JP4821145B2 - Image forming apparatus and image forming method - Google Patents

Description

  The present invention relates to an image forming apparatus having a plurality of developing units that store toner of the same color and an image forming method thereof.

  In electrophotographic image forming apparatuses that use toner to form images, such as printers, copiers, and facsimile machines, it has been proposed to use a plurality of developing units that store toner of the same color. Yes. For example, the image recording apparatus described in Patent Document 1 (corresponding to the “image forming apparatus” in the present specification) is an image recording apparatus in which a plurality of developing devices can be mounted, and the developing devices having different toner colors are mounted. In addition to being usable as a color printer, it can be used as a single-color printer by mounting a developing device of the same toner color.

  In this image recording apparatus, a developing device mounted in a predetermined position is preferentially used. However, when the remaining amount of toner in the developing device decreases to a predetermined value, the developing device is switched to another developing device. In another aspect of the image recording apparatus, when there are a plurality of usable developing devices, the one having the oldest mounting time is preferentially used.

Japanese Patent Laying-Open No. 2003-316106 (FIG. 4)

  In this type of image forming apparatus, since the same color toner is stored in each developing device, the quality of the image formed should be constant regardless of which developing device is used. However, in practice, there is a problem that the quality of the formed image varies due to the difference in the state of each developing device. In particular, when a relatively new developing device and an old developing device are mixed, when these are mixed, a large variation in image quality occurs.

  The present invention has been made in view of the above problems, and an object of the present invention is to stably form an image with good image quality in an image forming apparatus having a plurality of developing units that store toner of the same color and an image forming method thereof. And

In order to achieve the above object, an image forming apparatus according to the present invention includes a plurality of developing devices that store toners of the same color, a storage unit that stores information about usage history of each of the developing devices, and the developing device. A control unit that selectively uses and executes an image forming operation, and the control unit, for each of the plurality of developing devices, based on information on the usage history of the developing device stored in the storage unit, A first quality rank that can form an image with a predetermined target image quality when the developing device is used for the image forming operation, and an image can be formed, but the image quality may be inferior to the target image quality The quality rank determination is performed so as to rank a plurality of quality ranks including the second quality rank, and the number of developing devices determined as the first quality rank is small among the plurality of developing devices. When both with one, while performing the image forming operation using only the developing device belonging to the first quality ranks, in said plurality of developing devices, there is nothing of the first quality ranks, and the When there is at least one developing device determined to be the second quality rank, the image forming operation is performed using the developing device having the second quality rank .

According to another aspect of the present invention, there is provided an image forming method in which an image is formed by selectively using a plurality of developing units that store toner of the same color. Storing information relating to the usage history of the developing device in the storage means, and for each of the plurality of developing devices, the developing device is used for the image forming operation based on information relating to the usage history of the developing device stored in the storage means. A first quality rank capable of forming an image with a predetermined target image quality when used, and a second quality rank capable of forming an image but having a possibility of being inferior to the target image quality A step of performing a quality rank determination to rank the plurality of quality ranks, and a step of executing an image forming operation in a mode according to the determination result, Wherein when the first quality ranks with the determined developing device there is at least one, while performing the image forming operation using only the developing device belonging to the first quality ranks, in said plurality of developing units, When there is no developer of the first quality rank and there is at least one developing device determined to be the second quality rank, the image forming operation is executed using the developing device of the second quality rank. It is characterized by that.

  In the invention configured as described above, since it is possible to form an image in a mode according to the image quality assumed when each developing device is used, it is possible to stably form an image with good image quality. It becomes.

More specifically, when there is at least one developing device determined as a first quality rank capable of forming an image with a predetermined target image quality among the plurality of developing devices, the first quality rank The image forming operation may be executed using only the developing devices belonging to the above. By doing so, a developing device that deteriorates the image quality is not used, and an image can be formed with a predetermined target image quality. On the other hand, when there is no developer of the first quality rank among the plurality of developers and there is at least one developer determined to be the second quality rank, the developer of the second quality rank The image forming operation can be executed by using. In a state where there is no developing device of the first quality rank, there is a possibility that the desired image quality cannot be obtained, but image formation can be continued using the developing device of the second quality rank. In particular, in an apparatus that forms an image using a plurality of developing devices that store toner of the same color, the entire color image forming apparatus in which color misregistration due to variations in image quality for each color is a major problem Image density variations are allowed. Accordingly, by making it possible to use a developing device whose image quality is likely to deteriorate to some extent in an image forming operation, it is possible to effectively use the toner in the developing device. In addition, it is desirable to notify the user in advance that there is a possibility that the image quality may deteriorate.

  When there are a plurality of developing devices determined to have the first quality rank, it is desirable to execute the image forming operation using the developing devices while switching them based on a predetermined switching condition. In this type of image forming apparatus, when an image is formed using a developing device that is not used for a long time, periodic density unevenness may occur. By using the developing devices while switching the developing devices as described above, there is no bias in the developing devices used, and an image with stable image quality can be formed without causing such density unevenness.

  The image forming operation may further include positioning means for sequentially moving and positioning the plurality of developing devices from a predetermined standby position to a developing position, and using the first quality rank developing device positioned at the developing position. In the image forming apparatus that executes the above, when the developing device of the first quality rank is at the standby position, the developing device at the developing position is retracted and the developing at the standby position is based on a predetermined switching condition. The switching operation for moving the developer to the developing position may be performed, while the switching operation may not be performed when the developing device of the first quality rank is not in the standby position. When there is a developing device of the first quality rank in the standby position, it is possible to maintain good image quality over a long period of time by switching between the developing device currently in use and the developing device. On the other hand, if the developing device in the standby position is not in the first quality rank, the image quality may deteriorate if the developing device is used next time. In addition, it takes time to switch to the developing device of the first quality rank at another position, and the image forming throughput is reduced.

  The switching condition may be, for example, that the number of images formed by continuously using one developing device has reached a predetermined number. By switching the developing device for every fixed number of sheets, each developing device can be used without deviation, and deterioration in image quality caused by leaving any one of the developing devices can be suppressed.

  In addition, when there are a plurality of developing devices determined as the first quality rank, one of them is selected based on a predetermined selection condition, and the image forming operation is performed using the selected developing device. You may make it perform. In this way, by selecting the developing device to be used each time, it is possible to use each developing device evenly as in the case of switching the developing device described above. For example, it is possible to select the developing device determined to have the first quality rank that has the longest pause time after the end of the past use. As described above, by selecting the developing device having the longest pause time, any of the developing devices is not left for a long period of time, and each developing device can be used evenly.

