JPH0519601A - Image forming method - Google Patents

Abstract

(57) [Summary] [Purpose] The gradation is always stable and high image quality is maintained without being affected by the beam intensity of the exposure light, the light attenuation characteristics of the photoconductor, the development characteristics, the transfer characteristics, and the fixing characteristics. Provided is a digital image forming method that can be performed. The diameter of the LD beam L in the main scanning direction is set to 35 μm, and the LD beam L is irradiated onto the photosensitive drum to form an electrostatic latent image for one pixel. To scan. In this way, the area where the image density of the latent image exceeds the potential at which the image density is saturated becomes relatively large, and the area lower than the potential at which the image density saturates decreases. Therefore, the toner adhesion state and the like are stable, and the gradation expression is also stable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method applied to a printer, a digital copying machine, a facsimile and the like, and more particularly to an image forming method using an area gradation method.

[0002]

2. Description of the Related Art In a digital copying machine or the like, exposure light from a semiconductor laser (LD) is modulated in accordance with an optical writing signal obtained on the basis of image data to expose a charged photoconductor to expose the photoconductor. An electrostatic latent image is formed on. Next, toner is supplied to the electrostatic latent image to develop it, and a toner image is formed on the photoconductor. Then, the toner image on the photoconductor is transferred to the transfer paper, the toner image on the transfer paper is fixed, and the image formation is completed.

The example described below is a developing method (reverse development) in which toner is attached to a portion where the potential is lowered by exposure.
Is adopted. 9A and 9B show latent images formed by giving gradation to one pixel. Vh in the figure
Indicates the potential of the photoconductor charged by the charging charger, and Vl is the potential at which the image density is saturated.

A spot is formed by using a beam having a size of about one pixel, and (a) is formed by a pulse width modulation method, that is, by dividing the exposure time stepwise and changing the exposure intensity to give gradation within one pixel. 2B shows an electrostatic latent image, and FIG. 3B shows an electrostatic latent image formed by a power modulation method, that is, by changing the exposure intensity and giving gradation within one pixel. It is known that a digital copying machine has a writing density of 400 dpi and modulates exposure light to record one pixel in a range of three or more gradations.

In any of the above methods, gradation is expressed by using an area higher than the exposed portion potential Vl and lower than the non-exposed portion Vh, that is, an intermediate density area where the toner density is intermediate. The factors that influence the gradation expression such as the toner density in the intermediate density region are compared with the potential Vl at which the image density is saturated, the beam intensity of the exposure light, which is the electrophotographic characteristic, the light attenuation characteristic of the photoconductor, the developing characteristic, Transfer characteristics,
It has a drawback that it is easily affected by fixing characteristics and the like, and stable gradation expression cannot be performed.

In addition to the above gradation expression method, there is an area gradation method using a dither method or the like. In this area gradation method, a matrix of a predetermined size is divided, and this division is defined as one pixel,
A predetermined pixel in the matrix is irradiated with exposure light based on the binarized image data to form an electrostatic latent image, toner is supplied to the electrostatic latent image, and a predetermined number and a predetermined position in the matrix are supplied. The toner is attached to the pixels of (1) to express the intermediate gradation.

[0007]

FIG. 10A is a typical view of an electrostatic latent image formed by irradiating a photoreceptor charged with a charging potential Vh with a laser beam. 1 pixel is 2
When the gradation is expressed by a value and the area gradation method is used, the finer the image density, the smoother the expression becomes. The increase in the density is a decrease in the area of one pixel, and the stability of development is significantly deteriorated.

As shown in the figure, since there is a sufficient potential difference in the region d in which the image density of the electrostatic latent image exceeds the potential Vl, the beam intensity of the exposure light, which is the electrophotographic characteristic, and the light attenuation characteristic of the photoconductor are characteristic. , Development characteristics, transfer characteristics, fixing characteristics, etc., and less susceptible to changes over time. Therefore, the factors that affect the gradation expression, such as the toner adhesion amount, are constant. On the other hand, in the region f lower than the potential Vh, since there is no sufficient potential difference, it is easily affected by the change of the electrophotographic characteristics, and the factors that affect the gradation expression such as the toner adhesion amount are easily changed.

The latter method is used to form a latent image, and the area gradation method is used to increase the density for smoother halftone expression, which causes a relative increase in toner particle size per pixel.
Increases noise in the development process. Especially when the toner particle size is 50
This tendency is remarkable when developing pixels of twice or less. Therefore, as shown in FIG. 10B, the toner may scatter in a wide range around one pixel G and the gradation reproducibility may be deteriorated.

