JP2007193076A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve the improvement of throughput and the miniaturization of an image forming apparatus while preventing the occurrence of jamming in an image forming apparatus and an image forming method for consecutively forming images of a plurality of pages. <P>SOLUTION: When the images are consecutively formed by using one developing device (a), an interval d21 and the like of the respective images I21 to I26 are made the same. When the images are formed while switching the developing devices (b), (c) and (d), the developing device is switched at an interval between the images of fourth and fifth pages. When the risk of the occurrence of jamming is low, a mode (b) in which an interval d43 is locally narrowed is performed. When the risk is higher than it, a mode (c) or (d) in which the minimum interval gets larger is performed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image forming method for continuously forming images of a plurality of pages.

  As an image forming apparatus including a plurality of developing devices, a plurality of developing devices storing different color toners are used while switching to form a full-color image, or one of them is used as a monochrome device. An apparatus for forming an image is conventionally known. For example, in the image forming apparatus described in Patent Document 1, four developing units that store yellow (Y), magenta (M), cyan (C), and black (K) toners are mounted on a developing rotary, respectively. A color printing mode that forms a full-color image by superimposing the toner images of each color formed while switching between them on the intermediate transfer belt, and a single-color printing mode that forms a monochrome image by fixing the developing device to be used in black Is configured to do.

  Further, in this image forming apparatus, the image forming position on the intermediate transfer belt is different between the color printing mode and the single color printing mode. That is, in the color printing mode, since it is necessary to perform color superposition while switching the developing device, an area having a particularly wide image interval is provided for each turn of the intermediate transfer belt. On the other hand, in the monochromatic printing mode, it is not necessary to switch the developing device, so that the image interval is constant.

Japanese Patent Laying-Open No. 2005-003759 (FIGS. 1 and 6)

  In this type of image forming apparatus, it is desired to make the formation pitch of each image as short as possible in order to improve the throughput of image formation. Speaking of the above prior art, it is desirable to make the interval between the images arranged on the intermediate transfer belt as small as possible. As a result, the length of the intermediate transfer belt required to carry the same number of pages of images can be shortened, leading to a reduction in the size of the apparatus. However, if the interval between the images is reduced, the transport pitch of the recording material (recording paper, transparent sheet, etc.) to which those images are finally transferred is also shortened, and the transport timing variation or slippage of each recording material on the transport path. Jam (conveyance abnormality such as clogging of recording materials on the conveyance path or overlapping of recording materials) is likely to occur. Thus, in the prior art, there are conflicting problems of improving throughput, reducing the size of the apparatus, and preventing jamming, and it has been difficult to form an image while achieving both of these.

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems. In an image forming apparatus and an image forming method for continuously forming images of a plurality of pages, an improvement in throughput and a reduction in size of the apparatus are achieved while preventing jamming. It aims at providing the technology that can do.

  In order to achieve the above object, an image forming apparatus according to the present invention is configured such that a toner image can be carried on a surface thereof, and an endless image carrier that circulates in a predetermined direction. An image forming means for carrying on the surface of the carrier, a conveyance means for conveying a sheet-like recording material from the image carrier to a transfer position to which a toner image is transferred along a predetermined conveyance path, A detection unit that detects a conveyance state of the recording material; and a control unit that controls the operation of the image forming unit. When the control unit forms a plurality of pages of toner images successively, the detection result of the detection unit is Accordingly, the position of forming each toner image on the image carrier formed by the image forming means is changed.

  In order to achieve the above object, the image forming method according to the present invention sequentially forms a plurality of pages of toner images on an endless image carrier that circulates in a predetermined direction, and the image forming process. A transfer step of sequentially transferring the plurality of toner images formed in the process to each of the sheet-like recording materials conveyed one by one along a predetermined conveyance path, and the recording material on the conveyance path The toner image forming position on the image carrier is changed and set in the image forming step in accordance with the transport status.

  In these inventions, the formation position of each toner image on the image bearing member is dynamically changed according to the conveyance state of the recording material, so that both the prevention of jamming and the improvement of throughput and the miniaturization of the apparatus are achieved. Can be achieved. For example, under conditions where the recording material is transported very well, the throughput can be improved by reducing the interval between the toner images. On the other hand, if the recording material is not transported well, the interval can be increased. Jam generation can be suppressed. Further, for example, by setting the interval between the toner images to be small in advance and forming the toner image by increasing the interval only when the conveyance condition is not good, the image carrier can be made compact. .

  More specifically, for example, when the detection unit is configured to detect the presence or absence of jam of the recording material in the conveyance path, and the control unit detects a high frequency of jam detected by the detection unit. The positions where the toner images are formed may be set so that the intervals between the toner images on the image carrier are increased. Thereby, a high jam suppression effect can be obtained. On the contrary, if the risk of jamming is low, the throughput can be improved by reducing the interval between the toner images.

  In addition, for example, the detection unit is configured to detect the presence or absence of the recording material passing through a predetermined detection position on the conveyance path, and the control unit is configured to detect the recording obtained from the detection result of the detection unit. The toner image formation position may be set so that the interval between the toner images on the image carrier becomes wider as the time required for the material to pass the detection position is longer. Due to wear and dirt on the parts that make up the transport path, and variations in the quality of the recording material, it takes more time for the recording material to pass through a certain point on the transport path due to slippage on the transport path. There is a case. In such a case, it is considered that there is a high risk of the occurrence of a jam. Therefore, the occurrence of a jam can be prevented beforehand by widening the interval between the toner images.

  Note that the “conveyance path” here refers to the entire path from the position where the recording material is set in advance to the discharge position provided outside or inside the apparatus through the transfer position. It is not limited to the path to the transfer position. Therefore, the jam detecting means described above may detect a jam at any position on the transport path. Further, the detection position by the recording material detection means described above may be provided at any position on the transport path.

  Further, for example, when the image carrier is configured to be able to simultaneously carry the toner images for a plurality of pages on the surface thereof, the control unit may perform the image carrier according to the detection result of the detection unit. The number of toner images formed on the body may be set. In other words, by determining whether to form all or only a part of the toner image that can be simultaneously carried on the image carrier according to the detection result of the detection means, the jam suppression effect and the throughput maintenance effect can be achieved. Image formation can be performed while balancing.

  In addition, a plurality of developing units that store toner of the same color can be mounted, and a positioning unit that selectively moves and positions one of the mounted developing units to a predetermined developing position is further provided, As an image forming operation for continuously forming toner images of a plurality of pages, the means forms a toner image of a predetermined number of pages using one developing device, and then switches the developing device by the positioning device to perform the next development. A switching image forming operation for forming a toner image on the next page or later using a container may be executed.

  In this way, a large number of toner images can be formed continuously. In addition, since a toner image cannot be formed while the developing device is switched, in the conventional technique in which the toner image forming position is fixed, the throughput may be reduced by switching. It is also possible to prevent this by adjusting the formation position.

  For example, the distance by which the surface of the image carrier moves is changed while the switching time required for switching the developing device located at the developing position from one developing device to another developing device by the positioning means is changed. The control means, except for the interval between the toner images formed immediately before and after the switching of the developing device, among the intervals between adjacent toner images along the moving direction of the image carrier. Can be less than the switching distance. It is inevitable that an interval larger than the switching distance described above is opened between the toner images during the switching of the developing device, but there is no such restriction for other intervals, so by reducing this interval, A decrease in throughput can be suppressed.

