JP2003084526A - Multicolor image forming apparatus and its control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the multicolor image forming apparatus of a tandem system in which the density difference of an image to be generated at the time of forming a monochromatic image is made to be dissolved. SOLUTION: In a multicolor image forming apparatus in which a form conveying belt is made to be changeable (separately contactable) to a retraction state from the image forming state of a downstream side which does not participate in an image formation at the time of forming a monochromatic image, the initial electrification potential of a photoreceptor 6B is changed over by changing the confronting interval d between the screen grid 12 and the discharge electrode 13 of the electrifier 1B of the image forming station (B) of the uppermost stream at the time of forming a color image when entire image forming stations are used and at the time of forming the monochromatic image when only the image forming station (B) positioned at the uppermost stream is used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a toner image forming means for forming a toner image corresponding to each color and black for forming a color image or a monochromatic image, and toner of each color formed by the toner image forming means. The present invention relates to a multicolor image forming apparatus for transferring an image onto a recording medium, and more particularly to a multicolor image forming apparatus in which toner image forming units of respective colors are arranged in a tandem system and a control method thereof.

[0002]

2. Description of the Related Art A color image forming apparatus having a plurality of developing devices, each developing device forming a visible image (toner image) of a different color, and finally transferring the toner images by superimposing them on the same sheet. Have been proposed. Generally, a color image forming apparatus is roughly classified into a single drum type and a multi-stage drum type (tandem type).

FIG. 1 is a diagram schematically showing the construction of a main part of a single-drum type color image forming apparatus which carries out a multi-rotation process. In the color image forming apparatus shown in the figure, an initialization charging device 102, a recording head 103 for irradiating a laser beam, and four developing devices 104 a along the photoconductor 101 are provided along the peripheral surface of a single photoconductor 101.・ 104b ・ 104c ・ 104
A sheet winding semi-conductive drum 105 that forms a transfer portion is arranged facing the photoconductor 101.

Inside the cylinder of the paper winding semi-conductive drum 105, a corona discharger 106 that discharges a transfer current is disposed in the vicinity of the transfer portion. The sheet winding semi-conductive drum 105 winds a sheet, which is carried in from the lower left direction, in a counterclockwise direction as shown by an arrow A in FIG.

The photoconductor 101 has the same peripheral surface speed as the peripheral speed of the semi-conductive drum 105 around which the paper is wound, and the arrow C in FIG.
While rotating in the clockwise direction as indicated by, the toner image formed by the M (magenta: red dye) developing device 104a is first carried, and this toner image is carried by the corona discharger 106.
Transfer to paper by the discharge current of.

Next, the sheet winding semi-conductive drum 1 is again used.
05 rotates once, and in response to this, the photoconductor 101 carries the toner image formed by the C (cyan: blue with greenish tint) developing device 104b, and the corona discharger 106 transfers it onto the paper. Pile up. Furthermore, the paper winding semi-conductive drum 105 rotates once again, and in response to this, the photoconductor 101 carries the toner image formed by the Y (yellow: yellow) developing device 104c, and the corona discharger 106 carries it. Transfer them onto paper and stack them.

Finally, the paper winding semiconductive drum 1 is again wound.
05 rotates once, and in response to this, the photoconductor 101 carries the toner image formed by the Bk (black: black) developing device 104d, and the corona discharger transfers and superimposes it on the paper.

Then, when the transfer (overcoating) of the four color toner images is completed, the winding of the paper is released, and the paper is conveyed by the conveyor belt 107 to the fixing unit 108 arranged on the left side. At the same time, the transferred toner images of four colors that have been overlaid are heat-fixed on the paper surface.

As described above, the single-drum type color image forming apparatus has three primary colors of subtractive color mixture of M (magenta) toner, C (cyan) toner, and Y (for one sheet (one page) of paper. In order to transfer a total of four types of toner, that is, a yellow) toner and a Bk (black) toner dedicated to the printing of the black portion, in order to perform the transfer, it is necessary to perform printing (exposure recording, development, and transfer) individually for each toner. I have to. Therefore, the printing process is repeated four times for one page of paper, and the printing process takes a long time.

On the other hand, since the tandem type color image forming apparatus transfers four kinds of toner onto the paper in order in one step, the processing speed is almost four times that of the single drum type. is doing. For this reason, in recent years, the tandem type color image forming apparatus has become the mainstream in combination with the fact that the internal apparatus has been downsized and has been assembled (unitized) to be relatively inexpensive.

FIG. 2 is a cross-sectional view of main parts showing a schematic configuration of a color image forming apparatus to which the tandem system is applied. In the figure, for example, a Bk image forming station 200B
At k, the drum-shaped photoconductor 201Bk rotates in the direction of arrow D in the figure, and around it, at least in the order of rotation, the charger 202Bk, the writing optical system 203Bk, and the developing device 20.
4Bk and a cleaning device 205Bk are arranged.

In this color image forming apparatus, the charger 20
A laser beam is irradiated from the writing optical system 203Bk to the surface of the photoconductor between 2Bk and the developing device 204Bk, and an electrostatic latent image of Bk is formed on the photoconductor 201Bk.

Image forming stations 210C, 220M and 230Y, which are equivalent to the image forming station 200Bk centering around the photoconductor 201Bk, are arranged side by side along a sheet conveying belt 250 which is a sheet conveying means.

The paper conveying belt 250 is in contact with the photoconductor between the developing device and the cleaning device of each image forming station, and a transfer bias is applied to the surface (back surface) of the paper conveying belt 250 which is the back side of each photoconductor side. Transfer roller 206Bk, 216C, 226M, 236 for applying
Y is arranged. The image forming stations have the same configuration except for the toner color inside the developing device.

In the color image forming apparatus having the structure shown in FIG. 2, the image forming operation is performed as follows. First, in each image forming station, the photoconductor is charged by the charger, and then the writing optical system forms an electrostatic latent image corresponding to an image of each color to be created by laser light.

Next, a developing device forms an electrostatic latent image to form a toner image. The developing devices are Bk and
In the developing device that develops with C, M, and Y toners, the toner images of the respective colors formed on the four photoconductors are superposed on the paper.

The paper is fed from the paper feeding unit 240 onto the paper carrying belt 250 in the direction of arrow E in the figure at the same timing as the image formation on the photoconductor.

The sheet held on the sheet conveying belt 250 is conveyed and the toner image of each color is transferred at the contact position (transfer section) with each photoconductor. The toner image on the photoconductor is transferred onto the sheet by an electric field formed by the potential difference between the transfer bias applied to the transfer roller and the photoconductor.

The recording paper on which the four color toner images are superposed through the four transfer portions is conveyed to the fixing device 260, the toner is fixed, and the recording paper is discharged to a paper discharging portion (not shown). Further, the residual toner remaining on each photoconductor without being transferred by the transfer unit is collected by the cleaning device.

In the example shown in FIG. 2, the image forming unit Bk,
Although the colors are arranged in the order of C, M, and Y, the color order is not limited to this order, and the color order can be set arbitrarily.

As described above, the image forming stations have the same configuration except that the toner color inside the developing device is different. Therefore, the rotation speed of the developing device of each image forming station is also set to the same rotation speed.

However, in the tandem system, even when a single color image is formed, the paper comes into contact with the photosensitive drums which are the other three image bearing members, so that the toner of unnecessary color remaining on the photosensitive drums. Has adhered to the transfer material and deteriorates the image quality. Further, there is a problem that the life of the photoconductor and the paper transport belt is shortened due to unnecessary contact of the paper transport belt.

Therefore, for the purpose of maximizing the wear of the paper conveying belt and the photoconductor due to the contact with the photoconductor, and preventing the unnecessary mixing of the toner color at the time of forming a monochromatic image, Japanese Patent Laid-Open Publication No. Hei 6 (1994) -31868. -258914 and Japanese Unexamined Patent Publication No.
Japanese Patent Laid-Open No. 0-198121 discloses an image forming apparatus capable of tilting the angle of a belt loop downwardly of the belt loop and changing the shape of the belt loop so that the photoconductor and the paper transport belt are brought into contact with each other. Is disclosed.

That is, in these image forming apparatuses, when a monochromatic image is formed in which only the image forming station at the most downstream position is involved, the belt loop on the upstream side is tilted downward so that the photoconductors of the other image forming stations are not moved. It is configured to be separated from the paper transport belt.

However, with such a structure, there is a problem that a high image quality cannot be obtained for a color image which requires a delicate balance due to the density and properties of each toner. For example, in black, which is most often used as a single color,
Since the effect of masking other colors is high, when an image is formed in the image forming station on the most downstream side, each color image formed up to that point is masked, resulting in a darkened image. Therefore, the image processing station for forming a monochromatic image (particularly a monochrome monochromatic image) needs to be arranged on the most upstream side. Therefore, in the above-described related art, only the most upstream image forming station is involved, instead of the configuration in which only the most downstream image forming station is involved and the upstream side belt loop is tilted downward and separated during image formation. At the time of image formation, it is desirable that the downstream belt loops be similarly separated.

On the other hand, in the tandem system, during color image formation, a reverse transfer phenomenon occurs in which unfixed toner once transferred onto the sheet at the upstream image forming station is taken away at the immediately downstream station. However, there is a problem that a good color image cannot be obtained.

The reverse transfer phenomenon will be described below. In the tandem type color image forming apparatus, the toner image of the first color formed in the developing process of the most upstream image forming station is transferred to the sheet, but the second color toner image formed in the developing process of the subsequent image forming station is transferred. ~ When the toner image of the fourth color is transferred onto the sheet in a superimposed manner, the first image transferred first is transferred.
A phenomenon occurs in which a part of the color toner image is reversely transferred to the photoconductor side of the second to fourth color image forming stations.

Therefore, after the transfer of the toner image of the fourth color is completed at the most downstream image forming station, the toner adhesion amount of the toner image of the first color on the paper is smaller than the initial adhesion amount. It will be reduced to 10%. Then, the same phenomenon occurs in the second color toner image and the third color toner image.

Therefore, for example, if the steps of forming and transferring a toner image are performed four times, the final amount of each toner image deposited on the paper is the smallest for the toner image of the first color, "Amount of first color"<"Amount of second color"
The color balance is unbalanced, that is, “adhesion amount of third color” <adhesion amount of fourth color. For this reason, there is a problem that the color density becomes higher as the color becomes more downstream and the color image becomes more distant from the correct hue.

The reason why this reverse transfer phenomenon occurs is as follows. In the image forming process, when the toner adheres to the photoconductor from the developing roller in accordance with the image potential of the photoconductor (drum or belt), the toner does not necessarily have a uniform negative charge, but has a negative polarity. Weak ones and, conversely, those having a positive polarity charge are mixed.

In some cases, discharge is generated in the step of peeling from the photoconductor after transfer, and the polarity becomes positive afterwards. It is said that the reverse transfer phenomenon occurs when these return to the photoconductor little by little each time they receive a transfer charge of positive polarity in the subsequent transfer process.

Therefore, in the tandem system color image forming apparatus, when forming a color image, image forming conditions are set such that a large amount of toner is attached to the most upstream side image forming station in anticipation of the reverse transfer phenomenon. There is.

[0033]

However, in the color image forming apparatus having the structure in which the upstream belt loops are separated from each other as in the above-mentioned conventional example, when this condition is applied as it is even when forming a single color image, Since the reverse transfer phenomenon is avoided by separating the belt loops, a large amount of toner that has been previously attached remains attached to the paper, resulting in a problem that the image density becomes high.

In other words, this is a problem that occurs when the toner supply amount setting conditions set for the color image formation are applied as they are without any change during the monochromatic image formation. In other words, when forming a color image, toner (developer) from the most upstream image forming station is supplied in a large amount in anticipation of the reverse transfer phenomenon. This is because the reverse transfer phenomenon is sometimes avoided because the image forming stations other than the most downstream are evacuated, resulting in too much toner.

In normal use, even in a color image forming apparatus, the frequency of using monochrome image formation by only the Bk color image forming station is overwhelmingly higher than the case of performing color image formation. Since an image density higher than necessary becomes more conspicuous as a defect in image quality, a countermeasure has been demanded.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a tandem type multicolor image forming apparatus and a control method therefor capable of eliminating the image density difference that occurs when a single color image is formed. To do.

[0037]

The present invention has means for solving the above-mentioned problems as follows.

(1) A plurality of sets of image forming stations centering on the photoconductor are arranged along the sheet conveying direction, and are formed at each image forming station while the sheet is conveyed by the sheet conveying belt in the sheet conveying direction. Color images can be formed by sequentially superposing the colored images to form a single image. When forming a single-color image, the image is formed by partially using the image forming station on the upstream side in the paper conveyance direction. At the time of color image formation using all the image forming stations in the multi-color image forming apparatus in which the sheet conveying belt can be switched to the retracted state from the image forming station on the downstream side that is not involved in the image forming (can be separated from and contacted). When,
It is characterized in that the initialization charging potential Vo of the photoconductor of the most upstream image forming station is switched between the time of monochromatic image formation using only the most upstream image forming station.

In a tandem type multicolor image forming apparatus capable of forming a color image, a little more toner is supplied from the most upstream image forming station in anticipation of the reverse transfer phenomenon. However, since the amount of toner supplied is the same as that during color image formation during the formation of a monochromatic image, the reverse transfer phenomenon occurs due to the separation of the paper transport belt from the downstream image forming station during the monochromatic image formation. Although avoided, it is too much as a toner for forming a monochromatic image.

