JP2005077850A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To selectively apply high gradation and low gradation in one image to be formed image and to form high-quality images. <P>SOLUTION: The image forming apparatus A is equipped with four electrophotographic image forming parts Pa to Pd respectively, including development devices 2a, 2b, 2c and 2d. In each of the development devices 2a and 2b, a developer tank in which a black toner α having 10μm average particle diameter is stored is set and in each of the development devices 2c and 2d, a developer tank in which a black toner β having 6μm average particle diameter is stored is set. Then, the image forming part to be used is selected according to the information concerning the image to be formed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus including an image forming unit that performs electrophotographic image formation, and more particularly to a monochrome / color combination type image forming apparatus including a plurality of image forming units.

  In a conventional color image forming apparatus, as a method of forming a monochrome image, a method of forming an image by superposing developers (toners) of a plurality of different hues (for example, yellow, magenta, cyan) and a black developer are used. Image forming method. In both of these methods, although the color image forming apparatus includes a reading unit having a high gradation property for a color image, the formed image is a low gradation image similar to the conventional monochrome image forming apparatus. As a result, the color image forming apparatus cannot fully exercise its capabilities.

  In order to solve such problems, in recent years, means for forming an image having high gradation even in monochrome image formation by using image processing has been proposed.

For example, a wide range of charge amount of developer is stored in one developing tank, and a high gradation image can be formed by adjusting the charging potential of the photoreceptor in accordance with image information obtained by image processing. The structure which does is disclosed (refer patent document 1).
JP 2002-258600 A

  However, in this configuration, it is difficult to selectively use the developer in one developing tank, and when an image is repeatedly formed in such a configuration, an image with a biased gradation is formed in one of them. End up. In addition, there is a problem that high gradation and low gradation cannot be selectively applied in one image to be formed, and it is difficult to form a high-quality image.

  The problem to be solved by the present invention is to provide an image forming apparatus that can selectively apply high gradation and low gradation in one image to be formed, and can form a high-quality image. is there.

  The image forming apparatus of the present invention has the following configuration as means for solving the above-described problems.

  (1) An image forming apparatus including a plurality of electrophotographic image forming units, wherein at least two of the image forming units store developer having the same hue and different particle diameters. And an image forming unit to be used is selected according to information on an image to be formed.

  In this configuration, an image is formed using a developer having an optimum particle size in accordance with information relating to an image to be formed. Therefore, for example, when a developer having a small particle diameter is used for an image that requires high gradation, and a developer having a large particle diameter is used for an image having low gradation, one image to be formed is formed. High gradation and low gradation are selectively applied in the image, and a high-quality image is formed.

(2) The hue of the developer is black.

  In this configuration, since a plurality of types of black developer having different particle diameters are provided, the black gradation is improved and a high-quality monochrome image is formed. Further, when the number of image forming units provided is larger than the number of types of developers, the image forming units can be preliminarily stored by storing the developers in the developing tanks of the remaining image forming units. Play a role. Accordingly, the life cycle of the image forming unit such as the photoconductor and the developing tank can be extended. In addition, since the developer can be reserved, it is possible to cope with a sudden large amount of image formation, and convenience is improved.

(3) The developer includes a developer having a particle size of 8 μm to 12 μm and a developer having a particle size of 4 μm to 8 μm.

  In this configuration, the developer having a particle size of 8 μm to 12 μm is larger than the particle size of the developer generally used so far, so that it is possible to form an image with a smaller amount than before and the developer is saved. . Further, since the developer having a particle diameter of 4 μm to 8 μm is smaller than the particle diameter of a developer generally used conventionally, a higher-density, high-definition, multi-gradation image is formed than before.

(4) Each of the image forming units includes a latent image carrier, and the latent image carrier having a smaller particle diameter of the developer stored in the opposite developer tank has a lower charging potential.

  In this configuration, since the charging potential of the latent image carrier is adjusted according to the particle diameter of the developer, the developer can be transferred to the corresponding latent image carrier of each developer regardless of the particle diameter of the developer. The amount of adhering is made uniform.

(5) The information on the image to be formed is information for specifying the type of image.

