JP4191892B2 - Color image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a color image forming apparatus having an optical writing unit provided with a plurality of light emitting elements each emitting a light beam.
[0002]
[Prior art]
In recent years, laser beam printers, which are image forming apparatuses, have come to be widely used since high image quality can be obtained at high speed.
In such a laser beam printer, a photosensitive member formed in a drum shape or a belt shape is moved in the sub-scanning direction, and its surface is uniformly charged by a charger, and irradiated from a laser diode as a light emitting element. Then, exposure is performed by irradiating one laser beam scanned in the main scanning direction by a rotating polygonal mirror such as a polygon mirror to form an electrostatic latent image.
The electrostatic latent image is developed by a developing device to be a visible image, and is transferred to a transfer paper by a transfer device.
[0003]
In order to further increase the speed of this type of laser beam printer, it is necessary to increase the rotational speed of the rotary polygon mirror that scans the laser beam. However, the rotating polygon mirror has a problem in that when it is rotated at a high speed, rotation unevenness occurs, so that a stable high-quality image cannot be obtained.
In addition, there is a problem that noise is increased and durability is lowered.
In order to solve such problems, in recent years, a plurality of laser beams are emitted from a light source, and the laser beams are simultaneously scanned in the main scanning direction on the photosensitive member by a rotating polygon mirror such as a polygon mirror. A so-called multi-beam type laser beam printer has been proposed (see, for example, Japanese Patent Application No. 9-134824).
[0004]
Such a multi-beam type laser beam printer has an advantage that the rotational speed of the rotary polygon mirror can be reduced in accordance with the number of laser beams, while the speed can be increased.
For example, when the number of laser beams is two, the rotational speed of the rotary polygon mirror can be halved compared with a laser beam printer having one laser beam. Therefore, the rotating polygon mirror can reduce rotational unevenness, and can realize improved durability and reduced noise.
[0005]
By the way, the change of the light quantity of the laser beam in such a laser beam printer is generally performed by changing the duty or power or both of them according to the image data.
In order to improve the gradation of an image, it is effective to employ area gradation by dither processing.
For example, in the 2 × 2 dither as shown in FIG. 4, the dot growth with respect to the increase in the write data is based on the laser light as shown in FIG. Diode LD ₁ And LD ₂ Are emitted with the same data and the dots (1) and (2) are grown simultaneously, and FIG. 6 shows a laser diode LD. ₁ And LD ₂ The amount of light is LD ₁ > LD ₂ As shown in the figure, the amount of light is indicated by the size of the black circle. ₁ In some cases, the dot of (1) is preferentially grown by emitting only the light (LD) ₂ The dot (2) may be preferentially grown so that only the side emits light).
[0006]
[Problems to be solved by the invention]
However, as mentioned above, the laser diode LD ₁ , LD ₂ When scanning with two laser beams using the laser diode LD ₁ , LD ₂ When the respective light amounts are different from each other, the highlight has a different density for the reason described below.
[0007]
That is, in a full-color printer, a light emitting element used for writing an image for each page of each color plate is a laser diode LD. ₁ , LD ₂ If the light emitting element used for the image writing is selected for the purpose of giving priority to the image alignment, the laser diode LD is used. ₁ , LD ₂ The combination of yellow Y, magenta M, and cyan C used for the highlight of 2 becomes the cube of 2, and the laser diode LD is shown in FIG. ₁ And LD ₂ The amount of light is LD ₁ > LD ₂ As shown, there are eight types (both cyan C, magenta M, and yellow Y are shown in parallel for convenience of explanation, but are actually overlapped).
[0008]
In that case, since the achromatic gray uses three colors of yellow Y, magenta M, and cyan C, a slight light amount difference between the three colors becomes a potential difference on the photosensitive member, and the toner is changed according to the potential difference. Since a difference occurs in the amount of adhesion, a color difference occurs.
As a result, when full-color continuous printing is performed, the same writing data, but if the pages are different, a maximum of eight different color tones are obtained.
