JP2008276051A - Image forming apparatus and control method thereof - Google Patents

Abstract

An image forming apparatus capable of controlling the correction of an operation state of the image forming apparatus with higher accuracy and a control method thereof.
Photosensitive drums 11a to 11d and an intermediate transfer belt 31 onto which toner images formed on the photosensitive drums 11a to 11d are superimposed and transferred, and the toner images superimposed and transferred to the intermediate transfer belt 31 are recorded. An image forming apparatus that forms an image by transferring to a material P. The intermediate transfer belt 31 is provided with a line 129, and mark detection sensors 122 and 123 that detect the line 129 and detection information of the mark detection sensors 122 and 123. Therefore, the conveyance speed control unit 200 that corrects the conveyance speed of the intermediate transfer belt 31 and the timing period when the mark detection sensors 122 and 123 detect the mark do not overlap with the generation timing of the load fluctuation that occurs in the intermediate transfer belt 31. And an image formation timing control means for controlling the image formation timing as described above. Forming apparatus.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine, and a control method thereof. In particular, the present invention relates to an image forming apparatus employing an electrostatic recording method, an electrophotographic recording method, and the like, and a control method thereof.

  Conventionally, various types of color image forming apparatuses have been proposed. One of them is an intermediate transfer type color image forming apparatus. In this color image forming apparatus, an image forming unit is provided for each color developer, and each image forming unit forms a visible image of each color on a photosensitive drum as an image carrier. Then, these visible images are sequentially transferred (primary transfer) onto a conveyance body (intermediate transfer body) that performs endless circulation motion (circular drive), so-called intermediate transfer belt, and batch transfer (secondary transfer) to the supplied recording paper. After fixing, a color image is obtained.

  In such an intermediate transfer type image forming apparatus, a visible image of each color on the photosensitive drum is superimposed on the intermediate transfer belt to obtain one color image. For this reason, some image forming apparatuses include a correction unit that forms a registration correction pattern image of each color on an intermediate transfer belt and detects the amount of deviation to correct the color deviation.

  One of the main causes of color misregistration is due to the speed fluctuation of the intermediate transfer belt. Therefore, a technique for correcting the operation state of the image forming apparatus by detecting the speed of the intermediate transfer belt has been proposed. For example, two marks are provided on the intermediate transfer belt with an interval in the belt conveyance direction, the time for which this mark passes the mark detection sensor is detected, and the conveyance speed of the intermediate transfer belt is determined from the two mark passage times. There is something to calculate.

  In this case, when the temperature rise in the image forming apparatus, which is a factor of fluctuation in the speed of the intermediate transfer belt, occurs, the intermediate transfer belt also expands and the distance between the two marks also changes. For this reason, accurate belt speed fluctuations could not be detected.

  On the other hand, the following belt conveying apparatus is proposed (for example, refer patent document 1). That is, two sensors are arranged at intervals in the conveyance direction of the intermediate transfer belt, the conveyance speed of the intermediate transfer belt is detected from the time interval at which the two sensors detect the marks, and the conveyance speed becomes constant. Thus, the driving speed of the intermediate transfer belt is controlled.

Further, the conveyance speed of the intermediate transfer belt is detected from the time interval at which the two sensors detect the mark, the speed after color misregistration correction is stored, and the subsequent conveyance speed of the intermediate transfer belt is stored. There has been proposed a correction to match (see, for example, Patent Document 2).
Japanese Patent No. 3344614 JP 2005-156877 A

  In the conventional example, the conveyance speed of the intermediate transfer belt is constantly detected from the time interval at which the two sensors detect the mark, and the conveyance speed of the intermediate transfer belt is made constant by correcting the rotation speed of the driving roller of the intermediate transfer belt. To keep. According to this, for example, steady fluctuations in the conveyance speed of the intermediate transfer belt caused by expansion of the intermediate transfer belt due to temperature rise in the image forming apparatus or change in the outer diameter of the driving roller that drives the intermediate transfer belt can be corrected. is there.

  However, the intermediate transfer belt speed fluctuation includes not only the above-described steady speed fluctuation but also an instantaneous intermediate transfer belt conveyance speed fluctuation caused by a temporary load fluctuation. Examples of the load fluctuation that occurs in the intermediate transfer belt include a primary transfer operation in which a visible image formed on the photosensitive drum during the image forming process is transferred to the intermediate transfer belt.

