Patent Publication Number: US-7593665-B2

Title: Image forming apparatus with cleaning unit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional application of U.S. patent application Ser. No. 11/341,739, filed on Jan. 27, 2006, the entire contents of which are incorporated herein by reference and priority to which is claimed herein. The 11/341,739 application claimed the benefit of the date of the earlier filed Japanese Patent Applications No. 2005-024907 filed Feb. 1, 2005 and Japanese Application No. 2005-063927 filed Mar. 8, 2005, both of which are incorporated herein by reference, and priority to both of which is claimed herein. 

   BACKGROUND 
   This invention relates to an electrophotographic image forming apparatus. 
   An electrophotographic color image forming method used, for example, by a copying machine or printer forms a visible image by forming a toner image on a photoreceptor or belt-type transfer member by a toner image forming unit, transferring the toner image to a transfer material by an image transferring device, and fixing the transfer material. The toner left unused on the belt-type transfer member is removed by a cleaning device. 
   One of toner cleaning devices uses a bias roller such as a conductive brush roller to remove residual toner electrostatically. Usually, however, toner particles left on the belt-type transfer member have both positive and negative charges even when toner particles in the toner image forming unit, for example, in a developer are charged negatively. This is because the toner particles are charged oppositely to the charge polarity of the toner particles in the developer by the transferring electric field formed in the transferring unit. Therefore, it is impossible for such a cleaning device to remove the positively- and negatively-charged residual toner particles by a single brush roller. 
   To solve such a problem, a cleaning device has been proposed which has, for example, two brush rollers one of which has a positive cleaning voltage and the other has a negative cleaning voltage (for example, in Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) and Tokkaihei 6-332342 (Japanese Non-Examined Patent Publication)). 
   However, there is the possibility that such a cleaning device cannot completely remove toner particles to form toner patches for detection of image density on a belt-type transfer member in order to control the density and gray scale of a visible image. 
   This is because the toner patch formed on the belt-type transfer member remains non-transferred on the belt-type transfer member when the transferring unit is not working and because the toner is too much to be removed by the above cleaning unit. 
   Similarly, when a transfer material jams, the transferring unit stops and a lot of non-transferred toner particles remain on the belt-type transfer member. This causes a similar problem. 
   To overcome the abovementioned problems, there has been proposed another cleaning unit, which applies a specific and large cleaning bias voltage when a large amount of non-transferred toner happens to remain (for instance, set forth in Tokkai 2000-04079 (Japanese Non-Examined Patent Publication)). 
   However, since it is necessary for the above-mentioned cleaning unit to set the cleaning bias voltage at such a value that is appropriate for the most severe condition for removing the large amount of non-transferred toner, there have been problems that the abovementioned cleaning unit should have a high-power outputting capability with a power source having a large capacity, and therefore, a danger of electric current leak becomes a high-risk factor. Specifically, in a configuration in which a pair of plus and minus electrodes create an electric field, the cleaning bias voltage becomes very high when cleaning non-transferred toner. 
   Another cleaning device has been proposed which contains a bias voltage applying device and two cleaning brushes between which voltages of an identical polarity are changed (for example, in Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication)). 
   [Patent Documents 1]
         Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication)       

   [Patent Documents 2]
         Tokkaihei 6-332342 (Japanese Non-Examined Patent Publication)       

   [Patent Documents 3]
         Tokkai 2000-04079 (Japanese Non-Examined Patent Publication)       

   Further, the method of changing the same voltage between the cleaning brushes to the bias voltage applying unit and two cleaning brushes (for instance, set forth Tokkaihei 6-332342 (Japanese Non-Examined Patent Publication)). 
   [Patent Documents 4]
         Tokkaisho 60-170879 (Japanese Non-Examined Patent Publication)       

   Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) and Tokkaisho 60-170879 (Japanese Non-Examined Patent Publication) disclose an image forming apparatus which forms, on an intermediate transfer member, a patch image to control the image density and a patch image to correct timing of forming an image of each color (Y, M, C, and K) to form a color image. However, in such an image forming apparatus, an image formed on the intermediate transfer member must be cleaned after the image density is controlled or image timing is corrected. Further, if a transfer material jams before a toner image is transferred from the intermediate transfer member to the transfer material, a lot of toner (if any) on the intermediate transfer member cannot be removed by a single cleaning process and some toner may be left on the intermediate transfer member after the cleaning process. This toner left on the intermediate transfer member will cause image problems such as color mingling in image formation, stains on the back side of the transfer material, and insufficient image density control. 
   Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) also discloses a cleaning method which contains a bias voltage applying device and two cleaning brushes between which voltages of the same polarity are changed. However, the additional bias voltage applying device makes the configuration complicated. 
   To solve the above problem, it may be possible to conceive a method of providing two cleaning modes and changing polarities of the brush rollers to clean toner left non-transferred on the intermediate transfer member. The first cleaning mode forms an image according to normal image, transfers the toner image to a transfer material, and reverses the polarity of voltages applied to the two brush rollers to remove toner particles left on the intermediate transfer member. 
   The second cleaning mode applies voltages of a positive polarity to the two brush rollers to remove toner particles left on the intermediate transfer member since the non-transferred toner particles are charged negatively (which is the regular polarity). 
   However, we found that this cleaning method cannot be free from causing image problems and insufficient image density control that generated by the cleaning rollers of Tokkaihei 6-130875 (Japanese Non-Examined Patent Publication) and Tokkaisho 60-170879 (Japanese Non-Examined Patent Publication) since toner particles of a positive polarity brushed out by the brush rollers whose polarity was changed from negative to positive stuck to the intermediate transfer member and remained after the cleaning process. 
   SUMMARY 
   An embodiment of the present invention may provide an image forming apparatus which is equipped with two brush rollers to clean a belt-type transfer member, may completely remove toner particles left non-transferred on the belt-type transfer member, and may always present stainless images. 
   Further, an embodiment of the present invention may provide an image forming apparatus which can prevent image problems and insufficient image density control without any complicated unit and may completely remove normal residual toner particles and non-transferred toner particles such as patch image toner from the intermediate transfer member and a cleaning method thereof. 
   Accordingly, the present invention can be attained by image forming apparatus described as follows.
     (1) An image forming apparatus, comprising: a toner image forming unit to form a toner image on an image bearing member by employing toner; an intermediate transfer member driven to rotate; a primary transferring unit to transfer the toner image formed on the image bearing member onto the intermediate transfer member; a secondary transferring unit to transfer the toner image residing on the intermediate transfer member onto a transfer material; and a cleaning unit to clean residual toner remaining on the intermediate transfer member; wherein the cleaning unit includes: a conductive opposing roller that contacts an inner surface of the intermediate transfer member; a first conductive brush member and a second conductive brush member, both of which are pressed against the conductive opposing roller while putting the intermediate transfer member between them; an electric current path that is formed so as to allow an electric current to flow between the first conductive brush member and the second conductive brush member through the conductive opposing roller; a cleaning-voltage applying power source to apply a cleaning voltage having a polarity opposite to that of the residual toner onto the first conductive brush member or the second conductive brush member; and an opposing-roller potential controlling unit to control an electric potential state of the conductive opposing roller, so that an electric potential difference, between the first conductive brush member and the conductive opposing roller at a time when the secondary transferring unit is deactivated, is larger than that at a time when the primary transferring unit is activated.   (2) An image forming apparatus, comprising: a toner image forming unit to form a toner image on an image bearing member by employing toner; an intermediate transfer member driven to rotate; a primary transferring unit to transfer the toner image formed on the image bearing member onto the intermediate transfer member; a first cleaning unit to remove residual toner remaining on the image bearing member after a primary transferring operation is completed; a secondary transferring unit to transfer the toner image residing on the intermediate transfer member onto a transfer material; a second cleaning unit to clean residual toner remaining on the intermediate transfer member, wherein the second cleaning unit includes a first brush roller and a second brush roller disposed at a position downstream from the first brush roller in a moving direction of the intermediate transfer member, each of which is driven to rotate while contacting the intermediate transfer member so as to clean the intermediate transfer member; and a controller to control a voltage to be applied to the first brush roller and a voltage to be applied to the second brush roller, and to control the primary transferring unit; wherein the image forming apparatus are provided with a first mode in which image forming operations based on normal image data are conducted and a second mode in which patch image forming operations based on patch image data are conducted; and wherein, in the first mode, the controller applies a voltage having a negative polarity to the first brush roller, a voltage having a positive polarity to the second brush roller and a voltage having a predetermined polarity to the primary transferring unit, respectively, while in the second mode, the controller applies a voltage having a positive polarity to the first brush roller, a voltage having a positive polarity to the second brush roller and a voltage having the predetermined polarity same as that in the first mode to the primary transferring unit, respectively.   (3) A cleaning method for cleaning residual toner in an image forming apparatus, which includes: a toner image forming unit to form a toner image on an image bearing member by employing toner; an intermediate transfer member driven to rotate; a primary transferring unit to transfer the toner image formed on the image bearing member onto the intermediate transfer member; a first cleaning unit to remove residual toner remaining on the image bearing member after a primary transferring operation is completed; a secondary transferring unit to transfer the toner image residing on the intermediate transfer member onto a transfer material; and a second cleaning unit to clean residual toner remaining on the intermediate transfer member, wherein the second cleaning unit includes a first brush roller and a second brush roller disposed at a position downstream from the first brush roller in a moving direction of the intermediate transfer member, each of which is driven to rotate while contacting the intermediate transfer member so as to clean the intermediate transfer member; wherein the cleaning method is conducted in a first mode in which image forming operations based on normal image data are conducted or in a second mode in which patch image forming operations based on patch image data are conducted; the cleaning method comprising the steps of: applying a voltage having a negative polarity to the first brush roller and a voltage having a positive polarity to the second brush roller, respectively, in the first mode; applying a voltage having a positive polarity to the first brush roller and a voltage having a positive polarity to the second brush roller, respectively, in the second mode; applying a voltage having a positive polarity to the primary transferring unit in order to inversely transfer toner, ejected from the first brush roller to the intermediate transfer member, onto the image bearing member; and removing the toner inversely transferred onto the image bearing member by means of the first cleaning unit.   (4) An image forming apparatus, comprising: a toner image forming unit to form a toner image on an image bearing member by employing toner; an intermediate transfer member onto which the toner image is transferred; a primary transferring unit to transfer the toner image formed on the image bearing member onto the intermediate transfer member; a first cleaning unit to remove residual toner remaining on the image bearing member after a primary transferring operation is completed; a secondary transferring unit to transfer the toner image residing on the intermediate transfer member onto a transfer material; a second cleaning unit to clean residual toner remaining on the intermediate transfer member, wherein the cleaning unit includes a first brush roller and a second brush roller disposed at a position downstream from the first brush roller in a moving direction of the intermediate transfer member, each of which is driven to rotate while contacting the intermediate transfer member so as to clean the intermediate transfer member; and a controller to control a voltage to be applied to the first brush roller and a voltage to be applied to the second brush roller; wherein the image forming apparatus are provided with a first mode in which image forming operations based on normal image data are conducted and a second mode in which patch image forming operations based on patch image data are conducted; and wherein, in the first mode, the controller applies a voltage having a negative polarity to the first brush roller and a voltage having a positive polarity to the second brush roller, respectively, while in the second mode, the controller initially applies a voltage having a positive polarity to the first brush roller and the a voltage having a positive polarity to the second brush roller, respectively, and then, applies a voltage having a negative polarity to the first brush roller and a voltage having a positive polarity to the second brush roller.   (5) A cleaning method for cleaning residual toner in an image forming apparatus, which includes: a toner image forming unit to form a toner image on an image bearing member by employing toner; an intermediate transfer member driven to rotate; a primary transferring unit to transfer the toner image formed on the image bearing member onto the intermediate transfer member; a first cleaning unit to remove residual toner remaining on the image bearing member after a primary transferring operation is completed; a secondary transferring unit to transfer the toner image residing on the intermediate transfer member onto a transfer material; and a second cleaning unit to clean residual toner remaining on the intermediate transfer member, wherein the second cleaning unit includes a first brush roller and a second brush roller disposed at a position downstream from the first brush roller in a moving direction of the intermediate transfer member, each of which is driven to rotate while contacting the intermediate transfer member so as to clean the intermediate transfer member; wherein the cleaning method is conducted in a first mode in which image forming operations based on normal image data are conducted or in a second mode in which patch image forming operations based on patch image data are conducted; the cleaning method comprising the steps of: applying a voltage having a negative polarity to the first brush roller and a voltage having a positive polarity to the second brush roller, respectively, in the first mode; and applying at first a voltage having a positive polarity to the first brush roller and a voltage having a positive polarity to the second brush roller, and then, at a time before toner ejected from the first brush roller to the intermediate transfer member arrive at the first brush roller, applying a voltage having a negative polarity to the first brush roller and a voltage having a positive polarity to the second brush roller.   