In this structure, the image exposure amount and the developing bias are commonly set in both image forming modes, and only the initialization charging potential Vo for the photoconductor of the most upstream image forming station is switched according to the image forming mode. As a result, the optimum amount of toner attached to the photoconductor is adjusted.

That is, with the above configuration, the initialization charging potential Vo for the photoconductor of the most upstream image forming station is set.
, The photoconductor surface potential (bright portion potential) | VL | fluctuates accordingly, but the developing bias | VD | does not fluctuate. Therefore, the contrast potential | VC | (the potential difference between the developing bias VD and the exposing portion potential VL). , | VC | = | VD
Since −VL |) also changes, it is possible to adjust the amount of toner attached to the photoconductor.

Thus, the image density obtained at the image forming station located at the uppermost stream of the sheet conveying belt at the time of forming a color image and the image density obtained at the time of forming a single color image involving only the image forming station located at the uppermost stream of the sheet conveying belt. It is possible to eliminate the density difference that occurs between the generated image density and the image quality of a monochrome image.

Further, in such a tandem type multicolor image forming apparatus, when forming a single color image in which only the most upstream image forming station is involved, the sheet is conveyed from the downstream image forming station not involved in image formation. By separating the belts, it is possible to avoid the wear of the paper transport belt and the photoconductor as much as possible.

(2) During the color image formation and the monochromatic image formation, the switching of the initialization charging potential to the photoconductor of the most upstream image forming station along the sheet conveying belt is performed by the most upstream image forming station. This is performed by changing the facing distance between the screen grid of the charger and the discharge electrode.

If the distance between the screen grid (hereinafter referred to as a grid) of the charger and the discharge electrode is narrowed, the discharge electrode is easily discharged, so that the amount of negative charge attached to the photoconductor is increased, and | Vo | To increase. Along with this, | VL | also increases. Also, image exposure conditions and developing bias | VB |
Is fixed, | VC | decreases, and the amount of toner adhered to the photoconductor also decreases.

On the other hand, if the above-mentioned interval is widened, it becomes difficult for the discharge electrode to discharge, so that the amount of negative charges adhering to the photosensitive member decreases and | Vo | also decreases. Along with this, | VL | also decreases. Further, since the image exposure condition and the developing bias | VB | are fixed, | VC | increases, and the amount of toner adhered to the photoconductor also increases.

In this structure, by adjusting the facing distance between the grid and the discharge electrode in both image modes,
Since the amount of toner attached to the photoconductor can be adjusted,
An appropriate image density can be secured.

(3) The facing distance between the screen grid and the discharge electrode is changed by providing a moving means for moving the discharge electrode so that the discharge electrode can advance and retreat toward the photoreceptor.

The grid electrode may be moved in order to change the facing distance between the grid electrode and the discharge electrode. However, even in a color image forming apparatus, there are overwhelmingly more opportunities to perform black monochromatic printing than to perform color printing. With frequent black monochromatic printing, the initialization charging potential Vo of the photoconductor is reduced. Moving the grid electrode toward the photoreceptor to increase the gap between them reduces the interference with the photoreceptor.
This is not a good idea considering the increased risk of damage.

Therefore, by fixing the grid electrode and moving the discharge electrode, the initialization charging potential V of the photosensitive member is set.
By changing o, the charging potential can be switched more safely while avoiding the occurrence of interference or damage to the photoconductor.

It is also possible to change the voltage applied to the discharge electrode, but in the same manner as described above, the discharge is performed in order to increase the initialization charging potential Vo of the photoconductor with the overwhelmingly high frequency of black monochromatic printing. As the voltage applied to the electrodes increases, so does the discharge current.

As a result, the amount of ozone generated from the discharged portion increases, the characteristics of the photosensitive member deteriorate, and the image quality deteriorates. When the amount of ozone generated increases, the discharge products adhere to the surfaces of the discharge electrode and the grid electrode, leading to a decrease in the charging performance of the charger.

Therefore, it is less burdensome on the high-voltage power supply to change the initialization charging potential Vo of the photosensitive member by fixing the voltage applied to the discharge electrode and moving the discharge electrode as in this configuration. It is possible to suppress the performance deterioration of the photoconductor and the charger.

(4) The image exposure amount and the developing bias are the same in the color image formation and the monochromatic image formation, and the grid and the discharge in the image forming station arranged at the most upstream side of the paper transport belt are formed in the color image formation. It is characterized in that the facing interval with the electrode is set to be wider than that at the time of monochromatic image formation.

At the time of color image formation, the amount of toner adhering to the most upstream image forming station decreases due to the reverse transfer phenomenon.
It is necessary to supply a large amount of toner from the most upstream image forming station.

In this structure, when forming a color image, the amount of toner adhering is increased by expanding the above-mentioned interval as compared with the case of forming a single color image, and the image density difference generated by the toner color of the most upstream image forming station is reduced. Can be reduced.

(5) During the formation of a color image and the formation of a single color image, the switching of the initialization charging potential to the photoconductor of the most upstream image forming station along the sheet conveying belt is performed by the most upstream image forming station. It is characterized in that the screen grid of the charger is expanded and contracted in the longitudinal direction.

In this structure, the image exposure amount and the developing bias voltage are set to be common for the color image formation and the monochromatic image formation, and only the initialization charging potential Vo for the photoconductor of the most upstream image forming station is set. The optimum toner adhesion amount can be adjusted by expanding and contracting the screen grid of the charger of the image forming station in the longitudinal direction to change the screen grid according to the image forming mode.

That is, in order to change the amount of toner adhering to the photoconductor during the formation of a color image in which the reverse transfer phenomenon should be taken into consideration and during the formation of a single color image which need not be taken into consideration, only the initialization charging potential Vo is changed. However, even if the image exposure amount and the developing bias voltage are set in common in both modes, the contrast potential Vc (potential difference between the developing bias voltage VB and the exposure portion potential VL) is different. The amount of adhesion can be adjusted.

Therefore, the image density obtained at the image forming station located at the uppermost stream of the sheet conveying belt at the time of forming a color image and the image density obtained at the time of forming a single color image involving only the image forming station located at the uppermost stream of the sheet conveying belt. The density difference that occurs with the image density is eliminated,
It is possible to improve the image quality of a monochrome image.

(6) The initialization charging potential is switched by changing the expansion / contraction state of the screen grid of the charger.

When the aperture ratio of the screen grid of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio of the screen grid decreases,
The initialization charging potential Vo of the photoconductor is on the non-white background side.

In this configuration, by fixing the image exposure condition and the developing bias voltage and contracting the screen grid, the aperture ratio is lowered, the contrast potential VC is increased, and the amount of toner adhered to the photoconductor is large. Therefore, the image density can be secured.

(7) The present invention is characterized by comprising control means and drive means for adjusting the expansion / contraction state of the screen grid of the charger of the most upstream image forming station along the paper transport belt.

When the screen grid of the charger is stretched at both ends, the initialization charging potential Vo of the photoconductor is on the white background side, and as it contracts, the initialization charging potential Vo of the photoconductor is reduced.
Is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the screen grid is contracted, and the amount of toner adhered to the photosensitive member also becomes larger.

In this structure, by providing the control means and the drive means for appropriately adjusting the expansion / contraction state of the screen grid, the toner adhesion amount to the photoconductor can be appropriately adjusted, and a desired image can be obtained. The concentration can be obtained.

(8) The present invention is characterized in that the screen grid of the charger of the image forming station arranged at the uppermost stream of the sheet conveying belt is stretched during the formation of a monochromatic image, and the screen grid is contracted during the formation of a color image.

When the screen grid of the charger is stretched at both ends, the initialization charging potential Vo of the photoconductor is on the white background side, and as it contracts, the initialization charging potential Vo of the photoconductor is reduced.
Is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the screen grid contracts, the amount of toner adhered to the photosensitive member increases, and an appropriate image density can be secured.

(9) At the time of color image formation and monochromatic image formation, the switching of the initialization charging potential to the photoconductor of the most upstream image forming station along the paper transport belt is performed by the most upstream image forming station. This is performed by changing the aperture ratio of the screen grid of the charger.

When the aperture ratio of the screen grid of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio of the screen grid decreases,
The initialization charging potential Vo of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the aperture ratio of the screen grid is made lower, and the amount of toner adhered to the photoconductor becomes larger, so that an appropriate image density can be secured.

(10) A control means and a driving means for adjusting the aperture ratio of the screen grid of the charger of the most upstream image forming station along the sheet conveying belt to be increased or decreased are provided.

When the aperture ratio of the screen grid of the charger is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio decreases, the initialization charging potential Vo of the photoconductor is on the non-white background side. .

When the image exposure conditions and the developing bias voltage are fixed, the contrast potential Vc becomes larger when the aperture ratio of the screen grid is smaller, the amount of toner adhering to the photosensitive member also increases, and an appropriate image density can be secured. .

In this structure, since the control means and the driving means for appropriately adjusting the aperture ratio of the screen grid are provided, the toner adhesion amount to the photoconductor can be appropriately adjusted, and a desired image can be obtained. The concentration can be obtained.

(11) When a monochromatic image is formed, the aperture ratio of the screen grid of the charger of the image forming station disposed on the most upstream side of the paper transport belt is set higher than the aperture ratio during color image formation. To do.

When the aperture ratio of the screen grid of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio is low, the initialization charging potential Vo of the photoconductor is on the non-white background side. .

When the image exposure condition and the developing bias voltage are fixed, the contrast potential Vc increases when the aperture ratio of the screen grid is low, so that the amount of toner adhering to the photoconductor also increases and an appropriate image density can be secured.

Therefore, it is possible to reduce the difference in image density caused by the toner color of the most upstream image forming station between the color image formation and the monochromatic image formation.

(12) During the formation of a color image and the formation of a single color image, the initialization charging potential is switched to the photoconductor of the most upstream image forming station along the paper transport belt.
It is characterized in that the tension is applied in the longitudinal direction to both ends of the screen grid of the charger of the most upstream image forming station.

When the tension in the longitudinal direction applied to both ends of the screen grid of the charger is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the tension applied to both ends is decreased, The initialization charging potential Vo is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the tension applied to both ends of the screen grid is smaller, the amount of toner adhered to the photosensitive member is larger, and the proper image density is obtained. Can be secured.

(13) A control means and a driving means for adjusting the tension applied to both ends of the screen grid of the charger of the most upstream image forming station along the paper transport belt along the longitudinal direction to be increased or decreased are provided. To do.

When the tension applied to both ends of the screen grid of the charger along the longitudinal direction is large (when the aperture ratio of the screen grid is large), the initialization charging potential Vo of the photoconductor is on the white background side, and the both ends of the screen grid are As the applied tension (aperture ratio of the screen grid) decreases, the initialization charging potential Vo of the photoconductor becomes the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the aperture ratio of the screen grid is smaller, the amount of toner adhered to the photoconductor becomes larger, and an appropriate image density can be secured. .

In this structure, the amount of toner adhering to the photosensitive member is properly adjusted by providing the control means and the driving means for adjusting the tension applied to both ends of the screen grid along the longitudinal direction so that the tension can be increased or decreased. It is possible to obtain a desired image density.

(14) The tension in the longitudinal direction applied to both ends of the screen grid of the charger of the image forming station arranged on the most upstream side of the sheet conveying belt during the monochromatic image formation is higher than the tension applied during color image formation. It is characterized by setting.

When the tension in the longitudinal direction applied to both ends of the screen grid of the charger is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the applied tension becomes lower, the initialization charge of the photoconductor is reduced. The potential is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC increases when the tension applied to both ends of the screen grid is low, so that the amount of toner adhered to the photosensitive member increases and the image density can be secured. .

Therefore, it is possible to reduce the difference in image density caused by the toner color of the most upstream image forming station between the color image formation and the monochromatic image formation.

(15) Initialization charge potential Vo of the photoconductor
In order to switch among the image forming stations, a mesh grid electrode having a multiple structure is provided on the main surface of the charger of the image forming station arranged at the uppermost stream, and all image forming stations are used for color image formation and at the uppermost stream. And a means for making at least one of the grid electrodes movable relative to the other during monochromatic image formation using only the image forming station.

In this structure, at least one of the mesh-shaped grid electrodes of the multiple structure of the charger of the most upstream image forming station is moved relative to the other, thereby forming an image of the initialization charging potential Vo for the photoconductor. By changing according to the mode, it is possible to optimally adjust the toner adhesion amount.

That is, in order to change the amount of toner adhering to the photoconductor during the formation of a color image in which the reverse transfer phenomenon should be taken into consideration and during the formation of a single color image which need not be taken into consideration, the initialization charging potential Vo is set. If changed, the amount of toner attached to the photoconductor can be adjusted.

Therefore, for the toner color of the most upstream image forming station, the density difference generated between the image density obtained during color image formation and the image density obtained during monochromatic image formation is eliminated.

(16) Initialization charge potential Vo of the photoconductor
In order to switch between the two, the charger of the most upstream image forming station is provided with a plurality of grid electrode plates having a plurality of pores, and all image forming stations are used for color image formation and most upstream. At the time of single-color image formation using only the image forming station located at, it is possible to move at least one of the grid electrode plates of the charger while changing the overlapping area of the pores between the grid electrode plates. Characterize.

