JP5233259B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image.

In an electrophotographic image forming apparatus, an electrostatic latent image is formed by irradiating and exposing a photosensitive member charged to a predetermined potential, and electrostatic potential is generated by a potential difference from a developing bias applied to a developing roll. A process of developing the latent image with a developer is executed. However, when the toner image formed on the photosensitive member is transferred to a recording medium such as paper, the density of the toner image varies depending on the type of the recording medium. This is due to the material, surface roughness, thickness, etc. of the recording medium. For example, when comparing plain paper made of pulp fiber material and recording paper (so-called coated paper) with a coating layer formed of kaolin or calcium carbonate on the recording surface, image data with the same gradation value Even if the image is formed based on the image, the image formed on the coated paper tends to have a higher density than the image on the plain paper. As described above, for example, in Patent Document 1, ICC (International Color Consortium) color profile data corresponding to the paper type is used or gradation correction is performed so that there is no difference in color reproducibility depending on the type of recording paper. Thus, it is disclosed to correct color reproducibility.
JP-A-6-62249

The technique described in Patent Document 1 corrects the tone value of image data so that the color reproducibility matches the color reproduction of so-called offset printing (Japan color, etc.) on plain paper and coated paper. . Therefore, the density change amount with respect to the change amount of the toner area ratio (the coverage of the toner on the recording paper (medium) per unit area) is large in the low density range, and the density change amount with respect to the toner area ratio is small in the high density range. When coated paper showing such a tendency (so-called high gamma characteristic) is used, tone jump is more likely to occur than plain paper in which this relationship changes substantially linearly in the entire density range. Further, in coated paper that reproduces an image with a higher density than plain paper, the solid image portion (area ratio is 100%) represented by the input image signal is expressed in a halftone such as a so-called line screen, There is a problem that the texture of the image is different from that when plain paper is used, and there is also a deterioration in image quality in which characters and fine lines are interrupted.
The present invention has been made in view of such circumstances, and an object of the present invention is to suppress image quality deterioration such as tone jump and fine line breakage that are likely to occur in a recording medium provided with a coating layer.

In order to solve the above-described problems, the present invention charges the image carrier in accordance with a control parameter that determines the driving state of the self-image forming unit, and irradiates the charged image carrier with light according to image data. Image forming means for forming an electrostatic latent image, developing the electrostatic latent image with a developer containing a color material, transferring the developed image onto a recording surface of a recording medium, and fixing the image; Detecting means for detecting whether the recording medium is a recording medium having a coating layer on the recording surface or a recording medium having no coating layer, and the coating means by the detection means. When an image corresponding to image data is formed on the recording surface of the recording medium that is detected as having a coating layer, the density of the image formed on the recording surface on which the coating layer is provided And a note that the coating layer is not provided. By determining the control parameter as the difference between the density of the image is reduced in the case of the forming an image on the surface, reducing the amount of the coloring material to be transferred to said recording surface coating layer is provided When the control unit for forming the image on the image forming unit and the type of the recording medium are designated and the setting of the control parameter for the recording medium is instructed, a plurality of tests having a specific area ratio are performed. A test image formation control unit configured to form an image on a recording surface of the type of recording medium by using the image forming unit with different control parameters, and the control unit reads the plurality of test images. Each of the control parameters for determining the density and forming an image of the area ratio with the target density on the type of recording medium based on the determined densities of the plurality of test images. Remember, when forming an image according to the image data on the recording surface of the recording medium, the control parameter stored corresponding to the type of the recording medium is determined, and the image is recorded on the image forming means. to provide an image forming apparatus, characterized in that to form.

In the image forming apparatus of the present invention, the control unit may use the common target density regardless of the type of the recording medium.

  According to the present invention, it is possible to suppress the occurrence of image quality deterioration such as tone jump and fine line breakage that are likely to occur in a recording medium provided with a coating layer.

Embodiments of the present invention will be described below with reference to the drawings.
(1) Apparatus Configuration FIG. 1 is a diagram showing a configuration of an image forming apparatus 1 that is an embodiment of the present invention. The image forming apparatus 1 includes a test image signal generation circuit 11, a selector 12, an image reading unit 20, an image input device 30, a UI (User Interface) unit 40, an image forming unit 50, and a process control unit 60. With.

  The test image signal generation circuit 11 stores data for generating a test image signal representing a predetermined test image, and supplies the generated test image signal to the selector 12. Details of this test image will be described later. The selector 12 selects either the test image signal acquired from the test image signal generation circuit 11 or the image signal acquired from the image input device 30 and outputs the selected image signal to the image forming unit 50. The selector 12 outputs an image signal that is a basis of the toner image formed by the image forming unit 50.

  The image reading unit 20 is a so-called scanner, and includes a light source, an imaging lens, and a line sensor (not shown). The light source irradiates light on an object to be imaged such as a document. The imaging lens forms an image of reflected light from the object to be picked up at the position of the line sensor. The line sensor receives the imaged light, and generates and outputs an image signal corresponding to the light. This line sensor includes an image sensor that can image in three colors of R (red), G (green), and B (blue), and generates image signals of these three colors. Then, the image reading unit 20 performs A / D conversion processing on the image signal, and outputs this to the process control unit 60 as a read signal.

