Patent Publication Number: US-2018039212-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an electrophotographic image forming apparatus for forming an image on a recording medium. 
     Description of the Related Art 
     There is a known electrophotographic image forming apparatus of a so-called intermediate transfer type tandem configuration. Within the image forming apparatus of this sort, a plurality of photoconductors corresponding to colors of toners are disposed, and toner images formed on the respective photoconductors are primarily transferred onto an intermediate transfer member and are transferred secondarily from the intermediate transfer member to a recording medium to form an image thereon. 
     Japanese Patent Laid-open No. 2003-043770 discloses a printer configured to bring the respective photoconductors into contact with the intermediate transfer belt in a mode of forming a full-color image, while separating color photoconductors from the intermediate transfer belt in a mode of forming black monochrome image. Because the color photoconductors are not in contact with the intermediate transfer belt in the latter mode, deterioration otherwise caused by abrasion or contact pressure between the both members is reduced. 
     However, there has been a case where toner and other substances are firmly stuck on a surface of the photoconductors separated from the intermediate transfer member in such configuration in which a part of the photoconductors can be separated from the intermediate transfer belt like the printer in the above-described document. Then, due to such sticking of toner, there has been a case where a striped defective image is generated when the printer outputs a full-color image. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present disclosure provides an image forming apparatus arranged to be able to reduce caking of toner and others on an image bearing member in a configuration in which a part of image bearing members can be separated from an intermediate transfer member. 
     According to one aspect of the present invention, an image forming apparatus includes a first image bearing member configured to rotate while bearing a toner image, a second image bearing member configured to rotate while bearing a toner image, an endless intermediate transfer member configured to bear and convey a toner image transferred from either or both of the first and second image bearing members and to be transferred to a recording medium at a transfer portion, a switch mechanism configured to switch between a state in which the second image bearing member is in contact with the intermediate transfer member and a state in which the second image bearing member is separated from the intermediate transfer member, a cleaning member disposed in contact with the second image bearing member and configured to clean a surface of the second image bearing member along with rotation of the second image bearing member, a temperature detecting portion configured to detect temperature, and a control portion configured to execute an either mode of a first mode and a second mode, the first mode being a mode in which toner images are formed on the first and second image bearing members and are transferred to the intermediate transfer member so as to form an image on the recording medium in a state where the first and second image bearing members are in contact with the intermediate transfer member, the second mode being a mode in which a toner image is formed on the first image bearing member and is transferred to the intermediate transfer member so as to form an image on the recording medium in a state where the first image bearing member is in contact with the intermediate transfer member and the second image bearing member is separated from the intermediate transfer member, the control portion being configured to execute a rotation process in which rotation of the second image bearing member being in a stopped condition is started and then is stopped in a duration of the second mode in a case where detection result of the temperature detecting portion exceeds a predetermined temperature in the duration of the second mode. 
     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 of the present disclosure. 
         FIG. 2  is a schematic diagram illustrating a configuration of an image forming portion. 
         FIG. 3A  is a section view illustrating an all-contact condition in which all photosensitive drums of the respective image forming portion are in contact with an intermediate transfer belt. 
         FIG. 3B  is a section view illustrating a partial contact condition in which a part of the photosensitive drums are separated from the intermediate transfer belt. 
         FIG. 3C  is a section view illustrating an all-separation condition in which all of the photosensitive drums are separated from the intermediate transfer belt. 
         FIG. 4  is a section view illustrating a moving mechanism of a primary transfer roller. 
         FIG. 5  is a perspective view of a cam composing the moving mechanism. 
         FIG. 6A  is a plan view illustrating a shape of a holder of the primary transfer roller. 
         FIG. 6B  is a plan view of another shape of the holder of the primary transfer roller. 
         FIG. 7A  is a section view illustrating a main part of the moving mechanism in a color mode. 
         FIG. 7B  is a section view illustrating the main part of the moving mechanism in a monochrome mode. 
         FIG. 7C  is a section view illustrating the main part of the moving mechanism in the all-separation condition. 
         FIG. 8A  is a plan view illustrating the main part of the moving mechanism in the color mode. 
         FIG. 8B  is a plan view illustrating the main part of the moving mechanism in the monochrome mode. 
         FIG. 8C  is a plan view illustrating the main part of the moving mechanism in the all-separation condition. 
         FIG. 9  is a block diagram illustrating a control system of the image forming apparatus. 
         FIG. 10  is a flowchart illustrating a control process of the image forming apparatus of a first embodiment. 
         FIG. 11  is a graph indicating a relationship between a number of consecutively printed sheets and temperature within the apparatus in the monochrome mode. 
         FIG. 12  is a flowchart illustrating a control process of the image forming apparatus of a second embodiment. 
         FIG. 13  is a flowchart illustrating a control process of the image forming apparatus of a third embodiment. 
         FIG. 14  is a flowchart illustrating a comparative control process of the image forming apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An image forming apparatus of the present disclosure will be described with reference to the drawings. As illustrated in  FIG. 1 , the image forming apparatus  1  of the present disclosure includes an image forming portion  10  of a so-called intermediate transfer type tandem configuration, which includes four image forming units Pa, Pb, Pc, and Pd provided within an apparatus body  1 A. The image forming apparatus  1  is configured to output and form an image on a recording medium S based on image information read from a document or on image information inputted from an external device. It is noted that the recording medium S refers to, besides a plain paper, those including a special paper such as a coated paper, those having a special shape such as an envelope and an index paper, and those including a plastic film for an overhead projector and a cloth. 
     The image forming units Pa, Pb, Pc, and Pd are configured to form toner images of yellow (Y), magenta (M), cyan (C) and black (K), respectively. Because configurations of the respective image forming units are basically the same other than that colors of toners used in development are different, the following description will be explained by exemplifying the configuration of the yellow image forming unit Pa. 
