JP2001142280A - Image forming apparatus and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device and an image forming method by which a high-quality image is formed. SOLUTION: An operation sequence is set so that a secondary transfer roller 48 and a cleaner blade 491 do not abut on/separate from an intermediate transfer belt 41 until primary transfer is completed after a vertical synchronizing signal is outputted from a read sensor for vertical synchronization 40. That is secondary transfer processing and cleaning processing are executed after completing the primary transfer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus and an image forming method for forming a color image by superimposing toners of a plurality of colors. A transfer step of transferring a toner image onto a transfer medium such as a transfer belt, an intermediate transfer drum, and a transfer sheet is performed for each toner color to form a color image on the transfer medium.

[0002]

2. Description of the Related Art An example of this type of image forming apparatus is shown in FIG. This image forming apparatus forms a full-color image by superimposing four color toners of yellow (Y), cyan (C), magenta (M), and black (K). When an image signal is given to a control unit (not shown), each part of the engine unit E is controlled in accordance with a command from the control unit to correspond to an image signal on a sheet member S such as a transfer sheet, a copy sheet or an OHP sheet. To form an image.

In the engine section E, a toner image can be formed on the photosensitive member 21 of the image carrier unit 2. That is, the image carrier unit 2 includes a photoconductor 21 rotatable in the direction of the arrow in FIG. 21, and further includes a charging roller 22 as a charging unit, a developing unit around the photoconductor 21 along the rotation direction. Developing units 23Y, 23C, 2
3M, 23K, and a photoreceptor cleaner blade 24 are disposed, respectively.

In this apparatus, the charging roller 22 is connected to the photosensitive member 2
After the outer peripheral surface of the photoreceptor 21 is uniformly charged by contacting the outer peripheral surface of the photoconductor 1, the laser light L is emitted from the exposure unit 3 toward the outer peripheral surface of the photoconductor 21. As shown in FIG. 1, the exposure unit 3 includes a light emitting element 31 such as a semiconductor laser that is modulated and driven in accordance with an image signal, and a laser beam L from the light emitting element 31 is rotated by a high-speed motor 32. Incident on a polygon mirror 33 to be formed. The laser light L reflected by the polygon mirror 33 is transmitted to the lens 34 and the mirror 3
5, the photosensitive drum 21 is scanned in the main scanning direction (a direction perpendicular to the plane of the drawing) to form an electrostatic latent image corresponding to an image signal. Reference numeral 36 denotes a horizontal synchronization reading sensor for obtaining a synchronization signal in the main scanning direction, that is, a horizontal synchronization signal HSYNC.

The electrostatic latent image formed in this way is
Is developed with toner. That is, in this embodiment, as the developing unit 23, a developing unit 23Y for yellow, a developing unit 23C for cyan, a developing unit 23M for magenta, and a developing unit 23K for black are arranged along the photoconductor 21 in this order. Have been. These developing units 23Y and 23
Each of C, 23M, and 23K is configured to be able to freely contact and separate from the photoconductor 21, and the four developing units 23Y, 23M, 23C, and 23 according to instructions from the control unit.
B selectively contacts the photoconductor 21 and applies a high voltage to apply a toner of a selected color to the surface of the photoconductor 21 to apply electrostatic latent on the photoconductor 21. Reveals an image.

The toner image developed by the developing unit 23 is primarily transferred onto the intermediate transfer belt 41 of the transfer unit 4 in a primary transfer region R1 located between the black developing device 23K and the photosensitive member cleaner blade 24. You. A cleaner blade 24 for the photoconductor is disposed at a position in the circumferential direction (in the direction of the arrow in FIG. 21) from the primary transfer region R1, and remains on the outer peripheral surface of the photoconductor 21 after the primary transfer. Scrape off toner.

Next, the configuration of the transfer unit 4 will be described. In this embodiment, the transfer unit 4 includes a roller 4
2 to 47; an intermediate transfer belt 41 stretched over these rollers 42 to 47; a secondary transfer roller 48 for secondary transferring the intermediate toner image transferred to the intermediate transfer belt 41 to the sheet member S; Photoconductor 21 and intermediate transfer belt 4
1 and a photoreceptor / belt drive unit (not shown) for rotating and driving the photoconductor 1 in synchronization. When the color image is transferred to the sheet member S, the color images are formed by superimposing the toner images of the respective colors formed on the photoreceptor 21 on the intermediate transfer belt 41. The sheet member S is taken out from the cassette 61, the manual feed tray 62 or an additional cassette (not shown) by the paper feeding unit 63 and is conveyed to the secondary transfer area R2. Further, a full-color image is obtained by secondarily transferring a color image to the sheet member S.

After the secondary transfer, toner remaining on the outer peripheral surface of the intermediate transfer belt 41 is removed by a cleaner blade 491 provided on a belt cleaner 49. That is, the belt cleaner 49
Is disposed opposite to the roller 46 with the intermediate transfer belt 41 interposed therebetween. At an appropriate timing, the cleaner blade 491 comes into contact with the intermediate transfer belt 41 to scrape toner remaining on the outer peripheral surface thereof. Drop.

A sensor 40 for detecting a reference position of the intermediate transfer belt 41 is disposed near the roller 43, and a synchronization signal in a sub-scanning direction substantially orthogonal to the main scanning direction, that is, a vertical synchronization signal. It functions as a vertical synchronization reading sensor for obtaining VSYNC.

The sheet member S on which the toner image has been transferred by the transfer unit 4 as described above is subjected to secondary transfer along a predetermined paper feed path (two-dot chain line) by the paper feed unit 63 of the paper feed / discharge unit 6. The toner image is conveyed to the fixing unit 5 disposed downstream of the region R2, and the conveyed toner image on the sheet member S is fixed on the sheet member S. Then, the sheet member S is further conveyed to the paper discharge section 64 along the paper supply path, and then discharged to the standard paper discharge tray.

[0011]

In the image forming apparatus constructed as described above, in order to superimpose a plurality of color toner images while registering each other, the vertical synchronizing reading sensor 40 outputs a vertical synchronizing read sensor 40. The synchronization signal VSYNC is used. That is, each time the vertical synchronization signal VSYNC is output, a toner image is formed on the photoconductor 21 at a predetermined timing, and then the toner image is formed on the intermediate transfer belt 41 that rotates at a constant conveyance speed in synchronization with the photoconductor 21. Is primary-transferred, and by controlling the registration in this way, toner images of a plurality of colors are accurately superimposed. Therefore, the intermediate transfer belt 41 needs to be rotated and conveyed at a constant speed in synchronization with the photosensitive member 21 from the output of the vertical synchronization signal VSYNC to the completion of the primary transfer.

However, as described above, the secondary transfer roller 48 is attached to the intermediate transfer belt 41 at an appropriate timing.
And the cleaner blade 491 temporarily abuts. Therefore, the rotation of the intermediate transfer belt 41 is hindered by the contact, the intermediate transfer belt 41 is elastically extended, and a load is applied to a belt driving unit (not shown) that rotationally drives the intermediate transfer belt 41. As a result, the intermediate transfer belt 41 cannot be rotated and conveyed at a constant speed. As a result, the secondary transfer roller 48 or the cleaner blade 491
Are separated from each other, the toner images are displaced from each other between the toner colors, that is, a registration shift occurs, and the image quality may be deteriorated.

The present invention has been made in view of the above problems, and has as its object to provide an image forming apparatus and an image forming method capable of forming a high-quality image.

[0014]

According to a first aspect of the present invention, a primary transfer process of transferring a toner image formed on a photoreceptor to a rotationally driven transfer medium is repeated for a plurality of different toner colors. An image forming apparatus for forming a color image on the transfer medium by superimposing toner images of respective toner colors, wherein in order to achieve the above-mentioned object, the image forming apparatus is provided so as to be able to freely contact and separate from the transfer medium, A secondary transfer means for temporarily abutting on the medium and secondary transferring the color image on the transfer medium to a sheet member; and A cleaning unit that abuts the transfer medium by contact, and a control unit that controls an operation of separating and contacting the secondary transfer unit and the cleaning unit with the transfer unit, wherein the control unit includes: After the primary transfer process for the final toner color among the plurality of toner colors is completed, the secondary transfer unit is brought into contact with the transfer unit to execute the secondary transfer process, and the cleaning unit is transferred to the transfer unit. The cleaning unit is configured to execute the cleaning process by contacting the cleaning unit.

According to a second aspect of the present invention, a sensor for detecting a reference position of the transfer medium is provided, and based on a reference signal output from the sensor, a primary transfer process corresponding to the reference signal is executed. The control unit controls the secondary transfer unit and the cleaning unit with respect to the transfer medium at a timing excluding the period from when the reference signal is output to when the primary transfer process is completed. Each is configured to contact and separate.

Further, according to a third aspect of the present invention, the control unit may temporarily stop the transfer medium when the secondary transfer unit and the cleaning unit repeat the primary transfer process, if necessary. The contact / separation timing is changed so as to be in contact with each other, and at least two or more of the plurality of toner colors in the sub-scanning direction for each toner color during the primary transfer process. The center of the swing width is configured to match each other.

Further, in the invention according to claim 4, the primary transfer process of transferring the toner image formed on the photoreceptor to a rotationally driven transfer medium is repeated for a plurality of different toner colors, and An image forming method for forming a color image on the transfer medium by superimposing toner images, and in order to achieve the above object, a primary transfer process for a final toner color of the plurality of toner colors is completed,
After a color image is formed on the transfer medium, a secondary transfer unit is brought into contact with the transfer unit to secondarily transfer the color image on the transfer medium to a sheet member. The transfer medium is cleaned by contacting the transfer medium.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention utilizes the basic principle first discovered by the present inventor, and the following describes the basic principle of the present invention.
The present embodiment using the basic principle will be described in detail.