  Further, among the developers determined as the first quality rank, those that can be used immediately for the image forming operation and those that need to be subjected to a predetermined preparation operation prior to use in the image forming operation. Are mixed, it is preferable to use only the developer that is determined to be the first quality rank and that can be used immediately for the image forming operation. For example, as described above, if a certain developing device is left unused for a certain period of time without being used, the image quality may deteriorate when an image is formed using the developing device next time. This kind of image quality degradation can be recovered by performing a predetermined preparatory operation on the developing device prior to image formation. However, if there is a developer that can be used immediately, it is preferable to use the developer for the image forming operation in order to obtain better image quality.

  If any of the developing devices determined to have the first quality rank requires a preparatory operation, the necessary preparatory operation is performed on the developing devices, and image formation is performed using these developing devices. It is desirable to perform the operation. In the developing device that requires the first quality rank and the preparatory operation and the developing device of the second quality rank that will be described later, the former is preferentially used (after the preparatory operation) for the image forming operation. Is preferred. This is because a good image quality can be obtained by using the developing device of the first quality rank subjected to the preparatory operation.

  When there are a plurality of developing devices determined as the second quality rank, as in the case where there are a plurality of developing devices of the first quality rank, one of them is selected based on a predetermined selection condition, The image forming operation can be executed using the selected developing device.

  Further, when a plurality of images are continuously formed, it is preferable to form the plurality of images using only one developing device having the second quality rank. In a developing device of the second quality rank, the degree of image quality degradation may vary greatly depending on the state of each developing device. Therefore, when these developing units are switched and used, the image quality greatly fluctuates. On the other hand, when one developing device is continuously used without switching the developing device, even if the image quality gradually deteriorates due to the deterioration of the developing device, a large change in image quality occurs. This is prevented.

  FIG. 1 is a view showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. As will be described later, the apparatus 1 is an image forming apparatus in which four developing devices are mounted to form an image. In a state where developing devices storing different color toners are attached, a full color image using these developing devices and a monochrome image using one of these developing devices can be formed. Further, in a state where a developing device that stores toner of the same color is mounted, it functions as an image forming apparatus dedicated to monochrome images of the toner color. In the following, an embodiment in which the present invention is applied to an image forming apparatus dedicated to monochrome images and equipped with four developing units each storing black toner will be described.

  In this image forming apparatus 1, when an image forming command is given to the main controller 11 from an external device such as a host computer, the engine controller 10 controls each part of the engine unit EG in accordance with the command from the main controller 11. An image forming operation is executed, and a monochrome image corresponding to an image signal given from an external device is formed on the sheet S.

  In the engine unit EG, the photosensitive member 22 is provided to be rotatable in the arrow direction D1 in FIG. A charging unit 23, a rotary developing unit 4 and a cleaning unit 25 are arranged around the photosensitive member 22 along the rotation direction D1. The charging unit 23 is applied with a predetermined charging bias, and uniformly charges the outer peripheral surface of the photoconductor 22 to a predetermined surface potential. The cleaning unit 25 removes the toner remaining on the surface of the photosensitive member 22 after the primary transfer, and collects it in a waste toner tank provided inside. The photosensitive member 22, the charging unit 23, and the cleaning unit 25 integrally constitute the photosensitive member cartridge 2, and this photosensitive member cartridge 2 is detachably attached to the main body of the apparatus 1 as a whole.

  Then, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photosensitive member 22 charged by the charging unit 23. The exposure unit 6 exposes the light beam L onto the photosensitive member 22 in accordance with an image signal given from an external device, and forms an electrostatic latent image corresponding to the image signal.

  The electrostatic latent image thus formed is developed with toner by the developing unit 4. The developing unit 4 is configured as a support frame 40 that is rotatably provided around a rotation axis that is orthogonal to the paper surface of FIG. To 4Kd, and a rotary drive (not shown) for rotating them integrally. The developing unit 4 is controlled by the engine controller 10. Based on the control command from the engine controller 10, the developing unit 4 is rotationally driven in the direction D4 in FIG. 1, and the developing cartridges 4Ka to 4Kd are selectively brought into contact with the photosensitive member 22 or have a predetermined value. When positioned at a predetermined developing position facing each other with a gap, toner is applied to the surface of the photosensitive member 22 from the developing roller 44 provided in the developing cartridge. As a result, the electrostatic latent image on the photosensitive member 22 is developed with the toner in the developing cartridge positioned at the developing position.

  The toner image developed by the developing unit 4 as described above is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TR1. The transfer unit 7 includes an intermediate transfer belt 71 that is stretched over a plurality of rollers 72 to 75, and a drive unit that rotates the roller 73 to rotate the intermediate transfer belt 71 in a predetermined rotation direction D2. Yes. Then, the transfer unit 7 transfers the black toner image formed on the photosensitive member 22 onto the intermediate transfer belt 71 and is taken out one by one from the cassette 8 to the secondary transfer region TR2 along the transport path F. Secondary transfer is performed on the conveyed sheet S.

  At this time, in order to correctly transfer the image on the intermediate transfer belt 71 to a predetermined position on the sheet S, the timing of feeding the sheet S to the secondary transfer region TR2 is managed. Specifically, a gate roller 81 is provided on the transport path F on the front side of the secondary transfer region TR2, and the gate roller 81 rotates in accordance with the timing of the circumferential movement of the intermediate transfer belt 71. S is sent to the secondary transfer region TR2 at a predetermined timing.

  Further, the sheet S on which the monochrome image is thus formed is conveyed to the discharge tray portion 89 provided on the upper surface portion of the apparatus main body via the fixing unit 9, the pre-discharge roller 82 and the discharge roller 83. Further, when images are formed on both sides of the sheet S, when the rear end portion of the sheet S on which the image is formed on one side as described above is conveyed to the reversal position PR behind the pre-discharge roller 82. The rotation direction of the discharge roller 83 is reversed, whereby the sheet S is conveyed in the direction of the arrow D3 along the reverse conveyance path FR. Then, the sheet is again placed on the transport path F before the gate roller 81. At this time, the surface of the sheet S to which the image is transferred by contacting the intermediate transfer belt 71 in the secondary transfer region TR2 is first transferred. It is the opposite surface. In this way, images can be formed on both sides of the sheet S.

  A density sensor 60 is disposed in the vicinity of the roller 75. The density sensor 60 is provided to face the surface of the intermediate transfer belt 71 and measures the image density of the toner image formed on the outer peripheral surface of the intermediate transfer belt 71 as necessary. Based on the measurement result, this apparatus adjusts the operating conditions of each part of the apparatus that affects the image quality, for example, the developing bias applied to each developing cartridge, the intensity of the light beam L, and the like.

  The density sensor 60 is configured to output a signal corresponding to the image density of a region of a predetermined area on the intermediate transfer belt 71 using, for example, a reflection type photosensor. The CPU 101 can detect the image density of each part of the toner image on the intermediate transfer belt 71 by periodically sampling the output signal from the density sensor 60 while rotating the intermediate transfer belt 71.

