Patent Publication Number: US-2017351200-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile, and a multi-functional printer having multiple functions of these apparatuses. 
     Description of the Related Art 
     Conventionally, there is a known image forming apparatus with an intermediate transfer system in which a toner image formed on a photosensitive drum is primarily transferred to an intermediate transfer belt as an image bearing member and the toner image is then secondarily transferred from the intermediate transfer belt to a recording material. A transfer roller, i.e., a secondary transfer outer roller, abutting against an outer peripheral surface of the intermediate transfer belt is disposed at a secondary transfer portion in which the toner image is secondarily transferred onto the recording material, and the secondary transfer is performed by applying a transfer voltage to the transfer roller. 
     The transfer roller includes an elastic layer provided on an outer circumferential surface of a conductive shaft portion, and conductive agent such as an ion conductive agent is dispersed in the elastic layer to impart conductivity. Therefore, when an application time of a voltage to the transfer roller is increased by use, ions in the ion conductive agent are polarized so as to be biased to one side of a roller surface side (outside) and a shaft portion side (inside), and electric resistance tends to increase. Japanese Patent Laid-Open No. 2005-316200 has proposed such a configuration to suppress an increase in the electric resistance caused by polarization that a voltage is applied from a power feed roller, which serves as a rotary power feed member and disposed in contact with the surface of the transfer roller, to the transfer roller, and a toner image is transferred from an intermediate transfer belt to a recording material. 
     However, in the case of the configuration of the above-described document, if the toner adheres to the transfer roller from the intermediate transfer belt, the toner may also adhere to the power feed roller in contact with the transfer roller. When the toner adheres to the power feed roller, there is a possibility that unevenness of a current flowing from the power feed roller to the transfer roller occurs. In addition, there is a case where the toner adhered to the power feed roller re-adheres to the transfer roller to contaminate the recording material. 
     Therefore, it is conceivable to electrostatically clean the toner adhered to the power feed roller by applying a voltage of the same polarity as the toner to the power feed roller. In this case, however, since the toner is transferred from the power feed roller to the transfer roller, the transfer roller should also be cleaned to suppress re-adhesion of the toner to the recording material. As a result, when electrostatically cleaning the power feed roller, a cleaning time may be longer than when simply cleaning the transfer roller. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an image forming apparatus includes an image bearing member provided movable and configured to bear a toner image, a transfer roller including a conductive shaft portion and an outer portion which is formed on an outer circumference of the shaft portion and which contains conductive agent, the transfer roller forming a transfer portion in which the toner image borne on the image bearing member is transferred to a recording material, a rotary power feed member disposed in contact with the transfer roller and configured to feed a current to the transfer roller, a power supply configured to apply voltage to the rotary power feed member, a cleaning member disposed in contact with the rotary power feed member and configured to clean a surface of the rotary power feed member along with a rotation of the rotary power feed member, and an execution unit configured to execute a cleaning mode in a period other than a period during which the toner image of the image bearing member is transferred to the recording material in the transfer portion, the power supply applying only a cleaning voltage to the rotary power feed member while the image bearing member is moved and the transfer roller and the rotary power feed member are rotated in the cleaning mode, a polarity of the cleaning voltage being the same polarity as a normal charge polarity of toner. 
     According to another aspect of the present invention, an image forming apparatus includes an image bearing member provided movable and configured to bear a toner image, a transfer roller including a conductive shaft portion and an outer portion which is formed on an outer circumference of the shaft portion and which contains conductive agent, the transfer roller forming a transfer portion in which the toner image borne on the image bearing member is transferred to a recording material, a rotary power feed member disposed in contact with the transfer roller and configured to feed a current to the transfer roller, a power supply configured to apply voltage to the rotary power feed member, a cleaning member disposed in contact with the rotary power feed member and configured to clean a surface of the rotary power feed member along with a rotation of the rotary power feed member, and an execution unit configured to execute a cleaning mode in a period other than a period during which the toner image of the image bearing member is transferred to the recording material in the transfer portion, the power supply applying only a cleaning voltage to the rotary power feed member while the image bearing member is moved and the transfer roller and the rotary power feed member are rotated in the cleaning mode, a polarity of the cleaning voltage being an opposite polarity to a normal charge polarity of toner. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a configuration of an image forming apparatus according to a first embodiment. 
         FIG. 2  is a schematic diagram illustrating a cleaning configuration of a power feed roller according to the first embodiment. 
         FIG. 3  is a control block diagram of a secondary transfer bias according to the first embodiment. 
         FIG. 4A  is a schematic diagram illustrating a configuration of a secondary transfer portion according to a comparative example in a state upon a start of cleaning. 
         FIG. 4B  illustrates the comparative example in a state after a half rotation of a secondary transfer outer roller. 
         FIG. 4C  illustrates the comparative example in a state after one rotation of the secondary transfer outer roller 
         FIG. 4D  illustrates the comparative example in a state after one rotation of a power feed roller in addition to one rotation of the secondary transfer outer roller. 
         FIG. 5  is a diagram illustrating a relationship between a cleaning voltage and an application time of the cleaning voltage in the first embodiment and the comparative example. 
         FIG. 6  is a diagram illustrating a distribution of toner adhered to a secondary transfer outer roller and the power feed roller. 
         FIG. 7  is a schematic diagram illustrating a cleaning configuration of a power feed roller according to a second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Now, embodiments of the present disclosure will be described with reference to the attached drawings. 
