Patent Publication Number: US-11644778-B2

Title: Image forming apparatus and control method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/233,964 filed on Apr. 19, 2021, which is a continuation of U.S. Pat. No. 11,009,821 which issued from U.S. patent application Ser. No. 16/869,722 filed on May 8, 2020, which is a continuation application of U.S. Pat. No. 10,671,002 which issued from U.S. patent application Ser. No. 16/027,439 filed on Jul. 5, 2018 and which claims priority from Japanese Patent Application No. 2017-133272, which was filed on Jul. 7, 2017, the disclosures of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Technical Field 
     The following disclosure relates to an image forming apparatus including: a developing unit including a developing roller; and a developer cartridge storing developer. 
     Description of Related Art 
     There is known an image forming apparatus which includes: an image forming unit having a developing device; a developer cartridge removably mounted on the image forming unit; and a supplier configured to supply developer from the developer cartridge to the developing device. In the known apparatus, the developer is supplied from the developer cartridge to the developing device in accordance with use of the developer. Further, when the end of life of the image forming unit is detected, the developer stops being supplied from the developer cartridge to the developing device and a control is executed in which image formation is performed by the developer remaining in the developing device of the image forming unit. (The control will be hereinafter referred to as “use-up mode” where appropriate.) The use-up mode is selectable by a user&#39;s operation on an operation panel of the image forming apparatus. 
     SUMMARY 
     In the known technique, the use-up mode is executed with the developer cartridge kept mounted on the image forming unit, and there may arise a problem of unexpected execution of the use-up mode against a user&#39;s intention. In the case where the user makes an erroneous setting on the operation panel, the use-up mode is unintentionally executed, causing a risk of a deterioration in image quality and a malfunction of the developing device. 
     Accordingly, one aspect of the present disclosure relates to a technique of obviating erroneous execution of the use-up mode. 
     One aspect of the present disclosure provides an image forming apparatus, including: a printer housing; a photoconductor; a developing unit including a developing roller configured to form a developer image on the photoconductor; a developer cartridge storing developer, the developer cartridge being removably mountable on the printer housing, a supplier configured to supply the developer from the developer cartridge to the developing unit; and a controller configured to execute a first printing process of forming a developer image by use of the developer supplied from the developer cartridge, when the developer cartridge is being mounted on the printer housing, and a second printing process of forming a developer image by use of the developer remaining in the developing unit, when a usage amount of the developing unit is not less than a predetermined value and the developer cartridge is not being mounted on the printer housing, wherein the controller does not execute the second printing process when the usage amount of the developing unit is less than the predetermined value. 
     Another aspect of the present disclosure provides an image forming apparatus, including: a printer housing; a photoconductor; a developing unit including a developing roller configured to form a developer image on the photoconductor; a developer cartridge storing developer, the developer cartridge being removably mountable on the printer housing; a supplier configured to supply the developer from the developer cartridge to the developing unit; and a controller configured to execute a non-mount printing process of forming a developer image by use of the developer remaining in the developing unit when an index value that increases in accordance with use of the developing unit is not less than a predetermined value and the developer cartridge is not being mounted on the printer housing, wherein the controller does not execute the non-mount printing process when the index value that increases in accordance with use of the developing unit is less than the predetermined value. 
     Still another aspect of the present disclosure provides a control method of an image forming apparatus including a printer housing, a photoconductor, a developing unit including a developing roller configured to form a developer image on the photoconductor, a developer cartridge storing developer, the developer cartridge being removably mountable on the printer housing, and a supplier configured to supply the developer from the developer cartridge to the developing unit, the control method including: a first printing step in which, when the developer cartridge is being mounted on the printer housing, the developer is supplied by the supplier from the developer cartridge to the developing unit and a developer image is formed on the photoconductor; and a second printing step in which, when a usage amount of the developing unit is not less than a predetermined value and the developer cartridge is not being mounted on the printer housing, a developer image is formed on the photoconductor by the developer remaining in the developing unit, wherein execution of the second printing step is prohibited when the usage amount of the developing unit is less than the predetermined value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, advantages, and technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of an embodiment, when considered in connection with the accompanying drawings, in which: 
         FIG.  1    is a view showing an overall structure of a laser printer according to one embodiment; 
         FIG.  2    is a cross-sectional view of a developing unit and a developer cartridge; 
         FIG.  3    is a flowchart showing an operation of a controller; 
         FIG.  4    is a flowchart showing a cartridge confirming process; 
         FIG.  5    is a flowchart showing an error displaying process; and 
         FIG.  6    is a flowchart showing a part of the cartridge confirming process according to a modification. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     There will be explained in detail one embodiment referring to the drawings. In the following explanation, directions are defined based on directions indicated in  FIG.  1   . That is, a right side and a left side in  FIG.  1    are respectively defined as a front side and a rear side, and a back side and a front side of the sheet of  FIG.  1    are respectively defined as a right side and a left side. Further, an up-down direction in  FIG.  1    is defined as an up-down direction. 