     As illustrated in  FIG. 2 , the image forming unit Pa includes a photosensitive drum  1   a , a charging device  2 , a developing unit  4 , and a cleaning unit  6 . The apparatus body  1 A also includes an exposure unit  3  configured to scan the photosensitive drum  1   a  based on the image information. In response to a start of an image forming process, the photosensitive drum  1   a  is driven to rotate as indicated by an arrow R 1  with a predetermined processing speed, e.g., 100 mm/sec of circumferential speed. The surface of the photosensitive drum  1   a  is homogeneously electrified by proximity discharging of the charging device  2  including a charging roller  2   a  and charging cleaner  2   b , and then an electrostatic latent image is formed on the surface of the photosensitive drum  1   a  by the exposure unit  13 . The electrostatic latent image formed on the photosensitive drum  1   a  is visualized, i.e., developed, as a toner image by the toner supplied from a developer bearing member  4   a  of the developing unit  4 . 
     The toner image borne on the photosensitive drum  1   a  is primarily transferred onto an intermediate transfer belt  7  serving as an intermediate transfer member at a primary transfer portion N 1  formed between a primary transfer roller  5   a  serving as a transfer member and the photosensitive drum  1   a . At this time, toner particles of the toner image are adsorbed to the intermediate transfer belt  7  by a bias voltage, i.e., a primary transfer bias, applied to the primary transfer roller  5   a  from a primary transfer power source  82  through a voltage regulating portion  83 . Transfer residual toner left on the photosensitive drum  1   a  is collected by the cleaning unit  6  having a cleaning blade  6   a  and a collecting screw  6   b.    
     Toner images of the respective colors are formed similarly also on the photosensitive drums in the image forming units Pb, Pc, and Pd. The toner images formed on the respective photosensitive drums are primarily transferred onto the intermediate transfer belt  7  so as to be superimposed with each other by primary transfer rollers  5   b  through  5   d  disposed on an inner circumferential side of the intermediate transfer belt  7 . 
     As illustrated in  FIG. 1 , the intermediate transfer belt  7  is an endless belt member wound around a secondary transfer inner roller  8 , a tension roller  17  and a driven roller  18 . The intermediate transfer belt  7  is driven by the secondary transfer inner roller  8 , which serves as a driving roller, to rotate in a direction of an arrow R 7  along rotation of the photosensitive drums  1   a  through  1   d  in a condition in which an adequate tension is applied to the intermediate transfer belt  7  by the tension roller  17 . 
     A secondary transfer roller  14  is disposed downstream of the image forming units Pa through Pd in the rotation direction of the intermediate transfer belt  7  so as to face the secondary transfer inner roller  8  across the intermediate transfer belt  7 . A bias voltage serving as a secondary transfer bias is applied to the secondary transfer roller  14  from a secondary transfer power source  16  through a voltage regulating portion not illustrated. Thereby, the toner image borne on the intermediate transfer belt  7  are transferred collectively onto the recording medium S at a secondary transfer portion N 2  formed between the secondary transfer roller  14  and the secondary transfer inner roller  8 . Adhesive materials such as transfer residual toner left on the intermediate transfer belt  7  after passing through the secondary transfer portion N 2  are removed by a belt cleaning device  11 . 
     It is noted that the present embodiment adopts a reverse developing system, and the bias voltage applied in the abovementioned configuration is set in accordance to the system. That is, the charging bias voltage having the same polarity as that of the electrified toner, i.e., negative polarity, is applied to the charging roller  2   a  to electrify the photosensitive drums  1   a  through  1   d  with negative polarity. The charging bias voltage may be negative DC voltage alone, or DC voltage superimposed with AC voltage may be also used. The bias voltage having polarity inverse to that of the electrified toner, i.e., positive polarity, is applied to the primary transfer rollers  5   a  through  5   d  and the secondary transfer roller  14  to electrostatically attract the toner particles at the primary and secondary transfer portions N 1  and N 2 . 
     In parallel with such image forming process, a sheet feed portion provided in the apparatus body  1 A executes an operation of feeding the recording medium S toward the image forming portion  10 . The sheet feed portion includes sheet feed cassettes and feed units provided for each sheet feed cassette. The feed unit may be a retard roller type or a separation pad type and is configured to feed the recording medium S stacked on the sheet feed cassette while separating one by one. The recording medium S fed by the sheet feed portion is delivered to a registration roller pair  15  disposed right before the secondary transfer portion N 2 . The registration roller pair  15  corrects a skew of the recording medium S and also conveys the recording medium S to the secondary transfer portion N 2  while synchronizing with the advance of the image forming process in the image forming portion  10 . 
     The recording medium S on which the non-fixed toner image has been transferred at the secondary transfer portion N 2  is passed to the fixing device  20  while being guided by guide members  24  and  25 . The fixing device  20  is composed of a heating roller  201  serving as a fixing roller, which is heated by a heat source  203  such as a halogen heater, and a counter roller  202  serving as a pressure roller in pressure contact with the heating roller  201 . Then, the recording medium S is nipped at a fixing nip between the heating roller  201  and the counter roller  202  and is heated and pressurized so as to melt the toner and to fix the image onto the recording medium S. 
     Then, the recording medium S on which the toner image has been fixed by the fixing device  20  is passed to a discharge roller pair not shown to be discharged out to a discharge tray. In a case where duplex printing is to be carried out, the recording medium S is guided toward a reverse conveyance portion at a branch conveyance portion provided between the fixing device  20  and the discharge roller pair and is passed to a duplex conveyance portion in a condition in which a first surface, i.e., a front surface, is reversed to a second surface, i.e., a back surface, by the reverse conveyance portion. Then, the recording medium S is conveyed to the registration roller pair  15  by the duplex conveyance portion, and an image is transferred again to the back surface of the recording medium S at the secondary transfer portion N 2 . Then, the recording medium S is fixed at the fixing device  20  and is then discharged to the discharge tray. 
     The operation of the image forming apparatus  1  outputting a color image has been described in the abovementioned description, and the image forming apparatus  1  can also execute an operation of outputting a monochrome image by using the black image forming unit Pd. That is, the image forming apparatus  1  can execute an image forming operation of a color mode in which the image forming apparatus  1  outputs a full-color image by using the four image forming units Pa, Pb, Pc, and Pd, and an image forming operation of a monochrome mode in which the image forming apparatus  1  outputs a black monochrome image by using only the black image forming unit Pd. 
