Abstract:
An image forming apparatus of the present invention includes an intermediate image transfer belt passed over a plurality of support members and movable while carrying a toner image of preselected polarity transferred thereto. An electrode member contacts the inside surface of the belt and is applied with a preselected voltage for transferring the toner image from the belt to a recording medium. A contact member with high electric resistance contacts the belt at a position adjacent the electrode member and includes an insulating layer thereon.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an image forming apparatus of the type including an intermediate image transfer belt. 
   2. Description of the Background Art 
   A color image forming apparatus including an intermediate image transfer body implemented as a belt or a drum belongs to a family of conventional image forming apparatuses. In the color image forming apparatus, toner images of different colors are sequentially formed on an image carrier while being sequentially transferred to the intermediate image transfer body one above the other. This image transfer will be referred to as primary image transfer. The resulting composite toner image is transferred from the intermediate image transfer belt to a sheet or recording medium. This image transfer will be referred to as secondary image transfer and is effected by a secondary image transfer roller or body and a back electrode or roller facing it. The back electrode is electrically connected to the inside surface of the intermediate image transfer body. 
   Some other electrodes usually adjoin the back electrode for secondary image transfer and are also electrically connected to the inside surface of the intermediate image transfer body. Such other rollers include a back electrode facing a cleaning member assigned to the intermediate image transfer body. An electric field is formed between the cleaning member and the back roller, which face each other, so that the cleaning member can collect toner left on the intermediate image transfer body after secondary image transfer. 
   Another electrode contacting the intermediate image transfer body is a back electrode facing a charging member configured to invert the polarity of the toner left on the intermediate image transfer body after secondary image transfer. An electric field is also formed between the charging member and the back electrode, which face each other, in order to invert the polarity of the above residual toner and then cause the toner to again deposit on an image carrier at a primary image transfer position. 
   Still another electrode contacting the intermediate image transfer body is a tension roller supported by the frame of the apparatus for applying tension to the image transfer body. 
   In this connection, Japanese Patent Laid-Open Publication No. 10-49019 discloses an image forming apparatus in which a voltage of the same polarity as toner is applied to the inside surface of an intermediate image transfer drum. By this voltage, toner left on the intermediate image transfer drum after secondary image transfer is inverted in polarity and then caused to again deposit on an image carrier at a primary image transfer station. 
   In the image forming apparatus of the type including the intermediate image transfer body, a voltage subject to constant-current control is applied from the back electrode for secondary image transfer to the inside surface of the above image transfer body. At the same time, the secondary image transfer roller is grounded. As a result, an electric field for secondary image transfer is formed between the intermediate image transfer body and the secondary image transfer roller. This electric field varies little even when some current flows via a recording medium or even when the resistance of the intermediate image transfer body or that of the secondary image transfer roller varies, allowing a stable image to be formed on the recording medium. 
   However, when any one of the electrodes adjoining the back roller for secondary image transfer roller, as stated earlier, is grounded, a current fed to the back roller for secondary image transfer leaks to the other back roller via the intermediate image transfer body. As a result, a current flowing toward the recording medium becomes short. Further, when the tension roller contacting the inside surface of the intermediate image transfer body adjoins the back roller for secondary image transfer, the current fed from the back roller for secondary image transfer to the intermediate image transfer body leaks to the frame of the apparatus via the above image transfer body, again making the current flowing toward the recording medium short. 
   Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-102737, 10-39642, 2000-19854, 2001-166614 and 2002-251076 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image forming apparatus capable of preventing a current expected to form an electric field for secondary image transfer from leaking to an electrode or a member adjacent a back roller for secondary image transfer to thereby insure a stable image at all times. 
   An image forming apparatus of the present invention includes an intermediate image transfer belt passed over a plurality of support members and movable while carrying a toner image of preselected polarity transferred thereto. An electrode member contacts the inside surface of the belt and is applied with a preselected voltage for transferring the toner image from the belt to a recording medium. A contact member with high electric resistance contacts the belt at a position adjacent the electrode member. 