  Further, as shown in FIG. 2, each of the developing cartridges 4Ka to 4Kd is provided with memories 91 to 94 for storing data relating to the manufacturing lot and usage history of the developing cartridge, the remaining amount of the built-in toner, and the like. Further, each of the developing cartridges 4Ka to 4Kd is provided with wireless communication devices 49Ka, 49Kb, 49Kc, and 49Kd. Then, if necessary, these selectively communicate with the wireless communication device 109 provided on the main body side in a non-contact manner, and data is transmitted between the CPU 101 and each of the memories 91 to 94 via the interface 105. Various information such as consumable management for the developing cartridge is managed by sending and receiving. In this embodiment, non-contact data transmission / reception is performed using electromagnetic means such as wireless communication. However, a connector is provided on the main body side and each developing cartridge side, and the connector is mechanically fitted. By doing so, you may make it mutually transmit / receive data.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image given from an external device such as a host computer via the interface 112, and reference numeral 117 denotes a calculation result in the CPU 111 and other information. It is a RAM that temporarily stores data. Reference numeral 106 is a ROM for storing a calculation program executed by the CPU 101, control data for controlling the engine unit EG, and the like. Reference numeral 107 is a RAM for temporarily storing calculation results in the CPU 101 and other data. is there.

  By the way, in the apparatus configured as described above, when executing the image forming operation, it is necessary to determine which of the four developing devices to use based on an appropriate standard. This is because in a full-color image forming apparatus, a developing device to be used is uniquely determined according to the color of a toner image to be formed, whereas in a monochrome-only device, there are a plurality of developing devices that store toner of the same color.

In this embodiment,
(1) To stably form an image with good image quality;
(2) Shortening the time (first print time) from when an image formation command is received until an image is formed;
(3) increasing the throughput of image formation; and
(4) The control operation of the engine controller 10 is performed as follows for the purpose of reducing the frequency of a preparatory operation (described later) to the developing device to maintain the image quality as much as possible and efficiently performing the necessary preparatory operation. It is enacted as follows.

  That is, the engine controller 10 executes the image forming availability determination process shown in FIG. 4, monitors the state of each developing device, and determines whether or not the apparatus is ready to execute the image forming operation. I go from time to time. Further, if necessary, by performing a predetermined preparation operation (FIGS. 5 and 6) for each developing device, the apparatus can be ready to execute the image forming operation immediately when an image forming instruction is given. Is maintained. When an image formation command is given, the engine controller 10 executes the image forming operation shown in FIG. 7, and selects and uses the optimum developing device from among the developing devices. Form. Details of the operation will be described below.

  In this embodiment, the life rank of the developing device is determined based on the remaining amount of toner in each developing device and the total rotation time of the developing roller 44 provided in the developing device, and image formation is performed based on the result. In addition to determining whether or not the operation is possible, the image forming operation described below is made different. Therefore, prior to the description of the image formation possibility determination process, the rank classification of the developing devices will be described.

  FIG. 3 is a diagram showing rank classification of the developing devices. The deterioration of image quality due to the life of the developing device is caused not only when the remaining amount of toner decreases, but also due to the deterioration of the characteristics of the remaining toner. In other words, good image quality cannot be obtained if the remaining amount of toner is small, and even if a sufficient amount of toner remains in the developing device, good image quality will not be obtained if the toner is deteriorated. I can't get it. The total rotation time of the developing roller is a value indicating the degree of deterioration of the toner. When the value reaches a predetermined value (for example, 10,000 seconds), even if the remaining amount of toner is sufficient, The toner is likely to be deteriorated, and it can be said that the developing device is not suitable for use.

  In this embodiment, as shown in FIG. 3, when the remaining amount of toner is 5% or more of the initial amount and the developing roller rotation time is less than 70% of the predetermined value, the developing device is ranked 1. When image formation is performed using a rank 1 developer, the remaining amount of toner is sufficient and the degree of deterioration is slight, so that the necessary and sufficient image density does not occur due to insufficient density or blurring. It is expected to be obtained. On the other hand, when the remaining amount of toner is less than 1% of the initial amount or when the developing roller rotation time exceeds the predetermined value, the developing device is ranked 3. Rank 3 developers are no longer suitable for use in image forming operations. Further, in the middle of both ranks, that is, the toner remaining amount is 1% or more and less than 5% of the initial amount, and the developing roller rotation time is less than the predetermined value, or the toner remaining amount is 1% or more of the initial amount. When the developing roller rotation time is 70% or more and less than 100% of the predetermined value, the developing device is ranked 2. When image formation is performed using a rank 2 developing device, although image formation is possible, image defects such as insufficient density and blurring may occur. That is, rank 2 is a state in which an image can be formed, but the image quality may be deteriorated in some way. It should be noted that the rank to which each developing device belongs is preferably displayed on a display display (not shown) as needed.

In this way, the life rank of the developing device is ranked into three ranks: rank 1 that can form an image with a predetermined quality, rank 3 that is not suitable for use, and rank 2 that is positioned between them. The state of the device can be divided into three stages:
(I) A state where at least one developing device of rank 1 is mounted. In this state, a rank 1 developer can be used to form an image with the desired image quality;
(Ii) A state in which there is no rank 1 developer and at least one rank 2 developer is installed. In this state, it is possible to form an image using a rank 2 developer, but the desired image quality may not be obtained;
(Iii) A state in which only a rank 3 developer is mounted. In this state, the expected image quality can no longer be expected.

  Further, even in a rank 1 or 2 developing device, if the unused state continues for a long time, a phenomenon in which periodic density unevenness occurs in an image in an initial stage of image formation performed thereafter (in this specification, This phenomenon is sometimes referred to as “banding phenomenon”. Such density unevenness is caused by the fact that the toner is left on the developing roller, and the developing roller provided in the developing device is used before the image forming operation. It can be solved by rotating a predetermined amount. Therefore, in this embodiment, an operation of rotating a developing device provided in each developing device by a predetermined amount (this operation is referred to as “stirring operation” in this specification) is performed as necessary. Yes. More specifically, for each developing device, a pause time from the end of use in the last executed image forming operation and preparation operation to the present is measured, and the pause time is predetermined. When the value is reached, a flag indicating that a stirring operation is required for the developing device is set. The pause time can be defined as, for example, the elapsed time from when the rotation of the developing roller 44 provided in the developing device is stopped to the present time. For the developing device in which this flag is set, the stirring operation is executed at the timing described later, and the occurrence of the neglected banding phenomenon is suppressed. When the stirring operation is executed, the flag is reset.

