Abstract:
An image forming cartridge comprises a developing unit configured to hold toner including a developing section configured to use the toner for developing and a supplying section configured to provide the toner to the developing section and a toner cartridge attachable to the developing unit, wherein the toner cartridge contains toner and supplies toner to the developing unit and the toner cartridge includes a stand-by charging unit configured to perform stand-by charge to the toner in the toner cartridge.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. P2007-309602 filed on Nov. 30, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image forming cartridge used in an electrophotographic printer, an electrophotographic copier and the like, and to an image forming apparatus. 
         [0004]    2. Description of Related Art 
         [0005]    A modern electrophotographic image forming apparatus generally forms an image in the following manner. First, the electrophotographic image forming apparatus exposes a surface of a photosensitive drum uniformly charged by a charging device with an exposure head, and thereby projects a desired image, character or the like onto the surface of the photosensitive drum to form an electrostatic latent image thereon. Then, the electrophotographic image forming apparatus develops the electrostatic latent image with toner in a triboelectrically charged image forming cartridge to form a toner image on the photosensitive drum, which serves as an image carrier, and thereafter transfers and fixes this toner image onto a medium. 
         [0006]    In such an image forming apparatus, when a toner cartridge is replaced due to toner running low, a so-called fog noise is caused because new toner in the replaced new toner cartridge is not sufficiently charged. The fog noise is a phenomenon wherein toner particles attach to a portion that is not intended for developing. As a method for preventing this, Japanese Patent Application Publication No. 2002-258676 discloses a method of securing long times for triboelectrically charging toner in an image forming cartridge by incurring time to allow idling rotation of the image forming cartridge when a toner cartridge is replaced, for example. 
         [0007]    However, in the above-described image forming apparatus of the related art, the extra time for idling rotation of the image forming cartridge is incurred every time and immediately after replacement of the toner cartridge. This prolongs stand-by time and subsequent print times. 
       SUMMARY OF THE INVENTION 
       [0008]    An aspect of the invention provides an image forming cartridge that comprises: a developing unit configured to hold toner including: a developing section configured to use the toner for developing; and a supplying section configured to provide the toner to the developing section; and a toner cartridge attachable to the developing unit, wherein the toner cartridge contains toner and supplies toner to the developing unit, and the toner cartridge includes a stand-by charging unit configured to perform stand-by charge to the toner in the toner cartridge. 
         [0009]    Another aspect of the invention provides an image forming apparatus that comprises: an image carrier; a developing unit, comprising an exposure unit configured to form an electrostatic latent image on the image carrier; and a developing unit that makes the electrostatic latent image visible; 
         [0010]    a developer container attachable to the developing unit and configured to supply the developing unit with the developer, the developer container containing the developer; a replacement detector configured to detect replacement of the developer container; a stand-by charging unit configured to charge only the developer in the developer container; and a partition provided in a developer path from the developer container to the developing unit and configured to controllably open after stand-by charging of the developer upon detection of replacement of the developer container. 
         [0000]    Another aspect of the invention provides a toner cartridge comprising: an identification storing unit configured to store data identifying the toner cartridge; a stirring unit configured to stir the toner in the toner cartridge, thereby charging the toner; and a conductive or semiconductive member configured to stand-by charge the toner, wherein the toner cartridge supplies the toner to a developing unit attachable to the toner cartridge after stand-by charging the toner in the toner cartridge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a configurational view of an image forming apparatus according to a first embodiment; 
           [0012]      FIG. 2  is a configurational view of an image forming cartridge according to the first embodiment; 
           [0013]      FIG. 3  is a cross-sectional view of a developing unit and a toner cartridge according to the first embodiment; 
           [0014]      FIG. 4  is a configurational view of a stirring unit of the image forming apparatus according to the first embodiment; 
           [0015]      FIG. 5  is a configurational diagram of a control system of the image forming apparatus according to the first embodiment; 
           [0016]      FIG. 6  is an operational illustration of the image forming apparatus according to the first embodiment; 
           [0017]      FIG. 7  is an operational illustration of the image forming apparatus according to the first embodiment; 
           [0018]      FIG. 8  is an operational flowchart of the image forming apparatus according to the first embodiment; 
           [0019]      FIG. 9  is a cross-sectional view showing the configuration of a developing unit according to a second embodiment; 
           [0020]      FIG. 10  is a view showing a toner level detector of an image forming apparatus according to the second embodiment; 
           [0021]      FIG. 11  is a configurational diagram of a control system of the image forming apparatus according to the second embodiment; 
           [0022]      FIGS. 12A to 12D  illustrate detection of amounts of toner in an image forming apparatus according to the second embodiment; 
           [0023]      FIGS. 13A to 13D  illustrate detection of amounts of toner in an image forming apparatus according to the second embodiment; 
           [0024]      FIGS. 14A and 14B  show sensor output waveforms from the image forming apparatus according to the second embodiment; 
           [0025]      FIG. 15  exemplifies a relationship between toner level and sensor-on time of the toner level detector according to the second embodiment; and 
           [0026]      FIG. 16  is an operational flowchart of the image forming apparatus according to the second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0027]    Embodiments of the invention will be described by referring to the drawings. In the drawings, the same reference numerals denote components that are common in the drawings. Note that, the invention will be described using a printing apparatus to exemplify an image forming apparatus, but is not so limited. 
