Abstract:
An image forming apparatus includes an image forming portion with a plurality of image bearing members and a plurality of developing devices, a plurality of cartridges, a maintenance door allowing access to the image forming portion for a maintenance operation, a plurality of first doors, remainder detecting means for detecting information relating to amounts of the developers in the cartridges, locking means for locking the first doors, a controller, a moving mechanism, and a second door capable of opening and closing to cover the first doors. The first doors open and close respective openings for insertion of the cartridges into the main assembly independently from each other, without access via the maintenance door. The controller controls the locking means to release a locking of a first door corresponding to a cartridge which requires exchange, on the basis of a detection result of the remainder detecting means.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to image forming apparatuses, such as a printer, a copy machine, and a facsimile machine, and the like. 
     A toner supplying apparatus removably connects a toner cartridge, which contains toner, with the image assembly of an image forming apparatus, and delivers the toner from the toner cartridge into the toner hopper of the image forming apparatus. A toner supplying apparatus is employed by image forming apparatuses such as an electrophotographic printer, an electrophotographic copy machine, and an electrophotographic facsimile machine. An electrophotographic image forming apparatus is structured so that a latent image is written on its photosensitive drum with a beam of laser light, and is developed with toner. Thus, the developing device of an electrophotographic image forming apparatus is provided with a toner hopper which temporarily stores the toner to be supplied to the peripheral surface of the photosensitive drum, and also, supplies the developing device with the toner. 
     As a latent image is developed, the toner within the toner hopper is consumed. Thus, as the toner in the toner hopper is consumed, toner is delivered into the toner hopper from a toner cartridge which is in the image forming apparatus main assembly, and is removably mountable in the main assembly. 
     Further, as it is detected by a sensor or the like that the amount of the developer in a developer cartridge has reduced to a certain level, a message which prompts a user to replace the toner cartridge is displayed. Further, if the amount of the toner in the toner hopper of the main assembly of the image forming apparatus falls below a preset level, the image forming apparatus stops printing. Thereafter, as soon as the replacement of the toner cartridge in the main assembly of the image forming apparatus makes it possible to supply the toner hopper with toner, the image forming apparatus restores itself so that it can restart the interrupted image forming operation. 
     Some image forming apparatuses employ multiple toner cartridges. These image forming apparatuses are structured so that as the amount of toner in any of the toner cartridges reduces below a preset level, they display a message that prompts a user to replace the toner cartridge, and also, so that the multiple cartridges can be individually replaced. Generally speaking, it is unlikely for all the cartridges to be replaced at the same time. That is, usually, it is only one of the cartridges that needs to be replaced. However, image forming apparatuses which require a user to perform a cartridge replacement sequence to make them ready for cartridge replacement are problematic in that it is possible for a wrong toner cartridge, that is, a toner cartridge which does not need to be replaced, will be replaced. 
     Thus, it has been proposed to provide an image forming apparatus with an automatic developing device replacement apparatus, more specifically, an apparatus which automatically selects a proper developing device from among the multiple developing devices in a developing device storing apparatus, and automatically replaces the developing device, which is necessary to be replaced, with the proper replacement cartridge (for example, Japanese Laid-open Patent Application H03-50568). However, the provision of an image forming apparatus with the structural arrangement for moving a process cartridge, a developing device, and a toner cartridge, from the position in which they can function for image formation, to the position into and from which they are to be mounted or dismounted, respectively, leads to an increase in size the main assembly of the image forming apparatus, and also, an increase in the space necessary for the maintenance of the image forming apparatus, and therefore, is not desirable. 
     Thus, it has been proposed to provide each of the toner cartridge chambers with such a door that automatically opens, as it is unlocked, to inform a user that the corresponding cartridge needs to be replaced. It has also been proposed (disclosed) to provide each toner cartridge with a display so that a user can be given visual information that the cartridge has virtually run out of toner (U.S. Pat. No. 6,560,416). 
