JP2003202729A - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the occurrence of a difference in the fixed image density of a toner image transferred on the most upstream side even at either time of forming a color image or forming a monochromatic image. <P>SOLUTION: The press-contact force of respective transfer members 81BK, 81C, 81M and 81Y to a recording paper feeding belt 72 is adjusted in accordance with the time of forming the monochromatic image and the time of forming the color image by press-contact force adjusting means 93BK, 93C, 93M and 93Y so that the press-contact force of the transfer member to the recording paper feeding belt 72 is switched respectively at the time of forming the color image and at the time of forming the monochromatic image. Since the adjustment of the press-contact force has a relation that the intensity of an electric field is inversely proportional to a distance from an electric field generating part to an observation point, the opposed positions of the transfer members 81BK, 81C, 81M and 81Y to photoreceptive drums 75Bk, 75C, 75M and 75Y are changed so that a transfer electric field is changed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus also called a multi-stage drum system or a tandem system in which a plurality of photosensitive drums on which toner images of each color and black are individually formed are arranged in a recording paper conveying direction. .

[0002]

2. Description of the Related Art A color image forming apparatus used in a copying machine and a printing apparatus basically has a plurality of developing devices, and each developing device forms a toner image of a different color on the surface of a photosensitive drum. Then, these toner images are finally transferred onto the same recording paper in a superimposed manner, and then fixed to form a color image, and only the black toner image is recorded on the recording paper. It has a monochrome image forming mode that forms a black monochrome image after it is transferred to and fixed to each of these modes, and each of these modes can be selectively set by the user by selecting the operation panel. It is configured to be able to.

Such a color image forming apparatus is roughly classified into a multi-drum type color image forming apparatus 1 shown in FIG. 19 and a single drum type color image forming apparatus 2 shown in FIG.

FIG. 19 is a simplified side view showing a schematic structure of a conventional multi-stage drum type color image forming apparatus 1. In the multi-stage drum type color image forming apparatus 1, a paper feed cassette 5 in which a plurality of recording papers 4 are stacked and accommodated is detachably attached to a lower portion of an apparatus main body (not shown), and an image is provided above the paper feed cassette 5. The forming part 6 is arranged. In the image forming unit 6, a recording paper conveyance belt 7 that conveys the recording paper 4 is stretched around a driven roller 8 and a drive roller 9 and stretched in a conveyance direction A (left in FIG. 19). The four photoconductor drums 10Y, 10M, 10C and 10BK are provided in contact with the recording paper carrying surface of the recording paper carrying belt 7.

The photosensitive drums 10Y, 10M, 10C,
Cleaning devices 11Y and 11 are provided around 10BK.
M, 11C, 11BK, initialization chargers 12Y, 12M,
12C, 12BK, image exposure devices 13Y, 13M, 13C,
13BK, developing devices 14Y, 14M, 14C, 14B
K, developing rollers 15Y, 15M, 15C and 15BK, and a plurality of transfer brushes 16Y, 16M, 16C and 16BK under the upper stretched portion of the recording paper transport belt 7.
Are arranged corresponding to the respective photoconductor drums 10Y, 10M, 10C and 10BK.

The above-mentioned developing rollers 15Y, 15M, 15
C and 15BK are developing devices 14Y, 14M, 14C and 1
The developing devices 14Y, 14M, 14C, and 14BK are rotatably disposed in the lower opening of the 4BK, and yellow (Y), magenta (M), cyan (C), and black (BK) are provided in these developing devices 14Y, 14M, 14C, and 14BK. Each contains toner.

The recording paper 4 accommodated in the paper feed cassette 5 is
Sheets are taken out one by one from the top by the paper feed roller 17,
As shown by the arrow B, the paper is fed while being reversed to the rear of the apparatus (to the left in FIG. 19). The recording paper 4 is conveyed by the recording paper conveyance belt 7 which is circulated counterclockwise in FIG. 19, and the photosensitive drums 10Y, 10M, 10C,
10BK and each transfer brush 16Y, 16M, 16C, 16
Four transfer positions 18Y, 18M, 1 facing BK
8C and 18BK are sequentially passed in the transport direction A.

The respective photosensitive drums 10Y, 10M, 10C,
Each of 10BK rotates in the direction of arrow D, which is clockwise in FIG. 19, and the surface of the photosensitive layer on the outer periphery thereof is uniformly charged with high electric charges by the initialization chargers 12Y, 12M, 12C, and 12BK to initialize. Image exposure devices 13Y, 13M,
The electrostatic latent images are formed by being exposed by 13C and 13BK, and the toners are transferred by the developing rollers 15Y, 15M, 15C and 15BK to visualize the electrostatic latent images, that is, the electrostatic latent images are developed. Each transfer position 18Y,
18M, 18C, 18BK is rotated and conveyed.

Each photosensitive drum 10Y, 10M, 10C,
The tip of the toner image on 10BK is transferred to the transfer positions 18Y, 18
The recording paper 4 is conveyed at the timing of reaching M, 18C, and 18BK, and the print start position of the recording paper 4 is the transfer position 1
8Y, 18M, 18C, 18BK are sequentially reached.
The transfer brushes 16Y, 16M, 16C, 16BK apply charges supplied from a high voltage power source (not shown), that is, a transfer bias to the recording paper 4 via the recording paper conveyance belt 7, and the transfer brushes 16Y, 16M, 16B The toner images on the photoconductor drums 10Y, 10M, 10C, and 10BK are transferred to the recording paper 7. In this way, the four transfer positions 18
The recording paper 4 on which the toner images have been sequentially transferred by Y, 18M, 18C, and 18BK is carried into the fixing device 21 arranged on the left side of FIG. 19, and the heat of the fixing device 21 is removed. By the roller 22 and the pressure roller 23, the toner images of the respective colors Y, M, C, and BK that have been transferred in an overlapping manner are thermally fixed on the recording paper 4 and discharged to the outside of the machine.

FIG. 20 is a view showing the schematic arrangement of another conventional single-drum type color image forming apparatus 2. The single-drum type color image forming apparatus 2 includes an initializing charger 31, a recording head 33 for irradiating a laser beam 32, and four vertically movable developing devices 34M around a single photosensitive drum 31. , 34C, 34Y, 34BK, and a semi-conductive conveying drum 36 for supplying the recording paper 35 to the photosensitive drum 31. The transfer drum 36 and the photoconductor drum 31 have a transfer position 37 in a region closest to each other, and a transfer corona discharger 38 is provided inside the transfer drum 36 so as to face the transfer position 37.

As shown by the arrow E, the transport drum 36 has the recording paper 3 loaded from the lower left in FIG.
5 is wound counterclockwise as indicated by arrow F and conveyed. On the surface of the photosensitive drum 31, while rotating in the arrow G direction at the same peripheral speed as the peripheral speed of the conveying drum 36,
First, the developing device 34M forms a magenta M toner image, and this toner image is transferred to the recording paper 35 by the action of the electric field generated by the discharge of the transfer corona discharger 38.

Next, the conveying drum 36 rotates once again, and in synchronization with this, the photosensitive drum 31 forms a cyan C toner image of the developing device 34C, and the cyan C toner image is transferred to the transfer corona. It is transferred to the recording paper 35 by the action of the electric field due to the discharge of the electric device 38. Further, the transport drum 36
Is rotated once and the toner image of yellow Y is transferred onto the recording paper 35 in the same manner as when the toner image of cyan C is transferred onto the recording paper 35, and then the toner image of black BK is finally transferred. It is transferred onto the recording paper 35 in an overlapping manner. When the toner images of the respective colors M, C, Y, and BK are transferred to the recording paper 35 in this manner, the recording paper 35 is guided from the conveying drum 36 to the recording paper conveying belt 40, is supplied to the fixing device 41, and is heated on the paper surface. After being fixed, a color image is formed on the recording paper 35.

In the multi-drum type color image forming apparatus 1 shown in FIG. 19 described above, since four types of toner images are successively superposed and transferred onto the recording paper 4 in one step, the single drum type color image forming shown in FIG. The processing speed is almost four times that of the device 2. For this reason, in recent years, the multi-drum color image forming apparatus 1 is becoming the mainstream together with the fact that the internal apparatus has been downsized and unitized to be relatively inexpensive.

However, in the multi-stage drum type color image forming apparatus 1, the recording paper 4 has the photosensitive drums 10Y, 10M, 10C of the other colors Y, M, C even when the black BK single color image is formed.
Since each of the photoconductor drums 10Y,
There is a problem that unnecessary color toner remaining on 10M and 10C adheres to the recording paper 4 and deteriorates image quality. In addition, the photosensitive drums 10Y of other colors that are not moving,
When 10M and 10C come into contact with the moving recording paper conveyance belt 7, their relative speeds are high and they are easily damaged. In this way, while holding the recording paper 4,
From the photoconductor drum 10BK that transfers the black BK toner image, the photoconductor drums 10Y, 10M, and 10 of the respective colors Y, M, and C
There is a problem that the life of each of the photoconductor drums 10Y, 10M, 10C and the recording paper transport belt 7 is shortened due to unnecessary contact of the recording paper transport belt 7 that sequentially transports C.

Still another conventional technique for solving such a problem is disclosed in JP-A-6-258914 and JP-A-10-198121 and will be described with reference to FIG.

