JP2003091125A - Color image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain high image reproducibility by preventing the rise of image density in a monochromatic image forming mode by easily changing an image forming condition at an image forming part positioned on a most upstream side in the monochromatic image forming mode and a color image forming mode. SOLUTION: The initial potential by electrification of a photoreceptive drum 201a is changed on a white side by rotating an electrifying roller 202a in a counter direction through a driving device 15 in the monochromatic image forming mode in which a toner sticking amount by reverse transfer development is not reduced and the initial potential by electrification of the photoreceptive drum 201a is changed on a non-white side by rotating the electrifying roller 202a in a width direction through the driving device 15 at the time of a color image forming mode in which the toner sticking amount by the reverse transfer is reduced by a CPU 11 that is provided at the image forming part 200a on the most upstream side and that constitutes a driving control part 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has a plurality of image forming portions for forming images of mutually different colors by an electrophotographic method arranged in parallel in a conveying path of a transfer material onto which an image is transferred. The present invention relates to a color image forming apparatus that sequentially and multiplex transfers formed images onto a transfer material.

[0002]

2. Description of the Related Art As a color image forming apparatus such as a color copying machine or a color printer for forming a full-color image by an electrophotographic image forming process, a single photosensitive material is used with a transfer material being electrostatically adsorbed on its peripheral surface. A so-called single-drum type image forming apparatus provided with a body drum and a semi-conductive drum that rotates at a constant speed, and a so-called side-by-side arrangement of a plurality of image forming units that form images of mutually different colors in a transfer material transport path. A tandem type color image forming apparatus.

In a single-drum type image forming apparatus, the steps of charging, exposing, developing, transferring, and cleaning are repeatedly performed on a single photosensitive drum for each of a plurality of image colors, and a semiconductive drum is used. Since a color image is formed on the transfer material adsorbed on the peripheral surface of the sheet, there is a drawback that the processing time becomes long. On the other hand, the tandem type color image forming apparatus forms a plurality of image formations while one transfer material is conveyed once from the paper feed unit to the paper discharge unit via a conveyance member such as a conveyance belt. Since the images formed by each of the image forming units are sequentially transferred, there is an advantage that the processing speed is high. With the downsizing and cost reduction of the devices that form each image forming unit, the tandem type color image forming apparatus is mainstream. Is becoming.

However, in the tandem type color image forming apparatus, the transfer material is conveyed through the plurality of image forming sections in order even in the monochrome image forming mode such as a monochrome image, so that the transfer material forms a single image. Contacting the photoconductors of a plurality of image forming parts other than the image forming part of, the unnecessary color toner remaining on the surface of these photoconductors adheres to the transfer material,
The image forming state on the transfer material deteriorates. In addition, since the transfer material and the transport member also contact the surface of the photoconductor of the image forming unit that does not perform image formation in the single-color image forming mode, the photoconductor and the transport member are quickly worn, and the life of these parts is shortened. Become.

Therefore, in JP-A-6-258914 and JP-A-10-198121, it is unnecessary for the transfer material by separating the conveying member from the image forming section in which no image is formed in the single color image forming mode. There is disclosed a configuration in which the adhesion of toner of various colors and the shortening of the life of the photoconductor and the conveying member are prevented.

Since a monochrome image is often formed as a monochromatic image forming process, a conveying member is always provided on the photoconductor of the image forming unit that forms a black image among the plurality of image forming units. The conveying member is configured to be contactable and separable with respect to an image forming unit that forms an image of cyan (C), magenta (M), and yellow (Y) other than black (BK). Further, of the plurality of image forming units arranged in parallel along the transfer material conveyance direction, the conveying member is always brought close to only a single image forming unit, and the conveying member is brought into contact with or separated from other image forming units. In order to be flexible, the image forming unit (the image forming unit for forming the image of BK), which should be always close to the conveying member, is arranged on the most upstream side or the most downstream side in the conveying direction of the transfer material, and The simplest configuration method is to make the transport member swingable with the upstream side or the most downstream side as a fulcrum.

In this case, since the BK toner has a high concealing effect with respect to the toners of other colors, if the image forming portion for forming the BK image is arranged on the most downstream side in the transfer material conveying direction, the color image forming mode is set. At times, the colors of the C, M, and Y toners previously transferred onto the transfer material are hidden by the BK toners, and a blackened image is formed as a whole. Therefore, in order to improve the formation state of a full-color image, Japanese Patent Laid-Open Publication Nos. 6-258914 and 10-
Contrary to the configuration disclosed in Japanese Patent Publication No. 198121, B
The image forming unit for forming the K image should be arranged on the most upstream side in the transfer material conveyance direction.

On the other hand, in the color image forming mode in the tandem type color image forming apparatus, the reverse transfer phenomenon in which the unfixed toner transferred to the transfer material in the upstream image forming section is taken away in the downstream image forming section Occurs and the reproducibility of color tone deteriorates. This reverse transfer phenomenon occurs when the toner supplied from the developing device to the photoconductor during the developing process includes toner having an insufficient amount of charge or toner charged with the opposite polarity, and when the transfer material is peeled from the photoconductor. The toner on the transfer material may be charged to the opposite polarity due to the generated discharge, and it is considered that these defective toners are caused by the transfer electric field from the transfer material to the photoreceptor due to the transfer electric field during the transfer process.