  In the primary transfer portion, the photosensitive drum and the intermediate transfer belt form a nip portion. As a primary transfer operation, when a high voltage is applied to a primary transfer device disposed at a position facing the photosensitive drum across the intermediate transfer belt, the intermediate transfer is performed between the photosensitive drum and the intermediate transfer by an electric force (Coulomb force). The frictional force with the belt changes. As a result, load fluctuation occurs in the intermediate transfer belt.

  In such a case, if the instantaneous speed fluctuation caused by the load fluctuation is detected by the sensor and the driving roller speed of the intermediate transfer belt is corrected based on the detected conveyance speed, the speed of the intermediate transfer belt is changed abruptly. Can be. As a result, there is a problem that the conveyance speed of the intermediate transfer belt becomes unstable.

  The present invention has been made under such circumstances, and an object thereof is to provide an image forming apparatus capable of controlling the correction of the operation state of the image forming apparatus with higher accuracy and a control method therefor.

  The present invention has been made to solve the above-described problems, and includes the following configuration.

  (1) A plurality of image carriers and an intermediate transfer member onto which the developer images formed on the plurality of image carriers are superimposed and transferred, and the developer images superimposed and transferred onto the intermediate transfer member An image forming apparatus for forming an image by transferring to a recording medium, wherein a mark is provided on the intermediate transfer body, and a first mark detecting unit and a second mark detecting unit for detecting the mark, and the first mark Based on the detection information of the detection means and the second mark detection means, the conveyance speed correction means for correcting the conveyance speed of the intermediate transfer member, and the second mark detection from the timing when the first mark detection means detects the mark. Image formation for controlling the image formation timing so that the period until the timing at which the means detects the mark does not overlap with the timing at which the load fluctuation that occurs in the intermediate transfer member during image formation occurs An image forming apparatus comprising: the timing control means.

(2) a plurality of image carriers, an intermediate transfer member having a mark, and a developer image formed on each of the plurality of image carriers, and a first mark detector for detecting the marks And a second mark detection means, and a control method of an image forming apparatus for forming an image by transferring a developer image superimposed and transferred to the intermediate transfer member to a recording medium,
A conveyance speed correction step of correcting the conveyance speed of the intermediate transfer body based on detection information of the first mark detection means and the second mark detection means;
The period from the timing at which the first mark detection means detects the mark to the timing at which the second mark detection means detects the mark overlaps with the timing at which load fluctuations that occur in the intermediate transfer body during image formation occur. And an image forming timing control step for controlling the image forming timing so as not to occur.

  According to the present invention, it is possible to provide an image forming apparatus capable of controlling the correction of the operation state of the image forming apparatus with higher accuracy and a control method therefor.

  The best mode for carrying out the present invention will be described based on the following examples.

  However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. .

  The image forming apparatus according to the present embodiment will be described below with reference to the drawings. As an example of the image forming apparatus according to the present embodiment, a four-drum type color image forming apparatus that employs an electrophotographic method and includes a plurality of image forming units and uses an intermediate transfer belt as an intermediate transfer member; did.

  FIG. 1 is a schematic cross-sectional view of the image forming apparatus of this embodiment.

  The image forming apparatus 1 is provided with an image output unit 1P. The image output unit 1P is roughly divided into an image forming unit 10 (four image forming units (stations) a, b, c, and d having the same configuration are arranged in parallel), a paper feeding unit 20, an intermediate transfer unit 30, a fixing unit. It comprises a unit 40 and a control unit (not shown).

  Here, each part and unit will be described.

  Photosensitive drums 11a, 11b, 11c, and 11d (image bearing members) that are rotationally driven in the direction of arrow C in the drawing are pivotally supported at the center of the image forming unit 10. Each of the photoconductive drums 11a to 11d is opposed to the outer peripheral surface thereof, and in order of rotation, primary chargers 12a, 12b, 12c, and 12d, exposure units 13a, 13b, 13c, and 13d (optical system), a developing device. 14a, 14b, 14c, 14d are arranged.

