Patent Publication Number: US-11378895-B2

Title: Image forming apparatus with developing roller separation processing

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2020-051543 filed Mar. 23, 2020. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     This disclosure relates to an image forming apparatus. 
     BACKGROUND 
     An image forming apparatus having a separation mechanism is conventionally known. The separation mechanism is configured to cause a developing roller to contact or separate from a photosensitive drum. 
     SUMMARY 
     According to one aspect, this specification discloses an image forming apparatus. The image forming apparatus includes a photosensitive drum, a developing roller, a transfer member, a moving mechanism, a drive mechanism, and a controller. The developing roller is configured to move between a contact position at which the developing roller contacts the photosensitive drum and a separation position at which the developing roller is separated from the photosensitive drum. The transfer member is configured to form a transfer nip with the photosensitive drum. The moving mechanism is configured to move the developing roller between the contact position and the separation position. The drive mechanism is configured to drive the photosensitive drum. The controller is configured to: in a case where an inter-sheet time is shorter than or equal to a first time, maintain the developing roller in contact with the photosensitive drum during the inter-sheet time, the inter-sheet time being a time after a trailing end of a sheet corresponding to a particular page reaches the transfer nip and before a leading end of a sheet corresponding to a next page reaches the transfer nip in a case where a plurality of pages is printed successively; in a case where the inter-sheet time is longer than the first time and shorter than or equal to a second time longer than the first time, perform at least part of temporary separation processing during the inter-sheet time, the temporary separation processing being processing of controlling, in a state where the photosensitive drum is rotating, the moving mechanism and the drive mechanism to separate the developing roller from the photosensitive drum and subsequently bring the developing roller into contact with the photosensitive drum; and in a case where the inter-sheet time is longer than the second time, perform at least part of temporary stop processing during the inter-sheet time, the temporary stop processing being processing of controlling the moving mechanism and the drive mechanism to separate the developing roller from the photosensitive drum and then stop rotation of the photosensitive drum, and subsequently restart rotation of the photosensitive drum and then bring the developing roller into contact with the photosensitive drum. 
     According to another aspect, this specification also discloses an image forming apparatus. The image forming apparatus includes a photosensitive drum, a developing roller, a transfer member, a moving mechanism, a drive mechanism, and a controller. The developing roller is configured to move between a contact position at which the developing roller contacts the photosensitive drum and a separation position at which the developing roller is separated from the photosensitive drum. The transfer member is configured to form a transfer nip with the photosensitive drum. The moving mechanism is configured to move the developing roller between the contact position and the separation position. The drive mechanism is configured to drive the photosensitive drum. The controller is configured to: when performing duplex printing such that only one sheet formed with a toner image on at least one surface thereof is retained within the image forming apparatus, perform at least part of temporary stop processing during a time after a trailing end of a sheet corresponding to a particular page reaches the transfer nip and before a leading end of a sheet corresponding to a next page reaches the transfer nip, the temporary stop processing being processing of controlling the moving mechanism and the drive mechanism to separate the developing roller from the photosensitive drum and then stop rotation of the photosensitive drum and subsequently restart rotation of the photosensitive drum and then bring the developing roller into contact with the photosensitive drum; and when performing duplex printing such that two or more sheets formed with a toner image on at least one surface thereof are retained within the image forming apparatus, maintain the developing roller in contact with the photosensitive drum or perform at least part of temporary separation processing during a time after a trailing end of a sheet corresponding to a particular page reaches the transfer nip and before a leading end of a sheet corresponding to a next page reaches the transfer nip, the temporary separation processing being processing of controlling, in a state where the photosensitive drum is rotating, the moving mechanism and the drive mechanism to separate the developing roller from the photosensitive drum and subsequently bring the developing roller into contact with the photosensitive drum. 
     According to still another aspect, this specification also discloses an image forming apparatus. The image forming apparatus includes a photosensitive drum, a developing roller, a transfer member, a moving mechanism, a drive mechanism, a sheet sensor, and a controller. The developing roller is configured to move between a contact position at which the developing roller contacts the photosensitive drum and a separation position at which the developing roller is separated from the photosensitive drum. The transfer member is configured to form a transfer nip with the photosensitive drum. The moving mechanism is configured to move the developing roller between the contact position and the separation position. The drive mechanism is configured to drive the photosensitive drum. The sheet sensor is configured to detect a sheet at a position upstream of the transfer nip in a conveyance direction of the sheet. The controller is configured to, in a case where an inter-sheet time is shorter than a first time, maintain the developing roller in contact with the photosensitive drum during the inter-sheet time. The inter-sheet time is a time after a trailing end of a sheet corresponding to a particular page reaches the transfer nip and before a leading end of a sheet corresponding to a next page reaches the transfer nip in a case where a plurality of pages is printed successively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments in accordance with this disclosure will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a schematic diagram showing a color printer according to an embodiment of this disclosure; 
         FIG. 2A  is an explanatory diagram showing the states of separation/contact between photosensitive drums and developing rollers; 
         FIG. 2B  is a schematic diagram showing an example of a configuration of a moving mechanism; 
         FIGS. 3A to 3C  show the conveyance state of each sheet from the start of conveyance of a first sheet to the start of conveyance of a second sheet in a first duplex printing mode; 
         FIGS. 4A and 4B  show the conveyance state of each sheet from the formation of an image on a first surface of the second sheet to the start of conveyance of a third sheet; 
         FIGS. 5A and 5B  show the conveyance state of each sheet from the formation of an image on a first surface of the third sheet to the start of conveyance of a fourth sheet; 
         FIGS. 6A to 6C  are tables showing each processing to be executed between each page of duplex printing; 
         FIG. 7  is a flowchart showing the operation of a controller; 
         FIG. 8  is a flowchart showing temporary separation processing; and 
         FIG. 9  is a flowchart showing temporary stop processing. 