For example, in a tandem type color image forming apparatus having four image forming sections, the first color transferred to the transfer material in the image forming section arranged on the most upstream side in the conveying direction of the transfer material. When the toner is lost from the transfer material due to the reverse transfer phenomenon in each of the three image forming portions located on the downstream side, and the adhesion amount on the transfer material finally decreases by several tens of percent of the initial adhesion amount. There is.

Therefore, in the conventional tandem type color image forming apparatus, the image forming conditions in each of the plurality of image forming units are individually controlled, and the image forming unit located closer to the upstream side in the transfer material conveying direction is exposed to light. A large amount of toner is attached to the body to compensate for the toner lost from the transfer material due to the reverse transfer phenomenon.

[0011]

However, in the case where the conveying member can be freely moved toward and away from the image forming portion other than the image forming portion located on the most upstream side, reverse transfer is performed in the image forming portion where the conveying member is separated. Since the phenomenon does not occur, the toner transferred to the transfer material in the most upstream image forming section that forms an image in the single color image forming mode is transferred from the transfer material in the three downstream image forming sections in the color image forming mode. Although it is robbed, it is not robbed from the transfer material in the three image forming portions on the downstream side in the monochrome image forming mode. Therefore, if the image forming conditions in the image forming unit that forms an image in the single color image forming mode are the same as the image forming conditions in the color image forming mode, excess toner transferred onto the transfer material in the single color image forming mode is generated. There is a problem that the density of the image formed on the transfer material is not lost in the image forming portion on the downstream side and the image reproducibility is deteriorated.

An object of the present invention is to provide the most upstream side of the image forming sections other than the most upstream side of the plurality of image forming sections in the monochromatic image forming mode and the color image forming mode. When the conveying member can be freely moved toward and away from the image forming units other than the image forming unit located at the position, the image forming conditions in the image forming unit located at the most upstream side are set in the single color image forming mode and the color image forming mode. It is an object of the present invention to provide a tandem-type color image forming apparatus that can be easily changed by using, and can prevent an increase in image density in the single-color image forming mode and can maintain good image reproducibility.

[0013]

The present invention has the following structure as means for solving the above problems.

(1) A plurality of image forming portions for forming toner images of mutually different colors are provided by using individual photoconductors to which an initial charging potential is given by a charging roller, and each photoconductor is opposed to each photoconductor. In the single-color image forming mode, the transfer material conveying path that sequentially passes through the plurality of image forming portions is separated from the image forming portions except the image forming portion arranged on the most upstream side in the conveying direction of the transfer material, and the color image is formed. In the color image forming apparatus in which all the image forming units are brought close to each other in the forming mode, the peripheral speed difference between the charging roller and the photoconductor of the image forming unit arranged on the most upstream side is set to be the same as that in the single color image forming mode. It is characterized by having different values depending on time.

In this structure, the peripheral speed difference between the charging roller and the photoconductor that affects the initial charging potential of the photoconductor in the most upstream image forming unit causes a reverse transfer phenomenon in the downstream image forming unit. Different values are set in the non-monochromatic image forming mode and in the color image forming mode in which the reverse transfer phenomenon occurs. The difference in peripheral speed between the charging roller and the photoconductor affects the initial charging potential of the photoconductor applied from the charging roller.
Therefore, the toner adhesion amount on the transfer material due to the reverse transfer phenomenon at the downstream side image forming unit does not decrease in the single color image forming mode due to the reverse transfer phenomenon and the toner adhesion amount on the transfer material at the downstream side image forming unit. The initial charge potential of the photoconductor in the most upstream side image forming section changes in the color image forming mode, which causes a decrease in The density difference from the image forming mode is further suppressed.

(2) The peripheral speed difference between the charging roller and the photoconductor of the image forming portion disposed on the most upstream side is made larger in the monochromatic image forming mode than in the color image forming mode.

In this structure, the reverse transfer phenomenon occurs in the single-color image forming mode in which the peripheral speed difference between the charging roller and the photoconductor does not cause the reverse transfer phenomenon in the downstream image forming section in the most upstream image forming section. Which is larger than that in the color image forming mode. The larger the peripheral speed difference between the charging roller and the photoconductor, the higher the initial charging potential of the photoconductor applied from the charging roller. Therefore, in the single-color image forming mode in which the amount of toner adhered on the transfer material due to the reverse transfer phenomenon does not occur, the initial charging potential for the photoconductor is relatively high, and the toner adheres to the photoconductor during the image forming process of the reversal development method. The quantity is reduced. On the other hand, in the color image forming mode in which the amount of toner adhering to the transfer material is reduced due to the reverse transfer phenomenon, the initial charging potential for the photoconductor is relatively low, and the amount of toner adhering to the photoconductor in the image forming process of the reversal development method is made. Will be a lot. As a result, the density of the image formed in the most upstream side image forming unit is made uniform in the monochrome image forming mode and the color image forming mode.

(3) A plurality of image forming sections for forming toner images of mutually different colors are provided by using individual photoconductors to which an initial charging potential is given by the charging roller, and each photoconductor is opposed to each photoconductor. In the single-color image forming mode, the transfer material conveying path that sequentially passes through the plurality of image forming portions is separated from the image forming portions except the image forming portion arranged on the most upstream side in the conveying direction of the transfer material, and the color image is formed. In the color image forming apparatus that is brought close to all the image forming units in the forming mode, the rotation direction of the charging roller of the image forming unit arranged on the most upstream side is set to be the same in the single color image forming mode and the color image forming mode. It is characterized by the opposite direction.

