Patent Publication Number: US-6212351-B1

Title: Image transferring method and image forming apparatus for transferring toner image from image carrier to recording medium either via or carried on intermediate image transfer belt

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming method of the type transferring a toner image from an image carrier to a recording medium via an intermediate image transfer belt, or intermediate image transfer body, or transferring it from the image carrier to a recording medium carried on a transfer belt, or medium carrier, and a copier, printer facsimile apparatus or similar image forming apparatus for practicing the same. 
     An image forming apparatus of the type transferring a toner image from a photoconductive element or image carrier to an intermediate image transfer belt (primary transfer) is well known in the art. For the primary transfer, use may be made of an indirect bias application system, which applies a bias for image transfer indirectly to the belt. In the indirect bias application system, a bias roller for belt transfer is positioned downstream of a nip between the photoconductive element and the belt while a ground roller is positioned upstream of the nip. The above bias is applied to the bias roller in order to transfer a toner image from the photoconductive element to the belt. 
     The problem with the above image forming apparatus is that toner is scattered at the time of primary transfer of a toner image from the photoconductive element to the belt. Specifically, at the time of primary transfer, a toner image formed on the photoconductive element is not transferred to a preselected position on the belt, but is scattered around the preselected position and blurred. Particularly, such scattering of toner causes thin lines to lose sharpness. 
     One cause of the scattering of toner is so-called pretransfer, i.e., the transfer of toner from the photoconductive element to the belt occurring at a position upstream of the nip between the element and the belt in the direction of movement of the element, as well known in the art. Another cause is so-called retransfer, i.e., the transfer of toner from the belt back to the photoconductive element occurring at a position downstream of the above nip. More specifically, as for pretransfer, when the bias is applied to the bias roller, a potential slope occurs between the bias roller and the ground roller and forms an electric field even at the side upstream of the nip, causing the toner to move toward the belt away from the photoconductive element. As for retransfer, the toner image successfully transferred from the photoconductive element to the belt is disturbed by an electric field for image transfer formed at the side downstream of the nip. 
     Presumably, the above pretransfer and retransfer also occur when a toner image is directly transferred from the photoconductive element to an image transfer belt used to convey a recording medium. 
     It is a common practice with an image forming apparatus using the intermediate image transfer belt or the transfer belt to cause the belt to contact an object facing it by use of a pressing member. The pressing member presses the surface of the belt opposite to the surface expected to contact the object. However, with this kind of arrangement, it is likely that when the belt is left unused over a long time, both the belt and the object contacting each other over a long time are damaged, and the belt curls complementarily to the contour of the pressing member. The cur led portion of the belt would vary the mechanical contact condition and therefore the image transfer condition on entering the nip and would thereby bring about a defective image ascribable to, e.g., irregular image transfer. 
     The above problem arises not only in an image forming apparatus including the image transfer belt or the transfer belt, or image transfer body, to which a toner image is transferred from the image carrier, but also in an image forming apparatus including a belt, a pressing member for pressing the belt, and an object which the surface of the belt opposite to the surface pressed by the pressing member contacts. 
     Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-249842, 8-166731, 8-2409591, and 10-161440. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an image transferring method capable of obviating pretransfer and retransfer apt to occur during belt transfer, and an image forming apparatus for practicing the same. 
     It is another object of the present invention to provide an image forming apparatus capable of preventing a belt from curling. 
     It is another object of the present invention to provide an image forming apparatus capable of preventing a belt from curling and freeing the belt and an object which the belt is expected to contact from damage ascribable to a long time of contact. 
     In accordance with the present invention, in an image transferring method for discharging, at a nip between an image carrier and an intermediate image transfer belt moving while contacting the surface of the image carrier over a preselected distance, a charge deposited on the belt, depositing a transfer charge on the belt at a position downstream of the nip in the direction of movement of the belt, and transferring a toner image formed on the image carrier to the belt by an electric field formed at the nip, a discharging member for discharging the belt discharges, at the nip, the belt in contact with the surface of the belt opposite to the surface contacting the image carrier with a pressure between 0.05 N/cm 2  and 2 N/cm 2 . 
     Also, in accordance with the present invention, an image forming apparatus includes an image carrier, and an intermediate image transfer unit. The intermediate image transfer unit includes an intermediate image transfer belt movable while contacting the surface of the image carrier over a preselected distance, a discharging member for discharging a charge deposited on the belt at a nip between the belt and the image carrier, and a charge depositing member for depositing a transfer charge on the belt at a position downstream of the nip in the direction of movement of the belt. A toner image formed on the image carrier is transferred to the belt by an electric field formed at the nip. At the nip, the discharging member discharges the belt in contact with the surface of the intermediate image transfer belt opposite to the surface contacting the image carrier with a pressure between 0.05 N/cm 2  and 2 N/cm 2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a section showing an image forming apparatus embodying the present invention; 
     FIG. 2 is a view showing a photoconductive element included in the illustrative embodiment together with various units arranged around the element; 
     FIG. 3 is a view showing an alternative embodiment of the present invention; 
     FIGS. 4A and 4B are fragmentary views each showing a specific configuration of moving means included in the embodiment of FIG. 3; 
     FIG. 5 is a table listing biases to be selectively applied to a secondary transfer bias roller included in the embodiment of FIG.  3 ; 
     FIG. 6 is a view showing another alternative embodiment of the present invention; 
     FIG. 7 is a table listing biases selectively applied to a secondary transfer bias roller included in the embodiment of FIG. 6; and 
     FIG. 8 is a view showing a further alternative embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2 of the drawings, a preferred embodiment of the present invention is shown which is implemented as a full-color electrophotographic copier by way of example. As shown, the copier is generally mage up of a scanner section or color image reading device  1  and a printer section or color image recording device  2 . 
