JPH08123123A - Color image forming device - Google Patents

Abstract

PURPOSE: To achieve faithful color reproduction, character reproduction of high quality, reductions in size and cost, and a high speed output by making toner images of separated colors in each image forming unit different from each other in electrostatic-latent-image screen structure. CONSTITUTION: In a part higher than the middle of a color image forming device 1, a transfer drum 25 is set which is used to transfer toner images formed by two image-forming units 11 and 12 each of which forms toner images of two separate colors onto recording paper 20. The two image-forming units 11 and 12 are equipped with photoreceptor drums 21a and 21b, respectively, each having a photoconductive thin layer, such as an organic photosensitive layer, on its surface. By exposing the surfaces of the photoreceptor drums 21a and 21b to laser beam 35 for scanning, toner images of the separated colors in each of the image forming units 11 and 12 are made different from each other in electrostatic-latent-image screen structure. Thus, an average toner height on the paper 20 or intermediate transfer body can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention carries out uniform charging on a photosensitive medium, image exposure with a light beam, and development with a plurality of color toners a plurality of times to form toner images of a predetermined number of colors on the photosensitive medium. A xerographic system for forming a color image on a recording medium by sequentially transferring toner images of a predetermined number of colors formed by these image forming units on the recording medium. The present invention relates to a color image forming apparatus.

[0002]

2. Description of the Related Art In recent years, xerographic full-color image forming apparatuses have become widespread along with the colorization of document processing in offices and the like, but this type of full-color image forming apparatus is small, inexpensive and simple. It is required to simultaneously satisfy various characteristics such that a full-color image can be obtained at high speed with a wide range of structures and a wide range of paper can be used.

Conventionally, as a color image forming apparatus of this type, which is small, inexpensive, and has a simple structure, a full-color image can be obtained at high speed and a wide range of usable papers can be satisfied, for example, There is one disclosed in JP-A-4-204871. FIG. 19 shows the outline of the overall configuration of the color image forming apparatus according to Japanese Patent Laid-Open No. 4-204871. The schematic configuration and operation will be described below.

In the figure, 101a and 101b are organic photoconductors having the same diameter, which rotate in the arrow direction during operation. Reference numerals 102a and 102b are chargers, and
The surface of a and 101b is uniformly charged. 10
Reference numeral 3 denotes a laser optical system, which is a uniformly charged photoconductor 10.
Two types of laser beams 103 are provided on the surfaces of 1a and 101b
a and 103b are exposed to form a latent image. 1
Reference numerals 04M, 104C, 104Y and 104K are a magenta developing device, a cyan developing device, a yellow developing device and a black developing device, respectively. 106a and 106b are photoreceptors 101a,
A cleaner for cleaning the surface of 101b. Reference numeral 107 denotes a wide endless belt-shaped intermediate transfer member,
08 and support rollers 109 and 110, and during operation, driven by the drive roller 8 to rotate in the arrow direction at the same speed as the peripheral speed of the photoconductors 101a and 101b. The intermediate transfer member 107 is in light contact with the photosensitive member 101a at the first transfer portion Ta, and the transfer charger 111 is provided on the back surface thereof.
a, the toner image on the photosensitive member 101a is transferred to the intermediate transfer member 1.
The transfer charger 1 can be transferred to the 07 side, and is further in contact with the photoconductor 101b at the second transfer portion Tb.
By the action of 11b, the toner image on the photoconductor 101b can be transferred to the intermediate transfer body 107 side. A transfer charger 112 is provided opposite to the third transfer portion Tc of the support roller 110, and when the sheet moves while contacting the intermediate transfer body 107 at this portion, the intermediate transfer body 1 is formed.
The toner image on 07 is transferred to the paper. Numerals 113 and 114 are static eliminators, which neutralize the residual charges of the intermediate transfer member 107 by an appropriate amount.

When the operation starts, the photoconductors 101a, 10
1b and the intermediate transfer member 107 start rotating, and the photosensitive member 101
The surfaces of a and 101b are uniformly charged by the chargers 102a and 102b. Then, the photoconductors 101a and 101b
The laser beams 103a and 103b modulated by the respective color signals are irradiated on the surface of the, and an electrostatic latent image corresponding to the image is formed. The electrostatic latent image on the photoconductor 101a is developed by the magenta developing device 104M,
The magenta toner image formed on a is sequentially transferred to the intermediate transfer member 107 from the leading edge of the image as it rotates. on the other hand,
Similarly, the electrostatic latent image on the photoconductor 101b is also developed by the yellow developing device, and sequentially transferred from the front end of the magenta image onto the intermediate transfer body 107. Depending on the latent image writing timing at this time, the magenta toner and the yellow toner are positionally aligned and transferred. Photoconductor 101 after transfer
The residual toners of a and 101b are cleaned by the cleaners 106a and 106b to prepare for the next image formation.

The magenta and yellow toner images transferred to the intermediate transfer member 107 are discharged in accordance with the circulation.
After the charge is removed by 13, 114, the cyan toner image formed on the photoconductor 101a and the black toner image formed on the photoconductor 101b are sequentially superimposed and transferred. The toner image formed as described above is transferred and fixed on the paper at the same timing at the third transfer portion Tc to become a full-color image.

Further, as another conventional color image forming apparatus, there are Japanese Patent Laid-Open Nos. 2-105175 and 2-10.
Some are disclosed in Japanese Patent No. 5174. The color image forming apparatuses disclosed in these publications are intended to form a fine multicolor image without color shift at high speed. FIG. 20 shows an outline of the entire configuration of the color image forming apparatus disclosed in Japanese Patent Laid-Open No. 2-105175. The schematic configuration and operation will be described below with reference to the drawings.

In the figure, reference numerals 201a and 201b denote photosensitive drums having the same diameter.
A cleaner 202 is provided around each of 1a and 201b.
a, 202b, static elimination lamps 203a, 203b, chargers 204a, 204b, and exposure means 217Y, 217.
M, 217C and 217BK, and developing units 205Y and 205
M, 205C, 205BK and chargers 204a, 204
b are sequentially arranged. The transfer material passes through the registration roller 215 and is transferred to the transfer drum 21 by the adsorption charger 209.
6 is adsorbed on.

In the above structure, the photosensitive drum 201a is irradiated with a laser beam corresponding to the image information of the yellow component to form an electrostatic latent image corresponding to the yellow component, and then the developing unit 205Y forms a yellow toner image. To form. The image information of the magenta component stored in the buffer memory is output as a laser beam after a predetermined time, and the yellow and magenta toner images are simultaneously transferred onto the transfer material supplied at the timing of the image forming process. Next, after a predetermined time from the latent image formation time of the yellow component,
A laser beam corresponding to a cyan component is applied to the photosensitive drum 201b.
To form a latent image corresponding to the cyan component, and then the developing device 205C forms a cyan toner image.
The image information of the black component stored in the buffer memory is output as laser light after a predetermined time, and the cyan and black toner images are simultaneously transferred onto the transfer material supplied at the timing of the image forming process. In this way, four-color toner images are formed during one rotation of the transfer drum 216.

[0010]

However, the above-mentioned prior art has the following problems. That is, in the case of the color image forming apparatus according to Japanese Patent Laid-Open No. 4-204871, the toner images are sequentially transferred on the intermediate transfer member for each color, and therefore, the intermediate toner image is transferred during the transfer of the four color toner images. Since the transfer body is always in contact with the photosensitive body, there is a problem that the mechanical contact portion between the non-image area of the intermediate transfer body and the photosensitive body adversely affects the life of both of them. Further, in the case of the color image forming apparatus disclosed in JP-A-4-204871 and JP-A-2-105175, the toner images transferred to the intermediate transfer member or the transfer material and superposed on each other have the maximum density on the paper. 1 to 2 layers of toner images are required for each color, and the maximum height of the superimposed toner images is 50 to 70 μm in total. There are problems that the transfer efficiency is lowered in a portion having a low toner height corresponding to an image, and the image quality is significantly impaired due to the scattering of toner in the high density color character portion. Further, the toner images respectively superposed on the photoreceptor are
To obtain the maximum density on the paper, one to two toner layers are required for each color, and the total maximum height reaches 50 to 70 microns, so due to the influence of the toner layer charge and the decrease in the toner layer transmittance, There is a problem that it is difficult to accurately reproduce the color of the overlapped portion.

On the other hand, as a method of outputting a color image at high speed, a tandem type color image forming apparatus in which four photoconductors corresponding to yellow, magenta, cyan and black are arranged and the images are sequentially transferred to a paper sheet. It has been known.

However, this system has a problem in that it is difficult to reduce the size and cost because the device is large and the configuration is complicated, and the positional deviation control of the image is large.

Further, to explain the above-mentioned problems in image quality, in the case of the color image forming apparatus according to the above-mentioned JP-A-4-204871 and JP-A-2-105175, it is already formed on the photoreceptor. The toner image that is formed on the photoconductor is blocked by the toner image that interferes with the next image exposure, and the potential of the toner layer rises when the charged toner layer is stacked on the photoconductor. There is a problem that the latent image potential of the electric latent image is affected and the gradation and resolution are degraded. Similarly, in this type of color image forming apparatus, due to the charge of the toner layer already laminated on the photoconductor, the potential of the photoconductor does not decrease sufficiently even if image exposure is performed, and the potential of the exposed portion increases. However, there is also a problem that the resolution is lowered due to the dielectric properties of the toner layer already laminated on the photoreceptor.

In this way, by repeating charging, image exposure and development on one photoconductor for a plurality of colors, toner images are formed on the photoconductor in the state of being laminated on each other by a plurality of colors. The color electrophotographic method in which toner images of a plurality of colors formed on the photoconductor are sequentially transferred onto an intermediate transfer body or a transfer material can reduce the size and cost of the apparatus, and has less color misregistration. Although it has an excellent feature that it can provide a high quality color image in principle, at the same time, it causes deterioration of image quality due to increase of electric potential of toner layer, dielectric property of toner layer, and toner shielding effect. There is also a problem that the image quality of a color image is seriously adversely affected.

Among the problems that adversely affect the image quality of these color images, the problems associated with the increase in the potential of the toner layer (or the increase in the potential of the exposed portion due to the charge of the toner layer) are the toners of a plurality of colors on the photoreceptor. The superposition of images causes the largest deterioration in image quality among the adverse effects caused by the method of forming a color image. Therefore, it is extremely important to solve the problem caused by the increase in the potential of the toner layer.