  Further, the timing for executing the switching of the developing device, that is, how many pages of toner image are formed after using one developing device is controlled based on the detection result of the detecting means. It may be. By switching the developing unit between toner image formations, the conveyance pitch of the recording material may temporarily fluctuate, which may cause jamming. By controlling the switching timing according to the detection result of the detection means, it is possible to suppress the occurrence of jam.

(First embodiment)
FIG. 1 is a diagram showing a first embodiment of an image forming apparatus according to the present invention. 2 is an external perspective view of the image forming apparatus of FIG. FIG. 3 is a block diagram showing an electrical configuration of the image forming apparatus shown in FIG. This apparatus 1 forms a full color image (color printing mode) by superposing four color toners (developers) of yellow (Y), magenta (M), cyan (C), and black (K), or black The image forming apparatus forms a monochrome image (single color printing mode) using only the toner of (K). In this image forming apparatus 1, when a print command signal including an image signal is given from an external device such as a host computer to the main controller 11 via the interface 112, the CPU 111 of the main controller 11 is suitable for an operation instruction of the engine unit EG. The job data is converted into job data of the same format and sent to the engine controller 10. On the other hand, the engine controller 10 controls each part of the engine unit EG based on job data from the CPU 111 to selectively execute the color printing mode and the monochrome printing mode, and forms an image corresponding to the image signal 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 receives the charging bias from the charging control unit 103 and uniformly charges the outer peripheral surface of the photosensitive member 22 to a predetermined surface potential. Note that the photosensitive member 22, the charging unit 23, and the cleaning unit 25 integrally constitute the photosensitive member cartridge 2, and as shown in FIG. 2, the photosensitive member cartridge 2 is integrally attached to and detached from the apparatus 1 main body. It is free.

  In the image forming apparatus 1, as shown in FIG. 2, an openable / closable external cover 121 is provided on the side surface of the apparatus main body 120, and when the user or a service engineer opens the external cover 121, the apparatus main body 120. The side surface portion of the photosensitive member cartridge 2 is exposed through the opening portion 122 for the photosensitive member. Then, the lock lever 123 for fixing the photoconductor cartridge 2 is rotated in the arrow direction D4 to release the lock, and the photoconductor cartridge 2 can be pulled out along the (+ X) direction in FIG. Further, by inserting the photoconductor cartridge 2 in the (−X) direction of FIG. 2 through the photoconductor opening 122, a new photoconductor cartridge 2 can be mounted on the apparatus main body 120. Then, the photosensitive cartridge 2 is fixed by the lock lever 123. When the photoconductor cartridge 2 is thus mounted, the photoconductor opening 122 is substantially blocked by the side surface of the photoconductor cartridge 2.

  In the photoreceptor cartridge 2 mounted on the apparatus main body 120 as described above, the light beam L is irradiated from the exposure unit 6 toward the outer peripheral surface of the photoreceptor 22 charged by the charging unit 23. The exposure unit 6 exposes the light beam L onto the photosensitive member 22 in accordance with a control command given from the exposure control unit 102 to form an electrostatic latent image corresponding to the image signal. For example, when an image signal is given from an external device such as a host computer to the CPU 111 of the main controller 11 via the interface (I / F) 112, the CPU 101 of the engine controller 10 sends the image signal to the exposure control unit 102 at a predetermined timing. In response to this, a light beam L is emitted from the exposure unit 6 onto the photosensitive member 22, and an electrostatic latent image corresponding to the image signal is formed on the photosensitive member 22.

  The electrostatic latent image thus formed is developed with toner by the developing unit 4. In other words, in this embodiment, the developing unit 4 is configured to be detachably attached to the support frame 40 that is rotatably provided about the shaft center, a rotation drive unit that is not shown, and the support frame 40, and each color toner. Is provided with a yellow developing device 4Y, a magenta developing device 4M, a cyan developing device 4C, and a black developing device 4K. In order to attach and detach the developing devices 4Y, 4M, 4C, and 4K, the apparatus main body 120 is configured as follows. In other words, as shown in FIG. 2, the apparatus main body 120 is provided with a developing device opening 124 for attaching / detaching the developing devices 4Y, 4M, 4C, and 4K. An openable / closable internal cover 125 is provided so as to cover the developing device opening 124. The inner cover 125 is provided inside the outer cover 121. That is, since the outer cover 121 is formed so as to cover the developing device opening 124, the inner cover 125 cannot be opened when the outer cover 121 is closed. Conversely, the outer cover 121 cannot be closed unless the inner cover 125 is closed. When the user opens the inner cover 125, if the developing unit 4 is stopped at a predetermined attachment / detachment position, one of the mounted developing devices can be taken out through the developing device opening 124. . In addition, one developing device can be mounted through the developing device opening 124. Further, the developing devices 4Y, 4M, 4C, and 4K can be positioned at the following positions by operating the rotation driving unit.

  FIG. 4 is a schematic diagram showing the stop position of the developing unit 4. The developing unit 4 is rotationally driven in the arrow direction D5 based on a control command from the CPU 101, and is positioned and fixed at three types of positions shown in FIG. 4 by the CPU 101 and a rotary lock mechanism (not shown). The three types of positions are: (a) home position; (b) development position; (c) attachment / detachment position. Among them, (a) the home position is a position that is positioned when the image forming apparatus is in a standby state in which no image forming operation is performed, and as shown in FIG. All of the provided developing rollers 41Y and the like are in a state of being separated from the photosensitive member 22, and any of the developing devices 4Y,... Cannot be taken out through the developing device opening 124 provided in the apparatus main body 120. is there.

  Further, (b) the development position is a position that is positioned when the electrostatic latent image on the photosensitive member 22 is visualized with a selected toner color. As shown in FIG. 4B, the developing roller provided in one developing device (the developing roller 41Y provided in the yellow developing device 4Y in the example shown in FIG. 4) is disposed to face the photosensitive member 22, and has a predetermined value. By applying a developing bias, the electrostatic latent image is visualized with toner. Even at this developing position, any developing device cannot be taken out through the developing device opening 124. When the external cover 121 is opened during the image forming operation, the image forming operation is immediately stopped, and the developing unit 4 stops after moving to the home position.

  Further, (c) the attach / detach position is a position that can be taken only when the attach / detach operation of the developing device is performed. When the developing unit 4 is positioned at this attaching / detaching position, as shown in FIG. 4C, one developing device appears in the developing device opening 124 and can be taken out through the opening 124. FIG. 4C shows a state in which the black developing device 4 </ b> K appears in the developing device opening 124. In addition, a developing device can be newly attached to the support frame 40 to which the developing device is not attached. In this attaching / detaching position, the developing roller provided in any developing device is placed at a position separated from the photosensitive member 22. As described above, when the developing unit 4 is positioned at the attachment / detachment position, only one developing device appearing in the developing device opening 124 can be taken out. Therefore, the user does not inadvertently attach and detach the developing device and damage the apparatus.