Especially, the electrophotographic characteristics change depending on the environment,
Further, since it also changes with time, it becomes difficult to maintain high image quality, and in the end, it has the same drawbacks as the image forming method by the analog method using the intermediate density area for gradation expression. However, there is a problem that the advantages of the digital image forming method cannot be utilized.

The present invention has been made by paying attention to the above-mentioned conventional problems. In a system in which the size of one dot in area gradation is 1/50 or less of the toner particle size, the beam intensity of the exposure light is increased. An object of the present invention is to provide a digital image forming method that is not easily affected by changes in the light attenuation characteristics, developing characteristics, transfer characteristics, fixing characteristics, etc. of a photoconductor and that can always maintain high image quality.

[0012]

According to a first aspect of the present invention, a charging step of charging a photoconductor is performed, and writing is performed on the charged photoconductor by exposure light using an area gradation method based on image data. It has an exposure step of forming an electrostatic latent image and a developing step of supplying toner to the electrostatic latent image formed on the photoconductor to develop the electrostatic latent image.
In an image forming method for forming dots, exposure light of a beam spot smaller than the pixel size in the main scanning direction and larger than the pixel size in the sub scanning direction is used to form an electrostatic latent image for one pixel in the main scanning. The beam spot of the exposure light is scanned in the direction to perform exposure to form an electrostatic latent image,
In the developing step, a developer including a toner and a carrier is used, and the electrostatic latent image is developed under an electric field having a vibration component that vibrates the toner and does not vibrate the carrier. It is a forming method.

[0013]

According to the present invention, the photosensitive member is charged, and an area having a cross-sectional area of 50 times or more of the toner is defined as one pixel by the area gradation method based on the image data on the charged photosensitive member. The exposure light of the beam spot smaller than the pixel size and larger than the pixel size in the sub-scanning direction exposes the beam spot of the exposure light in the main scanning direction to form an electrostatic latent image for one pixel. Done. Next, a developer consisting of toner and carrier is used, and the toner is supplied to the electrostatic latent image formed on the photoconductor to develop the toner under an electric field having a vibration component that vibrates the toner and does not vibrate the carrier. Done.

[0014]

Embodiments of the present invention will be described with reference to the drawings. FIG. 6 shows a digital copying machine. In the figure, reference numeral 3
Reference numeral 1 denotes a photoconductor drum, and the photoconductor drum 31 rotates clockwise. Around the photosensitive drum 31, a charging charger 32, a developing device 33, and a transfer charger 35 are arranged. Reference numeral 38 is a conveying device, reference numeral 36
Indicates a fixing device.

Reference numeral 40 denotes an exposure optical system, and the detailed structure of the exposure optical system 40 is shown in FIG. In the figure, reference numeral 41 denotes a semiconductor laser, and a condenser lens 45, an aperture 49, a cylindrical lens 46, and a polygon mirror 42 in the laser light irradiation direction of the semiconductor laser 41.
Are arranged. Further, a lens 44 and a mirror 47 are arranged in the reflection direction of the polygon mirror 42. The photosensitive drum 31 is arranged in the reflection direction of the mirror 47. Reference numeral 48 denotes a photodetector for keeping the writing position constant.

FIG. 8 shows the detailed structure of the developing device 33.
In the figure, reference numeral 61 indicates a casing, and an opening 61a is formed in this casing 61. Reference numeral 52
Indicates a developing sleeve that rotates counterclockwise, a part of the peripheral surface of the developing sleeve 52 is exposed from the opening 61a, and the exposed portion faces the peripheral surface of the photosensitive drum 31. A magnet 53 is arranged in the developing sleeve 52. A two-component developer including a carrier and toner is contained in the casing 61.

The carrier is made of a magnetic material as a core material, and the core material is coated with a polymer, and has a particle size of 20 μm.
To 60 μm are used. The toner is made of a polymer containing a colorant and has a size of about 7.0 μm.

A bias voltage applying device (not shown) for applying a bias voltage to the developing sleeve 52 is provided.
This bias voltage applying device is adapted to apply an oscillating current on which a direct current is superimposed. DC current is -200
Used in the range of V to -500V, vibration component is 3KH
It is used in the range of 300 V (pp) to 3000 V (pp) for z or more. Further, the bias waveform may be any waveform as long as it is periodic, such as a triangular wave, a sine wave, and a rectangular wave.

