Patent Publication Number: US-11036163-B2

Title: Image forming apparatus that discharges developer

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus that uses a dry-type two-component developing method. 
     Description of the Related Art 
     An electrophotographic image forming apparatus employing a dry-type developing method using a two-component developer consisted of toner and a carrier is known. In this image forming apparatus, when a number of images with low coverage rates are formed, toner is excessively charged because a developing device for which toner consumption or toner replenishment is not performed is running for a long period of time. This presents a problem that the amount of developer put on a photosensitive body per unit area decreases and also presents a problem that an external additive attached to the toner falls off due to friction with the carrier, causing degradation of print quality. 
     For this reason, an image forming apparatus has been proposed which adds up the number of pixels in image information, and when an integrated value exceeds a threshold value when a predetermined number of sheets or more have been printed, forms a pattern image to consume toner in a developing device (Japanese Laid-Open Patent Publication (Kokai) No. 2007-264398). In this image forming apparatus, the pattern image (hereafter referred to as a discharge pattern) formed on a photosensitive body is not transferred to a recording medium but is collected by a removal means (cleaning unit) for removing toner on the photosensitive body. Accordingly, to prevent a toner image from being transferred from the photosensitive body to an intermediate transfer body, this image forming apparatus provides control such that a high voltage for primary transfer is opposite in polarity to a bias applied in a case where an image is formed on the recording medium. 
     The image forming apparatus disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2007-264398 switches the primary transfer bias from a bias for normal image formation (positive bias) to a reverse bias successively in each of stations after forming the discharge pattern described above. During the switching from the positive bias to the reverse bias, a force acting on the intermediate transfer body varies, and the behavior of the intermediate transfer body temporarily are unstable in a direction (width direction) perpendicular to a conveying direction of the intermediate transfer body. For example, in the image forming apparatus has stations for four colors, assuming that the switching to the reverse bias is started in the station for the first color when primary transfer of preceding images is being performed or transfer is getting started in the stations for the third and fourth ones of the four colors. This may cause color misregistration because transfer positions in the respective stations become misaligned in a main scanning direction (width direction). Moreover, when the primary transfer bias is switched from the reverse bias back to the positive bias successively in each station so as to switch back to normal image formation again, the behavior of the intermediate transfer body also are unstable for a predetermined period of time after the switching back to the positive bias. Primary transfer of succeeding images in this unstable state may cause color misregistration. 
     To prevent such color misregistration, it is necessary to wait until transfer of succeeding images is completed in all the stations before formation of discharge patterns ( 2001  in  FIG. 5A ). Also, to form succeeding images after the formation of the discharge patterns, it is necessary to wait until the primary transfer bias in all the stations is switched back to the positive bias and the behavior of the intermediate transfer body stabilizes ( 2003 ,  2004  in  FIG. 5A ). However, this would considerably increase the waiting time and decrease productivity of image formation. 
     In a monochrome mode (monochrome print mode), even if primary transfer to the intermediate transfer body is done with its behavior being unstable, no color misregistration would occur because a toner image is formed and transferred in only a station for one color. If the above described measures such as waiting for transfer of preceding images and waiting for the behavior of the intermediate transfer body to stabilize are taken across the board when discharge patterns are formed in this monochrome mode, the problem of decreased productivity would remain unsolved (see  FIG. 5B ). 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides an image forming apparatus including a first image forming unit configured to have a first photosensitive body and a first developing device storing a black developer and form a black image on the first photosensitive body with the black developer in the first developing device, a second image forming unit configured to have a second photosensitive body and a second developing device storing a color developer and form a color image on the second photosensitive body with the color developer in the second developing device, an intermediate transfer body on which the black image and the color image are formed, a transfer unit configured to transfer the black image and the color image formed on the intermediate transfer body to a sheet, a first cleaner configured to remove a first pattern image used to adjust an amount of electrostatic charge on the black developer in the first developing device, a second cleaner configured to remove a second pattern image used to adjust an amount of electrostatic charge on the color developer in the second developing device, and a controller configured to control the first image forming unit such that while a plurality of images are being sequentially formed, the first pattern image is formed in a first sheet-to-sheet area between a first black image and a second black image among the plurality of images on the first photosensitive body, and control the second image forming unit such that while the plurality of images is being sequentially formed, the second pattern image are formed in a second sheet-to-sheet area between a first color image and a second color image among the plurality of images on the second photosensitive body, wherein the first sheet-to-sheet area in a case where the first pattern image is formed without the second pattern image being formed is narrower than the first sheet-to-sheet area in a case where the first pattern image and the second pattern image are formed. 
     According to the present invention, it is possible to selectively maintain high image quality and improve efficiency. 
     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 cross-sectional view of an image forming apparatus. 
         FIG. 2  is a block diagram of a controller. 
         FIG. 3  is an enlarged view of a secondary transfer unit and its vicinity. 
         FIG. 4  is a view showing an arrangement of an operation display device. 
         FIGS. 5A to 5C  are time charts of discharge sequences. 
         FIG. 6  is a flowchart of a printing process. 
         FIG. 7  is a flowchart of a discharge execution determination process. 