       (Configuration) 
       [0028]      FIG. 1  is a configurational view of an image forming apparatus according to a first embodiment.  FIG. 2  is a configurational view of an image forming cartridge.  FIG. 3  is a cross-sectional view of a toner cartridge. 
       (Configuration of the Image Forming Apparatus) 
       [0029]    First, as shown in  FIG. 1 , image forming apparatus  1  according to the first embodiment includes cassette  102  configured to store paper sheet(s)  103 , picker roller  104  configured to send out paper sheets  103  one by one onto transport path  101 , image forming cartridge  100  configured to perform toner development on paper sheet  103  transported as shown by arrow A, fixing unit  105  configured to thermally fix the toner development on paper sheet  103 , and transport rollers  106  configured to send out paper sheet  103  on which the toner development is fixed, to the outside of image forming apparatus  1 . 
         [0030]      FIG. 2  shows a configuration of image forming cartridge  100  of image forming apparatus  1  according to the first embodiment and a peripheral portion thereof, including photosensitive drum  2 , charging roller  3 , exposure head  4 , developing roller  5 , feed roller  6 , toner restricting member  7 , transfer roller  8 , and cleaning device  9 . 
         [0031]    Photosensitive drum  2  includes a conductive support and a photosensitive layer. The conductive support is an organic photoreceptor in which a charge generation layer and a charge transport layer are sequentially laminated as photoconductive layers on a metal pipe formed of aluminum. Charging roller  3  is formed of a metal shaft and a semiconducting rubber layer. Exposure head  4  functions to expose a surface of photosensitive drum  2  with LED light or laser light. 
         [0032]    Developing roller  5  is formed of a metal shaft and a semiconducting urethane rubber member or the like. Feed roller  6  is formed of a metal shaft and a foamed urethane rubber member or the like. Toner restricting member  7  is a thin plate with, for example, a thickness of approximately 0.08 mm and a length in a longitudinal direction being substantially same as a width of an elastic body of developing roller  5 . The toner restricting member  7  is provided so that one end in the longitudinal direction would be fixed to an unillustrated frame and the other end would come in contact with developing roller  5  on a surface slightly inside from the tip end. 
         [0033]    Developing roller voltage supplying unit ( 23   a ) connects to developing roller  5 . Feed roller voltage applying unit ( 23   b ) connects to feed roller  6 . Both units are configured to charge respective rollers to a predetermined potential. In addition, an unillustrated voltage power supply is connected to charging roller  3  and transfer roller  8  so as to apply a predetermined voltage. These rollers rotate in directions shown by arrows B, C, and E in  FIGS. 2 and 3  via motor (not shown) as a driving source under control of controller  32  shown in  FIG. 5  described later. 
       (Configuration of the Developing Unit  10 ) 
       [0034]      FIG. 3  shows a cross-sectional view of developing unit  10  and toner cartridge  20  of image forming cartridge  100  shown in  FIG. 2 . Developing unit  10  includes developing roller  5 , feed roller  6 , toner restricting member  7 , and openable and closeable shutter  12  provided on a toner path between toner cartridge  20  and developing unit  10 . Image forming toner cartridge  100  may be attachable to and removable from image forming apparatus  1 . 