  Furthermore, as the developing device is used, its deterioration progresses and the image density fluctuates. Therefore, it may be necessary to change the operating conditions of each part of the apparatus when the image forming operation is performed using the developing device. Therefore, in this embodiment, when the information indicating the life of the developing device described above, that is, when either the remaining toner amount or the developing roller rotation time reaches a predetermined threshold, the operating conditions of the developing device are readjusted. A flag indicating that an operation (density control operation) for controlling the image density to a predetermined density is necessary is set. For the developing device in which this flag is set, the density control operation is executed at a timing described later. As a result, fluctuations in image density are suppressed. When the density control operation is executed, the flag is reset.

  There is not only one threshold value for the remaining toner amount and the developing roller rotation time that triggers the execution of the density control operation. That is, when the remaining amount of toner and the developing roller rotation time reach a certain threshold value and the density control operation is performed, the next threshold value is newly set. When the remaining amount of toner or the developing roller rotation time reaches the new threshold value, the density control operation is executed again. In this way, by repeatedly performing the density control operation a plurality of times while one developing device is attached to the apparatus and has reached the end of its life, it is possible to change the characteristics of the developing device over time. Correspondingly, the image quality can be maintained stably.

  Further, information indicating the state of the developing device, such as the remaining amount of toner and the developing roller rotation time, is stored in a storage unit provided in the developing device (for example, a non-volatile memory 91 provided in the developing device 4Ka). In addition, the CPU 101 can be configured to read / write the information as necessary. By doing so, information about each developing device is attached to the developing device itself, and when the developing device is once removed and remounted, or a developing device used in another apparatus is mounted. Even in this case, the state of each developing device can be appropriately managed.

  The above-described “preparation operation” is a general term for processing operations performed on the developing device prior to use in the image forming operation in order to make the developing device usable for image formation. The prepared stirring operation and concentration control operation are included in the “preparation operation”.

  Thus, in this embodiment, the developing devices are classified into ranks 1 to 3 according to the remaining amount of toner and the developing roller rotation time. In addition, for rank 1 or 2 developing devices, the following three categories can be used immediately for image forming operations, that is, those that do not require any preparatory operations (printable); agitation prior to image formation It is classified into those requiring operation (stirring operation required); and those requiring density control operation prior to image formation (necessary density control operation). However, a developing device in which flags indicating both a stirring operation and a required concentration control operation are set is classified into a required concentration control operation category. The reason will be described later.

  Based on these classifications, an image formation availability determination process is performed. This image formation possibility determination process and an image forming operation described later are configured based on the following basic idea. That is, in this apparatus, in order to achieve the above-described object (1) of stably forming an image with good image quality, as long as there is at least one rank 1 developer, only the rank 1 developer is used. Used for image formation. By so doing, there is no risk of insufficient density or blurring, and an image with good image quality can be stably formed. Since the possibility of causing image defects cannot be ruled out with the rank 2 developer, the rank 2 developer is not used unless there is no rank 1 developer capable of obtaining better image quality.

  When there are a plurality of rank-1 developing devices, the first developing device is selected each time to start image formation, and the developing devices are used uniformly while switching the developing devices as appropriate. To. If it is a rank 1 developer, the image quality is stable regardless of which developer is used, and there is little variation in image quality due to switching use. Further, by using each developing device evenly, there is no appearance of a developing device that is left unused for a long time, and it is possible to avoid the development device in the printable category from moving to the stirring required operation category. it can. Therefore, it is possible to prevent the occurrence of the above-mentioned neglected banding phenomenon and reduce the frequency of the preparation operation to be performed. In addition, this maximizes the period during which the apparatus 1 is in an image formable state. In addition, the effect of making the toner in each developing device uniform is produced next to the rotation of the developing unit 4 being performed periodically.

  In this case, if there is at least one of the rank 1 developing devices in the printable category, the stirring operation and the density control operation are not performed for the other developing devices. This is because it is possible to form an image with good image quality as long as there is one usable developing device, and it is not always necessary to make other developing devices usable. Further, this is suitable for the purpose (4) of efficiently performing the preparation operation. On the other hand, when there is no more printable category in the rank 1 developer, the developer is restored to a usable state by performing a predetermined preparation operation. This keeps the apparatus in a state where it can continue to form images with good image quality. Further, since the developing device is not used for image formation without performing the necessary preparatory operations, an image with inferior image quality is not formed. Further, as will be described later, a device for shortening the time required for the preparation operation has been devised.

  When the rank 1 developing device runs out, the image forming operation using the rank 2 developing device is enabled. In the rank 2 developing device, the remaining amount of toner or the deterioration of the characteristics is slightly advanced. Therefore, in the image forming operation using this, the quality of the image to be formed depends on the degree of the remaining amount of toner and the deterioration. May be reduced. In a full-color image forming apparatus, an image defect such as insufficient density or fading in one of the toner colors constituting the full-color image appears as a color difference or color unevenness of the entire image. On the other hand, since the apparatus 1 is a monochrome-only image forming apparatus, such a problem of color unevenness does not occur. In consideration of the actual usage in an image forming apparatus dedicated to monochrome images, this type of image forming apparatus is often used mainly for forming character images. In such applications, some image defects are often tolerated if the characters are sufficiently readable. Therefore, it is considered that an image quality apparatus for monochrome only has a wider allowable range of image quality than a full-color image forming apparatus. Therefore, in this embodiment, even when there is no rank 1 developer, the image forming operation is not prohibited, and the image forming operation by the rank 2 developer can be executed. By doing so, the toner remaining in the developing device can be used more effectively.

  In this case, however, the developer is not switched during the series of image forming operations. This is because the appearance of image defects differs depending on the state of each developing device, and therefore, when image formation is performed while switching the developing devices, the variation in image quality may increase depending on the developing device used. In particular, when an image formed using a rank 1 developer and an image formed using a rank 2 developer are mixed, the variation becomes large, resulting in confusion for the user. Also from this point, it is not preferable to mix the rank 1 developer and the rank 2 developer. Note that whether or not to perform the preparatory operation for the rank 2 developer can be the same as that for the rank 1 developer.

  The image formation feasibility determination process is not executed only at a specific timing, for example, immediately after the apparatus is turned on or when an image formation command is received, but at the time of waiting for an image formation command from the outside. It is also desirable to execute from time to time. By doing so, it is possible to cope with a change in the state of the apparatus that changes every moment even during standby. Since the necessary preparatory operations can be performed during standby, the image forming operation can be started without delay when an image forming command is given thereafter, and the above object (2) is achieved. Can do. Further, when a large number of images are continuously formed, the necessity for a preparation operation is not required during the process, and the possibility that the image formation is interrupted is low. (3) is achieved.