FIG. 21 is a diagram showing a schematic configuration of another conventional multi-stage drum type color image forming apparatus 3, FIG.
1 (1) is each photosensitive drum 50Y, 50M, 50C, 5
The state where the recording paper conveyance belt 51 is in contact with 0BK is shown.
FIG. 21 (2) shows each photoconductor drum 50Y, 50M, 50C.
1 shows a state in which the recording paper conveyance belt 51 is separated from. This conventional multi-stage drum type color image forming apparatus 3 is
The photoconductor drum 50BK that transfers the toner image of black Bk rotates in the direction of arrow H, and at least around it the rotation direction H
Along with the initialization charger 52Bk, the image exposure device 53Bk,
A developing device 54Bk and a cleaning device 55Bk are arranged. The image exposure device 53 is provided on the surface of the photoconductor drum 50BK between the charging device 52Bk and the developing device 54Bk.
Laser light is emitted from Bk, and an electrostatic latent image is formed on the photoconductor drum 50Bk.

The remaining photosensitive drums 50Y, 50
Around the M and 50C as well as the photoconductor drum 201Bk, along with the rotation direction G, the initialization chargers 52Y and 5Y.
2M, 52C, image exposure devices 53Y, 53M, 53C, developing devices 54Y, 54M, 54C, and cleaning devices 55Y, 55M, 55C are arranged.

The recording paper conveying belt 51 contacts the photosensitive drums 50Y, 50M, 50C and 50BK between the developing devices 54Y, 54M, 54C and 54BK and the cleaning devices 55Y, 55M, 55C and 55BK. Each of the photosensitive drums 50Y of the recording paper transport belt 51,
A transfer roller 57Y for applying a transfer bias is provided on the inner surface (back surface) on the back side of the 50M, 50C, and 50BK sides.
57M, 57C, and 57BK are arranged.

In such a multi-stage drum type color image forming apparatus 3, the image forming operation is performed as follows. First, the photosensitive drums 50Y, 50M, 50C, 5
0BK is each initialization charger 52Y, 52M, 52C, 52
It is charged by BK. Next, each image exposure device 53Y, 5
The photoconductor drums 50Y, 50M, 50C, 50BK are irradiated with laser light emitted from 3M, 53C, 53BK.
After the electrostatic latent images corresponding to the images of the respective colors are formed, the electrostatic latent images on the photoconductor drums 50Y, 50M, 50C and 50BK are developed by the developing devices 54Y, 54M, 54C and 54BK. , Toner images of the respective colors are individually formed.

The photosensitive drums 50Y, 50M, 50C,
The toner image of each color formed on 50 BK is recorded on the recording paper 58.
Overprinted on top. The recording paper 58 is the photosensitive drum 5
The toner image is transferred from the paper feed unit to the recording paper conveyance belt 51 in the direction of arrow H at the timing when the toner image is transferred from 0Y, 50M, 50C, and 50BK. The recording paper 58 held on the recording paper conveyance belt 51 is conveyed and the toner images of the respective colors are transferred onto the recording paper 58 at the contact positions with the respective photoconductor drums 50Y, 50M, 50C and 50BK, that is, at the transfer positions. . Each photoconductor drum 50Y, 50M, 50C,
The toner image on 50BK is transferred to the transfer rollers 57Y, 57M,
It is transferred onto the recording paper 58 by the electric field due to the potential difference between the transfer bias applied to 57C and 57BK and the photoconductor drums 50Y, 50M, 50C and 50BK.

The recording paper 58, on which the four color toner images have been superposed and transferred through the four transfer positions, is fixed by the fixing device 6.
The toner is conveyed to 0, the toner is fixed, and the sheet is discharged from the sheet discharge unit (not shown) to the outside of the machine. Further, the photosensitive drums 50Y, 50M, 50C, 50BK are not transferred at the respective transfer positions.
The residual toner remaining on the cleaning device 55Y, 55
Recovered by M, 55C, 55BK.

In the prior art shown in FIG. 21, the photoconductor drums 50Y, 50M, 50C, 50BK and the initialization chargers 52Y, 52 provided in association therewith.
M, 52C, 52BK, image exposure devices 53Y, 53M, 53
C, 53BK, developing devices 54Y, 54M, 54C, 54
BK, and cleaning devices 55Y, 55M, 55
Each image forming unit including C and 55 BK has a recording paper 58.
Bk, C, M, from the upstream side to the downstream side
Although they are arranged in the order of Y color, the order is not limited to this order, and the color order can be arbitrarily changed. Such a color change may be achieved only by changing the toner color stored in each of the developing devices 54Y, 54M, 54C and 54BK, and the other configurations are the same. Therefore, each developing device 54Y, 54
The rotation speeds of the developing rollers of M, 54C and 54BK are also set to the same rotation speed.

In the multi-drum type color image forming apparatus 3, however, the recording paper 58 comes into contact with the other three photoconductor drums 50Y, 50M and 50C during black monochromatic image formation. Drum 50Y, 50M, 50
Unnecessary color toner remaining on C adheres to the recording paper 58, causing a problem of degrading image quality. In addition, each photosensitive drum 50 of the recording paper conveyance belt 51
Unnecessary contact with Y, 50M, and 50C damages the photoconductor drums 50Y, 50M, and 50C and the recording paper conveyance belt 51, which causes a problem that the life is shortened.

In order to solve such a problem, in this conventional technique, as shown in FIG. 21 (2), the loop shape of the recording paper conveying belt 51 is hardly changed,
The entire recording paper conveyance belt 51 is angularly biased downward around the transfer position where the black BK photosensitive drum 50BK and the recording paper conveyance belt 51 contact each other, and the other photosensitive drums 50Y, 50M, 50C and the recording paper conveyance belt 51 are separated from each other, and when the black monochromatic image is formed in which only the black photosensitive drum 50BK is involved, the recording paper conveyance belt 51 is contacted by inclining downward so that the downstream side in the conveyance direction is opened. The other photosensitive drums 50Y, 50
The M and 50C can be separated from the recording paper conveyance belt 51.

However, there is a problem in that a high quality image cannot be obtained with a color image which requires a delicate balance in terms of the density and properties of the toners of the respective colors only with such a structure. For example, black, which is most often used as a single color, has a high masking effect (concealment effect) on other colors. Therefore, when the image forming unit on the most downstream side is set for black BK, an image is formed by that time. Moreover, the color images of the respective colors Y, M, and C are masked, resulting in a blackened image. Therefore, when the image forming unit for the black BK is arranged on the most upstream side, the black toner becomes the lowermost layer on the recording paper 58, and the influence of the black toner on the other color toners Y, M, and C is reduced, and the output image of the output image is reduced. Color reproducibility is improved.

Therefore, in the conventional technique shown in FIG. 21, when the image formation is performed in which only the image forming unit on the most downstream side in the conveying direction of the recording paper 58 is involved, the recording paper conveying belt 51 on the upstream side is inclined downward and separated. Instead of the above configuration, the downstream recording paper conveyance belt 51 is similarly separated during image formation in which only the most upstream image forming unit is involved.

On the other hand, in the multi-stage drum type color image forming apparatus 3, at the time of color image formation, the unfixed toner temporarily transferred onto the recording paper 58 by the image forming unit on the upstream side forms the image on the downstream side. There is a problem that a reverse transfer phenomenon occurs in which the toner is attached to the photoconductor drums 50C, 50M, and 50Y of the unit and taken away, and a good color image cannot be obtained.

The reverse transfer phenomenon will now be described. In a multi-stage drum type, that is, a tandem type color image forming apparatus, the toner image of the first color (for example, black BK) formed in the developing process by the most upstream image forming unit is transferred to the recording paper. The second to fourth colors formed in the developing process of the forming unit (for example, cyan C,
When the toner image of magenta M, yellow Y) is transferred onto the recording paper in an overlapping manner, a part of the toner image of the first color (for example, black BK) transferred previously is partially transferred to the second to fourth colors (for example, cyan). A phenomenon occurs in which the image is reversely transferred to the photosensitive drum of the image forming unit of C, magenta M, and yellow Y).

Therefore, after the transfer of the toner image of the fourth color (for example, yellow Y) is completed by the most upstream image forming unit, after the transfer of the first color (for example, black B) onto the recording paper.
The toner adhesion amount of the toner image K) is reduced by several tens of percent compared to the initial adhesion amount. And the same phenomenon is the second
It also occurs in the toner image of the color (for example, cyan C) and the toner image of the third color (for example, magenta M).

Therefore, the transfer process of the toner image is performed by, for example, black BK → cyan C → magenta M → yellow Y.
When the toner image of the first color BK is the smallest, the final amount of adhesion of each toner image to the recording paper is the smallest when the toner image of the first color BK is Q _BK < the color scheme unbalanced called adhesion amount Q _Y of adhesion amount Q _M <fourth color Y of adhesion amount Q _C <third color M second color C. For this reason, there is a problem in that the color density becomes higher toward the downstream side and the color image becomes more distant from the correct hue.

The reason why this reverse transfer phenomenon occurs is as follows. Depending on the image potential of the photoconductor drum (or belt) in the image forming process, when the toner adheres to the photoconductor drum from above the roller, the toner does not necessarily have a uniform negative charge, but a negative polarity. Some of them have a weak charge and the other of them have a positive charge. Further, in some cases, discharge is generated in the step of separating the recording paper from the photosensitive drum after the transfer, so that the polarity becomes positive later. Each time they receive a transfer charge of positive polarity in the subsequent transfer process, they gradually return to the photosensitive drum and a reverse transfer phenomenon occurs.

