JP2000267378A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the color slurring of plural color toner images transferred to an intermediate transfer body. SOLUTION: Before forming the yellow toner image of a 1st color on a photoreceptor drum 1, a yellow developing device 4Y of the 1st color to which developing bias is applied is positioned at a developing position D, and the drum 1 is rotated in a non-electrifying state and a non-exposing state, and an intermediate transfer drum 5 is rotated at least once, whereby a small quantity of yellow toner of the 1st color is uniformly fogged on the drum 5 through the drum 1 in this image forming device. Before and after the yellow toner image of the 1st color formed by the second rotation of the drum 5 passes a primary transfer part T1, a friction coefficient between the drums 1 and 5 is made nearly equal and the stable rotation of the drum 5 is maintained. Thus, the color slurring of plural color toner images transferred to the drum 5 is prevented, so that the high-quality color image is obtained in the device.

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 such as a copying machine, a printer, a facsimile, etc., which forms an image by an electrophotographic method, and more particularly, to an image forming apparatus which forms a color image by superimposing a plurality of color toner images. About.

[0002]

2. Description of the Related Art FIG. 6 is a schematic diagram showing a main part of a full-color image forming apparatus using a conventional intermediate transfer member (in the figure, an intermediate transfer drum).

In this image forming apparatus, at the time of image formation, an electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 101 is rotationally driven in the direction of an arrow (counterclockwise), and the surface thereof is charged by a charger 1.
After uniformly charging at 02, exposure L according to image information input by the exposure device 103 is performed to form an electrostatic latent image.

Developing units Y, M, C, and B containing yellow, magenta, cyan, and black toners, respectively.
The rotation of the rotatable developing device 104 having the K
The developing unit Y for the color eye (for example, yellow) is located at a developing position D facing the photosensitive drum 1, and the toner is adhered to the electrostatic latent image on the photosensitive drum 1 and developed as a yellow toner image.

The yellow toner image is transferred to the photosensitive drum 1
Is primary-transferred on the intermediate transfer body (intermediate transfer drum) 105 rotating in the direction of the arrow (clockwise) in contact with the primary transfer nip T1. Then, this primary transfer step is performed by another 3
The same process is performed for colors, that is, magenta, cyan, and black, and four color toner images are superimposed on the intermediate transfer member 105.

[0006] These four color toner images are collectively transferred onto a transfer material P conveyed from a paper feed cassette (not shown) by a secondary transfer member 106 at a secondary transfer nip T2.
By performing the next transfer, a full-color image can be obtained. The secondary transfer member 106 is brought into contact with the secondary transfer nip portion T2 of the intermediate transfer member 105 at a predetermined pressure by the pressing device 107 during the secondary transfer.

FIG. 7 is a diagram showing the relationship between the rotation of the intermediate transfer member 105 and the colors (yellow, magenta, cyan, and black) of the toner image to be primarily transferred during image formation.

In this image formation, the intermediate transfer member 10
The yellow, magenta, cyan, and black toner images are sequentially transferred (primary transfer) by the rotation of 5, and the full-color image is formed by superimposing the four color toner images by rotating the intermediate transfer body 105 four times. Thus, the images are collectively transferred to one (one page) transfer material P.

[0009]

By the way, in the above-mentioned conventional image forming apparatus, before the leading end of the yellow toner image of the first color is primarily transferred in the primary transfer nip portion T1, no toner is interposed. After the primary transfer, the change in friction between the photosensitive drum 101 and the intermediate transfer member 105 due to the interposition of the yellow toner causes the intermediate transfer member 105 to change.
Is slightly slipped and the peripheral speed of the intermediate transfer member 105 changes, thereby causing a problem that affects the full-color image transferred to the transfer material P.

That is, the position of the toner image is relatively shifted between the leading end and the trailing end of the first color yellow toner image transferred (primary transfer) onto the intermediate transfer member 105.
Hereinafter, this shift is referred to as “color shift”.

In the case of the magenta toner image of the second color, the yellow toner is interposed before the primary transfer in the primary transfer nip portion T1, and the yellow toner and the magenta toner are interposed after the primary transfer. Toner is interposed. Similarly, in the third color cyan toner image and the fourth color black toner image, toner is interposed before and after the primary transfer in the primary transfer nip T1.

