JPH079550B2 - Multicolor image forming method - Google Patents

Description

The present invention relates to a multicolor image forming method for forming a multicolor image by sequentially forming toner images having different color tones on an image support, and electrostatic recording and electrophotography. Used in fields such as recording.

[Prior art]

Conventionally, for example, in order to form a multicolor image by electrophotography, the copying process of charging, exposing, developing, and transferring is repeated for each component color, and the toner images of the respective colors are superposed and transferred onto the copy paper. I have to. For example, an electrostatic charge image is formed in each of the steps by using decomposition light of blue, green, red, etc. obtained through a color separation filter, and developed with yellow, magenta, cyan and, if necessary, black toner to form a toner image. Then, the toner image is formed and laminated on a recording paper and transferred to form a multicolor image. However, in such a multi-color image forming method, it is necessary to transfer the image onto the transfer body after each development of each color is completed. There is a problem that it is necessary to guarantee the accuracy of positional displacement.

Therefore, there is a multicolor image forming method that solves the above-mentioned drawbacks by superposing a plurality of toner images on the same photoreceptor and developing them, and the transfer step is completed at once. The toner image obtained by the development is disturbed, the toner in the developer in the former stage is mixed in the developer in the latter stage, or the toner image in the latter stage becomes difficult to adhere to the toner image portion in the former stage, and thus the multicolor image This causes adverse effects such as disturbed color balance.

[Object of the Invention]

The present invention has been proposed in view of the above circumstances, and an object thereof is to provide a multicolor image forming method capable of forming a multicolor image having high resolution and excellent color balance when reproducing a multicolor original document. And

[Structure of Invention]

(1) A multicolor image method for forming a toner image of four colors of yellow, magenta, cyan, and black as a dot image by repeating charging, image exposure, and reversal development using a dry toner on a photoconductor. Image data for latent image formation is created by converting at least a part of the density values common to the image data of yellow, magenta, and cyan to black image data by the calculation processing of the image data of magenta, cyan, and Multicolor image formation characterized in that dots at least partially overlap each other, gradation is expressed by the size of the dots, and the developing step in the image forming process for the second and subsequent colors is performed by non-contact development Method, and (2) In addition to the characteristics of (1) above, the distribution angles of at least two color toner images of the four color toner images are different from each other. Multi-color image forming method characterized in that multi-color image consisting of SEQ dot is formed,
Achieved by

Next, each of the technical elements constituting the present invention will be described.

First, a process for forming a multicolor image by transferring and fixing after forming a toner image on the photoconductor by charging, exposing and reversal developing will be described with reference to the flowchart of FIG. FIG. 1 shows changes in the surface potential of the photoconductor, and takes the case where the charging polarity is positive as an example. PH
Is the exposed portion of the photosensitive member, DA is the non-exposed portion of the photosensitive member, DUP is the amount of potential increase caused by the positively charged toner T adhering to the exposed portion PH in the first development, and CUP is generated by the second charging. exposure
Indicates the increase in PH potential.

The photoconductor is uniformly charged by a scorotron charger or the like to give a constant positive surface potential E. The surface potential E is lowered to near zero potential at the exposed portion PH by the first image exposure by the exposure source such as laser, cathode ray tube, LED or the like. Here, by applying a positive bias whose direct current component is substantially equal to the surface potential E of the unexposed portion to the developing device to perform development, the positively charged toner T in the developing device has a relatively low potential in the exposed portion PH. To form a first visible image. In the region where the visible image is formed, the potential is increased by DUP due to the adhesion of the positively charged toner T, but the potential is further increased by CUP by the second charging by the charger. As it rises, the initial surface potential E is obtained similarly to the non-exposed area DA. This charging step is a step necessary for averaging the potential contrast of the formed toner image and allowing the photoconductor to have photosensitivity by the next image exposure. Next, a second image exposure is performed on the surface of the photoconductor on which a uniform surface potential E has been obtained to form an electrostatic latent image,
A second visible image is obtained through the same developing operation. By repeating the above process, a multicolor toner image on the photoconductor is obtained, and the multicolor image is obtained by transferring the multicolor toner image onto the recording paper and further fixing it by heating or pressing. Here, the toner and charges remaining on the photoconductor are cleaned, and the photoconductor is prepared for the next multicolor image formation. In the multicolor image forming method, a charge eliminating step may be performed before charging every time. The exposure sources used for each image exposure may be the same or different.