Yet another conventional technique for preventing the deterioration of image quality due to such a reverse transfer phenomenon is disclosed in Japanese Patent Laid-Open No. 9-3191.
No. 79 is disclosed. According to this conventional technique, in a multi-stage drum type color image forming apparatus, when a color image is formed, in consideration of the reverse transfer phenomenon, a large amount of toner is supplied to the recording paper by the most upstream image forming unit. It is shown that the image forming condition is set.

In the conventional technique disclosed in Japanese Patent Laid-Open No. 9-319179, the image forming conditions set for the color image formation are not set for the single color image formation, but the image forming conditions suitable for the single color image formation are not set. If the conditions are shared even during the monochromatic image formation, the following problems occur. That is, in the multi-stage drum type color image forming apparatus 3 configured to separate the downstream recording paper carrying belt shown in FIG. 22, the reverse transfer phenomenon is avoided by separating the recording paper carrying belt when forming a single color image. However, a large amount of toner that has been attached in advance remains attached to the recording paper, which causes a problem that the image density becomes high.

[0034]

SUMMARY OF THE INVENTION An object of the present invention is to fix the image density of a toner image transferred on the most upstream side after fixing regardless of whether a color image is formed or a monochrome image is formed. An object of the present invention is to provide a color image forming apparatus capable of eliminating the difference.

[0035]

SUMMARY OF THE INVENTION According to the present invention, toner images of different colors are formed along a predetermined transporting path of a recording sheet, and the electrostatic latent images are visualized by adhering a developer to each other. A plurality of photosensitive drums, and a recording paper conveyance belt that carries recording paper and conveys the recording paper along the conveyance path, and is arranged at the most upstream side in the conveyance direction of the recording paper when forming a single-color image. The recording paper transport belt is brought into contact with only the photoconductor drums, and the recording paper transport belt is separated from the remaining photoconductor drums, and a transport unit is provided to bring all the photoconductor drums into contact with the recording paper transport belt during color image formation, For each photoconductor drum, the toner image formed on each photoconductor drum is provided on the recording paper so as to face the inner peripheral surface of the recording paper transport belt on the opposite side of the recording paper transport belt. Individual A plurality of transfer members to be transferred, a pressure contact force adjusting unit that adjusts the pressure contact force of each transfer member to the recording paper conveyance belt in accordance with color image formation and single color image formation, and toner transferred onto the recording paper. A color image forming apparatus comprising: a fixing unit that fixes an image on the recording paper.

According to the present invention, the recording paper carrying belt is arranged on the downstream side of the photoconductor drum arranged on the most upstream side in the recording paper carrying direction when the monochromatic image is formed by the carrying means. It can be separated from the drum and can be brought into contact with all the photoconductor drums at the time of color image formation. Further, the pressure contact force of each transfer member to the recording paper conveyance belt is adjusted by the pressure contact force adjusting means according to the time of forming the single color image and the time of forming the color image. The pressure contact force of the transfer member with respect to the recording paper conveyance belt can be switched for each. In this adjustment of the pressure contact force, since the strength of the electric field and the distance from the electric field generating portion to the observation point are in inverse proportion to each other, the transfer electric field is changed by changing the facing position of the transfer member with respect to the photosensitive drum. Is. The monochromatic toner image transferred to the recording paper at the time of forming the monochromatic image and the color toner image transferred to the recording paper at the time of forming the color image are fixed to the respective recording papers by the fixing means.

With such a structure, when a color image is formed in which the reverse transfer phenomenon should be taken into consideration, the pressure contact force of each transfer member to the recording paper conveyance belt is adjusted so that the pressure contact between each photosensitive drum and the recording paper is made. The force can be adjusted so that the reverse transfer phenomenon does not occur, and the amount of toner transferred to the recording paper can be set for each toner image of each color to optimize the color image density. Further, at the time of forming a single-color image in which the reverse transfer phenomenon does not have to be taken into consideration, the recording paper conveyance belt comes into contact with the photosensitive drum arranged on the most upstream side in the recording paper conveyance direction, and Since the recording paper conveying belt is not in contact with each photosensitive drum arranged on the downstream side of the photosensitive drum, the transfer member of the transfer member corresponding to the most upstream side photosensitive drum is transferred to the recording paper conveying belt. The pressure contact force is adjusted by the pressure contact force adjusting means, the amount of toner transferred onto the recording paper is adjusted so that the optimum image density can be obtained in a single color, and the toner image can be transferred onto the recording paper. .

In this way, not only the monochromatic image obtained at the time of forming a monochromatic image in which the photoconductor drum located on the most upstream side is involved, but also the photoconductor drum on the most upstream side is involved. The density difference generated between the image obtained during color image formation can be eliminated, the image quality of a single color image can be improved, and a high-quality single color image and a high-quality single color image can be obtained without causing the reverse transfer phenomenon during color image formation. A color image is formed.

Further, in the present invention, each pressure contact force adjusting means is configured such that the pressure contact force of each transfer member arranged on the downstream side of the transfer member arranged on the most upstream side with respect to the recording paper conveying belt at the time of color image formation. Is set to be larger than the pressure contact force of the transfer member arranged on the most upstream side with respect to the recording paper conveyance belt.

According to the present invention, each pressure contact force adjusting means adjusts the amount of toner transferred onto the recording paper, so that the pressure contact force of each transfer member against the recording paper conveying belt, and hence the pressure contact force against each photosensitive drum. The more specific adjustment direction of is set as follows. That is, during color image formation involving a plurality of photoconductor drums and during monochromatic image formation involving only the most upstream photoconductor drum, the pressure contact force during color image formation is greater than the pressure contact force during monochromatic image formation. Is also set to a large value, the transfer electric field is reduced to the transfer position where the photoconductor drum arranged on the most upstream side is in contact with the recording paper conveyance belt during monochromatic image formation, and To reduce the amount of toner attached to the recording paper from the drum,
In order to obtain the image density, it is possible to adjust to the optimum toner adhesion amount without excess or deficiency.

Therefore, when forming a color image,
Even if the amount of toner adhered to the most upstream photoconductor drum is set in consideration of the reverse transfer phenomenon, the recording paper conveyance belt is separated from the most downstream photoconductor drum, and the most upstream photoconductor drum is separated. When forming a single-color image in which transfer is performed only by itself, the amount of toner adhered to the most upstream side photoconductor drum, which was set to a large amount during the formation of the color image, will be reduced, and a density difference will occur with respect to the color image. It is possible to prevent useless transfer of toner and form a monochromatic image having a density difference that is less than that of a color image when forming a monochromatic image.

In this way, the density difference generated between a color image obtained by involving a plurality of photosensitive drums and a monochromatic image obtained by only involving the photosensitive drums located on the most upstream side is It is possible to surely solve the problem and further improve the image quality of the monochrome image.

Further, the present invention is characterized in that a toner image is formed by black toner on the photoconductor drum arranged on the most upstream side.

According to the present invention, since the toner image is formed on the photosensitive drum arranged on the most upstream side by the black toner, the black toner image becomes the lowermost layer on the recording paper when the color image is formed. , The influence on the toner images of other colors can be minimized. As a result, at the time of forming a color image, a black image obtained by involving only the photoconductor drum arranged on the most upstream side and a color image obtained by involving the photoconductor drum arranged on the downstream side are formed. The occurrence of the density difference is eliminated, and a high-quality image having no density difference can be obtained in both the black monochromatic image formation and the color image formation.

Further, according to the present invention, the pressing force adjusting means includes a transfer power source for applying a transfer voltage to the transfer member, and a direction in which the transfer member approaches and separates along the normal line direction at the transfer position of the photosensitive drum. And a driving means for biasing the bias.

According to the present invention, the recording paper conveying belt is elastically pressed toward each photoconductor drum by the conductive portion of the pressing member, and a transfer voltage is applied to the conductive portion from the transfer power source. The driving means biases the pressing member toward and away from each other along the normal line direction at the transfer position of the photosensitive drum, so that the pressing force of the pressing member to the recording paper conveyance belt during the formation of a single color image is as described above. The adjustment can be performed so that an optimum image having no difference in density from the color image at the time of forming the color image can be obtained.

As described above, the transfer voltage to the conductive portion of the transfer power source is controlled by the mechanical control for displacing the pressing position of the pressing member to the recording paper conveying belt in the normal direction to the transfer position of the photosensitive drum. By making it possible to adjust the transfer electric field without changing the transfer voltage, it is possible to avoid an increase in the power supply load on the transfer power supply, and thus the power load on the color image forming apparatus, due to the change in the high transfer voltage. .

Further, according to the present invention, the press contact force adjusting means includes a transfer power source for applying a transfer voltage to the transfer member, and the transfer member along the tangential direction at the transfer position of the photosensitive drum along the upstream side and the downstream side in the transport direction. Drive means for biasing to

According to the present invention, the recording paper conveying belt is elastically pressed toward each photoconductor drum by the conductive portion of the pressing member, and a transfer voltage is applied to the conductive portion from the transfer power source. The driving means biases the pressing member along the tangential direction at the transfer position of the photosensitive drum to the upstream side and the downstream side in the conveying direction, and the pressing force of the pressing member to the recording paper conveying belt is changed to a monochromatic image as described above. It can be adjusted so as to obtain an optimum image that does not cause a density difference from the color image at the time of forming the color image at the time of forming.

