , after the transfer paper  100  has been separated, is discharged by the transfer belt discharger  137 . Furthermore, the surface of the paper transfer belt  134  is cleaned by the transfer cleaning blade  132 . 
   In the following, description will be made on the operation of the control unit  160  which constitutes a characterizing portion of the third embodiment.  FIG. 10  is a block diagram illustrating the configuration of a controller  161  in the control unit  160 . The controller  161  has the same configuration as the second embodiment previously described with reference to  FIG. 4 . The controller  161  has an I/O interface  165  which is connected to the variable discharge power supply  159 ; a driving motor  124   a  coupled to a driving roller  124  for driving the intermediate transfer belt  121 ; and a mark sensor  124   b  for detecting a mark attached on the inner peripheral surface of the intermediate transfer belt  21  for detecting a rotating position. The variable discharge power supply  159  for applying the discharging brush  151  with a direct current voltage is turned ON/OFF at a timing that is set based on an output signal of the mark sensor  124   b . In the third embodiment, the control unit  160  variably controls the direct current voltage generated by the variable discharge power supply  159  in accordance with a potential on the surface of the intermediate transfer bent  121 , a surface moving speed of the same, and the temperature and humidity around the intermediate transfer belt  121 . 
   For controlling the direct current voltage of the variable discharge power supply  159  in accordance with the surface potential on the intermediate transfer belt  121 , data on a copy mode of the copier is supplied to a CPU  162  in the controller  161  of the control unit  160 . For controlling the direct current voltage of the variable discharge power supply  159  in accordance with the surface moving speed of the intermediate transfer belt  121 , data on the rotating speed of the driving motor  124   a  for driving the driving roller  124  is supplied to the CPU  162 . For controlling the direct current voltage of the variable discharge power supply  159  in accordance with temperature and humidity conditions around the intermediate transfer belt  121 , the CPU  162  is supplied with temperature data and humidity data through the I/O interface  165  from a temperature and humidity sensor  68  which is disposed near the position at which the discharging brush  151  contacts the intermediate transfer belt  121 , and connected to the I/O interface  165 . 
   Then, the CPU  162  in the controller  161  calculates an optimal discharging bias based on the various data supplied thereto, and forces the variable discharge power supply  159  to apply the discharging brush  151  with an optimal direct current voltage. 
   It should be understood that the copier according to the third embodiment can be used not only in the foregoing full-color copy mode but also in any other copy mode, as is the case of the aforementioned first embodiment. 
   Now, description will be made on one implementation of the present invention which uses the copier according to the third embodiment and a combination of the second, third and fourth examples in the second embodiment for controlling a discharging bias. 
   Explained first is an experiment conducted to reveal the relationship between a residual potential on the intermediate transfer belt and a discharging bias or a direct current voltage applied to the contact-type discharge member for removing the residual potential. In this experiment, the control unit  160  of the copier was not used. 
   The experiment involved measurements of affected images produced when an image forming process was executed with a residual potential maintained on the intermediate transfer belt  121 . It is desired that the intermediate transfer belt is discharged such that the surface potential is at zero volt on the intermediate transfer belt after the discharging. Actually, however, it is extremely difficult to bring the surface potential exactly to zero volt by the discharging. 
   Also, when the intermediate transfer belt is discharged insufficiently, the next primary transfer step is performed with a potential of the same polarity as that of a toner held on the intermediate transfer belt, resulting in an insufficient transfer bias and accordingly an incomplete transfer which will lead to an affected image. On the other hand, excessive discharging causes the intermediate transfer belt  121  to have a surface potential of the opposite polarity to the toner. The next primary transfer performed on the intermediate transfer belt  121  with the surface potential of the opposite polarity would result in a so-called pre-transfer where the primary transfer is performed before the primary transfer region, which leads to deteriorated dot reproductivity and consequently an affected image. To solve this problem, the inventors of the present invention and others measured the relationship between a surface potential Vb on the intermediate transfer belt  121  after discharging (hereinafter referred to as the “post-discharge potential Vb”) and affected images, and concluded in Table 2 below. 
   
     
       
         
             
             
           
             
                 
               TABLE 2 
             
           
          
             
                 
                 
             
             
                 
               Post-Discharge Potential Vb (volts) 
             
          
         
         
             
             
             
             
          
             
                 
               Vb &lt;−300 
               −300 Vb 300 
               Vb &gt;300 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               Affected Image Due 
               ◯ 
               ◯ 
               X 
             
             
                 
               to Pre-Transfer 
             
             
                 
               Affected Image Due 
               X 
               ◯ 
               ◯ 
             
             
                 
               to Insufficient 
             
             
                 
               Transfer 
             
             
                 
                 
             
          
         
       
     
   
   Thus, the measurements revealed that when the absolute value of the post-discharge potential Vb on the intermediate transfer belt  121  is at least 300 volts or less, images can be produced without affecting much by pre-transfer or insufficient transfer. 
   Keeping the foregoing measurement results in mind, the inventors of the present invention and others next conducted an experiment for revealing the relationship between a surface potential Va on the intermediate transfer belt  121  after a secondary transfer step has been completed and before the intermediate transfer belt  121  is discharged (hereinafter referred to as the “pre-discharge potential Va) and the post-discharge potential Vb, with a varying direct current voltage applied to the discharging brush roller.  FIG. 11A  is a graph showing the result of the experiment conducted at temperature of 23° C. and humidity of 65% (in a laboratory environment);  FIG. 11B  is a graph showing the result of the experiment conducted at temperature of 10° C. and humidity of 15% (in an low temperature and low humidity (L.L.) environment); and  FIG. 11C  is a graph showing the result of the experiment conducted at temperature of 27° C. and humidity of 80% (in a high temperature and high humidity (H.H.) environment). 
   In each of  FIGS. 11A ,  11 B,  11 C, it can be said that each plot is substantially linear when the pre-discharge potential Va on the intermediate transfer belt  121  is at −100 volts or less. From the results of the experiment represented by the graphs, the relationship between the pre-discharge potential Va, the post-discharge potential Vb, and a direct current voltage V applied to the discharging brush  151  can be expressed substantially by the following Equation 1:
 
 Vb =0.65 Va +(25 +V /2)  (Equation 1)
 
   The post-discharge potential Vb which meets the condition of producing images with less pre-transfer and with sufficient transfer must fall within a range expressed by:
 
−300 Va 300  (Equation 2)
 
   Therefore, the direct current voltage V applied to the discharging brush  151  for ensuring images with less pre-transfer and with sufficient transfer can be expressed by:
 
−1.3Va−650 V −1.3Va+550  (Equation 3)
 
   In this implementation, the intermediate transfer belt  121  is formed to have a thickness of 0.15 mm, a width of 368 mm, and an inner peripheral length of 565 mm, and a surface moving speed of the intermediate transfer belt  121  is set at 200 mm/s. Also, the surface layer of the intermediate transfer belt  121  is formed of an insulating layer having a thickness of approximately 1 μm. The intermediate layer of the intermediate transfer belt  121  is formed of polyvinylidene fluoride in a thickness of approximately 75 μm. The volume resistivity ρv of the intermediate layer is 9×10 12  Ωcm when measured using a resistance measuring instrument “High Rester IP” manufactured by Yuka Denshi at temperature of 25° C. and humidity of 45% with a voltage of 100 volts applied thereto for 10 seconds, and 6×10 12  Ωcm when measured in the same environment using the same instrument with a voltage of 500 volts applied thereto for 10 seconds. The base layer is formed of PVDF and titanium oxide in a thickness of approximately 75 μm. The volume resistivity ρv of the base layer is 7×10 7  Ωcm when measured in the same environment using the same instrument with a voltage of 100 volts applied thereto for 10 seconds. 
   The surface resistance on the surface of the surface layer of the intermediate transfer belt  121  is 10 13  Ωcm when measured with resistance measuring instrument “High Rester IP” manufactured by Yuka Denshi. Other than this resistance measuring instrument, the surface resistivity may be measured in accordance with the surface resistance measuring method described in JISK6911. 
   In this implementation, the primary transfer bias roller  122  may be a nickel plated metal roller, and the ground roller  123  may be a metal roller. Other rollers may be metal rollers or rollers made of any conductive resin. 
   The primary transfer bias roller  122  is applied with a direct current primary transfer bias at 1.5 kV for the first color (Bk) toner image; 1.7 kV for the second color (C) toner image; 1.9 kV for the third color (M) toner image; and 2.1 kV for the fourth color (Y) toner image. The width of the nip in the primary transfer region is set to be 10 mm. 
   The transfer unit  130  uses the secondary transfer bias controller  131  implemented by a roller having a surface layer made of a conductive sponge or a conductive rubber, and a core layer made of a metal or a conductive resin. The secondary transfer bias roller  131  is applied with a transfer bias that is a current regulated in a range of 10 to 50 μA. Appropriate values within this range are used depending on copy modes available to the copier and types of used transfer papers. Specific values for the regulated current for different types of papers and different modes are shown in Table 3 below. 
   
     
       
         
             
             
           
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
               Secondary Transfer Current (μA) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               Normal Paper (1C Mode) 
               25 
             
             
                 
               Normal Paper (4C Mode) 
               35 
             
             
                 
               Thick Paper (1C Mode) 
               14 
             
             
                 
               Thick Paper (4C Mode) 
               18 
             
             
                 
               Very Thick Paper (1C Mode) 
               16 
             
             
                 
               Very Thick Paper (4C Mode) 
               20 
             
             
                 
                 
             
          
         
       
     
   
   The paper transfer belt  134  is formed of a PVDF-based material having a volume resistivity ρv of 10 13  Ωcm in a thickness of 100 μm. The transfer paper discharger  136  and the transfer belt discharger  137  are implemented by dischargers which are applied with an alternate current voltage or with a combination of alternate current and direct current voltages. The transfer cleaning blade  132  is in contact with the surface of the paper transfer belt  134  on the opposite side of the third supporting roller  135   c.    
   The temperature and humidity sensor  68  described in the third example of the second embodiment is connected to the I/O interface  165  of the controller  161 . Data on temperature and humidity around the intermediate transfer belt  121  sensed by the temperature and humidity sensor  68  is supplied to the CPU  162  in the controller  161 . The CPU  162  is also supplied with information on a copy mode, in which the copier is to operate to produce the next copy, for controlling the discharging bias in accordance with the surface potential on the intermediate transfer belt  121 , more specifically, in accordance with the number of times toner images are transferred or superimposed onto the intermediate transfer belt  121 . Assume in this implementation that three types of copy modes, 1C mode, 2C mode and 4C mode, are available to the copier. The CPU  162  is further supplied with information on a transfer paper on which the copier produces a copy next time, more specifically, information for discriminating whether a normal paper, a thick paper, a very thick paper or an OHP sheet is used. Assume in this implementation that when the copier produces copies on normal papers at a normal speed, the speed of producing copies on thick papers, very thick papers and OHP sheets is one half of the normal speed. 
   As described above, this implementation employs a variable discharge power supply such as  159 , and the discharging bias applied to the discharging brush roller  151  is set as shown in the following Table 4, Table 5 and Table 6 in accordance with the aforementioned results of the experiments. 
   
