JP2000118036A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent omission of an image of a middle tone image due to carrier over by providing an exposing energy necessary for attenuating the charged surface potential in the optical potential attenuation characteristics of a photosensitive member by 50% and an exposing energy necessary for attenuating the charged surface potential by 10% within a specific value range. SOLUTION: In a scanning optical system for guiding a light beam synthesized by a synthesis means to a photosensitive member and scanning the photosensitive member in the main scanning direction, a first output energy is defined to be EE1, and a second light beam output energy is defined to be EE2. In proviso that an exposing energy necessary for attenuating the charged surface potential in the optical potential attenuation characteristics of the photosensitive member by 50% is E50 and an exposing energy necessary for attenuating the charged surface potential by 10% is E10, the formula E10/E50>=EE2/EE1-0.2 is satisfied. Specifically, the ratio of the exposing energies for attenuating the charged surface potential VH by 50% and by 10% in a potential attenuation curve is used as the optical potential attenuation characteristics.

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 used for an electrophotographic printer or copier.

[0002]

2. Description of the Related Art In recent years, in the field of printers and copiers, an electrostatic latent image is formed on a photoreceptor by irradiating the photoreceptor with a light beam modulated in accordance with image information representing characters and pictures. A so-called digital electrophotographic image forming apparatus which forms a visible image by developing the electrostatic latent image with toner is widely used.

In such a digital image forming apparatus, not only when forming a binary image of black and white but also when forming an image having a halftone, a light beam is turned on and off at a high speed, and a so-called halftone dot structure is formed. 2. Description of the Related Art A method of expressing a halftone by forming an electrostatic latent image having a line structure is conventionally known. This method has a relatively simple algorithm,
Since it can be realized at low cost, it has been widely used in digital electrophotographic printers and copiers.

[0004]

In such a system for expressing a halftone by forming an electrostatic latent image having a so-called halftone dot structure or line structure, a low density portion (highlight portion) in the image is used. ) To a high density portion, a latent image is formed with a constant light beam spot diameter and a constant number of dots or lines per unit length. For this reason, the exposure profile in the highlight portion has an intermediate (analog-like) exposure energy distribution in which the exposure energy does not change in a binary (digital) manner as in the case of ON / OFF and the contrast is reduced. It becomes an exposure profile, and furthermore, since the exposure amount itself is small, the reproducibility of dots and lines is poor, a grainy image with low granularity is obtained, and the gradation expression tends to be unstable due to changes in the temperature and humidity environment There's a problem.

In order to improve the contrast of an exposure profile in a highlight portion, it is conceivable to form a latent image by reducing the number of dots or lines per unit length. Although the reproducibility of dots and lines can be improved, image structures such as dots are more easily recognized, and the resolution of characters is also reduced, resulting in lower image quality.

A method of making the light beam spot diameter sufficiently small is also conceivable. However, according to the consideration of the imaging optical system, in the propagation of the Gaussian beam, the beam convergence angle is set to θ.
_beam , the wavelength is λ, and the refractive index is n, the minimum beam diameter ω
0 is obtained by the following equation.

Ω0 ≒ λ / (n · π · θ _beam ) Further, assuming that the diameter of the beam incident on the f-θ lens is D and the focal length of the f-θ lens is f, the beam convergence angle θ _beam is given by the following equation. Is required.

Θ _beam = tan ⁻¹ (D / (2 · f)) Therefore, in order to reduce the beam diameter, the wavelength λ must be shortened, or the beam diameter D incident on the f-θ lens, that is, the rotating polygon mirror It can be seen that the diameter of the incident beam to the beam should be increased.

In order to shorten the wavelength λ of the light beam, a method of using a semiconductor laser having a short wavelength is conceivable. However, a semiconductor laser having a short wavelength is about 680 nm. 780 nm, which is only about 12% improvement. Further, when a light source having a shorter wavelength is used, it is necessary to employ a technology such as an argon laser or a combination of a semiconductor laser and a wavelength conversion element, so that there is a problem that the apparatus becomes larger and the price increases.

In order to increase the diameter D of the incident beam on the f-.theta. Lens, an image forming optical system for condensing a light beam and forming a light beam spot on a photoreceptor becomes large and very precise and expensive. It is not suitable for practical use.