As described above, the transfer voltage to the conductive portion of the transfer power source is changed by the mechanical control in which the pressing position of the pressing member to the recording paper conveying belt is biased in the tangential direction at the transfer position of the photosensitive drum. By making it possible to adjust the transfer electric field without doing so, it is possible to avoid an increase in the power supply load to the transfer power supply, and thus the power load to the color image forming apparatus, due to the change in the high transfer voltage.

With this structure, the direction in which the pressing member is biased is set to the transfer position directly below the photosensitive drum.
This can be done either upstream or downstream in the transport direction, but if it is biased to the upstream side, the toner will be on the recording paper due to the effect of the transfer electric field even before the photosensitive drum on the most upstream side contacts the recording paper. Since the toner easily scatters, it is more effective to control the toner scattering, by biasing it to the downstream side.

[0052]

1 is a sectional view showing a simplified structure of a multi-stage drum type color image forming apparatus 70 according to an embodiment of the present invention. The multi-stage drum type color image forming apparatus 70 includes a recording paper conveyance belt 7 for conveying a recording paper 71.
2 is wound around a driving roller 73 and a driven roller 74 and stretched, and the lower end portions of the recording paper transport belt 72 are in contact with the recording paper transport surface of the upper stretched portion 72a of the recording paper transport belt 72. 75C, 75M, and 75Y are provided.

Photoreceptor drums 75BK, 75C, 75
Cleaning devices 76BK, 7 are provided around M, 75Y.
6C, 76M, 76Y, initialization charger 77BK, 77
C, 77M, 77Y, image exposure device 78BK, 78C, 78
M, 78Y, developing devices 79BK, 79C, 79M, 79
Y, and developing rollers 80BK, 80C, 80M, 80
Y is provided, and the transfer members 81BK and 8K are provided below the recording paper conveying belt 72 with the upper stretched portion 72a interposed therebetween.
1C, 81M, 81Y are provided.

Each of the developing rollers 80BK, 80C, 8 described above
0M and 80Y are the developing devices 79BK, 79C and 79
In the opening facing the downstream side in the transport direction A below the M and 79Y,
The photoconductor drums 75BK, 75C, 75M, and 75Y are provided rotatably around rotation axes parallel to the rotation axes, and these four developing devices 79BK, 79C, 79M,
79Y contains toners of black BK, cyan C, magenta M, and yellow Y, respectively.

A paper feed cassette 82 is detachably attached to the lower portion of the main body of the multi-drum type color image forming apparatus 70. In this paper feed cassette 82, a plurality of recording papers 7
Each of the recording sheets 71 is accommodated in a stacked state.
The sheets are taken out one by one from the uppermost portion by the sheet feeding roller 83, inverted to the rear of the apparatus (leftward in FIG. 1) indicated by the arrow B, and supplied to the recording sheet conveying belt 72. The recording paper 71 supplied to the recording paper conveyance belt 72 is a recording paper conveyance belt 72 that is circulated and driven counterclockwise in FIG.
The photoconductor drums 75BK, 75C, 75M, 75Y and the transfer members 8 are conveyed by the above-mentioned overhanging portion 72a.
1BK, 81C, 81M, and 81Y sequentially pass through four transfer positions 84BK, 84C, 84M, and 84Y facing each other.

Photoreceptor drums 75BK, 75C, 75
M and 75Y rotate clockwise in FIG. 1 in the direction of arrow D, and their outer peripheral surfaces have initialization chargers 77BK, 77C and 77.
High electric charges are uniformly applied by the M and 77Y to be initialized, and exposed by the image exposure devices 78BK, 78C, 78M and 78Y to form electrostatic latent images, and the developing rollers BK and
The toner is transferred by C, M, and Y and electrostatically adheres, and the electrostatic latent image formed by the exposure is for each color BK,
The toner images of C, M, and Y are visualized (developed), and these toner images are conveyed to the transfer positions BK, C, M, and Y by the rotation of the photoconductor drums 75BK, 75C, 75M, and 75Y. It

Photoreceptor drums 75BK, 75C, 75M,
The paper feed roller rotates at the timing when the front end of the toner image on 75Y reaches the transfer position, and the uppermost recording paper 71 is indicated by arrow B.
The recording paper is conveyed in the direction, and the print start position of the recording paper 71 sequentially reaches the transfer positions BK, C, M, and Y by the recording paper conveyance belt 72. The transfer members BK, C, M, and Y are connected to the recording paper 7 via the recording paper conveyance belt 72 by using charges supplied from a transfer power supply 91 shown in FIG.
The toner image on each of the photosensitive drums 75BK, 75C, 75M, and 75Y is transferred to the recording paper 71 by the potential difference from the transfer bias. 4 transfer positions B
The recording paper 71 on which the toner images are sequentially transferred in K, C, M, and Y is carried into a fixing unit 86 arranged on the left side outside the drawing, and the toner images are transferred by a heat roller 87 and a pressure roller 88. The recording paper 71 is heat-fixed on the surface of the recording paper 71 and discharged outside the apparatus.

FIG. 2 is a diagram showing a specific structure of the pressure contact force adjusting means 93BK provided in the image forming unit 90BK arranged on the most upstream side. The image forming unit 90BK, which is arranged on the most upstream side in the transport direction A of the recording paper 71 by the stretched portion 72a of the recording paper transport belt 72, includes the above-described photosensitive drum 75BK, cleaning device 76BK,
Initializing charger 77BK, image exposing device 78BK, developing device 7
9BK, developing roller 80BK, and transfer member 81BK
including. Photosensitive drums 75C, 75M and 75Y provided for the remaining colors C, M and Y, and a cleaning device 76
C, 76M, 76Y, initialization charger 77C, 77M, 7
7Y, image exposure devices 78C, 78M, 78Y, developing device 79
C, 79M, 79Y, and developing rollers 80C, 80
M, 80Y and transfer members 81C, 81M, 81Y constitute image forming units 90C, 90M, 90Y, respectively, and these image forming units 90C, 90M, 90Y are formed.
Are provided with the above-mentioned transfer members 81C, 81M, and 81Y, respectively, and the transfer members 81C, 81M, and 81Y are applied to the upper stretched portion 72a of the recording paper conveyance belt 72 by the pressure contact force adjusting means 93C, 93M, and 93Y. It is configured so that the pressure contact force of can be adjusted. In FIG. 2, the pressure contact force adjusting means 93BK of the image forming unit 90BK which forms a black toner image and transfers it onto the recording paper 71 is shown as a representative, but the other pressure contact force adjusting means 93C, 93M, 93Y are also the same. Shall be configured.

The pressure contact force adjusting means 93BK is attached to a substantially T-shaped mounting member 94 which is slidably contacted with the back surface of the recording paper transport belt 72 and fixed to the base end portion of the transfer member 81BK made of a conductive material. The member 94 is driven to be displaced up and down, and the distance X from the back surface of the upper stretched portion 72a of the recording paper conveyance belt 72 is measured.
Eccentric cam 95, a drive motor 96 for rotating the eccentric cam 95 by a predetermined rotation amount, and a drive motor 96
Drive control device 97 for controlling the rotation operation of the eccentric cam 95, an operation panel 98 provided in the main body of the device for an operator to input an operation command, and a host computer 99.
Including and The drive control device 97 is a ROM storing a control program for executing an electrophotographic process.
(Read Only Memory) and RAM (Random Access Memory) for temporarily storing information necessary for control.

A print command by the operator from the operation panel 98, for example, a command for executing either color printing or single color printing, or the host computer 99.
When an output command for either color printing or single color printing is input to the drive control unit 97 from the terminal via a terminal realized by a personal computer or the like connected to the multi-drum type color image forming apparatus 70, the drive control unit 97 Reference numeral 97 selectively drives a drive signal in the forward or reverse rotation direction to the drive motor 96 to drive the drive motor 96.
That is, when a color printing command is input to the drive control device 97, the rotation direction of the drive motor 96 is set to the transfer member 81.
BK, 81C, 81M, 81Y photoconductor drums 75B
When the contact pressure of the transfer members 81BK, 81C, 81M, and 81Y is selected in the approaching rotation direction by increasing the pressure contact force with respect to K, 75C, 75M, and 75Y, and the monochromatic print command is input, the drive motor 96 of the drive motor 96 is input. Transfer member 8 in the direction of rotation
1BK, 81C, 81M, 81Y photosensitive drum 75B
The pressure contact force with respect to K, 75C, 75M, and 75Y is reduced so that the rotation direction in which the transfer members 81BK, 81C, 81M, and 81Y are separated from the recording paper conveyance belt 72 is selected.

The pressing force for pressing the most upstream transfer member 81BK against the photosensitive drum 75BK is 550 g to 700 g.
It is suitable to be about g. When the pressing force of the transfer member 81BK is less than 550 g, transfer omission occurs frequently and 700 g
If it exceeds, the dot reproducibility is deteriorated. Table 1 shows an example of setting the pressing force to the drive control device 97.

[0062]

[Table 1]

The pressing force of the transfer member 81BK on the photoconductor drum 75BK in the most upstream image forming unit 90BK is set so that the pressing force during monochromatic image formation is lower than the pressing force during color image formation. Has been done.
Here, the total pressing force at both ends of the transfer member in Table 1 above is the total load applied to the ends of the transfer member 81BK.

