Abstract:
Disclosed is an image forming apparatus in which an increase in the service life is compatible with a reduction in apparatus size and cost and which provides high productivity in image formation. The image forming apparatus includes: an image bearing member; an image forming unit; an intermediate transfer device; a final transfer member; a potential gradient forming unit; and a cleaning unit; an upstream side cleaning roll, 
     a downstream side cleaning roll which is in contact with the intermediate transfer member on the downstream side of the upstream side cleaning roll with respect to a feedback direction in which the residual toner is fed back from the final transfer position to the primary transfer position, and a cleaning brush which is in contact with the intermediate transfer member on the downstream side of the upstream side cleaning roll and on the upstream side of the downstream side cleaning roll.

Description:
FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a printer, a facsimile, or a multifunction apparatus formed by combining them, and in particular, to an improvement in a cleaning technique for an image forming apparatus. 
     Up to now, electrophotographic (electrostatic transfer type) image forming apparatuses, such as copying machines and printers, have been widely known. In such image forming apparatuses, a toner image is transferred to a recording sheet by a final transfer member, and then the toner image is fixed to thereby realize on the recording sheet the toner image as a permanent image. The portion of the toner that is not transferred to the recording sheet has to be removed by a cleaning device in the image forming apparatus. As a method for removing such residual toner, there have been proposed a technique in which a blade is used (See, for example, JP6-148910A), a technique in which a brush is used (See, for example, JP 10-274889 A), and so on. 
     However, these conventional techniques have the following problem. 
     When a blade is used, cleaning is performed with the blade being held in press contact with an intermediate transfer member, so that wear of the intermediate transfer member progresses after use over a relatively short period of time, so that the service life of the intermediate transfer member is rather short. Further, a surface flaw of the intermediate transfer member generated through wear may cause disturbance in the toner image. In particular, when a spherical toner produced by polymerization is used as the toner for forming the toner image, it is difficult to maintain a satisfactory cleaning performance. 
     When a brush is used, in order to remove residual toner from the brush, it is necessary for the cleaning device to be provided with, apart from the brush, a detoner roll (a roll for transferring residual toner), a flicker member (a member for shaking off residual toner) or the like, which is rather disadvantageous in terms of space efficiency and cost. Instead of using a detoner roll or a flicker member, it might be possible to adopt an arrangement in which residual toner is temporarily retained by the brush and in which apart from the image formation mode, there is provided a special cleaning mode in which the temporarily retained toner is carried to a residual toner recovery portion by utilizing a potential gradient. However, that would involve frequent operation of the cleaning mode, thus impairing the productivity in continuous image formation. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned technical problem, and provides an image forming apparatus in which an increase in the service life of the intermediate transfer member is compatible with a reduction in apparatus size and cost and which provides high productivity in image formation. 
     Therefore, according to an aspect of the present invention, an image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member and which is in contact with the image bearing member at a primary transfer position; a final transfer member which is in contact with the intermediate transfer device at a final transfer position; a potential gradient forming unit which forms a potential gradient between the image bearing member, the intermediate transfer member, and the final transfer member so as to transfer the toner image to the final transfer position; and a cleaning unit which removes residual toner on the intermediate transfer member that is not finally transferred to a recording sheet at the final transfer position. In the apparatus, the cleaning unit includes: an upstream side cleaning roll which is in contact with the intermediate transfer member, a downstream side cleaning roll which is in contact with the intermediate transfer member on the downstream side of the upstream side cleaning roll with respect to a feedback direction in which the residual toner is fed back from the final transfer position to the primary transfer position, and a cleaning brush which is in contact with the intermediate transfer member on the downstream side of the upstream side cleaning roll with respect to the feedback direction and on the upstream side of the downstream side cleaning roll with respect to the feedback direction. 
     In this way, the blade is not held in press contact with the intermediate transfer member, so that wear of the intermediate transfer member does not progress easily, thus making the service life of the intermediate transfer member relatively long. Further, there is no need to provide a detoner roll (a roll for transferring residual toner), a flicker member (a member for shaking off residual toner) or the like for the cleaning brush, which is advantageous in terms of space efficiency and cost. Further, since this cleaning brush is placed between an upstream side cleaning roll and a downstream side cleaning roll, it is possible to remove residual toner without providing any special cleaning mode, and there is no fear of the productivity in image formation being impaired. 