     As shown in  FIG.  1   , a laser printer  100  is an image forming apparatus including a printer housing  120 , a feeder portion  130  configured to supply a sheet S, an image forming portion  140  configured to form an image on the sheet S, a controller  200 , and a motor  300 . A drive force of the motor  300  is transmitted to the feeder portion  130  and the image forming portion  140 . 
     The feeder portion  130  includes a sheet supply tray  131  removably mounted on a lower portion of the printer housing  120  and a conveyor mechanism  132  configured to convey the sheet S in the sheet supply tray  131  toward a transfer roller  183 . The conveyor mechanism  132  includes: a sheet supply mechanism  133  configured to separate the sheets S in the sheet supply tray  131  one by one and to convey the separated sheet S toward registration rollers  134 ; and the registration rollers  134  for properly positioning each position in the leading edge of the sheet S being conveyed. 
     The image forming portion  140  includes a scanner unit  150 , a process cartridge  160 , a transfer roller  183 , and a fixing device  170 . 
     The scanner unit  150  is provided in an upper portion of the printer housing  120 . The scanner unit  150  is an exposing device including a laser light emitter, a polygon mirror, lenses, and reflective mirrors (not shown). In the scanner unit  150 , a laser beam is applied to a surface of a photoconductor  181  (which will be explained) by high-speed scanning. 
     The process cartridge  160  includes a developing unit cartridge DC and a developer cartridge  2 . The process cartridge  160  is mountable on and removable from the printer housing  120  through an opening  122  which is opened and closed by a front cover  123  pivotably provided on a front wall of the printer housing  120 . 
     The developing unit cartridge DC includes the photoconductor  181 , a charger  182 , and a developing unit  1 . The photoconductor  181  is a cylindrical photoconductor drum and rotatably held by a frame of the developing unit cartridge DC. The photoconductor  181  may be configured to be attachable to and removable from the developing unit cartridge DC. 
     The developer cartridge  2  is mountable on and removable from the developing unit  1 . In other words, the developer cartridge  2  is mountable on and removable from the printer housing  120  together with the developing unit cartridge DC. The developing unit  1  and the developer cartridge  2  store single-component dry toner as developer. The developing unit  1  and the developer cartridge  2  will be later explained in detail. 
     The printer housing  120  is provided with a first sensor  101  configured to detect whether the developing unit cartridge DC is being mounted on the printer housing  120  and a second sensor  102 , as one example of a sensor, for detecting whether the developer cartridge  2  is being mounted on the printer housing  120 . The first sensor  101  and the second sensor  102  are optical sensors, for instance. Further, an operation panel  124 , as one example of a setting device, is provided on an upper surface of the printer housing  120 . It is possible to set, through the operation panel  124 , whether a second printing process (which will be later explained) is allowed to be executed. The operation panel  124  may be a touch panel or may be constituted by a display screen and push buttons. 
     The fixing device  170  includes a heating roller  171  and a pressure roller  172  pressed onto the heating roller  171 . 
     In the image forming portion  140 , the surface of the photoconductor  181  that rotates is uniformly charged by the charger  182  and subsequently exposed to a high-speed scanning of a laser beam from the scanner unit  150 . Thus, an electrostatic latent image based on image data is formed on the surface of the photoconductor  181 . 
     Subsequently, the toner is supplied from the developing unit  1  to the electrostatic latent image on the photoconductor  181 , so that a toner image is formed on the surface of the photoconductor  181 . Thereafter, the sheet S is conveyed between the photoconductor  181  and the transfer roller  183 , so that the toner image carried on the surface of the photoconductor  181  is transferred onto the sheet S. Then, in the fixing device  170 , the toner transferred onto to the sheet S is thermally fixed between the heating roller  171  and the pressure roller  172 . 
     A sheet sensor  103  is provided downstream of the fixing device  170  in a conveyance direction of the sheet S. The sheet sensor  103  is configured to detect passage of the sheet S discharged from the fixing device  170 . The sheet sensor  103  includes, for instance, a swing lever configured to swing by being pushed by the sheet S that is being conveyed and an optical sensor configured to detect swinging of the swing lever. In the present embodiment, the sheet sensor  103  is in an ON state while the sheet S is passing, namely, while the swing lever is being laid down by the sheet S. 
     A discharge roller R is disposed downstream of the sheet sensor  103 . The sheet S that has been subjected to thermal fixation of the toner by the fixing device  170  is conveyed to the discharge roller R and subsequently discharged onto a sheet discharge tray  121  from the discharge roller R. 
     As shown in  FIG.  2   , the developing unit  1  includes a housing  11 , a developing roller  12 , a supply roller  13 , a layer-thickness limiting blade  14 , and a first agitator  15 . The housing  11  stores the toner therein. The housing  11  supports the layer-thickness limiting blade  14  and rotatably supports the developing roller  12 , the supply roller  13 , and the first agitator  15 . 