     Switching of Contact Condition of Intermediate Transfer Belt 
     Next, an operation for switching contact conditions between the photosensitive drums  1   a  through  1   d  of the image forming units Pa, Pb, Pc, and Pd and the intermediate transfer belt  7  in accordance with the modes of the image forming operation will be described. 
     As illustrated in  FIG. 3A , the image forming operation is executed in a condition in which the respective photosensitive drums  1   a  through  1   d  are in contact with the intermediate transfer belt  7 , i.e., in an all-contact condition, in the color mode. In this case, the respective photosensitive drums  1   a  through  1   d  rotate with the intermediate transfer belt  7 , and toner images are formed in parallel by the four image forming units Pa through Pd. 
     Meanwhile, as illustrated in  FIG. 3B , the image forming operation is executed in a condition in which the black photosensitive drum  1   d  is in contact with the intermediate transfer belt  7  and the color photosensitive drums  1   a ,  1   b  and  1   c  are separated from the intermediate transfer belt  7 , i.e., in a partial contact condition, in the monochrome mode. In this case, while the photosensitive drum  1   d  rotates with the intermediate transfer belt  7  and a toner image is formed by the image forming unit Pd, the photosensitive drums  1   a ,  1   b  and  1   c  are stopped to rotate. During execution of the monochrome mode, the photosensitive drums  1   a ,  1   b  and  1   c  are separated from the intermediate transfer belt  7  to reduce wear otherwise caused by friction and deterioration otherwise caused by contact pressure of the both members. 
     The color mode corresponds to a first mode in the image forming operation and the monochrome mode corresponds to a second mode. The black photosensitive drum  1   d  corresponds to a first image bearing member that is in contact with the intermediate transfer member commonly in the first and second modes. Each color photosensitive drums  1   a ,  1   b  and  1   c  corresponds to a second image bearing member in contact with the intermediate transfer member in the first mode and separated from the intermediate transfer member in the second mode. 
     It is noted that as illustrated in  FIG. 3C , the image forming apparatus  1  is able to switch to a condition in which all of the photosensitive drums  1   a  through  1   d  are separated from the intermediate transfer belt  7 , i.e., in an all-separation condition. It is preferable to adopt this condition in replacing the intermediate transfer belt  7  for example because the intermediate transfer belt  7  is released from the photosensitive drums  1   a  through  1   d . Still further, there is a case where a patch density sensor  28  capable of detecting density of a toner patch formed by the respective image forming units Pa through Pd is disposed between the most downstream photosensitive drum  1   d  and the secondary transfer inner roller  8 . In this case, it is preferable to separate the intermediate transfer belt  7  from the patch density sensor  28  by simultaneously moving the driven roller  18  in the all-separation condition. 
     Moving Mechanism of Primary Transfer Roller 
     Next, a switch mechanism for switching contact condition between the photosensitive drums  1   a  through  1   d  and the intermediate transfer belt  7  will be described. As illustrated in  FIGS. 3A through 3C , the respective primary transfer rollers  5   a  through  5   d  are movable between contact positions where the respective primary transfer rollers  5   a  through  5   d  come into contact with the corresponding photosensitive drums  1   a  through  1   d  across the intermediate transfer belt  7  and separate positions where the respective primary transfer rollers  5   a  through  5   d  are separated from the corresponding photosensitive drums  1   a  through  1   d . That is, the intermediate transfer belt  7  is brought into contact with and is separated from the photosensitive drums  1   a  through  1   d  by movement of the primary transfer rollers  5   a  through  5   d.    
     As illustrated in  FIG. 4 , the moving mechanism  40  of the primary transfer rollers  5   a  through  5   d  includes slide members  29  and  30  movable in a horizontal direction with respect to the apparatus body and a cam member  27  configured to move the slider members. The slider members include a black slider (Bk slider, hereinafter)  29  configured to move the black primary transfer roller  5   d  and a color slider (CL slider, hereinafter)  30  configured to move the primary transfer rollers  5   a ,  5   b  and  5   c  of yellow, magenta and cyan. The Bk slider  29  and the CL slider  30  are provided separately. The Bk slider  29  corresponds to a first actuation portion configured to move the first transfer member, and the CL slider  30  corresponds to a second actuation portion configured to move the second transfer member. The cam member  27  engages with the Bk slider  29  and the CL slider  30 , respectively, in a condition in which the cam member  27  is supported by a cam shaft  27   a  connected with a cam driving motor not illustrated. 
     As illustrated in  FIG. 5 , the cam member  27  includes a boss portion  271  configured to engage with the cam shaft  27   a  so as not to rotate relatively, a first cam surface  272  configured to engage with an engage portion  290  of the Bk slider  29  and a second cam surface  273  configured to engage with an engage portion  300  of the CL slider  30  (see also  FIG. 8 ). The cam member  27  is configured such that the first and second cam surfaces  272  and  273  have different shapes from each other when viewed from an axial direction of the cam shaft  27   a  and such that the Bk slider and the CL slider  30  can be moved separately by controlling rotation angle of the cam shaft  27   a . In a case of an example illustrated in  FIG. 5 , the cam member  27  is set such that at least one of the Bk slider  29  and the CL slider  30  moves every time when the cam member  27  is rotated by 120 degrees. 
     As illustrated in  FIG. 4 , the Bk slider  29  and the CL slider  30  are connected with roller holders  25   a ,  25   b ,  25   c  and  25   d  configured to hold the respective primary transfer rollers  5   a  through  5   d . Among the roller holders  25   a ,  25   b ,  25   c  and  25   d , the yellow and magenta roller holders  25   a  and  25   b  are configured to swing along with the move of the slider members. 
     That is, as the roller holder  25   b  is illustrated in  FIG. 6A  as a typical example, the roller holders  25   a  and  25   b  are swing members, i.e., lever members, swingable centering on a swing shaft  250  supported by the apparatus body. Provided on one side and another side of the swing shaft  250  are a holding portion  251  configured to rotatably hold the primary transfer rollers  5   a  and  5   b  and a project portion  252  configured to engage with the CL slider  30 . Meanwhile, as illustrated in  FIG. 6B , the cyan and black roller holders  25   c  and  25   d  have a shape in which a holding portion  253  configured to rotatably hold the primary transfer rollers  5   c  and  5   d  and a project portion  254  configured to engage with the Bk/CL slide members  29  and  30  are coaxially disposed. 