The image input device 30 receives an image signal given to form an image from an external device such as a computer (not shown) via a network such as a LAN (Local Area Network), so that the image forming device 1 receives the image signal. Input an image signal. This image signal corresponds to the color of the toner formed by the image forming unit 50, and is divided into four color components of cyan (C), magenta (M), yellow (Y), and black (K). The gradation value of each pixel is included. The UI unit 40 is a user interface such as a touch panel that accepts an operation input by a user and can be notified including a display for the user. The recording paper on which an image is formed by the image forming apparatus 1 includes a recording paper made of pulp fibers (hereinafter referred to as “plain paper”), and a “coated paper” in which a coating layer is provided on the recording surface of the plain paper. There is. Details of the “coating layer” and each recording sheet will be described later. Also, plain paper is cheaper and more frequently used than coated paper.
When the user operates the UI unit 40 to instruct the type of recording paper (recording medium) on which a toner image is to be formed, the UI unit 40 outputs a paper type signal indicating the type of recording paper to the process control unit 60.

  The image forming unit 50 forms an image corresponding to the image signal supplied from the selector 12 on a recording sheet, and includes a paper feed tray 51, an image forming engine 52, an intermediate transfer belt 55, and a secondary transfer. Device 56 and a fixing device 57. The paper feed tray 51 is made up of paper feed trays 51a, 51b, and 51c, each of which contains a plurality of recording sheets cut into a predetermined size such as A4, and the stored sheets are one by one from the paper feed tray 51. The sheet is picked up and sequentially conveyed on a sheet conveyance path 600 connected to the sheet discharge port via the secondary transfer unit 56 and the fixing unit 57. Here, “plain paper” is stored in the paper feed tray 51a, “coated paper” is stored in the paper feed tray 51b, and different types of recording paper are stored in the paper feed tray 51c. The image forming engine 52 forms a toner image on the intermediate transfer belt 55 according to the image signals of C, M, Y, and K colors.

  Here, FIG. 2 is a diagram showing a detailed configuration of the image forming unit 50 (mainly the image forming engine 52) and the process control unit 60, and a configuration around it. In order to control (process control) the driving state of the image forming unit 50, the process control unit 60 obtains values suitable as various physical quantities (hereinafter referred to as “control parameters”) that determine the driving state. Note that the mechanisms for forming toner images of C, M, Y, and K in the image forming engine 52 have the same configuration, and therefore, only one of the mechanisms will be illustrated and described below.

The image forming engine 52 includes a charging device 502, a potential sensor 503, an exposure device 504, a developing device 505, a primary transfer device 506, a toner density sensor 508, a cleaner 509, and the like, which are provided so as to surround the rotating photosensitive drum 501. A laser driver 510, a charger power supply 511, a developing bias power supply 512, a temperature sensor 513, and an environment sensor 514 are provided.
The photoconductive drum 501 has, for example, two to three photoconductive layers including a charge generation layer and a charge transport layer made of OPC (Organic Photo Conductor) as a charge acceptor on a conductive substrate. This is an image carrier formed by the above method. The photosensitive drum 501 is rotated in the direction of the arrow A (clockwise direction) shown in the drawing by the drive mechanism (not shown) around the central axis. The charger 502 is a scorotron for primary charging that uniformly charges the surface of the photosensitive drum 501 and charges the surface of the photosensitive drum 501 to a predetermined charging potential. The potential sensor 503 is a sensor that detects the surface potential of the photosensitive drum 501 on the downstream side of the charging position of the charger 502 and on the upstream side of the developing position of the developing device 505 with respect to the arrow A direction. A surface potential signal indicating the surface potential is output to the process control unit 60.

  The exposure unit 504 irradiates the polygon mirror with laser light and irradiates the surface of the photosensitive drum 501 with the reflected light, thereby reducing the surface potential of the photosensitive drum 501 to a predetermined potential due to photoconductivity. To form an electrostatic latent image. The developing device 505 develops the electrostatic latent image using toner (coloring material) contained in the developer. At this time, the developing device 505 supplies an amount of toner according to the control by the process control unit 60. The scanning direction of the exposure light is equal to the axial direction of the photosensitive drum, and this direction is referred to as the “main scanning direction” of the image forming unit 50. The direction orthogonal to the main scanning direction is referred to as “sub-scanning direction” of the image forming unit 50.

  The primary transfer unit 506 is configured such that the toner image formed on the surface of the photosensitive drum 501 has a position (transfer position) between which itself and the photosensitive drum 501 sandwich the intermediate transfer belt 55 as the photosensitive drum 501 rotates. ), The toner image is transferred to the intermediate transfer belt 55. Specifically, a primary transfer current having a magnitude corresponding to a control parameter obtained by the process control unit 60 is supplied to the primary transfer unit 506, and a main roll that is charged to a potential (transfer potential) corresponding thereto. The electrostatic force is generated by the potential difference from the photosensitive drum 501, and the toner image is transferred by the action of the electrostatic force.

The toner density sensor 508 irradiates the surface of the photosensitive drum 501 on which the toner image is formed with detection light, detects the density of the toner image according to the amount of reflected light, and represents the detected density of the toner image. Output a signal. Here, “density” refers to optical density, and the toner density sensor 508 calculates the density D of the toner image by performing the calculation of the following formula (1).
D = −log (I / Io) (1)
Here, Io is the amount of detection light emitted by the toner concentration sensor 508, and I is the amount of reflected light received by the toner concentration sensor 508.