In this structure, the rotation direction of the charging roller, which affects the initial charging potential of the photoconductor in the most upstream image forming unit, does not cause the reverse transfer phenomenon in the downstream image forming unit. The directions are different in the monochrome image forming mode and the color image forming mode in which the reverse transfer phenomenon occurs. Therefore, the toner adhesion amount on the transfer material due to the reverse transfer phenomenon at the downstream side image forming unit does not decrease in the single color image forming mode due to the reverse transfer phenomenon and the toner adhesion amount on the transfer material at the downstream side image forming unit. The initial charge potential of the photoconductor in the most upstream side image forming section changes in the color image forming mode, which causes a decrease in The density difference from the image forming mode is suppressed.

(4) When the charging roller of the image forming portion disposed on the most upstream side is in a position facing the photoconductor in the monochromatic image forming mode, the moving direction of the surface of the charging roller is the moving direction of the surface of the photoconductor. In the color image forming mode, the charging roller is rotated so that the surface of the charging roller moves in the direction opposite to the moving direction of the surface of the photoconductor in the position facing the photoconductor.

In this structure, the rotation direction of the charging roller with respect to the photosensitive member in the most upstream side image forming section is
In the monochrome image forming mode in which the reverse transfer phenomenon does not occur in the image forming section on the downstream side, the moving direction of the surface of the charging roller is set to be the same as the moving direction of the surface of the photosensitive member at the position facing the photosensitive member, and the reverse transfer phenomenon In the color image forming mode in which the occurrence of the above occurs, the moving direction of the surface of the charging roller is opposite to the moving direction of the surface of the photoconductor at the position facing the photoconductor. Therefore, in the single-color image forming mode in which the amount of toner adhered on the transfer material due to the reverse transfer phenomenon does not occur, the initial charging potential for the photoconductor is relatively high, and the toner adheres to the photoconductor during the image forming process of the reversal development method. The quantity is reduced. On the other hand, in the color image forming mode in which the amount of toner adhering to the transfer material is reduced by the reverse transfer phenomenon, the initial charging potential for the photoconductor is relatively low, and the amount of toner adhering to the photoconductor in the image forming process of the reversal development method is Will be a lot. As a result, the density of the image formed in the most upstream side image forming unit is made uniform in the monochrome image forming mode and the color image forming mode.

(5) The image forming unit arranged on the most upstream side is an image forming unit for forming a black toner image.

In this structure, the peripheral speed difference and the rotation direction of the charging roller with respect to the photosensitive member in the image forming unit for forming the black toner image do not cause the reverse transfer phenomenon in the downstream image forming unit. Between the time and the color image forming mode in which a reverse transfer phenomenon occurs in the image forming unit on the downstream side. Therefore, an image that is most commonly formed in the single-color image forming mode and forms a black toner image that should be located in the lowermost layer on the transfer material in the color image forming mode and has the least influence on the toner images of other colors. In the forming section, there are a single-color image forming mode in which the reduction of the toner adhesion amount on the transfer material due to the reverse transfer phenomenon does not occur and a color image forming mode in which the reduction of the toner adhesion amount on the transfer material due to the reverse transfer phenomenon occurs. The initial charging potential of the photoconductor changes, and the difference in density of the black toner image between the monochrome image forming mode and the color image forming mode is suppressed.

[0024]

1 and 2 are views showing the configuration of the main part of a color image forming apparatus according to an embodiment of the present invention. FIG. 1 shows a state in which a transfer / conveying belt is located at a horizontal position. FIG. 2 shows a state in which the transfer / conveying belt is located at the inclined position.

Tandem type color image forming apparatus 100
As an example, as shown in FIG. 1, four image forming units 2
00a to 200d are arranged side by side in a line in the conveying direction of the recording paper (including transfer materials other than paper such as OHP film). Below the image forming units 200a to 200d, a transfer conveyance belt 250 stretched between a pair of rollers including a driving roller 251 and a driven roller 252 is arranged. The transfer / conveyance belt 250 conveys the recording sheet electrostatically attracted to the upper surface by the rotation of the rollers 251 and 252. A fixing device 260 for fixing the toner image transferred onto the recording sheet onto the recording sheet is disposed on the downstream side of the sheet conveying path by the transfer conveying belt 250.
A paper feed cassette 240 is disposed below the transfer / transport belt 250, and the recording papers stored in the paper feed cassette 240 are separated one by one and fed to the upstream side of the transfer / transport belt 250. To be done.

Each of the image forming sections 200a to 200d has the same structure. As an example, the image forming unit 200a includes the photosensitive drum 2 that is rotationally driven in the arrow direction.
01a, a contact type charging roller 202a for uniformly charging the surface of the photoconductor drum 201a by rotating the peripheral surface of the photoconductor drum 201a with its peripheral surface in contact with the surface of the photoconductor drum 201a,
An exposure unit 203a that exposes the surface of the photosensitive drum 201a to form an electrostatic latent image, a developing device 204a that visualizes the electrostatic latent image formed on the surface of the photosensitive drum 201a into a toner image, a photosensitive drum A transfer roller 205a for transferring the toner image carried on the surface of 201a to a transfer material, and a cleaner 206a for removing the toner remaining on the surface of the photosensitive drum 201a are sequentially arranged along the rotation direction of the photosensitive drum 201a. Is configured. With this configuration, the image forming process including the steps of charging, exposing, developing, transferring, and cleaning is performed on the surface of the photosensitive drum 201a.