Further, the potential of the toner layer actually formed on the photosensitive member largely changes depending on the weight of the developing toner per unit area. According to experiments conducted by the present inventors, the potential of the toner layer actually formed on the photosensitive member changes as shown in FIG. 21 depending on the weight of the developing toner per unit area. In the highlight region, which is particularly important for a high-quality color image, the toner concentration is low and the weight of the toner layer is small, so the toner layer potential on the photoconductor has a low value as shown in FIG. Therefore, in the highlight region where a small amount of toner layer is laminated on the photoconductor, the potential itself of the toner layer already formed on the photoconductor is low, but the latent image contrast itself is low, so that the image area of the second color The ratio of the influence on the potential of the image becomes relatively large, and it is very difficult to faithfully reproduce a predetermined color tone at a predetermined density in a highlight image around a high-density color character image even in a highlight area. In addition to the above, there is a problem that an image is missing.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and its object is to faithfully reproduce the toner multiple portions and the single-color portions, which are important for high-quality color images. It is an object of the present invention to provide a small-sized, inexpensive, high-speed color image forming apparatus that enables reproduction and high-quality character reproduction.

[0018]

According to a first aspect of the present invention, there is provided a color image forming apparatus comprising: a charging means for uniformly charging the surface of a photosensitive medium; and a photosensitive medium charged by the charging means. An exposure unit that forms an electrostatic latent image by scanning and exposing a light beam on the surface according to image information, and a plurality of units that selectively develop the electrostatic latent image formed on the photosensitive medium with toner of a plurality of colors. And two developing units, each of which is provided with two image forming units, and the photosensitive medium of each of the two image forming units is charged, exposed and developed a plurality of times
A plurality of color toner images are formed on each photosensitive medium in an overlapping state, and the plurality of color toner images formed on each photosensitive medium of the two sets of image forming units are sequentially transferred onto a recording medium or an intermediate transfer member. In a color image forming apparatus for forming a color image by transferring the image onto a screen, the screen structure of the electrostatic latent image formed by the exposing unit is divided between toner images of a plurality of colors in each image forming unit as shown in the figure. The feature is that they are different from each other.

In the color image forming apparatus according to the second aspect of the present invention, charging means for uniformly charging the surface of the photosensitive medium, and image information on the surface of the photosensitive medium charged by the charging means. An exposure unit capable of forming a plurality of electrostatic latent images by scanning and exposing a plurality of light beams according to
Two sets of image forming units each including a plurality of developing units for selectively developing the electrostatic latent image formed on the photosensitive medium with toner of a plurality of colors are provided, and each of the two sets of image forming units includes a photosensitive medium. On the other hand, the exposure and development steps are performed a plurality of times while the uniformly charged photosensitive medium makes one rotation, and toner images of a plurality of colors are formed on each photosensitive medium in a superimposed state.
In a color image forming apparatus for forming a color image by sequentially transferring toner images of a plurality of colors formed on each photosensitive medium of a pair of image forming units onto a recording medium or an intermediate transfer body, the color image forming apparatus is formed by the exposing unit. The electrostatic latent image screen structure is made different between the toner images of a plurality of colors in each image forming unit.

Further, a color image forming apparatus according to a third aspect of the present invention is the color image forming apparatus according to the first or second aspect, in which a plurality of scanning image forming devices are formed. The screen structure of the electrostatic latent image is different between the image forming units.

Still further, the color image forming apparatus according to claim 4 of the present invention is defined by claim 2 or claim 3.
The color image forming apparatus described in the item 1 is characterized in that the peripheral length of the photosensitive medium is shorter than the maximum image length in the process direction.

Further, the color image forming apparatus according to claim 5 of the present invention is the color image forming apparatus according to claim 1 or 4, wherein the developing means is yellow, magenta or cyan. , With black toner,
Developing means having cyan and black toners are distributed to different image forming units, developing means having yellow and magenta toners are distributed to different image forming units, and the developing means arranged in one image forming unit In the case where all the developing means are used, the developing means having cyan and black toner is characterized in that the developing is carried out in the subsequent step of the process.

Furthermore, the color image forming apparatus according to the sixth aspect of the present invention is the color image forming apparatus according to the fifth aspect, wherein the developing toner weights of the yellow and magenta toners are 0.8 mg / cm. ^The feature is that it is ² or less.

A color image forming apparatus according to a seventh aspect of the present invention is arranged in each image forming unit in the color image forming apparatus according to any one of the first to sixth aspects. The electrostatic latent image contrast (V _image -V _high ) of the first color of the developing means is 450 V or less.

[0025]

According to the present invention, the contact between the intermediate transfer member and the photosensitive member is reduced to half that of the conventional one per color print by the above-mentioned constitution, and the life of the apparatus and the reliability can be increased. Since the screen structure of the electrostatic latent image formed by scanning exposure differs in each image forming unit, the average toner height on the transfer material or the intermediate transfer member can be reduced, resulting in color reproduction and gradation. It is possible to provide a high-quality color image forming apparatus which is excellent in reproduction and can reproduce color characters well.

Further, since the screen structure is different between the colors in each image forming unit, the influence of the toner layer charge in the multiple development process can be reduced, and the stable gray color reproduction and the toner scattering can be realized by the highlight reproduction of the multiple colors. By enabling color character reproduction without
As a result, it is possible to provide a high-quality color image forming apparatus which is excellent in color reproduction and gradation reproduction.

Further, a charging means for uniformly charging the surface of the photosensitive medium, and a plurality of electrostatic beams by scanning and exposing a plurality of light beams on the surface of the photosensitive medium charged by the charging means according to image information. Exposure means capable of forming a latent image,
Two sets of image forming units each including a plurality of developing means for selectively developing the electrostatic latent image formed on the photosensitive medium with toner of a plurality of colors are provided. Since the toner images are formed in an overlapping manner, high-speed output equivalent to the tandem system using four photosensitive media is possible, and a low-cost and small-sized device can be realized.

Furthermore, by setting the circumference of the photosensitive medium to be shorter than the maximum recording image length in the above structure, it is possible to reduce the diameter of the photosensitive medium and further reduce the size of the color image forming apparatus. Can be provided.

Further, developing means having cyan and black are distributed to different image forming units, and developing means having yellow and magenta toner are distributed to different image forming units and arranged in one image forming unit. In general, the developing means having cyan and black develops in a later step of the process, so that the infrared laser exposure generally used from the toner layer formed on the photosensitive medium. For example, yellow and magenta toners, which have relatively high transmittance for infrared laser light, can be developed first, so exposure loss due to toner layer transmittance can be reduced, and color reproduction and gradation reproduction are excellent. Further, it is possible to provide a high-quality color image forming apparatus.

Furthermore, since the developing toner weight of the yellow and magenta toners is 0.8 mg / cm ² or less, the influence of the toner layer charge on the developing process of the next color can be reduced, and the color reproduction and gradation reproduction are excellent. Further, it is possible to provide a high-quality color image forming apparatus.

Further, the electrostatic latent image contrast (V _image) of the first color of the developing means arranged in each image forming unit is
Since −V _high ) is 450 V or less, even when the recharging step is performed, a rise in the toner layer potential in the recharging step can be reduced, and a high-quality color image forming apparatus excellent in color reproduction and gradation reproduction can be provided. Can be provided.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

FIG. 2 is a schematic diagram showing an embodiment of an electrophotographic color image forming apparatus according to the present invention.

In FIG. 2, reference numeral 1 denotes a main body of the electrophotographic color image forming apparatus, and an image reading device 3 for reading an image of an original 2 is provided at an upper end portion of the main body 1 of the color image forming apparatus. Are arranged. The image reading device 3 outputs an image of an original 2 placed on a platen glass 4 while being pressed by a platen cover 5 as a light source 6.
The reflected light image of the original 2 is scanned and exposed by the CCD sensor 10 through the first and second scanning mirrors 7 and 8 and the imaging lens 9, and the red (R ), Green (G), and blue (B) color material reflected light images of a predetermined dot density (for example, 16 dots / m).
It is designed to be read in m).

The image data of the original 2 read by the image reading device 3 is sent to an image processing device which will be described later, and in this image processing device, if necessary, shading correction and position deviation are performed on the image data. Predetermined image processing such as correction, lightness / color space conversion, gamma correction, frame erasing, color / moving editing, etc. is performed, and the image data of the original 2 is black (K), yellow (Y), magenta (M), It is converted into gradation data of four colors of cyan (C) (each 8 bits).

Further, two image forming units 11 and 12 respectively forming toner images of two colors are provided in a portion above the central portion in the color image forming apparatus main body 1, and these two image forming units 11 are formed. , Transfer drum 25 for transferring the toner image formed by
Is arranged. The two image forming units 11,
Reference numeral 12 includes photosensitive drums 21a and 21b each having a photoconductive thin layer such as an organic photosensitive layer on the surface thereof. The peripheral lengths of these photosensitive drums 21a and 21b are the maximum image length in the process direction. The diameter is set so as to be longer than the above, and the driving means (not shown) is driven to rotate in the arrow direction at the same speed. Around the photoconductor drums 21a, 21b, scorotron chargers 22a, 22b for uniformly charging the surfaces of the photoconductor drums 21a, 21b, and photoconductor drums 21a by a signal from an image processing device described in detail later, Laser exposure devices 23a and 23b for forming an electrostatic latent image on 21b, color developing devices 24a and 24b equipped with yellow and cyan color toners, respectively, and a color developing device 24c equipped with magenta and black color toners, 24d, cleaning devices 27c and 27d that are separated from each other during image formation and that contact the photosensitive drums 21a and 21b during cleaning, and static elimination lamps 28a and 28
b corresponds to the corresponding photoconductor drum 21a and photoconductor drum 2
It is arranged separately in 1b.

Further, the two image forming units 11,
Below the 12 photosensitive drums 21a and 21b,
A transfer drum 25 is disposed on the surfaces of the two photoconductor drums 21a and 21b so as to be in contact with each other at a predetermined timing. The toner images formed on the photoconductor drums 21 a and 21 b are transferred onto the transfer paper 20 held on the transfer drum 25,
The transfer corotrons 25a and 25b are sequentially transferred by charging. The transfer drum 25 is formed in a cylindrical shape by a dielectric film (not shown), and the dielectric film is reinforced by a metal plate called a tie bar and held in a drum shape. The transfer paper 20 is placed on the dielectric film forming the transfer drum 25.
Is electrostatically adsorbed by an adsorption corotron (not shown).