     
    
    
     DETAILED DESCRIPTION 
     In an image forming apparatus that performs development by causing a developing roller to contact a photosensitive drum, there is a problem that the developing roller and the photosensitive drum deteriorate if the developing roller is left in contact with the photosensitive drum during print processing. 
     In view of the foregoing, an aspect of an objective of this disclosure is to suppress deterioration of the developing roller and the photosensitive drum. 
     Hereinafter, an embodiment of this disclosure will be described in detail with reference to the drawings as appropriate. 
     As shown in  FIG. 1 , a color printer  1  as an example of an image forming apparatus is configured to perform color printing and monochrome printing and to form an image on both sides of a sheet S. The color printer  1  includes a main housing  2 , a supply unit  3 , an image forming unit  4 , a fixing device  8 , and a conveyance unit  9  arranged inside the main housing  2 . The main housing  2  has a discharge tray  21  on the upper surface thereof. 
     The supply unit  3  is provided in the lower part of the main housing  2 , and includes a supply tray  31  for accommodating the sheet S and a supply mechanism  32  for supplying the sheet S in the supply tray  31  to the image forming unit  4 . 
     The image forming unit  4  has a function of transferring a toner image to the sheet S to form an image, and includes an exposure device  5 , four process units  6 , and a transfer unit  7  as an example of a transfer member. 
     The exposure device  5  is arranged in the upper part of the main housing  2  and includes a light source (not shown), a polygon mirror, and so on. The exposure device  5  exposes the surface of a photosensitive drum  61  by irradiating the surface of the photosensitive drum  61  with light. 
     The process unit  6  includes the photosensitive drum  61 , a charger  62 , and a developing roller  63 . The process unit  6  further includes a drum cleaner  64  configured to contact the photosensitive drum  61  and clean toner and the like on the photosensitive drum  61 . Toner of respective colors of yellow, magenta, cyan and black is contained in the four process units  6 . 
     Regarding the process units  6 , the process units  6 Y,  6 M,  6 C, and  6 K containing toner of respective colors of yellow, magenta, cyan, and black are arranged in this order from the upstream in the conveyance direction of the sheet S. In the present specification and drawings, when the photosensitive drums  61 , the developing rollers  63 , and so on corresponding to the colors of toner are specified, the symbols Y, M, C, and K for yellow, magenta, cyan, and black, respectively, will be added. 
     As shown in  FIG. 2A , the developing roller  63  is configured to move between a contact position at which the developing roller  63  contacts the photosensitive drum  61  and a separation position at which the developing roller  63  is separated from the photosensitive drum  61 . The color printer  1  further includes a moving mechanism (separation mechanism)  300 , a motor MT as an example of a drive mechanism, and a controller  100 . The moving mechanism  300  is a mechanism that moves the developing roller  63  between the contact position and the separation position. The motor MT is a motor for driving the photosensitive drum  61 . The controller  100  has a CPU, a ROM, a RAM, and so on, and is configured to execute various processing in response to the reception of a print command and so on according to a program prepared in advance. The controller  100  controls the moving mechanism  300  and the motor MT. 
       FIG. 2B  shows an example of the configuration of the moving mechanism  300 . The developing roller  63  is rotatably supported by a developing roller support member  60 . A cam  98  and a spring  97  are provided at the main housing  2  side. The rotational position of the cam  98  is controlled by the controller  100 . The spring  97  urges the developing roller support member  60  in a direction from the upstream side toward the downstream side in the conveyance direction CD of the sheet S. A pressed portion  66  is provided at the developing roller support member  60 . The pressed portion  66  protrudes outward in the axial direction of the developing roller  63  from the developing roller support member  60 . When the pressed portion  66  (that is, the developing roller support member  60 ) is pressed by the cam  98  from the downstream side toward the upstream side in the conveyance direction CD, the developing roller support member  60  moves to a separation position where the developing roller  63  is separated from the photosensitive drum  61 , against the urging force of the spring  97 . When the pressure by the cam  98  is released, the developing roller support member  60  moves to a contact position where the developing roller  63  contacts the photosensitive drum  61  due to the urging force of the spring  97 . 