       (Configuration of the Toner Cartridge  20 ) 
       [0035]    On the other hand, toner cartridge  20  is a toner container that is attachable and removable to an upper portion of developing unit  10 . Toner cartridge  20  includes stirring unit  21  configured to stir toner in the toner cartridge, a metal plate  22  being set so as to cover an inner wall surface, and data recorder  24 , for example, a radio frequency identification (RFID) chip, in which information such as the product number of toner cartridge  20  is stored. Stand-by charging voltage applying unit VTC  23   c,  which may be included in image forming apparatus  1  is electrically connected to metal plate  22  to apply a voltage to metal plate  22 . In addition, if toner cartridge  20  or image forming cartridge is attachable and removable, an interface for electrically connecting stand-by charging voltage applying unit VTC  23   c  and metal plate  22  may be provided in each of stand-by charging voltage applying unit VTC  23   c  and metal plate  22 .  FIG. 4  shows stirring unit  21  configured to stir the toner in toner cartridge  20 . As shown in  FIG. 4 , stirring unit  21  is configured to rotate stirring bar  21   b  through arm  21   a,  under control of controller  32 , by causing rotation shaft  21   d  to rotate with its rotation axis as center of rotation in the direction shown by arrow D through gear  21   c  driven by an unillustrated driving source. Note that, the driving source of developing roller  5  and feed roller  6  may be the same or different. Here, it is preferable that the driving source and controller  32  are provided to image forming apparatus  1 . The stirring unit  21  may, of course, be directly rotated by a motor or the like without gear  21   c,  or the rotating direction may be reversed. Stirring bar  21   b  may, of course, have a plate form instead of a shaft form, or may have another form. 
         [0000]    In addition, as long as a method promotes toner charging, the configuration is not limited to the combination of the above-described metal plate and stirring bar. The combination and the shape may be of any type, such as a metal blade and a rubber roller, or a mesh metal plate and a rubber roller, as long as at least one is a conductive material or semiconducting material. 
       (Configuration of the Control System) 
       [0036]      FIG. 5  is a block diagram of a control system for image forming apparatus  1  according to the first embodiment. Data readout unit  31  is a device configured to read out information stored in data recorder  24  included in toner cartridge  20  in a noncontact manner. Controller  32  gives an operation command to shutter opening/closing driver  34 , image forming cartridge driver  35 , and voltage applying unit  23  in response to a print instruction, and analyses data obtained from data readout unit  31 . In addition, controller  32  has timer  33  configured to determine if a predetermined operation time of stirring bar  21   b  has expired. 
         [0037]    Shutter opening/closing driver  34  opens shutter  12  through an unillustrated solenoid in response to an instruction from controller  32 . In addition, image forming cartridge driver  35  drives developing roller  5 , feed roller  6 , and stirring bar  21   b  through an unillustrated motor in response to an instruction from controller  32 . Voltage applying unit  23 , which includes developing roller voltage apply init  23   a,  supply roller voltage applying unit  23   b  and stand-by charging voltage applying unit  23   c,  generates a high voltage for developing roller  5 , feed roller  6 , and metal plate  22  in the toner cartridge in response to an instruction from controller  32 . 
       (Operations) 
       [0038]    With the above-described configuration, image forming apparatus  1  according to the first embodiment will operate as follows. Image forming operations performed by this image forming apparatus  1  will be described with reference to the configurational views of  FIGS. 1 and 2 . 
       (Image Forming Operations) 
       [0039]    First, a surface of photosensitive drum  2  is uniformly charged to an arbitrary polarity and potential by a charging device, such as charging roller  3  or the like. According to a print instruction from a higher-level device, image data outputted from an unillustrated writing controller is sent to exposure head  4 . Then, an electrostatic latent image corresponding to an image pattern is formed on the surface of photosensitive drum  2 . 
         [0040]    Feed roller  6  to which a predetermined voltage is applied by feed roller voltage applying unit  23   b  comes in contact with developing roller  5 . By rotationally driving feed roller  6 , toner in a toner container of developing unit  10  is charged to a predetermined potential, and fed to developing roller  5 . 
         [0041]    The toner on developing roller  5  is triboelectrically charged by toner restricting member  7  coming in contact with developing roller  5 . A thickness of the toner is determined by the pressing force of toner restricting member  7  onto developing roller  5 . Developing roller  5  comes in contact with photosensitive drum  2  whose surface is charged by charging roller  3  to a predetermined voltage, and the toner charged by developing roller voltage applying unit  23   a  adheres onto the electrostatic latent image on photosensitive drum  2 . 