To solve this problem, Japanese Patent Laid-Open Publication No.
Japanese Patent Application Laid-Open No. 4 (1999) -1994 discloses a photosensitive member having a photo-potential decay characteristic in which the charging potential does not attenuate up to a certain exposure intensity and the charging potential abruptly attenuates when the exposure intensity exceeds a certain threshold value, that is, a so-called switching type photosensitive member. There is disclosed a method for improving the contrast of an exposure profile by using the method. Japanese Patent Application Laid-Open No. 2-282277 describes a halftone image reproducing method using a photoreceptor having the same photo-potential attenuation characteristics.

According to these methods, the charging potential is not attenuated at the low exposure intensity, so that the low exposure intensity portion, which is the bottom of the exposure profile, is cut off, and clear characters can be reproduced. The line reproducibility is also good, and an image without graininess having excellent graininess can be obtained.

However, according to this method, the charging potential changes abruptly beyond a dead area where the charging potential is not attenuated at a low exposure intensity, so that the gradation expression including the highlight portion becomes steep and the light beam irradiation is performed. If energy unevenness or the like is present, there is a problem that concentration unevenness is likely to become apparent. In order to suppress the steepness of the gradation expression and the appearance of density unevenness, it is necessary to make the light beam spot diameter sufficiently small, so that the imaging optical system is extremely precise and expensive as described above. In addition, the size of the apparatus is easily increased, which is not suitable for practical use.

As another solution to this problem, Japanese Patent Application Laid-Open No. 9-169136 discloses a method for forming an electrostatic latent image on the same scanning line while simultaneously scanning two light beams having different beam diameters. Exposure of image information is performed using the light beam having the smaller beam diameter of the two light beams as the first exposure, and at the same time, the inverted image is formed using the light beam having the larger beam diameter as the second exposure. A so-called bias exposure method has been proposed in which exposure is performed and an electrostatic latent image is formed by summing the first exposure and the second exposure.

In this bias exposure method, the inclination of the contour of the combined exposure image obtained by combining the first exposure image and the second exposure image becomes steep due to the difference in the beam diameter of the two light beams, and An extremely high-contrast electrostatic latent image that cannot be obtained by light beam exposure can be obtained. By combining this method with a switching type photoreceptor, it is possible to reproduce images without a grainy feeling with excellent graininess while having a practical light beam spot diameter, and the highlight part is smooth and excellent in uniformity Images can be obtained.

FIG. 12 is a schematic diagram for explaining a conventional bias exposure method.

FIG. 12 shows the driving timings A and B of the first light beam and the second light beam and the first light beam and the second light when forming a halftone image having a line structure. Exposure profile C in which the exposure image by the beam is synthesized
And an electrostatic latent image profile D formed based thereon
Are shown. The exposure timing of the first light beam and the exposure timing of the second light beam having an inversion relationship with the first light beam make the combined exposure profile C
, A fringe portion (shown by a broken line) is formed at a boundary between a portion where image formation is required and a portion where image formation is not required. The fringe portion is also transmitted to the electrostatic latent image. If the level is large, the reverse electric field (electric field that acts to prevent development) applied to the developer locally increases, and the fringe portion due to the reverse electric field causes the electrostatic latent image to move. It also appears in the image profile D. Due to this reverse electric field, an image defect such as image missing due to carrier adhesion to a photoconductor, which is called carrier over, may occur in a halftone image portion. Furthermore, depending on the conditions of the exposure beam and the photopotential attenuation characteristics of the photoconductor, it is necessary to set the charging potential of the photoconductor to a very high potential. It can also occur.

When this bias exposure method is applied to an image forming apparatus for forming a color image by transferring a visible image by a plurality of color toners so as to be sequentially superimposed on a transfer object, the bias exposure method is used repeatedly. There is also a problem that color unevenness is generated depending on the conditions of the exposure beam and the photosensitive member charging potential during the exposure.

The present invention has been made in view of the above circumstances, and has as its object to provide an image forming apparatus capable of preventing an image of a halftone image from being lost due to carrier over, and capable of obtaining an image having excellent graininess and having no roughness. Aim.

[0020]

In order to achieve the above object, an image forming apparatus according to the present invention comprises a photosensitive member on which an electrostatic latent image is formed, a photosensitive member for forming an electrostatic latent image, a charging means for charging the photosensitive member, and a charging member. Exposure means for forming an electrostatic latent image on the photosensitive member by scanning the photosensitive member with a light beam corresponding to an image signal in a predetermined scanning direction; and an electrostatic latent image formed on the photosensitive member. In an image forming apparatus including a developing unit that forms a toner image on a photoconductor by developing an image, the exposing unit generates a first light beam that is modulated on and off based on the image signal. Light beam generating means;
A second light beam that generates a second light beam that is on-off modulated contrary to the first light beam based on the image signal;
Light beam generating means, and the first light beam generating means generated by the first light beam generating means and the second light beam generating means.
Light beam combining means for combining the light beam and the second light beam into one light beam, and guiding the light beam combined by the combining means to the photosensitive member to scan the photosensitive member in the main scanning direction. And the output energy of the first light beam is EE1, the output energy of the second light beam is EE2, and the charged surface potential in the photopotential decay characteristic of the photoconductor is 50%. When the exposure energy required to attenuate is E50 and the exposure energy required to attenuate the charged surface potential by 10% is E10, E10 / E50 ≧ EE2 / EE1-0.2 is satisfied. .

