JPH0789244B2 - Color electrophotographic device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color electrophotographic apparatus that can be used in a color hard copy apparatus such as a color copying machine or a color printer.

2. Description of the Related Art In recent years, a plurality of toner images of different colors are formed on an electrophotographic photosensitive member (hereinafter referred to as a photosensitive member) by repeating charging, exposure, and development a plurality of times, and then the toner images are collectively transferred to a paper to perform color printing. A color electrophotographic method for obtaining an image has been actively studied. Unlike the conventional color electrophotographic method, this method has an advantage that the apparatus can be downsized without a transfer drum.

As a color electrophotographic apparatus of this kind, for example, Japanese Patent Application No.
There is a device proposed in -212927 specification. This conventional device will be described with reference to FIG.

After charging the initial potential of the photoconductor 2 to + 800V by the corona charger 1, the yellow image signal is exposed by the light emitting diode 3.
A negative electrostatic latent image (an electrostatic latent image in which the image area is exposed and the surface potential of the photoconductor is attenuated) is formed. Then, the electrostatic latent image is negative-positive reversal developed by the developing device 4 containing Y toner to form a yellow toner image on the photoconductor 1. At this time, a developing bias of +700 V is applied to the developing device 4 containing Y toner, but the other developing devices 5 and 6 are grounded and adjusted so that toner does not fly.
After developing with Y toner, the entire surface of the photoconductor 2 is irradiated with the discharge lamp 7 to erase the yellow electrostatic latent image.

Next, by the same method and conditions as the method for forming the yellow toner image (the initial potential of the photoconductor, the developing bias, etc.), the steps of charging, exposing, developing and removing light are repeated, and M, And the toner images of C are sequentially formed on the toner image of Y. This is a method in which after the formation of all the toner images is completed, the electrostatic latent image is erased by the discharging lamp 8 and the toner image is electrostatically transferred onto the recording paper 10 by the corona charger 9.

Problems to be Solved by the Invention In the method described in the previous section, there is a new step of forming a new electrostatic image by exposing through a toner layer formed on a photoconductor, which is a new step which is not in the conventional electrophotographic method. Contains.
Because of the step of exposing through the toner layer, there are many new problems that could not be considered in the conventional electrophotographic method. The cause is that the thickness of the toner layer attached to the photoconductor is not uniform, the shape of the toner is not constant, and is irregular. Hereinafter, the newly generated problems will be summarized and described.

In the following description, a photoconductor having a light attenuation characteristic as shown in FIG. 3 will be used. With this photosensitive member, the surface potential drops to 850 V from the initial surface potential of 850 V at an exposure dose of 0.8 μJ / cm ² .

First of all, the first problem is that it is difficult to overlap colors in the thin line portion or the solid edge portion. In the method of overlapping colors on the photoconductor, the solid edge part and fine line part, especially the line width
In a line drawing of 100 μm or less, it is difficult to overlap two colors, and color unevenness is likely to occur. In order to investigate the cause, the spectral transmittance (about 10 μm) of the toner used was measured (FIG. 4). As shown in the figure, both yellow and magenta transmit at 85% or more at the emission wavelength of 670 nm of the light emitting diode. Therefore, the amount of light required to expose the photoconductor to the residual potential by exposing it from above the yellow or magenta toner is about 20% higher than that without toner.
1.0 μJ / cm ² should be sufficient for calculation. However, when actually printed, the solid portions could be overlaid with color, but the thin line portions and the edge portions of the solid portions could not be overlaid with color, resulting in unevenness. To investigate the reason for this, the state of adhesion of the toner layer developed on the photoconductor by this non-contact development method was observed. The observation view is shown in FIG. In the thin line drawing portion and the solid edge portion (FIG. 5B),
It was found that due to the edge effect peculiar to the electrophotographic method, about 1.5 to 3 times the amount of toner adhered to the solid portion (FIG. 5A), and the toner layer thickness was increased particularly at that portion. Therefore, the transmission of light is poor at the edge of the image line,
The amount of light on the photoconductor therebelow was insufficient and the potential did not drop, and it was difficult to develop color-overlapping toner at the edge portion. The phenomenon that the amount of adhered toner increases at the edge portion of the image line is characteristically emphasized in the developing method used in this conventional example when a difference appears between the surface speed of the developing roller and the surface speed of the photoconductor. appear. Initially, even if the rotation speed is adjusted so that the edge enhancement of the image line does not occur as much as possible at the beginning, as the amount of toner inside the developing device decreases, toner is also re-ellocated on the rotating shaft. When the number of rotations of the developing roller fluctuates due to the occurrence of troubles such as an increase in the rotational load of penetration, an edge enhancement phenomenon appears instantly in the image. As described above, when the edge portion is emphasized, as a side effect, color overlapping becomes difficult and color unevenness occurs. In addition, as another problem, it was found that when the exposure amount was about 1 or 2 times smaller, the toner was scattered around the image line and bleeding occurred when observed carefully. When I investigated the cause, I found the following. As a matter of course, in the naked photoconductor portion, the light potential of the photoconductor is sufficiently attenuated to about 50 V when the light amount is 1.2 times the required light amount. However,
In the photoconductor with the toner layer attached, the light amount is attenuated by the toner layer, so the potential of the photoconductor is slightly attenuated. For example, in the case of the photoconductor shown in FIG. 3, if the toner layer is thickly attached like the edge portion, it will be about 400V.
It is thought that it has become. In this state, as shown in FIG. 6, a potential difference of 350V is generated between the bare photoconductor (50V) and the photoconductor with the toner layer (400V) in the peripheral area of the image line, and a new generation occurs. The toner is scattered laterally on the photoconductor along the lines of electric force (FIG. 7). Therefore, not only the problem that the color superimposition described above becomes difficult, but also a new problem that the toner image bleeds occurs.