     The developing roller  12  is configured to supply the toner to the electrostatic latent image formed on the photoconductor  181 . The developing roller  12  is rotatable about a rotation axis extending in a right-left direction. 
     The supply roller  13  is configured to supply, to the developing roller  12 , the toner in the housing  11 . The layer-thickness limiting blade  14  is a member for limiting a thickness of the toner on the developing roller  12 . 
     The first agitator  15  includes: a shaft portion  15 A rotatable about a first axis X1 which is a rotation axis parallel to the rotation axis of the developing roller  12 ; and an agitating blade  15 B fixed to the shaft portion  15 A. The housing  11  rotatably supports the shaft portion  15 A. The agitating blade  15 B is configured to rotate clockwise in  FIG.  2    together with the shaft portion  15 A, so as to agitate the toner in the housing  11 . 
     The developer cartridge  2  includes: a housing  21  in which the toner is stored; an auger  22 , as one example of a supplier, configured to supply the toner in the housing  21  to the developing unit  1 ; and a second agitator  23  configured to rotate clockwise in  FIG.  2    so as to agitate the toner in the housing  21 . 
     The auger  22  is rotatable about a rotation shaft  22 A extending in the right-left direction. The auger  22  is configured to rotate so as to convey the toner in the housing  21  in the axial direction. Specifically, the auger  22  is a screw auger including the rotation shaft  22 A and a plate  22 B helically provided around the rotation shaft  22 A. The plate  22 B of the auger  22  is formed integrally with the rotation shaft  22 A. 
     The housing  21  has an outlet  21 A through which the toner in the housing  21  is supplied to the developing unit  1  and a toner conveyor portion  21 B surrounding the auger  22 . The housing  11  of the developing unit  1  has an inlet  11 A which faces the outlet  21 A and through which the toner is supplied from the outlet  21 A. The outlet  21 A and the inlet  11 A are located below the auger  22  and on one end side of the auger  22  in the axial direction, specifically, on a downstream side in a conveyance direction of the toner conveyed by the auger  22 . In this configuration, when the auger  22  rotates, the toner is conveyed toward the one end side in the axial direction by the helical plate  22 B, so that the toner is supplied into the housing  11  through the outlet  21 A and the inlet  11 A. 
     The second agitator  23  includes a shaft portion  23 A parallel to the right-left direction and an agitating blade  23 B provided on the shaft portion  23 A. The second agitator  23  is coupled to the auger  22  via a predetermined number of gears not shown. 
     To the auger  22 , a drive force is input from a drive source provided on the printer housing  120 . The controller  200  controls transmission and cutoff of the drive force to the auger  22 . A mechanism for transmitting the drive force from the drive source to the auger  22  may be configured as follows, for instance. In a configuration in which the drive force is transmitted from the drive source to the developing unit  1 , the drive force transmitted to the developing unit  1  may be transmitted to the auger  22  via a clutch mechanism. In this case, the controller  200  is configured to control connection and disconnection of the clutch mechanism, thereby enabling transmission and cutoff of the drive force to the auger  22 . 
     The controller  200  includes a CPU, a RAM, a ROM, a nonvolatile memory, an ASIC, and an input/output circuit. The controller  200  executes control by executing various arithmetic processing based on a print instruction output from an external computer, signals output from the sensors  101 - 103 , and programs and data stored in the ROM, for instance. The controller  200  is configured to execute a developing process, a usage-amount obtaining process, a first printing process, a second printing process, a cartridge confirming process, and an error displaying process. 
     The developing process is a process of developing an electrostatic latent image on the photoconductor  181 . Specifically, in a state in which an appropriate voltage is applied to the developing roller  12 , the controller  200  executes an exposure process of blinking a laser beam scanned by the scanner unit  150  based on the image data in accordance with the print instruction. 
     The usage-amount obtaining process is a process of obtaining a toner usage amount Qu which is a usage amount of the toner in the developing process. In the usage-amount obtaining process, the controller  200  counts, by a dot counter, the number of dots of the image data in accordance with the print instruction. 
     In the case where the number of dots per unit area is not greater than a predetermined value, the number of dots may be regarded as the predetermined value. In a toner saving mode, for instance, the toner usage amount Qu may be calculated so as to be smaller by multiplying the number of dots by a coefficient less than 1. 
     The controller  200  has a function of executing the usage-amount obtaining process after a toner image corresponding to an image for one sheet S (for one page of the sheet S) has been formed on the photoconductor  181  in the developing process. Specifically, in the present embodiment, the controller  200  executes the usage-amount obtaining process after the state of the sheet sensor  103  has been switched from ON to OFF, namely, after the sheet S has passed through the fixing device  170 . 