  FIG. 4 is a flowchart showing the image forming availability determination process. In this process, first, it is determined whether or not the above-mentioned rank 1 is present among the four developing devices 4Ka to 4Kd (step S101). If there is at least one rank 1 developer, the process proceeds to step S102, and it is determined whether or not any rank 1 developer belongs to the printable category. If there is at least one developing device of the printable category, the image forming apparatus 1 has at least one developing device that has sufficient toner, little deterioration, and can be used immediately. Become. Therefore, if an image formation command is given at this time, it is possible to immediately execute an image forming operation using the developing device. In this case, the process proceeds to step S106, where the apparatus determines that image formation is possible and ends the image formation availability determination process.

  If it is determined in step S102 that there is no developing unit of the printable category, the process proceeds to step S103, and it is determined whether or not there is a stirrer operation category in the rank 1 developing unit. If there is at least one developing device in the stirring operation category, the stirring operation is executed for the developing device (step S104). The processing content of the stirring operation will be described later.

  In step S103, if there is no developing device in the stirring required operation category, this apparatus has at least one developing device of rank 1, and all of them belong to the required density control operation category. Therefore, in this case, subsequently, a density control operation described later is executed for these developing units (step S105).

  On the other hand, when it is determined in step S101 that there is no rank 1 developer, it is subsequently determined whether there is a rank 2 developer (step S107). Here, if there is no rank 2 developing device, all four developing devices are ranked 3. In this case, it is determined that the image forming operation is not possible because the developing device usable for image formation is not installed in this apparatus (step S112).

  When there is at least one rank 2 developing device, the same processing as in the case where there is a rank 1 developing device is performed. That is, if the rank 2 developer includes a printable category, it is immediately determined that an image can be formed (steps S108 and S106). If there is no printable category and a stirrer operation category is required, a stir operation is performed on the developing device (steps S109 and S110). On the other hand, if the density control operation category is only available, the density control operation is performed on the developing device (step S111), and the image forming operation is enabled (step S106).

  FIG. 5 is a flowchart showing the stirring operation. In the stirring operation, first, one of the developing devices in the stirring required operation category is positioned at a developing position facing the photosensitive member 22 (step S201). Subsequently, the developing roller 44 provided in the developing unit is rotationally driven for a predetermined time (step S202). At this time, no developing bias is applied to prevent toner scattering from the developing roller 44. By doing this, the neglected banding phenomenon caused by the continued stop state of the developing roller is eliminated, and the developing device belongs to a state where it can be used for an image forming operation, that is, a printable category. If there is a next developing device to be stirred, the above process is repeated. If there is no next developing device, the process is terminated (step S203).

  In this embodiment, a series of stirring operations are performed for the developers belonging to the same rank among the developing devices in the stirring required operation category. That is, in the process of step S106 in FIG. 4, even if there is a rank 2 and a stirrer required operation, the stirrer operation is performed only for the rank 1 and stirrer required developer unit. Then, the stirring operation is not performed. As described above, in this embodiment, if there is a rank 1 developer, an image is formed using the rank 1 developer, and therefore, it is not necessary to perform a stirring operation for the rank 2 developer at this point. In this way, unnecessary preparation operations can be omitted, and the processing time for shifting the apparatus to a state where image forming operations can be performed can be shortened, and the above objects (2) to (4) can be achieved. Will contribute.

  Further, the developing devices in which both the “stirring required operation” and “necessary concentration control operation” flags are set are classified into the required concentration control operation category, and at this time, the stirring operation is not performed on the developing device. The reason is as follows. As will be described later, the stirring operation is an operation that forms part of the concentration control operation, and when the concentration control operation is performed, the stirring operation is automatically executed. Therefore, it is not necessary to separately perform the developing devices that require the stirring operation and the density control operation, and only the density control operation may be performed. On the contrary, even if only the stirring operation is performed on the developing device that requires the density control operation, the developing device cannot be immediately printed. Since the density control operation requires a longer processing time than the stirring operation, there is a developing device that can be shifted to a printable category by executing only the stirring operation. At this time, the stirring operation for such a developing device is performed. It is preferable to achieve only the objects (2) and (3).

  FIG. 6 is a flowchart showing the density control operation. In this density control operation, in order to maintain an image formed by execution of the image forming operation with a constant image quality, a patch image is formed and the image density is detected while variously changing the operating conditions of each part of the apparatus. This is a process of adjusting the operating condition based on the detection result. In this process, the development bias and exposure power are adjusted as control factors that affect the image quality among the operation parameters that determine the operation conditions of each part of the apparatus. There are various other operating parameters that function as control factors, and many known techniques are known regarding the principle and control method of image quality control using them. Since these techniques can also be applied to the present embodiment, only the flow of processing will be briefly described here.

  First, one of the developing devices in the required density control operation category is positioned at a developing position facing the photosensitive member 22 (step S301). Subsequently, a patch image of a predetermined pattern such as a solid image is formed with each bias value while changing and setting the magnitude of the developing bias applied to the developing device in multiple stages (step S302). Subsequently, the image density of these patch images is detected by the density sensor 60 (step S303), and based on the detection result, the optimum value of the developing bias is calculated so that the patch image has a predetermined target density (step S303). S304).

  Subsequently, the exposure power is adjusted. The development bias is set to the optimum value obtained above (step S305), and a halftone image is formed as a patch image while changing and setting the exposure power in multiple stages (step S306). Subsequently, the image density of these patch images is detected by the density sensor 60 (step S307), and based on the detection result, an optimum value of exposure power is calculated so that the patch image has a predetermined target density (step S307). S308).

  In the developing device in which the density control operation is executed in this way, the optimum developing bias and exposure power are required in the developing device, and at the same time, the developing roller 44 is driven to rotate when the patch image is formed, thereby eliminating the neglected banding phenomenon. Is done. As a result, the developing device shifts from the required density control operation category to a printable category that can be used for the subsequent image forming operation. Thereafter, if there is a next developing device to be subjected to the density control operation, the above processing is repeated. If there is no next developing device, the processing is ended (step S309).

  By performing the image forming availability determination process configured as described above, in the apparatus 1, it is determined whether or not the apparatus is in a state capable of performing an image forming operation. Further, if necessary, a predetermined preparatory operation is performed on a developing device that requires a preparatory operation, and the apparatus is maintained in an image-formable state as long as at least one rank 1 or 2 developing device is mounted. Is done. When an image forming command is given from an external device, the image forming operation can be started immediately according to the command. On the other hand, when all the developing devices are ranked 3, it is determined that the image forming operation is impossible, so that it is possible to prevent an image having a poor image quality from being formed. When image formation is impossible, the user is notified of this, and reception of an image formation command from the outside is stopped.