A. Basic principle of the invention Conventionally, when the contacting means such as the secondary transfer roller or the cleaner blade is brought into contact with the intermediate transfer belt during the primary transfer process, a registration shift occurs and the image quality is reduced. The phenomenon of inviting was known. However, it has not been analyzed in detail what kind of technical matter it is based on, or the relationship between the separation / contact of the contact means and the amount of registration deviation. Therefore, FIG.
In order to improve the image quality by correcting the registration error based on the result of the detailed examination of the state of occurrence of the registration error when the first image forming apparatus is operated in the operation sequence shown in FIG. The resist control amount was determined.
Hereinafter, this will be described in detail with reference to FIGS.

FIG. 1 is a timing chart showing an example of an operation sequence in the image forming apparatus of FIG.
As shown in the figure, after the apparatus power is turned on or when the sleep mode of the image forming apparatus is released, the intermediate transfer belt 41 is rotated and conveyed, and the vertical synchronization reading sensor 40 is moved.
Outputs the vertical synchronization signal VSYNC intermittently. Then, the vertical synchronization signal VSYNC is output at timings VT1 to VT7,.
Each time is output, a yellow electrostatic latent image, a cyan electrostatic latent image, a magenta electrostatic latent image, and a black electrostatic latent image are formed on the photoreceptor 21 repeatedly in this order after a certain period of time. After the electrostatic latent image is formed, the developing units 23Y and 23
One of the developing devices M, 23C and 23B selectively comes into contact with the photoconductor 21 to make an electrostatic latent image on the photoconductor 21 visible,
The toner image is primarily transferred onto the intermediate transfer belt 41.
Accordingly, all the toner images of each color are formed at a predetermined position on the photoconductor 21, that is, a reference latent image forming position, and the intermediate transfer belt 41 that rotates in synchronization with the photoconductor 21 is formed.
Is primarily transferred at the same position (primary transfer processing for each toner color).

When the primary transfer process is repeated for four colors, the four color toner images are superimposed on the intermediate transfer belt 41 to form a color image. When the color image is obtained in this manner, the secondary transfer roller 48 abuts on the intermediate transfer belt 41 with the sheet member S interposed therebetween, and secondary-transfers the color image onto the sheet member S. At the same time, the cleaner blade 491 is activated according to the CB signal. The toner remaining in contact with the intermediate transfer belt 41 and remaining on the belt surface is removed. The sheet member S on which the color image is formed by repeating such an operation is sequentially discharged to the standard discharge tray.

This is an outline of the operation of the image forming apparatus according to the operation sequence shown in FIG. 1. The relationship between such an operation and the amount of registration deviation in the sub-scanning direction is examined. Gave different results. Such a difference is caused by the difference in the operation sequence. Hereinafter, the operation sequence for forming the first sheet (hereinafter, referred to as “first print sequence”) and the image formation for the second and subsequent sheets will be described. The operation sequence (hereinafter, referred to as “second printing sequence”) will be described separately. Also, in this type of apparatus, since there is a third print sequence associated with the idling process, this will also be described.

A-1. First Printing Sequence First, when the apparatus power is turned on (or the sleep mode of the image forming apparatus is released), the intermediate transfer belt 41
Are rotated and conveyed, and the vertical synchronization signal VSYNC is sequentially output from the vertical synchronization reading sensor 40 at timings VT1 to VT3. The yellow toner image Y1 is transferred to the intermediate transfer belt 41 as described above corresponding to the first timing VT1. Is primary-transferred, the cyan toner image C1 is primary-transferred onto the intermediate transfer belt 41 so as to overlap the yellow toner image Y1 corresponding to the timing VT2, and the magenta toner image M1 is further converted to the yellow toner image corresponding to the timing VT3. Y1
And an intermediate transfer belt 41 superimposed on the cyan toner image C1.
The primary transfer is performed. During this time, the cleaning process and the secondary transfer process of the intermediate transfer belt 41 are not performed, and the abutting unit (the secondary transfer roller 48 and the cleaner blade 49) is not used.
1) is separated from the intermediate transfer belt 41. Therefore, these three toner images Y1, C1, and M1 are all superimposed at the same position on the intermediate transfer belt 41, and are accurately registered in the sub-scanning direction. That is, FIG.
As shown in FIG. 3, these three toner images Y1, C1, M1
All of the transfer start positions coincide with the reference transfer start position, and their post-transfer end positions also coincide with the reference transfer post-end position. FIG. 1 (and FIG. 1 described later)
The dashed line in (0) indicates the primary transfer position where each toner image is transferred, and in the actual primary transfer process, the toner images are sequentially superimposed at this dashed line portion. For convenience, each toner image is shown separated in the vertical direction.

Next, at timing VT4, the vertical synchronizing signal VS
When the YNC is output, as shown in FIG.
Thereafter, a VIDEO signal is supplied to the exposure unit 3 to form an electrostatic latent image corresponding to the black toner image K1 at a predetermined reference latent image forming position in the same manner as other toner colors. I do. Then, the primary transfer processing is started at a point in time when a predetermined time T20 has elapsed from the output of the vertical synchronization signal VSYNC (timing VT4). At this point, the yellow toner image Y1 and the cyan toner image C
1 and the magenta toner image M1, the cleaner blade 491 is separated from the intermediate transfer belt 41, and as a result, as shown in FIG.
The transfer start position of No. 1 also coincides with the reference transfer start position similarly to the other toner images Y1, C1, M1. During the continuation of the separation, the conveyance speed V of the intermediate transfer belt 41 is constant, and the black toner image K1 is superposed while being accurately registered with the other toner images Y1, C1, and M1 that have already been primarily transferred. Go.

However, at a certain point in time when the black toner image K1 reaches the latter half of the primary transfer, that is, at the timing t
At 1, the CB signal for controlling the operation of the cleaner blade 491 rises from the L level to the H level, the cleaner blade 491 contacts the intermediate transfer belt 41, and the black toner image K1 is compared with the other toner images Y1, C1, and M1. In the sub-scanning direction. That is, the timing t
At 1, the cleaner blade 491 contacts the intermediate transfer belt 41, acts as a transport load for the intermediate transfer belt 41, and instantaneously extends the intermediate transfer belt 41 in the sub-scanning direction.
27 occurs. As a result, the resist displacement amount A in the (-) direction
A resist shift occurs only by 27.

After the timing t1, the cleaner blade 491 is kept in contact with the intermediate transfer belt 41 and the cleaning process of the intermediate transfer belt 41 is executed until the next CB signal rises again from the L level to the H level. However, the primary transfer processing of the black toner image K1 is continued in the contact state until timing t2. As a result, the registration deviation is further increased, and the final registration deviation amount of the black toner image K1 in the sub-scanning direction is A32 = A27 + A6, and as shown in FIG. It is shifted from the rear end position of the transfer by the shift amount A32 in the (-) direction. However, the symbol A6 corresponds to the belt elongation caused by the cleaner blade 491 continuing to contact the intermediate transfer belt 41 from the timing t1 to the timing t2 (that is, the time A7). Also, in the same figure (and a diagram showing the state of registration deviation described later), a thick solid line indicates the registration deviation of the toner image of the corresponding toner color, while a thick broken line indicates an aid for understanding the occurrence state of the registration deviation. Line.

As described above, for the first color image, as shown in FIG.
Only 1 deviates from the other toner images Y1, C1 and M1, and in particular, at the rear end of the color image, deviates by the registration deviation amount A32. More specifically, as shown in FIG. 3, with respect to the first black toner image, the registration deviation in the sub-scanning direction during the primary transfer is (+) and ( Each shift occurs in the range of the amount of deviation (A32 / 2) in the-) direction, which leads to a decrease in image quality. Incidentally, before the cleaner blade 491 abuts, the secondary transfer roller 48 also abuts on the intermediate transfer belt 41 and a similar registration shift occurs, but the corresponding registration shift amount is smaller than that of the cleaner blade 491. Here, in order to facilitate understanding of the basic principle of the present invention, a description will be given here ignoring registration deviation due to separation and contact of the secondary transfer roller 48 with the intermediate transfer belt 41.

A-2. Second printing sequence Such a registration error does not occur only on the first sheet, but also appears on the second color image. That is, in order to form the second yellow toner image Y2,
As shown in FIG. 4, at timing VT5, the vertical synchronizing signal VS
After a predetermined time T10 has elapsed from the output of YNC, a VIDEO signal for forming the yellow toner image Y2 is given to the exposure unit 3. Then, while an electrostatic latent image corresponding to the yellow toner image Y2 is formed on the photoreceptor 21, toner development is performed by the yellow developing unit 23Y.
Output of the vertical synchronization signal VSYNC (timing VT5)
After a lapse of a predetermined time T20 from the time t3
The primary transfer process is started.

However, some time after the output timing VT5 of the vertical synchronizing signal VSYNC, the cleaner blade 491 comes into contact with the intermediate transfer belt 41 as described above, and the intermediate transfer belt 41 instantaneously expands in the sub-scanning direction A27. Moreover, the contact state is changed to C as described later.
Since the B signal continues until it rises to the H level, the elongation in the sub-scanning direction increases as time elapses. And
At the primary transfer start timing t3, the registration deviation amount A30 in the sub-scanning direction is A30 = A27 + A9. However, the symbol A9 corresponds to belt elongation caused by the cleaner blade 491 continuing to contact the intermediate transfer belt 41 from the timing t1 to the timing t3 (that is, the time A10).