That is, yellow Y, cyan C, black B
When the color misregistration of K with respect to magenta M is evaluated, the color misregistration due to the leading end of the yellow toner image is maximized as described above. FIG. 8 shows an example of the evaluation of the color shift at this time.
Shown in

FIG. 8 is a diagram showing the evaluation results of color misregistration when transferred to A3 size paper (transfer material). The horizontal axis represents the length direction of A3 paper (0 to 420 mm), and the vertical axis represents the vertical axis. The color shift amount based on magenta M is shown. Further, when a colored graph or the like is output as an image on a white background in this state, the disturbance of the image due to the color shift of the yellow toner image at the leading end of the image is remarkable.

In this evaluation, the maximum value of the amount of color shift at the leading end of the yellow toner image was 125 μm, and the average of the amount of color shift was 45 μm. Also, 10 minutes from the leading edge of A3 paper
At around 0 μm, the color shift of the yellow toner image is stably improved.

Accordingly, an object of the present invention is to provide an image forming apparatus capable of obtaining a high quality image by reducing the amount of color misregistration.

[0016]

In order to achieve the above object, the present invention provides an image carrier, charging means for charging the image carrier, and exposing a surface of the image carrier charged by the charging means to light. Exposing means for forming an electrostatic latent image corresponding to the image information, developing means for forming a toner image by electrostatically attaching toner to the electrostatic latent image, and a developing means formed on the image carrier. An intermediate transfer member onto which the toner image is transferred, and a step of transferring the toner image formed on the image carrier to the intermediate transfer member by repeating a step of transferring a plurality of color toners on the intermediate transfer member. An image forming apparatus that forms an image and transfers the plurality of color toner images on the intermediate transfer body to a transfer material;
Before forming a toner image of a color on the image carrier, the intermediate transfer body is rotated at least once while the electric field is formed by the developing unit without operating the charging unit and the exposing unit. It is characterized by.

A first mode in which the intermediate transfer member is rotated a predetermined number of times to form the plurality of color toner images on the intermediate transfer member; and a first mode in which the first color toner image is formed on the image carrier. Before the operation, the charging unit and the exposing unit are not operated, and while the electric field is formed by the developing unit, the intermediate transfer body is rotated at least once, and then the intermediate transfer body is rotated a predetermined number of times. The image forming apparatus is characterized in that the image forming apparatus further comprises control means for switching between the second mode for forming the plurality of color toner images on the intermediate transfer member.

Further, before forming the first color toner image on the image carrier, the intermediate transfer is performed without operating the charging unit and the exposing unit and keeping an electric field formed by the developing unit. The first color toner is transferred from the image carrier to the intermediate transfer body during at least one rotation of the body.

Further, after the first color toner is transferred from the image bearing member to the intermediate transfer member, the first color toner image is transferred from the image bearing member to the intermediate transfer member. I have.

Further, the plurality of color toner images are a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image.

The first color toner image is a yellow toner image.

Further, the developing means includes a plurality of developing devices for forming the toner images of the plurality of colors on the image carrier, respectively.

Further, there is provided a moving means for moving the plurality of developing devices to a developing position.

Further, a plurality of the image carriers are provided for respectively carrying the toner images of the plurality of colors.

(Operation) According to the present invention, before the first color toner image is formed on the image carrier, the charging means and the exposure means are not operated and the electric field is formed by the developing means. By rotating the intermediate transfer member at least once, a small amount of toner of the first color can be intentionally transferred (fogged) onto the intermediate transfer member. As a result, since a small amount of toner is interposed before the leading end of the first color image is transferred onto the intermediate transfer body, a change in the peripheral speed of the intermediate transfer body due to the above-described slip is reduced, thereby suppressing color misregistration and achieving high image quality. Can be secured.

Further, according to the present invention, a high-quality color image can be ensured by making the first color toner intentionally transferred (fogged) onto the intermediate transfer body a low density and inconspicuous yellow toner. can do.

[0027]

Embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment> FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to a first embodiment of the present invention.

This image forming apparatus includes a photosensitive drum 1 as an image carrier, a charging roller 2, an exposing device 3, a developing device 4, an intermediate transfer drum 5 as an intermediate transfer member, a fixing device 6, and a secondary transfer member. A secondary transfer belt 8 and the like are provided.

The photosensitive drum 1 has a photoconductive layer (not shown) on a drum base (not shown) made of aluminum.
It is rotationally driven in the direction of arrow a (counterclockwise) at a predetermined peripheral speed (process speed). A cleaning device 7 is provided outside the photosensitive drum 1, and an intermediate transfer drum 5 is provided.
The primary transfer residual toner remaining on the photosensitive drum 1 without being primarily transferred to the photosensitive drum 1 is removed by the cleaning blade 7a.

The charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force, rotates following the rotation of the photosensitive drum 1, and turns the photosensitive drum 1 to a predetermined polarity and potential by applying a charging bias. Charges uniformly.

The developing device 4 includes a yellow (Y) developing device 4
Y, magenta (M) developing device 4M, cyan (C) developing device 4
C, a black (BK) developing device 4BK is mounted on the rotating body 4a, and the rotating body 4a is driven by a rotation driving device (not shown).
In the developing process, the yellow developing unit 4Y, the magenta developing unit 4M, the cyan developing unit 4C, and the black developing unit 4BK are arranged at the developing position D facing the photosensitive drum 1 in the order of rotation in the direction of the arrow b (clockwise). .

These developing units 4Y, 4M, 4C, 4BK
Is provided with a developing sleeve 4b that holds toner as a developer carrier and conveys the toner to a developing position D. At the time of development, a developing bias (a negative DC voltage, and if necessary, an AC voltage is superimposed on the developing sleeve 4b). May be applied, and the toner is electrostatically attached to the electrostatic latent image on the photosensitive drum 1 and developed as a toner image. The developing method may be a contact developing method in which the developing sleeve 4b is brought into contact with the photosensitive drum 1 for development, or a non-contact developing method in which the developing sleeve 4b is developed in contact with the photosensitive drum 1 without contact. In the present embodiment, the exposed portion potential portion on the surface of the photosensitive drum 1 is reversely developed with negatively charged negative toner to visualize the electrostatic latent image (to make it visible).

The intermediate transfer drum 5 has a primary transfer nip T
1, the photosensitive drum 1 contacts the surface of the photosensitive drum 1, and further contacts the surface of the secondary transfer belt 8 at the secondary transfer nip T <b> 2, and rotates in the direction of arrow c (clockwise). A primary transfer bias power supply 9 is connected to the intermediate transfer drum 5, and a predetermined primary transfer bias is applied to the core of the intermediate transfer drum 5 from the primary transfer bias power supply 9, whereby The formed toner image is primarily transferred onto the intermediate transfer drum 5 by the potential difference between the photosensitive drum 1 and the intermediate transfer drum 5 in the primary transfer nip T1.

The secondary transfer belt 8 includes a secondary transfer roller 10
And an endless belt that is stretched and suspended by the drive roller 11, and the upper surface of the belt moves in the direction of the arrow d by the rotation of the drive roller 11. The secondary transfer roller 10 has 2
A secondary transfer bias power supply 12 is connected to apply a predetermined secondary transfer bias to the secondary transfer roller 10.

The secondary transfer roller 10 and the driving roller 11
The transfer frame 13 is supported by bearings, and a pressure spring 14 and a pressure piece 15 are arranged on one end side of the transfer frame 13. The pressure piece 15 is supported by a pressure cam 16, and the secondary transfer belt 8 is rotated at a predetermined timing by the rotation of the pressure cam 16 by the rotation of an electromagnetic clutch (not shown) and a driving unit (not shown). And can be freely contacted / separated from the intermediate transfer drum 5. Secondary transfer belt 8
During the secondary transfer, the secondary transfer roller 10 is brought into contact with the intermediate transfer drum 5 at a predetermined secondary transfer pressure via the secondary transfer roller 10 by the spring force of the pressure spring 14 during the secondary transfer.

On the outer peripheral surface of the intermediate transfer drum 5, a roller-shaped multi-layer intermediate transfer member cleaning roller 17 is disposed so as to be able to contact / separate from the intermediate transfer drum 5. During the cleaning, a predetermined bias is applied from the bias power supply 18 to remove the secondary transfer residual toner remaining on the surface of the intermediate transfer body 5 without being transferred to the transfer material P. The intermediate transfer member cleaning roller 17 charges the secondary transfer residual toner on the intermediate transfer drum to the opposite polarity. The secondary transfer residual toner charged to the opposite polarity is charged on the photosensitive drum 1. At the same time as the primary transfer of the toner image onto the intermediate transfer drum 5, the toner image is transferred from the intermediate transfer drum 5 to the photosensitive drum 1 and removed by the cleaning device 7.