In the rear area of the developing sleeve 52, a separator 56, a counterclockwise rotating paddle wheel 55,
An agitator 54 that rotates in the clockwise direction is arranged.
Reference numeral 63 indicates a toner replenishing section, and this toner replenishing section 63
Toner 65 is stored in the inside. Toner supply unit 63
The toner 65 inside is replenished into the casing 61 by a toner replenishing roller 69.

Next, a basic image forming operation of this digital copying machine will be described. The photoconductor drum 31 rotates clockwise and is charged by the charging charger 32. Next, exposure is performed by the exposure optical system 40, and an electrostatic latent image is formed on the photosensitive drum 31. That is, the original image is read by an image reading device (not shown), and the image data is sent to an image processing device (not shown). Then, image processing such as gamma conversion and halftone reproduction processing is performed by the image processing device, and laser light is emitted from the semiconductor laser 41 based on the processed image information. This laser light passes through the condenser lens 45, the beam diameter is adjusted by the aperture 49, and the cylindrical lens 46
And is further reflected by the polygon mirror 42. Then, the laser light reflected by the polygon mirror 42 passes through the lens 44 and is reflected by the mirror 47 toward the photoconductor drum 31 to form an electrostatic latent image on the photoconductor drum 31.

The electrostatic latent image formed on the photosensitive drum 31 is developed by supplying toner from the developer carried on the developing sleeve 52 of the developing device 33. On the other hand, the transfer paper is sent to the area where the photoconductor drum 31 and the transfer charger 35 face each other by a paper feeding device (not shown), and the toner image on the photoconductor drum 31 is transferred onto the transfer paper by the action of the transfer charger 35. The transfer paper on which the toner image is transferred is sent to the fixing device 36 by the conveying device 38, and after the toner image on the transfer paper is fixed, the transfer paper is ejected to the outside of the machine.

In this embodiment, a threshold matrix 70 as shown in FIG. 4 is used and a dither method, which is one of the area gradation methods, is used, and image data for expressing gradation based on an original image. Are processed to be linear as shown in FIG. In addition to the dither method, an error diffusion method can be used.

The working conditions in the first embodiment are as follows: the charging potential of the photosensitive drum 31 is -400 V, the linear velocity of the photosensitive drum 31 is 150 mm / s, the writing density is 600 dpi, L.
7. The diameter of the D beam in the main scanning direction is 35 μm, the diameter in the sub scanning direction is 55 μm, the potential of the exposed portion is −20 V, and the toner particle size is 7.
0 μm (weight average), carrier particle diameter 50 μm (weight average), toner charge amount 18 μc / g, AC3 KHz as a developing bias, and a voltage obtained by superimposing DC-300 V on 1 KVpp are applied to the developing sleeve 52 and the photosensitive drum 31. The gap is 500 μm, and the linear velocity of the developing sleeve 31 is 180 mm / s. The gap between the developing sleeve 52 and the photosensitive drum 31 is 200 μm to 1000 μm.
It is possible to set within the range.

The working conditions in the second embodiment are that the charging potential of the photosensitive drum 31 is -400 V, the linear velocity of the photosensitive drum 31 is 150 mm / s, the writing density is 600 dpi, L.
Diameter of D beam in main scanning direction is 35 μm, diameter in sub scanning direction is 55 μm, potential of exposed portion is −20 V, toner particle size is 7.
0 .mu.m (weight average), carrier particle size 50 .mu.m (weight average), toner charge amount 18 .mu.c / g, and a developing bias voltage of AC3 KHz, 1, 5 KVpp superimposed with DC-300 V is applied, and the developing sleeve 52 and the photosensitive drum 3 are applied.
1, and the linear velocity of the developing sleeve 31 is 180 mm / s.

In the first and second embodiments, since the diameter of the LD beam in the main scanning direction is 35 μm, as shown in FIG. 2, the static image formed by irradiating the photoconductor drum 31 with this LD beam is used. The rising of the latent image is steep. Therefore, the area f lower than the potential Vh becomes smaller. Then, as shown in FIG. 1, to form an electrostatic latent image for one pixel, the spot L of the LD beam is scanned in the main scanning direction indicated by the arrow. In this way, the area d exceeding the potential Vl where the image density of the latent image is relatively saturated becomes large, and the area f lower than the potential Vl can be reduced.

