JP2006047789A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus always maintaining a high quality image formation by suppressing variation in hue even when a change in a state of the image forming apparatus such as an image carrier occurs in the image forming apparatus with at least one set of combination of developing apparatuses containing developers of same hue in different lightness with respect to a plurality of developing apparatuses. <P>SOLUTION: A developed image for image control is formed on the image carrier, the image density is detected by a density detecting means and image density control of a formed image is carried out according to the detected result in the image forming apparatus. The frequency of execution of image density control or the number of developer images for image control by which the image density control is executed differ for an image developed by the developing apparatus with high lightness in the contained developer and for the image developed by the developing apparatus with low lightness in the combination of the developing apparatuses containing the developers of the same hue in different lightness. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus having a plurality of developing units, and including a combination of developing units containing developers having the same hue and different brightness.

  2. Description of the Related Art Conventionally, particularly in an electrophotographic image forming apparatus, the level of needs has increased with recent advances, and the number of developer colors is increased compared to conventional image forming apparatuses using four color developers. The proposal has been partially realized. In addition to the conventional four colors of cyan, magenta, yellow and black, those corresponding to special colors such as red, blue, green, gold, silver and fluorescent colors have been proposed. Various things have been proposed, such as those with general light cyan (light cyan), light magenta (light magenta), and the like. The purpose of adding the number of colors is mainly to provide images differentiated in terms of image quality.

  In particular, a system that adds a developer containing a light color toner (light color toner) such as light cyan or light magenta as a developer generally aims at reducing graininess and appeals for photographic high image quality. Used as an image forming apparatus.

  For example, a multi-color image forming apparatus using a conventional developer of six colors, which is a combination of four colors of cyan, magenta, yellow, and black (dark toner) plus two colors of light cyan and light magenta (light toner). In general, a tandem type as shown in FIG. 11 or a one-image carrier type as shown in FIG. 2 can be considered.

  In the tandem type image forming apparatus shown in FIG. 11, each of the six image carriers 1 is associated with a developing device 4 loaded with a developer having different spectral characteristics for each image carrier 1, and these are arranged in series. Is. This configuration is a type that emphasizes productivity because the output speed of images can be made the same even when considered based on an image device of four colors.

  On the other hand, by associating six developing devices 41 to 46 with one image carrier 1 as shown in FIG. 2 and rotating the rotary unit 4a on which these developing devices 41 to 46 are mounted, There is an image forming apparatus that performs development while switching the developing devices 41 to 46 facing the image carrier 1.

  The image forming apparatus shown in FIGS. 2 and 11 employs an intermediate transfer method, and performs primary transfer to the intermediate transfer member 5a for each color, performs multiple transfer on the intermediate transfer member 5a, and transfers all six colors. At the stage when it is finished, it is configured to perform secondary transfer onto the transfer material. In this way, it is possible to output a six-color image while minimizing space.

  Here, the color and image quality of the developer will be described.

  Cyan, magenta, and yellow, which are the colors of the color developer used here, are generally said to be the three primary colors in the case of subtractive color mixing, and when these three colors are added, they become gray (achromatic color). There is a nature. All colors are basically expressed by changing the balance of cyan, magenta and yellow.

  Therefore, various ideas have been made to stably reproduce each color so that the balance of these three primary colors is not lost. For example, as shown in Patent Document 1, image formation is performed based on image densities detected for a plurality of image control developer images (patches) formed by each developer formed on an image carrier. By periodically executing image density control (calibration) for converting image forming data as a condition, and further adopting an image control method for controlling the conversion characteristics of the image forming data used for this calibration, More stable color reproduction is possible.

  However, particularly in an image forming apparatus having a plurality of color developing devices 41 to 46 for one image carrier 1 as shown in FIG. 2, the potential of the electrostatic latent image decreases due to the sensitivity fluctuation of the image carrier. , Development contrast may increase. In that case, the rate of change in the amount of toner developed by each of the light cyan, light magenta, and yellow developing devices is almost the same, but since the density of the toner is different, the three colors of light cyan, light magenta, and yellow are used. The light gray formed by mixing colors has a problem that the gray balance changes in the yellow direction because yellow is darker than the other two colors.

