JPH0518509B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a multicolor image forming apparatus that sequentially forms toner images of different colors on an image forming member to obtain a multicolor image, and is used in fields such as electrophotography. [Prior Art] Conventional Example A multicolor image forming method has been proposed in which a plurality of toner images are superimposed and developed on the same photoreceptor so that only one transfer step is required. In this method, there are disadvantages such as disturbing the toner image formed by the previous stage development during the latter stage development, or mixing toner from the toner developed in the previous stage into the latter stage developer, disrupting the color balance. It is desirable to adopt a method in which a bias with an alternating current component superimposed is applied to the developing device during the second and subsequent development to cause the toner to fly onto the electrostatic latent image formed on the image forming body. In this method, the developer layer does not rub the toner image formed up to the previous stage, so that image disturbance does not occur. The principle of this multicolor image forming method will be explained below with reference to the flowchart shown in FIG. FIG. 8 shows changes in the surface potential of the image forming body, taking as an example the case where the charging polarity is positive. PH is the exposed area of the image forming body, DA is the unexposed area of the image forming body, and DUP is the increase in potential caused by the attachment of the positively charged toner T ₁ to the exposed area PH during the first development. The image forming body is uniformly charged by a scorotron charger, and has a constant positive surface potential E as shown in (a). Next, first image exposure is applied using a laser, cathode ray tube, LED, or the like as an exposure source, and as shown in (b), the potential of the exposed portion PH decreases according to the amount of light. The electrostatic latent image thus formed is developed by a developing device to which a positive bias approximately equal to the surface potential E of the unexposed area is applied. As a result, as shown in (c), the positively charged toner T ₁ adheres to the exposed portion PH, which has a relatively low potential, and a first toner image is formed. The area where this toner image is formed has a potential increased by DUP due to the attachment of the positively charged toner _T1 , but does not have the same potential as the unexposed area DA.
Next, the surface of the image forming body on which the first toner image has been formed is charged a second time by a charger, and as a result,
A uniform surface potential E is obtained regardless of the presence or absence of toner _T1 . This is shown in (d). A second image exposure is performed on the surface of this image forming body to form an electrostatic latent image ((e)), and similarly to (c) above, a positively charged toner T of a different color from the toner T is formed. Development is performed to obtain a second toner image. This is shown in (f). By performing the above process multiple times, a multicolor toner image is obtained on the image forming body. A multicolor recorded image can be obtained by transferring this onto recording paper and further fixing it by heating or applying pressure. In this case, the image forming member is cleaned of toner and charges remaining on the surface and used for the next multicolor image formation. On the other hand, apart from this, there is also a method of fixing a toner image on an image forming body. In the method illustrated in Figure 8, at least (f)
It is desirable that the developing step is carried out in such a way that the developer layer does not come into contact with the surface of the image forming member. Note that in the multicolor image forming method, the second and subsequent charging steps can be omitted. If such charging is not omitted and charging is repeated each time, a static elimination process may be performed before charging. Furthermore, the exposure sources used for each image exposure may be the same or different. In the multicolor image forming method, toners of four colors, for example, yellow, magenta, cyan, and black, are often superimposed on an image forming member for the following reason. According to the principle of subtractive color method, yellow,
A black image should be obtained by overlapping the three primary colors of magenta and cyan, but the toners for the three primary colors in practical use do not have the ideal absorption wavelength range, and the position of the toner image of the three primary colors is Due to the misalignment, it is not only difficult to reproduce the clear black required for characters and lines using only these three primary color toners, but also the density tends to be insufficient even in color images. Therefore, it is conceivable to form a multicolor image using four colors, which are the three primary colors plus black, as described above. In electrophotography, image exposure means include gas or semiconductor laser light, LED, CRT,
Liquid crystals are used. In addition to the electrophotographic method described above, methods for forming latent images for multicolor image formation include methods in which an electrostatic latent image is formed by injecting charges directly onto the image forming body using a multi-needle electrode, and magnetic A method of forming a magnetic latent image using a head can be used. FIG. 9 is a block diagram of an example of a multicolor image forming system to which the above method is applied. The image input section optically scans the original with an image sensor through a filter and converts it photoelectrically to obtain image data.The image processing section performs shading correction, etc., and then converts this image data into a suitable image for recording. data. Hereinafter, this will be referred to as recorded data. In this system, binarization is performed. The recording section forms a latent image on the image forming body according to the binarized data,