  A cleaner 509 removes residual toner and paper dust on the surface of the photosensitive drum 501. The laser driver 510 controls the intensity (exposure amount) of light emitted by the exposure device 504 according to the control parameter obtained by the process control unit 60, and forms an electrostatic latent image on the surface of the photosensitive drum 501. The charger power supply 511 supplies the charger 502 with electric power for charging the photosensitive drum 501. The developing bias power supply 512 supplies power for applying a developing bias to the developing roll of the developing device 505. The amount of power supplied to the charger 502 and the developing device 505 is an amount corresponding to the control parameter obtained by the process control unit 60. The temperature sensor 513 is installed around the fixing device 57 and outputs a signal representing the detected temperature to the process control unit 60. The environmental sensor 514 detects the temperature and humidity in the image forming apparatus 1 and outputs a temperature and humidity signal indicating the detected value to the process control unit 60.

Here, returning to FIG.
The secondary transfer unit 56 includes the belt surface of the intermediate transfer belt 55 on which the toner images of each color are superimposed by the image forming engine 52 and the sheet conveyed from the sheet feeding tray 51 via the sheet conveying path 600. When the transfer position is reached, the toner image is transferred from the intermediate transfer belt 55 to the recording sheet by the action of the main roll. The fixing device 57 fixes the toner image transferred to the recording paper. Specifically, the fixing device 57 forms a contact region by sandwiching the sheet conveyance path 600 between the peripheral surfaces of a heating roll having a heating source and a pressure roll. When the recording paper on which the toner image has been transferred by the secondary transfer device 56 is moved to this transfer position, the fixing device 57 applies the toner image to the recording paper by the heating action by the heating roll and the pressure action by the pressure roll. To settle.
The above is the configuration of the image forming unit 50.

Next, the configuration of the process control unit 60 will be described with reference to FIG. 2 again.
The process control unit 60 includes a dot counter 601, a patch signal generation unit 602, a signal selector 603, a potential controller 604, a potential parameter determination unit 605, a development control controller 606, a transfer control controller 607, and a fixing control controller 608.

  The dot counter 601 counts the number of dots when the exposure unit 504 forms an electrostatic latent image on the surface of the photosensitive drum 501 based on the image signal acquired from the image input device 30. The dot counter 601 outputs the counted number of dots to the development controller 606, while outputting the acquired image signal as it is to the signal selector 603.

  The patch signal generation unit 602 generates a predetermined image signal (hereinafter referred to as “patch image signal”) and outputs the signal to the signal selector 603. The patch image signal is an image signal of raster data in which a plurality of rectangular images each having a density that gradually changes from the highest density to the lowest density are arranged in the sub-scanning direction. The width of the rectangular image in the main scanning direction is within a narrow range around the density detection point of the toner density sensor 508.

  The signal selector 603 selects either the image signal acquired from the dot counter 601 or the patch image signal acquired from the patch signal generation unit 602 and outputs the selected signal to the laser driver 510 of the image forming unit 50. Specifically, the signal selector 603 selects and outputs a patch image signal when an image selection signal described later is acquired, and selects the image signal acquired from the dot counter 601 otherwise. Output.

  The potential controller 604 controls driving of the laser driver 510, the charger power supply 511, and the developing bias power supply 512. Specifically, the potential controller 604 receives the paper type signal from the UI unit 40 and the potential sensor 503 each time an image signal is supplied from the signal selector 603 to the image forming unit 50 to form a toner image. The surface potential signal and the temperature / humidity signal from the environmental sensor 514 are respectively acquired. Based on the acquired signals, the potential controller 604 determines the actual value and the ideal value of the exposure amount of the laser diode of the exposure unit 504, the charging potential of the charging unit 502, and the developing bias potential of the developing unit 505. Determine the deviation. Then, the potential controller 604 drives the laser driver 510, the charger power supply 511, and the development bias power supply 512 based on the control parameters that compensate for the deviation, respectively, the exposure amount control signal, the charge potential control signal, and the development. Outputs a bias control signal.

At this time, the potential controller 604 controls to control the amount of color material per unit area (hereinafter simply referred to as “toner weight”) in accordance with the type of recording paper specified in accordance with the paper type signal. Each part is driven with parameters.
The toner weight of toner transferred from the developing unit 505 to the surface of the photosensitive drum 501 during development is determined by the potential difference between the surface potential of the photosensitive drum 501 and the developing bias of the developing unit 505, and the electrostatic force decreases as the potential difference decreases. The toner weight is reduced. For example, if the exposure amount is reduced while the development bias and the charging potential are kept constant, the potential of the exposed area, that is, the electrostatic latent image area, increases, so the potential difference between the electrostatic latent image and the development bias decreases. Also, the toner weight is reduced. Further, even when the developing bias is reduced while the charging potential and the exposure amount are kept constant, the potential difference between the developing bias and the electrostatic latent image is reduced, and the toner weight is reduced. In addition, if the charging potential is increased while the exposure amount and the developing bias are kept constant, the potential of the electrostatic latent image also increases accordingly, so the potential difference between the electrostatic latent image and the developing bias decreases, and the toner weight is Get smaller. As described above, the toner weight of the developed image on the photosensitive drum 501 changes according to the exposure amount, the charging potential, and the developing bias. The potential controller 604 drives each unit based on each control parameter according to the gradation value included in the image data and the paper type signal so as to form a toner image with a target toner weight. These control parameters are stored in advance in a memory (not shown) of the potential controller 604 in association with identification information indicating the type of recording paper.
In addition, the potential controller 604 outputs an image selection signal to the signal selector 603 at a timing when the deviation between the actual value of the exposure amount, the charging potential, and the developing bias potential and the ideal value is equal to or greater than the threshold value. The case where the deviation becomes large is a case where the driving states of the laser driver 510, the charger power supply 511, and the developing bias power supply 512 are different from the ideal drive states.