     Here, the image forming apparatus according to the present invention may be structured such that the upstream side cleaning roll and the downstream side cleaning roll are all formed of conductive members, and that the potential gradient forming unit applies to one cleaning roll a high cleaning roll bias voltage higher than the surface potential of the intermediate transfer member which is in contact with the cleaning roll, and applies to the other cleaning roll a low cleaning roll bias voltage lower than the surface potential of the intermediate transfer member which is in contact with the cleaning roll. 
     Further, the image forming apparatus according to the present invention maybe structured such that the potential gradient forming unit applies to the cleaning brush a cleaning brush bias voltage which is on the side closer to the side of the cleaning roll bias voltage to be applied to the upstream side cleaning roll as compared with the surface potential of the intermediate transfer member which is in contact with the cleaning brush. 
     Further, the image forming apparatus according to the present invention may be structured such that the cleaning unit is equipped with a cleaning blade which scrapes off residual toner adhering to the (upstream side and/or downstream side) cleaning roll(s) and a take-out mechanism which takes out the residual toner scraped off by the cleaning blade. Furthermore, the cleaning roll and the cleaning blade may be formed of metal. 
     Further, the present invention is applicable not only to a monochrome image forming apparatus, but also to a multicolor image forming apparatus. That is, it is possible to adopt a construction which has as the image bearing member plural image bearing members for different colors and has as the intermediate transfer device a single intermediate transfer member. It is also possible to adopt a construction which has as the image bearing member plural image bearing members for different colors and has as the intermediate transfer device a first upstream side intermediate transfer member which is in contact with a part of the plural image bearing members, a first downstream side intermediate transfer member which is in contact with the remaining ones of the plural image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member, the final transfer member being in contact with the second intermediate transfer member. 
     As a specific construction of the latter image forming apparatus, it is possible to adopt a construction such that the image forming apparatus includes: as the image bearing member four image bearing members for yellow, magenta, cyan, and black; and as the intermediate transfer device a first upstream side intermediate transfer member and a first downstream side intermediate transfer member respectively in contact with two of the four image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member, in which the final transfer member is in contact with the second intermediate transfer member. 
     In this image forming apparatus, the upstream side cleaning roll abuts against the second intermediate transfer member, and the downstream side cleaning roll may be exclusively in contact with the first upstream side intermediate transfer member or exclusively in contact with the first downstream side intermediate transfer member; there may be provided as the downstream side cleaning roll two downstream side cleaning rolls, one of which is in contact with the first upstream side intermediate transfer member and the other of which is in contact with the first downstream side intermediate transfer member. 
     Further, this image forming apparatus may have as the cleaning brush only an upstream side cleaning brush which is in contact with the second intermediate transfer member and a downstream side cleaning brush which is in contact with the first upstream side intermediate transfer member, or only a downstream side cleaning brush which is in contact with the first downstream side intermediate transfer member; or, it may have as the downstream side cleaning brush two cleaning brushes, one of which is in contact with the first upstream side intermediate transfer member and the other of which is in contact with the first downstream side intermediate transfer member. 
     Further, of the intermediate transfer member, the cleaning roll abutting against the intermediate transfer member, and the cleaning brush abutting against the intermediate transfer member, at least two may receive application of bias voltage from the same power source. Specifically, it is possible to adopt construction in which grounding is possible from the same power source to the intermediate transfer member, the cleaning roll abutting against the intermediate transfer member, and the cleaning brush abutting against the intermediate transfer member through an electrical resistor or a zener diode, or a construction in which bias voltage is supplied from the power source through an electrical resistor or a diode. 
     Further, the potential gradient forming unit can apply biases of the same polarity (positive or negative) to the intermediate transfer member, the cleaning roll abutting against the intermediate transfer member, and the cleaning brush abutting against the intermediate transfer member. It is possible to adopt a construction in which the cleaning member (the cleaning roll or the cleaning brush) does not come into contact with the final transfer member, or a construction in which the cleaning member (the cleaning roll or the cleaning brush) does not come into contact with the image bearing member. By thus constructing the image forming apparatus, it is only necessary to prepare a positive or a negative power source, which contributes to a reduction in apparatus size and is advantageous in terms of cost. 