The developing devices 204a to 204d provided in the respective image forming units 200a to 200d are sequentially black (B
K), cyan (C), magenta (M), and yellow (Y) toners are stored, and the exposure units 203a to 203d sequentially store image data of BK, C, M, and Y colors, respectively. Is entered. Therefore, in each of the image forming units 200a to 200d, the photoconductor drum 20
Toner images of BK, C, M, and Y colors are formed on the surfaces of 1a to 201d.

In the image forming units 200a to 200d, the photosensitive drums 201a to 201d and the transfer rollers 205a to
The transfer conveyance belt 250 passes between the transfer conveyance belt 250 and 205d.
Therefore, the transfer units 205a to 205d are opposed to the surfaces of the photoconductor drums 201a to 201d with the transfer conveyance belt 250 interposed therebetween. The transfer / conveying belt 250 has a rotary shaft of the transfer roller 205a of the image forming unit 200a as a center.
It is swingable between the horizontal position shown in FIG. 1 and the inclined position shown in FIG. 2 in which the downstream end in the transport direction of the recording sheet is displaced downward by a predetermined amount.

In the color image forming mode based on the color image information, each of the image forming units 200a to 200 is placed with the transfer / conveying belt 250 in the horizontal position shown in FIG.
Image data of each color of BK, C, M, and Y is input to the exposure units 203a to 203d of d. In the image forming apparatus using the electrophotographic method, when an electrostatic latent image is formed on the surface of the photoconductor by irradiating the image light modulated by the image data, it is possible to reduce the fatigue of the surface of the photoconductor due to the light irradiation. The reversal development method in which the image light is applied to a portion where the toner is to be adsorbed in the process and the white light portion of the image is not applied is the mainstream.

As a result, each of the image forming units 200a-20
At 0d, B is formed on the surfaces of the photosensitive drums 201a to 201d.
While the toner images of K, C, M, and Y are formed and the recording paper electrostatically attracted to the upper surface of the transfer / transport belt 250 passes through the image forming units 200a to 200d in order, BK,
The toner images of C, M, and Y colors are sequentially transferred onto the recording paper. The recording sheets are the image forming units 200a to 200d.
After the toner images of the colors BK, C, M, and Y are sequentially transferred in a state of being superposed while passing through the fixing device 2, the fixing device 2
Heated and pressurized at 60. As a result, the toner image is firmly fixed on the surface of the recording paper.

Here, since each toner image is not fixed on the surface of the recording sheet while being electrostatically adsorbed on the transfer / conveying belt 250 and passing through each of the image forming sections 200a to 200d, the sheet is conveyed. The unfixed toner image transferred on the recording paper in the image forming section on the upstream side in the direction of the image is transferred to the photosensitive drum of the image forming section on the downstream side, causing a reverse transfer phenomenon, and a good color image is formed on the recording paper. Can no longer be formed.

That is, the toner image of BK transferred onto the recording paper in the image forming portion 200a on the most upstream side in the paper conveying direction is
Before the toner image of C on the recording sheet is transferred from the photosensitive drum 201b onto the recording sheet in the second image forming unit 200b, it is transferred to the surface of the photosensitive drum 201b, and the same phenomenon occurs in the third image forming. It also occurs in the portion 200c and the fourth image forming portion 200d. Therefore, at the time when the transfer of the Y toner image in the image forming unit 200d on the most downstream side in the paper transport direction is completed, the amount of BK toner adhered to the recording paper is determined immediately after the completion of the transfer process in the image forming unit 200a. The amount of adhesion is reduced by about several tens of percent, and the reproducibility of color images deteriorates.

Therefore, in the color image forming mode in which the reverse transfer phenomenon occurs, the image forming section 20 arranged on the most upstream side.
The amount of BK toner adhered on the recording paper at 0a is
The reproducibility of the color image finally formed on the recording sheet can be improved by increasing the amount of the photosensitive drums 201b to 201d in the downstream image forming units 200b to 200d in advance. .

On the other hand, the transfer / conveying belt 250 is shown in FIG. 2 in the monochromatic image forming mode based on the monochrome image information for forming an image of only the BK toner image in the image forming portion 200a on the most upstream side in the delivery direction of the recording sheet. The image data of BK is input to the exposure unit 203a of the image forming unit 200a while being positioned at the inclined position.

As a result, a BK toner image is formed on the surface of the photosensitive drum 201a only in each image forming section 200a, and the recording paper electrostatically adsorbed on the upper surface of the transfer / conveying belt 250 passes through the image forming section 200a. In the meantime, BK
Toner image is transferred onto the recording paper. After that, the recording paper is not transferred to the other image forming unit 2 without transferring the toner image.
After passing through 00b to 200d in order, the fixing device 260 is heated and pressurized. As a result, the BK toner image is firmly fixed on the surface of the recording paper.