The toner in the developer is obtained by making the photoconductor and the developer layer for developing the electrostatic latent image formed on the photoconductor non-contact, and superposing an AC component on the DC bias applied to the developing device if necessary. There is a method of forming a multicolor image by adopting a method of flying and developing. In this method, the developer layer does not rub the toner image formed up to the previous stage, so that the image is not disturbed. Therefore, it is necessary to use the above non-contact developing method in the second and subsequent toner image formation.

In the multicolor image forming method of the present invention, at least a part of the four color toner images of yellow, magenta, cyan and black are laminated and formed on the photoreceptor by superposition. The reason is,
This is because if the color toners are not overlapped with each other, sufficient image density and resolution cannot be obtained. According to the principle of the subtractive color method, a black image should be obtained by superimposing the three primary colors of yellow, magenta, and cyan, but a practical toner for the three primary colors has an ideal absorption wavelength range. In addition, since it is difficult to precisely align the toner images of the three primary colors, even if the toner images of the respective colors are superposed, sufficient image density and resolution cannot be obtained, and only the three primary colors are used for characters and line drawings. It is difficult to reproduce the vivid black that is required by the etc.

Therefore, as described above, the toner images of four colors in which black is added in addition to the three primary colors are superposed to obtain a multicolor image closer to the original.

In the multicolor image forming method, a reversal developing method is used as a method for developing an electrostatic latent image. In the reversal development method, it suffices to expose only the toner image forming portion of the photoreceptor,
Since it is not necessary to expose the background portion without a gap as in the case of regular development, a latent image can be formed relatively easily on a photoconductor on which a toner image has already been formed. Further, there is an advantage that the photoreceptor is less fatigued and the life is extended. Further, since the second and subsequent charges are performed with the same polarity as the toner, there is no problem in electrostatic transfer.

Next, a dot image forming method will be described.

In the present invention, in the method of forming an electrostatic latent image by uniformly charging and imagewise exposing on the photoconductor, a method capable of multistep gradation expression is adopted as each of the latent image forming methods.
The gradation expression by such a method requires a large amount of image data because the latent image to be formed is a so-called multistep gradation. Therefore, it is also possible to use a binary gradation image data forming method in which each pixel is binarized and recorded, and a pseudo gradation expression is performed by its distribution, and a small amount of image data is required. In order to express the gradation of the image by the method of forming the binary gradation image data, for example, the second
The density pattern method shown in the figure and the dither method shown in FIG. 3 are used.

The density pattern method shown in FIG. 2 is a method of converting one pixel into a plurality of pixels. 1a is a manuscript, and each pixel has a gradation. Reference numeral 2a is a sample for taking out a pixel 5a having a typical density value of the matrix of the original document 1a and thresholding it, 3a is an M × N threshold density matrix corresponding to the sample, and 4a is The dither method shown in FIG. 3, which is a binarized pattern by comparing the threshold value matrix 3a with the sample 2a, is a method of converting one pixel into one pixel. The original 1b is divided into a density matrix of M × N pixels. 2b is a sample for threshold processing corresponding to the density matrix of the original document 1b, 3b is an M × N threshold density matrix corresponding to the sample 2b, and 4b is a value obtained by comparing the threshold density matrix 3b with the sample 2b. It is a quantized pattern.

In order to express a multicolor image having gradation by superimposing color toner images, the following two types of methods, that is, a method in which toner dots of different colors do not overlap each other, and at least a part of toner dots of different colors overlap There is a method. In this case, toner dots of different colors are formed at different positions in the pattern 4a or 4b in FIGS. 2 and 3. Therefore, toner dots of different colors are separated from each other and are two-dimensionally distributed, and a pseudo color mixture is formed on the recording paper.

In this case, since the dots are separated from each other, the influence of overlapping is small, but there is a drawback that sufficient image density and resolution cannot be obtained.

In this case, since the different-color toner dots are formed to be mixed in the pattern 4a or 4b, at least some of the different-color toner dots are formed to overlap each other. In the above case, development is performed by controlling the latent image potential and the developing bias, but the potential of the overlapped portion is insufficient, or the toner dots developed earlier are used for image exposure to form toner dots later. There is a problem that the desired latent image formation is not achieved, and as a result, the color tone of the preceding toner dot is excessively emphasized and the color balance of the multicolor image is lost. This problem is more prominent than in the case of, and becomes a serious obstacle in expressing the color balance. Even when the different-color toner dots do not overlap with each other, the same trouble occurs due to inevitable misalignment of the positions which occurs when forming a latent image of the different-color toner image, and dot spread.