     As illustrated in  FIG. 7A , the project portions  252  and  254  of the roller holders  25   a  through  25   d  are in contact with slope portions  291 ,  301  and  302  provided in the Bk/CL slide members  29  and  30  and move along the slope when the Bk/CL slide members  29  and  30  moves. Among the slope portions of the CL slider  30 , the slope portions  301  corresponding to the yellow and magenta, i.e., the lever type, roller holders  25   a  and  25   b  are inclined inversely from that of the slope portion  302  corresponding to the cyan roller holder  25   c . Accordingly, the move mechanism is configured such that the color primary transfer rollers  5   a ,  5   b  and  5   c  move in a same direction with respect to the photosensitive drums  1   a ,  1   b  and  1   c  when the CL slider  30  moves in right and left directions in  FIG. 7A . 
     As illustrated in  FIGS. 7A through 7C , the BK/CL slide members  29  and  30  are movable between right and left positions, respectively. Here, the right and left positions refer to right and left side positions in  FIG. 7A through 7C  within a moving ranges of the BK/CL slide members  29  and  30 . Depending on setting of an inclination direction of the slope portions  301  and  302 , the color primary transfer rollers  5   a ,  5   b  and  5   c  are held at the contact positions when the CL slider  30  is located at the left position, and are held at the separate positions when the CL slider  30  is located at the right position. The black primary transfer roller  5   d  is held at the contact position when the Bk slider  29  is located at the left position, and is held at the separate position when the Bk slider  29  is located at the right position. 
     It is noted that the Bk slider  29  is provided with a push-up portion  292  configured to abut with a swing arm  21  holding the driven roller  18  (see also  FIG. 4 ). The swing arm  21  brings the driven roller  18  into pressure contact with an inner circumferential surface of the intermediate transfer belt  7  by a resilient force of a spring member  22  and positions the intermediate transfer belt  7  along a body rail not illustrated. The push-up portion  292  pushes up the swing arm  21  by resisting against urging force of the spring member  22  corresponding to the operation, caused by the slope portion  291 , of moving the black primary transfer roller  5   d  from the contact position to the separate position. 
     With such arrangement described above, the moving mechanism  40  moves the primary transfer rollers  5   a  through  5   d  from the contact position to the separate position corresponding to the modes of the image forming operation. As illustrated in  FIGS. 7A and 8A , the CL slider  30  is positioned at the left position and the Bk slider  29  is positioned at the left position by the cam member  27  in the color mode. Then, because all of the primary transfer rollers  5   a  through  5   d  are held at the contact positions, the respective photosensitive drums  1   a  through  1   d  come into contact with the intermediate transfer belt  7 . 
     As illustrated in  FIGS. 7B and 8B , in the monochrome mode, the CL slider  30  is positioned at the right position and the Bk slider  29  is positioned at the left position by the cam member  27 . Then, because the color primary transfer rollers  5   a ,  5   b  and  5   c  are held at the separate positions and the black primary transfer roller  5   d  is held at the contact position, only the black photosensitive drum  1   d  is in contact with the intermediate transfer belt  7 . 
     As illustrated in  FIGS. 7C and 8C , if the CL slider  30  is positioned at the right position and the Bk slider  29  is positioned at the right position by the cam member  27 , all of the primary transfer rollers  5   a  through  5   d  are held at the separate position. Therefore, an all-separation condition in which the respective photosensitive drums  1   a  through  1   d  are separated from the intermediate transfer belt  7  is realized. In this state, the swing arm  21  is pushed up by the push-up portion  292  and the driven roller  18  recedes upward (see also  FIG. 3C ). 
     It is noted that positions similar to those in the monochrome mode are set as home positions of the primary transfer rollers  5   a  through  5   d  in the present embodiment. That is, only the black primary transfer roller  5   d  is located at the contact position and the other primary transfer rollers  5   a ,  5   b  and  5   c  are held at the separate position (see  FIGS. 7B and 8B ) during standby period for a printing job. However, the other positions, e.g., the separate positions of all of the primary transfer rollers  5   a  through  5   d , may be set as home positions. 
     Toner Sticking in Monochrome Mode 
     As described above, in the monochrome mode, the image forming operation is executed by using the black photosensitive drum  1   d  while the color photosensitive drums  1   a ,  1   b  and  1   c  are separated from the intermediate transfer belt  7 . However, in a case where the condition in which the rotation of the photosensitive drums  1   a ,  1   b  and  1   c  is stopped in the monochrome mode continues for a certain period of time, there is a case where adhesive materials including toner are firmly stuck on the surface of the drums. That is, toner and external additives such as wax accumulated in a nip portion between the cleaning blade  6   a  (see  FIG. 2 ) and the photosensitive drums  1   a ,  1   b  and  1   c  may be pressed against and fixed to the surface of the drums by contact pressure, i.e., nip pressure, of the cleaning blade  6   a.    
     It was confirmed that such toner sticking likely occurs particularly in a case where temperature within the apparatus body increases by conducting consecutive printing, or consecutive feeding, of consecutively forming images on a large number of recording media. Still further, it has been known that as an operation time of the image forming apparatus  1  increases, a drum surface wears due to friction between the photosensitive drum and other members such as the cleaning blade, so that surface roughness of the photosensitive drum changes, increases normally. It is conceivable that chance of occurrence of toner sticking on the drum surface changes due to such factors. 
     If a printing job of a color image is inputted in the condition in which there is toner stuck on the surface of the photosensitive drums  1   a ,  1   b  and  1   c  due to such factors, a defective image may be created. That is, a striped defective image may appear in the toner images outputted from the color image forming units Pa, Pb and Pc because the image forming operation of such toner image is performed in the condition in which toner sticking has occurred along the contact portion of the cleaning blade  6   a.    
     Here, according to the present embodiment, an operation of temporarily rotating the color photosensitive drums  1   a ,  1   b  and  1   c  is executed in a case where a predetermined condition is met in a duration of the monochrome mode in order to avoid such unfavorable result of toner sticking. A specific exemplary arrangement of the image forming apparatus  1  will be described below along a flowchart executed by a control portion. 