At this time, since the transfer / transport belt 250 is located at the inclined position, the upper surface of the transfer / transport belt 250 contacts the surface of the photosensitive drum 201a only in the most upstream side image forming section 200a, and Image forming unit 20
0b to 200d, the photosensitive drums 201b to 20
Separated from 1d. As a result, the recording paper and the transfer / transport belt 250 do not come into contact with the photoconductor drums 201b to 201d of the image forming units 200b to 200d, and wear of the photoconductor drums 201b to 201d and the transfer transport belt 250 is suppressed.

However, by locating the transfer / conveying belt 250 at the inclined position, the image forming section 200 on the downstream side is formed.
In the single-color image forming mode in which the conveyance path of the recording paper is formed at a position separated from b to 200d, the reverse transfer phenomenon does not occur with respect to the BK toner image formed in the most upstream side image forming unit 200a. Therefore, in the monochrome image forming mode, B on the recording paper in the image forming unit 200a is
If the K toner adhesion amount is made the same as the adhesion amount in the color image forming mode in consideration of the decrease due to the reverse transfer phenomenon,
In the monochrome image forming mode, the density of the monochrome image formed on the recording paper becomes too high, and the image reproducibility deteriorates.

Therefore, in the color image forming apparatus 100 according to this embodiment, the peripheral speed ratio of the charging roller 202a to the photosensitive drum 201a in the BK image forming portion 200a disposed on the most upstream side is controlled to control the photosensitivity. The initial charging potential of the body drum 201a can be changed between the single color image forming mode and the color image forming mode with a simple configuration, and the amount of BK toner adhered onto the recording paper in the image forming unit 200a can be changed. It is relatively small in the single-color image forming mode in which the reverse transfer phenomenon does not occur, while it is relatively large in the color image forming mode in which the reverse transfer phenomenon occurs, in both the single-color image forming mode and the color image forming mode image. This makes it possible to maintain good image reproducibility.

Specifically, the color image forming apparatus 100
Is the circumferential speed of the charging roller 202a relative to the photoconductor drum 201a by reversing the rotation direction of the charging roller 202a in the BK image forming unit 200a between the single color image forming mode and the color image forming mode. By changing the ratio, the initial charging potential of the photosensitive drum 201a is changed.

FIG. 3 is a view showing the arrangement of the main parts of the most upstream image forming section in the color image forming apparatus. In the color image forming apparatus 100, the BK image forming unit 200a arranged on the most upstream side in the recording sheet conveyance direction.
Is a photosensitive drum 2 that rotates in the direction of arrow A during image formation.
01a, the charging roller 202a is rotatably supported in the directions of arrows B and C. This charging roller 202a
Is supplied with a rotational force from a motor 16 that rotates in both forward and reverse directions. The motor 16 rotates the rotating shaft in the forward or reverse direction according to the drive voltage applied from the drive device 15.
A charging voltage is applied from the power supply device 14 to the charging roller 202a.

The power supply device 14 and the drive device 15 are connected to the CPU 11 which constitutes the drive control unit 10. CP
The U11 includes a ROM 12 and a RAM 13, and is a main CP that controls the entire color image forming apparatus 100 as a whole.
It is connected to U21 and an operation panel unit 22 equipped with a key switch and a display. The CPU 11 outputs control data to the power supply device 14 and the drive device 15 based on the control data input from the main CPU 21 and the operation data of the key switches read from the operation panel unit 22.

That is, when the drive start command is input from the main CPU 21 at a predetermined timing, the CPU 11 selects either the single color image forming mode or the color image forming mode from the operation state of the key switch on the operation panel section 22. Whether or not the control data for reading the rotation direction and the rotation speed of the charging roller 202a according to the selected mode is output to the driving device 15 and applied to the charging roller 202a according to the selected mode. The control data indicating the charging voltage is output to the power supply device 14.

The drive unit 15 rotates the rotation shaft of the motor 16 in the forward or reverse direction according to the control data supplied from the CPU 11 to rotate the charging roller 202a in the arrow B or C direction. Here, as shown in FIG. 4A, the charging roller 202a rotates in the arrow B direction, which is the same direction as the rotation direction of the photosensitive drum 201a in the arrow A direction,
The rotation direction in which the peripheral surface of the charging roller 202a moves in the direction opposite to the peripheral surface of the photosensitive drum 201a at the position facing the surface of the photosensitive drum 201a is referred to as the counter direction. On the other hand, as shown in FIG. 4B, the charging roller 202a rotates in the direction of arrow C, which is opposite to the direction of rotation of the photosensitive drum 201a in the direction of arrow A, and charges at the position facing the surface of the photosensitive drum 201a. The rotation direction in which the peripheral surface of the roller 202a moves in the same direction as the peripheral surface of the photoconductor drum 201a is referred to as the with direction.

When the other elements such as the charging voltage and the rotation speed are the same, the charging roller 2 is attached to the photosensitive drum 201a.
When 02a is rotated in the counter direction, the photosensitive drum 2
The contact probability between the peripheral surface of 01a and the peripheral surface of the charging roller 202a is improved, and a discharge current easily flows between them, which is compared with the case where the charging roller 202a is rotated in the width direction with respect to the photosensitive drum 201a. As a result, the initial charging potential of the photosensitive drum 201a becomes high.