Here, the exposure energies E50 and E50, which are important indices representing the photo-potential decay characteristics of the photoreceptor, are shown.
The ratio E10 / E50 to 10 will be described with reference to the drawings.

FIG. 1 is a graph showing the photo-potential decay characteristics of the photoreceptor.

In the potential decay curve shown in FIG. 1, the exposure energy E50 required to attenuate the charged surface potential VH by 50% to the surface potential V50 and the charged surface potential VH by 10% attenuate the surface. The photopotential decay characteristic of this photoreceptor can be expressed by the ratio E10 / E50 to the exposure energy E10 required for setting the potential V10.

[0024]

Embodiments of the present invention will be described below.

FIG. 2 is a schematic configuration diagram of an embodiment of the image forming apparatus of the present invention.

In this embodiment, an image forming apparatus according to the present invention is applied to a color copying machine, and a photosensitive drum 1 having an electrostatic latent image formed on a surface thereof, and an electrostatic latent image for charging the photosensitive drum 1 are provided. Image forming charger 2, a light beam scanning device that forms an electrostatic latent image on photosensitive drum 1 by scanning charged photosensitive drum 1 with a light beam corresponding to an image signal in a predetermined scanning direction. 20 and a rotary developing device 3 for forming a toner image on the photosensitive drum 1 by developing the electrostatic latent image formed on the photosensitive drum 1 with toner.

The charger 2 for forming an electrostatic latent image in this image forming apparatus corresponds to a charging means according to the present invention, and the light beam scanning device 20 corresponds to an exposure means according to the present invention. In addition, the rotary developing device 3 corresponds to the developing means according to the present invention.

The light beam scanning device 20, ie, the exposure means according to the present invention, includes a first light beam generating means, a second light beam generating means, a light beam synthesizing means, and a scanning optical system. . Details of the exposure means (light beam scanning device 20) will be described later.

Next, the basic operation of the image forming apparatus will be described.

The photosensitive drum 1 rotating in the direction of arrow A is
After being uniformly charged by the electrostatic latent image forming charger 2, it is repeatedly scanned in the main scanning direction by a plurality of light beams from the light beam scanning unit 20. The photosensitive drum 1 is exposed by this light beam scanning, and an electrostatic latent image is formed on the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is transported to a developing position D facing the rotary developing device 3. The rotary developing device 3 includes four developing units 3a that respectively store yellow, cyan, magenta, and black toners. As the developing unit 3a, a developing unit of a reversal developing system using two-component magnetic brush development is used. The average particle size of the toner is 7 μm. Each time the rotary developing device 3 develops the electrostatic latent image corresponding to each color, the rotary developing device 3 rotates so that the developing unit 3a corresponding to the color is set at the developing position D, and the toner corresponding to the color is used. The electrostatic latent image on the photosensitive drum 1 is developed. At this time, the developing roll 3b of the developing unit 3a where development is performed
Is applied with a bias voltage to suppress toner adhesion to the background portion of the electrostatic latent image. As a result of the development, the toner image formed on the photosensitive drum 1 is carried on the photosensitive drum 1 and is conveyed to a transfer position T facing the transfer drum 4.

The transfer drum 4 rotating in the direction of arrow B has
A paper suction charger 4a for suctioning the recording paper P transported from the paper tray 11 via the paper transport path 12 to the outer periphery of the transfer drum 4, and transfers the toner image formed on the photosensitive drum 1 A transfer charger 4b for electrostatic transfer on the recording paper P at the position T, a peeling charger 4c for removing electrostatic attraction between the recording paper P and the transfer drum 4, and a transfer drum 4 A peeling claw 4d for peeling off the recording paper P, and a charge eliminator 4e for discharging the transfer drum 4 after the recording paper has been peeled off are provided.