Therefore, in the conventional color image forming apparatus, two light emitting elements for emitting laser beams are provided, and writing of each color plate image using the two laser beams is started based on a signal detected by the intermediate transfer mark. There is something like that.
[0009]
At that time, the light emitting element used for writing out the image for each color plate is selected such that the light emitting element having the smallest positional deviation in the sub-scanning direction of the formed image is selected. Accordingly, in the case of the two-beam writing method at 600 dpi, the positional deviation of one dot 42 μm can be reduced at the maximum as compared with the case where no light emitting element is selected.
On the contrary, when writing is always performed using the same light emitting element without selecting the light emitting element (laser diode) performed in order to give priority to the alignment, the color tone is stable, but the above position Misalignment increases.
In other words, until now, in a color image forming apparatus provided with a plurality of light emitting elements, it is necessary to select one of the importance of the accuracy of positional deviation and the importance of the color stability of highlights. It was.
[0010]
The present invention has been made in view of the above problems, and even a color image forming apparatus capable of speeding up by using a plurality of laser beams emitted from a plurality of light emitting elements is conspicuous in an image. It is an object of the present invention to prevent the occurrence of misalignment and to prevent color variation.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention scans the surface of the photosensitive member, the charging unit uniformly charging the photosensitive member, and the surface of the photosensitive member uniformly charged by the charging unit. In a color image forming apparatus comprising a plurality of light emitting elements that emit a plurality of light beams that form an image,
When creating a black plate that forms a black image, select from multiple light emitting elements. The one where the positional deviation is smaller with respect to the target center in the sub-scanning direction Select a light emitting element and export using the selected light emitting element. , Color Edition A writing light emitting element determining means is provided that always writes out all colors to be formed using the same light emitting element at the time of image formation.
[0012]
Further, in the color image forming apparatus, the writing light emitting element determining means selects and writes the light emitting element to be written out from the plurality of light emitting elements for the first color when the color plate image is formed. After the color, the same light emitting element as that used for the first color writing may be used to continue writing until the end of the repeat job.
Further, in the color image forming apparatus, an intermediate transfer body is provided, on which a plurality of different color images formed in stages on the photosensitive body are sequentially transferred in a superimposed state, One job When there are multiple pages Is Until all the images on the multiple pages are formed Is , Each of the above multiple pages On page The export at the time of black version image creation in , It is preferable that writing is performed using the same light emitting element as that selected when the first black plate screen is formed.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is an overall configuration diagram showing an embodiment of a color copying machine which is a color image forming apparatus according to the present invention, and FIG. 2 is a perspective view showing an optical writing unit included in the color copying machine.
The electrophotographic color copying machine as the color image forming apparatus shown in FIG. 1 includes a color scanner 1, a color printer 2, and a paper supply bank 3.
The color scanner 1 forms an image of a document (not shown) set on the contact glass 4 on a color sensor 7 through an illumination lamp 6, a mirror group 9, and a lens 15 to obtain color image information of the document. For example, red, green, and blue (hereinafter referred to as R, G, and B) color separation lights are read and converted into electrical image signals.
[0015]
The color sensor 7 includes, for example, R, G, and B color separation means and a photoelectric conversion element such as a CCD, and simultaneously reads three color images obtained by color-separating an original image.
Then, based on the R, G, B color separation image signal intensity levels obtained by the color scanner 1, color conversion processing is performed by an image processing unit (not shown), and black (hereinafter referred to as Bk) and cyan (hereinafter referred to as “Bk”). C), magenta (hereinafter referred to as M), and yellow (hereinafter referred to as Y) color image data.
[0016]
The operation of the color scanner 1 for obtaining the Bk, C, M, Y color image data is as follows.
In response to a scanner start signal that takes the operation and timing of the color printer 2 to be described later, the optical system including the illumination lamp 6 and the mirror group 9 moves to the left in FIG. Scan and obtain color image data of four colors in one scan.
