Patent Publication Number: US-2023152732-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2021-186885, filed on Nov. 17, 2021, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure relate to an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction peripheral having at least two of copying, printing, and facsimile functions. 
     Related Art 
     One type of image forming apparatus such as a copier or a printer perform start-up operations such as a warm-up operation after receiving a print-start command and before starting printing. In addition, the image forming apparatus corrects image forming conditions in order to maintain a constant image density. 
     SUMMARY 
     This specification describes an improved image forming apparatus that includes an image bearer, a developing device, a fixing device, and circuitry. The developing device contains a developer and develops a latent image on the image bearer with the developer. The developing device includes a stirrer to stir the developer. The fixing device fixes a toner image onto a sheet. The circuitry performs start-up operations after receiving a print start command and before starting a printing operation. The start-up operations includes starting a warm-up operation of the fixing device and starting, during the warm-up operation, a stirring operation in which the stirrer stirs the developer. 
     This specification further describes an improved image forming apparatus that includes an image bearer, a developing device, and circuitry. The developing device contains a developer and develops a latent image on the image bearer with the developer. The developing device includes a stirrer to stir the developer. The circuitry performs start-up operations after receiving a print start command and before starting a printing operation. The start-up operations includes starting a stirring operation in which the stirrer stirs the developer. The circuitry adjusts a timing to start the stirring operation based on a cumulative image area rate of images printed in a previous job. 
     This specification still further describes an improved image forming apparatus that includes an image bearer, a developing device, and circuitry. The developing device contains a developer and develops a latent image on the image bearer with the developer. The developing device includes a stirrer to stir the developer. The circuitry performs start-up operations after receiving a print start command and before starting a printing operation. The start-up operations includes starting a stirring operation in which the stirrer stirs the developer. The circuitry starts the printing operation in response to a change amount of a toner charge amount of the developer stirred being equal to or smaller than a predetermined value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG.  1    is a schematic view of a configuration of an image forming apparatus according to a first embodiment of the present disclosure; 
         FIG.  2    is a partially enlarged view of an image forming device and a block diagram that relates to the image forming device in the image forming apparatus of  FIG.  1   ; 
         FIG.  3    is a timing chart illustrating start-up operations before starting a printing operation; 
         FIG.  4    is a graph illustrating a relation between a stirring time for which a stirrer stirs developer in a developing device and a toner charge amount of the developer; 
         FIG.  5 A  is a timing chart illustrating start-up operations before starting the printing operation in a comparative embodiment; 
         FIG.  5 B  is a graph illustrating a relation between a stirring time for which the stirrer stirs the developer in the developing device and the toner charge amount of the developer in the comparative embodiment; 
         FIG.  6    is a flowchart of start-up operations performed by the image forming apparatus according to a second embodiment of the present disclosure; 
         FIG.  7    is a graph illustrating a relation between a stirring time for which the stirrer stirs the developer in the developing device and the toner charge amount of the developer in cases of different cumulative image area rates; 
         FIG.  8    is a timing chart of start-up operations performed by the image forming apparatus according to a third embodiment of the present disclosure; 
         FIG.  9    is a graph illustrating a relation between the stirring time for which the stirrer stirs the developer in the developing device and the toner charge amount of the developer in cases of different rotation speeds of the stirrer; and 
         FIG.  10    is a timing chart of start-up operations performed by the image forming apparatus according to a fourth embodiment of the present disclosure. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     Referring now to the drawings, embodiments of the present disclosure are described below. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Identical reference numerals are assigned to identical components or equivalents and a description of those components is simplified or omitted. 
     A first embodiment is described below. 
     With reference to  FIGS.  1  and  2   , a description is given of the overall configuration and operations of an image forming apparatus  100 . 
       FIG.  1    is a schematic view of a configuration of a printer as an example of the image forming apparatus.  FIG.  2    is a partially enlarged view of an image forming device and a block diagram that relates to the image forming device in the image forming apparatus of  FIG.  1   . 
     As illustrated in  FIG.  1   , the image forming apparatus  100  includes an intermediate transfer belt  8  in the body of the image forming apparatus  100 . The intermediate transfer belt  8  functions as an intermediate transferor. The image forming apparatus  100  further includes image forming devices  6 Y,  6 M,  6 C and  6 K, respectively corresponding to the colors of yellow, magenta, cyan, and black. The image forming devices  6 Y,  6 M,  6 C and  6 K are arranged in parallel, facing the intermediate transfer belt  8 . 
     On the exterior of the body of the image forming apparatus  100 , an operation display panel  95  is disposed. The operation display panel  95  displays information relating to printing operations (that is, image forming operations) and allows a user to perform operations relating to the printing operations. 