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
       FIG. 1  shows an explanatory schematic diagram of the first embodiment of an image forming apparatus embodied in the present invention; 
       FIG. 2  shows an explanatory schematic diagram of a cleaning device equipped in an image forming apparatus embodied in the present invention, illustrating an enlarged peripheral view of the cleaning device; 
       FIG. 3  shows an explanatory schematic diagram of a second cleaning device equipped in an image forming apparatus embodied in the present invention, and the electric configuration thereof; 
       FIG. 4  shows a flow chart of a process executed by a color image forming apparatus to clean an intermediate transfer member; 
       FIG. 5  shows a flow chart of another cleaning process executed by a color image forming apparatus to clean an intermediate transfer member; and 
       FIG. 6  shows an enlarged schematic diagram for explaining another example of a cleaning device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the drawings, embodiments of the present invention will be detailed in the following. 
     FIG. 1  shows an explanatory schematic diagram of the first embodiment of the image forming apparatus embodied in the present invention. 
   The image forming apparatus, serving as an image forming apparatus for forming a color image, employs the intermediate transferring method, so to speak, which includes the steps of: forming each of a plurality of toner images having different color onto each of a plurality of image bearing members; sequentially transferring each of the unicolor toner images onto a common belt-type transfer member so as to overlap the unicolor toner images with each other on the belt-type transfer member; and then, transferring the full color toner image, formed on the belt-type transfer member, onto a transfer material P at a time as a transferring operation. 
   The image forming apparatus is provided with a belt-type transfer member  17 , which is made of an endless type belt and is circularly moved in a direction indicated by an arrow shown in  FIG. 1 . In an arranging area of toner image forming units disposed at the outer circumferential region of the belt-type transfer member  17 , four toner image forming units  30 Y,  30 M,  30 C,  30 K, for forming a yellow toner image, a magenta toner image, a cyan toner image, a black toner image, respectively, are disposed in such a manner that these are arrayed along the moving direction of the belt-type transfer member  17  while sequentially separating from each other. The belt-type transfer member  17  is threaded on various kinds of rollers including a conductive opposing roller  17   a  detailed later (hereinafter, referred to as an opposing roller  17   a , for simplicity),  17   b ,  17   c  and  17   d , so that the belt-type transfer member  17  is circularly moved while being contacted image bearing members  10 Y,  10 M,  10 C,  10 K by pushing actions of primary transferring devices  14 Y,  14 M,  14 C,  14 K in the toner image forming units  30 Y,  30 M,  30 C,  30 K, respectively. 
   The belt-type transfer member  17  is made of the endless belt, having semiconductivity, mass resistivity in a range of 1×10 8 -1×10 10  Ωcm and surface resistivity in a range of 1×10 4 -1×10 12  Ω/cm 2 . The surface resistivity is measured by applying a voltage of 100 V for 10 seconds under the environment of room temperature and room humidity (temperature: 20±1° C., humidity: 50±2%) by means of the resistivity measuring instrument (Hiresta IP, manufactured by Yuka Electronic Co.). 
   It is preferable that the belt-type transfer member  17  is made of polyimide, such as, for instance, a heat curing polyimide, a modification polyimide, etc. 
   Further, the moving velocity of the belt-type transfer member  17  is set at a value in a range of, for instance, 200-500 mm/sec. 
   The toner image forming units  30 Y for forming a toner image of color Y (Yellow) is provided with an image bearing member  10 Y being a photoreceptor drum to be rotated. In the peripheral space along the circumferential surface of the image bearing member  10 Y, a charging device  11 Y, an exposing device  12 Y and a developing device  13 Y for developing a yellow toner image by using developing agent for color Y (Yellow) are arranged in a rotating direction of the image bearing member  10 Y according to this order. Further, a cleaning device  18 Y having a cleaning blade for cleaning the image bearing member is disposed at a downstream side of a primary transferring device  14 Y, which is disposed at a downstream position of the developing device  13 Y in the rotating direction of image bearing member  10 Y. 
   Further, the density detecting sensor (not shown in the drawings), for detecting density of the toner image formed on the image bearing member  10 Y, is disposed at a position downstream from the developing device  13 Y and upstream from the primary transferring device  14 Y. 
   For instance, the image bearing member  10 Y is provided with a photosensitive layer, which is coated on a drum-shaped metal base member and is made of a resin material containing an organic photoconductive material. In  FIG. 1 , the image bearing member  10 Y is arranged in such a manner that the longitudinal direction of the photoreceptor drum is extended in a direction perpendicular to the paper surface. 
   The charging device  11 Y includes, for instance, a scorotron charger having a grid electrode and a discharging electrode, while the exposing device  12 Y includes, for instance, a laser beam irradiating device. 
   The developing device  13 Y includes a developing sleeve, which rotates and which incorporates a magnet to retain developing agent while rotating, and a voltage applying device  15 Y for applying a DC bias voltage and/or an AC bias voltage to a gap between the image bearing member  10 Y and the developing sleeve. In this connection, it is needless to say that the developing devices  13 M,  13 C,  13 K also include voltage applying devices  15 M,  15 C,  15 K in the same manner as abovementioned, respectively. 
   The primary transferring device  14 Y is constituted by a primary transferring roller  141 Y that is provided so as to form a primary transferring region in a state of press-contacting the surface of the image bearing member  10 Y while putting the belt-type transfer member  17  between them, and a transfer-current supplying device (not shown in the drawings) including, for instance, a constant current source coupled to the primary transferring roller  141 Y. The yellow toner image, residing on the image bearing member  10 Y, is electrostatically transferred onto the belt-type transfer member  17  by supplying a primary transferring current outputted from the transfer-current supplying device to the primary transferring roller  141 Y. The abovementioned method is called as the contact-transferring method. In this connection, it is needless to say that the primary transferring devices  14 M,  14 C,  14 K are also constituted by primary transferring rollers  141 M,  141 C,  141 K, in the same manner as abovementioned, respectively. 
   The cleaning blade for cleaning the image bearing member, provided in the cleaning device  18 Y, is made of an elastic material, such as, for instance, a polyurethane rubber, etc. The base portion of the cleaning blade is supported by a supporting member, while the leading edge portion of the cleaning blade contacts the surface of the image bearing member  10 Y. Further, the cleaning blade is extended from the base portion in a counter direction, opposite to the rotating direction of the image bearing member  10 Y at the contacting point. 
   Each configuration of the toner image forming units  30 M,  30 C,  30 K is the same as that of the toner image forming units  30 Y for forming a toner image of color Y (Yellow), except that the developing agent includes each of magenta toner, cyan toner and black toner, instead of yellow toner. 
   A secondary transferring device  14 S is disposed at a position downstream from the toner image forming unit  30 K for forming a toner image of color K (Black). The secondary transferring device  14 S is constituted by a secondary transferring roller  141 S that is provided so as to form a transferring region in a state of press-contacting the backup roller  17   d  while putting the belt-type transfer member  17  between them, and a transfer-current supplying device (not shown in the drawings) coupled to the secondary transferring roller  141 S. The full color toner image, formed on the belt-type transfer member  17 , is transferred onto a conveyed transfer material P by supplying a transferring current outputted from the transfer-current supplying device to the secondary transferring roller  141 S. The above-mentioned method is called as the contact-transferring method. 
   When a toner patch is formed on the belt-type transfer member  17  to control the density of a visible image and gradation or when a transfer material P jams in the transfer path, the transfer operation stops. In such a case, the secondary transferring device  14 S receives a Toner Patch Formation signal or a Jam Reset signal from a central processing unit (which is not shown in the drawings) and becomes inactive. 
   Next, as the first embodiment of the present invention, a cleaning device  18 S will be detailed in the following. 