         FIG. 8  is a flowchart of a discharge sequence process. 
         FIG. 9  is a flowchart of a discharge pattern forming process. 
         FIG. 10  is a flowchart of a discharge pattern station process. 
         FIGS. 11A to 11C  are views showing display examples of a mode setting screen, a setting menu screen, and a selection screen, respectively. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to the embodiment. The image forming apparatus  100  has a cabinet  101  and an operation display device  180 . The cabinet  101  houses various mechanisms constituting an engine unit. The image forming apparatus  100  is an electrophotographic color image forming apparatus for which a dry-type developing method using a two-component developer consisted of toner and a carrier is adopted. 
     Letters Y, M, C, and K used in the following description are abbreviations of yellow, magenta, cyan, and black, respectively. The engine unit has four stations  120 ,  121 ,  122 , and  123  for Y, M, C, and K, respectively. The stations  120 ,  121 ,  122 , and  123  are image forming units that form images by transferring toner to a recording sheet  110 . The stations  120 ,  121 ,  122 , and  123  are consisted of substantially common parts, and hence an arrangement of only the station  120  will be described as a typical example. 
     Photosensitive drums  105 , which are photosensitive bodies, are electrically charged to a uniform surface potential by primary electrostatic chargers  111 . Latent images (electrostatic images) are formed on the photosensitive drums  105  by laser light output from lasers  108 . Developing devices  112  develop the electrostatic images with color materials (toner) to form toner images on the photosensitive drums  105  (photosensitive bodies). The toner images (visible images) are transferred onto an intermediate transfer belt  106 , which is an intermediate transfer body, by primary transfer rollers  107  which are first transfer units. The visible images formed on the intermediate transfer belt  106  are transferred onto the recording sheet  110 , which has been conveyed from a housing cassette  113 , by a secondary transfer belt  114 . 
     A fixing process mechanism has a first fixing device  150  and a second fixing device  160  and heats and pressurizes the toner images transferred onto the recording sheet  110  and fixes them on the recording sheet  110 . The first fixing device  150  includes a fixing roller  151  for applying heat to the recording sheet  110 , a pressurization belt  152  for bringing the recording sheet  110  into pressure contact into the fixing roller  151 , and a first post-fixing sensor  153  that detects completion of fixing. The fixing roller  151  is a hollow roller and has a heater therein. The second fixing device  160  is disposed downstream of the first fixing device  150  in a direction in which the recording sheet  110  is conveyed. The second fixing device  160  gives a gloss to and reliably fixes the toner images fixed on the recording sheet  110  by the first fixing device  150 . As with the first fixing device  150 , the second fixing device  160  has a fixing roller  161 , a pressurization belt  162 , and a second post-fixing sensor  163 . Some types of the recording sheet  110  do not need to pass through the second fixing device  160 . In this case, for the purpose of reducing energy consumption, the image forming apparatus  100  passes the recording sheet  110  through a conveying path  130  without causing it to pass through the second fixing device  160 . 
     For example, when a setting that gives a large amount of gloss to the recording sheet  110  has been made or when the recording sheet  110  is a thick sheet which needs a large amount of heat so as to be fixed, the recording sheet  110  that has passed through the first fixing device  150  is conveyed to the recording sheet  110  as well. On the other hand, when the recording sheet  110  is a thin sheet or an ordinary sheet and the setting that gives a large amount of gloss to the recording sheet  110  has not been made, the recording sheet  110  is conveyed to the conveying path  130  detouring around the second fixing device  160 . Whether to convey the recording sheet  110  to the second fixing device  160  or cause the recording sheet  110  to detour the second fixing device  160  is controlled by switching a flapper  131  controlled by a motor control unit  312  ( FIG. 2 ), to be described later. 
     All of flappers  132 ,  133 , and  134  are guiding members for switching conveying paths under the control of the motor control unit  312 . The flapper  132  guides the recording sheet  110  to an output path  135  or to an output path  139  leading to outside. A leading end of the recording sheet  110  guided to the output path  135  passes through an inversion sensor  137  and is conveyed to an inversion unit  136 . When the inversion sensor  137  detects a trailing end of the recording sheet  110 , the conveying direction for the recording sheet  110  is switched. The flapper  133  guides the recording sheet  110  to an output path  138  for double-sided image formation or to the output path  135 . The flapper  134  guides the recording sheet  110  to the output path  139  leading to outside. The recording sheet  110  conveyed to the output path  139  is output from the image forming apparatus  100 . 
     Next, referring to  FIG. 2 , a description will be given of an arrangement of a controller that performs a role in controlling the entire image forming apparatus  100 .  FIG. 2  is a block diagram of the controller. As shown in  FIG. 2 , the controller has a CPU circuit unit  900 , which incorporates a CPU  901 , a ROM  902 , and a RAM  903 . The CPU  901  integratedly controls an image control unit  922 , a printer control unit  931 , and a display control unit  941  in accordance with control programs stored in the ROM  902 . The RAM  903  temporarily holds control data and is used as a work area for computation processes involved in control by the CPU  901 . 