     In accordance with the present invention, it is possible to provide an image forming apparatus in which an increase in the service life of the intermediate transfer member and a reduction in apparatus size and cost are mutually compatible and which involves as little deterioration in productivity in image formation as possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail based on the following figures, wherein: 
     FIG. 1 is a schematic sectional view of a full color printer according to Embodiment 1 of the present invention; 
     FIG. 2 is a main-portion sectional view of the full color printer of Embodiment 1 of the present invention; 
     FIG. 3 is a block diagram illustrating a potential gradient control system of the full color printer of Embodiment 1 of the present invention; 
     FIG. 4 illustrates how a toner image as an output image moves in the full color printer of Embodiment 1 of the present invention; 
     FIG. 5 illustrates how residual toner that has not been transferred moves in the full color printer of 1 of the present invention; and 
     FIG. 6 illustrates how residual toner that has not been transferred moves in a full color printer according to Embodiment 2 of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail. 
     Embodiment 1 
     FIG. 1 shows a tandem type full color printer (image forming apparatus) according to Embodiment 1 of the present invention. FIG. 2 shows a main image forming portion of the full color printer (image forming apparatus) shown in FIG.  1 . 
     This full color printer is roughly composed of an image forming portion, an intermediate transfer device, a final transfer roll  40 , a fixing device  6 , and a sheet feeding portion. 
     The image forming portion is composed of four image forming units (image forming means)  1 Y through  1 K for yellow (Y), magenta (M), cyan (C), and black (K), and an exposure device  15 . The image forming units  1 Y through  1 K are respectively composed of four photosensitive drums (image bearing members)  10 Y through  10 K, charging rolls (contact type charging members)  11 Y through  11 K which are respectively in contact with the photosensitive drums  10 Y through  10 K, developing devices  12 Y through  12 K respectively opposed to the photosensitive drums  10 Y through  10 K, and photosensitive brush rolls  13 Y through  13 K which are respectively in contact with the photosensitive drums  10 Y through  10 K. 
     Here, regarding the arrangement of the members around the photosensitive drum  10 , a charging roll  11 , a developing device  12  (the developing sleeve of a developing device), a first intermediate transfer roll (described below), and a photosensitive brush roll  13  are arranged around each photosensitive drum  10  from the upstream side to the downstream side with respect to the rotating direction of the photosensitive drum  10 . The photosensitive brush roll  13  is formed by winding a flocked tape of conductive resin spirally around a roll-shaped base member of metal (stainless steel) 
     A DC voltage of approximately −900 V is applied to the photo sensitive drums  10 Y through  10 K by the charging rolls  11 Y through  11 K, whereby the drums are uniformly charged to approximately −380 V; when electrostatic latent images are written thereto by the exposure device  15 , the surface potential thereof is reduced to approximately −70 V. 
     Each of the developing devices  12 Y through  12 K is a magnetic-brush-contact, two-component development type developing device equipped with a developing roll, a developer amount regulating member, a developer carrying member, and an auger for carrying and agitating developer. The amount of developer regulated by the developer amount regulating member and carried to the developing portion is approximately 30 to 40 g/m 2 ; at this time, the charge amount of the toner existing on the developing roll is approximately −20 to −40 μC/g. An AC+DC developing voltage is applied to these developing devices  12 Y through  12 K to execute development; this developing voltage is composed of an AC component of approximately 4 kHz and 1.6 kVpp, and a DC component of approximately −300 V. 
     The intermediate transfer device is equipped with a first upstream side intermediate transfer roll (first upstream side intermediate transfer member)  20   a  which is in contact with the photosensitive drums  10 Y and  10 M at a primary transfer position, a first downstream side intermediate transfer roll (first downstream side intermediate transfer member)  20   b  which is in contact with the photosensitive drums  10 C and  10 K at a primary transfer position, a second intermediate transfer roll  30  which is in contact with the two first intermediate transfer rolls  20   a  and  20   b  at a secondary transfer position respectively, and a toner sensor  8  which detects optically and in a non-contact fashion the presence and density of a toner image on the second intermediate transfer roll  30 . 