While yellow, cyan, magenta, and black toner images are sequentially formed on the photosensitive drum 1, one recording sheet is attracted to the transfer drum 4 and is rotated together with the transfer drum 4. The transfer drum 4 is synchronized with the successive arrival of the toner images of each color on the body drum 1 at the transfer position T.
The recording paper P attracted to the recording paper P is also conveyed to the transfer position T, and is electrostatically transferred by the transfer charger 4b so that toner images of each color are sequentially superimposed on one recording paper P. When the transfer of the four color toner images of yellow, cyan, magenta, and black onto the recording paper P is completed, the recording paper P attracted to the transfer drum 4 is transferred to the transfer drum 4 by the peeling charger 4c.
Is removed from the transfer drum 4 by the peeling claw 4d, fixed by the fixing device 9, and then carried out of the apparatus. After the transfer, the photosensitive drum 1 is cleaned by a cleaner 5 to remove the remaining toner, is discharged by the light irradiated from the pre-exposure device 6, and when the next electrostatic latent image is continuously formed, the electrostatic latent image is removed. The surface of the photosensitive drum 1 is charged by the image forming charger 2.

On the other hand, the transfer drum 4 after the recording paper P is peeled off is discharged by the discharging charger 4e, and when the next image formation is continuously performed, similarly to the above, by the paper suction charging device 4a. The next recording paper P is attracted onto the transfer drum 4.

FIG. 3 is a schematic configuration diagram of a light beam scanning device in the image forming apparatus according to the first embodiment.

FIG. 4 is a sectional view in the main scanning direction of the light beam scanning device shown in FIG.

FIG. 5 is a schematic diagram showing the operation and configuration of the image signal control device of the light beam scanning device shown in FIG.

FIG. 6 is a schematic view showing a process in which a combined exposure image is obtained by the image forming apparatus according to the first embodiment.

As shown in FIGS. 3 and 4, the light beam scanning device 20 includes a laser diode 21a that generates a first light beam L1 that is turned on and off based on an image signal, and a laser diode 21a based on the image signal. A laser diode 21b for generating a second light beam L2 that is reciprocally on-off modulated with respect to the first light beam L1, and a first light beam L1 and a second light beam L2 generated from the laser diode 21a and the laser diode 21b, respectively. Light beam L2
A beam splitter 23 that combines the light into a single light beam;
The cylinder lens 24, the reflection mirror 25, the polygon mirror 26, and the f-θ lens guide the light beam synthesized by the beam splitter 23 to the photosensitive drum 1 and scan the photosensitive drum 1 in the main scanning direction indicated by the arrow A. 27, and a scanning optical system 29 including a cylinder mirror 28 and the like.

In this image forming apparatus, the first light beam L
EE1, the output energy of the second light beam L2 is EE2, and the exposure energy required to attenuate the charged surface potential by 50% in the photopotential decay characteristic of the photosensitive drum 1 is E50. Exposure energy required to attenuate the charged surface potential by 10% is E10
Then, a relationship of E10 / E50 ≧ EE2 / EE1-0.2 is established.

The laser diode 21a in this embodiment corresponds to the first light beam generating means according to the present invention, and the laser diode 21b corresponds to the second light beam generating means according to the present invention. The beam splitter 23 corresponds to a light beam combining unit according to the present invention.

Next, the operation of the light beam scanning device will be described with reference to FIGS. 3, 4, 5 and 6.

In the present embodiment, binary raster data is processed as image information, and will be referred to as image data in the following description.

When image data 33 having a waveform as shown in FIG. 5A is input to the image signal controller 32 shown in FIG. 5B, the amplifier 32a has a waveform corresponding to the image data 33. An image signal 34a is output, and an inverted image signal 34b having a waveform obtained by inverting the image data 33 is output from the inverting amplifier 32b.

The image signal 34a output from the amplifier 32a in the image signal control device 32 is applied to the laser driving device 31a.
(See FIGS. 3 and 4), the laser diode 21a driven by the laser driver 31a emits a first light beam L1 on / off modulated based on the image signal 34a to the collimator lens 22a. Is done. Also, the image signal 3 output from the inverting amplifier 32b
When 4b is input to the laser driving device 31b, the laser diode 21 driven by the laser driving device 31b
b, the second light beam L2, which is on-off modulated reciprocally to the first light beam L1, is output from the collimator lens 22b.
Is emitted.