     The first printing process is a process in which, when the developer cartridge  2  is being mounted on the printer housing  120 , the toner is supplied by the auger  22  from the developer cartridge  2  to the developing unit  1  and an image is formed on the sheet S. In other words, the controller  200  executes the first printing process of forming an image (developer image) by use of the toner supplied from the developer cartridge  2  when the developer cartridge  2  is being mounted on the printer housing  120 . In the first printing process, the controller  200  executes a supplying process of supplying the toner from the developer cartridge  2  to the developing unit  1  by driving the auger  22  in accordance with use of the toner. Specifically, the controller  200  executes the supplying process when an increase amount Qu 1  of the toner usage amount Qu from a time point of execution of a preceding supplying process up to a current time point becomes equal to or greater than a threshold TH 1 . 
     The controller  200  has a function of supplying a predetermined amount of the toner to the developing unit  1  in the supplying process. In the supplying process, the controller  200  causes the auger  22  to rotate a predetermined number of times. Specifically, in the supplying process, the controller  200  causes the auger  22  to rotate at a predetermined rotational speed for a predetermined length of time. 
     In the present embodiment, the increase amount Qu 1  of the toner usage amount Qu is updated to a value obtained by subtracting the threshold TH 1  every time the supplying process is executed. Further, the toner usage amount Qu is counted as a total toner usage amount Qus and reset to an initial value every time the developer cartridge  2  is replaced. 
     The second printing process is a process in which an image is formed on the sheet S by the toner remaining in the developing unit  1  when a usage amount of the developing unit  1  is not less than a predetermined threshold and the developer cartridge  2  is not being mounted on the printer housing  120 . In other words, the controller  200  executes the second printing process of forming an image (developer image) by use of the toner remaining in the developing unit  1  when the usage amount of the developing unit  1  is not less than the predetermined threshold and the developer cartridge  2  is not being mounted on the printer housing  120 . That is, the second printing process is a process for using up the toner remaining in the developing unit  1  when the usage amount of the developing unit  1  reaches a value corresponding to an end of its life. In the following explanation, the second printing process will be also referred to as a use-up mode where appropriate. When the usage amount of the developing unit  1  is not less than the predetermined threshold, the first printing process may be prohibited from being executed. 
     In the present embodiment, the number of rotations Nd of the developing roller  12  is illustrated as one example of the usage amount of the developing unit  1 . With an increase in the number of rotations Nd counted from a brand-new state of the developing roller  12 , wear of the developing roller  12  and the layer-thickness limiting blade  14  proceeds, making it difficult to maintain the quality of a printed image. In the present embodiment, the controller  200  determines that the usage amount of the developing unit  1  is not less than the threshold, namely, the developing unit  1  reaches the end of its life, when the number of rotations Nd of the developing roller  12  is not less than a first threshold N 1 . Further, the controller  200  has a function of notifying that the developing unit  1  needs to be replaced when the usage amount of the developing unit  1  is not less than the predetermined threshold. 
     In the second printing process, the controller  200  has a function of stopping an operation of the auger  22 , specifically, a function of prohibiting transmission of the drive force to the auger  22 . Further, the controller  200  is configured not to execute the usage-amount obtaining process and the supplying process in the second printing process. 
     The controller  200  has a function of enabling the second printing process to be executed on condition that execution of the second printing process is allowed through the operation panel  124 . In other words, the controller  200  is configured to execute a setting accepting process of accepting a setting as to whether the execution of the second printing process is allowed and to execute the second printing process when a setting to allow the execution of the second printing process is being established. The controller  200  has a function of prohibiting the execution of the second printing process on condition that a developing unit cartridge DC whose usage amount is less than the predetermined threshold is mounted after the second printing process has been executed. The present embodiment illustrates, as the developing unit cartridge DC whose usage amount is less than the predetermined threshold, a brand-new developing unit cartridge DC, namely, a developing unit cartridge DC in which the number of rotations Nd of the developing roller  12  is 0. 
     The cartridge confirming process is a process for confirming a state of each of the developer cartridge  2  and the developing unit cartridge DC as consumables. In the cartridge confirming process, the controller  200  mainly sets, to  1 , error flags (which will be later explained) depending upon the state of the developer cartridge  2  and the state of the developing unit cartridge DC. 
     The error displaying process is a process of displaying, on the operation panel  124 , a message corresponding to the kind of the error flag. In the error displaying process, the controller  200  is configured to display a message to permit the user to select the second printing process, i.e., “use-up mode”. 
     There will be next explained an operation of the controller  200  in detail. 
     In response to turn-on of the laser printer  100  or in response to closing of the front cover  123 , the controller  200  starts a control shown in the flow chart of  FIG.  3   . The controller  200  repeatedly executes the control of  FIG.  3    in a period in which the laser printer  100  is kept turned ON. 
     In the control of  FIG.  3   , the controller  200  executes the cartridge confirming process (S 1 ). As shown in  FIG.  4   , in the cartridge confirming process, the controller  200  initially determines whether the developing unit cartridge DC is being mounted on the printer housing  120  (S 21 ). 