The transfer sheet 20 is, as shown in FIG.
Paper is fed by a paper feed roll 43 from a plurality of paper feed cassettes 40, 41, 42 arranged in the lower part of the image forming apparatus body 1, and from a manual feed tray 44 arranged on the side surface outside the image forming apparatus body 1. Can also be fed,
The fed transfer paper 20 is transported to the surface of the transfer drum 25 by the transport rollers 45. And
The transfer sheet 20 is held on the surface of the transfer drum 25 in a state of being electrostatically adsorbed on the surface of the transfer drum 25 by charging of an adsorption corotron (not shown).

Further, the transfer paper 20 on which the toner images of a predetermined number of colors are transferred from the photosensitive drums 21a and 21b.
Is peeled from the surface of the transfer drum 25 by the charging of a peeling corotron (not shown), and then conveyed to a fixing device 26. The fixing device 26 fixes the toner image on the transfer paper 20 by heat and pressure, and the paper discharge tray. The image is discharged onto the sheet 29 and the color image forming process is completed.

Incidentally, in FIG. 3, reference numeral 25c denotes a static elimination corotron for static elimination of the transfer drum 25.

Further, the scorotron chargers 22a, 2
As 2b, it is desirable to use a scorotron charger using two or more discharge wires so that stable and uniform charging can be performed even when forming a second color image.

FIG. 3 is a schematic configuration diagram showing a laser exposure device used in the color image forming apparatus of FIG.

The laser exposure devices 23a and 23b are
As shown in FIG. 3, the semiconductor laser 31 includes a laser control circuit, a collimator lens 32, a polygon mirror 33, and an fθ lens 34. Also, if necessary, a laser beam 3 may be provided by using a reflection mirror.
The optical path 5 may be folded. The semiconductor laser 31 is turned on / off by being modulated by a built-in laser control circuit based on the exposure data 36, and emits a laser beam 35 which is modulated by a so-called pulse width modulation method. The laser beam 35 emitted from the semiconductor laser 31 is used by the collimator lens 3
2 is irradiated onto the surface of the polygon mirror 33 via the optical axis 2 and is reflected and deflected by the surface of the polygon mirror 33 rotating at a high speed. 21
Scanning exposure is performed along (a, 21b axial direction).

By the way, in this embodiment, a plurality of electrostatic latent image screen structures formed by scanning exposure of the laser beam 35 on the photosensitive drums 21a and 21b are provided in the image forming units 11 and 12, respectively. The color toner images are configured to be different from each other.

That is, in this embodiment, image data 3 which is image-exposed by the laser exposure devices 23a and 23b is used.
6 is generated by the image processing apparatus as shown in FIG. The image processing apparatus includes image data 50 having gradation information for each pixel to be printed, which is sent from the image reading apparatus 3 or the host computer described above.
Are input as 8-bit gradation information of R, G, B or L ^* , a ^* , b ^* , respectively. Next, these image data 50 are converted into Y, M, and C image data 52 by the conversion table 51. A conversion matrix may be used instead of a table for this conversion. Further, the image data 52 is converted into yellow (Y), magenta (M), cyan (C), and black (K) by the undercolor removal circuit 53, and characters are formed by superimposing Y, M, and C. A black image area such as a background portion is converted into black (K) image data. As described above, in the under color removal circuit 53, Y, M, and C image data 52
Processing for converting a black image area by superimposing M and C into black (K) image data is performed. Further, the MTF correction circuit 54 performs MTF correction. Next, the image signals 56 of the respective colors, which have been subjected to the gradation conversion as shown in FIG. 5 by the gradation conversion circuit 55 according to the developing characteristics, reach the screen generation circuit 57. The screen generation circuit 57 is configured to use a screen having a parallel line structure. However, it goes without saying that a screen having another structure such as a dot screen other than the parallel line structure may be used. The image signal 36 output from the screen generation circuit 57 is sent to the laser control circuit 58.

As described above, as a method of reducing the weight of the developing toner itself, by removing the undercolor, the toner of a predetermined weight can be replaced with black to further improve. That is, if the ratio of removal of the undercolor is increased, the total height of the developing toner for each color is decreased, and the toner layer potential can be further suppressed.

FIG. 6 shows an example of the screen generation circuit.

In FIG. 6, reference numeral 61 is an input I for inputting the image data 56 output from the gradation conversion circuit or the like of the image processing apparatus.
/ F circuit, 62 is an 8-bit quantized data 63 (25
D / A converter having a function of decomposing into 6 gradations, 6
A buffer circuit 4 temporarily stores the 8-bit quantized data 63 output from the D / A converter 62.
Is a triangular wave generating circuit that generates a triangular wave based on the reference clock 66, and 67 is the 8-bit quantized data 63 temporarily stored in the buffer circuit 64 and the triangular wave signal 68 output from the triangular wave generating circuit 65. A comparator 70 for comparison, a buffer circuit 70 for temporarily storing the image data 69 output from the comparator 67, and a laser control circuit 58 for controlling the semiconductor laser 31 based on the image data 36 output from the buffer circuit 70. .

Reference numeral 71 is a phase control circuit, and this phase control circuit 71 controls the phase of the triangular wave signal 68 output from the triangular wave generation circuit 65 for each line. By this phase control circuit 71, it is possible to form a parallel line screen that extends at a predetermined angle with respect to the main scanning direction at a pitch according to the cycle of the triangular wave signal 68.

Then, the screen generation circuit 57 is
Image data 56 having gradation information for each pixel to be printed is output from the I / F circuit 6 from the gradation conversion circuit of the image processing apparatus.
1 is input via the input image data 6
1 is D / A converted based on a reference clock 66 by a D / A converter 62 having an 8-bit quantization resolution, and the image data 63 converted into this analog signal is
It is sent to the comparator 67 via the buffer circuit 64. Further, the triangular wave generation circuit 65 generates a triangular wave signal 68 in synchronization with the reference clock 66, and the triangular wave signal 68 is generated.
Is also sent to the comparator 67. This comparator 67
Compares the triangular wave signal 68 with the image data 63 converted into an analog signal and outputs a pulse having a length proportional to the gradation information of the image data 63, and this output is the buffer circuit 7.
Laser control circuit 5 as exposure data 36 via 0
8 is input.

By doing so, the screen generating circuit 57 uses a parallel line screen that extends in the main scanning direction of the laser beam 35 and about 60 degrees in the main scanning direction at a pitch according to the cycle of the triangular wave signal 68. The exposure data 36 is generated. FIGS. 7A and 7B are schematic diagrams showing the main scanning direction of the laser beam 35 and the electrostatic latent image of the parallel line screen extending about 60 degrees with respect to the main scanning direction. The semiconductor laser 21a or 21b connected to the laser control circuit 58 is turned on by the exposure data 36 “00 to FF” of the parallel line screen generated by the screen generation circuit 57 (corresponding to 8-bit resolution 0 to 255 steps). Then, the surface of the photosensitive drum 21a or 21b where the laser beam 35 is irradiated is discharged.
A potential contrast as shown in FIG. The potential contrast formed on these photosensitive drums 21a or 21b becomes an electrostatic latent image. Here, for example, a line of 200 lines / inch is formed,
Of course, the line pitch may be set to another value by changing the period of the triangular wave. Further, in FIG. 7, the electrostatic latent image is schematically represented in a uniform strip shape, but in reality, the intensity distribution of the laser beam 35 is a Gaussian distribution and the limit of the frequency response of the laser control circuit 58. As a result, the edge shape of the electrostatic latent image becomes a smooth edge shape.

In this embodiment, as described above, the screen structure of the electrostatic latent image formed by scanning and exposing the laser beam 35 on the photosensitive drums 21a and 21b is used in the image forming units 11 and 12, respectively. The toner images of a plurality of colors are configured to be different from each other. That is, the two color toner images of yellow and cyan are formed on the photoconductor drum 21a in a state of being superimposed on each other, but the electrostatic latent image corresponding to the yellow image is shown in FIG. ), The laser beam 35
A line screen extending about 60 degrees with respect to the main scanning direction of the laser beam 35 is used for the electrostatic latent image corresponding to the cyan color image, as shown in FIG. 7A. A parallel line screen that stretches to is used. On the other hand, toner images of two colors, magenta and black, are formed on the photoconductor drum 21b in a state of being superimposed on each other, but an electrostatic latent image corresponding to a magenta image is shown in FIG. 7), a line screen extending about 60 degrees with respect to the main scanning direction of the laser beam 35 is used, and the electrostatic latent image corresponding to the black image is as shown in FIG. 7A. In addition, a parallel line screen that extends in the main scanning direction of the laser beam 35 is used.

The figure shows the configuration of the color developing device.

The color developing devices 24a, 24b, 24
c and 24d are color developing devices 24a and 24b, respectively, around the photosensitive drum 21a.
4d is substantially fixed and held around the photosensitive drum 21b via a tracking roll or the like. As shown in FIG. 9, each of the color developing devices 24a, 24b, 24c, and 24d has a housing 74 that accommodates a two-component developer 73 made up of toner and carrier, and adsorbs the developer 73 on its surface to rotate it. By doing so, a cylindrical developing carrier 75 that conveys the developer 73, a developer regulating member 76 that regulates the amount of the developer 73 adhering to the surface of the developing carrier 75, and the developer 73 that rotates to stir the developer 73. And carrying developer carrier 7
5 is provided with a screw auger 77 for supplying the developer 73.

The developer carrying member 75 has a built-in magnet roll 78 fixed so as not to rotate, and a developing sleeve 79 rotatably supported around the magnet roll 78. The developing sleeve 79 has, for example, a diameter of 16
mm, developing sleeve 79 and photosensitive drums 21a and 21b
0.5 mm apart from the developer 7 on the developing sleeve 79.
The thickness of the three layers is set to 0.3 mm, the peripheral speed is set to 200 mm / s, and the developer 73 is set to the photosensitive drums 21a, 21.
What develops in the state which does not contact b is used. This is a precondition for preventing the toner image previously developed on the photoconductor drums 21a and 21b during the overlapping development from being disturbed in the subsequent developing process.