     The image control unit  922  carries out a variety of processes on a digital image signal input from a computer  905  via an external I/F  904 , converts the digital image signal into a video signal, and outputs the video signal to the printer control unit  931 . Processing operations performed by the image control unit  922  are controlled by the CPU circuit unit  900 . The CPU circuit unit  900  forms images and makes various adjustments, to be described later, via the printer control unit  931 . 
     Connected to the printer control unit  931  are a high-voltage control unit  311  for controlling various high voltages, the motor control unit  312  for driving various motors, and an I/O control unit  313  for controlling I/O (input and output) to and from various sensors. The high-voltage control unit  311  provides control to apply biases to the primary transfer rollers  107  in the respective stations used in the image forming apparatus  100 , a secondary transfer roller  1061  (secondary transfer unit) inside the secondary transfer belt  114 , a bias roller  1142  (to be described later with reference to  FIG. 3 ), and so forth. The motor control unit  312  controls a plurality of motors, flappers, and so forth used in the image forming apparatus  100 . Conveying rollers and others are connected to the respective motors. Sensors including a conveyance sensor are connected to the I/O control unit  313 , and the CPU  901  is notified of changes in sensor signals via the I/O control unit  313  and the printer control unit  931 . 
     The high-voltage control unit  311  applies a predetermined bias to the primary transfer roller  107  ( FIG. 1 ) provided in each of the stations  120  to  123 . To feed a transfer current for transferring a toner image on the photosensitive drum  105  to the intermediate transfer belt  106 , the high-voltage control unit  311  applies a positive bias (for example, about +2000 V) to the primary transfer roller  107 . An electrostatic force arising from the transfer current transfers the toner image to the intermediate transfer belt  106 . On the other hand, in the present embodiment, “discharge control” is carried out so as to discharge a degraded developer by consuming the developer. When this discharge control is carried out, the CPU circuit unit  900  provides control to collect and remove the toner image with a drum cleaner  109 , which is a removal unit, without transferring the toner image formed on the photosensitive drum  105  to the intermediate transfer belt  106 . Accordingly, the high-voltage control unit  311  applies a reverse bias (for example, about −500 V) to the primary transfer roller  107 . 
     The secondary transfer belt  114  is subjected to a cleaning process by a cleaning mechanism, to be described later with reference to  FIG. 3 . However, there is a limit to the density of a toner image on the secondary transfer belt  114  which can be cleaned, and if toner remaining without being completely removed contaminates the secondary transfer belt  114 , for example, the reverse side of a sheet tends to become dirty. To avoid this, in the discharge control according to the present embodiment, the CPU circuit unit  900  forms a discharge pattern, which is a pattern image for the discharge control, on the photosensitive drum  105 . Then, the CPU circuit unit  900  provides control to apply the reverse bias to the primary transfer roller  107  so that the discharge pattern remaining without being transferred to the intermediate transfer belt  106  can be removed. In addition to this, there may be cases where the discharge pattern cannot be completely removed by passing it through the drum cleaner  109  only once, the CPU circuit unit  900  provides control so as not to start image formation for a next page until the photosensitive drum  105  makes one more full rotation. 
       FIG. 3  is an enlarged view of the secondary transfer unit and its vicinity. In the secondary transfer unit, the secondary transfer roller  1061  is inscribed in the intermediate transfer belt  106 . A predetermined bias (for example, about −3000 V) is applied to the secondary transfer roller  1061 . An outer roller  1143   a  and tension rollers  1143   b ,  1143   c , and  1143   d  are inscribed in the secondary transfer belt  114 . The outer roller  1143   a  is electrically grounded. The outer roller  1143   a  faces the secondary transfer roller  1061  across the intermediate transfer belt  106 . A toner image on the intermediate transfer belt  106  (i.e. on the transfer body) is transferred onto the recording sheet  110 , which is a recording medium, through an electrostatic force arising from a predetermined transfer current fed from the outer roller  1143   a  to the secondary transfer roller  1061 . 
     A cleaner fur  1141  is provided on an outer peripheral side of the secondary transfer belt  114 . A bias roller  1142  is provided in contact with the cleaner fur  1141 , and a cleaner blade  1145  is provided in contact with the bias roller  1142 . A predetermined bias (for example, about +1000 V) is applied to the bias roller  1142 . The cleaner fur  1141 , the bias roller  1142 , and the cleaner blade  1145  constitute a cleaning mechanism for collecting and removing toner remaining on the intermediate transfer belt  106  after being transferred to the secondary transfer belt  114 . Here, the toner on the intermediate transfer belt  106  includes a measurement image for use in auto registration control and remaining toner that remains on the intermediate transfer belt  106  without being completely transferred to the recording sheet  110  during image formation. The auto registration control is to correct for a shift in the timing of image writing in the stations and adjust the tilt of an image. 
       FIG. 4  is a view showing an arrangement of the operation display device  180 . A start key  602  for starting an image forming operation and a stop key  603  for causing an image forming operation to stop are disposed on the operation display device  180 . Further, keys  604  to  612  and  614  of a numeric keypad for setting numerals and others, an ID key  613 , a clear key  615 , a reset key  616 , and so forth are disposed on the operation display device  180 . The operation display device  180  also has a display unit  620 , on which a touch panel is formed so that software keys can be created on a screen. Selection of an image formation mode (color/monochrome) and setting of an operating mode in the monochrome mode (productivity prioritized (productivity priority mode)/image positional accuracy prioritized (image quality priority mode)) are implemented by input operations through the operation display device  180  by a user. The productivity priority mode is a mode in which priority is given to printing efficiency, and the image quality priority mode is a mode in which priority is given to image quality. 