The fixing device 6 has a fixing roller 6a and a pressure roller 6b.
The transfer material P is heated and pressurized while the transfer material P on which the unfixed toner image has been transferred to the fixing nip portion is fixed, so that the toner image is fixed.

Next, the image forming operation of the above-described image forming apparatus in this embodiment will be described.

First, the photosensitive drum 1 is driven to rotate at a predetermined peripheral speed (process speed) in the direction of arrow a. Also,
Similarly, the intermediate transfer drum 5 is rotated in the direction of arrow c at a predetermined peripheral speed.

Then, the developing device 4 rotates to move the first color yellow developing device 4Y to the developing position D, and a developing bias is applied to the yellow developing device 4Y. The rotation of the intermediate transfer drum 5 at this time is the first rotation. At this time, no charging bias is applied to the charging roller 2, and no image exposure is performed by the exposure device 3. That is, no image formation is performed on the intermediate transfer drum 5 and the drum is idle.

Therefore, in this state, the yellow developing device 4
The same developing bias as in normal image formation is applied to Y (if necessary, the developing sleeve 4 of the yellow developing device 4Y).
The bias (voltage) applied to b may be smaller than the normal developing bias), and the yellow developing device 4Y is located at the developing position D, so that a small amount of yellow toner adheres to the photosensitive drum 1. It is further uniformly attached (covered) to the surface of the intermediate transfer drum 5 at the primary transfer nip T1. At this time, no voltage is applied to the metal core of the intermediate transfer drum 5, but a voltage (positive polarity) may be applied as necessary.

Thereafter, when the image forming operation is started, the photosensitive drum 1 is charged to a predetermined polarity and potential by the charging roller 2 to which a predetermined charging bias is applied. Then, an image exposure L by a laser beam is given to the charged photosensitive drum 1 by the exposure device 3 to form an electrostatic latent image corresponding to the first color yellow toner image of the target color image. Next, the electrostatic latent image is developed by the yellow developing device 4Y with the first color yellow toner to form a yellow toner image.

The yellow toner image formed and carried on the photosensitive drum 1 passes through the primary transfer nip T1 between the photosensitive drum 1 and the intermediate transfer drum 5, and the pressure in the primary transfer nip T1 is reduced. The primary transfer (intermediate transfer) is performed on the outer peripheral surface of the intermediate transfer drum 5 by the electric field generated by the primary transfer bias applied to the intermediate transfer drum 5 from the primary transfer bias power supply 9. The intermediate transfer drum 5 at this time
Is the second rotation.

At this time, on the intermediate transfer drum 5 before the leading end of the yellow toner image passes through the primary transfer nip portion T1, a small amount of yellow toner fogged when the intermediate transfer drum 5 idles is interposed. are doing. Further, the yellow toner image is interposed even after the leading end of the yellow toner image has passed through the primary transfer nip portion Tl. This means that the friction coefficient between the photosensitive drum 1 and the intermediate transfer drum 5 becomes substantially equal before and after the leading end of the yellow toner image passes through the primary transfer nip T1.

Next, the magenta developing device 4M is moved to the developing position D, and the second color magenta toner image formed and carried on the photosensitive drum 1 in the same manner as the first color yellow toner image is transferred to the intermediate transfer drum. Transfer onto 5 The rotation of the intermediate transfer drum 5 at this time is the third rotation.

Thereafter, similarly, the cyan developing device 4C and the black developing device 4BK are sequentially moved to the developing position D, and the third color cyan toner image is obtained at the fourth rotation of the intermediate transfer drum 5.
At the fifth rotation of the intermediate transfer drum 5, the fourth color black toner image is sequentially superimposed and transferred onto the intermediate transfer drum 5, and a full-color image is formed. At this time, a primary transfer bias applied from a primary transfer bias power supply 9 for sequentially superimposing transfer of the first to fourth color toner images from the photosensitive drum 1 to the intermediate transfer member 5 has a polarity opposite to that of the toner. is there. The secondary transfer belt 8 and the intermediate transfer member cleaning roller 17 are separated from the intermediate transfer member 5 in the process of sequentially superimposing and transferring the first to fourth color toner images from the photosensitive drum 1 to the intermediate transfer drum 5. .

Then, in synchronization with the formation of the full-color image, the secondary transfer belt 8 abuts on the intermediate transfer drum 5 by the spring force of the pressure spring 14 by the rotation of the pressure cam 16, and the paper feed cassette ( Transfer material P such as paper (not shown)
Is fed to a secondary transfer nip T2 between the intermediate transfer drum 5 and the secondary transfer belt 8 through a pair of registration rollers 19 and a pre-transfer guide 20.