     Referring to  FIG.  2   , the image forming device  6 Y that forms a yellow toner image includes a photoconductor drum  1 Y, a charging device  4 Y, a developing device  5 Y, a cleaning device  2 Y, a lubricant applicator  3 , and a discharging device. The photoconductor drum  1 Y functions as an image bearer. The charging device  4 Y, the developing device  5 Y, the cleaning device  2 Y, the lubricant applicator  3 , and the discharging device are disposed around the photoconductor drum  1 Y. A series of image forming processes including charging, exposure, developing, primary transfer, cleaning, and electrical discharge processes is performed on the photoconductor drum  1 Y. Accordingly, a yellow image is formed on the surface of the photoconductor drum  1 Y. 
     The other three image forming devices  6 M,  6 C, and  6 K also have almost the same configuration as the image forming device  6 Y corresponding to yellow, except a configuration that the toner colors used are different. Due to such a configuration, a description below is given of the image forming device  6 Y alone and descriptions of the other three image forming devices  6 M,  6 C, and  6 K are appropriately omitted. 
     Referring to  FIG.  2   , the photoconductor drum  1 Y as the image bearer is rotated counterclockwise by a main motor  91 . The charging device  4 Y uniformly charges the surface of the photoconductor drum  1 Y in the charging process. 
     The photoconductor drum  1 Y is rotated further until reaching a position opposite to and facing an exposure device  7 . The exposure device  7  irradiates the surface of the photoconductor drum  1 Y with a laser light beam L emitted from the exposure device  7  at this position and scans the surface of the photoconductor drum  1 Y in a width direction, which is a main scanning direction orthogonal to the drawing sheets on which  FIGS.  1  and  2    are drawn. By performing the above-described operation, the exposure device  7  forms or writes an electrostatic latent image corresponding to the color of yellow on the surface of the photoconductor drum  1 Y in the exposure process. 
     After the electrostatic latent image is formed on the surface of the photoconductor drum  1 Y, the photoconductor drum  1 Y is rotated further and reaches a position facing the developing device  5 Y. At the position, the developing device  5 Y develops the electrostatic latent image into a visible toner image of yellow in the developing process. 
     Thereafter, the surface of the photoconductor drum  1 Y reaches a position opposite a primary transfer roller  9 Y and the intermediate transfer belt  8 , and the toner image formed on the photoconductor drum  1 Y is transferred to a surface of the intermediate transfer belt  8  at this position in the primary transfer process. After the primary transfer process, a certain amount of untransferred toner remains on the photoconductor drum  1 Y. 
     When the surface of the photoconductor drum  1 Y reaches a position facing the cleaning device  2 Y, a cleaning blade  2 a collects the untransferred toner from the photoconductor drum  1 Y into the cleaning device  2 Y in the cleaning process. 
     The cleaning device  2 Y includes the lubricant applicator  3  for applying lubricant onto the photoconductor drum  1 Y. The lubricant applicator  3  includes a lubricant supply roller  3   a,  a solid lubricant  3   b,  and a compression spring  3   c.  The lubricant supply roller  3   a  rotating clockwise in  FIG.  2    scrapes a small amount of lubricant from the solid lubricant  3   b  and applies the lubricant to the surface of the photoconductor drum  1 Y. Applying the lubricant to the surface of the photoconductor drum  1 Y prevents the photoconductor drum  1 Y and the cleaning blade  2   a  from wearing and deteriorating. 
     Finally, the surface of the photoconductor drum  1 Y reaches a position facing the discharging device, and the discharging device removes residual potentials from the photoconductor drum  1 Y. 
     Thus, a series of image forming processes performed on the surface of the photoconductor drum  1 Y is completed. 
     The above-described image forming processes are performed in the image forming devices  6 M,  6 C, and  6 K similarly to the image forming device  6 Y for yellow. In other words, the exposure device  7  disposed above the image forming devices  6 M,  6 C, and  6 K emits respective laser light beams L based on respective image data, toward a photoconductor drum  1 M of the image forming device  6 M, a photoconductor drum  1 C of the image forming device  6 C and a photoconductor drum  1 K of the image forming device  6 K. Specifically, the exposure device  7  includes a light source to emit the laser light beams L, multiple optical elements, and a polygon mirror that is rotated by a motor. The exposure device  7  scans, with the laser light beams L, the photoconductor drums  1 M,  1 C, and  1 K via the multiple optical elements while deflecting the laser light beams L with the polygon mirror. 
     Subsequently, developing devices  5 M,  5 C, and  5 K develop electrostatic latent images into visible magenta, cyan, and black toner images, respectively, in the development process. The magenta, cyan, and black toner images respectively formed on the photoconductor drums  1 M,  1 C, and  1 K are primarily transferred onto the intermediate transfer belt  8  such that the magenta, cyan, and black toner images are superimposed one atop another. Thus, a color toner image is formed on the intermediate transfer belt  8 . 
     The intermediate transfer belt  8  serving as an intermediate transferor is entrained around and supported by the multiple rollers and is formed into an endless loop. As a drive motor drives and rotates the drive roller, the intermediate transfer belt  8  is rotated in a direction indicated by arrow in  FIG.  1   . 