     First Embodiment 
     A first embodiment will be described with reference to  FIGS. 1 to 6 . First, a schematic configuration of an image forming apparatus of the present embodiment will be described with reference to  FIG. 1 . Image Forming Apparatus 
     An image forming apparatus  100  is a so-called tandem type intermediate transfer full color printer in which a plurality of image forming portions  10   a,    10   b,    10   c , and  10   d  are arranged along a direction of rotation (direction of movement) of an intermediate transfer belt  56 . Such an image forming apparatus  100  forms a full color image on a recording material S such as a sheet (paper, OHP sheet, or the like) using an electrophotographic method in accordance with an image signal transmitted from an external device such as a personal computer, an image signal from a document reading device, or the like. 
     The image forming portions  10   a,    10   b,    10   c,  and  10   d  respectively have the photosensitive drums  50   a,    50   b,    50   c , and  50   d  that rotate in directions of arrows of  FIG. 1 . Surfaces of the photosensitive drums  50   a,    50   b,    50   c,  and  50   d  are respectively charged by charging rollers  51   a,    51   b,    51   c , and  51   d.  Electrostatic latent images are formed on the charged photosensitive drums  50   a,    50   b,    50   c,  and  50   d  by exposing units  52   a,    52   b,    52   c,  and  52   d.  The electrostatic latent images on the photosensitive drums  50   a,    50   b,    50   c , and  50   d  are respectively visualized as toner images by developing units  53   a,    53   b,    53   c,  and  53   d  containing toners of respective color components. In the present embodiment, the developing units  53   a,    53   b,    53   c,  and  53   d  respectively use a two-component developer including nonmagnetic toner and carrier having magnetism, and a normal charge polarity of the toner is negative. It is noted that it is also possible to use a one-component developer. 
     Primary transfer rollers  54   a,    54   b,    54   c,  and  54   d  are disposed at positions facing the photosensitive drums  50   a,    50   b,    50   c,  and  50   d  with the intermediate transfer belt is interposed therebetween, and respectively configure primary transfer portions T 1   a , T 1   b , T 1   c , and T 1   d . A primary transfer bias is applied to the primary transfer rollers  54   a,    54   b,    54   c,  and  54   d  and thereby the toner images of respective colors formed on the photosensitive drums  50   a ,  50   b,    50   c,  and  50   d  are successively superimposed, and are primarily transferred on the intermediate transfer belt  56 . The toner remaining on the photosensitive drums  50   a,    50   b ,  50   c,  and  50   d  after the primary transfer is removed by drum cleaning units  55   a,    55   b,    55   c,  and  55   d.  The image forming portions  10   a,    10   b,    10   c,  and  10   d  are disposed in the order of yellow (Y), magenta (M), cyan (C), and black (K) from upstream to downstream in the rotation direction of the intermediate transfer belt  56 . 
     Meanwhile, the recording material S accommodated in a recording material storage cassette (not illustrated) is conveyed from a supply roller  66  to a secondary transfer portion T 2  in accordance with a timing of a toner image forming. Then, a secondary transfer bias is applied to the secondary transfer portion T 2  and thereby the toner images having been superimposed and transferred on the intermediate transfer belt  56  are collectively transferred (secondarily transferred) in the secondary transfer portion T 2 . Detailed configurations of the secondary transfer portion T 2  will be described later. The toner or paper dust remaining on the intermediate transfer belt  56  without being transferred to the secondary transfer portion T 2  is removed by a belt cleaning unit  65 . 
     The belt cleaning unit  65 , which serves as an image-bearing-member cleaning member, is disposed at a position downstream of the secondary transfer portion T 2  and upstream of the most upstream primary transfer portion T 1   a , i.e., that of the image forming portion  10   a,  with respect to the direction of rotation of the intermediate transfer belt, so as to face a tension roller  63 . The belt cleaning unit includes a blade abutting against the intermediate transfer belt  56  at this position and the surface of the intermediate transfer belt  56  is cleaned as the intermediate transfer belt moves. 
     Next, the recording material S is conveyed to a fixing unit (not illustrated). The toner on the recording material S is melted and mixed by being heated and pressed in the fixing unit and is fixed to the recording material S as a full color image. Thereafter, the recording material S is discharged out of the apparatus. In this manner, a sequential image forming process is completed. An operation of each unit is controlled by a control unit  110 , which serves as an execution unit. Intermediate Transfer Belt 
     The intermediate transfer belt  56 , which is an example of an image bearing member, is an endless belt of film and rotates (moves) while carrying the toner image primarily transferred from each of the photosensitive drums  50   a,    50   b,    50   c,  and  50   d.  Such an intermediate transfer belt  56  is made of a resin such as polyimide or polyamide or an alloy thereof, or one which is obtained by containing an antistatic agent such as carbon black in various rubbers or the like in an appropriate amount. Therefore, it is formed to have a surface resistivity of 1×109 to 5×1013 ohms per square and a thickness thereof is, for example, approximately 0.04 to 0.5 mm. 
     The intermediate transfer belt  56  is stretched by idler rollers  60 ,  61 , and  67 , the tension roller  63 , and a secondary transfer inner roller  62 . The tension roller  63  applies a tension of, for example, approximately 3 to 12 kilogram-forces (about 29 to 118 N) to the intermediate transfer belt  56 . The secondary transfer inner roller  62  is rotationally driven by a motor (not illustrated) to rotate the intermediate transfer belt  56  at a predetermined speed. 