At this time, the secondary transfer bias power supply 12
A secondary transfer bias is applied to the next transfer roller 10, and the full-color image superimposedly transferred (primary transfer) on the intermediate transfer drum 5 is collectively transferred onto the transfer material P.

The transfer material P on which the full-color image has been transferred is separated by curvature at the downstream side in the transport direction of the secondary transfer belt 8 and is transported between the fixing roller 6a and the pressure roller 6b of the fixing device 6 and heated. The paper is pressed, the full-color image is fixed on the surface, and the paper is discharged onto a paper discharge tray (not shown).

The secondary transfer residual toner remaining on the intermediate transfer drum 5 without being subjected to the secondary transfer is changed to the opposite polarity by the intermediate transfer member cleaning roller 17 to which the bias is applied, and the photosensitive drum 1 Electrostatically attracted to
The surface of the intermediate transfer drum 5 is cleaned. The secondary transfer residual toner adsorbed on the photosensitive drum 1 is then removed from the cleaning device 7.
Is collected by the cleaning blade 7a.

FIG. 2 is a diagram showing the rotation of the intermediate transfer drum 5 and the state of transfer on the intermediate transfer drum 5 during the above-mentioned image formation. FIG. 2 shows the transfer of a full-color image onto one (one page) transfer material P. In the formation, the intermediate transfer drum 5 is rotated five times, and is idlely rotated (one rotation) before the transfer of the first color.

FIG. 3 is a diagram showing an evaluation result of color misregistration when a full-color image formed by the above-described image forming apparatus of the present embodiment is transferred to an A3-size sheet (transfer material). The horizontal axis is the length direction of A3 paper (0 to 420
mm), and the vertical axis indicates the amount of color shift based on magenta M.

As is apparent from this evaluation, the maximum value of the color shift amount at the leading end of the yellow toner image of the first color is 75 μm.
m, the average of the amount of color misregistration is 32 μm.

As described above, in this embodiment, before the yellow toner image of the first color is transferred to the intermediate transfer body 5, the yellow toner image is rotated once (idle rotation).
The first color yellow developing device 4Y is moved to the developing position D, and a developing bias is applied, so that a small amount of yellow toner is transferred via the photosensitive drum 1 rotating (one rotation) in an uncharged state and a non-exposed state. The toner is uniformly covered on the intermediate transfer drum 5.

Then, at the second rotation of the intermediate transfer drum 5,
Before and after the first color yellow toner image to be formed passes through the primary transfer nip portion Tl, the friction coefficients between the photosensitive drum 1 and the intermediate transfer drum 5 become substantially equal, and as described in the above-described conventional example, the yellow toner image becomes yellow. Since the change in the peripheral speed of the intermediate transfer drum 5 due to the slip caused by the friction difference before and after the leading end of the toner image passes through the primary transfer nip portion T1, the color shift at the leading end of the yellow toner image can be greatly reduced. As a result, a high-quality image could be obtained.

In the first embodiment of the present invention, a small amount of toner of the first color irrelevant to the target full-color image is intentionally applied to the intermediate transfer drum 5 at the first rotation (idle rotation). Because of the fogging configuration, the small amount of fogging toner may not be yellow toner and may be any of magenta, cyan, and black. However, as in the present invention, four colors (yellow, magenta, cyan, and black) are used. ) Is the lightest and least noticeable yellow
By setting the color toner, that is, the toner to be covered on the intermediate transfer member 5, it is possible to hold a high-quality image.

In the image forming step of the present invention, when a full-color image is formed on one transfer material P, at least the intermediate transfer drum 5 rotates five times after an image forming operation signal is input. This means that the throughput (the number of output sheets per minute) is reduced to 4/5 as compared with the conventional image forming process in which at least four rotations are performed after the input of the image forming operation signal. For this reason, when outputting a general image, the "normal mode" in which the intermediate transfer drum 5 rotates four times is used as in the conventional case. By forming the image by switching to the “high image quality mode” in which the transfer drum 5 is rotated five times, the image is formed in the “high image quality mode” only when it is desired to obtain a high quality image with greatly suppressed color shift. The formation can take place. Such mode switching is performed by a CPU as control means (not shown).
Is controlled by

Second Embodiment FIG. 4 is a schematic configuration diagram showing an image forming apparatus according to a second embodiment of the present invention. The same members as those of the image forming apparatus according to the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and overlapping description will be omitted.