  The photoreceptor drums 11a to 11d are given a uniform charge amount of charge on their surfaces by the primary chargers 12a to 12d. Next, light beams such as laser beams modulated according to the recording image signal are exposed on the photosensitive drums 11a to 11d by the exposure units 13a to 13d, and electrostatic latent images are formed on the photosensitive drums 11a to 11d. Is done. Each electrostatic latent image is visualized as a toner image (developer image) by developing devices 14a to 14d that respectively store toners (developer) of four colors of yellow, cyan, magenta, and black.

  The photosensitive drums 11 a to 11 d and the intermediate transfer belt 31 abut on the downstream side of the developing devices 14 a to 14 d in the rotation direction of the photosensitive drum, and the toner images formed on the photosensitive drums 11 a to 11 d are transferred to the intermediate transfer belt 31. Primary transfer portions Ta, Tb, Tc, and Td to be transferred are formed. On the downstream side of the photosensitive drum rotation direction of the primary transfer portions Ta to Td, the toner remaining on the photosensitive drums 11a to 11d is scraped off by the cleaning devices 15a, 15b, 15c, and 15d, and the photosensitive drums 11a to 11d. The surface of is cleaned.

  Through the processes described above, toner image formation with each toner is sequentially performed.

  On the other hand, the paper feed unit 20 includes cassettes 21a and 21b and a manual feed tray 27 for storing the recording material P (recording medium). The cassettes 21a and 21b and the manual feed tray 27 are provided with pickup rollers 22a, 22b and 26 for feeding the stored recording materials P one by one. The recording material P sent out from each of the pickup rollers 22 a, 22 b, 26 is conveyed by the paper feed roller pair 23 through the paper feed guide 24. Then, the recording material P is conveyed to registration rollers 25 a and 25 b for sending the recording material P to the secondary transfer unit Te in accordance with the image forming timing in the image forming unit 10.

  The intermediate transfer unit 30 has an intermediate transfer belt 31 (intermediate transfer member). Further, the intermediate transfer unit 30 includes a driving roller 32 that transmits a driving force to the intermediate transfer belt 31 and a backup roller 62 provided to face the registration mark detection sensors 60 and 61. Further, a tension roller 33 that applies an appropriate tension to the intermediate transfer belt 31 by an urging force of a pressure mechanism such as a spring (not shown), and a position that opposes a secondary transfer device 36 that will be described later with the intermediate transfer belt 31 interposed therebetween. And a secondary transfer inner roller 34. The intermediate transfer belt 31 is wound around a driving roller 32, a backup roller 62, a tension roller 33, and a secondary transfer inner roller 34. As the material of the intermediate transfer belt 31, for example, PI [polyimide] or PVdF [polyvinylidene fluoride] is selected.

  A primary transfer plane A is formed between the drive roller 32 and the backup roller 62. The driving roller 32 is formed by coating the surface of a metal roller with rubber (urethane or chloroprene) having a thickness of several millimeters, thereby preventing slippage with the intermediate transfer belt 31. The drive roller 32 is rotationally driven by a pulse motor (not shown). The alignment of the tension roller 33 urged by a pressure mechanism (not shown) can be adjusted, and the meandering of the intermediate transfer belt 31 can be corrected.

  In the primary transfer portions Ta to Td where the photosensitive drums 11a to 11d and the intermediate transfer belt 31 face each other, the primary transfer devices 35a, 35b, and 35c are located at positions facing the photosensitive drums 11a to 11d with the intermediate transfer belt 31 interposed therebetween. , 35d are arranged. Further, a secondary transfer device 36 is disposed at a position facing the secondary transfer inner roller 34 with the intermediate transfer belt 31 in between, and a secondary transfer portion Te is formed.

  A cleaning device 50 for cleaning the toner image carrying surface of the intermediate transfer belt 31 is disposed downstream of the secondary transfer portion Te in the conveyance direction of the intermediate transfer belt 31 (in the direction of arrow B in the figure). The cleaning device 50 includes a cleaner blade 51 and a waste toner box 52 that stores waste toner. As the material of the cleaner blade 51, polyurethane rubber or the like is used.

  The fixing unit 40 includes a fixing roller 41a having a heat source such as a halogen heater inside, and a pressure roller 41b in pressure contact with the fixing roller 41a. The pressure roller 41b may also be provided with a heat source. The fixing unit 40 also feeds the recording material P discharged from the fixing roller 41a and the pressure roller 41b to the conveyance guide 43 for guiding the recording material P to the nip portion between the fixing roller 41a and the pressure roller 41b. An inner discharge roller 44, an outer discharge roller 45, and the like.