To solve the above problem, in a multicolor image forming method for sequentially forming a plurality of toner images having different color tones on an image support to obtain a multicolor image, the toner image has a dot structure and toner dots of different colors are used. Is overlapped, and a gradation is expressed by the size of the dots, thereby achieving a multicolor image forming method. By doing so, the latent image is formed in a cohesive form. As a result, the toner can be stably formed even when the toner is layered. Further, in order to avoid the influence of superposition, it is desirable that the dots of each of these colors be arranged at different distribution angles. By doing so, a multicolor image having high resolution and good color balance is formed.

Next, a process of converting at least a part of the density values common to the yellow, magenta, and cyan color image data into black image data will be described.

When forming a multicolor image by overlapping toner images,
It is preferable to replace at least a part of the yellow, magenta and cyan components with the black component. As a result, the amount of toner on the photosensitive member decreases, and the potential of the toner image portion,
It is possible to prevent the color balance from being lost due to the light shielding effect at the time of exposure of the toner image portion.

In the multicolor image forming method of the present invention, for example, an output signal of an image pickup device that scans a multicolor original, a transmission signal from another device such as a facsimile, or data stored in a recording device is used as image data. The image data is typically three primary color data Yi, Mi, yellow, magenta, and cyan.
It consists of Ci and black data BKi. When performing such multicolor image formation, the image data is input to the arithmetic processing unit of the color correction section shown in FIG. 4, and the desired four color data is calculated, for example, by the following arithmetic expression [I]. Formula [I] However, Ym.Mm.Cm is the data after calculation Yi, Mi, Ci is the input image data α ₁ , α ₂ , α ₃ , α ₄ , β ₁ , β ₂ , β ₃ , β ₄ are the development conditions etc. A color correction coefficient based on an external factor, min [Yi, Mi, Ci], indicates a color tone having the minimum density value among the three primary colors of yellow, magenta, and cyan.

Next, in order to understand the arithmetic expression [I], α _{1 to} α ₄ and β _{1 to} β
_The case where ₄ is all 1 will be described below as an example. If the color tone of the minimum density value is cyan (Ci) as shown in FIG. 5, the black component is obtained according to the principle of the primary color method by collecting together the ones obtained by subtracting the density corresponding to this cyan from each of the three primary colors. To be This black component is added to the black data BKi to form black image data as shown in FIG. Further, the remainder obtained by subtracting the cyan density equivalent amount from each of the three primary colors is used as the image data of the three primary colors as shown in FIG. Thus, the color balance at the time of development can be improved, the toner consumption can be saved, and the toner amount in the overlapping portion can be reduced.

Four-color data Ym, M color-corrected by the arithmetic processing unit shown in FIG.
m, Cm.Bkm are compared with a threshold matrix described later to obtain binarized four-color data Yo, Mo, Co, BKo. This data is stored in the memories My, Mm, Mc, and Mbk, but is output to the exposure system in response to a command from the control unit, and an electrostatic charge image is formed on the photoconductor. This electrostatic charge image includes yellow, magenta, cyan. The development is preferably performed in a non-contact manner by four types of developing devices respectively containing black toner. Thus, toner images of four colors are laminated and formed on the photoconductor, and transferred and fixed on the recording paper fed in time with the photoconductor to form a multicolor image.

Next, gradation expression based on the size of dots will be described.

In the present invention, a multi-color image is formed on the photoconductor as a dot image, and dots in which at least some of the dots of different colors overlap each other are formed.

When at least a part of the toner images are overlapped, the toner image previously formed on the photoconductor and the toner image formed later are not overlapped and overlapped with each other at the time of exposure for the subsequent toner image. There is a problem that the potential of the exposed portion becomes higher than that of the non-toner portion because the toner image does not reach the photoreceptor sufficiently and the previous toner layer has a potential. For this reason, the adhesion of the toner becomes unstable, and the color balance is easily lost.

On the other hand, the present invention overcomes the above problems by the following means.

That is, gradation is expressed by the size of dots, in other words, gradation is expressed by making small dots at low density and large dots at high density.