     
       
         
             
             
           
             
                 
               TABLE 4 
             
             
                 
                 
             
             
                 
               Discharging Bias V (volts) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               1C Mode (Normal Speed) 
               0 
             
             
                 
               2C Mode (Normal Speed) 
               0 
             
             
                 
               4C Mode (Normal Speed) 
               50 
             
             
                 
               1C Mode (Half Speed) 
               0 
             
             
                 
               2C Mode (Half Speed) 
               0 
             
             
                 
               4C Mode (Half Speed) 
               50 
             
             
                 
                 
             
             
                 
               Note: 
             
             
                 
               at temperature of 23° C. and humidity of 65% 
             
          
         
       
     
   
   
     
       
         
             
             
           
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
               Discharging Bias V (volts) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               1C Mode (Normal Speed) 
               0 
             
             
                 
               2C Mode (Normal Speed) 
               50 
             
             
                 
               4C Mode (Normal Speed) 
               350 
             
             
                 
               1C Mode (Half Speed) 
               0 
             
             
                 
               2C Mode (Half Speed) 
               50 
             
             
                 
               4C Mode (Half Speed) 
               350 
             
             
                 
                 
             
             
                 
               Note: 
             
             
                 
               at temperature of 10° C. and humidity of 15% 
             
          
         
       
     
   
   
     
       
         
             
             
           
             
                 
               TABLE 6 
             
             
                 
                 
             
             
                 
               Discharging Bias V (volts) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               1C Mode (Normal Speed) 
               0 
             
             
                 
               2C Mode (Normal Speed) 
               0 
             
             
                 
               4C Mode (Normal Speed) 
               50 
             
             
                 
               1C Mode (Half Speed) 
               0 
             
             
                 
               2C Mode (Half Speed) 
               0 
             
             
                 
               4C Mode (Half Speed) 
               50 
             
             
                 
                 
             
             
                 
               Note: 
             
             
                 
               at temperature of 27° C. and humidity of 80% 
             
          
         
       
     
   
   Also in this implementation, four copies are produced from a single original sheet using normal papers of A4 size. It should be noted that in this implementation, the intermediate transfer belt  121  is formed with an image surface area for accommodating two images to speed the image formation. During the image formation, the discharging brush roller  151  is applied with a discharging bias at a timing which is controlled in accordance with a timing chart as illustrated in  FIG. 12 . The discharging brush roller  151  is moved into and out of contact with the intermediate transfer belt  121  in association with timings at which the belt cleaning blade  129  which is moved into and out of contact with the intermediate transfer belt  121 . As illustrated in  FIG. 12 , the discharging bias is controlled in the following manner. The discharging bias is applied when the surface of the intermediate transfer belt has moved by 24 mm after the discharging brush roller  151  had been brought into contact with the intermediate transfer belt  121 . Then, the applied discharging bias is removed slightly before the discharging brush  151  is moved out of contact with the intermediate transfer belt  121 . 
   [Embodiment 4] 
   Next, a fourth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied. 
     FIG. 13  schematically illustrates the configuration of a main portion in a printer unit of the copier according to the fourth embodiment. In general, the illustrated copier, which is intended for a reduction in cost, differs from the copier according to the second embodiment only in the following aspects. Therefore, similar constituent members in the fourth embodiment are designated the same reference numerals as those in the second embodiment, and description thereon is omitted. 
   In the fourth embodiment, an intermediate transfer belt  221  forming part of an intermediate transfer unit  220  has a middle resistance intermediate layer with a volume resistivity ρv in a range of 10 8  Ωcm to 10 11  Ωcm. Also, the intermediate transfer belt  221 , as a whole, has a volume resistivity ρv in a range of 10 10  Ωcm to 10 12  Ωcm. Further, the intermediate transfer belt  221  is made to have a surface resistivity on the surface side in a range of 10 7  Ωcm to 10 14  Ωcm. More specifically, the intermediate layer is formed of PVDF and titanium oxide with the volume resistivity ρv of 5×10 12  Ωcm when measured using the aforementioned resistance measuring instrument “High Rester IP” manufactured by Yuka Denshi at temperature of 25° C. and humidity of 45% with a voltage of 100 volts applied thereto for 10 seconds, and 2×10 11  Ωcm when measured in the same environment using the same instrument with a voltage of 500 volts applied thereto for 10 seconds. The surface layer of the intermediate transfer belt  221  is formed of an insulating layer having a thickness of approximately 1 μm. The base layer is formed of PVDF and titanium oxide in a thickness of approximately 75 μm. The volume resistivity ρv of the base layer is 7×10 7  Ωcm when measured in the same environment using the same instrument with a voltage of 100 volts applied thereto for 10 seconds. In addition, a surface moving speed of the intermediate transfer belt  221  is set at 156 mm/s. The use of the intermediate transfer belt  221  having such a middle resistance can prevent uneven charging from occurring on the surface of the intermediate transfer belt  221  after primary transfer. Additionally, in the fourth embodiment, a driving roller  224  in the intermediate transfer unit  220  is disposed downstream of a secondary transfer region and upstream of a primary transfer region in a direction of movement of the intermediate transfer belt  221 . Then, a belt cleaning blade  129  is disposed opposite to the driving roller  224 , so that the driving roller  224  may also serve as the cleaning opposed roller  127  that is used in the third embodiment. 
   A primary transfer bias roller  122  is applied with a direct current primary transfer bias at 1.7 kV for the first color (Bk) toner image; 1.8 kV for the second color (C) toner image; 1.9 kV for the third color (M) toner image; and 2.0 kV for the fourth (Y) toner image. 
   The fourth embodiment employs, as a transfer means, a secondary bias roller  231  disposed opposite to a secondary transfer opposed roller  126  in the intermediate transfer unit  220 , instead of the transfer unit used in the third embodiment. Thus, a fed transfer paper  100  is sandwiched between a secondary transfer bias roller  234  and the intermediate transfer belt  221 , and conveyed to pass between a pair of fixing rollers  145   a  of a fixing unit  145 . The configuration as described results in a reduction in the number of constituent members required for the secondary transfer step and accordingly a reduced cost as compared with the third embodiment. 
   The secondary transfer bias roller  231  is implemented by a roller made of conductive rubber, and is applied with a transfer bias that is a regulated current having values as shown in Table 7 below. 
                           TABLE 7                       Secondary Transfer Current (μA)                                                    Normal Paper (1C Mode)   10           Normal Paper (4C Mode)   18           Thick Paper (1C Mode)    8           Thick Paper (4C Mode)   10           Very Thick Paper (1C Mode)   non           Very Thick Paper (4C Mode)   non                        
[Embodiment 5]
 