     Primary Transfer Roller 
     The primary transfer rollers  54   a,    54   b,    54   c,  and  54   d  are provided inside the intermediate transfer belt  56 , and the material thereof is formed of a metal roller such as sulfur and sulfur compound free cutting steel (SUM) or stainless steel (SUS). A voltage (primary transfer bias) of an opposite polarity to a normal charge polarity of the toner is applied to the primary transfer rollers  54   a,    54   b ,  54   c,  and  54   d.  Therefore, a primary transfer contrast, which is a potential difference between surface potentials of the photosensitive drums  50   a,    50   b,    50   c,  and  50   d,  and potentials of the primary transfer rollers  54   a,    54   b,    54   c , and  54   d,  is formed. A predetermined primary transfer contrast is formed in each of the primary transfer portions T 1   a  to T 1   d . Therefore, the toner images of the photosensitive drums  50   a  to  50   d  are electrostatically attracted to the intermediate transfer belt  56  respectively and sequentially, and the toner images superimposed on the intermediate transfer belt  56  are formed. Moreover, the primary transfer rollers  54   a,    54   b,    54   c,  and  54   d  have a straight shape in a thrust direction and a roller diameter is approximately 6 to 10 mm. 
     Secondary Transfer Portion 
     The secondary transfer portion T 2  is formed by a secondary transfer outer roller  64 , which is an example of a transfer roller, abutting against a toner image bearing surface (outer circumferential surface) of the intermediate transfer belt  56 . More specifically, the secondary transfer inner roller  62  is disposed so as to nip the intermediate transfer belt  56  with the secondary transfer outer roller  64 , and the intermediate transfer belt  56  and the secondary transfer outer roller  64  form a nip portion at which the recording material is nipped. Therefore, the toner image is transferred from the intermediate transfer belt  56  onto the recording material passing through the nip portion. 
     In addition, a current is supplied from a power feed roller  68 , which serves as a rotary power feed member, to the secondary transfer outer roller  64  to transfer the toner image from the intermediate transfer belt  56  to the recording material. That is, the power feed roller  68  is configured to rotate while being in contact with the secondary transfer outer roller  64  at a position different from the secondary transfer portion T 2 , and is capable of feeding the current to the secondary transfer outer roller so as to transfer the toner image at the secondary transfer portion T 2 . The power feed roller  68 A is configured to be applied a voltage from a high voltage power supply  120 , i.e., a power supply. 
     Here, the secondary transfer inner roller  62  is formed by providing ethylene/propylene/diene rubber (EPDM) around a metal core. The secondary transfer inner roller is formed so that a roller diameter is 20 mm and a rubber thickness is 0.5 mm, and the hardness is set to, for example, 70° in Asker C Scale. 
     On the other hand, the secondary transfer outer roller  64  has a metal core  64   a  serving as a conductive shaft portion and an elastic layer  64   b  serving as an outer portion. The elastic layer  64   b  includes conductive agent and is formed on an outer circumference of the metal core  64   a.  The secondary transfer outer roller  64  is configured by providing the elastic layer  64   b,  which is made of nitrile rubber (NBR) or EPDM containing conductive agent such as a metal complex or carbon, around the metal core  64   a.  The secondary transfer outer roller  64  is formed so that the roller diameter is 24 mm and the thickness of the elastic layer  64   b,  i.e., sponge layer, is 6 mm. 
     The power feed roller  68  is positioned in contact with an opposite side to the secondary transfer outer roller  64  with respect to the secondary transfer inner roller  62 . More specifically, the power feed roller  68  is positioned to abut against the secondary transfer outer roller  64  at a position that is shifted by approximately 180° in the direction of rotation of the secondary transfer outer roller  64  with respect to an abutting position of the secondary transfer outer roller  64  against the intermediate transfer belt  56 . The position of the power feed roller  68  may be any other position as long as the position is different from the abutting position of the secondary transfer outer roller  64  and the intermediate transfer belt  56 . 
     The both ends of the power feed roller  68  in a rotation axial direction are pressed by springs (not illustrated) so that the power feed roller  68  abuts against the secondary transfer outer roller  64 . In the present embodiment, a spring pressure for the power feed roller  68  is set to a total pressure of 500 gram-force (about 4.9 N). The high voltage power supply  120  supplies power to form an electric field used for the secondary transfer or various controls to the secondary transfer portion T 2 . In the present embodiment, a constant voltage power supply is used as the high voltage power supply  120 . 
     In the image forming operation, the secondary transfer outer roller  64  rotates following the traveling of the intermediate transfer belt  56 . Furthermore, the power feed roller  68  follows the rotational drive of the secondary transfer outer roller  64 . After various controls are performed and when the recording material P is sent from the supply roller  66  to the secondary transfer portion T 2 , a secondary transfer bias having an opposite polarity to a normal charge polarity of the toner is applied to the power feed roller  68 , in order to secondarily transfer the toner image formed on the intermediate transfer belt  56  onto the recording material P. In the present embodiment, since the toner has a negative charge polarity, a positive bias is applied as the secondary transfer bias. 
     Power Feed Roller 
     Here, the power feed roller  68  will be described in detail. The power feed roller  68  has a configuration in which a metal roller of which an exemplary material is SUM or SUS, i.e., stainless steel, is coated with a conductive resin containing a conductive substance. A diameter of the metal roller is approximately 4 to 15 mm and a thickness of the conductive resin is 1 to 200 μm. If the diameter of the metal roller is smaller than that, deflection occurs when pressurized, which may cause that a voltage is not uniformly applied in a longitudinal direction (rotation axial direction), or that cracking or peeling of the conductive resin occurs. On the other hand, if the diameter of the metal roller is greater than that, the material cost increases, and the size and weight of the power feed roller  68  will increase. Therefore, it is preferable that the diameter of the metal roller is within the range described above. 