  On the other hand, when the development contrast is reduced by changing the sensitivity of the image carrier, the gray balance changes to the blue side.

  That is, if the saturation balance of two or more developers constituting secondary colors having the same hue and different densities, such as cyan and light cyan and magenta and light magenta, is different, the image forming condition varies. However, there is a problem that a hue change is likely to occur.

This phenomenon is caused by executing image control as shown in Patent Document 1, and the density of each color is controlled at a relatively early timing and the hue change is subtracted, but this hue change appears for a while after the potential change. .
JP 2003-228201 A

  An object of the present invention is to change the state of an image forming unit such as an image carrier in an image forming apparatus having at least one combination of developers having different hues with the same hue among a plurality of developers. It is an object of the present invention to provide an image forming apparatus that suppresses a change in hue and always maintains high-quality image formation even if the occurrence of the image occurs.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image carrier on which an electrostatic latent image is formed on a surface, a plurality of developing units that develop the electrostatic latent image to form a developer image, and an image of the developer image. Density detecting means for detecting density, and at least two of the plurality of developing devices are a set of combinations of developing devices in which developers having the same hue and different brightness are accommodated, An image forming apparatus in which the developer image is transferred to a transfer material,
The developer image for image control is formed by the developing device, the image density of the developer image for image control is detected by the density detecting means, and the image density of the formed image is controlled according to the detection result. In the forming device,
In the combination of the developing devices in which developers having the same hue and different lightness values are stored, an image forming operation performed by a developing device having a high lightness value of the stored developer and an image forming operation performed by a low developing device. In contrast, the present invention provides an image forming apparatus characterized in that the frequency at which the image density control is performed or the number of developer images for image control on which the image density control is executed is different.

  The image forming apparatus of the present invention has a plurality of developing units, and includes a combination of developing units that store developers having the same hue and different brightness levels. For image forming operations performed in a developing device and for image forming operations performed in a low developing device, the frequency of performing image density control, or the developer for image control in which image density control is executed Since the number of images is different, even if the state of the image forming unit such as the image carrier changes, it is possible to suppress the hue change and always maintain high-quality image formation.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
In this embodiment, a color using a rotary 4a mounted with one image carrier 1 and a plurality of developing units 4 (developing units 41 to 46) provided around the image carrier 1 shown in FIG. The present invention is applied to an image forming apparatus.

  This embodiment has a digital color image reader unit 101 at the top and a digital color image printer unit 102 at the bottom. The reader unit 101 reads image information from the outside, converts it into an image signal, and transmits it to the printer unit 102. The printer unit 102 controls each image forming unit based on this image information, and a desired image is obtained.

  In the reader unit 101, a document 30 as external information is placed on a document table glass 31 and exposed and scanned by an exposure lamp 32, whereby a reflected light image from the document 30 is condensed on a full-color CCD sensor 34 by a lens 33. A color separation image signal is obtained. The color-separated image signal is subjected to image processing by a video processing unit (not shown) through an amplification circuit (not shown) and sent to the printer unit 102 via an image memory (not shown).

  In addition to the signal from the reader unit 101, an image signal from a computer, an image signal from a FAX, and the like are similarly sent to the printer unit 102.

  Based on a signal from the reader unit 101, an image forming operation is performed in the printer unit 102. In the printer unit 102, the photosensitive drum 1 as an image carrier is rotatably supported in the direction of the arrow in the figure, and around the photosensitive drum 1, a pre-exposure lamp 11 as an image forming unit, a corona primary charger 2, a laser. The exposure optical system 3, the potential sensor 12, the rotary developing device holding unit (rotary) 4a, and the six developing devices 4 (41 to 46) mounted on the holding unit 4a and loaded with toner having different spectral characteristics. A transfer device 5 and a cleaning device 6 are arranged. In this specification, the developing devices 41 to 46 are collectively referred to as the developing device 4.

  The developing device 4 contains a cyan developing device 41 containing dark cyan toner as a developer, a magenta developing device 42 containing dark magenta toner, a yellow developing device 43 containing yellow toner, and black toner. There are a black developing device 44, a light cyan developing device 45 containing light cyan toner, and a light magenta developing device 46 containing light magenta toner.