When the intermediate transfer belt 41 has passed through the belt cleaner 49 for about one rotation, the entire belt is cleaned and the cleaning process is completed. At a timing t4, the CB signal rises again from the L level to the H level, The cleaner blade 491 is used for the intermediate transfer belt 41
Separate from. During the period from the start of the primary transfer (timing t3) to the separation of the cleaner blade 491 (timing t4), the cleaner blade 491 keeps contacting the intermediate transfer belt 41, during which the intermediate transfer belt 41 is moved to A12 (= t4−t3). The registration deviation further increases by extending by the extension amount A11 in the sub-scanning direction, and the registration deviation amount immediately before the timing t4 becomes the displacement amount A35 in the (-) direction.

On the other hand, at this timing t4, the cleaner blade 491 separates from the intermediate transfer belt 41. Accordingly, the load applied to the intermediate transfer belt 41 is released contrary to the time of the contact.
Is reduced by the contraction amount A26, and the registration deviation amount in the sub-scanning direction is reduced by A26. As described above, for the second color image, the transfer start position of the yellow toner image Y2 is greatly shifted from the reference transfer start position. In addition, as the primary transfer progresses, the displacement increases, and when the cleaner blade 491 separates at the timing t4 during the primary transfer, the resist displacement decreases. That is, as shown in FIG. 4, the second yellow toner image Y2
With respect to (3), the registration deviation in the sub-scanning direction during the primary transfer is the deviation amount (A26 / A) in each of the (+) and (-) directions in the sub-scanning direction centering on the swing center AC2.
This occurs within the range of 2), and causes deterioration in image quality.

The transfer start position of the cyan toner image C2, which is primarily transferred after the yellow toner image Y2, is shifted from the reference transfer start position under the influence of the separation and contact of the cleaner blade 491. This phenomenon will be described with reference to FIG.

In order to form the second cyan toner image C2, after a lapse of a predetermined time T10 from the output of the vertical synchronizing signal VSYNC at timing VT6, a VIDEO signal for forming the cyan toner image C2 is transmitted to the exposure unit. 3
Given to. Then, while forming an electrostatic latent image corresponding to the cyan toner image C2 on the photoconductor 21, the toner is developed by the cyan developing unit 23C. Further, the primary transfer process is started at a point in time when a predetermined time T20 has elapsed from the output of the vertical synchronization signal VSYNC (timing VT6), that is, at timing t5.

Here, at the output timing VT6 of the vertical synchronization signal VSYNC, the cleaner blade 491 is in contact with the intermediate transfer belt 41 as described above, and the timing t4 (the CB signal rises again from the L level to the H level). This contact state is maintained until the time A14. Therefore, the intermediate transfer belt 41 extends by A13 between the timing VT6 and the timing t4.
On the other hand, when the cleaner blade 491 separates from the intermediate transfer belt 41 at the timing t4, the load applied to the intermediate transfer belt 41 is released and the Shrinks by A26. Thereafter, the separated state is maintained until the CB signal next rises from the L level to the H level again. As a result, at the primary transfer start time (timing t5) of the cyan toner image C2, the registration deviation amount A34 in the sub-scanning direction is A34 = A26-A13.

As described above, the second cyan toner image C2
With respect to the above, the registration deviation in the sub-scanning direction during the primary transfer has an amplitude amount of 0 around the oscillation width center AC3. A parallel shift is performed in the sub-scanning direction (+) by the shift amount A34, which causes a decrease in image quality.

When the primary transfer of the cyan toner image C2 is completed as described above, the toner image formation of the magenta toner image M2 and the primary transfer process are performed. During this process, the cleaner blade 491 causes the intermediate transfer to be performed. Belt 41
In the same manner as the first sheet, there is no registration shift in the sub-scanning direction, and the shift amount is zero. Therefore, with respect to the magenta toner image M2, the registration deviation in the sub-scanning direction during the primary transfer is determined by setting the axis of the amount of registration deviation to zero (the dashed line AC0 in FIGS. The amount is also zero. For this reason, in the image forming apparatus that forms an image in the operation sequence shown in FIG. 1, the magenta toner image is used as the reference toner image, and the transfer start position and the transfer end position thereof are referred to as “reference transfer start position” and “reference transfer position”, respectively. Transfer end position ".

When the primary transfer of the magenta toner image M2 is completed, the image formation of the third black toner image and the primary transfer process are performed. The blade 491 comes into contact with the intermediate transfer belt 41 to extend the intermediate transfer belt 41 by A32, and a registration shift occurs in the (-) direction in the sub-scanning direction. A profile (hereinafter, simply referred to as a "profile") indicating a change in the amount of registration deviation with respect to the operation sequence is the same as that shown in FIG. , And in the (+) and (−) directions in the sub-scanning direction, respectively, within the range of the amount of deviation (A32 / 2), thereby deteriorating the image quality.

Further, following the third color image,
In the case of successively forming color images on the second and subsequent sheets, the same registration deviation as in the above-described second sheet occurs.

A-3. Third printing sequence Further, in this type of image forming apparatus, the intermediate transfer belt 4
1 may run idle. For example, if the interval between image data from an external device such as a host computer is longer than a certain value, the intermediate transfer belt 41 is idled.
At this time, the cleaner blade 491 is
1 is in a contact state. When a new image formation is to be started, the intermediate transfer belt 41 is rotated to start image formation. However, when the first yellow toner image is primarily transferred, the second and subsequent sheets shown in FIG. The same registration deviation as in the case of the cyan toner image occurs.

That is, as shown in FIG. 6, when the image formation is restarted and the intermediate transfer belt 41 is driven to rotate, the vertical synchronization signal VSYNC is output from the vertical synchronization reading sensor 40 at the timing VT01, and the timing VT01 is obtained. After a predetermined time A14 from when the cleaner blade 491 separates from the intermediate transfer belt 41, the primary transfer of the yellow toner image is started. For this reason, the transfer start position is shifted in the (+) direction by the shift amount A34 for the same reason as in the case of the cyan toner image C2 in the above “A-2. Second print sequence”. In other words, the registration deviation in the sub-scanning direction during the primary transfer has an amplitude of 0 centering on the center of oscillation width AC3. The image is parallel-shifted in the scanning direction (+) by the shift amount A34, which causes a decrease in image quality.

Since the subsequent primary transfer of the cyan and magenta toner images is always executed with the cleaner blade 491 being separated from the intermediate transfer belt 41, no registration deviation occurs. As in the case of the first and second print sequences, the cleaner blade 491 and the secondary transfer roller 48 abut on the intermediate transfer belt 41 during the primary transfer, and a registration shift of the shift amount A32 occurs in the (-) direction. I do.

As described above, when the contacting means such as the cleaner blade 491 comes into contact with the intermediate transfer belt 41 while the primary transfer process is repeated, the predetermined amount of registration deviation depends on the timing of the contact / separation. appear. This profile itself is determined by the device configuration and operating conditions.The profile itself does not change unless the device configuration or operation sequence is changed, but the transfer start position is moved in the sub-scanning direction according to the amount of registration deviation. By doing so, registration deviation from the reference toner image can be reduced to zero or suppressed. For example, as for the cyan toner image C2, as shown in FIG. 5, the transfer start position of the cyan toner image C2 has a shift amount A34 in the (+) direction with respect to the reference transfer start position. Is not seen, the transfer start position of the cyan toner image C2 is shifted from the reference transfer start position by the amount of registration deviation A34 (-).
By controlling so as to shift in the direction, the amount of registration deviation can be made zero.

Therefore, prior to the actual image forming process, the same analysis as described above is performed in advance from the apparatus configuration and operation sequence to derive the amount of registration deviation, and it is necessary to reduce or eliminate the amount of registration deviation. In the actual image forming process, the transfer start position is corrected in the sub-scanning direction based on the registration control amount (e.g., A34 in the case of cyan). By adjusting the center of fluctuations AC1 to AC4 of the toner colors (Y, C, K) other than magenta) to the center of fluctuation AC0 of the reference toner color, it is possible to suppress registration deviation and form a high-quality image. it can.

B. First Embodiment Image forming apparatuses according to the first embodiment and other embodiments use the above-described "basic principle of the invention", and have the same mechanical configuration as that of FIG. On the other hand, it has remarkable features not found in the prior art in its electrical configuration and image forming method. Therefore, in the following, the description of the mechanical configuration is omitted, and the electrical configuration and the image forming method will be mainly described in detail.

B-1. Electrical Configuration FIG. 7 is an electrical configuration diagram showing an embodiment of an image forming apparatus according to the present invention, and has a configuration common to all embodiments. In the image forming apparatus, when an image signal is provided from an external device such as a host computer to the main controller 11 of the control unit 1, the engine controller 12 is configured as shown in FIG. By controlling each part of the engine unit E, an image corresponding to the image signal is formed on the sheet member S.

The engine controller 12 has a CPU 1
And a horizontal synchronization signal HSYNC from the horizontal synchronization reading sensor 36 as an input signal from the engine unit E.
Also, the vertical synchronization signal VSYNC is output from the vertical synchronization read sensor 40.
And a temperature sensor 51 provided in the fixing unit 5.
, Respectively.
In addition, based on these input signals and various information,
The CPU 121 supplies a photoconductor / belt drive control circuit 122 with a photoconductor / belt drive unit 41a that rotationally drives the photoconductor 21 and the intermediate transfer belt 41 in synchronization with each other. Belt drive control circuit 1
A photoreceptor / belt driving unit 41a using a DC motor as a driving source is controlled by 22 to control the rotational speed of the photoreceptor 21 and the transport speed V of the intermediate transfer belt 41 by acceleration / deceleration.
Further, the CPU 121 establishes and sets a registration control amount described later.
A storage process, a sequence flag update process, a registration control amount establishment process, and the like are executed.