     The scanner section  1  includes a lamp  4  for illuminating a document  3  laid on a glass platen. The resulting reflection from the document is incident to a color image sensor  7  via mirrors  5   a ,  5   b  and  5   c  and a lens  6 . The color image sensor  7  separates color image information incident thereto to, e.g., a blue (B), a green (G) and a red (R) component and transforms the B, G and R components to a B, a G and an R electric signal, respectively. To read the three colors at the same time, the image sensor  7  includes color separating means and CCDs (Charge Coupled Devices) or similar photoelectric transducers. An image processing section, not shown, executes color conversion with the B, G and R image signals on the basis of intensity level to thereby output black (Bk), cyan (C), magenta (M) and yellow (Y) color image data. More specifically, in response to a start signal associated with the operation of the printer section  2 , the above scanning optics scans the document in a direction indicated by an arrow A in FIG. 1 so as to output the color image data. In the illustrative embodiment, every time the optics sans the document, image data of one color is output. Therefore, to output the Bk, C, M and Y color image data, the optics scans the same document four consecutive times. 
     The printer section  2  includes an optical writing unit or exposing means  8  and a photoconductive drum  10  which is a specific form of an image carrier. The optical writing unit  8  converts the color image data output from the scanner section  1  to optical signals so as to sequentially form electrostatic latent images on the drum  10 . The optical writing unit  8  may be implemented by a semiconductor laser  8   a , a controller for controllably driving the laser  8   a , a polygonal mirror  8   b , a motor  8   c  for driving the mirror  8   b , an f/θ lens  8   d , and a mirror  8   e . The drum  10  is rotated in a direction indicated by an arrow B in FIG. 1, i.e., counterclockwise. 
     Arranged around the drum  10  are a drum cleaning unit or drum cleaning means  11 , a discharge lamp or discharging means  12 , a charger or charging means  13 , a potential sensor or potential sensing means  14 , a Bk developing unit  15 , a C developing unit  16 , an M developing unit  17 , a Y developing unit  18 , a density pattern sensor or density sensing means  19 , and an intermediate image transfer unit  20 . The cleaning unit  11  includes a blade  11   a , a brush roller or applying means  11   b  for applying a lubricant to the drum  10 , and a precleaning discharger  11   d . The Bk, C, M and Y developing units  15 - 18  constitute developing means. 
     The blade  11   a  is constantly held in contact with the drum  10  for removing toner left on the drum  10  after primary transfer. The brush roller  11   b  also constantly held in contact with the drum  10  applies a lubricant to the surface of the drum  10  in order to enhance the cleaning ability of the cleaning unit  11 . Specifically, when a drive mechanism, not shown, connected to the shaft of the brush roller  11   b  causes the roller  11   b  to rotate, the roller  11   b  shaves off solid zinc stearate  11   c  and applies the resulting fine powder of zinc stearate to the drum  10 . 
     The developing units  15 - 18  respectively include paddles  15   a - 18   a , toner content sensors  15   b - 18   b , and developing sleeves  15   c - 18   c . The paddles  15   a - 18   a  each play the role of agitating means for agitating a developer while scooping it up. The toner content sensors  15   b - 18   b  each are toner content sensing means responsive to the toner content of a developer. The developing sleeves  15   c - 18   c  each are a developer carrier for causing the ear of a developer formed thereon to contact the surface of the drum  10 . While the copier is in a stand-by state, the developing units  15 - 18  each maintain the ear of the developer deposited on the respective developing sleeve in an inoperative position. 
     The intermediate image transfer unit  20  includes an intermediate image transfer belt or intermediate image transfer body  21  passed over a primary transfer bias roller or charge depositing means  22 , a ground roller or primary pretransfer discharging means  23 , a drive roller or drive means  24 , and a driven roller  25 . The primary transfer bias roller  22  is connected to a primary transfer power supply  28 . A motor, not shown, is drivably connected to the intermediate image transfer belt  21 . 
     The above belt  21  has a laminate structure made up of a surface layer, an intermediate layer, and a base layer although not shown specifically. The belt  21  is positioned such that the surface layer contacts the drum  10  while the base layer is remotest from the drum  10 . An adhesive layer, not shown, intervenes between the intermediate layer and the base layer. The belt  21  has a volume resistivity of 10 11  Ωcm to 10 14  Ωcm, preferably 10 12  Ωcm to 10 13  Ωcm or more preferably 10 13  Ωcm, as measured by a method prescribed by JIS (Japanese Industrial Standards) K6911. 
     In the illustrative embodiment, the surface layer and intermediate layer of the belt  21  each have a high resistance while the base layer has a medium volume resistivity of 10 8  Ωcm to 10 11  Ωcm. This configuration is, of course, only illustrative. 
     A belt cleaning unit  29  adjoins the belt  21  and includes, like the drum cleaning unit  11 , a blade  29   a  and a brush roller  29   b  for applying a lubricant implemented by solid zinc stearate to the belt  21 . The blade  29   a  contacts the belt  21  in an orientation counter to the direction in which the belt  21  moves, as illustrated. The brush roller  29   b  faces the surface of the belt  21  at a position upstream of the position where the blade  29   a  contacts the belt  21  in the direction of movement of the belt  21 . A gear, not shown, is mounted on the shaft of the brush roller  29   b  and rotated to, in turn, rotate the roller  29   b . As a result, the brush roller  29   b  shaves off the solid zinc stearate and applies the resulting fine powder to the belt  21 . Moving means, not shown, selectively brings the blade  29   a  and brush roller  29   b  into or out of contact with the belt  21 . 