   A cleaning device  18 S to remove residual toner from the belt-type transfer member  17  is provided in a downstream side of the secondary transferring device  14 S along the movement of the belt-type transfer member. As shown in  FIG. 2 , the cleaning device  18 S is equipped with an opposing roller  17   a  which is in contact with the inner surface of the belt-type transfer member  17 , and first and second brush roller systems. The first brush roller system  21  contains a first conductive brush roller  22  (hereinafter also called “first brush roller”) which is in contact with the outer surface of the belt-type transfer member  17 , a first flicker rod  23  which is a toner recovering roller in contact with the first brush roller  22 , and a scraper  24  which is in contact with the first flicker rod  23 . The second brush roller system  25  contains a second conductive brush roller  26  (hereinafter also called “second brush roller”) which is another conductive brush roller in contact with the outer surface of the belt-type transfer member  17  and located in the upstream side of the first brush roller  22  along the movement of the belt-type transfer member  17 , a second flicker rod  27  which is a toner recovering roller in contact with the second brush roller  26 , and a plate-shaped scraper  28  which is in contact with the second flicker rod  27 . 
   The first brush roller  22  is pressed against the opposing roller  17   a  with the belt-type transfer member  17  therebetween. The first brush roller  22  has bristles such as conductive nylon bristles densely implanted on the outer surface of the roller body. The brush bristles have a diameter of, for example, 5 to 8 deniers, a length of, for example, 2 to 5 mm, an electric resistance of, for example, 1×10 9  to 1×10 11  Ω, a Young&#39;s modulus of, for example, 4,900 to 9,800 N/mm 2 , and an implantation density (number of bristles per unit area) of, for example, 50 to 200 kilo bristles per square inch. 
   The first brush roller  22  is disposed so that the bristles may bite the belt-type transfer member  17  by 1 mm. This “bite quantity of bristles” of the first brush roller  22  means the maximum length of bristles which enter (overlap) the belt-type transfer member space when the belt-type transfer member  17  is removed. 
   The first flicker rod  23  in the first brush roller system  21  is to remove toner particles from the first brush roller  22  and the diameter thereof is, for example, 8 to 30 mm. It is made of, for example, stainless steel. The first brush roller  22  is disposed so that the rod  23  may bite the first brush roller  22  by 1 mm. This “bite quantity to the first brush roller” means the maximum length of bristles which enter the first flicker rod space when the first flicker rod  23  is removed. 
   The scraper  24  is a plate to mechanically remove toner from the first flicker rod  23 . The scraper  24  is, for example, 0.05 mm thick. 
   The second brush roller  26  is pressed against the opposing roller  17   a  with the belt-type transfer member  17  therebetween. 
   The second brush roller  26 , second flicker rod  27 , and scraper  28  in the second brush roller system  25  are the same in materials as the first brush roller  22 , first flicker rod  23 , and scraper  24  in the first brush roller system  21 . The bite quantity of the second brush roller  26  to the belt-type transfer member  17  is 1 mm and the bite quantity of the second flicker rod to the second brush roller  26  is 1 mm. 
   The brush rollers  22  and  26  are made to rotate, for example, at a speed of 100 to 250 mm/sec oppositely (clockwise in  FIG. 2 ) to the movement of the belt-type transfer member respectively at points where the brush rollers  22  and  26  touch the belt-type transfer member  17 . The flicker rods  23  and  27  are made to rotate in the same direction as the brush rollers  22  and  26  rotate (counterclockwise in  FIG. 2 ). 
   This cleaning device  18 S has a circuit path  33  which flows a current to the conductive rollers  22  (first brush roller),  17   a  (opposing roller), and  26  (second brush roller). A power supply  29  is provided to apply a cleaning voltage of a polarity opposite to the charge polarity of toner in the developers  13 Y,  13 M,  13 C and  13 K (hereinafter called “developing polarity of toner”) to the first brush roller  22  via the first flicker rod  23 . When a cleaning voltage is applied to first brush roller  22 , a cleaning current I flows to the first brush roller  22 , the opposing roller  17   a , and the second brush roller  26  in that order through the current path  33 . 
   The brush rollers  22  and  26  brush off residual toner from the belt-type transfer member  17  and remove them electrostatically. When the developing polarity of the used toner is negative, the first brush roller  22  in the current path  33  has a function to remove negatively-charged toner particles among toner particles left on the belt-type transfer member  17  and the second brush roller  26  has a function to remove positively-charged toner particles among the toner particles. 
   The magnitude of a cleaning voltage to be applied to the first flicker rod  23  by the cleaning voltage applying device  29  is, for example, +200 to +1000 V when the developing polarity of toner is negative. 
   The opposing roller  17   a  also works as a roller to support and stretch the belt-type transfer member  17 . The roller  17   a  is a hard roller made of an aluminum core bar and the outer diameter is, for example, 20 to 80 mm. 
   The image forming apparatus is equipped with a mechanism  36  to control the potential of the opposing roller  17   a . The mechanism  36  contains a changeover switch which applies, to the opposing roller  17   a , the same potential as the second brush roller  26  when the switch is made. When the secondary transferring device  14 S becomes inactive, the mechanism  36  is controlled to be active. In other words, when the secondary transferring device  14 S becomes inactive, the control section  31  controls so that the potential difference V 1   off  between the opposing roller  17   a  and the first brush roller  22  when the secondary transferring device  14 S becomes inactive may be greater than the potential difference V 1   on  between the opposing roller  17   a  and the first brush roller  22  when the secondary transferring device  14 S is active. 
   In the cleaning device  18 S, for example, the second brush roller  26  is earthed to the ground potential. When the control mechanism  36  becomes active, the opposing roller  17   a  is also earthed to the ground potential. 
   &lt;Toner&gt; 
   It is preferable that a mass average particle size of the toner to be employed in the image forming apparatus aforementioned is in a range of 4-7 μm. By employing the toner having the mass average particle size in a range of 4-7 μm, it becomes possible to reduce such toner that have an excessive adhesive property or a weak adhesive force for the transfer material P in a fixing process performed by a fixing apparatus (not shown in the drawings), resulting in a long time stability of the developing efficiency. Further, since the high transferring efficiency can be achieved, it also becomes possible not only to improve the image quality of a halftone image area, but also to form a visual image in which the image quality of fine lines and that of dots are improved. 
   Incidentally, hereinafter, the mass average particle size of the toner is measured by employing the “Coulter Counter TA II” or the “Coulter Multi-sizer” (both manufactured by Coulter Co.). 
   The abovementioned toner is acquired by polymerizing the polymerization monomer in the water-type agent. For instance, fine polymerized particles are manufactured by employing an emulsion polymerization method or by emulsion-polymerizing the monomer in the liquid including emulsion liquid being a necessary addition agent, and then, the abovementioned toner are manufactured by employing the method of adding and associating an organic solvent, a flocculent, etc. Further, the abovementioned toner can be also manufactured by employing the method of mixing and associating a releasing agent, a coloring agent, etc., being necessary constituents of the toner, with the monomer, or by employing the method of dispersing constituents of the toner, such as the releasing agent, the coloring agent, etc., into the monomer, and then, emulsion-polymerizing them, etc. Incidentally, the term of “association” means that a plurality of resin particles and a plurality of coloring agent particles fuse into each other. Further, the water-type agent, defined in the present invention, contains water at least 50%-by-mass. 
   An example of such the method for manufacturing the toner includes the steps of: adding various kinds of constituents, such as the coloring agent, the releasing agent, the charge controlling agent, the polymerizing initiation agent, etc., as needed, into the polymerization monomer; dissolving or dispersing the various kinds of constituents into the polymerization monomer by using a homogenizer, a sand mill, a sand grinder, an ultrasound dispersing machine, etc.; dispersing the polymerization monomer, in which the various kinds of constituents are dissolved or dispersed, in the water-type agent including a dispersing stabilizer into oil particles each of which has a desired dimension as a toner particle; heating them in a reacting apparatus to accelerate the polymerizing reaction; and after the polymerizing reaction is completed, adjusting the toner by removing the dispersing stabilizer, by filtering, by washing, and further, by drying. 
   It is preferable that the sphericity of the toner mentioned in the above is in a range of 0.94-0.98. The sphericity of the toner is calculated by employing the following equation 1, after analyzing the 500 toner-particle images, which are randomly sampled from toner particle images magnified 500 hundred times by the scanning type electronic microscope (SEM), by employing the Scanning Image Analyzer (manufactured by Japan Electronic Co. Ltd.).
 