   
     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 drawing in which: 
       FIG. 1  is a fragmentary view showing a conventional image forming apparatus including an intermediate image transfer belt; 
       FIG. 2  is a view showing an image forming apparatus applicable to a first to a third embodiment of the present invention; 
       FIG. 3  is a fragmentary view showing an intermediate image transfer body and members associated therewith and representative of the first embodiment of the present invention; 
       FIG. 4  is a fragmentary view showing the second embodiment of the present invention; 
       FIG. 5  is a view similar to  FIG. 4 , showing the third embodiment of the present invention; and 
       FIG. 6  is a fragmentary view showing a fourth embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   To better understand the present invention, brief reference will be made to a conventional image forming apparatus, shown in FIG.  1 . As shown, the image forming apparatus includes an intermediate image transfer belt (simply belt hereinafter)  426  passed over an electrode roller  450  for primary image transfer, a drive roller  444 , a back roller or back electrode  446 , and a back roller or back electrode  448 . The back roller  450  faces a photoconductive drum or image carrier  402  while the back roller  446  faces a secondary image transfer roller  454 . The back roller  448  faces a cleaning unit  452  configured to clean the surface of the belt  426 . 
   A bias for primary image transfer is applied to the electrode roller  450  for transferring a toner image formed on the drum  402  to the belt  426 . Such image transfer is repeated color by color with the result that a composite color image is formed on the belt  426 . A bias for secondary image transfer, which is of the same polarity as toner, is applied to the back roller  446 , for thereby transferring the color image from the belt  426  to a sheet or recording medium P. 
   More specifically, the bias applied to the back roller  446  forms an electric field for secondary image transfer between the belt  426  and the sheet P. Even when the resistance of the sheet P or that of the belt  426  or the secondary image transfer roller  454  varies, the above electric field varies little and allows a stable image to be formed on the sheet P. 
   The back roller for cleaning  448  is generally formed of stainless steel or similar conductive metal and electrically connected to the frame of the apparatus. 
   The conventional apparatus described above operates in a satisfactory manner so long as the resistance of the belt  426  is sufficiently high. However, assume that the resistance of the belt  426 , particularly its inner surface contacting the back roller  446 , is lowered to about 10 10  Ω·cm in terms of surface resistivity due to resistance shift ascribable to the varying environment or the deterioration of current feed. Then, a current fed to the back roller  446  leaks to the back roller  448 , which adjoins the back roller  446 , via the belt  426 . As a result, a current flowing toward the sheet P and therefore the electric field for secondary image transfer becomes short, lowering image quality. 
   Preferred embodiments of the image forming apparatus in accordance with the present invention will be described hereinafter. 
   First Embodiment 
   Referring to  FIGS. 2 and 3 , an image forming apparatus embodying the present invention is shown and implemented as a color copier by way of example. In  FIGS. 2 and 3 , structural elements identical with the structural elements shown in  FIG. 1  are designated by identical reference numerals. 
   As shown in  FIG. 2 , the color copier includes an optical writing unit  400 . The writing unit  400  converts color image data received from a color scanner  200  to a corresponding optical signal and scans a photoconductive drum or image carrier  402  with the optical signal for thereby forming a latent image on the drum  402 . The writing unit  400  includes a laser diode  404 , a polygonal mirror  406 , a motor  408  assigned to the polygonal mirror  406 , an f/θ lens  410 , and a mirror  412 . 
   The drum  402  is rotatable counterclockwise, as indicated by an arrow in FIG.  2 . Arranged around the drum  402  are a drum cleaning unit  414 , a quenching lamp  416 , a potential sensor  420 , a revolver type developing unit (revolver hereinafter)  422 , a density pattern sensor  424 , and an intermediate image transfer belt or body (simply belt hereinafter)  426 . The revolver  422  is positioned such that one of a plurality of developing sections thereof is located at a developing position where it faces the drum  402 ; a developing section  438  is shown as facing the drum  402  in FIG.  2 . 
   More specifically, the revolver  422  includes a black, a cyan, a magenta and a yellow developing section  428 ,  430 ,  432  and  434  and a drive section, not shown, for causing such drive sections to revolve. The developing sections  428  through  434  are identical in configuration except for the color of toner. 
   In a stand-by condition, the revolver  422  is positioned such that the black developing section  428  faces the drum  402 . On the start of a copying cycle, the color scanner  200  starts reading black image data from a document at preselected timing. The writing unit  400  starts forming a latent image (black latent image hereinafter) on the drum  402  with a laser beam modulated in accordance with the image data. 
