Patent Publication Number: US-9417547-B2

Title: Image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2014-234608, filed on Nov. 19, 2014. The contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus. 
     Image forming apparatuses are known to remove toner adhering to an intermediate transfer belt using a cleaning device. 
     For example, an image forming apparatus is disclosed in which the voltage that is applied to a transfer roller in forced toner discharge is of a polarity opposite to the voltage that is applied to the transfer roller in printing. This image forming apparatus applies a voltage of 1 kV to the transfer roller in the forced toner discharge, which is 200 V higher than an electrical breakdown voltage of 800 V at a region where an image is transferred. 
     The above-described image forming apparatus is disclosed to be capable of quickly and sufficiently cleaning the intermediate transfer belt using a cleaning device therein in the forced toner discharge. 
     SUMMARY 
     An image forming apparatus according to a first aspect of the present disclosure is an image forming apparatus for forming an image on a recording medium. The image forming apparatus includes a plurality of photosensitive drums, a plurality of development sections, a plurality of primary transfer rollers, an intermediate transfer belt, and a voltage application section. The plurality of development sections are provided in one-to-one correspondence with the photosensitive drums and supply toners to the respective photosensitive drums to from toner images each having a different color on the respective photosensitive drums. The plurality of primary transfer rollers are disposed opposite to the photosensitive drums in one-to-one correspondence. The intermediate transfer belt is held between the photosensitive drums and the primary transfer rollers. The toner images are transferred onto the intermediate transfer belt such that the toner images are superimposed on one another for forming the image. The voltage application section applies voltage to a plurality of voltage application regions between each of the photosensitive drums and the corresponding one of the primary transfer rollers. In forced toner discharge from the development sections, the voltage application section applies voltage of the same polarity to each voltage application region other than a most downstream voltage application region located most downstream in a traveling direction of the intermediate transfer belt among the plurality of voltage application regions and applies, to the most downstream voltage application region, voltage of a polarity opposite to the polarity of the voltage that is applied to each voltage application region other than the most downstream voltage application region. 
     An image forming apparatus according to a second aspect of the present disclosure is an image forming apparatus for forming an image on a recording medium. The image forming apparatus includes a plurality of photosensitive drums, a plurality of development sections, a plurality of primary transfer rollers, an intermediate transfer belt, and a voltage application section. The plurality of development sections are provided in one-to-one correspondence with the photosensitive drums and supply toners to the respective photosensitive drums to from toner images each having a different color on the respective photosensitive drums. The plurality of primary transfer rollers are disposed opposite to the photosensitive drums in one-to-one correspondence. The intermediate transfer belt is held between the photosensitive drums and the primary transfer rollers. The toner images are transferred onto the intermediate transfer belt such that the toner images are superimposed on one another for forming the image. The voltage application section applies voltage to a plurality of voltage application regions between each of the photosensitive drums and the corresponding one of the primary transfer rollers. In forced toner discharge from the development sections, the voltage application section applies voltage of the same polarity to each voltage application region other than a most downstream voltage application region located most downstream in a traveling direction of the intermediate transfer belt among the plurality of voltage application regions and applies substantially no voltage to the most downstream voltage application region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view illustrating configuration of an image forming apparatus according to an embodiment of the present disclosure. 
         FIG. 2  is a side view illustrating configuration of an image forming unit and a transfer section illustrated in  FIG. 1 . 
         FIG. 3  is a side view illustrating configuration of a voltage application section that applies voltage to primary transfer rollers illustrated in  FIG. 2 . 
         FIG. 4  is a side view of the image forming unit illustrated in  FIG. 2 , illustrating toner behavior when images are formed on an intermediate transfer belt. 
         FIG. 5  is a side view illustrating operation of the voltage application section illustrated in  FIG. 3  according to a first embodiment. 
         FIG. 6  is a side view illustrating operation of the voltage application section illustrated in  FIG. 3  according to a second embodiment. 
         FIG. 7  is a side view illustrating operation of the voltage application section illustrated in  FIG. 3  according to a first comparative example. 
         FIGS. 8A to 8E  are tables for evaluation of results of cleaning performed by a blade according to configurations illustrated in  FIGS. 5 to 7 .  FIG. 8A  shows results of the cleaning according to the first comparative example.  FIG. 8B  shows results of the cleaning according to the second embodiment.  FIGS. 8C to 8E  show results of the cleaning according to the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings ( FIGS. 1 to 8E ). It should be noted that elements in the drawings that are the same or equivalent are labelled using the same reference signs and explanation thereof is not repeated. 
     First, an image forming apparatus  1  according to an embodiment of the present disclosure will be described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating configuration of the image forming apparatus  1  according to the present embodiment. In the present embodiment, the image forming apparatus  1  is a color copier. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  is an apparatus that forms an image on recording paper P and includes a housing  10 , a paper feed section  2 , a conveyance section L, a toner replenishment unit  3 , an image forming unit  4 , a transfer section  5 , a fixing section  7 , and an ejecting section  8 . 
     The paper feed section  2  is disposed in a lower part of the housing  10  and feeds the recording paper P to the conveyance section L. The paper feed section  2  can contain a plurality of sheets of recording paper P and picks up an uppermost sheet of recording paper P to feed the sheets of recording paper P to the conveyance section L one sheet at a time. Hereinafter, the recording paper P will be referred to as paper P in order to facilitate description. 