The color image data is stored in a memory (not shown), and the color image data of four colors is sequentially obtained by extracting the color image data at a predetermined timing.
Each color image is sequentially visualized by the color printer 2 each time, and the visualized images of the respective colors are superimposed to obtain a final four-color full-color image.
[0017]
The color printer 2 includes a photosensitive drum 5 that is an image carrier, an optical writing unit 8, a revolver developing unit 14, an intermediate transfer unit 16, a secondary transfer unit 17, a fixing device 38, and the like. Yes.
The photosensitive drum 5 rotates in the direction indicated by the arrow B, and around the photosensitive drum cleaning device 10, the charge eliminating lamp 13, and the charger 12 that is a charging unit that uniformly charges the surface of the photosensitive drum 5, respectively. A potential sensor and a development density pattern detector (not shown), the above-described revolver development unit 14, intermediate transfer unit 16, and secondary transfer unit 17 are arranged.
[0018]
The optical writing unit 8 scans the surface of the photosensitive drum 5 uniformly charged by the charger 12 to form an electrostatic latent image thereon, and emits a plurality of light beams as shown in FIG. Laser diode LD which is a plurality of light emitting elements ₁ , LD ₂ The semiconductor laser 35 having
The optical writing unit 8 includes a laser light emission drive control unit (not shown), a polygon mirror 46, a motor 47 for rotating the polygon mirror 46, an f / θ lens 48 (both see FIG. 1), a reflection mirror 49, and the like. Have.
The optical writing unit 8 converts the color image data obtained from the color scanner 1 shown in FIG. 1 into an optical signal, performs optical writing corresponding to the image of the document on the photosensitive drum 5, and performs the photosensitive operation. An electrostatic latent image is formed on the body drum 5.
[0019]
In this optical writing unit 8, two laser diodes LD ₁ , LD ₂ Two laser beams generated from the laser beam are deflected and scanned by the polygon mirror 46, reflected by the reflecting mirror 49, and irradiated onto the photosensitive drum 5.
Thus, in this optical writing unit 8, the two laser diodes LD shown in FIG. ₁ , LD ₂ Two laser beams L from ₁ , L ₂ Thus, the photosensitive drum 5 is simultaneously scanned in the main scanning direction over two lines.
[0020]
The revolver developing unit 14 shown in FIG. 1 includes a Bk developing unit 14B, a C developing unit 14C, an M developing unit 14M, a Y developing unit 14Y, and each of these developing units. It is comprised by the revolver rotation drive part etc. which are rotated later.
A developing bias in which an AC voltage is superimposed on a negative DC voltage is applied to each developing sleeve of the revolver developing unit 14 by a developing bias power source (not shown), and each developing sleeve is a metal of the photosensitive drum 5. The base layer is biased to a predetermined potential. In the revolver developing unit 14, when the copying machine main body is in a standby state, the developing unit 14B is set 30 degrees before the developing position.
In this color copying machine, when the copy operation is started, the color scanner 1 starts reading Bk color image data at a predetermined timing, and the light by the laser beam of the optical writing unit 8 based on the color image data. Writing starts. Thereby, formation of a black electrostatic latent image (hereinafter referred to as a Bk electrostatic latent image) is started.
[0021]
In order to enable development from the leading edge of the Bk electrostatic latent image, before the leading edge of the electrostatic latent image reaches the Bk developing position, the developing device 14B is moved to the developing position and the Bk developing sleeve starts to rotate. Then, the Bk electrostatic latent image is developed with Bk toner. Thereafter, the developing operation of the Bk electrostatic latent image area is continued, but when the rear end portion of the Bk electrostatic latent image passes the Bk developing position, the developing device for the next color is quickly brought to the developing position. Then, the revolver developing unit 14 is rotated. The rotational movement is completed at least before the leading edge of the electrostatic latent image based on the next image data reaches the developing position.