  In this configuration, for example, an image forming unit to be used is selected according to the type of image, such as a photographic image, a halftone image, or a character image, and an optimum particle size according to the type of image is selected. An image is formed using a developer. Therefore, for example, a developer having a small particle size is used for a type of image that requires high gradation such as a photographic image or a halftone image, and a developer having a large particle size for a type of image having a low gradation. Is used. In this way, high gradation and low gradation are selectively applied within one image to be formed, and a high-quality image is formed.

(6) The information relating to the image to be formed is information for distinguishing the contour portion of the image from the portion other than the contour portion of the image, and for image formation of the contour portion of the image and image formation of the portion other than the contour portion of the image. An image forming unit including a developing tank in which a developer having a particle diameter smaller than that of a developer stored in a developing tank included in the image forming unit to be used is used.

  In this configuration, a developer having a small particle size is used for image formation at the contour portion, and a developer having a large particle size is used for image formation at portions other than the contour portion. Therefore, a high-definition and high-definition image is formed at the contour portion, and the quality is improved. On the other hand, in a portion other than the contour portion, an image of the same quality is formed with a smaller amount of developer than when a developer having a small particle diameter is used, and the developer is saved.

(7) Image forming units to be used in accordance with information about the image to be formed by storing developers having the same hue and different particle diameters in the developing tanks respectively included in at least two image forming units of the electrophotographic system It is characterized by selecting.

  In this configuration, an image is formed using a developer having an optimum particle size in accordance with information relating to an image to be formed. Therefore, for example, when a developer having a small particle diameter is used for an image that requires high gradation, and a developer having a large particle diameter is used for an image having low gradation, one image to be formed is formed. High gradation and low gradation are selected in the image, and a high-quality image is formed.

(8) The image to be formed is a monochrome image.

  In this configuration, since a developer having an optimum particle size is used according to information relating to a monochrome image, the gradation is improved and a high-quality monochrome image is formed. Further, when the number of image forming units provided is larger than the number of types of developers, the image forming units can be preliminarily stored by storing the developers in the developing tanks of the remaining image forming units. Play a role. As a result, the life cycle of the image forming unit such as the developing tank can be extended. In addition, since the developer can be reserved, it is possible to cope with a sudden large amount of image formation, and convenience is improved.

  According to the present invention, the following effects can be obtained.

  (1) By forming an image using a developer having an optimum particle size according to information relating to the image to be formed, for example, a developer having a small particle size is used for an image that requires high gradation, It can be said that a developer having a large particle diameter is used for an image with low tonality, and high gradation and low gradation can be selectively applied within one image to be formed. A quality image can be formed.

  (2) By providing a plurality of types of black developer having different particle diameters, the black gradation can be improved and a high-quality monochrome image can be formed. Further, when the number of image forming units provided is larger than the number of types of developers, the image forming units can be preliminarily stored by storing the developers in the developing tanks of the remaining image forming units. Play a role. As a result, the life cycle of the image forming unit such as the photoconductor and the developing tank can be prolonged. In addition, since the developer can be reserved, it is possible to cope with a sudden large amount of image formation, and convenience is improved.

  (3) By using a developer having a particle size of 8 μm to 12 μm, the particle size of this size is larger than the particle size of a developer generally used so that an image can be obtained in a smaller amount than before. It becomes possible to form and save developer. In addition, by using a developer having a particle size of 4 μm to 8 μm, the particle size of this size is smaller than the particle size of the developer generally used so far, so it has higher density and higher definition than before. A multi-tone image can be formed.

  (4) By adjusting the charging potential of the latent image carrier in accordance with the particle diameter of the developer, regardless of the particle diameter of the developer, each developer can be applied to the corresponding latent image carrier. The amount of adhesion can be made uniform, and the quality of the image can be improved.

  (5) A type that requires high gradation by selecting an image forming unit to be used according to the type of image and forming an image using a developer having an optimum particle size according to the type of image. A developer having a small particle size is used for the image of 1, and a developer having a large particle size is used for a type of image having low gradation. As described above, high gradation and low gradation can be selectively applied in one image to be formed, and a high-quality image can be formed.