     The conveyance section L conveys the paper P fed by the paper feed section  2  to the ejecting section  8  through the transfer section  5  and the fixing section  7 . 
     The toner replenishment unit  3  is a container for replenishing the image forming unit  4  with toner and includes four toner cartridges  3   c ,  3   m ,  3   y , and  3   k . The toner cartridge  3   c  contains a cyan toner. The toner cartridge  3   m  contains a magenta toner. The toner cartridge  3   y  contains a yellow toner. The toner cartridge  3   k  contains a black toner. 
     Hereinafter, the toner cartridges  3   c ,  3   m , and  3   y  may be referred to as color toner cartridges  31 , and the toner cartridge  3   k  may be referred to as a black toner cartridge  32 . 
     The transfer section  5  includes an intermediate transfer belt  54 . The transfer section  5  transfers toner images formed by the image forming unit  4  from the intermediate transfer belt  54  onto paper P. Configuration of the transfer section  5  will be described later with reference to  FIG. 2 . 
     The image forming unit  4  forms toner images on the intermediate transfer belt  54 . The image forming unit  4  receives the color toners from the respective color toner cartridges  31  and the black toner from the black toner cartridge  32 . More specifically, the image forming unit  4  includes four image forming sections  4   c ,  4   m ,  4   y , and  4   k . The image forming section  4   c  receives the cyan toner from the toner cartridge  3   c . The image forming section  4   m  receives the magenta toner from the toner cartridge  3   m.  The image forming section  4   y  receives the yellow toner from the toner cartridge  3   y.  The image forming section  4   k  receives the black toner from the toner cartridge  3   k.  Configuration of the image forming unit  4  will be described later with reference to  FIG. 2 . 
     The fixing section  7  has a pair of rollers including a heating roller  71  and a pressure roller  72  that fix toner images formed on the paper P by the transfer section  5 . The heating roller  71  and the pressure roller  72  apply heat and pressure to the paper P. Thus, the unfixed toner images transferred onto the paper P by the transfer section  5  are fixed by the fixing section  7 . 
     The ejecting section  8  ejects the paper P having the toner images fixed thereon to the outside of the apparatus. 
     Next, configuration of the image forming unit  4  and the transfer section  5  will be described with reference to  FIG. 2 .  FIG. 2  is a side view illustrating configuration of the image forming unit  4  and the transfer section  5 . As illustrated in  FIG. 2 , the image forming unit  4  includes the four image forming sections  4   c ,  4   m ,  4   y , and  4   k.    
     The image forming sections  4   c ,  4   m ,  4   y , and  4   k  each include a light exposure device  41 , a photosensitive drum  42 , a development section  43 , a charging roller  44 , and a cleaning blade  45 . The four image forming sections  4   c ,  4   m ,  4   y , and  4   k  each supply a toner of a different color and otherwise have substantially the same configuration. Herein, therefore, the configuration of the image forming section  4   c  that receives the cyan toner will be described, and description of the configuration of the other image forming sections  4   m ,  4   y , and  4   k  will be omitted. 
     The image forming section  4   c  has a light exposure section  41   c  ( 41 ), a photosensitive drum  42   c  ( 42 ), a development section  43   c  ( 43 ), a charging roller  44   c  ( 44 ), and a cleaning blade  45   c  ( 45 ). 
     The charging roller  44   c  charges the photosensitive drum  42   c  to a specific electric potential. The light exposure section  41   c  performs light exposure on the photosensitive drum  42   c  by irradiating laser light thereto to form an electrostatic latent image on the photosensitive drum  42   c . The development section  43   c  has a development roller  431   c . The development roller  431   c  supplies the cyan toner to the photosensitive drum  42   c  to develop the electrostatic latent image into a toner image. Thus, a cyan toner image is formed on a peripheral surface of the photosensitive drum  42   c.    
     The cleaning blade  45   c  has an edge (an upper edge in  FIG. 2 ) in rubbing contact with the peripheral surface of the photosensitive drum  42   c . The edge of the cleaning blade  45   c  in rubbing contact with the peripheral surface of the photosensitive drum  42   c  removes residual cyan toner on the peripheral surface of the photosensitive drum  42   c.    
     The transfer section  5  transfers the toner image onto paper P (see  FIG. 1 ). The transfer section  5  includes four primary transfer rollers  51  ( 51   c ,  51   m ,  51   y , and  51   k ), a secondary transfer roller  52 , a drive roller  53 , the intermediate transfer belt  54 , and a driven roller  55 . 
     The transfer section  5  transfers toner images formed on the photosensitive drums  42  ( 42   c ,  42   m ,  42   y , and  42   k ) of the respective image forming sections  4   c ,  4   m ,  4   y,  and  4   k  onto the intermediate transfer belt  54  such that the toner images are superimposed on one another. The transfer section  5  subsequently transfers the superimposed toner images from the intermediate transfer belt  54  to the paper P (see  FIG. 1 ). 