As described above, at least a part of the yellow, magenta, and cyan color components is converted to the black component by the undercolor removal technique to form a toner image, thereby overcoming the above-mentioned adverse effects and achieving good color balance. I succeeded in obtaining a color image.

Due to the above, large dots are formed in the high density portion. As a result, the toner is stably attached to the latent image, and an image with good color balance can be stably obtained.

Moreover, the amount of toner adhered to the same position is reduced. This effectively alleviates the problems caused by toner superposition.

In a further preferred embodiment, as shown in FIG. 7, the distribution angles of the dots are made different between the respective colors to prevent the occurrence of moire and succeed in forming a very good color image.

FIG. 7 shows one mode of dot formation of toner images of yellow, magenta, cyan and black.

The dot image shown in Figure 7b is created using the threshold matrix of Figure 7a. When continuous tone image data is processed by the threshold matrix of FIG. 7a, for example, in the case of a density of 1, “0” and “1” which are approximately at the center of each of the 3 × 3 matrices arranged in parallel. Only black and white others. When the density is 8, “0”, “2”, “4”, “6” and “8” in the left matrix are “1”, “3”, “5” and “7” in the right matrix. "Is black. In the case of the maximum density, that is, density 17, all the dots in the matrix are black. As can be seen from the above, when the density is 1, it becomes a small dot, and when the density is high, it becomes a large dot as an aggregate of small dots, and the dot size corresponds to the gradation. Become. That is, by the matrix of the centralized threshold matrix of FIG. 7A, a dot image is formed as a small dot at low density and as a large dot at high density.

Note that the black circles in FIGS. 6b, 7b, 8b, and 9b represent the centers of the dots, and the arrows indicate the direction in which the dots become larger as the density increases. For example, in the 3 × 3 matrix in the left half of FIG. 7A, black dots become larger clockwise as 0 → 2 → 4 → 6 → 8. Similarly, in the 3 × 3 matrix in the right half of FIG. 6A, black dots become larger in the counterclockwise direction. 7 (b), 7 (c), and 7 (d) are dot images in which, when viewed as a completed image as a whole, the dots become larger in accordance with the increase in density, as in FIG. 7 (a). 7 (b), (c), and (d), the dot becomes larger around the black circle in the direction of the arrow, and in the high-density portion,
The dot grows to fill the white circle in the direction of the arrow.

The angle of the dot distribution in (a) is 90 °. Similarly, there are dot images such as (b), (c), and (d). When forming each image of black, yellow, magenta, and cyan, moire is generated by forming black with (a), yellow with (b), magenta with (c), and cyan with (d). It is possible to prevent the deterioration of the color balance and obtain a good color image. In this example, the dot distribution angle is 90 ° in (a) (θ _{1 in} 6c figure), 45 ° in (b) (θ _{2 in} 7c), 26.6 ° in (c) (θ _{3 in} 8c),
In (d), the distribution angle of each color dot is set to -26.6 ° (θ _{3 in} 9c). The distribution angles of the dots may be different for all the colors, or may be different for the two colors.

Next, a specific example of the present invention will be described.

Specific description will be given below with reference to the drawings of the first embodiment. FIG. 8 is a sectional view of a multicolor image forming apparatus for explaining the present embodiment, and FIG. 9 is a laser device applied to the image forming apparatus of FIG. FIG. 10 shows a developing device applied to the image forming apparatus of FIG. In FIG. 8, a photosensitive drum 11 is a selenium photosensitive member having a diameter of 120 mm which rotates in a direction of an arrow at a linear velocity of 120 mm / sec.
A uniform charge of 0 V is applied. This uniform charge is imagewise exposed by the following imagewise exposure means to form an electrostatic charge image.

That is, the data Yi, Mi, Ci, BKi previously stored in the storage device
Is input as image data to the arithmetic processing unit of FIG. 4, and is arithmetically operated by the arithmetic expression [I] to obtain color-corrected data Ym, Mm, Cm, BKm, and the comparing unit of FIG. The binarized data Yo, Mo, C compared with the threshold matrix of
o, BKo is obtained. These data are stored in the memory My, Mm, M
Although it is stocked in c and Mbk, it is output to the exposure system according to a command from the control unit. In this embodiment, the laser device 14 shown in FIG. 9 is used as an exposure system, and the laser light L from the device 14 is imagewise exposed to form an electrostatic latent image corresponding to each color on the photoconductor 11. It

Details of the laser device 14 are shown in FIG. Source 33
The helium-neon laser output from the
The signal is input to the acousto-optic modulator (AOM) 34 via 8 and is modulated by the binarized image data here. This modulated laser light is a mirror scanner consisting of a rotating octahedron.
The light is polarized by 34, and the surface of the photoconductor 11 is scanned at a constant speed through the image forming f-θ lens 36 to perform image exposure. Reference numeral 39 is an inspector for checking the characteristics of the laser light L.