     An image transfer unit or image transferring means  30  also adjoins the belt  21  and includes a secondary transfer bias roller  31  facing the drive roller  24 , a cleaning blade  32 , and a moving mechanism  33  for selectively moving the unit  30  into or out of contact with the belt  21 . 
     The printer section  2  further includes a pick-up roller  41  for feeding, via a registration roller pair  42 , a paper or similar recording medium  100  toward a secondary image transfer region between the secondary transfer bias roller  31  and the portion of the belt  21  contacting the drive roller  24 . Paper cassettes  43   a ,  43   b  and  43   c  each are loaded with papers  100  of particular size. A manual feed tray  40  is available for feeding OHP (OverHead Projector) sheets, thick sheets and other special sheets by hand. The printer section  2  additionally includes a conveyor unit  44 , a fixing unit or fixing means  45  including a heat roller  45   a  and a press roller  45   b , and a copy tray  46 . 
     The operation of the illustrative embodiment will be described on the assumption that it sequentially forms a Bk, a C, an M and a Y toner image in this order by way of example. On the start of the copying operation, the scanner section  1  reads a document laid on the glass platen. The optical writing unit  8  scans the surface of the drum  10  with a laser beam based on the resulting Bk image data, thereby forming a Bk latent image on the drum  10 . The Bk developing unit  15  develops the Bk latent image with Bk toner to thereby form a Bk toner image. To insure the development of the Bk latent image, the developing sleeve  15   a  of the Bk developing unit  15  is caused to start rotating before the leading edge of the Bk latent image arrives at a developing position assigned to the developing unit  15 . That is, the developer deposited on the developing sleeve  15  is held in an operative position before the leading edge of the Bk latent image arrives at the above developing position. As soon as the trailing edge of the Bk latent image moves away from the developing position, the developer on the sleeve  15   a  is immediately brought to the inoperative position, rendering the developing unit  15  inoperative. This is completed at least before the leading edge of a C latent image to be developed next arrives at the developing position of the Bk developing unit  15 . To render the developer on the sleeve  15   a  inoperative, the sleeve  15   a  may be rotated in the direction opposite to the direction for development. 
     The Bk toner image formed on the drum  10  is transferred from the drum  10  to the surface of the belt  21  moving at the same speed as the drum  10  (primary transfer). 
     In parallel with the primary transfer of the Bk toner image, the scanner section  1  again reads the same document at a preselected timing in order to produce C image data. The optical writing unit  8  scans the drum  10  in accordance with the C image data to thereby form a C latent image on the drum  10 . The C developing unit  16  develops the C latent image so as to form a C toner image. The developing sleeve  16   a  of the C developing unit  16  is caused to start rotating after the trailing edge of the Bk latent image has moved away from a developing position assigned to the developing unit  16 , but before the leading edge of the C latent image arrives at the developing position. As soon as the trailing edge of the C latent image moves away from the developing position, the developer on the sleeve  16   a  is immediately brought to the inoperative position, rendering the developing unit  16  inoperative. This is completed at least before the leading edge of an M latent image to be developed next arrives at the developing position of the C developing unit  16 . The C toner image is transferred from the drum  10  to the belt  21  over the Bk toner image existing on the belt  21  (primary transfer). 
     The above procedure is repeated with an M latent image and a Y latent image also. As a result, the Bk and C toner images and an M and a Y toner image are sequentially transferred from the drum  10  to the belt  21  one above the other in this order, forming a full-color toner image on the belt  21 . 
     During the interval between the primary transfer of one toner image and that of the next toner image, e.g., the primary transfer of the first or Bk toner image and that of the second or C toner image, the belt  21  is driven by any one of conventional systems including a constant speed forward system, a skip forward system, and a reciprocation or quick return system. If desired, to increase the copy speed, any one of the above drive systems may be selected in accordance with the copy size, or a plurality of them may be efficiently combined. 
     Briefly, the constant forward system is such that the belt  21  is driven forward at a low speed during primary transfer. The skip forward system is such that after the forward movement effected for the primary transfer in the same manner as in the constant forward system, the belt  21  is released from the drum  10  and then caused to skip forward to a primary transfer start position at a high speed. The reciprocation or quick return system is such that after the belt  21  has been released from the drum  10  in the same manner as in the skip forward system, it is returned in the reverse direction to a primary transfer start position at a high speed. 
     The belt  21  carrying the full-color image thereon conveys the image to the secondary image transfer region in order to transfer it to the paper  100  (secondary transfer). Usually, the moving mechanism  33  presses the secondary transfer bias roller  31  against the belt  21  at a timing for transferring the toner image to the paper  100 . Subsequently, a preselected bias for secondary transfer is applied to the bias roller  31  in order to form an electric field in the secondary image transfer region. As a result, the toner image is transferred from the belt  21  to the paper  100 . Specifically, the paper  100  is fed from one of the paper cassettes  43   a - 43   c  designated by the operator via an operation panel, not shown, to the secondary image transfer region via the registration roller pair  42 . The registration roller pair  42  drives the paper  100  toward the secondary image transfer region such that the leading edge of the paper  100  meets the leading edge of the toner image formed on the belt  21 . 
     The conveyor unit  44  conveys the paper  100  carrying the full-color toner image thereon to the fixing unit  45 . The fixing unit  45  fixes the toner image on the paper  100  with the heat roller  45   a  and press roller  45   b . The paper or copy  100  is then driven out to the copy tray  46 . 
     After the primary transfer, the drum cleaning blade  11   a  removes the toner left on the drum  10 , and then the brush roller  11   b  applies zinc stearate to the cleaned surface of the drum  10 . 