  That is, the image forming apparatus according to the present exemplary embodiment has a combination of magenta and cyan having the same hue and different lightness, and low-lightness toner (dark toner), dark magenta and dark cyan, and high-lightness toner ( As the light toner, there are two types, dark toner developing devices 41 and 42 that store light magenta and light cyan, and light toner developing devices 45 and 46, respectively. Here, the yellow developing device 43 and the black toner developing device 44 having the same hue and not having different lightness are also dark toner developing devices.

  Note that developers having the same hue and different lightness are usually toners having the same spectral characteristics and different amounts of color components (pigments) contained in a toner based on resin and color components (pigments). Say. The toner having a high lightness means a toner having a relatively low density among a combination of toners having the same hue and different lightness.

  The same hue means that the color developing components (pigments) have the same spectral characteristics as described above, but generally they are not exactly the same, such as magenta, cyan, yellow, black, etc. In addition, a range that can be called the same color in terms of the normal color concept.

In the present invention, a toner having the same hue and high brightness has an optical density after fixing of less than 1.0 per 0.5 mg / cm ² of toner on the transfer material. When the toner amount is 0.5 mg / cm ² , the optical density after fixing is 1.0 or more.

In this embodiment, dark cyan, dark magenta, yellow, and black, which are dark toners, have an optical density of 1.6 after fixing when the amount of toner applied on the transfer material is 0.5 mg / cm ^2. The amount of pigment is adjusted so that In addition, light cyan and light magenta, which are light color toners, are designed such that the applied density is 0.5 mg / cm ² and the optical density after fixing is 0.9. The two gradations of toner are mixed well to reproduce the gradation of each color toner.

  Further, the developer contained in each developing device 4 in this embodiment is a two-component developer used by mixing toner and carrier, but there is no problem even if the toner is a one-component developer. In addition, as the toner types, combinations with different brightness are provided for two colors of cyan and magenta. However, the present invention is not limited to these two types, and the brightness is different for all colors for cyan only, magenta only, yellow only, and the like. It does not matter if a combination is provided. In other words, any hue may have a combination with different brightness. Accordingly, the number of the developing devices 4 is six in this embodiment, but is not limited thereto.

  The image forming operation in the printer unit 102 will be described. First, the photosensitive drum 1 is rotated in the direction of the arrow, and the surface of the photosensitive drum 1 after being neutralized by the pre-exposure lamp 11 is uniformly charged by the primary charger 2 (charging process). ). In the laser exposure optical system 3, the image signal from the reader unit 102 is converted into an optical signal by a laser output unit (not shown), and the laser beam converted into the optical signal is reflected by the polygon mirror 3a, and the lens 3b and each The light is projected onto the surface of the photosensitive drum 1 through the reflection mirror 3c. Thus, each separation color is exposed by the laser exposure optical system 3 to form an electrostatic latent image on the photosensitive drum 1 (latent image forming step). Next, the rotary 4a is rotated, the predetermined developing device 4 is moved to the developing position on the photosensitive drum 1, the developing device 4 is operated, and the latent image on the photosensitive drum 1 is developed. A developer image (toner image) based on a resin and a pigment is formed (development process).

  Further, as shown in FIG. 2, the toner in the developing device 4 is a toner ratio in each developing device 4 from toner accommodating portions (hoppers) 61 to 66 for each color arranged between and next to the laser exposure optical system 3. (Or toner amount) is replenished as needed at a desired timing so as to keep constant.

  Note that a photo sensor 100 as density detecting means is disposed on the photosensitive drum 1 so that the amount of toner on the photosensitive drum 1 can be detected.

  The toner image formed on the photosensitive drum 1 is primarily transferred by the transfer device 5 to an intermediate transfer member (intermediate transfer belt) 5a constituting the toner image (primary transfer step). The toner remaining after the primary transfer process is removed by the cleaning device 6 disposed around the photosensitive drum 1, the rotary 4 a rotates, and the developing device 4 containing the toner of the next color is moved to a portion facing the photosensitive drum 1. The above-described charging process to primary transfer process are executed again, and the toner image of the next color is transferred on the toner image previously transferred onto the intermediate transfer belt 5a. Incidentally, here, the transfer device 5 is constituted by the intermediate transfer belt 5a, its winding roller, and members around it, and the primary transfer process from the photosensitive drum 1 and the secondary transfer process to the transfer material are performed. ing.