As described above, in the color image forming apparatus 100 according to this embodiment, the surface of the photosensitive drum 201a is irradiated with the image light on the portion where the toner should be adsorbed in the developing process, and the white background portion of the image is imaged. A reversal development method that does not illuminate light is used. Therefore, the charging roller 202
When a is rotated in the counter direction, the photosensitive drum 201
As indicated by the broken line in FIG. 5, the initial charging potential V ₀₀ of “a” moves to the white background side as compared with the case where the charging roller 202a indicated by the solid line in the figure is rotated in the width direction. As a result, when the charging roller 202a rotates in the counter direction, the absolute value of the contrast potential V _C (difference between the developing bias potential V _B and the exposure portion potential _VL ) becomes smaller than that in the rotation in the width direction, and the photoconductor is rotated. The amount of toner attached to the surface of the drum 201a is reduced.

In the color image forming apparatus 100 described above, the image forming section 2 for forming a BK toner image on the most upstream side.
The charging roller 202a of the image forming unit 200a
charging rollers 202b to 202d included in b to 200d
Similarly, for example, a rubber material such as natural rubber, EPDM, urethane, chloroprene, or silicone rubber in which a conductive material such as carbon black or metal powder for resistance adjustment is dispersed around a conductive core metal. , Injection molding, die molding or cutting to integrally form a roller-shaped resistance value of 1
It is about 0 ³ to 10 ⁷ Ω · cm.

In the first embodiment, as shown in FIG. 6, the drive control section 10 changes only the rotation direction of the charging roller 202a between the single color image forming mode and the color image forming mode. . That is, the drive controller 10 rotates the charging roller 202a in the counter direction in the monochromatic image forming mode and in the with direction in the color image forming mode. The rotation speed of the charging roller 202a is 480 during rotation in the counter direction and during rotation in the with direction.
It is constant at rpm.

Therefore, when the diameter of the photosensitive drum 201a is Dmm, the diameter of the charging roller 202a is dmm, and the rotation speed of the photosensitive drum 202a is N rpm, the peripheral speed difference mm / sec between the photosensitive drum 201a and the charging roller 202a is In the color image forming mode (when rotating in the width direction), (NDπ / 60) − (480dπ / 60), and in the monochrome image forming mode (when rotating in the counter direction), (NDπ / 60) + ( 480 dπ / 60).

As a result, for the BK toner image formed on the photoconductor drum 201a, the initial charging potential of the photoconductor drum 201a is set in the single-color image forming mode in which the reverse transfer phenomenon does not occur in the image forming units 200b to 200d on the downstream side. Moves to the white side. As a result, it is possible to reduce the amount of toner adhered to the exposed portion of the photosensitive drum 201a in the developing step, and it is possible to prevent the density of the monochrome image formed on the recording sheet from becoming too high.

On the other hand, for the BK toner image formed on the photosensitive drum 201a, the image forming section 200b on the downstream side is formed.
In the color image forming mode in which the reverse transfer phenomenon occurs at 200 d to 200 d, the initial charging potential of the photoconductor drum 201a moves to the non-white background side, and the photoconductor drum 20 in the developing process.
It is possible to increase the amount of toner attached to the exposed portion in 1a. As a result, it is possible to prevent the adhesion amount of the BK toner image formed in the lowermost layer on the recording sheet from decreasing.

In the second embodiment, as shown in FIG. 7, the drive controller 10 changes the rotation direction and the rotation speed of the charging roller 202a between the monochromatic image forming mode and the color image forming mode. I have to. That is, the drive control unit 1
0 rotates the charging roller 202a in the counter direction at 1000 rpm in the single-color image forming mode, and in the width direction at 480 rpm in the color image forming mode.

Therefore, assuming that the diameter of the photosensitive drum 201a is Dmm, the diameter of the charging roller 202a is dmm, and the rotation speed of the photosensitive drum 202a is N rpm, the peripheral speed difference mm / sec between the photosensitive drum 201a and the charging roller 202a is In the color image forming mode (when rotating in the width direction), (NDπ / 60) − (480dπ / 60), and in the monochrome image forming mode (when rotating in the counter direction), (NDπ / 60) + ( 1000dπ / 60).

Also in this case, for the BK toner image formed on the photoconductor drum 201a, the initial charging of the photoconductor drum 201a is performed in the single-color image forming mode in which the reverse transfer phenomenon does not occur in the image forming units 200b to 200d on the downstream side. The potential moves to the white background side, the amount of toner adhered to the exposed portion of the photosensitive drum 201a can be further reduced in the developing process, and it is possible to prevent the density of the monochrome image formed on the recording paper from becoming too high. .

On the other hand, with respect to the BK toner image formed on the photosensitive drum 201a, the image forming section 200b on the downstream side is formed.
In the color image forming mode in which the reverse transfer phenomenon occurs at 200 d to 200 d, the initial charging potential of the photoconductor drum 201a moves to the non-white background side, and the photoconductor drum 20 in the developing process.
It is possible to increase the amount of toner adhered to the exposed portion of 1a and prevent the amount of adhered toner image of BK formed in the lowermost layer on the recording sheet from decreasing.

By changing only the rotation speed of the charging roller 202a between the monochromatic image forming mode and the color image forming mode, the peripheral speed difference of the charging roller 202a with respect to the photosensitive drum 201a is changed, and the reverse transfer phenomenon is caused. It is formed on the recording paper in the monochromatic image forming mode in which the reverse transfer phenomenon does not occur while preventing the adhesion amount of the toner image of BK formed on the lowermost layer on the recording paper in the occurring color image forming mode from being reduced. It may be possible to prevent the density of the monochrome image from becoming too high.