     In a repeat copy mode, the scanner section  1  having output the Y or fourth color image data for the first copy starts the Bk or first color step for the second copy at a preselected timing. The printer section  2  forms a Bk latent image for the second copy on the drum  10 . After the secondary transfer of the first full-color toner image from the belt  21  to the first paper  100 , a Bk toner image for the second copy is transferred from the drum  10  to the portion of the belt  21  having been cleaned by the cleaning blade  29   a.    
     In a three-color or a two-color copy mode, the illustrative embodiment operates in the same manner as in the above full-color or four-color mode except for the colors of toner. In a one-color copy mode, the developer stored in designated one of the developing units  15 - 18  is constantly held operative until a desired number of copies have been produced. In this case, the belt cleaning blade  29   a  and image transfer unit  30  are held in contact with the belt  21  while the belt  21  is held in contact with the drum  10 . In this condition, the belt  21  is driven forward at a preselected speed. 
     Part of the above construction and operation unique to the illustrative embodiment will be described more specifically hereinafter. As shown in FIG. 2, the primary transfer bias roller  22  is positioned downstream of a nip between the drum  10  and the belt  21 , i.e., a primary image transfer region. The power supply  28  applies a preselected bias for primary transfer to the bias roller  22 . The ground roller or discharging means  23  connected to ground is pressed against the inner surface of the belt  21  by a preselected pressure, so that the belt  21  is pressed against the drum  10 . The ground roller  23  therefore forms the start point of the nip between the drum  10  and the belt  21 . 
     It is noteworthy that the primary transfer bias roller  22  and ground roller  23  supporting the belt  21  replace a separate charge depositing member and a separate discharging member otherwise located at the above nip, thereby saving cost and space. 
     Further, in the illustrative embodiment, by simply connecting the ground roller  23  to ground, it is possible to discharge the charge deposited on the belt  21  by the primary transfer bias roller  22 . Consequently, the charge deposited on the belt  21  substantially does not migrate or migrates little to the side upstream of the start point of the nip between the belt  21  and the drum  10 . That is, the charge does not exist or exists little on the belt  21  upstream of the above nip. It follows that an electric field effecting the toner image transferred to the belt  21  does not exit at the side upstream of the nip. This, coupled with the fact that the belt  21  and drum  10  pressed against each other by the ground roller  23  press the toner entered the nip, causes the toner to cohere on the belt  21 . 
     As stated above, despite the bias applied to the bias roller  22  located downstream of the nip in the direction of movement of the belt  21 , no electric fields causative of pretransfer are formed at the upstream side. In addition, because the toner coheres at the nip, the toner image is disturbed little and prevented from being retransferred to the drum  10  even when subjected to an electric field at the downstream side. The ground roller  23  should preferably be pressed against the belt  21  by a pressure of 0.05 N/cm 2  or above. Should the pressure be excessively low, the effect achievable with the cohesion of the toner would be lost. 
     On the other hand, should the pressure pressing the ground roller  23  against the belt  21  be excessively high, both the adhesion of the toner to the drum  10  and the adhesion of the same to the belt  21  would increase. If the adhesion of the toner to the drum  10  increases, it is likely that the toner remains on the drum  10  and results in a vermicular image. In light of this, the above pressure should preferably be 2 N/cm 2  or below. 
     To increase the adhesion of the toner to the belt  21 , the drum  10  and belt  21  each may be formed of a particular material, or the amount of zinc stearate to be applied to the drum  10  and belt  21  may be adjusted. This, however, cannot fully obviate vermicular images because the adhesion is sometimes partly inverted. 
     A separate discharging member may be located at the above nip and implemented by any one of a brush, a blade and a roller. In such a case, a roller is preferable in consideration of damage to the belt  21  and the movement of the discharging member caused by the movement of the belt  21 . Further, because the separate discharging member would reduce the substantial image transfer region upstream of the discharge position, compared to the ground roller  23  forming the start point of the nip. The separate discharge member should therefore be positioned as close to the start point of the nip as possible. This is successful to form a relatively broad substantial image transfer region and therefore to increase the image transfer efficiency. 
     As stated above, the illustrative embodiment obviates pretransfer and retransfer of a toner image and thereby insures attractive images free from toner scattering. 
     Reference will be made to FlG.  3  for describing an alternative embodiment of the present invention also implemented as a full-color electrophotographic copier. This embodiment also includes the scanner section, not shown, and basically operates in the same manner as the previous embodiment. This embodiment differs from the previous embodiment mainly in the construction and operation of the printer section. As shown, the printer section includes the drum  10 . Arranged around the drum  10  are the optical writing unit, not shown, a drum cleaning unit or drum cleaning means  111 , the charger  13 , a revolver type developing unit (revolver hereinafter)  110 , and an intermediate image transfer unit or intermediate image transferring means  120 . The drum cleaning unit  111  includes a cleaning blade  111   a  and a brush roller  111   b  for applying a lubricant or solid zinc stearate  111   c  to the drum  10 . The printer section additionally includes an image transfer unit or image transferring means  130  and a fixing unit or fixing means  145  including a heat roller  145   a  and a press roller  145   b  as well as the paper feed section and controller described in relation to the previous embodiment. 
     The drum cleaning blade  111   a  is constantly held in contact with the drum  10  for cleaning the surface of the drum  10  after the primary transfer. The brush roller  111   b  is also held in contact with the drum  10  for applying the lubricant  111   c  to the drum  10  in order to enhance the cleaning ability. Specifically, when the brush roller  111   b  is caused to rotate by a drive mechanism, not shown, connected to the shaft of the roller  111   b , the roller  111   b  shaves off the lubricant  111   c  and applies the resulting fine lubricant powder to the surface of the drum  10 . 