In this structure, the image exposure amount and the developing bias voltage are set to be common for the color image formation and the monochromatic image formation, and at least one of the grid electrode plates of the charger of the most upstream image forming station is set. By moving the sheet between the grid electrode plates while changing the overlapping area of the pores, only the initialization charging potential Vo with respect to the photoconductor is changed according to the image forming mode. Adjustments can be made.

That is, in order to change the amount of toner adhering to the photoconductor during the formation of a color image in which the reverse transfer phenomenon should be taken into consideration and during the formation of a single color image which does not need to be taken into consideration, only the initialization charging potential Vo is changed. If the image exposure amount and the developing bias voltage are common in both modes, the contrast potential VC (the potential difference between the developing bias voltage VB and the exposure portion potential VL) is different, so that the toner on the photoconductor is changed. The amount of adhesion can be adjusted.

Therefore, the image density obtained at the image forming station located at the uppermost stream of the sheet conveying belt at the time of color image formation, and the image density obtained at the time of monochromatic image formation involving only the image forming station located at the uppermost stream of the sheet conveying belt. The density difference that occurs with the image density is eliminated,
It is possible to improve the image quality of a monochrome image.

(17) At the time of color image formation and single-color image formation, the initialization charging potential Vo is switched to the photoconductor of the most upstream image forming station along the paper transport belt by switching the most upstream image forming station. This is performed by changing the aperture ratio between the grid electrode plates of the charger.

When the aperture ratio between the grid electrode plates of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio between the grid electrode plates decreases,
The initialization charging potential of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the facing distance between the shield electrodes is smaller, and the amount of toner adhered to the photoconductor is larger, so that the image density can be secured.

(18) It is characterized by comprising grid electrode plate position adjusting means for adjusting the aperture ratio of the grid electrode plate of the charger of the most upstream image forming station along the paper transport belt.

When the aperture ratio between the grid electrode plates of the charger is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio decreases, the initialization charging potential Vo of the photoconductor is on the non-white background side. Becomes

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the aperture ratio between the grid and the electric gutter plates is smaller, and the amount of toner adhered to the photosensitive member is larger, thus ensuring the image density. it can.

In this structure, by providing the grid electrode plate position adjusting means for adjusting the aperture ratio of the grid electrode plate so that it can be increased or decreased, it is possible to properly adjust the toner adhesion amount to the photosensitive member, and it is possible to obtain a desired value. The image density can be obtained.

(19) A control means and a drive means for adjusting the aperture ratio between the grid electrode plates of the charger of the most upstream image forming station along the paper transport belt to be adjustable.

When the aperture ratio between the grid electrode plates of the charger is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio decreases, the initialization charging potential Vo of the photoconductor is on the non-white background side. Becomes

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the aperture ratio between the grid electrode plates is smaller, and the amount of toner adhering to the photoconductor becomes larger, so that the image density can be secured. .

In this structure, by providing the control means and the driving means for adjusting the aperture ratio of the grid electrode plate so that it can be increased or decreased, it is possible to properly adjust the amount of toner adhered to the photosensitive member and to obtain a desired image. The concentration can be obtained.

(20) At the time of monochromatic image formation, it is necessary to set the aperture ratio between the grid electrode plates of the charger of the image forming station arranged at the most upstream side of the paper transport belt to be higher than the aperture ratio at the time of color image formation. Characterize.

When the aperture ratio between the grid electrode plates of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side,
As the aperture ratio decreases, the initialization charging potential V of the photoconductor
o is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes large when the aperture ratio between the grid electrode plates is low, so that the amount of toner adhering to the photoconductor also increases and the image density can be secured.

Therefore, it is possible to reduce the difference in image density caused by the toner color of the most upstream image forming station between the color image formation and the monochromatic image formation.

(21) During color image formation and single-color image formation, the initialization charging potential is switched to the photoconductor of the most upstream image forming station along the paper transport belt.
This is performed by changing the facing interval of the shield electrodes of the charger of the most upstream image forming station.

In this structure, the initial charging potential Vo of the photoconductor is adjusted by changing the facing interval of the shield electrodes of the chargers of the image forming stations arranged in the most upstream, so that the time of the color image formation can be improved. It is possible to control the optimum toner adhesion amount, which varies depending on when a single-color image is formed.

That is, at the time of color image formation, in consideration of a reverse transfer phenomenon in which unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station is taken away at the downstream station. , It is necessary to increase the amount of toner.

In the multicolor image forming apparatus of the present invention, the image exposure amount and the developing bias voltage are set in common, and the initialization charging potential Vo for the photoconductor of the most upstream image forming station is set.
By changing only the image formation mode,
At the time of color image formation that should consider the reverse transfer phenomenon of toner,
It is possible to change the amount of toner adhered to the photoconductor at the time of forming a single-color image that does not need to be taken into consideration (specifically, the amount of toner adhered at the time of forming a single-color image is smaller than that at the time of forming a color image).

That is, when the initialization charging potential Vo is changed, the contrast potential VC (the potential difference between the developing bias voltage VB and the exposure portion potential VL is changed even if the image exposure amount and the developing bias voltage are unchanged in both modes). ) Are different, it is possible to adjust the amount of toner adhered to the photoconductor.

Therefore, at the time of color image formation, the image density obtained from the image forming station located on the most upstream side,
It is possible to improve the image quality of a single-color image by eliminating the density difference generated between the image densities obtained from the image forming stations located on the same uppermost stream side when forming a single-color image.

(22) At the time of monochromatic image formation, only the image forming station located on the most upstream side of the paper carrying belt is used, and the paper carrying belt is retracted to the other unused image forming stations located on the downstream side. It is characterized in that the image formation is carried out.

In this configuration, when a monochrome image is formed, the unused image forming station on the downstream side is separated from the paper transport belt, so that it is not necessary to consider the reverse transfer phenomenon, and suitable image formation can be performed, and The wear of the conveyor belt can be reduced.

(23) The initialization charging potential is set to a potential on the non-white background side when forming a color image as compared to when forming a monochromatic image.

In this structure, the contrast potential V
Since C (the potential difference between the developing bias voltage VB and the exposed portion potential VL) is larger during color image formation than during monochromatic image formation, the toner adhesion amount to the photoconductor during monochromatic image formation is determined by the color image. It can be reduced compared to the time of formation.

That is, by setting the initialization charging potential of the photoconductor at the time of color image formation to a potential on the non-white background side from the initialization charging potential of the photoconductor at the time of monochromatic image formation, the images in both image forming modes are formed. Even if the exposure conditions are common settings,
The surface potential (bright portion potential VL) of the photoconductor after image exposure is a potential on the non-white background side when forming a color image as compared to when forming a monochrome image.

Since the developing bias voltage VB is set in common in both image forming modes, the contrast potential VC (potential difference between the developing bias voltage VB and the exposure portion potential VL) is smaller in color image formation. It is larger than when a monochromatic image is formed.

Therefore, the amount of toner adhering to the photosensitive member and thus the image density can be adjusted, and the image at the time of color image formation and the image at the time of monochromatic image formation involving only the image forming station located at the uppermost stream of the sheet conveying belt. The density difference can be eliminated.

(24) The image forming station located on the most upstream side of the paper transport belt is provided with a moving means for adjusting the facing distance between the shield electrodes of the charger.

In this structure, when the facing distance between the shield electrodes of the charger is widened by the moving means, the initialization charging potential Vo of the photoconductor is on the white background side. As the facing distance is narrowed, the initialization charging potential of the photoconductor is reduced. Vo moves to the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the facing distance between the shield electrodes is smaller. Therefore, the amount of toner adhering to the photoconductor can be adjusted by appropriately adjusting the facing distance between the shield electrodes during color image formation and during monochrome image formation, and as a result, the image density during both image formation can be made uniform. Can be kept at

(25) When a single-color image is formed, the facing distance between the shield electrodes of the chargers of the image forming stations located on the most upstream side of the paper transport belt is set wider than that during color image formation.

When the opposing distance between the shield electrodes of the charger is widened, the initialization charging potential Vo of the photoconductor moves to the white background side,
As the facing interval becomes narrower, the initialization charge potential V of the photoconductor is increased.
o moves to the non-white background side.

When the image exposure conditions and the developing bias voltage are fixed, the contrast potential VC increases as the facing distance between the shield electrodes decreases, and the amount of toner adhered to the photoconductor also increases.

In this structure, by appropriately adjusting the facing distance of the shield electrodes between the color image formation and the monochromatic image formation, the image density difference of the toner color of the image forming station located on the most upstream side can be reduced. It is possible to reduce the amount and form a uniform image.

(26) The image forming station located on the most upstream side is an image forming station provided with black toner.

In this structure, since the black image forming station, which has a great influence on other colors, is located on the most upstream side, black toner becomes the lowermost layer during color image formation, and influences on other colors. Can be minimized.

Furthermore, the black toner image forming station eliminates the density difference between the image density obtained when forming a color image and the image density obtained when forming a black single color image, and when the color image is formed, An image with high image quality can be obtained both during the formation of a black monochrome image.

Further, even in the case of a multicolor image forming apparatus, in actual use, there are much more opportunities to form a black single color image than to form a color image. At the time of this monochromatic image formation, the black toner arranged at the most upstream position has a smaller amount of toner adhesion to the photoconductor than at the time of color image formation, so an adequate amount of toner is sufficient to obtain image density. Supplied. Therefore, the black toner, which is located on the most upstream side and consumes the most amount of black toner, is not wasted, so that the black toner consumption amount can be suppressed as compared with the conventional multicolor image forming apparatus.

(27) A plurality of image forming stations equipped with a charging device, a writing optical system, a developing device and a cleaning device are provided around the photoconductor, and a paper carrying belt for carrying a transfer material on which an image is formed. Therefore, when forming a single-color image, only the image forming station located on the most upstream side of the paper transport belt is used, and the image is formed by retracting the paper transport belt to the other unused image forming stations located on the downstream side. In a method of controlling a multicolor image forming apparatus, a facing interval of shield electrodes of a charger provided in the most upstream image forming station is changed between the time of color image formation and the time of single color image formation, whereby It is characterized in that the initialization charging potential is adjusted.

In this structure, by optimizing the initialization charging potential Vo of the photosensitive member by widening the facing interval of the shield electrodes of the charger, the optimum toner which changes between the color image formation and the monochromatic image formation. The amount of adhesion can be controlled.

That is, at the time of color image formation, in consideration of the reverse transfer phenomenon in which the unfixed toner once transferred onto the sheet (transfer material) at the most upstream image forming station is taken away at the downstream station. , It is necessary to increase the amount of toner. On the other hand, at the time of monochromatic image formation, the unused image forming station on the downstream side is separated from the paper transport belt, so it is not necessary to consider the above-mentioned reverse transfer phenomenon. If the amount is the same, it is difficult to perform optimal image formation.

Therefore, the image exposure amount and the developing bias voltage are set in common, and only the initialization charging potential Vo for the photoconductor of the most upstream image forming station is changed according to the image forming mode, whereby the toner reverse transfer is performed. The amount of toner adhering to the photoconductor can be changed between the color image formation in which the phenomenon should be taken into consideration and the monochromatic image formation which does not need to be taken into consideration (specifically, the color image formation is performed during the monochromatic image formation. The amount of toner adhered is reduced compared to the time).

That is, when the initialization charging potential Vo is changed, the contrast potential VC (developing bias voltage VB and the exposure portion potential VL is different even if the image exposure amount and the developing bias voltage are unchanged in both modes). ) Are different, it is possible to adjust the amount of toner adhered to the photoconductor.

Therefore, the image density obtained from the image forming station located on the most upstream side during color image formation,
It is possible to improve the image quality of a single-color image by eliminating the density difference generated between the image densities obtained from the image forming stations located on the same uppermost stream side when forming a single-color image.

(28) The initialization charging potential is set to a potential on the non-white background side when forming a color image as compared to when forming a single color image.

In this structure, the contrast potential V
Since C (the potential difference between the developing bias voltage VB and the exposed portion potential VL) is larger during color image formation than during monochromatic image formation, the toner adhesion amount to the photoconductor during monochromatic image formation is determined by the color image. It can be reduced compared to the time of formation.

That is, by setting the initialization charging potential Vo of the photoconductor during color image formation to a potential on the non-white background side relative to the initialization charging potential of the photoconductor during monocolor image formation, Even if the image exposure conditions are common, the photoconductor surface potential (bright portion potential VL) after image exposure is
The potential at the time of forming a color image becomes higher than that at the time of forming a single-color image. Since the developing bias voltage VB is set in common in both image forming modes, the contrast potential VC (the potential difference between the developing bias voltage VB and the exposure portion potential VL) is more monochromatic during image formation. It will be bigger than time.

Therefore, the amount of toner adhering to the photosensitive member and thus the image density can be adjusted, and the image at the time of forming a color image and the image at the time of forming a single color image in which only the image forming station located at the most upstream side of the paper transport belt is involved The density difference can be eliminated.

(29) When a monochromatic image is formed, the facing distance between the shield electrodes of the chargers of the image forming stations located on the most upstream side of the sheet conveying belt is set wider than that when forming a color image.

When the opposing distance between the shield electrodes of the charger is widened, the initialization charging potential Vo of the photoconductor moves to the white background side,
As the facing interval becomes narrower, the initialization charge potential V of the photoconductor is increased.
o moves to the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC increases as the facing distance between the shield electrodes decreases, and the amount of toner adhering to the photoconductor also increases.