     When it is determined at S 21  that the developing unit cartridge DC is not being mounted (No), the controller  200  sets a drum non-mount flag F 1  to 1, the drum non-mount flag F 1  indicating that the developing unit cartridge DC is not being mounted (S 22 ). On the other hand, when it is determined at S 21  that the developing unit cartridge DC is being mounted (Yes), the controller  200  sets the drum non-mount flag F 1  to 0 (S 23 ). 
     After S 23 , the controller  200  determines whether the developing unit cartridge DC currently mounted on the printer housing  120  is brand new (S 24 ). The determination as to whether the developing unit cartridge DC is brand new may be made based on information in a memory provided in the developing unit cartridge DC, for instance. Alternatively, the determination may be made such that a movable member movable between a brand-new cartridge position and a used cartridge position is provided for the developing unit cartridge DC and a position of the movable member is detected by a sensor provided in the printer housing  120 , for instance. 
     When it is determined at S 24  that the developing unit cartridge DC is not brand new (No), the controller  200  determines whether a use-up flag Fu is 0 (S 25 ), the use-up flag Fu indicating that the use-up mode is selected by a user. When it is determined at S 25  that the use-up flag Fu is 0 (Fu=0), namely, the use-up mode is not selected (Yes), the controller  200  determines whether the number of rotations Nd of the developing roller  12  is not less than the first threshold N 1 , so as to determine whether the developing unit cartridge DC is at the end of its life, specifically, whether the developing unit  1  is at the end of its life (S 26 ). 
     When it is determined at S 26  that the number of rotations Nd of the developing roller  12  is not less than the first threshold N 1  (Nd≥N 1 ), the controller  200  sets a life flag F 2  to 1 (S 27 ), the life flag F 2  indicating that the developing unit cartridge DC is at the end of its life. When it is determined at S 24  that the developing unit cartridge DC is brand new (Yes), the controller  200  resets counters related to the developing unit cartridge DC (S 28 ). Specifically, at S 28 , the controller  200  resets a counter for counting the number of rotations Nd of the developing roller  12 , for instance. 
     After S 28 , the controller  200  sets the use-up flag Fu and the life flag F 2  to 0 (S 29 , S 30 ). After S 30 , S 27 , S 22  or when a negative determination is made at S 25 , S 26 , the controller  200  determines whether the use-up flag Fu is 0 (S 31 ). 
     When it is determined at S 31  that the use-up flag Fu is 0 (Fu=0) (Yes), the controller  200  determines whether the developer cartridge  2  is being mounted on the printer housing  120  via the developing unit cartridge DC (S 32 ). When it is determined at S 32  that the developer cartridge  2  is not being mounted (No), the controller  200  sets a box non-mount flag F 3  to 1 (S 33 ), the box non-mount flag F 3  indicating that the developer cartridge  2  is not being mounted. 
     When it is determined at S 32  that the developer cartridge  2  is being mounted (Yes), the controller  200  sets the box non-mount flag F 3  to 0 (S 34 ). After S 34 , the controller  200  determines whether the developer cartridge  2  is brand new (S 35 ). The determination as to whether the developer cartridge  2  is brand new may be made in a manner similar to that in the determination as to whether the developing unit cartridge DC is brand new. That is, the determination may be made based on information in a memory provided in the developer cartridge  2 , for instance. Alternatively, the determination may be made such that a movable member configured to be movable between a brand-new cartridge position and a used cartridge position is provided for the developer cartridge  2  and a position of the movable member may be detected by a sensor provided in the printer housing  120 . 
     When it is determined at S 35  that the developer cartridge  2  is brand new (Yes), the controller  200  resets counters related to the developer cartridge  2  (S 36 ). Specifically, at S 36 , the controller  200  resets a counter for counting the total toner usage amount Qus, for instance. 
     After S 36  or a negative determination (No) is made at S 35 , the controller  200  determines whether the developer cartridge  2  is empty (S 37 ). For instance, the determination as to whether the developer cartridge  2  is empty may be made by determining whether the total toner usage amount Qus is not less than a predetermined threshold. Alternatively, the determination may be made based on a detection result by a sensor configured to detect a toner amount in the developer cartridge  2 . 
     When it is determined at S 37  that the developer cartridge  2  is empty (Yes), the controller  200  sets an empty flag F 4  to 1 (S 38 ), the empty flag F 4  indicating that the developer cartridge  2  is empty. On the other hand, when it is determined at S 37  that the developer cartridge  2  is not empty (No), the controller  200  sets the empty flag F 4  to 0 (S 39 ). After S 38 , S 39 , S 33 , the controller  200  ends the present control. 
     Returning back to  FIG.  3   , after the cartridge confirming process, the controller  200  determines whether any of the error flags is 1 (S 2 ). Specifically, the controller  200  determines at S 2  whether any of the flags F 1 -F 4  is 1. 