  Then, the charging process to the primary transfer process are performed for each color, and the respective toner images are sequentially superimposed on the intermediate transfer belt 5a. The intermediate transfer belt 5a constituting the transfer device 5 is driven by a driving roller 51, and the transfer cleaning device 50 is configured to be able to contact and separate from the driving roller 51 at a position facing the intermediate transfer belt 5a.

  Further, in the transfer device 5, a detection sensor 53 that detects the positional deviation and density of the image transferred from each photosensitive drum 1 is disposed opposite to the driven roller 52. Depending on the detection result of the detection sensor 53. An image forming apparatus main body control unit (not shown) corrects image density, toner replenishment amount, image writing timing, image writing start position, and the like as needed.

  The transfer cleaning device 50 cleans the residual toner on the intermediate transfer belt 5a after being transferred to the transfer material by being pressed by the driving roller 51 after the necessary color has been superimposed on the intermediate transfer belt 5a.

  On the other hand, the transfer material is conveyed one by one from the storage units 71, 72, 73, and 74 by the paper feeding units 81, 82, 83, and 84, and the skew is corrected by the registration rollers 85. At a desired timing, the toner image on the intermediate transfer belt 5a is conveyed to the secondary transfer portion 54 that transfers it to a transfer material.

  The toner image is transferred onto the transfer material by the secondary transfer unit 54 (secondary transfer process), and the transfer material passes through the conveyance unit 86 and is fixed by the heat roller fixing device 9 (fixing process). It is discharged as an image formed product to a paper tray or a post-processing apparatus.

  On the other hand, the intermediate transfer belt 5a after the secondary transfer is cleaned with the transfer residual toner by the transfer cleaning device 50 as described above, and again used for the primary transfer process of the toner images of the respective colors.

  When images are formed on both sides of the transfer material, the conveyance path guide 91 is driven immediately after the transfer material passes through the fixing device 9, and the transfer material is once guided to the reverse path 76 via the conveyance path 75. Due to the reverse rotation of the reversing roller 87, the rear end of the feeding roller 87 is used as the head, and the sheet is withdrawn in the direction opposite to the feeding direction and sent to the duplex conveyance path 77. After that, the paper passes through the double-sided conveyance path 77, the skew feeding correction and timing are performed by the double-sided conveyance roller 88, and conveyed to the registration roller 85 at a desired timing, and the image is again formed on the other surface by the image forming process described above. Transcript.

  A block diagram of the image processing unit 200 of the image forming apparatus is shown in FIG. An image signal sent from the outside, read from the reader unit 102 or on a network (not shown), is transmitted to the image processing unit 200 as the input signal 200a. The image processing unit 200 may be provided in the reader unit 101 or the printer unit 102. This input signal is color-separated into six colors of dark cyan, dark magenta, yellow, light cyan, light magenta, and black by the direct mapping color conversion processing unit 201, and the image data is converted into image density by the gamma conversion processing unit 202. Corresponding signal conversion is performed, and halftone processing is performed by the halftone processing unit 203, and then the laser unit 204 that drives the laser optical system 3 that is a latent image forming unit of the printer unit 102 is passed through the printer unit 102. Then, image exposure is performed and image formation is started.

Here, the so-called gamma characteristic in which the development contrast potential (difference between the latent image potential and the DC component value of the development bias voltage) in the image forming apparatus is taken on the horizontal axis and the image density outputted on the vertical axis is shown in FIG. As shown in the graph, the graph has a gentle curve. Note that light cyan and light magenta, which are light color toners, have low image density even at the same development contrast. Specifically, as described above, the dark toners of dark cyan, dark magenta, and yellow are adjusted so that the toner amount is 0.5 mg / cm ² and the density is 1.6. Further, light cyan and light magenta, which are light color toners, are adjusted so that the toner amount is 0.5 mg / cm ² and the density is 0.9.