   Next, a fifth embodiment of the present invention will be described in connection with an image forming apparatus which employs a transfer material carrier such as a belt for carrying and conveying a transfer material such as a paper, an OHP sheet or the like, and to which the present invention is applied. 
     FIG. 14  schematically illustrates the configuration of a transfer unit equipped in the copier according to the fifth embodiment. In the fifth embodiment, the present invention is utilized in a transfer material carrier for carrying and conveying a transfer material rather than in an intermediate transfer unit as in the foregoing embodiments. 
   The transfer unit  330  has a paper transfer belt  332  for carrying and conveying a transfer material such as a paper, an OHP sheet or the like; a cleaning blade  331  for cleaning the surface of the paper transfer belt  332 ; a ground roller  335   a  positioned at one end of a paper feed unit, not shown, and serving as a pre-transfer discharging means; a transfer bias roller  334  as a charge supply means; a transfer power supply  338  connected to the transfer bias roller  334 ; a tension roller  335   b  positioned at one end of a fixing unit, not shown; a cleaning opposed roller  335   c  disposed opposite to the cleaning blade  331  for aiding the cleaning blade  331  in cleaning the surface of the paper transfer belt  332 ; a transfer paper discharger  336 ; and a discharge roller  251  which is a contact-type discharge member. The paper transfer belt  332  used in the fifth embodiment may be formed of a middle resistance material having a volume resistivity ρv in a range of 10 8  Ωcm to 10 11  Ωcm. It should be understood that the transfer unit  330  is not limited to this configuration, and that in an alternative, the transfer unit  330  may employ, for example, a member of a different shape such as a drum instead of the paper transfer belt  332 . 
   The copier according to the fifth embodiment forms a toner image on a photosensitive drum  10  serving as an image carrier in a well known electronic photographic process, and transfers the toner image to a transfer material, or a transfer paper  100  in this embodiment, fed into a transfer region defined by a transfer nip formed between the photosensitive drum  10  and the paper transfer belt  332 . In this event, an intermediate transfer belt may also be used as the image carrier instead of the photosensitive drum  10 . 
   The ground roller  335   a  is disposed downstream of the transfer region in a direction of movement of the paper transfer belt  332  (hereinafter referred to as the “paper transfer belt moving direction). On the other hand, the transfer bias roller  334  is disposed upstream of the transfer region in the paper transfer belt moving direction. The transfer bias roller  334  is applied with a predetermined transfer bias from the transfer power supply  338 , whereby a transfer electric field is formed in the transfer region. Then, a toner image on the photosensitive drum  10  is transferred to the transfer paper  100  which is carried on and conveyed by the paper transfer belt  332 . Then, the transfer paper  100 , which has received the toner image transferred thereto, passes through a separation region in which the transfer paper  100  is discharged by the transfer paper discharger  336  to separate the transfer paper  100  from the paper transfer belt  332 . Then, the transfer paper  100  separated from the paper transfer belt  332  is conveyed to a fixing unit, not shown. 
   An area of the paper transfer belt  332 , from which the transfer paper  100  has been separated, is cleaned by the cleaning blade  331 . The discharge roller  251  is disposed downstream of the cleaning blade  331  in the paper transfer belt moving direction, such that the paper transfer belt  332  is discharged by the discharge roller  251 . The discharge roller  251  is grounded through the discharge power supply  259 . The discharger roller  251  is applied by the discharge power supply  259  with a direct current or an alternate current discharging bias, or a combination of direct current and alternate current discharging biases. In this event, when a direct current power supply for applying a direct current voltage is employed as the discharge power supply, a reduction in cost is expected. The fifth embodiment employs a regulated direct current power supply as the discharge power supply  259 . 
   It is also possible to employ a variable discharge power supply to vary a direct current voltage applied to the discharge roller  251  in accordance with a variety of factors which may cause variations in an optimal value for the discharging bias, in a manner similar to the discharging of the intermediate transfer belt in the aforementioned embodiments. 
   While the fifth embodiment has been described as employing a photosensitive drum as an image carrier for an illustrative purpose, the present invention can be applied to other image carriers having different structures, for example, to an endless photosensitive belt which is passed over two rollers for endless movement. In addition, while the fifth embodiment employs a bias roller as a charge supply means for use in the primary transfer or the secondary transfer for an illustrative purpose, other members of different shapes such as a blade, a brush and so on may also be employed instead. Similarly, while the fifth embodiment employs a ground roller as a pre-transfer discharging means for an illustrative purpose, other members of different shapes such as a blade, a brush and so on may also be employed instead. 
   Also, while the aforementioned first to fourth embodiments have each employed an intermediate transfer belt as an intermediate transfer body for an illustrative purpose, the present invention can be applied to other intermediate transfer bodies having configurations different from the foregoing, for example, an intermediate transfer drum, an intermediate transfer roller, and so on. Further, the electric characteristic such as the surface resistivity or the like, structure, thickness and so on of the intermediate transfer belt may be selected as appropriate depending on specific applications which may require different image forming conditions. 
   Furthermore, the foregoing embodiments have illustrated a developing unit which employs a reverse developing mode in which the photosensitive drum is negatively charged, and a two-component based developer is used. The present invention, however, is not limited to any specific developer or the polarity of a charging potential on the photosensitive drum, and can be applied to any image forming apparatus which may employ a one-component based developer, a normal developing mode, or the like. 
   [Embodiment 6] 
   Referring next to  FIGS. 15 to 17 , a sixth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier” that is an image forming apparatus in which the present invention is applied. 
     FIG. 15  is a cross-sectional view schematically illustrating the configuration of the copier according to the sixth embodiment, and  FIG. 16  is an enlarged view schematically illustrating the structure around a photosensitive drum  10  in the copier of  FIG. 15 . The following description will be mainly focused on only those portions which are different from the first embodiment illustrated in  FIGS. 1 and 2 . 
   A photosensitive drum cleaning unit  450  has, within a casing  452 , a fur brush  451  as a cleaning member; a residual toner recovering container  455  for containing the toner remaining after first transfer and swept away by the fur brush  451 ; and a flicker  456  for removing residual toner attached on the fur brush. The photosensitive drum cleaning unit  450  is provided for cleaning the surface of the photosensitive drum  10  after primary transfer. 
   Around an intermediate transfer belt  21 , there are disposed a belt cleaning unit  460 , and a transfer unit  30 . The belt cleaning unit  460  is provided with a cleaning blade  461  disposed within a casing  462 , and a cleaning contact/separation mechanism  463  for moving the cleaning blade  461  into and out of contact with the intermediate transfer belt  21  as required. 
   In addition, the transfer unit  30  has a secondary transfer bias controller  34  opposing a driving roller  24  of the intermediate transfer unit  20 ; a cleaning blade  31  disposed within a casing  32 ; and a transfer contact/separation mechanism  33 . The transfer contact/separation mechanism  33  enables the secondary transfer bias controller  34  to come into contact with and separate away from the intermediate transfer belt  21 . 
   During a time period in which a complete toner image is formed on the intermediate transfer belt  21 , specifically, during a time period from the time the first color (Bk) toner image had been transferred to the intermediate transfer belt  21  to the time the fourth color (Y) toner image has been transferred to the same, the cleaning blade  461  of the belt cleaning unit  460  and the secondary bias roller  34  of the transfer unit  30  are separated away from the intermediate transfer belt  21  by the respective contact/separation mechanisms ( 463 ,  33 ) associated therewith. 
   While the operation of the copier has been described in connection with a copy mode for producing full-color copies, the same description is applicable to other copy modes, i.e, a three-color copy mode and a two-color copy mode, except that used colors and associated mechanisms are different. For a single-color copy mode, a developer in a developing device associated with a selected color is maintained to form a sleeve or ear, i.e., the developing device is maintained in operative state until a predetermined number of copies have been produced. Also, with the belt cleaning unit  460  and the transfer unit  30  maintained in contact with the intermediate transfer belt  21  and with the intermediate transfer belt  21  maintained in contact with the photosensitive drum  10 , the intermediate transfer belt  21  is driven in the forward direction at a constant speed for producing copies. 
   In the following, description will be made on the configurations and operations of characterizing portions of the sixth embodiment, i.e., the photosensitive drum cleaning unit  450  for cleaning the photosensitive drum  10 ; the cleaning unit  460  for cleaning the intermediate transfer belt  21 ; and the cleaning blade  31  for cleaning the secondary transfer bias roller  34  of the transfer unit  30 . 
   First, the photosensitive drum cleaning unit  450  will be described in terms of its configuration with reference to  FIG. 16 . Essentially, the photosensitive cleaning unit  450  of the sixth embodiment simultaneously cleans and discharges the photosensitive drum  10 . The sixth embodiment employs the fur brush  451  which is formed of a conductive roller and a conductive brush sheet wrapped around the roller. 
   The conductive roller, serving as a core bar of the fur brush  451 , is connected to a ground  454 . To improve a discharging efficiency, the fur brush  451  is fabricated such that the resistance between a portion contacting the photosensitive drum  10 , i.e., the brush tip and the ground  454  is equal to or lower than 10 8  Ω, and preferably, equal to or lower than 10 7  Ω. While the sixth embodiment employs a fur brush as a cleaning member, other cleaning members well known in the art may also be used, for example, a cleaning blade, a combination of a cleaning blade and a fur brush, and so on. In addition, the fur brush  451  with a narrowest possible gaps between edges of the wrapped conductive brush sheet would have an improved cleaning performance and eliminate uneven discharging because of a more uniform bristle density in the axial direction of the fur brush  451 . 
   Description will next be made on the operation of the photosensitive drum cleaning unit  450 . After a toner image formed on the photosensitive drum  10  has been transferred to the intermediate transfer belt  21 , the toner remaining on the photosensitive drum  10  after the primary transfer is brought into a cleaning region defined between the photosensitive drum  10  and the fur brush  451 . Then, the residual toner is swept away from the photosensitive drum  10  by the rotating fur brush  451 . It should be noted the fur brush  451  is driven to rotate at a speed relative to the intermediate transfer belt  21  in order to prevent the discharging performance from degrading and bristles of the fur brush  451  from lying down. In the sixth embodiment, the fur brush  451  is rotated in the direction reverse to the intermediate transfer belt  21 . 
