JPH0318182B2 - - Google Patents

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a color image forming apparatus that forms a color image by sequentially forming toner images on an image support, and is applicable to the fields of electrostatic recording and electrophotography. used.

[Prior art]

Conventionally, to form a color image using electrophotography, for example, the steps of charging, image exposure, development, and transfer are repeated for each component color to obtain toner images of each color superimposed on recording paper. There is. That is, in order to form a color image using yellow, magenta, cyan, and black toners using light modulated by color information data from a color original, the above steps are repeated four times to obtain a color toner image. It will be done like this. However, in such a color image forming method, it is necessary to transfer each color toner to a transfer body each time development is completed, which causes problems such as an increase in the size of the device, a complicated recording process, and a large loss of time. be. Furthermore, since the toner images of each color are transferred onto the recording paper in separate steps, there are problems such as transfer misalignment and the formation of a blurred color image.

Therefore, there is a color image forming method that solves the above problem by superimposing and developing a plurality of toner images on the same photoreceptor so that only one transfer step is required. However, even with this method, there are disadvantages such as the toner image obtained by the previous stage development being disturbed during the latter stage development, and the color balance of the final image being disrupted due to the toner from the previous stage being mixed into the latter stage developer. be.

Therefore, for example, Japanese Patent Application Laid-open No. 144452/1983 proposes to adopt a method of developing the photoreceptor and the spikes of the developer layer in a non-contact manner. In this developing method, an alternating current bias is applied, and the action of the alternating current bias causes the toner in the developer to fly toward the photoreceptor, thereby performing non-contact development.

The principle of the image forming method described in the publication using the above development method will be explained below. The flowchart in FIG. 1 shows changes in the surface potential of the photoreceptor when it is positively charged and developed with positive toner. PH is the exposed area of the photoreceptor, DA is the unexposed area of the photoreceptor, DUP is the increase in potential caused by the positively charged toner T adhering to the exposed area PH during the first development, and CUP is the increase in potential due to the second charging. exposed area
Indicates the increase in PH potential.

The photoreceptor is uniformly charged by a scorotron charger and given a constant surface potential E.
This surface potential E is determined by the first image exposure from an exposure source such as a laser, cathode ray tube, or light emitting diode.
The potential decreases to near zero potential at the exposed portion PH. Here, the developing device is developed by applying a positive bias whose DC component is approximately equal to the surface potential E of the unexposed area, so that the positively charged toner in the developing device is transferred to the exposed area PH, which has a relatively low potential. , and a first visible image is formed. In the area where the visible image is formed, the potential increases by DUP due to the adhesion of the positively charged toner, but then a second charging is performed by the Scorotron charger, which further increases the surface potential. Increase by CUP,
Almost the same surface potential E as in the non-exposed area DA is obtained. Next, a second imagewise exposure is performed on the surface of the photoreceptor on which a uniform surface potential E has been obtained to form an electrostatic latent image, and a second visible image is formed through a similar development operation. can get.

By repeating the above operations, toner images are superimposed one after another on the photoreceptor, and a color toner image is obtained. Further, this color toner image is transferred to recording paper and fixed under pressure or heat to form a color image. Here, the toner and charge remaining on the photoreceptor are cleaned and neutralized, and the photoreceptor is prepared for the next color image formation. Note that in the color image forming method, the second and subsequent charging steps can be omitted. When charging is repeated each time without omitting such charging, a static elimination step can be included before charging, and the exposure source used for each image exposure may be the same or different.

By the way, in a color image forming method, when a color tone is expressed by superimposing the three primary colors of yellow, magenta, and cyan, a black component should not be necessary based on the principle of the subtractive color method. However, when expressing sharp images such as characters and line drawings, it becomes necessary to emphasize black compared to the three primary colors, and in such cases, it is especially important to emphasize black, which is formed by superimposing the three primary colors. This is insufficient. The reasons for this are that the three primary color toners that are in practical use do not have the ideal light absorption wavelength range, and that it is impossible to precisely align the three primary color toners, resulting in some misalignment. It is assumed that this is based on the following. Furthermore, even in the additive color method in which colors are reproduced without overlapping the three primary colors at the same position on the screen, image density may be insufficient for the same reason. Therefore, when forming a color image, a developing device containing black toner is usually prepared.