     When it is determined at S 2  that any of the error flags is 1 (Yes), the controller  200  executes the error displaying process (S 14 ). In the error displaying process shown in  FIG.  5   , the controller  200  initially determines whether the life flag F 2  is 1 (S 51 ). When it is determined at S 51  that the life flag F 2  is not 1 (No), the controller  200  displays, on the operation panel  124 , a message corresponding to the kind of the error flag currently set at 1 (S 52 ). In the case where the drum non-mount flag F 1 , as the error flag, is 1, for instance, the controller  200  displays, on the operation panel  124 , a message indicating that the developing unit cartridge DC is not being mounted, e.g., a message saying “Developing unit cartridge DC is not being mounted”. 
     When it is determined at S 51  that the life flag F 2  is 1 (F 2 =1) (Yes), the controller  200  displays, on the operation panel  124 , a message to prompt the user to replace the developing unit cartridge DC and a message that the use-up mode is executable (S 53 ). Specifically, in the case where the operation panel  124  is a touch panel, for instance, the controller  200  displays, on the screen, a button for executing the use-up mode. 
     After S 53 , the controller  200  determines whether the use-up mode is selected based on a signal from the operation panel  124  (S 54 ). When it is determined at S 54  that the use-up mode is selected (Yes), the controller  200  sets the use-up flag Fu to 1 (S 55 ) and determines whether the developer cartridge  2  is mounted (S 56 ). 
     When it is determined at S 56  that the developer cartridge  2  is being mounted, namely, an affirmative determination (Yes) is made at S 56 , the controller  200  displays, the operation panel  124 , an error message indicating that the developer cartridge  2  is being mounted (S 57 ). Specifically, the controller  200  displays, on the operation panel  124 , an error message saying “Remove the developer cartridge  2  from the developing unit cartridge DC for execution of the use-up mode.”, for instance. 
     When it is determined at S 56  that the developer cartridge  2  is not being mounted, namely, a negative determination (No) is made at S 56 , the controller  200  sets the life flag F 2  to 0 so as to clear the error flag (S 58 ). After S 58 , S 57 , S 52  or when a negative determination (No) is made at S 54 , the controller  200  ends the present control. 
     Returning back to  FIG.  3   , when the controller  200  determines at S 2  that no error flags are 1 (No), the controller  200  executes a standby process (S 3 ). In the standby process, the controller  200  controls the agitators  15 ,  23  to rotate or controls the temperature of the heating roller  171  to rise to a standby temperature lower than the temperature in printing. 
     After S 3 , the controller  200  determines whether a print instruction is received (S 4 ). When it is determined at S 4  that the print instruction is received (Yes), the controller  200  starts a printing operation (S 5 ). After S 5 , the controller  200  executes an image forming process in which an image is formed on the sheet S (S 6 ). 
     After S 6 , the controller  200  determines whether the use-up flag Fu is 0 (S 7 ). When it is determined at S 7  that the use-up flag Fu is 0 (Fu=0) (Yes), the controller  200  determines whether a leading edge of the sheet S has passed the sheet sensor  103  (S 8 ). 
     When it is determined at S 8  that the leading edge of the sheet S has passed the sheet sensor  103  (Yes), the controller  200  increments the dot counter (S 9 ). After S 9 , the controller  200  calculates the increase amount Qu 1  of the toner usage amount Qu from the time point of the preceding supplying process (S 10 ). 
     After S 10 , the controller  200  determines whether the increase amount Qu 1  is not less than the threshold TH 1  (S 11 ). When it is determined at S 11  that the increase amount Qu 1  is not less than the threshold TH 1  (Qu 1 ≥TH 1 ) (Yes), the controller  200  executes the supplying process in which the toner in the developer cartridge  2  is supplied to the developing unit  1  (S 12 ). 
     After S 12  or when a negative determination is made at S 11 , S 7 , the controller  200  determines whether printing of the number of pages instructed in the print instruction has been completed (S 13 ). 
     When it is determined at S 13  that printing is not yet completed (No), the control flow returns to S 6 . When it is determined at S 13  that printing has completed (Yes), when a negative determination (No) is made at S 4 , or after S 14 , the controller  200  ends the present control. 
     There will be next explained an example of the operation of the controller  200 . In an initial state in the following explanation, each of the flags is set at 0. 
     When a brand-new process cartridge  160  is mounted on the printer housing  120  and the front cover  123  is closed, the controller  200  ends the cartridge confirming process shown in  FIGS.  3  and  4    without setting any of the error flags to 1 in the cartridge confirming process. Accordingly, the controller  200  makes a negative determination (No) at S 2  and proceeds to S 3  and then to S 4 . When it is determined at S 4  that the print instruction is received (Yes), the controller  200  executes the first printing process shown in S 5 -S 13 . 