For such a shortage of light quantity, a method of increasing the light quantity of the light source can be considered, but it has been found that it is not enough to simply give an excessive light quantity to the photoconductor. For example, when the light amount of the light source is increased, the life of the light source is naturally shortened, and the reliability of the device is impaired. In addition to that, there are other image quality problems. This will be described with reference to FIG.

For example, when the photoconductor is irradiated with an exposure dose of 5 μJ / cm ² (about 12 times the required exposure dose), the thick image line in the portion where the toner is thickly adhered to the photoconductor such as the fine line portion causes a thick image line. Occurred.
For example, the lines to be reproduced to 100 μm in red were thickened to around 200 μm, and the kanji written in Mincho type were completely destroyed. When the cause of this was investigated, the following was found. this is,
The first cause was that the toner used was amorphous. Due to the irregular shape of the toner and the different particle sizes, the incident light is scattered in the toner layer and exposed or spreads to thicken the electrostatic latent image. , The line drawing got fat.

It was also found that when the naked photoconductor is directly irradiated with such an intense amount of light, a memory effect appears on the photoconductor, which hinders the formation of the next electrostatic latent image.

As described above, it has been found that the step of exposure is extremely difficult in the electrophotographic method for obtaining a color image by directly superposing the toner layer on the photoconductor.

An object of the present invention is to provide a color electrophotographic apparatus that obtains a color image in which the color superimposition of fine line images is good.

Means for Solving the Problems The present invention relates to a light amount of a light source for image exposure in a color electrophotographic apparatus in which a plurality of toner images of different colors are formed on a photoconductor by repeating a cycle of charging, exposing and developing a plurality of times. But,
The color electrophotographic apparatus is characterized in that the amount of light is 1.6 times or more of the amount of light required to reduce the processed surface potential of the bare photoconductor by 500 V.

Effect According to the present invention, even if a large amount of toner adheres to the edge of the image line, the light amount of the light source is emitted with a margin, so that the potential of the photoconductor under the toner is sufficiently lowered. Toners of different colors can be accurately overlaid and developed from above. Further, the adverse effects such as the thickening of the image line and the occurrence of bleeding due to the excessive light amount are not caused.

EXAMPLES In the present invention, a developing method is used in which the toner is not in contact with the photosensitive member and is directly developed by an electric field. However, when an attempt is made to develop a sufficient amount of toner on the photosensitive member by this developing method, image areas of the photosensitive member are And, the contrast potential of the non-image area needs to be about 500 V or more. Therefore, the potential of the photoconductor is 500V from the initial surface potential.
It becomes essential to decrease. At this time, when the amount of toner adhering to the photoconductor by this developing method was examined, it was found that the amount of adhering to the solid portion was 1.5 to 3 times in the thin line image and the edge portion of the solid portion. If you try to attenuate the photoconductor through this toner layer until it gets the contrast potential of 500V required for development, the amount of light from the light source that exposes the photoconductor will change from the initial surface potential of the photoconductor with no toner attached. The amount of light required is 1.6 times or more the amount of light required to drop 500V. However, if this light source has too much light,
It causes a strong memory effect on the photoconductor and causes the image to store a memory of the previous image. Therefore, it is desirable to suppress the light quantity of the light source to 10 times or less. As mentioned above, the toner adhesion amount is not so large in the solid part, and therefore the light amount may be about 1.1 times that of the bare part, so basically the part where the light amount must be 1.6 times or more is It is sufficient that only the boundary area portion of the drawing line portion is used. Further, in the present invention, the photoconductor and the developing roller are not in contact with each other, which is particularly effective in the case of a developing means for applying a DC voltage between the developing roller and the photoconductor to cause the toner to fly and develop. At that time, the developing condition is that the thickness of the toner on the developing roller is
At 100 μm or less, the gap between the developing roller and the photoconductor surface is 300
It is particularly effective for images printed with a thickness of less than μm. In this developing method, the developing roller is rotated in the opposite direction to the photosensitive member, that is, when the surface of the photosensitive member and the surface of the developing roller are in the same traveling direction and the moving speed of the developing roller is 0.7% of the moving speed of the photosensitive member. When the range is set from 2 times to 1.3 times, no significant edge effect appears in the image, so it is particularly effective. In the present invention, a strong light source is desirable because the photoreceptor must be exposed from above the toner, and a light emitting diode (wavelength 670 nm) or a semiconductor laser (wavelength 800 nm) is preferable. At this time, the toner that does not transmit the wavelengths of these light sources must be subjected to the image forming process after the color superposition order. Therefore, it is necessary that the order of charging, exposing, and developing is yellow, magenta, and cyan, or magenta, yellow, and cyan. Of course, when using the black toner, it is needless to say that the black toner may come in any order because it is not necessary to overlap the colors.