     When a small amount of toner is consumed by forming a predetermined number of images (coverage rates are low), the image forming apparatus  100  suspends image formation to carry out the discharge control to refresh by consuming developers. For example, it is assumed that images with patterns of low coverage rates such as a yellow coverage rate of 2.0%, a magenta coverage rate of 1.0%, a cyan coverage rate of 1.5%, and a black coverage rate of 6.0% are sequentially formed. When the average coverage rate of any color is less than “2.0%”, the image forming apparatus  100  carries out the discharge control such that developers (toner) are discharged so that the average coverage rate can be 2.0%. In the above example, since the magenta coverage rate is 1.0% and the cyan coverage rate is 1.5%, the image forming apparatus  100  discharges 1.0% magenta toner and discharges 0.5% cyan toner. Namely, the image forming apparatus  100  forms discharge patterns so that toner corresponding in amount to a predetermined number of sheets×1.0% can be discharged from a magenta developing device, and toner corresponding in amount to the predetermined number of sheets×0.5% can be discharged from a cyan developing device. In a sequence of the discharge control, the image forming apparatus  100  suspends image formation and discharges degraded toner by forming a discharge pattern. A bias applied to the primary transfer roller  107  is opposite in polarity to that of a bias for normal image formation based on a print job so that a toner image discharged as the discharge pattern can be removed by the drum cleaner  109 . 
     Next, referring to flowcharts of  FIGS. 6 to 10  and  FIGS. 4, 5, and 11 , a detailed description will be given of the discharge control (toner discharge sequences) by the CPU  901 . 
       FIGS. 5A to 5C  are time charts of discharge sequences.  FIG. 5A  shows a discharge sequence in a mixed color mode (color mode).  FIGS. 5B and 5C  show discharge sequences in cases where the operating mode in the monochrome mode is the productivity priority mode and the image positional accuracy priority mode, respectively. It should be noted that in  FIGS. 5B and 5C , an exposure in the stations where no image or discharge patterns is formed is described as “exposure (dummy)”. 
       FIG. 6  is a flowchart of a printing process. The process in this flowchart is implemented by the CPU  901  reading out programs stored in the ROM  902  into the RAM  903  and executing them. Whether or not to carry out a discharging process changes between page printing and page printing. In the process in  FIG. 6 , the CPU  901  act as a control unit and a setting unit. 
     First, in step S 101 , the CPU  901  stands by until it receives a request for page printing based on a print job, that is, a print request, and when it receives the print request, the process proceeds to step S 102 , in which the CPU  102  in turn carries out a discharge execution determination process ( FIG. 7 ), to be described later. It should be noted that image data as well as the print job is transferred from an external apparatus (a computer, a server, or a scanner) to the CPU  102 . The image data includes data created using, for example, PDL (page-description language). In the discharge execution determination process ( FIG. 7 ), FLAG which is a variable indicating whether or not execution of the discharge control is necessary is set to TRUE indicating that the execution is necessary or FALSE indicating that the execution is unnecessary. The variable FLAG is stored in the RAM  903 . 
     In step S 103 , the CPU  901  determines whether or not a value of the variable FLAG is TRUE. When the value of the variable FLAG is not TRUE, the process proceeds to step S 106  because the execution of the discharge control is unnecessary. On the other hand, when the value of the variable FLAG is TRUE, the process proceeds to step S 104  because the execution of the discharge control is necessary. In the step S 104 , the CPU  901  carries out a discharge sequence process ( FIG. 8 ), to be described later. In the discharge execution determination process ( FIG. 7 ), discharge amount integrated values which are variables indicating required discharge amounts are calculated for the respective colors and stored in the RAM  903 . The CPU  901  carried out the discharge control in the step S 104 , and hence in step S 105 , the CPU  901  clears the discharge amount integrated values for all the stations stored in the RAM  903  to zero. In step S 106 , the CPU  901  carries out a page printing process, and in step S 107 , the CPU  901  determines whether or not the print job has completely been processed. When the CPU  901  determines that the print job has not completely been processed, the process returns to the step S 101 , and then the CPU  901  determines that the print job has completely been processed, the CPU  901  ends the process in  FIG. 6 . 
       FIG. 7  is a flowchart of the discharge execution determination process which is carried out in the step S 102  in  FIG. 6 . In step S 201 , the CPU  901  resets a value of the variable FLAG to FALSE, and in step S 202 , the CPU  901  resets a value of a variable color to an initial value “1”, the value of a variable color indicating an index of a color for which the discharge control is to be carried out. The variable color is stored in the RAM  903 . It is assumed here that the variable color and the colors have the following relationships: 1:Y, 2:M, 3:C, and 4:K. For example, the initial value “1” of the variable color represents yellow. 