     When the number of rotations Nd of the developing roller  12  becomes equal to or greater than the first threshold N 1  after long-term use of the process cartridge  160 , the controller  200  makes an affirmative determination (Yes) at S 26  in the cartridge confirming process and sets the life flag F 2  to 1 (S 27 ). After S 27 , the controller  200  executes processes in the order of S 31 : Yes, S 32 : Yes, S 34 , S 35 : No, S 37 : No, and S 39 . Thus, the controller  200  ends the cartridge confirming process. 
     When the life flag F 2  is set to 1 in the cartridge confirming process, the controller  200  makes an affirmative decision (Yes) at S 2  in  FIG.  3    and executes the error displaying process (S 14 ). As shown in  FIG.  5   , in the error displaying process, the controller  200  makes an affirmative decision (Yes) at S 51  and displays, on the operation panel  124 , the message to prompt the user to replace the developing unit cartridge DC and the message indicating that the use-up mode is executable (S 53 ). 
     When the user selects the use-up mode by an operation on the operation panel  124 , the controller  200  makes an affirmative determination (Yes) at S 54  and sets the use-up flag Fu to 1 (S 55 ). After S 55 , the controller  200  determines whether the developer cartridge  2  is not being mounted (S 56 ). When the developer cartridge  2  is kept mounted on the developing unit cartridge D, the controller  200  makes a negative determination (No) at S 56  and displays, on the operation panel  124 , the message prompting the user to remove the developer cartridge  2  (S 57 ). 
     The user prompted by the message opens the front cover  123 , removes the process cartridge  160  from the printer housing  120 , and removes the developer cartridge  2  from the developing unit cartridge DC. The user thereafter mounts only the developing unit cartridge DC on the printer housing  120  and closes the front cover  123 . 
     In response to closing of the front cover  123 , the controller  200  again starts the control shown in  FIG.  3    and executes the cartridge confirming process (S 1 ). In the cartridge confirming process, the controller  200  executes processes in the order of S 21 : Yes, S 23 , and S 24 : No. Thereafter, the controller  200  makes a negative determination (No) at both of S 25  and S 31  because the use-up flag Fu is being set at 1. Thus, the cartridge confirming process is ended. In this case, S 32 -S 39  are skipped because of negative determination (No) at S 31 , so that the box non-mount flag F 3  is not set to 1 at S 33 . In the use-up mode, therefore, printing can be performed even in a state in which the developer cartridge  2  is not mounted. 
     As shown in  FIG.  3   , after the cartridge confirming process, the controller  200  proceeds to S 2 . Because the life flag F 2  is kept at 1, the controller  200  makes an affirmative determination (Yes) at S 2  and executes the error displaying process (S 14 ). As shown in  FIG.  5   , in the error displaying process, the controller  200  executes processes in the order of S 51 : Yes and S 53 . When the user selects the use-up mode, the controller  200  proceeds to S 56  after execution of processes in the order of S 54 : Yes and S 55 . Because the developer cartridge  2  is already removed from the developing unit cartridge DC, the controller  200  makes an affirmative decision (Yes) at S 56  and sets the life flag F 2  back to 0 so as to clear the error flag (S 58 ). 
     The life flag F 2  is set back to 0 in the error displaying process, so that the controller  200  makes a negative determination (No) at S 2  in  FIG.  3   . Subsequently, the controller  200  proceeds to S 3  and then to S 4 . When it is determined at S 4  that the print instruction is received (Yes), the controller  200  proceeds to S 5  and then to S 6 . Subsequently, the controller  200  proceeds to S 7 . 
     On this occasion, because the use-up flag Fu is 1, the controller  200  makes a negative determination (No) at S 7  and proceeds to S 13 . That is, when the use-up flag Fu is 1, the controller  200  executes a second printing process shown in S 5 -S 7 , and S 13 , namely, the use-up mode. Thus, the toner remaining in the developing unit  1  can be efficiently used up. 
     The present embodiment offers the following advantageous effects. 
     On condition that the developer cartridge  2  is not being mounted, an image is formed by the toner remaining in the developing unit  1 , namely, the use-up mode is executed, thereby preventing or reducing erroneous execution of the use-up mode. When the use-up mode is executed, there may be caused some troubles. For instance, the print quality may be deteriorated due to shortage of the toner or the wear of the developing roller  12  and the layer-thickness limiting blade  14  may rapidly proceed. In the present embodiment, the use-up mode is executed by user&#39;s operations to remove the developer cartridge  2  and mount only the developing unit cartridge DC. Thus, a shift to the use-up mode is clear by the user&#39;s intention. 
     Owing to provision of the second sensor  102  configured to detect whether the developer cartridge  2  is being mounted on the printer housing  120 , it is possible to appropriately determine, based on the signal from the second sensor  102 , whether the developer cartridge  2  is being mounted. 
     On condition that the execution of the second printing process is allowed by the operation panel  124 , the second printing process becomes executable. In other words, when a setting to allow the execution of the second printing process is being established, the controller  200  executes the second printing process. In this configuration, the second printing process is executed only when the user explicitly allows the second printing process to be executed, preventing the second printing process from being unintentionally executed. 