  This gamma characteristic is likely to change due to various factors, thereby causing a change in color and tending to impair the stability of the image. As an example, assuming that the development bias voltage with respect to the image density fluctuates by 20V and the development contrast potential increases by 20V, as shown in FIG. Shift to. In FIG. 5, the graph indicated by the solid line is shifted upward to become a graph indicated by the dotted line. The amount of change differs depending on the development contrast. FIG. 6 is a graph showing only the difference density ΔC (ΔC1 (dark color toner), ΔC2 (light color toner)) with respect to the development contrast potential. As shown, there is a change that gradually increases to a certain development contrast and gradually decreases as the development contrast increases.

  For example, when the development contrast potential fluctuates, the density fluctuation amount ΔC2 of light cyan and light magenta, which are light color toners, becomes smaller than the density fluctuation amount ΔC1 of dark cyan, dark magenta, and yellow, which are dark color toners, and is more stable. The characteristic will be shown. However, colors expressed by a mixture of light color toner and dark color toner, such as light cyan and dark magenta or yellow, and light magenta and dark cyan or yellow, have different hues due to different density change amounts ΔC. It will be reproduced greatly deviated.

  Therefore, in the image forming apparatus, as described above, an image control developer image (on the photosensitive drum 1 or the intermediate transfer belt 5a is formed on the photosensitive drum 1 or the intermediate transfer belt 5a at a timing other than the normal image forming time when the normal image forming process is executed. Patch) is formed, the image density is detected by the density detecting means, and based on the detection result, the density signal used for gamma conversion is optimized, that is, the image shown in FIG. Control for correcting the LUT, which is a relationship table between the density signal and the image density, provided in the gamma conversion unit 202 in the processing unit 200 is executed. In this case, as the patch, a gradation patch having a plurality of density portions from a low density to a high density is formed, and in this embodiment, this gradation patch is formed on the photosensitive drum 1 and is detected as density detection. Gradation control detected by the photosensor 100 as means is performed.

  The patch is signal-processed image information based on the test pattern stored in the storage unit of the image forming apparatus control unit, and in this embodiment, the gradation patch pattern, and is transmitted to the laser driver 204, and the laser exposure optical system 3 And a latent image for image control (patch latent image) formed on the photosensitive drum 1 is developed by the developing device 4 to be formed.

  In this embodiment, LUT correction and gradation control are performed in the process according to the flowchart shown in FIG. That is, when a plurality of images are continuously formed, a non-image region between the image portions on the photosensitive drum 1 or a non-image region immediately after the image formation in the case of forming one image, that is, a transfer material. A patch is formed in a non-image forming area outside the image area to be transferred, the image density as the toner amount is detected by the photosensor 100 (S101, S102), and a difference Δc between the detected amount and the target value is calculated. (S103) Based on the amount of deviation between the detected amount and the target value, the LUT information in the gamma conversion unit 202 is corrected (S104), and control is performed to keep the density stable. This control is performed by a CPU which is an image forming apparatus main body control unit (not shown).

  Here, in S103, a target value is determined in advance, and by obtaining the density difference Δc from the target value, appropriate control can be performed according to the environment of the image forming apparatus, and it is less affected by disturbances such as noise. Taste stabilization can be performed with high accuracy, but it is not always necessary to perform processing for obtaining the density difference Δc, and the LUT may be directly corrected from the relationship between the detection result of the photosensor 100 and the density signal. .

  Here, as in the present embodiment, in an image forming apparatus having developers with the same hue and different lightness, the method described in Patent Document 1, that is, tone control of all colors with the same frequency ( In the image density control method, a hue change occurs.

  This phenomenon will be described with reference to FIG. This shows the density variation amount ΔC of each color when gradation control is performed once for eight image formations by normal image formation when the development contrast potential changes continuously. As described above, when gradation control is performed at the same frequency for light cyan and light magenta light toner and dark cyan, dark magenta, and yellow dark toner, the image density change amount ΔC is dark toner with respect to the development contrast. Is high, a difference for each color occurs in the density change within the gradation control period, resulting in a change in color.

  Therefore, as a feature of the present invention, the density fluctuation amount ΔC within the gradation control period is controlled to be equal.

  Specifically, the tone control cycle of the dark color toner is set to once every four sheets, which is 1/2 of the light color toner. In the configuration of this embodiment, since the photo sensor 100 is provided on the photosensitive drum 1, the patch can be formed once for each color at maximum.