Further, in this embodiment, the thickness is 0.3 mm.
In order to form the thin developer layer 73 on the developing sleeve 79, a magnetic powder-dispersed polymer carrier having an average particle diameter of 40 μm and a saturation magnetization of 50 emu / g is used. In addition, the carrier is used to charge the toner to a negative polarity,
30 wt% of styrene-acrylic copolymer, 68 wt% of magnetite, and 2 wt% of nigrosine were kneaded and pulverized to obtain a carrier having a positive charging property.

On the other hand, regarding the toner, the yellow toner is 90 wt% polyester and 10 wt% benzine derivative.
% Was kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. The cyan toner was kneaded and pulverized with 90 wt% of polyester and 10 wt% of copper phthalocyanine to obtain colored particles having an average particle diameter of 7 μm. Magenta toner is polyester 90wt
%, And polytungstophosphoric acid 10 wt% were kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. For the black toner, 80 wt% of styrene-acryl copolymer, 10 wt% of styrene-butadiene rubber, and 10 wt% of carbon black were kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. Also,
With respect to these toners, hydrophobic silica was externally added to the colored particles in an amount of 5 wt% to obtain negatively charged toner. When the charge amount Q of the toner was measured for these developers, it was about -15 μC / g.

Further, each color developing device 24a, 24b, 2
The developing bias applied to 4c and 24d is set to DC + AC voltage so as not to disturb the toner image formed on the photosensitive drum and to obtain a desired high image quality, -650 V as a DC component, and AC component as an AC component. Frequency 6 kHz,
V _PP = 1.6 kV was applied. On the other hand, the transfer drum 25
As described above, as described above, those using a corotron or a transfer roll that directly transfers an image to a sheet adsorbed and conveyed on a cylindrical resin film are used. The transfer drum 25 is provided with a charge eliminating discharger for eliminating charge on the film or the paper as required.

27a and 27b are cleaning devices using blades, which are usually separated from the photosensitive drums 21a and 21b, and when a cleaning cycle is started, the cam mechanism makes contact with the photosensitive drums 21a and 21b to remove residual toner. To remove.

Based on the above configuration, the color image forming apparatus according to this embodiment is configured to form a color image as follows. That is, in the color image forming apparatus configured as described above, as shown in FIG.
First, if necessary, the photoconductor drums 21a and 21b that have been sufficiently photo-erased by the static elimination lamps 28a and 28b, or the photoconductor drums 21a and 21b in the initial state and the transfer drum 25 are rotated in the arrow direction after the operation is started. First, the surface of the photoconductor drums 21a and 21b is the scorotron charger 2
It is uniformly charged to, for example, -700V by 2a and 22b. At this time, it is needless to say that the charging potential and the charging polarity of the photosensitive drum 21 can be set to a voltage and a polarity different from the above values depending on the developing efficiency and other conditions.

Next, as described above, the scorotron charger 22 is used.
Photosensitive drum 21 uniformly charged by a and 22b
Laser exposure devices 23a and 23a are provided on the surfaces of a and 21b.
Image exposure corresponding to the image data 36 of the first color is performed by b, and the electrostatic latent image of the first color is formed. At this time, the exposure time of each of the photoconductor drums 21a and 21b is
The photoconductor drums 21a and 21b are controlled at the timing when the image areas transferred onto the sheet 20 adsorbed on the transfer drum 25 exactly overlap each other. Specifically, a state in which the image signals are synchronized based on the position detection signals output at the positions of the respective photoconductor drums 21a and 21b by a photocoupler (not shown) disposed inside the transfer drum 25. It is controlled by outputting.

As a result, a yellow color composed of a line screen extending toward the photosensitive drum 21a by the laser exposure device 23a in a direction inclined by about 60 degrees with respect to the main scanning direction as shown in FIG. 7B. To form an electrostatic latent image. The potential corresponding to the maximum input coverage of the portion except the edge portion of the electrostatic latent image thus formed was -200V.

Next, the electrostatic latent image of the first color formed on the photosensitive drum 21a as described above is reversed by the yellow color developing device 24a by the negatively charged yellow toner of the first color. It is developed. As a result, the yellow toner adheres to the electrostatic latent image formed on the photoconductor drum 21a substantially along the line shape. The developing characteristics of the yellow developing device 24a used at this time are shown in FIG. A maximum density of about 2.0 was obtained for a development contrast of 450V [-200-(-650V)].

Thereafter, on the surface of the other photosensitive drum 21b, there is a line screen extending by the laser exposure device 23b in a direction inclined by about 30 degrees with respect to the main scanning direction, as shown in FIG. 7B. An electrostatic latent image corresponding to the magenta color is formed. The potential corresponding to the maximum input coverage of the portion except the edge portion of the electrostatic latent image thus formed was -200V.

Next, the electrostatic latent image of the first color formed on the photoconductor drum 21b as described above is inverted by the magenta color developing device 24b by the negatively charged magenta toner of the first color. It is developed. As a result, magenta toner adheres to the electrostatic latent image formed on the photoconductor drum 21b substantially along the line shape. The developing characteristics of the magenta developing device 24a used at this time are shown in FIG. Development contrast 450V [-200-(-650
V)] a maximum concentration of about 2.0 was obtained.

In the above process, the transfer sheet 20 held on the transfer drum 25 is transferred to the respective photosensitive drums 21.
Rotating in a non-contact state with the surfaces of a and 21b,
The yellow and magenta toner images formed on the photoconductor drums 21a and 21b rotate while being held on the photoconductor drums 21a and 21b as they are.

After that, the photosensitive drums 21a and 21b
Enters the second color image forming step. Photoconductor drum 21
The a is recharged to -700V from the photoconductor drum 21a including the yellow toner layer by the scorotron charger 22a. Next, on the surface of the photosensitive drum 21a,
Image exposure is performed by the laser exposure device 23a according to the image data 36 of the second color formed by a parallel line screen extending in the main scanning direction as shown in FIG. 7A, and an electrostatic latent image of the second color is formed. It is formed. The potential corresponding to the maximum input coverage of the portion of the electrostatic latent image formed in this manner excluding the edge portion was −250 V on the yellow toner and −200 V on the monochrome region (on the photoconductor). Similarly, the electrostatic latent image of the second color formed on the photosensitive drum 21a is subjected to the same development parameters as the first color by the cyan color of the second color so that the cyan toner adheres (reverse development). To develop. As a result, the cyan toner adheres to the electrostatic latent image formed on the photoconductor drum 21b substantially along the line shape. The development characteristics of the Cyan developing device 24a used at this time are shown in FIG. Since the output density is saturated at a developing contrast of about 400 V or more as shown in FIG. 10, a superposed image corresponding to almost the same density as that of a monochrome portion was obtained even in the overlapping portion of yellow and cyan.

On the other hand, the other photosensitive drum 21b is recharged to -700V from above the photosensitive drum 21b including the magenta toner layer by the scorotron charger 22b. Next, on the surface of the photoconductor drum 21b, image exposure according to the image data 36 of the second color, which is composed of a line screen extending in the main scanning direction by the laser exposure device 23b as shown in FIG. 7A, is performed. Is applied to form a second color electrostatic latent image. The potential corresponding to the maximum input coverage of the portion of the electrostatic latent image formed in this manner excluding the edge portion was −250 V on the magenta toner and −200 V on the monochrome region (on the photoconductor). Similarly, the electrostatic latent image of the second color formed on the photoconductor drum 21b is
The second color black toner is used with the same development parameters as the first color, and black toner is attached (reverse development).
Develop as you would. As a result, the photosensitive drum 21
Black toner adheres to the electrostatic latent image formed on b substantially along the line shape. The developing characteristics of the black developing device 24a used at this time are shown in FIG. Since the output density is saturated at a development contrast of about 400 V or more as shown in FIG. 10, a superposed image corresponding to almost the same density as the monochrome portion was obtained even in the magenta and black overlapping portions.

In the above steps, the photosensitive drum 21a
Two toner images of two colors, yellow and cyan, are formed on top of each other in a state of being superimposed on each other, and two magenta and black toner images are formed on the other photosensitive drum 21b.
The color toner images are formed in a superimposed manner.

Next, in the transfer cycle, the toner images formed on the photoconductor drums 21a and 21b are first transferred onto the photoconductor drum 21a by the baffle of the transfer corotron 25a facing the photoconductor drum 21a. When they come into contact with each other, a voltage is applied to the transfer corotron 25a at the same time, and the yellow and cyan toner images are collectively transferred onto the transfer paper 20. The baffle of the transfer corotron 25a is a film made of synthetic resin for concentrating the discharge from the transfer corotron 25a at the transfer position, and the baffle is such that the paper 20 held on the surface of the transfer drum 25 is a photosensitive member. It is driven by a solenoid or the like so as to come into contact with and separate from the surface of the drum 21a. Further, the transfer drum 25 rotates, and based on the position detection signal,
The baffle of the transfer corotron 25b facing the photoconductor drum 21b causes the paper 20 to lightly contact the photoconductor drum 21b, and at the same time, a voltage is applied to the transfer corotron 25b.
The magenta and black toner images are collectively transferred onto the transfer paper 20. In this way, a full-color image can be formed on the transfer paper 20 held on the transfer drum 25 by transferring twice.

Further, the paper 20 separated from the transfer drum 25 is fixed by passing through the fixing device 26, and a color image is formed.

The toner remaining on the photosensitive drums 21a and 21b and the history of charging are recorded in the cleaning devices 27a and 2a.
The image forming cycle is completed by the continuous removal by 7b and the discharge lamps 28a, 28b.

According to the above-described embodiment, since the screen structures of the electrostatic latent images of the first and second colors on the photoconductor drums 21a and 21b are different, the photoconductor drums 21a and 21b are different from each other.
It is possible to significantly reduce the overlap of the toner layers on 21b,
It is possible to reduce the influence of the toner layer transmittance and the influence of the toner layer charge at the time of overlapping exposure. As a result, the reproduction in the highlight region and the reproduction of color characters are improved, and a high quality image can be obtained.