     Specifically, the controller  100  executes a color mode, a monochrome mode, and an all separation mode. In the color mode in which color printing is performed, the controller  100  performs control such that all the developing rollers  63 Y,  63 M,  63 C,  63 K contact the corresponding photosensitive drums  61 Y,  61 M,  61 C,  61 K. In the monochrome mode in which the controller  100  performs monochrome printing, the controller  100  performs control such that only the developing roller  63 K for black contacts the photosensitive drum  61 K, and the other three colors developing rollers  63 Y,  63 M,  63 C separate from the corresponding photosensitive drums  61 Y,  61 M,  61 C. That is, the photosensitive drum  61 K and the developing roller  63 K are examples of a first photosensitive drum and a first developing roller, respectively, which are used for both monochrome printing and color printing. The photosensitive drums  61 Y,  61 M,  61 C and the developing rollers  63 Y,  63 M,  63 C are examples of second photosensitive drums and second developing rollers, respectively, which are used only for color printing. Further, when cleaning the photosensitive drums  61 , for example, the controller  100  executes the all separation mode to separate all the developing rollers  63 Y,  63 M,  63 C,  63 K from the corresponding photosensitive drums  61 Y,  61 M,  61 C,  61 K, respectively. 
     As shown in  FIG. 1 , the transfer unit  7  includes a drive roller  71 , a follow roller  72 , a conveyance belt  73 , and four transfer rollers  74 . The conveyance belt  73  is an endless belt, and is stretched between the drive roller  71  and the follow roller  72 . Inside the conveyance belt  73 , the transfer rollers  74  are arranged so as to sandwich the conveyance belt  73  with the corresponding photosensitive drums  61 . 
     The transfer unit  7  forms, together with each photosensitive drum  61  therebetween, transfer nips NY, NM, NC, and NK for transferring a toner image on each photosensitive drum  61  to the sheet S. Specifically, the transfer nips NY, NM, NC, and NK are formed between each photosensitive drum  61  and the conveyance belt  73 . 
     A cleaning member  75  configured to clean toner on the conveyance belt  73  of the transfer unit  7  is arranged below the transfer unit  7 . The cleaning member  75  includes a cleaning roller  75 A that contacts the conveyance belt  73  to collect the toner on the conveyance belt  73 , and a waste toner accommodating portion  75 B that stores the toner collected by the cleaning roller  75 A. 
     The charger  62  charges the surface of the photosensitive drum  61 . After that, the exposure device  5  exposes the surface of the photosensitive drum  61  to form an electrostatic latent image based on image data on the surface of the photosensitive drum  61 . 
     The developing roller  63  supplies toner to the electrostatic latent image formed on the photosensitive drum  61 . As a result, a toner image is formed on the photosensitive drum  61 . After that, when the sheet S is conveyed between the photosensitive drum  61  and the transfer roller  74  by the conveyance belt  73 , the toner image on the photosensitive drum  61  is transferred onto the sheet S. 
     A sheet sensor SS for detecting the presence or absence of the sheet S is provided upstream of the image forming unit  4  in the conveyance direction of the sheet S. The sheet sensor SS includes a swing lever that is pushed by the conveyed sheet S and swings, for example, and an optical sensor that detects the swing of the swing lever. In this embodiment, it is assumed that the sheet sensor SS is ON while the sheet S is passing, that is, when the swing lever is tilted by the sheet S, and that the sheet sensor SS is OFF while the sheet S is not passing, that is, when the swing lever is not tilted by the sheet S. The relationship between the posture of the swing lever and ON and OFF of the sheet sensor SS may be reversed. 
     The fixing device  8  is a device that thermally fixes the toner image on the sheet S. The fixing device  8  includes a heating roller  81  and a pressure roller  82  that sandwiches the sheet S with the heating rollers  81 . 
     The conveyance unit  9  is configured to convey the sheet S discharged from the fixing device  8  to the outside of the main housing  2  or to the image forming unit  4  again. The conveyance unit  9  includes a first conveyance path  91 , a second conveyance path  92 , a reconveyance path  93 , a first conveyance roller  94 , a second conveyance roller  95 , a first switchback roller SR 1 , and a second switchback roller SR 2 , a plurality of reconveyance rollers  96 , a first flapper FL 1 , and a second flapper FL 2 . 
     The first conveyance path  91  is a path that guides the sheet discharged from the fixing device  8  toward the discharge tray  21 . The second conveyance path  92  is a path that guides the sheet discharged from the fixing device  8  toward the discharge tray  21  by a route different from the first conveyance path  91 . The reconveyance path  93  is a path that guides the sheet S drawn into the main housing  2  by the first switchback roller SR 1  and so on described later, to the supply mechanism  32  on the upstream side of the image forming unit  4 . The reconveyance rollers  96  convey the sheet S in the reconveyance path  93  toward the supply mechanism  32 , and is provided on the reconveyance path  93 . 
     The first conveyance roller  94  is provided in the fixing device  8 . The first conveyance roller  94  conveys the sheet S on which the toner image is thermally-fixed toward the second flapper FL 2 . 
     The second conveyance roller  95 , the first switchback roller SR 1  and the second switchback roller SR 2  are configured to rotate in the forward and reverse directions. The second conveyance roller  95 , the first switchback roller SR 1 , and the second switchback roller SR 2  convey the sheet S to the outside of the main housing  2  during forward rotation, specifically toward the discharge tray  21 , and draw the sheet S into the main housing  2  during reverse rotation. 
     The second conveyance roller  95  and the first switchback roller SR 1  are provided on the first conveyance path  91 . The first switchback roller SR 1  is arranged closer to the discharge tray  21  than the second conveyance roller  95  is. The second switchback roller SR 2  is provided on the second conveyance path  92 . 