The first light beam L1 and the second light beam L2 are converted into parallel light beams by the collimator lenses 22a and 22b, respectively, and are incident on the beam splitter 23. From the beam splitter 23, a light beam obtained by combining the first light beam L1 and the second light beam L2 is emitted to the cylinder lens 24. This light beam is converged in the sub-scanning direction by the cylinder lens 24 and is reflected by the reflection mirror 2.
After passing through 5, the light enters a polygon mirror 26 rotating in the direction of arrow B. The polygon mirror 26 scans the photosensitive drum 1 in the main scanning direction A with a light beam. Polygon mirror 26
Is adjusted by the f-θ lens 27 so that the scanning angle and the scanning distance are proportional to each other, and then condensed in the sub-scanning direction by the cylinder mirror 28 to form an image on the photosensitive drum 1. I do.

FIG. 6 shows an image signal (a), an inverted image signal (b), an exposure image (c) formed on a photosensitive drum by a first light beam, and a photosensitive drum by a second light beam. An exposure image (d) formed thereon and a combined exposure image (e) combined from the first light beam and the second light beam
Are shown.

In the light beam scanning device 20 (see FIGS. 3 and 4) of the image forming apparatus of this embodiment, the photosensitive drum 1
The optical parameters are set such that the beam diameter of the first light beam L1 emitted from the laser diode 21a on the image forming surface is smaller than the beam diameter of the second light beam L2 emitted from the laser diode 21b. Designed. As a result, as shown in FIG. 6E, the inclination of the image contour portion 36 of the combined exposure image is steep, and an extremely high-contrast electrostatic latent image is obtained.

FIG. 7 is a schematic structural view of a developing unit constituting a rotary developing device used in the image forming apparatus of the first embodiment.

This developing unit carries the developer on a surface in a housing 62 containing a developer in which magnetic particles and toner are mixed, and rotates the developer in a direction of arrow B to move the developer to a developing position D. Cylindrical developing roll 61 to be conveyed
And a developer regulating member 65 that regulates the amount of the developer adhering to the surface of the developing roll 61; and screw augers 63 and 64 that rotate and stir and transport the developer to supply the developer to the developing roll 61. And

The developing roll 61 is composed of a sleeve 61_1 rotatably supported and a fixed magnet roll 61_2 disposed inside the sleeve 61_1. The magnet roll 61_2 has a plurality of magnetic poles 66a, 66b,..., 66e, and forms a magnetic brush of magnetic particles on the surface of the sleeve 61_1 by a magnetic field formed between adjacent magnetic poles. . At the developing position D, the sleeve 61_1 is located between the sleeve 61_1 and the photosensitive drum 1 rotating in the direction of arrow A.
The developer is conveyed to the developing position D at a distance of 5 mm, and is conveyed to the developing position D when the amount of the developer is regulated by the developer regulating member 65 by rotating in the direction of arrow B.

The sleeve 61_1 of the developing roll 61 has
A developing bias voltage in which an alternating current is superimposed on a direct current is applied from a direct current superimposed alternating voltage power supply 68, and the toner having a charge is charged on the photosensitive drum 1 by an electric field formed at a developing position D close to the photosensitive drum 1. Is developed by adhering to the electrostatic latent image. The applied voltage is set so that the DC component has the same polarity as the photoconductor charging potential.
The AC component of the developing bias voltage has an amplitude value of 1.2 KV in peak-to-peak value and a frequency of 6 kHz.
A rectangular wave is used as the waveform.

The developer used in this embodiment is a mixture of a toner having an average particle diameter of 7 μm and magnetic particles (ferrite carrier) having an average particle diameter of 50 μm.
The toner density is set to 7%.

Next, in the present invention, the output energy EE1 of the first light beam and the output energy EE1 of the first light beam, which are important parameters that greatly affect the exposure profile, particularly the inclination of the contour of the combined exposure image and the strength of the fringe, are considered. The ratio EE2 / EE1 of the light beam to the output energy EE2 will be described with reference to the drawings.

FIG. 8 shows the output energy E of the first light beam.
The ratio EE between E1 and the output energy EE2 of the second light beam
FIG. 2 is a diagram for explaining 2 / EE1.

The potential decay curve shown in FIG. 8 (a) shows that the potential decay is gentle for light input with weak exposure energy, but the potential decay curve becomes steeper as the exposure energy increases, and the inflection point The so-called S-shaped photopotential decay characteristic is shown in which the potential decay becomes gentle again when the exposure energy further increases beyond.