     However, U.S. Pat. No. 6,560,416 suffers from the following problem. That is, in the case of the structural arrangement disclosed in U.S. Pat. No. 6,560,416, the door of the chamber for the toner cartridge necessary to be replaced automatically opens without a request from an operator. Therefore, it is possible for the door to be temporarily left open, which is undesirable. 
     It is possible to provide an image forming apparatus with a display which offers visual information regarding the toner cartridge, instead of equipping the development cartridge chamber door with a locking means which allows the door, which corresponds to the developer cartridge with virtually no toner, to automatically open, as disclosed in U.S. Pat. No. 6,560,416, to inform a user that the toner cartridge is virtually out of toner. 
     In the case of this structural arrangement, the door was not provided with a locking means. Therefore, a wrong door was sometimes opened by a user, which resulted in the interruption of the on-going image forming operation. Further, it required an image forming apparatus to be provided with multiple displays, which increases the apparatus in cost. On the other hand, in the case of the structural arrangement which prevents the cost increase by not employing multiple displays, and equips each door with a locking means to prevent the replacement of toner cartridge(s) which does not need to be replaced, a user has to look for the unlocked door, which is a waste of time. 
     SUMMARY OF THE INVENTION 
     Thus, the primary object of the present invention is to provide an image forming apparatus which does not have multiple displays which correspond to multiple developer cartridges, one-for-one, and yet, enables an operator to easily find a developer cartridge, in the apparatus, which needs to be replaced, and also, is structured so that it does not occur that a developer cartridge chamber door opens without a confirmation from a user, and therefore, is left open in the state in which the corresponding cartridge is accessible. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising a plurality of cartridges for supplying developers, said cartridges being detachably mountable to a main assembly of the apparatus; a plurality of first doors, provided correspondingly to cartridges, respectively, for opening and closing inserting openings for permitting insertion of said cartridges into said main assembly of the apparatus, independently from each other; remaining amount detecting means for detecting information relating to an amount of the developer contained in the cartridges; locking means for locking said first doors; a controller for controlling said locking means, on the basis of a result of detection of said remaining amount detecting means, to unlock only said first door corresponding to the cartridge to be exchanged; a moving mechanism for moving said first door from a position closing to an opening position with the unlocking of said locking means; and a second door movable between a closing position for preventing access to said first doors and an opening position for permitting access to said first doors; opening and closing detecting means for detecting opening and closing of said second door; wherein said controller unlocks said first door on the basis of the results of detection of said opening and closing detecting means and said remaining amount detecting means. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the image forming apparatus in the first embodiment of the present invention. 
         FIG. 2  is a perspective view of the image forming apparatus in the first embodiment of the prevent invention, which is in the state in which the developer cartridge chamber door of the apparatus is open, and a development cartridge is being mounted into the developer cartridge chamber. 
         FIG. 3  is a perspective view of the developer cartridge chamber and developer cartridge chamber door of the image forming apparatus in the first embodiment, which are in the state in which the door is fully open. 
         FIG. 4  is a drawing for showing the state of engagement between the developer cartridge chamber door and the apparatus for locking (or unlocking) the door. 
         FIG. 5  is a drawing for showing the state of disengagement between the developer cartridge chamber door and the apparatus for locking (or unlocking) the door. 
         FIG. 6  is a flowchart of the development cartridge replacement sequence. 
         FIG. 7  is a flowchart of the development cartridge replacement sequence of the image forming apparatus, whose development cartridge chambers doors are equipped with a means for detecting whether the external door is open or closed. 
         FIG. 8  is a schematic sectional view of a typical electrophotographic image forming apparatus, and shows the structure of the apparatus. 
         FIG. 9  is a perspective view of the means for detecting whether the external door is open or closed. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the appended drawings. It should be noted here that the measurements, materials, and shapes of the structural components of the image forming apparatuses in the following embodiments of the present invention, and the positional relationship among them, are not intended to limit the present invention in scope, unless specifically noted. 