         [0042]    Then, under transport control of controller  32 , the toner on photosensitive drum  2  is transferred by an electric field with transfer roller  8  onto paper sheet  103 , which is send out by picker roller  104  and transported from the inside of cassette  102  through conveyance path  101 . Thereafter, the transferred toner on paper sheet  103  is fixed by fixing unit  105  and discharged from a discharging port by transport rollers  106 . Note that, the toner residual on photosensitive drum  2  is removed by cleaning device  9  after the transfer is completed. 
         [0043]    In the above-described operations of image forming, when a toner level is sufficiently high in the toner container of developing unit  10  and there is no need to replace a toner cartridge, as shown in  FIG. 2  or  3 , toner cartridge  20  is set on the upper portion of developing unit  10  with shutter  12  open. Here, the toner in toner cartridge  20  and the toner in developing unit  10  are in a stirred state with set, desirable potentials. Thereby, a fog noise and the like caused by an insufficiently-charged new toner is avoided. Thus, an electrostatic latent image can be stably formed on photosensitive drum  2 . 
         [0044]    In contrast, when toner cartridge  20  is replaced, new toner in toner cartridge  20  is mixed into developing unit  10 . Thereby, the electric potential of the toner in developing unit  10  is not at a desirable level. Thus, a fog noise and the like are created. 
       (Operations of Stand-By Charging) 
       [0045]    To resolve such inconveniences, the image forming apparatus according to the first embodiment operates as follows. This operation will be described with reference to  FIG. 6  in which shutter  12  is closed, with reference to  FIG. 7  in which shutter  12  is opened. 
         [0046]    First, when toner cartridge  20  is replaced, an unillustrated toner cartridge lock bar of toner cartridge  20  is rotated in a predetermined direction to manually close a shutter (not shown) of toner cartridge  20 . Thereafter, toner cartridge  20  is pulled upward from the apparatus  1  and then detached. At that time, shutter  12 , which is in the upper portion of developing unit  10 , also is closed. 
         [0047]    Next, new toner cartridge  20  is inserted into image forming apparatus  1  and the unillustrated toner cartridge lock bar of toner cartridge  20  is rotated in a direction opposite to the direction in which the shutter is closed. Thereby, the shutter of toner cartridge  20  is opened. As shown in  FIG. 6 , when new toner cartridge  20  is placed, controller  32  reads out, for example, the product number of the new toner cartridge from data recorder  24  provided in the upper portion of toner cartridge  20 , and detects that the product number before replacement is different from the new product number. Thereby, controller  32  detects that toner cartridge  20  is replaced. 
         [0048]    Note that, the detection of whether toner cartridge  20  is replaced may be performed preferably when power to image forming apparatus  1  is turned on or when an unillustrated body cover of image forming apparatus  1  is closed after changing toner cartridge  20 . 
         [0049]    Next, when a printing instruction is made, developing unit  10  performs a regular printing operation. At the same time, toner cartridge  20  stirs toner by rotating stirring bar  21   b  of stirring unit  21  while voltage is applied to metal plate  22  via stand-by charging voltage applying unit  23   c.  Stirring bar  21   b  of stirring unit  21  rotates while coming into contact with metal plate  22 , to which a voltage is applied by stand-by charging voltage applying unit  23   c,  so that stand-by charging to a developer is promoted. 
         [0050]    At that time, shutter  12  remains closed. Thereby, only new toner in replaced toner cartridge  20  is independently stirred and charged without mixing with the toner in developing unit  10 . 
         [0051]    Voltage VTC applied from stand-by charging voltage applying unit  23   c  to metal plate  22  preferably may have the same polarity as the polarity of toner charging applied to developing roller  5  and feed roller  6 . In addition, voltage VTC preferably maybe substantially the same level as, for example, voltage VSB to be applied to feed roller  6 . 
         [0052]    Next, as for triboelectrically charging the toner in toner cartridge  20 , a stirring operation is performed for a predetermined period and thereafter, as shown in  FIG. 7 , shutter  12  is opened by an instruction from controller  32 . Then, the toner in toner cartridge  20  is put into developing unit  10  and mixed. 
         [0053]    Note that, after shutter  12  is opened, it is better to terminate voltage to metal plate  22  to avoid deteriorating toner charging characteristics by unnecessarily increasing the toner charging amount in toner cartridge  20 . 