     Further, the first upstream side intermediate transfer roll  20   a  is equipped with a cleaning device (cleaning unit)  21   a . This first upstream side cleaning device  21   a  is equipped with a metal (stainless steel) cleaning roll (downstream side cleaning roll)  210   a  which is in contact with the first upstream side intermediate transfer roll  20   a , a metal (stainless steel) cleaning blade  211   a  abutting against the cleaning roll  210   a , a screw auger (conveyance mechanism)  214   a  which carries residual toner scraped off by the cleaning blade  211   a  to a toner recovery box (not shown) outside the upstream side cleaning device  21   a , an intermediate transfer brush roll (downstream side cleaning brush)  213   a  which is in contact with the intermediate transfer roll  20   a  in the vicinity of the upstream side of the cleaning roll  210   a  with respect to the rotating direction of the first upstream side intermediate transfer roll  20   a , and a cleaner housing (accommodating member)  212   a  accommodating the cleaning roll  210   a , the cleaning blade  211   a , the screw auger  214   a , and the intermediate transfer brush roll  213   a . The intermediate transfer brush roll  213   a  is formed by spirally winding a flocked conductive resin tape around a roll-shaped metal (stainless steel) base member. 
     Similarly, the first downstream side intermediate transfer roll  20   b  is equipped with a cleaning device (cleaning unit)  21   b . This first upstream side cleaning device  21   b  is equipped with a metal (stainless steel) cleaning roll (downstream side cleaning roll)  210   b  which is in contact with the first downstream side intermediate transfer roll  20   b , a metal (stainless steel) cleaning blade  211   b  abutting the cleaning roll  210   b , a screw auger (conveyance mechanism)  214   b  which carries residual toner scraped off by the cleaning blade  211   b  to a toner recovery box (not shown) outside the upstream side cleaning device  21   b , an intermediate brush roll (downstream side cleaning brush)  213   b  which is in contact with the intermediate transfer roll  20   b  in the vicinity of the upstream side of the cleaning roll  210   b  with respect to the rotating direction of the first downstream side intermediate transfer roll  20   b , and a cleaner housing (accommodating member)  212   b  accommodating the cleaning roll  210   b , the cleaning blade  211   b , the screw auger  214   b , and the intermediate transfer brush roll  213   b . The intermediate transfer brush roll  213   b  is formed by spirally winding a flocked conductive resin tape around a roll-shaped metal (stainless steel) base member. 
     The second intermediate transfer roll  30  is equipped with a cleaning device (cleaning unit)  31 . This second cleaning device  32  is equipped with a metal (stainless steel) cleaning roll (upstream side cleaning roll)  310  in contact with the second intermediate transfer roll  30 , a metal (stainless steel) cleaning blade  311  abutting the cleaning roll  310 , a screw auger (conveyance mechanism)  314  which carries residual toner scraped off by the cleaning blade  311  to a toner recovery box (not shown) outside the upstream side cleaning device  31 , an intermediate transfer brush roll (upstream side cleaning brush)  313  which is in contact with the intermediate transfer roll  30  in the vicinity of the downstream side of the cleaning roll  310  with respect to the rotating direction of the second intermediate transfer roll  30 , and a cleaner housing (accommodating member)  312  accommodating the cleaning roll  310 , the cleaning blade  311 , the screw auger  314 , and the intermediate transfer brush roll  313 . The intermediate transfer brush roll  313  is formed by spirally winding a flocked conductive resin tape around a roll-shaped metal (stainless steel) base member. 
     Regarding the arrangement of the members around the first upstream side intermediate transfer roll  20   a , the photosensitive drum  10 M, the photosensitive drum  10 Y, the second intermediate transfer roll  30 , the intermediate transfer brush roll  213   a , and the cleaning roll  210   a  are arranged around the first upstream side intermediate transfer roll  20   a  from the upstream side to the downstream side with respect to the rotating direction of the first upstream side intermediate transfer roll  20   a . Further, regarding the arrangement of the members around the first downstream side intermediate transfer roll  20   b , the photosensitive drum  10 K, the photosensitive drum  10 C, the second intermediate transfer roll  30 , the intermediate transfer brush roll  213   b , and the cleaning roll  210   b  are arranged around the first downstream side intermediate transfer roll  20   b  from the upstream side to the downstream side with respect to the rotating direction of the first downstream side intermediate transfer roll  20   b . Further, regarding the arrangement of the members around the second intermediate transfer roll  30 , the first upstream side intermediate transfer roll  20   a , the first downstream side intermediate transfer roll  20   b , the toner sensor  8 , the final transfer roll  40 , the cleaning roll  310 , and the intermediate transfer brush roll  313  are arranged around the second intermediate transfer roll  30  from the upstream side to the downstream side with respect to the rotating direction of the second intermediate transfer roll  30 . 