Develop to a specific color. After performing the above steps one color at a time to form a multicolor image on the image forming body, the image is transferred to recording paper to obtain a recorded matter. FIG. 10 is an explanatory diagram of an example of the binarization part of the image processing section. The image data is compared in size with reference data prepared in advance pixel by pixel in a comparator. As a result, recording data in which all pixels are binarized is formed. [Problems to be Solved by the Invention] However, in the conventional multicolor image forming methods described above, when black development is included, the amount of black toner deposited is too small, and the color balance of the recorded image tends to be disrupted. . In addition, in a multicolor image forming method in which toner images are superimposed, it is difficult for the next toner to overlap on top of the toner image on the previous image forming body, resulting in insufficient color reproduction and text and lines, which are important elements of black image data. There was a problem in that it was difficult to obtain clear, high-contrast images because the density in such areas was low and the images became blurred or interrupted. The reason for this is thought to be that when toner images are superimposed, it becomes difficult for the toner to overlap the previous toner due to light shielding and toner layer potential. In particular, the effect was large on the toner image that was deposited last. [Means for Solving the Problems] (Objective of the Invention) The present invention solves the problems of the multicolor image forming method in which toner images are superimposed on an image forming body as described above, and improves color reproduction. An object of the present invention is to provide a multicolor image forming method capable of recording an image with good quality, clarity, and high contrast. [Structure of the Invention] The present invention includes a charger, an image exposure device, a developing device having yellow, magenta, cyan, and black toners, a transfer device, and a cleaning device arranged around the photoreceptor, and By repeating an image forming step multiple times in which a latent image is formed on the body by charging by the charger and image exposure by the image exposure device, and a toner image is formed by reversal development by the developer, A multicolor image forming method in which a plurality of toner images are superimposed on the photoreceptor, in which optical information is input into color separation and image exposure is performed based on the image data that has been subjected to arithmetic processing to form a latent image on the photoreceptor. At the same time, the final image forming step for forming the toner image is a black toner image, and the image data corresponding to the black toner image is emphasized more than the image data corresponding to toner images of other colors. The above object is achieved by a multicolor image forming method characterized by the following. [Function] Hereinafter, the present invention will be specifically explained based on the drawings. FIG. 1 shows a block diagram of a system according to the invention. In the image input section, the original is optically scanned by an image sensor through a filter, and this is photoelectrically converted. The image processing unit performs preprocessing such as correction of shading development, which causes the peripheral areas of the image to become dark due to the characteristics of the image sensor and lens aberrations. Convert data into data suitable for recording. In this system, binarization is performed. The recording section forms a latent image on the image forming body according to the binarized data and develops it into a specific color. After performing the above steps for each color to form a multicolor image on the image forming body, the image is transferred to recording paper to obtain a recorded matter. As an example of emphasis, a specific example of an edge emphasis method is shown in FIG. A is an operator called a Labrasian, which performs an operation of subtracting the sum of data on the upper, lower, left, and right sides of the pixel of interest from a value four times the value of the pixel of interest. Furthermore, standardization is performed as necessary. By doing this, the difference between adjacent pixels is emphasized, resulting in a sharp image. In other words, if each pixel data of the input image is expressed as d _ij as shown in Fig. 2B, the output pixel data is
D _ij is D _i,j = (4d _i,j-1 −d _i,1-j −d _i+1,j −d _i,j+1 )/
It becomes 8. Here, i and j are pixel coordinates and are integers. Various operators can be used, not just the Laplacian. For example, the system shown in FIG. 2C has simple processing and high calculation speed. Generally, the method using operators performs calculations expressed by the following formula. D _i,j =〓a _k,,1 d _ik,j-1 where a _k,1 represents an element of the operator. Note that edge enhancement is not limited to the method using operators, but may also employ methods such as performing high frequency modulation using a digital filter or the like. Figure 3A shows an example of a device that implements this method. R is an image input unit that reads the optical information of the original;
P is an image processing section that performs arithmetic processing on the read data, Q is a recording section that forms a multicolor image based on the processed data, and M is a storage section that stores image data. In R, the original 14 is optically scanned by the image exposure lamp 15, and the projected light is passed through a filter 16 for each color, a reflecting mirror 17, and a lens 18.