In this structure, since the facing distance between the shield electrodes is appropriately adjusted between the color image formation and the monochromatic image formation, the difference in the image density of the toner color of the image forming station located on the most upstream side is reduced. Therefore, uniform image formation can be performed.

[0154]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) This embodiment corresponds to claims 1 to 4. The multicolor image forming apparatus shown in this embodiment and each of the following embodiments is, for example, as shown in FIGS.
As shown in FIG. 2, a tandem type multicolor image forming apparatus having a configuration in which a belt loop on the downstream side is inclined downward and separated when forming a single color image in which only the most upstream image forming station (B) is involved. is there. The description of the same components of the same device as those described in the section of the related art will be omitted, and only the differences will be described below.

As shown in FIG. 9 (a), the Bk image forming station (B) is located at the most upstream position along the running direction of the sheet conveying belt 7 due to the demand for improving the color reproducibility during color printing. The image forming stations (C), (M), and (Y) for the colors C, M, and Y are arranged in descending order of hiding power with respect to other toner colors.

Further, when performing black monochromatic printing in which only the Bk image forming station (B) at the most upstream position is involved, the photoreceptors 6B ... By contact with the image forming stations not involved in the image forming operation, and the paper. Unnecessary wear of the conveyor belt 7
In order to avoid the abrasion, as shown in FIG. 9B, the downstream side of the paper transport belt 7 is retracted while being slightly inclined downward (counterclockwise).

In this embodiment, attention is paid to the initialization charging potential Vo of the photoconductor 6B provided by the charger 1B of the most upstream Bk image forming station (B), and the value is switched (changed). This eliminates the difference in image density between the color image formation and the monochromatic image formation. In FIG. 9, 2B ...
B ... is a developing device, 4B ... is a transfer roller, 5B ... is a cleaning device, 8 is a fixing device, 10 is a reflective sensor,
10a is a light emitting element, and 10b is a light receiving element.

FIG. 3 shows the Bk image forming station (B).
FIG. 3 is a cross-sectional view showing the configuration of the main part around the photoconductor 6B of FIG.
By changing the facing distance d between the discharge electrode 12 and the discharge electrode 13, the initialization charging potential Vo of the photoconductor 6B can be changed as shown in FIG.

Here, when the facing distance d between the screen grid 12 and the discharge electrode 13 is narrowed, as shown in FIG.
The discharge electrode 13 is easily discharged, and the amount of negative charges attached to the photoconductor 6B increases, so that | Vo | also increases.

On the other hand, if the opposing distance d between the screen grid 12 and the discharge electrode 13 is widened, the discharge electrode 13 becomes less likely to discharge, and the amount of negative charges adhering to the photoconductor 6B decreases, so that | Vo | also decreases. To do.

Here, the photoreceptor surface potential (bright portion potential) | VL | after the image exposure of the photoreceptor 6B increases with an increase of | Vo | and decreases with a decrease of | Vo |.

Further, as shown in FIG. 5, the developing bias V
The potential difference between D and VL is the contrast potential | VC | (|
VC | = | VD-VL |).

Further, when the image exposure amount and | VD | are fixed, | VC | decreases with an increase in | VL |
| VC | increases with the decrease of | VL |.

That is, when the facing distance d is narrowed, | Vo |
Is increased, and along with that, | VL | is also increased.
Decreases. On the other hand, when the facing distance d is widened, | Vo | decreases, and along with this, | VL | also decreases, but | VC | increases.

Further, the potential difference between VD and VL (that is, V
Due to the electric field formed from C), the toner is transferred to the photoconductor 6B.
Attached to. Therefore, the contrast potential | VC |
Is expressed as the image density.

In FIG. 5, Voo is the photosensitive drum surface potential (bright portion potential) irradiated by the laser beam driven by the digitized 0-level image signal driving the laser. In addition, the fog removal potential (the non-image portion potential V of the photoconductor)
The potential difference of oo with respect to the developing bias potential VB becomes Vback) is to completely remove the fog of the image white background portion (non-image portion) irradiated with the 0 level light amount.
Further, at the time of monochromatic formation, the initialization charging potential V of the photoconductor 6B is
o moves to the dotted line position (white background side).

FIG. 6 is a graph showing the relationship between the facing distance d between the screen grid 12 and the discharge electrode 13 and the copy density (image density). If the facing distance d is narrowed, then | VC.
Although | decreases and the image density decreases, | VC | increases and the image density increases when the facing distance d is increased.

Therefore, this relationship is applied to the charger (1B) of the most upstream image forming station (B),
By changing the facing distance d and adjusting | Vo | between the color image formation and the monochromatic image formation, the image density difference generated with respect to the toner color of the most upstream image forming station (B) is reduced. You can do it.

For example, the screen grid 1 in the charger (1B) of the most upstream image forming station (B) is set as follows in the formation of a color image and the formation of a single color image.
Vo is changed by adjusting the facing distance d between the discharge electrode 13 and the discharge electrode 13. In both image forming modes, the voltage Vg applied to the screen grid 12 is fixed to Vg = -600 [V].

[0171]

[Table 1]

As shown in Table 1, when Vo in color is set to a non-white background side than Vo in single color, VL also approaches the non-white background (for the photoconductor 6B having a negative charging polarity). In the reversal development method in which toner having a negative charging polarity is applied to develop, the relative polarity of the bright portion potential VL after exposure of the electrostatic latent image approaches the plus side of the opposite polarity). If the conditions such as the image exposure amount, the developing bias VD, and the transfer efficiency are the same in both image forming modes, | VC |
Will increase.

Therefore, in the color mode, it is possible to make a difference in the direction in which the amount of toner transferred to the photoconductor 6B increases as compared with the case of a single color, and it is possible to make a difference in the amount of toner transferred to the paper. .

Further, in this case, the VL for a single color is closer to the white background than the VL for a color (the relative polarity of the bright portion potential Vo after the electrostatic latent image is exposed is minus the normal charging polarity). If the conditions such as the image exposure amount, the developing bias VD, and the transfer efficiency are the same in both image forming modes, the toner transfer amount to the photoconductor 6B decreases in the single color mode than in the color mode. It is possible to make a difference in the direction and also make a difference in the amount of toner transferred to the paper.

Another example will be described below with reference to FIGS. 7 and 8. In this case, as shown in FIG. 7, in order to charge the photoconductor 6B by the charger 1B, a moving means 14 for moving the discharge electrode 13 for corona discharge in the radial direction of the photoconductor 6B is provided. Then, the facing distance d between the grid 12 and the discharge electrode 13 of the charger 1B is adjusted.

The moving means 14 is composed of an electrode holding material 15 fixed in the case of the charger 1B and a drive device for moving the discharge electrode 13 forward and backward toward the photoconductor 6B. This drive device includes a drive control device 16 for instructing the movement of the discharge electrode in accordance with the switching of the image forming mode according to an output command of the operation panel 23 of the image forming device main body or the host computer 24, and the drive control device. Shaft drive device 1 for driving a motor in response to a command from 16
7 and a motor 1 driven by a shaft drive device 17.
8 and a driving force transmission mechanism that is connected to the motor 18 and drives the discharge electrode 13. The discharge electrode 13 is movably insulated and held by the electrode holding material 15.

FIG. 8 is a perspective view for explaining the internal structure of the charger 1B (driving force transmitting mechanism of the sawtooth electrode 13). The charger 1B includes a charger case 19 and a sawtooth electrode unit holding member (electrode holding member). ) 15, a sawtooth electrode unit (described later), a sawtooth electrode (discharge electrode) 13, a unit drive gear 20, and a gear shaft 21.

The charger case 19 is formed of a long metal plate, and has a C-shaped channel (having a U-shaped cross section) which is composed of a top plate surface and both side surfaces and is open at the bottom. The sawtooth electrode unit holding material 15 is a strip-shaped plate, and has one long side fixed to the top plate surface of the charger case 19 at right angles and parallel to the side surfaces.

The sawtooth electrode unit is formed by integrally forming a sawtooth electrode 13 having a plurality of sawtooth-shaped sharp edges on one side of a long metal plate with two insulating plates sandwiched from both sides. It is held so that it can slide on the surface of the sawtooth electrode unit holding member 15.

A rack 22 is formed on the non-sliding surface of the sawtooth electrode unit, and the rack 22 and the unit drive gear 20 mesh with each other. Unit drive gear 20
Is fixed to the gear shaft 21, and the gear shaft 2
The sawtooth electrode unit can be reciprocally driven in the radial direction of the photoconductor 6B by rotating 1 in either of the arrow directions. That is, the sawtooth electrode 13 can be moved forward and backward with respect to the photoconductor 6B.

In FIG. 7, the charger 1B has a driving force transmission mechanism for the sawtooth electrode 13 as shown in FIG. 8, and the sawtooth electrode 13 is arranged to face the photoconductor 6B. The screen grid 12 is provided between the photoconductor 6B and the charger 1B, and the distance d from the sharp end of the sawtooth electrode 13 is provided.
Separated.

A power source (grid power source) having a negative polarity voltage V2 is connected to the screen grid 12,
The charger case 19 is also connected to this grid power supply. A negative polarity voltage V1 is applied to the sawtooth electrode 13.
A power source (high voltage power source) of (V1 <V2) is connected.
The reason why V1 <V2 is set is that negative charges are discharged to the grid electrode 12 side and the photoreceptor 6B is negatively charged.

Further, the drive control device 16 is selectively operated via the operation panel 23 of the main body of the image forming apparatus or the host computer 24 so that the type of the image forming mode, either the color image forming mode or the single color image forming mode, can be selected. Is input to. Further, a command from the drive control device 16 is input to the shaft drive device 17.

Next, the shaft drive device 17 drives the drive motor 18, and the saw electrode 13 is driven through the drive transmission mechanism.
Transmits the driving force to. Then, the sawtooth electrode 13 to which the driving force is transmitted is reciprocally driven (moves in the vertical direction) in the radial direction (normal direction) of the photoconductor 6B.

The moving means 14 shown in FIG. 7 drives the drive motor 18 and the drive transmission mechanism via the shaft drive device 17, moves the saw electrode 13 via the drive transmission mechanism, and moves the saw electrode 13 and the grid. The distance d from 12 is adjusted. At this time, the drive control device 16 controls the drive timing so that the sawtooth electrode 13 does not move while the charger 1B is charging the photoconductor 11.

In this example, the driving when the charger 1B has the saw electrode 13 has been described, but if the discharge electrode (saw electrode) 13 can be reciprocally moved in the radial direction of the photosensitive drum 6B. Alternatively, another configuration may be used (for example, a wire electrode charger).

With the configuration described above, as shown in FIG. 4, the initialization charging potential Vo of the photoconductor 6B can be changed. As a result, the image density can be adjusted as shown in FIG.

Therefore, using this structure, the facing distance d between the screen grid 12 and the discharge electrode 13 in the black image forming station (B) is adjusted to be wide in the color mode and narrow in the single color mode. As a result, it is possible to suppress the occurrence of an image density difference between the color mode and the monochrome image.

(Embodiment 2) This embodiment corresponds to claims 5 to 14.

The present multicolor image forming apparatus is of the same tandem system as that of the first embodiment (see FIG. 9), and the description of the same components will be omitted and only the differences will be described.

In this embodiment, in particular, by changing the aperture ratio of the screen grid (grid electrode) 12 of the charger 1B of the most upstream image forming station (B) (see FIG. 9), it is applied to the photoconductor 6B. The initialized charging potential Vo is changed and adjusted.

Specifically, the image forming station (B)
10, which shows the configuration around the photoconductor 6B, the initialization charging potential Vo of the photoconductor 6B can be changed by changing the aperture ratio of the screen grid 12 of the charger 1B as shown in FIG. it can.

When the aperture ratio of the screen grid 12 of the charger 1B decreases, the initialization charging potential Vo of the photoconductor 6B decreases, and conversely, when the aperture ratio of the screen grid 12 increases, the initialization charging potential of the photoconductor 6B decreases. Vo increases.

Here, when the image exposure amount and the developing bias voltage are fixed, when the initialization charging potential Vo of the photoconductor 6B is large, the photoconductor surface potential VL (light portion potential) after the image exposure. Becomes higher (see FIG. 5). Contrast potential V
C (a potential difference between the developing bias voltage VB and the exposed portion potential VL) is expressed as an image density.

FIG. 12 shows the relationship between the aperture ratio of the screen grid 12 and the image density.
When the aperture ratio is low, the image density is high, and when the screen grid 12 has a high aperture ratio, the image density is low.

Therefore, this relationship is applied to the charger 1B of the most upstream image forming station (B), and the most upstream image forming station (B) that occurs during color image formation and single color image formation. It is only necessary to reduce the difference in image density that occurs with respect to the toner colors.

For example, as shown in Table 2, the aperture ratio of the screen grid 12 in the charger 1B of the most upstream image forming station (B) is adjusted between the full-color image formation and the single-color image formation, and the photoconductor is adjusted. 6B initialization charging potential V
change o. In both image forming modes, the voltage Vg applied to the charger grid is fixed at Vg = -600 [V].

[0198]

[Table 2]

Since the initialization charging potential Vo in color is set to a non-white background side than the initialization charging potential Vo in single color, the light portion potential VL after exposure also approaches the non-white background side (regular charging). In a reversal development method in which a toner having a negative charging polarity is applied to a photoconductor having a negative polarity to perform development, the relative polarity of the bright portion potential Vo after exposing the electrostatic latent image is Even if the conditions such as the image exposure amount, the developing bias voltage, and the transfer efficiency are the same in both image forming modes, there is a difference in the direction in which the toner transfer amount to the photoconductor 6B increases. The amount of toner transferred to the paper can be made different.