     It is noted that the following control process is executed by the control portion including a main control circuit  100  as illustrated in  FIG. 9 . The main control circuit  100  serving as the control portion integrally controls the image forming apparatus  1  includes a central processing unit (CPU)  101  serving as an execution portion of a control program, a read only memory (ROM)  102  and a random access memory (RAM)  103 , which serve as storage portions. The ROM  102  and the RAM  103  temporarily and/or permanently store data such as control programs and setting information for printing jobs. 
     The main control circuit  100  receives input signals from: a temperature and humidity sensor S 1  configured to detect temperature and humidity within the image forming apparatus  1 ; a timer S 2  capable of counting an elapsed time in each step of the control process; and a durability counter S 3  configured to record a number of images outputted from the image forming apparatus  1 . The temperature and humidity sensor S 1  is an exemplary temperature detecting portion capable of detecting temperature change within the apparatus. The timer S 2  and the durability counter S 3  may be replaced by software systems whose functions are achieved by the CPU  101 . 
     The main control circuit  100  is also capable of controlling driving motors M 1  through M 4  in accordance with the control process. The ITB (intermediate transfer belt) driving motor M 1  drives the secondary transfer inner roller to rotate the intermediate transfer belt  7 . The drum driving motors M 2  and M 3  are provided respectively for the color photosensitive drums  1   a ,  1   b  and  1   c  and the black photosensitive drum  1   d  and rotate the corresponding photosensitive drums. The cam driving motor M 4  rotates the cam member  27  through an intermediary of the cam shaft  27   a.    
     It is noted that the cam driving motor M 4  is composed of a stepping motor for example, and the CPU  101  can detect a rotation phase of the cam member  27  from a driving amount of the cam driving motor M 4 . However, another detection mechanism such as a sensor directly detecting position of the intermediate transfer belt  7  may be disposed as long as it is an arrangement capable of detecting whether the photosensitive drums  1   a  through  1   d  are in contact with the intermediate transfer belt  7 . 
     First Embodiment 
     The control process of a first embodiment will be described at first along a flowchart in  FIG. 10 . In response to an input of a print job to the image forming apparatus  1 , the CPU  101  starts rotating the black photosensitive drum  1   d  and the intermediate transfer belt  7  in Step S 101  and determines whether an image to be outputted is a color image or a monochrome image in Step S 102 . 
     Monochrome Mode 
     The monochrome mode is selected when the monochrome image is to be outputted, i.e., Y in Step S 102 , and forming a toner image in the black image forming unit Pd is started in Step S 103 . At this time, the color primary transfer rollers  5   a ,  5   b  and  5   c  are located at the separate position, and the color photosensitive drums  1   a ,  1   b  and  1   c  are stopped their rotation. In the monochrome mode, while monitoring whether temperature within the apparatus detected by the temperature and humidity sensor S 1 , i.e., in-apparatus temperature T, exceeds a predetermined temperature T 1  in Step S 104 , the CPU  101  continues the printing job in Step S 111 . 
     In a case where the in-apparatus temperature T reaches a value equal to or more than the predetermined temperature T 1 , i.e., Y in Step S 104 , the CPU  101  shifts the mode to a toner sticking avoiding mode in Step S 105  and judges whether a number of consecutively printed sheets P exceeds a predetermined number of sheets P 1  in Step S 106 . Here, the number of consecutively printed sheets P is a number of the recording media S on which images are formed after when the in-apparatus temperature T has reached the predetermined temperature T 1  in the monochrome mode. In a case where the number of consecutively printed sheets P is less than the predetermined number of sheets P 1 , i.e., N in Step S 106 , the CPU  101  leaves from the toner sticking avoiding mode and continues the normal printing job. 
     In a case where the number of consecutively printed sheets P is equal to or more than the predetermined number of sheets P 1 , i.e., Y in Step S 106 , the CPU  101  executes the process of temporarily rotating the color photosensitive drums  1   a ,  1   b  and  1   c . That is, after driving the corresponding driving motor M 3  for a certain period of time in Step S 107 , the CPU  101  executes the process of stopping the rotation in Step S 108 . While the driving time of the driving motor M 3  is arbitral, it is preferable to set the driving time to be less than one rotation in terms of a rotation amount of the photosensitive drums  1   a ,  1   b  and  1   c , e.g., 60 msec, since it is sufficient for avoiding toner sticking. The photosensitive drums  1   a ,  1   b  and  1   c  are driven to rotate while being kept separate from the intermediate transfer belt  7 . 
     In the case where the CPU  101  executes the rotation of the photosensitive drums  1   a ,  1   b  and  1   c , the CPU  101  resets the value of the number of consecutively printed sheets P in Step S 109  and continues the printing job in Step S 110 . Then, when output of the images of the number of sheets inputted as the printing job has ended, i.e., Y in Step S 110  and Y in Step S 111 , the CPU  101  stops the rotation of the photosensitive drum  1   d  and the intermediate transfer belt  7  in Step S 112  to finish the printing job. 
     Color Mode 
     Meanwhile, in a case where the color image is to be outputted, i.e., N in Step S 102 , the CPU  101  starts to rotate the color photosensitive drums  1   a ,  1   b  and  1   c  in Step S 121  and makes the primary transfer rollers  5   a ,  5   b  and  5   c  approach the corresponding photosensitive drums through the moving mechanism  40  in Step S 122 . When it is confirmed that the primary transfer rollers  5   a ,  5   b  and  5   c  have reached the contact positions, i.e., Y in Step S 123 , the respective image forming units Pa, Pb, Pc and Pd start to form toner images in Step S 124 . 
     When an output of the number of color image sheets inputted as the printing job is finished in Step S 125 , the CPU  101  moves the color primary transfer rollers  5   a ,  5   b  and  5   c  toward the separate positions in Step S 126 . Then, after confirming that the respective primary transfer rollers  5   a  through  5   d  are located at their home positions, i.e., Y in Step S 127 , the CPU  101  stops the rotation of the photosensitive drums  1   a  through  1   d  and the intermediate transfer belt  7  in Step S 128  to finish the printing job. 