In the color image forming apparatus 100 according to the above embodiment, the toner images of the respective colors are transferred from the image forming units 200a to 200d to the recording paper conveyed on the transfer / conveying belt 250. A color image forming apparatus in which a transfer belt is used instead of the transfer conveyance belt 250, the toner images of the respective colors are transferred from the image forming units 200a to 200d to the transfer belt, and the toner images are transferred from the transfer belt to recording paper again. Also in this, this invention can be implemented similarly.

[0057]

According to the present invention, the following effects can be obtained.

(1) In the most upstream side image forming section, the peripheral speed difference between the charging roller and the photosensitive body, which affects the initial charging potential of the photosensitive body, is determined by a single color which does not cause the reverse transfer phenomenon in the downstream side image forming section. By setting different values in the image forming mode and in the color image forming mode that causes the reverse transfer phenomenon, a monochromatic image formation in which the toner adhesion amount on the transfer material does not decrease due to the reverse transfer phenomenon in the downstream image forming unit The initial charging potential of the photoconductor in the most upstream image forming section is changed in the mode and in the color image forming mode in which the amount of toner adhered on the transfer material decreases due to the reverse transfer phenomenon in the downstream image forming section. However, it is possible to suppress the density difference between the image formed in the most upstream side image forming unit in the single-color image forming mode and the color image forming mode. As a result, it is possible to prevent the amount of toner images formed on the most upstream side image forming portion on the transfer material from decreasing due to the reverse transfer phenomenon in the color image forming mode, and to prevent the toner amount on the most upstream side in the single color image forming mode. It is possible to prevent the toner amount formed on the transfer material from being excessively adhered on the transfer material, and the reproducibility of the image on the transfer material is the monochromatic image forming mode or the color image forming mode. Regardless of whether or not it can be maintained at all times.

(2) In the most upstream image forming section, the peripheral speed difference between the charging roller and the photoconductor is set in the single color image forming mode in which the reverse transfer phenomenon does not occur in the downstream image forming section.
By making it larger than that in the color image forming mode that causes the reverse transfer phenomenon, in the single color image forming mode in which the reduction of the toner adhesion amount on the transfer material due to the reverse transfer phenomenon does not occur,
The initial charge potential on the photoconductor can be made relatively high, and the amount of toner adhered to the photoconductor can be reduced in the image forming process of the reversal development method. On the other hand, in the color image forming mode in which the amount of toner adhering to the transfer material decreases due to the reverse transfer phenomenon, the initial charging potential for the photoconductor can be made relatively low, and the toner for the photoconductor is used in the image forming process of the reversal development method. Can be increased. As a result, the density of the image formed in the most upstream side image forming section can be made uniform in the monochromatic image forming mode and the color image forming mode, and the reproducibility of the image on the transfer material can be improved. Regardless of whether it is the formation mode or the color image formation mode, it can always be maintained well.

(3) In the most upstream side image forming section, the direction of rotation of the charging roller affecting the initial charging potential of the photosensitive element with respect to the photosensitive element is set to the monochromatic image in which the reverse transfer phenomenon does not occur in the downstream image forming section. A single-color image forming mode in which the toner adhesion amount on the transfer material does not decrease due to the reverse transfer phenomenon in the image forming unit on the downstream side by setting different directions in the forming mode and the color image forming mode in which the reverse transfer phenomenon occurs The initial charging potential of the photoconductor in the most upstream side image forming section is changed depending on the time and the color image forming mode in which the amount of toner adhering to the transfer material decreases due to the reverse transfer phenomenon in the downstream side image forming section. It is possible to suppress the density difference between the image formed in the most upstream side image forming unit in the monochrome image forming mode and the color image forming mode. As a result, it is possible to prevent the amount of toner images formed on the most upstream side image forming portion on the transfer material from decreasing due to the reverse transfer phenomenon in the color image forming mode, and to prevent the toner amount on the most upstream side in the single color image forming mode. It is possible to prevent the toner amount formed on the transfer material from being excessively adhered on the transfer material, and the reproducibility of the image on the transfer material is the monochromatic image forming mode or the color image forming mode. Regardless of whether or not it can be maintained at all times.

(4) In the most upstream image forming unit, the rotation direction of the charging roller with respect to the photosensitive member is set to the position facing the photosensitive member in the monochrome image forming mode in which the reverse transfer phenomenon does not occur in the downstream image forming unit. The direction of movement of the surface of the charging roller should match the direction of movement of the surface of the photoconductor, and in the color image forming mode in which a reverse transfer phenomenon occurs, the movement direction of the surface of the charging roller is the direction of the movement of the surface of the photoconductor at the position facing the photoconductor In the single-color image forming mode, in which the amount of toner adhered on the transfer material does not decrease due to the reverse transfer phenomenon, the initial charging potential for the photoconductor is set relatively high and the reversal development is performed. It is possible to reduce the amount of toner adhering to the photoconductor in the image forming processing of the system. On the other hand, in the color image forming mode in which the amount of toner adhering to the transfer material is reduced by the reverse transfer phenomenon, the initial charging potential for the photoconductor is made relatively low, and the amount of toner adhering to the photoconductor in the image forming process of the reversal development system is made. Can do a lot. As a result, it is possible to make the density of the image formed in the image forming portion on the most upstream side uniform in the single-color image forming mode and the color image forming mode, and to form the most upstream image in the color image forming mode. Of the toner image formed on the transfer material of the toner image formed on the most upstream side image forming portion in the single-color image forming mode, while preventing the amount of adhesion of the toner image formed on the transfer material on the transfer material from being reduced by the reverse transfer phenomenon. It is possible to prevent the adhesion amount from becoming excessive, and it is possible to always maintain good image reproducibility on the transfer material regardless of whether it is in the single-color image forming mode or the color image forming mode.