The light is introduced into the image sensor 19 via the image sensor 19, photoelectrically converted, and then introduced into the laser optical system 10 via the image processing section P and storage section M. A multicolor image is formed in the recording section Q in the following manner. The surface of the image forming body 1 having a photoconductive layer is uniformly charged by a scorotron charging electrode 2. Subsequently, the image forming body is irradiated with image exposure light L from the laser optical system 10.
In this way, an electrostatic latent image is formed. This electrostatic latent image is developed by a developing device A containing yellow toner. The image forming body 1 on which the toner image has been formed is uniformly charged again by the scorotron charging electrode 2 and subjected to image exposure L. The formed electrostatic latent image is developed by a developing device B containing magenta toner. As a result, a two-color toner image of yellow toner and magenta toner is formed on the image forming body 1. Thereafter, the cyan toner and the black toner are developed in a similar manner, and a four-color toner image is formed on the image forming member 1. The four-color toner image is charged by the charging electrode 9 and transferred to the recording paper at the transfer pole 4.
Transferred to P _a . The recording paper P _a is separated from the image forming body 1 by a separation pole 5 and fixed by a fixing device 6 .
On the other hand, the image forming body 1 is cleaned by the removing electrode 7 and the cleaning device 8. The cleaning device 8 is a cleaning blade 8
1 and a fur brush 82. These are kept out of contact with the image forming body 1 during image formation, and when a multicolor image is formed on the image forming body 1, they come into contact with the image forming body 1 and scrape off residual toner after transfer. Thereafter, the cleaning blade 81 separates from the image forming body 11, and a little later, the fur brush 82 separates from the image forming body 1. The fur brush 82 is a cleaning brake 81
When the toner leaves the image forming body 1, it functions to remove the toner remaining on the image forming body 1. The laser optical system 10 is shown in FIG. In the figure,
21 is a semiconductor laser oscillator, 22 is a rotating polygon mirror, and 23 is a θ lens. In this multicolor image forming apparatus, one color is developed each time the image forming member 1 rotates once, but AC voltage continues to be applied to developing devices that are not in use. In addition,
During image formation, none of the charging electrodes 2 and above, the paper feed, the paper conveyance, and the cleaning device 8 act on the image forming body 1. Next, desirable development conditions for carrying out the present invention will be explained. The four developing devices used in the multicolor image forming apparatus shown in FIG. 3A may be of the same or similar construction. As an example, a sectional view of the first developing device A is shown in FIG. The developer De is conveyed in the direction of the arrow by the rotation of the sleeve 42 and the magnetic roll 41 having 12 poles. The developer De is formed into a developer layer with a constant thickness by the spike control blade 43 during transportation. A stirring screw 45 is provided in the developer reservoir 44 to sufficiently stir the developer De. When the developer De in the developer reservoir 44 is consumed, the toner supply roller 46 rotates and toner T is replenished from the toner hopper 47. A power source 48 for applying a developing bias is