     The primary transfer roller  51   c  is disposed opposite to the photosensitive drum  42   c  with the intermediate transfer belt  54  therebetween. The primary transfer roller  51   c  can be pressed into contact with the photosensitive drum  42   c  with the intermediate transfer belt  54  therebetween and separated from the photosensitive drum  42   c  by a drive mechanism (not shown). In a normal state, the primary transfer roller  51   c  is in pressed contact with the photosensitive drum  42   c  with the intermediate transfer belt  54  therebetween. Like the primary transfer roller  51   c , the other primary transfer rollers  51   m ,  51   y , and  51   k  are each in pressed contact with the corresponding photosensitive drum  42  ( 42   m ,  42   y , or  42   k ) with the intermediate transfer belt  54  therebetween. 
     The drive roller  53  is disposed opposite to the secondary transfer roller  52  and drives the intermediate transfer belt  54 . 
     The intermediate transfer belt  54  is an endless belt wound around the four primary transfer rollers  51 , the drive roller  53 , and the driven roller  55 . The intermediate transfer belt  54  is driven by the drive roller  53  to rotate in a counterclockwise direction as indicated by arrows F 1  and F 2  in  FIG. 2 . The intermediate transfer belt  54  has an outer surface in contact with the peripheral surface of each of the photosensitive drums  42  ( 42   c ,  42   m ,  42   y , and  42   k ). The primary transfer rollers  51  ( 51   c ,  51   m ,  51   y , and  51   y ) transfer toner images from the photosensitive drums  42  ( 42   c ,  42   m ,  42   y , and  42   k ) to the outer surface of the intermediate transfer belt  54 . 
     Specifically, the intermediate transfer belt  54  is a seamless belt made from a resin such as polyimide, polycarbonate, and polyvinylidene fluoride. 
     The driven roller  55  is driven to rotate by the rotation of the intermediate transfer belt  54 . A blade  56  is disposed at a location opposite to the driven roller  55  with the intermediate transfer belt  54  therebetween. The blade  56  removes residual toner on the outer surface of the intermediate transfer belt  54 . 
     The secondary transfer roller  52  is pressed against the drive roller  53 . The secondary transfer roller  52  and the drive roller  53  in such an arrangement form a nip N therebetween. The secondary transfer roller  52  and the drive roller  53  transfer the toner images from the intermediate transfer belt  54  to paper P (see  FIG. 1 ) while the paper P is passing through the nip N. 
     Next, “forced toner discharge” will be described. The following process is performed in order to adjust the charge state of the toners in the development sections  43  ( 43   c ,  43   m ,  43   y , and  43   k ) (or in order to prevent deterioration of the toners). That is, the toners in the development sections  43  ( 43   c ,  43   m ,  43   y , and  43   k ) are each forced out to the photosensitive drums  42  ( 42   c ,  42   m ,  42   y , and  42   k ) at a predetermined timing to form toner images. 
     Unused toners in amounts that make up for the amounts of the toners forced out from the development sections  43  ( 43   c ,  43   m ,  43   y , or  43   k ) are supplied from the toner replenishment unit  3  (the toner cartridges  3   c ,  3   m ,  3   y , and  3   k ). Thus, the charge state of the toners in the development sections  43  ( 43   c ,  43   m ,  43   y , and  43   k ) is adjusted. The above-described process is referred to as “forced toner discharge”. The predetermined timing is for example every time a predetermined number of pages (for example, 10 pages) are printed or every time a predetermined period of time (for example, 1 minute) elapses. 
     In the case of the “forced toner discharge”, as in the case of printing, toner images formed on the respective photosensitive drums  42  ( 42   c ,  42   m ,  42   y , and  42   k ) of the image forming sections  4   c ,  4   m ,  4   y , and  4   k  are transferred onto the intermediate transfer belt  54  such that the toner images are superimposed on one another. 
     Since discharging of the respective toners from the development sections  43  ( 43   c ,  43   m ,  43   y , and  43   k ) is timed so that the toner images are transferred to be superimposed on one another on the intermediate transfer belt  54 , the time taken for the “forced toner discharge” can be reduced. 
     However, the toner images are not transferred from the intermediate transfer belt  54  to paper P by the secondary transfer roller  52  and the drive roller  53  in the case of the “forced toner discharge”. More specifically, paper P is not conveyed to the secondary transfer roller  52  and the drive roller  53  in the case of the “forced toner discharge”. Furthermore, the secondary transfer roller  52  and the drive roller  53  do not apply heat to the toners on the intermediate transfer belt  54  in the case of the “forced toner discharge”. Accordingly, the toners transferred from the photosensitive drums  42  ( 42   c ,  42   m ,  42   y , and  42   k ) to the intermediate transfer belt  54  are conveyed to the blade  56  and removed by the blade  56  in the case of the “forced toner discharge”. 
     Next, a voltage application section  6  that applies voltage between each primary transfer roller  51  and the corresponding photosensitive drum  42  will be described with reference to  FIG. 3 .  FIG. 3  is a side view illustrating configuration of the voltage application section  6  that applies voltage to each primary transfer roller  51  illustrated in  FIG. 2 . The voltage application section  6  includes a first voltage application section  61  and a second voltage application section  62 . 