     Examples of the conductive substance contained in the conductive resin includes carbon black and carbon fiber. As a method of forming the conductive resin, the conductive substance described above is dissolved and dispersed in a suitable organic solvent to obtain a coating solution for a surface layer. Then, the coating solution is applied to the outer periphery of the metal roller by a method such as ring coating, dip coating, or spray coating, and drying is performed for the purpose of removing the organic solvent. Drying in an environment of approximately 30 to 60 degrees Celsius is desirable so as not to induce a radical reaction. Thereafter, ultraviolet ray curing is performed using an ultraviolet irradiation machine to obtain the power feed roller  68  described above. In the present embodiment, the metal roller of SUS having a diameter of 8 mm is coated with the conductive resin of 10 μm by dip coating. As the conductive resin, a material obtained by adding perfluoropolyether and zinc antimonate to acrylic resin is used. 
     Cleaning Configuration of Power Feed Roller 
     In the present embodiment, a roller cleaning unit  70  for cleaning the surface of the power feed roller  68  is provided. The roller cleaning unit  70  is of a so-called blade cleaning system using an elastic blade  71  as illustrated in  FIG. 2 . More specifically, the roller cleaning unit  70  includes the elastic blade  71  as a cleaning member, a support member  72 , a collecting container  73 , and a collecting sheet  74 . 
     The elastic blade  71  is not particularly limited as long as it is an elastic body, and for example, fluororubber, EPDM or the like is used. In the present embodiment, the elastic blade  71  is made of polyurethane rubber, and abuts against the surface of the power feed roller  68  at a position different from the abutting position of the surface of the power feed roller  68  against the secondary transfer outer roller  64 , in a so-called counter direction, with a contact angle of 130°. In  FIG. 2 , the power feed roller  68  rotates in a direction of an arrow and the elastic blade  71  extends so as to confront the direction of rotation. That is, at least a distal end portion of the elastic blade  71  abuts against the surface of the power feed roller  68 , and at least a part of the abutting surface of the elastic blade  71  facing the surface of the power feed roller  68  is gradually separated from the surface of the power feed roller  68  along a downstream direction in the direction of rotation of the power feed roller  68 . 
     In addition, the distal end portion of the elastic blade  71  abutting against the power feed roller  68  is extended along the rotation axial direction of the power feed roller  68 , and a contact width of the elastic blade  71  in the longitudinal direction is equal to or greater than a width of the maximum image in the rotation axial direction capable of being formed on each photosensitive drum. In addition, an abutting pressure of the elastic blade  71  against the power feed roller  68  is regulated by setting spring pressure of the springs (not illustrated) to  200  gram-force (about 1.96 N), which are disposed at both end portions of the roller cleaning unit  70  in the longitudinal direction (rotation axial direction of the power feed roller  68 ). 
     A base end portion of the support member  72  is fixed to the collecting container  73  and the elastic blade is supported on the distal end portion thereof. Material of the support member  72  is not particularly limited, and an example thereof includes a rigid metal, an elastic metal, plastic, ceramic, or the like. For example, the support member  72  may be made of an untreated steel sheet, a steel sheet subjected to a surface treatment such as zinc phosphate treatment, or chromate treatment, or the like. In assembling the elastic blade  71 , it is preferable to treat the support member  72  with a solvent to perform degreasing. 
     The collecting container  73  recovers the toner removed from the power feed roller  68  by the elastic blade along with the rotation of the power feed roller  68 . Accordingly, a portion of the collecting container  73  facing the power feed roller  68  is opened, and the elastic blade  71  is supported on an upper portion of the opening portion via the support member  72 . The collecting sheet  74  is supported at a lower portion of the opening portion of the collecting container  73 , and the distal end portion thereof abuts against the power feed roller  68 . With this configuration, the collecting sheet  74  receives the toner removed by the elastic blade  71  so as not to fall outside, and collects the toner in the collecting container  73 . High Voltage Control of Secondary Transfer Portion 
     Next, an outline of a high voltage control of the secondary transfer portion will be described with reference to  FIG. 3 . The control unit  110  as control means, i.e., execution unit, is provided with a Central Processing Unit (CPU)  111  which performs the high voltage control. Furthermore, a Read Only Memory (ROM)  112   a  is provided in a memory  112 . A program corresponding to a control procedure or the like is stored in the ROM  112   a.  The CPU  111  is adapted to control each portion while reading the program. In addition, the memory  112  also has a Random Access Memory (RAM)  112   b  in which operation data or input data is stored. The CPU  111  performs control with reference to data stored in the RAM  112   b  based on the program described above or the like. 
     In addition, the control unit  110  is connected to an environment detection sensor  113  which detects an environment condition such as a temperature or humidity inside the apparatus. The memory  112  stores a high voltage output table  114  that is a relationship between the temperature and humidity, and a voltage to be applied to the power feed roller  68 . The high voltage power supply  120  is controlled by the CPU  111  with reference to the high voltage output table  114 , and applies the secondary transfer bias or a transfer cleaning bias, which is described below, to the power feed roller  68 . 
     Secondary Transfer Cleaning Mode 
     The present embodiment is configured to execute a secondary transfer cleaning mode as an example of a cleaning mod. The secondary transfer cleaning mode, in which the transfer cleaning bias is applied to the power feed roller  68 , is executed at a timing when the toner image is not transferred to the recording material at the secondary transfer portion T 2 . The timing of executing such a secondary transfer cleaning mode is after the completion of the jam process or after the execution of the adjustment mode such as toner concentration adjustment and toner image positional deviation adjustment. The jam process is, for example, a process for removing a jammed recording material in a case where a jam occurs in which the recording material is jammed in one of conveyance paths within the image forming apparatus during the image forming operation. In this case, there is a possibility that a jam occurs in a state where the toner image is placed on the intermediate transfer belt  56 . After the jam process, a large amount of toner on the intermediate transfer belt  56  may adhere to the secondary transfer outer roller  64 . 