  The control unit is composed of a control board 70 for controlling the operation of each part and the mechanism in the unit, a motor drive board (not shown), and the like.

  Next, the operation of the image forming apparatus will be described.

  When the image forming operation start signal is issued, first, for example, the recording material P is sent out one by one from the cassette 21a by the pickup roller 22a. The recording material P is guided between the paper feed guides 24 by the paper feed roller pair 23 and conveyed to the registration rollers 25a and 25b. At this time, the registration rollers 25a and 25b are stopped, and the leading edge of the recording material P abuts against a nip portion formed by the registration rollers 25a and 25b.

  Thereafter, the registration rollers 25a and 25b start rotating in accordance with the timing at which the image forming unit 10 starts image formation. The rotation start timing of the registration rollers 25a and 25b is set so that the recording material P and the toner image primarily transferred from the image forming unit 10 to the intermediate transfer belt 31 are exactly the same at the secondary transfer unit Te. .

  On the other hand, when an image forming operation start signal is issued, the image forming unit 10 forms a toner image on the photosensitive drum 11d on the most upstream side in the rotation direction of the intermediate transfer belt 31 through the process described above. The formed toner image is primarily transferred to the intermediate transfer belt 31 at the primary transfer portion Td by the primary transfer device 35d to which a high voltage is applied. Then, the toner image primarily transferred to the intermediate transfer belt 31 is conveyed to the next primary transfer portion Tc. On the next photosensitive drum 11c on the downstream side in the transport direction of the intermediate transfer belt 31, image formation is performed with a delay by a time during which the toner image is transported between the stations. The toner image formed on the photosensitive drum 11c is transferred with the resist aligned on the image transferred from the photosensitive drum 11d to the intermediate transfer belt 31. Then, the same process is repeated, and finally the four color toner images are superimposed and primarily transferred to the intermediate transfer belt 31. The superimposed color toner image is conveyed to the secondary transfer portion Te.

  Thereafter, when the recording material P enters the secondary transfer portion Te and contacts the intermediate transfer belt 31, a high voltage is applied to the secondary transfer device 36 in accordance with the passing timing of the recording material P. Then, the four color toner images formed on the intermediate transfer belt 31 by the process described above are transferred to the recording material P. The recording material P to which the toner image has been transferred is guided to the nip portion between the fixing roller 41 a of the fixing unit 40 and the pressure roller 41 b by the conveyance guide 43. The toner image is fixed on the recording material P by the heat of the fixing roller 41 a and the pressure roller 41 b and the pressure of the nip, and the recording material P on which the toner image is fixed is conveyed by the inner discharge roller 44 and the outer discharge roller 45. Discharged outside the machine.

  Next, control for detecting the conveyance speed of the intermediate transfer belt 31 and correcting the conveyance speed to a predetermined value will be described.

  The image forming apparatus 1 has a problem that color misregistration occurs due to temperature fluctuations in the apparatus. Among the factors of color misregistration due to temperature fluctuation, the factor that most affects color misregistration is a change in diameter due to temperature fluctuation of the driving roller 32 of the intermediate transfer belt 31. When the outer diameter of the driving roller 32 changes due to the temperature rise, the driving roller 32 is driven at a constant rotation speed, so that the conveyance speed of the intermediate transfer belt 31 changes constantly. For this reason, the image forming position in each image forming unit and the position of the image of the other color conveyed by the intermediate transfer belt 31 do not match, and a color misregistration image is generated.

  In order to prevent this, in this embodiment, a speed detector 120 (speed detector) for detecting the conveyance speed of the intermediate transfer belt 31 is provided. As illustrated in FIG. 2, the speed detection unit 120 holds a mark detection sensor 122 (second mark detection unit) and a mark detection sensor 123 (first mark detection unit) on the support member 121, and the intermediate transfer belt 31. It is arrange | positioned facing the inner surface of. Further, a line 129 (mark) previously formed of immortal ink is provided on the inner surface of the intermediate transfer belt 31, and the mark detection sensors 122 and 123 are configured to change the output signal when the line 129 passes. Has been.