       FIG. 1  is a perspective view of the image forming apparatus (which hereafter may be sometimes referred to as “apparatus main assembly”) in the first preferred embodiment of the present invention, and depicts the structure of the apparatus.  FIG. 2  is a perspective view of the image forming apparatus in the first embodiment of the prevent invention, which is in the state in which the external door  102  and developer cartridge chamber door  2  are open, and a development cartridge is being mounted.  FIG. 3  is a perspective view of the developer cartridge chamber and developer cartridge chamber door of the image forming apparatus in the first embodiment, which are in the state in which the door is fully open.  FIG. 4  is a drawing for showing the state of engagement between the developer cartridge chamber door  2  and the apparatus for locking (or unlocking) the door  2 .  FIG. 5  is a drawing for showing the state of disengagement between the developer cartridge chamber door  2  and the apparatus for locking (or unlocking) the door. 
     The developer supplying apparatus  12  depicted in  FIG. 2  is structured to selectively and removably mount one-by-one the multiple developer cartridges  1 , which are full of the developer to be supplied to the main assembly of the developing apparatus  14 , into the image forming apparatus main assembly  100 . Referring to  FIG. 8 , the image forming apparatus main assembly  100  forms images with the use of an electrophotographic method. It has four photosensitive drums  13  ( 13 K,  13 C,  13 M, and  13 Y), which are the members on which an electric static latent image is borne. It has also four developing apparatuses  14  ( 14 K,  14 C,  14 M, and  14 Y), which are means for developing the electrostatic latent images on the photosensitive drums  14  into visible images, that is, images formed of toner (which hereafter will be referred to simply as “toner images”), one-for-one. Further, the image forming apparatus main assembly  100  has the developer supplying apparatus  12 , a transferring apparatus  15 , etc. The developer supplying apparatus  12  supplies the developing apparatuses  14  (developing apparatus main assembly) with developers. The transferring apparatus  15  is a means for transferring the toner images formed by the developing apparatuses  14 , onto a sheet P of recording medium. 
     Next, referring to the flowchart in  FIG. 6 , the development cartridge replacement sequence carried out by the developer supplying apparatus  12  in this embodiment will be described with reference to the replacement of the black developer cartridge  1 . As an image forming process continues (Step  602 ), the developer in the black developer cartridge  1  is eventually consumed virtually in entirety (Step  602 ). As the developer in the black developer cartridge  1  in the image forming apparatus main assembly  100  is consumed virtually in entirety, the state of this developer cartridge is detected by a developer amount detecting portion  21 , which is a means for detecting the amount of the developer in a developer cartridge; it is detected that the black developer cartridge  1  has run out of developer (Step  603 ). Then, a user is given this information through an information displaying means such as a liquid crystal panel (Step  604 ); as the controller  11  detects, through the detecting portion  21  that the developer in the black developer cartridge  1  has been consumed virtually in entirety, it displays on the display  101  that the black developer cartridge  1  needs to be replaced. 