     In addition, the present embodiment is, as illustrated in  FIG. 1 , provided with a detection sensor  57  as detection member configured to detect the toner image on the intermediate transfer belt  56 , i.e., on the image bearing member. The detection sensor  57  is disposed at a position upstream of the secondary transfer portion T 2  and downstream of image forming portions so as to face the surface of the intermediate transfer belt  56 . In the adjustment mode, a patch image as an adjustment toner image is formed in each image forming portion, is transferred onto the intermediate transfer belt  56 , and is detected by the detection sensor  57 . Therefore, the image forming conditions such as the concentration adjustment of the toner image or the positional deviation of the toner image of each image forming portion are adjusted based on a detection result by the detection sensor  57 . Since the patch image is not transferred to the recording material at the secondary transfer portion T 2 , a large amount of toner on the intermediate transfer belt  56  may adhere to the secondary transfer outer roller  64  after execution of such adjustment mode. 
     In any case, when a large amount of toner passes through the secondary transfer portion T 2  without the recording material P, a large amount of toner adheres to the secondary transfer outer roller  64 . This is because the toner passes through the secondary transfer portion T 2  without the recording material P and toner adhesion is likely to occur on the secondary transfer outer roller  64 . In a case where the next image forming process is performed while the toner having adhered on the secondary transfer outer roller  64 , backside contamination may occurs, in which the toner adheres to a back surface of the recording material passing through the secondary transfer portion T 2 . Therefore, in a case where there is a possibility that a large amount of toner adheres to the secondary transfer outer roller  64 , the control unit  110 , i.e., execution unit, performs the secondary transfer cleaning mode for cleaning the toner having adhered to the secondary transfer outer roller  64 . Before explaining the secondary transfer cleaning mode of the present embodiment, the secondary transfer cleaning mode of the comparative example will be described with reference to  FIGS. 4A to 4D . 
     Comparative Example 
     In a comparative example, a roller cleaning unit for cleaning the power feed roller  68  is not provided and the transfer cleaning bias is applied to the power feed roller  68  to clean the toner adhered to the secondary transfer outer roller  64 . As illustrated in  FIG. 4A , in a case where the secondary transfer outer roller  64  and the power feed roller  68  are contaminated with toner (pointed by “t”), the secondary transfer cleaning mode is executed. In addition, in the case of the comparative example, the power feed roller  68  is disposed so that an abutting position of the power feed roller  68  and the secondary transfer outer roller  64  is positioned at a position shifted by 180° with respect to an abutting position of the secondary transfer outer roller  64  and the intermediate transfer belt  56  in the direction of the rotation of the secondary transfer outer roller  64 . 
     In the secondary transfer cleaning mode, the intermediate transfer belt  56  is rotationally driven. Then, the secondary transfer outer roller  64  and the power feed roller  68  are rotated. In this state, first, a voltage (transfer cleaning bias) having the same polarity, i.e., negative polarity, as the normal charge polarity of the toner is applied to the power feed roller  68 , and whereby the toner t of negative polarity adhered to the secondary transfer outer roller  64  is transferred onto the intermediate transfer belt  56 . In addition, the toner t adhered to the power feed roller  68  is transferred onto the secondary transfer outer roller  64  and then transferred further to the intermediate transfer belt  56  by applying the voltage. 
       FIG. 4B  shows how toner adheres to the secondary transfer outer roller  64  and the power feed roller  68  after the secondary transfer outer roller  64  is half-turned. After the half rotation of the secondary transfer outer roller  64 , the half of the secondary transfer outer roller is cleaned and the remaining half is in a state where the toner contamination remains. In addition, the toner contamination remains on the power feed roller  68  over the whole circumference. 
     This is because toner contamination occurs in both the secondary transfer outer roller  64  and the power feed roller  68  during the operation in  FIGS. 4A to 4B . That is, during the secondary transfer outer roller  64  rotates by the half rotation, a voltage is applied between the power feed roller  68  and the secondary transfer outer roller  64  while the power feed roller  68  is in contact with the secondary transfer outer roller  64  causing toner contamination. Therefore, the voltage is applied in a state where a large amount of the toner existing in the nip portion between the secondary transfer outer roller  64  and the power feed roller  68 . In such a case, a shortage of the voltage for moving the toner from the power feed roller to the secondary transfer outer roller  64  occurs, and the toner is unlikely to move from the power feed roller  68  to the secondary transfer outer roller  64 . 
     In addition, due to the contact-friction between the secondary transfer outer roller  64  and the power feed roller  68 , the toner on the secondary transfer outer roller may adhere to the power feed roller  68  with non-electrostatic adhesive force. Therefore, while the secondary transfer outer roller  64  rotates by the half rotation, the toner contamination is likely to remain on the entire circumference of the power feed roller  68 . For this reason, cleaning of the power feed roller  68  is substantially started after the toner t 0 , which had been at an upstream position ( FIG. 4A ) from the abutting position of the secondary transfer outer roller  64  against the power feed roller  68  in the rotation direction of the secondary transfer outer roller  64  at the time when the voltage application is started, has moved to the intermediate transfer belt  56  ( FIG. 4B ). In the comparative example, cleaning of the power feed roller  68  is started after the half rotation of the secondary transfer outer roller  64 . 