     Setting of Threshold Values of Temperature and Number of Consecutively Printed Sheets 
     Here, setting of the conditions for executing the rotation process of the color photosensitive drums  1   a ,  1   b  and  1   c  in the monochrome mode will be described.  FIG. 11  is a graph representing a relationship between a number of consecutively printed sheets and temperature within the apparatus in a duration of the monochrome mode. The in-apparatus temperature is temperature within a casing of the image forming apparatus  1  measured by the temperature and humidity sensor S 1 , and color and black drum temperatures are temperatures measured around the photosensitive drums  1   a  through  1   d.    
     As indicated in the graph, the in-apparatus temperature and the color drum temperature increase along with an increase of the number of consecutively printed sheets within a range of 0 to 1,000 sheets and are almost constant within a range exceeding 1,000 sheets. Still further, when it was checked whether occurrence of toner sticking on the surface of the photosensitive drums  1   a ,  1   b  and  1   c , it was confirmed that it is not necessary so much to consider toner sticking within a range of less than 2,000 sheets of consecutively printed sheets. 
     Then, in the present embodiment, the in-apparatus temperature of 37 degrees Celsius is set as a threshold, i.e., the predetermined temperature T 1  is set 37 degrees Celsius, as a rough indication that the color photosensitive drums  1   a ,  1   b  and  1   c  have reached the roughly constant temperature range. Still further, because toner sticking occurs in a case where a certain period of time elapses in a condition in which the in-apparatus temperature is high, a threshold of number of consecutively printed sheets is set at 1,000 sheets, i.e., the predetermined number of sheets P 1 =1,000 sheets. However, it is preferable to appropriately change such threshold values corresponding to such characteristics as liability to an increase of the in-apparatus temperature and tendency to aggregate of the toner. 
     An advantages of the present embodiment will be described below by comparing with a comparative control process. As illustrated in  FIG. 14 , this comparative control process is different from the control process of the present embodiment in that the processes of the toner sticking avoiding mode, i.e., Steps S 104  through S 108  in  FIG. 10 , are not executed. Accordingly, when the output of images of the predetermined number of sheets is finished in Step S 113 , the CPU  101  stops the photosensitive drum  1   d  and the intermediate transfer belt  7  in Step S 114  regardless of fluctuation of the in-apparatus temperature and other condition in the monochrome mode and finishes the printing job. 
     In such configuration as described above, however, if a printing job requesting output of more than 2,000 monochrome images is inputted, there is a possibility that toner sticking occurs on the color photosensitive drums  1   a ,  1   b  and  1   c . Even if the printing job is that of less than 2,000 sheets, there is a possibility that toner sticking occurs if monochrome mode printing jobs are repeatedly inputted or if ambient temperature is high. 
     In contrast, according to the present embodiment, the operation of temporarily rotating the photosensitive drums  1   a ,  1   b  and  1   c  is executed as illustrated in Steps S 107  and S 108  in  FIG. 10  in the case where consecutive printing of more than the predetermined number of sheets P 1  is executed in the condition in which the in-apparatus temperature T exceeds the predetermined temperature T 1 . In other words, the rotation process is executed in the monochrome mode in a condition in which a detection result of the temperature and humidity sensor S 1  exceeds the predetermined temperature during execution of the monochrome mode. Thereby, the possibility of causing toner sticking is reduced because the photosensitive drums  1   a ,  1   b  and  1   c  is relatively moved with respect to the cleaning blade  6   a  before the toner accumulated at the contact portion of the cleaning blade  6   a  is firmly stuck on the drum surface. That is, in addition to prolonging lives of the color photosensitive drums  1   a ,  1   b  and  1   c  by means of separating the color photosensitive drums  1   a ,  1   b  and  1   c  from the intermediate transfer belt  7  in the monochrome mode, it is possible to reduce the possibility of causing defective images otherwise caused by toner sticking. 
     It is noted that the present embodiment has been described such that the rotation of the photosensitive drums  1   a ,  1   b  and  1   c  is executed in the case where the temperature condition and the condition of the number of consecutively printed sheets are simultaneously met. That is, the present embodiment has been described such that the CPU  101  executes the rotation process of the photosensitive drums based on the detection result of the temperature and humidity sensor S 1  and on the signals from the durability counter S 3 . It is noted that the CPU  101  may make such decision based on either one condition of the temperature and the number of consecutively printed sheets. For instance, it is conceivable to avoid toner sticking with such a configuration that the photosensitive drums  1   a ,  1   b  and  1   c  are rotated when the number of consecutively printed sheets in the monochrome mode becomes more than the predetermined number of sheets, e.g., more than 2,000 sheets. Still further, if readiness of causing toner sticking fluctuates due to wear of the photosensitive drums  1   a ,  1   b  and  1   c , the condition of executing the rotation process may be changed based on a signal from the durability counter S 3 . 
     Still further, the arrangement of using the cleaning blade  6   a  that is arranged to abut against the photosensitive drum  1   a  from a counter direction of the rotation direction, i.e., an arrow R 1  in  FIG. 2 , of the photosensitive drum  1   a  has been described as the cleaning member configured to clean the image bearing member in the present embodiment. However, toner sticking may occur even in a case where a blade member that is arranged to abut with the photosensitive drum from a trailing direction opposite from the counter direction and where a cleaning member of different shape and material other than the rubber blade is used. Accordingly, the technology of the present disclosure is also applicable to the case where such cleaning member is used. 
     Second Embodiment 
     A control process of a second embodiment will be described next along a flowchart in  FIG. 12 . The present embodiment is different from the first embodiment in that a continuous operating time τ of the image forming portion  10  is adopted instead of the number of consecutively printed sheets. 
     In response to an input of a print job to the image forming apparatus  1 , the CPU  101  starts to rotate the black photosensitive drum  1   d  and the intermediate transfer belt  7  in Step S 201  and determines whether an image to be outputted is a color image or a monochrome image in Step S 202 . 