In this way, the pressing force of the transfer member 81BK on the photosensitive drum 75BK is set, and the transfer member 81BK is set.
When the transfer voltage applied to BK is 2.5 kV, the most upstream image forming unit 90BK depends on the image forming mode.
It has been confirmed by experiments by the inventor of the present invention that it is possible to effectively suppress the difference in image density that occurs with the toner color.

FIG. 3 is an enlarged sectional view near the transfer position when a flexible sheet is used for the transfer member 81BK.
[Fig. 8] is a perspective view showing an appearance of a transfer member 81BK. The flexible sheet used as the transfer member 81BK includes a sheet-shaped member 101 and a support member 102 that supports the sheet-shaped member 101. In this embodiment, the sheet-shaped member 101 is made of nylon synthetic resin as a material, and carbon is dispersed as a conductive agent. This electrically conductive nylon resin is formed into a sheet shape and is set to have an electric resistance value of, for example, 1 × 10 ⁶ to 1 × 10 ⁷ Ω. The amount of carbon dispersed in the synthetic resin is 3 to 30% by weight of carbon with respect to 100% by weight of resin in the case of a nylon resin. When the amount of carbon exceeds 30% by weight, it becomes difficult to uniformly disperse the carbon, and carbon agglomerates are formed here and there to locally develop a portion having a low electric resistance, which makes it unusable. turn into. On the other hand, if the electric resistance value is set to 1 × 10 ⁷ Ω or more, the amount of carbon is too small, and in this case also, it becomes difficult to disperse the carbon uniformly. You can do it, and you will not be able to use it.

Next, the supporting member 102 is made of a flexible polyethylene (abbreviated as PET) film, and is adhered to the back surface of the sheet-shaped member 101 by using an adhesive or an adhesive material such as a double-sided tape. They are stacked and elastically support the sheet-like member 101 from the back surface.

In the transfer member 81BK made of such a flexible sheet, the lower end of the support member 102 on the back side is
As shown in FIG. 3, it is attached to and supported by a mounting member 94 having a substantially T-shaped cross section. This mounting member 94
One end of a conductive tape is attached to the lower end of the sheet-like member 101, which is the surface side of the transfer member 81BK attached and supported at the tip of the conductive tape, and the other end of the conductive tape is attached to the attachment member 9
4 is attached to the lower surface of the mounting member 94 around the lower end portion of the laterally inclined T-shaped inclined portion 94a, and the wiring 103 for supplying a transfer current is connected to the portion attached to the lower surface.

The inclination of the inclined portion 94a of the mounting member 94 is a vertical line (the upper stretched portion 72a of the recording paper conveyance belt 72).
Is formed at an angle θ of about 35 ° on the downstream side in the transport direction A of the recording paper transport belt 72. As a result, the transfer sheet is disposed on the downstream side in the circulating movement direction of the recording paper conveyance belt with an inclination of about 35 °.

When the sheet-like transfer member 81BK is in a free state where no bending pressure is applied to the leading end of the sheet-shaped transfer member 81BK due to the sliding friction with the recording paper conveying belt 72, the leading end is shown by a virtual line in FIG. As described above, the length is set to be higher than the conveyance surface of the recording paper conveyance belt 72. The transfer member 81BK is in sliding contact with the recording paper conveyance belt 72 as shown by the phantom line in FIG. 3, and the sliding contact portion is in contact with the photosensitive drum 75BK and the recording paper conveyance belt 72.
It is arranged so as to be located immediately below the contact portion with.

FIG. 5 is a cross-sectional view showing the contact state of the transfer member 81BK with the recording sheet conveying belt 72, and the vicinity of the transfer position 84BK. FIG. 5A shows the contact state of the transfer member 81BK during color image formation. 5 (2) shows a contact state of the transfer member 81BK at the time of forming a monochromatic image. The distance X between the lower surface of the recording paper conveyance belt 72 and the upper surface of the transfer member 81BK is set so that the distance X2 at the time of forming a single color image is wider than the distance X1 at the time of forming a color image. That is, the bending pressure of the transfer sheet is weaker when forming a single-color image than when forming a color image, that is,
In order to reduce the pressure contact force to the recording paper conveyance belt 72, in other words, the photosensitive drum 75BK and the recording paper conveyance belt 7
It is set so that the nipping force between the recording sheet 71 and the recording sheet 71 is weakened. By weakening the pressure contact force, the transfer electric field is weakened, and thus the image density is lowered.

This is the photosensitive drum 75B at the transfer position.
It is considered that there are a physical one due to the contact pressure (pressure contact force) to K and an electrostatic one due to being converted into a part of the charging polarity of the toner by the transfer electric field at the time of transfer. There is. Then, in order to eliminate the physical influence of the contact pressure, the pressure contact force of the transfer sheet to the recording paper conveyance belt is set to be weak.

As shown in FIG. 6A, the transfer member 81B.
The larger the pressure contact force of K, the stronger the transfer electric field. Therefore, as shown in FIG. 6B, the transfer member 8
The higher the pressure contact force of 1BK, the higher the image density.

Further, as shown in FIG. 7A, the transfer electric field becomes weaker as the deviation amount of the transfer member 81BK becomes larger. Therefore, as shown in FIG. 7B, the image density decreases as the amount of bias of the transfer electric field increases. Table 2 below shows an example of setting the distances X1 and X2.

[0074]

[Table 2]

When the distance between the transfer member 81BK and the back surface of the recording paper conveyance belt 72 is set as described above and the transfer voltage applied to the transfer member 81BK is set to 2.5 kV, the highest level is selected depending on the image forming mode. The image density difference caused by the toner color of the image forming unit 90BK can be effectively suppressed.

In the above structure, the pressure contact force or the opposed position of the transfer member 81BK is arbitrarily set by the pressure contact force adjusting means for adjusting the pressure contact force or the opposed position of the transfer member 81BK with respect to the photosensitive drum 75BK in the most upstream image forming unit 90BK. Can be set to. Therefore, according to the present embodiment, it is possible to obtain the best pressure contact force or the facing position in any of the printing modes of color printing or single color printing, and obtain the optimum transfer state for each printing mode. Is possible.

In another embodiment of the present invention, the image forming unit 90B is used as the pressure contact force adjusting means and the transfer member.
When K is shown as a representative example, FIG. 8 (1) and FIG. 8 (2)
Actuator 105 and compression spring 10 as shown in FIG.
The conductive brush 107 may be vertically displaced by 6 and the conductive brush 109 may be vertically displaced by the eccentric cam 108 shown in FIGS. 9 (1) and 9 (2). . The parts corresponding to those in the above-described embodiment are designated by the same reference numerals, and although not shown, the structures corresponding to the drive control means 97, the operation panel 98 and the host computer 99 are similarly provided. To do. The parts corresponding to those in the above-described embodiment are designated by the same reference numerals.

The actuator 105 and the compression spring 106 constitute a pressure contact force adjusting means, and the conductive brush 10
Reference numeral 7 constitutes a transfer member. Further, the eccentric cam 108 constitutes a pressure contact force adjusting means, and the conductive brush 109 constitutes a transfer member. Here, FIG. 8 (1) shows the actuator 1
FIG. 8 shows a state at the time of color image formation in which 05 is retracted downward and the compression spring 106 is extended to reduce the pressure contact force.
(2) shows a state during monochromatic image formation in which the actuator 105 is extended upward and the compression spring 106 is compressed to increase the pressure contact force. Further, FIG. 9 (1) shows a case of forming a color image in which the pressure contact force of the conductive brush 109 is increased by the cam surface having the maximum radius of the eccentric cam 108, and FIG. 9 (2) shows the cam surface having the minimum radius of the eccentric cam 108. Conductive brush 1
09 shows the time of monochromatic image formation in which the pressure contact force of 09 is reduced. The conductive brush 109 may be realized by a fixed brush made of metal that is hardly deformed in the range where the pressure contact force changes. Also with such a configuration, it is possible to achieve the same effect as that of the above-described embodiment.

Further, in another embodiment of the present invention, FIG.
As shown in FIG. 0, a conductive rubber roller 112 is rotatably provided, and the rubber roller 112 is connected to an actuator 113.
The pressure contact force may be adjusted by finely displacing the recording paper conveying belt 71 with respect to the upper stretched portion 71a by 30 ° to 45 ° as shown in FIG.
The proximal end of the flexible conductive film 114 tilted at a certain angle α may be finely displaced up and down by the actuator 115 to adjust the pressure contact force, as shown in FIG. As described above, the brush roll 116 in which a large number of conductive fibers are radially arranged is rotatably provided, and the brush roll 116 is finely displaced up and down by the actuator 117 to adjust the pressure contact force. Of course, as shown in FIG. 13, a so-called mohair-like brush block 118 in which a conductive brush is planted perpendicularly to the upper stretched portion 72a of the recording paper conveyance belt 72 is moved up and down by an actuator 119. The pressure contact force may be adjusted by finely displacing it.

With the configurations shown in FIGS. 10 to 13, the same effects as those of the above-described embodiments can be achieved. It should be noted that parts corresponding to those of the above-described embodiments are designated by the same reference numerals, and although not shown, configurations corresponding to the drive control means 97, the operation panel 98, and the host computer 99 are similarly provided. To do.