  When the patch image formed on the recording paper by the image forming unit 50 is read by the image reading unit 20, the potential parameter determination unit 605 acquires a read signal indicating the reading result, and the control parameter determined based on the read signal Is output to the potential controller 604 in association with the paper type signal. The function and operation of the potential parameter determination unit 605 will be described in detail later.

  The development controller 606 has a function of feeding back the density of the toner image detected by the toner density sensor 508 to the toner supply amount of the developer 505 when a toner image corresponding to the patch image signal is formed. Specifically, the development controller 606 counts the dot count value supplied from the dot counter 601 and outputs an image selection signal to the signal selector 603 when the accumulated value exceeds a preset threshold value. . This threshold value is set based on the number of dots determined according to the number of output sheets (for example, 30 sheets) that requires calibration of the toner supply amount of the developing device 505. In other words, the image selection signal is output at a timing at which it is preferable that the image forming process is performed a predetermined number of times or more and the optimum toner supply amount is reset.

  When the signal selector 603 that has acquired the image selection signal outputs the patch image signal to the image forming unit 50, the image forming unit 50 sequentially causes the toner images of rectangular images of respective densities corresponding to the patch image signal to the photosensitive drum 501. It is formed. The density of the formed toner image is read by the toner density sensor 508 and supplied to the development controller 606 as a toner density signal. The development controller 606 calculates a deviation between the actual density value at the density detection point and the ideal density value stored in its own memory based on the toner density signals sequentially supplied from the toner density sensor 508, and the deviation is calculated. It is determined whether or not it falls within a preset allowable range. When it is determined that the allowable range has been exceeded, the development control controller 606 then generates a toner supply control signal that compensates for the deviation when forming a toner image according to the image signal input by the image input device 30. The amount of toner supplied to the developing device 505 and supplied from the toner cartridge to the developing device 505 is increased or decreased. For example, if the actual value of the density at the density detection point is below the lower limit of the allowable range, a toner supply control signal instructing to increase the toner supply amount is supplied, and if it exceeds the allowable range, the toner supply amount is decreased. A toner supply control signal instructing to do so is output.

  The transfer controller 607 supplies a primary transfer current control signal corresponding to the paper type signal to the primary transfer unit 506 and controls the primary transfer current flowing through the primary transfer unit 506. Here, the magnitude of the primary transfer current controlled by the transfer controller 607 is set in advance in association with identification information representing the type of recording paper. As a result, the toner weight of the toner transferred from the photosensitive drum 501 to the intermediate transfer belt 55 can be controlled. Specifically, the surface potential of the photosensitive drum 501 and the transfer potential of the primary transfer unit 506 are opposite in polarity, and the electrostatic force generated between them becomes smaller as the primary transfer current value becomes smaller. Toner weight is reduced (transfer rate is reduced). On the other hand, as the primary transfer current increases, these potential differences increase, and the toner weight of the toner transferred to the intermediate transfer belt 55 increases (transfer rate increases).

The fixing controller 608 outputs a fixing control signal for controlling the fixing temperature, fixing speed, and fixing pressure to the fixing device 57 based on the sheet type signal and the fixing roll temperature acquired from the temperature sensor 513. The control parameter corresponding to the fixing control signal is set in advance to increase the fixing temperature, decrease the fixing speed, or increase the fixing pressure, for example, for a recording sheet having a large thickness. The fixing controller 608 controls a control parameter for fixing conditions according to the toner weight, which will be described in detail later.
As described above, the process control unit 60 outputs the control parameters according to the paper type and the paper property to the image forming unit 50 to control the toner weight of the toner image formed on the recording paper.

(2) Types of Recording Paper Next, FIG. 3 is a diagram for explaining the structure of plain paper and coated paper and an image structure made of toner that is superimposed on these recording papers during development.
As shown in FIG. 3A, when a toner image is formed on plain paper, the image structure has a “fiber layer” composed of a large number of fine thread-like pulp fibers entangled with each other. It consists of a “toner layer” configured by superposing toner on a fiber layer. As can be seen from the figure, in the “fiber layer”, there are voids between the pulp fibers. Further, the pulp fiber is exposed on the recording surface of plain paper, and the surface roughness is very large. Here, the “surface roughness” refers to the centerline average roughness, and the size of plain paper is about 10 μm to 20 μm.

  On the other hand, as shown in FIG. 3B, when the toner image is formed on the coated paper, the image structure is composed of the “fiber layer” and the “coating layer” superimposed on both sides of the “fiber layer”. And a “toner layer” composed of toner superimposed on the “coating layer” on the upper surface. The “coating layer” is a layer formed by a coating material formed on the surface of plain paper (fiber layer). As this coating material, for example, a paint containing a pigment such as calcium carbonate or kaolin is used. It is done. This “coating layer” is provided to reduce the surface roughness of the surface of the recording paper, and has a higher gloss than that of plain paper. When a toner image is formed, a “toner layer” is formed by superimposing toner on the coating layer. The “coating layer” exposed on the surface of the coated paper is applied such that the surface roughness (centerline average roughness) is much smaller than that of plain paper, and the size is about 1 μm. ~ 3 μm.