  In this image forming apparatus, the development position and the attachment / detachment position described above are set for each of the four developing devices 4Y, 4M, 4C, and 4K. Therefore, the stop position of the development unit 4 has one home position. It is 9 places including convenience.

  The developing unit 4 configured as described above is controlled by a developing device controller 104 as shown in FIG. Based on a control command from the developing device controller 104, the developing unit 4 is driven to rotate, and the developing devices 4Y, 4M, 4C, and 4K selectively abut against the photosensitive member 22 or have a predetermined gap. Is positioned at a predetermined development position facing each other. Further, by applying a developing bias from the developing device controller 104 to the developing roller 41 of the developing device positioned at the developing position, the toner carried on the surface of the developing roller 41 from the developing roller 41 to the photosensitive member 22 is transferred. Supply. As a result, the electrostatic latent image on the photosensitive member 22 is visualized with the selected toner color.

  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 stretched around a plurality of rollers 72 to 75, and a drive unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined movement direction D2 by rotationally driving the roller 73. It has.

  FIG. 5 is a development view showing the configuration of the intermediate transfer belt. As shown in FIG. 5, the intermediate transfer belt 71 is an endless belt formed by joining substantially rectangular sheet bodies at a seam 711. In the figure, an arrow 713 indicates the rotation axis direction. The intermediate transfer belt 71 has a protrusion 714 provided on one end side (the upper side in FIG. 5) in the rotation axis direction 713, and also has a transfer prohibition area 715 and a transfer permission area 716. The transfer prohibition area 715 is set from one end to the other end in the rotation axis direction 713 within a predetermined size range on both sides of the joint 711. On the other hand, the transfer permission area 716 is located at the center of the surface of the intermediate transfer belt 71 and is set to a rectangular area excluding one end and the other end in the rotation axis direction 713. The toner image is primarily transferred.

  Further, as shown in FIG. 9A, an A3 size toner image TI having a long side direction in the rotational drive direction D2 can be transferred to the transfer permission area 716. Further, as shown in FIG. 4B, a toner image of A4 plate size or less, for example, A4 plate size, A5 plate size, B5 plate size, or the like having a short side direction in the rotation driving direction D2 is formed around the intermediate transfer belt 71. Two pieces can be transferred to the transfer permission area 716. FIG. 5B shows an A4 size toner image TI. In this embodiment, two toner images TI of A4 size or smaller in the rotational drive direction D2 are arranged side by side in the transfer permission area 716. However, as will be described later, the arrangement relationship is different for each print mode. In addition, the position of the toner image on the intermediate transfer belt 71 is controlled.

  The vertical synchronization sensor 77 is composed of, for example, a photo interrupter having a light emitting portion (for example, LED) and a light receiving portion (for example, a photodiode) disposed to face each other, and is disposed on one end side in the rotational axis direction 713 of the rotating intermediate transfer belt 71. The detection signal is output by detecting the passage of the protrusion 714. A detection signal output from the vertical synchronization sensor 77 every time the protrusion 714 is detected is used as a vertical synchronization signal that serves as a reference for an image forming operation by the CPU 101 of the engine controller 10. That is, the image forming operation is executed by the operation of each unit of the engine unit EG in synchronization with the vertical synchronization signal.

  When forming a full-color image, the developing devices 4Y, 4M, 4C, and 4K are sequentially positioned at the developing position, and the electrostatic latent image on the photosensitive member 22 is developed to form a toner image of the toner color. In this way, a color image is formed by superimposing the four color toner images on the intermediate transfer belt 71 while switching the developing devices 4Y, 4M, 4C, and 4K (color printing mode). Then, the color images formed in this way are secondarily transferred onto the sheet S taken out one by one from the cassette 8 and conveyed along the conveyance path F to the secondary transfer region TR2.

  On the other hand, when a monochrome image is transferred to the sheet S, an image forming operation is executed only for the black color to form a monochrome image (monochrome print mode), and the secondary transfer region TR2 is transferred to the secondary transfer region TR2 in the same manner as in the case of a color image. The image is transferred to the conveyed sheet S to obtain a monochrome image.

  In this embodiment, 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, as shown in FIG. 1, a gate roller 81 is provided on the transport path F on the front side of the secondary transfer region TR2. A sheet detection sensor 88 is provided on the front side of the gate roller 81 on the conveyance path F. The sheet detection sensor 88 is composed of, for example, a reed switch or a photo interrupter, and detects whether or not the sheet S exists at a detection position on the conveyance path F. Based on the output of the sheet detection sensor 88, the CPU 101 can grasp whether the sheet S has reached or passed the detection position at a prescribed timing. Then, by rotating the gate roller 81 in accordance with the timing of the circumferential movement of the intermediate transfer belt 71, the sheet S is fed into the secondary transfer region TR2 at a predetermined timing.

  Further, the sheet S on which the color image or 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 120 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.

  In FIG. 3, 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. 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.

  In addition, the image forming apparatus 1 is configured to be able to perform an image forming operation in a state where four developing devices that store toner of the same color are mounted. That is, the developing devices mounted on the developing unit 4 have substantially the same shape, and for example, another black developing device can be mounted instead of the magenta developing device 4M. The same applies to other toner colors. For example, only four black developing devices can be attached to the developing unit 4. Which toner color the installed developer corresponds to is identified by, for example, storing it in a memory provided in each developer, or making the outer shape of the developer partially different for each color. Can be possible.

  In a state where four developing devices of the same toner color are mounted, the image forming apparatus 1 operates as an image forming apparatus dedicated to a monochrome image with the toner color. At this time, by forming an image while appropriately switching the four developing devices, this apparatus can continuously form a large amount of monochrome images. That is, even if one developing device runs out of toner, it is possible to continue image formation using another developing device. Further, when the developing device is switched, the entire developing unit 4 rotates, so that the toner in the developing device is agitated, and an image can be formed using fresh toner, which is advantageous in terms of image quality. . Also, it is known that when an image is formed using a developing device that has been left for a long time, the quality of the image initially formed may be inferior, but the developing device is switched periodically. This solves this problem.

  FIG. 6 is a flowchart showing an image forming operation in this apparatus. The image forming operation is executed when a print command signal is given from an external device. In this operation, first, the use mode of the apparatus is determined (step S101). This determination is made based on the type of the developing unit mounted on the developing unit 4. That is, when the four developing units mounted on the developing unit 4 are all compatible with different toner colors, the apparatus determines that the color use mode is used, while the four developing units are all compatible with the same color. If it is, it is determined as the monochrome use mode.

  First, the operation in the color use mode will be described. In this case, based on the instruction content of the print command signal, it is determined whether the image type, that is, the image to be formed is a color image or a monochrome image (step S102). Here, if the image to be formed is a color image, a necessary color image is formed by operating the engine unit EG in the color printing mode (step S103). The color printing mode is an operation mode in which the toner images of the respective colors are sequentially formed while the four developing devices are sequentially positioned at the development position, and the toner images are superimposed on the intermediate transfer belt 71.

  On the other hand, if the image to be formed is a monochrome image, one of the four developing units (generally, the black developing unit 4K) is moved to the developing position, and all the necessary units are used by using the developing unit. The image is formed. The operation of forming an image using only one of the four developing devices in this way is referred to as “monochrome single print mode” in this specification (step S111).