Of the electrostatic latent images corresponding to the respective colors, the electrostatic latent image corresponding to yellow is formed by irradiating the laser light modulated by the yellow data. The yellow data is binarized using the threshold matrix 7a shown in FIG.

The electrostatic latent image corresponding to the yellow is the first developing device 15
To be developed, and a first toner image (yellow toner image) is formed on the photoconductor 11. The first toner image is not transferred to the recording paper P, and the photosensitive member 11 is charged with + 600V again by the scorotron charger 2. Then, the laser light is modulated by the magenta data binarized by using the threshold matrix 8a shown in FIG. 7, and the modulated laser light is irradiated on the photoconductor 11 to form an electrostatic latent image.
This electrostatic latent image is developed by the second developing device 16,
A second toner image (magenta toner image) is formed. In the same manner as described above, the threshold matrices 9a and 6a are sequentially used and sequentially developed by the third developing device 17 and the fourth developing device 18, and the third toner image (cyan toner image) and the fourth toner image (black toner image) are obtained. A toner image) is formed, and a four-color toner image sequentially formed on the photoconductor 11 is formed. These four-color toner images are discharged by the charge remover 19 and transferred onto the recording paper P supplied from the paper supply device 20c by the action of the transfer electrode 24. here
Reference numeral 23 is a paper feed roller, and 22 is a guide plate. The recording paper P carrying the transferred toner image is separated from the photoconductor 11 by the separation electrode 25, is conveyed by the guide 26 and the conveyance belt 27, is carried into the fixing roller 28, is heated and fixed, and is discharged to the paper discharge tray 29. .

On the other hand, the photoconductor 11 that has completed the transfer is discharged by the static eliminator 31 that was not used during the toner image formation, and then the toner remaining on the surface was released during the toner image formation by the blade 32 of the cleaning device 30. Are removed by the above process so that the next multicolor image formation is not hindered.

Next, four types of developing devices are used as the developing device used in the multicolor image forming apparatus of FIG. 8, but these may have the same or similar structure, and a cross-sectional view of the first developing device 15 is representative. Is shown in FIG. The developer D has a magnetic roll 41 having 6 poles at a speed of 1000 rpm and is in the direction of arrow F and has a diameter of 30 mm.
The sleeve 42 is rotated in the direction of arrow G at a linear velocity of 120 mm / SEC to be conveyed in the direction of arrow G. The developer D is a two-component developer, and the spike-up restraining blade 43 is in the middle of conveyance.
Due to this, the thickness is regulated and a 0.5 mm thick developer layer is formed. A stirring screw 45 is provided in the developer pool 44 so that the developer D is sufficiently stirred, and when the developer D in the developer pool 44 is consumed, the toner supply roller 45 rotates. As a result, the toner T is replenished from the toner hopper 47.

Next, the gap between the sleeve 42 and the photoconductor drum 11 is set to 0.8 mm, and a DC power source 48 for applying a developing bias is provided between the sleeve 42 and the photosensitive drum 11 in order to carry out reversal development. And vibrate the developer D
11 is provided in series with an AC power supply 49 and a DC power supply 48 so that the power can be sufficiently supplied to 11. R is a protective resistance. Here, the developing bias has a DC component of +500 V and an AC component of 2 KHz.
And the effective value is 1.5KV. In the developing device 15, the toner T in the developer carried by the sleeve 42 is provided with a charge amount of 20 μc / g before reaching the developing area E.

On the other hand, developers used in such machines include two-component developers composed of toner and carrier and one-component developers composed of toner only. The two-component developer requires management of the amount of toner with respect to the carrier, but has the advantage that the triboelectric charge control of toner particles can be easily performed. Further, especially in a two-component developer composed of a magnetic carrier and a non-magnetic toner, it is not necessary to include a large amount of a black magnetic substance in the toner particles, and thus it is possible to use a color toner that does not cause color turbidity due to the magnetic substance. It is possible to form a clear color image.