  Here, the interval of once every eight sheets, which is the gradation control cycle of the light color toner, can be arbitrarily set by the operator from an operation unit (not shown) connected to the CPU in the image forming apparatus, and the optimum color required by the operator It is possible to adjust the balance between stability and running cost.

  Increasing the frequency of gradation control improves stability, but increases the amount of patch to be formed and increases the amount of waste toner, which affects the running cost of toner and the cost of a waste toner container (not shown). .

  Taking these problems into consideration, the numerical value of 8 has adopted a value that would be well-balanced as the initial setting value. In the image forming apparatus control section, the dark toner gradation control period is halved with respect to the light toner gradation control period set by the operator. Here, the reason for setting it to 1/2 is obtained from the ratio of the density characteristics of the toner. As described above, since light cyan and light magenta have half the density characteristics with the same applied toner amount with respect to dark cyan, dark magenta, and yellow, they are calculated from the ratio.

  Therefore, if the ratio between the density characteristics of the dark toner and the density characteristics of the light toner is different, it is desirable to set the gradation control cycle ratio at that ratio. As a minimum, if the relationship between the density characteristics of each color and the tone control cycle ratio is “the tone control cycle of the dark color> the tone control cycle of the light color”, the amount of color change The purpose of reducing the amount can be achieved, and it is sufficient if the relationship is satisfied.

  A control process when gradation control is performed during continuous image formation in this embodiment is shown in the flowchart of FIG.

  When the count value of a number counter (not shown) provided in the image forming apparatus main body control unit that counts the number of image formations for each sheet from the start of image formation becomes 4 (S1), dark cyan that is a dark color toner image. Then, gradation control is performed for the dark magenta and yellow toner images (S2).

  After the gradation control of the dark color toner image, the number of sheets is further counted every time the image is formed. When the count value reaches 8 (S3), the gradation control of light cyan and light magenta, which are light color toner images, is performed. (S4). When the gradation control of the light color toner image is completed, the count value of the number counter is returned to 0 (S5).

  After that, every time the image is formed (S6), the number counter counts the number of images formed (S7), and when it becomes a multiple of 4 (S1), dark color toner gradation control (S2) is carried out to a multiple of 8. When this happens (S3), image formation is performed until the specified number of sheets is reached (S8) while performing dark toner gradation control and light color toner gradation control.

  By performing such control, it can be read that the transition of the density fluctuation amount of each color is the same as shown in FIG. However, although there is a difference in the amount of fluctuation within the gradation control period, the maximum value of the difference is halved compared to the case where the gradation control interval is the same period for each developing device 4 (FIG. 8).

  The transition of the difference in the amount of density fluctuation is shown in the case where the gradation control is performed for the dark toner and the light toner in the same period, and in the case where the gradation control period is changed in both as in the present embodiment a2. 10 shows. As shown in FIG. 10, the difference in the density fluctuation amount is suppressed, and as a result, the hue fluctuation of the multi-order color formed by the mixed color of the dark toner and the light toner is reduced.

  Further, the photosensor 100 is not limited to this position, and may be on the intermediate transfer belt 5a. In that case, since it is necessary to form a single-color patch image on the intermediate transfer belt 5a, the maximum frequency of gradation control is determined by the number of patch images that can be formed within the sheet interval of the image forming apparatus. Become.

  That is, the image control frequency is the number of times of image control and can be said to be the number of patch images to be formed.

  According to the present invention, the density control frequency of a developer having a high brightness developer in the combination of developers having different brightness with the same hue among cyan, magenta, and yellow is the same hue and low brightness. The density control frequency is less than that of developers with developers and developers without the same hue and different combinations, and the frequency can be set by the operator. It is possible to reduce color fluctuations with a minimum running cost.

  The present invention is not limited to the image forming apparatus having the configuration as shown in FIG. 2, but a tandem type as shown in FIG. 11 or a direct transfer type that directly transfers an image from a photosensitive drum to a transfer material. It can also be applied to.

  Further, the hue and number of the developer contained in the developing device are not limited to this, and the hue having a combination with different brightness is not limited to magenta or cyan.

  In addition, the dimensions, materials, shapes, and relative positions of the components of the image forming apparatus described above are not intended to limit the scope of the present invention to those unless otherwise specified. Absent.