   As mentioned above, the residual toner swept away by the fur brush  451  is received by the residual toner recovering container  455  within the casing  452 . Also, residual toner attached on the fur brush  451  is removed therefrom by the flicker  456  in contact with the fur brush  451 . The removed residual toner is received by the residual toner recovering container  455 . A recovering roller  453  is disposed inside the residual toner recovering container  455 . The recovering roller  453  is applied by a power supply, not shown, with a bias for attracting the residual toner. In this way, since the residual toner received by the residual toner recovering container  455  is collected by the recovering roller  453 , contamination inside the copier is obviated. 
   It should be noted that the fur brush  451  also discharges the photosensitive drum  10  simultaneously with the cleaning when it comes in contact therewith. Specifically, since the fur brush  451  is made of conductive materials and is grounded, the fur brush  451 , when in contact with the photosensitive drum  10 , causes a residual charge on the surface of the photosensitive drum  10  to flow into the fur brush  451 . In this way, the residual charge on the photosensitive drum  10  can be removed so that the photosensitive drum  10  is discharged. 
   Next, the belt cleaning unit  460  will be described in terms of the structure with reference again to  FIG. 16 . The belt cleaning unit  460  of the sixth embodiment has the ability of simultaneously cleaning and discharging the intermediate transfer belt  21  Unlike the photosensitive drum cleaning unit  450 , the belt cleaning unit  460  employs a conductive cleaning blade  461  as a cleaning member. The cleaning blade  461  is formed of a plate-shaped member which contacts the intermediate transfer belt  21  over its entire width. 
   The cleaning blade  461  is connected to a ground  464 , and is fabricated such that the resistance between a portion contacting the intermediate transfer belt  21 , i.e., the blade tip and the ground  464  is equal to or lower than 10 8  Ω, and preferably, equal to or lower than 10 7  Ω. While the sixth embodiment employs a cleaning blade as a cleaning member, other cleaning members well known in the art may also be used, as is the case of the photosensitive drum cleaning unit  450 . 
   Description will next be made on the operation of the belt cleaning unit  460 . Generally, after a toner image on the intermediate transfer belt  21  transferred from the photosensitive drum  10  (primary transfer) has been transferred to a transfer paper  100  (secondary transfer), the residual toner remaining on the intermediate transfer belt  21  is introduced into a cleaning region defined between the intermediate transfer belt  21  and the cleaning blade  461 . Then, the residual toner is removed from the intermediate transfer belt  21  by the cleaning blade  461  pressed against the intermediate transfer belt  21 , falls into the casing  461  and remains therein. 
   The cleaning blade  461  simultaneously discharges the intermediate transfer belt  21  when they are in contact. Specifically, as the cleaning blade  461  comes into contact with the intermediate transfer belt  21 , a residual charge on the intermediate transfer belt  21  after secondary transfer, negatively charged due to the discharging which occurs when a transfer paper is separated therefrom, flows into the cleaning blade  461  connected to the ground  464 . In this way, the residual charge on the intermediate transfer belt  21  can be removed to discharge the intermediate transfer belt. 
   Next, a modification to the sixth embodiment will be described with reference to  FIG. 17 .  FIG. 17  is an enlarged view illustrating the configuration of the photosensitive drum and associated components therearound in the copier of the sixth embodiment including the modification. The illustrated copier is substantially similar to the sixth embodiment except that discharge power supplies ( 458 ,  468 ) are connected to the fur brush  451  of the photosensitive drum cleaning unit  450  and to the cleaning blade  461  of the belt cleaning unit  460 , respectively, for applying respective discharging biases. 
   A bias applied to the fur brush  451  of the photosensitive drum cleaning unit  450  may be selected from a direct current or an alternate current bias, or a combination of direct current and alternate current biases as the case may be. Such a discharging bias promotes a residual charge existing on the photosensitive drum  10  to flow into the fur brush  451 , thus allowing for efficient discharging. In this way, the photosensitive drum  10  can be stably discharged even when the surface moving speed of the photosensitive drum  10  is increased, for example, in order to perform the image formation at a higher speed. 
   The cleaning blade  461  of the belt cleaning unit  460  is also applied with a discharging bias as described above, and similar effects can be produced thereby. It is further possible to apply a discharging bias to the cleaning blade  31  in the transfer unit  30  to discharge the secondary transfer bias roller  34 , in a manner similar to the cleaning blade  461  of the belt cleaning unit  460 . 
   [Embodiment 7] 
   Referring next to  FIG. 18 , a seventh embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”.) in which the present invention is applied.  FIG. 18  schematically illustrates the configuration of a main portion of a printer unit in the copier according to the seventh embodiment. The illustrated copier includes a scanner unit, not shown, which has the same configuration as that of the sixth embodiment. The seventh embodiment differs from the sixth embodiment in a belt cleaning unit for cleaning an intermediate transfer belt in the printer unit. Since the copier of the seventh embodiment performs image forming operations basically in the same manner as the sixth embodiment, description on those parts that are constructed and operated in a manner similar to the sixth embodiment is omitted. 
   In  FIG. 18 , a belt cleaning unit  560  of the seventh embodiment has a fur brush  561  and cleaning blade  567  disposed within a casing  562  as cleaning members; and cleaning contact/separation mechanism  563  for moving the fur brush  561  and the cleaning blade  567  into and out of contact with an intermediate transfer belt  21  as required. The fur brush  561  has the ability of simultaneously cleaning and discharging the intermediate transfer belt  21 , and is the same as the fur brush  451  of the photosensitive cleaning unit  450  in the aforementioned sixth embodiment. The cleaning blade  567 , in turn, is disposed downstream of the fur brush  561  in a belt moving direction, and, unlike the fur brush  561 , only cleans the intermediate transfer belt  21  without discharging it. 
   The cleaning blade  567  is connected to a cleaning power supply for applying the same with a cleaning bias. This cleaning bias has a polarity which repels that of the residual toner remaining on the intermediate transfer belt  21 . Specifically, since the residual toner has the negative polarity, the cleaning blade  567  is applied with a cleaning bias of negative polarity The applied cleaning bias produces a repellent force to disperse a portion of the residual toner remaining on the intermediate transfer belt  21  after secondary transfer from a cleaning region, before the cleaning blade  567  comes in contact with the intermediate transfer belt  21 , so that the residual toner is partially removed before cleaning The remaining residual toner, not affected by the repellent force, is introduced into the cleaning region as it is, and removed by the cleaning blade  567 . 
   In the manner described above, the intermediate transfer belt  21  is cleaned by the cleaning blade  567  after the amount of residual toner remaining thereon has been previously reduced. In this way, even if a large amount of residual toner, for example, due to a jammed transfer paper  100 , must be removed from the intermediate transfer belt  21  by the cleaning blade  567 , it is possible to completely remove the residual toner from the intermediate transfer belt  21 . The residual toner dispersed by the repelling force of the cleaning bias is received on the inner wall of the casing  562 , and accumulated within the casing  562 . The residual toner removed by the cleaning blade S 67 , in turn, falls into the casing  562  by the gravity and accumulated therein. 
   As appreciated, the seventh embodiment employs a combination of the fur brush  561  having the discharging ability and the cleaning blade  567  as cleaning members. Alternatively, the intermediate transfer belt  121  may be cleaned by a conventional cleaning blade without discharging ability and separately providing a discharging means, or by utilizing an intermediate transfer belt having such a volume resistivity ρv that does not require discharging, or the like. Further, the copier according to the seventh embodiment can be utilized not only in the foregoing full-color copy mode but also in any other copy mode, as is the case of the sixth embodiment. 
   [Embodiment 8] 
   Referring next to  FIG. 19 , an eighth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”) in which the present invention is applied.  FIG. 19  schematically illustrates the configuration of a main portion of a printer unit in the copier according to the eighth embodiment. The illustrated copier includes a scanner unit, not shown, which has the same configuration as that of the sixth embodiment, and performs image forming operations basically in the same manner as the copier of  FIG. 15 . The eighth embodiment differs from the sixth embodiment mainly in the structure and operation of the printer unit.  FIG. 19  corresponds to  FIG. 9 , so that the following description will be mainly focused on only those portions which are different from  FIG. 9 . 
   A photosensitive drum cleaning unit  550  has a fur brush  551  and a cleaning blade  557 , and is provided for cleaning the surface of a photosensitive drum  10  after primary transfer. It should be noted that the fur brush  551  and the cleaning blade  557  in the photosensitive drum cleaning unit  550  are identical in structure to the fur brush  561  and the cleaning blade  567  in the belt cleaning unit  560 . 
   Around an intermediate transfer belt  121 , there are disposed a belt cleaning unit  660 , and a transfer unit  130  which can be moved into and out of contact with the intermediate transfer belt  121  by respective contact/separation mechanisms, not shown, associated therewith. 
   An image surface area on the intermediate transfer belt  121 , on which a Bk toner image formed in the aforementioned process has been transferred, is again returned to a primary transfer region, as is the case of the sixth embodiment. In this event, the cleaning blade  661  of the belt cleaning unit  660  is moved away from the intermediate transfer belt  121  by a mechanism, not shown, associated therewith so as not to disturb the toner image. Further, a first supporting roller  135   a  and a secondary transfer bias roller  131  in the transfer unit  130  are also moved by associated transfer contact/separation mechanisms, not shown, such that the secondary transfer bias roller  131  is moved away from the intermediate transfer belt  121 . In this event, a secondary transfer power supply  139  connected to the secondary transfer bias roller  131  is inhibited from applying a voltage. 
   The above-mentioned state is held until the toner image transferred to the intermediate transfer belt  121  is transferred to a transfer paper  100 . 
   In the following, description will be made on the a cleaning opposed roller  127 , opposing the cleaning blade  661  through the intermediate transfer belt  121 , which constitutes a characterizing portion of the eighth embodiment. It should be noted that the cleaning blade  661  has the ability of simultaneously cleaning and discharging the intermediate transfer belt  121 , and has the same structure as the cleaning blade  461  of the belt cleaning unit  460  in the aforementioned sixth embodiment. 
   The cleaning opposed roller  127  of the eighth embodiment is connected to a cleaning power supply  140  for applying a cleaning bias. This cleaning bias has a polarity which generates an electric field that causes a residual toner remaining on the intermediate transfer belt  121  to separate therefrom. Specifically, since the residual toner has the negative polarity, the cleaning opposed roller  127  is applied with a cleaning bias of negative polarity. Such a cleaning bias applied to the cleaning opposed roller  127  results in the formation of the above-mentioned electric field in a region in front of the cleaning blade  661 , i.e., in a region upstream of the cleaning blade  661  in a belt moving direction. Consequently, a portion of residual toner remaining on the intermediate transfer belt  121  after secondary transfer is removed by the electric filed, before it is introduced into a cleaning region. The remaining residual toner, not affected by the electric field, is introduced into the cleaning region as it is, and removed by the cleaning blade  661 . 