   Before the leading edge of the black latent image arrives at the developing position, a sleeve included in the black developing section  428  starts being rotated to develop the black latent image from the leading edge to the trailing edge. As a result, a toner image of negative polarity is formed on the drum  402 . As soon as the trailing edge of the black latent image moves away from the developing position, the revolver  422  is caused to rotate to bring the next image forming section to the developing position. This rotation completes at least before the leading edge of a latent image derived from the next color image data arrives at the developing position. 
   On the other hand, when the copying cycle begins, a motor, not shown, causes the drum  402  to rotate counterclockwise while another motor, not shown, causes the belt  5426  to turn clockwise, as viewed in FIG.  2 . While the belt  426  is in movement, a black (Bk), a cyan (C), a magenta (M) and a yellow (Y) toner image are sequentially formed on the drum  402  while being sequentially transferred to the belt  426  one above the other, completing a full-color image. This is the primary image transfer mentioned earlier. 
   The belt  426  is passed over an electrode roller  450  for primary image transfer, a drive roller  444 , a back roller or electrode roller for secondary image transfer  446 , and a back roller  448 A. The electrode roller  450  faces the drum  402  while the back roller  446  faces a secondary image transfer roller or body  454 . The back roller  448 A faces a cleaning unit  452  configured to clean the surface of the belt  425 . The back roller for secondary image transfer  446  transfers the full-color image from the belt  426  to a sheet or recording medium. 
   A sheet bank  456  includes sheet cassettes  458 ,  460  and  462  loaded with stacks of sheets different in size from sheets stacked on a sheet cassette  464  disposed in the apparatus body. A particular pickup roller  466  is associated with each of the sheet cassettes  458  through  464  and pays out the sheets from the associated sheet cassette toward a registration roller pair  470  one by one. A manual feed tray  468  is also mounted on the apparatus body for allowing OHP (OverHead Projector) films, thick sheets or similar special sheets-to be fed by hand. 
   When image formation begins, a sheet is fed from designated one of the sheet cassettes  458  through  464  to the registration roller pair  470  and stopped for a moment thereby. The registration roller pair  470  starts conveying the sheet at such timing that the leading edge of the sheet meets the leading edge of the toner image being conveyed by the belt  426  at the back roller  446 . 
   A bias for secondary image transfer, which is of the same polarity as toner, is applied to the back roller for secondary image transfer  446 . When the sheet laid on the belt  426  is conveyed below the back roller  446 , the toner image is transferred from the belt  426  to the sheet. This is the secondary image transfer. Subsequently, the sheet with the toner image is quenched, separated from the belt  426 , and then handed over to a belt conveyor  472 . The belt conveyor  472  conveys the sheet to a fixing unit  470  of the type using a belt. The fixing unit  470  fixes the toner image on the sheet with heat and pressure. The sheet coming out of the fixing unit  470  is driven out to a tray, not shown, as a full-color copy. 
   Reference will be made to  FIG. 3  for describing an intermediate image transfer mechanism unique to the illustrative embodiment. In the illustrative embodiment, the belt  426  is made up of a base layer, an intermediate layer and a surface layer sequentially laminated in this order. The base layer, which is 50 μm to 100 μm thick, is formed of polyimide resin with carbon dispersed therein. This composition frees an image from expansion and contraction. The intermediate layer, which is 100 μm to 300 μm thick, is formed of urethane, chloroprene or similar elastic rubber whose resistance is adjusted by carbon or titanium oxide. The surface layer, which is 1 μm to 20 μm thick, is formed of fluorocarbon resin, PVDF or similar material having parting ability. The belt  426  has surface resistivity of 10 10  Ω·cm to 10 12  Ω·cm on the inner surface, volume resistivity of 10 10  Ω·cm to 10 13  Ω·cm, and surface resistivity of 10 10  Ω·cm to 10 14  Ω·cm on the outer surface. 
   The electrode roller  450 , facing the drum  402 , is made up of a metallic core  450   a  and an elastic layer  450   b  covering the core  450   a  and having low or medium resistance. A positive voltage controlled to a preselected current value is applied to the core  450   a , so that the toner image of negative polarity is transferred from the drum  402  to the belt  426 . 
   The secondary image transfer roller  454  is made up of a core  454   a , an elastic intermediate layer, and a surface layer. The intermediate layer is formed of chloroprene or NBR rubber in which carbon or titanium oxide is dispersed for the adjustment of resistance. The surface layer, which is 1 μm to 20 μm thick, is formed of fluorocabon resin or PVDF having parting ability. The core  454   a  is electrically connected to the frame of the apparatus and not applied with a voltage. 