connected to the sleeve 42 . On the other hand, there are two types of developer: a two-component developer mainly composed of non-magnetic toner and a magnetic carrier, and a certain amount developer composed only of magnetic toner, and either of them may be used in the present invention. In particular, two-component developers require control of the amount of toner relative to the carrier, but they do not require the toner to contain a large amount of black magnetic material, which can easily control the toner's charge, so color toner without color turbidity can be produced. can,
There are advantages such as As an example of the developer, a preferable configuration example of the two-component developer as described above is given. Toner Thermoplastic resin (binder) 80-90Wt% Examples: polystyrene, styrene acrylic polymer, polyester, polyvinyl butyral, epoxy resin, polyamide resin, polyethylene, ethylene vinyl acetate copolymer, etc., or mixtures of the above Pigments (coloring) agent) 0-15Wt% Example: Black: Carbon black Yellow: Benzidine derivative Magenta: Rhodamine B lake, Carmine 6B
etc. Cyan: phthalocyanine, sulfonamide dielectric dye, etc. Charge control agent 0-5Wt% Plastoner: Nigrosine-based electron-donating dye,
Alkoxylated amines, alkylamides, chelates, pigments, quaternary ammonium salts, etc. Negative toners: Electron-accepting organic compounds, chlorinated paraffins, chlorinated polyesters, polyesters with excess acid groups, chlorinated copper phthalocyanines, etc. Examples of fluidizing agents : Colloidal silica, hydrophobic silica, silicone varnish, metal soap, nonionic surfactant, etc. Cleaning agent (prevents toner filming on the photoreceptor) Examples: Fatty acid metal salt, oxidized silicon acid with an organic group on the surface, Fluorine surfactants, etc. Fillers (Improves image surface gloss, reduces raw material costs) Examples: Calcium carbonate, clay, talc, pigments, etc. In addition to these materials, to prevent fogging on the image surface and toner scattering. , magnetic powder may be contained. Such magnetic powder has a particle size of 0.1
~1 mm of triferric tetraoxide, r-ferric oxide, chromium dioxide, nickel ferrite, iron alloy powder, etc. are used, and the content is 5 to 70 Wt%. The resistance of toner varies greatly depending on the type and content of magnetic powder, but in order to retain sufficient charge, it must have a resistance of 10 ³ Ωcm or more, preferably 10 ¹² Ωcm or more, and the amount of magnetic material must be 55 Wt or less. , 30Wt% to maintain even more vivid colors
It is desirable to do the following. In addition, resins suitable for pressure fixing toner, which is plastically deformed and fixed to paper with a force of approximately 20 kg/cm, include:
Adhesive resins such as wax, polyolefins, ethylene vinyl acetate copolymer, polyurethane, and rubber are used. A toner can be made using the above-mentioned materials by a conventionally known manufacturing method. In this apparatus, in order to obtain a more preferable image, it is desirable that the toner particle size (weight average) be approximately 50 μm or less. Further, in order to prevent toner scattering and improve transportability, a thickness in the range of 1 to 30 μm is preferable. Note that the weight average particle diameter is a value measured with a Coulter Counter (manufactured by Coulter Inc.). In addition, the specific resistance of the particles is determined by placing the particles in a container with a cross-sectional area of 0.50 cm ² and tapping them, then applying a load of 1 kg/cm ² on the packed particles to make the thickness about 1 mm.