  In this embodiment, the speed detection unit 120 is arranged between the secondary transfer inner roller 34 and the drive roller 32 as shown in FIG. 2, but any position where the conveyance speed of the intermediate transfer belt 31 can be detected. It may be placed anywhere in the device. However, since color misregistration occurs in the primary transfer portions Ta to Td where the toner images are transferred from the photosensitive drums 11a to 11d to the intermediate transfer belt 31, the speed detection unit 120 of the intermediate transfer belt 31 is located near the primary transfer portion. It is more preferable to arrange them.

  Next, the operation of the speed detection unit 120 will be described.

  As shown in FIG. 2, when the intermediate transfer belt 31 is conveyed, the line 129 passes through the opposing positions of the two mark detection sensors 122 and 123. At that time, line detection timing information (detection information) having a time interval corresponding to the distance between the sensors from the mark detection sensors 122 and 123 is output to the conveyance speed control unit 200 (conveyance speed correction means) as shown in FIG. Is done. The conveyance speed control unit 200 is provided in a control unit, for example.

  The line detection timing information output from the mark detection sensors 122 and 123 is input to the speed calculation circuit 126 of the transport speed control unit 200. The speed calculation circuit 126 calculates the conveyance speed of the intermediate transfer belt 31 from the difference Δt [s] in line detection timing between the mark detection sensors 122 and 123 and the design distance L [mm] between the mark detection sensors 122 and 123. The Then, based on the calculated conveyance speed, the correction amount calculation circuit 127 determines a conveyance speed correction amount that provides a predetermined conveyance speed. Based on the conveyance speed correction amount calculated by the correction amount calculation circuit 127, the control circuit 128 corrects the driving speed of the driving roller 32.

  The design distance L [mm] between the mark detection sensors 122 and 123 is an important parameter for calculating the conveyance speed of the intermediate transfer belt 31. When the design distance L [mm] between the mark detection sensors 122 and 123 has an error, accurate conveyance speed detection cannot be performed. Therefore, the design distance L [mm] between the mark detection sensors 122 and 123 needs to be managed with high accuracy. Therefore, in the present embodiment, the design distance L [mm] between the mark detection sensors 122 and 123 is managed by providing the mark detection sensors 122 and 123 on the support member 121.

  Next, image formation timing control in this embodiment will be described.

  As described above, by detecting the conveyance speed of the intermediate transfer belt 31 and correcting the conveyance speed, it is possible to correct a steady conveyance speed fluctuation of the intermediate transfer belt 31 due to temperature fluctuations in the apparatus. However, the momentary conveyance speed fluctuation due to the load fluctuation of the intermediate transfer belt 31 is also detected by the mark detection sensors 122 and 123 to correct the conveyance speed. In this case, the conveyance speed is based on the momentary conveyance speed fluctuation. The correction amount is determined. For this reason, the rotational speed of the driving roller 32 is rapidly changed, and the conveyance speed of the intermediate transfer belt 31 becomes unstable.

  As described above, the load fluctuation generated in the intermediate transfer belt 31 during the image forming job is a load fluctuation due to a change in the frictional force between the photosensitive drums 11a to 11d and the intermediate transfer belt 31 during the primary transfer operation of the toner image. is there. However, the load fluctuation occurrence timing of the intermediate transfer belt 31 that occurs during the image forming process (at the time of image formation) can be known in advance from the control sequence of the image forming process.

  Therefore, an unillustrated image formation timing control unit (image formation timing control means) prevents the mark detection sensors 122 and 123 from detecting the line 129 so that the timing of load fluctuation of the intermediate transfer belt 31 does not overlap. The image formation timing is controlled. The image forming timing unit is provided, for example, in the control unit.

  Image formation timing control in this embodiment will be described with reference to the flowchart of FIG. This processing is executed by a CPU (not shown) of the image forming apparatus using a RAM (not shown) as a work area while controlling each part based on a control program stored in a ROM (not shown). is there.

  When the job is started (step S401), the drive motors for the photosensitive drums 11a to 11d and the intermediate transfer belt 31 are turned on (step S402). In this state, since the conveyance speed of the intermediate transfer belt 31 is unknown, the image forming process is set in a standby state.