   The back roller  446 , contacting the inner surface of the belt  426 , is formed of stainless steel and provided with surface roughness of 2 μm or below. When the sheet P is nipped between the belt  426  and the secondary image transfer roller  454 , a negative voltage controlled to a preselected current value is applied to the back roller  446 . As a result, the toner image of negative polarity is transferred from the belt  426  to the sheet P. 
   The cleaning unit  452  includes a rotary brush  452   a , a blade  452   b , and a screw  452   c . The brush  452   a  is rotated to coat zinc stearate or similar lubricant on the outer surface of the belt  426  or form an electric field for cleaning, thereby collecting the toner from the belt  426 . The blade  452   b  scrapes off the toner left on the belt  426 . The screw  452   c  conveys the toner thus collected by the brush  452   a  and blade  452   b . Only one of the brush  452   a  and blade  452   b  may be used, if desired. 
   The back roller  448 A associated with the cleaning unit  452  is made up of a metallic core  448 A- 1  and a high-resistance layer  448 A- 2  covering the core  448 A- 1 . The high-resistance layer  448 A- 2  is 0.5 μm to 5 μm thick and provided with surface roughness of 2 μm or below. The high-resistance layer is formed of POM (polyacetal) resin in which barium titanate is dispersed as a conduction filler, and provided with volume resistivity of 11 11  Ω·cm to 10 14  Ω·cm. Particularly, when the blade  452   b  is used as a cleaning blade alone, the high-resistance layer  448 A- 2  is formed of non-elastic resin in order to preserve the expected cleaning ability even when the back roller  448 A is displaced or deformed. 
   For the surface layer of the back roller  448 A, use may be made of insulative resin not containing a conduction filler. Further, even when the back roller  448  has metallic conductivity, but is not grounded via the frame of the apparatus (floating state), it effectively obviates the leak of a current from the back roller  446 . However, when charge accumulates in such a surface layer, the surface layer is likely to form a strong electric field and cause defects having a diameter of about 1 mm each to appear in an image. This is presumably because the charge is released to the brush  452   a  of the cleaning unit  452 . In addition, noise ascribable to such discharge is apt to cause the apparatus to malfunction or to prevent it from meeting electromagnetic wave standards. 
   Second Embodiment 
   An alternative embodiment of the present invention will be described with reference to FIG.  4 . The structural elements of this embodiment identical with those of the previous embodiment are designated by identical reference numerals and will not be described in detail. This is also true with other embodiments to follow. 
   As shown in  FIG. 4 , a back roller for cleaning  448 , like the conventional back roller  448 , is implemented as a metal conductor or similar low-resistance member. In the illustrative embodiment, a resistor  453  is connected between the back roller  448  and the frame of the apparatus and should preferably have resistance of 30 MΩ to 3 GΩ. This resistance reduces the leak current preventing effect if too low or raises the voltage of the back roller  448  to thereby bring about abnormal discharge stated earlier if too high. When the resistance is adequate, the voltage of the back roller  448  is about 1 kV to 4 kV in absolute value and directly acts on opposite ends of the resistor  453 . The resistor  453  therefore must be highly voltage-resistant. 
   Further, the voltage of the back roller  448  mentioned above is closer to the voltage applied to the back roller  446 , i.e., a difference in potential between the back rollers  446  and  448  is reduced. This also contributes to the reduction of leak current. 
   Third Embodiment 
   Another alternative embodiment of the present invention will be described with reference to FIG.  5 . As shown, the back roller  448  associated with the cleaning unit  452  is also implemented as a metal conductor or similar low-resistance member. In the illustrative embodiment a Zener diode or similar constant-voltage device  455  is connected between the back roller  448  and the frame of the apparatus. If the voltage of the constant-voltage device  455  is close to the voltage applied to the back roller  446 , then it is possible to reduce the leak current. 
   The voltage of the constant-voltage device  455  should preferably be about 1 kV to 4 kV in absolute value or may be made higher than the voltage applied to the back roller  446  in order to practically obviate the leak current. Such a high voltage, however, is apt to bring about abnormal discharge stated earlier. In light of this, the above voltage should preferably be about 4 kV in absolute value or must be about 7 kV or below. 
   The advantages of the first to third embodiments described above are also achievable with an image forming apparatus of the type transferring toner images of different colors from a plurality of drums to an intermediate image transfer body one above the other, and then transferring the resulting color image to a recording medium. 