       FIG. 4C  illustrates the remaining adhered toner on the secondary transfer outer roller  64  and the power feed roller  68  after the secondary transfer outer roller  64  has rotated once. In the operation of  FIGS. 4B to 4C , the toner adhered to the power feed roller  68  is transferred onto the secondary transfer outer roller  64 , and the power feed roller  68  is cleaned. Therefore, as illustrated in  FIG. 4C , when cleaning is performed for one rotation of the secondary transfer outer roller  64 , the toner t remains on the secondary transfer outer roller  64  over an area corresponding one round of the power feed roller  68 . 
     Therefore, as illustrated in  FIG. 4D , cleaning of a time of the one rotation of the power feed roller  68  from the state of  FIG. 4C  is further performed, as for the negatively charged toner, cleaning of both the secondary transfer outer roller and the power feed roller  68  can be completed. Therefore, in order to clean the toner charged to one of the negative polarity and the positive polarity, a cleaning time corresponding to one rotation of the power feed roller  68  is required in addition to a cleaning time corresponding to one rotation of the secondary transfer outer roller  64 . The toner transferred onto the intermediate transfer belt  56  is cleaned by the belt cleaning unit  65 . 
     Here, the toner adhered to the secondary transfer outer roller  64  and the power feed roller  68  is a mixture of toner that is electrically positively charged, i.e., positively charged toner, and toner that is negatively charged, i.e., negatively charged toner. That is, although the toner used for image forming process is basically charged to a negative polarity, the polarity of a part of the toner is reversed in some cases, since a voltage having an opposite polarity (positive polarity) to the normal charge polarity of the toner is applied as the secondary transfer bias at the secondary transfer portion T 2 . 
     Therefore, when the secondary transfer outer roller  64  and the power feed roller  68  are cleaned, the toner can be pulled back onto the intermediate transfer belt  56  by using this electric characteristic. More specifically, the positively charged toner is transferred onto the intermediate transfer belt  56  by applying a bias having a direction from the secondary transfer outer roller  64  to the secondary transfer inner roller  62 , that is, by applying a voltage having the positive polarity to the secondary transfer outer roller  64 . On the other hand, the negatively charged toner is transferred onto the intermediate transfer belt  56  by applying a bias having a direction from the secondary transfer inner roller  62  to the secondary transfer outer roller  64 , that is, by applying a voltage having the negative polarity to the secondary transfer outer roller  64 . 
     In this case, by making the secondary transfer outer roller  64  electrically floating, i.e., not connected to the Earth, biases in the same directions as that between the secondary transfer inner roller  62  and the secondary transfer outer roller  64  are applied between the secondary transfer outer roller  64  and the power feed roller  68 . Therefore, it is possible to clean the secondary transfer outer roller  64  and the power feed roller  68  at the same time with one bias power supply. 
     In the configuration of the comparative example, voltages of both polarities are applied as a transfer cleaning bias in order to clean the secondary transfer outer roller  64  and the power feed roller  68  to which the toner having both polarities is adhered. Therefore, as indicated by a broken line (Y) in  FIG. 5 , the application time of the transfer cleaning bias in the comparative example is a time twice a time obtained by adding the time of one rotation of the power feed roller  68  to the time of one rotation of the secondary transfer outer roller  64 . More specifically, when the secondary transfer cleaning mode is started, a transfer cleaning bias having the negative polarity for a time corresponding to one rotation of the power feed roller  68  is applied in addition to one rotation of the secondary transfer outer roller  64 , so that the negatively charged toner adhered to the secondary transfer outer roller  64  and the power feed roller  68  is removed. Next, the transfer cleaning bias having the positive polarity is applied for the same time, so that the positively charged toner adhered to the secondary transfer outer roller  64  and the power feed roller  68  is removed. As described above, in the case of the comparative example, since the voltages having both polarities are applied respectively for a time which obtained by adding one rotation of the power feed roller  68  to one rotation of the secondary transfer outer roller  64 , a long time is required for cleaning the secondary transfer outer roller  64  and the power feed roller  68 . 
     Here, it is known that the distribution of the charge amount of the toner adhered to the secondary transfer outer roller  64  and the power feed roller  68  is as illustrated in  FIG. 6 . In the secondary transfer portion T 2 , a bias having an opposite polarity to that of the toner is applied (for example, −1 μA) when a jam occurs or the like. In this case, the distribution of the toner adhered to an upstream portion, from the abutting position against the power feed roller  68 , of the secondary transfer outer roller  64  is as indicated by the curve (A) in  FIG. 6 . That is, the distribution of the toner on the upstream portion from the power feed roller  68  of the secondary transfer outer roller  64  is composed of positively charged toner and negatively charged toner, which are respectively electrostatically and physically attracted from the intermediate transfer belt  56  to the secondary transfer outer roller  64 . 
     In addition, in this case, the distribution of the toner adhered to the power feed roller  68  is as indicated by the curve (B) in  FIG. 6 . That is, the distribution of the toner of the power feed roller  68  is composed of positively charged toner and a small amount of negatively charged toner which are respectively electrostatically and physically attracted from the secondary transfer outer roller  64  to the power feed roller  68 . 
     Further, in this case, the distribution of the toner adhered to the downstream portion, from the abutting position against the power feed roller  68 , of the secondary transfer outer roller  64  is as indicated by the curve (C) in  FIG. 6 . That is, the distribution of the toner on the downstream portion of the power feed roller  68  of the secondary transfer outer roller  64  is composed of a small amount of the positively charged toner and the negatively charged toner remaining without being electrostatically attracted by the power feed roller  68 . 
     In other words, in the upstream portion from the power feed roller  68  of the secondary transfer outer roller  64 , it is in a state where positively and negatively charged toner is mixed (curve (A)). On the other hand, in the downstream portion from the power feed roller  68  of the secondary transfer outer roller  64 , the negatively charged toner occupies a large amount (curve (C)). Further, on the power feed roller  68 , it is in a state where positively charged toner occupies a large amount (curve (B)). 