To further explain, according to the above embodiment,
Since the screen structures of the first and second color electrostatic latent images on the respective photoconductor drums 21a and 21b are different, the positions of the line structures of the first and second colors are shifted from each other as shown in FIG. The photosensitive drums 21a, 21
To simultaneously solve the problems caused by the rise of the toner potential due to the formation of a plurality of toner images on b, the dielectric properties of the toner layer, and the problems of the toner shielding effect as follows. You can

FIG. 11 shows a cross section of an overlapping portion of the yellow toner image 81 and the cyan toner image 82 formed on the photosensitive drum 21a. In the figure, 83 indicates the center of the toner image. As shown in FIG. 11, the yellow toner image 71 and the magenta toner image 72 do not overlap each other in a low density region of the toner density, that is, in a highlight portion (or, as shown in the figure, there is also an extremely highlighted portion that does not substantially overlap each other). It can be seen that the yellow toner image 71 and the cyan toner image 72 gradually overlap with each other in the middle density region and the high density region.

At this time, although the heights of the toner images 71 and 72 are different in each density range, the toner image (yellow + cyan) formed by the electrostatic latent image formed by overlapping and exposing the dots of the conventional method and the present embodiment are used. FIG. 12 shows a comparison with the height of the toner image obtained by shifting the phase of the line of the second color by. In FIG. 12, the toner image is actually formed on the photosensitive drum 21 using the parameters described in the above embodiment, and the electrostatic latent image of each color of the conventional system is the same using the same parameters as this embodiment. This is a comparison with a conventional example in which a toner image is actually formed on the photoconductor drum 21 by a method of overlapping at the position.

As is apparent from FIG. 12, the height of the toner image is lower in this embodiment in which the structure of the line screen is different from that in the conventional example, and this tendency tends to occur as the developing toner weight increases. It can be seen that is also remarkable. Also, when the weight of the developing toner is large, the overlap of the toner images increases, so it is easy to predict that the toner image height will be the same as the conventional method that does not shift the lines due to dot overlap.
Actually, as shown in FIG. 13, 2 of yellow and Sian
Toner weight 1.3, which is near the saturation point of toner weight for color
Differences are also seen around mg / cm ² . As shown in FIG. 13, the toner image at this time has little overlap in the highlight portion where the weight of the developing toner is small, so that the dielectric effect due to the height (thickness) of the toner image can be removed, and a high toner image It can be seen that a light image can be recorded.

As is apparent from FIG. 13, in this embodiment, even when two color toner images of yellow and cyan are formed on the photosensitive drum 21, the height of the toner image and the toner layer potential are both. It is lower than that of the conventional method, and it is possible to prevent the deterioration of the image quality due to the deterioration of the resolution and gradation due to the increase of the toner potential, the dielectric property of the toner layer, and the toner shielding effect. It can be seen that it is possible to form a halftone image. This effect is remarkable in the medium density range where the amount of the toner layer on the photoconductor drum 21 is relatively small, particularly in the low density range, and it is possible to faithfully reproduce the highlight area which is particularly important for a high quality color image. Becomes

Further, according to this embodiment, the rise of the toner potential, the dielectric property of the toner layer and the toner shielding effect are comprehensively solved, which is important for a good halftone image, especially a high quality color image. It is possible to faithfully reproduce the highlight area, and as a result, stable gradation reproduction can be obtained without causing defects such as pseudo contours due to discontinuous changes in gradation.

Further, in the above-described embodiment, a line screen extending about 60 degrees with respect to the main scanning direction is used for the first color or the first laser beam, and the main scanning direction is used for the second color or the second laser beam. I used a line screen that stretches in the direction of, but the reverse is also possible,
Other combinations of angles may be used. Further, different angles may be used for the photosensitive drums 21a and 21b. Furthermore, it is desirable to combine screen structures having a large difference in angle.

Although the transfer drum 25 is used as the transfer means in the above embodiment, the image may be transferred twice using an intermediate transfer member, or another transfer device may be used.

Second Embodiment FIG. 14 shows a second embodiment of the present invention. The same parts as those in the first embodiment will be designated by the same reference numerals. A plurality of electrostatic latent images can be formed by scanning and exposing two charging means for uniformly charging the surface and the surface of the photosensitive medium charged by these charging means with a plurality of light beams according to image information. 2 sets of image forming units each including an exposing unit and a plurality of developing units for selectively developing the electrostatic latent image formed on the photosensitive medium with toners of a plurality of colors.

FIG. 14 is a schematic block diagram showing the second embodiment of the electrophotographic color image forming apparatus according to the present invention.

In FIG. 14, reference numerals 21a and 21b denote photoconductor drums having a photoconductive thin layer such as an organic photoconductor layer on the surface thereof. These photoconductor drums 21a and 21b are respectively moved in a direction of an arrow by a driving means not shown. It is designed to be rotated. Around the photoconductor drums 21a and 21b, scorotron chargers 22a, 22b and 22c and 22d for uniformly charging the surfaces of the photoconductor drums 21a and 21b, and a signal from an image processing device, which will be described in detail later, are exposed. Laser exposure devices 23a and 23b for forming electrostatic latent images on body drums 21a and 21b, color developing devices 24a and 24b equipped with yellow and cyan color toners, and color equipped with magenta and black color toners, respectively. Arranged are developing devices 24c and 24d, cleaning devices 27c and 27d having blades that are separated from each other during image formation and contact the photosensitive drums 21a and 21b during cleaning, and static elimination lamps 28c and 28d. The forming units 11 and 12 are configured.

Further, the two image forming units 11,
Below the 12 photosensitive drums 21a and 21b,
A transfer drum 25 that can contact the surfaces of the two photosensitive drums 21a and 21b is arranged. The photoconductor drum 21
The toner images formed on a and 21b are transferred onto the transfer drum 25.
The transfer corotron 25 is placed on the transfer paper 20 held on top.
It is sequentially transferred by charging a and 25b. The transfer drum 25 is formed in a cylindrical shape by a dielectric film (not shown), and the dielectric film is reinforced by a metal plate called a tie bar and held in a drum shape. The transfer sheet 20 is electrostatically adsorbed on the dielectric film forming the transfer drum 25 by an adsorption corotron (not shown).

FIG. 15 is a schematic diagram showing a laser exposure apparatus used in the color image forming apparatus of FIG.

The laser exposure devices 23a and 23b are
As shown in FIG. 15, a dual beam semiconductor laser 31 (or two semiconductor lasers) capable of simultaneously emitting two laser beams 35a and 35b capable of independently modulating by incorporating a laser control circuit, and a collimator lens 3 are provided.
2, a polygon mirror 33, and an fθ lens 34. Further, the optical path may be folded by using a reflection mirror as needed. The semiconductor laser 31 uses the two exposure data 36a and 36b.
On the basis of the pulse width modulation method, two laser beams 35a and 35b which are modulated by a built-in laser control circuit and individually turned on and off and modulated by a so-called pulse width modulation method are emitted. The two laser beams 35a and 35b emitted from the semiconductor laser 31 are collimator lens 3
2 is irradiated onto the surface of the polygon mirror 33 via the light beam 2 and is reflected and deflected by the surface of the polygon mirror 33 rotating at a high speed. 21a,
21b is scanned and exposed along the axial direction), and electrostatic latent images corresponding to respective image data are formed on the photosensitive drums 21a, 2b.
It is formed on 1b.

In this embodiment, the photosensitive drum 21 described above is used.
An electrostatic latent image formed by scanning and exposing the laser beams 35a and 35b on a and 21b is formed using a screen having a parallel line structure. An image signal for electrostatic latent image formation using a screen having this parallel line structure is generated by a screen generation circuit shown below based on digital image data having gradation information for each pixel to be printed and a reference clock. To be done.

More specifically, the image data imagewise exposed by the laser exposure devices 23a and 23b is generated by an image processing device as shown in FIG. That is, in this image processing device, image data 50 having gradation information for each pixel to be printed, which is sent from the above-described image reading device or host computer, is stored in R, G, B or L ^* ,
Each of a ^* and b ^* is input as 8-bit gradation information. Next, the image data 50 is converted into yellow (Y), magenta (M), and cyan (C) image data 52 by the conversion table 51. A conversion matrix may be used instead of a table for this conversion. Further, these image data 52 are converted into yellow (Y), magenta (M), cyan (C), and black (K) by the undercolor removal circuit 53, and Y, M, and C are converted.
A black image area such as a character or a background portion due to the superposition of is converted into black (K) image data. As described above, the undercolor removal circuit 53 performs processing for converting the black image area by superimposing Y, M, and C on the Y, M, and C image data into black (K) image data. It Further, the MTF correction circuit 54 performs MTF correction. Next, the image signal of each color subjected to the gradation conversion as shown in FIG. 5 by the gradation conversion circuit 55 according to the developing characteristics reaches the screen generation circuit 57. The screen generation circuit 57 is configured to use a screen having a parallel line structure. However, it goes without saying that a screen having another structure such as a dot screen other than the parallel line structure may be used. The image signal 36 output from the screen generation circuit 57 is sent to the laser control circuit 58.

As described above, as a method of reducing the weight of the developing toner itself, it is possible to further improve it by replacing the toner of a predetermined weight with black by removing the undercolor. That is, if the ratio of the undercolor removal is increased, the total developing toner height of each color is decreased, and the toner layer potential can be kept low.

FIG. 6 shows an example of the screen generation circuit.

In FIG. 6, reference numeral 61 is an I / F circuit for inputting the image data 56 output from the gradation converting circuit 55 of the image processing apparatus, and 62 is image data input via the I / F circuit 61. 56 to 8-bit quantized data 63
A D / A converter having a function of decomposing into (256 gradations), 64 is output from the D / A converter 62 8
A buffer circuit for temporarily storing the bit quantized data 63, a triangular wave generating circuit 65 for generating a triangular wave based on a reference clock 66, and 67 for the 8-bit quantized data 63 temporarily stored in the buffer circuit 64. , A comparator for comparing the triangular wave signal 68 output from the triangular wave generating circuit 65, 70 is a buffer circuit for temporarily storing the image data 69 output from the comparator 67, 5
8 is image data 3 output from the buffer circuit 70.
6 is a laser control circuit for controlling the semiconductor laser 31 on the basis of FIG.

Reference numeral 71 denotes a phase control circuit. The phase control circuit 71 controls the phase of the triangular wave line by line.
This makes it possible to form a parallel line screen that extends at a predetermined angle with respect to the main scanning direction at a pitch that corresponds to the cycle of the triangular wave.