Concrete Example 1 Hereinafter, a concrete example of the present invention will be described in detail with reference to FIG.

The developing units 11, 12 and 13 are non-contact type non-magnetic one-component developing units that fly toner by a DC electric field. A thin layer of toner is formed on the developing rollers 14, 15 and 16 made of aluminum by a blade 17. It is configured to be formed. The developing device 11 contains yellow (Y), the developing device 12 contains magenta (M), and the developing device 13 contains cyan (C) insulating toner. Then, each developing unit was installed opposite to the periphery of the photoconductor 18 while keeping the gap (development gap) between the developing rollers 14, 15, 16 and the photoconductor 18 constant. Each developing device is provided with a separation / contact mechanism that is close to the photoconductor during development and is separated during non-development. The specifications and developing conditions of the developing device and the physical properties of the toner are shown below.

Development device specifications and development conditions Development roller diameter: 16 mm Development roller peripheral speed: 130 mm / s Development roller rotation direction: Opposite direction to photoconductor Toner layer thickness on development roller: 30 μm Development roller rotation direction: Photosensitive Opposite direction to body 18 Development gap: 150 μm at development, 700 μm at non-development Physical properties of toner: Toner charge: +3 μC / g Average particle size: 10 μm Amorphous Se− with a diameter of 100 mm sensitized in the infrared region as a photoconductor The Te photosensitive drum 18 was used and rotated at a peripheral speed of 150 mm / s. Charge this photoreceptor 18 to a charger 19 (corona voltage: + 7k
V) was charged to a surface potential of + 800V. Next, output 5m
A semiconductor laser 20 with W and a wavelength of 780 nm was emitted. At this time, the intensity of the laser beam on the surface of the photoconductor was 2 mW. At this time, 0.8 mW on the surface of the photoconductor was sufficient to reduce the potential of the surface of the bare photoconductor 18 from + 800V to + 50V. Therefore, the amount of laser light used is 2.5 times the required amount of light. Using this semiconductor laser 20, a negative yellow signal was exposed on the photoconductor 18 to form an electrostatic latent image. The latent image is reversely developed by the yellow developing device 11 in the developing state in which + 700V is applied to the developing roller 14, and then the photoconductor 18 is passed through the non-developing magenta developing device 12 and the cyan developing device 13 to obtain the yellow toner. Formed an image. At this time, the yellow toner layer thickness on the photoconductor was from 1 to 2.

Next, the photoconductor 18 was charged to + 850V by the corona charger 19 again. After that, a signal light corresponding to magenta was exposed on the photoconductor 18 by the semiconductor laser 20 to form a magenta electrostatic latent image. Then, the photoconductor 18 is passed through the yellow developing device 11 in the non-developing state, the magenta developing device 12 in the developing state to which the developing roller 15 + 800V is applied, and the cyan developing device 13 in the non-developing state to form a magenta toner image. . At this time, the toner layer thicknesses of yellow and magenta, which were superposed on the photoreceptor, were 2 to 4 layers.

Next, the photoconductor 18 was charged to + 950V again by the corona charger 19. Then, the semiconductor laser 20 exposed signal light corresponding to cyan to form a cyan electrostatic latent image. Here, the surface potential of the image area formed on the M toner image is +
The surface potential of the image area formed on the image in which 100V, Y toner and M toner were superposed was + 200V, and the contrast potential of the latent image was 750V. Next, the photoreceptor 18 is a non-developed yellow developing device 11, a magenta developing device 12, and a developing roller cyan developing device in which + 900V is applied to the developing roller 16.
It was passed through 13 to form a cyan toner image.

The color toner image thus obtained on the photoconductor 18 was transferred onto the paper 22 by the transfer charger 21 and then thermally fixed. on the other hand,
After the transfer, the surface of the photoconductor 18 was cleaned by pressing the fur brush 23 against the photoconductor 18.