FIG. 14 is a simplified sectional view showing a color image forming apparatus 70a according to still another embodiment of the invention, and FIG. 15 is an image forming unit 9 arranged on the most upstream side.
It is the simplified side view which shows the concrete structure of the press contact force adjustment means 93BK1 with which 0BK is equipped. The color image forming apparatus 70a of the present embodiment is similar to each of the embodiments shown in FIGS. 1 to 13, and corresponding parts are designated by the same reference numerals. The color image forming apparatus 70a according to the present embodiment has four image forming units 90BK, 90C, 90M and 90Y, like the color image forming apparatus 70 according to the above-described embodiments.
The recording sheet conveying belt 71 and the fixing device 86 are included.

In the image forming unit 90BK for black transfer on the most upstream side in the transport direction A, the photosensitive drum 7 having a right cylindrical shape is provided.
5BK rotates in the direction of arrow D, and an initialization charging device 77BK, an image exposure device 78BK, a developing device 79BK, and a cleaning device 76BK are arranged around the 5BK in the rotation direction D. An image exposure device 78BK is provided on the surface of the photoconductor drum 75BK between the initialization charging device 77BK and the developing device 79BK.
Is irradiated with laser light from the above to form an electrostatic latent image on the photosensitive drum 75BK.

Image forming units 90C, 90M and 90Y similar to the image forming unit 90BK centering around the photoconductor drum 75BK are arranged on the upper stretched portion 72a of the recording paper conveying belt 72 along the conveying direction A. They are juxtaposed. The recording paper conveyance belt 72 is used for each image forming unit 90.
BK, 90C, 90M, 90Y developing device 79BK, 7
9C, 79M, 79Y and cleaning device 76BK, 7
Between 6C, 76M and 76Y, photosensitive drum 75B
K, 75C, 75M, and 75Y are in contact with the photosensitive drums 75BK, 75C, and 75 of the recording paper transport belt 72.
Transfer rollers 81BK1 and 81C, which are transfer members for applying a transfer bias, are provided on the back surfaces of the M and 75Y sides.
1, 81M1 and 81Y1 are arranged in contact with the recording paper conveyance belt 72 in this order. Each image forming unit 9
0BK, 90C, 90M, and 90 have the same configuration except for the toner color inside the developing device.

The color image forming apparatus 70a adjusts the pressure contact force of the transfer rollers 81BK1, 81C1, 81M1, 81Y1 to the recording paper conveying belt 72 according to the color image formation and the monochromatic image formation. The pressure contact force adjusting means 93BK1, 93C1, 93M1, 93Y1 are provided. The pressure contact force adjusting means 93BK of the image forming unit 90BK, which is typically arranged on the most upstream side, will be described.
The transfer roller 8 that comes into contact with the recording paper conveyance belt 72 in a state in which 2 is elastically pressed toward the photoconductor drum 75BK.
An eccentric cam 121BK for pressing 1BK1, a transfer power source 122 for applying a transfer voltage to the transfer roller 81BK1, and
A drive motor 96 that is a drive unit that biases the eccentric cam 121BK toward the upstream side and the downstream side in the transport direction A along the tangential direction at the transfer position of the photosensitive drum 75BK, a drive control device 97, an operation panel 98, and a host. And a computer 99.

During color image formation, the photoconductor drums 75BK, 75C, 75M and 75Y of the image forming units 90BK, 90C, 90M and 90Y sandwich the recording paper conveyance belt 72 and transfer rollers 81BK1, 81C1 and 8B.
It is in contact with 1M1 and 81Y1. Therefore, when the recording paper 71 is conveyed by the recording paper conveyance belt 72, the toner images of the respective colors are superimposed and transferred from the photoconductor drums 75BK, 75C, 75M, 75Y of the image forming units 90BK, 90C, 90M, 90Y, The toner image is fixed on the recording paper 71 by the fixing means. As a result, a color image is formed on the recording paper 71.

On the other hand, at the time of forming a monochromatic image, the recording paper conveying belt 72 is not the uppermost image forming unit 90.
It is angularly displaced in the arrow K direction from C, 90M, and 90Y and separated. Therefore, when the recording paper 71 is conveyed by the recording paper conveyance belt 72, the image forming unit 90 on the most upstream side.
The black toner image is transferred only by the BK photosensitive drum 75BK and is fixed by the fixing means. As a result, a black monochromatic image is formed on the recording paper 71.

Here, in the present embodiment, the amount of toner adhered to the recording paper 71 by the image forming unit 90BK arranged in the most upstream is optimized for each of the color image formation and the monochromatic image formation. The transfer electric field is switched to.

Generally, the photosensitive drums 75BK, 75
The toner images of the respective colors on C, 75M, and 75Y are transferred by the transfer voltage applied to the transfer rollers 81BK, 81C, 81M, and 81Y and the transfer electric field formed by the potential difference between the photosensitive drums 75BK, 75C, 75M, and 75Y. Transferred onto recording paper. Therefore, by changing the transfer electric field,
The amount of toner adhered to the recording paper can be adjusted. As a means for changing the transfer electric field, the transfer roller 81BK,
There is a method of changing the transfer voltage applied to 81C, 81M and 81Y. However, since the transfer voltage is very high, the method of switching the transfer voltage increases the burden on the high-voltage system and should be avoided, and the transfer voltage should be kept constant in each image forming unit. Therefore, in the color image forming apparatus 70a of the present embodiment, as a specific method for changing the transfer electric field, the following method that can be relatively easily performed without affecting the overall balance of the system is adopted. There is.

In the present embodiment, the relationship between the facing positions of the photosensitive drum 75BK of the image forming unit 90BK and the transfer roller 81BK of the image forming unit 90BK located in the most upstream side in the recording paper conveying direction is changed according to the image forming mode to form a color image. When,
The transfer electric field is set to be different from that at the time of monochromatic image formation in which only the most upstream image forming unit is involved. Since the strength of the electric field and the distance (distance from the electric field generating portion to the observation point) are in inverse proportion, the transfer electric field can be changed by changing the facing position of the transfer roller 81BK with respect to the photosensitive drum 75BK. .

Specifically, the most upstream image forming unit 9
With respect to 0BK, the toner adhesion amount at the time of forming a single color image is set to be smaller than that at the time of forming a color image. This is because the image forming conditions are set such that a large amount of toner of the most upstream side image forming unit 90BK is supplied in anticipation of the reverse transfer phenomenon during color image formation. That is, if monochromatic image formation is performed with only the most upstream image forming unit 90BK with this setting, excess toner will adhere to the recording paper.

As shown in FIG. 14, the transfer rollers 81BK, 81C, 81M and 81Y are made to face each other substantially directly below the photosensitive drums 75BK, 75C, 75M and 75Y at the time of color image formation, and the recording paper is conveyed at the time of single color image formation. Bias in the downstream direction. Transfer rollers 81BK, 81C,
81M and 81Y are photosensitive drums 75BK, 75C and 75
It is preferable that the amount of deviation when it is deviated from the position directly below M and 75Y in the recording paper conveyance direction is about 0.25 to 1.0 mm.

This is because when the deviation amount is less than 0.25 mm, the transfer electric field becomes larger than necessary, and as a result, the toner adhesion amount becomes excessive and the dot reproducibility deteriorates. On the other hand, when the deviation amount is 1.0 mm or more, the transfer electric field is insufficient, and as a result, a sufficient toner adhesion amount cannot be obtained and the density of the solid portion is reduced.

Hereinafter, the transfer rollers 81BK, 81C, 81
M, 81Y photoconductor drums 75BK, 75C, 75M,
An example of setting the deviation amount for 75Y will be described.

[Setting Example 1] During color image formation ... Photosensitive drums 75BK, 75C, 7
5M, 75Y immediately below (lowermost point position) 0.25 mm When forming a single-color image ... Photosensitive drums 75BK, 75C, 75
0.75 mm from the position right below M and 75Y (the lowest point position) When the transfer voltage applied to the transfer roller is fixed at 2.5 kV, if the transfer roller is biased by the above amount, the most upstream image forming unit B The transfer electric field is as follows: [kV / m] during color image formation ... 303 × 10 ³ [kV / m] during monocolor image formation, and relates to the toner color of the most upstream image forming unit generated in the image forming mode. The image density difference can be effectively suppressed.

[Setting Example 2] During color image formation ... Photosensitive drums 75BK, 75C, 7
5M, 75Y immediately below (lowermost point position) 0.25 mm When forming a single-color image ... Photosensitive drums 75BK, 75C, 75
1.0 mm below the lower part of M, 75Y (the lowest point position) When the transfer voltage applied to the transfer roller is fixed at 2.5 kV, if the transfer roller is biased by the above amount, the most upstream image forming unit B The transfer electric field is as follows: 1.0 × 10 ⁴ [kV / m] when forming a color image, and 2.5 × 10 ³ [kV / m] when forming a single color image. It is possible to effectively suppress the image density difference relating to the toner color of the image forming unit.

[0096]

[Table 3]

From the above results, the relationship between the deviation amount d of the transfer roller 81BK and the transfer electric field E (toner transfer amount) and the relationship between the deviation amount d and the image density ID (toner transfer amount) are shown in FIG. And, as shown in FIG. 16 (2), the relationship is inversely proportional. In either case of setting example 1 or setting example 2, the black developer is arranged at the position of the image forming unit 90BK at the most upstream position, and the other three primary colors C,
It is desirable to arrange M and Y on the downstream side. As a result, the black toner image is the first layer (lowermost layer) in the stacking order after transfer.
Therefore, the color reproducibility can be improved.