     Four primary transfer rollers  9 Y,  9 M,  9 C, and  9 K nip the intermediate transfer belt  8  together with the four photoconductor drums  1 Y,  1 M,  1 C, and  1 K to form the four primary transfer nips between the intermediate transfer belt  8  and the photoconductor drums  1 Y,  1 M,  1 C, and  1 K, respectively. A transfer voltage (i.e., a primary transfer bias) having a polarity opposite to a polarity of toner is applied to each of the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K. 
     The intermediate transfer belt  8  travels in the direction indicated by arrow in  FIG.  1    and sequentially passes through the primary transfer nips formed by the four primary transfer rollers  9 Y,  9 M,  9 C, and  9 K. Thus, the toner images formed on the respective photoconductor drums  1 Y,  1 M,  1 C, and  1 K are primarily transferred onto the intermediate transfer belt  8  in a manner of being superimposed one atop another to form a composite color toner image on the intermediate transfer belt  8  in the primary transfer process. 
     Subsequently, the intermediate transfer belt  8  bearing the composite color toner image reaches a position opposite a secondary transfer belt  72  (and a secondary transfer roller  70 ). At this position, a secondary transfer backup roller  22  sandwiches the intermediate transfer belt  8  and the secondary transfer belt  72  with the secondary transfer roller  70  to form an area of contact, herein called a secondary transfer nip (as a transfer nip), between the intermediate transfer belt  8  and the secondary transfer belt  72 . At the secondary transfer nip, the composite color toner image (or four-color toner image including yellow, magenta, cyan, and black colors) is secondarily transferred from the intermediate transfer belt  8  onto a sheet P serving as a recording medium conveyed to the position of the secondary transfer nip, in a secondary transfer process. At this time, untransferred toner that is not transferred onto the sheet P remains on the surface of the intermediate transfer belt  8 . 
     Thereafter, the intermediate transfer belt  8  reaches a position opposite the intermediate transfer belt cleaner. At this position, the intermediate transfer belt cleaner removes substances such as the untransferred toner adhering to the surface of the intermediate transfer belt  8 . 
     Thus, a series of transfer processes performed on the surface of the intermediate transfer belt  8  is completed. 
     With reference to  FIG.  1   , the sheet P is conveyed from a sheet feeder  26  disposed in a lower portion of the body of the image forming apparatus  100  to the secondary transfer nip as the transfer nip via a feed roller  27  and a registration roller pair  28 . 
     Specifically, the sheet feeder  26  contains a stack of multiple sheets P such as sheets of paper stacked on one on another. The feed roller  27  is rotated counterclockwise in  FIG.  1    to pick up and feed an uppermost sheet P of the plurality of sheets P toward a portion between rollers of the registration roller pair  28  via a first sheet conveyance passage K 1 . 
     The sheet P conveyed to the registration roller pair  28  (that is a conveyance roller pair) temporarily stops at a position of the roller nip between the rollers of the registration roller pair  28  that has stopped rotating. Subsequently, the registration roller pair  28  rotates to convey the sheet P to the secondary transfer nip, timed to coincide with the arrival of the composite color toner image on the intermediate transfer belt  8 . Thus, the desired color toner image is transferred onto the sheet P. 
     After the composite color toner image is secondarily transferred onto the sheet P at the secondary transfer nip, the secondary transfer belt  72  entrained around and supported by the secondary transfer roller  70  and a separation roller  71  conveys the sheet P. After the sheet P is separated from the secondary transfer belt  72 , a conveyance belt  60  conveys the sheet P to a fixing device  80 . In the fixing device  80 , a fixing roller  81  and a pressure roller  82  apply heat and pressure to the sheet P to fix the composite color toner image on the sheet P, which is a fixing process. 
     The sheet P is conveyed through a second conveyance passage K 2  and ejected by an ejection roller pair to the outside of the image forming apparatus  100 . The sheets P ejected by the ejection roller pair to the outside of the image forming apparatus  100  are sequentially stacked as output images on a stack tray. 
     Thus, a series of image forming processes (i.e., printing operations) of the image forming apparatus  100  is completed. 
     The fixing device  80  includes the fixing roller  81  as a fixing rotator, a heater  85 , the pressure roller  82  as a pressure rotator, and a temperature sensor to detect a temperature (a surface temperature) of the fixing roller  81 . The heater  85  is secured inside the hollow core of the fixing roller  81 . 
     When the image forming apparatus  100  is powered on, a power supply supplies power to the heater  85 . A controller  90  as circuitry controls the power supplied to the heater  85 , that is, an output of the heater  85 . Radiant heat from the heater  85  heats the fixing roller  81 , and the fixing roller  81  applies heat to the toner image on the sheet P entering a fixing nip between the fixing roller  81  and the pressure roller  82 . 
     The controller  90  controls the output of the heater  85  based on the detection result of a surface temperature of the fixing roller  81  (specifically, a temperature of the outer circumferential surface of the fixing roller  81 ) detected by the temperature sensor. The temperature sensor is disposed opposite (facing) the outer circumferential surface of the fixing roller  81 . The above-described control of the power supplied to the heater  85  adjusts the temperature of the fixing roller  81  (that is, a fixing temperature) to a desired temperature (that is, a target control temperature). 