(3) Relationship Between Recording Paper Type and Toner Image Density Next, the relationship between the recording paper type and the toner image density will be described.
FIG. 4 is a graph showing experimentally obtained relationships between the toner area ratio and the density of the toner image when the same toner image is formed on plain paper and coated paper. Hereinafter, this relationship is referred to as “gradation characteristics”.
As shown in FIG. 4, the density of the toner image formed on the coated paper (illustrated by a broken line) is almost equal to the density of the toner image formed on the plain paper (indicated by a broken line) in almost the entire area of 0% to 100% area ratio. It can be seen that it is higher than that shown in the solid line). For example, attention is paid to the density of each toner image having an area ratio of 100%. The density (target density) Dmax1 of the toner image formed on the plain paper is approximately “1.75” even though the toner weights when the toner images are formed on these recording papers are substantially the same. On the other hand, the density Dmax2 of the toner image formed on the coated paper is about “2.0”, and it can be seen that the density of the coated paper is about “0.25”. Further, referring to the gradation characteristics of these recording papers, the plain paper shows a substantially linear gradation characteristic, while the coated paper has a linear density with respect to an increase in the toner area ratio. It ’s not really big. Specifically, the density change amount with respect to the toner area ratio is large in the low density region, and the density change amount with respect to the toner area ratio tends to be small in the high density region. As described above, when the gradation characteristics differ depending on the type of recording paper, the color reproducibility differs depending on the recording paper on which the toner image is formed.

The reason for this will be described with reference to FIG.
When the “toner layer” superimposed on the plain paper of the image structure shown in FIG. 3A is viewed microscopically, although a uniform amount of toner has been transferred to each position on the paper surface. In other words, the toner amount distribution varies greatly depending on each angular position. This is because the toner enters the gaps between the pulp fibers of the fiber layer and is absorbed by the fiber layer as shown in FIG. Accordingly, the amount of toner visible to humans is reduced, and the density of the toner image is reduced. Further, the surface of the toner layer is uneven due to uneven distribution of the toner amount on the plain paper.

  On the other hand, as shown in FIG. 3B, in the coated paper, the toner is superimposed on the surface of the coated layer with almost no toner absorbed. Therefore, the toner layer is composed of a uniform amount of toner at each position on the smooth coating layer. That is, the thickness of the toner layer is uniform at each position, and the surface of the toner image is also flat.

  As described above, when toner images are formed on plain paper and coated paper with the same toner weight, the density of plain paper is significantly lower than that of coated paper. Furthermore, since the surface of the “toner layer” formed on plain paper is not flat, when the toner layer is irradiated with light, the intensity of the specular reflection component of the reflected light is small and the intensity of the irregular reflection component is small. growing. In this case, according to the equation (1), since I / Io is reduced, the density D of the toner image is further reduced. Therefore, when the image forming apparatus 1 attempts to form a toner image on coated paper, the control parameters optimally set so that the toner image can be formed on the plain paper at a target density are used as they are. The density of the toner image on the paper becomes considerably high.

For this reason, the density of the toner image varies depending on the type of recording paper. To correct this, a look-up table corresponding to the type of recording paper is applied to the gradation value of the image signal. A method of correcting the gradation characteristics is known.
According to this method, the density Dmax2 of the toner image having a toner area ratio of 100% on the coated paper is brought close to the density of the toner image having the toner area ratio of 100% on the plain paper (hereinafter referred to as “target density”) Dmax1. As described above, the gradation characteristics can be corrected. However, in this correction by image processing, as indicated by an arrow R1 in FIG. 4, the target density Dmax1 represented by an area ratio of 100% on plain paper is expressed by an area ratio of about 50% on coated paper. become. In this case, although the density of the toner image can be matched between the respective recording sheets, the texture is different due to the difference in the toner area ratio. For example, when a toner image that represents a character represented by a solid image with an area ratio of 100% on plain paper is expressed in a halftone such as a line or halftone dot on a coated paper, This results in an uncomfortable feeling, which results in image quality degradation. Furthermore, when characters and fine lines with a density equivalent to 100% of the area ratio on plain paper are reproduced, the density of the toner image matches that of plain paper, but there is an image defect that the characters and fine lines are interrupted. Sometimes.

For example, when the density of the toner image is expressed by 256 gradations (8 bits), the density corresponding to the area ratio is expressed in the range of 0% to 100% in the plain paper. . On the other hand, in the coated paper, each density is expressed with the maximum density as the target density Dmax1, and as can be seen from FIG. 4, each density is approximately in accordance with the area ratio within the range of 0% to 50% area ratio. Will be expressed. That is, when a toner image is formed on plain paper, the density of the toner image can be expressed only with an area ratio in the range of about 50% compared to the case of coated paper, and therefore the number of density gradations is about 50. % (The number of gradations is about “128”). This decrease in the number of gradations causes a tone jump in which the change in density of the toner image with respect to the change in area ratio increases. Due to this tone jump, for example, a smooth color change in a gradation image cannot be expressed, a level difference occurs, and an image defect such as a pseudo contour may occur.
As described above, when the conventional technique is used to correct the gradation characteristics so as to match, the coated paper having the coating layer deteriorates the image quality or causes image defects.

(4) Density Correction by Controlling Toner Weight In order to correct the density of the toner image so as not to cause image defects and image quality degradation as described above, the image forming apparatus 1 uses control parameters according to the type of recording paper. Process control is performed to control the toner weight when forming a toner image.
Here, FIG. 5 shows a change in toner image density with respect to a change in toner weight during image formation based on image data representing an image with a 100% area ratio (so-called solid image) formed on plain paper and coated paper. It is the graph showing what calculated | required the relationship between and experimentally.
As shown in FIG. 5, when comparing the density of the toner images formed on the plain paper and the coated paper within the range of the illustrated toner weight, the coated paper is about 20% higher than the plain paper. I understand that. Therefore, if the toner weight of the toner image formed on the coated paper is reduced according to the characteristics shown in the figure, the difference in density between the coated paper and the plain paper can be almost eliminated, and the difference in the respective gradation characteristics is also reduced. be able to. Specifically, as shown in the figure, when forming a toner image with a target density Dmax1 (= 1.75), if the toner weight of the toner with respect to plain paper is M1≈4.8 g / m ² , the area ratio The target density Dmax1 is reached at 100%. On the other hand, as shown by the arrow R2, in the coated paper, if the toner weight is reduced to M2≈3.7 g / m ² , the density can be made to coincide with the target density Dmax1 at an area ratio of 100%.