In this embodiment, as described above, the most upstream image forming unit 90BK is used depending on the image forming mode.
By adjusting the facing position of the transfer roller 81BK with respect to the photosensitive drum 75BK, the transfer electric field (toner transfer amount) is changed, and a color image is formed and a monochrome image is formed.
Although it is possible to prevent an image density difference from occurring with respect to the toner color of the most upstream image forming unit 90BK, adjustment of the facing position is performed by the pressure contact force adjusting means 93BK, 93C, 93M, 93Y as follows.

FIG. 17 is a perspective view showing a simplified mechanical structure of the press contact force adjusting means 93BK. Since the pressure contact force adjusting means 93BK, 93C, 93M, 93Y have the same configuration, the black pressure contact force adjusting means 93B is typically used.
K will be described. Referring also to FIG. 15, a pressure contact force adjusting unit 93BK for moving the transfer roller 81BK forward and backward along the recording paper conveyance direction A is provided to adjust the facing position of the transfer roller 81BK and the photosensitive drum 75BK. The pressure contact force adjusting means 93BK includes a drive control device 97, a drive motor 96, a counter shaft 133, an eccentric cam 95, and a bearing 13.
4,135 and spring 136 are included.

As in the above-described embodiments, the drive control device 97 transfers the transfer roller 81BK when the operator switches the print mode according to an output command from the operation panel 98 of the image forming apparatus main body or the host computer 99. Is to command the movement of the. The drive motor 96 is a drive source that rotates in both forward and reverse directions upon receiving a command from the drive control device 97. The counter shaft 133 is connected to the output shaft of the drive motor 97 and transmits the driving force to the eccentric cam 95. The eccentric cam 95 is integrally provided on the counter shaft 133 and presses the transfer roller 81BK. The transfer power supply 91 applies a transfer voltage to the transfer roller 81BK. One bearing 1
34 rotatably supports the counter shaft 133, and the other bearing 135 rotatably supports the shaft of the transfer roller 81BK so as to be movable in the transport direction A. Spring 13
6 elastically supports the transfer roller 81BK.

Next, the operation of reciprocating movement of the transfer roller 81BK will be described. The drive control device 97 includes an operation panel 98 of the image forming apparatus main body and a host computer 99.
The type of the image forming mode, either the color image forming mode or the monochromatic image forming mode, is selectively input via. Further, the drive motor 96 includes a drive controller 9
The command from 7 is input. Next, the drive motor 96 rotates, and the drive force is transmitted to the transfer roller 81BK via the eccentric cam 95. Then, the transfer roller 81BK to which the driving force is transmitted is moved in the recording paper conveyance direction A (that is, the photoconductor drum 7).
It reciprocates along the tangential direction of 5BK.

As described above, the pressure contact force adjusting means 93BK shown in FIG. 17 is driven by the drive motor 96 via the drive control device 97.
And the counter shaft 133 are driven to move the transfer roller 81BK via the eccentric cam 130, thereby adjusting the facing position between the transfer roller 81BK and the photosensitive drum 75BK. At this time, the drive control device 97
The drive timing of the transfer roller 81BK is controlled so that the transfer roller 81BK does not move during the image forming operation.

In this embodiment, the driving in the case where the eccentric cam 130 adjusts the position where the transfer roller 81BK and the photosensitive drum 75BK face each other has been described, but the transfer roller 81BK moves in the recording paper conveyance direction A. Other configurations may be used as long as they can move back and forth along.

For example, as shown in the perspective view of still another embodiment of the present invention in FIG. 18, it is configured to include a drive control device 97, a drive motor 96, a counter shaft 133, a rack 140, and a pinion gear 141. May be. The parts corresponding to those in the embodiment of FIG. 17 are designated by the same reference numerals.

In the case of the embodiment of FIG. 18, the bearing 135 for supporting the transfer roller 81BK is fixed to the rack 140 and the transfer roller 81BK. Also, rack 1
40 and the pinion gear 141 mesh with each other.

According to this structure, the drive motor 96
When is rotated in either the normal or reverse direction, the pinion gear 141 also rotates via the counter shaft 133. When the pinion gear 141 is rotated, the rack 14 is screwed.
0 moves back and forth. When the rack 140 moves, the transfer roller 81BK fixed integrally with the rack 140 also moves.

According to such an embodiment, in the most upstream image forming unit 90BK, the photosensitive drum 75
The position of the transfer roller 81BK with respect to BK can be changed. As a result, the most upstream image forming unit 9
The image density can be adjusted by changing the transfer electric field of 0BK. Therefore, the most upstream image forming unit 90B
Photoconductor drum 75BK and transfer roller 81BK in K
By adjusting the relationship of the facing position with the, it is possible to suppress the occurrence of an image density difference between the color mode and the monochrome image. That is, in the case of the single color mode, the amount of toner transferred from each of the photoconductor drums 75BK, 75C, 75M, and 75Y to the recording paper can be reduced by adjusting the facing positions to be separated from each other.

In another embodiment of the present invention, the brush roll 116 shown in FIG. 12 can be used as the transfer member instead of the transfer rolls 81BK, 81C, 81M and 81Y. The brush roll 116 is formed in the shape of a roller as a whole by forming an elastic layer (conductive rubber) having a medium resistance of 10 ⁶ to 10 ¹² Ω · cm around a metal cored bar. Specifically, a core metal made of a metal material such as a stainless steel rod capable of ensuring sufficient strength and a resin material such as polyurethane rubber, silicone rubber, and a elastomeric resin of a styrene-butadiene copolymer pattern around the core metal are used. Formed as a foam having a thickness of about 100 μm, and further as a conductivity-imparting agent for the resin material, an inorganic substance such as carbon black, carbon fiber, or graphite particles, and / or
Alternatively, it is composed of an elastic member capable of supplying electric charges in which an organic conductive agent is mixed and dispersed.

The surface hardness of the transfer member is measured with a rubber hardness meter, and the Asker C scale hardness is about 20 ° to 60 ° (J
IS-K6301) is suitable. If the surface hardness of the transfer member is less than 20 ° on the Asker C scale, the amount of the transfer nip becomes excessive and toner scattering occurs. (Phenomenon that is too soft and has a large compression set and cannot be put to practical use.) Conversely, if the surface hardness exceeds 60 ° Asker C scale, secure a sufficient transfer nip amount. Cannot be performed, resulting in defective transfer. (If it is too hard, the photoconductor drums 75BK, 75C, 7
The 5M and 75Y are damaged, and a reaction phenomenon causes wear of the bearing. )

In yet another embodiment of the present invention, the rubber roller 112 shown in FIG. 10 may be used instead of the transfer roller 81BK, or the conductive film 114 shown in FIG. 11 may be used. Alternatively, the brush block 118 shown in FIG. 13 may be used.

Since the toner transfer amount transferred to the recording paper 71 can be adjusted as described above, the density generated between the image density during color image formation and the image density during monochromatic image formation. The difference can be eliminated.

The direction in which the transfer member is biased can be either upstream or downstream in the recording paper transport direction A from directly below the photosensitive drums 75BK, 75C, 75M, and 75Y, but it is biased upstream. In this case, the photoconductor drums 75BK, 75C, 75M, 75 of the most upstream image forming unit
Since toner easily scatters on the recording paper 71 due to the influence of the transfer electric field before Y and the recording paper 71 come into contact with each other, biasing the toner to the downstream side is effective for toner scattering control.

Furthermore, when a color image is formed (that is, in a multicolor image forming mode), the black toner becomes the lowermost layer and the influence on the toner images of other colors is minimized, so that a preferable color image can be formed. Further, the image density obtained for the image forming unit 90BK of the black toner BK located at the uppermost stream of the recording paper transport belt 72 during color image formation, and the image density of the black toner BK located at the uppermost stream of the recording paper transport belt 72. It is possible to eliminate the density difference that occurs between the image density obtained at the time of forming a black monochromatic image in which only 90BK is involved. Therefore, during both color image formation and black monochromatic image formation,
It is possible to obtain a high quality image.

Further, even in the tandem type color image forming apparatus, there are far more opportunities to form an image of a single black color than an opportunity to form an image of color, but in this case also, in consideration of the reverse transfer phenomenon, The black toner supply amount is set to be large.

However, according to this structure, when a black monochromatic image is formed, the transfer electric field is reduced with respect to the black developing device arranged at the most upstream position, so that the photoconductor drum 75BK can be formed more than when a color image is formed. , 75C, 75M, 75Y
Since the adjustment is performed in the direction of reducing the amount of toner adhered to the toner, an appropriate amount of toner, which is sufficient to obtain the image density, is supplied, and unnecessary toner consumption does not occur.

[0116]

According to the present invention, the transfer electric field is changed by changing the facing position of the transfer member with respect to the photosensitive drum, and the recording of each transfer member is performed at the time of color image formation in which the reverse transfer phenomenon should be taken into consideration. A monochromatic image obtained by forming a monochromatic image in which the photoconductor drum located on the most upstream side is involved by adjusting the pressure contact force to the paper conveying belt 72, and not only the most upstream photoconductor drum but also the downstream photoconductor. The density difference that occurs with the image obtained when forming a color image that also involves the drum is eliminated, and the image quality of a single color image can be improved,
A high-quality single-color image and color image are formed without causing a reverse transfer phenomenon at the time of forming a color image.