     The first voltage application section  61  controls voltage to be applied between each of three primary transfer rollers  51  ( 51   c ,  51   m , and  51   y ) located upstream in a traveling direction of the intermediate transfer belt  54  and a corresponding one of the photosensitive drums  42  ( 42   c ,  42   m , and  42   y ). More specifically, the first voltage application section  61  controls the voltage to be applied between the primary transfer roller  51   c  and the photosensitive drum  42   c , between the primary transfer roller  51   m  and the photosensitive drum  42   m , and between the primary transfer roller  51   y  and the photosensitive drum  42   y . A region between the primary transfer roller  51   c  and the photosensitive drum  42   c , a region between the primary transfer roller  51   m  and the photosensitive drum  42   m , and a region between the primary transfer roller  51   y  and the photosensitive drum  42   y  are each equivalent to one “voltage application region”. 
     The second voltage application section  62  controls voltage to be applied between the primary transfer roller  51   k  located most downstream in a traveling direction of the intermediate transfer belt  54  and the photosensitive drum  42   k . A region between the primary transfer roller  51   k  and the photosensitive drum  42   k  is equivalent to one “voltage application region” that is the “most downstream voltage application region”. 
     In the present embodiment, a configuration is described in which the voltage application section  6  includes the first voltage application section  61  and the second voltage application section  62 . However, a different configuration may be employed so long as the voltage application section  6  includes the second voltage application section  62 . In other words, a different configuration may be employed so long as the voltage to be applied between the most downstream primary transfer roller  51   k  and the photosensitive drum  42   k  can be controlled independently from the voltage to be applied between the three other primary transfer rollers  51  and the three other photosensitive drums  42 . For example, a configuration may be employed in which the voltage application section  6  controls the voltage to be applied between the four primary transfer rollers  51  and the four photosensitive drums  42  independently. That is, the voltage to be applied between the four primary transfer rollers  51  and the four photosensitive drums  42  refers to the voltage to be applied between the primary transfer roller  51   c  and the photosensitive drum  42   c , the voltage to be applied between the primary transfer roller  51   m  and the photosensitive drum  42   m , the voltage to be applied between the primary transfer roller  51   y  and the photosensitive drum  42   y , and the voltage to be applied between the primary transfer roller  51   k  and the photosensitive drum  42   k.    
     Next, the voltage to be applied by the first voltage application section  61  and the second voltage application section  62  for forming an image on paper P will be described.  FIG. 4  is a side view of the image forming unit  4  and the transfer section  5  illustrated in  FIG. 2 , illustrating toner behavior when an image is formed on paper P. In the present embodiment, a configuration is described in which positively charged toners are used to form an image on paper P. 
     Next, the toner behavior will be described with reference to  FIG. 4 . In  FIGS. 4 to 7 , “TP 1 ” to “TP 5 ” represent positively charged toners, and “TM 1 ” to “TM 5 ” represent negatively charged toners. Charges QP 1 , QP 2 , QP 3 , QP 4 , and QP 5  of the respective toners TP 1 , TP 2 , TP 3 , TP 4 , and TP 5  satisfy the relationship represented by the expression (1). A “charge” referred to herein means an absolute value of an electric charge.
 
QP1&lt;QP2&lt;QP3&lt;QP4&lt;QP5   (1)
 
     The charges QM 1 , QM 2 , QM 3 , QM 4 , and QM 5  of the respective toners TM 1 , TM 2 , TM 3 , TM 4 , and TM 5  satisfy the relationship represented by the following expression (2).
 
QM1&lt;QM2&lt;QM3&lt;QM4&lt;QM5   (2)
 
     As illustrated in  FIG. 4 , the photosensitive drum  42   c  needs to have a higher electric potential than the primary transfer roller  51   c  in order to cause the toner TP 1  included in the toner image formed on the photosensitive drum  42   c  to adhere to the intermediate transfer belt  54 . In other words, the first voltage application section  61  controls the voltage to be applied between the primary transfer roller  51   c  and the photosensitive drum  42   c  so that the photosensitive drum  42   c  has a higher electric potential than the primary transfer roller  51   c.    
     A voltage applied between a primary transfer roller  51  and a corresponding photosensitive drum  42  so that the photosensitive drum  42  has a higher electric potential than the primary transfer roller  51  is referred to as “positive polarity voltage”. A voltage applied between a primary transfer roller  51  and a corresponding photosensitive drum  42  so that the photosensitive drum  42  has a lower electric potential than the primary transfer roller  51  is referred to as “opposite polarity voltage”. 
     In  FIGS. 4 to 7 , a plus (+) sign given to a photosensitive drum  42  together with a minus (−) sign given to a primary transfer roller  51  indicates that the voltage therebetween is “positive polarity voltage”. A minus (−) sign given to a photosensitive drum  42  together with a plus (+) sign given to a primary transfer roller  51  indicates that the voltage therebetween is “opposite polarity voltage”. 
     In order to cause the toner TP 1  included in the toner image formed on the photosensitive drum  42   m  to adhere to the intermediate transfer belt  54 , the first voltage application section  61  applies positive polarity voltage between the primary transfer roller  51   m  and the photosensitive drum  42   m . Likewise, in order to cause the toner TP 1  included in the toner image formed on the photosensitive drum  42   y  to adhere to the intermediate transfer belt  54 , the first voltage application section  61  applies positive polarity voltage between the primary transfer roller  51   y  and the photosensitive drum  42   y . In order to cause the toner TP 1  included in the toner image formed on the photosensitive drum  42   k  to adhere to the intermediate transfer belt  54 , the second voltage application section  62  applies positive polarity voltage between the primary transfer roller  51   k  and the photosensitive drum  42   k.    