  In addition, the optimum toner weight of the toner used for the coated paper varies depending on the color of the toner, the color of the recording paper, the fixing conditions that determine the glossiness, and the surface roughness of the recording paper. It has been found that even if the toner weight used for paper is reduced by about 5% to 40%, almost the same color can be reproduced. In the electrophotographic image forming apparatus, whether to use plain paper or coated paper, the target color reproduction of so-called offset printing (Japan color, etc.) is targeted. The same. Therefore, by reducing the toner weight of the toner when using coated paper compared to when using plain paper, the toner image can be brought closer to the target density without causing image defects such as tone jumps or image quality degradation. The density can be corrected.

Therefore, when the optimum toner weight is determined according to the relationship between the toner weight and the density according to the type of recording paper as shown in FIG. 5, the process control unit 60 reduces the toner weight according to the determined toner weight. Further, by controlling the control parameters and correcting the density of the toner image, the tone characteristics can be made to coincide on each recording sheet. In order to realize the optimum toner weight, control parameters for the exposure amount, the charging potential, the developing bias, and the primary transfer current are stored in the process control unit 60 for each type of recording paper.
Therefore, at the time of image formation, when the selector 12 outputs the image signal acquired from the image input device 30 to the image forming unit 50, while the use of the coated paper is detected by the process control unit 60, it is normal. Each control parameter is determined so that a toner image is formed so that the toner weight can be reduced as compared to when using paper. However, in this case, each control parameter is determined so that the difference between the density of the image formed on the coated paper and the density of the image formed on the plain paper becomes small.

  FIG. 6 is a graph showing experimentally obtained gradation characteristics of plain paper and coated paper when the toner weight of plain paper is M1 and the toner weight of coated paper is reduced from M1 to M2. . In FIG. 6, a curved line P1 indicated by a broken line is a gradation characteristic when a toner image is formed on the recording paper while the toner weight M1 is maintained (before density correction) when the coated paper is used. A curve P2 indicated by a one-dot chain line is a gradation characteristic when the toner image is formed on the recording paper by reducing the toner weight M2 (after correcting the density) when the coated paper is used. A curve P3 indicated by a solid line is a gradation characteristic when a toner image is formed on a recording sheet with a toner weight M1 when plain paper is used. As can be seen from the figure, the gradation characteristics after correction of the toner weight illustrated by the curve P2 substantially match the gradation characteristics of the plain paper illustrated by the solid line P3, and these differences are illustrated by the curve P1. It is smaller than the difference from the gradation characteristics before correction in the coated paper. In this way, by correcting the density Dmax2 of the coated paper so that it substantially matches the target density Dmax1, the image forming apparatus 1 can detect the toner image formed on the plain paper and the toner image formed on the coated paper. The toner image can be formed so that there is no difference in the texture of the toner.

FIG. 7 is a graph showing the measurement result of the relationship between the lightness L * and the glossiness of the toner image formed on the coated paper. In the figure, the solid line graph shows the relationship when a toner image is formed as the toner weight M1 of the toner used on plain paper (before density correction), and the broken line graph is described above with reference to FIG. Thus, the relationship when the toner weight is reduced to M2 (after density correction) is shown.
As can be seen from FIG. 7, the lower the lightness L *, the more the reduction in the glossiness of the toner image due to the reduction in the toner weight, while the lower the lightness L *, the lower the glossiness. Due to such a phenomenon, when the toner image is formed with the toner weight M2 being reduced, the difference between the maximum value and the minimum value of the glossiness of the toner image when a toner image with low to high brightness is formed is large. The “gloss difference” indicating the thickness becomes smaller. Specifically, when the toner weight is M1, the maximum glossiness value is approximately “65”, the minimum value is approximately “35”, and the glossiness difference is approximately “30”. On the other hand, when the toner weight is reduced to M2, the maximum value of the glossiness is about “48”, the minimum value is about “35”, and the difference in glossiness is about “13”. That is, in this example, the difference in glossiness is reduced to nearly one third. In general, in a toner image, the smaller the amount of change in glossiness with respect to the change in brightness (that is, the smaller the difference in glossiness), the less likely it is to feel uncomfortable when a human sees the image. Therefore, when using coated paper, it can be said that reducing the toner weight compared to using plain paper contributes to improving the image quality when using coated paper from the viewpoint of glossiness. it can.

Further, reducing the toner weight means that transfer defects such as toner scattering (so-called blur) and transfer omission that are conspicuous on coated paper, fogging that the toner adheres to an area corresponding to the background, development on the surface of the photosensitive drum 501, and so on. It is also possible to improve development defects such as an edge effect caused by a potential difference between the developed area and the undeveloped area. Further, it is possible to improve fixing defects such as blisters in which bubbles are generated on the paper surface and offsets in which toner is left behind in the fixing device 57 and the like.
In addition to these, the image forming apparatus 1 has an effect of realizing energy saving when fixing a toner image. Specifically, when the toner weight is reduced, the fixing control controller 608 of the process control unit 60 specifies this according to the paper type signal, and the fixing of the fixing device 57 according to the amount by which the toner weight is reduced. Control speed, fusing temperature or fusing pressure. Specifically, the greater the amount of toner reduced, that is, the smaller the toner weight of the toner that forms the toner image, the smaller the heat energy required for fixing, so the fixing speed of the fixing device 57 is increased. Or the fixing temperature can be lowered, or the fixing pressure can be reduced. Thereby, the toner image can be reliably fixed.