The potential decay curve shown in FIG.
It shows a so-called J-shaped photopotential decay characteristic in which the potential attenuates almost in proportion to the amount of incident light. When a photoconductor having this J-shaped photopotential attenuation characteristic is used, the output energy EE1 of the first light beam and the output energy EE1 of the first light beam are increased in order to secure a sufficient image contrast by increasing the inclination of the outline of the exposed image. Ratio EE2 / E of output energy of second light beam to EE2
It is necessary to increase E1 to some extent. To this end, as shown in FIG.
It must be set high, such as H '. However, if the charging potential of the photoconductor is too high, the fringe in the electrostatic latent image becomes too large, and carrier over is likely to occur.
In addition, non-uniformity of sensitivity and non-uniformity of charging property in the surface of the photoreceptor become remarkable, and the image is output as an uneven density. On the other hand, in the case of the photosensitive member having the S-shaped photopotential decay characteristic shown in FIG. 8A, it is not necessary to set the charging potential higher than the photosensitive member having the J-shaped photopotential decay characteristic. Since the fringe does not become excessive and carrier over does not occur, the image forming apparatus of the present invention has
It is preferable to use a photoreceptor exhibiting a V-shaped photopotential decay characteristic.

[0057]

Next, an embodiment of the present invention will be described.

First, the four types of photosensitive members used in this embodiment will be described.

FIG. 9 is a sectional view showing the structure of the photosensitive member used in this embodiment.

Of these four types of photoconductors, photoconductors A and
B and C are photoconductors that exhibit a so-called S-shaped photopotential decay characteristic, in which the potential decay is relatively unlikely to occur for a weak light input and the photopotential decay curve for the input exposure energy has an inflection point. Further, the photosensitive member D generally shows a potential decay almost proportional to the amount of incident light,
This is a function-separated type photoconductor having a charge generation layer and a charge transport layer using an organic semiconductor and exhibiting a so-called J-shaped photopotential decay characteristic.

FIG. 9A is a sectional view of the photosensitive members A, B, and C used in this embodiment. As shown in FIG. 9A, these photoconductors include a conductive support 101,
It is composed of a charge generation layer 102, a light potential attenuation control layer 103, and a charge transport layer 104.

Here, the photopotential decay control layer is a layer in which charge transporting domains are dispersed in an electrically inactive matrix, and is composed of two phases, an electrically inactive phase and a charge transporting phase. It is homogeneous. As a specific example of the photopotential attenuation control layer, a material in which a material having a charge transporting property is dispersed in a microcrystalline (charge transporting domain) state in an insulating resin (electrically inactive matrix) may be mentioned. In addition, the charge transport layer is formed of a charge transport matrix, and is a homogeneous system including one charge transport phase. Specific examples of the charge transport layer include a material in which a material having charge transport ability is dissolved in a binder resin in a molecular state and a charge transportable polymer.

Next, the photosensitive member A will be described in detail.

As the conductive support 101 of the photosensitive member A,
A cylindrical aluminum pipe was used. Charge generation layer 10
No. 2 is composed of a dichlorotin phthalocyanine pigment which is a photoconductive pigment and a polyvinyl butyral resin, each of which is mixed and dispersed in a solvent at a ratio of 2: 1 and applied to the above-mentioned aluminum pipe surface to form a 0.1. 2 μm
Was formed. Photoelectric attenuation control layer 10
3 consists of hexagonal selenium fine particles and vinyl chloride-vinyl acetate copolymer resin, and these are mixed and dispersed in a solvent,
By coating on the charge generation layer 102, a 2 μm photopotential decay control layer 103 was formed. The volume ratio of the hexagonal selenium fine particles in the light potential attenuation control layer was about 30%. The average particle diameter of the hexagonal selenium fine particles is 0.1.
It was 05 μm. Next, a coating solution obtained by dissolving a compound having a molecular weight of 80,000, which is a polymer charge transporting material and having a repeating unit represented by the following structural formula, in a solvent is applied to the above-mentioned photopotential attenuation control layer 103 by dip coating. Coating ・
After drying, a charge transport layer 104 having a thickness of 20 μm was formed.

[0065]

Embedded image

Next, the photosensitive member B will be described.

Photoconductor B was changed to photoconductor A except that the volume ratio of the hexagonal selenium microparticles in the photopotential attenuation control layer was changed from 30% to 35% by changing the amount of hexagonal selenium microparticles added.
Is the same as

Next, the photosensitive member C will be described.

Photoconductor C was changed except that the volume ratio of the hexagonal selenium microparticles in the photopotential attenuation control layer was changed from 30% to 40% by changing the amount of hexagonal selenium microparticles added.
Is the same as

FIG. 9B is a sectional view of the photosensitive member D used in the present embodiment.