Generally, in order to form a color image by electrophotography, there is a regular development method in which an electrostatic charge image on a photoreceptor is developed using a toner of a different polarity.
There is a reversal development method in which the image is developed with toner of the same polarity as the electrostatic charge image. In the reversal development method, only the area to which the toner adheres needs to be exposed, and there is no need to expose the background area without gaps as in the case of the regular development method, so the optical system does not require strict mechanical precision, and the photosensitive It has the advantage that it causes less fatigue on the body and lasts a long time, and that electrostatic transfer is easy because the charge on the photoreceptor and the toner that adheres to it have the same polarity.

Therefore, a reversal development method is often employed in recording apparatuses using a laser, cathode ray tube, light emitting diode, or the like as an exposure source. However, when forming a color toner image on a photoreceptor by employing the reversal development method, there are the following problems. That is, imagewise exposure light is difficult to pass through the area to which toner has adhered in the previous stage of development, and the surface potential of the photoreceptor does not decrease, so that toner is difficult to adhere in the latter stage of development.
In the case of the additive color method, a similar problem occurs because perfect alignment and complete development according to the electrostatic charge image are difficult. Therefore, even if an attempt is made to express various tones by sequentially developing toners of the three primary colors of yellow, magenta, and cyan, a desired color image cannot be formed because the color balance is disrupted or the image is distorted around the edges.

[Purpose of the invention]

The present invention has been proposed in view of the above-mentioned circumstances, and an object of the present invention is to correct and transform the color tone of image data according to an external command, and to form a color image using the converted data. It is an object of the present invention to provide an image forming apparatus that is well-balanced and forms a clear color image without image disturbance.

[Structure of the invention]

The above objects include a means for color correcting image data consisting of color information, a means for forming a latent image on an image support based on the result of color correction by the means;
and a plurality of developing means for reversingly developing the latent image formed by the means with toners of different colors, and repeating charging, image exposure, and reversal development to form a plurality of different toner images on the image support. In an apparatus that forms a color image by sequentially forming a color image, the color correction means compares and calculates the color tone of the image data using an external command, and based on the result, creates image data of yellow, magenta, and cyan. This is achieved by a color image forming apparatus characterized by having an arithmetic processing section for converting into a black component.

〔Example〕

Examples of the present invention will be described in detail below based on the drawings, but the embodiments of the present invention are not limited thereto. 2 to 7 are diagrams for explaining this embodiment, in which FIG. 2 is a sectional view of main parts of a color image forming apparatus, and FIG. 3 is a sectional view of a developing device of the color image forming apparatus shown in FIG. 2. It is.

FIG. 4 is a block diagram of the arithmetic processing unit in the color correction means, and FIGS. 5 and 6 are diagrams explaining the algorithm of the arithmetic processing in the arithmetic processing unit of FIG.
FIG. 7 is a timing chart illustrating the operation timing of the image forming device.

FIG. 2 shows an apparatus that records, as image data, output signals of an image sensor that scans a document, transmission signals from other devices, data in a memory, and the like.
The drum-shaped photoreceptor 1 is a selenium photoreceptor with a diameter of 120 mm that rotates in the direction of the arrow at a circumferential speed of 120 mm/sec, and a scorotron charger 2 applies a uniform charge of +600 V to the photoreceptor. Then yellow, magenta,
Among the four color image data of cyan and black, yellow data is input to the laser device 3 first. The laser light L _Y modulated by the laser device 3 and reflected by the rotating polygon mirror is image-exposed onto the photoreceptor 1 through the imaging lens 4, thereby forming an electrostatic charge image. This electrostatic charge image is developed by a first developing device A, and a first toner image (yellow toner image) is formed on the photoreceptor 1.
is formed.

Without transferring this toner image to recording paper, the photoreceptor 1 is charged again by the scorotron charger 2, and a toner image based on the magenta information data is formed. That is, an electrostatic charge image is formed by the image exposure of the laser beam L _M , and a second toner image (magenta toner image) is formed by the second developing device B. Thereafter, similarly, the results of image exposure with laser light L _C based on cyan information and development by third developing device C, and image exposure of laser light L _B based on black information and development with fourth developing device D, A toner image (cyan toner image) and a fourth toner image (black toner image) are formed. In this way, the first to fourth toner images are superimposed on the photoreceptor 1 to form a multicolor toner image.