  The color print mode and the monochrome single print mode described above are well-known operations that are widely performed in image forming apparatuses having a general configuration, and thus detailed description thereof is omitted here.

  Next, an operation when the apparatus is in the monochrome use mode, that is, when the toners stored in the four developing devices are all the same color (generally black) will be described. In this case, the engine unit EG is operated in a monochrome four-print mode in which a monochrome image is formed while appropriately switching the four developing devices (step S121). Even if the print command signal corresponds to a color image, the main controller 11 converts the color image signal into a monochrome image signal, and then the engine unit EG forms a monochrome image corresponding to the image signal. A specific operation in the monochrome four-print mode will be described below.

  FIG. 7 is a flowchart showing the monochrome four-print mode. First, the degree of jam risk is determined (step S201). In this print mode, the mode of switching the developing device is varied depending on the risk of jam occurrence. The reason is as follows. In this printing mode, as will be described later, a plurality of pages of monochrome images are formed while switching the developing device. However, when the developing device is switched, the interval between the images on the intermediate transfer belt 71 is temporarily irregular. Along with this, the conveyance timing of the sheet S also becomes irregular temporarily and jamming is likely to occur. Therefore, the mode of switching the developing device is changed according to the risk of jam occurrence at that time.

  The risk of jam occurrence can be determined, for example, by how many jams have occurred during the most recent conveyance of a certain number of sheets (for example, 100 sheets). Whether or not a jam has occurred can be detected from the output of the sheet detection sensor 88 provided on the conveyance path F. That is, it can be said that a jam has occurred when the sheet S does not reach the detection position at the correct timing, or when the sheet S has not yet passed after a predetermined time has passed. If the past jam occurrence frequency is high, it can be considered that the current jam occurrence risk is high.

  Further, for example, the passage time of the sheet S at the detection position may be obtained from the output of the sheet detection sensor 88, and the jam risk degree may be determined from the result. That is, the sheet S may be transported in a state deviating from the original transport speed depending on the surrounding environment such as roller wear and dirt on the transport path F, temperature and humidity, and the quality of the sheet S. In particular, when the sheet S slides on the conveyance path F, it may take a longer time to pass through the detection position. In such a case, jamming due to variations in the conveyance speed between the sheets is likely to occur. Therefore, if the sheet conveyance delay is large, it can be said that the risk of jam occurrence is high. According to this method, it is possible to avoid the risk of jam occurrence even if jam does not actually occur.

  When it is determined that the risk of jam occurrence is high, the engine unit EG is operated in the monochrome single print mode described above to form an image (step S202). This is because, in the monochrome single print mode, since the developer is not switched, the interval between images can be kept wide and constant, and the occurrence probability of jam can be kept low.

  On the other hand, if the risk of jam occurrence is medium or low, the number of continuously formed pages is set according to the risk (step S211). The number of continuously formed pages is the maximum number of pages of images that are continuously formed using one developing device. Here, as an example, the number of continuously formed pages is 8 when the risk of jam occurrence is medium, and the number of continuously formed pages is 16 when the risk of jam occurrence is low.

  Subsequently, the developing unit 4 is rotated to move and position one of the four developing units to the developing position facing the photoconductor 22 (step S212), and an image for one page is generated using the developing unit. Is formed (step S213). As a method of selecting the developing device to be used at this time, for example, (1) select the one that can be moved to the developing position earliest, and (2) select the one that has the longest unused period among the four. (3) Select the toner with the smallest amount of toner, (4) Select the toner with the largest amount of toner. According to the method (1), the first print time can be minimized. Further, according to the method (2), it is possible to prevent deterioration in image quality due to a specific developing unit being left for a long time. According to the method (3), since a developing device with a small amount of toner is used preferentially, it is possible to prevent a plurality of developing devices from running out of toner one after another. Furthermore, according to the method (4), it is possible to level the remaining amount of toner in each developing device.

  When an image for one page is formed, it is determined whether there is an image for the next page or later to be formed (step S214). If there is no next page image, the operation is terminated. On the other hand, when there is an image to be subsequently formed, it is determined whether or not the number of pages of images continuously formed using the developing unit has reached the previously set number of continuously formed pages ( Step S215). If the number of continuously formed pages has not been reached, the process returns to step S213 to form the image of the next page, while if the number of continuously formed pages has been reached, the developing device is switched (step S216). That is, the developing unit 4 is rotated by 90 degrees, and the developing unit used so far is retracted from the developing position, and the developing unit adjacent to the developing unit is newly positioned at the developing position. Then, the process returns to step S213, and the image of the next page is formed using the developing device newly positioned at the developing position.

  That is, in this monochrome four-print mode, the developer is switched every time the number of pages of an image formed using one developer reaches the number of continuously formed pages. By doing so, each developing device is used evenly, and it is possible to prevent the deterioration of image quality caused by leaving the developing device unattended. Further, the toner in the developing device is agitated by the rotation of the developing unit 4, and an image can be formed with good image quality over a long period of time. As a result, a stirring mechanism to be provided in the developing device for stirring the toner can be omitted, and the developing device can be reduced in size and capacity.

  Since the image formation is temporarily stopped while the developing device is switched, a decrease in the throughput of the image formation becomes a problem when the frequency of switching is increased. Therefore, in this embodiment, the throughput is prevented from being lowered by devising the image forming position on the intermediate transfer belt 71.

  FIG. 8 is a diagram showing the arrangement of images on the intermediate transfer belt. In this drawing, the arrangement of the images in the case where the operation of forming two A4 size images on the circumferential surface of one turn of the intermediate transfer belt 71 is performed over a plurality of turns is shown for each print mode. This figure is a schematic diagram in which the intermediate transfer belt 71 is developed in a plane. For each print mode, this figure shows an image arrangement for three rotations of the intermediate transfer belt 71.

(A) In color printing mode (FIG. 8 (a))
In the color printing mode, since the toner images of the respective colors are superimposed on the intermediate transfer belt 71, it is necessary to switch the developing device every time the intermediate transfer belt 71 makes one revolution. Therefore, first, for example, two toner images Ik1 and Ik2 made of black toner are formed on the intermediate transfer belt 71 with a predetermined distance d11, and then two toner images Ic1 and Ic2 made of cyan toner are formed as black toner images Ik1 and Ik2. At this time, a gap d12 wider than the gap d11 is provided between the second black toner image Ik2 and the first cyan toner image.

This interval d12 is a length determined according to the time required for switching the developing devices. That is, the time required for switching the developing device, that is, the time from when image formation by the previous developing device becomes impossible for switching to the time when image formation by the next developing device becomes possible is tex, and the surface of the intermediate transfer belt 71 If the moving speed of v is v,
d12 ≧ dex = v · tex
The relationship must be established. Here, dex is a distance that the surface of the intermediate transfer belt 71 moves during the switching of the developing device, and is a minimum distance that can be taken as an interval between toner images of different colors.