  FIG. 7 is a flowchart showing the image forming operation. If it is determined by the image formation feasibility determination process that the apparatus is in a state where image formation is possible, the apparatus is in a standby state for accepting an image formation command from the outside (step S401). When an image formation command is given, it is first determined whether or not there is a rank 1 developing device (step S402). If there is at least one rank 1 developer, the developer to be used is selected with reference to a first table (FIG. 8) to be described later (step S403). If there is no rank 1 developer, The developing device to be used is selected with reference to the second table (FIG. 9) (step S404). In this embodiment, a combination of the current state (rank and category) of each developing device and the last developing device (hereinafter simply referred to as “last developing device”) used in the previous image forming operation. The next developing device to be used is selected.

  FIG. 8 is a diagram showing a first table for selecting a developing device. If there is a rank 1 developer and the device is imageable, there should be at least one developer that is in the rank 1 and printable category. Such a developing device is used for the image forming operation. When there is only one corresponding developing device, it is used. When there are a plurality of developing devices, the developing device to be used next is determined by the last developing device. More specifically, among the corresponding developing devices, the developing device located on the upstream side in the rotational direction of the rotary developing unit 4 as viewed from the last developing device and closest to the last developing device is Used for image formation.

  For example, when all four developing devices are in the rank 1 and printable category (case No. 1 in FIG. 8), if the last developing device is the developing device 4Ka, the next image forming operation is performed. The developing device 4Kb located on the upstream side is used. Similarly, when the last developing devices are the developing devices 4Kb, 4Kc, and 4Kd, the developing devices 4Kc, 4Kd, and 4Ka that are respectively installed at positions adjacent to the upstream side of the last developing device are used. . For example, when only the developing device 4b among the four developing devices does not satisfy the above requirement (rank 1 and printable category) (case No. 5 in FIG. 8), the developing devices 4Ka and 4Kc satisfying the requirement. And 4Kd are selected. In this case, if the last developing unit is 4Ka or 4Kb, the developing unit 4Kc located closest to the upstream side of the developing unit is used next. The other cases can be considered similarly.

  By selecting the developing devices in this way, when there are a plurality of rank 1 developing devices, the developing devices used for image formation are changed each time. As a result, any of the developing devices is not left unattended for a long time.

  FIG. 9 is a diagram showing a second table for selecting a developing device. Also in the second table used when there is no rank 1 developer, the developer to be used is based on the current developer state and the last developer as in the case of using the first table. It comes to choose. However, in this case, the developing device to be used next is selected from the developing devices of the rank 2 and printable category.

  Returning to FIG. 7, the description of the image forming operation will be continued. After selecting the developing device as described above, the selected developing device is moved and positioned to the developing position facing the photosensitive member 22 (step S405). Then, the developing unit is used to form the first page of the image corresponding to the image formation command (step S406). If all the images to be formed are present (step S407), the developing unit 4 is moved to the home position (step S411), and the image forming operation is ended.

  On the other hand, if there is an image to be formed next, the image is continuously formed. Prior to that, whether the number of images continuously formed by the developing device currently used has reached a predetermined number. It is determined whether or not (step S408). If it has not reached the predetermined number (here, 8), the process returns to step S406 to form the next image. When the predetermined number has been reached, it is determined whether or not there is a next developing device that can be switched and used (step S409). If there is a next developing device, the rotary developing unit 4 is rotated 90 degrees to the next. While the developing device is positioned at the developing position (step S410), if there is no next developing device, the developing device being used is continuously positioned at the developing position, and the process returns to step S406 to form the next image.

  The determination criteria in step S409 are as follows. When the developing device at the upstream adjacent position of the developing device currently in use in the rotation direction of the developing unit 4 is in the rank 1 and printable category, the developing device can be used as the next developing device. The determination result in this case is “YES”. In all other cases, “NO” is set. For example, if the developing device currently in use is the developing device 4Ka and the developing device 4Kb in the upstream adjacent position is rank 1 and the printable category, the determination result in step S409 is “YES”. On the other hand, if the developing device 4Kb adjacent to the upstream side of the developing device 4Ka currently in use is not in the rank 1 and printable category, even if there is another developing device in the rank 1 and printable category, step S409 is executed. The determination result in is “NO”.

  If there is a rank 1 and printable category developer at a position adjacent to the upstream side of the currently used developer, a new rank 1 and printable category developer can be moved to the development position only by rotating the developing unit 4 by 90 degrees. Can be moved. Therefore, the developing device can be switched without reducing the image forming throughput. In addition, since the next developing device belongs to rank 1 and the printable category, the image quality can be maintained satisfactorily.

  By switching the developing device in a series of image forming operations, the following effects can be obtained. If one developing device is continuously used, the ratio of old toner increases around the developing roller in the developing device, and the image quality gradually decreases. During this time, in other developing units, since the toner is left on the surface of the developing roller 44, the banding phenomenon tends to occur. On the other hand, when the developing unit 4 is rotated to switch the developing device, the toner in the developing device is agitated and uniformed, and the image quality can be maintained. In addition, by not biasing the developing devices to be used, it is possible to suppress the occurrence of the leaving banding phenomenon in each developing device.

  On the other hand, when a developing device at a position adjacent to the upstream side of the developing device currently in use belongs to a life rank other than rank 1 or a category other than the printable category, image formation is performed using the developing device. The image quality may fluctuate greatly before and after the switching. Further, even if the developing device belonging to the rank 1 and printable category is located at a position other than the position adjacent to the upstream side of the developing device currently in use, the developing unit 4 is rotated 180 degrees or more in order to switch to the developing device. Since it is necessary to rotate, it takes time to switch and causes a decrease in throughput. Therefore, in these cases, the developing device is not switched and the currently used developing device is used continuously.

  Note that after a series of image forming operations are completed, a developing device is newly selected when an image forming operation corresponding to a new image forming command is performed. In some cases, a preparatory operation is performed during standby until a new image formation command is given, and one of the developing devices newly shifts to the printable category. Therefore, a developing device that was not used in the previous image forming operation may be selected and used in the next image forming operation.

  As described above, in the image forming operation of this embodiment, the next developing device to be used is determined based on the current state of each developing device and the last used developing device in the previous image forming operation. In addition, when there are a plurality of usable developing devices, they are used while being switched. Therefore, all the usable developing devices are used alternately, and the use is not biased to only a part of the developing devices.

  In addition, by selecting a developing device to be used for image formation based on the above-described criteria, when executing an image forming operation, among the developing devices in the printable category, the previous image forming operation or preparation operation The one having the longest pause time from the end of use in is selected and used. As described above, the neglected banding phenomenon is likely to occur when the pause time is long. However, as in this embodiment, the developer having the longest pause time among the developers that can be used for the image forming operation is the next image. By using it in the forming operation, it is possible to prevent the neglected banding phenomenon that occurs when the developing device is further neglected.