  (6) By using a developer having a small particle diameter for image formation at the contour portion and using a developer having a large particle diameter for image formation at portions other than the contour portion, the contour portion has high density and high definition. Image can be formed and the quality can be improved.On the other side than the contour portion, an image of the same quality can be formed with a smaller amount of developer than when a developer having a small particle diameter is used. Can be saved.

  (7) By forming an image using a developer having an optimum particle size in accordance with information on the image to be formed, for example, a developer having a small particle size is used for an image requiring high gradation, It can be said that a developer having a large particle diameter is used for an image with low tonality, and high gradation and low gradation can be selectively applied within one image to be formed. A quality image can be formed.

  (8) By using a developer having an optimum particle size according to information relating to a monochrome image, the gradation can be improved and a high-quality monochrome image can be formed. Further, when the number of image forming units provided is larger than the number of types of developers, the image forming units can be preliminarily stored by storing the developers in the developing tanks of the remaining image forming units. Play a role. As a result, the life cycle of the image forming unit such as the developing tank can be prolonged. In addition, since the developer can be reserved, it is possible to cope with a sudden large amount of image formation, and convenience is improved.

  Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus A is a monochrome / color combination type image forming apparatus configured to be able to form multicolor and single color images on a predetermined sheet (recording paper) based on image data input from the outside. It is. The image forming apparatus A includes photosensitive drums 3a, 3b, 3c, and 3d as latent image carriers, chargers 5a, 5b, 5c, and 5d, developing devices 2a, 2b, 2c, and 2d, and cleaner units 4a and 4b. , 4c and 4d, four electrophotographic image forming portions Pa to Pd are configured. The four image forming units Pa to Pd are arranged in tandem.

  A black toner (developer) is accommodated in the developing tank in the developing device 2a, and a black image is formed by the image forming portion Pa. Similarly, cyan, magenta, and yellow toners are respectively stored in the developing tanks in the developing units 2b, 2c, and 2d, and cyan, magenta, and yellow images are formed by the image forming units Pb, Pc, and Pd, respectively. As a result, the image forming apparatus A can handle color images using black, cyan, magenta, and yellow.

  In the image forming unit Pa, the photosensitive drum 3a rotates counterclockwise in FIG. Around the photosensitive drum 3a, a charger 5a, a developing device 2a, an intermediate transfer roller 6a, and a cleaner unit 4a are arranged in this order from the upstream side along the rotation direction of the photosensitive drum 3a.

  The charger 5a is a roller type and is disposed so as to contact the surface of the photosensitive drum 3a. The charger 5a charges the surface of the photosensitive drum 3a to a predetermined uniform potential.

  An exposure unit 1 is disposed below the photosensitive drum 3a. In this embodiment, a laser scanning unit (LSU) including a laser irradiation unit and a reflection mirror is used as the exposure unit 1. The exposure unit 1 exposes the photosensitive drum 3a by irradiating the photosensitive drum 3a charged by the charger 5a with laser light based on the input image data, and converts the image data onto the surface of the photosensitive drum 3a. Forming an electrostatic latent image based thereon;

  The developing device 2a visualizes the electrostatic latent image formed on the photosensitive drum 3a with black toner stored in a developing tank in the developing device 2a. The intermediate transfer roller 6a will be described later. The cleaner unit 4a removes and collects toner remaining on the surface of the photosensitive drum 3a after being developed by the developing device 2a and transferred to an intermediate transfer belt 7 described later. The other image forming units Pb, Pc, and Pd are similarly configured.

  An intermediate transfer belt unit 8 is disposed above the photosensitive drum 3a. The intermediate transfer belt unit 8 includes an intermediate transfer belt 7, an intermediate transfer belt drive roller 71, an intermediate transfer belt driven roller 72, an intermediate transfer belt tension mechanism 73, intermediate transfer rollers 6a, 6b, 6c, and 6d, and an intermediate transfer belt cleaning unit. 9 is included.