The intermediate transfer unit 16 includes an intermediate transfer belt 18 that is an intermediate transfer member stretched between a plurality of rollers. The intermediate transfer belt 18 is formed on the photosensitive drum 5 in a stepwise manner. A plurality of visible images of different colors are sequentially transferred to the superimposed state.
[0022]
Around the intermediate transfer belt 18, a belt cleaning blade 51 for cleaning the intermediate transfer belt 18 and a lubricant application brush (not shown), which is a lubricant application means, are also arranged to face each other.
The intermediate transfer belt 18 is provided with a position detection mark (not shown) on its inner peripheral surface (or an outer peripheral surface).
The position detection mark may be provided on the outer peripheral surface side of the intermediate transfer belt 18, but in this case, the position detection mark needs to be provided so as to avoid the passing area of the belt cleaning blade 51. If the arrangement is difficult, the position detection mark may be provided on the inner peripheral surface side of the intermediate transfer belt 18 as in this embodiment.
[0023]
The optical sensor 52 that functions as a mark detection sensor for detecting the position detection mark is located between the bias roller 53 and the drive roller 54 around which the intermediate transfer belt 18 is stretched, and the inner surface of the intermediate transfer belt 18. It is provided in the position which opposes.
In addition to the bias roller 53 and the drive roller 54, the intermediate transfer belt 18 is stretched between a plurality of rollers as shown in the figure.
Each roller that stretches the intermediate transfer belt 18 is made of a conductive material, and each roller other than the bias roller 53 is grounded.
The transfer bias roller 53 has a transfer bias controlled to a predetermined current or voltage according to the number of superimposed toner images by a primary transfer power source (not shown) controlled at a constant current or constant voltage. It is to be applied.
[0024]
The intermediate transfer belt 18 is rotated clockwise in FIG. 1 by a driving roller 54 that is rotated clockwise in FIG. 1 by a driving motor (not shown).
The intermediate transfer belt 18 is made of a semiconductor or an insulator and has a single layer or a multilayer structure. The intermediate transfer belt 18 is formed larger than the maximum sheet passing size so that the toner image formed on the photosensitive drum 5 is superimposed on the belt surface.
The secondary transfer unit 17 has a secondary transfer belt 55 stretched between three support rollers, and a part of the secondary transfer belt 55 can be pressed against the intermediate transfer belt 18.
The secondary transfer belt 55 is rotated in the counterclockwise direction in FIG. 1 by the rotation of the drive roller in the support roller that stretches the secondary transfer belt 55.
[0025]
A secondary transfer bias roller 56 serving as a secondary transfer unit is provided inside the secondary transfer belt 55, and the secondary transfer bias roller 56 places the secondary transfer belt 55 between the intermediate transfer belt 18. It can be pinched.
A transfer bias having a predetermined current is applied to the secondary transfer bias roller 56 by a secondary transfer power source (not shown) under constant current control. The secondary transfer belt 55 is moved to a position shown in FIG. 1 where the secondary transfer belt 55 is in pressure contact with the intermediate transfer belt 18 and a position separated from the intermediate transfer belt 18 by a separation / contact mechanism (not shown). Yes. The secondary transfer bias roller 56 is moved in the same direction in conjunction with the separation / contact operation of the secondary transfer belt 55.
[0026]
On the left side of the secondary transfer belt 55 in FIG. 1, a transfer paper neutralization charger 57 as a transfer material neutralization unit and a belt neutralization charger 58 as a transfer material carrier neutralization unit are arranged to face each other. Yes.
The transfer paper neutralization charger 57 neutralizes the charge held on the transfer paper so that the transfer paper can be satisfactorily separated from the secondary transfer belt 55 with the strength of the transfer paper itself. . The belt neutralization charger 58 neutralizes charges remaining on the secondary transfer belt 55.
Further, the secondary transfer belt 55 is not shown in FIG. 1, but a cleaning blade is in contact with the belt surface. The cleaning blade functions to remove the adhering matter adhering to the surface of the secondary transfer belt 55 for cleaning.