     Each of the first intermediate transfer rolls  20   a  and  20   b  is formed by providing a silicone rubber layer (conductive elastic layer) on a metal pipe, and forming thereon a high release coating layer (resistance layer having a higher electric resistance compared to the conductive elastic layer); while the acceptable resistance value thereof normally ranges from 10 5  to 10 9  Ω, in this example, it is approximately 10 8  Ω. And, the requisite surface potential for transferring toner images from the photosensitive drums  10 Y through  10 K to the first intermediate transfer rolls  20   a  and  20   b  normally ranges from approximately +250 through 500 V, and an optimum potential value can be set according to the toner charging condition, the ambient temperature, the humidity, etc. Like the first intermediate transfer rolls  20   a  and  20   b , the second intermediate transfer roll  30  is formed by providing a silicone rubber layer (conductive elastic layer) on a metal pipe and forming thereon a high release coating layer (resistance layer having a higher electric resistance compared to the conductive elastic layer); while the acceptable resistance value thereof normally ranges from 10 8  to 10 12  Ω, in this example, it is approximately 10 11  Ω (That is, it exhibits a resistance value higher than that of the first intermediate transfer rolls  20   a  and  20   b ). And, the requisite surface potential for transferring toner images from the first intermediate transfer rolls  20   a  and  20   b  to the second intermediate transfer roll  30  normally ranges from approximately +600 through 1200 V, and an optimum potential value can be set according to the toner charging condition, the ambient temperature, the humidity, etc. 
     The final transfer roll (final transfer member)  40  is in contact with the second intermediate transfer roll  30  at the final transfer position, and formed by providing an urethane rubber layer on a metal pipe and providing thereon a resin coating layer; while the acceptable resistance value thereof normally ranges from 10 6  to 10 9  Ω, in this example, it is approximately 10 8  Ω (That is, it exhibits a resistance value lower than that of the second intermediate transfer roll  30 ). And, the transfer voltage to be applied to this final transfer roll  40  in order to transfer a toner image from the second intermediate transfer roll  30  to the sheet S (recording sheet) normally ranges from approximately +1200 through 5000 V, and an optimum voltage value can be set according to the ambient temperature, the humidity, the kind of sheet S (the resistance value thereof, etc.), etc. In this example, the constant current system is adopted, and approximately +6 μA is applied under normal temperature and normal humidity to obtain a substantially appropriate final transfer voltage of approximately +1600 to 2000 V. 
     Further, (unlike the first intermediate transfer rolls  20   a  and  20   b  and the second intermediate transfer roll  30 ), the final transfer roll  40  is not caused to abut against the cleaning roll (cleaning member). Further, (exclusive of the time when replacing the image forming unit and inclusive of the times when the apparatus is in the image formation mode, the process control mode, and the cleaning mode), the final transfer roll  40  comes into contact with the second intermediate transfer roll  30 , and requires no special retracting mechanism or the like. 
     The surface roughness (Rz) of the final transfer roll  40  may be 20 [μm(Rz)] or less, for example, 10 [μm(Rz)], and the surface roughness (Rz) of the first and second intermediate transfer rolls  20   a ,  20   b , and  30  may be 10 [μm(Rz)] or less, for example, 1 [μm(Rz)]. Further, the final transfer roll  40  exhibits a higher degree of surface roughness (Rz) than the first and second intermediate transfer rolls  20   a ,  20   b , and  30 . It is desirable that the surface roughness of these rolls be not more than the average grain size of the toner forming the toner image. 