Since the writing density is 600 dpi, it is possible to obtain a good image when judged by human eyes. According to a study published in Japan Hardcopy '89, human vision is 10 cycles / mm.
If there is a spatial wavelength of, only two values can be discriminated. Therefore, when performing binary recording, it may not be determined that the image is smooth at 400 dpi, but it may be determined that the image is sufficiently smooth at 600 dpi. A sample of silver salt image was prepared on photographic paper and sensory evaluation was conducted, and the results supporting this were obtained. It is also possible to prevent the image from becoming thick when fixing the image on the transfer paper.

At the time of development, a developer consisting of toner and carrier is used, and since a bias having a vibration component that vibrates the toner and does not vibrate the carrier is applied, the toner in the developer on the developing sleeve 52 is removed. It vibrates violently and becomes a floating state, so that it becomes possible to reliably supply the toner to a fine portion of the electrostatic latent image. Due to the increase in writing density for performing smooth gradation expression, it is necessary to develop with a larger toner for one pixel area than in the conventional case. By applying the bias having the vibration component, this requirement can be satisfied, and in addition to the above-described effect of latent image formation under the condition that the size of one pixel is 50 times the toner particle size or less, which is difficult for development, As a result, the toner adhesion state of the pixel G becomes stable as shown in FIG.

Conventionally, the absolute value of the charging potential of a normal photoconductor is 1 KV to 800 V. This is because a large potential difference is required to express an intermediate gradation, and a large amount of toner needs to adhere to the photoconductor in order to obtain a sufficient toner density. On the other hand, in this embodiment, the photosensitive drum 31 is charged to −400 V and gradation is expressed based on the binarized image information.
Therefore, the potential can be sufficiently attenuated with a small exposure amount, and due to the above-described effect at the time of development, the toner can be sufficiently adhered and a sufficient image density can be obtained. The potential of the photoconductor is not limited to -400V, and the potential difference between the image portion and the non-image portion can be 500V or less in absolute value.

The working conditions in the third embodiment are as follows: the charging potential of the photosensitive drum 31 is −400 V, the linear velocity of the photosensitive drum 31 is 150 mm / s, the writing density is 600 dpi, L.
Diameter of D beam in main scanning direction is 35 μm, diameter in sub scanning direction is 55 μm, potential of exposed portion is −20 V, toner particle size is 7.
0 μm (weight average), carrier particle size 50 μm (weight average), a voltage of DC-300V superimposed is applied, and the gap between the developing sleeve 52 and the photosensitive drum 31 is 500 μm.
m, the linear velocity of the developing sleeve 31 is 180 mm / s, the toner charge amount is 20 μc / g, and the developing bias is DC-30.
When a voltage of 0 V is applied, the gap between the developing sleeve 52 and the photosensitive drum 31 is 500 μm, the linear velocity of the developing sleeve 31 is 180 mm / s, the number of magnetic poles of the magnet roller 53 is 8 poles, and the magnetic force is 800 gauss. It is rotated clockwise at 1300 rpm by a rotating device (not shown).

The working conditions in the fourth embodiment are as follows: the charging potential of the photosensitive drum 31 is -400 V, the linear velocity of the photosensitive drum 31 is 150 mm / s, the writing density is 600 dpi, L.
Diameter of D beam in main scanning direction is 35 μm, diameter in sub scanning direction is 55 μm, potential of exposed portion is −20 V, toner particle size is 7.
0 μm (weight average), carrier particle size 50 μm (weight average), DC-300V superimposed voltage is applied, and the gap between the developing sleeve 52 and the photosensitive drum 31 is 800 μm.
m, the linear velocity of the developing sleeve 31 is 180 mm / s, the toner charge amount is 20 μc / g, and the developing bias is DC-30.
When a voltage of 0 V is applied, the gap between the developing sleeve 52 and the photosensitive drum 31 is 500 μm, the linear velocity of the developing sleeve 31 is 180 mm / s, the number of magnetic poles of the magnet roller 53 is 6, and the magnetic force is 1000 gauss. It is rotated clockwise at 1500 rpm by a rotating device (not shown).

The magnetic force of the magnet roller 53 is preferably about 800 gauss to 1000 gauss, and the rotation speed is 1
300 rpm to 1500 rpm is preferred. The gap between the developing sleeve 52 and the photosensitive drum 31 is 200 μm.
It is possible to set in the range from 1 to 1000 μm.

