JPH10142881A - Multicolor image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high-quality image without causing the irregularity of an image formed to be developed in advance without depending on the kind of a photoreceptor and the ability of a reelecttifier by specifying the relation of the exposure of a non-image part and an image part in a state where a toner image is not formed and in a state where it is formed. SOLUTION: After the multicolor toner image is formed by repeating the digital exposure of every pixel and development with toner having mutually different color, it is transferred altogether. The 1st exposure in pixel units at the non-image part and the image part in the state where the toner image is not formed, and the 2nd exposure in pixel units at the non-image part and the image part in the state where the toner image is formed are provided. The 2nd exposure in pixel units is larger than the 1st exposure both at the non-image part and the image part, and the difference of the exposure in pixel units at the non-image part is nearly equal to that at the image part. Thus, the irregularity of the image formed in advance is not caused at the second and succeeding developing time and trouble such as density thinning and reverse fogging is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a laser beam printer and an electrostatic recording apparatus, and more particularly to a multicolor image forming apparatus capable of performing multicolor printing.

[0002]

2. Description of the Related Art Recent images include visible images of different colors,
In many cases, different information is synthesized and formed on one sheet of paper, and an image forming apparatus having a plurality of developing devices housed in advance is on the market.

In particular, there are many techniques that disclose a technique of developing a multicolor image with two or more developing devices during one rotation of a latent image carrier and simultaneously transferring the image to paper. For example, U.S. Pat. No. 4,572,651 which discloses a technique for performing development while keeping an electric field constant with a DC bias in both developing devices.
No. 4,416,533. These documents mainly disclose a method for forming a latent image and do not suggest a problem during development.

On the other hand, US Pat. No. 4,349,268 and Japanese Patent Application Laid-Open No. 56-1444 published earlier in Japan.
In Japanese Patent Application Laid-Open No. 52-125, an AC developing bias is used in non-contact development for the second color development, and in Japanese Patent Application Laid-Open No. 56-12650, a DC bias is used in the non-contact development, and the developed image of the first color is developed for the second color. There is disclosed a technique for preventing the material from being disturbed by rubbing with an agent. Incidentally, Japanese Patent Application Laid-Open No. 56-144452
In the publication, there is no description about the potential of the developed image of the first color.

As described above, in the conventional multi-developed image forming apparatus, a technique for performing the next development so as not to disturb the previously developed image has been known.

Similarly, US Pat. No. 4,660,961 discloses a technique for increasing the potential of a latent image of a previously developed image. By uniformly charging the latent image carrier with the same polarity as the developer, the latent image potential of the developed image of the first color can be made substantially the same as that of the non-developed portion, so that the development speed of the second color is greatly improved. It was possible to prevent the image formed on the sample from being disturbed. This has been actively studied in recent years in a one-pass multicolor printing image forming apparatus, particularly in a technique called a negative-negative recharging method.

In recent years, particularly in the field of high-speed machines, a multicolor image forming apparatus using a highly durable amorphous silicon as a photosensitive member has been studied.

[0008]

However, the above-mentioned conventional example has the following problems.

Depending on the type of photoreceptor, its electrostatic capacity is so large that it cannot be charged sufficiently by recharging. In some cases, it is not possible to provide a recharger having a sufficient capacity due to restrictions on miniaturization and cost of the apparatus. As described above, the latent image potential of the developed image of the first color cannot be made substantially the same as that of the non-developed portion even in the negative recharging process described above due to the photoreceptor and device configuration conditions. Problems such as disturbing the formed image and low density and fogging in the second developed image have occurred.

FIG. 11 illustrates an example of a conventional image forming process, in which (1) to (6) show each step, and each figure schematically shows the surface potential of the photosensitive member.

In (1), the photoreceptor is charged with a corona charger.
For example, it is charged to + 400V. Next, in (2), a first exposure is performed on the photoreceptor based on the first image signal, and the surface potential of the exposed portion is attenuated to, for example, +50 V at the maximum to form a first electrostatic latent image.

Next, in (3), a developing bias voltage (for example, +300 V; indicated by a broken line) is applied to the sleeve of the first developing device to reversely develop the exposed portion.