     The revolver  110  includes a Bk developing section  115 , a C developing section  116 , an M developing section  117 , and a Y developing section  118 . The revolver  110  is rotatable to bring any one of the developing sections  115 - 118  to a developing position where the developing unit faces the drum  10 . 
     The intermediate image transfer unit  120  includes an intermediate image transfer belt or intermediate image transfer body  121  passed over a primary transfer bias roller  122 , a ground roller or primary transfer predischarging means  123 , a drive roller or belt driving means  124 , a tension roller  125 , a secondary transfer counter roller  126 , and a cleaning counter roller  127 . A primary transfer power source  128  is connected to the primary transfer bias roller  122 . All the rollers over which the belt  121  is passed are electrically conductive, and all the rollers other than the bias roller  122  are connected to ground. The power source  128  applies a preselected bias subjected to constant current or constant voltage control to the bias roller  122 . The belt  121  is identical with the belt  21  of the previous embodiment except that it has a volume resistivity of 10 12  Ωcm to 10 14  Ωcm, preferably 10 13  Ωcm. The surface layer of the belt  121  has a surface resistance of 10 7  Ω/cm 2  to 10 14  Ω/cm 2 . 
     A belt cleaning blade  129   a  and a brush roller  129   b  for applying a lubricant or zinc stearate  129   c  to the belt  121  adjoin the belt  121 . A moving mechanism, not shown, selectively moves the blade  129   a  and brush roller  129   b  into or out of contact with the belt  121 . Another moving mechanism, not shown, moves the image transfer unit  130  into and out of contact with the belt  121 . 
     The image transfer unit  130  includes a belt or recording medium carrier  134  for effecting secondary transfer. A belt cleaning blade  132  cleans the surface of the belt  134 . A secondary transfer bias roller  131  faces the secondary transfer counter roller  126  included in the intermediate image transfer unit  120 . A secondary transfer power source  139  is connected to the bias roller  131 . The belt  134  is passed over a first support roller  135   a  located at a paper inlet end, a second support roller  135   b  adjoining the fixing unit  145 , and a third support roller  135   c  facing the belt cleaning blade  132 . The image transfer unit  130  additionally includes a paper discharger  136  and a belt discharger  137 . The belt  134  is formed of PVDF (polyvinyl idene fluoride) and has a volume resistivity as high as 10 13  Ωcm or above. If desired, the belt  134  may be replaced with a drum or any other suitable member. 
     The operation of the illustrative embodiment will be described on the assumption that a Bk, a C, an M and a Y toner image are sequentially formed in this order. Before the start of an image forming cycle, the drum  10  is rotated counterclockwise, i.e., in a direction indicated by an arrow C in FIG. 3, and the charger  13  starts corona discharge. For example, the charger  13  uniformly charges the drum  10  to a preselected negative potential. The belt  121  of the intermediate image transfer unit  120  is driven at the same speed as the drum  10  in a direction indicated by an arrow D in FIG. 3, i.e., clockwise. 
     The scanner section outputs color image data at a preselected timing as in the previous embodiment. The optical writing unit scans the charged surface of the drum  10  with a laser beam in accordance with Bk image data by, e.g., raster exposure. As a result, a Bk latent image is electrostatically formed on the drum  10 . The Bk developing section  115  of the revolver  110  develops the Bk latent image with toner charged to negative polarity (reversal development), thereby forming a Bk toner image. 
     The Bk toner image is transferred from the drum  10  to the belt  121  by an electric field formed in the primary image transfer region. The electric field is formed by a charge deposited on the belt  121  by the primary transfer bias roller  122 . For example, the power source  128  for primary transfer applies a bias of 1.5 kV to the bias roller  122  for the Bk or first color toner image, a bias of 1.6 kV to 1.8 kV for the C or second color toner image, a bias of 1.8 kV to 2.0 kV for the M or third color toner image, and a bias of 2.0 kV to 2 kV for the Y or fourth color toner image. The drum cleaning blade  111   a  removes the toner left on the drum  10  after the primary transfer, and then the brush roller  111   b  applies the lubricant  111   c  to the drum  10 . 
     The portion of the belt  121  carrying the Bk toner image is again returned to the primary transfer region as in the previous embodiment. At this time, the belt cleaning blade  129   a  and brush roller  129   b  are released form the belt  121  so as not to disturb the toner image. Also, the first support roller  125   a  and secondary transfer bias roller  131  are so moved as to release the bias roller  131  from the belt  121 . At this instant, the application of the bias from the power source  139  to the bias roller  131  is interrupted. This condition is maintained until the secondary transfer of a full-color toner image from the belt  121  to the paper  100 . 
     After the primary transfer of the Bk toner image to the belt  121 , the scanner section again reads the same document to output C image data. The optical writing unit forms a C latent image with a laser beam in accordance with C image data as in the previous embodiment. The C developing section  116  of the revolver  110  develops the C latent image to thereby produce a C toner image on the drum  10 . 
     In the illustrative embodiment, after the trailing edge of the Bk latent image has moved away from the developing position, the revolver  110  is immediately rotated. This rotation of the revolver  110  is completed before the leading edge of the C latent image arrives at the developing position where the C developing section  116  is positioned. In this condition, the developing section  116  develops the C latent image with C toner. 
     The above procedure is repeated with an M latent image and a Y latent image also. As a result, the Bk and C toner images and an M and a Y toner image are sequentially transferred from the drum  10  to the belt  121  one above the other, completing a full-color toner image. 
     The belt  121  carrying the full-color toner image conveys the toner image to the secondary image transfer region. At this instant, the secondary transfer bias roller  131  is brought into contact with the belt  121 . Subsequently, a preselected bias for secondary transfer is applied to the bias roller  131  so as to form an electric field in the secondary transfer region. As a result, the full-color toner image is transferred from the belt  121  to the paper  100 . Again, the paper  100  is fed such that the leading edge of the paper  100  meets the leading edge of the toner image at the secondary image transfer region. 