   In one implementation, the fur brush  551  of the photosensitive cleaning unit  550  is formed of a metal roller and a conductive brush sheet wrapped around the metal roller. The conductive brush sheet is made of acrylic fiber dispersed with carbon and having a size of 6.5 deniers, and wrapped around the metal roller such that a gap between edges of the wrapped sheet is 1 mm or less. The fur brush  551  has a filling density of 100,000 per square inch. The resistance from the brush tip to the ground of the fur brush  551  is set at 10 6  Ω. For the cleaning blade  557  of the photosensitive drum cleaning unit  550 , a known one is used. 
   A cleaning blade  661  having a discharging ability is used for the belt cleaning unit  660 . The cleaning blade  661  is formed of a conductive material. The casing  662  of the belt cleaning unit  660  includes a blade mount  662  extending from the inner wall surface of the casing  662 , to which an electrode member  662   b  is secured, as can be seen in  FIG. 20 . The electrode member  662   b  is connected to a discharge power supply  668  and also to a ground  664 . Further, the cleaning blade  661  is securely adhered on the electrode member  662   b  with a conductive adhesive  662   c.    
   In the manner described above, the intermediate transfer belt  121  is cleaned by the cleaning blade  661  after the amount of residual toner remaining thereon has been previously reduced by the electric field. In this way, even if a large amount of residual toner, for example, due to a jammed transfer paper  100 , must be removed from the intermediate transfer belt  121  by the cleaning blade  567 , it is possible to completely remove the residual toner from the intermediate transfer belt  121 . The residual toner removed by the electric field is received on the inner wall of the casing  662 , and accumulated within the casing  662 . The residual toner removed by the cleaning blade  661 , in turn, falls into the casing  662  by the gravity and accumulated therein. 
   As appreciated, the eighth embodiment employs a cleaning blade having a discharging ability as a cleaning member. Alternatively, the intermediate transfer belt  121  may be cleaned only by the cleaning blade  661 , for example, by separately providing a discharging means, by utilizing an intermediate transfer belt having such a volume resistivity ρv that does not require discharging, or the like. Further, the copier according to the eighth embodiment can be utilized not only in the foregoing full-color copy mode but also in any other copy mode, as is the case of the sixth embodiment. 
   [Embodiment 9] 
   Referring next to  FIG. 21 , a ninth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied. 
     FIG. 21  schematically illustrates the configuration of a main portion of a printer unit in the copier according to the ninth embodiment. In general, the illustrated copier is intended for a reduction in cost, and differs from the copier according to the eighth embodiment only in the following aspects. Therefore, similar constituent members in the ninth embodiment are designated the same reference numerals as those in the eighth embodiment, and description thereon is omitted.  FIG. 21  corresponds to  FIG. 11  so that the following description will be mainly focused on only those portions which are different from  FIG. 11 . 
   A cleaning opposed roller  227  in the ninth embodiment also functions as a belt driving means. A cleaning blade  761  is disposed opposite to the cleaning opposed roller  227  through an intermediate transfer belt  221 . 
   In the following, a belt cleaning unit  760 , which constitutes a characterizing portion of the ninth embodiment, will be described with reference to  FIG. 22 . 
     FIG. 22  is an enlarged view illustrating that a cleaning blade  761  of the belt cleaning unit  760  opposes the cleaning opposed roller  227  of the intermediate transfer unit  220 . The cleaning opposed roller  227  is grounded. 
   The cleaning blade  761  has the ability of simultaneously cleaning and discharging the intermediate transfer belt  221 . The cleaning blade  761  has a cleaner  761  positioned on the upstream side in a belt moving direction E of the intermediate transfer belt  221 ; a discharger  761   b  positioned on the downstream side of the belt moving direction E; and an insulating layer  761   c  interposed between the cleaner  761   a  and the discharger  761   b  for insulating them. 
   The cleaner  761   a  is connected to a cleaning power supply  769  for applying the cleaner  761   a  with a bias of a polarity which repels that of residual toner  200  remaining on the intermediate transfer belt  221 . The discharger  761   b  in turn is connected to a discharge power supply  768 . Specifically, since the residual toner  200  is negatively charged, the cleaning power supply  769  applies the cleaner  761   a  with a bias of the same polarity as that of the residual toner  200 . On the other hand, the discharge power supply  768  applies the discharger  761   b  with a bias of the opposite polarity to that of the residual toner  200 . 
   By thus applying the cleaner  761   a  and the discharger  761   b  with respective biases, it is possible to maintain the cleaning performance by previously removing a portion of the residual toner before cleaning and simultaneously discharge the intermediate transfer belt  221 . Stated another way, even when a large amount of residual toner is deposited on the intermediate transfer belt  221 , the residual toner can be completely removed and the intermediate transfer belt  221  can be discharged by a single member. In addition, by completely removing the residual toner by the action of the cleaner  761   a , the discharging performance of the discharger  761   b  can be improved, thereby making it possible to accomplish stable and effective discharging. 
   As a modification to the ninth embodiment, the cleaning opposed roller  227  may be modified to have a similar structure to the cleaning opposed roller  127  in the eighth embodiment, with the result that the cleaning performance can be further improved. Consequently, this leads to a further improvement in the discharging performance of the discharger  761   b  and more stable and effective discharging. 
   [Embodiment 10] 
   Next, a tenth embodiment of the present invention will be described in connection with an image forming apparatus which employs a transfer material carrier such as a belt for carrying and conveying a transfer material such as a paper, an OHP sheet or the like, and to which the present invention is applied. 
     FIG. 23  schematically illustrates a transfer unit of a copier according to the tenth embodiment. In the tenth embodiment, the present invention is utilized in a transfer material carrier for carrying and conveying a transfer material rather than in an intermediate transfer unit as in the foregoing embodiments.  FIG. 23  corresponds to  FIG. 12  so that the following description will be mainly focused on only those portions which are different from  FIG. 12 . 
   An area of the paper transfer belt  332 , from which a transfer paper  100  has been separated, is moved to a cleaning region defined between the cleaning blade  331  and the cleaning opposed roller  335   c . After completion of a normal transfer step, there is a bit of contaminants such as paper dusts, rather than toner, attached on the paper transfer belt  332 . Such contaminants may be sufficiently removed by any conventional cleaning member. However, for example, if a jammed transfer paper  100  results in a toner image on the photosensitive drum  10  transferred to the paper transfer belt  332 , instead of a transfer paper, an excessively large amount of toner must be removed by a cleaning member. This problem is particularly grave in a full color image forming apparatus. In this case, an amount of toner exceeding the cleaning capability of the cleaning blade  331  will be introduced into the cleaning region, so that a conventional cleaning member is not capable of completely removing such a large amount of toner. As a result, a portion of the toner, too much for the cleaning member to remove, may cause troubles such as an insufficient transfer bias or the like in the next transfer step. 
   In the tenth embodiment, however, a portion of toner is previously removed making use of an electric action, before cleaning, to reduce the amount of toner introduced into the cleaning region, such that the cleaning blade  331  removes only the remaining toner, in a manner similar to the ninth embodiment. In this way, since the amount of toner remaining on the paper transfer belt  332  is previously reduced before cleaning, even a large amount of toner can be completely removed from the paper transfer belt  332 . 
   The foregoing tenth embodiment utilizes an electric field as a means for removing a portion of residual toner before the paper transfer belt  332  comes in contact with the cleaning blade  331 . However, when a magnetic toner is employed in some copiers, a magnetic field generating means may be used to form a magnetic field which can remove a residual magnetic toner. 
   [Embodiment 11] 
   Referring next to  FIGS. 24 to 27 , an eleventh embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied. 
     FIG. 24  is a cross-sectional view schematically illustrating the configuration of the copier according to the eleventh embodiment, and  FIG. 25  is an enlarged view schematically illustrating the structure around a photosensitive drum in the copier of  FIG. 24 . The following description will be mainly focused on only those portions which are different from the first embodiment illustrated in  FIGS. 1 ,  2 . 
   Around an intermediate transfer belt  21 , there are disposed a lubricant coating unit  850 , a cleaning belt (belt cleaning unit)  460 , and a transfer unit  30 . The lubricant coating unit  850  has a lubricant coating brush roller  851 , a lubricant container  852 , and a contact/separation mechanism  853  for moving the lubricant coating brush roller  851  into and out of contact with the intermediate transfer belt  21 . The lubricant coating brush roller  851  is associated with the contact/separation mechanism  853 , so that it is driven by the contact/separation mechanism  853  to come into and out of contact with the intermediate transfer belt  21 . 
   The cleaning unit  460  has a brush roller  465  and a cleaning blade  461  as cleaning members, and a cleaning unit contact/separation mechanism  463 . The cleaning unit contact/separation mechanism  463  enables the cleaning unit  460  to come into and out of contact with the intermediate transfer belt  21 . 
   While the seventh embodiment employs a combination of the cleaning blade  461  and the brush roller  465  as cleaning members, they may be used in separation, or other known cleaning members may also be used. 
   During a time period in which a complete toner image is formed on the intermediate transfer belt  21 , specifically, during a time period from the time the first color (Bk) toner image had been transferred to the intermediate transfer belt  21  to the time the fourth color (Y) toner image has been transferred to the same, the lubricant coating unit  850 , the cleaning unit  460 , and the transfer unit  30  are separated away from the intermediate transfer belt  21  by the respective contact/separation mechanisms ( 853 ,  463 ,  33 ) associated therewith. 
   While the operation of the copier has been described in connection with a copy mode for producing full-color copies, the same description is applicable to other copy modes, i.e, a three-color copy mode and a two-color copy mode, except that used colors and associated mechanisms are different. For a single-color copy mode, a developer in a developing device associated with a selected color is maintained to form a sleeve or ear, i.e., the developing device is maintained in operative state until a predetermined number of copies have been produced. Also, with the lubricant coating unit  850 , the cleaning unit  460 , and the transfer unit  30  maintained in contact with the intermediate transfer belt  21  and with the intermediate transfer belt  21  maintained in contact with the photosensitive drum  10 , the intermediate transfer belt  21  is driven in the forward direction at a constant speed for producing copies. 
   In the following, description will be made on the structure and operation of the lubricant coating unit  850  which constitutes a characterizing portion of the eleventh embodiment.  FIG. 26  is a cross-sectional view schematically illustrating the structure of the lubricant coating unit  850  according to the eleventh embodiment, and  FIG. 27  is a front view of a lubricant coating brush roller  851  in the lubricant coating unit  850 . The lubricant coating unit  850  is disposed downstream of a secondary transfer region and upstream of a primary transfer region in a belt moving direction, and downstream of the cleaning blade  461  and upstream of the primary transfer region in the belt moving direction. 