     The apparatus main assembly  100  has multiple (four in this embodiment) developer cartridge chambers  16 , which are independent from each other, and each of which has a door  2  which can be independently opened or closed from the other doors  2 . More specifically, the apparatus main assembly  100  has a door  2 K for a black developer cartridge chamber  16 K, a door  2 C for a cyan developer cartridge chamber  16 C, a door  2 M for a magenta developer cartridge chamber  16 M, and a door  2 Y for a yellow developer cartridge chamber  16 Y. The apparatus main assembly  100  is also provided with multiple (four in this embodiment) door locking apparatuses  10  which are means for keeping the developer cartridge chamber doors  2  locked, one-for-one. Next, referring to  FIG. 3 , each of the developer cartridge chamber doors  2  is provided with a hook  2   a , whereas the apparatus main assembly  100  is provided with multiple latches  3  (one for each hook  2   a ), which engage with the hook  2   a  of the door  2 . The latches  3 , which are door locking means, are rotationally supported by bearings  4  ( FIG. 4 ), one-for-one, with which the apparatus main assembly  100  is provided. Each latch  3  keeps the corresponding developer cartridge chamber door  2 , that is, the first door, locked. The latch  3  is under the pressure from a spring  5  ( FIG. 4 ), which works in the direction to rotate the latch  3  as indicated by an arrow mark in  FIG. 4 . Thus, the latch  3  remains in the locking position shown in  FIG. 4 . Further, the apparatus main assembly  100  is provided with multiple (four in this embodiment) solenoids  6  and multiple (four in this embodiment) linkages  7 . Each solenoid  6  is attached to a specific portion of the apparatus main assembly  100 , and functions as a part of a door controlling mechanism. The linkage  7  is in connection with the solenoid  6 , and is slid by the solenoid  6  in the direction indicated by an arrow mark B in  FIG. 4 . As the solenoid  6  is turned on by the controller  11 , the linkage  7  is slid against the resiliency of a return spring  8 , and presses on the end portion of the latch  3 , causing the latch  3  to rotate against the resiliency of the spring  5  in the direction to disengage from the hook  2   a  of the door  2 . Consequently, the latch  3  disengages from the hook  2   a , allowing thereby the developer cartridge chamber door  2  to open. That is, in this embodiment, the locking apparatus  10 , which is a door locking means, is in engagement with the developer cartridge chamber door  2 , and is moved by the solenoid  6  which keeps the door  2  closed unless it is activated. The developer cartridge chamber door  2  is always under the pressure applied thereto from an unshown pressure applying means in the direction to open the door  2 . Therefore, as the latch  3  is disengaged from the hook  2   a , the developer cartridge chamber door  2  automatically opens. However, the image forming apparatus main assembly  100  is provided with the external door  102 , that is, the second door, which is large enough to cover all the multiple developer cartridge chamber doors  2 . Therefore, while the external door  102  remains closed, the developer cartridge chamber doors  2  are kept closed by the external door  102 . Incidentally, the means for keeping the developer cartridge chamber door  2  pressed in the opening direction, may be structured to utilize the weight of the door itself, or to employ a pressure applying means such as a spring (Step  605 ). 
     The controller  11  determines, based on the information sent from the developer amount detecting portion  21 , whether or not the amount of the developer in the developer cartridge  1  is greater than a preset value, that is, whether or not it is unnecessary to replace the developer cartridge  1 . If the controller  11  determines that it is unnecessary to replace the developer cartridge  1 , it keeps the developer cartridge chamber door  2  locked by the locking apparatus  10  as a locking means, preventing the door  2  from opening regardless whether the external door  102  is open or closed. On the other hand, if it determines based on the information from the developer amount detecting portion  21  that the developer in the developer cartridge has been consumed virtually in entirety, it unlocks only the developer cartridge chamber door  2  which corresponds to the developer cartridge  1 , with the use of the locking apparatus  10 , which also corresponds to this developer cartridge  1 . That is, the controller  11  controls the latch  3 , which is a locking means, based on the amount of the developer in the developer cartridge  1 , which is detected by the developer amount detecting portion  21 , which is the means for detecting the amount of developer remaining in the developer cartridge  1 ; the controller  11  controls the latch  3  so that it unlocks (unlatches) only the developer cartridge chamber door  2 , that is, the first door, which corresponds to the developer cartridge  1 , that is, the cartridge which needs to be replaced, among multiple developer cartridges in the apparatus main assembly  100 . As for the solenoid  6  which is the locking mechanism moving means, it causes the latch  3  which is a locking means, to unlatch from the hook  2   a  of the developer container slot door  2  which is the first door, allowing the door  2  to move from its closed position, to its open position. 