Sphericity=“circumferential length of a circle derived from circle equivalent diameter”/“circumferential length of a projected particle image”  (1)
 
   As for the toner whose sphericity is lower than 0.94, the unevenness of the particles are getting large. Accordingly, such the toner particles are liable to be destructed, and since the toner particles are not uniformly charged in each of the developing devices  13 Y,  13 M,  13 C,  13 K, it is impossible to form a good visual image. On the other hand, as for the toner whose sphericity is greater than 0.98, the cleaning efficiency is getting deteriorated, since the each particle is getting close to the true sphere. 
   In the image forming apparatus embodied in the present embodiment, by employing the developing agent, which includes the small-sized spherical toner manufactured by the aforementioned method and whose shape fulfill the specific condition, it becomes possible not only to improve the image quality of a halftone image area, but also to form a visual image in which the image quality of fine lines and that of dots are improved. 
   The aforementioned toner can be employed for either one component developing agent or two component developing agent. 
   When employed for one component developing agent, the non-magnetized one component developing agent, or the magnetized one component developing agent, in which magnetic particles in a range of 0.1-0.5 μm are included with toner, can be cited as an applicable example. 
   When employed for two component developing agent mixed with carrier, materials, such as an iron, a ferrite, a magnetite, an alloy of these metal and aluminum, an alloy of these metal and lead, etc., can be conventionally and preferably employed as the magnetic carrier particles, and specifically, the ferrite particles are preferable. It is preferable that the mass average particle diameter of the abovementioned magnetic carrier particles is in a range of 15-100 μm, and more preferable, in a range of 25-80 μm. The mass average particle diameter of the carrier particles can be measured by employing the laser diffraction sensor HELOS (manufactured by Sympatec Co. Ltd.) as a representative measuring instrument provided with a wet dispersing unit. 
   In the image forming apparatus, the image forming operations are conducted as follow. 
   In each of the toner image forming units  30 Y,  30 M,  30 C,  30 K, each of the image bearing member  10 Y,  10 M,  10 C,  10 K is driven to rotate. Each of the image bearing members  10 Y,  10 M,  10 C,  10 K is charged at a predetermined polarity, for instance, a negative polarity, by the charging device  11 Y,  11 M,  11 C,  11 K. Next, on an image forming area of the surface of each image bearing member on which a toner image is to be formed, an electronic potential of an irradiated portion (an exposed region) is lowered by an exposing action performed by each of the exposing device  12 Y,  12 M,  12 C,  12 K so as to form an electrostatic latent image corresponding to the original image on each of image bearing members  10 Y,  10 M,  10 C,  10 K. Then, in each of the developing devices  13 Y,  13 M,  13 C,  13 K, the reverse developing operation is performed in such a manner that toner charged at, for instance, a negative polarity, namely, the same as that of the surface potential of each of the image bearing members  10 Y,  10 M,  10 C,  10 K, are attached to the electrostatic latent image formed on each of the image bearing members  10 Y,  10 M,  10 C,  10 K, to form a unicolor toner image corresponding to each of colors Y, M, C, K. 
   Further, each of the unicolor toner images is sequentially transferred onto the primary transferring area on the belt-type transfer member  17  by each of the primary transferring device  14 Y,  14 M,  14 C,  14 K, so that the unicolor toner images of colors Y, M, C, K overlap with each other to form a full color image on the belt-type transfer member  17 . 
   Then, the color toner image, formed on the belt-type transfer member  17 , is transferred onto the transfer material P by applying a transferring voltage, adjusted at an appropriate amplitude by the transfer-voltage supplying device, to the secondary transferring roller  141 S of the secondary transferring device  14 S. Successively, in the fixing process, a fixing device fixes the color toner image onto the transfer material P, to form a full color image. 
   In each of the toner image forming units  30 Y,  30 M,  30 C,  30 K, residual toner, remaining on each of the image bearing members  10 Y,  10 M,  10 C,  10 K after passing through the primary transferring region, are removed by the image bearing member cleaning blade equipped in each of the cleaning devices  18 Y,  18 M,  18 C,  18 K. 
   Toner left on the belt-type transfer member  17  after passing through the secondary transfer region are removed by the cleaning device  18 S. 
   Specifically, in a normal service status, when the secondary transferring device  14 S is made active, the cleaning voltage applying device  29  applies a cleaning voltage to the first brush roller  22 . With this, a cleaning current flows through the current path  33 . Consequently, the first brush roller  22  electrostatically removes toner particles which are charged negatively. The second brush roller  26  electrostatically removes toner particles which are charged positively. 
   The flicker rods  23  and  27  respectively scrape off toner from the first and second brush rollers  22  and  26 . (Toner particles are moved to the flicker rods  23  and  27  by difference in the electric potentials.) Toner particles on the flicker rods  23  and  27  are scraped off the scrapers  24  and  28  into a recovery tray for recovery and recycling. 
   Meanwhile, when the secondary transferring device  14 S is made inactive, the mechanism  36  to control the potential of the opposing roller is switched to earth the opposing roller  17   a  to the ground potential. Consequently, the potential difference between the opposing roller  17   a  and the first brush roller  22  becomes greater. This status is very effective to remove a lot of non-transferred residual toner. In this status, the cleaning voltage applying device  29  applies a cleaning voltage to the first brush roller  22 . With this, a cleaning current flows through the current path  33 . Finally, the first brush roller  22  electrostatically removes a lot of non-transferred residual toner which passed through the secondary transfer region. 
   In accordance with the above image forming apparatus, when the secondary transferring device  14 S is made inactive, the potential difference becomes greater between the opposing roller  17   a  and the first brush roller  22  which receives a cleaning voltage whose polarity is opposite to the developing polarity of the toner. This increases the performance of the cleaning device  18 S to remove toner whose polarity is opposite to the developing polarity of the toner. This can facilitate the cleaning device  18 S to accomplish a preferable cleaning performance. Therefore, the cleaning device  18 S can utilize its preferable cleaning performance also upon the non-transferred toner left on the belt-type transfer member. In other words, non-transferred toner can be completely removed from the belt-type transfer member and images can be free from stains. 
   The present invention is embodied in the above description, but it should be understood that the above-described embodiment is not limited by any of the details of the foregoing description. Variations may be made by one skilled in the art without departing from the spirit and scope of the invention. 
   For example, as far as the potential difference between the first brush roller  22  and the opposing roller  17   a  is set to a desired value when the secondary transferring device  14 S is made inactive, the opposing roller  17   a  need not be earthed to the ground potential when the secondary transferring device  14 S is made inactive. 
   The configuration of the cleaning device is not limited to the configuration of the above embodiment as far as one of the first and second brush rollers electrostatically removes positively-charged toner and the other electrostatically removes negatively-charged toner. 
   For example, the configuration can be modified so that a power supply for the second brush roller may be provided to apply a cleaning voltage whose polarity is opposite to the polarity of toner to the second brush roller. In this configuration, a current flows through the second brush roller  26 , the opposing roller  17   a , and the first brush roller  22  in that order along the current path  33 . The second brush roller  26  removes the negatively-charged toner and the first brush roller  22  removes the positively-charged toner. 
   Second Embodiment 
   The configuration of the second embodiment of the image forming apparatus is the same as that of the first embodiment of the image forming apparatus, except the cleaning device, a configuration of which will be detailed in the following. 
   As shown in  FIG. 6 , the cleaning device  48 S is constituted by an opposing roller  17   a  disposed in such a manner that the opposing roller  17   a  contacts an inner surface of the belt-type transfer member  17 , a conductive brush roller  42  (hereinafter, referred to as a brush roller  42 , for simplicity) contacting outer surface of the belt-type transfer member  17  and serving as a conductive brush member, a flicker rod  43  disposed in a state of contacting the brush roller  42  and serving as a toner recovery roller, a brush roller assembly  41  including a scraper  44  disposed in a state of contacting the flicker rod  43 , and a conductive fixed brush member  46  (hereinafter, referred to as a fixed brush member  46 , for simplicity) contacting outer surface of the belt-type transfer member  17  at a position located upstream from the brush roller  42  in a moving direction of the belt-type transfer member  17  and serving as another conductive brush member. 
   The brush roller  42  is equipped in such a manner that the brush roller  42  press-contacts the opposing roller  17   a  while putting the belt-type transfer member  17  between them. Further, the configuration of the brush roller  42  is the same as that of the first brush roller  22  and the second brush roller  26  employed in the first embodiment, and a penetration amount against the belt-type transfer member  17  is set at 1 mm. 
   The fixed brush member  46  is equipped in such a manner that the fixed brush member  46  press-contacts the opposing roller  17   a  while putting the belt-type transfer member  17  between them. For instance, the fixed brush member  46  has bristles such as conductive nylon bristles densely implanted on the outer surface of the roller body. The brush bristles have a diameter of, for example, 5 to 8 deniers, a length of, for example, 2 to 5 mm, an electric resistance of, for example, 1×10 4  to 1×10 6  Ω, a Young&#39;s modulus of, for example, 4,900 to 9,800 N/mm 2 , and an implantation density (number of bristles per unit area) of, for example, 50 to 200 kilo bristles per square inch. Further, a penetration amount against the belt-type transfer member  17  is set at 1 mm. 
   The configuration of the flicker rod  43  is the same as those of the first flicker rod  23  and the second flicker rod  27  employed in the first embodiment. Further, a penetration amount against the brush roller  42  is set at 1 mm. Still further, the configuration of the scraper  44  is the same as those of the scrapers  24 ,  28  employed in the first embodiment. 
   The brush rollers  42  is made to rotate, for example, at a speed of 100 to 250 mm/sec in a direction opposite to the moving direction of the belt-type transfer member  17  (clockwise in  FIG. 6 ) at a point where the brush rollers  42  contact the belt-type transfer member  17 . The flicker rod  43  is made to rotate in the same direction as the brush roller  42  rotates (counterclockwise in  FIG. 6 ). 