Reference numeral 36 denotes a registration roller pair which feeds transfer paper as a transfer material between the secondary transfer belt 55 and the intermediate transfer belt 18 at a position where the secondary transfer bias roller 56 is located at a predetermined timing.
[0027]
In this color copying machine, when the image forming operation is started, the photosensitive drum 5 is rotated in a direction indicated by an arrow B in FIG. 1 by a drive motor (not shown), and the intermediate transfer belt 18 is rotated by the drive roller 54 in FIG. It rotates in the turning direction.
Then, the charger 12 uniformly charges the surface of the photosensitive drum 5 with a negative charge to a predetermined potential by corona discharge. Next, based on the belt mark detection signal, the optical writing unit 8 performs raster exposure of the charged surface on the photosensitive drum 5 with laser light based on the Bk color image signal obtained from the color scanner 1 at a predetermined timing. Do. When this raster image is exposed, the charge proportional to the amount of exposure light disappears from the exposed portion of the uniformly charged surface of the photosensitive drum 5, thereby forming a Bk electrostatic latent image.
[0028]
The negatively charged Bk toner on the Bk developing roller of the Bk developing device 14B comes into contact with the Bk electrostatic latent image, so that the toner does not adhere to the portion where the charge on the photosensitive drum 5 remains. Then, the toner is attracted to a portion having no charge, that is, an exposed portion, and a visible image of a Bk toner image (black toner image) similar to the electrostatic latent image is formed. The Bk toner image formed on the photosensitive drum 5 is transferred (belt transfer of the toner image) to the surface of the intermediate transfer belt 18 that is driven at a constant speed in contact with the photosensitive drum 5.
Then, some untransferred residual toner remaining on the surface of the photosensitive drum 5 after the Bk toner image is transferred to the intermediate transfer belt 18 is prepared for the next image formation on the photosensitive drum 5. Cleaning is performed by the cleaning device 10.
[0029]
Thereafter, the photosensitive drum 5 proceeds to the next C image forming process, and laser light writing is performed by the optical writing unit 8 based on the C image data read by the color scanner 1, so that the surface of the photosensitive drum 5 is formed. A C electrostatic latent image based on cyan image data is formed.
Then, the revolver developing unit 14 is rotated before the trailing edge of the previous Bk electrostatic latent image passes through the developing position and the leading edge of the C electrostatic latent image reaches the developing position. Developing machine 14C is set at the developing position. Then, the C electrostatic latent image is developed with C toner, and a C toner image (cyan toner image) is formed on the photosensitive drum 5.
Thereafter, the development of the C electrostatic latent image area is continued. When the rear end of the C electrostatic latent image passes the development position, the revolver developing unit 14 is rotated again to develop the next magenta. The machine 14M is moved to the developing position.
[0030]
The rotational movement of the revolver developing unit 14 is completed before the leading end of the next M electrostatic latent image reaches the developing position.
Then, the image forming process for each color of M (magenta) and Y (yellow) is performed in the same manner as in the case of Bk (black) and C (cyan).
In this way, the Bk toner image, the C toner image, the M toner image, and the Y toner image sequentially formed on the photosensitive drum 5 are sequentially aligned on the same surface on the intermediate transfer belt 18. In this state, the toner image is formed by superimposing four colors at the maximum.
[0031]
On the other hand, at the time when the above-described image forming operation is started, the transfer paper is fed from the transfer paper cassette or the manual feed tray of any selected paper feed stage in the paper supply bank 3, and the transfer paper is transferred to the registration roller pair. Waiting at 36 nips.
Then, the leading edge of the transfer sheet is aligned with the timing at which the leading edge of the superimposed toner image carried on the intermediate transfer belt 18 comes into contact with the secondary transfer portion where the intermediate transfer belt 18 and the secondary transfer belt 55 are in contact with each other. The registration roller pair 36 conveys the transfer paper at a timing that coincides with the leading edge of the image, whereby registration of the transfer paper and the toner image is performed.