Since the light portion potential VL at the time of monochromatic image formation is on the white background side than the light portion potential VL at the time of color image formation (the relative polarity of the light portion potential Vo after exposing the electrostatic latent image is Even if the conditions such as the image exposure amount, the developing bias voltage, and the transfer efficiency are the same in both image forming modes, the amount of toner transfer to the photoconductor decreases. The amount of toner transferred to the paper can be made different.

In another example, as described above, by adjusting the aperture ratio of the screen grid 12 of the most upstream image forming station (B) according to the image forming mode, the initialization charging of the photoconductor 6B is performed. The potential Vo is changed so as to suppress the occurrence of an image density difference in the toner color of the most upstream image forming station (B) between the color image formation and the monochromatic image formation, but the screen grid 12 of the charger 1B. The aperture ratio of is adjusted as follows.

FIG. 13A is a plan view showing the charger 1B in the most upstream image forming station (B) with the opening to be directed toward the surface of the photoconductor facing upward.
FIG. 3B is a sectional view showing the internal structure of the charger 1B as viewed from the front in the same state.

The frame body of the charger 1B includes a charger casing 31 constituting front and rear side surfaces, and the charger casing 3
The end block 32 is provided at each of left and right end portions that also have a function as a span member and a positioning member for the charge wire.

In the illustrated example, the upper end of the charger casing 31 is bent outward to form a guided portion when the charger 1B is attached to and detached from the main body of the copying machine. The charge wire 33 and the screen grid 12 are installed between the end blocks 32.

In the illustrated example, the left and right ends of the charge wire 33 are fixed to the left and right ends of the pins (locking members).
It is locked to 51. A part of the left end portion of the charge wire 33 is a spring 33a for keeping the charge wire 33 at an appropriate tension.

Next, the screen grid 12 and the attachment mechanism for attaching the screen grid 12 to the left and right end blocks 32 of this embodiment will be described in detail. The screen grid 12 is configured to be stretchable in the longitudinal direction, and the aperture ratio is changed by this stretching.

In the illustrated example, the left and right flat plate portions 41, 41 of the screen grid 12 are connected to each other by four rows of meandering portions 42 which are extendable and contractible in the longitudinal direction, and the adjacent meandering portions 42, 42 form a part. The connecting portions 43 are connected to each other.

Further, the left and right flat plate portions 41, 41 have the structure shown in FIG.
As shown in FIG. 4 (a), locking holes 41a are formed respectively. Note that FIG. 14A is a plan view showing a state in which the screen grid 12 is in a normal length in which no tension T is applied from the outside, and FIG. 14B is a view in which the meandering portion is formed by applying tension T in a longitudinal stretching direction. 42 is a plan view showing a state in which 42 extends in the longitudinal direction to some extent. FIG.

As is clear from these figures, when the length of the screen grid 12 in the longitudinal direction, for example, the distance A between the two locking holes is increased, the opening around the meandering portion 42 is enlarged, The aperture ratio of the screen grid 12 is increased.

On the contrary, for example, if the tension T is made small and the distance A is made relatively small from the state of FIG. 14B, the opening around the meandering portion contracts, and the aperture ratio of the screen grid 12 becomes smaller. Becomes smaller. In this way, the aperture ratio changes due to the expansion and contraction in the longitudinal direction.

The attachment mechanism of the screen grid 12 of this example is mounted on the right end block 32 for positioning the right end of the screen grid 12 at a fixed position by locking the right locking hole 41a of the screen grid 12. The hook 52 is formed, and a movable attachment mechanism 53 for positioning the left end of the screen grid 12 in a positionally adjustable manner.

This movable mounting mechanism 53 has an aperture ratio adjusting plate 54 in which a pin 54a for locking the left locking hole 41b is planted, and an elongated hole 54b is formed in the longitudinal direction, and this elongated hole. 54b for penetrating 54b and for positioning the aperture ratio adjusting plate 54.

The pin 56 planted on the left end block 32 so as to penetrate the elongated hole 54b also facilitates the work of attaching the aperture ratio adjusting plate 54 to the left end block 32, and the opening 56 The rate adjusting plate 54 is positioned in the lateral direction.

A gear portion (rack portion) 54c is formed on one edge portion of the aperture ratio adjusting plate 54.
The pinion 60 meshing with 4c is the output shaft 70 of the motor 70.
It is attached directly to a.

The motor 70 is a drive unit for automatically changing the position of the aperture ratio adjusting plate 54. Rotation of the motor 70 in either forward or reverse directions causes the pinion 60 to rotate.
The position of the aperture ratio adjusting plate 54 is changed via the, and thereby the length of the grid 12 in the longitudinal direction is changed to change the grid aperture ratio.

In the above structure, the screen grid aperture adjusting plate 5 for determining the total length of the screen grid 12
Screen grid 1 by changing the position of 4
It is possible to arbitrarily set the aperture ratio of 2.

Therefore, according to the charging device of this example, the best screen grid aperture ratio can be obtained in any printing mode of full-color printing or single-color printing,
It is possible to obtain the optimum initialization charging potential Vo of the photoconductor 6B for each printing mode.

Next, the control of the motor 70 and the like will be described with reference to FIG. In the figure, 73 is a high-voltage power supply that supplies a current to the charge wire 33, and 74 is a control with a ROM that stores and stores control programs and a RAM that temporarily stores and stores information necessary for control. It is a department. In addition to the charger 1B, a well-known image forming unit (not shown) for executing an electrophotographic process is disposed around the photoconductor 6B.

Then, for example, a print command by the operator via the operation panel 25 of the color image forming apparatus main body (either full-color printing or single-color printing is instructed by pressing the operation button), or the color image forming apparatus is connected. Terminal 2 such as a personal computer
An output instruction of either full-color printing or single-color printing via 6 is input to the control unit 74 of the image forming apparatus, and this control unit 74 is connected to the motor 70 of the charger 1B of the most upstream image forming station (B). Has been done.

With such a configuration, the forward / reverse rotation direction of the motor 70 is selected and driven according to the content of the print command for full-color printing or single-color printing input to the control unit 74. That is, when a full-color print command is input to the control unit 74, the rotation direction of the motor 70 is selected to be the contraction direction of the longitudinal dimension in order to reduce the aperture ratio of the screen grid 12, and the single-color print command is input. Is selected in the longitudinal direction in order to increase the aperture ratio of the screen grid 12 by rotating the motor 70.

The tendency relationship between the stretchable state applied to the screen grid 12 of the most upstream image forming station (B), the aperture ratio and the tension along the longitudinal direction, and the surface potential of the photoconductor and the level of the image density are summarized. And as shown in Table 3 below.

[0222]

[Table 3]

(Embodiment 3) The present embodiment provides claim 15.
Through 20.

This multicolor image forming apparatus is of the tandem system (see FIG. 9) similar to the first and second embodiments, and the description of the same components will be omitted and only the differences will be described.

Also in this embodiment, similarly, attention is paid to the initialization charging potential Vo of the photoconductor 6B provided by the charger 1B of the most upstream image forming station (B) (see FIG. 9).

FIG. 16 is a schematic cross-sectional view of the periphery of the photoconductor 6B of the image forming station (B), in which the photoconductor is changed by changing the aperture ratio of the screen grid (grid electrode) 12 of the charger 1B. The initialization charging potential Vo of 6B can be changed (see FIG. 11).

As the screen grid 12, for example, stainless wire material or tungsten wire material is 1 to 3 mm.
It is possible to use one that is stretched at a predetermined interval, or one in which a stainless steel plate having a plate thickness of about 0.1 mm is meshed by etching into halftone dots or multiple parallel slits, but in recent years, assembling work of the charger 1B is possible. The latter type is often used in consideration of sex.

When the aperture ratio of the screen grid 12 of the charger 1B is lowered (see FIG. 11), the initialization charging potential Vo of the photoconductor 6B is decreased, and conversely, when the aperture ratio of the screen grid 12 is increased, the photoconductor 6B is reduced. Initialization charge potential Vo of
Will increase.

Here, when the image exposure amount and the developing bias voltage are fixed, as shown in FIG. 5, when the initialization charging potential Vo of the photoconductor 6B is larger, the surface of the photoconductor after the image exposure is larger. The potential VL (light portion potential) becomes high. Then, the contrast potential VC (potential difference between the developing bias voltage VB and the exposed portion potential VL) is expressed as the image density.

Regarding the relationship between the aperture ratio of the screen grid 12 and the image density (see FIG. 12), when the aperture ratio of the screen grid 12 is low, the image density is high, and when the aperture ratio of the screen grid 12 is high. Image density is low.

Therefore, this relationship is applied to the charger 1B of the most upstream image forming station (B), and the most upstream image forming station (B) occurs during full-color image formation and single-color image formation. It is only necessary to reduce the difference in image density that occurs with respect to the toner color in the above.

For example, as shown in Table 4 below, the aperture ratio of the screen grid 12 in the charger 1B of the most upstream image forming station (B) is adjusted for forming a full-color image and a single-color image, and the exposure is performed. The initialization charging potential Vo of the body 6B is changed.

In both image forming modes, the applied voltage Vg to the screen grid 12 is Vg = -600.
It is fixed to [V]. Further, since the scorotron charger as described above has a poor discharge efficiency, sufficient charging cannot be performed unless the aperture ratio of the screen grid 12 is raised to about 80 to 90%.

[0234]

[Table 4]

Since the initialization charging potential Vo in full color is set to the non-white background side than the initialization charging potential Vo in single color, the light portion potential VL after exposure also approaches the non-white background side (regular charging). In the reversal development method in which a toner having a negative charging polarity is applied to a photosensitive member having a negative polarity to develop, a bright portion potential Vo after exposing an electrostatic latent image is obtained.
The relative amount of toner is closer to the positive side of the opposite polarity), the amount of toner transferred to the photoconductor 6B is the same even when the conditions such as the image exposure amount, the developing bias voltage, and the transfer efficiency are the same in both image forming modes. It is possible to make a difference in the increasing direction and also make a difference in the amount of toner transferred to the transfer material.

Since the light portion potential VL at the time of forming a single color image is on the white background side relative to the light portion potential VL at the time of forming a full-color image (the relative polarity of the light portion potential Vo after exposing the electrostatic latent image is
Even if the conditions such as the image exposure amount, the developing bias voltage, and the transfer efficiency are the same in both image forming modes, the amount of toner transfer to the photoconductor decreases. The amount of toner transferred to the transfer material can be made different.

In this embodiment, as described above, the most upstream image forming station (B) is used depending on the image forming mode.
By adjusting the aperture ratio of the screen grid 12 of No. 3, the initialization charging potential Vo of the photoconductor 6B is changed, and the toner color of the most upstream image forming station (B) is changed between the full-color image formation and the single-color image formation. The generation of the image density difference is suppressed, but the aperture ratio of the screen grid 12 of the charger 1B is adjusted as follows.

In FIG. 17 (a), a 0.1 mm-thick (t) stainless steel plate material is etched at a rightward inclination angle of 60 ° with respect to the horizontal at 0.8 mm pitch intervals so as to leave 0.1 mm-wide thin lines. Then, a slit-shaped grid electrode plate with one side inclination is formed. In this case, the aperture ratio of the grid electrode plate alone is 88.9% (see Table 5).

In FIG. 17 (b), a 0.1 mm thick (t) stainless steel plate material is left at a pitch of 1.7 mm at a tilt angle of 60 ° with respect to the horizontal at intervals of 1.7 mm pitch, as fine lines with a width of 0.1 mm are left. Etching is performed to form the slit-shaped grid electrode plate 12 that is inclined on one side. In this case, the aperture ratio of the grid electrode plate alone is 94.4% (see Table 5).

Here, the shape and the parallel form of a large number of small mesh-shaped openings in the grid electrode plate 12 are not particularly limited, and the shape of the openings is circular (see FIG. 2).
0) or a honeycomb shape (see FIG. 21). Note that, in FIGS. 20 and 21, as will be described below, reference numerals for the case where two grid electrode plates 12 (121, 122) are stacked (fixed side and movable side) are given.

[0241]

[Table 5]

Next, as shown in FIG. 18A, the same grid electrode plates 121 and 122 as described above are stacked, one grid electrode plate 121 is fixed, and the other grid electrode plate 122 is made movable. , This plurality of grid electrode plates 12
The area in which the openings overlap with each other (opening area) varies between 1 and 122.

For example, when two grid electrode plates of FIG. 17 (a) are overlapped, the aperture ratio is 88.9% (maximum) when the two electrodes are completely overlapped, and as shown in FIG. 18 (b). In the state shown in FIG. 18 (c), the movable side is slid after the two partially overlap each other.
It can be changed up to 3.3% (minimum).

Similarly, when two grid electrode plates of FIG. 17 (b) are overlapped, the aperture ratio between the two is 94.4% (FIG. 18 (a) in the state where the two electrodes are completely overlapped. It can be changed from (maximum) to 91.6% (minimum) of the state shown in FIG. 18C in which the movable side is slid.

FIGS. 19 (a) and 19 (b) show a position adjusting mechanism and the like in the case of constructing the grid of the scorotron charger 1B by stacking two grid electrode plates 121 (fixed side) and 122 (movable side). An example is shown.