After the first development, recharging is performed in (4). As described above, depending on the structure of the photoreceptor and the charger, the first developed image area is suppressed while the potential rise of the first non-image area is suppressed. Can not be given a sufficient charge.
It can only be charged to 50V. Therefore, in (5), exposure corresponding to the second image information is performed, and in (6), the developing bias voltage (for example, +300) is applied to the sleeve of the second developing device.
V: indicated by a broken line), the first development is performed.
The second developer is also developed in the developing section, and color mixing occurs.

If the bias voltage of the second developing device is set to, for example, +200 V in order to prevent color mixing, a sufficient second developing density cannot be obtained, and the potential difference between the developing bias and the first developed non-image area is also 200 V. Therefore, the reversal toner having the polarity opposite to the desired polarity is developed.

Further, toner of a desired polarity is developed unless the photosensitive member potential is higher than the developing bias potential by about 100 V. For this reason, in this example, the potential of the second developing non-image portion in both the first developing portion and the first non-developing portion is higher than the developing bias by 100.
It is desirable to set the voltage to 400 V, which is higher by V. That is, after recharging,
It is necessary to converge the drum surface potential at the second development position between the first developed non-image portion and the first developed image portion.

As a means for solving the above problem, it is conceivable to make the structure and bias of the recharger appropriate. FIG. 12 is a graph of experimental results showing the current applied to the recharger corotron wire and the drum surface potential of the first developed non-image portion and the first developed image portion after recharging. From this figure, it can be seen that it is difficult for the corotron recharger to converge the first developed non-image portion and the first developed image portion (hereinafter referred to as recharge convergence).

Next, it is conceivable to use a scorotron charger as the recharger, reduce the drum direction current flowing in the first developed non-image area, and secure the drum direction current flowing in the first developed image area. FIG. 13 is a graph of experimental results showing the voltage applied to the grid and the drum surface potential of the first developed non-image portion and the first developed image portion after recharging.

As can be seen from the figure, when the voltage applied to the grid is substantially the same as that of the first developed non-image area, the first developed image area is kept in a state where the potential of the first developed non-image area after recharging is suppressed. It can be seen that the toner can be charged most and the convergence is improved. However, with such a grid voltage, a large amount of current in the drum direction is required to sufficiently increase the potential of the first developed image area after recharging. It would be very disadvantageous in terms of space. These disadvantages are even more disadvantageous in an amorphous silicon drum having a large relative dielectric constant and being difficult to be charged.

As a further alternative, after the first development, the second
A technique has been proposed in which a process before exposure is uniformly exposed to make the latent image potential of the first color developed image substantially the same as that of the non-developed portion. However, the above technique is disadvantageous in terms of miniaturization and cost of the apparatus because a uniform exposure means must be provided in the process after the first development and before the second exposure.

Accordingly, an object of the present invention is to disturb previously developed images during the second and subsequent developments, and to prevent the second and subsequent developed images from being produced, irrespective of the type of photoreceptor and the capability of the recharger. An object of the present invention is to provide a small-sized and low-cost multicolor image forming apparatus which does not have problems such as low density and reversal fog.

[0021]

The above object is achieved by a multicolor image forming apparatus according to the present invention. In summary, the present invention provides a method for charging an electrostatic latent image carrier, digital exposure for each pixel,
And repeatedly developing toners of different colors from each other,
After forming a multicolor toner image on the electrostatic latent image carrier,
In a multicolor image forming apparatus that performs a batch transfer onto a transfer material, an exposure unit that performs digital exposure exposes the electrostatic latent image carrier in a state where a toner image is not formed on a first non-pixel basis. An image portion and an image portion exposure amount, and a second non-image portion and an image portion exposure amount in pixel units for exposing the electrostatic latent image carrier in a state where a toner image of at least one color has already been formed. And the second exposure amount in the pixel unit of the non-image portion is larger than the first exposure amount, and the second exposure amount in the pixel unit of the image portion is larger than the first exposure amount in the pixel unit. A multicolor image forming apparatus characterized in that the difference in the amount of exposure in pixel units of the non-image portion is substantially equal to the difference in the amount of exposure in pixel units of the image portion. is there.

At the time of charging in the state where the toner image of at least one color is formed, the toner image is formed based on the toner image surface potential of the portion where the toner image of at least one color is already formed. It is preferable to charge the photosensitive member so that the absolute value of the potential is larger in the photosensitive member surface potential in the non-existing portion.