Then, the screen generation circuit 57 is
Image data 56 having gradation information for each pixel to be printed is output from the I / F circuit 6 from the gradation conversion circuit 55 of the image processing apparatus.
1 is input, the input image data 5
6 is D / A converted based on a reference clock 66 by a D / A converter 62 having an 8-bit quantization resolution, and the image data 63 converted into this analog signal is
It is sent to the comparator 67 via the buffer circuit 64. Further, the triangular wave generating circuit 65 generates a triangular wave in synchronization with the reference clock 66, and this triangular wave signal is also sent to the comparator 67. The comparator 67 compares the triangular wave signal 68 with the image data 63 converted into an analog signal, outputs a pulse having a length proportional to the gradation information of the image data 63, and this output passes through the buffer circuit 70. And is input as exposure data 36 to the laser control circuit 58.

By doing so, in the screen generating circuit 57, the exposure data using a parallel line screen that extends in the main scanning direction of the laser beam 35 and about 30 degrees in the main scanning direction at a pitch according to the cycle of the triangular wave. 36 is generated. 7 (a) and 7 (b) are schematic views of the main scanning direction of the laser beam 35 and the electrostatic latent image of the parallel line screen extending about 30 degrees with respect to the main scanning direction. Exposure data 36 "00-FF" of the parallel line screen generated by the screen generation circuit (8-bit resolution 0
(Corresponding to 255 steps), the laser control circuit 5
The semiconductor laser 21 connected to the switch 8 is turned on, the surface of the photosensitive drum 21 in the laser-irradiated portion is discharged, and a potential contrast as shown in FIG. The potential contrast formed on the photosensitive drum 21 becomes an electrostatic latent image. Here, for example,
Although a line of 200 lines / inch is formed, the pitch of the line may be set to another value by changing the period of the triangular wave. Further, in FIG. 7, the electrostatic latent image is schematically represented as a uniform strip shape, but in reality, the intensity distribution of the laser beam 35 is a Gaussian distribution and the frequency response limit of the laser control circuit 51 is limited. As a result, the edge shape of the electrostatic latent image becomes a smooth edge shape.

FIG. 9 shows the structure of the color developing device.

The color developing devices 24a, 24b, 24
c and 24d are color developing devices 24a and 24b, respectively, around the photosensitive drum 21a.
4d is substantially fixed and held around the photosensitive drum 21b via a tracking roll or the like. As shown in FIG. 9, each of the color developing devices 24a, 24b, 24c, and 24d has a housing 74 that accommodates a two-component developer 73 made up of toner and carrier, and adsorbs the developer 73 on its surface to rotate it. By doing so, a cylindrical developing carrier 75 that conveys the developer 73, a developer regulating member 76 that regulates the amount of the developer 73 adhering to the surface of the developing carrier 75, and the developer 73 that rotates to stir the developer 73. And carrying developer carrier 7
5 is provided with a screw auger 77 for supplying the developer 73.

The developer carrying member 75 contains a magnet roll 78 fixed so as not to rotate, and a developing sleeve 79 rotatably supported around the magnet roll 78. The developing sleeve 79 has, for example, a diameter of 16
mm, developing sleeve 79 and photosensitive drums 21a and 21b
0.5 mm apart from the developer 7 on the developing sleeve 79.
The thickness of the three layers is set to 0.3 mm, the peripheral speed is set to 200 mm / s, and the developer 73 is set to the photosensitive drums 21a, 21.
What develops in the state which does not contact b is used. This is a precondition for preventing the toner image previously developed on the photoconductor drums 21a and 21b during the overlapping development from being disturbed in the subsequent developing process.

Further, in this embodiment, the thickness is 0.3 mm.
In order to form the thin developer layer 73 on the developing sleeve 79, a magnetic powder-dispersed polymer carrier having an average particle diameter of 40 μm and a saturation magnetization of 50 emu / g is used. In addition, the carrier is used to charge the toner to a negative polarity,
30 wt% of styrene-acrylic copolymer, 68 wt% of magnetite, and 2 wt% of nigrosine were kneaded and pulverized to obtain a carrier having a positive charging property.

On the other hand, regarding the toner, the yellow toner is 90 wt% polyester and 10 wt% benzine derivative.
% Was kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. The cyan toner was kneaded and pulverized with 90 wt% of polyester and 10 wt% of copper phthalocyanine to obtain colored particles having an average particle diameter of 7 μm. Magenta toner is polyester 90wt
%, And polytungstophosphoric acid 10 wt% were kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. For the black toner, 80 wt% of styrene-acryl copolymer, 10 wt% of styrene-butadiene rubber, and 10 wt% of carbon black were kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. Also,
With respect to these toners, hydrophobic silica was externally added to the colored particles in an amount of 5 wt% to obtain negatively charged toner. When the charge amount Q of the toner was measured for these developers, it was about -15 μC / g.

Further, each color developing device 24a, 24b, 2
The developing bias applied to 4c and 24d is set to DC + AC voltage so as not to disturb the toner image formed on the photosensitive drum and to obtain a desired high image quality, -650 V as a DC component, and AC component as an AC component. Frequency 6 kHz,
V _PP = 1.6 kV was applied.

On the other hand, as the transfer drum 25, as described above, the one using a corotron or a transfer roll for directly transferring an image to a sheet sucked and conveyed by a cylindrical resin film is used. The transfer drum 25 is provided with a charge eliminating discharger for eliminating charge on the film or the paper as required.

27a and 27b are cleaning devices using blades, which are normally separated from the photoconductor drums 21a and 21b, and when a cleaning cycle starts, the cam mechanism makes contact with the photoconductor drums 21a and 21b to remove residual toner. To remove.

Based on the above-mentioned structure, the color image forming apparatus according to the second embodiment forms a color image as follows. That is, in the color image forming apparatus configured as described above, first, as shown in FIG. 14, first, the photoconductor drums 21a and 21b, which have been sufficiently photo-erased by the static elimination lamp 28 as necessary, or the photoconductors in the initial state. Body drums 21a and 21b and transfer drum 2
5 starts rotating in the direction of the arrow after the operation starts, and the surfaces of the photoconductor drums 21a and 21b are covered with the scorotron charger 22a,
22b uniformly charges to, for example, -700V. At this time, it is needless to say that the charging potential and the charging polarity of the photosensitive drum 21 can be set to a voltage and a polarity different from the above values depending on the developing efficiency and other conditions.

Next, as described above, the scorotron charger 22 is used.
Photosensitive drum 21 uniformly charged by a and 22b
Laser exposure devices 23a and 23a are provided on the surfaces of a and 21b.
Image exposure corresponding to the image data 36 of the first color is performed by b, and the electrostatic latent image of the first color is formed. At this time, the exposure time of each of the photoconductor drums 21a and 21b is
The photoconductor drums 21a and 21b are controlled at the timing when the image areas transferred onto the sheet 20 adsorbed on the transfer drum 25 exactly overlap each other. Specifically, a photocoupler (not shown) located inside the transfer drum 25 is used to synchronize the image signals based on the position detection signals output at the positions of the photoconductor drums 21a and 21b. Output. In addition, the photosensitive drum 2
A counter counts the time elapsed between the two beam positions set on 1a and 21b to synchronize the image positions.

As a result, an electrostatic latent image corresponding to yellow is formed on the photosensitive drum 21a by the laser exposure device 23a, which is composed of a line screen extending in a direction inclined by about 60 degrees with respect to the main scanning direction. did. The potential corresponding to the maximum input coverage of the portion except the edge portion of the electrostatic latent image thus formed was -200V.

Next, the electrostatic latent image of the first color formed on the photosensitive drum 21a as described above is reversed by the yellow toner of the first color negatively charged by using the yellow color developing device 24a. It is developed. As a result, the yellow toner adheres to the electrostatic latent image formed on the photoconductor drum 21a substantially along the line shape. The developing characteristics of the yellow developing device 24a used at this time are shown in FIG. A maximum density of about 2.0 was obtained for a development contrast of 450V [-200-(-650V)].

Subsequently, the photosensitive drum 21a is
The second color image forming process is started. The photoconductor drum 21
The surface of a is -7 from the surface of the photosensitive drum 21a including the yellow toner layer uniformly by the scorotron charger 22a.
It is recharged to 00V. Next, this photosensitive drum 21a
An electrostatic latent image of the second color is formed on the surface of the line 2 by a line screen extending in the main scanning direction by the second laser beam emitted from the laser exposure device 23a.
The potential corresponding to the maximum input coverage of the portion except the edge of the electrostatic latent image thus formed is -250 V on the yellow toner and -200 on the monochrome region (on the photosensitive drum).
It was V.

Similarly, the electrostatic latent image of the cyan color of the second color was developed with the same development parameters as the first color, and the electrostatic latent image was developed so that the cyan toner adheres (reverse development). As a result,
The electrostatic latent image formed on the photoconductor drum 21a includes
The cyan toner adheres along the line shape. The development characteristics of the Cyan developing device 24a used at this time are shown in FIG. As shown in the figure, since the output density is saturated at a developing contrast of about 400 V or more, a superposed image corresponding to almost the same density as that of a monochromatic portion was obtained even in the overlapping portion of yellow and cyan.

Next, with respect to the photosensitive drum 21b, a magenta color formed by a line screen extending in a direction inclined by about 60 degrees with respect to the main scanning direction by the first laser beam emitted from the laser exposure device 23b. To form an electrostatic latent image. The potential corresponding to the maximum input coverage of the portion except the edge portion of the electrostatic latent image thus formed was -200V.

Next, the electrostatic latent image of the first color formed on the photosensitive drum 21b as described above is inverted by the magenta color developing device 24b by the negatively charged magenta toner of the first color. It is developed. As a result, magenta toner adheres to the electrostatic latent image formed on the photoconductor drum 21b substantially along the line shape. The developing characteristics of the magenta developing device 24a used at this time are shown in FIG. Development contrast 450V [-200-(-650
V)] a maximum concentration of about 2.0 was obtained.