     The image forming apparatus  100  in the first embodiment includes a contact-separation mechanism  93  for the primary transfer rollers (see  FIG.  2   ) that vertically moves the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K. During normal printing operations, the contact-separation mechanism  93  for the primary transfer rollers moves the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K to positions illustrated in  FIG.  1    so as to come into contact with the photoconductor drums  1 Y,  1 M,  1 C, and  1 K via the intermediate transfer belt  8 , respectively. In contrast, when printing is not performed, the contact-separation mechanism  93  for the primary transfer rollers moves the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K downward from the positions illustrated in  FIG.  1    to release contacts between the photoconductor drums  1 Y,  1 M,  1 C, and  1 K and the intermediate transfer belt  8  in order to prevent elastic distortion of the intermediate transfer belt  8  and the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K. 
     The image forming apparatus  100  in the first embodiment includes a contact-separation mechanism  94  (see  FIG.  2   ) for a secondary transfer device  69  that vertically moves the secondary transfer device  69 . During normal printing operations, the contact-separation mechanism  94  for the secondary transfer device  69  moves the secondary transfer device  69  to a position illustrated in  FIG.  1    so as to come into contact with the secondary transfer backup roller  22  via the intermediate transfer belt  8 . In contrast, when printing is not performed, the contact-separation mechanism  94  for the secondary transfer device  69  moves the secondary transfer device  69  downward from the position illustrated in  FIG.  1    to release contacts between the secondary transfer device  69  and the intermediate transfer belt  8  in order to prevent elastic distortion of the intermediate transfer belt  8 , the secondary transfer roller  70 , a secondary transfer belt  72 , and the secondary transfer backup roller  22 . 
     Next, a detailed description is provided of a configuration and operation of the developing device  5 Y of the image forming device  6 Y with reference to  FIG.  2   . 
     The developing device  5 Y includes a developing roller  51 Y as a developer bearer disposed opposite the photoconductor drum  1 Y, a doctor blade  52 Y disposed opposite the developing roller  51 Y, two conveying screws  55 Y as stirrers disposed in developer containers, and a toner concentration sensor  56 Y to detect concentration of toner in a developer. The developing roller  51 Y includes a magnet and a sleeve. The magnet is fixed inside the developing roller  51 Y. The sleeve rotates about the magnet. The developer containers contain the developer G that is a two-component developer including carrier (carrier particles) and toner (toner particles). 
     The developing device  5 Y configured as described above operates as follows. 
     The sleeve of the developing roller  51 Y rotates in the direction indicated by arrow illustrated in  FIG.  2   . The developer G held on the developing roller  51 Y by the magnetic field generated by the magnet moves along the circumference of the developing roller  51 Y (in the direction of arc) as the sleeve rotates. The percentage (concentration) of toner in the developer (ratio of toner to carrier) in the developing device  5 Y is constantly adjusted within a predetermined range. Specifically, in response to detection of low toner concentration by the toner concentration sensor  56 Y disposed in the developing device  5 Y, driving a supply roller  59  disposed inside a toner container  58  to rotate supplies fresh toner (new toner) from the toner container  58  into the developing device  5 Y so that the toner concentration falls within the given range. 
     The two conveying screws  55 Y as the stirrers stir and mix the developer G with the toner supplied from the toner container  58  to the developer container while circulating the developer G in the two developer containers separated each other. In this case, the developer G moves in the direction perpendicular to the surface of the sheet on which  FIG.  2    is drawn. The toner in developer G is charged by friction with carrier and electrostatically attracted to the carrier. Then, the toner is carried on the developing roller  51 Y together with the carrier by a magnetic force generated on the developing roller  51 Y. 
     The developer G borne on the developing roller  51 Y is transported in the direction indicated by arrow in  FIG.  2    to the doctor blade  52 Y. At this position, the doctor blade  52 Y adjusts the amount of the developer G on the developing roller  51 Y to an appropriate amount. Thereafter, the developer G on the developing roller  51 Y is conveyed to a position opposite the photoconductor drum  1 Y (i.e., a developing area). In the developing area, the toner is attracted to the latent image formed on the photoconductor drum  1 Y by an electric field generated in the developing area. Thereafter, the developer G remaining on the developing roller  51 Y is conveyed to an upper portion of the developer container along with rotation of the sleeve of the developing roller  51 Y, where the developer G is separated from the developing roller  51 Y. 
     The electric field in the developing area is generated by a development bias applied to the developing roller  51 Y by a development power supply  97  and a surface potential (in other words, a latent image potential) formed on the surface of the photoconductor drum  1 Y in the charging process and the exposure process. 
     A developing motor  92  drives the developing roller  51 Y as the developer bearer, the two conveying screws  55 Y as the stirrers to rotate them in the directions indicated by arrows in  FIG.  2   . Specifically, a driving force of the developing motor  92  is transmitted to the developing roller  51 Y and the two conveying screws  55 Y via a gear train. 