     Then, in step S 203 , the CPU  901  obtains information on an image for one page to be printed this time. Specifically, based on a result of analysis on image data, the CPU  901  determines information on the total number of pixels in the image for one page to be printed this time and the number of dots to be printed (on-dot number) in each station and obtains them as page information (image data). It is assumed that the number of dots to be printed in each station is stored as array type variables videoCnt [color] arranged on the RAM  903 . Then, the CPU  901  calculates an image density of a color designated by the variable color. The CPU  901  calculates the image density and stores a result of the calculation in the RAM  903 . The density [%] corresponds to a coverage rate of the image for one page to be printed this time.
 
Density [%]=(videoCnt[color]×100)/the total number of pixels  (Equation 1)
 
     In step S 205 , the CPU  901  determines whether or not a threshold value Th 1  is equal to or greater than the density (coverage rate) calculated in the step S 204  (the threshold value Th 1 ≥the density). Here, the threshold value Th 1  is a fixed value which is a target value representing a targeted coverage rate of “2.0%”. The threshold value Th 1 , however, may be changed by a maintenance person or changed according to installation environments. As a result of the determination in the step S 205 , when the threshold value Th 1 ≥the density does not hold, the CPU  901  determines that it is unnecessary to update the discharge amount integrated value with respect to the station for which the density has been calculated this time, and hence the process proceeds to step S 209 . On the other hand, when the threshold value Th 1 ≥the density holds, the CPU  901  determines that it is necessary to update the discharge amount integrated value with respect to the station for which the density has been calculated this time, and hence the process proceeds to step S 206 . 
     In the step S 206 , the CPU  901  updates the discharge amount integrated value [color] using an equation 2 below based on the density (coverage rate) of the image for one page to be printed this time and stores the updated discharge amount integrated value [color] in the RAM  903 .
 
Discharge amount integrated value[color]=discharge amount integrated value [color]+{(threshold value Th1−density)×the total number of pixels)}/100   (Equation 2)
 
     The discharge amount integrated value [color] corresponds to the total number of pixels which is a shortfall in a target coverage rate. In other words, the discharge amount integrated value [color] corresponds to a value obtained by adding up differences between the number of pixels forming an electrostatic image per page and a target value. Here, “the total number of pixels” represents the total number of pixels on a page targeted this time. 
     Then, in step S 207 , the CPU  901  determines whether or not the discharge amount integrated value [color] is equal to or greater than a threshold value Th 2  (the threshold value Th 2 ≤the discharge amount integrated value [color]). Here, the threshold value Th 2  is a value corresponding to the total number of pixels in a discharge pattern and is a fixed value. The threshold value Th 2  may also be changed by a maintenance person or changed according to installation environments. It should be noted that when a discharge amount integrated value from a previous discharge has reached the threshold value Th 2 , it is determined that a developer has degraded. When the CPU  901  determines that the threshold value Th 2 ≤the discharge amount integrated value [color] does not hold, the process proceeds to the step S 209 . On the other hand, when the threshold value Th 2 ≤the discharge amount integrated value [color] holds, the CPU  901  determines that the developer has degraded, and hence the CPU  901  executes step S 208 , followed by the process proceeding to the step S 206 . In the step S 208 , the CPU  901  sets the variable FLAG to TRUE. Thus, whether or not to carry out the discharge control is determined based on the discharge amount integrated value [color]. 
     In the step S 209 , the CPU  901  adds one to the variable color so that the variable color can be a value for a next color. Then, in step S 210 , the CPU  901  determines whether or not the variable color has exceeded the number of stations. When the variable color has not exceeded the number of stations, the process returns to the step S 204  because there is an unprocessed station regarding the page to be printed this time. On the other hand, when the variable color has exceeded the number of stations, processing on all the stations has been completed for the page to be printed this time, and hence the CPU  901  ends the process in  FIG. 7 . 
       FIG. 8  is a flowchart of the discharge sequence process which is carried out in the step S 104  in  FIG. 6 . In the following description, a page on which an image is formed immediately before certain discharge control is designated as a preceding page N, and a page on which an image is formed immediately after the discharge control is designated as a succeeding page N+1. Thus, the discharge control is carried out between the preceding page N and the succeeding page N+1.  FIGS. 5 and 11  will also be referred to in the following description as the need arises. 
     First, in step S 301 , the CPU  901  determines whether or not an image formation mode for a print job this time is the monochrome mode. Here, the user can set the image formation mode from the display unit  620  of the operation display device  180  in  FIG. 4 . When the user depresses a “color selection key”  621  which is a software key on the display unit  620 , a mode setting screen in  FIG. 11A  is displayed. On this mode setting screen, when the user depresses “monochrome”, the monochrome mode can be set. When the user depresses “full color”, the mixed color mode can be set. Alternatively, the image formation mode can also be set when a print job is submitted from the computer  906 . 