  In other words, the distance d12 is the distance between the second black toner image Ik2 and the first cyan toner image Ic1 and between the second cyan toner image Ic2 and the first magenta toner image Im1. Must be greater than dex. On the other hand, between the first black toner image Ik1 and the second black toner image Ik2, between the first cyan toner image Ic1 and the second cyan toner image Ic2, and between the first magenta toner image Im1 and the first magenta toner image Im1. There is no such restriction with respect to the second magenta toner image Im2, but if the interval is too short, the interval in the transport path F of the sheet S to which these toner images are to be transferred becomes short and jamming is likely to occur. turn into. Further, in order to send the sheet S to the secondary transfer region TR2 in synchronization with the rotation of the intermediate transfer belt 71, the gate roller 81 is stopped once every time the sheet S is sent, and the sheet S is moved to the secondary transfer region TR2. The gate roller 81 needs to be rotated again in accordance with the rotation timing of the intermediate transfer belt 71. Therefore, the interval d11 should be determined from the viewpoint of safety against jamming and operation control of the gate roller 81. In terms of design, the peripheral length of the intermediate transfer belt 71 is determined from these intervals and the image size.

  Further, from the viewpoint of image quality, it should be avoided to carry a toner image on and near the joint 711 of the intermediate transfer belt 71, and it is desirable to switch the toner color across the joint 711. .

(B) In the monochrome single print mode (FIG. 8B)
In the monochrome single print mode, the toner images I21 to I26 are continuously formed without switching the developing device. Therefore, it is not necessary to widen the interval between toner images for switching. Accordingly, the distance d21 between the first toner image I21 and the second toner image I22 formed in the first rotation of the intermediate transfer belt 71 and the first formed in the second rotation of the toner image I22. The distance d22 to the toner image I23 can be arbitrarily determined without being restricted by the distance dex, but it is more preferable that d21 = d22. The reason is as follows.

  In the color printing mode, the four color toner images are superimposed on the intermediate transfer belt 71 and then transferred to the sheet S. Therefore, the number of sheets S sent to the conveyance path F is every four turns of the intermediate transfer belt 71. Two. Therefore, as shown in FIG. 8A, jamming is unlikely to occur even if the interval between toner images is reduced. On the other hand, in the monochrome single mode, the two sheets S are successively conveyed per rotation of the intermediate transfer belt 71, so that the risk of jamming when the interval is narrowed is much higher than in the color printing mode. Therefore, it is desirable to make the intervals between the toner images as wide as possible to prevent jamming. Further, by making the interval between the toner images constant, it is possible to make the sheet conveyance cycle constant, so that it is possible to obtain an effect that the operation control of the apparatus becomes easier.

  In the color print mode and the monochrome single print mode, a toner image of two pages is formed on the intermediate transfer belt 71 for each rotation of the intermediate transfer belt 71. However, the risk of jam occurrence If it is known that the toner image is high, the toner image formed for each turn may be changed to one page. By doing so, the throughput is reduced, but the interval between the toner images on the intermediate transfer belt 71 is widened, and the conveyance interval of the sheet S is also widened, so that jamming can be effectively suppressed.

(C) Monochrome four-print mode (Fig. 8 (c))
In the monochrome four-print mode in which an image is formed while switching the developing device, different considerations are required for both the color print mode and the monochrome single-print mode. This is because the developer is switched at a timing different from those in these print modes. Here, if an image is arranged in the same manner as in the color printing mode described above, there is no problem with the switching operation because the timing at which the developing device can be switched is provided for each rotation of the intermediate transfer belt 71.

  However, when this is done, the risk of jamming increases. This is because two sheets S are transported per rotation of the intermediate transfer belt 71 as in the monochrome single-print mode. Therefore, from the viewpoint of jam suppression, it is desirable to make the interval between the toner images as wide as possible. On the other hand, if the interval between the toner images is to be increased uniformly, the interval necessary for switching the developing device cannot be ensured. However, if the peripheral length of the intermediate transfer belt 71 is increased, the throughput is reduced even in other printing modes, and the apparatus becomes large and expensive.

  Therefore, in the monochrome four-print mode of this embodiment, as shown in FIG. 8C, when the developer is not switched, the toner image interval is widened as in the monochrome single-print mode. In the round immediately before the switching, the interval I33 between the first toner image I33 and the second toner image I34 in the round is reduced as in the color printing mode. That is, the toner image I34 is formed next by forming the toner image I34 forward (leftward in the drawing) from the region that should be originally formed (the region indicated by the dotted line in FIG. 8C) if there is no switching. The distance d34 from the toner image I35 is increased. In this way, the distance d34 between the toner images I34 and I35 can be set to the distance dex. More precisely, the distance d34 is used to provide the distance d34 on the intermediate transfer belt 71. Developers can be switched using a period during which no toner image is formed on the body 22. As a result, in the monochrome four-print mode, it is possible to obtain the same throughput as other print modes while suppressing the occurrence of jam.

  FIG. 8C shows a case where the developing device is switched between the fourth toner image I34 and the fifth toner image I35, but in the actual apparatus, the eighth and ninth are switched. The developer is switched between the toner images (when the number of continuously formed pages is 8) or between the 16th and 17th toner images (when the number of continuously formed pages is 16).

  Here, the interval d33 between the toner images I33 and I34 can be set to the same value as the interval d11 between the toner images Ik1 and Ik2 in the color printing mode, for example. Further, the interval d34 between the toner images I34 and I35 can be set to the same value as the interval d12 between the toner images Ik2 and Ic1 in the color printing mode, for example. Further, the interval d31 between the toner images I31 and I32 and between the toner images I35 and I36 can be set to the same value as the interval d21 between the toner images I21 and I22 in the monochrome single print mode, for example. Further, the interval d32 between the toner images I32 and I33 can be set to the same value as the interval d22 between the toner images I22 and I23 in the monochrome single print mode, for example. When all of these are applied, the arrangement of the image on the intermediate transfer belt 71 in the monochrome four-print mode is the same as that in the color print mode in the round immediately before the switching of the developing device, and in the other rounds, the monochrome one is printed. The layout is the same as the print mode. In other words, at this time, in the monochrome four-print mode, the image layout in the color print mode and the image layout in the monochrome single-print mode are properly used. In this way, there are only two image arrangements and sheet conveyance timings on the intermediate transfer belt 71, so that the design of the intermediate transfer belt 71 and the control of the conveyance system are facilitated.

  In the monochrome four-print mode, the risk of jamming is increased by reducing the interval between the toner images I33 and I34. However, the interval in this case is about the same as the minimum interval in the color printing mode, and the developing device is switched between four turns (when the number of continuously formed pages is eight) or eight turns (when the number is 16). ), The risk of jamming is not extremely high compared to other printing modes.

  However, it is true that the higher the frequency of switching, the higher the risk of jamming. In consideration of this, in this embodiment, as described above, a measure is taken so that the frequency of switching is low or not switched when the apparatus is in a state of easily causing a jam.