     In the fixing device  6 , a heating roll  62  and a pressurizing roll  61  are held in press contact with each other to form a fixing nip. Arranged in the heating roll  62  is a halogen lamp (not shown) serving as the heat source; at the time of fixing, the surface of the heating roll  62  is heated to a predetermined fixing temperature. Further, on the downstream side of the fixing nip with respect to the direction in which the sheet S is transported, there is arranged fixing/discharge roll pair  63   a  and  63   b.    
     The sheet feeding portion is formed along the transport path (indicated by the dotted line) P for the sheet S extending from the sheet feeding tray  50  to the discharge tray  70 . The sheet feeding tray  50  accommodates plural sheets S, and from the sheet feeding tray  50  to the downstream side of the transport path, there are sequentially arranged a roll pair formed by a pick-up roll  51   a  and a retarding roll  51   b , a pair of transport rolls  52   a  and  52   b , a pair of registration rolls  53   a  and  53   b , and (on the downstream side of the final transfer roll  40  and the fixing device  6 ) a pair of discharge rolls  54   a  and  54   b.    
     FIG. 3 is a block diagram illustrating the potential control system of this full color printer  1 . According to the situation the color printer  1  is in, that is, according to whether the printer is in the print mode (image formation mode), the process control mode, or the cleaning mode, the potential control portion (potential gradient forming unit)  9  controls the voltages to be applied to the charging roll  11 , the photosensitive brush roll  13 , the first intermediate transfer rolls  20   a  and  20   b , the cleaning roll  210 , the intermediate transfer brush roll  213 , the second intermediate transfer roll  30 , the cleaning roll  310 , the intermediate transfer brush roll  313 , and the final transfer roll  40 , with the result that it forms an appropriate potential gradient according to the situation the full color printer  1  is in between the charging roll  11 , the photosensitive brush roll  13 , the first intermediate transfer rolls  20   a  and  20   b , the cleaning roll  210 , the intermediate transfer brush roll  213 , the second intermediate transfer roll  30 , the cleaning roll  310 , the intermediate transfer brush roll  313 , and the final transfer roll  40 . In this embodiment, by using a zener diode, the bias voltage supplied from the same power source is applied to plural members after being transformed into appropriate bias voltages. 
     The operation of this full color printer  1  will now be described. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 V(40) 
                 V(310) 
                 V(313) 
                 V(213) 
                 V(210) 
                 V(11) 
               
               
                   
               
             
             
               
                 Print 
                 2000 
                 1200 
                 1300 
                 800 
                 100 
                 −900 
               
               
                   
               
             
          
         
       
     
     Given in Table 1 are the bias voltages to be applied, in the print mode, by the potential control portion  9  of this embodiment to the charging rolls  11 , the cleaning roll  210 , the intermediate transfer brush roll  213 , the cleaning roll  310 , the intermediate transfer roll  313 , and the final transfer roll  40 . 
     FIG. 4 illustrates how the toner image as the output image moves. In the drawing, the solid-line arrow indicates the movement path of the toner image as the output image. The toner used in this embodiment is negatively charged toner. That is, the normal-polarity toner is negatively charged, and the reverse-polarity toner is positively charged. 
     Yellow, magenta, cyan, and black toner images are respectively formed on the photosensitive drums  10 Y through  10 K by the image forming units  1 Y through  1 K. That is, the surface of each photosensitive drum  10  is uniformly charged by the charging roll  11 , and a laser beam R corresponding to the output image is applied from the exposure device  15  to the surface of the photosensitive drum  10  after the charging, forming an electrostatic latent image on the photosensitive drum  10  through a difference in potential between the exposed portions and the unexposed portions. The developing device  12  imparts toner selectively to this electrostatic latent image to thereby form a toner image on the photosensitive drum  10 . 
     Then, a magenta toner image is primarily transferred to the first upstream side intermediate transfer roll  20   a  from the photosensitive drum  10 M for magenta. Further, a yellow toner image is primarily transferred to the first upstream side intermediate transfer roll  20   a  from the photosensitive drum  10 Y for yellow, and superimposed on the magenta toner image. Similarly, a black toner image is primarily transferred to the first downstream side intermediate transfer roll  20   b  from the photosensitive drum  10 K for black. Further, a cyan toner image is primarily transferred to the first downstream side intermediate transfer roll  20   b  from the photosensitive drum  10 C for cyan. 