   The lubricant coating unit  850  is mounted to an arm  853   a  extending from the contact/separation mechanism  853 . A solid lubricant  855  and a spring  856  are contained in the lubricant container  852  of the lubricant coating unit  850 . The solid lubricant  855  may be, for example, a plate formed of fine particles of zinc stearate. The solid lubricant  855  is urged by the spring  856  toward the lubricant coating brush roller  851  to be in contact therewith. The lubricant coating brush roller  851  can be driven by a driving means, not shown, for rotation. When the lubricant  855  is actually coated on the intermediate transfer belt  21  after secondary transfer, the lubricant coating brush roller  851  is rotated to scrape off the solid lubricant  855 . The lubricant thus scraped off is transformed into powder which is then coated on the intermediate transfer belt  21 . 
   In the eleventh embodiment, the lubricant coating brush roller  851  also functions as a discharging member. Specifically, the lubricant coating brush roller  851  is brought into contact with the intermediate transfer belt  21  to coat the lubricant thereon and simultaneously discharge the intermediate transfer belt  21 . In this event, the lubricant coating brush roller  851  is driven to rotate in the same direction as the intermediate transfer belt  21  in order to prevent the discharging performance from degrading and its brush bristles from lying down. In addition, the lubricant coating brush roller  851  is controlled such that it rotates at a line velocity higher than that of the intermediate transfer belt  21  in a discharge region in which the lubricant coating brush roller  851  contacts the intermediate transfer belt  21 . 
   As illustrated in  FIG. 27 , the lubricant coating brush roller  851  is formed of a conductive roller and a conductive fabric sheet  851   b  having brush bristles  851   a  wrapped around the conductive roller. In this event, the brush roller  851  with a narrowest possible gaps between edges of the wrapped conductive brush sheet would eliminate uneven discharging because of a more uniform bristle density in the axial direction of the lubricant coating brush roller  851 . The conductive fabric sheet  851   b  is made, for example, of acrylic fiber dispersed with carbon, or the like. The conductive roller, serving as a core bar of the lubricant coating brush roller  851 , is connected to a ground  854 . To improve a discharging efficiency, the lubricant coating brush roller  851  is fabricated such that the resistance between a portion contacting the intermediate transfer belt  21 , i.e., the tip of the brush bristles  851   a  and the ground  854  is equal to or lower than 10 8  Ω, and preferably, equal to or lower than 10 7  Ω. Essentially, this resistance means the resistance of the conductive fabric sheet since the roller serving as a core bar of the lubricant coating brush roller  851  is conductive. 
   Next, a modification to the eleventh embodiment will be described with reference to  FIG. 28 .  FIG. 28  is an enlarged view schematically illustrating a modified structure around a photosensitive drum  10  in the copier of the eleventh embodiment. The modification basically has substantially the same configuration as the eleventh embodiment, and differs from the eleventh embodiment only in that in the modification, the lubricant coating brush roller is connected to a discharge power supply  859  for applying a discharging bias, whereas in the eleventh embodiment, the lubricant coating brush roller  851  is simply grounded. Since the discharging bias permits a residual charge existing on the intermediate transfer belt  21  to flow into the lubricant coating brush roller  851 , thus allowing for efficient discharging. In this way, the photosensitive drum  10  can be stably discharged even when the surface moving speed of the photosensitive drum  10  is increased, for example, in order to perform the image formation at a higher speed. 
   [Embodiment 12] 
   Referring next to  FIGS. 29 to 31A  and  31 B, a twelfth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied.  FIG. 29  schematically illustrates the configuration of a main portion of a printer unit in the copier according to the twelfth embodiment. The illustrated copier includes a scanner unit, not shown, which has the same configuration as that of the eleventh embodiment, and performs image forming operations basically in the same manner as the copier of  FIG. 24 . The twelfth embodiment differs from the sixth embodiment mainly in the structure and operation of the printer unit.  FIG. 29  corresponds to  FIG. 9 , so that the following description will be mainly focused on only those portions which are different from  FIG. 9 . 
   Around an intermediate transfer belt  121 , there are disposed a lubricant coating unit  850  identical to that used in the eleventh embodiment, and a transfer unit  130 . These components can be moved into and out of contact with the intermediate transfer belt  121  by respective contact/separation mechanisms, not shown, associated therewith. 
   The image forming surface on the intermediate transfer belt  121 , on which the Bk toner image has been transferred, is again returned to the primary transfer region, similarly to the eleventh embodiment. In this event, the lubricant coating brush roller  851  and the cleaning blade  170  are moved away from the intermediate transfer belt  121  by respective contact/separation mechanisms associated therewith so as not to disturb the toner image. 
   In the following, description will be made on the contact/separation mechanism for moving the lubricant coating brush roller  851  into and out of contact with the intermediate transfer belt  121 , and the contact/separation mechanism for moving the cleaning blade  170  into and out of contact with the intermediate transfer belt  121 , which constitute characterizing portions of the twelfth embodiment. Like the eleventh embodiment, the lubricant coating brush roller  851  also functions as a discharging member, and for this purpose, is connected to a discharge power supply  859  for applying the same with a discharging bias. Thus, the lubricant coating brush roller  851 , when in contact with the intermediate transfer belt  121 , can coat a lubricant on the intermediate transfer belt  121  and simultaneously discharge the intermediate transfer belt  121 . 
   First, the structures of the contact/separation mechanisms will be described with reference to  FIG. 30 .  FIG. 30  schematically illustrates the structure of the contact/separation mechanism  173  which functions to move not only the lubricant coating brush roller  851  but also the cleaning blade  170  into and out of contact with the intermediate transfer belt  121 . The contact/separation mechanism  173  includes the lubricant coating unit  850 ; the cleaning blade  170 ; a first contact/separation cam  850   c  for moving the lubricant coating brush roller  851  into and out of contact with the intermediate transfer belt  121 ; a second contact com  170   c  for moving the cleaning blade  170  into and out of contact with the intermediate transfer belt  121 ; and driving units, not shown, connected to these cams. 
   The lubricant coating unit  850  is supported at one end of a first bracket  850   a . The first bracket  850   a  is supported for pivotal movements about a first bracket pivot shaft  850   b . The other end of the first bracket  850   a , opposite to the one end at which the lubricant coating unit  850  is supported, abuts to a cam surface of the first contact/separation cam  850   c . In this event, the other end of the first bracket  850   a  is urged by a spring, not shown, toward the cam surface. The cleaning blade  170 , in turn, is secured to a second bracket  170   a  at one end thereof. The second bracket  170   a  is supported for pivotal movements about a second bracket pivot shaft  170   b . The other end of the second bracket  170   a , opposite to the one end at which the cleaning blade  170  is secured, abuts to a cam surface of the second contact/separation cam  170   c . In this event, the other end of the second bracket  170   a  is urged by a spring, not shown, toward the cam surface. 
   The first contact/separation cam  850   c  is secured to a first cam shaft  850   d  connected to the driving unit. To the first cam shaft  850   d , a first gear  850   e  is also secured at the end on the front side on the drawing. The second contact/separation cam  170   c  is secured to a second cam shaft  170   d . To the second cam shaft  170   d , a second gear  170   e  is also secured at the end on the front side on the drawing. The first gear  850   e  and the second gear  170   e  have the same number of teeth, and are meshed with each other on the same plane. 
   Next, the operation of the contact/separation mechanism  173  will be described with reference to  FIGS. 31A and 31B .  FIG. 31A  and  FIG. 31B  are enlarged views schematically illustrating a main portion of the contact/separation mechanism  173  when the lubricant coating brush roller  851  and the cleaning blade  170  are out of contact with the intermediate transfer belt  121 , and when the lubricant coating brush roller  851  and the cleaning blade  170  are into contact with the intermediate transfer belt  121 , respectively. 
   In  FIG. 31A , the lubricant coating brush roller  851  and the cleaning blade  170  are separated from the intermediate transfer belt  121 . From the illustrated state, the first cam shaft  850   d  is rotated over 180° by a motor, not shown, disposed in the driving unit. This causes the first contact/separation cam  850   c  to also rotate over 180°, and the cam surface thereof to lift the other end of the first bracket  850   a , thus bringing the lubricant coating brush roller  851  into contact with the intermediate transfer belt  121 . In addition, the rotation of the first cam shaft  850   d  also causes the second cam shaft  170   d  to rotate over 180° by way of the first gear  850   e  and the second gear  170   e . The rotation of the second cam shaft  170   d  causes the second contact/separation cam  170   c  to lift the other end of the second bracket  170   a  to bring the cleaning blade  170  into contact with the intermediate transfer belt  121 . In this way, the state illustrated in  FIG. 31A  proceeds to the state illustrated in  FIG. 31B . 
   Further, when the motor is driven to rotate the first cam shaft  850   d  over another 180°, the lubricant coating brush roller  851  and the cleaning blade  170  are moved out of contact with the intermediate transfer belt  121  because the first bracket  850   a  and the second bracket  170   a  have their other ends urged toward the cam surface of the first contact/separation cam  850   c  and the cam surface of the second contact/separation cam  170   c , respectively. In this way, the state illustrated in  FIG. 31B  proceeds to the state illustrated in  FIG. 31A . 
   As an additional feature, by adjusting the angle at which the first contact/separation cam  850   c  is secured to the first cam shaft  850   d  and the angle at which the second contact/separation cam  170   c  is secured to the second cam shaft  170   d , it is possible to arbitrarily set an interval between a timing at which the cleaning blade  170  is moved into contact with the intermediate transfer unit  121  and a timing at which the lubricant coating brush roller  851  is subsequently moved into contact with the intermediate transfer unit  121 . 
   In the twelfth embodiment, the foregoing angles are set such that after the cleaning blade  170  has been brought into contact with the intermediate transfer belt  121 , the lubricant coating brush roller  851  is brought into contact with the intermediate transfer belt  121  at a timing the contacted surface of the intermediate transfer belt  121  passes a position at which the lubricant coating brush roller  851  is designed to contact the intermediate transfer belt  121 . In this way, since the surface discharged by the lubricant coating brush roller  851  in contact therewith has been cleaned, a less amount of contaminants or the like will attach to the lubricant coating brush roller  851 . 
   In one implementation, the lubricant coating brush roller  851  is formed of a metal roller and a conductive fabric sheet wrapped around the metal roller. The conductive fabric sheet is made of acrylic fiber dispersed with carbon and having a size of 6.5 deniers, and wrapped around the metal roller such that gap G between edges of the wrapped sheet is 1 mm or less. The lubricant coating brush roller  851  has a filling density of 100,000 per square inch. The resistance from the brush tip to the ground  854  of the lubricant coating brush roller  851  is set at 10 6  Ω. 