     Monochrome Mode 
     The CPU  101  selects the monochrome mode in a case where the monochrome image is to be outputted, i.e., Y in Step S 202 , and starts to form a toner image in the black image forming unit Pd in Step S 203 . At this time, the color primary transfer rollers  5   a ,  5   b  and  5   c  are located at the separate positions, and the color photosensitive drums  1   a ,  1   b  and  1   c  are held in a condition in which their rotation is stopped. In the monochrome mode, while monitoring whether the in-apparatus temperature T detected by the temperature and humidity sensor S 1  exceeds the predetermined temperature T 1  in Step S 204 , the CPU  101  continues the printing job in Step S 211 . 
     In a case where the in-apparatus temperature T reaches a value equal to or more than the predetermined temperature T 1 , i.e., Y in Step S 204 , the CPU  101  shifts the mode to a toner sticking avoiding mode in Step S 205  and judges whether the continuous operating time τ exceeds a predetermined time  11  in Step S 206 . Here, the continuous operating time τ is a time during which the image forming portion  10  operates after when the in-apparatus temperature T has reached the predetermined temperature T 1  in the monochrome mode. The predetermined time τ 1  is a value set as a rough indication of a time until when toner sticking occurs in the condition in which the in-apparatus temperature T exceeds the predetermined temperature T 1  by taking aggregability of toner and other conditions into consideration. The predetermined time τ 1  is set at 30 minutes for example. In a case where the continuous operating time τ is less than the predetermined time τ 1 , i.e., N in Step S 206 , the CPU  101  leaves from the toner sticking avoiding mode and continues the normal printing job. 
     In a case where the continuous operating time τ is more than the predetermined time τ 1 , i.e., Y in Step S 206 , the CPU  101  executes the process of temporarily rotating the color photosensitive drums  1   a ,  1   b  and  1   c  being in the stopped condition. That is, after an elapse of a predetermined period of time after driving the corresponding driving motor M 3  in Step S 207 , the CPU  101  executes the process of stopping the rotation in Step S 208 . The driving time of the driving motor M 3  in thus rotating the photosensitive drums  1   a ,  1   b  and  1   c  for the predetermined period of time is set at 60 msec, for example. In the case where the CPU  101  executes the rotation of the photosensitive drums  1   a ,  1   b  and  1   c , the CPU  101  resets the value of the continuous operating time τ in Step S 209  and continues the printing job in Step S 210 . Then, when the output of images of the number of sheets inputted as the printing job ends, i.e., Y in Step S 210  and Y in Step S 211 , the CPU  101  stops rotation of the photosensitive drum  1   d  and the intermediate transfer belt  7  in Step S 212  to finish the printing job. 
     Color Mode 
     Meanwhile, in a case where the color image is to be outputted, i.e., N in Step S 202 , the CPU  101  starts to rotate the color photosensitive drums  1   a ,  1   b  and  1   c  in Step S 221  and makes the primary transfer rollers  5   a ,  5   b  and  5   c  approach the corresponding photosensitive drums through the moving mechanism  40  in Step S 222 . When it is confirmed that the primary transfer rollers  5   a ,  5   b  and  5   c  have reached the contact positions, i.e., Y in Step S 223 , the respective image forming units Pa, Pb, Pc and Pd start to form toner images in Step S 224 . 
     When the output of the number of color image sheets inputted as the printing job is finished in Step S 225 , the CPU  101  moves the color primary transfer rollers  5   a ,  5   b  and  5   c  toward the separate positions in Step S 226 . Then, after confirming that the respective primary transfer rollers  5   a  through  5   d  are located at their home positions, i.e., Y in Step S 227 , the CPU  101  stops rotation of the photosensitive drums  1   a  through  1   d  and the intermediate transfer belt  7  in Step S 228  to finish the printing job. 
     According to the present embodiment, the operation of temporarily rotating the photosensitive drums  1   a ,  1   b  and  1   c  is executed as illustrated in Steps S 207  and S 208  in  FIG. 12  in the case where the time of more than the predetermined time τ 1  has elapsed in the condition in which the in-apparatus temperature T exceeds the predetermined temperature T 1 . Accordingly, it is possible to reduce the possibility of causing toner sticking and to reduce the possibility of producing defective images in the same manner with the image forming apparatus  1  of the first embodiment. 
     It is noted that although the present embodiment has been described such that the rotation of the photosensitive drums  1   a ,  1   b  and  1   c  is executed in the case where the temperature condition and the condition of the continuous operating time are simultaneously met, the CPU  101  may execute the similar process by the sole condition of the continuous operating time. That is, it is conceivable to avoid toner sticking with such a configuration that the photosensitive drums  1   a ,  1   b  and  1   c  are rotated when an elapsed time from start of execution of the monochrome mode, i.e., the continuous operating time, becomes more than a predetermined time, e.g., more than 60 minutes. 
     Third Embodiment 
     A control process of a third embodiment will be described next along a flowchart in  FIG. 13 . The present embodiment is different from the first and second embodiments described above in that a process for avoiding toner sticking is inserted, i.e., Step S 325 , is inserted before executing a succeeding printing job in a color mode when a predetermined condition is met in a duration of a preceding printing job in a monochrome mode. 
     In response to an input of a print job to the image forming apparatus  1 , the CPU  101  starts to rotate the black photosensitive drum  1   d  and the intermediate transfer belt  7  in Step S 301  and determines whether an image to be outputted is a color image or a monochrome image in Step S 302 . 
     Monochrome Mode 
     The CPU  101  selects the monochrome mode in the case where the monochrome image is to be outputted, i.e., Y in Step S 302 , and starts to form a toner image in the black image forming unit Pd in Step S 303 . At this time, the color primary transfer rollers  5   a ,  5   b  and  5   c  are located at the separate positions, and the color photosensitive drums  1   a ,  1   b  and  1   c  are held in a condition in which their rotation is stopped. In the monochrome mode, while monitoring whether the in-apparatus temperature T detected by the temperature and humidity sensor S 1  exceeds the predetermined temperature T 1  in Step S 304 , the CPU  101  continues the printing job in Step S 311 . 