After that, the photoconductor drum 21b is recharged to -700V from above the photoconductor drum 21b including the magenta toner layer by the scorotron charger 22a. Next, on the surface of the photosensitive drum 21b, image exposure according to the image data 36 of the second color which is a line screen extending in the main scanning direction by the second laser beam emitted by the laser exposure device 23b. Is applied to form a second color electrostatic latent image. The potential corresponding to the maximum input coverage of the portion other than the edge portion of the electrostatic latent image formed in this way is −25 on the magenta toner.
The voltage was 0 V and -200 V in the monochrome region (on the photoconductor). Similarly, the electrostatic latent image of the second color formed on the photoconductor drum 21b is transferred to the first color by the black toner of the second color.
Development is performed with the same development parameters as for the color, and development is performed so that black toner adheres (reverse development). As a result, the cyan toner adheres to the electrostatic latent image formed on the photoconductor drum 21b substantially along the line shape. The developing characteristics of the black developing device 24a used at this time are shown in FIG.
As shown in the figure, since the output density is saturated at a development contrast of about 400 V or more, a superposed image corresponding to almost the same density as that of a monochrome portion was obtained even in the overlapping portion of magenta and black.

Transfer is performed in parallel with the above steps. In the toner images formed on the photoconductor drums 21a and 21b, first, the paper is brought into light contact with the photoconductor drum 21a by the baffle of the transfer corotron 25a facing the photoconductor drum 21a, and at the same time, a voltage is applied to the transfer corotron 25a. The yellow, yellow and cyan toner images are transferred onto the transfer paper 20.
It is transcribed all at once on top. Further, the transfer drum 25 rotates and, based on the position detection signal, the baffle of the transfer corotron 25b facing the photoconductor drum 21b causes the paper to lightly contact the photoconductor drum 21b, and at the same time, a voltage is applied to the transfer corotron 25b, and the magenta The black toner image and the black toner image are collectively transferred onto the transfer paper 20. In this way, a full-color image can be formed on the transfer paper 20 held on the transfer drum 25 by transferring twice. At this time, the resin film and the paper 20 carried on the resin film may be always in contact with the photosensitive drum, or may be in contact only when necessary. In this way, a full-color image can be formed by transferring the sheet 20 twice.

Further, the paper 20 separated from the transfer drum 25 is fixed by passing through the fixing device 26, and a color image is formed.

The toner remaining on the photoconductor drums 21a and 21b and the history of charging are recorded in the cleaning devices 27a and 2a.
The image forming cycle is completed by the continuous removal by 7b and the discharge lamps 28a, 28b.

According to the above embodiment, since the screen structures of the first and second color electrostatic latent images on the respective photoconductor drums are different, it is possible to greatly reduce the overlap of the toner layers on the photoconductor drums. In addition, it is possible to reduce the influence of the toner layer transmittance and the influence of the toner layer charge at the time of overlapping exposure. As a result, the reproduction in the highlight region and the reproduction of color characters are improved, and a high quality image can be obtained.

Further, in the above-described embodiment, the line screen extending for about 30 degrees with respect to the main scanning direction is used for the first color or the first laser beam, and the second color or the second color is used.
Although the parallel line screen extending in the main scanning direction by the laser beam was used, the reverse thereof may be used, or a combination of other angles may be used. Further, different angles may be used for the photosensitive drums 21a and 21b. further,
It is desirable to combine screen structures having a large difference in angle.

Although the transfer drum is used as the transfer means in the above embodiment, the image may be transferred twice using an intermediate transfer member, or another transfer device may be used.

Embodiment 3 This Embodiment 3 can be applied to any one of the configurations of FIGS. 1 to 10 showing Embodiment 1 and Embodiment 2, but instead of the transfer drum as the transfer means. It is particularly effective when an intermediate transfer member is used. This intermediate transfer body
After the toner images formed on the photoconductor drums 21a and 21b are once transferred, these toner images are transferred to a sheet. The difference in the image forming process will be described below. That is, in the third embodiment, the laser beam 35 emitted from the laser exposure device 23a is moved to the photoconductor drum 21a in the direction of about 60 degrees with respect to the main scanning direction as in the procedure of the above-described embodiment. A yellow electrostatic latent image composed of a stretching line screen and a cyan electrostatic latent image were formed.

Next, the electrostatic latent image was developed so that the negatively charged first-color yellow toner and cyan toner would adhere (reverse development). As a result, toner was adhered almost along the line shape of the electrostatic latent image.

The laser beam 3 emitted from the laser exposure device 23b is applied to the photosensitive drum 21b.
5, an electrostatic latent image of magenta and black consisting of a line screen extending in the main scanning direction was formed.

Next, the electrostatic latent image on the photosensitive drum 21b was developed such that the negatively charged magenta of the first color and the black toner adhere (reverse development). As a result, toner was adhered almost along the line shape of the electrostatic latent image.

The transfer process, which is performed in parallel with the above process or sequentially, is performed in the same manner as in the above two embodiments.

Further, the paper 20 separated from the transfer drum 25 is fixed by passing through the fixing device 26, and a color image is formed.

The toner remaining on the photoconductor drums 21a and 21b and the history of charging are continuously removed by the cleaning devices 27a and 27b and the discharge lamps 28a and 28b, and the image forming cycle is completed.

According to the above embodiment, since the screen structures of the images formed on the two photoconductor drums 21a and 21b are different, it is possible to greatly reduce the overlap of the toner layers transferred onto the intermediate transfer body. As a result, the reproduction in the highlight region and the reproduction of color characters are improved, and a high quality image can be obtained.

Further, in the above embodiment, the photosensitive drum 21
A line screen extending in the direction of about 60 degrees with respect to a was used for a, and a line screen extending for the photoconductor drum 21b in the direction of main scanning was used. Any combination of angles may be used. Furthermore, it is desirable to combine screen structures having a large difference in angle.

The other structure and operation are the same as those of the above-mentioned embodiment, and the description thereof will be omitted.

Embodiment 4 This Embodiment 4 is applied in the same configuration as the above Embodiment 2 or Embodiment 3, and is denoted by reference numeral 21 in FIG.
a and 21b, the photosensitive drum having a photoconductive thin layer such as an organic photosensitive layer on its surface is the photosensitive drum 21a,
The diameter is selected so that the circumference of 21b is shorter than the maximum image length in the process direction. The other configurations and the image forming process were the same as those in the second or third embodiment.

According to the above embodiment, it is possible to form a two-color image during one rotation of the photosensitive drums 21a and 21b and successively transfer the images while using the multiple development process.
As a result, by reducing the outer diameters of the photoconductors 21a and 21b, further miniaturization is possible, and an image forming apparatus that does not interfere even in an office environment can be realized.

Fifth Embodiment This fifth embodiment is applied to any of the configurations disclosed in FIGS. 1 to 10 showing the first to fourth embodiments. The difference in the image forming process will be described below.
That is, in this embodiment, the yellow electrostatic latent image and the cyan electrostatic latent image are applied to the photosensitive drum 21a by the laser beam 35 emitted from the laser exposure device 23a in the same manner as in the above-described embodiment. Form an image.

Next, the electrostatic latent image was developed so that the negatively charged first color yellow toner and cyan toner would adhere (reverse development). As a result, toner was adhered almost along the line shape of the electrostatic latent image.

On the photoconductor drum 21b, magenta and black electrostatic latent images were formed by the laser beam emitted from the laser exposure device 23b.

Next, the electrostatic latent image on the photosensitive drum 21b was developed so that the negatively charged magenta of the first color and the black toner adhere (reverse development). As a result, toner was adhered almost along the line shape of the electrostatic latent image.

The transfer step and the subsequent steps which are carried out in parallel or sequentially with the above steps are carried out in the same manner as in the above embodiment.

FIG. 16 shows changes in the transmittance of the yellow, magenta, cyan, and black toners used in the above-mentioned examples with respect to the weight of the developing toner. The transmittances of cyan and black are lower than those of yellow and magenta, and the optical effect of the toner layer on the photoconductor drum can be improved by making image formation on these photoconductor drums a later process. Could be significantly reduced. As a result, the reproduction in the highlight region and the reproduction of color characters are improved, and a high quality image can be obtained.

Further, in the above embodiment, the photosensitive drum 21
Although Sian toner is used as a post-process for a and black toner is used as a post-process for the photoconductor drum 21b, they may be reversed.

Sixth Embodiment In the sixth embodiment, as in the fifth embodiment, the configuration of any one of FIGS. 1 to 10 is applied. The difference in the image forming process will be described below. That is, in this embodiment, the laser beam 35 emitted from the laser exposure device 23a is applied to the photosensitive drum 21a.
A yellow electrostatic latent image and a cyan electrostatic latent image were formed in the same manner as in the above-described example.

Next, the first color yellow toner and the cyan toner negatively charged with respect to this electrostatic latent image were subjected to reversal development so that the weight of the developing toner would be 0.8 mg / cm ² or less. As a result, toner was adhered almost along the line shape of the electrostatic latent image.

On the photosensitive drum 21b, a magenta and black electrostatic latent image was formed by the laser beam emitted from the laser exposure device 23b.

Next, the magenta toner of the first color negatively charged with respect to the electrostatic latent image on the photosensitive drum 21b and the black toner have a developing toner weight of 0.8 mg / cm.
Reversal development was performed so that it was ² or less. As a result, toner was adhered almost along the line shape of the electrostatic latent image.

The transfer step and the subsequent steps which are carried out in parallel or sequentially with the above steps are carried out in the same manner as in the above-mentioned embodiment.

FIG. 17 shows changes in the toner layer potential with respect to the developing toner weights of the yellow, magenta, cyan and black toners used in the above examples. From this, the toner layer potential rises as the weight of the developing toner increases, and the difference between the weight of the developing toner of the monochromatic color and the weight of the developing toner of the superposing portion is the density difference, and the desired density of the superposing portion is satisfied while satisfying the density of the single color. It has been found that the color difference ΔE≈2 that can be detected by a general human being with respect to the above color can satisfy the developing toner weight of 0.8 mg / cm ² or less.

The toner layer potential in the vicinity of the developing toner weight of 0.8 mg / cm ² is 50 to 60 V, and it can be understood from the developing characteristics of FIG. 9 that the density is almost satisfactory. By these, the toner layer potential generated by the toner layer on the photoconductor drum can be reduced, and as a result, the reproduction in the highlight region and the reproduction of color characters are improved, and a high quality image can be obtained.