     The belt  134  of the image transfer unit  130  conveys the paper  100  carrying the full-color toner image to a position where the paper discharger  136  is located. The paper discharger  136  discharges the paper  100  and thereby peels off the paper  100  from the belt  134 . The paper  100  peeled off is conveyed toward the fixing unit  145 . In the fixing unit  145 , the heat roller  145   a  and press roller  145   b  fix the toner image on the paper  100  with heat and pressure. Subsequently, the paper or copy  100  is driven out to a copy tray not shown. 
     After the secondary transfer, the belt cleaning unit  129   a  is brought into contact with the belt  121  in order to remove the toner left on the belt  121 , and then the brush roller  129   b  applies the fine powder of the lubricant  129   c  to the belt  121 . 
     After the separation of the paper  100  from the belt  134 , the belt discharger  137  discharges the belt  134 , and then the belt cleaning blade  132  cleans the surface of the belt  134 . 
     Part of the construction and operation unique to the illustrative embodiment is as follows. As shown in FIG. 3, the primary transfer bias roller or charge depositing means  122  is positioned downstream of the nip between the drum  10  and the belt  121  in the direction of movement of the belt  121 , as in the previous embodiment. Also, the ground roller or discharging means  123  is positioned upstream of the above nip and presses the belt  121  against the drum  10  with a pressure between 0.05 N/cm 2  and 2 N/cm 2 . Therefore, the illustrative embodiment is also successful to obviate pretransfer and retransfer and therefore to insure attractive images free from toner scattering. 
     As shown in FIG. 3, the belt  121  is constantly pressed against the drum  10  by the ground roller  123 . This may bring about a problem that when the belt  121  is not driven over a long time, the drum  10  and belt  121  are apt to suffer from damage, and the belt  121  is apt to curl along the circumference of the ground roller  123 . The cur led portion of the belt  121  would vary the mechanical contact condition and therefore image transfer condition on entering the nip, resulting in a defective image ascribable to, e.g., irregular image transfer. 
     In light of the above, the illustrative embodiment additionally includes moving means for selectively moving the ground roller  123  into or out of contact with the belt  121 . The moving means may be implemented by, e.g., a cam device or a solenoid mechanism. Specifically, as shown in FIG. 4A, on the stop of rotation of the belt  121 , the moving means moves the ground roller  123  away from the belt  121  in response to a signal received from control means not shown. As a result, the belt  121  is released from the drum  10  and from the ground roller  123 . Alternatively, as shown in FIG. 4B, the ground roller  123  may be moved at least to a position where it does not press the belt  121 , but contacts the belt  123 . With this configuration, it is possible to prevent the belt  121  from being constantly pressed against the drum  10  and therefore to minimize damage to the belt  121  and drum  10 . Moreover, the belt  121  is prevented from curling along the circumference of the ground roller  123  even when held inoperative over a long time, thereby solving the above defective image problem. 
     While a conventional support roller for supporting the belt  121  has a diameter great enough to obviate the curling of the belt  121 , the illustrative embodiment including the above moving means is practicable with a roller having a relatively small diameter. In the illustrative embodiment, use is made of a roller having a diameter of 30 mm. Because a mechanism for mounting and dismounting the intermediate image transfer unit  120  is usual ly arranged between the opposite runs of the belt  121  together with other mechanisms, the roller diameter should preferably be as small as possible. 
     A series of experiments were conducted with the illustrative embodiment under the following conditions. The intermediate transfer belt  121  was 0.15 mm thick and 268 mm wide and had an inner peripheral length of 565 mm. The belt  121  was driven at a speed of 200 mm/sec. Further, the belt  121  had an about 1 μm thick surface layer formed of an insulating material and an about 75 μm thick intermediate layer formed of PVDF. The intermediate layer had a volume resistivity of 9×10 12  Ωcm when a voltage of 100 V was applied for 10 seconds or a volume resistivity of 6×10 12  Ωcm when a voltage of 500 V was applied for 10 seconds, as measured at a temperature of 25° C. and a humidity of 45% by a resistance measuring device Hirester IP available from Yuka Denshi. In addition, the belt  121  had an about 75 μm thick base layer formed of PVDF and titanium oxide. The base layer had a volume resistivity of 7×10 7  Ωcm when a voltage of 100 V was applied for 10 seconds, as measured in the above environment by the same measuring device. 
     The surface layer of the belt  121  had a surface resistance of 10 13  Ω/cm 2  as measured by the above measuring device. To measure the surface resistance, use may be made of a measuring method prescribed by JIS (Japanese Industrial Standards) K6911 in place of the above measuring device. 
     The primary transfer bias roller  122  was implmented by a metal roller plated with nickel while the ground roller  123  was implemented by a metal roller. The other rollers were formed of metal or conductive resin. The bias roller  122  was applied with a DC voltage of 1.5 kV for the Bk or first color toner image, a DC voltage of 1.7 kV for the C or second color toner image, a DC voltage of 1.9 kV for the M or third color toner image, and a DC voltage of 2.1 kV for the Y or fourth color toner image. The primary image transfer region had a nip width of 10 mm. 
     In the image transfer unit  130 , the secondary transfer bias roller  131  had a surface layer formed of conductive sponge or conductive rubber and a core layer formed of metal or conductive resin. A particular transfer bias subjected to constant current control was applied to the bias roller  131  for each of different kinds of papers, as shown in FIG.  5 . The secondary image transfer belt  134  was formed of PVDF and had a volume resistivity of 10 13  Ωcm and a thickness of 100 μm. 