The image forming apparatus of the present embodiment uses an intermediate transfer belt 30 as an intermediate transfer member, a primary transfer roller 31 for primary transfer, and a secondary transfer roller 10 as a secondary transfer member. It is. Other configurations are the same as in the first embodiment.

The intermediate transfer belt 30 includes a primary transfer roller 3
1, is stretched between the driving roller 32 and the driven roller 33, and is driven to rotate in the direction of arrow c at a predetermined peripheral speed by driving of the driving roller 32 by a driving means (not shown).

The secondary transfer roller 10 has a transfer frame 13
, And a pressure spring 14 and a pressure piece 15 are arranged on one end side of the transfer frame 13. The pressure piece 15 is supported by a pressure cam 16,
The next transfer roller 10 can be freely contacted / separated with the intermediate transfer belt 30 at a predetermined timing by the rotation of the pressure cam 16 by the rotational drive of an electromagnetic clutch (not shown) and a driving unit (not shown). The secondary transfer roller 10 is brought into contact with the secondary transfer nip T2 between the intermediate transfer belt 30 and the intermediate transfer belt 30 at a predetermined secondary transfer pressure by the spring force of the pressure spring 14 during the secondary transfer. A primary transfer bias power supply 9 is connected to the primary transfer roller 31, and a secondary transfer bias power supply 12 is connected to the secondary transfer roller 10.

In this embodiment, as in the first embodiment, first, the first color yellow developing device 4Y is positioned at the developing position D during the first rotation of the intermediate transfer belt 30, and the yellow developing device By applying a developing bias to 4Y, a small amount of yellow toner adheres to the photosensitive drum 1 rotating (one rotation) in a non-charged state and a non-exposed state, so that the yellow toner is further intermediated in the primary transfer nip portion T1. The toner is uniformly adhered (wears) on the surface of the transfer belt 30. The following image forming operation is the same as in the first embodiment.

As described above, also in the present embodiment, the photosensitive drum 1 and the intermediate transfer belt before and after the leading end of the yellow toner image of the first color in the second rotation of the intermediate transfer belt 30 passes through the primary transfer nip portion T1. Since the friction coefficients between the intermediate transfer belts 30 can be made substantially equal, the intermediate transfer belt 30 can perform stable image formation without a change in peripheral speed, and greatly reduce color misregistration to obtain high-quality images. Can be obtained.

Also, in the present embodiment, as described in the first embodiment, by switching between the "normal mode" and the "high image quality mode" to form an image, color misregistration is greatly reduced. The image formation can be performed in the above-mentioned “high image quality mode” only when it is desired to obtain a high quality image suppressed to a minimum. Such mode switching is controlled by a CPU (not shown) as control means.

<Embodiment 3> FIG. 5 is a schematic configuration diagram showing an image forming apparatus according to Embodiment 3 of the present invention. Members having the same functions as those of the image forming apparatus according to the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and overlapping descriptions will be omitted.

The image forming apparatus according to the present embodiment includes four image forming units. Each image forming unit has a photosensitive drum 1Y, 1M, 1C, and 1BK, respectively.
Charging rollers 2Y, 2M, 2C, 2BK and exposure device 3
Y, 3M, 3C, and 3BK, a yellow developing device 4Y, a magenta developing device 4M, a cyan developing device 4C, and a black developing device 4.
BK etc. are provided. The yellow developing device 4Y, the magenta developing device 4M, the cyan developing device 4C, and the black developing device 4BK store yellow, magenta, cyan, and black toners, respectively.

Each of the photosensitive drums 1Y, 1M, 1C, and 1BK has a primary transfer nip T1Y, T1M, T1C, T1.
BK contacts the intermediate transfer belt 30. The intermediate transfer belt 30 includes a driving roller 31 and driven rollers 32 and 33.
The driving roller 31 is driven by a driving unit (not shown) to rotate at a predetermined peripheral speed in a direction indicated by an arrow c.

The secondary transfer roller 10 has a transfer frame 13
, And a pressure spring 14 and a pressure piece 15 are arranged on one end side of the transfer frame 13. The pressure piece 15 is supported by a pressure cam 16,
The next transfer roller 10 can be freely contacted / separated with the intermediate transfer belt 30 at a predetermined timing by the rotation of the pressure cam 16 by the rotational drive of an electromagnetic clutch (not shown) and a driving unit (not shown).