The toner image on the photoreceptor 1 is charged before transfer by a charger 5, and is transferred by the action of a transfer device 9 onto a recording paper P fed from a paper feed device 6 by a paper feed roll 7 and a guide 8. The recording paper P to which this toner image has been transferred is separated from the photoreceptor 1 by the action of a separator 10, and is conveyed by a conveyor belt 12 via a guide 11 and sent to a heat roll 13. After being heat-fixed here, it is discharged to the discharge tray 14. On the other hand, after the transfer of the photoreceptor 1 has been completed, the static electricity is removed by the static eliminator 16 that was not used during the toner image formation, and the toner remaining on the surface is removed from the blade 17 of the cleaning device 15 that was not used during the toner image formation. removed by

The developing device A used here is shown in FIG. Note that the developing devices B, C, and D basically have the same structure as the developing device A. The developer K is transported by a magnetic roll 21 having six magnetic poles at a speed of 1000 rpm in the direction of arrow F, and by a sleeve 22 with a diameter of 30 mm at a circumferential speed of 120 rpm.
By being rotated in the direction of arrow G at mm/sec,
It is transported in the direction of arrow G. The developer K is a two-component developer, and during transportation, the spike control blade 23
The thickness is regulated, and a developer layer with a thickness of 0.5 mm is formed. The developer reservoir 24 contains developer K.
A stirrer 25 is provided to ensure sufficient stirring of the developer, and when the toner in the developer in the developer reservoir 24 is consumed, toner T is quantitatively replenished from a hopper 27 by a toner supply roller 26. Ru. Next, the gap d between the sleeve 22 and the photoreceptor 1 is
The length is 0.8 mm, and a DC power supply 28 is provided between the two to apply a developing bias to perform reversal development. In addition, an AC power source 29 is provided in series with the DC power source 28 in order to vibrate the developer K in the development area E so that the developer K can be sufficiently supplied to the photoreceptor 1 and to enable non-contact development. . R
is the protective resistance. Here, the developing bias is
DC component is +500V, AC component is 2KHz, effective value
It is assumed to be 1.0KV. Also, the developer K in the developing device A is
The toner in the developer K reaches the development area E.
It is conveyed so as to impart a triboelectric charge of 20 μc/g. Note that the DC component of the developing bias in developing devices B to D is all +500V,
Regarding the AC component, in the case of developing devices B and C,
The effective value is 1.8 KV at 2 KHz, and in the case of developing device D, the effective value is 1.5 KV at 2 KHz.

In this embodiment, since the developer K is conveyed to the photoreceptor 1 without contacting it, the toner is caused to fly to the latent image surface by an alternating current bias. Here, due to the constantly changing phase of the alternating current, an electric force toward the photoreceptor 1 and an electric force in the opposite direction act on the toner particles between the photoreceptor 1 and the developing device (for example, B, C, D). do. Of these, the latter may cause the toner on the photoreceptor 1 to return to the developing device, causing toner of a different color to be mixed into the developer in the developing device.

As a countermeasure to this problem, the following measures can be taken when toner images are superimposed.

(i) Use toners with increasing charge amounts in order.

(ii) Sequentially reduce the amplitude and/or period of the AC component of the developing bias.

(iii) Move the developing device that is not in use away from the photoreceptor 1.

(iv) Gradually increase the toner supply amount.

(v) Gradually increase the latent image potential contrast.

(vi) Gradually increase the gap d between the photoreceptor 1 and the developer layer.

(vii) Apply a bias (bias with the same polarity as the toner) to the developing device that is not in use to prevent toner of a different color from being mixed in.

Next, as the developer preferably used in the above-mentioned developing device, there are those described below.

There are two-component developers consisting of toner and carrier, and one-component developers consisting only of toner.
Two-component developers require control of the amount of toner relative to the carrier, but have the advantage that triboelectric charging of toner particles can be easily controlled. In addition, especially with two-component developers consisting of a magnetic carrier and non-magnetic toner, it is not necessary to contain a large amount of black magnetic material in the toner particles, so it is possible to use color toner that does not cause color turbidity due to magnetic material. It has the advantage of being able to form clear color images.

It is particularly preferable that the two-component developer used in the present invention is composed of a magnetic carrier and a non-magnetic toner.

The composition of the toner is generally as follows.

Thermoplastic resin: Binder 80-90wt% Examples: Polystyrene, styrene acrylic polymer, polyester, polyvinyl butyral, epoxy resin, polyamide resin, polyethylene, ethylene vinyl acetate copolymer, etc. are often used as a mixture.