     The toner container  58  is detachably (replaceably) attached on the developing device  5 Y in the image forming apparatus  100 . Specifically, when the fresh toner contained in the toner container  58  is consumed to be empty, the toner container  58  with no toner is removed from the developing device  5 Y in the image forming apparatus  100  and is replaced with a new toner container  58  with fresh toner. 
     The configuration and operation of the image forming apparatus  100  according to the first embodiment are described in further detail below. 
     As described above with reference to  FIGS.  1  and  2   , the image forming apparatus  100  according to the first embodiment includes the developing device  5 Y that develops the latent image formed on the surface of the photoconductor drum  1 Y as the image bearer and the fixing device  80  that fixes a transferred and unfixed toner image onto the sheet P. The developing device  5 Y includes the conveying screws  55 Y as the stirrers that stir the developer G in the developing device  5 Y. 
     With reference to  FIG.  3   , the following describes start-up operations of the image forming apparatus  100  according to the first embodiment. The controller  90  performs the start-up operations after a timing at which the controller  90  receives a print start command and before starting the printing operation. After the controller  90  receives the print start command, the controller  90  starts a warm-up operation of the fixing device  80 . During the warm-up operation, the controller  90  starts a stirring operation that stirs the developer G in the developing device  5 Y. The stirring operation is performed by the two conveying screws  55 Y as stirrers. 
     Specifically, an operator such as a user operates the operation display panel  95  to input various printing conditions such as the number of sheets to be printed and a printing mode (color mode, monochrome mode, or the like) and presses a print start button. Then, the controller receives the print start command and starts the start-up operations until printing is started. The start-up operations are preparation operations of main parts of the image forming apparatus  100  to perform a favorable printing operation. 
     Specifically, the controller  90  starts the warm-up operation of the fixing device  80 , as the first start-up operation. In the warm-up operation, the controller  90  controls a power source to supply electric power to the heater  85  to raise the temperature of the fixing roller  81  to a desired temperature (that is, a fixing temperature). The longer the image forming apparatus  100  is left and the cooler the fixing roller  81 , the longer the warm-up operation to raise the temperature of the fusing roller  81 . Therefore, the controller  90  preferentially performs the warm-up operation before other preparation operations. In particular, the controller  90  in the first embodiment starts the warm-up operation of the fixing device  80  immediately after receiving the print start command as illustrated in  FIG.  3   . 
     After the electric power is supplied to the heater  85  to some extent, the controller  90  drives the fixing device  80 . While the fixing roller  81  and the pressure roller  82  rotate, the controller  90  controls the electric power supplied from the power source to the heater  85  based on the result detected by the temperature sensor that detects the temperature of the fixing roller  81  to uniform temperatures of the fixing roller  81  in the circumferential direction of the fixing roller  81 . 
     While the fixing device  80  is activated as described above, the main motor  91  starts to rotationally drive the photoconductor drums  1 Y,  1 M,  1 C, and  1 K. Subsequently, the contact-separation mechanism  93  of the primary transfer rollers moves the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K and the intermediate transfer belt  8 , which are separated from the photoconductor drums  1 Y,  1 M,  1 C, and  1 K, to bring the intermediate transfer belt  8  into contact with the photoconductor drums  1 Y,  1 M,  1 C, and  1 K as illustrated in  FIGS.  1  and  2   . Next, the contact-separation mechanism  94  for the secondary transfer device moves the secondary transfer device  69  that is separated from the intermediate transfer belt  8  to bring the secondary transfer device  69  into contact with the intermediate transfer belt  8  as illustrated in  FIG.  1   . 
     Then, the controller  90  completes the start-up operations and starts the printing operation set by the operator. 
       FIG.  5 A  is a timing chart of start-up operations according to a comparative embodiment. In  FIG.  5 A , the controller  90  activates the fixing device  80  and starts rotating the photoconductor drums  1 Y,  1 M,  1 C, and  1 K after the warm-up operation of the fixing device  80  is completed. Subsequently, the controller  90  starts rotating the conveying screws  55 Y as the stirrers to start stirring the developer G in the developing device  5 Y. In contrast, the controller  90  according to the first embodiment starts, during the warm-up operation of the fixing device  80 , rotating the conveying screws  55 Y as the stirrers to start stirring the developer G in the developing device  5 Y. Specifically, the controller  90  starts the warm-up operation of the fixing device  80  in response to receiving the print start command and, after a few moments, starts driving the developing motor  92  to start driving the developing device  5 Y (that is, rotating the conveying screws  55 Y and the developing roller  51 Y). In other words, the timing of start of stirring the developer G in the developing device  5 Y according to the first embodiment illustrated in  FIG.  3    is faster than the timing of start of stirring the developer G in the developing device  5 Y according to the comparative embodiment illustrated in  FIG.  5 A . The timing of start of stirring the developer G in the first embodiment is accelerated from the timing of start of stirring the developer in the comparative embodiment. That is to say, a stirring time to stir the developer G during the start-up operations in the first embodiment is longer than that in the comparative embodiment. 