     Next, behavior of the toner TP 1  on the surface of the photosensitive drum  42   c  will be described. As illustrated in  FIG. 4 , positive polarity voltage is applied between the primary transfer roller  51   c  and the photosensitive drum  42   c , and therefore the toner TP 1  on the surface of the photosensitive drum  42   c  is caused to adhere to the intermediate transfer belt  54 . The toner TP 1  becomes charged to a slightly higher degree to be toner TP 2  due to the positive polarity voltage as passing between the primary transfer roller  51   c  and the photosensitive drum  42   c.    
     Positive polarity voltage is applied between the primary transfer roller  51   m  and the photosensitive drum  42   m . Accordingly, the toner TP 2  on the intermediate transfer belt  54  becomes charged to a much higher degree to be toner TP 3  due to the positive polarity voltage as passing between the primary transfer roller  51   m  and the photosensitive drum  42   m . Likewise, positive polarity voltage is applied between the primary transfer roller  51   y  and the photosensitive drum  42   y . Accordingly, the toner TP 3  on the intermediate transfer belt  54  becomes charged to a much, much higher degree to be toner TP 4  due to the positive polarity voltage as passing between the primary transfer roller  51   y  and the photosensitive drum  42   y . Likewise, positive polarity voltage is applied between the primary transfer roller  51   k  and the photosensitive drum  42   k . Accordingly, the toner TP 4  on the intermediate transfer belt  54  becomes charged to a very high degree to be toner TP 5  due to the positive polarity voltage as passing between the primary transfer roller  51   k  and the photosensitive drum  42   k.    
     Like the toner TP 1  on the photosensitive drum  42   c  caused to adhere to the intermediate transfer belt  54 , the toners TP 1  on the surfaces of the other photosensitive drums  42   m ,  42   y , and  42   k  are caused to adhere to the intermediate transfer belt  54 . Thus, the toner images of the respective colors formed by the toners TP 1  on the respective photosensitive drums  42  ( 42   c ,  42   m ,  42   y , and  42   k ) are sequentially transferred onto the intermediate transfer belt  54  such that the toner images are superimposed on one another, and subsequently the superimposed toner images are transferred by the secondary transfer roller  52  from the intermediate transfer belt  54  to paper P (see  FIG. 1 ). 
     When positive polarity voltage is applied to each primary transfer roller  51  and the corresponding photosensitive drum  42  in the forced toner discharge as in printing, as described above, the toner TP 5  charged to a very high degree is caused to adhere to the intermediate transfer belt  54 . Accordingly, adhesion between the toner TP 5  and the intermediate transfer belt  54  is very strong, and therefore the toner TP 5  adhering to the intermediate transfer belt  54  may not be removed by the blade  56 . 
     First Embodiment 
     Next, operation of the voltage application section  6  according to the first embodiment will be described with reference to  FIG. 5 .  FIG. 5  is a side view illustrating operation of the voltage application section  6  illustrated in  FIG. 3  according to the first embodiment. In the first embodiment, as illustrated in  FIG. 5 , the first voltage application section  61  applies opposite polarity voltage, and the second voltage application section  62  applies positive polarity voltage. More specifically, the first voltage application section  61  applies opposite polarity voltage with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y , and the second voltage application section  62  applies positive polarity voltage with respect to the most downstream primary transfer roller  51   k.    
     Next, the toner behavior in the first embodiment will be described. As illustrated in  FIG. 5 , opposite polarity voltage is applied between the primary transfer roller  51   c  and the photosensitive drum  42   c , and therefore the toner TP 1  on the surface of the photosensitive drum  42   c  is not caused to adhere to the intermediate transfer belt  54 . In the present embodiment, positively charged toners are used to form an image on paper P as described above, but the photosensitive drum  42   c  also has some negatively charged toner thereon. The negatively charged toner TM 1  on the surface of the photosensitive drum  42   c  is caused to adhere to the intermediate transfer belt  54 . The toner TM 1  becomes charged to a slightly higher degree to be toner TM 2  due to the opposite polarity voltage as passing between the primary transfer roller  51   c  and the photosensitive drum  42   c.    
     Opposite polarity voltage is applied between the primary transfer roller  51   m  and the photosensitive drum  42   m . Accordingly, the toner TM 2  on the intermediate transfer belt  54  becomes charged to a much higher degree to be toner TM 3  due to the opposite polarity voltage as passing between the primary transfer roller  51   m  and the photosensitive drum  42   m . Likewise, opposite polarity voltage is applied between the primary transfer roller  51   y  and the photosensitive drum  42   y . Accordingly, the toner TM 3  on the intermediate transfer belt  54  becomes charged to a much, much higher degree to be toner TM 4  due to the opposite polarity voltage as passing between the primary transfer roller  51   y  and the photosensitive drum  42   y.    
     Although not illustrated in  FIG. 5 , the toners TM 1  on the surfaces of the photosensitive drums  42   m  and  42   y  are caused to adhere to the intermediate transfer belt  54  in the same way as the toner TM 1  on the surface of the photosensitive drum  42   c  caused to adhere to the intermediate transfer belt  54 . 