     The monochrome mode is a mode in which a monochrome image is formed using only one of the multiple stations. The mixed color mode is a mode in which a mixed color image is formed using two or more of the stations. It should be noted that although in the present embodiment, black is used in the monochrome mode, this is not limitative, but any one color other than black is used in the monochrome mode. Moreover, the number of stations for use in the mixed color mode has only to be two or more, and the number of stations which the image forming apparatus  100  has may be five or more. The user can designate colors for use in the mixed color mode. It should be noted that in an image forming operation performed by the image forming apparatus  100  in the monochrome mode using black, the stations other than the black station operates in the same manner as in the mixed color mode except that image data received from the image control unit  922  is blank data. 
     When the CPU  901  determines in the step S 301  that the image formation mode is not the monochrome mode, this means that the image formation mode is the mixed dolor mode, and hence the process proceeds to step S 302 . On the other hand, when the CPU  901  determines in the step S 301  that the image formation mode is the monochrome mode, the process proceeds to step S 306 , in which the CPU  901  in turn determines whether or not an operating mode in the monochrome mode is “productivity prioritized”. When the CPU  901  determines that the operating mode is not “productivity prioritized”, this means that the operating mode is “image positional accuracy prioritized”, and hence the process proceeds to the step S 302 , and on the other hand, when the CPU  901  determines that the operating mode is “productivity prioritized”, the process proceeds to the step S 307 . 
     When the process proceeds from the step S 301  to the step S 302 , the discharge sequence ( FIG. 5A ) in the mixed color mode is performed in the steps S 302  to S 305 . When the process proceeds from the step S 306  to the step S 302 , the discharge sequence ( FIG. 5B ) with image positional accuracy prioritized in the monochrome mode is performed in the steps S 302  to S 305 . When the process proceeds from the step S 306  to the step S 307 , the discharge sequence ( FIG. 5C ) with productivity prioritized in the monochrome mode is performed in the steps S 307  and S 308 . A sequence mode in which the steps S 303  to S 305  are performed is a “first mode”, and a sequence mode in which the steps S 307  and S 308  are performed is a “second mode”. Thus, the CPU  901  selectively sets the first mode and the second mode. It should be noted that as far as insertion of blanks (waiting periods) is concerned, the discharge sequences in  FIGS. 5A and 5B  are the same. Also, as far as insertion of the blanks is concerned, the discharge sequence in  FIG. 5B  is the same as in a conventional monochrome mode. 
     In the step S 302 , the CPU  901  waits until exposure on the preceding page N by the lasers  108  is completed in all the stations (the station  123  for black at which exposure is performed last in the present embodiment). As a result, a succeeding image completion wait  2001  is inserted as a blank before formation of discharge patterns. When exposure on the preceding page N by the lasers  108  is completed in all the stations, the CPU  901  carries out a discharge pattern forming process ( FIG. 9 ), to be described later (step S 303 ), followed by the process proceeding to the step S 304 . 
     The steps S 304  and S 305  are processing steps for providing blanks between the formation of the discharge patterns and the formation of the image on the succeeding page N+1. Specifically, the CPU  901  provides control to insert a transfer switching wait  2003  and a belt stabilization wait  2004  shown in  FIG. 5A or 5B  as the blanks. Namely, in the step S 304 , the CPU  901  waits until completion of a discharge pattern station process in the black station  123 , to be described later (completion of step S 506  in  FIG. 10  in step S 407  in  FIG. 9 ). As a result, the cleaning wait  2002  and the transfer switching wait  2003  are inserted as the blanks as illustrated in  FIGS. 5A and 5B . After that, when the discharge pattern station process in the black station  123  is completed, the CPU  901  waits for a predetermined stabilization wait time T (for example, two seconds) to elapse (step S 305 ). As a result, the belt stabilization wait  2004  is inserted as the blank as illustrated in  FIGS. 5A and 5B . When the stabilization wait time T has elapsed, the process in  FIG. 8  is ended. 
     In the step S 307 , the CPU  901  carries out the discharge pattern forming process ( FIG. 9 ), to be described later, followed by the process proceeding to the step S 308 . The step S 308  is a processing step for waiting until a time to form the image on the succeeding page N+1 comes after the discharge patterns are formed. Specifically, in the step S 308 , the CPU  901  waits for a cleaning process (step S 505  in  FIG. 10  in step S 401  in  FIG. 9 ) to be completed in the yellow station  120  for which the order of operation is the first. As a result, the cleaning wait  2002  is inserted as a blank between the formation of the discharge patterns and the formation of the image on the succeeding page N+1 as illustrated in  FIG. 5C . When the cleaning process is completed, the process in  FIG. 8  is ended. 
     Here, the two modes consisting of the productivity prioritized mode and the image positional accuracy prioritized mode are provided in the monochrome mode for reasons below. First, in the image forming apparatus  100 , when biases for the primary transfer rollers  107  are switched, the intermediate transfer belt  106  is shifted several dozen μm at the maximum in a main scanning direction. Color misregistration never occurs in image formation using a single station as in the monochrome mode. On the other hand, an image may be shifted several dozen μm at the maximum with respect to the recording sheet  110 . This amount of shift is small in terms of the order of accuracy required for an image position with respect to the recording sheet  110 , but the image positional accuracy prioritized mode is offered for users who request for higher accuracy. Even in the monochrome mode, the same image positional accuracy as in the past is realized by securing the waiting time (belt stabilization wait  2004 ) as in the past before the intermediate transfer belt  106  is stabilized after the discharge patterns are formed. 