     The first flapper FL 1  is rotatable between a first position P 1  shown by the double-dot chain line and a second position P 2  shown by the solid line. When the first flapper FL 1  is located at the first position P 1 , the first flapper FL 1  guides the sheet S discharged from the fixing device  8  toward the second flapper FL 2 . When the first flapper FL 1  is located at the second position P 2 , the first flapper FL 1  prevents the sheet S drawn into the main housing  2  by the first switchback roller SR 1  or the second switchback roller SR 2  from being conveyed toward the fixing device  8 . 
     The second flapper FL 2  is rotatable between a third position P 3  shown by the solid line and a fourth position P 4  shown by the double-dot chain line. When the second flapper FL 2  is located at the third position P 3 , the second flapper FL 2  guides the sheet S discharged from the fixing device  8  toward the first switchback roller SR 1 . When the second flapper FL 2  is located at the fourth position P 4 , the second flapper FL 2  guides the sheet S discharged from the fixing device  8  toward the second switchback roller SR 2 . 
     The first flapper FL 1  is urged from the first position P 1  toward the second position P 2  by a spring (not shown). When the sheet S discharged from the fixing device  8  pushes the first flapper FL 1  against the urging force of the spring, the first flapper FL 1  rotates from the second position P 2  to the first position P 1 . Then, when the trailing end of the sheet S separates from the first flapper FL 1 , the first flapper FL 1  rotates from the first position P 1  to the second position P 2  by the urging force of the spring. The second flapper FL 2  is located at the third position P 3  in a state where no printing is performed. The second flapper FL 2  is configured to be switched between the third position P 3  and the fourth position P 4  by the controller  100 . 
     The controller  100  is configured to execute duplex (double-sided) print processing of performing printing on both sides of the sheet S. 
     The duplex print processing executed by the controller  100  includes two modes, that is, a first duplex printing mode in which the second flapper FL 2  is controlled and duplex printing is performed by using the two conveyance paths  91  and  92 , and a second duplex printing mode in which duplex printing is performed by using only the first conveyance path  91  while the second flapper FL 2  is not moved so as to remain at the third position P 3 . Alternatively, the controller  100  may be set so as to perform the first duplex printing mode in a case where the apparatus includes the second conveyance path  92  and the second flapper FL 2 , whereas the controller  100  may be set so as to perform the second duplex printing mode in a case where the apparatus does not include the second conveyance path  92  and the second flapper FL 2 . 
     Specifically, when the duplex print processing is performed on three or more sheets S in the first duplex printing mode, as shown in  FIGS. 3A and 3B , the controller  100  controls the image forming unit  4  to form an image on the first surface of the first sheet SH 1  picked up from the supply tray  31 , after which this sheet SH 1  is conveyed to the first conveyance path  91 . Thereafter, as shown in  FIG. 3C , the controller  100  controls the first switchback roller SR 1  and so on to reverse the conveyance direction of (switchback) the first sheet SH 1  to convey the first sheet SH 1  to the reconveyance path  93 . In  FIGS. 3A to 3C , the sheet shown by the broken line shows a sheet after the switchback. The sheet shown by the solid line shows a sheet before the switchback. 
     Next, the controller  100  picks up the second sheet SH 2  from the supply tray  31  to convey the second sheet SH 2  to the image forming unit  4 . Thereafter, as shown in  FIG. 4A , the controller  100  controls the image forming unit  4  to form an image on the first surface of the second sheet SH 2  while conveying the first sheet SH 1  on the reconveyance path  93  to convey this sheet SH 2  to the first conveyance path  91 . 
     Thereafter, as shown in  FIG. 4B , the controller  100  controls the first switchback roller SR 1  and so on to reverse the conveyance direction of (switchback) the second sheet SH 2  while conveying the first sheet SH 1  on the reconveyance path  93  and convey the second sheet SH 2  to the reconveyance path  93 . Next, the controller  100  picks up the third sheet SH 3  from the supply tray  31  to send the sheet SH 3  to the downstream side of the first sheet SH 1  in the conveyance direction. 
     Thereafter, as shown in  FIG. 5A , the controller  100  switches the second flapper FL 2  from the third position P 3  to the fourth position P 4 . As a result, the third sheet SH 3  for which an image is formed on the first surface by the image forming unit  4  is conveyed to the second conveyance path  92 . 
     After the third sheet SH 3  is conveyed to the second conveyance path  92 , as shown in  FIG. 5B , the controller  100  switches the second flapper FL 2  from the fourth position P 4  to the third position P 3 . Thereafter, the controller  100  controls the second switchback roller SR 2  to reverse the conveyance direction of (switchback) the third sheet SH 3  and controls the image forming unit  4  to form an image on the second surface of the first sheet SH 1 . The first sheet SH 1  for which images are formed on both surfaces is discharged to the discharge tray  21  through the first conveyance path  91 . 