     Positive polarity voltage is applied between the primary transfer roller  51   k  and the photosensitive drum  42   k . Accordingly, the toner TM 4  on the intermediate transfer belt  54  is caused to adhere to the photosensitive drum  42   k . The positively charged toner TP 1  on the photosensitive drum  42   k  is caused to adhere to the intermediate transfer belt  54  due to the positive polarity voltage applied between the primary transfer roller  51   k  and the photosensitive drum  42   k . Furthermore, the toner TP 1  caused to adhere to the intermediate transfer belt  54  becomes charged to a slightly higher degree to be toner TP 2  due to the positive polarity voltage as passing between the primary transfer roller  51   k  and the photosensitive drum  42   k.    
     Thus, as a result of the opposite polarity voltage applied with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y , and the positive polarity voltage applied with respect to the most downstream primary transfer roller  51   k , the intermediate transfer belt  54  has the toner TP 2  charged to a slightly higher degree thereon. Since the toner TP 2  is not charged to a very high degree, adhesion (Coulomb&#39;s force) between the toner TP 2  and the intermediate transfer belt  54  is not high. The toner TP 2  adhering to the intermediate transfer belt  54  can therefore be removed easily by the blade  56 . As a result, occurrence of poor cleaning of the intermediate transfer belt  54  in the forced toner discharge can be reduced. 
     Second Embodiment 
     Next, operation of the voltage application section  6  according to the second embodiment will be described with reference to  FIG. 6 . The second embodiment of the voltage application section  6  is different from the first embodiment in that the polarity of the voltage to be applied by the first voltage application section  61  and the polarity of the voltage to be applied by the second voltage application section  62  are reversed. 
       FIG. 6  is a side view illustrating operation of the voltage application section  6  illustrated in  FIG. 3  according to the second embodiment. In the second embodiment, as illustrated in  FIG. 6 , the first voltage application section  61  applies positive polarity voltage and the second voltage application section  62  applies opposite polarity voltage. More specifically, the first voltage application section  61  applies positive polarity voltage with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y,  and the second voltage application section  62  applies opposite polarity voltage with respect to the most downstream primary transfer roller  51   k.    
     Next, the toner behavior in the second embodiment will be described. As illustrated in  FIG. 6 , positive polarity voltage is applied between the primary transfer roller  51   c  and the photosensitive drum  42   c , and therefore the toner TP 1  on the surface of the photosensitive drum  42   c  is caused to adhere to the intermediate transfer belt  54 . The toner TP 1  becomes charged to a slightly higher degree to be toner TP 2  due to the positive polarity voltage as passing between the primary transfer roller  51   c  and the photosensitive drum  42   c.    
     Positive polarity voltage is applied between the primary transfer roller  51   m  and the photosensitive drum  42   m . Accordingly, the toner TP 2  on the intermediate transfer belt  54  becomes charged to a much higher degree to be toner TP 3  due to the positive polarity voltage as passing between the primary transfer roller  51   m  and the photosensitive drum  42   m . Likewise, positive polarity voltage is applied between the primary transfer roller  51   y  and the photosensitive drum  42   y . Accordingly, the toner TP 3  on the intermediate transfer belt  54  becomes charged to a much, much higher degree to be toner TP 4  due to the positive polarity voltage as passing between the primary transfer roller  51   y  and the photosensitive drum  42   y.    
     Although not illustrated in  FIG. 6 , the toners TP 1  on the surfaces of the photosensitive drums  42   m  and  42   y  are caused to adhere to the intermediate transfer belt  54  in the same way as the toner TP 1  on the surface of the photosensitive drum  42   c  caused to adhere to the intermediate transfer belt  54 . 
     Since opposite polarity voltage is applied between the primary transfer roller  51   k  and the photosensitive drum  42   k , the toner TP 4  on the intermediate transfer belt  54  is caused to adhere to the photosensitive drum  42   k . Likewise, since opposite polarity voltage is applied between the primary transfer roller  51   k  and the photosensitive drum  42   k , the toner TM 1  on the photosensitive drum  42   k  is caused to adhere to the intermediate transfer belt  54 . Furthermore, the toner TM 1  caused to adhere to the intermediate transfer belt  54  becomes charged to a slightly higher degree to be toner TM 2  due to the opposite polarity voltage as passing between the primary transfer roller  51   k  and the photosensitive drum  42   k.    
     Thus, as a result of the positive polarity voltage applied with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y , and the opposite polarity voltage applied with respect to the most downstream primary transfer roller  51   k , the intermediate transfer belt  54  has the toner TM 2  charged to a slightly higher degree thereon. Since the toner TM 2  is not charged to a very high degree, adhesion (Coulomb&#39;s force) between the toner TM 2  and the intermediate transfer belt  54  is not strong. The toner TM 2  adhering to the intermediate transfer belt  54  can therefore be removed easily by the blade  56 . As a result, occurrence of poor cleaning of the intermediate transfer belt  54  in the forced toner discharge can be reduced. 
     FIRST COMPARATIVE EXAMPLE 
     Next, operation of the voltage application section  6  according to the first comparative example will be described with reference to  FIG. 7 .  FIG. 7  is a side view illustrating operation of the voltage application section  6  illustrated in  FIG. 3  according to the first comparative example. In the first comparative example, as illustrated in  FIG. 7 , the first voltage application section  61  and the second voltage application section  62  apply opposite polarity voltage. More specifically, the first voltage application section  61  applies opposite polarity voltage with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y , and the second voltage application section  62  applies opposite polarity voltage to the most downstream primary transfer roller  51   k.    