The secondary transfer roller 10 causes the secondary transfer roller 10 to move between the secondary transfer roller 10 and the intermediate transfer belt 30 by the spring force of the pressure spring 14 during the secondary transfer.
It is brought into contact with the next transfer nip portion T2 at a predetermined secondary transfer pressure.
The primary transfer bias power supply 9 is connected to the intermediate transfer belt 30, and the secondary transfer bias power supply 12 is connected to the secondary transfer roller 10.

In the present embodiment, similarly to Embodiments 1 and 2, a developing bias is first applied to the first color yellow developing device 4Y during the first rotation of the intermediate transfer belt 30, and a small amount of yellow is applied. By attaching the toner to the photosensitive drum 1Y rotating (one rotation) in a non-charged state and a non-exposed state, the toner is further uniformly attached to the surface of the intermediate transfer belt 30 at the primary transfer nip portion T1Y ( Wear). At this time, the photosensitive drums 1M, 1C, and 1BK also rotate (one rotation) in a non-charged state.

The photosensitive drums 1Y, 1M, 1C, and 1BK in the second rotation rotating in the direction of the arrow (clockwise) are charged to predetermined polarities and potentials by the charging rollers 2Y, 2M, 2C, and 2BK.

Then, the charged photosensitive drum 1Y is subjected to image exposure LY by a laser beam from the exposure device 3Y via the reflection mirror 3a, and an electrostatic latent image corresponding to a yellow color component image of a target color image is provided. Is formed. Next, the electrostatic latent image is developed with the yellow toner of the developing device 4Y to form a yellow toner image. Similarly, magenta, cyan, and black toner images are formed on the photosensitive drums 1M, 1C, and 1BK by the magenta developing device 4M, the cyan developing device 4C, and the black developing device 4BK, respectively.

Then, the photosensitive drums 1Y, 1M, 1C, 1B are placed on the intermediate transfer belt 30 rotating in the direction of arrow c.
From K, yellow, magenta, cyan, and black toner images are respectively transferred to primary transfer nip portions T1Y, T1M, T
In 1C and T1BK, the images are superposed and transferred in this order to form a full-color image.

Then, in synchronization with the formation of this full-color image, the secondary transfer roller 10 abuts on the intermediate transfer belt 30 by the spring force of the pressure spring 14 by the rotation of the pressure cam 16, and the paper cassette ( (Not shown), a transfer material P such as paper is transferred between the intermediate transfer belt 30 and the secondary transfer roller 10 through a pre-transfer guide 20 and a pair of registration rollers 19.
The sheet is fed to the next transfer nip T2.

At this time, the secondary transfer bias power supply 12
A secondary transfer bias is applied to the next transfer roller 10, and the full-color image superimposedly transferred (primary transfer) on the intermediate transfer belt 30 is collectively transferred onto the transfer material P.

The transfer material P on which the full-color image has been transferred is fixed to the fixing roller 6a of the fixing device 6 and the pressure roller 6b.
The sheet is conveyed in between, heated and pressed, a full-color image is fixed on the surface, and the sheet is discharged onto a sheet discharge tray (not shown).

The image forming apparatus of the present embodiment described above
A plurality of photosensitive drums 1Y, 1M, 1C, and 1BK are arranged in series, and primary transfer nip portions T1Y and T1 are respectively provided.
M, T1C, and T1BK are the most severe configurations for color misregistration due to their positional accuracy.
Applying a developing bias to Y to remove a small amount of yellow toner,
The photosensitive drum 1Y rotating in a non-charged state and a non-exposed state
Over the intermediate transfer belt 30 via the.

Thus, also in the present embodiment, similarly to the first embodiment, before and after the leading end of the yellow toner image of the first color in the second rotation of the intermediate transfer belt 30 passes through the primary transfer nip portion T1Y. Can be made substantially equal in the friction coefficient between the photosensitive drum 1 and the intermediate transfer belt 30.
The intermediate transfer belt 30 can perform stable image formation without a change in peripheral speed, and can greatly reduce color misregistration to obtain a high-quality image.

Also, in the present embodiment, as described in the first embodiment, by switching between the "normal mode" and the "high image quality mode" to form an image, color misregistration is greatly reduced. The image formation can be performed in the above-mentioned “high image quality mode” only when it is desired to obtain a high quality image suppressed to a minimum. Such mode switching is controlled by a CPU (not shown) as control means.