  The intermediate transfer belt 7 is formed endlessly using a film having a thickness of about 100 μm to 150 μm, and is disposed so as to be in contact with the surfaces of the photosensitive drums 3 a to 3 d. The intermediate transfer belt 7 is stretched around an intermediate transfer belt drive roller 71, an intermediate transfer belt driven roller 72, an intermediate transfer belt tension mechanism 73, and intermediate transfer rollers 6a to 6d, and is driven to rotate in the direction of arrow B.

  The intermediate transfer rollers 6 a to 6 d are disposed in contact with the back side of the intermediate transfer belt 7 when viewed from the photosensitive drums 3 a to 3 d and are rotatably supported by the intermediate transfer roller mounting portion of the intermediate transfer belt tension mechanism 73. The intermediate transfer rollers 6a to 6d are made by coating the surface of a metal shaft such as stainless steel having a diameter of 8 mm to 10 mm with a conductive elastic agent such as EPDM or urethane foam. A high-voltage transfer bias having a polarity (+) opposite to the toner charge polarity (-) is applied to the intermediate transfer rollers 6a to 6d, and the conductive elastic agent functions as a transfer electrode, so that the intermediate transfer belt 7 is uniformly applied with a high voltage transfer bias.

  When a transfer bias is applied to the intermediate transfer belt 7 by the intermediate transfer rollers 6a to 6d, each color toner image (monochromatic toner image) visualized on the respective photosensitive drums 3a to 3d is transferred onto the intermediate transfer belt 7. Are sequentially superimposed and transferred to form a color toner image (multicolor toner image).

  In this way, the multicolor toner image formed on each of the image forming portions Pa to Pd and then stacked on the intermediate transfer belt 7 is conveyed by the rotation of the intermediate transfer belt 7, and the intermediate transfer belt 7 and the sheet are conveyed. It is conveyed to the contact position.

  A transfer roller 11 facing the intermediate transfer belt driving roller 71 is disposed at a contact position between the intermediate transfer belt 7 and the sheet. Either the transfer roller 11 or the intermediate transfer belt drive roller 71 is formed of a hard material such as metal, and the other is formed of a soft material such as an elastic roller (elastic rubber roller, foaming resin roller, or the like). As a result, the transfer roller 11 and the intermediate transfer belt drive roller 71 are constantly pressed against each other with a predetermined strength. Further, a high voltage having a polarity (+) opposite to the charging polarity (−) of the toner is applied to the transfer roller 11, whereby the multicolor toner image on the intermediate transfer belt 7 is transferred onto the sheet.

  An intermediate transfer belt cleaning unit 9 is disposed at a position facing the intermediate transfer belt driven roller 72. The intermediate transfer belt cleaning unit 9 includes, for example, a cleaning blade as a cleaning member. The cleaning blade is set so as to contact the intermediate transfer belt 7 at a position where the intermediate transfer belt 7 is supported by the intermediate transfer belt driven roller 72 from the back side. The toner remaining on the intermediate transfer belt 7 without being transferred onto the sheet is removed and collected by the intermediate transfer belt cleaning unit 9 to prevent toner color mixing in the next step.

  A sheet feeding tray 10 is disposed below the image forming apparatus A and stores sheets for forming images. Further, a paper discharge tray 15 is disposed on the upper part of the image forming apparatus A, and a sheet on which an image has been formed is placed face down (in a state where the image is directed downward).

  A pickup roller 16a, a registration roller 14, a transfer roller 11, a fixing unit 12, and a pair of conveyance rollers 25a, 25b, and 25c are disposed in a substantially vertical sheet conveyance path S from the sheet feed tray 10 to the sheet discharge tray 15.

  The pickup roller 16 a is disposed at the end of the paper feed tray 10 and functions as a drawing roller that feeds sheets from the paper feed tray 10 to the paper transport path S one by one. The sheet supplied from the sheet feeding tray 10 to the sheet conveyance path S is sent to the registration roller 14 while being promoted / assisted by a conveyance roller pair 25 a formed by small rollers, and is temporarily held by the registration roller 14. . Then, the sheet is fed between the transfer roller 11 and the intermediate transfer belt driving roller 71 at the timing when the leading edge of the multicolor toner image on the intermediate transfer belt 7 and the leading edge of the sheet are aligned by the registration roller 14.