     The paper discharger  136  and belt discharger  137  each were applied only with an AC voltage or an AC+DC voltage from a power supply not shown. The cleaning blade  132  contacted the portion of the secondary transfer belt  134  contacting the third support roller  135   c  in a counter orientation. 
     In FIG. 3, the primary transfer bias roller  122  was located downstream of the nip between the drum  10  and the intermediate transfer belt  121  in the direction of movement of the belt  121 . The ground roller  123  connected to ground was pressed against the belt  121  by a pressure between 0.05 N/cm 2  and 2 N/cm 2 , so that the belt  121  was pressed against the drum  10 . Under the above conditions, the illustrative embodiment successfully obviated pretransfer at the downstream side and retransfer at the upstream side and thereby produced desirable images. 
     Another alternative embodiment of the illustrative embodiment is shown in FIG.  6  and also implemented as a full-color electrophotographic copier. This embodiment is directed mainly toward a low cost construction. Because this embodiment is similar to the embodiment of FIG. 3 except for the following, identical structural elements are designated by identical reference numerals. 
     As shown in FIG. 6, this embodiment includes an intermediate image transfer unit  220  including an intermediate image transfer belt  221 . The belt  221  has an overall volume resistivity of 10 10  Ωcm to 10 12  Ωcm. Specifically, the belt  221  includes an intermediate layer having a medium volume resistivity of 10 8  Ωcm to 10 11  Ωcm, and a surface layer having a surface resistance of 10 7  Ω/cm 2  to 10 14  Ω/cm 2 . With the belt  221  having a medium resistance, it is possible to free the surface of the belt  221  from irregular charging after the primary transfer. 
     A drive roller  224  included in the intermediate image transfer unit  220  is located downstream of the secondary image transfer region, but upstream of the primary image transfer region, in the direction of movement of the belt  221 . A belt cleaning blade  229   a  faces the drive roller  224 . In this sense, the drive roller  224  plays the role of the cleaning counter roller  127  of the previous embodiment at the same time. The reference numerals  229   b  and  229   c  designate a brush roller and a lubricant, respectively. 
     A secondary bias roller  231  and a power supply  802  constitute image transferring means and replace the image transfer unit of the embodiment shown in FIG.  3 . The bias roller  231  faces the secondary transfer counter roller  126  of the intermediate image transfer unit  220 . This configuration reduces the number of parts necessary for the secondary transfer and thereby reduces the cost, compared to the embodiment shown in FIG.  3 . 
     In the illustrative embodiment, the secondary transfer bias roller  231  and belt  221  directly nip the paper  100  fed to the secondary image transfer position and drive it toward the heat roller  145   a  and press roller  145   b.    
     Part of the above construction and operation particular to this embodiment will be described hereinafter. As shown in FIG. 6, a ground roller  223  is so positioned as to contact the belt  221  although the former does not press the latter. This prevents the belt  221  from wrapping around the ground roller  223  and therefore prevents it from curling along the circumference of the ground roller  223  even when left inoperative over a long time. This embodiment not only achieves the same advantages as the embodiment of FIGS. 1 and 2, but also obviates defective images ascribable to the variation of image transfer condition. 
     A series of experiments were conducted with the above embodiment under the following conditions. The structural members except for ones to be described hereinafter are identical with the structural members of the embodiment of FIG.  3 . The belt  221  had an intermediate layer formed of PVDF and titanium oxide and had a volume resistivity of 5×10 2  Ωcm when applied with a voltage of 100 V for 10 seconds or a volume resistivity of 2×10 11  Ωcm when applied with a voltage of 500 V for 10 seconds, as measured at a temperature of 25° C. and a humidity of 45% by Hirester mentioned earlier. The surface layer and base layer of the belt  221  were identical with the surface layer and base layer of the belt  121  of the previous embodiment. The belt  221  was moved at a speed of 156 mm/sec. 
     The bias roller  122  was applied with a DC voltage of 1.7 kV for the Bk or first color toner image, a DC voltage of 1.8 kV for the C or second color toner image, a DC voltage of 1.9 kV for the M or third color toner image, and a DC voltage of 2.0 kV for the Y or fourth color toner image. The bias roller  231  for secondary transfer was formed of conductive rubber. As shown in FIG. 7, a particular bias subjected to constant current control was applied to the bias roller  231  for each of different kinds of papers. 
     As shown in FIG. 6, a primary transfer bias roller  222  was located downstream of the nip between the drum  10  and the belt  121  in the direction of movement of the belt  121 . The ground roller  223  was located upstream of the above nip to press the belt  221  toward the drum  10  with a pressure between 0.05 N/cm 2  and 2 N/cm 2 . Under these conditions, the illustrative embodiment successfully obviated pretransfer at the downstream side and retransfer at the upstream side. 
     A further alternative embodiment of the present invention will be described hereinafter which is applicable to an image forming apparatus of the type including a belt for conveying a paper, OHP sheet or similar recording medium. As shown in FIG. 8, the illustrative embodiment is applied to the drum or image carrier  10  in place of the intermediate image transfer body shown and described. In FIG. 8, the reference numeral  311   a  designates a cleaning blade while the reference numerals  335   a  and  225   b  designate support rollers. In the illustrative embodiment, a toner image is formed on the drum  10  by a conventional electrophotographic process. The toner image is transferred to the paper  100  at the nip between the drum  10  and a belt  334  included in an image transfer unit  330 . 