The engine controller 12 has a dedicated control circuit for controlling the transfer unit 4, a transfer roller separation / contact control circuit 123 and a belt cleaner separation / contact control circuit in addition to the photoconductor / belt drive control circuit 122. 1
24 is further provided. The transfer roller separation / contact control circuit 123 controls the secondary transfer roller drive unit 48 a based on a command signal from the CPU 121 to separate and contact the secondary transfer roller 48 with the intermediate transfer belt 41 at an appropriate timing. On the other hand, the belt cleaner separation / contact control circuit 124 controls the belt cleaner drive unit 49a by giving the CB signal to the belt cleaner drive unit 49a based on the command signal from the CPU 121, and transfers the cleaner blade 491 to the intermediate transfer at an appropriate timing. The belt 41 is brought into contact with and separated from the belt 41.

Reference numeral 113 in the figure is an image memory provided in the main controller 11 for storing an image provided from an external device such as a host computer via the interface 112, and reference numeral 125 is an engine unit E. Is a RAM for temporarily storing control data for controlling the operation, calculation results in the CPU 121, and the like, and a reference numeral 126 is a ROM for storing a calculation program executed by the CPU 121 and the like.

B-2. Next, the operation of the image forming apparatus according to this embodiment will be described. FIG. 8 is a flowchart showing the operation of the image forming apparatus. In this image forming apparatus, when the apparatus power is turned on, prior to actual image forming processing, a registration control amount establishing step (step S1) is executed to automatically establish three types of registration control amounts and store the same. RAM as a part
125. In this embodiment, the following resist control amounts Ra, Rb, Rc, as three types of resist control amounts,
Ra: For example, as shown in FIGS. 2 and 3, the cleaner blade 491 abuts during the primary transfer process, and is used to correct a registration shift generated by completing the primary transfer process in the abutted state. Registration control amount, Rb: As shown in FIG. 4, for example, the cleaner blade 491 contacts before the start of the primary transfer, and the primary transfer process is continued in the contact state, and the cleaner blade 49
Rc: for example, as shown in FIG. 5 and FIG. 6, the cleaner blade 491 contacted before the start of the primary transfer is separated,
Thereafter, a registration control amount for correcting a registration deviation generated when performing the primary transfer processing in the separated state is established. The details of the operation for automatically setting the three types of registration control amounts (step S1) will be described later.

Thus, the three types of resist control amounts Ra, R
When the establishment of b and Rc (step S1) is completed, the process waits for a print request from an external device such as a host computer (step S2). When there is a print request, it is determined whether the print mode is monochrome printing or color printing (step S3). If it is determined that the printing mode is monochrome printing, normal image forming processing is performed without registration control. Execute and return to step S2. On the other hand, if it is determined in step S3 that the printing is color printing, the three sequence flags F
A sequence flag corresponding to the printing sequence state is selectively set from 0, F1, and F2 (identification variable setting step: step S4). Here, three sequence flags F0, F1, and F2 are prepared. These are identification variables corresponding to the first to third print sequences, respectively. This is because it can be associated with the control amounts Ra, Rb, and Rc.

FIG. 9 is a flowchart showing the updated contents of the sequence flag in FIG. In this sequence flag updating process, it is determined whether or not the print content is the first color print (step S41). Then, when it is determined that it is the first sheet, that is, when it is detected that the first print sequence is executed, a sequence flag F0 is set (step S42). On the other hand, in step S41,
If it is determined that the image is the second or later, step S43
Then, it is determined whether or not the idling process is being performed.

In the case where the idling process is not performed, that is, in the case of continuous printing, since the second printing sequence is executed, the sequence flag F1 is set (step S4).
4). On the other hand, if the idling process is being performed, the third print sequence is executed, so the sequence flag F2
Is set (step S45).

As described above, the printing sequence is detected by the sequence flag updating process (step S4), and the corresponding sequence flag is set and updated. Each of the sequence flags F0, F1, and F2 is controlled by the registration control amount. And the following associations are made.

<Sequence Flag F0: First Print Sequence> The first print sequence is the first color print as shown in FIG. 9, that is, the first color print performed after the power is turned on or the sleep mode is canceled. This is for forming an image. When the power is turned on or the sleep mode is released, no toner remains on the intermediate transfer belt 41 and the primary transfer process can be executed as it is.
During the primary transfer of each of the cyan and magenta toner images, both the cleaner blade 491 and the secondary transfer roller 48 are separated from the intermediate transfer belt 41, and no registration deviation occurs during these primary transfers. . In contrast,
During the primary transfer of the black toner image, the cleaner blade 491 and the secondary transfer roller 48 come into contact with the intermediate transfer belt 41 to cause a registration shift.

Therefore, in the first print sequence, the flag F0 is set, and as shown in Table 1, the registration control amounts of the yellow toner image Y1, the cyan toner image C1, and the magenta toner image M1 correspond to the sequence flag F0. Is set to “0”, while the control amount Ra is set as the registration control amount of the black toner image K1.

[0056]

[Table 1] <Sequence Flag F1: Second Printing Sequence> The second printing sequence is for the case where color printing of the second and subsequent sheets is continuously performed as shown in FIG. Thus 2
After the first sheet, the transfer start position of the yellow toner image shifts in the sub-scanning direction as described in detail with reference to FIG. The amount of registration shift changes. During the primary transfer of the cyan toner image, the transfer start position is shifted in the sub-scanning direction as described with reference to FIG. Moreover, for the black toner image, similarly to the first sheet,
During the primary transfer, the cleaner blade 491 and the secondary transfer roller 48 come into contact with the intermediate transfer belt 41 to cause a registration shift.

Therefore, in the second printing sequence, the flag F1 is set, and as shown in Table 1, the control amount Rb is set as the registration control amount of the yellow toner image Y2 in accordance with the sequence flag F1, and the cyan toner is set. Image C2
The control amount Rc is set as the registration control amount of the magenta toner image M2, "0" is set as the registration control amount of the magenta toner image M2, and the control amount Ra is set as the registration control amount of the black toner image (K2).

<Sequence Flag F2: Third Printing Sequence> The third printing sequence is a color printing for the second and subsequent sheets as shown in FIG. is there. As described above, when the next n-th image formation (n ≧ 2) starts when the idle rotation process exists, as described above, before the primary transfer of the yellow toner image, the cleaner blade 491 causes the intermediate transfer belt to move. 41, the transfer start position shifts in the sub-scanning direction (FIG. 6). Then, since the primary transfer of the subsequent cyan and magenta toner images is always performed with the cleaner blade 491 being separated from the intermediate transfer belt 41, no registration deviation occurs, but the first and second toner images of the last black toner image are formed. As in the case of the two-print sequence, during the primary transfer, the cleaner blade 491 and the secondary transfer roller 48 come into contact with the intermediate transfer belt 41, and a registration shift occurs.

Therefore, in this printing sequence, the flag F2 is set, and as shown in Table 1, a control amount Rc is set as the registration control amount of the yellow toner image corresponding to the sequence flag F2, and the cyan toner image and the control amount are set. “0” is set as the registration control amount of the magenta toner image, and the control amount Ra is set as the registration control amount of the black toner image.

When the updating of the sequence flag corresponding to the printing sequence is completed as described above, a registration control amount corresponding to the sequence flag is set (registration control amount setting step: step S5), and each toner image is primary-printed. At the time of transfer, the photosensitive member 21 is subjected to acceleration / deceleration control during a predetermined acceleration / deceleration possible period to shift the latent image forming position by a registration control amount in the sub-scanning direction with respect to the reference latent image forming position. The intermediate transfer belt 4 to which the primary transfer is performed by this
The transfer position of the toner image on the reference numeral 1 also moves by the registration control amount in the sub-scanning direction. For example, when printing the first color image, the sequence flag F0 corresponding to the first print sequence is set in step S4, so that the yellow toner image Y1, the cyan toner image C1,
“0” is set as the registration control amount of the magenta toner image M1, and the control amount Ra is set as the registration control amount of the black toner image K1. Therefore, the yellow toner image Y1, the cyan toner image C1, and the magenta toner image M1 are all formed at predetermined positions on the photoconductor 21, that is, the reference latent image forming positions, as in the conventional example. Intermediate transfer belt 41 that rotates in synchronization
Is primarily transferred at the same position. As a result, FIG.
0, these three toner images Y1, C1, M
All of the transfer start positions 1 correspond to the reference transfer start positions,
Moreover, those transfer rear end positions also coincide with the reference transfer rear end position.

On the other hand, for the black toner image K1,
Since the control amount Ra is set as the registration control amount, as shown in FIG. 11, the acceleration / deceleration possible period T is determined based on the vertical synchronization signal VSYNC output at the timing VT4.
At timing t11 of 11, the photosensitive member 21 is accelerated / decelerated to move the black toner image latent image forming position to the reference latent image forming position on the (+) side in the sub-scanning direction by a control amount Ra (= A32 / 2). Shift shift. Here, the “acceleration / deceleration possible period” refers to a period during which the VIDEO signal is at the H level and the exposure processing is stopped. Also, this acceleration / deceleration possible period T
In FIG. 11, the previous toner image (magenta toner image M
1) The primary transfer process is ongoing, but in this embodiment, the drive of the intermediate transfer belt 41 is controlled in synchronization with the photoconductor 21, so that the acceleration and deceleration control of the photoconductor 21 and the intermediate transfer belt 41 are performed in parallel. As a result, no disturbance occurs in the toner image primarily transferred.