     In the first embodiment, the conveying screws  55 Y as the stirrers start stirring the developer G in the developing device  5 Y in the start-up operations and continue stirring the developer G after the start of printing operation. 
     In other words, the developing motor  92  starts driving in the start-up operations and continues the driving until the printing operation is completed after the start of the printing operation without being interrupted. 
     As described above, the start-up operations according to the first embodiment having a sufficient time to stir the developer G during the warm-up operation of the fixing device  80  stabilize a toner charge amount of the developer G contained in the developing device  5 Y immediately after the start of printing. As a result, the image forming apparatus according to the first embodiment prevents a disadvantage that the image density varies in images printed immediately after the start of printing. 
     The following describes the above-described effect according to present the embodiment in detail. 
     In order to stabilize the image density in printed images, the states of the toner in the developer G, such as toner concentration, toner charge amount, and toner deterioration state, are controlled. Under a constant developing potential, which means a constant developing electrical field formed by the difference between the developing bias and the latent image potential, the toner charge amount is inversely proportional to the amount of toner developed on the photoconductor drum  1 Y that represents an image density on the photoconductor drum  1 Y. Accordingly, decreasing variations in the toner charge amount decreases variations in the image density. 
     Sufficiently stirring the developer G is important to decrease variations in the toner charge amount. In particular, after the developing device is stopped and left, the toner charge amount of the developer G rapidly increases immediately after the start of stirring, reaches a maximum, and then gradually decreases to converge to a stable state, as illustrated in  FIG.  4   . Start-up operations according to the comparative embodiment not having the sufficient time to stir the developer G results in starting the printing operation before the toner charge amount sufficiently decreases to converge to the stable state as illustrated in  FIG.  5 B , causing the image density in the image printed immediately after the start of printing operation to be lower than the image density in the image printed thereafter. 
     In contrast, the start-up operations according to the first embodiment having the sufficient time to stir the developer G results in starting the printing operation after the toner charge amount sufficiently decreases to converge to the stable state as illustrated in  FIG.  4   , causing the image density in the image printed immediately after the start of printing operation to be stable. Setting the time to stir the developer G in the start-up operations of the first embodiment to be longer than that of the comparative embodiment within the time for the warm-up operation of the fixing device  80  does not cause a disadvantage that a first print time (that is, a time until the image forming apparatus  100  starts the printing operation) becomes longer. 
     In the start-up operations according to the first embodiment, the developing motor  92  rotates the conveying screws  55 Y as the stirrers and the developing roller  51 Y to stir the developer G. 
     However, the developing roller  51 Y may not be rotated in the start-up operations, and only the conveying screws  55 Y as the stirrers may be rotated to stir the developer G. In this case, the image forming apparatus  100  includes another driver to drive and rotate the conveying screws  55 Y as the stirrers in addition to the driver to drive and rotate the developing roller  51 Y. 
     As described above, the image forming apparatus  100  according to the first embodiment includes the photoconductor drum  1 Y as the image bearer, the developing device  5 Y, and the fixing device  80 . The developing device  5 Y includes the conveying screws  55 Y as the stirrers to stir the developer G inside the developing device  5 Y and develops the latent image formed on the surface of the photoconductor drum  1 Y into the toner image. The toner image is transferred to the sheet P. The fixing device  80  fixes the transferred and unfixed toner image onto the sheet P. In addition, the image forming apparatus  100  according to the first embodiment includes the controller  90  as the circuitry. The controller  90  performs the start-up operations after the controller  90  receives the print start command. The start-up operations includes the warm-up operation of the fixing device  80 . During the warm-up operation, the controller  90  starts the stirring operation that stirs the developer G in the developing device  5 Y, which is performed by the conveying screws  55 Y as stirrers. 
     The above-described configuration and operations can stabilize the image density in the images printed immediately after the start of printing. 
     A second embodiment is described below. 
     Similar to the image forming apparatus  100  according to the first embodiment, the image forming apparatus  100  according to the second embodiment includes the developing device  5 Y including the conveying screws  55 Y as the stirrers. 
     With reference to  FIG.  6   , the following describes start-up operations of the image forming apparatus  100  according to the second embodiment. The controller  90  performs the start-up operations after the controller  90  receives the print start command and before starting the printing operation. The controller  90  stores a cumulative image area rate that is a sum of image area rates of all images sequentially printed before receiving the print start command, that is, the cumulative image area rate in a previous print job. Based on the cumulative image area rate, the controller  90  adjusts a timing to start the stirring operation that stirs the developer G in the developing device  5 Y, which is performed by the two conveying screws  55 Y as stirrers during the start-up operations. 
     Specifically, based on a small cumulative image area rate in the previous job, the controller  90  sets the timing to start stirring the developer G during the start-up operations to be earlier than the timing set based on a large cumulative image area rate. 