In the third and fourth embodiments, the magnet roller 53 rotates clockwise, that is, in the direction opposite to the developing sleeve 52. Therefore, the magnetic poles of the magnet roller 53 sequentially pass through the developing area, which is the area where the photosensitive drum 31 and the developing sleeve 52 face each other, the carrier on the developing sleeve 52 is agitated, and the separation of the toner from the carrier is promoted. Be in a floating state. Therefore, the efficiency with which the toner is supplied to the electrostatic latent image is increased, and the toner can be surely supplied to the fine portion of the electrostatic latent image.
Therefore, the gradation reproducibility of one pixel is improved.

The working conditions in the fifth embodiment are as follows: the charging potential of the photosensitive drum 31 is -400 V, the linear velocity of the photosensitive drum 31 is 150 mm / s, and the writing density 60 in the main scanning direction.
0 dpi, write density in the sub-scanning direction is 800 dpi,
The diameter of the LD beam in the main scanning direction is 35 μm, the diameter in the sub scanning direction is 35 μm, the potential of the exposed portion is −20 V, the toner particle diameter is 5.2 μm (weight average), the carrier particle diameter is 50 μm (weight average), and the toner charge amount is 18 μc / g, as a developing bias, a voltage of AC 3 KHz, 1 KVpp and DC-300 V superposed is applied, and the developing sleeve 52 and the photoconductor drum 31.
And the linear velocity of the developing sleeve 31 is 180 mm / s.

In the fifth embodiment, the size of the pixel in the main scanning direction is 42 μm and the size in the sub scanning direction is 31 μm.
The pixels are larger in the main scanning direction than in the sub scanning direction. Therefore, it was possible to efficiently use the power of the laser light by using an LD beam having a circular spot and obtain a higher density image.

Although one laser beam is used in the above embodiment, a plurality of laser beams may be used. By using a plurality of laser beams, high-density recording can be realized without increasing the rotation speed of the polygon mirror. Although a digital copying machine is shown in the above embodiment, the present invention can be applied to a printer and a facsimile.

[0036]

As described above, according to the present invention, a region of 50 times or less the toner cross-sectional area is defined as one dot, and a beam spot smaller than the pixel size in the main scanning direction and larger than the pixel size in the sub scanning direction is formed. In order to form an electrostatic latent image for one pixel using the exposure light, the beam spot of the exposure light is scanned in the main scanning direction to perform exposure to form an electrostatic latent image. Is used to develop the electrostatic latent image under an electric field having a vibration component that vibrates the toner and does not vibrate the carrier. Therefore, the beam intensity of the exposure light and the light attenuation characteristics of the photoconductor are used. It is possible to form an image by a digital method that can always express gradation and maintain high image quality without being affected by development characteristics, transfer characteristics, fixing characteristics, and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing movement of an LD beam spot used in an image forming method according to an embodiment of the present invention in a main scanning direction.

FIG. 2 is a diagram showing a potential state of an electrostatic latent image formed on a photosensitive drum by an image forming method according to an example of the present invention.

FIG. 3 is an enlarged view of a toner image formed by the image forming method according to the exemplary embodiment of the present invention.

FIG. 4 is a diagram showing a threshold matrix used in the image forming method according to the embodiment of the present invention.

FIG. 5 is a graph showing the relationship between image data and image density in the image forming method according to the embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of a digital copying machine according to an embodiment of the present invention.

7 is a diagram showing a configuration of an optical system installed in the digital copying machine of FIG.

8 is a diagram showing a configuration of a developing device mounted in the digital copying machine of FIG.

FIG. 9 is a diagram showing a potential state of an electrostatic latent image formed on a photosensitive drum by an image forming method according to a conventional example.

FIG. 10 is a diagram for explaining a conventional problem.

[Explanation of symbols]

 31 Photoreceptor Drum 33 Developing Device 40 Optical System 52 Developing Sleeve 53 Magnet Roller L LD Beam Spot G Pixel

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Claims (1)

Claim: What is claimed is: 1. A charging step for charging a photoconductor, and an electrostatic latent image is written on the charged photoconductor by exposure light using an area gradation method based on image data. It has an exposure step of forming and a developing step of supplying toner to the electrostatic latent image formed on the photoconductor to develop the latent image.
In an image forming method of forming an area equal to or less than twice as large as one dot, exposure light of a beam spot smaller than the pixel size in the main scanning direction and larger than the pixel size in the sub scanning direction is used to form an electrostatic latent image for one pixel. To form an electrostatic latent image, a beam spot of exposure light is scanned in the main scanning direction to form an electrostatic latent image. In the developing step, a developer composed of toner and carrier is used to vibrate the toner. ,
An image forming method, wherein an electrostatic latent image is developed under an electric field having a vibration component that does not vibrate a carrier.