JPH0318181B2 - - 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. ing. 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, resulting in 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 development being disturbed during the subsequent development, and the toner from the previous stage being mixed into the developer of the latter stage, disrupting the color balance of the final image. .

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 decreases to near zero potential at the exposure portion PH by the first image exposure using an exposure source such as a laser, a cathode ray tube, or a light emitting diode.
Here, by applying a positive bias whose DC component is approximately equal to the surface potential E of the unexposed area to the developing device to perform development, the positively charged toner in the developing device is transferred to the exposed area PH, which has a relatively low potential. It becomes attached 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 positively charged toner, but then a second charge is applied by the scorotron charger, which further increases the surface potential by CUP. The non-exposed area
A surface potential E almost similar to that of DA can be obtained. Next, a second
A second imagewise exposure is performed to form an electrostatic latent image, and a second visible image is obtained through a similar development operation.

By repeating the above operations, toner images are successively superimposed on the photoreceptor to obtain a color toner image. This color toner image is then 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 performed 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, especially the black formed by the superposition of the three primary colors. This is insufficient. The reasons for this are that the three primary color toners that have been put into 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. Also, 3
Even in an additive color method in which color reproduction is performed without overlapping primary colors at the same position on the screen, insufficient image density may occur for the same reason. Therefore, when forming a color image, a developing device containing black toner is usually prepared.

Generally, to form a color image using electrophotography, there are two methods: a regular development method in which an electrostatic charge image on a photoreceptor is developed using toner of a different polarity, and a toner of the same polarity as the electrostatic charge image. There is a reversal development method for developing. In the case of reversal development, it is only necessary to expose the area to which the toner adheres, and there is no need to expose the background area without gaps as in the case of regular development, so strict mechanical precision is not required for the optical system. ,
This has the advantage that the photoreceptor suffers less fatigue, has a longer life, and has greater durability. Also, since the second and subsequent charging is done with the same polarity as the toner,
The advantage of ease of electrostatic transfer is also recognized.
Therefore, the reversal development method is often employed in recording apparatuses using laser light, cathode ray tubes, light emitting diodes, or the like as exposure sources.

However, when forming a color toner image on a photoreceptor by employing the reversal development method, there are the following problems. That is, in the area to which toner is attached during the first stage development, it is difficult for the imagewise exposure light to pass through and the surface potential is not lowered sufficiently, so there is a problem that the toner is difficult to adhere during the second stage development. In the case of the additive color method, a similar problem occurs because it is difficult to achieve perfect alignment and complete development according to the electrostatic charge image. Therefore, even if the three primary colors of yellow, magenta, and cyan are sequentially developed to express various tones, problems such as color balance will be disrupted and the image will be distorted around the edges will occur, making it impossible to form a desired color image. .

[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 organize a plurality of input color information data in an arithmetic processing unit so that the number of input color information data is small compared to the number of color information data. To provide a color image forming apparatus capable of forming a clear color image without image disturbance, maintaining good color balance, and forming a color image with several color toners.

[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 information, and based on the result, an arithmetic processing unit for converting image data of yellow, magenta, and cyan into a black component. This is achieved by a color image forming apparatus having the following. Also,
In particular, the present invention provides a color image forming apparatus in which the arithmetic processing unit subtracts data indicating the lowest density value of image data having yellow, magenta, and cyan components from each color data and uses this as a black component. It is something.

〔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 8 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. Next, among the four color image data of yellow, magenta, cyan, and black, yellow data is input to the laser device 3 first. Laser light _L
image exposure to form an electrostatic charge image. This electrostatic charge image is developed by the first developing device A, and a first toner image (yellow toner image) is formed on the photoreceptor 1.

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, in the same manner, as a result of image exposure with the laser beam L _C based on cyan information and development by the third developing device C, and image exposure of the laser beam L _B based on the black information and development with the fourth developing device D, the third 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 its thickness is regulated by the spike control blade 23 during conveyance to form a 0.5 mm thick developer layer. A stirrer 25 is provided in the developer reservoir 24 to sufficiently stir the developer K. When the toner in the developer in the developer reservoir 24 is consumed, the toner is removed by the toner supply roll 26. Toner T is quantitatively replenished from the hopper 27.

Next, the gap d between the sleeve 22 and the photoreceptor 1 is 0.8
mm, and a DC power supply 28 is provided between them 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 developing area E and to enable sufficient supply of the developer K to the photoreceptor 1 and to enable non-contact development. There is. 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. Furthermore, 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 K, the effective value is 1.5 KV at 2 KHz.

In this embodiment, since the developer D is conveyed to the photoreceptor 1 without contacting it, the toner is caused to fly to the latent image surface using 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 mix into 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, polyvinylbutyral, 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-based 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 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, 4
Carrier particles with a weight average particle diameter of 50 μm were used for both colors. The weight average particle size of the toner and carrier is measured using a Coulter counter (manufactured by Coulter).