At the positions after the second charging and before the exposure,
It is preferable that charging is performed such that the surface potential of the portion where the toner image is not formed is larger than the surface potential of the portion where the toner image of at least one color or more is formed.

In the multicolor image formation, a plurality of color toners can be superposed in each pixel. It is preferable that the photosensitive layer of the electrostatic latent image carrier be made of a material having a relative dielectric constant ε of 8 or more. It is preferable that the electrostatic latent image carrier uses amorphous silicon for its photosensitive layer. The charger used for the recharging is preferably a corotron.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a multicolor image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

Embodiment 1 FIG. 1 shows an example of a two-color image forming apparatus according to the present invention. The photoconductor (photosensitive drum) 1 is provided with a photoconductive layer on a cylindrical conductive substrate, and is rotatably supported in the direction of arrow R1 in the figure. And, around the photosensitive drum 1,
A first scorotron charger 2, which uniformly charges the surface of the photosensitive drum 1 in order along the rotation direction, reads a document, and based on a first image signal proportional to the density of one color image divided into two colors. Exposing the photosensitive drum 1 with a laser beam 3 to form a first electrostatic latent image, and a first developing device 4 for forming a first toner image by attaching toner to the first electrostatic latent image. Is arranged.

A second scorotron charger (hereinafter, referred to as a recharger) 5 for charging the photosensitive drum 1 after carrying the first toner image, a second image proportional to the density of the other color image separated Exposure with laser light 6 based on the signal,
A second exposure means for forming a second electrostatic latent image and a second developing device 7 for forming a second toner image by attaching toner to the second electrostatic latent image are arranged.

Further, a corona transfer charger 8 for transferring the color superimposed image formed on the photosensitive drum 1 onto a transfer paper P as a transfer material, and a transfer paper P on which the color superimposed image is transferred to the photosensitive drum 1
And a cleaning device 13 for removing residual toner on the photosensitive drum 1 after transferring a color superimposed image, a pre-exposure (lamp) 30 for removing residual charges on the photosensitive drum 1, and the like. Have been.

The transfer paper P on which the color superimposed image has been transferred is
After being separated from the photosensitive drum 1, the sheet is conveyed to a fixing device 12, where a toner image on the surface is fixed, a desired print image is formed, and the image is discharged outside the image forming apparatus main body.

The image scanner section 18 scans the original 15 placed on the original glass table 14 by scanning with an illumination lamp 16 and converts image information into an electric signal by a photoelectric conversion element 19. The reflected light from the document scanned by the mirrors 17a, 17b, 17
c, led by the lens 17d, red, green,
An image is formed on the photoelectric conversion element 19 having a built-in blue filter.

The electrical signals output by the photoelectric conversion element 19 for each of the red, green, and blue components are A / A
After being digitized by the D converter 21, it is sent to a signal processing unit 22 as a color separation unit, and is converted into an image signal proportional to the image density of each of the red and black components.

The red image signal (first image signal) and the black image signal (second image signal) are sent to a laser driver 24 as a signal generator, and a laser is generated according to the red and black image signals. 20 light emission is turned on / off. The laser light 3 emitted according to the red signal writes a first electrostatic latent image on the photosensitive drum 1 via the polygon mirror 28 and the mirror 17e as first image information, and adds a certain amount to the amount corresponding to the black signal. The emitted laser light 6 is used as second image information as polygon mirror 28 and mirror 17.
f, a second electrostatic latent image is written on the photosensitive drum 1 via 17g.

In this embodiment, an amorphous silicon drum is used as the photosensitive drum 1. This is because the amorphous silicon drum has features such as high durability and long life.
However, in the case of amorphous silicon drum, OPC
In particular, in the negative recharge type one-pass two-color image forming apparatus as in this embodiment, the latent image potential of the developed image of the first color is recharged to substantially the same potential as that of the non-developed part. (Hereinafter referred to as recharge convergence).