[0081]

As described above, according to the present invention, 1
Before forming the color toner image on the image carrier, without operating the charging unit and the exposing unit, while the electric field is formed by the developing unit, the intermediate transfer body is rotated at least once,
By uniformly covering a small amount of toner of the first color on the intermediate transfer member via the image carrier, the toner image of the first color to be formed is transferred to the primary transfer portion at the second rotation of the intermediate transfer member. Before and after the passage, the coefficient of friction between the image carrier and the intermediate transfer member can be made substantially equal, and stable rotation of the intermediate transfer member can be maintained.

As a result, the color shift at the tip of the first color toner image is greatly reduced, and a high quality image can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating rotation of an intermediate transfer drum and a transfer state on the intermediate transfer drum during image formation according to the first embodiment.

FIG. 3 is a diagram showing evaluation results of color misregistration in the first embodiment.

FIG. 4 is a schematic configuration diagram illustrating an image forming apparatus according to a second embodiment of the present invention.

FIG. 5 is a schematic configuration diagram showing an image forming apparatus according to Embodiment 3 of the present invention.

FIG. 6 is a schematic configuration diagram showing an image forming apparatus in a conventional example.

FIG. 7 is a diagram illustrating a rotation of an intermediate transfer drum and a transfer state on the intermediate transfer drum during image formation according to a conventional example.

FIG. 8 is a diagram showing evaluation results of color misregistration in a conventional example.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum (image carrier) 2 charging roller 3 exposing device 4 developing device (developing means) 4a rotating body (moving means) 4Y yellow developing device 4M magenta developing device 4C cyan developing device 4BK black developing device 5 intermediate transfer drum (intermediate) (Transfer member) 6 Fixing device 6a Heat roller 6b Pressure roller 8 Secondary transfer belt 10 Secondary transfer roller 30 Intermediate transfer belt (intermediate transfer member) 31 Primary transfer roller

Claims (9)

[Claims] 1. An image carrier, a charging unit for charging the image carrier, and an exposure for exposing a surface of the image carrier charged by the charging unit to form an electrostatic latent image corresponding to image information. Means, developing means for electrostatically attaching toner to the electrostatic latent image to form a toner image, and an intermediate transfer member to which the toner image formed on the image carrier is transferred. Repeating the step of transferring the toner image formed on the image carrier to the intermediate transfer body to form a plurality of color toner images on the intermediate transfer body, and forming the plurality of color toner images on the intermediate transfer body. In an image forming apparatus for transferring a toner image to a transfer material, an electric field is formed by the developing unit without operating the charging unit and the exposing unit before forming a first color toner image on the image carrier. The intermediate transfer body is rotated at least once That, the image forming apparatus characterized by. 2. A first method for forming the plurality of color toner images on the intermediate transfer member by rotating the intermediate transfer member a predetermined number of times.
And before the toner image of the first color is formed on the image carrier, the intermediate transfer body is formed without forming the electric field by the developing unit without operating the charging unit and the exposing unit. Having at least one rotation, and a control means for switching the intermediate transfer body to a second mode in which the intermediate transfer body is rotated a predetermined number of times to form the plurality of color toner images on the intermediate transfer body. The image forming apparatus according to claim 1, wherein: 3. Before forming the first color toner image on the image bearing member, the charging unit and the exposing unit are not operated, and the electric field is formed by the developing unit.
3. The image forming apparatus according to claim 1, wherein the first color toner is transferred from the image carrier to the intermediate transfer body during at least one rotation of the intermediate transfer body. 4. 4. After the first color toner is transferred from the image carrier to the intermediate transfer member, the first color toner image is transferred from the image carrier to the intermediate transfer member. The image forming apparatus according to claim 3, wherein 5. The image forming apparatus according to claim 1, wherein the plurality of color toner images are a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image. 6. The image forming apparatus according to claim 5, wherein the first color toner image is a yellow toner image. 7. The image forming apparatus according to claim 1, wherein the developing unit includes a plurality of developing units that respectively form the plurality of color toner images on the image carrier. 8. The image forming apparatus according to claim 7, further comprising moving means for moving the plurality of developing devices to a developing position. 9. The image forming apparatus according to claim 1, wherein a plurality of the image carriers are provided to carry the toner images of the plurality of colors, respectively.