Additionally, there are other problems such as charges being easily injected into the carrier particles by 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 that the bias voltage is not applied sufficiently. In order to prevent this from occurring, the carrier should have an insulating resistivity of 10 ⁸ Ωcm or more, preferably 10 ¹³ Ωcm or more, and more preferably 10 ¹⁴ Ωcm or more, and also have a particle size of the above-mentioned resistivity. Good. In this example, a resin-dispersed carrier with a magnetization of 50 e.mu and a resistivity of 10 ¹⁴ Ωcm 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. It is determined by applying a voltage that generates an electric field, reading the value of the current 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, for spherical magnetic carrier particles, resin-coated carrier particles should be selected as spherical as possible and coated with resin. It is produced by using fine particles of a magnetic material, forming dispersed resin particles and then subjecting them to a spheroidizing treatment using hot air or hot water, or directly forming spherical dispersed resin particles by a spray drying method.

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 predetermined size, and arithmetic processing is performed for each region.
That is, the yellow, magenta, cyan, and black density data Yi, Mi, Ci, and Bi are processed according to the calculation processing algorithm described later and converted into Yo, Mo, Co, and Bo, and are stored in the memories My, Mm, Mc, and Mb. is stored in When all the image data to be recorded has been processed, the density data stored in the memory is retrieved according to a specification from the control unit, and the exposure system and the corresponding developing device are driven for each color based on 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 Yi indicating the minimum concentration value among the input data is set as the data Yi,
Subtract it from Mi and Ci and replace it with black. This can be expressed as a formula as follows.

Formula Yo=Yi−min(Yi, Mi, Ci) Mo=Mi−min(Yi, Mi, Ci) Co=Ci−min(Yi, Mi, Ci) Bo=Bi+min(Yi, Mi, Ci) Figure 6 5 shows the state in which color density data of each color is assigned to each pixel in one divided area (this area consists of 4×4 pixels) in FIG. FIG. 6A shows input data assigned as is, and FIG. 6B shows data converted by the above-mentioned arithmetic expressions. When comparing FIGS. 6A and 6B, since a considerable number of the three primary color data are replaced with black, the amount of toner deposited during development is reduced overall. As a result, the first effect is that the amount of consumed toner can be saved. Secondly, as mentioned above, the disadvantage that toner images are difficult to overlap at the same position when reversal development is repeated on the same photoconductor is that the density of toner adhering to the photoconductor 1 is low. This has the effect of not causing a serious hindrance to color reproduction. Therefore, toner consumption is small and clear color images with excellent color balance can be obtained.

In the arithmetic processing based on the algorithm, any input data may be used as long as it includes color information. For example, in the case of a television image that is scanned by an electron beam according to the transmitted signal and shows the brightness of the three additive primary colors, blue, green, and red, the difference between the level of each of the three primary colors and its saturation amount is calculated. As a result, the density levels of the three subtractive primary colors yellow, magenta, and cyan are converted. Furthermore, the Y, M, and C analog output signals of an image sensor, etc. may be used as input data for arithmetic processing as they are, and furthermore, the analog signals may be digitized or other data may be added as necessary and used as input data. is also possible. The above color information may be multicolor information of three 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 time (seconds) of each imaging cycle, and the vertical axis represents the operation of each device. In the figure, a bias of +500V is applied to the developing device that is not performing development during the four developing steps to prevent toner of a different color from being mixed in, and a bias of -300V is applied just before and after the development to prevent toner from scattering. A bias is applied.
Further, the magnetic roll 21 and sleeve 22 are 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, by performing arithmetic processing to convert common three primary color density values in input data into black components as a color correction means of a color image forming apparatus, toner consumed in development can be saved and The disadvantages that occur when a color image is formed by superimposing multicolor toner images on a body are eliminated, and effects such as good color balance and a clear image without image disturbance can be obtained.

[Brief explanation of drawings]

Fig. 1 is a flowchart explaining the principle of image forming method in a conventional color image forming apparatus;
The figure is a sectional view of a main part of a color image forming apparatus according to 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. 5 and 6 are diagrams explaining the algorithm of the calculation processing in the calculation processing section of FIG.
The figure shows a timing chart representing the operational timing of the imaging equipment. 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, Yi, Mi,
Ci, Bi...Yellow, magenta, cyan, black input data, Yo, Mo, Co, Bo...Yellow,
Magenta, cyan, black conversion data, 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. The apparatus for forming an image is characterized in that the color correction means has a calculation processing section for comparing and calculating the color information and converting yellow, magenta, and cyan image data into black components based on the results. color image forming device. 2. The color image forming apparatus according to claim 1, wherein data indicating the lowest density value of image data having yellow, magenta, and cyan components is subtracted from the respective color data, and this is used as a black component. . Formula Yo=Yi−min(Yi, Mi, Ci) Mo=Mi−min(Yi, Mi, Ci) Co=Ci−min(Yi, Mi, Ci) Bo=Bi+min(Yi, Mi, Ci) [In the formula Yi, Mi, Ci, Bi are input data indicating the density values of yellow, magenta, cyan, and black that are input to the calculation processing unit, and Yo, Mo, Co, and Bo are the yellow, magenta, and cyan converted by the calculation processing unit. , data indicating the black density value, min (Yi, Mi, Ci) is 3
This data indicates the minimum density value among the color data Yi, Mi, and Ci. 3. The color image forming apparatus according to claim 1 or 2, wherein at least the second and subsequent developing means of the plurality of developing means are means for developing in a non-contact manner. 4. The color image forming apparatus according to claim 1, 2 or 3, wherein the plurality of developing means are means for performing reversal development.