     As illustrated in  FIG.  6   , the controller receives the print start command in step  51 , reads the cumulative image area rate in the previous job in step S 2 , and determines the timing for starting the rotation of the conveying screws  55 Y, that is, the timing for starting to stir developer, during the start-up operations based on the cumulative image area rate in step S 3 . In step S 4 , the controller  90  performs the start-up operations including the operation that starts at the determined timing. After completing the start-up operations, the controller  90  starts the printing operation in step S 5 . 
     The reason why the above-described control is performed is as follows. As illustrated in  FIG.  7   , an increase in the toner charge amount after the stirrers start stirring the developer G in a case of the small cumulative image area rate in the previous print job is larger than that in a case of the large cumulative image area rate. The larger the increase in the toner charge amount, the longer the time until the toner charge amount stabilizes. For this reason, based on the small cumulative image area rate in the previous job, the controller  90  according to the second embodiment sets the timing to start stirring the developer G to be earlier than the timing set based on the large cumulative image area rate so as to take a long time for stirring the developer G during the start-up operations. As a result, the toner charge amount after the start of the printing operation is stabilized regardless of the cumulative image area rate in the previous print job. 
     The image area rate is obtained by dividing an image area by an area in which the image can be formed and proportional to the amount of toner consumed in the developing device  5 Y and the number of pixels of latent images written by the exposure device  7 . In the second embodiment, the controller  90  acquires the number of pixels from the exposure device  7 , calculates the cumulative image area rate, stores the cumulative image area rate in a memory in the controller  90 , and adjusts the timing to start stirring the developer G based on the cumulative image area rate. 
     As described above, the image forming apparatus  100  according to the second embodiment includes the photoconductor drum  1 Y as the image bearer and the developing device  5 Y. The developing device  5 Y includes the conveying screws  55 Y as the stirrers to stir the developer G inside the developing device  5 Y and develops the latent image formed on the surface of the photoconductor drum  1 Y into the toner image. The controller  90  performs the start-up operations after the controller  90  receives the print start command and before starting printing. The controller  90  stores the cumulative image area rate that is the sum of image area rates of all images sequentially printed before receiving the print start command, that is, the cumulative image area rate in the previous print job. Based on the cumulative image area rate, the controller  90  adjusts the timing to start the stirring operation that stirs the developer G in the developing device  5 Y, which is performed by the two conveying screws  55 Y as stirrers during the start-up operations. 
     The above-described configuration and operations can stabilize the image density in the images printed immediately after the start of printing. 
     A third embodiment is described below. 
     As illustrated in  FIG.  8   , the image forming apparatus  100  according to the third embodiment is different from the image forming apparatus  100  according to the first embodiment in that the start-up operations include a time for which the rotation speed of the conveying screw  55 Y as the stirrer is slower than the rotation speed of the conveying screw  55 Y during the printing operation. 
     Specifically, the developing motor  92  is a variable rotation speed motor, and the controller  90  controls the developing motor  92  so that the rotation speed of the conveying screw  55 Y during the time in the start-up operations is slower than the rotation speed of the conveying screw  55 Y during the printing operation. 
     The reason why the above-described control is performed is as follows. As illustrated in  FIG.  9   , the slower the rotation speed of the conveying screw  55 Y, the shorter the time for which the toner charge amount increases to a maximum value after the start of stirring the developer G, which shortens a time to stabilize the toner charge amount. For this reason, the controller  90  in the third embodiment sets the rotation speed of the conveying screw  55 Y for the time during the start-up operations to be smaller than the rotation speed of the conveying screw  55 Y during the printing operation to stabilize the toner charge amount at an early stage. As a result, the time taken for the start-up operations is reduced. 
     Thus, similar to the above-described embodiments, above-described configuration and operations in the image forming apparatus  100  according to the third embodiment can stabilize the image density in the images printed immediately after the start of printing. 
     In addition, as illustrated in  FIG.  8   , the controller  90  in the third embodiment controls the main motor  91  to drive and rotate the photoconductor drum  1 Y as the image bearer together with the developing roller  51 Y as the developer bearer and controls the development power supply  97  to apply the developing bias to the developing roller  51 Y when the conveying screws  55 Y as the stirrers are driven to rotate during the start-up operations. 
     In other words, in the start-up operations, the controller  90  controls the developing motor  92  to start driving the conveying screws  55 Y to start stirring the developer. At the same time, the controller  90  controls the development power supply  97  to supply the developing bias to the developing roller  51 Y that is driven to rotate and controls the main motor  91  to start rotating the photoconductor drum  1 Y. 
     Performing the above-described control consumes toner of the developer G in the developing device  5 Y (the toner is adhered to the photoconductor drum  1 Y). As a result, the above-described control can reduce the deterioration of the toner of the developer G compared with the control that does not consume the toner of the developer G even when the developer G is stirred for a long time during the start-up operations. The deterioration of the toner causes a vertical streak image (mainly caused by lubricant unevenly applied to the photoconductor drum  1 Y). The above-described control can prevent such a disadvantage. 
     A fourth embodiment is described below. 
     Similar to the image forming apparatus  100  according to the above-described embodiments, the image forming apparatus  100  according to the fourth embodiment includes the developing device  5 Y including the conveying screws  55 Y as the stirrers. 