     During this, the controller  100  picks up the fourth sheet SH 4  from the supply tray  31  to supply this sheet SH 4  to a position between the first sheet SH 1  and the second sheet SH 2 . Thereafter, the controller  100  similarly inserts a new sheet S between the two sheets S for which images are formed on the first surfaces or switches the second flappers FL 2  alternately, thereby reversing the conveyance direction of (switchback) the sheet S for which an image is formed only on the first surface through the second conveyance path  92  and discharging the sheet S for which images are formed on both surfaces to the discharge tray  21  through the first conveyance path  91 . 
     As shown in  FIGS. 3A to 3C  and  FIG. 4A , when the duplex print processing is performed on three or more sheets S in the second duplex printing mode, the controller  100  performs a processing similar to the first duplex printing mode to form images on each first surface of the first sheet SH 1  and the second sheet SH 2  and reverse the conveyance direction of (switchback) the sheets in the first conveyance path  91 . Thereafter, the controller  100  forms an image on the second surface of the first sheet SH 1 . Next, the controller  100  picks up the third sheet SH 3  from the supply tray  31  to supply this third sheet SH 3  to a position between the first sheet SH 1  and the second sheet SH 2 , and forms an image on the first surface of the third sheet SH 3 . 
     The controller  100  discharges the first sheet SH 1  for which images are formed on both surfaces to the discharge tray  21  through the first conveyance path  91 , and subsequently reverses the conveyance direction of (switchback) the third sheet SH 3  in the first conveyance path  91 . Thereafter, the controller  100  similarly inserts a new sheet S between two sheets S for which images are formed on the first surfaces to thereby discharge the sheets S for which images are formed on both surfaces to the discharge tray  21  through the first conveyance path  91 , and reverses the conveyance direction of (switchback) the sheet S for which an image is formed only on the first surface in the first conveyance path  91 . 
     The controller  100  is configured to execute temporary separation processing and temporary stop processing. The temporary separation processing is processing to control the moving mechanism  300  and the motor MT to separate the developing roller  63  from the photosensitive drum  61  and thereafter bring the developing roller  63  into contact with the photosensitive drum  61  while the photosensitive drum  61  is rotating. Specifically, in the temporary separation processing, the controller  100  controls the moving mechanism  300  while continuing the driving of the motor MT, thereby causing the developing roller  63  separate from the photosensitive drum  61  and subsequently contact the photosensitive drum  61 . 
     The temporary stop processing is processing to control the moving mechanism  300  and the motor MT to separate the developing roller  63  from the photosensitive drum  61  and then stop the rotation of the photosensitive drum  61 , and subsequently restart the rotation of the photosensitive drum  61  and then bring the developing roller  63  into contact with the photosensitive drum  61 . Specifically, in the temporary stop processing, the controller  100  controls the moving mechanism  300  to separate the developing roller  63  from the photosensitive drum  61  and subsequently stops the motor MT. Thereafter, the controller  100  restarts the driving of the motor MT and then brings the developing roller  63  into contact with the photosensitive drum  61 . In the temporary stop processing, the controller  100  is configured to, in accordance with the stop and restart of the rotation of the photosensitive drum  61 , stop and restart the rotation of the drum cleaner  64 , the drive roller  71 , the transfer roller  74 , and the cleaning roller  75 A. 
     The controller  100  is configured to perform, for each color, contact and separation processing of the developing roller  63  in the temporary separation processing or the temporary stop processing described above. The contact and separation processing of the developing roller  63  performed for each color is similar processing. Thus, the following section will describe contact and separation processing of the developing roller for yellow as a typical example and will not repeat the description for other colors if the same description applies. 
     In order to print a plurality of pages successively, the controller  100  executes the temporary separation processing or the temporary stop processing described above depending on an inter-sheet time TS which is the time after the trailing end of the sheet S corresponding to a particular page reaches the transfer nip NY and before the leading end of the sheet S corresponding to the next page reaches the transfer nip NY. Specifically, the controller  100  is configured to, when the inter-sheet time TS is shorter than or equal to a first time T 1 , execute maintaining processing to maintain a state where the photosensitive drum  61 Y is rotating and the developing roller  63 Y is in contact with the photosensitive drum  61 Y during the inter-sheet time TS. 
     The controller  100  is configured to, when the inter-sheet time TS is longer than the first time T 1  and is shorter than or equal to a second time T 2  which is longer than the first time T 1 , execute at least a part of the temporary separation processing during the inter-sheet time TS. That is, the entirety of the temporary separation processing may be executed during the inter-sheet time TS. Alternatively, a part of the temporary separation processing may be executed in a period before or after the inter-sheet time TS and the remaining part of the processing may be executed during the inter-sheet time TS. 
     Specifically, the controller  100  executes the temporary separation processing after the development of an image to be transferred to a particular page is finished and before the development of an image to be transferred to the next page is started. That is, the controller  100  is configured to separate the developing roller  63 Y from the photosensitive drum  61 Y after the development by the developing roller  63 Y of an electrostatic latent image corresponding to the particular page on the photosensitive drum  61 Y is finished, and to bring the developing roller  63 Y into contact with the photosensitive drum  61 Y before the development by the developing roller  63 Y of an electrostatic latent image corresponding to the next page on the photosensitive drum  61 Y is started. 
     The controller  100  executes at least a part of the temporary stop processing during the inter-sheet time TS when the inter-sheet time TS is longer than the second time T 2 . That is, the entirety of the temporary stop processing may be executed during the inter-sheet time TS. Alternatively, a part of the temporary stop processing may be executed in a period before or after the inter-sheet time TS, and the remaining part of the processing may be executed during the inter-sheet time TS. 