   In the cleaning device  48 S, a current path  53 , through which an electric current serially flows into the brush roller  42 , the opposing roller  17   a  and the fixed brush member  46 , is formed. In addition, there is also provided a power supply  49  for applying a cleaning voltage having a polarity opposite to the developing charge polarity of toner to the brush roller  42  through the flicker rod  43 . Accordingly, when a cleaning voltage is applied to the flicker rod  43 , a cleaning current I flows into the brush roller , the opposing roller  17   a  and the fixed brush member  46  in that order through the current path  53 . 
   By employing the current path  53 , when a developing polarity of toner to be employed is, for instance, a negative polarity, since the fixed brush member  46  applies electric charge onto toner charged at a positive polarity among the residual toner remaining on the belt-type transfer member  17 , the polarity of the toner originally charged at positive can be converted to negative. On the other hand, the brush roller  42  has a capability of totally removing the toner charged at the negative polarity on the belt-type transfer member  17 , namely all of the residual toner on the belt-type transfer member  17 . 
   When a developing polarity of toner to be employed is, for instance, a negative polarity, the amplitude of the cleaning voltage applied to the flicker rod  43  by the power supply  49  for applying the cleaning voltage is in a range of, for instance, +200-+1000 volts. 
   The image forming apparatus is equipped with a mechanism  36  to control the potential of the opposing roller  17   a . The mechanism  36  contains a changeover switch which applies, to the opposing roller  17   a , the same potential as that of the fixed brush member  46  when the switch is made. When the secondary transferring device  14 S becomes inactive, the mechanism  36  is controlled to be active. In other words, when the secondary transferring device  14 S becomes inactive, the control section  31  controls so that the potential difference between the opposing roller  17   a  and the brush roller  42  when the secondary transferring device  14 S becomes inactive may be greater than the potential difference between the opposing roller  17   a  and the first brush roller  42  when the secondary transferring device  14 S is active. 
   In the cleaning device  48 S, for example, the fixed brush member  46  is earthed to the ground potential. When the control mechanism  36  becomes active, the opposing roller  17   a  is also earthed to the ground potential. 
   In the image forming apparatus, the cleaning operation of the residual toner on the belt-type transfer member  17  is performed by the cleaning device  48 S. Concretely speaking, in a normal state, namely, when the secondary transferring device  14 S is active, the power supply  49  for applying a cleaning voltage applies the cleaning voltage onto the brush roller  42  so as to flow the cleaning current into the current path  53 . Then, the polarity of the toner initially charged at a positive polarity is converted to the negative by applying electric charge onto the toner by means of the fixed brush member  46 . Accordingly, all of the toner charged at a negative polarity, namely, all of the residual toner remaining on the belt-type transfer member  17 , are electrostatically removed. 
   Toner particles removed by the brush roller  42  are moved to the flicker rod  43 . Toner particles on the flicker rods  43  are further scraped off by the scraper  44  into a recovery tray (not shown in the drawings) for recovery and recycling. 
   Meanwhile, when the secondary transferring device  14 S is made inactive, the mechanism  36  to control the potential of the opposing roller is switched to earth the opposing roller  17   a  to the ground potential. Consequently, the potential difference between the opposing roller  17   a  and the brush roller  42  becomes greater. This status is very effective to remove a lot of non-transferred residual toner. In this status, the power supply  49  for applying a cleaning voltage applies a cleaning voltage to the first brush roller  42 . With this, a cleaning current flows through the current path  53 . Finally, the brush roller  42  electrostatically removes a lot of non-transferred residual toner which passed through the secondary transfer region, without transferring the non-transferred residual toner onto the transfer material P. 
   In accordance with the above image forming apparatus, when the secondary transferring device  14 S is made inactive, the potential difference becomes greater between the opposing roller  17   a  and the brush roller  42  which receives a cleaning voltage whose polarity is opposite to the developing polarity of the toner. This increases the performance of the cleaning device  18 S to remove toner whose polarity is opposite to the developing polarity of the toner. This can facilitate the cleaning device  48 S to accomplish a preferable cleaning performance. Therefore, the cleaning device  48 S can utilize its preferable cleaning performance also upon the non-transferred toner left on the belt-type transfer member. In other words, non-transferred toner can be completely removed from the belt-type transfer member  17  and images can be free from stains. 
   The present invention is embodied in the above description, but it should be understood that the above-described embodiment is not limited by any of the details of the foregoing description. Variations may be made by one skilled in the art without departing from the spirit and scope of the invention. 
   For example, as far as the potential difference between the brush roller  42  and the opposing roller  17   a  is set to a desired value when the secondary transferring device  14 S is made inactive, the opposing roller  17   a  need not be earthed to the ground potential when the secondary transferring device  14 S is made inactive. 
   The configuration of the cleaning device is not limited to the configuration of the above embodiment as far as the residual toner can be electrostatically removed from the belt-type transfer member  17 , by actions of two conductive brush members. 
   For instance, a configuration of the cleaning device, in which a power supply for applying a cleaning voltage is provided for the brush roller so that the power supply for applying a cleaning voltage applies the cleaning voltage having a polarity opposite to that of the toner, would be also applicable. In this configuration, the cleaning current flows through the current path  53  in order of the fixed brush member  46 , the opposing roller  17   a  and the brush roller  42  so that the fixed brush member  46  applies electric charge onto the toner to convert its polarity into a positive polarity. As a result, the toner charged into a positive polarity, namely, all of the residual toner are completely removed by means of the brush roller  42 . 
   EXAMPLES 
   The following examples are included to confirm the effects of this invention. However, it is to be understood that the invention is not intended to be limited to the specific embodiments. 
   Example 1 
   An image forming apparatus of this invention was produced according to the configuration of  FIG. 1 . This image forming apparatus (variation of “8050” manufactured by Konica Minolta Business Technologies Co., Ltd.) has the following specific configuration. 
   (1) The developer is of a 2-component developing method. 
   (2) The developing agent contains toner of negative charging characteristics. 
   (3) The belt-type transfer member is an endless polyimide semi-conductive resin belt having a surface resistivity of 1×10 11  Ω/cm 2 , a volume resistivity of 1×10 9  Ω.cm, and a peripheral length of 861 mm. The belt is moved at a speed of 220 mm/sec and tensioned at 49N. 
   (4) The cleaning device is equipped with an opposing roller  17   a  which is in contact with the inner surface of the belt-type transfer member, and first and second brush roller systems. The first brush roller system contains a first conductive brush roller which is pressed against the opposing roller with the belt therebetween, a first flicker rod which is in contact with the first brush roller, and a plate-like scraper which is in contact with the first flicker rod. The second brush roller system contains a second brush roller which is pressed against the opposing roller with the belt-type transfer member therebetween in the upstream side of the first brush roller along the movement of the belt-type transfer member, a second flicker rod which is in contact with the second brush roller, and a scraper which is in contact with the second flicker rod, and a cleaning-voltage applying device connected to the first brush roller. The second brush roller is kept at the ground potential and a current path is provided to flow a cleaning current from the cleaning voltage applying device to the first brush roller, the opposing roller, and the second brush roller in that order. The components of the cleaning device are described in detail below. 
   (4-1) 
   The opposing roller  17   a  is a hard roller made of an aluminum core bar and the outer diameter is 30 mm. 
   (4-2) 
   The first and second brush rollers respectively have bristles such as conductive nylon bristles densely implanted on the outer surface of the roller body. The brush bristles have a diameter of 6 deniers, an electric resistance of 1×10 10  Ω, a length of 5 mm, a Young&#39;s modulus of 9,800 N/mm 2 , and an implantation density of 100 kilo bristles per square inch. The rollers are rotated at a speed of 220 mm/sec and the bite quantity of the bristles of the brush rollers to the belt-type transfer member is 1 mm. 
   (4-3) 
   The first and second flicker rods in the cleaning device are respectively made of a stainless-steel rod of 16 mm in outer diameter. The rods are rotated at a speed of 220 mm/sec and the bite quantity of the rollers to the bristles of the brush rollers is 1 mm. 
   (4-4) 
   The scraper in the cleaning device is a stainless-steel plate of 0.05 mm thick. 
   The image forming apparatus, which is described in the foregoing and in which a cleaning voltage of +500 V is applied to the first brush roller in the cleaning device, is employed for the actual image forming test described as follow. Further, an opposing roller potential control mechanism for switching an opposing roller potential of the opposing roller is equipped in the image forming apparatus. The opposing roller potential control mechanism is made to be active at a time when the secondary transferring device is turned into a deactivate state, so as to switch the electric potential of the opposing roller to the ground potential. The following actual image forming test is conducted in the state of activating the opposing roller potential control mechanism. The result of the actual image forming test is listed in Table 1. 
   (Actual Image Forming Test) 
   The surface potential of the organic photoconductive material in each toner image forming unit is made −700 V in the non-exposed area and −100 V in the exposed area. A toner image is formed on the belt-type transfer member and the formed visible image is transferred to a transfer material (by the activated secondary transferring device). A toner patch is made on the belt-type transfer member but not transferred to the transfer material (without activating the secondary transferring device). After the above operations, the belt-type transfer member is cleaned and checked for residual toner particles. 
   Comparative Example 1 
   The configuration of the image forming apparatus of this example is the same as the configuration of the image forming apparatus of Embodiment 1 but the cleaning device of this example is not equipped with the opposing roller potential control mechanism to keep the potential of the opposing roller at a working potential even when the secondary transfer device is made inactive. The actual image forming test of this example is the same as that of Embodiment 1. The result is listed in Table 1. 
   Comparative Example 2 
   The configuration of the image forming apparatus of this example is the same as the configuration of the image forming apparatus of Embodiment 1 but the cleaning device of this example is not equipped with the opposing roller potential control mechanism and the opposing roller is always earthed to the ground potential. The actual image forming test of this example is the same as that of Embodiment 1. The result is listed in Table 1. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Opposing roller 
                 