The latent image formed on the photosensitive member 21 as described above is visualized by the developing unit 23K, and the black toner image K1 is primarily transferred onto the intermediate transfer belt 41. As a result, as shown in FIG. 10, the transfer start position of the black toner image K1 is shifted from the reference transfer start position by the registration control amount Ra in the (+) direction.

Then, as shown in FIG. 11, the CB for controlling the operation of the cleaner blade 491 at the timing t1 when the primary transfer process proceeds and reaches the latter half thereof.
The signal rises from the L level to the H level, the cleaner blade 491 abuts on the intermediate transfer belt 41, and the black toner image K1 shifts in the sub-scanning direction with respect to the other toner images Y1, C1, and M1. Further, the contact state is continued until timing t2. As a result, the registration deviation further increases, but the registration deviation amount in the sub-scanning direction of the final black toner image K1 is shifted in the (-) direction (A32 / 2). That is, by moving the transfer start position of the black toner image K1 in the (+) direction with respect to the reference transfer start position by the registration control amount Ra, the center of the swing width AC1 for the black color is set for the magenta color which is the reference toner color. Of the swing center AC0 of
By doing so, the center of the deviation width of the registration shift in the sub-scanning direction for each toner color during the primary transfer process for all the toner colors coincides with each other.

As a result, in this embodiment, as shown in FIG. 10, the black toner image K1 is
In the (+) direction on the transfer start side with respect to C1 and M1 (A32 /
2) and at the rear end side of the transfer, in the (-) direction by (A32 / 2), and the maximum deviation amount is half that in the case where no registration control is performed (FIGS. 2 and 3).

When the formation of the first color image is completed while suppressing the registration deviation based on the registration control amount in this way, it is determined in step S7 whether or not the printing has been completed, and it is determined that the printing has been completed. In this case, the process returns to step S2 and waits for the next print request. On the other hand, if it is determined that printing has not been completed, the process returns to step S3, and the same processing as described above is repeated. Therefore, as described above, when the second color image is formed following the formation of the first color image (second printing sequence), the process proceeds from step S3 to step S4, and the flag F1 is set as the sequence flag. After the setting, a high-quality image can be formed by suppressing the registration deviation as described below.

That is, in step S5, a registration control amount corresponding to the sequence flag F1 is set. That is, the control amount Rb is set as the registration control amount of the yellow toner image Y2, the control amount Rc is set as the registration control amount of the cyan toner image C2, and the magenta toner image M
In addition, “0” is set as the registration control amount of No. 2, and the control amount Ra is set as the registration control amount of the black toner image K2. Then, registration control is executed for each toner image.

First, for the yellow toner image Y2,
Since the control amount Rb is set as the registration control amount, as shown in FIG. 12, the acceleration / deceleration possible period T is determined based on the vertical synchronization signal VSYNC output at the timing VT5.
At timing t11 of 11, the photosensitive member 21 is accelerated and decelerated to shift the latent image formation position of the yellow toner image toward the (+) side in the sub-scanning direction by the control amount Rb with respect to the reference latent image formation position. Then, this latent image is visualized by the developing device 23Y.

At timing t 1, the CB signal rises from L level to H level, and the separated cleaner blade 491 contacts the intermediate transfer belt 41. Thereafter, the transfer process of the yellow toner image Y2 is performed while the amount of registration shift changes according to the profile indicated by the thick solid line in FIG. The maximum deviation amount with respect to the toner image (magenta toner image M2) is greatly reduced as compared with the case where no registration control is performed (FIG. 4).

As described above, in this embodiment, the photosensitive member 2
The transfer start position of the second yellow toner image Y2 is adjusted by shifting the latent image formation position on the reference image 1 in the sub-scanning direction with respect to the reference latent image formation position by the registration control amount Rb. As a result, the center of the fluctuation width AC2 for the yellow color is matched with the center of the fluctuation width AC0 for the magenta color which is the reference toner color. Therefore, the amount of deviation from the reference toner image (magenta toner image M2) can be suppressed within the range of the fluctuation width (A26 / 2).

Following the yellow toner image Y2, primary transfer processing of the cyan toner image C2 is performed, and a control amount Rc is set as a registration control amount of the cyan toner image C2. Therefore, as shown in FIG. 13, the rotation speed of the photoconductor 21 and the transport speed V of the intermediate transfer belt 41 are temporarily reduced at the timing t11 of the acceleration / deceleration possible period T11 based on the vertical synchronization signal VSYNC output at the timing VT6. In this case, the rotation amount of the photoconductor 21 and the conveyance amount of the intermediate transfer belt 41 are reduced by the registration control amount Rc as compared with the case where the sheet is rotated and conveyed at a constant speed (in the case of the reference toner image, that is, the magenta toner image). . As a result, the latent image forming position on the photoconductor 21 shifts by the registration control amount Rc in the sub-scanning direction with respect to the reference latent image forming position.

Then, the latent image formed on the photosensitive member 21 as described above is visualized by the developing device 23C, and the cyan toner image C2 is primarily transferred onto the intermediate transfer belt 41. Therefore, the amount of registration deviation (A26) due to the separation and contact of the cleaner blade 491 and the amount of shift Rc of the toner image C2 on the photoconductor 21 match, and the cyan toner image C2
Transfer start position coincides with the reference transfer start position.

The timing t before the primary transfer of the cyan toner image C2 to the intermediate transfer belt 41 is started.
4, the CB signal rises from the L level to the H level, and the cleaner blade 491 in contact with the intermediate transfer belt 4
Since it is separated from 1, no misregistration occurs during the primary transfer process. For this reason, the transfer end position of the cyan toner image C2 coincides with the transfer end position.

As described above, in this embodiment, the photosensitive member 21 and the intermediate transfer belt 4 are controlled based on the registration control amount Rc.
By controlling acceleration / deceleration of 1, the swing center AC3 for cyan is matched with the swing center AC0 for magenta, which is the reference toner color. Therefore, the amount of deviation from the reference toner image (magenta toner image M2) can be suppressed to zero.

The primary transfer processing of the magenta toner image M2 is executed following the cyan toner image C2. The transfer start position and the transfer end position of M2 coincide with the reference transfer start position and the transfer end position, respectively.

Thus, the three color toner images Y2, C2, M
When Step 2 is completed, next, primary transfer processing of the final toner color, that is, the black toner image K2 is executed. In this primary transfer process, as in the case of the first black toner image K1, the position at which the latent image is formed on the photoconductor 21 is determined by the registration control amount R.
By shifting by b in the sub-scanning direction, the center of fluctuation AC1 for the black color coincides with the center of fluctuation AC0 for the reference toner color, magenta.

Therefore, the reference toner image is shifted by (A32 / 2) in the (+) direction on the transfer start side with respect to the reference toner image.
On the trailing end side of the transfer, there is a shift in the (-) direction by (A32 / 2), and the maximum shift amount is half that in the case where the resist control is not performed (FIGS. 2 and 3).

As described above, also for the second sheet, for all the toner colors, the center of the deviation width of the registration shift in the sub-scanning direction for each toner color during the transfer process coincides with each other. The transfer start position of the toner image is corrected by synchronously controlling the rotation speed of the photoconductor 21 and the conveyance speed of the intermediate transfer belt 41 based on the registration control amount corresponding to the registration control amount. As a result, the cyan toner image C2 can be completely registered with the magenta toner image M2 as the reference toner image, and the yellow toner image Y2 and the black toner image K2 cannot be completely registered with the reference toner image. Also minimizes the amount of misregistration,
High quality image formation is possible.

When the sequence flag F2 is set, the control amount Rc is set as the registration control amount for the yellow toner image Yn, and "0" is set as the registration control amount for the cyan toner image Cn and the magenta toner image Mn. Is set, and the control amount Ra is set as the registration control amount of the black toner image Kn. Then, registration control is executed for each toner image.

First, for the yellow toner image Yn,
Since the control amount Rc is set as the registration control amount, as shown in FIG. 14, the acceleration / deceleration possible period T is determined based on the vertical synchronization signal VSYNC output at the timing VT01.
At the timing t11 of 11, when the rotation speed of the photoconductor 21 and the conveyance speed V of the intermediate transfer belt 41 are temporarily reduced, when the rotation conveyance is performed at a constant speed (in the case of the reference toner image, ie, the magenta toner image) In comparison, the rotation amount of the photoconductor 21 and the conveyance amount of the intermediate transfer belt 41 are reduced by the registration control amount Rc. As a result, the latent image forming position on the photoconductor 21 shifts by the registration control amount Rc in the sub-scanning direction with respect to the reference latent image forming position.

Then, the latent image formed on the photosensitive member 21 as described above is made visible by the developing unit 23Y, and the yellow toner image Yn is primarily transferred onto the intermediate transfer belt 41. Therefore, the amount of registration deviation (A26) due to the separation and contact of the cleaner blade 491 and the shift amount Rc of the toner image Yn on the photoconductor 21 match, and the yellow toner image Yn
Transfer start position coincides with the reference transfer start position.

The CB signal rises from L level to H level at a timing t4 before the primary transfer process of the yellow toner image Yn to the intermediate transfer belt 41 is started.
Since the cleaner blade 491 in contact with the intermediate transfer belt 41 is separated from the intermediate transfer belt 41, no registration deviation occurs during the primary transfer process. For this reason, the transfer end position of the yellow toner image Yn coincides with the transfer end position.