         [0054]    With the operations described above, the toner in toner cartridge  20  is mixed into the toner in developing unit  10 . Thereby, a printing operation will proceed normally as shown in  FIG. 2  or  3 . 
       (Detailed Operations of Stand-By Charging) 
       [0055]    The above-described operations will be described in further detail with reference to the flowchart of  FIG. 8 . Note that the following description exemplifies the case wherein the driving source of stirring unit  21  is the same as the driving source of developing roller  5  or feed roller  6 . In this example, stirring bar  21   b  can rotate only when printing is performed or at start-up, namely, when developing roller  5  or feed roller  6  rotates. 
         [0056]    First, the product number of the toner cartridge or the like is read out from data recorder  24  (step S 1 ) to determine whether toner cartridge  20  is changed (step S 2 ). 
         [0057]    Then, if it is determined that toner cartridge  20  is not replaced, the step returns to step S 1  to repeat steps S 1  and S 2 . A regular printing operation is performed during that time. 
         [0058]    In contrast, if it is determined that toner cartridge  20  is replaced, a timer value of a total operation time Tsum of stirring bar  21   b  is cleared (step S 3 ) and a timer value of an operation time Tk of stirring bar  21   b  in a series of printing operations is further cleared (step S 4 ). 
         [0059]    Then, measurement of the operation time Tk of stirring bar  21   b  in the series of printing operations is started, and a voltage is applied to metal plate  22  in toner cartridge  20  to start stand-by charging of the new toner in toner cartridge  20  (step S 5 ). 
         [0060]    After the series of printing operations are finished, the total operation time Tsum of stirring bar  21   b  is calculated (step S 6 ), and controller  32  determines whether the total operation time Tsum exceeds a predetermined time (for example, one minute) (step S 7 ). 
         [0061]    When the total operation time Tsum does not reach the predetermined time, processes from steps S 4  to S 6  are repeated for every series of printing operations. When the total operation time Tsum reaches the predetermined time, it is determined that the toner in toner cartridge  20  is sufficiently triboelectrically charged. Thus, the timer values of the operation time Tk and the total operation time Tsum are cleared (step S 8 ). 
         [0062]    Thereafter, application of voltage to metal plate  22  in toner cartridge  20  terminates and shutter  12  opens, so that the toner in toner cartridge  20  and the toner in developing unit  10  would be mixed (step S 9 ). Then, the process performed when the toner cartridge is replaced is finished. 
         [0063]    In the description above, an example is described wherein the driving source of the rotation driver of stirring bar  21   b  is the same as the driving source of developing roller  5  and feed roller  6 , and stirring bar  21   b  rotates only during the printing operations as in steps S 3  to S 6 . However, when stirring bar  21   b  is rotationally driven by another driving source, stirring bar  21   b  can rotate regardless of the printing operations. Thus, shutter  12  may be opened after stirring bar  21   b  rotates for a reference period, which is set in advance. 
         [0064]    In addition, the above-described embodiment describes the case wherein shutter  12  is opened based on a time in which stirring bar  21   b  rotates. However, shutter  12  maybe opened based on, not the time, but, for example, a toner potential in the toner cartridge, which is detected by toner potential measuring unit  25  provided in toner cartridge  20 . For example, the toner potential, which is charged by stirring in the toner cartridge  20  is measured by a general potential measuring method. When the measured potential reaches a predetermined value, shutter  12  may be opened by stopping the voltage application to metal plate  22 . Note that, measurement of the potential merely requires a gold-plated electrode (with a diameter of approximately 6 mm) that is vibrated with a distance of several mm from a measuring surface. The surface potential at that time is measured by a vibration capacitance measurement. 
         [0065]    In addition, the embodiment above describes the case wherein toner cartridge  20  is attachable and removable to developing unit  10  and thereafter stirring bar  21   b  rotates for charging the toner by stirring. However, toner cartridge  20  may be attachable and removable to developing unit  10  after becoming charged by stirring for a predetermined period by providing another charging device before being attached to developing unit  10 . 
         [0066]    Moreover, the embodiment above describes the case wherein the toner is charged by stirring by rotating stirring bar  21   b  via application of voltage to metal plate  22  for a predetermined period. However, if there is no need to uniformly charge the toner, voltage may be applied to metal plate  22  with time monitoring of steps S 3  to S 7 , without rotating stirring bar  21   b . Alternatively, Stand-by charging may be applicable by rotating stirring bar  21   b  in contact with metal plate  22  to stir toner in toner cartridge  20  instead of applying voltage to metal plate  22 . 