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

Charge control agent 0-5wt% Plastoner: Many nigrosine-based electron-donating dyes, as well as metal salts of naphthenic acid or higher fatty acids, alkoxylated amines, alkylamides,
Chelates, pigments, quaternary ammonium salts, etc.

Negative toner: Electron-accepting organic complexes are effective, as well as chlorinated paraffin, chlorinated polyester, polyester with excess acid groups, chlorinated copper phthalocyanine, etc.

Fluidizer Examples: Typical examples include colloidal silica and hydrophobic silica, and other examples include silicone varnish, metal soap, and nonionic surfactants.

Cleaning agent Prevents toner filming on the photoreceptor.

Examples: fatty acid metal salts, oxidized silicon acids with organic groups on the surface, and fluorine surfactants.

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

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

In addition to these materials, a magnetic material may be included to prevent fogging and toner scattering.

As magnetic powder, 0.1 to 1 μm triiron tetroxide, γ
- Ferric oxide, chromium dioxide, nickel ferrite, iron alloy powder, etc. have been proposed, but at present, triiron tetroxide is often used and has a 5%
Contains ~70wt%. The resistance of the toner varies considerably depending on the type and amount of magnetic powder, but in order to obtain sufficient resistance, it is preferable that the amount of magnetic material is 55 wt% or less. Furthermore, in order to maintain clear colors as a color toner, it is desirable that the amount of magnetic material be 30 wt% or less.

Other resins suitable for pressure fixing toners include adhesive resins such as wax, polyolefins, ethylene-vinyl acetate copolymers, polyurethane, and rubber, so that they can be plastically deformed and adhered to paper with a force of about 20 kg/cm. etc. are selected. Capsule toners can also be used.

A toner can be made using the above-mentioned materials by a conventionally known manufacturing method.

In the structure of the present invention, in order to obtain a more preferable image, it is desirable that the particle size of these toners is usually about 50 microns or less in weight average particle size from the viewpoint of resolution. In this method, there is no theoretical limit to the toner particle size, but from the viewpoint of resolution, toner scattering, and conveyance, it is usually preferable to use a particle size of about 1 to 30 microns. In this embodiment, toners having a weight average particle size of 10 μm are used for all four colors.

In addition, in order to create delicate points or lines or increase gradation, magnetic carrier particles are particles made of magnetic particles and resin, such as resin dispersion systems of magnetic powder and resin, or resin-coated magnetic particles. More preferably, it is spherical. Weight average particle size is preferably 50 μm or less, particularly preferably 30 μm or less
Particles of 5 μm or more are preferred.

In this example, carrier particles with a weight average grain size of 50 μm were used for all four colors. The weight average particle diameters of the toner and carrier were measured using a Coulter Counter (manufactured by Coulter).

In addition, the problem of the carrier particles being easily injected with charges due to the bias voltage, which hinders good image formation, and the carrier particles tending to adhere to the surface of the image bearing member, and the problem of not applying enough bias voltage can be solved. To prevent this from occurring, the resistivity of the carrier should be at least 10 ⁸ Ωcm, preferably at least 10 ¹³ Ωcm,
More preferably, the material has an insulating property of 10 ¹⁴ Ωcm or more, and also has a resistivity of 10 14 Ωcm or more and a grain size as described above. In this example, a resin-dispersed carrier with a magnetization of 50 e.mu and a resistivity of 10 ¹⁴ Ω or more was used.
Further, the specific resistance of the carrier is measured by the following measuring method. That is, after the particles are placed in a container with a cross-sectional area of 0.50 cm ² and tapped, a load of 1 Kg/cm ³ is applied to the packed particles, and a voltage of 10 ^{2 to 5} V/cm is applied between the load and the bottom electrode. Applying a voltage that produces an electric field,
It is determined by reading the current value flowing at that time and performing a predetermined calculation. At this time, the thickness of the carrier particles is approximately 1 mm.

A method for manufacturing such a finely divided carrier is to use a magnetic material and a thermoplastic resin as described for toner, and coat the surface of the magnetic material with the resin, or coat the particles with a resin containing fine magnetic particles dispersed therein. It can be obtained by preparing the particles and selecting the particle size using a conventionally known average particle size selection means.
The carrier is shaped into a spherical shape in order to improve the agitation performance of the toner and carrier and the transportability of the developer, as well as to improve the charge control performance of the toner and make it difficult for toner particles to coagulate with each other or toner particles and carrier particles. However, in the case of resin-coated carrier particles, it is desirable to select spherical magnetic particles as spherical as possible and coat them with resin, and to obtain spherical magnetic carrier particles. It is manufactured by using fine particles of magnetic material as much as possible, by performing a spheroidization treatment with hot air or hot water after forming dispersed resin particles, or by directly forming spherical dispersed resin particles by spray drying. Ru.