As described above, in this embodiment, the photosensitive member 21 and the intermediate transfer belt 4 are controlled based on the registration control amount Rc.
By controlling the acceleration / deceleration of 1, the swing center AC4 for the yellow color is matched with the swing center AC0 for the magenta color, which is the reference toner color. For this reason, the amount of deviation from the reference toner image (magenta toner image Mn) can be suppressed to zero.

Following the yellow toner image Yn, the primary transfer processes of the cyan toner image Cn and the magenta toner image Mn are sequentially performed. In these transfer processes, the separation and contact of the cleaner blade 491 and the secondary transfer roller 48 are performed. The center of the fluctuation width for both toner colors coincides with each other, and the transfer start position and the transfer end position of both toner images Cn and Mn correspond to the reference transfer start position and the transfer end position, respectively.

Thus, the three color toner images Yn, Cn, Mn
Is completed, the primary transfer process of the final toner color, that is, the black toner image Kn is executed. In this primary transfer process, as in the case of the first and second print sequences, the photosensitive member 21 and the intermediate transfer belt 41 are accelerated / decelerated based on the registration control amount Rc, so that the center of the swing width AC1 for the black color is obtained. The center of the fluctuation width AC0 for the magenta color, which is the reference toner color, is matched. Accordingly, the reference toner image is shifted by (A32 / 2) in the (+) direction on the transfer start side and by (A32 / 2) in the (-) direction on the trailing end side of the transfer. This is half of the case where the resist control is not performed (FIGS. 2 and 3).

As described above, also in the color printing after the idling process, for all the toner colors, each of the toner colors in the sub-scanning direction in the sub-scanning direction during the transfer process is coincident with each other so that the centers of the deviation widths of the registration deviations coincide with each other. The transfer start position of the toner image is corrected by controlling the acceleration and deceleration of the photoconductor 21 and the intermediate transfer belt 41 based on the registration control amount Rc for each toner color. As a result, the yellow toner image Y
n, the cyan toner image Cn and the magenta toner image (reference toner image) Mn can be completely registered, and the black toner image Kn can be completely registered with the reference toner image even if it cannot be completely registered. It can be minimized, and high quality image formation becomes possible.

B-3. Operation and Effect As described above, according to the first embodiment, the contact means (the secondary transfer roller 48 and the cleaner blade 49) with the intermediate transfer belt 41 as the transfer medium during the repetition of the transfer processing.
Since the separation and abutment of 1) is performed, higher throughput and higher throughput can be obtained than in the related art in which the secondary transfer process and the cleaning process are performed after the primary transfer is completed. Further, as in the prior art, the intermediate transfer belt 41 is used.
Does not need to be set longer than necessary, and the device size can be reduced.

Of course, the contact means is the intermediate transfer belt 41
, The registration deviation occurs in the sub-scanning direction for black, yellow, and cyan colors in the sub-scanning direction for each toner color during the primary transfer process. By matching the center of the fluctuation width AC0 with respect to the magenta color as the reference toner color, registration deviation between all the toner colors is suppressed to a minimum, and a high-quality color image is obtained.

Further, in this embodiment, the following operation and effect can be obtained. In other words, as described above, the cleaner blade 491 abuts to generate a registration shift. In order to suppress the shift amount, for example, it is considered that the Young's modulus of the intermediate transfer belt 41 is increased to suppress elastic elongation at the time of the abutment. However, this limits the usable belt material and increases the cost. Also, the design and
It cannot be applied to manufactured equipment as it is,
Equipment improvement is required. On the other hand, according to the above-described embodiment, the registration deviation can be suppressed without depending on the device configuration, and the image quality can be improved.

B-4. FIG. 15 is a flowchart showing a process of automatically establishing a registration control amount. First, the following initial setting conditions are set in advance based on the apparatus configuration and operation sequence of the image forming apparatus according to the first embodiment, and are stored in the ROM 126. Then, as shown in FIG.
Based on the signal, (1) a period T1 in which the cleaner blade 491 and the secondary transfer roller 48 abut on the intermediate transfer belt 41, and (2) a period in which the cleaner blade 491 and the secondary transfer roller 48 keep abutting on the intermediate transfer belt 41. T2,
(3) Cleaner blade 491 and secondary transfer roller 48
Is separated from the intermediate transfer belt 41 by a period T3, and
(4) Cleaner blade 491 and secondary transfer roller 48
Is a registration control amount establishing job (step S1) in which the cycle T4 in which the
1) is repeated a predetermined number of times, for example, 20 times (step S)
12).

The initial conditions are: A2: process speed (peripheral speed of the intermediate transfer belt 41), A7: time from contact of the cleaner blade 491 to completion of primary transfer of the black toner image (see FIGS. 3 and 11). A8: Time required for the intermediate transfer belt 41 to make one rotation A10: Time from contact of the cleaner blade to start of primary transfer of the yellow toner image (see FIGS. 4 and 12), A12: From the transfer start position of the yellow toner image A14: Time from VSYNC signal to cleaner blade separation (see FIGS. 5 and 13), A17: Time from VSYNC signal to cleaner blade contact in cycle T1. Interval (see FIG. 16), A18: Time interval from VSYNC signal to cleaner blade separation in cycle T3 (see FIG. 16), To have.

In this embodiment, the charging bias and the primary transfer bias are always set to the ON state while the registration control amount establishing job (step S11) is repeatedly executed. Although not shown in FIG. 21, a static elimination lamp is provided between the primary transfer region R1 and the photosensitive member cleaner blade 24, and the static elimination lamp is also set to be always ON. .
Further, while the secondary transfer roller 48 is in contact with the intermediate transfer belt 41, a secondary transfer bias is applied to obtain a registration control amount in a state close to actual printing.

When 20 actual measured values are obtained for each of the periods T1 to T4, the average value T1 is obtained.
(av) to T4 (av) are calculated (step S1)
3). Further, the registration control amounts Ra, Rb, and Rc are obtained by calculation based on the following equations, respectively (step S:
14). The reason will be described separately.

<Registration Control Amount Ra> As shown in FIG. 3, the black toner image K1 is transferred to the intermediate transfer belt 41.
During the primary transfer, the contact of the cleaner blade 491 is started. For example, even when the primary transfer of the black toner image K1 of A3 size is completed, the contact of the cleaner blade 491 is continued. A registration shift amount A32 occurs in the sub-scanning direction. The resist displacement A32 is the sum of the two elongations A6 and A27. That is, A32 = A6 + A27.

Here, the contact elongation A6 is the contact elongation generated when the intermediate transfer belt 41 is rotated and conveyed while the cleaner blade 491 is in contact, and the elongation A27 is
This is the instantaneous elongation (elasticity + slippage) when the cleaner blade 491 contacts the intermediate transfer belt 41.

First, the elongation A6 will be examined. The periodic difference A1 is generated by the contact of the cleaner blade 491, and the periodic difference A1 can be obtained by the following equation: A1 = (T2 (av) -T4 (av)) × A2 × 1000 . Then, the black toner image K1
During the primary transfer, the cleaner blade 491 is in contact only for a predetermined time A7, so the contact elongation A6 is A6 = A1 × A7 / A8.

On the other hand, the instantaneous elongation A27 can be obtained by comparing the periods T1 and T4. That is, the instantaneous elongation A27 can be obtained by the following equation: A27 = (T1 (av) -T4 (av)) * A2 * 1000-A15. However, as shown in FIG. 16, the elongation A15 does not increase during the period T1.
Numeral 1 is the elongation due to the contact for a predetermined time A17, and this elongation A15 can be obtained by A15 = A1 × (A8−A17) / A8.

Accordingly, the amount of registration deviation A32 can be obtained by A32 = A6 + A27. By shifting the transfer start position in advance in the sub-scanning direction with respect to the reference transfer start position by half the value, the black toner image K1 can be obtained. Can be minimized. Therefore, in this embodiment, the resist control amount Ra is set to Ra = A32 / 2.

<Registration Control Rb> As shown in FIG. 4, when the yellow toner image Y2 is primarily transferred onto the intermediate transfer belt 41 following the primary transfer of the black toner image K1, the yellow toner image is brought into contact with the cleaner blade. A in the sub-scanning direction during the time A10 until the start of the primary transfer A
30 (= A27 + A9) has occurred. After the primary transfer is started, the elongation A11 occurs because the cleaner blade 491 is in contact with the intermediate transfer belt 41, but the cleaner blade 491 separates from the intermediate transfer belt 41 immediately before the completion of the primary transfer, Shrinkage A26 occurs. Accordingly, as shown in the figure, when the contraction A26 is larger than the elongation A11, the resist control amount Rb is set to Rb = A35-A26 / 2 where A35 = A30 + A11, while the opposite case (A26 < In A11), the registration deviation of the yellow toner image can be minimized by setting the registration control amount Rb to Rb = A35-A11 / 2.

Here, the elongation at the start of the primary transfer A30
As described above, A30 = A27 + A9, where the elongation A9 is the elongation caused by the intermediate transfer belt 41 being rotated and conveyed only for the time A10 while the cleaner blade 491 is in contact with the following equation. A9 = A1 × A10 / A8

The elongation A11 is the elongation caused by the cleaner blade 491 being in contact with the intermediate transfer belt 41 even after the primary transfer is started, and can be obtained by the following equation: A11 = A1 × A12 / A8 it can.