(5) The peripheral speed difference or the rotating direction of the charging roller in the image forming portion for forming a black toner image is set to be the same as that in the monochrome image forming mode in which the reverse transfer phenomenon does not occur in the downstream image forming portion. By changing between the color image forming mode in which the reverse transfer phenomenon occurs in the image forming section on the downstream side, the most commonly formed image is formed in the single color image forming mode. In the image forming unit that forms the black toner image that should be located in the lowermost layer that has the least effect on the toner image of the color, the single-color image forming mode that does not reduce the toner adhesion amount on the transfer material due to the reverse transfer phenomenon , In the color image forming mode, which reduces the amount of toner adhered on the transfer material due to the reverse transfer phenomenon, the black toner is changed by changing the initial charging potential of the photoconductor. It can be at the single-color image forming mode and suppress the density difference between the color image forming mode.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a main part of a color image forming apparatus according to an embodiment of the present invention, showing a state in which a transfer / conveying belt is located at a horizontal position.

FIG. 2 is a view showing a state in which the transfer / conveying belt is also located at an inclined position.

FIG. 3 is a diagram showing a configuration of a main part of an image forming unit on the most upstream side in the color image forming apparatus.

FIG. 4 is a diagram illustrating a rotation direction of a charging roller in an image forming unit on the most upstream side of the color image forming apparatus.

FIG. 5 is a diagram illustrating a charging potential of a photosensitive drum in an image forming unit on the most upstream side of the color image forming apparatus.

FIG. 6 is a diagram showing a rotating state of a charging roller in an image forming portion on the most upstream side of the color image forming apparatus according to the first embodiment of the present invention.

FIG. 7 is a diagram illustrating a rotation state of a charging roller in an image forming unit on the most upstream side of a color image forming apparatus according to a second embodiment of the present invention.

[Explanation of symbols]

10-Drive control unit 14-Power supply 15-Drive 100-color image forming apparatus 200a-200d-Image forming section 201a to 201d-photosensitive drum 202a to 202d-charging roller 250-Transfer Conveyor Belt

Continued front page    (72) Inventor Osamu Morikuni             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company (72) Inventor Masatsugu Nakamura             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company (72) Inventor Toru Yoshioka             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company F-term (reference) 2H030 AB02 AD02 AD05 AD07 AD08                       AD16 BB23 BB44 BB53 BB63                 2H200 FA03 FA16 FA17 GA12 GA23                       GA34 GA44 GA47 GA59 GB12                       GB22 GB23 GB25 HA03 HA29                       HB12 HB22 JA02 JA29 JB06                       JB25 PA11 PA12 PA19 PA26

Claims (5)

[Claims] 1. A plurality of image forming units for forming images of toner images of different colors by using individual photoconductors to which an initial charging potential is applied by a charging roller, and a plurality of image forming units facing each photoconductor. In the single-color image forming mode, the transfer material conveying path that sequentially passes through the image forming portions of the above is separated from the image forming portions other than the image forming portion arranged on the most upstream side in the conveying direction of the transfer material to form a color image. In the color image forming apparatus in which all the image forming units are brought close to each other in the mode, the difference in peripheral speed between the charging roller and the photoconductor of the image forming unit arranged on the most upstream side is set in the single color image forming mode and the color image forming mode. A color image forming apparatus having different values for and. 2. The peripheral speed difference between the charging roller and the photoconductor of the image forming portion arranged on the most upstream side is made larger in the monochromatic image forming mode than in the color image forming mode. The color image forming apparatus described in 1. 3. A plurality of image forming units for forming images of toner images of different colors by using individual photoconductors to which an initial charging potential is applied by a charging roller, and a plurality of image forming units facing each photoconductor. In the single-color image forming mode, the transfer material conveying path that sequentially passes through the image forming portions of the above is separated from the image forming portions other than the image forming portion arranged on the most upstream side in the conveying direction of the transfer material to form a color image. In the color image forming apparatus in which all the image forming units are brought close to each other in the mode, the rotation directions of the charging rollers of the image forming units arranged on the most upstream side are opposite in the monochrome image forming mode and the color image forming mode. A color image forming apparatus characterized by a direction. 4. The moving direction of the surface of the charging roller is the same as the moving direction of the surface of the charging roller when the charging roller of the image forming portion disposed on the most upstream side is in a position facing the photosensitive member in the single-color image forming mode. 4. When the color image forming mode is set, the charging roller is rotated such that the moving direction of the surface of the charging roller is opposite to the moving direction of the surface of the photosensitive member in the position facing the photosensitive member. The color image forming apparatus described in 1. 5. The image forming unit arranged on the most upstream side,
The color image forming apparatus according to claim 1, which is an image forming unit that forms a black toner image.