Then, the transfer paper is superimposed on the toner image on the intermediate transfer belt 18 and passes through the secondary transfer portion.
[0032]
At this time, the superimposed toner images on the intermediate transfer belt 18 are collectively transferred onto the transfer sheet by a transfer bias generated by a voltage applied to the secondary transfer bias roller 56 by the secondary transfer power source.
The transfer paper transported to the left in FIG. 1 by the secondary transfer belt 55 is neutralized when it passes through the transfer paper neutralization charger 57. Therefore, the secondary paper that has been adsorbed by static electricity due to the neutralization is removed. It is peeled off from the transfer belt 55 and conveyed toward the fixing device 38.
The superimposed toner images on the transfer paper are melted and fixed by the fixing device 38, and thereafter stacked on the paper discharge tray 39 outside the main body of the apparatus by a pair of discharge rollers to obtain a full color copy.
[0033]
On the other hand, the surface of the photosensitive drum 5 after the toner image is transferred to the intermediate transfer belt 18 is cleaned by the photosensitive member cleaning device 10 and is uniformly discharged by the discharging lamp 13. Further, the surface of the intermediate transfer belt 18 after transferring the toner image in which each color is superimposed on the transfer paper is also contacted with the surface of the intermediate transfer belt 18 by a separation mechanism (not shown). Cleaning is performed by the blade 51.
When there is a repeat copy, the operation of the color scanner 1 and the image formation on the photosensitive drum 5 are performed at a predetermined timing following the first color (Y) image formation process for the first color of the second color. The image forming process (Bk) is performed.
[0034]
The intermediate transfer belt 18 has two sheets in the area cleaned by the belt cleaning blade 51 on the surface following the batch transfer process of the first four-color superimposed toner image (in the case of a full car) to the transfer paper. The Bk toner image of the eye is transferred onto the belt. Thereafter, the operation is the same as that of the first sheet.
The above is the case of the copy mode for obtaining a four-color full-color copy. In the case of the three-color copy mode and the two-color copy mode, the same operation as described above is performed for the designated color and number of times. .
In the case of the single color copy mode, only the predetermined color developing machine of the revolver developing unit 14 is in the developing operation state until the predetermined number of sheets is completed, and the belt cleaning blade 51 is pressed against the intermediate transfer belt 18. The copy operation is performed at the same position.
[0035]
The image forming operation of the color copying machine has been described above. The control of the operation of each unit at that time is all performed by the control device 50 shown in FIG.
The control device 50 includes a central processing unit (CPU) having various determination and processing functions, a ROM storing each processing program and fixed data, a RAM serving as a data memory storing processing data, and an input / output circuit (I / O).
[0036]
In addition to the above-described control of the image forming operation of the entire color copying machine, the control device 50 includes a plurality of light emitting elements included in the optical writing unit 8 during black plate formation for forming a black (Bk) single color image. Laser diode LD ₁ , LD ₂ The laser diode to be written out is selected, and the selected laser diode is used for writing. When a color image other than a single color is formed, colors other than black (Bk) are always displayed for all colors to be formed. Control is also performed to write out using the same laser diode.
That is, in this embodiment, the control device 50 functions as a writing light emitting element determining unit.
[0037]
Incidentally, as in the optical writing unit 8 shown in FIG. ₁ , LD ₂ (This embodiment shows an example in the case of two, but it may be three or more.) When there is a laser beam irradiated from each laser diode on the photosensitive drum 5, In many cases, the amount of light is not completely the same due to the influence of a lens or the like.
Therefore, when there are a plurality of laser diodes as light emitting elements in this way, depending on the laser diode used for image writing, there is a positional shift between the color images to be superimposed when a color image in which two or more colors are superimposed is formed. It is easy to occur. In addition, the color tone is different for each screen (page), which may cause inconvenience.