In order to improve the recharge convergence, in the negative recharge system, it is necessary to charge more in the first image developing section than in the non-image first image section, that is, to apply more drum direction current. There is. The corona current discharged from the recharge discharge wire is divided into a drum direction current flowing in the drum direction and a shield direction current flowing in the charger shield. When the corona current is constant, it is considered that the ratio of the current in the drum direction to the current in the shield direction depends on the resistance in each direction. Although the resistance in the shield direction does not usually change, the resistance in the drum direction changes depending on the surface potential and the presence / absence of toner. That is, since the current in the drum direction at the time of recharging changes depending on the state of the first development, the charging potential also becomes the first potential.
It changes depending on the state of development.

FIG. 6 shows experimental results showing the relationship between the first development state and the current in the drum direction. 400 V and 150 V, respectively, assuming the potential on the toner in the first developed non-image area and the image area.
V and 150 V without toner were compared. First, 4
Comparing 00V and 150V without toner, 150V having a smaller drum surface potential causes more drum current to flow for the same corona current, and the ratio is (40%).
0V) :( 150V) ≒ 4.5: 5.5.

Next, comparing the surface potential and the presence or absence of toner, a comparison is made between the presence of toner and the presence of toner. In the case of the same corona current, a smaller current in the drum direction flows, and the ratio is (no toner): (no toner) ) ≒ 6: 4. That is, in the negative recharge system, a larger amount of current flows in the drum direction from the point of surface potential to the first developed image portion (eg, 150 V on the toner) than the first developed non-image portion (eg, 400 V), and This is a direction that is advantageous for convergence, but is a direction that is disadvantageous for recharge convergence from the viewpoint of the presence or absence of toner.

The total recharge convergence is determined by both of these factors. In the present experimental system, the graph shows (first developed non-image part) :( first developed image part) ≒ 5.5:
It is 4.5 and it can be seen that it does not converge. From the graph, it is assumed that when more corona current is applied, this relationship is reversed and eventually converges.However, a larger charger is required, which is very disadvantageous in terms of cost and body space. It is considered to be.

Although these relations are slightly different depending on the kind, amount or potential setting of the toner, it can be said that these relations are fundamental problems in the negative recharge system.

As a means for solving this problem, there is a method using a recharger having a high charging ability. However, they are not only expensive but also require more space and a large power supply. Not efficient.

Further, after the recharging, before the second exposure, another exposure means is provided, and the latent image potential of the developed image of the first color can be made substantially the same as the non-developed portion by performing uniform exposure. Although it is conceivable, this method also requires another exposure means,
It was not efficient in terms of cost and body space.

In this embodiment, attention is paid to the second exposure means. That is, by providing the role of the uniform exposure unit after the recharging as described above to the second exposure unit, a new member is not used, and a multicolor image forming apparatus advantageous in cost and space is provided. can do. Further, since it is not necessary to converge immediately after the recharging, the recharging can be performed without difficulty, and the recharger itself can be reduced in cost, space, and current and voltage applied to the recharger.

FIG. 2 illustrates an image forming process in the two-color image forming mode of the present embodiment. (1) to (6) show each step, and in each figure, the surface potential of the photosensitive member is schematically shown. Is shown.

In (1), the photoreceptor is charged to, for example, +400 V by a corona charger. Then, in (2), the first exposure of image information is performed, and the surface potential of the exposed portion is attenuated to, for example, +50 V. One electrostatic latent image is formed.

Next, in (3), a developing bias voltage (for example, +300 V: indicated by a broken line) is applied to the sleeve of the first developing device to reversely develop the exposed portion. After the first development (4), recharging is performed. 600 V which is larger than the desired second development position target potential of 400 V is applied to the grid, and the first development non-image portion is controlled to be charged to, for example, 600 V. At that time, the first developing unit is charged to, for example, 500V. The grounds and method for setting this potential will be described later.

Next, when performing the exposure according to the second image information in (5), a constant exposure amount (for example, the first developed non-image area is attenuated by 200 V) over the entire surface as compared with the second developed single color. Exposure is large). At this time, in the first developing section, the exposure for the fixed exposure amount does not attenuate as much as the potential decay in the first developing non-image section, and for example, attenuates by only 100 V. This is because the first developer scatters the light without transmitting it, and its transmittance was 50%.