The scorotron charger 1B includes a charger casing 8l constituting a side surface (side plate), a span member of the charger casing 8l, and a discharge wire 83.
And end blocks 82 provided at both left and right ends which also function as positioning members.

A discharge wire 83, a fixed side grid electrode plate 121, and a movable side grid electrode plate 122 are provided between the left and right end blocks 82, 82. A spring 83a for keeping the discharge wire 83 at a proper tension is connected to one end of the discharge wire 83.

Reference numerals 121a and 122a are openings (pores) formed in the grid electrode plates 121 and 122, respectively.
Indicates. By sliding at least one of the two grid electrode plates 121 and 122 with respect to the other by the grid electrode plate position adjusting means (driving means) 84, the area where the openings 121a and 122a overlap (opening). By changing the area), the aperture ratio of the scorotron charger 1B as a grid can be changed.

In this example, the grid electrode plate position adjusting means 84 is constituted by a combination of the eccentric cam 85 and the movable side grid electrode plate 122 which is pressed in the longitudinal direction of the charger case via a coil spring or the like. Then, the movable side grid electrode plate 122 is driven by the driving unit 90 such as a stepping motor.
The movement control is performed.

The aperture ratio adjusting mechanism will be described. A stepping motor 90, which is a drive unit for automatically changing the position of the movable grid electrode plate 122, is mounted on one end block 82. In the illustrated example, the eccentric cam 85 is directly attached to the output shaft 90a of the stepping motor 90.

By the rotation of the stepping motor 90, the holding frame body of the movable side grid electrode plate 122 whose one end is pressed by the coil spring 87 and the like via the eccentric cam 85 is guided by the guide frame bodies 88, 88 (FIG. 19 (c). )), The position of the movable side grid electrode plate 122 is changed while sliding in the longitudinal direction of the charger case 81 along with the fixed side grid electrode plate 121.
The aperture ratio with the movable grid electrode plate 122 can be changed.

In this example, the outer grid electrode plate (122) of the two stacked grid electrodes 121, 122 is the movable side, and the aperture ratio is adjusted by sliding the grid electrode plate (122). However, on the contrary, the outer grid electrode (122) is fixed to the inner grid electrode plate (12).
It is also possible to adjust the aperture ratio by making 1) the movable side.

Next, the control of the stepping motor 90 and the like will be described (see FIG. 15). For example, a print command by the operator via the operation panel (25) of the color image forming apparatus main body (either full-color printing or single-color printing is instructed by pressing the operation button), or a personal computer connected to the color image forming apparatus. An output instruction of either full-color printing or single-color printing via a terminal device (26) such as the above is input to a control unit (74) of the image forming apparatus, and the control unit (74) outputs the most upstream image forming station (B). ) Is connected to the stepping motor 90 (70) of the charger 1B.

With this configuration, the stepping motor 90 can be operated in accordance with the content of the print command for full-color printing or single-color printing input to the control section (74).
The rotation direction of (70) is selected. That is, when a full-color print command is input to the control unit (74), the rotation direction of the stepping motor 90 (70) is selected to reduce the aperture ratio of the screen grids 121 and 122, and the single-color print command is input. In this case, the screen grids 121 and 122 are selected in the direction of increasing the aperture ratio.

The tendency relationship between the aperture ratio given to the grid electrode plates 121, 122 of the charger 1B of the most upstream image forming station (B) and the surface potential of the photoconductor, the adhesion amount and the image density is shown. The summary is shown in Table 6 below.

[0256]

[Table 6]

(Embodiment 4) The present embodiment is claimed in claim 21.
Through 29.

In this embodiment, in order to eliminate the difference in the image density of the toner in the most upstream image forming station (B), which occurs between the full-color image formation and the single-color image formation, the shield electrode of the charger 1B is eliminated. Attention is paid to the facing interval of the, and the interval is changed.

As shown in FIGS. 22 (a) and 22 (b), the interval (shield case width) between the shield electrodes of the charger LB is set as follows.
Suppose you changed from W ₁ to W ₂ . Then, FIG.
As shown in the graph, the charging potential of the photoconductor 6B greatly changes.

This is because as the facing distance between the shield electrodes of the charger 1B becomes wider, the opening toward the photoconductor 6B becomes wider, and the charge applied from the charger 1B to the photoconductor 6B increases. . In this way, by changing the facing interval of the shield electrodes of the charger 1B, it becomes possible to change the initialization charging potential Vo of the photoconductor 6B.

Further, when the initialization charging potential Vo of the photoconductor drum 6B is changed, as shown in FIG. 5, when the image exposure amount and the developing bias voltage are fixed, the photoconductor 6B is fixed.
When the initialization charging potential Vo is large, the photosensitive member surface potential VL (light portion potential) after image exposure becomes higher. As a result, when the opposing distance between the shield electrodes of the charger 1B is widened during the formation of a single color, the initialization charging potential Vo of the photoconductor 6B becomes
It moves to the dotted line position (white background side) shown in FIG.

As a result, the absolute value of the contrast potential VC (potential difference between the developing bias voltage VB and the exposed portion potential VL) becomes smaller than that at the time of forming a full-color image on the non-white background side indicated by the solid line. The toner adhesion amount can be reduced. Therefore, the facing distance between the shield electrodes and the image density have an inverse proportional relationship in which the image density decreases as the facing distance between the shield electrodes increases, as shown in FIG.

For example, when forming a full-color image and during forming a single-color image, as shown in Table 7 below, the facing distance W between the shield electrodes in the charger IB of the image forming station (B) located on the most upstream side is adjusted. Then, it is assumed that the initialization charging potential Vo of the photoconductor 6B is changed. In both image forming modes, the voltage Vg applied to the charger grid is Vg =
It is fixed at -600 (V).

[0264]

[Table 7]

From Table 7, if the facing distance between the shield electrodes is changed from 9.0 mm in the full color mode to 11.0 mm in the single color mode, the initialization charging potential Vo of the photoconductor 6B is reduced by 35V. Therefore, the amount of toner attached to the photoconductor 6B is reduced, and it is possible to prevent a difference in image density between the two modes.

At the time of forming a full-color image, the initialization charging potential Vo is set to the non-white background side with respect to the initialization charging potential Vo in the case of a single color. Therefore, the light portion potential VL after exposure also approaches the non-white background side. In other words, in the reversal development method in which the toner having the negative charge polarity is applied to the photoconductor 6B having the negative charge polarity to develop the electrostatic latent image, the relative potential of the bright portion potential Vo after the exposure is changed. Positive polarity approaches plus of opposite polarity.

Therefore, even if the conditions such as the image exposure amount, the developing bias voltage, and the transfer efficiency are the same in both image forming modes,
By changing the facing interval of the shield electrodes of the charger 1B and adjusting the initialization charging potential Vo of the photoconductor 6B, it is possible to control the toner adhesion amount to the photoconductor 6B. As a result, it is possible to make a difference in the amount of toner transferred to the paper.

On the other hand, when forming a monochromatic image, the light portion potential VL
However, it moves to the white background side from the light portion potential VL at the time of full-color image formation (the relative polarity of the light portion potential VL after exposing the electrostatic latent image increases on the negative side of the normal charging polarity).

Therefore, even if the conditions such as the image exposure amount, the developing bias voltage, and the transfer efficiency are the same in both image forming modes,
The amount of toner adhered to the photoconductor 6B can be controlled so as to be reduced, and the amount of toner transferred to the paper can be reduced as compared with that during full-color image formation.

Next, a concrete means (moving means 49) for changing the facing interval of the shield electrodes of the charger 1B.
Will be described below. In the multicolor image forming apparatus according to the present exemplary embodiment, the charger 1B is provided in the charger case 19, the sawtooth electrode 13, and the sawtooth electrode unit holding material 15 in order to adjust the facing distance between the shield electrodes of the charger 1B shown in FIG. , Rack r, unit drive gear (pinion) p, and gear shaft s.

The charger case 19 is formed in a U-shape by stacking two long L-shaped metal mold members (shield electrodes) 19a and 19b on one side of the L-shape. The aligned side portions are slid so that the facing distance W between the shield electrodes 19a and 19b can be adjusted.

The sawtooth electrode unit holding material 15 is a strip-shaped plate and is made of a metal mold material 19a.
In the center of the top plate surface of the metal mold member 19a, which is placed underneath, of the l9b, the sawtooth electrode 13 is formed at a right angle to the top plate surface.
One of the long sides of is fixed. A rack r is formed on both ends of the upper surface of the other L-shaped metal mold material 19b, and the rack r and the unit drive gear p mesh with each other.

The unit drive gear p is fixed to the gear shaft s, and by rotating the gear shaft s forward or backward in the direction of arrow X, the metal mold material 19b is moved in a direction to increase or decrease the facing distance W between the shield electrodes. Can be slid.

Further, as shown in FIG. 26, the present multicolor image forming apparatus is provided with a drive control device 45, a shaft drive device 46 and a drive transmission mechanism 47 as means for driving the unit drive gear p. . The drive control device 45, the shaft drive device 46, the drive transmission mechanism 47, the motor 48 described below, and the like constitute the moving means 49.

The drive control device 45 commands the movement of the shield electrode in accordance with the switching of the image forming mode in response to the operator's operation panel of the image forming device or the output command of the host computer. The shaft drive device 46 receives a command from the drive control device 45, drives the motor 48, and transmits power to the drive transmission mechanism 47. Drive transmission mechanism 4
7 is a motor 4 driven by the shaft drive device 46.
8 transmits the driving force to the shield electrodes 19a and 19b,
The spacing is adjusted as described above.

Further, a screen grid 12 is provided between the photoconductor 6B and the charger IB, and the sawtooth electrode 13 is provided.
A predetermined interval is set between the sharp end of the photoconductor and the photoconductor 6B. The screen grid 12 and the charger case 19 are both connected to a power source (grid power source) having a negative polarity voltage V2, and the sawtooth electrode 13 has a power source (high voltage power source) having a negative polarity voltage V1 (V1 <V2). Are connected.

A process of adjusting the adhering amount of toner by changing the facing interval of the shield electrodes using the above-mentioned plurality of members will be described below. First, the drive control device 45
An image forming mode of either a full-color image forming mode or a single-color image forming mode is selectively commanded / input from an operation panel of the image forming apparatus main body or a host computer (not shown).

Next, a command from the drive control device 45 is input to the shaft drive device 46 to drive the motor 48. The motor 48 reciprocally drives (moves in the horizontal direction) the gap between the opposing shield electrodes 19a and 19b in the wide and narrow directions via the drive transmission mechanism 47.

With this structure, when forming a full-color image, image formation is performed without changing the distance between the shield electrodes 19a and 19b.

On the other hand, when forming a monochromatic image, the image is formed by widening the gap between the shield electrodes 19a and 19b. At that time, the shield electrodes 19a, 19
The driving force for expanding the distance b is the shaft driving device 4
6. The shield electrode 19b is moved by being transmitted to the unit drive gear p through the drive transmission mechanism 47. The drive controller 45 controls the drive timing so that the shield electrode 19b does not move while the charger 1B is charging the photoconductor 6B.

The multicolor image forming apparatus according to the present embodiment is configured as described above, and the gap between the shield electrodes 19a and 19b is adjusted to adjust the initialization charging potential Vo of the photoconductor 6B to form a monochromatic image. The amount of black toner attached to the photoconductor 6B can be appropriately adjusted depending on the time and the time of forming a full-color image.

In the present embodiment, an example in which the shield electrode 19b is moved during the formation of a single color image with reference to the time of full color image formation has been described. On the contrary, full color image formation is performed with reference to the time of formation of a single color image. Similar effects can be obtained by driving the shield electrode 19b at times. Further, in the present embodiment, the case where the charger has a saw electrode as the discharge electrode has been described, but the same can be applied to the case where the discharge electrode is a wire electrode charger.

Furthermore, in the present embodiment, an example in which the image forming station (B) located on the most upstream side of the paper transport belt 7 is provided with black toner has been described, but the present invention is not limited to this. Toners of other colors may be provided in the image forming station on the most upstream side, or the image forming stations other than the most upstream side may be provided with a charger capable of adjusting the facing interval of the shield electrodes. In any of the above cases, it is possible to obtain the same effect as that obtained in the present embodiment, and it is possible to adjust the toner adhesion amount.

The multicolor image forming apparatus and the control method of the multicolor image forming apparatus of the present invention have been described above based on the first to fourth embodiments, but the present invention is limited to the first to fourth embodiments. However, the invention is not restricted, and design changes, improvements, etc. can be freely made without departing from the spirit of the present invention, and the combinations between the embodiments can be freely made.

[0285]

As is apparent from the above description, the present invention has the following effects.

(1) In both color and monochrome image forming modes,
By setting the image exposure amount and the developing bias in common, and switching only the initialization charging potential Vo for the photoconductor of the most upstream image forming station according to the image forming mode, the optimum amount of toner adhesion to the photoconductor is adjusted. Image density obtained at the image forming station located at the uppermost stream of the paper transport belt during color image formation, and at the time of monochromatic image formation involving only the image forming station located at the uppermost stream of the paper transport belt. It is possible to eliminate the density difference that occurs between the generated image density and the image quality of a monochrome image.

Further, in the case of forming a monochromatic image in which only the most upstream image forming station is involved, by separating the sheet conveying belt from the image forming station on the downstream side not involved in image formation, the sheet conveying belt and the photosensitive member are separated. It is possible to avoid the wear of the maximum.