  For example, consider a case where all the developing devices are in the rank 1 and printable category, and image data for 20 sheets is given as the first image formation command. For example, if an image forming operation is started using the developing device 4Ka, switching to the developing device 4Kb is performed when eight images are formed by the developing device 4Ka. When eight more images are formed, switching to the developing device 4Kc is performed, and the remaining four images are formed by the developing device 4Kc, and a series of image forming operations is completed. Thereafter, when a new image formation command is given, the developing device 4Kd is used when the next image forming operation is executed with reference to the first table (FIG. 8). The developing unit 4Kd is a developing unit having the longest unused time (resting time) among the four developing units.

  At this time, it is assumed that the remaining amount of toner in the developing device 4Ka is reduced due to the previous image forming operation and the developing device has moved to rank 2. In this case, neither switching from the developing unit 4Kd to the developing unit 4Ka or switching from the developing unit 4Kd to the developing unit 4Kb is performed, and the developing unit 4Kd is continuously used in a series of image forming operations. Then, when the next image formation command is given, referring to the first table (FIG. 8), since the case No. 9 and the last used developing device is the developing device 4Kd, rank 1 and printing are performed. Among the possible developing devices 4Kb, 4Kc, and 4Kd, the developing device 4Kb is used for the next image formation. This developing device 4Kb has the longest rest time after being used first among the three usable developing devices.

  As described above, in this embodiment, each developing device is basically switched and used along the rotation direction of the developing unit 4, and the positional relationship of each usable developing device mounted on the developing unit 4, The next developing device to be used is selected based on the last used developing device in the previous image forming operation or preparation operation. In other words, the first and second tables are created so that the developer having the longest downtime among the usable ones is selected based on the information of the state of each developer and the last used developer. ing. In this way, it is possible to select the developing device having the longest downtime as a result without measuring the downtime of each developing device.

  As described above, in this embodiment, in the image forming apparatus dedicated to monochrome images equipped with four developing units each storing black toner, the image formation feasibility determination process (FIG. 4) is performed at any time. The state of the device is monitored, and based on the result, it is determined whether or not the apparatus is ready to perform an image forming operation. Then, by performing a predetermined preparation operation as necessary, the apparatus is maintained in a state where an image forming operation can be performed as much as possible. Specifically, if the lifetime of the developing device belongs to a relatively new rank 1 and there is at least one developing device that can be immediately used for the image forming operation without performing the preparatory operation, the apparatus can perform the image forming operation. to decide. In this state, when an image formation command is given, the image forming operation can be started without delay.

  In addition, when there is a rank 1 developing device but a preparation operation is required, the necessary preparation operation is performed to shift the developing device to a usable state. In this case, as a preparatory operation, when a developing device that can be used by an agitation operation that can finish the processing in a short time and a developing device that requires a longer density control operation are mixed. Only the stirring operation for the necessary developing device is executed. As a result, the processing time of the preparation operation can be shortened, and the apparatus can quickly recover to a state in which an image forming operation can be performed. Also, even if there is a developing device that requires a preparatory operation, if there is at least one other usable developing device, the preparatory operation is not performed at that time, and the minimum necessary when there is no usable developing device. Perform proper preparatory actions. Therefore, according to the image formation feasibility determination process of this embodiment, it is possible to perform the preparation operation efficiently while maintaining the apparatus in a state where the image formation operation can be performed as much as possible, while omitting useless preparation operations. Become.

  The preparatory operation includes a stirring operation that eliminates the neglected banding phenomenon by rotating the developing roller 44 attached to the developing device by a predetermined amount with respect to the developing device that has been used first and the rest time after that exceeds a predetermined value. In addition, it is possible to execute a density control operation for setting the operating conditions of the apparatus when using the developing device for the developing device whose toner remaining amount or developing roller rotation time has reached a predetermined value, and if necessary, Execute. When both a developing device that requires a stirring operation and a developing device that requires a density control operation exist, only the stirring operation with a shorter processing time is executed. By doing so, it is possible to shorten the time during which the image forming operation cannot be executed for the execution of the preparation operation.

  Also, even if there is no rank 1 developer and there is a rank 2 developer whose deterioration has progressed slightly, if there is something that can be used immediately, it is necessary if there is something that requires preparatory action. After performing various preparation operations, the apparatus determines that an image forming operation is possible. Therefore, image formation can be performed by effectively using the toner in the developing device.

  In the image forming operation of this embodiment, the developing device to be used is selected based on the state of each developing device and the last used developing device in the previous operation, and the developing device thus selected is used. Form an image. Therefore, since the image is formed by always selecting and using the best developing device, it is possible to stably form an image with good image quality in this embodiment. Specifically, among the developers that can be used immediately, the developer that has the longest pause time since it was used first is selected. Further, when there are a plurality of developing devices of rank 1 and printable category, image formation is performed while switching between them. Therefore, there is no bias in the use of the developing device, and the specific developing device is not used for a long time and is not left. As a result, in this embodiment, it is possible to suppress the occurrence of density unevenness (left banding phenomenon) that occurs when a developer that has been left for a long time is used, thereby preventing fluctuations in image quality and suppressing the phenomenon. Therefore, it is possible to minimize the execution frequency of the preparatory operation.

  Further, since the developing device is not switched when forming an image using the rank 2 developing device, there is no variation in image quality due to a difference in the state of the developing device.

  In this embodiment, since the image forming operation determination process is performed, the apparatus is immediately maintained in a state where the image forming operation can be performed. Therefore, when the image forming command is actually given, the image forming operation is performed without delay. It can be carried out. Therefore, in the image forming operation of this embodiment, an image can be formed in a short first print time. In addition, since the frequency of execution of the preparation operation is kept low, it is possible to minimize a decrease in throughput of image formation due to execution of the preparation operation during the image forming operation.

  As described above, in this embodiment, the engine controller 10 functions as the “control unit” of the present invention, and the rotary developing unit 4 functions as the “positioning unit” of the present invention. In this embodiment, the position where the developing unit faces the photosensitive member 22 (the position where the developing unit 4Ka is located in FIG. 1) corresponds to the “developing position” of the present invention, while the developing position in the rotation direction of the developing unit 4 The upstream adjacent position (position where the developing device 4Kb is located in FIG. 1) which is 90 degrees upstream from the position corresponds to the “standby position” of the present invention. The life ranks 1 and 2 in this embodiment correspond to the “first and second quality ranks” of the present invention, respectively. Further, in this embodiment, the remaining amount of toner and the developing roller rotation time of each developing device correspond to “information relating to the usage history of the developing device” of the present invention.