The grounds and method for setting the recharging potential in the step (4) as described above will be described with reference to FIG.
The surface potential after the second exposure is fixedly added and the exposure amount is 0.25 μJ. The surface potential after exposure is the second development position target potential of 400 V. The first development non-image portion recharged target potential is a known drum sensitivity of 800 V.
It was 600 V assuming a straight line of / μJ. Next, also from the known toner layer transmittance of 50%, the amount of light reaching the drum to the first image developing unit is 0.125 μJ. From this, it is understood that the target potential after recharging the first developed image portion may be set to 500 V in the same manner as in the method described above.

In this embodiment, a semiconductor laser is used as the second exposure means. However, no complicated processing is required between the second developing single-color mode and the two-color mode. As shown in FIG. 4, the amount of laser light is determined by the laser drive current. Therefore, in the two-color mode, the second exposure is achieved by adding a constant offset current ΔI to the drive current in the second developing single-color mode.

That is, the second image signal is also weakly exposed to the off portion, and the on portion is also exposed to an additional amount of exposure substantially equivalent thereto, so that the potential of the first developed image portion is 4
00V, the potential of the first developed non-image portion is also 400 V,
When the second image signal is on, the first developed non-image portion is 50
Exposure to V FIGS. 5A and 5B show the relationship between the drum surface potential and the exposure amount in the second exposure step. (A) is a conventional example, and (b) is a present example.

Thereafter, as shown in FIG. 2 (6), a reverse development was performed by applying a bias of 300 V to the second developing sleeve in the second developing step.

It is preferable to apply a photosensitive member having a relative permittivity ε of 8 or more to the above-described apparatus, and the effect is remarkable. In addition, the effect is obtained when amorphous silicon is used for the photosensitive member. Is big.

As described above, according to the present embodiment, regardless of the type of the photoreceptor and the ability of the recharger, the second developer can be mixed into the first developing unit and the first and second non-image The second image density can be obtained without being developed in the
A compact, low-cost two-color image forming apparatus was obtained.

Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIGS. In this embodiment, the present invention is applied to the first developer and the second developer.
This is an example corresponding to an image forming method of developing the developer at the same location to obtain a third color.

Even if the first developed image area is exposed in the same manner as in Example 1, the surface potential cannot be sufficiently lowered from the viewpoint of the permeability of the first developed toner, and the second developed density is not sufficient. Was enough. Therefore, in the present embodiment, regardless of ON / OFF of the second exposure, the exposure amount that is weak on the entire surface is added,
The exposure amount when the second exposure was turned on was increased so that the potential of the first developed image portion was reduced to 50 V.

At this time, a larger amount of light is applied to the drum surface in the first developed non-image area and the second signal ON area, but the amorphous silicon photosensitive member used in this embodiment is more exposed than the OPC photosensitive member. Since the characteristic of the amount vs. potential hardly changes above a certain exposure amount as shown in FIG. 7, the potential of the first non-image portion and second exposure portion was 45 V, which was almost the same as that of the first developing portion and the second exposure portion. .

Although the difference in the density of the second development is almost no problem even with such a difference, for example, as shown in FIG. 8, the relationship of the density with respect to the development contrast potential is different from the potential of the first development and second exposure part. Further improvement can be achieved by setting the second developing device so as to exhibit a characteristic that is hardly affected by the potential difference of the first non-image second exposure unit. This development characteristic can be changed by, for example, changing the waveform of the development bias or the rotation speed of the development sleeve.

As a result, the first image second non-exposed portion is
V, the first non-image portion, the second non-exposure portion can be 400 V, the first non-image second exposure portion can be 45 V, and the first image, second exposure portion can be 50 V, which is applied to the sleeve of the second developing device. By setting the bias voltage to 300 V, the second developer can
A two-color image formation that obtains a sufficient second image density without being mixed into the second developing unit and is not developed into the second image non-image part, and also obtains a sufficient density even when two colors are superimposed. did it.

Third Embodiment Next, a third embodiment of the present invention will be described with reference to FIGS.

In this embodiment, a corotron charger was used as the recharger instead of the scorotron charger. Since the corotron charger does not have a grid, the structure is simple and the cost is easy, and the charge generated by the discharge wire does not flow into the grid, so that the charging efficiency is good. FIG. 9 is a graph showing the relationship between the wire applied current and the drum surface potential of the corotron charger and the scorotron charger under the same conditions.
FIG. 10 is a graph showing the relationship between the wire applied voltage and the drum surface potential. From these results, it can be seen that using the corotron charger requires only a small current and voltage to obtain the same drum surface potential, that is, the charging efficiency is better.