As shown in FIG. 9B, this photosensitive member D
Are the conductive support 101, the intermediate layer 105, the charge generation layer 1
06 and the charge transport layer 107.

As the conductive support 101 of the photosensitive member D,
A cylindrical aluminum pipe was used. Also, the middle layer 1
For 05, a methoxymethylolated nylon resin having a thickness of 0.2 μm was used. The charge generation layer 106 of the photoreceptor A was the same as the charge generation layer 1 except that the hydroxygallium phthalocyanine pigment was used instead of the dichlorotin phthalocyanine pigment.
Same as 02. The charge transport layer 107 is the same as the charge transport layer 104 of the photoconductor A.

FIG. 10 is a graph showing the measurement results of the photo-potential decay characteristics of the photoconductors A, B, C, and D.

As shown in FIG. 10A, the photosensitive members A,
In B and C, the potential decay is gradual with respect to the light input having a low exposure energy, but the potential decay curve gradually becomes steeper as the exposure energy increases, and again when the exposure energy increases beyond the inflection point. It has a so-called S-shaped photopotential decay characteristic in which the potential decay becomes gentle.

As shown in FIG. 10B, the photosensitive member D is
It has a so-called J-shaped photopotential decay characteristic that shows potential decay almost proportional to the amount of incident light.

The photosensitive members A, B, C, and D described above are
Color copier Acolor6 manufactured by Fuji Xerox Co., Ltd.
30 is modified to adjust the ratio of the output energy EE1 of the first light beam to the output energy EE2 of the second light beam: EE2 / EE1, using an image forming apparatus having the configuration shown in FIG. A formation experiment was performed to evaluate the occurrence of carrier over in a halftone image. Table 1 shows the experimental conditions.

[0077]

[Table 1]

Table 2 shows the experimental results.

[0079]

[Table 2]

FIG. 11 shows E based on the experimental results shown in Table 2.
10 is a graph summarizing the relationship between 10 / E50 and EE2 / EE1.

As is clear from Table 2 and FIG. 11, the output energy of the first light beam is EE1 and the output energy of the second light beam is EE2. When the exposure energy required to attenuate the surface potential by 50% is E50 and the exposure energy required to attenuate the charged surface potential by 10% is E10, the relationship of E10 / E50 ≧ EE2 / EE1-0.2 is obtained. In each of Examples 1 to 12 of Table 2 that are satisfied, that is, in the region below the upward-sloping broken line shown in FIG. 11, no carrier over occurs in the halftone image. On the other hand, when the relationship between E10 / E50 and EE2 / EE1 is the data shown as Comparative Examples 1 to 4 in Table 2 in which E10 / E50 <EE2 / EE1-0.2,
That is, in the region above the dashed line ascending to the right as shown in FIG. 11, the structure of the electrostatic latent image in the halftone image portion is large (the change in the surface charge distribution is large), and carrier over is remarkably generated, resulting in uneven density. growing.

As described above, E10 / E50 and EE2 / E50
By satisfying the relationship of E10 / E50 ≧ EE2 / EE1-0.2 between E1 and E1, the occurrence of carrier over in a halftone image is prevented while having a practical light beam diameter of 30 μm or more. Thus, a good gradation image can be obtained.

In the above description, as a developing means, a rotary developing device provided with a plurality of developing units for sharing each of a plurality of electrostatic latent images and developing with each color toner to form a toner image of each color. 3 (see FIGS. 2 and 7) and a transfer means for transferring a plurality of toner images formed by the plurality of developing units onto a predetermined transfer receiving member so as to overlap each other. However, the present invention is not limited to only an image forming apparatus having a developing unit and a transfer unit corresponding to a plurality of colors. However, when the image forming apparatus is configured as the image forming apparatus having the developing unit and the transfer unit corresponding to a plurality of colors as described above, it is possible to uniformly reproduce an image over a plurality of colors. This is preferable because a great effect can be obtained in that unevenness is not generated.

[0084]

As described above, according to the image forming apparatus of the present invention, it is possible to prevent the halftone image from being lost due to carrier over without using a specially small light beam spot diameter, and to provide a rough image. It is possible to realize an image forming apparatus capable of obtaining an image with no feeling and excellent in granularity.

[Brief description of the drawings]

FIG. 1 is a graph showing photo-electric potential attenuation characteristics of a photoconductor.

FIG. 2 is a schematic configuration diagram of an embodiment of an image forming apparatus of the present invention.

FIG. 3 is a schematic configuration diagram of a light beam scanning device in the image forming apparatus according to the first embodiment.