     Specifically, the controller  100  executes the temporary stop processing after the development of an image to be transferred to the particular page is finished and before the development of an image to be transferred to the next page is started. Specifically, the controller  100  separates the developing roller  63 Y from the photosensitive drum  61 Y and stops the rotation of the photosensitive drum  61 Y and so on, after the development by the developing roller  63 Y of the electrostatic latent image corresponding to the particular page on the photosensitive drum  61 Y is finished. Thereafter, before the development by the developing roller  63 Y of the electrostatic latent image corresponding to the next page on the photosensitive drum  61 Y is started, the controller  100  restarts the rotation of the photosensitive drum  61 Y and so on and brings the developing roller  63 Y into contact with the photosensitive drum  61 Y. 
     The controller  100  determines, based on information from the sheet sensor SS, the timing to start the temporary separation processing or the temporary stop processing described above. That is, the controller  100  starts the temporary separation processing or the temporary stop processing based on the information from the sheet sensor SS such that at least a part of the temporary separation processing or the temporary stop processing is executed during the inter-sheet time TS. 
     As shown in  FIGS. 6A to 6C , the temporary stop processing, the temporary separation processing, and the maintaining processing are set depending on a page interval in the duplex print processing. A first table shown in  FIG. 6A  shows the settings of each processing when duplex printing is performed on a sheet S having a particular length in the first duplex printing mode. A second table shown in  FIG. 6B  shows the settings of each processing when duplex printing is performed on a long recording paper (which is a sheet S having a length longer than the particular length) in the first duplex printing mode. A third table shown in  FIG. 6C  shows the settings of each processing when duplex printing is performed on a sheet S having the particular length in the second duplex printing mode. 
     The mode “21DX” shown in the “duplex mode” column of the first table is a mode for executing the duplex print processing on both surfaces of one sheet S. In the 21DX mode, the controller  100  controls the image forming unit  4  to form an image on the first surface of a particular sheet S and subsequently form an image on the second surface of the particular sheet S. In the 21DX mode is a mode for executing the duplex printing in which only one sheet S for which a toner image is formed on at least one surface is retained within the apparatus. In the 21DX mode, because the inter-sheet time TS is longer than the second time T 2 , the processing executed in this mode is set to the temporary stop processing. 
     Similarly, for each page interval in the duplex printing, appropriate time, as the inter-sheet time TS, is set for each page interval depending on the number of the sheets S retained in the apparatus, and so on. Thus, in each table, the inter-sheet time TS is set depending on the page interval, and the processing suitable for the inter-sheet time TS is set. That is, the maintaining processing is set for a page interval for which the inter-sheet time TS is shorter than or equal to the first time T 1 . The temporary separation processing is set for a page interval for which T 1 &lt;TS≤T 2  (TS is longer than T 1  and shorter than or equal to T 2 ) is satisfied. The temporary stop processing is set for a page interval for which TS&gt;T 2  (TS is longer than T 2 ) is satisfied. 
     Specifically, the mode “2413DX” shown in the “duplex mode” column of the first table is a mode for executing duplex printing on both surfaces of two sheets S. In the 2413DX mode, the controller  100  controls the image forming unit  4  to print an image corresponding to the second page on the first surface of the first sheet S picked up from the supply tray  31 , and to subsequently print an image corresponding to the fourth page on the first surface of the second sheet S picked up by the supply tray  31 . Thereafter, the controller  100  controls the image forming unit  4  to print an image corresponding to the first page on the second surface of the inverted first sheet S and to print an image corresponding to the third page on the second surface of the inverted second sheet S. In the 2413DX mode, the inter-sheet time TS for each page interval is set to be longer than the first time T 1  and shorter than or equal to the second time T 2 . Thus, the temporary separation processing is set as the processing executed in each page interval in this mode. 
     The mode “2461DX” shown in the “duplex mode” column of the first table is a mode for executing the duplex printing on both surfaces of three or more sheets S. In this mode, the duplex printing is performed on the second page, the fourth page, the sixth page, and the first page in this order, and subsequently the printing is performed in order of an even number, an odd number, and an even number. At the end of the duplex printing, a plurality of odd number pages is printed successively. The reason is that there is no need any more to insert a new sheet S between the two sheets S for which the first surfaces are already printed, for example. In this mode, different inter-sheet time TS is set depending on the page interval. In the following description, page intervals such as “the interval between the second page and the fourth page” will be simply denoted as “page interval 2-4” and other page intervals also will be simply denoted in a similar manner. 
     Specifically, in this mode, the inter-sheet time TS is set to T 1 &lt;TS≤T 2  for the page interval 2-4, the page interval 4-6, and the page interval between the second-from-last page and the last page. For the other page intervals, the inter-sheet time TS is set to a time which is shorter than or equal to the first time T 1 . Thus, the temporary separation processing is set as the processing executed on the page interval 2-4, the page interval 4-6, and the page interval between the second-from-last page and the last page, while the maintaining processing is set as the processing executed on the other page intervals. 