     The primarily transferred magenta and yellow toner images are secondarily transferred to the second intermediate transfer roll  30 . The primarily transferred black and cyan toner images are also secondarily transferred to the second intermediate transfer roll  30 , where the cyan toner image is superimposed on the magenta and yellow toner images secondarily transferred previously to thereby form a full color toner image on the second intermediate transfer roll  30 . 
     The full color toner image and the black toner image, which are secondarily transferred, reach the nip portion between the second intermediate transfer roll  30  and the final transfer roll  40 . In synchronism with the timing of their reaching, the sheet S as the recording sheet is transported to the nip portion from the registration roll pair  53   a  and  53   b  (See FIG.  1 ), and the full color toner image and the black toner image are tertiarily (finally) transferred to the sheet S. 
     Thereafter, this sheet S passes the nip portion of the heating roll  62  and the pressurizing roll  61  of the fixing device  6  (See FIG.  1 ). In this process, by the action of the heat and pressure imparted from the rolls  61  and  62 , the full color toner image and the black toner image are fixed to the sheet S to become a permanent image. Thereafter, the sheet S is discharged onto the discharge tray  70  by the discharge roll pair  54   a  and  54   b  to complete the full color image formation. 
     FIG. 5 illustrate show residual toner, which is not transferred and is remaining on the intermediate transfer roil, and the final transfer roll moves. 
     The potential of the cleaning roll  310  is higher than that of the second intermediate transfer roll  30 , so that negatively charged normal-polarity residual toner adheres to the cleaning roll  310 . On the other hand, the potential of the intermediate transfer brush roll  313  is higher than that of the second intermediate transfer roll  30 , so that lowly and negatively charged normal-polarity residual toner that has failed to adhere to the cleaning roll  310  adheres to the intermediate transfer brush roll  313 . 
     The potential of the cleaning roll  210   a  is lower than that of the first upstream side intermediate transfer roll  20   a , so that positively charged reverse-polarity residual toner adheres to the cleaning roll  210   a . On the other hand, the potential of the intermediate transfer brush roll  213   a  is higher than that of the first upstream side intermediate transfer roll  20   a , so that negatively charged normal-polarity residual toner adheres to the cleaning roll  210   a . Similarly, the potential of the cleaning roll  210   b  is lower than that of the first downstream side intermediate transfer roll  20   b , so that positively charged reverse-polarity residual toner adheres to the cleaning roll  21   b . On the other hand, the potential of the intermediate transfer brush roll  213   b  is higher than that of the first downstream side intermediate transfer roll  20   b , so that negatively charged normal-polarity residual toner adheres to the cleaning roll  210   b.    
     Modification 
     In the full color printer  1  of Embodiment 1, it is also possible to omit the cleaning roll  210   b.    
     Embodiment 2 
     While the full color printer  1  of Embodiment 1 has as the intermediate transfer device plural (three) intermediate transfer members: the first upstream side intermediate transfer roll  20   a , the first downstream side intermediate transfer roll  20   b , and the second intermediate transfer roll  30 , the monochrome printer  1  of this embodiment has as the intermediate transfer device only one intermediate transfer roll  20 . The components that are the same as those of the full color printer  1  of Embodiment 1 are indicated by the same reference numerals, and a description thereof will be omitted. 
     In the following, the operation of this monochrome printer  1  will be described. 
     FIG. 6 illustrates how residual toner, which is not transferred and is remaining on the intermediate transfer roll, and the final transfer roll moves. Since the potential of the cleaning roll  210   a  (the upstream side cleaning roll) is higher than that of the intermediate transfer roll  20 , negatively charged normal-polarity residual toner adheres to the cleaning roll  210   a . On the other hand, the potential of the intermediate transfer brush roll  213  is still higher than that of the intermediate transfer roll  20 , so that lowly charged normal-polarity residual toner that has failed to adhere to the cleaning roll  210   a  adheres to the intermediate transfer brush roll  213 . Since the potential of the cleaning roll (downstream side cleaning roll)  210   b  is lower than that of the intermediate transfer roll  20 , positively charged reverse-polarity residual toner adheres to the cleaning roll  210   b . The magnitudes of the biases to be applied to the cleaning roll  210   a  and the cleaning roll  210   b  may be reversed.