  The sheet on which the toner image is transferred by the transfer roller 11 is conveyed to the fixing unit 12. The fixing unit 12 includes a heat roller 31 and a pressure roller 32. The heat roller 31 and the pressure roller 32 are pressed against each other with a predetermined pressing force, and rotate with the sheet interposed therebetween.

  The temperature of the heat roller 31 is set to maintain a predetermined temperature by a control unit (not shown) based on a signal from a temperature detector (not shown). Then, by heating and pressing the sheet with the heat roller 31 and the pressure roller 32, the toner of the multicolor toner image transferred onto the sheet is melted, mixed, pressed against the sheet, and thermally fixed to the sheet. Is done.

  The sheet on which the multicolor toner image is fixed is conveyed by a pair of conveyance rollers 25b and 25c, and is discharged onto the paper discharge tray 15 with the multicolor toner image facing downward.

  The image forming apparatus A requires an opening / closing operation like the paper feed tray 10 when the user forms an image on a desired small number of sheets, separately from the cassette type paper feed tray 10 that stores predetermined sheets in advance. A manual feed tray 20 on which sheets can be set. In the manual feed tray 20, a pickup roller 16b similar to the pickup roller 16a disposed in the paper feed tray 10 is disposed. The sheets placed on the manual feed tray 20 are supplied to the sheet conveyance path S one by one by the pickup roller 16b, and reach the registration rollers 14 while being promoted and assisted by a plurality of conveyance roller pairs 25d, 25e, and 25f. Thereafter, the sheet is discharged to the sheet discharge tray 15 through the same process as the sheet supplied from the sheet feed tray 10.

  At the time of single-sided image formation in which an image is formed only on one side of a sheet, image formation is completed after the above-described process. However, in the case of double-sided image formation in which an image is formed on both sides of a sheet, the following operation is performed. With the multicolor toner image fixed on one side of the sheet and the trailing edge of the sheet being chucked by the pair of transport rollers 25c, the transport roller pair 25c rotates in the reverse direction. Then, after the sheet is conveyed to the conveying roller pair 25g and 25h, the back side image is formed through the registration roller 14, the transfer roller 11, and the fixing unit 12, and is discharged to the paper discharge tray 15.

  If the image to be formed is not full-color, only the image forming unit including toner of the required hue among the image forming units Pa to Pd operates, and the toner image is transferred onto the intermediate transfer belt 7. The transferred toner image is transferred to a sheet by the transfer roller 11, fixed by the fixing unit 12, and discharged to the paper discharge tray 15.

  Next, the main part of the present invention will be described. FIG. 2 is an enlarged view showing a main part of the image forming apparatus A shown in FIG.

  In the case of forming a color image in the image forming apparatus A, for example, a developing tank containing black toner is set in the developing device 2a, and a developing tank containing cyan toner is set in the developing device 2b. A developing tank containing magenta toner is set in the device 2c, and a developing tank containing yellow toner is set in the developing device 2d. The photosensitive drums 3a to 3d and the intermediate transfer rollers 6a to 6d are in contact with the intermediate transfer belt 7 from both the front and back surfaces. When color image formation is performed, an image is formed using toner in the four developing tanks.

  Next, a case where a monochrome image is formed by the image forming apparatus A will be described. Here, two types of black toners having different particle diameters are used. A developing tank storing toner having a large particle diameter is set in the developing device 2a, and a developing tank storing toner having the same particle diameter as the developing device 2a is set as a spare for the developing device 2a. A developing tank storing toner having a small particle diameter is set in the developing device 2c, and a developing tank storing toner having the same particle diameter as that of the developing device 2c is set as a spare for the developing device 2c. .

  FIG. 3 shows a particle size distribution diagram when the particle sizes of two types of toners used as an example in this case are measured by an espert measuring device. The vertical axis represents the number of toner particles, and the horizontal axis represents the toner particle diameter.