[0038]
However, in the color copying machine according to this embodiment, as described above, the control device 50 has a plurality of laser diodes LD included in the optical writing unit 8 during black plate formation for forming a black (Bk) color image. ₁ , LD ₂ The laser diode to be written out is selected, and the selected laser diode is used for writing. When a color image other than a single color is formed, colors other than black (Bk) are always displayed for all colors to be formed. Since control is performed so that writing is performed using the same laser diode, the positional deviation of the laser beam spot in the sub-scanning direction during black (Bk) plate formation is generally more noticeable than in the case of a color image. Can be reduced to an inconspicuous level. In the case of a color image, color variation can be suppressed.
[0039]
That is, in this color copying machine, when a black (Bk) plate image that is conspicuous when a positional deviation occurs compared to the case of forming a color image, priority is given to the alignment of the laser beam spot on the photosensitive drum 5. For example, LD whose positional deviation is smaller with respect to the target center in the sub-scanning direction. ₁ Select.
In addition, between the three colors C, M, and Y, the laser diode used for writing is the laser diode LD. ₁ , LD ₂ As shown in FIG. 3 showing the selection result of the writing LD of the present invention, all the laser diodes LD are selected. ₂ It is fixed to.
When a color image other than a single color is formed, the first color (C color in this example) is a laser diode LD. ₁ , LD ₂ The laser diode to be written out is selected and written, and the M (magenta) and Y (yellow) colors after the second color use the same laser diode as the laser diode used in the first color. The writing may be continued until the end of the repeat job. In this case, the control device 50 shown in FIG. 1 determines the laser diode used for writing.
[0040]
In this way, the laser diode to be used for writing is selected with priority given to alignment until the first color (C) of the color, and the subsequent M (magenta) and Y (yellow) colors are C. By writing using the same laser diode as the color, it is possible to suppress the positional deviation in the black (Bk) plate and to suppress the color variation of the color image.
Further, the color copying machine described in FIG. 1 is an intermediate in which a plurality of visible images of different colors (Bk, C, M, Y) formed in stages on the photosensitive drum 5 are sequentially transferred to a superimposed state. For example, if a job has 10 pages, the transfer belt 18 is used. For example, when all 10 pages of image formation are completed, a plurality of screens within the same image formation cycle (between each page) ) Is the same laser diode as the laser diode selected in forming the first black plate screen (in this example, the laser diode LD). ₂ The control device 50 performs control so that writing is performed using.
In this way, it is not necessary to determine the laser diode (light emitting element) to be selected for alignment for the second and subsequent screens, so that the control is simplified accordingly.
[0041]
Further, in this color copying machine, in the single color mode in which image formation is performed using a single color formed by mixing two different colors at a predetermined mixing ratio, each of the two colors to be mixed is a laser. Diode LD ₁ , LD ₂ The control device 50 controls so that the laser diodes to be written out are selected and written out.
Examples of the single color used in the single color mode include green and blue, and the green is formed by mixing two colors of C and Y at a predetermined mixing ratio. Blue is formed by mixing two colors of C and M at a predetermined mixing ratio.
In this way, in the case of a single color in which two different colors are mixed at a predetermined mixing ratio to make a single color, it is necessary to prioritize the alignment between the two colors over the delicate color, so the writing is performed. The laser diode to be used is a laser diode (in this embodiment, an LD with a smaller positional deviation in the sub-scanning direction). ₁ ) Is selected.
[0042]
In addition, in this color copying machine, when forming an image using the two colors of single color and black, two laser diodes LD are used when forming a black plate screen. ₁ , LD ₂ A laser diode (in this embodiment LD ₂ The control device 50 performs control so as to select and write out.
For example, even in the black color + green two-color mode, the laser diode having the smaller positional deviation in the sub-scanning direction is selected in the black color, and C (cyan) is selected in the green color. In order to prioritize the alignment between two colors of Y (yellow), a laser diode with a smaller positional deviation in the sub-scanning direction is selected, and writing is performed by causing the selected laser diode to emit light.