(2) By adjusting the facing distance between the grid and the discharge electrode in the color image forming mode and the monochromatic image forming mode, the amount of toner adhered to the photosensitive member can be properly adjusted, so that an appropriate image can be obtained. The concentration can be secured.

(3) Since the discharge electrode is moved (while the voltage applied to the discharge electrode is fixed), changing the initialization charging potential Vo of the photosensitive member puts less burden on the high-voltage power supply, and the photosensitive member and the charging member are charged. It is possible to suppress the performance of the container.

(4) In the color image formation, the facing interval between the grid and the discharge electrode is set wider than that in the single color image formation. Therefore, the toner adhesion amount is increased as compared with the single color image formation, and the most upstream image formation is performed. It is possible to reduce the difference in image density caused by the toner color of the station.

(5) By expanding and contracting the screen grid in the longitudinal direction, the initialization charging potential Vo of the charger of the most upstream image forming station is switched, so that the optimum toner adhesion amount can be adjusted and Between the image density obtained at the image forming station located at the uppermost stream of the paper transport belt during image formation and the image density obtained at the time of monochromatic image formation involving only the image forming station located at the uppermost stream of the paper transport belt. The density difference that occurs is eliminated, and the image quality of a single-color image can be improved.

(6) Since the initialization charging potential for the most upstream photosensitive member is changed by changing the expansion / contraction state of the screen grid of the charger, the aperture ratio of the screen grid of the charger is high. In this case, the initialization charging potential Vo of the photoconductor is on the white background side, and the initialization charging potential of the photoconductor is on the non-white background side as the aperture ratio of the screen grid decreases.

When the image exposure conditions and the developing bias voltage are fixed, the contrast potential VC becomes larger when the facing distance between the shield electrodes is smaller, and the amount of toner adhered to the photosensitive member is larger, so that the image density can be secured.

(7) Since the control means and the driving means for adjusting the expansion / contraction state of the screen grid of the charger of the most upstream image forming station along the paper transport belt are provided, the screen grid of the charger is extended at both ends. In this case, the initialization charging potential Vo of the photoconductor is on the white background side, and as it contracts, the initialization charging potential Vo of the photoconductor is on the non-white background side. When the image exposure condition and the developing bias voltage are fixed,
When the screen grid contracts, the contrast potential VC becomes larger, the amount of toner attached to the photoconductor also increases, and an appropriate image density can be secured.

(8) When a monochromatic image is formed, the screen grid of the charger of the image forming station located at the uppermost stream of the paper transport belt is stretched, and the screen grid is contracted when a color image is formed. When the sheet is stretched, the initializing charge potential Vo of the photoconductor is on the white background side, and as it contracts, the initializing charge potential of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the screen grid contracts, the amount of toner adhered to the photosensitive member increases, and an appropriate image density can be secured.

(9) At the time of color image formation and monochromatic image formation, the initialization charging potential is switched to the photoconductor of the most upstream image forming station along the paper transport belt by changing the aperture ratio of the screen grid of the charger. When the aperture ratio of the screen grid of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio of the screen grid decreases,
The initialization charging potential of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger and the amount of toner adhered to the photosensitive member becomes larger when the facing distance between the shield electrodes is small, so that an appropriate image density is secured. it can.

(10) Since the control means and the driving means for adjusting the aperture ratio of the screen grid of the charger of the most upstream image forming station along the sheet conveying belt are adjustable, the aperture of the screen grid of the charger is provided. When the ratio is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio decreases, the initialization charging potential of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger and the amount of toner adhering to the photoconductor becomes larger when the aperture ratio of the screen grid is small, so that an appropriate image density can be secured. .

(11) When a monochromatic image is formed, the aperture ratio of the screen grid of the charger of the image forming station disposed on the most upstream side of the paper transport belt is set higher than the aperture ratio during color image formation. When the aperture ratio of the screen grid is high, the initialization charging potential V of the photoconductor is
o is on the white background side, and the initialization charging potential of the photoconductor is on the non-white background side as the aperture ratio decreases.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes large when the aperture ratio of the screen grid is low, so that the amount of toner adhering to the photoconductor also becomes large and an appropriate image density can be secured. It is possible to reduce the difference in image density caused by the toner color of the most upstream image forming station between the color image formation and the monochromatic image formation.

(12) The switching of the initialization charging potential for the photoconductor of the most upstream image forming station along the paper transport belt at the time of color image formation and monochromatic image formation,
Since this is performed by changing the tension in the longitudinal direction applied to both ends of the screen grid of the charger, when the tension in the longitudinal direction applied to both ends of the screen grid is large, the initialization charging potential Vo of the photoconductor is set to the white background side. Therefore, as the tension applied to both ends is decreased, the initializing charging potential of the photoconductor becomes the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger when the tension applied to both ends of the screen grid is smaller, the amount of toner adhered to the photosensitive member is larger, and the proper image density is obtained. Can be secured.

(13) Since the control means and the drive means for adjusting the tension applied to both ends of the screen grid of the charger of the most upstream image forming station along the sheet conveying belt along the longitudinal direction to be increased or decreased are provided, When the tension applied to both ends of the screen grid along the longitudinal direction is large (when the aperture ratio of the screen grid is large), the initialization charging potential Vo of the photoconductor is on the white background side, and the tension applied to both ends of the screen grid (screen grid As the aperture ratio) of the photoconductor decreases, the initializing charging potential of the photoconductor becomes a non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger and the amount of toner adhering to the photosensitive member becomes larger when the aperture ratio of the screen grid is small, so that an appropriate image density can be secured. .

(14) The tension in the longitudinal direction applied to both ends of the screen grid of the charger of the image forming station arranged on the most upstream side of the sheet conveying belt at the time of monochromatic image formation is set higher than the tension applied at the time of color image formation. Therefore, when the tension in the longitudinal direction applied to both ends of the screen grid is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the applied tension is low, the initialization charging potential of the photoconductor is not It will be on the white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC increases when the tension applied to both ends of the screen grid is low, so that the amount of toner adhered to the photoconductor also increases and an appropriate image density is obtained. Can be secured.

Therefore, it is possible to reduce the difference in image density caused by the toner color of the most upstream image forming station between the color image formation and the monochromatic image formation.

(15) A mesh grid electrode having a multiple structure is provided on the main surface of the charger of the most upstream image forming station, and all image forming stations are used for color image formation and the most upstream position. Since there is a means for making at least one of the grid electrodes relatively movable with respect to the other when forming a single-color image using only the image forming station, By moving the grid electrodes of the image forming station relative to each other and changing the initialization charging potential Vo for the photoconductor in accordance with the image forming mode, it is possible to optimally adjust the toner adhesion amount,
For the toner color of the most upstream image forming station, the density difference generated between the image density obtained during color image formation and the image density obtained during monochromatic image formation is eliminated.

(16) At least one of a plurality of grid electrode plates having a plurality of pores of the charger of the image forming station arranged in the most upstream is set to have an area where the pores overlap between the grid electrode plates. Since the initialization charging potential Vo of the photosensitive member is switched by moving the photosensitive member while changing it, the image exposure amount and the developing bias voltage are set to be common between the color image forming and the monochromatic image forming, and the most upstream image forming station. Initializing charging potential Vo for the photoconductor
By changing only the image formation mode,
The optimum amount of toner adhesion can be adjusted, and only the image density obtained for the image forming station located at the uppermost stream of the paper transport belt during color image formation and the image forming station located at the uppermost stream of the paper transport belt. It is possible to improve the image quality of a monochromatic image by eliminating the density difference that occurs with the image density obtained during the formation of the involved monochromatic image.

(17) During the formation of a color image and the formation of a monochromatic image, the initialization charging potential Vo is switched between the grid electrode plates of the charger by switching the initialization charging potential Vo to the photoconductor of the most upstream image forming station along the paper transport belt. Since it is performed by changing the aperture ratio, when the aperture ratio between the grid electrode plates of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio between the grid electrode plates decreases, the photoconductor is reduced. The initial charging potential of is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger and the amount of toner adhered to the photoconductor becomes larger when the facing distance between the shield electrodes is small, so that an appropriate image density is secured. it can.

(18) Since the grid electrode plate position adjusting means for adjusting the aperture ratio of the grid electrode plate of the charger of the most upstream image forming station along the sheet conveyance belt is adjustable, the space between the grid electrode plates of the charger is provided. When the aperture ratio of is large, the initialization charging potential Vo of the photoconductor is on the white background side,
As the aperture ratio decreases, the initializing charging potential of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger and the amount of toner adhered to the photosensitive member becomes larger when the aperture ratio between the grid and the grid plates is small, so that an appropriate image density is obtained. Can be secured.

(19) Since the control means and the driving means for adjusting the aperture ratio of the grid electrode plate of the charger of the most upstream image forming station along the sheet conveying belt are adjustable, the grid electrode plate of the charger is provided. When the aperture ratio is large, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio decreases, the initialization charging potential of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC becomes larger and the amount of toner adhered to the photosensitive member becomes larger when the aperture ratio between the grid electrode plates is small, so that an appropriate image density is obtained. Can be secured.

(20) At the time of monochromatic image formation, the aperture ratio between the grid electrode plates of the charger of the image forming station arranged at the uppermost stream of the paper transport belt is set higher than that at the time of color image formation. When the aperture ratio between the grid electrode plates of the charger is high, the initialization charging potential Vo of the photoconductor is on the white background side, and as the aperture ratio is low, the initialization charging potential of the photoconductor is on the non-white background side.

When the image exposure condition and the developing bias voltage are fixed, the contrast potential VC increases when the aperture ratio between the grid electrode plates is low, so that the amount of toner adhering to the photoconductor also increases and the image density can be secured.

Therefore, it is possible to reduce the difference in image density caused by the toner color of the most upstream image forming station between the color image formation and the monochromatic image formation.

(21) By changing the facing interval of the shield electrode of the charger of the image forming station arranged in the most upstream side, and adjusting the initialization charging potential Vo of the photoconductor, the most upstream side in the color image formation. The density difference between the image density obtained from the image forming station located at the same position and the image density obtained from the image forming station located at the same uppermost stream side during monochromatic image formation is eliminated, and the image quality of a single color image is improved. Can be planned.

(22) When a monochrome image is formed, an unused image forming station on the downstream side is separated from the paper transport belt, so that it is not necessary to consider the reverse transfer phenomenon, so that more preferable image formation can be performed.

(23) Since the initialization charging potential of the photoconductor at the time of color image formation is set to a potential on the non-white background side than the initialization charging potential of the photoconductor at the time of monochromatic image formation, images in both image forming modes are formed. Even if the exposure conditions are set to be common, the surface potential (bright portion potential VL) of the photoconductor after image exposure becomes a potential on the non-white background side when forming a color image as compared to when forming a single color image.

Since the developing bias voltage VB is set in common in both image forming modes, the contrast potential VC (potential difference between the developing bias voltage VB and the exposure portion potential VL) is smaller in color image formation. It is larger than when a monochromatic image is formed.

Therefore, the amount of toner adhering to the photosensitive member and thus the image density can be adjusted, and the image at the time of color image formation and the image at the time of monochromatic image formation involving only the image forming station located at the uppermost stream of the sheet conveying belt. The density difference can be eliminated.

(24) Since the facing distance between the shield electrodes of the charger is adjusted by the moving means, if the facing distance is widened, the initialization charging potential Vo of the photoconductor will be on the white background side,
When the facing interval is narrowed, the initialization charging potential of the photoconductor moves to the non-white background side. Therefore, the amount of toner adhering to the photoconductor can be properly adjusted by appropriately adjusting the facing distance between the shield electrodes during color image formation and during single-color image formation, and as a result, the image density during both image formation can be increased. Can be kept uniform.

(25) At the time of single-color image formation, since the facing distance between the shield electrodes of the charger of the most upstream image forming station is widened, the initialization charging potential Vo of the photoconductor moves to the white background side. Therefore, it is possible to reduce the image density difference of the toner color of the image forming station located on the most upstream side between the color image formation and the monochromatic image formation, and to perform uniform image formation.

(26) Since the black image forming station, which has a great influence on other colors, is located on the uppermost stream side, black toner becomes the lowermost layer during color image formation, and the influence on other colors is minimized. It can be suppressed to the limit.

Further, the black toner image forming station eliminates the density difference generated between the image density obtained during color image formation and the image density obtained during black monochromatic image formation, and during the color image formation An image with high image quality can be obtained both during the formation of a black monochrome image.

Further, even in the case of a multicolor image forming apparatus, in actual use, there are far more opportunities to form a black single color image than to form a color image. At the time of this monochromatic image formation, the black toner arranged at the most upstream position has a smaller amount of toner adhesion to the photoconductor than at the time of color image formation, so an adequate amount of toner is sufficient to obtain image density. Supplied. Therefore, the black toner, which is located on the most upstream side and consumes the most amount of black toner, is not wasted, so that the black toner consumption amount can be suppressed as compared with the conventional multicolor image forming apparatus.

(27) Since the image exposure amount and the developing bias voltage are set in common and only the initialization charging potential Vo for the photoconductor of the most upstream image forming station is adjusted according to the image forming mode, the toner reverse transfer is performed. The amount of toner adhering to the photoconductor can be changed between the color image formation in which the phenomenon should be taken into consideration and the monochromatic image formation in which it does not need to be taken into consideration (specifically, the full color image formation is performed during the monochromatic image formation. The amount of toner adhered is reduced compared to the time).