The two-component developer used in the present invention is particularly preferably composed of a magnetic carrier as a carrier and a non-magnetic toner as a toner.

The toner composition is generally as follows.

Thermoplastic resin: Binder 80 to 90 wt% Example: Polystyrene, styrene-acrylic polymer, polyester, polyvinyl butyral, epoxy resin, polyamide resin, polyethylene, ethylene vinyl acetate copolymer, etc. are often mixed and used.

 Pigment: Colorant 0 to 15 wt% Example: Black: Carbon black Cyan: Copper phthalocyanine, sulfonamide dielectric dye Yellow: Benzidine derivative Magenta: Rhodamine B lake, Carmine 6B, etc.

Charge control agent 0-5 wt% Plus Toner: Many Nigrosine electron donating dyes,
In addition, alkoxylated amines, alkylamides, chelates, pigments, quaternary ammonium salts, etc.

Minus Toner: Electron-accepting organic complex is effective, and in addition, chlorinated paraffin, chlorinated polyester, polyester with excess acid groups, chlorinated copper phthalocyanine, etc.

Superplasticizer Example: Colloidal silica and hydrophobic silica are typical.
In addition, there are silicone varnish, metal soap, nonionic surfactant, etc.

 Cleaning agent Prevents toner filming on the photoconductor.

Example: fatty acid metal salt, silicon oxide having organic group on the surface,
There are fluorochemical surfactants.

 Filler The purpose is to improve the surface gloss of images and reduce raw material costs.

Examples: Calcium carbonate, clay, talc, pigments, etc.

In addition to these materials, a magnetic material for preventing fogging and toner scattering may be contained.

As the magnetic powder, 0.1 to 1 μm of ferrosoferric oxide, r-ferric oxide, chromium dioxide, nickel ferrite, iron alloy powder, etc. have been proposed, but at present, ferrosoferric oxide is often used. It is contained in an amount of 5 to 70 wt% with respect to the toner. The toner resistance changes considerably depending on the type and amount of magnetic powder,
In order to obtain sufficient resistance, the amount of magnetic material is preferably 55 wt% or less. Further, in order to maintain a vivid color as a color toner, it is desirable that the magnetic substance layer is 30 wt% or less.

About 20 kg as a resin suitable for toner for pressure fixing
Adhesive resins such as wax, polyolefins, ethylene-vinyl acetate copolymer, polyurethane, and rubber are selected so that they can be plastically deformed by a force of about / cm to adhere to paper. Capsule toner can also be used.

By using the above materials, a toner can be manufactured by a conventionally known manufacturing method.

In the constitution of the present invention, in order to obtain a more preferable image, it is desirable that the weight average particle diameter of these toner particles is usually about 50 μm or less in consideration of the resolution. In the present means, there is no theoretical limitation on the toner particle size, but usually about 1 to 30 μm is preferably used in view of the resolution, the toner scattering and the transportation. In this embodiment, toner having a weight average particle diameter of 10 μm is used for all four colors.

Further, in order to increase fine dots or lines or gradation, the magnetic carrier particles are particles composed of magnetic particles and resin, for example, a resin dispersion system of magnetic powder and resin or resin-coated magnetic particles, Particles having a weight average particle diameter of preferably 50 μm or less, particularly preferably 30 μm or less and 5 μm or more, which are spherical, are more preferable. In this example, carrier particles having a weight average particle diameter of 50 μm were used for all four colors. The weight average particle diameters of the toner and carrier were measured with a Coulter counter (manufactured by Coulter).

In addition, there is no problem that carrier particles, which hinders good image formation, are easily injected with a bias voltage and carriers are easily attached to the surface of the image carrier, and that bias voltage is not sufficiently applied. Therefore, the resistivity of the carrier is 10 ⁸ Ωcm or more, preferably 10 ¹³ Ωcm or more, more preferably 10 ¹⁴ Ωcm or more of insulating properties, further, these resistivity, the particle size as described above. Good. In this embodiment, a resin dispersion type carrier having a magnetization of 50 I.mu and a specific resistance of 10 ¹⁴ Ωcm or more was used. The specific resistance of the carrier is measured by the following measuring method.
That is, the particles are placed in a container having a cross-sectional area of 0.50 cm ² and tapped, and then a load of 1 kg / cm ³ is applied to the packed particles, and a load of 10 ² to ⁵ V / cm is applied between the load and the bottom electrode. It is determined by applying a voltage that causes an electric field, reading the current value flowing at that time, and performing a predetermined calculation. At this time, the thickness of the carrier particles is about 1 mm.