Next, the functions of the arithmetic processing section in the color image forming apparatus of the present invention will be explained with reference to FIG.
Input image data is divided into regions of a predetermined size, and arithmetic processing is performed for each region. In other words, density data for yellow, magenta, cyan, and black.
Y _i , M _i , C _i , B _i are processed according to the arithmetic processing algorithm described later and converted into Y ₀ , M ₀ , C ₀ , B ₀ , and the memories M _y , M _n , M _c , M Stored in _B. When all the image data to be recorded has been subjected to arithmetic processing, the density data stored in the memory is retrieved according to a specification from the control section, and the exposure system and the corresponding developing device are driven for each color by the data. As a result, a color toner image is formed on the photoreceptor 1.

Next, the algorithm for the arithmetic processing will be explained with reference to FIGS. 5 and 6. FIG. 5 is a histogram showing the sum of color density levels for each color in one divided area. For example, if the input data is the data shown in Figure 5 A,
Three primary colors of equal density level: yellow, magenta,
Utilizing the fact that when cyan is mixed, it becomes black, convert A to B in Figure 5. That is, the portion of C _i indicating the minimum concentration value among the input data is divided into the data Y _i ,
Subtract it from M _i and C _i and replace it with the black component. Processing the image data in this way reduces the amount of toner that adheres to the photoreceptor 1, which has the advantage of greatly reducing the amount of toner consumed. However, as described above, when toner images are sequentially formed on a photoconductor by reversal development, there is a problem that subsequent toner images are difficult to overlap on the previous toner image, and the color tone of the previous toner image is emphasized. .

In this embodiment, in order to solve this problem, the image data arithmetic processing method is not fixed to the density level replacement, but parameters for the arithmetic processing are externally designated. Such parameters may be specified by, for example, detecting the composition of the developer, the bias voltage to the developing device, the order of development of each color toner, etc., and feeding back the results, or by using experimentally determined data. It may also be based on manual operation by an operator.

The above-described image data arithmetic processing method can be expressed as follows.

Formula B ₀ = α ₁ B _i + β ₁ min (Y _i , M _i , C _i ) Y ₀ = α ₂ Y _i + β ₂ min (Y _i , M _i , C _i ) M ₀ = α ₃ M _i + β ₃ min (Y _i , M _i , C _i ) C ₀ = α ₄ C _i + β ₄ min (Y _i , M _i , C _i ) In the formula, B _i , Y _i , M _i , and C _i are input to the arithmetic processing section Input data for each color indicating the density values of black, yellow, magenta, and cyan to be converted, and B ₀ , Y ₀ , M ₀ , and C ₀ are each color indicating the density values of black, yellow, magenta, and cyan converted by the calculation processing unit. The data, min(Y _i , M _i , C _i ) is data indicating the minimum density value among the three primary color data Y _i , M _i , C _i , α ₁ , α ₂ , α ₃ , α ₄ , β ₁ , β ₂ , β ₃ , β ₄
is a parameter converted by an external command.

The histogram of each color density level of the input data Y _i , M _i , C _i , B _i is shown in _FIG . Data Y ₀ obtained by arithmetic processing with α ₄ and β ₁ to β ₄ all set to 1,
A histogram of each color density level of M ₀ , C ₀ , and B ₀ is shown in FIG. 5B. Also, parameter β ₂ is 1.5
The histogram when specified is shown in FIG. 5C. (However, Y ₀ ' indicates the yellow conversion data when β ₂ = 1.5.) The figures A, B, and C in Figure 6 correspond to the figures A, B, and C in Figure 5.
Figure 6A corresponds to each figure in Figure 6A, where the input data in Figure 5A is assigned as is, and Figure 6B and C are the data converted by the arithmetic processing unit. It is something. Comparing A, B, and C in FIG. 6, it is clear that the amount of toner attached to the photoreceptor 1 is significantly smaller in B and C, which have undergone data conversion, than in A. Particularly in FIG. 6C, it can be seen that the yellow toner image that is developed first is suppressed and the color balance is optimized.