That is, when the initialization charging potential Vo is changed, the contrast potential VC (the potential difference between the developing bias voltage VB and the exposure portion potential VL) is maintained even if the image exposure amount and the developing bias voltage are unchanged in both modes. Is different,
The amount of toner attached to the photoconductor can be adjusted. Therefore, the density difference generated between the image density obtained from the image forming station located on the most upstream side during color image formation and the image density obtained from the image forming station located on the same most upstream side during monochromatic image formation is As a result, the image quality of a monochrome image can be improved.

(28) Since the initialization charging potential of the photosensitive member is set to a potential on the non-white background side during color image formation as compared with that during monochromatic image formation, the contrast potential VC (developing bias voltage VB and exposure portion potential VL) is set. The potential difference) is larger when a color image is formed than when a single color image is formed. Therefore, the amount of toner adhered to the photoconductor during the formation of a single color image can be smaller than that during color image formation.

Therefore, the amount of toner adhering to the photosensitive member and thus the image density can be adjusted, and the image at the time of color image formation and the image at the time of monochromatic image formation involving only the image forming station located at the uppermost stream of the sheet conveying belt. The density difference can be eliminated.

(29) At the time of single-color image formation, since the facing distance between the shield electrodes of the chargers of the image forming stations located on the most upstream side of the paper transport belt is set wider than that at the time of color image formation, the photoconductor is initialized. The charging potential Vo moves to the white background side, and when the color image is formed and when the monochrome image is formed,
The image density difference of the toner color at the image forming station located on the most upstream side is reduced, and uniform image formation can be performed.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a general configuration of a conventional single-drum type color image forming apparatus.

FIG. 2 is an explanatory diagram of a general configuration of a conventional tandem type color image forming apparatus.

FIG. 3 is a configuration diagram of a main part of the first embodiment of the present invention.

FIG. 4 is a graph showing the relationship between the spacing between the screen grid and discharge electrodes and the surface potential of the photoconductor.

FIG. 5 is a graph showing how the surface potential of the photoconductor changes.

FIG. 6 is a graph showing the relationship between the image density and the distance between the screen grid and the discharge electrodes.

FIG. 7 is a control system diagram for driving the discharge electrode.

FIG. 8 is a perspective view showing an internal structure of the charger.

FIG. 9 is an explanatory diagram of a configuration of the tandem-type multicolor image forming apparatus.

FIG. 10 is a configuration diagram of main parts of a second embodiment of the present invention.

FIG. 11 is a graph showing the relationship between the aperture ratio of the screen grid and the surface potential of the photoconductor.

FIG. 12 is a graph showing the relationship between the screen grid aperture ratio and image density.

FIG. 13 is an explanatory diagram of a mounting structure of the screen grid.

FIG. 14 is an explanatory diagram of a stretched state of the screen grid.

FIG. 15 is a control system diagram for changing the aperture ratio of the screen grid.

FIG. 16 is a configuration diagram of a main part of a third embodiment of the present invention.

FIG. 17 is a plan view showing an example of the grid pattern.

FIG. 18 is an explanatory diagram of a changing state of the aperture ratio of the screen grid in which the two sheets are stacked.

FIG. 19 is a sectional view of a position adjusting mechanism of the screen grid.

FIG. 20 is an explanatory diagram of different patterns of the same screen grid.

FIG. 21 is an explanatory diagram of another pattern of the screen grid.

FIG. 22 is a configuration diagram of main parts of a fourth embodiment of the present invention.

FIG. 23 is a graph showing the relationship between the shield electrode case width and the photoconductor potential.

FIG. 24 is a graph showing the relationship between the shield electrode case width and the image density.

FIG. 25 is a main part configuration diagram of the shield electrode.

FIG. 26 is an explanatory diagram of a mechanism for adjusting the facing distance of the shield electrodes.

[Explanation of symbols]

(B) (C) (M) (Y) -Image forming station 1B-Charger 6B-photoreceptor 7-Paper transport belt 12-screen grid 13-discharge electrode 14-Transportation means 19a, 19b-shield electrode 49-Transportation means 53-Drive means 74-Control means 84-Drive means 121a, 122a-pores d-opposing distance

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 15/16 103 G03G 15/16 103 21/00 384 21/00 384 21/14 372 (72) Inventor Tomoe Matsuoka 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Prefecture (72) Inventor Eiji Saikou 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka (72) Inventor Shoichi Fujita Osaka 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Prefecture (72) Inventor Hiroshi Sakita 22-22, Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture (72) Inventor Masatsugu Nakamura Osaka-shi, Osaka 22-22 Nagaike-cho, Abeno-ku, Sharp Corporation (72) Inventor Mitsuru Tokuyama 22-22, Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture (72) Akio Tanimura Mihoko 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka F-term (reference) within Sharp Corporation 2H027 EA01 EC06 ED03 ED24 ED30 EE02 EF09 FA28 FA37 2H030 AA03 AB02 AD02 AD07 AD08 AD17 BB13 BB23 BB34 BB34 BB44 BB44 BB44 FA19 GA12 GA23 GA34 GA44 GA47 GB12 GB22 HA12 HA29 HB03 HB14 HB21 HB26 HB29 HB33 HB48 JA02 JB06 JB23 JB29 JB37 NA02 PA03 PA10 PA12 PA13 PA23 PA26

Claims (29)

[Claims] 1. A plurality of image forming stations centering around a photoconductor are arranged along the paper transport direction, and each of the image forming stations is formed while the paper is transported by the paper transport belt in the paper transport direction. A color image can be formed by sequentially superimposing color images to form one image, and when forming a single-color image, the image is formed by partially using the image forming station on the upstream side in the paper conveyance direction. In the multi-color image forming apparatus in which the paper transport belt can be switched to the retracted state (separation / contact can be performed) from the image forming station on the downstream side that is not involved in image formation, Initialization charging of the photoconductor of the most upstream image forming station during single-color image formation using only the most upstream image forming station Multi-color image forming apparatus characterized by switching the position Vo. 2. The charging of the most upstream image forming station is performed by switching the initialization charging potential with respect to the photoconductor of the most upstream image forming station along the paper transport belt during color image formation and single color image formation. The multi-color image forming apparatus according to claim 1, wherein the multi-color image forming apparatus is performed by changing a facing distance between the screen grid of the container and the discharge electrode. 3. The multicolor image forming apparatus according to claim 2, wherein the facing distance between the screen grid and the discharge electrode is changed by a moving unit that moves the discharge electrode so as to move toward and away from the photoconductor. . 4. A grid and a discharge electrode in an image forming station arranged at the most upstream side of a sheet conveying belt during color image formation, with the same image exposure amount and developing bias when forming a color image. 4. The multicolor image forming apparatus according to claim 2, wherein the facing interval with respect to is set to be wider than that during single-color image formation. 5. The charging of the most upstream image forming station is performed by switching the initialization charging potential for the photoconductor of the most upstream image forming station along the paper transport belt during color image formation and single color image formation. The multicolor image forming apparatus according to claim 1, wherein the screen grid of the container is expanded and contracted in the longitudinal direction. 6. The multicolor image forming apparatus according to claim 5, wherein the initialization charging potential is switched by changing the expansion / contraction state of the screen grid of the charger. 7. A multicolor image forming apparatus according to claim 6, further comprising a control unit and a driving unit for adjusting the expansion / contraction state of the screen grid of the charger of the most upstream image forming station along the paper transport belt. apparatus. 8. The screen grid of a charger of an image forming station disposed at the uppermost stream of the paper transport belt is stretched during monochromatic image formation, and the screen grid is contracted during color image formation. The multicolor image forming apparatus described in 1. 9. The charging of the most upstream image forming station is performed by switching the initialization charging potential to the photoconductor of the most upstream image forming station along the paper transport belt during color image formation and single color image formation. The multicolor image forming apparatus according to claim 5, wherein the multicolor image forming apparatus is performed by changing the aperture ratio of the screen grid of the container. 10. The apparatus according to claim 9, further comprising control means and drive means for adjusting the aperture ratio of the screen grid of the charger of the most upstream image forming station along the paper transport belt. Color image forming apparatus. 11. When a monochromatic image is formed, the aperture ratio of the screen grid of the charger of the image forming station disposed on the most upstream side of the paper transport belt is set higher than the aperture ratio during color image formation. The multicolor image forming apparatus according to claim 9. 12. The charging of the most upstream image forming station is performed by switching the initialization charging potential to the photoconductor of the most upstream image forming station along the paper transport belt during color image formation and single color image formation. The multi-color image forming apparatus according to claim 5, wherein the multi-color image forming apparatus is performed by changing a longitudinal tension applied to both ends of the screen grid of the container. 13. A control means and a drive means for adjusting the tension applied to both ends of the screen grid of the charger of the most upstream image forming station along the sheet conveying belt in the longitudinal direction so that the tension can be increased or decreased. Claim 12
The multicolor image forming apparatus described in 1. 14. The tension in the longitudinal direction applied to both ends of the screen grid of the charger of the image forming station arranged at the uppermost stream of the paper transport belt during monochromatic image formation is set higher than the tension applied during color image formation. The multicolor image forming apparatus according to claim 15, wherein the multicolor image forming apparatus comprises: 15. A mesh-shaped grid electrode having a multiple structure is provided on the main surface of the charger of the image forming station arranged in the most upstream to switch the initialization charging potential Vo of the photoconductor, and all image forming stations are provided. A means for relatively moving at least one of the grid electrodes with respect to the other when forming a color image using the image forming apparatus and when forming a single color image using only the image forming station located at the uppermost stream. The multicolor image forming apparatus according to claim 1. 16. The charging device of the image forming station arranged in the uppermost stream is provided with a plurality of grid electrode plates having a plurality of pores in order to switch the initialization charging potential Vo of the photoconductor. At least one of the grid electrode plates of the charger is placed between the grid electrode plates at the time of color image formation using the image forming station and at the time of monochromatic image formation using only the most upstream image forming station. The multicolor image forming apparatus according to claim 1, wherein the multi-color image forming apparatus is moved while changing the overlapping area of the pores. 17. The initialization charging potential Vo is switched to the photoconductor of the most upstream image forming station along the paper transport belt during color image formation and monochromatic image formation.
17. The multicolor image forming apparatus according to claim 15, wherein the multicolor image forming apparatus is performed by changing the aperture ratio between the grid electrode plates of the charger of the most upstream image forming station. 18. The grid electrode plate position adjusting means for adjusting the aperture ratio of the grid electrode plate of the charger of the most upstream image forming station along the paper transport belt so as to be adjustable. Multi-color image forming apparatus. 19. The control means and the drive means for adjusting the aperture ratio between the grid electrode plates of the charger of the most upstream image forming station along the paper transport belt so that the aperture ratio can be increased or decreased. Multi-color image forming apparatus. 20. When a monochromatic image is formed, the aperture ratio between the grid electrode plates of the charger of the image forming station disposed at the uppermost stream of the paper transport belt is set higher than the aperture ratio during color image formation. The multicolor image forming apparatus according to claim 17. 21. The charging of the most upstream image forming station is performed by switching the initialization charging potential to the photoconductor of the most upstream image forming station along the paper transport belt during color image formation and single color image formation. The multi-color image forming apparatus according to claim 1, wherein the multi-color image forming apparatus is performed by changing a facing distance between shield electrodes of the container. 22. When a monochromatic image is formed, only the image forming station located on the most upstream side of the paper carrying belt is used, and the paper carrying belt is retracted from other unused image forming stations located on the downstream side. 22. The multicolor image forming apparatus according to claim 21, wherein the multicolor image forming apparatus is configured to form an image. 23. The initializing charging potential is set to a potential on a non-white background side when a color image is formed, as compared with when a monochromatic image is formed. The multicolor image forming apparatus described in 1. 24. The image forming station located on the most upstream side of the paper transport belt is provided with a moving means for adjusting the facing distance of the shield electrodes of the charger. Multicolor image forming apparatus. 25. When a monochromatic image is formed, the facing distance between the shield electrodes of the chargers of the image forming stations located on the most upstream side of the paper transport belt is set wider than that during color image formation. The multicolor image forming apparatus described in 1. 26. The image forming station located on the most upstream side is an image forming station provided with black toner, according to claim 1, 5, 15, 16, 21 to 25. Multicolor image forming apparatus. 27. A plurality of image forming stations provided with a charging device, a writing optical system, a developing device and a cleaning device around a photoconductor, and a paper carrying belt for carrying a transfer material on which an image is formed. During monochromatic image formation, only the image forming station located on the most upstream side of the paper carrying belt is used, and the paper carrying belt is retracted to the other unused image forming stations located on the downstream side to form an image. In a method of controlling a multi-color image forming apparatus, a facing interval of shield electrodes of a charger provided in the most upstream image forming station is changed between forming a color image and forming a single color image, whereby A method for controlling a multicolor image forming apparatus, which comprises adjusting an initialization charging potential. 28. The method of controlling a multicolor image forming apparatus according to claim 27, wherein the initialization charging potential is set to a potential on a non-white background side when forming a color image as compared to when forming a single color image. 29. When a monochromatic image is formed, the facing distance between the shield electrodes of the chargers of the image forming stations located on the most upstream side of the paper transport belt is set wider than that during color image formation. 5. A method for controlling a multicolor image forming apparatus according to item 1.