Further, the contraction A26 is caused by the cleaner blade 4
The reference numeral 91 indicates the separation from the intermediate transfer belt 41, which can be obtained by comparing the periods T3 and T4. That is, it can be obtained based on the following equation: A26 = A25− (T3 (av) −T4 (av)) × A2 × 1000. A25 in the formula is
As shown in FIG. 16, it is the elongation in the cycle T3, which can be obtained by the following equation: A25 = A1 × A18 / A8.

<Registry Control Amount Rc> As shown in FIG. 5, when the cyan toner image is primarily transferred to the intermediate transfer belt 41 following the primary transfer of the yellow toner image, the VSYNC signal VT6 serving as a reference for the primary transfer is used. Is output, the cleaner blade 491 is in contact with the intermediate transfer belt 41, and thereafter, the intermediate transfer belt 41 remains in contact with the intermediate transfer belt 41 for a time A14 before the primary transfer of the cyan toner image is started. Is rotated and conveyed, so that elongation A13 occurs. That is, the elongation A13 is A13 = A1 × A14 / A8.

When the cleaner blade 491 separates from the intermediate transfer belt 41, the above-mentioned <registration control amount Rb
As described in the section, shrinkage A26 occurs. Therefore, at the start of the primary transfer of the cyan toner image, the amount of registration deviation A34 (= A13-A26) occurs, but no deviation occurs in the sub-scanning direction during the primary transfer. Therefore, in this embodiment, by shifting the transfer start position in the sub-scanning direction by this value (registration shift amount A34) in advance, the registration shift of the cyan toner image can be suppressed to zero.
c is set to Rc = A34.

As described above, in the registration control amount setting process, the secondary transfer bias is applied while the secondary transfer roller 48 is in contact with the intermediate transfer belt 41. Is not an essential requirement for establishing the second transfer bias, a secondary transfer bias may not be applied, or a bias having a polarity opposite to that of the secondary transfer bias may be applied. In each case, the following effects can be obtained. That is, when the secondary transfer bias is not applied, the process of establishing the resist control amount can be simplified. Also,
When a secondary transfer bias is applied, the secondary transfer roller 4
8, the load applied to the intermediate transfer belt 41 and the photoconductor / belt drive unit 41a approaches the actual printing state, and the registration control amount can be accurately obtained. Further, when a bias of the opposite polarity is applied, the toner adhered to the secondary transfer roller 48 is returned to the intermediate transfer belt 41 side to clean the secondary transfer roller 48, and the back transfer of the sheet member by the secondary transfer roller 48 is performed. And a good printing result can be obtained.

In the above-described registration control amount setting process, the primary transfer bias is applied to the intermediate transfer belt 41 to obtain the registration control amount in a state close to actual printing.
The resist control amount can be accurately obtained.

Further, in the above-described registration control amount establishment processing, the registration control amount establishment job (step S11) is repeated 20 times from the start of driving (step S12), and 20 measured values of the periods T1 to T4 are measured. The resist control amount is obtained based on these actually measured values. However, immediately after the start of driving, the intermediate transfer belt 41
May not be stable, and if the registration control amount is obtained based on the actually measured periods T1 to T4 in such a state, the accuracy of the registration control amount may be reduced. Therefore, in order to solve such a problem, the intermediate transfer belt 41 is rotated and conveyed a predetermined number of times from the start of driving, and after its operation is stabilized, each cycle T1 to T4
May be measured, and the resist control amount may be determined based on the measured values. In this way, the resist control amount can be determined with high accuracy.

C. Second Embodiment By the way, in the first embodiment, in order to adjust the transfer start position in accordance with the registration control amount, the photosensitive member 21 and the intermediate transfer belt 41 are controlled at a variable speed in synchronization with each other. The latent image forming position on 21 is shifted in the sub-scanning direction in accordance with the registration control amount. Here, photoconductor 2
As a method of shifting the latent image forming position on the photosensitive drum 1, the exposure timing can be controlled in addition to the photoconductor / belt drive control. Further, the photoconductor / belt drive control and the exposure timing control may be combined. This embodiment will be described with reference to FIGS.

FIG. 17 is a flowchart showing the operation of the second embodiment of the image forming apparatus according to the present invention. In the second embodiment, when a registration control amount according to each sequence flag is set in the same manner as in the first embodiment (step S4), the photoconductor 2 is set based on the registration control amount.
The variable speed of the intermediate transfer belt 41 and the intermediate transfer belt 41 is controlled in the variable speed possible period T11 (step S6), and the exposure start timing is advanced or delayed, so that the latent image forming position on the photoconductor 21 is moved in the sub-scanning direction. The shift is performed (step S8).

The combination of the photoconductor / belt drive control (step S6) and the exposure timing control (step S8) is effective when the registration control amount is relatively large. This is because, for example, the yellow toner image Y2 and the cyan toner image C in the second printing sequence
In the transfer of No. 2 and the transfer of the yellow toner image Yn in the third print sequence, the amount of registration control is relatively large. Body 21
It is necessary to set a large change rate of the rotation speed and the belt conveyance speed V, and the accuracy of the photoconductor / belt drive control decreases and the motor load increases.

On the other hand, when the yellow toner image Y2 in the second print sequence is transferred, as shown in FIG.
By setting the exposure timing control so as to be shifted by one dot line in the sub-scanning direction, that is, in the (+) direction by the line interval Re in the sub-scanning direction, the latent image forming position is shifted by the photoconductor / belt drive control. Move amount ΔRb
(<Rb).

When the cyan toner image C2 of the second print sequence is transferred, as shown in FIG. 19, one dot line in the sub-scanning direction is previously controlled by exposure timing control.
In other words, by setting the shift in the (−) direction by the line interval Re in the sub-scanning direction, the shift movement amount of the latent image forming position by the photoconductor / belt drive control can be reduced by ΔRc (<Rc).
Can be suppressed.

Further, when the yellow toner image Yn of the third print sequence is transferred, as shown in FIG. 20, the exposure timing control preliminarily controls one dot line in the sub-scanning direction, that is, only the line interval Re in the sub-scanning direction. −), The amount of shift movement of the latent image formation position by the photoconductor / belt drive control is set to ΔRc (<
Rc). Therefore, it is possible to prevent an excessive load from being applied to the motor that rotationally drives the intermediate transfer belt 41, and to perform the photoconductor / belt drive control with high accuracy.

In the second embodiment, the latent image formation position on the photosensitive member 21 is shifted in the sub-scanning direction by one dot line Re by exposure timing control (step S8), but the registration control amount is large. In this case, the exposure timing may be controlled so as to shift by a plurality of dot lines.

In the second embodiment, the exposure timing control and the photoconductor / belt drive control are combined in order to perform the registration control. However, the latent image forming position on the photoconductor 21 is controlled only by the exposure timing control. May be shifted in accordance with the resist control amount.

D. Third Embodiment In the above embodiment, after the power of the apparatus is turned on, a resist control amount establishing step (step S1) is executed to automatically establish three types of resist control amounts Ra, Rb, and Rc.
The registration control amount corresponding to the printing sequence is set by updating and setting the sequence flag corresponding to the printing sequence by updating the sequence flag stored in the AM 125 and updating the sequence flag (step S4). The three types of registration control amounts Ra, Rb, and Rc obtained in step S1) may be stored in a table format corresponding to the print sequence.

That is, one-to-one for three printing sequences
The sequence flags F0, F1, and F2 are provided correspondingly, but as shown in Table 1, the sequence flags and the resist control amounts corresponding to the print sequences corresponding to the respective sequence flags are associated with each other. The information may be stored in the RAM 125. In this case, when the sequence flag corresponding to the print sequence is set by the update processing of the sequence flag (step S4), the registration control amount corresponding to the sequence flag is collectively read from the table in the RAM 125, and the registration control amount is read. By correcting the transfer start position of the toner image for each toner color based on the above, the same effect as in the above embodiment can be obtained.

E. Fourth Embodiment By the way, in the first to third embodiments, the secondary transfer roller 48 and the cleaner blade 491 are separated from and brought into contact with the intermediate transfer belt 41 as a transfer medium during the repetition of the primary transfer process, and the separation is performed. Although a high-quality color image is obtained by correcting the registration deviation caused by the contact, the first to third embodiments are implemented by implementing the fourth embodiment instead of the first to third embodiments. Although the throughput is reduced as compared with the third embodiment, registration deviation can be completely eliminated and a high-quality color image can be formed.

In the fourth embodiment, the vertical synchronizing signal VSY
An operation sequence is set so that the secondary transfer roller 48 and the cleaner blade 491 do not come into contact with or come into contact with the intermediate transfer belt 41 from the output of the NC until the completion of the primary transfer. That is, after the primary transfer is completed, the secondary transfer process and the cleaning process are performed. In addition to the four primary transfer processes, the intermediate transfer belt 41 is idled at least once or more, and during the idle rotation process, A secondary transfer process, a belt cleaning process, and the like are performed.
Therefore, no registration deviation occurs, and a high-quality image can be obtained.

F. Others The present invention is not limited to the above-described embodiment, and various changes other than those described above can be made without departing from the gist of the present invention. For example, in the above embodiment, the magenta color is used as the reference toner color, and the center of the fluctuation width of the other toner colors (yellow, cyan, and black) is matched with the fluctuation center of the magenta color. The color may be used as the reference toner color. However, in this embodiment, the four toner colors are performed in the order of yellow (Y), cyan (C), magenta (M), and black (K), and the magenta toner image is primary-transferred third. As described above, the contact means (the secondary transfer roller 48 and the cleaner blade 491)
The magenta color is the most suitable for the reference toner color because it is least affected by the separation and contact of the toner. Also, without providing the reference toner color, the center of the fluctuation width for all the toner colors is set at an appropriate position, for example, a straight line A as shown in FIG.
C00 (“registration shift amount in the sub-scanning direction = k”) may be used.