Such a method of producing a finely divided carrier is carried out by coating the surface of the magnetic material with a resin by using the magnetic material and the thermoplastic resin described in the toner or by dispersing the particles with a resin in which magnetic material fine particles are dispersed and contained. It can be obtained by selecting the particle size of the obtained particles by a conventionally known average particle size selecting means. Then, in order to improve the stirring property of the toner and the carrier and the transport property of the developer, and to improve the charge controllability of the toner to prevent the toner particles from agglomerating and the toner particles and the carrier particles from aggregating, the carrier is spherical. Although it is desirable to make the magnetic carrier particles spherical, in the resin-coated carrier particles, the magnetic particles should be spherical as much as possible and the resin should be coated thereon. It is manufactured by using magnetic fine particles to form spherical resin particles after forming dispersed resin particles by hot air or hot water, or by directly forming spherical dispersed resin particles by a spray drying method. When a four-color image with gradation was formed under the above conditions, a clear image with high image power, excellent gradation, and good color balance was obtained. Also, clear recording was obtained for characters and line drawings.

〔The invention's effect〕

As described above, according to the present invention, since the toner image is formed by the reversal development using the dry toner, the charging can be performed without disturbing the toner image. It is possible to prevent the color balance from being lost due to the stacking of toner images due to the use of the undercolor removal technique. Since the image is a dot image and the gradation is expressed by the size of the dots, it is possible to prevent the color balance from being disturbed when a multicolor image is formed by the laminated toner image.
By using non-contact development for forming the toner images of the second and subsequent colors, it is possible to perform development without disturbing the toner image on the photoconductor first. I was able to achieve it effectively.

[Brief description of drawings]

FIG. 1 is a flow chart for explaining the principle of a conventionally known multicolor image forming method, FIG. 2 is a density pattern method which is one of the gradation expression methods of a multicolor image, and FIG. 3 is a dither method. FIG. 4 is a block diagram of the image data correction system, and FIG.
FIG. 6 shows a histogram of density data of primary colors, FIG. 6 shows a histogram of density data after color correction, and FIG. 7 shows a threshold matrix of different color dot mixture type. FIG. 8 is a sectional view of the multicolor image forming apparatus of Embodiment 1, FIG. 9 is a sectional view of a laser apparatus applied to the apparatus of FIG. 8, and FIG.
The drawing shows a sectional view of a developing device applied to the device of FIG. 6a, 7a, 8a, 9a …… 4 color threshold matrix 6b, 7b, 8b, 9b …… 4 color dot pattern 6c, 7c, 8c, 9c …… 4 color dot angle 11 …… Photoconductor 12 …… Scorotron charger 14 …… Exposure device 15,16,17,18 …… Developing device 49 …… AC bias power supply 48 …… DC bias power supply T …… Toner, D …… Developer, P …… Recording paper Yi, Mi, Ci, BKi …… Image data from external memory Ym, Mm, Cm, BKm …… Data after color correction in the arithmetic processing unit Yo, Mo, Co, BKo …… Binarized data My, Mm, Mc, Mbk …… Data memory for each color

 ─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-58-81381 (JP, A) JP-A-52-118302 (JP, A) JP-A-57-87399 (JP, A) JP-A-59- 52969 (JP, A)

Claims (2)

[Claims] 1. A multicolor image forming method, wherein a toner image of four colors of yellow, magenta, cyan and black is formed as a dot image by repeating charging, image exposure and reversal development using a dry toner on a photoreceptor. , Yellow, magenta, and cyan image data are calculated to convert at least a part of the density values common to the yellow, magenta, and cyan image data into black image data, and latent image forming image data are created, which are different from each other. Multicolor, characterized in that at least a part of color dots are formed to overlap each other, gradation is expressed by the size of the dots, and the developing step in the image forming process for the second and subsequent colors is performed by non-contact development. Image forming method. 2. A multicolor image formed by dots in which at least two color toner images of the four color toner images are arranged at different distribution angles from each other, and a multicolor image is formed. Multicolor image forming method.