     Specifically, in the image transfer unit  330 , a transfer bias roller or charge depositing means  331  is located downstream of the above nip in the direction of movement of the belt  334 . A power supply, not shown, applies a preselected bias for image transfer to the bias roller  331 . As a result, an electric field is formed at the nip between the drum  10  and the belt  334 , so that a toner image is transferred from the drum  10  to the paper  100  being conveyed by the belt  334 . The belt  334  has a medium volume resistance of 10 8  Ωcm to 10 11  Ωcm. 
     Part of the above construction unique to the illustrative embodiment is as follows. As shown in FIG. 8, the bias roller  331  is located downstream of the nip, as stated above. A ground roller or discharging means  333  is connected to ground and located upstream of the above nip in such a manner as to press the belt  334  toward the drum  10  with a pressure between 0.05 N/cm 2  and 2 N/cm 2 . In this condition, the ground roller  333  pressed against the belt  334  causes the belt  334  to contact the drum  10  and thereby forms the start point of the nip. 
     In this embodiment, the ground roller  333  discharges the charge deposited on the belt  334  by the bias roller  331 . Therefore, the charge deposited on the belt  334  substantial ly does not migrate or migrates little to the side upstream of the start point of the nip. That is, the charge does not exist or exists little on the belt  334  upstream of the above nip. It follows that an electric field effecting the toner image transferred to the belt  334  does not exit at the side upstream of the nip. This, coupled with the fact that the belt  334  and drum  10  pressed against each other by the ground roller  333  press the toner entered the nip, causes the toner transferred to the paper  100  to cohere. 
     As stated above, even when the bias is applied to the bias roller  331  located downstream of the nip in the direct ion of movement of the belt  334 , pretransfer does not occur because no electric fields are formed at the upstream side. In addition, the toner image is disturbed little by the downstream electric field because the toner coheres at the nip, obviating retransfer. 
     All the embodiments shown and described insure attractive images free from toner scattering by obviating pretransfer and retransfer. The characterizing parts of the illustrative embodiments may be replaced with each other. 
     While each illustrative embodiment has been shown and described as including a ground or discharging means connected to ground, a bias opposite in polarity to the transfer charge may alternatively be applied to the ground roller so long as it does not effect the transfer charge required at the nip. 
     The bias roller or charge depositing means of any one of the illustrative embodiments may be replaced with any other suitable charge depositing means. 
     The embodiments described with reference to FIGS. 1-6 each use a secondary transfer bias roller as secondary transfer charge depositing means. The secondary transfer bias roller may, of course, be replaced with a blade, brush or similar secondary transfer charge depositing means. The embodiments described with reference to FIGS. 3 and 6 each are operable even in a copy mode other than the full-color copy mode like the embodiment of FIG.  1 . 
     In all the illustrative embodiments, the photoconductive drum  10  may be replaced with any other suitable image carrier, e.g., a photoconductive belt passed over two or more rollers. 
     In the embodiments of FIGS. 1-6, the intermediate transfer belt may have any suitable electrical characteristic including a surface resistance, structure and thickness matching with image forming conditions. 
     In the embodiments shown and described, the drum or image carrier  10  is charged to negative polarity while the developing means effects reversal development by using a two-ingredient type developer, i.e., a toner and carrier mixture. If desired, the drum  10  may be charged to positive polarity, and the developing means may use a single ingredient type developer, i.e., toner or may effect positive development. 
     In summary, the present invention achieves the following various unprecedented advantages. 
     (1) A charge deposited on an intermediate image transfer belt is discharged by a discharging member at a nip between an image carrier and the belt. This prevents the influence of an electric field for image transfer from extending to the side upstream of the nip in the direction of movement of the belt and thereby obviates pretransfer, i.e., the transfer of toner from the image carrier to the belt at the upstream side. The discharging member contacts the belt with a pressure between 0.05 N/cm 2  and 2 N/cm 2 , so that the belt and image carrier contact with each other with a pressure high enough to cause the toner to cohere at the nip. As a result, a toner image once transferred from the image carrier to the belt is disturbed little by the above electric field at the side downstream of the nip. This successful ly obviates pretransfer and retransfer causative of toner scattering. Should the above pressure be excessively high, the toner would cohere to an excessive degree and would remain on the image carrier at the time of image transfer, resulting in a vermicular image. The pressure of 2 N/cm 2  or below solves such a problem. This advantage is also achievable when the intermediate transfer belt is replaced with a transfer belt or recording medium carrier. 
     (2) By simply connecting the discharging member to ground, it is possible to reduce a charge deposited on the belt. 
     (3) The discharging member discharges the belt in the vicinity of the start point of the above nip. Therefore, an image transfer region upstream of the discharging position and contributing to image transfer is broadened, compared to a case wherein the discharging member is located downstream of the start point of the nip. It follows that higher image transfer efficiency is achievable. 
     (4) Because the belt is not wrapped around the discharging member, the belt is prevented from curling along the circumference of the discharging member even when left unused over a long time. A curled belt would vary the image transfer condition and would thereby bring about a defective image ascribable to, e.g., irregular image transfer. 
     (5) Moving means is capable of moving the discharging member to a position where the discharging member does not press the belt, but contacts the belt, or a position where it is spaced from the belt. This also achieves the above advantage (4), and in addition reduces damage to the belt and image carrier otherwise pressed against each other. This is also true when the discharging member is replaced with a pressing member. 
     (6) A roller member, as distinguished from a brush or a blade, reduces damage to the belt even when it exerts a high pressure against the belt. In addition, the roller member does not follow the rotation of the belt when the belt is driven. 
     (7) Support rollers supporting the belt play the role of a discharging member and a charge depositing member at the same time. This makes it needless to arrange a separate discharging member and a separate charge depositing member and thereby simplifies the construction. 
     (8) The belt is not wrapped around a pressing member. This is also successful to achieve the above advantage (4). 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.