  In this embodiment, toner α having an average particle size of 6 μm is used as a toner having a small particle size, and toner β having an average particle size of 10 μm is used as a toner having a large particle size. As shown in FIG. 3, it can be seen that the toner α is distributed with a smaller particle size as a whole with respect to the toner β.

  When toners having different particle diameters are used for image formation of the same area, it is apparent that the toner consumption is smaller for toners having a smaller particle diameter. Therefore, the charging potential of the photosensitive drums 3c and 3d using the toner α having a small particle diameter is set lower than the charging potential of the photosensitive drums 3a and 3b using the toner β having a large particle diameter. Thereby, the adhesion amount of the toner α having a small particle diameter and the toner β having a large particle diameter to the photosensitive drums 3a to 3d can be made the same.

  As the toner α, a toner having a particle diameter of 4 μm to 8 μm is preferably employed, and as the toner β, a toner having a particle diameter of 8 μm to 12 μm is preferably employed. If a toner having a particle size of 4 μm to 8 μm is used, it is smaller than the particle size of a conventionally used toner, so that a higher-definition, high-definition and multi-gradation image can be formed. In addition, if a toner having a particle size of 8 μm to 12 μm is used, the toner particle size is larger than that of a conventionally used toner, so that it is possible to form an image with a smaller amount than the conventional one, and the toner can be saved.

  FIG. 4 is a flowchart showing main operations of the image forming apparatus A. When an image formation request is made to the standby image forming apparatus A (S1), a control unit (not shown) determines whether the requested image is a monochrome image or a color image (S2). If it is determined that the image is a monochrome image, it is determined whether or not the two types of black toners α and β having different particle diameters are stored in the developing units 2a to 2d as shown in FIG. If it is determined (S3) and the two types of black toners α and β are not stored, a predetermined developing tank is set so as to include black toners α and β having different particle diameters. (S4).

  When it is confirmed that the black toners α and β having different particle diameters are provided, the input image data is analyzed by the control unit, and is decomposed into a character image, a halftone image, and a photographic image. The information for specifying the type is acquired by the control unit (S5). Further, the thickness of the character of the decomposed character image is confirmed, and the character is decomposed into a contour portion of the character and a portion other than the contour portion (S6). When the image processing in S5 and S6 is completed, the image decomposed in S5 and S6 is further subjected to image processing (S7) in order to process the image data to be transmitted to each image forming unit. In the image processing here, the contour portions of the photographic image, halftone image, and character image are processed so as to be transmitted to an image forming unit including a developing device 2c including a developing tank that stores toner having a small particle diameter. Portions other than the outline portion of the character image are processed so as to be transmitted to an image forming unit including a developing device 2a including a developing tank that stores toner having a large particle diameter.

  The image data processed in this way is transmitted via the exposure unit 1 to the optimum image forming unit selected according to the information relating to the image as described above. In other words, optimal image formation selected according to information specifying the type of image, such as a character image, halftone image, or photographic image, or information such as whether the character is a contour portion of the character image or a portion other than the contour portion Is transmitted to the part via the exposure unit 1. The image data transmitted to each image forming unit via the exposure unit 1 is formed as an electrostatic latent image on each photosensitive drum 3a, 3c, and is visualized with a corresponding toner and transferred onto a sheet. Through the image forming process such as the process and the fixing process, the image formation is completed (S11), and again enters the standby state.

  If it is determined in S2 that the image is not a monochrome image but a color image, it is determined whether each image forming unit has toner for forming a color image (S8), and it is determined that it is not provided. If this happens, it waits for the developer tank containing the predetermined toner to be set (S9). When it is determined that the toner for forming a color image is provided, the input image data is analyzed and transmitted to an image forming unit including a developing device including a developing tank for storing toner of each hue. Image processing is performed (S10). Then, the process proceeds to S11, and a color image is formed on the sheet through the same process as that for a monochrome image.