6 is a flowchart illustrating an example of an image density control process in continuous image formation according to the present invention. 1 is a schematic configuration diagram illustrating an example of an image forming apparatus. It is a block diagram which shows an example of operation | movement of the image process part in image formation. 6 is a graph showing an example of gamma characteristics of light color toner and dark color toner. 6 is a graph showing when the gamma characteristics of light color toner and dark color toner change. 6 is a graph showing the amount of density variation between light color toner and dark color toner. It is a flowchart which shows an example of the density | concentration control process which concerns on this invention. It is a graph which shows the image density fluctuation amount with respect to the number of image formation in the comparative example of this invention. 6 is a graph showing an example of an image density fluctuation amount with respect to the number of image formations according to the present invention. 6 is a graph showing an example of a change in the difference in density variation between dark toner and light color toner and a comparative example with respect to the number of formed images according to the present invention. It is a schematic block diagram which shows the other example of an image forming apparatus.

Explanation of symbols

1 Photosensitive drum (image carrier)
4, 41-46 Developer 5 Transfer device 100 Photo sensor (density detection means)

Claims (8)

An image carrier on which an electrostatic latent image is formed; a plurality of developers that develop the electrostatic latent image to form a developer image; and a density detector that detects an image density of the developer image. And at least two of the plurality of developing devices are a set of developing devices in which developers having the same hue and different brightness are accommodated, and the developer image is used as a transfer material. An image forming apparatus to be transferred,
The developer image for image control is formed by the developing device, the image density of the developer image for image control is detected by the density detecting means, and the image density of the formed image is controlled according to the detection result. In the forming device,
In the combination of the developing devices in which developers having the same hue and different lightness values are stored, an image forming operation performed by a developing device having a high lightness value of the stored developer and an image forming operation performed by a low developing device. In contrast, the image forming apparatus is characterized in that the frequency at which the image density control is performed or the number of developer images for image control on which the image density control is executed is different.   The image density control frequency or the image control to be executed with respect to the image forming operation in the developer containing a developer having a high lightness among the combinations of the developers containing the developers having the same hue and different lightness The number of developer images is compared with the image density control for the image forming operation in a developer that contains a developer having a low hue with the same hue or a developer that does not have a combination with a developer having a different brightness with the same hue. 2. The image forming apparatus according to claim 1, wherein the number of the image forming apparatuses is small.   3. The image forming apparatus according to claim 1, wherein the toner constituting the developer having the same hue and different brightness has the same spectral characteristics and different amounts of the pigments contained therein. The developer having the same hue and high brightness has an optical density after fixing of less than 1.0 when the toner amount on the transfer material is 0.5 mg / cm ² , and the developer having low brightness is the toner on the transfer material. 4. An image forming apparatus according to claim 1, wherein the optical density after fixing is 1.0 or more per 0.5 mg / cm < ² >.   The hue of the developer contained in the plurality of developing devices is any one of yellow, magenta, cyan, and black. Among them, the hue of the developer having a combination of the same hue and different brightness is cyan and magenta. When the lighter developer is light cyan and light magenta, and the lighter developer is dark cyan and dark magenta, the density control frequency is dark cyan, light cyan, light cyan, and light magenta. The image forming apparatus according to claim 1, wherein dark magenta and yellow are more common.   The frequency of the image density control executed for the image formed by each of the developing units or the ratio between the developing units of the number of developer images for image control is the density of the developer contained in the developing unit. 6. The image forming apparatus according to claim 1, wherein the ratio is the same as the ratio.   7. The image control developer image is formed on a medium on which the developer image is carried and in a non-image forming area outside an image area transferred to a transfer material. The image forming apparatus according to any one of the above.   An operation unit capable of externally setting the frequency of image density control or the number of image control developer images to be executed for an image developed by a developer containing a developer having a predetermined brightness; The frequency of the image density control or the number of the developer images for image control to be executed with respect to an image developed by a developer containing a developer having the same hue and different brightness as a developer having a predetermined brightness 8. The image forming apparatus according to claim 1, wherein the image forming apparatus is automatically set in the image forming apparatus in response to an input from the operation unit.

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