However, the corotron charger shown in FIG.
As shown in (1), since there is no potential convergence effect, the potential of the first non-image portion is further increased if the first developing unit is charged to a desired potential during recharging. Not suitable.

However, in the method of obtaining a good two-color image without converging the potentials of the first developing section and the first non-image section immediately after recharging as in the present invention, the recharger is a corotron charger. It may be.

The image forming process is the same as that of the first embodiment, and therefore will not be described. It is necessary to change the setting and the second exposure setting.

As an example, the first developed non-image portion is, for example, 80
Control is performed so as to be charged to 0V. At that time, the first developing section
It is charged to 00V. Next, when performing exposure in accordance with the second image information, a larger exposure is performed over the entire surface by a fixed exposure amount (for example, an exposure amount that attenuates the first developed non-image portion by 400 V). At this time, in the first developing section, the exposure for the fixed exposure amount does not attenuate as much as the potential decay in the first developing non-image section, and for example, only 200 V attenuates. This is because the first developer scatters light without transmitting it.

As a result, even when the second image signal is off, the exposure is performed, and the potential of the first developed image portion becomes 400 V and the potential of the first developed non-image portion becomes 400 V. Further, when the second image signal is on, the first developed non-image portion is exposed to 50V. By applying a bias of 300 V to the second developing sleeve in the subsequent developing step, the second developer is prevented from being mixed into the first developing portion and being not developed into the first and second non-image portions. Thus, a two-color image was formed to obtain a sufficient second image density.

A good two-color image could be obtained by using a corotron charger having a simple structure, a low cost, and a good charging efficiency as in this embodiment.

Although the present invention has been described with reference to the above embodiments, the present invention can of course be applied to configurations other than those described in the above embodiments.

For example, in the above embodiment, the positively charged aS
Although the description has been made with reference to the reversal development of the positive polarity toner using the i (amorphous silicon)
A drum may be used. Since the OPC drum is normally used with negative charging, in this case, the toner may be reversely developed using a negative toner.

Alternatively, with regard to the superposition of the toner images of the respective colors according to the present invention, the second toner image is superimposed on the first toner image within each pixel. A mechanism for registering the toner image is required. In order to simplify this, superposition of two colors in each pixel is not controlled, and the image processing side responds by setting the red image and the black image in a non-mixed state microscopically, Alternatively, it may be possible to create a shade of an image with an area gradation or a desired red-black mixed color image.

Although the pre-transfer charger for adjusting the charge amounts of the two-color toner images on the drum before the transfer was not described in detail, AC or D using a corotron or scorotron was used.
It is known that the charge amount is controlled by C. Of course, it is also known that attenuating the photoreceptor potential in combination with pre-transfer exposure (not shown) is effective for transfer separation of a two-color image, which is a subsequent step.

In the pre-exposure before the charge is removed from the photosensitive member at the end of the process, it is necessary to appropriately set the wavelength, intensity and the like so as to prevent the ghost of the previous image.

Although the above embodiment has been described with reference to a two-color image forming apparatus, the present invention can be applied to a multi-color image forming apparatus.

Further, in this embodiment, a two-color toner image is formed on the photoconductor by one rotation of the photoconductor. However, in an image forming apparatus for forming a plurality of toner images on the photoconductor by multiple rotations of the photoconductor. Can also be applied.

In any case, those skilled in the art can easily infer that these can be used in combination with the present invention.

[0073]

As is apparent from the above description, according to the present invention, the exposure means for performing digital exposure exposes the electrostatic latent image carrier on which no toner image has been formed. A first non-image part and an image part exposure amount of the unit;
A second pixel-by-pixel exposure that exposes the electrostatic latent image carrier in a state where at least one color toner image is already formed.
The second exposure amount in pixel units of the non-image portion is larger than the first exposure amount, and the second exposure amount in pixel units of the image portion is The first exposure amount is larger than the first exposure amount, and the difference in the exposure amount in pixel units in the non-image area is substantially equal to the difference in the exposure amount in pixel units in the image area. Regardless of the power of the charger,
During the second and subsequent developments, the previously developed image may be disturbed,
A small-sized, low-cost multicolor image forming apparatus capable of obtaining a high-quality image free from problems such as low density and reversal fog in the second and subsequent developed images can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of a two-color image forming apparatus according to the present invention.