Further, according to the present invention, each press contact force adjusting means adjusts the press contact force of each transfer member to the recording paper conveying belt 72 in order to adjust the toner transfer amount onto the recording paper. Since the pressure contact force at the time of color image formation is set to be larger than the pressure contact force at the time of single color image formation at the time of color image formation involving the body drum and at the time of single color image formation involving only the most upstream photosensitive drum. During monochromatic image formation, the transfer electric field is reduced with respect to the transfer position where the photoconductor drum arranged on the most upstream side contacts the recording paper transport belt, and the transfer from the photoconductor drum to the recording paper is reduced compared to when a color image is formed. The toner adhesion amount can be reduced and the toner adhesion amount can be adjusted to the optimum amount without excess or deficiency in order to obtain the image density, and useless transfer of toner that may cause a density difference to a color image can be prevented. , Monochrome image shape During, it is possible to form a monochrome image without density difference in comparison with the color image.

Further, according to the present invention, since the toner image is formed on the photosensitive drum arranged on the most upstream side by the black toner, the black toner image becomes the lowermost layer on the recording paper when the color image is formed. The least influence on toner images of other colors, and at the time of color image formation, the black image obtained by only the photoconductor drum arranged on the most upstream side and the photoconductor arranged on the downstream side. The occurrence of a density difference between the color image obtained by the involvement of the drum is eliminated, and a high-quality image having no density difference can be obtained during both black monochromatic image formation and color image formation. .

Further, according to the present invention, the transfer voltage to the conductive portion of the transfer power supply is controlled by the mechanical control for displacing the pressing position on the recording paper conveying belt in the direction normal to the transfer position of the photosensitive drum. By adjusting the transfer electric field without changing the transfer voltage, it is possible to avoid an increase in the power supply load to the transfer power supply and therefore to the color image forming apparatus due to the change in the high transfer voltage. it can.

Further, according to the present invention, the transfer voltage to the conductive portion of the transfer power source is controlled by the mechanical control in which the pressing position on the recording paper conveying belt is biased in the tangential direction at the transfer position of the photosensitive drum. By making it possible to adjust the transfer electric field without changing the transfer voltage, it is possible to avoid an increase in the power supply load on the transfer power supply, and thus the power load on the color image forming apparatus, due to the change in the high transfer voltage. . Further, when the transfer position immediately below the photoconductor drum is biased to the upstream side, the toner is easily scattered on the recording paper due to the influence of the transfer electric field before the most upstream photoconductor drum and the recording paper come into contact with each other. Therefore, it is possible to further prevent toner scattering by biasing the toner to the downstream side.

[Brief description of drawings]

FIG. 1 is a sectional view showing a simplified configuration of a multi-stage drum type color image forming apparatus 70 according to an embodiment of the present invention.

FIG. 2 is an image forming unit 90B arranged on the most upstream side.
It is a figure which shows the concrete structure of the press contact force adjustment means 93BK with which K is equipped.

FIG. 3 is an enlarged cross-sectional view in the vicinity of a transfer position when a flexible sheet is used for the transfer member 81BK.

FIG. 4 is a perspective view showing the appearance of a transfer member 81BK.

FIG. 5 is a cross-sectional view near a transfer position 84BK showing a contact state of a transfer member 81BK with a recording paper conveyance belt 72,
FIG. 5A shows a transfer member 81B during color image formation.
The contact state of K and the contact state of the transfer member 81BK at the time of forming the monochromatic image in FIG.

6 (1) is a graph showing the relationship between the pressure contact force of the transfer member and the transfer electric field, and FIG. 6 (2) is a graph showing the relationship between the pressure contact force of the transfer member and the image density.

FIG. 7 (1) is a graph showing the relationship between the deviation amount of the transfer member and the transfer electric field, and FIG. 7 (2) is a graph showing the relationship between the deviation amount of the transfer member and the image density.

FIG. 8 is a diagram showing a pressure contact force adjusting means according to another embodiment of the present invention, FIG. 8 (1) shows a state in which the pressure contact force is lowered, and FIG. 8 (2) shows an increase in the pressure contact force. Shows the state.

9A and 9B are views showing a pressure contact force adjusting means of another embodiment of the present invention, FIG. 9A shows a state in which the pressure contact force is increased, and FIG. 9B shows a state in which the pressure contact force is decreased. Shows the state.

FIG. 10 is a diagram showing a pressure contact force adjusting means according to another embodiment of the present invention.

FIG. 11 is a diagram showing a pressure contact force adjusting means according to another embodiment of the present invention.

FIG. 12 is a diagram showing a pressure contact force adjusting means according to another embodiment of the present invention.

FIG. 13 is a view showing a pressure contact force adjusting means according to another embodiment of the present invention.

FIG. 14 is a partial cross-sectional view showing a color image forming apparatus 70a according to still another embodiment of the present invention, and FIG. 15 is a pressure contact force adjusting means provided in the image forming unit 90BK arranged on the most upstream side. It is a simplified side view showing a specific configuration of 93BK1.

FIG. 15 is an image forming unit 90 arranged on the most upstream side.
It is the simplified side view which shows the concrete structure of the press contact force adjustment means 93BK1 with which BK is equipped.

16 (1) is a graph showing a change in the transfer electric field with respect to the deviation amount of the transfer member, and FIG. 16 (2) is a graph showing a change in the image density with respect to the deviation amount of the transfer member.

FIG. 17 is a perspective view showing a simplified mechanical structure of a press contact force adjusting means 93BK.

FIG. 18 is a perspective view showing a simplified mechanical configuration of a pressure contact force adjusting means according to another embodiment of the present invention.

FIG. 19 is a simplified side view showing a schematic configuration of a conventional multi-stage drum type color image forming apparatus 1.

FIG. 20 is a diagram showing a schematic configuration of another conventional single-drum type color image forming apparatus 2.

FIG. 21 is a diagram showing a schematic configuration of another conventional multi-stage drum type color image forming apparatus 3; FIG. 21 (1) is a diagram showing a recording paper conveyance belt for each photoconductor drum 50Y, 50M, 50C, 50BK. FIG. 21 (2) shows a state in which 7 contacts.
Shows a state in which the recording paper conveyance belt 51 is separated from each of the photosensitive drums 50Y, 50M and 50C.

[Explanation of symbols]

70, 70a Multi-drum type color image forming apparatus 71 recording paper 72 Recording paper conveyor belt 73 Drive roller 74 Driven roller 75BK, 75C, 75M, 75Y Photosensitive drum 76BK, 76C, 76M, 76Y Cleaning device 77BK, 77C, 77M, 77Y Initialization charger 78BK, 78C, 78M, 78Y Image exposure device 79BK, 79C, 79M, 79Y Developing device 80BK, 80C, 80M, 80Y Development roller 81BK, 81C, 81M, 81Y Transfer brush 93BK, 93C, 93M, 93Y Pressure contact force adjusting means

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroshi Kono             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company (72) Inventor Masayasu Narimatsu             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company (72) Inventor Kaori Takeshita             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company (72) Inventor Tatsuya Tanaka             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company F-term (reference) 2H030 AA06 AA07 AB02 AD05 AD07                       AD17 BB44 BB46 BB53 BB63                 2H200 FA16 GA12 GA23 GA34 GA47                       GB12 GB25 HA03 HB12 HB14                       JA02 JA23 JB07 JB13 JB45                       JB46 LA17 LA24 LA38 MA02                       MA20 MB06

Claims (5)

[Claims] 1. A plurality of photosensitive drums, which are provided along a predetermined conveying path of a recording paper, and on which toner images of different colors are formed by visualization due to adhesion of a developer to an electrostatic latent image. , Has a recording paper conveyance belt that carries recording paper and conveys it along the conveyance path, and conveys the recording paper only to the photoconductor drum arranged on the most upstream side in the conveyance direction of the recording paper when forming a single-color image. While the belt is in contact, the recording paper transport belt is separated from the remaining photoconductor drums, and when the color image is formed, a transporting unit that brings the recording paper transport belts into contact with all the photoconductor drums, and A plurality of transfer members provided on the inner peripheral surface of the recording paper transport belt on the side opposite to the photosensitive drums with respect to the recording paper transport belt and individually transferring the toner image formed on each photosensitive drum to the recording paper. , A pressure contact force adjusting means for adjusting the pressure contact force of each transfer member with respect to the recording paper conveyance belt depending on whether a color image is formed or a single color image is formed, and the toner image transferred onto the recording paper is transferred onto the recording paper. A color image forming apparatus comprising: a fixing unit for fixing. 2. The pressure contact force adjusting means is configured such that when a color image is formed, the pressure contact force of each transfer member arranged on the downstream side of the transfer member arranged on the most upstream side with respect to the recording paper conveyance belt is 2. The color image forming apparatus according to claim 1, wherein the pressure is set to be larger than the pressure contact force of the transfer member arranged on the most upstream side with respect to the recording paper conveyance belt. 3. The color image forming apparatus according to claim 1, wherein a toner image is formed by black toner on the photoconductor drum arranged on the most upstream side. 4. The pressing force adjusting means comprises a transfer power source for applying a transfer voltage to the transfer member, and a drive for biasing the transfer member in directions toward and away from each other along the normal line direction at the transfer position of the photosensitive drum. 4. The color image forming apparatus according to claim 1, further comprising a means. 5. The pressing force adjusting means biases the transfer power source for applying a transfer voltage to the transfer member and the transfer member to the upstream side and the downstream side in the transport direction along the tangential direction at the transfer position of the photosensitive drum. The color image forming apparatus according to claim 1, further comprising a driving unit.