In the above-described embodiment, the center of the fluctuation width is set to coincide with each other for all the toner colors.
The image quality can be improved by matching the centers of the fluctuation widths of at least two of the toner colors.

In the above embodiment, the photosensitive member 21 and the intermediate transfer belt 41 are connected to the same photosensitive member / belt driving unit 41.
a, the two are driven in synchronization with each other, but a photoconductor drive unit for driving and controlling the photoconductor 21 and a belt drive unit for driving and controlling the intermediate transfer belt 41 are provided. The intermediate transfer belt 41 and the intermediate transfer belt 41 may be driven synchronously.

When the photoconductor drive unit and the belt drive unit are separately provided as described above, the photoconductor 21 is driven to rotate at a constant speed, and no toner image is formed on the intermediate transfer belt 41. During the period in which the region is located in the primary transfer region R1 (the period in which the primary transfer is not performed), the transfer start position may be adjusted by controlling only the intermediate transfer belt 41 at a variable speed based on the registration control amount.

Further, in the above embodiment, the resist control amounts Ra, Rb, Rc are automatically established in the resist control amount establishing step (step S1) after the power supply of the apparatus is turned on.
Although it is stored in M125, it is not essential to execute the resist control amount establishing step every time the apparatus power is turned on.
Execution conditions of the resist control amount establishing step can be set arbitrarily. Also, instead of executing the resist control amount establishing step, a resist control amount is obtained in advance,
You may comprise so that it may memorize | store in ROM126. Further, the registration control amount may be corrected during the operation of the image forming apparatus, and after the correction, the registration control may be performed based on the corrected registration control amount. Further, a storage unit for storing the resist control amount is provided, and the RAM 125 and the R
In addition to the OM 126, a nonvolatile memory such as an EEPROM which can rewrite and read data and information may be used.

In the above-described embodiment, the sequence flags F0, F1, and F2 are set as identification variables in one-to-one correspondence with the three types of printing sequences. An identification variable may be used.

Further, in the above embodiment, the printing sequence is divided into three types, and the identification variables corresponding to the respective printing sequences are respectively set. However, the number of divided printing sequences is not limited to this. If the number of divisions is two or more, the same operation and effect as in the above-described embodiment, that is, it is not necessary to newly obtain the resist control amount every time the sequence changes, and excellent controllability can be obtained.

In the above embodiment, a DC motor is employed as a drive source for driving the intermediate transfer belt 41 to rotate.
Although the registration control is performed by controlling the acceleration and deceleration of the DC motor based on the registration control amount, the registration control is performed by using a pulse motor such as a stepping motor instead of the DC motor and performing pulse drive control based on the registration control amount. It may be.

The image forming apparatus according to the above-described embodiment prints an image given from an external device such as a host computer via the interface 112 on a sheet member such as copy paper, transfer paper, paper, and a transparent sheet for OHP. However, the present invention can be applied to an electrophotographic color image forming apparatus such as a copying machine or a facsimile apparatus, that is, an image forming apparatus that forms a color image by superimposing a plurality of color toners.

Further, in the above embodiment, the transfer step of transferring the toner image formed on the photoreceptor 21 onto the intermediate transfer belt 41 is executed for each toner color to form a color image on the intermediate transfer belt 41. However, the toner image is transferred to a transfer medium other than the intermediate transfer belt (a transfer drum, a transfer belt, a transfer sheet, an intermediate transfer drum, an intermediate transfer sheet, a reflective recording sheet, a transparent storage sheet, etc.), and a color image is formed. The present invention can also be applied to an image forming apparatus that forms the image.

[0129]

As described above, according to the present invention, the secondary transfer processing and the cleaning processing are executed after the primary transfer processing for the final toner color among the plurality of toner colors is completed. Therefore, it is possible to eliminate registration deviation due to the separation and contact of the secondary transfer unit and the cleaning unit with respect to the transfer medium, and to form a high-quality color image.

Further, if necessary, the separation / contact timing may be changed so that the secondary transfer unit and the cleaning unit temporarily contact the transfer medium when the primary transfer process is repeated. In this case, registration deviation occurs when the secondary transfer unit and the cleaning unit come into contact with the transfer medium. However, at least two or more of the plurality of toner colors are subordinate to each toner color during the transfer process. Since the centers of the deviation widths of the registration shift in the scanning direction are made to coincide with each other, the registration shift between the toner colors can be suppressed, and a high-quality color image can be obtained.

[Brief description of the drawings]

FIG. 1 is a timing chart showing an example of operation timing in an image forming apparatus to which the present invention is applied.

FIG. 2 is a diagram schematically illustrating a registration state of each toner image when a primary transfer process is performed at the operation timing of FIG. 1 without performing registration control.

FIG. 3 is a diagram illustrating a registration deviation state when a black toner image is transferred without performing registration control.

FIG. 4 is a diagram illustrating a registration deviation state when a yellow toner image is transferred without performing registration control.

FIG. 5 is a diagram illustrating a registration deviation state when a cyan toner image is transferred without performing registration control.

FIG. 6 is a diagram illustrating a state of registration deviation when a yellow toner image is transferred without performing registration control.

FIG. 7 is an electrical configuration diagram showing an embodiment of the image forming apparatus according to the present invention.

FIG. 8 is a flowchart illustrating an operation of the image forming apparatus according to the first embodiment.

FIG. 9 is a flowchart showing the updated contents of the sequence flag in FIG. 8;

10 is a diagram schematically illustrating a registration state of each toner image when a primary transfer process is performed at the operation timing of FIG. 1 while performing registration control.

11 is a diagram illustrating registration control contents when a black toner image is transferred in the image forming apparatus illustrated in FIG. 8;

12 is a diagram illustrating registration control contents when a yellow toner image is transferred in the image forming apparatus illustrated in FIG. 8;

13 is a diagram illustrating registration control contents when a cyan toner image is transferred in the image forming apparatus illustrated in FIG. 8;

FIG. 14 is a diagram illustrating registration control contents when a yellow toner image is transferred in the image forming apparatus illustrated in FIG. 8;

FIG. 15 is a flowchart illustrating processing for automatically establishing a registration control amount.

FIG. 16 is a timing chart showing the contents of a registration control amount establishment job.

FIG. 17 is a flowchart illustrating an operation of the second embodiment of the image forming apparatus according to the present invention.

18 is a diagram showing registration control contents when a yellow toner image is transferred in the image forming apparatus shown in FIG.

19 is a diagram illustrating registration control contents when a cyan toner image is transferred in the image forming apparatus illustrated in FIG. 17;

20 is a diagram illustrating registration control contents when a yellow toner image is transferred in the image forming apparatus illustrated in FIG. 17;

FIG. 21 is a diagram showing a configuration of an image forming apparatus as a background art of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 control unit (control means) 4 transfer unit 11 main controller 12 engine controller (control means) 21 photoconductor 40 vertical synchronization reading sensor 41 intermediate transfer belt (transfer medium) 48 secondary transfer roller Reference numeral 121: CPU (control means) 123: Transfer roller separation / contact control circuit 124: Belt cleaner separation / contact control circuit 491: Cleaner blade AC00, AC0 to AC4: Deflection center VSYNC: Vertical synchronization signal

Claims (4)

[Claims] A primary transfer process of transferring a toner image formed on a photoreceptor to a transfer medium that is driven to rotate in a sub-scanning direction is repeated for a plurality of different toner colors so that toner images of the respective toner colors are superimposed. An image forming apparatus for forming a color image on the transfer medium together with the transfer medium, the color image being provided on the transfer medium so that the color image can be temporarily contacted with the transfer medium to form a sheet on the transfer medium. A secondary transfer unit that performs secondary transfer to a member, a cleaning unit that is provided to be able to contact and separate from the transfer medium, and temporarily contacts the transfer medium to clean the transfer medium; And a control means for controlling the operation of separating and contacting the cleaning means with the transfer means, wherein the control means is configured to control one of the plurality of toner colors for the final toner color. After the transfer process is completed, performing the secondary transfer process by bringing the secondary transfer unit into contact with the transfer unit and performing the cleaning process by bringing the cleaning unit into contact with the transfer unit. Characteristic image forming apparatus. 2. A sensor for detecting a reference position of the transfer medium, wherein a primary transfer process corresponding to the reference signal is executed based on a reference signal output from the sensor. 2. The control unit causes the secondary transfer unit and the cleaning unit to contact and separate from the transfer medium at a timing except after the output from the reference signal until the completion of the primary transfer process. The image forming apparatus as described in the above. 3. The control unit may separate, if necessary, such that the secondary transfer unit and the cleaning unit temporarily contact the transfer medium when the primary transfer process is repeated. The contact timing is changed, and, for at least two or more of the plurality of toner colors, the center of the fluctuation width of the registration shift in the sub-scanning direction for each of the toner colors during the primary transfer processing coincides with each other. The image forming apparatus according to claim 1, wherein: 4. A primary transfer process for transferring a toner image formed on a photoreceptor to a rotationally driven transfer medium is repeated for a plurality of toner colors different from each other, and the transfer is performed by superimposing toner images of respective toner colors. In the image forming method for forming a color image on a medium, after a primary transfer process for a final toner color among the plurality of toner colors is completed and a color image is formed on the transfer medium, a secondary transfer unit is provided. The color image on the transfer medium is secondarily transferred to a sheet member by contacting the transfer unit, and the transfer medium is cleaned by contacting a cleaning unit to the transfer unit. Forming method.