     In a case where the in-apparatus temperature T reaches a value equal to or more than the predetermined temperature T 1 , i.e., Y in Step S 304 , the CPU  101  shifts the mode to the toner sticking avoiding mode in Step S 305  and judges whether the number of consecutively printed sheets P or the continuous operating time τ exceeds the predetermined threshold value, i.e., the predetermined number of sheets P 1  or the predetermined time  11 , in Step S 306 . Here, the number of consecutively printed sheets P, the continuous operating time τ and their threshold can be set in the same manner with the first and second embodiments described above. In a case where the number of consecutively printed sheets P and the continuous operating time τ are both less than the threshold values, i.e., N in Step S 306 , the CPU  101  leaves from the toner sticking avoiding mode and continues the normal printing job. 
     In a case where the number of consecutively printed sheets P or the continuous operating time τ is more than the threshold value, i.e., Y in Step S 306 , the CPU  101  executes the process of temporarily rotating the color photosensitive drums  1   a ,  1   b  and  1   c . That is, after driving the corresponding driving motor M 3  for a certain period of time τn Step S 307 , the CPU  101  executes the process of stopping the rotation in Step S 308 . The driving time of the driving motor M 3  is set at 60 msec, for example. In the case where the CPU  101  executes the rotation of the photosensitive drums  1   a ,  1   b  and  1   c , the CPU  101  resets the value of the number of consecutively printed sheets P in Step S 309  and continues the printing job in Step S 310 . Then, when the output of images of the number of sheets inputted as the printing job ends, i.e., Y in Step S 310  and Y in Step S 311 , the CPU  101  stops rotation of the photosensitive drum  1   d  and the intermediate transfer belt  7  in Step S 312  to finish the printing job. 
     Color Mode 
     Meanwhile, in a case where the color image is to be outputted, i.e., N in Step S 302 , the CPU  101  starts to rotate the color photosensitive drums  1   a ,  1   b  and  1   c  in Step S 321 . Here, the CPU  101  determines whether a potential occurrence of toner sticking in a duration of a standby period by making reference to information in the previous printing job. That is, the CPU  101  determines whether the previous printing job has been executed in the monochrome mode in Step S 322 , whether the in-apparatus temperature has exceeded the predetermined temperature T 1  in Step S 323 , and whether a predetermined standby time τ 2  has elapsed from completion of the previous job to the input of the present job in Step S 324 . 
     Then, in a case where these conditions are all met, the CPU  101  idly rotates the color photosensitive drums  1   a ,  1   b  and  1   c  for a certain period of time τn Step S 325  before moving the primary transfer rollers  5   a ,  5   b  and  5   c  to the contact positions. The period during which the photosensitive drums  1   a ,  1   b  and  1   c  are idly rotated is set such that stuck toner or nearly toner in a state close to stuck can be fully removed by the cleaning blade  6   a . At least such period is set to be longer than a period of idly rotating the drums in a case where the abovementioned conditions are not met. 
     After that, the CPU  101  makes the primary transfer rollers  5   a ,  5   b  and  5   c  approach the corresponding photosensitive drums through the moving mechanism  40  in Step S 326 . When it is confirmed that the primary transfer rollers  5   a ,  5   b  and  5   c  have reached the contact positions, i.e., Y in Step S 327 , the respective image forming units Pa, Pb, Pc and Pd start to form toner images in Step S 328 . 
     When the output of the number of color image sheets inputted as the printing job is finished in Step S 329 , the CPU  101  moves the color primary transfer rollers  5   a ,  5   b  and  5   c  toward the separate positions in Step S 330 . Then, after confirming that the respective primary transfer rollers  5   a  through  5   d  are located at their home positions, i.e., Y in Step S 331 , the CPU  101  stops rotation of the photosensitive drums  1   a  through  1   d  and the intermediate transfer belt  7  in Step S 332  to finish the printing job. 
     According to the present embodiment, the operation of temporarily rotating the photosensitive drums  1   a ,  1   b  and  1   c  is executed as illustrated in Steps S 307  and S 308  in  FIG. 13  in the case where the number of images more than the predetermined number of sheets P 1  has been outputted or the time of more than the predetermined time τ 1  has elapsed in the condition in which the in-apparatus temperature T exceeds the predetermined temperature T 1 . Accordingly, it is possible to reduce the possibility of occurrence of toner sticking and to reduce the possibility of producing defective images in the same manner with the image forming apparatus  1  of the first and second embodiments. 
     In addition to that, the CPU  101  executes idling of the photosensitive drums  1   a ,  1   b  and  1   c  in Step S 325  when a certain condition is met in a case where the printing job is to be executed in the color mode after the monochrome mode in the present embodiment. This configuration makes it possible to remove or reduce adhesive materials by the cleaning blade  6   a  by the idling operation even if toner is stuck or becomes close to a stuck condition in a duration of a standby period. Thereby, the possibility of causing toner sticking may be reduced further. 
     While the arrangement in which the belt member is used as the intermediate transfer member has been described in the first through third embodiments, such arrangement may be replaced by an arrangement in which an intermediate transfer member of a drum member, for example, is used as long as the image bearing member on which the toner image is primarily formed can be made into contact with/separate from the intermediate transfer member on which the toner image is to be transferred. Still further, although the abovementioned embodiments have been described such that the monochrome image is formed in the first mode and the color image is formed in the second mode, another operation may be carried out the first and second modes. For instance, the first mode may be a mode of outputting a glossy image by using transparent toner in addition to colored toner, and the second mode may be a mode of outputting an image without using the transparent toner. 
     Still further, while the moving mechanism  40  capable of collectively switching the contact positions between the color photosensitive drums  1   a ,  1   b  and  1   c  and the intermediate transfer belt  7  has been described as one exemplary switch mechanism in the above embodiments, a switch mechanism configured to bring an individual photosensitive drum into contact with/separate from the intermediate transfer belt may be used. Still further, in a case where a part of the image bearing members is switched to be used/not to be used depending on modes of image forming operations, the switch mechanism may be what switches the contact/separate conditions of, at least, such part of the image bearing members with respect to the intermediate transfer member. 
     OTHER EMBODIMENTS 
     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. 2016-152177, filed on Aug. 2, 2016, which is hereby incorporated by reference wherein in its entirety.