     As illustrated in  FIG. 7 , opposite polarity voltage is applied between the primary transfer roller  51   c  and the photosensitive drum  42   c , and therefore the toner TP 1  on the surface of the photosensitive drum  42   c  is not caused to adhere to the intermediate transfer belt  54 . In the present embodiment, positively charged toners are used to form an image on paper P as described above, but the photosensitive drum  42   c  also has some negatively charged toner thereon. Since opposite polarity voltage is applied between the primary transfer roller  51   c  and the photosensitive drum  42   c , the negatively charged toner TM 1  on the photosensitive drum  42   c  is caused to adhere to the intermediate transfer belt  54 . The toner TM 1  becomes charged to a slightly higher degree to be toner TM 2  due to the opposite polarity voltage as passing between the primary transfer roller  51   c  and the photosensitive drum  42   c.    
     Opposite polarity voltage is applied between the primary transfer roller  51   m  and the photosensitive drum  42   m . Accordingly, the toner TM 2  on the intermediate transfer belt  54  becomes charged to a much higher degree to be toner TM 3  due to the opposite polarity voltage as passing between the primary transfer roller  51   m  and the photosensitive drum  42   m . Likewise, opposite polarity voltage is applied between the primary transfer roller  51   y  and the photosensitive drum  42   y . Accordingly, the toner TM 3  on the intermediate transfer belt  54  becomes charged to a much, much higher degree to be toner TM 4  due to the opposite polarity voltage as passing between the primary transfer roller  51   y  and the photosensitive drum  42   y . Likewise, opposite polarity voltage is applied between the primary transfer roller  51   k  and the photosensitive drum  42   k . Accordingly, the toner TM 4  on the intermediate transfer belt  54  becomes charged to a very high degree to be toner TM 5  due to the opposite polarity voltage as passing between the primary transfer roller  51   k  and the photosensitive drum  42   k.    
     Thus, as a result of the opposite polarity voltage applied between the upstream primary transfer rollers  51  and the photosensitive drums  42 , the toner TM 5  charged to a very high degree is caused to adhere to the intermediate transfer belt  54 . Accordingly, in a configuration in which opposite polarity voltage is applied between the primary transfer rollers  51  and the photosensitive drums  42  in the forced toner discharge, the toner TM 5  adhering to the intermediate transfer belt  54  may not be removed by the blade  56 . 
     Next, the relationship between occurrence of poor cleaning of the intermediate transfer belt  54  and the polarity of the voltage to be applied by the first voltage application section  61  and the polarity of the voltage to be applied by the second voltage application section  62  will be described with reference to  FIGS. 8A to 8E .  FIGS. 8A to 8E  are tables for evaluation of results of the cleaning performed by the blade  56  according to the configurations illustrated in  FIGS. 5 to 7 .  FIG. 8A  shows results of the cleaning according to the first comparative example.  FIG. 8B  shows results of the cleaning according to the second embodiment.  FIGS. 8C to 8E  show results of the cleaning according to the first embodiment illustrated in  FIG. 5 . 
     The left column of each of the tables shown in  FIGS. 8A to 8E  shows the polarity of the voltage applied with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y , and the values of the electric current (μA) flowing through the primary transfer rollers  51   c ,  51   m , and  51   y . The middle column shows the polarity of the voltage applied to the most downstream primary transfer roller  51   k  and the values of the electric current (μA) flowing through the primary transfer roller  51   k . The electric current flowing through each primary transfer roller  51  is positive when flowing from the primary transfer roller  51  to the corresponding photosensitive drum  42 . The right column shows occurrence of poor cleaning of the intermediate transfer belt  54  by the blade  56 . The letter “B” indicates that poor cleaning occurred. The letter “A” indicates that poor cleaning did not occur. 
     First, the results of the cleaning according to the first comparative example will be described with reference to  FIG. 8A . In the first comparative example, all the primary transfer rollers  51  receive opposite polarity voltage. In this configuration, as shown in  FIG. 8A , poor cleaning of the intermediate transfer belt  54  occurred when the values of the electric current flowing through the primary transfer rollers  51  were in a range of 1 μA to 5 μA. 
     Next, the results of the cleaning according to the second embodiment will be described with reference to  FIG. 8B . In the second embodiment, the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  receive positive polarity voltage, and the most downstream primary transfer roller  51   k  receives opposite polarity voltage. In this configuration, as shown in  FIG. 8B , poor cleaning of the intermediate transfer belt  54  did not occur when the absolute values of the electric current flowing through the primary transfer rollers  51  were in a range of 1 μA to 5 μA. 
     It is therefore possible to reduce occurrence of poor cleaning of the intermediate transfer belt  54  in the forced toner discharge by applying positive polarity voltage with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y,  and opposite polarity voltage with respect to the most downstream primary transfer roller  51   k.    
     Preferably, as shown in  FIG. 8B , positive polarity voltage is applied with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  so that the absolute values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m ,  51   y  are within the range of 1 μA to 5 μA, and opposite polarity voltage is applied with respect to the most downstream primary transfer roller  51   k  so that the value of the electric current flowing through the most downstream primary transfer roller  51   k  is within the range of 1 μA to 5 μA. 