               Performance to 
             
             
                 
               potential 
               Performance to 
               remove non- 
             
             
                 
               control 
               remove 
               transferred patch 
             
             
                 
               mechanism 
               residual toner 
               toner 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               Embodiment 
               Provided 
               Good 
               Good 
             
             
               1 
             
             
               Comparative 
               Not provided 
               Good 
               Bad 
             
             
               example 1 
             
             
               Comparative 
               Not provided 
               Bad 
               Good 
             
             
               example 2 
             
             
                 
             
          
         
       
     
   
   Example 2 
   A cleaning device, detailed in the following, was employed as the example 2. The actual image forming test was conducted under conditions same as those for the example 1, except that the cleaning voltage of 500 volts was applied to the brush roller in the cleaning device of the example 2. 
   The cleaning device of the example 2 is constituted by an opposing roller disposed in such a manner that the opposing roller contacts the inner surface of the belt-type transfer member, a brush roller pressing the opposing roller while putting the belt-type transfer member between them, a flicker rod disposed in a state of contacting the brush roller, a brush roller assembly including a plate-shaped scraper disposed in a state of contacting the flicker rod, a fixed brush member pressing the opposing roller while putting the belt-type transfer member between them at a position located upstream from the brush roller in a moving direction of the belt-type transfer member, and a cleaning voltage applying power source coupled to the brush roller assembly. Further, the fixed brush member is kept at the ground potential and a current path is provided to flow a cleaning current from the cleaning voltage applying device to the brush roller, the opposing roller, and the fixed brush member in that order. 
   The fixed brush member of the cleaning device has bristles such as conductive nylon bristles densely implanted on the bottom surface of the fixed brush substrate. The brush bristles have a diameter of 6 deniers, an electric resistance of 1×10 5  Ω, a length of 5 mm, a Young&#39;s modulus of 9,800 N/mm 2 , and an implantation density of 100 kilo bristles per square inch. Further, the penetration amount for the belt-type transfer member is set at 1 mm. 
   The structures of the opposing roller, the brush roller, the flicker rod and the scraper are the same as those of the opposing roller, the first brush roller, the first flicker rod and the scraper employed in the example 1, respectively. 
   Comparative Example 3 
   The configuration of the image forming apparatus of this example is the same as the configuration of the image forming apparatus of Example 2 but the cleaning device of this example is not equipped with the opposing roller potential control mechanism to keep the potential of the opposing roller at a working potential even when the secondary transfer device is made inactive. The actual image forming test of this example is the same as that of Example 1. The result is listed in Table 2. 
   Comparative Example 4 
   The configuration of the image forming apparatus of this example is the same as the configuration of the image forming apparatus of Example 1 but the cleaning device of this example is not equipped with the opposing roller potential control mechanism and the opposing roller is always earthed to the ground potential. The actual image forming test of this example is the same as that of Example 2. The result is listed in Table 1. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               Opposing roller 
                 
               Performance to 
             
             
                 
               potential 
               Performance to 
               remove non- 
             
             
                 
               control 
               remove 
               transferred patch 
             
             
                 
               mechanism 
               residual toner 
               toner 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               Embodiment 
               Provided 
               Good 
               Good 
             
             
               2 
             
             
               Comparative 
               Not provided 
               Good 
               Bad 
             
             
               example 3 
             
             
               Comparative 
               Not provided 
               bad 
               Good 
             
             
               example 4 
             
             
                 
             
          
         
       
     
   