   It should be noted that the lubricant coating brush roller  851  need not be brought into contact with a completely cleaned surface, and an amount of contaminants not affecting a formed image may be regarded to fall within a tolerable range. Thus, depending on specific applications of the copier of the twelfth embodiment, it may be sufficient that a timing at which the lubricant coating brush roller  851  is brought into contact with the intermediate transfer belt  121  is controlled such that an uncleaned area of the intermediate transfer belt  121  contacted by the lubricant coating brush roller  851  is at least smaller than the case where the lubricant coating brush roller  851  and the cleaning blade  170  are simultaneously brought into-contact with the intermediate transfer belt  121 . 
   While in the twelfth embodiment, the lubricant coating brush roller  851  and the cleaning blade  170  are controlled by a single contact/separation mechanism, an individual contact/separation mechanism may be provided for each of them. Further, the copier according to the twelfth embodiment can be utilized not only in the foregoing full-color copy mode but also in any other copy mode, as is the case of the eleventh embodiment. 
   [Embodiment 13] 
   Referring next to  FIG. 32 , a thirteenth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied. 
     FIG. 32  schematically illustrates the configuration of a main portion of a printer unit in the copier according to the thirteenth embodiment. In general, the illustrated copier is intended for a reduction in cost, and differs from the copier according to the twelfth embodiment only in the following aspects. Therefore, similar constituent members in the thirteenth embodiment are designated the same reference numerals as those in the twelfth embodiment, and description thereon is omitted.  FIG. 32  corresponds to  FIG. 11  so that the following description will be mainly focused on only those portions which are different from  FIG. 11 . 
   For feeding a transfer paper  100  in the thirteenth embodiment, the fed transfer paper  100  is directly sandwiched between a secondary transfer bias roller  231  and an intermediate transfer belt  221 , and conveyed to pass between a pair of fixing rollers  145   a  of a fixing unit  145 . 
   In the following, description will be made on the structures and operations of a lubricant coating unit  850  and a cleaning blade  170  which constitute characterizing portions of the thirteenth embodiment. The lubricant coating unit  850  and the cleaning blade  170  in the thirteenth embodiment are substantially similar to the correspondents in the twelfth embodiment in basic structure, and differs in their positions. 
   In the thirteenth embodiment, a lubricant coating brush roller  851  of the lubricant coating unit  850  is also disposed opposite to a driving roller  224  which also serves as a cleaning opposed roller associated with the cleaning blade  170 . Since the driving roller  224  has a ground  222  connected to a casing, the driving roller  224  also serves as a grounding member disposed opposite to the lubricant coating brush roller  851 . As a result, an electric field is concentrically formed between the lubricant coating brush roller  851  and the driving roller  224 . The electric field thus formed allows for stable discharging of not only a charge on the surface of the intermediate transfer belt  121  but also a charge internal to the intermediate transfer belt  121 , so that the entire intermediate transfer belt  121  can be uniformly discharged. The lubricant coating brush roller  851  and the cleaning blade  170  may be disposed opposite to another supporting roller instead of the driving roller  224 . 
   The driving roller  224  opposing the lubricant coating brush roller  851  is formed of a metal roller coated with conductive rubber thereon. The resistance from the surface of the driving roller  224  to the ground  222  is set at 10 7  Ω. 
   [Embodiment 14] 
   Next, a fourteenth embodiment of the present invention will be described in connection with an image forming apparatus which employs a transfer material carrier such as a belt for carrying and conveying a transfer material such as a paper, an OHP sheet or the like, and to which the present invention is applied. 
     FIG. 33  schematically illustrates a transfer unit of a copier according to the fourteenth embodiment. In the fourteenth embodiment, the present invention is utilized in a transfer material carrier for carrying and conveying a transfer material rather than in an intermediate transfer unit as in the foregoing embodiments.  FIG. 33  corresponds to  FIG. 12  so that the following description will be mainly focused on only those portions which are different from  FIG. 12 . 
   A transfer unit  330  has a paper transfer belt  332  for carrying a transfer paper  100 ; a transfer cleaning blade  331  for cleaning the surface of the paper transfer belt  332 ; a ground roller  335   a  positioned at one end of a sheet feed unit, not shown; a transfer bias roller  334  as a charge supply means; a transfer power supply  338  connected to the transfer bias roller  334 ; a tension roller  335   b  positioned at one end of a fixing unit, not shown; a cleaning opposed roller  335   c  opposing the transfer cleaning blade  331 ; a transfer paper discharger  336 ; and a lubricant coating unit  350  for coating a lubricant on the surface of the paper transfer belt  332 . 
   A transfer paper  100 , on which a toner image has been transferred as described above, is discharged by the transfer paper discharger  336 , and passes a separation region where the transfer paper  100  is separated from the paper transfer belt  332 , and conveyed to the fixing unit, not shown. After the transfer paper  100  has been separated from the paper transfer belt  332 , the transfer cleaning blade  331  removes contaminants such as paper dusts from the surface of the paper transfer belt  332 . In this event, a lubricant coating brush roller  351  disposed in the lubricant coating unit  350  coats a lubricant on the cleaned surface of the paper transfer belt  332  in order to reduce a friction between the cleaning blade  331  and the paper transfer belt  332 . The lubricant coating brush roller  351  is disposed upstream of a transfer region and downstream of the separation region in a direction in which the paper transfer belt advances, and preferably, upstream of the transfer region and downstream of the transfer cleaning blade  331  in the paper transfer belt advancing direction. 
   [Embodiment 15] 
   Referring next to  FIGS. 34 to 42 , a fifteenth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied.  FIG. 34  is a cross-sectional view schematically illustrating the configuration of the copier according to the fifteenth embodiment, and  FIG. 35  is an enlarged view schematically illustrating the structure around a photosensitive drum in the copier of  FIG. 34 . The following description will be mainly focused on only those portions which are different from the first embodiment illustrated in  FIGS. 1 ,  2 . 
   Referring first to  FIG. 35 , description will be made on the structure and operation of a corona charger  960  which constitutes a characterizing portion of the fifteenth embodiment. The corona charger  960  is disposed downstream of a secondary transfer region and upstream of a primary transfer region in a direction of movement of an intermediate transfer belt  21 . The corona charger  960  is connected to a discharge power supply  969  for applying the same with a direct current voltage for discharging the intermediate transfer belt  21 . 
   Since the corona charger  960  can discharge the intermediate transfer belt  21  in a non-contact manner, the mechanism associated with discharging the intermediate transfer belt  21  can be simplified as compared with a contact-type discharger. This is because a contact-type discharger requires a contact/separation mechanism for forming a color image using two or more colors. Specifically, while a toner image of each color is transferred to the intermediate transfer belt  21 , the contact/separation mechanism is required to move a discharging member of the contact-type discharger out of contact with the intermediate transfer belt  21 , and after secondary transfer is completed, the contact/separation mechanism is again required to move the discharging member into contact with the intermediate transfer belt  21 . However, since the corona charger  960  introduces the generation of ozone, it is not preferably in view of environmental protection. From this point of view, when a contact-type discharger is used, a discharging brush, a discharging blade, or the like may be used as a discharging member of the contact-type discharger. 
   Next, a modification to the fifteenth embodiment will be described with reference to  FIG. 36 .  FIG. 36  is an enlarged view schematically illustrating a modified structure around a photosensitive drum in the copier of the fifteenth embodiment. This modification employs a contact-type discharger instead of the corona charger  960 , and a discharging brush  961  as a discharging member of the contact-type discharger. An intermediate transfer belt  21   a  used in this modification has a volume resistivity ρv of 10 12  Ωcm or less. A ground roller  23  for tensioning the intermediate transfer belt  21   a  is positioned such that the resistance from a portion of the intermediate transfer belt  21   a  contacting the discharging brush  961  to a ground  962  of the ground roller  23  is 10 8  Ω or less, and preferably 10 7  Ω or less. In this modification, the discharging brush  961  is not connected to a discharge power supply for applying the same with a discharging bias. 
   The volume resistivity ρv of the intermediate transfer belt  21   a  is set to be 10 12  Ωcm or less such that a charge can move within the intermediate transfer belt  21   a . In this way, a residual charge remaining within the intermediate transfer belt  21   a  after secondary transfer, which cannot be discharged by the discharging brush  961 , can move to the ground  962  of the ground roller  23 , thus preventing a residual potential from affecting an image to be formed next time. In this case, even without applying a discharging bias, the surface potential on the intermediate transfer belt  21   a  can be driven to −100 volts or less. 
   Next, another modification to the fifteenth embodiment will be described with reference to  FIG. 37 .  FIG. 37  is an enlarged view schematically illustrating a modified structure around a photosensitive drum in the copier of the fifteenth embodiment. While this modification is substantially similar to the foregoing modification, there are several differences between them. First, the discharging brush  961  is connected to a discharge power supply  969  for applying the same with a discharging bias. Also, an intermediate transfer belt  21   b  used in the second modification has a volume resistivity ρv in a range of 10 11  Ωcm to 10 14  Ωcm. Further, a conductive plate  963  is disposed, as a grounding member, opposite to the discharging brush  61  through the intermediate transfer belt  21   b . As the grounding member, a conductive roller or the like, for example, may be used instead of the conductive plate  963 . The conductive plate  963  is in contact with the rear surface of the intermediate transfer belt  21   b  with which the discharging brush  961  comes into contact. The structure as described enables an electric field to be concentrically formed between the discharging brush  961  and the conductive plate  963 . The electric field thus formed allows for stable discharging of not only a charge on the surface of the intermediate transfer belt  21   b  but also a charge internal to the intermediate transfer belt  21   b , so that the entire intermediate transfer belt  21   b  can be uniformly discharged. 
     FIG. 38  shows relationships between a discharging bias applied to the discharging brush  961  and the surface potential on the intermediate transfer belt  21   b  after discharging (hereinafter referred to as the “post-discharge potential”) when the conductive plate  963  is disposed opposite to the discharging brush  961 , and when the conductive plate  963  is not disposed. In  FIG. 38 , solid lines connecting two circles each indicate a range of variations in the surface potential on the intermediate transfer belt  21   b  when associated discharging biases are applied. As is apparent from this graph, for discharging the intermediate transfer belt  21   b  to be at such a surface potential that will not affect an image to be formed next time, i.e., in a range of −100 to +100 volts, a far less discharging bias is required for the discharging when the conductive plate  963  is disposed opposite to the discharging brush  961 . In addition, it can be seen that variations in potential on the intermediate transfer belt  21   b  after discharging are smaller when the conductive plate  963  is disposed. 
   Further, also in this modification, the resistance between a contacting portion of the conductive plate  963  with the intermediate transfer belt  21   b  and the ground  964  of the conductive plate  963  is preferably 10 8  Ω or less, and preferably 10 7  Ω or less to improve the discharging efficiency. 