By doing in this way, it is possible to form a single color and black two-color mode image with a high position system.
[0043]
【The invention's effect】
As described above, according to the present invention, the following effects can be obtained.
According to the color image forming apparatus of claim 1, from among the plurality of light emitting elements during black plate image formation. The one where the positional deviation is smaller with respect to the target center in the sub-scanning direction Select a light emitting element and export using the selected light emitting element. , Color Print image Since all the colors to be formed are always written out using the same light emitting element at the time of image formation, a black plate that is prone to misalignment can be made into an image with a high position system, and the color you want to keep the color tone from changing Color fluctuations can be suppressed in images.
According to the color image forming apparatus of claim 2. , Color Print image At the time of image formation, the first color is written by selecting a light emitting element to be written out from a plurality of light emitting elements, and the first color is written after the second color. To export Since the same light emitting element as the used light emitting element is used and writing is continuously performed until the repeat job end, it is possible to form an image with high positional accuracy up to the first color.
[0044]
According to the color image forming apparatus of claim 3, One job When there are multiple pages Is , Until all of the image formation on the plurality of pages is completed Is Each of the plurality of pages On page For black plate image creation, the same light emitting element as the light emitting element selected when forming the first black screen is used for writing. Since it is not necessary to determine the light emitting element to be selected, control is simplified accordingly.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram showing an embodiment of a color copying machine which is a color image forming apparatus according to the present invention.
FIG. 2 is a perspective view showing an optical writing unit included in the color copying machine.
FIG. 3 is a schematic diagram showing a laser diode used for writing out each color of an image to be formed.
FIG. 4 is an explanatory diagram showing a 2 × 2 dither matrix.
FIG. 5 shows a laser diode LD as an example of how to grow dots. ₁ And LD ₂ FIG. 6 is an explanatory diagram for explaining a case where dots are grown simultaneously with the same data.
FIG. 6 shows LD as an example of how to grow dots. ₁ It is explanatory drawing for demonstrating the case where only the light is emitted and the dot grows preferentially.
FIG. 7 is an explanatory diagram for explaining that a maximum of eight types of color differences occur in highlights when a laser diode used for writing an image for each page of each color plate is not fixed.
[Explanation of symbols]
5: Photosensitive drum 8: Optical writing unit
12: Charger (charging means)
18: Intermediate transfer belt (intermediate transfer member)
50: Control device (writing light emitting element determining means)
LD ₁ , LD ₂ : Laser diode (light emitting element)

Claims (3)

A photoconductor, a charging unit for uniformly charging the photoconductor, and a plurality of light beams for forming an electrostatic latent image by scanning the surface of the photoconductor uniformly charged by the charging unit, respectively. In a color image forming apparatus provided with a plurality of light emitting elements,
When forming a black plate for forming a black image, a light emitting element having a smaller positional deviation with respect to the target center in the sub-scanning direction is selected from the plurality of light emitting elements, and the selected light emitting element is used. Prevent dumping Te, a color image forming apparatus characterized in that a writing light emitting element determination means for performing writing using always the same light emitting element for all colors to be formed during color plane image imaging. The color image forming apparatus according to claim 1, wherein the writing light emitting element determining unit selects a light emitting element to be written out from the plurality of light emitting elements when writing a color plate image, and performs writing. the second or subsequent color color image forming apparatus, characterized in that the means for continuously performed Export using the same light-emitting element and a light-emitting element used for writing the first color to repeat job end. The color image forming apparatus according to claim 1, further comprising an intermediate transfer body on which a plurality of different color images formed stepwise on the photoconductor are sequentially transferred in a superposed state, and one job includes a plurality of pages. in some cases, until said image forming of a plurality of pages is completed, the writing at the time of definitive black plate imaging for each page of said plurality of pages, a light emitting element selected in forming the first black plate screen A color image forming apparatus, wherein writing is performed using the same light emitting element.

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