     Secondary Transfer Cleaning Mode in the Present Embodiment 
     Based on insights as described above, in the secondary transfer cleaning mode of the present embodiment, the high voltage power supply  120  applies only the voltage of one of the positive polarity and the negative polarity to the power feed roller  68  while the secondary transfer outer roller  64  and the power feed roller  68  are rotated. As described above, the execution of the secondary transfer cleaning mode is performed, for example, after completion of the jam process or after the execution of the adjustment mode and before the start of the next image formation, and in a period other than the period during which the toner image is transferred in the secondary transfer portion T 2 . For example, a transfer cleaning bias is applied to the power feed roller  68  after the completion of the jam process while rotating the photosensitive drum or the intermediate transfer belt without performing an image forming operation such as latent image formation. Otherwise, the secondary transfer cleaning mode may be executed at the time of pre-rotation upon starting an image forming job, at the time of post-rotation upon finishing the image forming job, or at the time of a paper interval after a predetermined number of images has been output. 
     It is noted that an image forming job is a period from the start of image forming operation based on a print signal that commands image forming on the recording material to the completion of the image forming operation. More specifically, it refers to a period which is from the pre-rotation after receiving the print signal (input of the image forming job) to the post-rotation, and which includes the image forming period and the paper interval (non-image formation) period. In addition, the pre-rotation is a period which is for preparatory operations before image forming process and in which the rotation of the photosensitive drum and the intermediate transfer belt is started, output of various voltages are sequentially started, and the various voltages are adjusted. The post-rotation is a period which is for operations after image formation and in which the various voltages are successively lowered while the rotation of the photosensitive drum and the intermediate transfer belt is continued, and finally the rotation of the photosensitive drum and the intermediate transfer belt is stopped. The paper interval is a period between the recording material and the recording material that sequentially pass through the secondary transfer portion T 2 . 
     In the present embodiment, during the execution of the secondary transfer cleaning mode, only voltage of one polarity is applied for a time that is equal to a time required for one rotation of the secondary transfer outer roller  64  or more and that is less than a time obtained by adding the time of one rotation of the power feed roller  68  to one rotation of the secondary transfer outer roller  64 . More specifically, as indicated by a solid line (Z) in  FIG. 5 , a negative voltage having the same polarity as the normal charge polarity of the toner is adopted as a cleaning voltage and applied for a time corresponding to the one rotation of the secondary transfer outer roller  64 . 
     In the present embodiment, since the roller cleaning unit  70  for cleaning the surface of the power feed roller  68  is provided, the toner adhered to the power feed roller  68  is removed by the roller cleaning unit  70 . Therefore, it is possible to eliminate, or reduce at the least, the toner transferred from the power feed roller  68  to the secondary transfer outer roller  64 . As described above, since the positively charged toner occupies a large amount in the adhered toner on the power feed roller  68 , almost all of the positively charged toner adhered to the power feed roller  68  can be removed by the roller cleaning unit  70 . Of course, the negatively charged toner attached to the power feed roller  68  can also be removed by the roller cleaning unit  70 . It is noted that if the negatively charged toner slips through the roller cleaning unit  70 , the negatively charged toner is transferred to the secondary transfer outer roller  64  by the application of the voltage described above and is transferred to the intermediate transfer belt  56  as follows. 
     Since the voltage having the negative polarity is applied to the secondary transfer outer roller  64  via the power feed roller  68 , the negatively charged toner adhered to the secondary transfer outer roller  64  is transferred to the intermediate transfer belt  56  during one rotation of the secondary transfer outer roller  64 . As described above, since the negatively charged toner occupies a large amount at the downstream portion of the power feed roller  68  of the secondary transfer outer roller  64 , most of the toner is transferred from the secondary transfer outer roller  64  to the intermediate transfer belt  56 . The toner transferred to the intermediate transfer belt  56  is cleaned by the belt cleaning unit  65 . 
     Further, the positively charged toner at the upstream portion from the power feed roller  68  of the secondary transfer outer roller  64  is transferred to the power feed roller  68  by voltage application, and the toner transferred to the power feed roller  68  is then cleaned by the roller cleaning unit  70 . 
     As described above, in the present embodiment, it is possible to perform cleaning of the secondary transfer outer roller  64  and the power feed roller  68  only by applying the voltage having one polarity as the transfer cleaning bias for a time required for one rotation of the secondary transfer outer roller  64 . Therefore, the operation time of the secondary transfer cleaning mode is greatly shortened as compared with the comparative example. That is, it is possible to shorten the cleaning time of the secondary transfer outer roller  64  and the power feed roller  68 . 
     Since secondary transfer cleaning mode is executed after the jam process or after the control mode, and before the next image formation is started, if the operation time of the secondary transfer cleaning mode is long, it takes time to start the next image forming process and causes decrease of the productivity of the image forming apparatus. In contrary, in the present embodiment, as described above, the operation time of the secondary transfer cleaning mode is shortened, and thereby the productivity can be improved. 
     Second Embodiment 
     A second embodiment will be described with reference to  FIG. 7 . In the present embodiment, a cleaning member for cleaning a power feed roller  68  is a brush. Since other configurations and operations are the same as those of the first embodiment described above, portions different from the first embodiment will be mainly described below. 