     Each processing is similarly set in the second table and the third table. The mode “2416DX” in the second table and the third table is a mode to execute the duplex printing on both surfaces of three or more sheets S. In this mode, the printing is performed on pages in an order different from that in the “2461DX” in the first table, since a sheet S longer than the particular length is conveyed or the second conveyance path  92  is not used. 
     The 2413DX mode, the 2461DX mode, or the 2416DX mode described above is a mode for executing the duplex printing in which two or more sheets S for which a toner image is formed on at least one surface is retained within the apparatus. 
     When the temporary separation processing is executed in a case where the monochrome printing is performed successively after the color printing is performed, the controller  100  separates all of the developing rollers  63  from the corresponding photosensitive drums  61  and subsequently brings only the monochrome developing roller  63 K into contact with the photosensitive drum  61 K in a state where all of the photosensitive drums  61  are rotating. When the temporary stop processing is executed in a case where the monochrome printing is performed successively after the color printing is performed, the controller  100  separates all of the developing rollers  63  from the corresponding photosensitive drums  61  and then stops the rotation of all of the photosensitive drums  61 , and subsequently restarts the rotation of all of the photosensitive drums  61  and then brings only the monochrome developing roller  63 K into contact with the photosensitive drum  61 K. 
     When the temporary separation processing is executed in a case where the color printing is performed successively after the monochrome printing is performed, the controller  100  separates the monochrome developing roller  63 K from the photosensitive drums  61 K and subsequently brings all of the developing rollers  63  into contact with the corresponding photosensitive drums  61  in a state where all of the photosensitive drums  61  are rotating. When the temporary stop processing is executed in a case where the color printing is performed successively after the monochrome printing is performed, the controller  100  separates the monochrome developing roller  63 K from the photosensitive drum  61 K and then stops the rotation of all of the photosensitive drums  61 , and subsequently restarts the rotation of all of the photosensitive drums  61  and then brings all of the developing rollers  63  into contact with the corresponding photosensitive drums  61 . 
     When the temporary separation processing is executed in a case where the monochrome printing is performed successively after the monochrome printing is performed, the controller  100  separates the monochrome developing roller  63 K from the photosensitive drum  61 K and subsequently brings only the monochrome developing roller  63 K into contact with the photosensitive drum  61 K in a state where all of the photosensitive drums  61  are rotating. When the temporary stop processing is executed in a case where the monochrome printing is performed successively after the monochrome printing is performed, the controller  100  separates the monochrome developing roller  63 K from the photosensitive drum  61 K and then stops the rotation of all of the photosensitive drums  61 , and subsequently restarts the rotation of all of the photosensitive drums  61  and then brings only the monochrome developing roller  63 K into contact with the photosensitive drum  61 K. 
     When the temporary separation processing is executed in a case where the color printing is performed successively after the color printing is performed, the controller  100  separates all of the developing rollers  63  from the corresponding photosensitive drums  61  and subsequently brings all of the developing rollers  63  into contact with the corresponding photosensitive drums  61  in a state where all of the photosensitive drums  61  are rotating. When the temporary stop processing is executed in a case where the color printing is performed successively after the color printing is performed, the controller  100  separates all of the developing rollers  63  from the corresponding photosensitive drums  61  and then stops the rotation of all of the photosensitive drums  61 , and subsequently restarts the rotation of all of the photosensitive drums  61  and then brings all of the developing rollers  63  into contact with the corresponding photosensitive drums  61 . 
     Next, the following section will describe the operation of the controller  100  in detail. The controller  100  repeatedly executes the processing shown in  FIG. 7 . 
     In the processing shown in  FIG. 7 , the controller  100  firstly determines whether a print instruction has been received (S 1 ). In response to determining in S 1  that no print instruction has been received (S 1 : No), then the controller  100  completes this processing. 
     In response to determining in S 1  that a print instruction has been received (S 1 : Yes), the controller  100  determines whether duplex printing is to be performed based on the print instruction (S 2 ). In response to determining in S 2  that duplex printing is to be performed (S 2 : Yes), the controller  100  starts the duplex print processing (S 3 ). 
     After S 3 , the controller  100  controls the moving mechanism  300  and the motor MT based on the number of pages and the tables of  FIGS. 6A to 6C  and so on (S 4 ). For example, when the printing mode is the first duplex printing mode and the sheet S has a particular length, the controller  100  selects the first table of  FIG. 6A . Then, the controller  100  determines which duplex mode is to be used for printing based on the number of pages to be printed. For example, in a case where the number of pages is four pages, the controller  100  selects the 2413DX mode as the duplex mode. In the 2413DX mode, the inter-sheet time TS between each page is longer than the first time T 1  and is shorter than or equal to the second time T 2 . Thus, the controller  100  performs the temporary separation processing which is set to each page interval in the 2413DX mode, as shown in the tables of  FIGS. 6A and 6C . 
     Returning to  FIG. 7 , after S 4 , the controller  100  determines whether the duplex print processing is completed for all pages (S 5 ). In response to determining in S 5  that the duplex print processing is not yet completed (S 5 : No), the controller  100  returns to the processing of S 4 . 