   Next, a further modification to the fifteenth embodiment will be described with reference to  FIGS. 39 and 40 .  FIG. 39  is an enlarged view schematically illustrating a modified structure around a photosensitive drum in the copier of the fifteenth embodiment, and  FIG. 40  is a front view of a discharging brush roller for use in the copier. This modification employs a discharging brush roller  965  instead of the discharging brush  961  in the foregoing modifications. The discharging brush roller  965  is formed of a conductive roller and a conductive fabric sheet  965   b  having brush bristles  965   a  wrapped around the conductive roller. The conductive fabric sheet  965   b  is made, for example, of acrylic fiber dispersed with carbon, or the like. In a contact-type discharger having the discharging brush roller  965  of this modification, the discharging brush roller  965  may be fabricated such that the resistance between a portion contacting the intermediate transfer belt  21 , i.e., the tip of the brush bristles  986   a  and a ground  966  connected to the discharging brush roller  965  is 10 8  Ω or less, and preferably, 10 7  Ω or less to improve a discharging efficiency. Essentially, this resistance means the resistance of the conductive fabric sheet since the roller serving as a core bar of the discharging brush roller  965  is conductive. 
     FIG. 41  is a graph showing the relationship between a filling density of the discharging brush roller  965  and an evaluation of potential unevenness on the intermediate transfer belt  21  after discharging. The evaluation of potential unevenness is made on a five-level basis, where the least potential unevenness is evaluated as level  5 . Generally, the potential unevenness on the surface of the intermediate transfer belt  21  evaluated as level  3  or higher will not affect an image to be formed next time. It can therefore be understood from the graph that the filling density of the discharging brush roller  965  is preferably 20,000 per square inch or more. With the discharging brush roller  965  thus formed, an increased number of bristles can be in contact with the surface of the intermediate transfer belt  21  per unit area, so that the potential unevenness can be effectively suppressed on the intermediate transfer belt  21  after discharging. 
   When the discharging brush roller  965  is fabricated, the conductive fabric sheet  965   b  is wrapped around the conductive roller as mentioned above, in which case the potential unevenness also varies largely depending on a gap G between edges of the wrapped conductive fabric sheet  965   b .  FIG. 42  shows the relationship between the gap G between edges of the wrapped conductive fabric sheet  965   b  and the evaluation of potential unevenness on the surface of the intermediate transfer belt  21  after discharging. The same five-level evaluation is also applied in  FIG. 42 . To achieve level  3  or higher for the evaluation of potential unevenness, the discharging brush roller  965  should be fabricated such that the gap G is at least 2 mm or less. In other words, the gap G of 2 mm or less is effective in suppressing the potential unevenness on the surface of the intermediate transfer belt  21  after discharging. 
   In this modification, the discharge coating brush roller  965  is driven to rotate in the same direction as the intermediate transfer belt  21  in order to prevent the discharging performance from degrading and its brush bristles from lying down. In addition, the discharging brush roller  965  is controlled such that it rotates at a line velocity higher than that of the intermediate transfer belt  21  in a discharge region in which the discharging brush roller  965  contacts the intermediate transfer belt  21 . 
   [Embodiment 16] 
   Referring next to  FIG. 43 , a sixteenth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied.  FIG. 43  schematically illustrates the configuration of a main portion of a printer unit in the copier according to the sixteenth embodiment. The illustrated copier includes a scanner unit, not shown, which has the same configuration as that of the fifteenth embodiment, and performs image forming operations basically in the same manner as the copier of  FIG. 34 . The sixteenth embodiment differs from the fifteenth embodiment mainly in the structure and operation of the printer unit.  FIG. 34  corresponds to  FIG. 9 , so that the following description will be mainly focused on only those portions which are different from  FIG. 9 . 
   Around an intermediate transfer belt  121 , there are disposed a lubricant coating and discharging brush roller  1065  for coating a lubricant on and discharging the surface of the intermediate transfer belt  121 ; a belt cleaning blade  170 ; and a transfer unit  130 . These components can be moved into and out of contact with the intermediate transfer belt  121  by respective contact/separation mechanisms, not shown, associated therewith. 
   As mentioned above, the image forming surface on the intermediate transfer belt  121 , on which a Bk toner image has been transferred, is again returned to the primary transfer region. In this event, the lubricant coating and discharging brush roller  1065  and the belt cleaning blade  170  are moved away from the intermediate transfer belt  121  by respective contact/separation mechanisms associated therewith so as not to disturb the toner image. 
   After secondary transfer, residual toner remaining on the surface of the intermediate transfer belt  121  is removed by pressing the belt cleaning blade  170  against the intermediate transfer belt  121  by the associated contact/separation-mechanism, not shown. Then, a lubricant contained in a lubricant container  1066  is coated on the surface of the intermediate transfer belt  121  by the lubricant coating and discharging brush roller  1065  pressed against the intermediate transfer belt  121  by the associated contact/separation mechanism, not shown, in order to improve the cleaning performance and the secondary transfer operability. The lubricant for use in this application may be, for example, a plate formed of fine particles of zinc stearate. 
   In the following, description will be made on the structures and operations of the lubricant coating and discharging brush roller  1065  and the belt cleaning blade  170  which constitute characterizing portions of the sixteenth embodiment. In the sixteenth embodiment, the lubricant coating and discharging brush roller  1065  functions to coat a lubricant on and discharge the surface of the intermediate transfer belt  121 . It should be noted that while the lubricant coating and discharging brush roller  1065  may be substantially similar to the discharging brush roller  965  previously described in the third modification of the fifteenth embodiment, the conductive fabric sheet forming the brush bristles has the resistance of approximately 10 6  Ω. The lubricant coating and discharging brush roller  1065  is connected to a variable discharge power supply  1069  for applying the same with a discharging bias. Also, the lubricant coating and discharging brush roller  1065  is disposed upstream of a primary transfer region and downstream of the belt cleaning blade  170  in a direction of movement of the intermediate transfer belt  121 . 
   The lubricant coating and discharging brush roller  1065  is formed of a metal roller and a conductive fabric sheet wrapped around the metal roller. The conductive fabric sheet is made of acrylic fiber dispersed with carbon and having a size of 6.5 deniers, and wrapped around the metal roller such that a gap G between edges of the wrapped sheet is 1 mm or less. The lubricant coating and discharging brush roller  1065  has a filling density of 100,000 per square inch. The resistance from the brush tip to the ground of the lubricant coating and discharging brush roller  1065  is set at 10 6  Ω. 
   A direct current voltage applied to the lubricant coating and discharging brush roller  1065  is calculated in accordance with the temperature and humidity around the intermediate transfer belt  121 , a surface potential on the intermediate transfer belt  121  based on the copy mode information, and a surface moving speed of the intermediate transfer belt  121  based on transfer paper information, and the variable discharge power supply  1069  is controlled to generate the calculated direct current voltage. Specifically, the discharging bias is set as shown in Tables 4, 5, and 6. 
   Since the structure and operation of the contact/separation mechanisms associated with the lubricant coating and discharging brush roller  1065  and the belt cleaning blade  170  are identical to those previously described with reference to  FIGS. 30 ,  31 A,  31 B, description thereon is omitted here. 
   [Embodiment 17] 
   Referring next to  FIG. 44 , a seventeenth embodiment of the present invention will be described in connection with a full color electronic photocopier (hereinafter simply referred to as the “copier”), that is, an image forming apparatus in which the present invention is applied. 
     FIG. 44  schematically illustrates the configuration of a main portion of a printer unit in the copier according to the seventeenth embodiment. In general, the illustrated copier is intended for a reduction in cost, and differs from the copier according to the sixteenth embodiment only in the following aspects. Therefore, similar constituent members in the seventeenth embodiment are designated the same reference numerals as those in the sixteenth embodiment, and description thereon is omitted.  FIG. 44  corresponds to FIG.  11  so that the following description will be mainly focused on only those portions which are different from  FIG. 11 . 
   Description will be made on the structures and operations of a lubricant coating and discharging brush roller  1065  and a belt cleaning roller  170  which constitute characterizing portions of the seventeenth embodiment. Like the foregoing sixteenth embodiment, the lubricant coating and discharging brush roller  1065  is used also as a discharging member. The seventeenth embodiment differs from the sixteenth embodiment in the positioning of the lubricant coating and discharging brush roller  1065  and the belt cleaning roller  170 . 
   Specifically, the lubricant coating and discharging brush roller  1065  is disposed opposite to a driving roller  224  which also serves as a cleaning opposed roller associated with the cleaning blade  170 , as can be seen in  FIG. 44 . 
   The lubricant coating and discharging brush roller  1065  is formed of a metal roller and a conductive fabric sheet wrapped around the metal roller. The conductive fabric sheet is made of acrylic fiber dispersed with carbon and having a size of 6.5 deniers, and wrapped around the metal roller such that a gap G between edges of the wrapped sheet is 1 mm or less. The lubricant coating and discharging brush roller  1065  has a filling density of 100,000 per square inch. 
   The driving roller  224  opposing the lubricant coating and discharging brush roller  1065  is formed of a metal roller coated with conductive rubber thereon. The resistance from the surface of the driving roller  224  to the ground  222  is set at 10 7  Ω. 
   [Embodiment 18] 
   Next, an eighteenth embodiment of the present invention will be described in connection with an image forming apparatus which employs a transfer material carrier such as a belt for carrying and conveying a transfer material such as a paper, an OHP sheet or the like, and to which the present invention is applied. 
     FIG. 45  schematically illustrates a transfer unit of a copier according to the eighteenth embodiment. In the eighteenth embodiment, the present invention is utilized in a transfer material carrier for carrying and conveying a transfer material rather than in an intermediate transfer unit as in the foregoing embodiments.  FIG. 45  corresponds to  FIG. 12  so that the following description will be mainly focused on only those portions which are different from  FIG. 12 . 
   A paper transfer belt  332  of the eighteenth embodiment is passed over a ground roller  335   a  positioned at one end of a sheet feed unit, not shown, and serving as a pre-transfer discharging means, and a tension roller  335   b  positioned at one end of a fixing unit, not shown. 
   A transfer paper  100 , on which a toner image has been transferred as described above, is separated from the paper transfer belt  332 , and conveyed to the fixing unit, not shown. The paper transfer belt  332 , after the transfer paper  100  has been separated therefrom, passes a region in which a discharging brush  361  disposed opposite to the tension roller  335   b  is brought into-contact therewith, and is discharged by the discharging brush  361 . 
   Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.