     On the display unit  620  of the operation display device  180  in  FIG. 4 , “productivity prioritized” or “image positional accuracy prioritized” is selected. When the user depresses a setting  622  which is a software key, a setting menu screen shown in  FIG. 11B  is displayed. When the user further depresses a “select operation mode in monochrome mode” key, a selection screen shown in  FIG. 11C  is displayed, and “productivity prioritized” or “image positional accuracy prioritized” is selectable on this selection screen. It should be noted that in the present embodiment, “productivity prioritized” is set as a default. It should be noted that the step S 306  may be dispensed with. For example, in the monochrome mode, productivity may always be prioritized, and the process may proceed to step S 307 . 
       FIG. 9  is a flowchart of the discharge pattern forming process in each station. This process is carried out in the step S 303  or S 307  in  FIG. 8 . First, in the step S 401 , the CPU  901  starts a discharge pattern station process ( FIG. 10 ) in the yellow station  120 . Next, in step S 402 , the CPU  901  waits for a station-to-station passage time to elapse. Namely, the CPU  901  waits for the discharge pattern station process to be started in the magenta station  121 . Here, the station-to-station passage time is calculated by dividing a distance between adjacent stations by a process speed time. For example, the station-to-station passage time between the Y and M stations is calculated by dividing a distance between the Y and M stations by the process speed time. 
     When the station-to-station passage time between the Y and M stations has elapsed, the CPU  901  starts the discharge pattern station process ( FIG. 10 ) in the magenta station  121 . Next, in step S 404 , the CPU  901  waits for a station-to-station passage time between the M and C stations to elapse. When the station-to-station passage time between the M and C stations has elapsed, the CPU  901  starts the discharge pattern station process ( FIG. 10 ) in the cyan station  122 . Then, in step S 406 , the CPU  901  waits for a station-to-station passage time between the C and K station to elapse. When the station-to-station passage time for passage between the C and K stations has elapsed, the CPU  901  starts the discharge pattern station process ( FIG. 10 ) in the station black  123  and ends the process in  FIG. 9 . 
       FIG. 10  is a flowchart of the discharge pattern station process in one station, which is consisted of steps of discharge pattern exposure, primary transfer, and cleaning in one station. The discharge pattern station process is started in each of steps S 401 , S 403 , S 405 , and S 407  in  FIG. 9  and may be carried out in parallel in the four stations. 
     In the following description with reference to  FIG. 10 , a station in which a discharge pattern is to be formed is referred to as a station to be processed. It should be noted that in the process in  FIG. 10 , an operation performed in stations other than the station to be processed in the monochrome mode is a “dummy exposure”. In this case, the CPU  901  performs an exposure based on image data corresponding to a blank sheet. 
     First, in step S 501 , the CPU  901  starts an exposure for forming a discharge pattern on the photosensitive drum  105 . Next, in step S 502 , the CPU  901  waits for a leading end of the discharge pattern on the photosensitive drum  105  to reach a primary transfer position (a position at which the photosensitive drum  105  and the primary transfer roller  107  face each other). When the leading end of the discharge pattern on the photosensitive drum  105  has reached the primary transfer position, the CPU  901  controls the voltage control unit  311  in step S 503  to apply a reverse bias to the primary transfer roller  107 . As a result, the discharge pattern (the toner image on the photosensitive drum  105 ) remains on the photosensitive drum  105  without being transferred to the intermediate transfer belt  106 . 
     Then, in step S 504 , the CPU  901  waits for a trailing end of the discharge pattern on the photosensitive drum  105  to pass the primary transfer position, and when the trailing end of the discharge pattern on the photosensitive drum  105  has passed the primary transfer position, the process proceeds to the step S 505 . In the step S 505 , the CPU  901  waits for the photosensitive drum  105  to make one full rotation so as to clean the photosensitive drum  105  by removing the toner remaining as the discharge pattern on the photosensitive drum  105  with the drum cleaner  109 . Upon detecting that a time period required for the full rotation of the photosensitive drum  105  has elapsed, the CPU  901  determines that the remaining toner has been removed, and hence the process proceeds to the step S 506 . As a result, the cleaning wait  2002  is inserted as the blank. It should be noted that in the step S 308  in  FIG. 8 , upon detecting that the process in the step S 505  has been completed in the yellow station  120 , the CPU  901  provides control to determine that the result is positive (yes). 
     In the step S 506 , the CPU  901  applies a predetermined bias (positive bias) to the primary transfer roller  107  by controlling the high voltage control unit  311  so that the primary transfer roller  107  goes back to its original state before formation of the discharge pattern. It should be noted that in the step S 304  in  FIG. 8 , upon detecting that the process in the step S 506  has been completed in the black station  123 , the CPU  901  provides control to determine that the result is positive (yes). 
     The control according to the flowcharts of  FIGS. 6 to 10  described above will now be described again with reference to  FIGS. 5B and 5C  from the standpoint of discharge control and image formation timings. 