(5) Setting of control parameters In the above description, plain paper and coated paper have been described as examples. However, other types of recording paper that do not have control parameters set in the image forming apparatus 1 are used. In some cases. As described above, the optimum toner weight is determined by conditions such as the surface roughness, thickness, color (whiteness), and fixing conditions of the recording paper. Therefore, the process control unit 60 performs recording to form a toner image. It is necessary to set a control parameter for each paper type. Also for plain paper or coated paper for which control parameters are set, the density of the toner image when the toner area ratio is 100% is made higher or lower than the target density Dmax1 according to the user's request. There is also a case. In such a case, it is necessary to add or update control parameters used by the process control unit 60. In order to meet such a demand, the image forming apparatus 1 has the functions described below.

Here, FIG. 8 is a diagram for explaining the relationship between the test image and the control parameters. The test images T1, T2,... T9 are square images each represented by an area ratio of 100%.
For example, when the user uses the UI unit 40 to instruct setting of control parameters for a type of recording paper that is accommodated in the paper feed tray 51c and for which no control parameters are set, the selector 12 causes the test image signal generation circuit 11 to operate. The test image signal is acquired from the image data and supplied to the image forming unit 50. The image forming unit 50 forms an image corresponding to the acquired test image signal on the recording paper stored in the paper feed tray 51c. Here, test images T1 to T9 surrounded by broken lines in FIG. 8 are sequentially formed in the sub-scanning direction. Further, during the formation of these test images T1 to T9, the image forming unit 50 changes the exposure amount in steps as described on the right side of each test image, thereby generating an electrostatic latent image. Form. The exposure amount at this time represents the amount of light with respect to this exposure amount, assuming that the exposure amount when forming a toner image on plain paper is 100%.

  Subsequently, the user sets the recording paper on which the test images T <b> 1 to T <b> 9 are formed in the image reading unit 20, and instructs the reading through the UI unit 40. In response to this, the image reading unit 20 reads the recording sheet, generates an image signal (reading signal) indicating the reading result, and outputs the image signal to the potential parameter determination unit 605 of the process control unit 60. This read signal represents each gradation value of R, G, and B.

The potential parameter determination unit 605 performs rasterization processing and color space conversion processing on the read signal represented in the RGB color space, converts the read signal into a raster format image signal represented in the CMYK color space, and converts the read signal into a gradation value. Accordingly, the density of each color of C, M, Y, and K is obtained. The potential parameter determination unit 605 outputs the potential parameter determined based on the density of each color of C, M, Y, and K to the potential controller 604 in association with the paper type signal. The “potential parameter” here represents the exposure amount. For example, the densities of the test images T1 to T9 obtained from the test image reading result by the image reading unit 20 are “2.0”, “1.95”, and “1.90”, respectively, as shown in FIG. ”,“ 1.80 ”,“ 1.70 ”,“ 1.60 ”,“ 1.50 ”,“ 1.40 ”,“ 1.30 ”. Here, when the density of the toner image at the area ratio of 100% is to be set to “1.70”, the exposure amount may be set to “80%” as shown in the figure, so that the user indicates that. Then, the potential parameter determination unit 605 associates the paper type signal with the control parameter indicating that the exposure amount is set to “80%”, and outputs it to the potential controller 604. The potential controller 604 stores recording sheet identification information corresponding to the acquired sheet type signal and the potential parameter in association with each other. Thereafter, when forming a toner image on a recording sheet corresponding to the sheet type signal, the potential controller 604 sets a control parameter based on the potential parameter.
In this way, when the control parameter is not set, for example, when using a sheet-like material made of plastic such as thick paper or an OHP (OverHead Projector) sheet, or when changing the target density in various ways, it is optimal. By setting control parameters, the density of the toner image can be corrected so as to form the toner image with a target gradation characteristic.

  According to the embodiment described above, the toner image is formed by setting the control parameter that determines the driving state of the image forming unit 50 so as to reduce the toner weight to the optimum amount according to the type of recording paper. The density can be corrected so that the densities of the toner images formed on various types of recording paper are matched without causing image degradation or image defects.

(6) Modifications The above embodiment may be modified as follows. Specifically, the following modifications are mentioned, for example. These modifications can be appropriately combined with each other.
In the above-described embodiment, the process control unit 60 controls the control parameters for all of the exposure amount, the charging potential, the developing bias, and the transfer potential according to the type of the recording paper. However, at least one of these parameters is controlled. By controlling one of them, it is possible to control (reduce) the weight of toner transferred to the recording paper. Any control parameter may be controlled as long as the toner weight can be reduced so as to reduce the difference in gradation characteristics when forming a toner image on each recording sheet. For example, even if only the primary transfer current is changed, for example, an amount of toner corresponding to the current value is transferred to the intermediate transfer belt 55, so that the toner weight of the toner superimposed on the recording paper can be changed.