  The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above embodiment, a maximum of four developing cartridges can be mounted on the support frame 40 of the developing unit 4 and the developing cartridges 4Ka to 4Kd are mounted at all mounting positions. The present invention can also be applied to an apparatus in which the number of mountable developing devices is different from this, or only a part of the mountable developing devices is mounted.

  Further, for example, in the image formation feasibility determination processing of the above embodiment, when there are a plurality of developing devices that require the same rank and the same preparatory operation, the preparatory operations for these developing devices are continuously performed. However, the present invention is not limited to this, and for example, a preparatory operation for only one of them may be performed. This is because, as described above, at least one developer that can be used is sufficient. Further, for example, the preparatory operation may be sequentially performed on the developing devices, and it may be determined that the apparatus is capable of the image forming operation when the preparatory operation is completed for the first developing device. This is because the image forming operation can be performed as the entire apparatus when the preparatory operation is completed for one developing device, and by doing so, it is possible to respond quickly by an image forming command. For a developing device that has not completed the preparatory operation, the preparatory operation may be continued after completion of the image forming operation. If there is another usable developing device, the preparatory operation at this point is omitted. Also good.

  In the above-described embodiment, the state of the developing device is determined for each developing device based on the two parameters of the remaining toner amount and the developing roller rotation time. The information is not limited to these. Further, the state of the developing device may be determined based on only one of these two pieces of information or in combination with other information.

  In the above embodiment, the present invention is applied to an image forming apparatus in which the monochrome image color is black. However, the present invention can also be applied to an apparatus that forms a monochrome image using other toner colors.

  In the above-described embodiment, the image forming operation by the rank 2 developer is permitted. However, the user's preference and necessity regarding whether to permit or prohibit the image formation using the rank 2 developer. You may make it selectable according to.

  In the above embodiment, the four developing cartridges 4Ka to 4Kd having the same shape are used. However, developing cartridges having different shapes may be used. In the above embodiment, the present invention is applied to a so-called rotary type image forming apparatus in which the rotary developing unit 4 is arranged for one photoconductor 22. The present invention can also be applied to an elevator type image forming apparatus in which a developing cartridge moves up and down and develops with each developing cartridge.

  Further, the present invention is not limited to the configuration of the above-described embodiment. For example, an apparatus for forming an image of a specific color by installing a development unit equipped with a plurality of development cartridges having toner of a specific color, or a transfer other than an intermediate transfer belt The present invention can be applied to an apparatus including a medium (transfer drum, transfer sheet, etc.), and also to other image forming apparatuses such as a copying machine and a facsimile apparatus.

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. The figure which shows rank division of a developing device. 6 is a flowchart showing image formation availability determination processing. The flowchart which shows stirring operation. The flowchart which shows density | concentration control operation. 5 is a flowchart showing an image forming operation. FIG. 4 is a diagram showing a first table for selecting a developing device. The figure which shows the 2nd table for selecting a developing device.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 4 ... Rotary developing unit (positioning means), 4Ka, 4Kb, 4Kc, 4Kd ... Developing device, 10 ... Engine controller (control means), 22 ... Photoconductor, 44 ... Developing roller, 60 ... Density sensor, 71 ... Intermediate transfer belt EG ... Engine part

Claims (10)

A plurality of developing devices for storing toners of the same color;
Storage means for storing information relating to the usage history of each of the developing devices;
Control means for selectively performing the image forming operation using the developing device,
For each of the plurality of developing units, the control unit is configured to determine a predetermined target image quality when the developing unit is used for the image forming operation based on information on the usage history of the developing unit stored in the storage unit. Quality that is classified into a plurality of quality ranks including a first quality rank that can form an image and a second quality rank that can form an image but may be inferior to the target image quality When at least one of the plurality of developing devices is determined to be the first quality rank, the image forming operation is performed using only the developing devices belonging to the first quality rank. while performing,
If there is no developer of the first quality rank among the plurality of developers and there is at least one developer determined to be the second quality rank, the developer of the second quality rank is used. Then , the image forming apparatus is configured to execute the image forming operation .   2. The control unit according to claim 1, wherein when there are a plurality of developing devices determined as the first quality rank, the control unit executes the image forming operation using the developing devices while switching based on a predetermined switching condition. The image forming apparatus described. A positioning unit that sequentially moves and positions the plurality of developing units from a predetermined standby position to a developing position;
The control means is configured to perform the image forming operation using the developing device of the first quality rank positioned at the developing position;
When the developing device of the first quality rank is at the standby position, switching is performed to retract the developing device at the developing position and move the developing device at the standby position to the developing position based on a predetermined switching condition. While doing the action
The image forming apparatus according to claim 1, wherein the switching operation is not performed when the developing device having the first quality rank is not present at the standby position.   4. The image forming apparatus according to claim 2, wherein the switching condition is that the number of images formed by continuously using one developing device has reached a predetermined number.   When there are a plurality of developing devices determined as the first quality rank, the control unit selects one of the developing devices based on a predetermined selection condition, and uses the selected developing device for the image. The image forming apparatus according to claim 1, wherein the image forming apparatus executes a forming operation.   The image forming apparatus according to claim 5, wherein the selection condition is to select a developing unit determined to be the first quality rank, which has the longest pause time after the end of the past use. Among the developing devices determined to have the first quality rank, those that can be used immediately for the image forming operation and those that need to be subjected to a predetermined preparation operation prior to use in the image forming operation are mixed. When you are
The image forming apparatus according to claim 2, wherein the control unit uses only a developer that is determined to be the first quality rank and is immediately usable for the image forming operation. When there are a plurality of developing devices determined to be the second quality rank, the control unit selects one of the developing devices based on a predetermined selection condition, and uses the selected developing device for the image. The image forming apparatus according to claim 1 , wherein the image forming apparatus executes a forming operation. When continuously formed a plurality of images, said control means, the image formation according to claim 1 or 8 for forming an image of said plurality using only one developing unit of the second quality ranks apparatus. In an image forming method of forming an image by selectively using a plurality of developing units that store toner of the same color from each other,
Storing information on the usage history of each of the developing devices in a storage means;
For each of the plurality of developing devices, an image is formed with a predetermined target image quality when the developing device is used for the image forming operation based on information on the usage history of the developing device stored in the storage unit. Performing a quality rank determination that ranks a plurality of quality ranks including a first quality rank that can be performed and a second quality rank that can form an image but may be inferior in image quality to the target image quality When,
A step of performing an image forming operation in a mode according to the determination result,
When there is at least one developer determined as the first quality rank among the plurality of developers, the image forming operation is executed using only the developers belonging to the first quality rank ,
If there is no developer of the first quality rank among the plurality of developers and there is at least one developer determined to be the second quality rank, the developer of the second quality rank is used. Then, the image forming operation is executed .

Images