     Then, the user is to open the external door  102  following the instruction given by the above-mentioned display  101 . As the user opens the external door  102  (Step  606 ), the developer cartridge chamber door  2  automatically opens (Step  607 ). In other words, as the external door  102  is opened, the developer cartridge chamber door  2  which corresponds to the developer cartridge  1  to be replaced, automatically opens, making it possible for the development cartridge  1  to be replaced. The controller  11  detects, through the door position detecting means  9  for detecting the position of the developer cartridge chamber door  2  shown in  FIG. 3 , that the developer cartridge chamber door  2  is open (Step  608 ), and turns off the solenoid  6 . Thus, the linkage  7  is slid back by the return spring  8 , being thereby separated from the end portion of the latch  3 . Thus, the latch  3  returns to the door locking position by being rotated by the spring  5  in the direction indicated by an arrow mark A in  FIG. 4  (Step  609 ). 
     As the developer cartridge chamber door  2  is allowed to automatically open, by the opening of the external door  102  which is initiated by the user, the user is to remove the development cartridge  1  in the development cartridge chamber in the apparatus main assembly  100 , and insert a new developer cartridge ( 1 ) into the chamber (Step  610 ). Then, the user is to close the developer cartridge chamber door  2  (Step  611 ). Then, the user is to close the external door  102  (Step  612 ). As the developer cartridge chamber door  2  is closed (rotationally moved), the hook  2   a  of the door  2  causes the latch  3 , which is in the locking position, to rotate against the resiliency of the spring  5 . As the developer cartridge chamber door  2  is rotated far enough to place its hook  2   a  in the locking (hooking) position, the latch  3  is moved back into its locking position by the resiliency of the spring  5 , and engages with the hook  2   a , making thereby it impossible for the developer cartridge chamber door  2  to open ( FIG. 5 ). 
     Incidentally, even if an attempt is made to open the other developer cartridge chamber doors  2 , which did not open when the external door  102  was opened, the corresponding latches  3  and hooks  2   a  remain locked with each other. Therefore, these doors  2  cannot be opened. Further, not only is the apparatus main assembly  100  structured to allow only the developer cartridge chamber door  2  which corresponds the developer cartridge  1 , that is, the cartridge which needs to be replaced, to automatically open, but also, the developer cartridge chamber doors  2  are not provided with a knob or handle, that is, a portion to grasp to open the doors  2 , to visually indicate that the developer cartridges behind these doors  2  cannot be replaced. 
     As the developer cartridge chamber door  2  is closed, the door position detecting means  9  detects that the developer cartridge chamber door  2  is in the closed position (Step  613 ). As soon as it is detected that the developer cartridge chamber door  2  has been moved back into its closed position, the apparatus main assembly  100  begins to supply its developer hopper (unshown) with the developer from the developer cartridge  1 , with the use of the unshown developer discharging means, becoming ready again for printing (Step  614 ). 
     The apparatus main assembly  100  is structured as described above. Thus, when there remains a sufficient amount of developer in the development cartridge  1 , that is, when the development cartridge  1  does not need to be replaced, the developer cartridge chamber door  2  which corresponds to this development cartridge  1  remains locked, being therefore prevented from automatically opening. On the other hand, as the development cartridge  1  reduces in the amount of the developer therein, that is, as it becomes necessary for the development cartridge  1  to be replaced, the controller  11  makes it possible for only the developer cartridge chamber door  2  which corresponds to the developer cartridge  1  to automatically open. Therefore, it is possible to prevented the problem that a development cartridge ( 1 ) which does not need to be replaced is erroneously replaced by a user. 
     In the embodiment described above, the apparatus main assembly  100  is structured so that the developer cartridge chamber door locking apparatus  10  was enabled to unlock (or lock) each of the multiple (four in this embodiment) developer cartridge chamber doors  2 , independently from the other, based on the information (amount of remaining developer) detected by the developer amount detecting portion  21 . However, the same effects as those achievable by the above described embodiment can also be achieved by providing the apparatus main assembly  100  with a door position detecting means  20 , shown in  FIG. 9 , which functions as a means for detecting whether the external door  102  is in the open or closed position. Incidentally, the door  2 K in  FIG. 9  is the door of the black toner cartridge chamber.  FIG. 7  shows the flowchart of the developer cartridge replacement sequence for this apparatus main assembly  100 . The image forming apparatus main assembly  100  whose developer cartridge replacement sequence flowchart is shown in  FIG. 7  is advantageous over the image forming apparatus main assembly  100  whose developer cartridge replacement sequence flowchart is shown in  FIG. 6 , in that the developer cartridge chamber doors  2  of the former do not collide with the external door  102 , and therefore, do not generate collisional noises. 