In the above-described embodiment, the potential parameter determining unit 605 determines the exposure amount control parameter so that the density at the area ratio of 100% matches the target density Dmax1 according to the test image reading result. A potential and a developing bias may be set. Since the toner weight changes depending on the potential difference between the surface potential of the photosensitive drum 501 and the developing bias, the process control unit 60 may control the charging potential or the developing bias instead of the exposure amount. Further, the process control unit 60 may be configured to control a control parameter for the transfer control controller 607 to optimally control the transfer rate.
However, in order to control the toner weight, it is most effective to control the exposure amount among the above control parameters. Therefore, in the embodiment, the exposure amount is corrected according to the result of reading the test image.

In the embodiment described above, whether the recording paper on which the toner image is to be formed is coated paper or plain paper is detected by a user operation on the UI unit 40, but the type of the recording paper is specified by other methods. May be. For example, invisible information (yellow dots or the like) corresponding to the type of each recording sheet is embedded, and the recording sheet stored in the paper feed tray 51 or the recording sheet conveyed through the sheet conveyance path 600 can be read. A sensor is provided. At the time of image formation, the sensor may read this type information, and each unit may detect the type of the recording paper according to the read result. In addition, when the image input device 30 receives image data from an external device, a paper type signal may also be acquired, thereby detecting the paper type. When a toner image is formed on a recording sheet of a type designated by the user of the external device, an instruction for designating the type of paper can also be received.
The image reading unit 20 generates an RGB format image signal. However, the image reading unit 20 only needs to be able to detect the densities of the test images T <b> 1 to T <b> 9, and thus has the same configuration as the toner density sensor 508 instead of the image reading unit 20. An apparatus may be employed. In addition, a colorimeter that obtains colorimetric values in the CIELAB color space may be employed instead of the image reading unit 20. In this case, the potential parameter determination unit 605 converts from the CIELAB color space to the CMYK color space. Of course, as long as it can be converted into a color space corresponding to the color of the toner, the reading result of the test image may be expressed in a color space other than this.

  In the embodiment described above, when forming an image on coated paper, the image forming apparatus 1 reduces the toner weight determined based on the use of plain paper, thereby reducing the density difference between the images formed on the respective paper. The toner image density was corrected so as to be small. Such a method is effective because the surface roughness of the surface of the coated paper becomes smaller than the surface roughness of the plain paper by providing the coating layer. Therefore, as the coating material used for the coating layer, materials other than those described above can be used as long as a recording surface smoother than the recording surface of plain paper can be obtained. For example, various pigments, various resins, rubber latex or polymer materials.

1 is a diagram illustrating a structure of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a detailed configuration of an image forming unit and a process control unit and a configuration around the configuration. FIG. 3 is a diagram illustrating the structure of plain paper and coated paper and an image structure made of toner that is superimposed on these recording papers during development. 5 is a graph showing experimentally obtained relationships (tone characteristics) between the area ratio of toner on plain paper and coated paper and the density of the toner image. 6 is a graph showing an experimentally obtained relationship between a change in toner image density and a change in toner weight during image formation based on image data representing an image with a toner area ratio of 100%. It is a graph showing the experimentally obtained gradation characteristics of plain paper and coated paper It is a graph showing the measurement result of the relationship between the brightness and glossiness of the toner image formed on the coated paper. It is a figure explaining the relationship between a test image and a control parameter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Control part, 11 ... Test image signal generation circuit, 12 ... Selector, 20 ... Image reading part, 30 ... Image input device, 40 ... UI part, 50 ... Image forming part, 501 ... Photosensitive Body drum 502 ... charger 503 ... potential sensor 504 ... exposure device 505 ... developing device 506 ... developing device 508 ... toner density sensor 509 ... cleaner 51, 51a, 51b, 51c ... feed tray 510 ... Laser driver, 511 ... Charger power supply, 512 ... Development bias power supply, 513 ... Temperature sensor, 514 ... Environmental sensor, 52 ... Image forming engine, 55 ... Intermediate transfer belt, 56 ... Secondary transfer device, 57 ... Fixer , 600... Paper transport path, 601... Dot counter, 602. Patch signal generation unit, 603... Signal selector, 604. Over data determining section, 606 ... developing controller, 607 ... transfer controller, 608 ... fixing controller, 60 ... process control unit.

Claims (2)

In accordance with a control parameter that determines the driving state of the self-image forming unit, the image holding member is charged, and an electrostatic latent image is formed by irradiating the charged image holding member with light according to image data. Image forming means for developing the electrostatic latent image with a developer, transferring the developed image onto a recording surface of a recording medium, and fixing the image;
Detecting means for detecting whether the recording medium to which the image is transferred is a recording medium having a coating layer on the recording surface or a recording medium having no coating layer;
When an image corresponding to image data is formed on the recording surface of the recording medium that is detected by the detection means as having the coating layer, the recording surface is provided with the coating layer. The control parameter is determined so as to reduce the difference between the density of the image to be formed and the density of the image when the image is formed on a recording surface where the coating layer is not provided. Control means for reducing the amount of the color material transferred to the recording surface provided with the layer and causing the image forming means to form the image;
When the type of the recording medium is designated and the setting of the control parameter for the recording medium is instructed, a plurality of test images having a specific area ratio are made different from each other in the control parameter, and the image forming unit Test image formation control means for forming on the recording surface of the type of recording medium by
The control means includes
The density obtained by reading the plurality of test images is obtained, and the control parameter for forming the area ratio image at the target density on the recording medium of the type is stored based on the obtained densities of the plurality of test images. When an image corresponding to image data is formed on the recording surface of the recording medium, the stored control parameter corresponding to the type of the recording medium is determined, and the image is formed on the image forming unit. An image forming apparatus. The control means includes
The image forming apparatus according to claim 1, wherein the common target density is used regardless of the type of the recording medium.

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