FIG. 4 is a sectional view of the light beam scanning device shown in FIG. 3 in a main scanning direction.

5 is a schematic diagram showing an operation and a configuration of an image signal control device of the light beam scanning device shown in FIG.

FIG. 6 is a schematic diagram illustrating a process in which a combined exposure image is obtained by the image forming apparatus according to the first embodiment.

FIG. 7 is a schematic configuration diagram of a developing unit included in a rotary developing device used in the image forming apparatus according to the first embodiment.

FIG. 8 is a diagram for explaining a ratio EE2 / EE1 between the output energy EE1 of the first light beam and the output energy EE2 of the second light beam.

FIG. 9 is a cross-sectional view illustrating a structure of a photoconductor used in the present embodiment.

FIG. 10 is a graph showing measurement results of photo-electric potential attenuation characteristics of photoconductors A, B, C, and D.

FIG. 11 shows the experimental results shown in Table 2 obtained by comparing E10 / E50 and E10.
6 is a graph summarizing the relationship of E2 / EE1.

FIG. 12 is a schematic diagram for explaining a conventional bias exposure method.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum 2 electrostatic latent image forming charger 3 rotary developing device 3a developing unit 3b developing roll 4 transfer drum 4a paper suction charger 4b transfer charger 4c peeling charger 4d peeling claw 4e discharging charger 5 Cleaner 6 Pre-exposure device 9 Fixing device 11 Paper tray 12 Paper transport path 20 Light beam scanning device 21a, 21b Laser diode 22a, 22b Collimator lens 23 Beam splitter 24 Cylinder lens 25 Reflection mirror 26 Polygon mirror 27 f-θ lens 28 Cylinder mirror 29 Scanning optical system 31a, 31b Laser driving device 32 Image signal control device 32a Amplifier 32b Inverting amplifier 33 Image data 34a Image signal 34b Inverted image signal 36 Image contour portion 61 Developing roll 61_1 Sleeve 61_2 Magnet roll 62 Housing 63, 64 Screw auger 65 Developer regulating member 66 a, 66 b,..., 66 e Magnetic pole 68 DC superimposed AC voltage power supply 101 Conductive support 102 Charge generation layer 103 Light potential attenuation control layer 104 Charge transport layer 105 Intermediate layer 106 Charge generation layer 107 Charge transport layer

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H04N 1/23 103 G03G 15/04 120 5C074 // G02B 26/10 F term (reference) 2C262 AA05 AA17 AB07 BB18 BB30 BB31 BB37 BB38 GA35 GA36 GA39 GA43 2C362 AA09 AA54 AA66 BA64 BA67 CA12 CA18 CB59 CB80 2H030 BB02 BB13 BB21 BB41 BB71 2H045 AA01 BA22 CA63 CB41 2H076 AB05 AB06 AB22 AB34 AB75 DA06 DA21 5C074 AA02 BB03 FB03 BB03

Claims (2)

[Claims] 1. A photosensitive member which moves in a predetermined direction and on which an electrostatic latent image is formed, charging means for charging the photosensitive member, and light corresponding to an image signal in a predetermined scanning direction on the charged photosensitive member. Exposure means for forming an electrostatic latent image on the photoconductor by scanning with a beam, and development for forming a toner image on the photoconductor by developing the electrostatic latent image formed on the photoconductor An image forming apparatus comprising: a first light beam generating unit configured to generate a first light beam that is on / off-modulated based on the image signal; and the second unit configured to generate the first light beam based on the image signal. A second light beam that generates a second light beam that is reciprocally on-off modulated with the first light beam
A light beam generating means, and a light beam combining means for combining the first light beam and the second light beam respectively generated from the first light beam generating means and the second light beam generating means into one light beam. A scanning optical system that guides the light beam synthesized by the synthesizing means to the photoconductor and scans the photoconductor in the main scanning direction; and sets the output energy of the first light beam to EE1, The output energy of the light beam of No. 2 is EE2, and the charged surface potential in the photopotential decay characteristic of the photosensitive member is 5
E10 / E50 ≧ EE2 / EE1-0.2, where E10 is the exposure energy required to attenuate 0% and E10 is the exposure energy required to attenuate the charged surface potential by 10%. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the developing unit includes a plurality of developing units that share each of the plurality of electrostatic latent images and develop each of the plurality of electrostatic latent images with toner of each color to form a toner image of each color. 2. The image forming apparatus according to claim 1, further comprising a transfer unit configured to transfer the plurality of toner images thus formed onto a predetermined transfer receiving member so as to overlap each other.