  As described above, in this embodiment, as an operation mode for forming a monochrome image, an operation mode in which a single developing device is used and toner images are formed on the intermediate transfer belt 71 at equal intervals (monochrome one image). Printing mode) and an operation mode (monochrome four-printing mode) in which the developing devices are switched at regular intervals while being basically equally spaced. Furthermore, in the monochrome four-print mode, the toner image formation position on the eighth page and the toner image formation position on the 16th page are changed, respectively. The arrangement is different. These operation modes are selectively used according to the risk of jam occurrence. In other words, an operation mode with a high jam suppression effect (a mode in which the interval between toner images is wide) is selected under conditions where jamming is likely to occur, while an operation with high throughput is achieved under conditions where the risk of jamming is low. Select a mode (a mode in which the interval between toner images is narrow). Therefore, in this embodiment, an image can be efficiently formed while balancing the jam suppression effect and the throughput maintenance effect. Further, when determining the length of the intermediate transfer belt 71, it is not necessary to provide an extra length for switching the developing device, so that the apparatus can be made compact.

  The present invention is particularly effective in an image forming apparatus having an intermediate transfer belt that is sized for the purpose of simultaneously carrying a plurality of pages of a toner image of a certain size at a predetermined interval. Play. In the above embodiment, when the developing device is switched every two pages, the arrangement of the toner images is the same as in the color printing mode (FIG. 8A), but the effect of the present invention is as follows. This is particularly noticeable in an apparatus configured to switch the developing device after forming a toner image having more than the number of pages (2 in this embodiment) that can be simultaneously carried on the intermediate transfer belt by using one developing device. Become. In the above embodiment, the number of pages (number of continuously formed pages) of the toner image formed using one developing device is 8 or 16, and thus the effect of the present invention is remarkably exhibited.

  As described above, in this embodiment, the developing unit 4 functions as the “positioning unit” of the present invention. Further, the intermediate transfer belt 71 functions as an “image carrier” of the present invention. Further, the photosensitive member 22, the developing unit 4, the exposure unit 6, the engine controller 10 and the like function as an “image forming unit” of the invention. In addition, the rollers 81 to 83 and the like constituting the conveyance path F function as a “conveying means” of the present invention as a whole. Further, the sheet detection sensor 88 functions as the “detection means” of the present invention. Further, the transport path F and the reverse transport path FR along which the sheet S as the “recording material” of the present invention is transported correspond to the “transport path” of the present invention. In this embodiment, the distance dex given as the product of the time tex required for switching the developing device and the moving speed v of the intermediate transfer belt 71 corresponds to the “switching distance” of the present invention.

(Second Embodiment)
Next, a second embodiment of the image forming apparatus according to the present invention will be described. The second embodiment is different from the first embodiment described above in the arrangement of the toner image on the intermediate transfer belt 71 in the monochrome four-print mode. That is, in the second embodiment, as will be described below, the toner image formation position on the intermediate transfer belt 71 is varied depending on the risk of occurrence of a jam and other conditions. Since the other points are the same as those in the first embodiment, description thereof is omitted.

  FIG. 9 is a diagram showing the arrangement of toner images in the second embodiment. However, FIG. 9A is the same as FIG. 8B, and shows the arrangement of the toner image in the monochrome single print mode as a comparative example for showing the characteristics of the arrangement of the toner image in each example. ing.

  In the arrangement shown in FIG. 9B, the toner images I41 to I44 from the first page to the fourth page are formed at the same positions as in the monochrome single print mode shown in FIG. Accordingly, the interval d41 between the toner images is the same as the interval d21 in the monochrome single print mode. Then, after the toner image I44 is formed, the developing device is switched. At this time, the distance d42 from the next toner image I45 must be larger than the distance d41 between other toner images. Therefore, in this embodiment, the interval necessary for switching is created by moving the position of the toner image I45 formed immediately after switching to the rear (rightward in the figure). As a result, the interval d43 with the toner image I46 on the sixth page is slightly reduced.

  In the arrangement shown in FIG. 9C, the toner images I51 to I53 and I56 of the first to third and sixth pages are formed at the same position as in the monochrome single-print mode. Therefore, the interval d51 between the toner images is the same as the interval d21 in the monochrome single print mode. On the other hand, the toner image I54 of the fourth page is moved forward (leftward in the figure) and the toner image I55 of the fifth page is moved rearward, thereby widening the distance d53 between them. During this time, the developer is switched. As a result, the interval d52 between the toner images I53 and I54 and the interval d54 between the toner images I55 and I56 are slightly narrowed, but the amount of decrease in the interval is smaller than the amount of decrease in the interval d43 in FIG. 9B. That's it.

  In the arrangement shown in FIG. 9D, the developer can be switched by widening the interval d64 between the toner image I64 of the fourth page and the toner image I65 of the fifth page. In order to secure this interval, the intervals d61, d62,... Of the toner images I61 to I64 and I65 to I66 are made small little by little. More preferably, d61 = d62 = d63 = d65. According to this arrangement, since the intervals between the toner images are evenly spaced, the intervals between the toner images are not locally reduced.

  In the monochrome four-print mode of the second embodiment, one of these arrangements is selected according to the state of the apparatus, and a toner image is formed with the selected arrangement. First, when it is assumed that the risk of jam occurrence is low, a toner image is formed with the arrangement shown in FIG. In this arrangement, the arrangement of the toner image does not change until immediately before the changeover of the developing device (same as the monochrome single print mode). For example, even when the developing device in use becomes out of toner, when the toner out is found. Subsequent toner images can be continuously formed by switching the developing device. In addition, it is possible to cope with a case where it is not determined in advance at which page the developing device is switched.

  If the risk of jamming is slightly higher than this, the arrangement shown in FIG. In this arrangement, since the minimum distances d52 and d54 of the toner images are larger than the minimum distance d43 in the arrangement shown in FIG. If the risk of jamming is further increased, the arrangement shown in FIG. 9D has a higher jamming suppression effect because the interval between toner images is constant except when the developing device is switched.

  As described above, in the apparatus of the second embodiment, a plurality of patterns having different toner image intervals are prepared in advance as the arrangement of the toner images on the intermediate transfer belt 71, and these patterns are prepared according to the situation. Adopt one of the following. By so doing, it is possible to efficiently form a large number of toner images while suppressing the occurrence of jamming.

(Third embodiment)
Next, a third embodiment of the image forming apparatus according to the present invention will be described. In the third embodiment, as described below, the arrangement of the toner image on the intermediate transfer belt 71 in the monochrome four-print mode is different from the arrangement in the first embodiment and the second embodiment described above. Yes.

  FIG. 10 is a flowchart showing a monochrome four-print mode in the third embodiment. In this print mode, first, the number of continuous formation laps is set (step S401). The “number of continuous forming laps” is a numerical value indicating how many turns the intermediate transfer belt 71 is rotated to switch the developing device. Here, the number of continuous formation rounds is 2, that is, the developing device is switched every time the intermediate transfer belt 71 makes two rounds. However, the present invention is not limited to this. In the previous embodiment, the “continuous number of sheets” is set, but as will be described later, in this embodiment, the number of pages of the toner image formed is the operation mode even if the number of rotations of the intermediate transfer belt 71 is the same. Therefore, the switching timing is controlled not by the number of pages but by the number of laps. Of course, it may be controlled by the number of pages.