As a result, the color density of the composite color of the solid areas of red, green and blue is 1.5.
As described above, a clear color image in which the colors of line images were accurately overlapped was obtained.

Comparative Example 1 The output of the semiconductor laser 20 was measured using the device described in FIG.
2% (from the surface of the photoconductor) to 10% of the required exposure of the photoconductor 18
When I dropped it to 0.9mW, I got a solid edge and 100
In a fine line drawing of μm or less, it became difficult to overlap red, green, and blue-violet, and color unevenness occurred on the drawing line.

Specific Example 2 Next, using the apparatus of Specific Example 1, a light emitting diode array (output 1 dot 7 μW) was used as a light source instead of the semiconductor laser 20, and a rod lens array (Nippon Sheet Glass Co., Ltd. Selfoc lens) was used. Examples used in combination with an array, SLA-20) will be described. The amount of exposure energy on the photoreceptor surface was 2.2 μJ / cm ² . At this wavelength, the exposure energy required to lower the potential of the photoconductor to the residual potential is 0.4 μJ / cm ² . Therefore, a 5-fold exposure was given. When printed under this condition, the color density of the composite color of the solid areas of red, green, and blue is 1.5 or more, and the Y, M, C three-color overlapping density is 1.7 or more, and the line images are clear with overlapping colors. A color image was obtained.

Comparative Example 2 The light emitting diode array (1 dot 7 μW) was also used as the light source using the apparatus of the specific example 2, and this time, a higher resolution but darker rod lens array (Nippon Sheet Glass Co., Ltd., SELFOC) was used. Lens array, SLA-9)
Used in combination with. The amount of exposure energy on the surface of the photoreceptor was 0.4 μJ / cm ² . This is the exposure amount at which the potential drops completely if the photoreceptor is bare. When printing was performed under these conditions, it was difficult to overlap red, green, and blue-purple in a solid edge portion and a fine line image of 100 μm or less, and color unevenness occurred in the image line.

EFFECTS OF THE INVENTION According to the present invention, it is possible to obtain a color electrophotographic apparatus capable of superimposing colors not only on solid areas but also on fine line drawings and stably obtaining a color image having a high color density of a composite color.

[Brief description of drawings]

FIG. 1 is a principle diagram of a color electrophotographic apparatus in one embodiment of the present invention, FIG. 2 is a principle diagram of a conventional color electrophotographic apparatus, and FIG. 3 is a light attenuation characteristic of a photoconductor used in the present invention. FIG. 4 is a characteristic diagram in which the spectral transmittance of the toner used in the present invention is measured, and FIG. 5 is a diagram schematically showing how toner adheres to the solid portion of the present invention. FIG. 7 and FIG. 7 are diagrams for explaining the blurring of the image line when the exposure amount is small, and FIG. 8 is a diagram for explaining the thickening of the image line when the excessive exposure amount is used. 11, 12, 13 …… Developing device, 14, 15, 16 …… Developing roller, 18
...... Photoreceptor, 19 ...... Charger, 20 ...... Light source

Claims (9)

[Claims] 1. An apparatus used in a color electrophotographic method for forming a plurality of toner images of different colors on a photoconductor by repeating a cycle of charging, exposing and developing a plurality of times, wherein the toner image on the photoconductor is A color electrophotographic apparatus, which is a light source in which the light amount during through-image exposure is 1.6 times or more the light amount required to reduce the initial surface potential of a bare photoreceptor by 500V. 2. The color electrophotographic apparatus according to claim 1, wherein the required light quantity is 10 times or less. 3. A developing method is a DC electric field flying type non-contact developing method in which a DC voltage is applied between a developing roller carrying a thin layer of toner and a photoconductor to fly the toner. The color electrophotographic apparatus according to claim 1. 4. The color electrophotographic apparatus according to claim 3, wherein the thickness of the toner on the developing roller is 100 μm or less, and the gap between the developing roller and the photosensitive member surface is 300 μm or less. 5. The method according to claim 3, wherein the rotation direction of the developing roller is opposite to that of the photoconductor, and the moving speed of the developing roller is 0.7 to 1.3 times the moving speed of the photoconductor. The described color electrophotographic apparatus. 6. The color electrophotographic apparatus according to claim 1, wherein the light source is a semiconductor laser. 7. A color electrophotographic apparatus according to claim 1, wherein the light source is an emitting diode. 8. The charging, exposing and developing cycle is yellow.
The color electrophotographic apparatus according to claim 6 or 7, wherein the order is magenta-cyan. 9. The color electrophotographic apparatus according to claim 6 or 7, wherein the cycle of charging, exposing and developing is in the order of magenta, yellow and cyan.