   As stated above, the first to third embodiments reduce a current to leak from the back roller for secondary image transfer to the cleaning back roller via the intermediate image transfer body, thereby insuring stable images. 
   Fourth Embodiment 
     FIG. 6  shows still another alternative embodiment of the present invention, particularly the polarities of toner grains and power supply devices for applying voltages to various electrodes. It is to be noted that the amount of toner grains shown in  FIG. 6  is not proportional to the actual amount, but is qualitatively representative of a difference in amount. 
   As shown in  FIG. 6 , conventional image forming means forms a toner image of negative polarity, or regular polarity, on a photoconductive drum or image carrier  501 . More specifically, a charger, not shown, uniformly charges the surface of the drum  501  being rotated. An optical writing unit, not shown, scans the charged surface of the drum  501  in accordance with image data color by color. The resulting latent images are developed to produce, e.g., a Y, an M, a C or a Bk toner image  502 . 
   An intermediate image transfer belt (simply belt hereinafter)  503  is passed over a drive roller  504 , a back roller for secondary image transfer  505 , a back roller for cleaning  506 , and a tension roller  507 . The belt  503  is held in contact with the drum  501  at a primary image transfer position. A drive source, not shown, causes the belt  503  to move at the same peripheral speed as the drum  501 . A spring  508  constantly presses the tension roller  507  against the inner surface of the belt  503 , thereby applying tension to the belt  503 . 
   A back roller for primary image transfer  509  is made up of a metallic core and a low- or medium-resistance elastic layer covering the core, although not shown specifically, and electrically connected to the inner surface of the belt  503 . A power supply device  510  applies a positive voltage controlled to a constant current value to the core of the back roller  509 , thereby forming an electric field for primary image transfer between the above core and the conductive core of the drum  501 . The toner images of negative polarity sequentially formed on the drum  501  are sequentially transferred to the belt  503  one above the other, completing a full-color image. After the primary image transfer, a cleaning unit, not shown, cleans the surface of the drum  501  to thereby prepare it for the next image forming cycle. 
   A cleaning roller or cleaning member  511  is assigned to the belt  503 . A moving mechanism, not shown, maintains the cleaning roller  511  released from the belt  503  while the primary image transfer for forming the full-color image is under way. After the secondary transfer of the full-color image from the belt  503  to a sheet or recording medium  516 , the moving mechanism moves the cleaning roller  511  into contact with the belt  503  so as to clean the belt  503 . Toner grains of positive polarity  512 , which the cleaning roller  511  has failed to remove from the belt  503 , are again transferred to the drum  501  at the primary image transfer position. 
   The back roller for secondary image transfer  505  is formed only of metal and electrically connected to the inner surface of the belt  503 . A power supply device  513  applies a negative voltage controlled to a constant current value to the back roller  505 . A secondary image transfer roller or body  514  is made up of a metallic core  514   a  and a low- or medium-resistance elastic layer  514   b  covering the core  514   a  and electrically connected to the inner surface of the belt  503 . The core  514   a  is connected to ground. 
   The sheet  516  is fed from a sheet feeding device, not shown, to a secondary image transfer position between the belt  503  and the secondary image transfer roller  514 . A power supply device  513  applies a negative voltage controlled to a preselected current value to the inner surface of the belt  503  via the back roller  505 , thereby forming an electric field for secondary image transfer. In this condition, a full-color toner image of negative polarity  517  is transferred from the belt  503  to the sheet  516 . An AC power supply  522  applies an AC voltage to a discharge needle  521 , so that the sheet  516  is discharged and thereby separated from the belt  503 . Subsequently, the sheet  516  with the toner image is conveyed to a fixing unit, not shown, and has the toner image fixed thereby. The sheet is then driven out to a tray not shown. 
   The electric field for secondary image transfer causes positive charge to be induced from the core  514   a  of the secondary image transfer roller  514 , so that charge is injected into the toner grains of the toner image during secondary image transfer. As a result, residual toner grains  518  left on the belt  503  after the secondary image transfer are partly of positive polarity and partly of negative polarity. 