It is determined from the current value that flows when an electric field of 10 ² to ^{10 5} V/cm is generated between the load and the bottom electrode. Carrier Basically, the materials listed as constituent materials of toner are used. Carrier particles are mainly composed of magnetic particles and resin, and in order to improve resolution and gradation reproducibility, they are preferably spherical and have a weight average particle diameter of 50 μm or less, particularly preferably 5 μm or more and 30 μm. The following are preferred. In addition, in order to prevent the charge from being injected by the bias voltage and the carrier from adhering to the surface of the image forming body, or the charge forming the latent image from disappearing, the resistivity of the carrier should be 10 ⁸ Ωcm or more. Preferably 10 ¹³ Ω
It is preferable to use an insulating material of at least 10 cm, more preferably at least 10 ¹⁴ Ωcm. Such carriers are made by coating the surface of a magnetic material with a resin, or by dispersing fine magnetic particles in a resin and sorting the resulting particles using a known particle size selection means. Furthermore, when making the carrier spherical, proceed as follows. Resin-coated carrier: Select spherical magnetic particles. Magnetic powder dispersion carrier: After forming the dispersed resin, spherical treatment is performed using hot air or hot water, or a spherical dispersed resin is directly formed by spray drying. Next, the developing method will be explained. In the present invention, it is desirable to use a non-contact development method in which the developer layer on the sleeve does not rub against the surface of the image forming body for at least the second and subsequent development. In the non-contact developing method, when there is no developing bias between the image forming body 1 and the sleeve 42, the thickness of the developer layer in the developing area is set to be smaller than the gap between the image forming body 1 and the sleeve 42. Set. In this case, when the amplitude of the alternating current component of the developing bias applied to the sleeve is V _Ac (V), the frequency is (Hz), and the distance between the photoreceptor and the sleeve is d (mm), development with a one-component developer is 0.2≦V _Ac /(d・)≦1.6 Development with a two-component developer is preferably carried out under the following conditions: 0.2≦V _Ac /(d・) [(V _Ac /d)−1500}/≦1.0. The above desirable conditions were determined for the following reasons. The present inventor conducted an experiment in which another toner image was superimposed on the image forming body 1 on which a toner image had been formed by non-contact development while changing conditions. Figure 5 shows
These are the results using a one-component developer. The diagram shows the frequency of the AC component on the horizontal axis and the amplitude (half between peaks) of the electric field (potential difference divided by distance) on the vertical axis. Areas where the effect is not sufficient, areas where the toner image already formed on the image forming body is likely to be destroyed, are sufficiently developed, and
This is an area in which already formed toner images are less likely to be destroyed, and these areas are particularly well developed. Moreover, FIG. 6 shows the results using a two-component developer. The symbols in the figure represent the same meanings as in FIG. As the two-component developer, one under the preferable conditions described above was used. As a result of the above, the above-mentioned preferable developing conditions were obtained, but in the case of a single-component developer, the toner image was not destroyed under the condition of 0.4≦V _Ac /(d・)≦1.2. ,
Furthermore, if 0.6≦V _Ac /(d·)≦1.0 is satisfied, sufficient image density can be obtained. Furthermore, when using a two-component developer, the toner image will not be destroyed under the condition of 0.5≦V _Ac /(d・) {(V _Ac /d)−1500}/≦1.0.
Furthermore, if 0.5≦V _Ac /(d·) {(V _Ac /d)−1500}/≦0.8 is satisfied, sufficient image density can be obtained. [Examples] The present invention will be specifically explained below with reference to Examples, but the embodiments of the present invention are not limited thereto. 1 to 4 and 7 are diagrams for explaining this embodiment, and FIG. 7 is a timing chart of the image forming process of this embodiment. Further, Tables 1 and 2 show setting conditions necessary to carry out this embodiment. The multicolor image of this embodiment is formed using the multicolor image forming apparatus shown in FIG. 3A based on the image forming system shown in FIG. That is, the projection light that optically scans the original 11 in the image input section R of the apparatus is photoelectrically converted and undergoes shading correction and shading correction in the image processing section P.
The black component image data is edge-emphasized and binarized by the operator shown in FIG. This recorded data is taken out at the time of recording and is stored in the laser optical system 1 of the recording section Q.