     Next, the development cartridge replacement sequence carried out by the developer supplying apparatus  12  in this apparatus main assembly  100  will be described with reference to the replacement of the developer cartridge  1 . As an electrophotographic image formation process is continued by the image forming apparatus main assembly  100  (Step  702 ), the developer in the developer cartridge  1  is eventually consumed virtually in entirety (Step  702 ). As the developer in the developer cartridge  1  is consumed virtually in entirety, the state of the developer cartridge  1 , in terms of the amount of the developer therein, is detected by a developer amount detecting portion  21 , which is a means for detecting the amount of the developer in a developer cartridge, and is in the image forming apparatus main assembly  100 ; it is detected that the developer cartridge  1  has run out of developer (Step  703 ). Then, a user is given this information through an information displaying means such as a liquid crystal panel (Step  704 ); as the controller  11  detects, through the detecting portion  21  that the developer in the developer cartridge  1  has been consumed virtually in entirety, it displays on the display  101  that the developer cartridge  1  needs to be replaced. As the user opens the external door  102  following the instruction given by the display (Step  705 ), and it is detected, by the door position detecting means  20  for detecting whether or not the external door  102  is open, that the external door  102  is open (Step  706 ), the controller  11  unlocks the developer cartridge chamber door  2 , allowing thereby the developer cartridge chamber door  2  to automatically open (Step  707 ). 
     That is, in this embodiment, the apparatus main assembly  100  is provided with the means  20  for detecting whether or not the external door  102  is open. Thus, as it is detected by the developer amount detecting portion  21  that the developer in the developer cartridge  1  has just been consumed virtually in entirety, and also, it is detected by the external door position detecting means  20  that the external door  102  is in the open position, the controller  11  causes the locking apparatus  10  to unlock only the developer cartridge chamber door  2  which corresponds to this developer cartridge  1 , allowing the door  2  to automatically open to make it possible for the developer cartridge  1  to be replaced. 
     The controller  11  detects, through the door position detecting means  9  for detecting the position of the developer cartridge chamber door  2  shown in  FIG. 3 , that the developer cartridge chamber door  2  is open (Step  708 ), and turns off the solenoid  6 . Thus, the linkage  7  is slid back by the return spring  8 , being thereby separated from the end portion of the latch  3 . Thus, the latch  3  is rotated by the spring  5  in the direction indicated by an arrow mark A in  FIG. 4 , returning thereby to the door locking position (Step  709 ). 
     Then, the user is to remove the development cartridge  1  in the development cartridge chamber in the apparatus main assembly  100 , insert a new developer cartridge ( 1 ) into the developer cartridge chamber (Step  710 ), close the developer cartridge chamber door  2  (Step  711 ), and then, close the external door  102  (Step  712 ). As the developer cartridge chamber door  2  is closed (rotationally moved), the hook  2   a  of the door  2  causes the latch  3 , which is in the locking position, to rotate against the resiliency of the spring  5 . As the developer cartridge chamber door  2  is rotated far enough to place its hook  2   a  in the locking position, the latch  3  is moved back into its locking position by the resiliency of the spring  5 , and engages with the hook  2   a , making it thereby impossible for the developer cartridge chamber door  2  to open ( FIG. 5 ). 