   The back roller for cleaning  506  is formed only of metal. A power supply device  519  applies a negative voltage of the same size as the voltage applied to the back roller  505  to the back roller  506 . More specifically, the power supply device  519  applies a negative voltage of the same size as the output voltage of the power supply device  513  in accordance with a signal output from a voltage sensor  524 , which is responsive to the output voltage of the power supply device  513 . Alternatively, the power supply device  513  may be branched to apply a single negative voltage to both of the back rollers  505  and  506 . 
   The cleaning roller  511  is made up of a metallic core  511   a  and a low- or medium-resistance elastic body  511   b  covering the core  511   a . The core  511   a  is connected to ground. The voltage applied from the power supply device  519  to the back roller  506  forms an electric field for cleaning between the back roller  506  and the core  511   a  of the cleaning roller  511 . Consequently, among the residual toner grains  518  on the belt  503 , the toner grains of negative polarity are transferred from the belt  503  to the cleaning roller  511 . A cleaning blade  520  removes such toner grains from the cleaning roller  11 . 
   The electric field for cleaning causes positive charge to be induced from the core  511   a  of the cleaning roller  511 , which is connected to ground. The positive charge is injected into the toner grains during cleaning for thereby inverting their polarity. In this sense, the cleaning roller  511  plays the role of a charging member for inverting the polarity of the toner grains left on the belt  503  while the back roller  506  plays the role of a back electrode. 
   As stated above, the toner grains of negative polarity are transferred two times by the secondary image transfer and cleaning while being subjected to charge injection two times. Consequently, only the toner grains of positive polarity are left on the belt  503  after the cleaning step and again transferred to the drum  501  by the electric field for primary image transfer. 
   The tension roller  507 , intervening between the back rollers  505  and  506 , is a member contacting the belt  503  at a position closest to the back roller  505 . In the illustrative embodiment, the tension roller  507  is made up of a metallic core and a polycarbonate or similar insulative resin layer covering the core. The insulative resin therefore electrically isolates the belt  505  from the frame of the apparatus. This prevents a current from leaking from the tension roller  507  via the belt  503  and effecting the secondary image transfer. 
   The drive roller  504  is remote from the back roller  505  and therefore causes a minimum of current to leak therefrom via the belt  503 . 
   As stated above, in the illustrative embodiment, a voltage of the same polarity as a voltage applied to the back roller  505  for secondary image transfer  505  is applied to the back roller for cleaning  506  adjacent the roller  505 . This reduces the leak of a current, which forms an electric field for secondary image transfer, to the electrode  506  via the belt  503 . In this condition, a voltage subject to constant current control is applied from the electrode  505  to the inner surface of the belt  503 , forming the electric field for secondary image transfer between the belt  503  and the secondary image transfer roller  514 . Therefore, even if some current flows via the sheet  516  or even if the resistance of the belt  503  or that of the secondary image transfer roller  514  varies, the electric field for secondary image transfer varies little. This successfully prevents the current to the sheet  516  from becoming short and thereby allows a stable image to be formed on the sheet  516  without fail. 
   Because the voltage applied to the back roller for cleaning  506  is opposite in polarity to the toner, an electric field for transferring the residual toner from the belt  503  to the cleaning roller  511  can be surely formed even when the cleaning roller  511  is grounded. This, coupled with a small potential difference between the two electrodes  505  and  506 , reduces the leak of a current fed from the electrode  505  to the electrode  506  via the belt  503 . Further, even when the cleaning roller  511  is grounded, charge opposite in polarity to the toner is induced and injected into the toner for thereby inverting the polarity of the residual toner. The toner thus inverted in polarity again deposits on the drum  501 . 
   In the illustrative embodiment, the portion where the belt  503  and tension roller  507  contact each other is electrically isolated from the frame of the apparatus. This obviates current leak from the tension roller  507  via the belt  503 . 
   Moreover, the same voltage of the same polarity is applied to both of the back electrode  505  and back electrode adjacent thereto, so that the leak of the current, which forms the electric field for secondary image transfer, to the electrode  506  via the belt  503  is practically obviated. The electric field therefore varies little even if some current flows via the sheet  516  or even if the resistance of the belt  503  or that of the secondary image transfer roller  514  varies. 
   If desired, the cleaning member and back electrode for cleaning and the charging member and back electrode for charging may be configured independently of each other. The illustrative embodiment is also applicable to an image forming apparatus of the type transferring toner images of different colors from a plurality of photoconductive drums to an intermediate image transfer belt one above the other, and then transferring the resulting color image from the belt to a recording medium. 
   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.