  As shown in FIG. 5, in the process of S5 of FIG. 4, the input image data is classified into a character image X, a halftone image Y, and a photographic image Z. For the halftone image Y and the photographic image Z, An image is formed using toner α having a small particle diameter. Further, as shown in FIG. 6, the character image X is further classified into a character contour portion and a portion other than the contour portion according to the thickness of the character, and the contour portion includes the halftone image Y and the photographic image. Similar to Z, an image is formed using toner α having a small particle diameter. In addition, an image is formed on a portion other than the contour portion using the toner β having a particle diameter larger than that of the toner α used in the contour portion.

  In the case of forming a monochrome image with a conventional image forming apparatus, the photosensitive drums 3b, 3c, 3d are separated from the intermediate transfer belt 7, and a monochrome image is formed only on one photosensitive drum 3a using black toner. It was. In the case of forming a monochrome image with a single photosensitive drum, the optimum toner particle diameter and charge amount vary depending on the sensitivity of the photosensitive drum, so the gradation tends to be low. According to the image forming apparatus A, since a monochrome image is formed by the plurality of photosensitive drums 3a to 3d using toners having different particle diameters, high gradation can be realized.

  Conventionally, images are formed in the same color tone regardless of the type of image such as a character image, a halftone image, and a photographic image. However, according to the image forming apparatus A, information for specifying the type of image is used. The image forming unit to be used is selected according to the toner image, and the particle size of the toner to be used is also selected. Therefore, the high gradation property and the low gradation property are selectively selected according to the image information in one image to be formed. Can be applied to. Specifically, by using the toner α with a small particle diameter for the image formation of the halftone image, the photographic image, and the outline of the character image, a high-density, high-definition, multi-gradation image can be formed. The quality is improved. On the other hand, by using toner β with a large particle diameter for image formation in areas other than the outline, an image of the same quality can be formed with a smaller amount of toner than when a toner with a small particle diameter is used, saving toner. it can.

  The chargers 5a, 5b, 5c, and 5d may be brush-type or charger-type chargers. The exposure unit 1 can also be configured using a write head in which light emitting elements such as EL and LEDs are arranged in an array. Further, in this embodiment, the intermediate transfer rollers 6a, 6b, 6c, and 6d adopt a roller shape, but a brush or the like may be used.

1 is a schematic diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is an enlarged view showing a main part of the image forming apparatus shown in FIG. 1. FIG. 6 is a particle size distribution diagram of toner measured by an Yspart measuring device. 3 is a flowchart showing main operations of the image forming apparatus shown in FIG. 1. It is a schematic diagram showing the type of image to be formed. It is explanatory drawing showing the outline part and part other than an outline part of a character image.

Explanation of symbols

A Image forming apparatus Pa to Pd Image forming portion α, β Toner 2a to 2d Developer 3a to 3d Photosensitive drum 4a to 4d Cleaner unit 5a to 5d Charger

Claims (8)

An image forming apparatus including a plurality of electrophotographic image forming units,
Among the image forming units, at least two image forming units each include developing tanks containing developers having the same hue and different particle diameters, and the image forming unit to be used is selected according to information on the image to be formed. An image forming apparatus.   The image forming apparatus according to claim 1, wherein the hue of the developer is black.   The image forming apparatus according to claim 1, wherein the developer includes a developer having a particle diameter of 8 μm to 12 μm and a developer having a particle diameter of 4 μm to 8 μm.   The image forming unit includes a latent image carrier, and a latent image carrier having a smaller particle diameter of developer stored in a developing tank facing the toner is set to have a lower charging potential. The image forming apparatus according to claim 3.   5. The image forming apparatus according to claim 1, wherein the information on the image to be formed is information for specifying a type of image.   The information relating to the image to be formed is information for distinguishing the contour portion of the image from the portion other than the contour portion of the image, and is used for image formation of the contour portion of the image and image formation of the portion other than the contour portion of the image. 6. The image forming unit including a developing tank in which a developer having a particle diameter smaller than the particle diameter of the developer stored in a developing tank included in the image forming unit is used. An image forming apparatus according to claim 1.   Developers having the same hue and different particle diameters are respectively stored in the developing tanks respectively included in at least two image forming units of the electrophotographic system, and the image forming unit to be used is selected according to information on the image to be formed. An image forming method.   The image forming method according to claim 7, wherein the image to be formed is a monochrome image.

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