     In response to determining in S 5  that the duplex print processing is completed (S 5 : Yes), the controller  100  ends this processing. In response to determining in S 2  that the duplex print processing is not to be performed (S 2 : No), the controller  100  performs simplex (single-sided) print processing of printing only one surface of the sheet S (S 6 ) and ends this processing. 
     As shown in  FIG. 8 , in the temporary separation processing, the controller  100  firstly controls the moving mechanism  300  to separate all of the developing rollers  63  from the corresponding photosensitive drums  61  (S 21 ). Specifically, when the previous printing mode is monochrome printing, in S 21  the controller  100  separates the monochrome developing roller  63 K from the photosensitive drum  61 K so that all of the developing rollers  63  are located at the separation position. When the previous printing mode is color printing, in S 21  the controller  100  separates all of the developing rollers  63  from the corresponding photosensitive drums  61 . 
     After S 21 , the controller  100  determines whether the current printing mode is color printing (S 22 ). In response to determining in S 22  that the current printing mode is color printing (S 22 : Yes), the controller  100  controls the moving mechanism  300  to bring all of the developing rollers  63  into contact with the corresponding photosensitive drums  61  after the elapse of a first particular time (S 23 ) and ends this processing. The first particular time is a time having a certain length in order to suppress the deterioration of the developing roller  63  and is set through experiments, simulations, and so on. 
     In response to determining in S 22  that the current printing mode is not color printing (that is, that the current printing mode is monochrome printing) (S 22 : No), the controller  100  controls the moving mechanism  300  to bring only the monochrome developing roller  63 K into contact with the photosensitive drum  61 K after the elapse of the first particular time (S 24 ) and ends this processing. 
     As shown in  FIG. 9 , in the temporary stop processing, the controller  100  firstly controls the moving mechanism  300  to separate all of the developing rollers  63  from the corresponding photosensitive drums  61  (S 41 ). Specifically, when the previous printing mode is monochrome printing, in S 41  the controller  100  separates the monochrome developing roller  63 K from the photosensitive drum  61 K such that all of the developing rollers  63  are located at the separation position. When the previous printing mode is color printing, in S 41  the controller  100  separates all of the developing rollers  63  from the corresponding photosensitive drums  61 . 
     After S 41 , the controller  100  stops all of the photosensitive drums  61 , the drum cleaner  64 , and so on (S 42 ). After S 42 , the controller  100  restarts the driving of all of the photosensitive drums  61 , the drum cleaner  64 , and so on, after the elapse of a second particular time since the photosensitive drum  61  and so on are stopped (S 43 ). The second particular time is the time having a certain length in order to suppress the deterioration of the developing roller  63  and the photosensitive drum  61  and is set through experiments, simulations, and so on. 
     After S 43 , the controller  100  determines whether the current printing mode is the color printing (S 44 ). In response to determining in S 44  that the current printing mode is the color printing (S 44 : Yes), the controller  100  controls all of the developing rollers  63  to contact the corresponding photosensitive drums  61  (S 45 ) and ends this processing. 
     In response to determining in S 44  that the current printing mode is not the color printing (that is, that the current printing mode is the monochrome printing) (S 44 : No), the controller  100  controls only the monochrome developing roller  63 K to contact the photosensitive drum  61 K (S 46 ) and ends this processing. 
     The following effects can be obtained in the embodiment. 
     In a case where the inter-sheet time TS is longer than the first time T 1 , by executing the temporary separation processing or the temporary stop processing, the developing roller  63  is temporarily separated from the photosensitive drum  61  and the photosensitive drum  61  is temporarily stopped. Thus, deterioration of the developing roller  63  and the photosensitive drum  61  can be suppressed. 
     Further, in the embodiment, the temporary separation processing or the temporary stop processing is executed depending on the number of sheets retained in the apparatus in duplex printing, so that the developing roller  63  is temporarily separated from the photosensitive drum  61  and the photosensitive drum  61  is temporarily stopped. Thus, deterioration of the developing roller  63  and the photosensitive drum  61  can be suppressed. 
     While the disclosure has been described in detail with reference to the above aspects thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the claims. 
     In the moving mechanism of the above-described embodiment, the rotary motion of the motor is converted into a reciprocating motion by using a cam and cam follower mechanism, and the developing roller is reciprocated between the contact position and the separation position. The mechanism that converts the rotary motion of the motor into the reciprocating motion is not limited to the cam and cam follower mechanism. Other mechanisms that convert a rotary motion into a reciprocating motion, such as a crank-link mechanism, may be used. 
     In the above-described embodiment, the motor MT is exemplified as the drive mechanism, but the present disclosure is not limited to this. The drive mechanism may include, for example, a motor and a clutch for switching transmission of a driving force from the motor to the photosensitive drum. In this case, by switching the connection state of the clutch, the rotation and stop of the photosensitive drum may be switched. 
     In the above-described embodiment, this disclosure is applied to the color printer  1 . However, this disclosure is not limited to this, and may be applied to another image forming apparatus, such as a monochrome printer, a copier, and a multifunction peripheral. 
     In the above-described embodiment, the transfer unit  7  including the conveyance belt  73  has been exemplified as the transfer member, but the present disclosure is not limited to this. The transfer member may be, for example, a transfer roller that contacts the photosensitive drum. 
     The elements described in the above-described embodiments and modifications may be combined as appropriate.