FIG. 2 is a schematic diagram illustrating a state of a drum surface potential in a two-color image forming process according to the first exemplary embodiment.

FIG. 3 is a graph for explaining the basis and method for determining a target potential after recharging in Example 1.

FIG. 4 is a graph showing a relationship between a driving current and a light amount of a second image exposure laser of Example 1.

FIGS. 5A and 5B are schematic diagrams of a conventional example (a) and an example 1 (b) showing a relationship between a drum surface potential and a second image exposure amount in a second image exposure step of a two-color image forming process.

FIG. 6 is a graph showing an experimental result showing a relationship between a first developing state and a current in a drum direction.

FIG. 7 is a graph showing the relationship between the exposure amount of the amorphous silicon photoconductor and the OPC photoconductor used in Example 2 and the photoconductor surface potential.

FIG. 8 is a graph showing the relationship between the development contrast potential and the density of the second developing device used in Example 2.

FIG. 9 is a graph showing a relationship between a wire applied current and a drum surface potential of a corotron charger and a scorotron charger used in Example 3.

FIG. 10 is a graph showing a relationship between a drum surface potential and a wire applied voltage of a corotron charger and a scorotron charger used in Example 3.

FIG. 11 is a schematic diagram showing a state of a drum surface potential in a conventional two-color image forming process.

FIG. 12 is a graph showing an experimental result of a drum surface potential with respect to a current applied to a wire of a corotron recharger in a conventional two-color image forming process.

FIG. 13 is a graph showing experimental results of a drum surface potential with respect to a voltage applied to a scorotron recharger grid in a conventional two-color image forming process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum (electrostatic latent image carrier) 2 First scorotron charger 3 First exposure 4 First developing device 5 Second charger 6 Second exposure 7 Second developing device 20 Laser

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI G03G 15/08 503 B41J 3/00 B (72) Inventor Takao Honda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. Inside

Claims (7)

[Claims] 1. A method for forming a multicolor toner image on an electrostatic latent image carrier by repeatedly charging an electrostatic latent image carrier, digitally exposing each pixel, and developing toners of different colors from each other. A multicolor image forming apparatus that performs a batch transfer onto a transfer material, wherein the exposing means for performing digital exposure exposes the electrostatic latent image carrier in a state where a toner image is not formed on a first-per-pixel basis. Exposure amount of non-image portion and image portion, and exposure of second non-image portion and image portion in pixel units for exposing the electrostatic latent image carrier in a state where a toner image of at least one color has already been formed. And a second pixel unit of the non-image portion.
Is larger than the first exposure amount, the second exposure amount in the pixel unit of the image unit is larger than the first exposure amount in the pixel unit, and the difference between the exposure amount in the pixel unit of the non-image unit and A multicolor image forming apparatus, wherein a difference between exposure amounts in pixel units of the image portion is substantially equal. 2. The method according to claim 1, wherein when the latent image carrier on which the at least one color toner image is formed is charged, a potential of the toner image surface of a portion where the at least one color toner image has already been formed is determined. 2. The multicolor image forming apparatus according to claim 1, wherein the charging is performed such that the absolute value of the potential of the photosensitive member surface at a portion where the toner image is not formed becomes larger. 3. The surface potential of the portion where the toner image of at least one color is not formed is higher than the surface potential of the portion where the toner image of at least one color is formed at the position after the second charging and before the exposure. 2. The multicolor image forming apparatus according to claim 1, wherein the charging is performed so that the absolute value of the potential becomes large. 4. The multicolor image forming apparatus according to claim 1, wherein the multicolor image formation is performed by superposing a plurality of color toners in each pixel. 5. The multicolor image forming apparatus according to claim 1, wherein the electrostatic latent image carrier uses a material having a relative dielectric constant ε of 8 or more for a photosensitive layer. 6. A multicolor image forming apparatus according to claim 1, wherein said electrostatic latent image carrier uses amorphous silicon for a photosensitive layer. 7. The multicolor image forming apparatus according to claim 1, wherein the charger used for recharging is a corotron.