       Effects of the First Embodiment 
       [0067]    As described above, in the image forming apparatus according to the first embodiment, both the metal plate, which can carry out stand-by charging and the stirring unit are provided in the toner cartridge. Furthermore, the controllable shutter is provided in the developing unit at its junction portion with the toner cartridge. In this configuration, when the toner cartridge is replaced, the shutter is opened after completion of stand-by charging of the toner in the toner cartridge, which is independently charged from the toner in the developing unit. Accordingly, provision of a new idling time for charging the new toner by stirring is not required. 
       Second Embodiment 
       [0068]      FIG. 9  shows the configuration of a developing unit according to a second embodiment as a cross-sectional view of developing unit  10 . This second embodiment has and added toner level detector  30 , which is configured to detect a residual amount of toner in developing unit  10 . Other configurations are same as in the image forming apparatus according to the first embodiment, and the descriptions thereof are omitted for simplification. 
       (Configuration of Toner Level Detector) 
       [0069]      FIG. 10  is a perspective view of toner level detector  30 . As shown in  FIG. 10 , toner level detector  30  includes toner level detection bar  30   b  formed in the shape of a crankshaft, gear  30   d  rotated by an unillustrated driver, and protruding member  30   c  attached to gear  30   d.  Rotational shaft  30   e  of toner level detection bar  30   b  is engaged into the center portion of gear  30   d , but is separately supported from gear  30   d.  Thereby, rotational shaft  30   e  can be rotated by being engaged in one direction in relation to gear  30   d.    
         [0070]    That is, when gear  30   d  rotates in the direction of arrow F in  FIG. 10 , protruding member  30   c  presses arm  30   a  of toner level detection bar  30   b  to cause toner level detection bar  30   b  to rotate in the direction of arrow F. 
         [0071]    Note that, a driving source for rotationally driving developing roller  5  or feed roller  6  may be configured to rotate toner level detection bar  30   b,  like the driving source for driving stirring unit  21 , or another driving source such as a motor may be provided for rotating toner level detection bar  30   b.    
       (Configuration of the Control System) 
       [0072]      FIG. 11  is a configurational diagram of a control system of the image forming apparatus according to the second embodiment. As shown in  FIG. 11 , the control system of the image forming apparatus according to the second embodiment is configured to connect toner level detection unit  30  to controller  32 . The toner level in developing unit  10  is detected by toner level detector  30  using a method to be described later and is inputted to controller  32 . 
         [0073]    Then, the charging by stirring performed by stirring bar  21  is made to the toner in the toner cartridge  20  in response to the toner level detection result from toner level detector  30 . Thereafter, timer  33  in controller  32  measures whether a predetermined time has passed to control opening/closing of shutter  12 . 
       (Operations) 
       [0074]    First, the operation of toner level detector  30 , which is configured to detect a toner level in developing unit  10 , will be described in detail by referring to  FIGS. 12A to 12D  and  FIGS. 13A to 13D . 
       (Operations of the Toner Level Detector) 
       [0075]    These figures show operation of toner level detection bar  30   b  in respective cases where the residual amount of toner in developing unit  10  is high and where it is low. In toner level detector  30 , toner level detection bar  30   b  is configured to drop by its own weight while rotating. The amount of toner amount is detected via a skip time due to changes in the drop due to the toner amount. 
         [0076]    That is, consider a case where the level of residual amount of toner is high, as shown in  FIGS. 12A and 12B . When gear  30   d  rotates in the F direction, toner level detection bar  30   b  rotates in the toner together with protruding member  30   c.  Thereafter, toner level detection bar  30   b  comes to the highest point as shown by the broken line in  FIG. 12C . Then, only toner level detection bar  30   b  rotates due to its weight and drops at the position shown by the solid line in  FIG. 12C . After that, toner level detection bar  30   b  stops until protruding member  30   c  comes thereto as shown in  FIG. 12D , and rotates by being pushed by protruding member  30   c  to return to the state shown in  FIG. 12A . 