In the arithmetic processing using the algorithm, any input data may be used as long as it includes color information. For example, when a television image is transmitted, the luminance levels of blue, green, and red are the three additive primary colors, but if you take the difference between the saturation amount of each color level, the three subtractive primary colors yellow, magenta, and Converted to cyan density level. In addition, the analog output signals of yellow, magenta, and cyan from an image sensor, etc. can be used as input data for calculation processing as they are, or they can be digitized or other data can be added as input data if necessary. be.

The color information in the color image forming apparatus described above may include three colors or four or more colors.

The operation timing of each image forming device in the color image forming apparatus of this embodiment is shown in FIG. The horizontal axis represents the elapsed time (seconds) of each image forming cycle, and the vertical axis represents the operation of each device. In the figure, the rotation of the photoreceptor drum 1 is shown up to 4 cycles,
Charging, image exposure, developing device A,
B, C, and D are operated sequentially with proper timing to form a color image. During the development process in the four cycles mentioned above, a bias of +500V is applied to the developing device that is not performing development to prevent toner of a different color from getting mixed in, and immediately before and after development to prevent toner from scattering. A bias of –300V is applied. Moreover, the magnetic roll 21 and sleeve 22 are
It is controlled to rotate only during development. When a four-color image was formed under the above conditions, a clear color image with good color balance and no image disturbance was obtained.

〔Effect of the invention〕

As explained above, as a color correction means of a color image forming apparatus, multicolor toner images are superimposed on a photoreceptor by calculating and converting data from color information according to commands from external information to form a color image. The disadvantages that occur when forming images are eliminated, and effects such as good color balance and clear images without image disturbance can be obtained.

[Brief explanation of the drawing]

Figure 1 is a flowchart explaining the principle of image formation in a conventional color image forming apparatus;
The figure is a sectional view of a main part of a color image forming apparatus of the present invention.
FIG. 3 is a sectional view of the developing device in the image forming apparatus of FIG. 2, and FIG. 4 is a block diagram of the arithmetic processing section in the color correction means. Figures 5 and 6 are the 4th
FIG. 7, which is a diagram illustrating an algorithm of arithmetic processing in the arithmetic processing section shown in the figure, shows a timing chart showing the operation timing of the image forming device. 1... Drum-shaped photoreceptor, 2... Scorotron charger, 3... Laser device, 4... Imaging lens, 5
...Pre-transfer charger, 9...Transfer electrode, 28...DC bias, 29...AC bias, A, B,
C, D...Developing device, K...Developer, Y _i , M _i ,
C _i , B _i ...Yellow, magenta, cyan, black input data, Y ₀ , M ₀ , C ₀ , B ₀ ... Yellow, magenta, cyan, black conversion data, α ₁ , α ₂ , α ₃ ，
α ₄ , β ₁ , β ₂ , β ₃ , β ₄ ...parameter, T ... toner, P ... recording paper.

Claims (1)

[Scope of Claims] 1. A means for color correcting image data consisting of color information, a means for forming a latent image on an image support based on the result of color correction by the means, and a latent image formed by the means. It has a plurality of developing means for reversing developing a latent image with toners of different colors, and by repeating charging, image exposure, and reversal development to sequentially form a plurality of different toner images on the image support. In the image obtaining device, as the color correction means,
The color tone of the image data is compared with the color information based on an external command, and based on the result, yellow, yellow,
A color image forming apparatus characterized by having an arithmetic processing section for converting magenta and cyan image data into black components. 2. The color image forming apparatus according to claim 1, wherein the arithmetic processing unit performs arithmetic processing based on the following formula. Formula B ₀ = α ₁ Bi + β ₁ min (Yi, Mi, Ci) Y ₀ = α ₂ Yi−β ₂ min (Yi, Mi, Ci) M ₀ = α ₃ Mi − β ₃ min (Yi, Mi, Ci) C ₀ = α ₄ Ci − β ₄ min (Yi, Mi, Ci) In the formula, Bi, Yi, Mi, Ci are input data indicating the density values of black, yellow, magenta, and cyan input to the calculation processing unit, B ₀ , Y ₀ , M ₀ , C ₀ are data indicating the density values of black, yellow, magenta, and cyan converted by the arithmetic processing unit, and min (Yi, Mi, Ci) is the three-color data Yi, Mi, Ci Data indicating the minimum concentration values among them, α ₁ , α ₂ , α ₃ , α ₄ , β ₁ , β ₂ , β ₃ , and β ₄ are parameters converted by external commands.