     On condition that the developing unit cartridge DC in a brand-new state is mounted, the use-up flag Fu is set to 0, namely, the second printing process is prohibited from being executed. This configuration prevents or reduces an adverse influence on the bran-new developing unit cartridge DC which would be caused if the second printing process were executed with the brand-new developing unit cartridge DC mounted. 
     It is to be understood that the present disclosure is not limited to the details of the illustrated embodiment but may be embodied otherwise as described below. In the following explanation, the same reference signs as used in the illustrated embodiment are used to identify components and steps similar to those in the illustrated embodiment, and a detailed explanation thereof is dispensed with. 
     In the illustrated embodiment, when the developing unit cartridge DC in a brand-new state, namely, the developing unit  1  in a brand-new state, is mounted, the use-up flag Fu is set to 0 (S 29 ) and the second printing process is prohibited. The present disclosure is not limited to this configuration. The controller  200  may be configured to prohibit the execution of the second printing process on condition that the developing unit  1  whose usage amount is less than the predetermined threshold is mounted on the printer housing  200 . In the case where the developing unit  1  is equipped with a memory and includes a counter for counting the usage amount, the second printing process may be prohibited when the developing unit  1  the life of which still remains is mounted upon replacement. 
     Specifically, S 71  shown in  FIG.  6    may be added to the flowchart of  FIG.  4   .  FIG.  6    illustrates a portion around S 26  in the cartridge confirming process shown in  FIG.  4   . In this modification, when the controller  200  makes a negative determination (No) at S 26 , the controller  200  executes S 71 . At S 71 , the controller  200  sets the life flag F 2  to 0. After S 71 , the controller  200  proceeds to S 31  in  FIG.  4   . 
     In the illustrated embodiment, the controller  200  is configured to determine that the usage amount of the developing unit  1  is not less than the predetermined threshold when the number of rotations Nd of the developing roller  12  is not less than the first threshold N 1 . The present disclosure is not limited to this configuration. In other words, in the illustrated embodiment, the number of rotations Nd of the developing roller  12  is set as one example of an index value that increases in accordance with use of the developing unit  1 , and it is determined whether the usage amount of the developing unit  1  is not less than the predetermined threshold based on the number of rotations Nd. The index value is not limited to the number of rotations Nd. For instance, the number of printed pages in the developing unit  1  currently being mounted on the printer housing  120  may be set an one example of the index value that increases in accordance with use of the developing unit  1 , and the controller  200  may be configured to determine that the usage amount of the developing unit  1  is not less than the predetermined threshold when the number of printed pages in the developing unit  1  is not less than a second threshold. Further, a print time in the developing unit  1  currently being mounted on the printer housing  120  or a drive time of a motor for driving the developing unit  1  may be set as one example of the index value that increases in accordance with use of the developing unit  1 , and the controller  200  may be configured to determine that the usage amount of the developing unit  1  is not less than the predetermined threshold when the print time in the developing unit  1  is not less than a third threshold. Thus, the usage amount of the developing unit  1  may be construed as the index value that increases in accordance with use of the developing unit  1 . Further, the usage amount of the developing unit  1  may be construed as one example of an index value which increases or decreases in accordance with use of the developing unit  1  and which is used for determining a replacement timing of the developing unit  1 . In this respect, the determination at S 26  may be made based on whether the index value for determining the replacement timing of the developing unit  1  is larger or smaller than a value set in advance. 
     In the case where the developing unit  1  and the developing unit cartridge DC are constituted as one component as in the illustrated embodiment, the controller  200  may be configured to determine that the usage amount of the developing unit  1  is not less than the predetermined threshold when the number of rotations of the photoconductor  181  is not less than a fourth threshold. 
     In the illustrated embodiment, the operation panel  124  is illustrated as one example of the setting device. The present disclosure is not limited to this configuration. For instance, the setting device may be an external device such as a computer configured to output the print instruction to the laser printer  100 . 
     In the illustrated embodiment, as one example of the developer cartridge, the developer cartridge  2  mountable on and removable from the printer housing  120  indirectly via developing unit cartridge DC is illustrated. The present disclosure is not limited to this configuration. For instance, the developer cartridge may be directly mountable on and removable from the printer housing  120 . 
     While the present disclosure is applied to the laser printer  100  in the illustrated embodiment, the present disclosure may be applied to other image forming apparatus such as a copying machine and a multi-function peripheral (MFP). 
     In the illustrated embodiment, the sheet S such as thick paper, a post card, or thin paper is illustrated as one example of the sheet. The sheet may be an OHP sheet. 
     In the illustrated embodiment, the photoconductor  181  is a cylindrical photoconductor drum. The photoconductor  181  may be a belt-like photoconductor. 
     The elements explained in the illustrated embodiment and the modification may be suitable combined.