0. In recording in the recording section Q, first, a scorotrostatic charger 2 uniformly charges an image forming body 1 having a photoconductive photosensitive layer (to be described later), which rotates in the direction of the arrow. Next, the charged surface is subjected to imagewise exposure L of a laser beam modulated by the recording data input to the laser optical system 10 to form an electrostatic latent image. This electrostatic latent image is developed into a toner image by the developing device shown in FIG. 4 containing the two-component developer based on the reversal development flowchart shown in FIG. This image forming process is repeated by rotating the image forming body four times for each color in the order of yellow, magenta, cyan, and black, thereby forming a multicolor toner image in which the toner images of each color are superimposed. At this time, the contrast and edges of black component line drawings, character images, etc. are emphasized to form a clear black image. This multicolor toner image is easily transferred by the action of the pre-transfer charger 9 and the exposure lamp 3, and then is placed on the recording paper Pa fed from the paper feeder 11 by the paper feed roller 12 and the timing roller 13 at a transfer pole. It is transferred by the action of 4. This recording paper Pa is separated by the action of the separation electrode 5 and is heated and fixed by the fixing device 6. On the other hand, after the image forming body 1 after transfer is neutralized by the pre-cleaning static eliminator 7, residual toner is cleaned by the blade 81 and fur brush 82 of the cleaning device 8, and the image forming body 1 is prepared for the next image formation. Tables 1 to 3 show the image forming conditions, developer, and setting values of the developing conditions necessary for the multicolor image formation.

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〔Effect of the invention〕

As explained above, since the present invention emphasizes the edges of specific color components such as black, it is possible to obtain a clear multicolor image without losing line information such as characters. Furthermore, in this case, even if the image is controlled so that the amount of black toner attached does not become too large and the image becomes dark, the text will not be blurred or blown out, and it will be possible to reproduce the image clearly and with high contrast.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a multicolor image forming system according to the present invention, FIG. 2 is a diagram showing an example of an edge enhancement method, FIG. Figure 3B is a cross-sectional view showing the laser optical system, Figure 4 is a cross-sectional view of a developing device suitable for the present invention, Figure 5 is a graph showing preferred developing conditions when using a one-component developer, and Figure 6 is A graph showing preferred development conditions when using a two-component developer,
FIG. 7 is a timing chart of the image forming process of the embodiment. FIG. 8 is a flowchart explaining the general principle of a multicolor image forming method, FIG. 9 is a block diagram of a conventional multicolor image forming system, and FIG.
The figure is a block diagram of the binarization method. 1... Image forming body, 2... Scorotron charger, 3...
Exposure lamp, 6... Fixing device, 7 Pre-cleaning static eliminator, 8... Cleaning device, 81... Blade, 8
2... Fur brush, 9... Pre-transfer charger, 10... Laser optical system, 11... Paper feeding device, 12 Paper feeding roller, 1
3... Timing roller, 14... Document, 15... Image exposure lamp, 16... Filter, 18... Lens, 19
...imaging device, R...image input section, P...image processing section,
Q...Image recording section, M recording data storage section, A...
B,,C,D...Each developing device, L...Image exposure, 21...Laser oscillator ((laser light source)), 22...Rotating polygon mirror,
23...Fθ lens, 41...Magnetic roll, 42...Sleeve, 43...Standing control blade, 44...Developer reservoir, 45...Agitation screw, 46...Toner supply roller.

Claims (1)

[Claims] 1. A charger, an image exposure device, and
A developing device having yellow, magenta, cyan, and black toners, a transfer device, and a cleaning device are arranged, and a latent image is formed on the photoreceptor by charging by the charger and image exposure by the image exposure device. A multicolor image forming method in which a plurality of toner images are superimposed on the photoreceptor by repeating an image forming step of forming a toner image by reversal development using the developer, the optical information is A latent image is formed on the photoreceptor by image exposure based on the image data which has been subjected to arithmetic processing, and the final image forming step for forming the toner image is a black toner image. A method for forming a multicolor image, characterized in that the image data corresponding to a toner image of another color is emphasized more than the image data corresponding to a toner image of another color. 2. The method according to claim 1, wherein the process of emphasizing the image data corresponding to the black toner image in the final image forming step emphasizes the contrast of the image data. Multicolor image forming method. 3. The method according to claim 1, wherein the process of emphasizing the image data corresponding to the black toner image in the final image forming step emphasizes edges of the image data. Multicolor image forming method.