     Next, the results of the cleaning in a configuration according to the first embodiment in which no voltage is applied with respect to the most downstream primary transfer roller  51   k  will be described with reference to  FIG. 8C . In the first embodiment, the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  receive positive polarity voltage, and the most downstream primary transfer roller  51   k  receives no voltage (or receive positive polarity voltage). In this configuration, as shown in  FIG. 8C , poor cleaning of the intermediate transfer belt  54  did not occur when the absolute values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  were in a range of 1 μA to 5 μA. 
     It is therefore possible to reduce occurrence of poor cleaning of the intermediate transfer belt  54  in the forced toner discharge by applying opposite polarity voltage with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y,  and no voltage with respect to the most downstream primary transfer roller  51   k.    
     Preferably, as shown in  FIG. 8C , opposite polarity voltage is applied with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  so that the values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m ,  51   y  are within the range of 1 μA to 5 μA. 
     Next, the results of the cleaning in a configuration according to the first embodiment in which positive polarity voltage is applied with respect to the most downstream primary transfer roller  51   k  will be described with reference to  FIGS. 8D and 8E . In the first embodiment, the three upstream primary transfer rollers  51   c ,  51   m,  and  51   y  receive opposite polarity voltage, and the most downstream primary transfer roller  51   k  receives positive polarity voltage (or no voltage). In this configuration, as shown in  FIG. 8D , poor cleaning of the intermediate transfer belt  54  did not occur when the value of the electric current flowing through the most downstream primary transfer roller  51   k  was “−3 μA”, and the values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  were within the range of 1 μA to 5 μA. Furthermore, as shown in  FIG. 8E , poor cleaning of the intermediate transfer belt  54  did not occur when the value of the electric current flowing through the most downstream primary transfer roller  51   k  was “−5 μA”, and the values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  were within the range of 1 μA to 5 μA. 
     It is therefore possible to reduce occurrence of poor cleaning of the intermediate transfer belt  54  in the forced toner discharge by applying opposite polarity voltage with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y,  and positive polarity voltage with respect to the most downstream primary transfer roller  51   k.    
     Preferably, as shown in  FIGS. 8D and 8E , opposite polarity voltage is applied with respect to the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  so that the values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m ,  51   y  are within the range of 1 μA to 5 μA. Preferably, positive polarity voltage is applied to the most downstream primary transfer roller  51   k  so that the absolute value of the electric current flowing through the most downstream primary transfer roller  51   k  is no greater than 5 μA. 
     The embodiments of the present disclosure have been described with reference to the drawings so far. However, the present disclosure is not limited to the above embodiments and may be practiced in various forms without deviating from the essence thereof (for example, as explained below in sections (1) to (5)). The drawings schematically illustrate elements of configuration in order to facilitate understanding and properties of elements of configuration illustrated in the drawings, such as thickness, length, and number thereof, may differ from actual properties thereof in order to facilitate preparation of the drawings. Furthermore, properties of elements of configuration described in the above embodiments, such as shapes and dimensions, are merely examples and are not intended as specific limitations. Various alterations may be made so long as there is no substantial deviation from the effects of the present disclosure. 
     (1) The first and second embodiments are described for a configuration in which the image forming apparatus  1  includes the four primary transfer rollers  51   c ,  51   m,    51   y , and  51   k , and the four photosensitive drums  42   c ,  42   m ,  42   y , and  42   k . However, the present disclosure is not limited to the configuration. The image forming apparatus  1  may include any number of primary transfer rollers and photosensitive drums so long as the number is two or more. For example, the number may be two, three, or five or more. 
     (2) The first embodiment is described for a configuration in which the values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m,  and  51   y  are within the range of 1 μA to 5 μA, and the absolute value of the electric current flowing through the most downstream primary transfer roller  51   k  is no greater than 5 μA. However, the present disclosure is not limited to the configuration. The values of the electric current may be out of the above-specified ranges. For example, the values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  may be 6 μA, and the absolute value of the electric current flowing through the most downstream primary transfer roller  51   k  may be 6 μA. 
     (3) The second embodiment is described for a configuration in which the absolute values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  are within the range of 1 μA to 5 μA, and the value of the electric current flowing through the most downstream primary transfer roller  51   k  is within the range of 1 μA to 5 μA. However, the present disclosure is not limited to the configuration. The values of the electric current may be out of the above-specified ranges. For example, the absolute values of the electric current flowing through the three upstream primary transfer rollers  51   c ,  51   m , and  51   y  may be 6 μA, and the value of the electric current flowing through the most downstream primary transfer roller  51   k  may be 0.5 μA. 
     (4) The first and second embodiments are described for a configuration in which discharging of each of the toners in the development sections  43  ( 43   c ,  43   m ,  43   y,  and  43   k ) is timed with transferring of each toner image onto the intermediate transfer belt  54  such that the toner images are superimposed on one another. However, the present disclosure is not limited to the configuration. For example, the discharging may be performed such that the toner images are not superimposed on one another. In such a configuration, control for superimposing the toner images is not necessary. 
     (5) The first and second embodiments are described for a configuration in which positively charged toners are used to form an image on paper P. However, the present disclosure is not limited to the configuration. Negatively charged toners may be used to form an image on paper P.