         [0077]    When the residual amount of toner is low, as shown in  FIGS. 13A and 13B , toner level detection bar  30   b  rotates together with protruding member  30   c,  and then comes to the highest point as shown by the broken line in  FIG. 13C . Thereafter, only toner level detection bar  30   b  rotates due to its weight and drops as shown by the solid line in  FIG. 13C . Here, toner level detection bar  30   b  stops in a position close to a substantially bottom portion until protruding member  30   c  comes thereto, and rotates again by being pushed by protruding member  30   c  to return to the state shown in  FIG. 13A . 
         [0078]    Based on the above-described operations of toner level detection bar  30   b,  toner level detection sensor  30   s,  such as a photointerrupter, provided under toner level detector  30  detects that toner level detection bar  30   b  is at a position close to the bottom portion and measures a detected duration. 
         [0079]      FIGS. 14A and 14B  are output waveforms of toner level detection sensor  30   s.  In  FIGS. 14A and 14B , Is denotes a time period required for toner level detection bar  30   b  to make one rotation and each of t 1  and t 2  indicates the time period when sensor is turned on by toner level detection bar  30   b  passing by toner level detection sensor  30   s.  When the level of residual amount of toner is high, as shown in  FIG. 14A , a sensor-on time of toner level detection sensor  30   s  is short (Low side), while when it is low, as shown in  FIG. 14B , the sensor-on time of toner level detection sensor  30   s  is longer. 
         [0080]      FIG. 15  is a graph in which a relationship between the residual amount of the toner in developing unit  10  and sensor-on time of toner level detector  30  is plotted. Here, the time period Ts required for the toner level detection bar to make one rotation is set to 4.64 seconds. As shown in  FIG. 15 , the sensor-on time gradually decreases as the residual amount of toner changes from approximately 5 g to approximately 20 g. It can be seen that, for example, the sensor-on time is approximately 2 seconds when the residual amount of the toner is 10 g. 
         [0081]    As described above, the residual amount of toner in developing unit  10  can be detected from the sensor-on time of toner level detector  30 , which varies depending on the residual amount of the toner. 
       (Operations of Stand-By Charging) 
       [0082]    If shutter  12  is opened when the level of residual amount of the toner in developing unit  10  is low, a large amount of new toner enters developing unit  10  from toner cartridge  20 . In contrast, if the level of residual amount of the toner in developing unit  10  is high, the amount of new toner entering developing unit  10  from toner cartridge  20  is small even if shutter  12  is opened. 
         [0083]    Accordingly, consider a case where the level of residual amount of the toner in developing unit  10  is high, and the toner in toner cartridge  20  is mixed into developing unit  10  without stand-by charging. Here, if the mixed new toner is accumulated in a portion far above developing roller  5 , for example if the new toner is mixed in a range of approximately 5 mm below shutter  12 , toner to be consumed for image forming can be supplemented by the toner in developing unit  20 , which is sufficiently charged. Thus, an effect of a fog noise on a printed image is small. 
         [0084]    However, if the level of residual amount of toner in developing unit  10  is low and the upper portion of developing roller  5  protrudes from the toner accumulation, the new toner mixed from toner cartridge  20  is soon used for image formation. Thus, the effect of fog noise on a printed image is increased. 
         [0085]    Judging from the foregoing description, the lower the level of residual amount of the toner in developing unit  10 , the greater the need for sufficiently electrostatically charging new toner mixed from toner cartridge  20   
         [0086]    For this reason, the image forming apparatus according to the second embodiment is configured so that when toner cartridge  20  is replaced, toner level detector  30  detects the residual level the toner in developing unit  10 . This detected amount is compared with a reference amount that is set in advance. Then, if the residual level of toner is equal to or less than the reference amount, the toner in toner cartridge  20  is charged by stirring. 
       (Detailed Operations of Stand-By Charging) 
       [0087]    The operations of stand-by charging will be described in detail below by referring the operational flowchart in  FIG. 16 . Note that, steps S 11 , S 12 , and S 14  to S 20  are substantially the same as those of the first embodiment, and the descriptions thereof are omitted. 
         [0088]    First, toner cartridge data is read from data recorder  24  (step S 11 ). Then, controller  32  determines whether toner cartridge  20  is replaced (step S 12 ). If the toner cartridge is found not replaced, the step returns to step S 11  to repeat steps S 11  and S 12 . For that time, a regular printing operation is performed. 
         [0089]    In contrast, if the toner cartridge is found replaced, the level of residual amount of the toner in developing unit  10  is detected by toner level detector  30 , and it is determined whether the detected residual amount is equal to or less than the toner level reference value, which is set in advance (step S 13 ). 