  Subsequently, the rotary developing unit 4 is rotated to position one of the four developing devices at the developing position (step S402), and a toner image for one page is formed at the normal position on the intermediate transfer belt 71 (step S402). Step S403). The “normal position” is the same position as the position where a toner image is formed in the monochrome single-print mode in which the developer is not switched. If there is no page to be formed next, the operation is terminated (step S404).

  If there is an image after the next page, it is determined whether or not the current number of laps has reached the previously set number of consecutive laps (step S405). If the number of rotations has not reached the number of consecutive formation cycles, the process returns to step S403 to form a toner image for the next page. If the number of laps has reached the number of continuously formed laps, it is necessary to switch the developing device during the lap. Therefore, it is determined how much the rotary developing unit 4 should be rotated for switching the developing device (step S406).

  In the monochrome four-print mode, when switching from one developing device to the next developing device, the developing device to be newly positioned at the developing position can be selected from the other three. In this case, the simplest method is to use a developing device mounted at a position adjacent to the upstream side of the current developing device in the rotation direction of the developing unit 4. In this case, the developing device can be switched by rotating the developing unit 4 by 90 degrees. On the other hand, there are cases where it is desired to use a developing device mounted at a position other than the upstream adjacent position. For example, there is a case where the developing device installed in the upstream adjacent position has run out of toner, or the toner of a specific developing device is preferentially used up. In such a case, it is necessary to rotate the developing unit 4 by 180 degrees or 270 degrees. Thus, when the rotation amount of the developing unit 4 increases, the time required for switching also increases. In this embodiment, the subsequent operation is changed according to the length of time required for switching the developing device, that is, depending on the rotation amount of the developing unit 4 at the time of switching. To be able to cope with it. This concept can be similarly applied to all devices that switch and use at least three developing devices.

  When the rotation amount of the rotary developing unit 4 required for switching is 90 degrees, that is, when switching to the developing device in the upstream adjacent position, the operation is the same as that in the monochrome four-print mode in the first embodiment. That is, as shown in FIG. 8C, by changing the formation position of the toner image immediately before the switching forward from the normal position, the interval with the next formed toner image is widened (step S407). Then, after switching the developing device (step S408), the toner image on the next page and thereafter is continuously formed.

  On the other hand, when a rotation of 180 degrees or 270 degrees is required, a longer switching time is required, so that it is not possible to cope only by adjusting the toner image formation position. Therefore, in this case, instead of changing the toner image forming position, the area for one page of the toner image on the intermediate transfer belt 71 is set as a blank area in which the toner image is not carried. As a result, a large interval is created between the toner images by combining one page of the toner image and the portions between the images on both sides thereof. Using this interval, the developing device is switched (step S411). Subsequently, toner images on the next page and thereafter are formed.

  FIG. 11 is a view showing the arrangement of toner images in the third embodiment. When the rotation amount of the developing unit 4 necessary for switching the developing device is 90 degrees, as shown in FIG. 11A, the toner image immediately before the switching is in front of the normal position (position indicated by the broken line) (in the drawing). It is formed by changing to the left). The arrangement of the toner images I31 to I36 and their distances d31 and d32 are the same as in the monochrome four-print mode in the first embodiment. In this operation, the switching of the developing device is limited to the one at the adjacent position, but the throughput is high.

  On the other hand, when the rotation amount of the developing unit 4 is 180 degrees or 270 degrees, the toner images I71 to I75 are formed at the normal positions as shown in FIG. A blank page Iv is provided at a position where a toner image is to be formed at the end of the circumference (the number of consecutive formations), and no toner image is formed here. The intervals d71 and d72 between the toner images and the blank pages are the same as the intervals d31 and d32 in the monochrome single print mode. As a result, a large interval d70 is generated between the toner images I73 and I74. It is possible to perform a large switching of the rotation amount. In this operation, the throughput is lowered, but the degree of freedom in switching the developing devices is high.

  As described above, in this embodiment, one of the operations in which the arrangement of toner images on the intermediate transfer belt 71 is different from each other is selected and executed according to the amount of rotation of the developing unit 4 necessary for switching the developing device. . In this way, each developing device can be used efficiently with a high degree of freedom in switching the developing devices while minimizing a decrease in throughput. Further, since the interval between the toner images is not reduced even when the developing device is switched, the jam suppression effect is high. In determining the length of the intermediate transfer belt 71, only the size of the toner image and the interval necessary for jamming have to be considered, and it is not necessary to provide an extra interval for switching. It can be configured in a small size.

  In this embodiment, when there is a high risk of jamming, the toner image originally formed on the intermediate transfer belt 71 by two pages is reduced to one page to widen the interval between the toner images, and the jam is further reduced. You may make it prevent reliably.

(4) Fourth Embodiment Next, a fourth embodiment of the image forming apparatus according to the present invention will be described. The monochrome four-print mode in the fourth embodiment is common to some operations in the third embodiment described above in that the developer is switched by providing a blank page without changing the toner image formation position. ing. However, in the fourth embodiment, it is selected whether the blank page is for one page or two pages.

  FIG. 12 is a flowchart showing a monochrome four-print mode in the fourth embodiment. Among these, the processing in each step of steps S501 to S506 is the same as the processing of steps S401 to S406 in the operation of the third embodiment (FIG. 10), and thus the description thereof is omitted. When the rotation amount of the developing unit 4 necessary for switching the developing device is 90 degrees, as in step S411 in the third embodiment, one blank page is provided, and the developing device is switched during this period. On the other hand, when the rotation amount of the developing unit 4 is 180 degrees or 270 degrees, two blank pages are continuously provided, and the developing unit 4 is switched during this period (step S511). Such an operation is particularly effective when the interval between toner images is originally set narrow or the size of the toner image is small.

  FIG. 13 is a view showing the arrangement of toner images in the fourth embodiment. Here, a case is considered where three pages of toner images corresponding to the B5 size are formed on the intermediate transfer belt 71. In such an arrangement, the intervals d81 to d83 between the toner images I81, I82,... Are smaller than those in the above-described embodiments, and the sizes of the toner images I81, I82,. However, since the time required for switching the developing device is determined regardless of the size and interval of the toner image, there is a possibility that the developing device cannot be switched simply by adjusting the toner image forming position. In addition, it may not be possible to cope with switching with a large amount of rotation just by providing one blank page. Therefore, in this embodiment, when the rotation amount of the developing unit 4 for switching the developing device is 90 degrees, a blank page Iv for one page is provided as shown in FIG. The interval d80 is secured. On the other hand, when the rotation amount of the developing unit 4 for switching the developing device is 180 degrees or 270 degrees, as shown in FIG. 13B, each of the toner images I91 to I97 is different from the case of FIG. Although formed at the same position (d91 = d81, d92 = d82, d93 = d83), blank pages Iv1, Iv2 for two pages are provided between the toner images I94, I95 before and after switching, so a larger interval d90. Can be freed up.

  As described above, in this embodiment, operation modes having different consecutive numbers of blank pages provided for switching of developing devices are provided, and these are selectively executed. By doing so, as in the case of the third embodiment, each developing device can be used efficiently with a high degree of freedom in switching the developing devices while minimizing a decrease in throughput.

  The present invention is not limited to the above-described embodiments, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, combinations of operation modes are not limited to those shown in the above embodiments. The various toner image arrangements shown in the embodiments may be appropriately combined and executed.