     With reference to  FIG.  10   , the following describes start-up operations of the image forming apparatus  100  according to the fourth embodiment. The controller  90  performs the start-up operations after the controller  90  receives the print start command and before starting printing. In the start-up operations, the controller  90  starts the stirring operation that stirs the developer G in the developing device  5 Y, which is performed by the conveying screws  55 Y as stirrers. Subsequently, the controller  90  starts the printing operation after a change amount in the toner charge amount of the toner of the developer G is equal to or smaller than a predetermined value X. 
     As illustrated in  FIG.  10   , after the controller  90  receives the print start command in step S 11 , the controller  90  starts the start-up operations in step S 12  and starts rotating the conveying screws  55 Y to start stirring the developer during the start-up operations in step S 13 . Subsequently, the controller  90  determines whether the change in the toner charge amount of the toner of the developer G is equal to or smaller than the predetermined value X in step S 14 . When the change amount in the toner charge amount of the toner of the developer G is equal to or smaller than the predetermined value X, the toner charge amount is stable, and the image density is stable. Therefore, the controller  90  completes the start-up operations and starts the printing operation in step S 15 . 
     The toner charge amount of the toner of the developer G in the developing device  5 Y may be directly detected by an electrostatic sensor or the like disposed in the developing device  5 Y or may be indirectly detected by developing a toner image for a toner charge detection on the photoconductor drum  1 Y and detecting the image density of the toner image. 
     Thus, the above-described configuration and operations in the image forming apparatus  100  according to the fourth embodiment can stabilize the image density in the images printed immediately after the start of printing. 
     In the above-described embodiments, the image forming apparatus  100  includes the intermediate transfer belt  8  as an intermediate transferor, the secondary transfer roller  70 , and the secondary transfer belt  72 , as a transfer device, but the present disclosure is not limited to this. Alternatively, the present disclosure may be applied to an image forming apparatus using a direct transfer system. The direct transfer system does not include an intermediate transferor such as an intermediate transfer belt or an intermediate transfer drum. The image forming apparatus using the direct transfer system includes the developing device, a photoconductor such as the photoconductor drum on which the developing device develops the toner image, and a transfer device such as a transfer roller or a transfer belt to transfer the toner image on the photoconductor drum onto the sheet conveyed to a position of the photoconductor drum. 
     In the above-described embodiments, the image forming apparatus  100  includes the secondary transfer roller  70  and the secondary transfer belt  72  as the transfer device, but the present disclosure is not limited to this. The present disclosure may be applied to the image forming apparatus not including the transfer belt but including the secondary transfer roller as the transfer device. 
     In the above-described embodiments, the present disclosure is applied to the image forming apparatus  100  that forms color image. Alternatively, the present disclosure may also be applied to an image forming apparatus that forms a monochrome image alone. 
     In the above-described embodiments, a developing device such as the developing device  5 Y includes a two component developer including toner and carrier, but the developing device may include a one component developer including only toner. In the developing device including the one component developer, a developing roller as the developer bearer may be in contact with the photoconductor drum as the image bearer. 
     In the above-described embodiments, the developing device  5 Y includes two conveying screws  55 Y as the stirrers horizontally arranged in parallel and the doctor blade  52 Y disposed above the developing roller  51 Y. However, the configuration of the developing device is not limited to the above-described configurations. The present disclosure may be applied to other developing devices such as a developing device including one stirrer or three or more stirrers, a developing device including multiple stirrers vertically arranged, or a developing device including the doctor blade disposed below the developing roller. 
     In the above-described embodiments, the heater  85  is used as the heater in the fixing device  80 , but the heater in the fixing device is not limited to this. The heater may be an electromagnetic induction coil or a resistive heat generator. 
     Two or more of the configurations and controls in the above embodiments may be combined as appropriate. 
     In such configurations, effects similar to those described above are attained. 
     The above-described embodiments are illustrative and do not limit this disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements at least one of features of different illustrative and exemplary embodiments herein may be combined with each other at least one of substituted for each other within the scope of this disclosure and appended claims. Further, features of components of the embodiments, such as the number, the position, and the shape are not limited the embodiments and thus may be preferably set. 
     The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention. Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above. 
     The functionality of the elements disclosed herein may be implemented using circuitry or processing circuitry which includes general purpose processors, special purpose processors, integrated circuits, application specific integrated circuits (ASICs), digital signal processors (DSPs), field programmable gate arrays (FPGAs), conventional circuitry and/or combinations thereof which are configured or programmed to perform the disclosed functionality. Processors are considered processing circuitry or circuitry as they include transistors and other circuitry therein. In the disclosure, the circuitry, units, or means are hardware that carry out or are programmed to perform the recited functionality. The hardware may be any hardware disclosed herein or otherwise known which is programmed or configured to carry out the recited functionality. When the hardware is a processor which may be considered a type of circuitry, the circuitry, means, or units are a combination of hardware and software, the software being used to configure the hardware and/or processor.