   As is evident from the results of the table 1 and the table 2, in the image forming apparatuses of Example 1 and Example 2 in both of which the potential of the opposing roller is earthed to the ground potential when the secondary transferring device is made inactive, the belt-type transfer member is completely cleaned without no residual toner after cleaning off toner particles left on the belt-type transfer member while the secondary transferring device is made active and after cleaning off a lot of non-transferred toner particles left on the belt-type transfer member while the secondary transferring device is made inactive. 
   Contrarily, in the image forming apparatuses of Comparative example 1 and Comparative example 3 in both of which the potential of the opposing roller is kept at a working potential (the potential when the secondary transferring device is active), the belt-type transfer member is clean without no residual toner after cleaning the normal residual toner particles, but has some residual toner particles after cleaning non-transferred toner particles. In other words, the image forming apparatuses of Comparative example 1 and Comparative example 3 have a cleaning problem on the belt-type transfer member. 
   Further, in the image forming apparatuses of Comparative example 2 and Comparative example 4 in both of which the opposing roller is always earthed to the ground potential, the belt-type transfer member is preferably cleaned without any toner particle thereon after removal of non-transferred toner particles but has some toner particles thereon after removal of residual toner particles, resulting in an occurrence of the cleaning defect of the belt-type transfer member. 
   In accordance with the image forming apparatus which is the first embodiment of this invention, toner particles left on the belt-type transfer member is basically cleaned off by either of two conductive brush rollers in the cleaning device independently of their charge polarities. Further, also when the secondary transferring device is made inactive, the cleaning device can clean off non-transferred toner effectively. Therefore, the image forming apparatus of this configuration can always assure complete clean-off of non-transferred toner from the belt-type transfer member and offer stain-less images. 
   Next will be described a cleaning device  206 A which is a third embodiment (namely, Embodiment 3) of this invention. The configuration of Embodiment 3 is the same as the configuration shown in  FIG. 1  but the cleaning device  206 A is used instead of the cleaning device cleaning device  18 S. Therefore, the explanation of the image forming apparatus excluding the cleaning device  206 A is omitted here. 
   The second cleaning device  206 A will be described below referring to  FIG. 3 .  FIG. 3  shows a schematic diagram of the second cleaning device  206 A and the electric configuration thereof. The first brush roll  261  has a brush  261 A (approx. 16 mm in outer diameter and resistivity of 1×10 10  to 1×10 11  Ω) of conductive acrylic fibers which is 6 deniers in diameter on the core bar. The first brush roll  261  is connected to a power supply  231  via a polarity changeover switch  231   a  that turns a ground point of the power supply  231  to either a contact point A or a contact point B so as to change a polarity (either positive or negative polarity) of the output voltage of the power supply  231 . The first brush roll  261  is in contact with the surface of the intermediate transfer member  17  by an overlap of 1 mm (a difference of the radius of the brush roll minus the distance between the center of rotation of the brush roll and the surface of the belt at which the brush roll touches the belt) and rotates at a speed of 300 revolutions per minute along the movement of the intermediate transfer member  17 . An aluminum roller  17   a  whose surface is conductive and earthed to the ground is provided oppositely to the first brush roll  261  with the intermediate transfer member  17  therebetween. A stainless-steel flicker rod  263  of 16 mm in diameter is provided in contact with the brush  261 A of the first brush roll  261  with an overlap (bite quantity) of 1 mm and rotates at a speed of 300 revolutions per minute in a direction opposite to the direction of the first brush roll  261  to remove toner from the brush  261 A. A stainless-steel scraper  265  of 0.05 mm thick is applied to touch the surface of the flicker rod  263  against the rotation of the flicker rod  263  to scrape off toner from the surface of the flicker rod  263  into a toner recovery section  267 . The second brush roll  262  has a brush  262 A (approx. 16 mm in outer diameter and resistivity of 1×10 10  to 1×10 11  Ω) of conductive acrylic fibers which is 6 deniers in diameter on the core bar. The second brush roll  262  is connected to a power supply  232  to apply a voltage of positive polarity. The second brush roll  262  is located in the downstream side of the first brush roll  261  along the movement of the intermediate transfer member  17 . The second brush roll  262  is in contact with the surface of the intermediate transfer member  17  by an overlap of 1 mm and rotates at a speed of 300 revolutions per minute in the direction opposite to the rotational direction of the first brush roll  261 . The second brush roll  262  is also pressed against the roller  17   a  with the intermediate transfer member  17  therebetween. A stainless-steel flicker rod  264  of 16 mm in diameter is provided in contact with the brush  262 A of the second brush roll  262  with an overlap (bite quantity) of 1 mm and rotates at a speed of 300 revolutions per minute in a direction opposite to the direction of the second brush roll  262  to remove toner from the brush  262 A. A stainless-steel scraper  266  of 0.05 mm thick is applied to touch the surface of the flicker rod  264  against the rotation of the flicker rod  264  to scrape off toner from the surface of the flicker rod  264  into a toner recovery section  267 . A power supply  233  is provided to apply a voltage to the first transfer rollers  141 Y and  141 K. The control device  230  containing a CPU (central processing unit, not shown in  FIG.3  ), work memory, and other parts works to read a program into the work memory and collectively control respective components of the image forming apparatus  100  of  FIG. 1  according to the program. The control device  230  controls not only the operation of the power supplies  231 ,  232 , and  233  but also the execution of the first mode to form images according to the normal image data and the second mode to form patch images according to patch image data. 
   In the first mode, the second cleaning device  206 A applies a voltage of negative polarity to the first brush roller  261  and a voltage of positive polarity to the second brush roller  262 . In normal printing, this first mode is used to remove residual toner from the intermediate transfer member  17 . This is because some of residual toner particles are charged positively and others are charged negatively and voltages of different polarities must be applied to the first and second brush rollers  261  and  262 . However, a single cleaning process in the first mode is not enough to remove a patch image from the intermediate transfer member  17  because the patch image uses a lot of toner. To remove a lot of patch image toner, the second mode uses that the polarity of toner before the secondary transferring is equal to the polarity given by the developing devices  13 Y,  13 M,  13 C ad  13 K and steady, switches the polarity of the voltage from positive to negative by the control device  230 , applies the voltage of the selected polarity to the first brush roller  261 , applies a voltage of positive polarity to the second brush roller  262 . In other words, the first and second brush rollers  261  and  262  respectively have polarities opposite to those of the residual toner particles. With this, lots of residual toner particles are removed. 
   In this case, when the polarity of the first brush roller  261  is switched from negative to positive, the positively-charged toner particles are flicked towards the intermediate transfer member  17  because the first brush roller  261  and the toner particles have the same polarity. Further, since the second brush roller  262  is also charged positively, toner particles move towards the toner image forming unit  30 Y through the bristles of the second brush roller  262 . 
   When a color image is formed, a voltage of positive polarity is applied to the primary transfer roller  141 Y. The primary transfer roller  141 Y is pressed against the image bearing member  10 Y to transfer toner back to the image bearing member  10 Y. In the present embodiment, the primary transfer roller  141 Y to be employed is disposed at a position nearer to the second brush roller  262  and downstream in the moving direction of the intermediate transfer member, so as to effectively conduct the cleaning operation. Then, the image bearing member  10 Y is cleaned by the image bearing member cleaning device  18 Y. When a black image is formed, a voltage of positive polarity is applied to the primary transfer roller  141 K to transfer toner back to the image bearing member  10 K. The image bearing member  10 K is cleaned by the image bearing member cleaning device  18 K. 
   The cleaning device in the second mode in accordance with Embodiment 2 can completely clean off toner particles from the intermediate transfer member regardless of whether the toner particles are normal residual toner particles or non-transferred patch image toner particles by applying a voltage of positive polarity to the first brush roller  261  and a voltage of positive polarity to the second brush roller  262 , applying a voltage of positive polarity to the primary transfer roller  141 Y, transferring toner particles (which are flicked from the first brush roller  261  to the intermediate transfer member  17 ) back to the image bearing member  10 Y, and cleaning off positively-charged toner particles which are back-transferred by the cleaning device  18 Y. 
   Referring to  FIG. 4  and  FIG. 5 , will be described a cleaning method of the image forming apparatus of this embodiment.  FIG. 4  shows a flow chart of a process executed by a color image forming apparatus to clean the intermediate transfer member  17 . The flow chart of  FIG. 4  assumes that the color image forming apparatus is powered on and the control device  230  is set to automatically start the first or second mode. When the first mode is selected (YES at step S 01 ), steps S 02  and S 03  follow. Step S 02  applies a voltage of negative polarity selected by the control device  230  from the power supply  231  to the first brush roller  261  and a voltage of positive polarity from the power supply  232  to the second brush roller  262 . Step S 03  rotates the first and second brush rollers  261  and  262  by a motor (not shown in drawings) and the flicker rods  263  and  264  by a motor (not shown in drawings) simultaneously. When the first mode is not selected (NO at step S 01 ), steps S 04  and later follow. Step S 04  selects the second mode. Step S 05  applies a voltage of positive polarity selected by the control device  230  from the power supply  231  to the first brush roller  261  and a voltage of positive polarity from the power supply  232  to the second brush roller  262 . Step S 06  rotates the first and second brush rollers  261  and  262  by a motor (not shown in drawings) and the flicker rods  263  and  264  by a motor (not shown in drawings) simultaneously. Step S 07  applies a voltage of a selected polarity to the primary transfer roller  141 Y from the power supply  233  to make toner particles flicked by the first brush roller  261  and toner particles passing through bristles of the first brush roller  262 . Step S 08  rotates the image bearing member  10 Y, presses the primary transfer roller  141 Y against the image bearing member  10 Y to transfer toner particles from the intermediate transfer member  17  back to the image bearing member  10 Y, and actuates the cleaning device  18 Y to clean the image bearing member  10 Y. At Step S 07 , in this case, when a black image is formed, it is possible to apply a voltage of a selected polarity to the primary transfer roller  141 Y, transfer toner particles back to the image bearing member  10 K which is always rotating in contact therewith, and clean the image bearing member  10 K by the cleaning device  18 K. 
   The cleaning device in the second mode in accordance with this embodiment can completely clean off toner particles from the intermediate transfer member regardless of whether the toner particles are normal residual toner particles or non-transferred patch image toner particles by applying a voltage of positive polarity to the first brush roller  261  and a voltage of positive polarity to the second brush roller  262 , applying a voltage of positive polarity to the primary transfer roller  141 Y, transferring toner particles (which are flicked from the first brush roller  261  to the intermediate transfer member  17 ) back to the image bearing member  10 Y, and cleaning the image bearing member  10 Y by the image bearing member cleaning device  18 Y.  FIG. 5  shows a flow chart of another cleaning process in accordance with this invention. Similar as in  FIG. 4 , the flow chart of  FIG. 5  assumes that the color image forming apparatus is powered on and the control device is set to automatically start the first or second mode. 
   Step S 09  checks whether the first mode is selected. When the first mode is selected (YES at step S 09 ), steps S 10  and S 11  follow. Step S 10  applies a voltage of negative polarity selected by the control device  230  from the power supply  231  to the first brush roller  261  and a voltage of positive polarity from the power supply  232  to the second brush roller  262 . Step S 11  rotates the first and second brush rollers  261  and  262  by a motor (not shown in drawings) and the flicker rods  263  and  264  by a motor (not shown in drawings) simultaneously. 
   When the first mode is not selected (NO at step S 12 ), steps S 13  and later follow. Step S 12  selects the second mode. Step S 13  applies a voltage of positive polarity selected by the control device  230  from the power supply  231  to the first brush roller  261  and a voltage of positive polarity from the power supply  232  to the second brush roller  262 . Step S 14  rotates the first and second brush rollers  261  and  262  by a motor (not shown in drawings) and the flicker rods  263  and  264  by a motor (not shown in drawings) simultaneously. Step S 15  conveys toner particles flicked by the first brush roller  261  and toner particles passing through bristles of the first brush roller  262  from the intermediate transfer member  17 . Before the toner particles reach the first and second brush rollers  261  and  262 , the control device  230  checks a timing whether switching to the first mode is required. When switching to the first mode is required (YES at Step S 15 ), Step  16  selects the first mode and Step S 17  applies a voltage of negative polarity to the first brush roller  261  from the power supply  231  and a voltage of positive polarity to the second brush roller  262  from the power supply  232 . to the first brush roller  261  and a voltage of positive polarity from the power supply  232  to the second brush roller  262 . Step S 14  rotates the first and second brush rollers  261  and  262  by a motor (not shown in drawings) and the flicker rods  263  and  264  by a motor (not shown in drawings) simultaneously. Step S 15  conveys toner particles flicked by the first brush roller  261  and toner particles passing through bristles of the first brush roller  262  from the intermediate transfer member  17 . Before the toner particles reach the first and second brush rollers  261  and  262 , the control device  230  checks a timing whether switching to the first mode is required. When switching to the first mode is required (YES at Step S 15 ), Step  6  selects the first mode and Step S 17  applies a voltage of negative polarity to the first brush roller  261  from the power supply  231  and a voltage of positive polarity to the second brush roller  262  from the power supply  232 . 
   Next, toner particles on the intermediate transfer member  17  are cleaned off by the cleaning device  206 A. When determined that the timing is not a timing for switching to the first mode (NO at Step S 15 ), control is returned to Step S 15  and the control device checks the timing whether mode switching is required again. 
   According to the present embodiment mentioned in the above, at first, the cleaning operation in the second mode is conducted, and then, the cleaning operation in the first mode is conducted successively. Accordingly, even for the patch image or the residual toner image at the time of jamming, in which a relatively large amount of toner is remained as the residual toner, it becomes possible to surly conduct the cleaning operation. 
   While the preferred embodiments of the present invention have been described using specific term, such description is for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit and scope of the appended claims.