Embodiment 7 This Embodiment 7 can be applied to any one of the configurations shown in FIGS. 1 to 17 showing Embodiments 1 to 6. The difference in the image forming process will be described below. That is, in this embodiment, the yellow electrostatic latent image and the cyan electrostatic latent image are applied to the photosensitive drum 21a by the laser beam emitted from the laser exposure device 23a in the same manner as in the above-described embodiment. Was formed so that the potential corresponding to the maximum input coverage was −250V.

Next, the electrostatic latent image was developed so that the negatively charged first color yellow toner and cyan toner would adhere (reverse development). As a result, toner was adhered almost along the line shape of the electrostatic latent image.

Further, with respect to the photosensitive drum 21b, the electrostatic latent image of magenta and black is adjusted by the laser beam emitted from the laser exposure device 23b so that the potential corresponding to the maximum input coverage becomes −250V. Formed.

Next, the electrostatic latent image on the photosensitive drum 21b was developed so that the negatively charged magenta of the first color and the black toner adhere (reverse development). As a result, toner was adhered almost along the line shape of the electrostatic latent image.

The latent image contrasts (V) of the yellow, magenta, cyan and black toners used in the above embodiment are
FIG. 18 shows changes in the toner layer potential with respect to _image : image portion potential- _Vhigh : photoconductor charging potential. From this, the toner layer potential increases as the latent image contrast increases, and since the monochromatic developing toner weight is about 50 to 60 V, it is generally higher than the desired color in the overlapping portion while satisfying the monochromatic density. It is necessary for the latent image contrast to be 450 V or less in order to satisfy the color difference ΔE≈2 that can be detected by a human being.

From this, it can be understood from the developing characteristics of FIG. 10 that the density is almost satisfactory. By these,
The toner layer potential generated by the toner layer on the photosensitive drum can be reduced. As a result, the reproduction in the highlight region and the reproduction of color characters are improved, and a high quality image can be obtained.

The present invention can be similarly applied to both wet development and dry development. Further, it is desirable to use a toner having an average particle size of 7 μm or less in the color developing device in order to achieve high image quality, but it goes without saying that a toner having an average particle size larger than this may be used.

Further, in the above-mentioned embodiment, the gradation information of R, G, B or L ^* a ^* , b ^* is used as the image data, but Y, M, C, K which is the image output itself or You may use Y, M, and C image data. Further, even data having no gradation or a sleek printer that does not perform image processing itself will not impair the characteristics of the present invention.

[0153]

The present invention has the above-described structure and operation, and is provided with two sets of image forming units provided with a photosensitive medium and the like, and each image forming unit forms toner images of a plurality of colors. The transfer of the toner image from each of these image forming units to the transfer drum, the intermediate transfer body, etc. may be performed twice, and the contact between the photoconductor and the transfer drum, the intermediate transfer body, etc. is half that of the conventional one per color print. It is possible to extend the life of the device and increase the reliability.

In the color image forming apparatus according to the first or second aspect of the present invention, the screen structure of the electrostatic latent image formed by scanning exposure has a plurality of toners in each image forming unit. Since the images differ from each other, the average toner height on the paper or the intermediate transfer member can be reduced. As a result, a high-quality color image forming apparatus that is excellent in color reproduction, gradation reproduction, and color characters can be obtained. Can be provided.

Further, claim 1 or 2 of the present invention
In the color image forming apparatus described in the paragraph (1), since the screen structure is different between the colors in each image forming unit, it is possible to reduce the influence of the toner layer charge in the multiple development process, and it is possible to achieve stable gray reproduction with excellent highlight reproduction of multiple colors. Since color characters can be reproduced without balance and toner scattering, as a result, it is possible to provide a high-quality color image forming apparatus excellent in color reproduction and gradation reproduction.

Further, in the color image forming apparatus according to the second aspect of the present invention, the toner image is formed on the photosensitive member in the two image forming units each including the charging means, the two light beams and the two developing means. Since they are formed in an overlapping manner, high-speed output equivalent to that of a tandem system using four photoconductors is possible, and a low-cost and small-sized device can be realized.

Further, in the color image forming apparatus according to the fourth aspect of the present invention, in the above structure, the peripheral length of the photosensitive member is set shorter than the maximum recording image length, whereby the color image can be further miniaturized. A forming device can be provided.

Furthermore, in the color image forming apparatus according to the fifth aspect of the present invention, the developing means having cyan and black toners are distributed to different units, and the developing means having yellow and magenta toners are respectively provided. When the developing means distributed to different units and arranged in one unit all perform development, the developing means having cyan and black toner is configured to perform development in a post-process step, so that it is formed on the photoconductor. It is possible to provide a high-quality color image forming apparatus that can reduce the exposure loss due to the toner layer transmittance when performing infrared laser exposure from the formed toner layer, and that is excellent in color reproduction and gradation reproduction. .

Furthermore, in the color image forming apparatus according to the sixth aspect of the present invention, since the developing toner weights of the yellow and magenta toners are 0.8 mg / cm ² or less, the development of the next color by the charge of the toner layer is performed. It is possible to provide a high-quality color image forming apparatus that can reduce the influence on the process and is excellent in color reproduction and gradation reproduction.

Further, in the color image forming apparatus according to claim 7 of the present invention, the latent image contrast (V) of the first color of the developing means arranged in each unit is
_{Since image} −V _high ) is 450 V or less, it is possible to provide a high-quality color image forming apparatus capable of reducing an increase in toner layer potential in the recharging step and excellent in color reproduction and gradation reproduction.

[Brief description of drawings]

1A and 1B are schematic explanatory views of a screen structure that is a main part of a color image forming apparatus according to the present invention.

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

FIG. 3 is a perspective configuration diagram showing a laser exposure apparatus.

FIG. 4 is a block diagram showing an image processing apparatus.

FIG. 5 is a graph showing conversion characteristics of image signals.

FIG. 6 is a block diagram showing a screen generation circuit.

7A and 7B are explanatory views showing screen images of electrostatic latent images, respectively.

FIG. 8 is a graph showing an electrostatic latent image.

FIG. 9 is a block diagram showing a color developing device.

FIG. 10 is a graph showing the relationship between development contrast and output density.

FIG. 11 is a schematic diagram showing how toner images are superposed on a photosensitive drum.

FIG. 12 is a graph showing the relationship between the developing toner weight and the toner image height.

FIG. 13 is a graph showing the relationship between the developing toner weight and the toner layer potential.

FIG. 14 is a schematic configuration diagram showing a second embodiment of the color image forming apparatus according to the present invention.

FIG. 15 is a perspective configuration diagram showing a laser exposure apparatus.

FIG. 16 is a graph showing the relationship between the weight of developing toner and the transmittance.

FIG. 17 is a graph showing the relationship between the developing toner weight and the toner layer potential.

FIG. 18 is a graph showing the relationship between the developing potential and the toner layer potential.

FIG. 19 is a schematic configuration diagram showing a conventional color image forming apparatus.

FIG. 20 is a schematic configuration diagram showing a conventional color image forming apparatus.

FIG. 21 is a graph showing the relationship between the conventional developing toner weight and the toner layer potential.

[Explanation of reference numerals] 21a, 21b Photoreceptor drums, 22a, 22b Scorotron charger, 23a, 23b Laser exposure device,
24a, 24b, 24a, 24b color developing device, 25
Transfer drum.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takuto Tanaka, 2274 Hongo, Ebina, Kanagawa Prefecture, Fuji Xerox Co., Ltd. (72) Haruyuki Namba, 2274, Hongo, Ebina, Kanagawa Prefecture, Fuji Xerox Co., Ltd.

Claims (7)

[Claims] 1. A charging means for uniformly charging the surface of a photosensitive medium, and an electrostatic latent image is formed on the surface of the photosensitive medium charged by the charging means by scanning and exposing a light beam according to image information. Two image forming units, each of which includes an exposing unit for exposing the electrostatic latent image formed on the photosensitive medium and a plurality of developing units for selectively developing the electrostatic latent image formed on the photosensitive medium with toner of a plurality of colors. Charging, exposing, and developing steps are performed plural times on each photosensitive medium to form toner images of a plurality of colors on each photosensitive medium, and the toner images are formed on each photosensitive medium of the two image forming units. In a color image forming apparatus for forming a color image by sequentially transferring the formed toner images of a plurality of colors onto a recording medium or an intermediate transfer member, a screen structure of an electrostatic latent image formed by the exposing unit is provided. Each picture A color image forming apparatus characterized in that a plurality of color toner images in an image forming unit are different from each other. 2. A charging unit for uniformly charging the surface of the photosensitive medium, and a plurality of electrostatic beams by scanning and exposing a plurality of light beams on the surface of the photosensitive medium charged by the charging unit according to image information. Two sets of image forming units each including an exposure unit capable of forming a latent image and a plurality of developing units for selectively developing the electrostatic latent image formed on the photosensitive medium with toner of a plurality of colors are provided. A state in which toner images of a plurality of colors are superposed on each photosensitive medium of the pair of image forming units by performing the exposure and development processes a plurality of times while the photosensitive medium uniformly charged is rotated once. In a color image forming apparatus for forming a color image by sequentially transferring the toner images of a plurality of colors formed on the photosensitive mediums of the above two sets of image forming units onto a recording medium or an intermediate transfer member. The exposure means A color image forming apparatus, wherein the screen structure of the electrostatic latent image formed by the toner image of each of the plurality of colors in each image forming unit is different from each other. 3. The color image forming apparatus according to claim 1 or 2, wherein a screen structure of a plurality of electrostatic latent images formed by scanning exposure is different between the image forming units. A color image forming apparatus characterized by the above. 4. The color image forming apparatus according to claim 2, wherein the peripheral length of the photosensitive medium is shorter than the maximum image length in the process direction. 5. The color image forming apparatus according to claim 1, wherein the developing means has yellow, magenta, cyan, and black toners, and the developing means has cyan and black toners, respectively. When the developing units having yellow and magenta toners are respectively distributed to different image forming units while being distributed to different image forming units, and all the developing units arranged in one image forming unit perform development,
A color image forming apparatus, characterized in that a developing means having cyan and black toner develops in a later step of the process. 6. The color image forming apparatus according to claim 5, wherein the developing toner weights of the yellow and magenta toners are 0.8 mg / cm ² or less. 7. The color image forming apparatus according to claim 1, wherein the electrostatic latent image contrast (V _image −) of the first color of the developing unit arranged in each image forming unit. V _high ) is 450 V or less, a color image forming apparatus.