         [0090]    For example, when the reference value of the residual amount of the toner is 10 g, 2 seconds is set as a reference time for sensor-on when the residual amount of the toner is 10 g. Then, the detected sensor-on time is compared with the reference time. If the sensor-on time is shorter than the reference time, it is determined that the residual amount of the toner is larger than 10 g. In contrast, if the sensor-on time is longer than the reference time, it is determined that the residual amount of the toner is smaller than 10 g. 
         [0091]    If it is determined by the above-described determination method that the residual amount of the toner in developing unit  10  is larger than the reference value, then shutter  12  is opened without charging the toner in toner cartridge  20  by stirring with stirring unit  21  (step S 21 ). Then, the process is completed to perform a regular printing operation. 
         [0092]    In contrast, if it is determined that the residual amount of the toner in developing unit  10  is small, the timer value of total operation time Tsum of stirring bar  21   b  and the timer value of operation time Tk of stirring bar  21   b  in a series of printing operations are cleared (steps S 14  and S 15 ). 
         [0093]    Then, measurement of the operation time Tk of stirring bar  21   b  in the series of printing operations is started, and a voltage is applied to metal plate  22  in toner cartridge  20  to start stand-by charging of new toner in toner cartridge  20 . If the series of printing operations are finished, a total operation time Tsum of stirring bar  21   b  is calculated. After that, it is determined whether total operation time Tsum exceeds the reference time, which is set in advance, for example, approximately one minute (steps S 16  to S 18 ). 
         [0094]    If total operation time Tsum does not reach the reference time, processes from steps S 15  to S 17  are repeated for each series of printing operations. If it is determined that total operation time Tsum reaches the reference time, then the toner in toner cartridge  20  is deemed sufficiently electrostatically charged. Thus, the timer values of operation time Tk and total operation time Tsum are cleared. After that, voltage application to metal plate  22  in toner cartridge  20  is terminated to open shutter  12 . Thereby, the toner in toner cartridge  20  and the toner in developing unit  10  are mixed (steps S 19  and S 20 ) . Then, the current process is completed. 
         [0095]    Note that, in the above-described second embodiment, only one reference value for residual amount of toner in developing unit  20  used in step S 13  is provided as a determination reference for determining whether electrostatic charging is performed before the new toner in toner cartridge  20  is mixed into the toner in developing unit  10 . However, multiple reference values can be set for the residual amount of toner. An electrostatic charging time of the new toner in toner cartridge  20  by stirring is changed depending on the residual amount of the toner in developing unit  10  by changing the reference time of the total operation time Tsum of stirring bar  21   b  for each reference value. 
         [0096]    In addition, in the description of the embodiment above, a method for detecting residual amount of toner is described by using, as an example, the method for detecting residual amount of the toner in developing unit  10  by utilizing sensor-on time of toner level detector  30 , which changes according to the residual amount of toner. However, other toner level detection methods, such methods for detecting toner weight, or the like may be used. 
       Effects of the Second Embodiment 
       [0097]    As described above, in the image forming apparatus according to the second embodiment, the toner level detector configured to detect the level of residual amount of the toner in developing unit is provided to detect a residual amount of the toner by the toner level detector when a toner cartridge is replaced. The stand-by charging time of the new toner in the toner cartridge changes based on the detected residual amount of toner. Thereby, an optimum stand-by time can be set. Thus, deterioration of toner charging characteristics can be prevented by greatly increasing the amount of toner charging. 
         [0098]    The present invention is widely applicable to image forming apparatuses, such as multifunctional printers, facsimiles, copiers, and the like, in addition to electrophotographic printers. 
         [0099]    The image forming apparatus according to the present embodiment includes a replacement detector configured to detect replacement of a developer container, a stand-by charger configured to perform stand-by charging on only a developer in the developer container, and a partition, which is provided on a developer path from the developer container to the developing unit and which is controlled to open and close. In addition, the image forming apparatus is configured for opening of the partition after the stand-by charging unit performs the stand-by charging on the developer in the developer container when the replacement detector detects that the developer container is replaced. Accordingly, there is no need to provide a new idling time for charging the new toner by stirring. 
         [0100]    As described above, the image forming cartridge and image forming apparatus according to the present embodiment can suppress the generation of fog noise and shorten waiting time for printing even when the toner cartridge is replaced. 
         [0101]    The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.