     A roller cleaning unit  80  according to the present embodiment includes a brush  81  that rotates while rubbing a surface of the power feed roller  68 , a collecting container  82 , and a scraping plate  83 . The brush  81  as a cleaning member is configured of a rotation shaft  81   a,  an elastic body layer  81   b  provided around the rotation shaft  81   a,  and a plurality of bristles  81   c  provided on an outer peripheral surface of the elastic body layer  81   b.  Both ends of the rotation shaft  81   a  are supported by the collecting container  82  so as to be substantially parallel to a rotation axis of the power feed roller  68 , and the rotation shaft  81   a  is a shaft that is rotated by drive from a motor (not illustrated). 
     The elastic body layer  81   b  is a layer made of an elastic body formed substantially uniformly with a material for fixing the brush bristles  81   c  on the peripheral surface of the rotation shaft  81   a.  It is preferable that the elastic body layer  81   b  is a layer made of an elastic body having a hardness of 90 degrees (JIS A) or less and a thickness of 1 mm or more. In the present embodiment, a layer of natural rubber is used as the elastic body layer  81   b.    
     It is preferable to use the brush bristles  81   c  that is harder than the elastic body layer  81   b  or brush bristles having the same hardness as the elastic body layer  81   b.  In the present embodiment, a diameter of the rotation shaft  81   a  is 6 mm, a thickness of the elastic body layer  81   b  is 2 mm and the elastic body layer  81   b  is uniformly provided around the rotation shaft  81   a,  and the brush bristles  81   c  are fixed to the elastic body layer  81   b  with a length of 5 mm. 
     The collecting container  82  collects the toner scraped off by the brush bristles  81   c.  In addition, the collecting container  82  supports the brush  81  so that a distance between the rotation axis of the brush  81  and the rotation axis of the power feed roller  68  becomes a predetermined distance (1.5 mm in the present embodiment). 
     The scraping plate  83  is disposed inside the collecting container  82  so that its tip enters the brush bristles  81   c  of the brush  81  and scrapes off the toner adhered to the brush bristles  81   c  into the collecting container  82 . In the present embodiment, the scraping plate  83  is an elastic plate, and its tip elastically comes into contact with the surface of the elastic body layer  81   b  of the brush  81 . 
     Such a roller cleaning unit  80  is rotated in the counterclockwise direction in  FIG. 7  by the motor described above in a state where the brush bristles  81   c  is in contact with the power feed roller  68 . That is, the brush  81  is rotated in a counter direction in which the direction of rotation is reversed at a position where the power feed roller  68  and the brush bristles  81   c  are in contact with each other. 
     As described above, the brush bristles  81   c  come into contact with the power feed roller  68 , thereby scraping off the toner particles adhered onto the power feed roller  68  and cleaning the power feed roller  68 . The toner entered a root portion of the brush bristles  81   c  of the brush  81  is scraped off when the brush bristles  81   c  come into contact with the scraping plate  83 . Otherwise, the elastic body layer  81   b  fixing the brush bristles  81   c  is deformed by being in contact with the scraping plate  83  with the rotation of the brush  81 , and the toner is discharged when the elastic deformation of the root portion of the brush bristles  81   c  is released. That is, a part of the elastic body layer  81   b,  which is elastically deformed by being in contact with the scraping plate  83 , is deformed at the time of being elastically restored by slipping through the scraping plate  83  by rotation. Therefore, the toner intruded into the root portion of the brush bristles  81   c  is discharged. 
     Also in the case of the present embodiment, as similar to the first embodiment, the voltage having the negative polarity, the same polarity as the normal charge polarity of the toner, is applied to the power feed roller for a time required for one rotation of the secondary transfer outer roller  64  while rotating the brush  81  when the secondary transfer cleaning mode is executed. It is noted that when the secondary transfer cleaning mode is not being executed, the brush  81  may be driven by the rotation of the power feed roller  68  without being rotated by the motor. 
     Other Embodiments 
     The image forming apparatus of the present disclosure is applicable to a copying machine, a printer, a facsimile, multi-function printer having multiple functions of these apparatuses, or the like. 
     Further, the cleaning member for cleaning the rotary power feed member may be anything other than using an elastic blade or a brush and as long as it can clean the rotary power feed member, that is, a cleaning roller may be used for example. 
     In addition, the transfer cleaning bias applied to the rotary power feed member in executing the secondary transfer cleaning mode may be opposite to the normal charge polarity of the toner. That is, in a case where the normal charge polarity of the toner is negative, a positive voltage may be adopted as a cleaning voltage and applied to the rotary power feed member. 
     In a case where a voltage of positive polarity, which is opposite to that of the toner, is applied, toner with reversed polarity (positively charged toner) is transferred to the intermediate transfer belt  56 , and the toner having the same polarity as the charge polarity is transferred from the secondary transfer outer roller  64  to the power feed roller  68 . Therefore, similarly to the above embodiments, it is possible to remove toners of both polarities in a short time. In short, the toner of one polarity is transferred to the intermediate transfer belt  56  and removed by the belt cleaning unit  65 , the toner of the other polarity is transferred to the power feed roller and removed by the roller cleaning unit  70  or  80 . Therefore, the secondary transfer portion T 2  can be cleaned in a short time. Further, the time for applying the transfer cleaning bias may be longer than the time for the transfer roller to make one rotation. 
     Still further, similar to the secondary transfer outer roller, in the case where the primary transfer roller is formed as a roller having an elastic layer including a conductive agent at outer portion thereof, a power feed roller may be in contact with the primary transfer roller to apply a current. In this case, the photosensitive drum serves as an image bearing member. That is, the present disclosure is also applicable to a direct transfer system in which a toner image formed on a photosensitive drum is directly transferred to a recording material. 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-111921, filed on Jun. 3, 2016, which is hereby incorporated by reference herein in its entirety.