     As the developer cartridge chamber door  2  is closed, the door position detecting means  9  detects that the developer cartridge chamber door  2  is in the closed position (Step  713 ). As soon as it is detected that the developer cartridge chamber door  2  has been moved back into its closed position, the apparatus main assembly  100  begins to supply its developer hopper (unshown) with the developer from the developer cartridge  1 , with the use of the unshown developer discharging means, becoming thereby ready again for printing (Step  714 ). That is, the apparatus main assembly  100  in this embodiment has the door position detecting means  20  for detecting whether or not the external door  102  (second door) is open. The controller  11  unlocks the pertinent developer cartridge chamber door  2  (first door) after it detects, based on the information from the door position detecting means  20 , that the external door  102  is open. 
     In this embodiment, the solenoid  6  is used as the means for causing the locking apparatus  10  to unlock the developer cartridge chamber doors  2 . However, this embodiment is not intended to limit the present invention in terms of the method for driving the locking apparatus  10 . For example, the image forming apparatus main assembly  100  may be structured so that the locking apparatus  10  is driven by a rotational driving means which engages with the developer cartridge chamber door  2  to prevent the door  2  from opening. The effects of the structural arrangement that employs the rotational driving means to disengage the latch  3  and hook  2   a  from each other are the same as those obtainable with the structural arrangement which employs the solenoid  6 . In other words, the present invention is also applicable to an image forming apparatus which employs a rotational driving means to disengage its latch  3  and hook  2   a  from each other, which is obvious. 
     In this embodiment, the developer cartridge chamber doors  2  and external door  102  are different from the door  103  for removing jammed recording medium or the like, and the maintenance door  104  for image forming portions, which are shown in  FIGS. 1 and 2 . Therefore, it is unnecessary to open these doors  103  and  104  to replace any of the multiple developer cartridges in the apparatus main assembly  100 , and therefore, it is unnecessary to interrupt an on-going black-and-white image forming operation to replace any of the multiple developer cartridges other than the black developer cartridge in the apparatus main assembly  100 . In other words, even when it is detected that one of the developer cartridges  1  other than the black developer cartridge  1 , is virtually out of developer, and therefore, is replaced by a user by opening the external door  102 , the formation of images with the use of black developer, and the outputting of the black-and-white prints, can be continued. 
     That is, even if it is detected by the developer amount detecting portion  21  that at least one of the multiple developer cartridges  1  other than the black developer cartridge  1  is virtually out of developer, and therefore, a user opens the external door  102 , and replaces the developer cartridge  1  which is virtually out of developer, the image forming apparatus  100  does not stop the on-going image forming operation, as long as the on-going image forming operation uses only the black developer. 
     Further, the apparatus main assembly  100  is provided with the developer hopper (unshown) of a large capacity, in which the developer is stored for immediate consumption. Thus, the image forming apparatus main assembly  100  is enabled to carry out a printing operation even while the developer cartridge(s)  1  is replaced by opening the external door  102 . That is, the developer cartridge(s)  1  can be replaced even while the apparatus main assembly  100  is forming images. Incidentally, even if an image forming apparatus is provided with a developer hopper of a large capacity, the image forming apparatus will stop the on-going image forming operation as soon as the developer hopper runs out of developer, which is obvious. 
     Referring to  FIG. 2 , in this embodiment, one of the lateral walls of the image forming apparatus main assembly  100  is provided with the developer cartridge chamber doors  2  and external door  102  so that the developer cartridges  1  are to be inserted into, or pulled out of, the apparatus main assembly  100 , in the side-to-side direction (from front side of drawing to rear side in  FIG. 2 ). Further, the developer cartridge chamber doors  2  are not provided with a knob, a handle, or the like. 
     The present invention is applicable to any image forming apparatus, such as a printer, a copy machine, a facsimile machine, etc., which is structured to employ replaceable developer cartridges. 
     The present invention makes it possible to provide an image forming apparatus which does not have multiple displays which correspond to multiple developer cartridges, one-for-one, and yet, enables an operator to easily find a developer cartridge, in the apparatus, which needs to be replaced, and also, is structured so that it does not occur that a developer cartridge chamber door opens without a confirmation from a user, and therefore, is left open in the state in which the corresponding cartridge is accessible. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 207865/2009 filed Sep. 9, 2009 which is hereby incorporated by reference.