  In the above embodiment, the sheet detection sensor 88 provided in the vicinity of the gate roller 81 in front of the secondary transfer region TR2 on the transport path F detects the presence or absence of a sheet or the occurrence of a jam at the position. Yes. However, the present invention is not limited to this, and other positions on the conveyance path, for example, positions immediately after exiting the cassette 8, such as between the gate roller 81 and the secondary transfer area TR2, and behind the secondary transfer area TR2. The presence / absence of a sheet or the occurrence of a jam may be detected based on the output of one sensor or a combination of outputs of a plurality of sensors. The jam suppressing effect according to the present invention is effective for a jam occurring at any position on the transport path.

  In the fourth embodiment (FIG. 13) described above, three pages of toner images are formed for one turn of the intermediate transfer belt 71, and when two blank pages are provided, they are arranged on the same turn. Although the two image forming areas are used as blank pages, the two blank pages may be provided in two image forming areas sandwiching the joint 711 of the intermediate transfer belt 71. Further, in this embodiment, when it is determined that the risk of jam occurrence is high, the number of toner images formed on the intermediate transfer belt 71 is reduced from 3 to 2 to widen the interval between the toner images, and the jam suppression effect is enhanced. It may be.

  In addition, each of the above embodiments is an apparatus including a single cassette 8. In this type of apparatus, a plurality of cassettes can be mounted, and a sheet S can be supplied from a manual feed tray. The present invention can be applied to these apparatuses. However, in such an apparatus, due to the difference in the lengths of the respective conveyance paths, the likelihood of jamming varies depending on where the sheet is supplied. Therefore, the arrangement of the toner images on the intermediate transfer belt 71 may be varied depending on the conveyance path. In general, it is considered that a jam due to a conveyance delay is likely to occur as the conveyance path is long. Therefore, it is preferable to select an operation mode in which the jam suppression effect is high as the conveyance path is long.

  In each of the above embodiments, the present invention is applied to an image forming apparatus provided with an intermediate transfer belt 71 as an image carrier. However, such an intermediate transfer member is not provided, and the photosensitive member is directly applied to the recording material. The present invention can also be applied to an apparatus configured to transfer a toner image. In this case, the photoreceptor functions as the “image carrier” of the present invention. Further, for example, the present invention can be applied to an apparatus including an intermediate transfer member other than the intermediate transfer belt, for example, an intermediate transfer drum or an intermediate transfer sheet.

  In addition to the monochrome four-print mode, the image forming apparatus according to each of the above embodiments is configured to be able to execute a color print mode and a monochrome one-print mode. For the purpose of the present invention, the color print mode In addition, the monochrome single print mode is not essential. Therefore, the present invention can be applied to any apparatus that includes a plurality of developing devices of the same color and forms a monochrome image of a plurality of pages while switching between them even if these printing modes are not provided. Further, the toner color of the monochrome image is not limited to black, and the number of developing devices is not limited to four.

  Further, the present invention is not limited to the electrophotographic image forming apparatus as in each of the above embodiments, but is applied to other systems, for example, an apparatus that forms an image by flying toner onto a transfer medium. Is possible.

1 is a diagram illustrating a first embodiment of an image forming apparatus according to the present invention. FIG. FIG. 2 is an external perspective view of the image forming apparatus in FIG. 1. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. FIG. 3 is a schematic diagram showing a stop position of the developing unit 4. FIG. 3 is a development view illustrating a configuration of an intermediate transfer belt. 6 is a flowchart showing an image forming operation in this apparatus. The flowchart which shows monochrome 4 print mode. FIG. 4 is a diagram illustrating an arrangement of images on an intermediate transfer belt. FIG. 10 is a diagram illustrating an arrangement of toner images according to a second embodiment. 10 is a flowchart illustrating a monochrome four-print mode in the third embodiment. FIG. 10 is a diagram illustrating an arrangement of toner images in a third embodiment. 10 is a flowchart illustrating a monochrome four-print mode in a fourth embodiment. FIG. 10 is a diagram illustrating an arrangement of toner images according to a fourth embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 4 ... Developing unit (positioning means, image forming means), 6 ... Exposure unit (image forming means), 10 ... Engine controller (image forming means), 22 ... Photoconductor (image forming means), 71 ... Intermediate transfer belt (image) Carrier), 81 to 83... Roller (conveying means), 88... Sheet detecting sensor (detecting means)

Claims (8)

An endless image carrier configured to carry a toner image on its surface and moving around in a predetermined direction;
Image forming means for forming a toner image and carrying the toner image on the surface of the image carrier;
Conveying means for conveying the sheet-like recording material along a predetermined conveying path from the image carrier to a transfer position to which the toner image is transferred;
Detecting means for detecting a conveyance state of the recording material on the conveyance path;
A control means for controlling the operation of the image forming means,
When the toner image of a plurality of pages is continuously formed, the control unit changes a formation position of each toner image on the image carrier formed by the image forming unit according to a detection result of the detection unit. An image forming apparatus. The detection means is configured to detect the presence or absence of a jam of the recording material in the transport path,
The control unit sets the toner image formation position so that the interval between the toner images on the image carrier becomes wider as the frequency of the jam detected by the detection unit increases. The image forming apparatus described. The detection means is configured to detect the presence or absence of the recording material passing through a predetermined detection position on the transport path,
The control means is configured so that the interval between the toner images on the image carrier increases as the time required for the recording material obtained from the detection result of the detection means to pass through the detection position is longer. The image forming apparatus according to claim 1, wherein a formation position of the toner image is set.   The image carrier is capable of simultaneously carrying a plurality of pages of the toner image on the surface thereof, and the control unit forms a toner image formed on the image carrier in accordance with a detection result of the detection unit. The image forming apparatus according to claim 1, wherein the number is set. A plurality of developing units that store toner of the same color can be mounted, and further includes positioning means that selectively moves and positions one of the mounted developing units to a predetermined developing position,
As the image forming operation for continuously forming toner images of a plurality of pages, the control unit forms a predetermined number of pages of toner images using one developing unit, and then switches the developing unit by the positioning unit to perform the next. 5. The image forming apparatus according to claim 1, wherein a switching image forming operation for forming a toner image on the next page and thereafter is executed using the developing unit. The switching distance is the distance that the surface of the image carrier moves while the switching time required for switching the developing unit located at the developing position from one developing unit to another developing unit by the positioning means has elapsed. When
The control means includes the switching distance other than the interval between the toner images formed immediately before and after the switching of the developing device among the intervals between adjacent toner images along the moving direction of the image carrier. The image forming apparatus according to claim 5, wherein the image forming apparatus is less than 6.   The image forming apparatus according to claim 5, wherein the control unit controls the timing of executing the switching of the developing device based on a detection result of the detection unit. An image forming step of sequentially forming a plurality of pages of toner images on an endless image carrier that circulates in a predetermined direction;
A transfer step of sequentially transferring the plurality of toner images formed in the image forming step to each of the sheet-like recording materials conveyed one by one along a predetermined conveyance path;
An image forming method comprising: changing and setting a formation position of the toner image on the image carrier in the image forming step according to a conveyance state of the recording material on the conveyance path.

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