     First, based on whether the sequence mode is the first mode or the second mode, the CPU  901  controls the start timing of discharge control with respect to the end timing of formation of electrostatic images in image formation (the preceding page N) immediately before the discharge control. Specifically, in the first mode, the CPU  901  starts the discharge control after waiting until formation of electrostatic images for image formation (the preceding page N) immediately before the discharge control is completed in all the stations as shown in  FIG. 5B . As a result, the preceding image completion wait  2001  is provided between the preceding page N and the discharge control, and hence satisfactory image quality is maintained. On the other hand, in the second mode, the CPU  901  starts the discharge control without waiting for the formation of electrostatic images during image formation (the preceding page N) immediately before the discharge control to be completed in all the stations as shown in  FIG. 5C . In particular, the CPU  901  starts the discharge control when the formation of the electrostatic image for the preceding page N is completed in the first station  120 . As a result, the preceding image completion wait  2001  is omitted, which increases productivity. 
     Thus, when productivity is prioritized in the monochrome mode, the CPU  901  provides control such that intervals between images formed based on a print job and succeeding discharge patterns for discharge control are shorter than in the case where the mixed color mode is set. Moreover, when productivity is prioritized in the monochrome mode, the CPU  901  provides control such that intervals between images formed based on a print job and succeeding discharge patterns for discharge control are shorter than in the case where the image positional accuracy is prioritized in the monochrome mode. 
     It should be noted that even in the case where the preceding image completion wait  2001  is not provided, there may be a time lag between the time when the formation of the electrostatic image for the succeeding page N is completed in the first station  120  and the time when the discharge control is started in the first station  120 . 
     Moreover, based on whether the sequence mode is the first mode or the second mode, the CPU  901  controls the start timing of formation of electrostatic images in image formation (the succeeding page N+1) immediately after discharge control with respect to the end timing of the discharge control. Specifically, in the first mode, as shown in  FIG. 5B , the CPU  901  waits until the bias applied to the primary transfer rollers  107  is switched in the first station  120  from one for discharge control (reverse bias) to one for image formation (positive bias) immediately after the discharge control. Moreover, after switching the bias, the CPU  901  starts the formation of the electrostatic images for the succeeding page N+1 immediately after the discharge control after waiting for a predetermined time period (stabilization waiting time period T) to elapse. As a result, the transfer switching wait  2003  and the belt stabilization wait  2004  are provided between the discharge control and the succeeding page N+1, and hence satisfactory image quality is maintained. On the other hand, in the second mode, the CPU  901  starts the formation of the electrostatic images for the succeeding page N+1 without waiting for the bias applied to the primary transfer rollers  107  to be switched in the first station  120  from the reverse bias to the positive bias immediately after the discharge control ( FIG. 5C ). In particular, the CPU  901  starts the formation of the electrostatic image for the succeeding page N+1 in the first station  120  when removal of the developer on the photosensitive drum  105  is completed in the first station  120 . As a result, the transfer switching wait  2003  and the belt stabilization wait  2004  are omitted, which increases productivity. 
     Thus, when productivity is prioritized in the monochrome mode, the CPU  901  provides control such that intervals between discharge patterns and succeeding images formed based on a print job are shorter than in the case where the mixed color mode is set. Moreover, when productivity is prioritized in the monochrome mode, the CPU  901  provides control such that intervals between discharge patterns and succeeding images formed based on a print job are shorter than in the case where the image positional accuracy is prioritized in the monochrome mode. 
     It should be noted that even in the case where the transfer switching wait  2003  or the like is not provided, there may be a time lag between the time when removal of the developer on the photosensitive drum  105  is completed in the first station  120  and the time when the formation of the electrostatic image for the succeeding page “N+1” is started in the first station  120 . 
     According to the present embodiment, when productivity is prioritized in the monochrome mode, intervals between images formed based on a print job and succeeding discharge patterns for discharge control are shorter than in the case where the mixed color mode is set. Also, intervals between discharge patterns and succeeding images formed based on a print job are shorter than in the case the mixed color mode is set. 
     On the other hand, when productivity is prioritized in the monochrome mode, intervals between images formed based on a print job and succeeding discharge patterns for discharge control are shorter than image positional accuracy is prioritized in the monochrome mode. Also, intervals between discharge patterns and succeeding images formed based on a print job are shorter than in the case image positional accuracy is prioritized in the monochrome mode. 
     Namely, at the time of image formation based on a print job, the CPU  901  forms images on the recording sheet  110 , and at the time of discharge control for consuming degraded developers by discharging the developers, suspends the image formation based on the print job and causes the developers  112  to discharge the developers. Based on the sequence mode, the CPU  901  controls the start timing of discharge control with respect to the end timing of formation of electrostatic images in image formation immediately before the discharge control. Also, based on the sequence mode, the CPU  901  controls the start timing of formation of electrostatic images in image formation immediately after discharge control with respect to the end timing of formation of electrostatic images in the discharge control. Therefore, maintenance of image quality accuracy or improvement of efficiency can be selected. 
     It should be noted that although in the control according to the present embodiment, intervals between discharge patterns and images formed based on a print job are distances, they may be time intervals. 
     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. 2018-017362, filed Feb. 2, 2018, which is hereby incorporated by reference herein in its entirety.