  Then, after the conveyance speed of the intermediate transfer belt 31 becomes a desired speed, it is confirmed whether or not the mark detection sensor 123 has detected the line 129 provided on the intermediate transfer belt 31 (step S403). When the mark detection sensor 123 detects the line 129, it is checked whether or not the mark detection sensor 122 has detected the line 129 provided on the intermediate transfer belt 31 (step S404). Then, after the mark detection sensor 123 and the mark detection sensor 122 detect the line 129, the image forming process is started after a certain timing (step S405). That is, the image forming process is not started at the timing when the line 129 is detected by the mark detection sensors 122 and 123. As described above, the timing at which the load fluctuation occurs after the start of the image forming process can be assumed in advance. Therefore, if the detection by the mark detection sensors 122 and 123 is finished before the load fluctuation occurs, the image forming process is controlled to start before the line 129 is detected by the mark detection sensors 122 and 123. You can also

  Then, after the image forming process is started, through each image forming step (step S406), the recording material P on which the toner image is fixed is obtained. Thereafter, the drive motors of the photosensitive drums 11a to 11d and the intermediate transfer belt 31 are turned off (step S407), and the job is ended (step S408).

  By controlling the start timing of the image forming process as described above, the load fluctuation occurrence timing of the intermediate transfer belt 31 generated during the image forming process does not overlap (different) within the detection timing of the line 129 by the mark detection sensors 122 and 123. You can For this reason, it is possible to avoid the conveyance speed of the intermediate transfer belt 31 from becoming unstable. Further, since the conveyance speed of the intermediate transfer belt 31 can be corrected to a predetermined value before the image forming process is started, a reduction in color misregistration can be expected.

  As described above, the image forming apparatus according to the present exemplary embodiment performs control so that the mark passing time between the two mark detection sensors is different from the load fluctuation timing generated in the intermediate transfer belt during the image forming process. As a result, it is possible to avoid an unstable transfer speed of the intermediate transfer belt due to instantaneous load fluctuations and to realize an image forming apparatus with reduced color misregistration.

Schematic sectional view of the image forming apparatus of Example 1 Schematic configuration diagram of speed detector Block diagram of control unit of image forming apparatus Flowchart explaining image formation timing control

Explanation of symbols

P Recording material (compatible with recording media)
11a, 11b, 11c, 11d Photosensitive drum (corresponding to image carrier)
31 Intermediate transfer belt (compatible with intermediate transfer members)
122 Mark detection sensor (corresponding to the second mark detection means)
123 Mark detection sensor (corresponding to the first mark detection means)
129 lines (corresponding to the mark)
200 Conveying speed control unit (corresponding to conveying speed correcting means)

Claims (4)

A plurality of image carriers and an intermediate transfer member onto which the developer images formed on the plurality of image carriers are superimposed and transferred, and the developer images superimposed and transferred onto the intermediate transfer member are used as recording media An image forming apparatus for transferring and forming an image,
The intermediate transfer member is provided with a mark,
First mark detection means and second mark detection means for detecting the mark;
A conveyance speed correction unit that corrects a conveyance speed of the intermediate transfer member based on detection information of the first mark detection unit and the second mark detection unit;
The period from the timing at which the first mark detection means detects the mark to the timing at which the second mark detection means detects the mark overlaps with the timing at which load fluctuations that occur in the intermediate transfer body during image formation occur. An image forming apparatus comprising: an image forming timing control unit that controls an image forming timing so as not to occur. The image forming apparatus according to claim 1,
The load fluctuation generated in the intermediate transfer member is caused by a change in frictional force between the image carrier and the intermediate transfer member, which occurs when the developer image is transferred from the image carrier to the intermediate transfer member. An image forming apparatus, comprising: The image forming apparatus according to claim 1, wherein
The image forming timing control unit controls the image forming operation to start after the first mark detecting unit detects the mark and then the second mark detecting unit detects the mark. Image forming apparatus. A plurality of image carriers, an intermediate transfer member having a mark, onto which a developer image formed on each of the plurality of image carriers is superimposed and transferred, a first mark detection unit for detecting the mark, and a second mark A method of controlling an image forming apparatus, comprising: a mark detection unit;
A conveyance speed correction step of correcting the conveyance speed of the intermediate transfer body based on detection information of the first mark detection means and the second mark detection means;
The period from the timing at which the first mark detection means detects the mark to the timing at which the second mark detection means detects the mark overlaps with the timing at which load fluctuations that occur in the intermediate transfer body during image formation occur. And an image forming timing control step for controlling the image forming timing so as not to occur.

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