JPH0895337A - Color image forming device - Google Patents

Abstract

PURPOSE: To enable faithful gray reproduction in a toner multiple part important for a high quality color image, while preventing the rising of an exposure potential caused by a toner charge and the shielding effect of a toner layer. CONSTITUTION: In this color image forming device forming plural color component toner images overlapped/arranged at least in a part., on an image carrier 1 in such a manner that electrification, an exposure and development are repeated several times and then, transferring the toner images altogether to an image acceptor 6 by a batch transfer means 5, the batch transfer means 5 has the addition of a transfer efficiency control means 7 controlling transfer efficiency so that a transfer is performed to obtain almost the same toner weight per unit area in each color component toner image in a black toner image formed by the color superimposing of plural respective color component toner images.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus using a xerographic system (electrophotographic system), and more particularly, to uniformly charging an image carrier having a photosensitive medium on its surface and scanning with a light beam. A type in which exposure and development with a plurality of color toners are performed a plurality of times to form toner images of a predetermined number of colors on an image carrier, and then the toner images are collectively transferred to a predetermined image receptor by a collective transfer means. The present invention relates to improvement of a color image forming apparatus.

[0002]

2. Description of the Related Art Generally, as a basic recording system of color xerography (color electrophotography) which can provide low cost and high printing quality, repeating electrification, image exposure and development on a photosensitive member for the required number of colors. A method is known in which a multicolor toner image is formed on a photoconductor by the method described above, and then the multicolor toner image on the photoconductor is collectively transferred to a recording medium (for example, JP-A-59-121348). ).

However, in the color image forming apparatus of the type which forms a multicolor toner image on the photoconductor as described above, the document "Color Electrophotographic Process-Toner Image Overlay on Photoconductor-" (NIP- 7, Japan Ha
There is a problem that the latent image potential is affected by the toner shielding effect and the toner potential, as described in rdcopy '89). Similarly, regarding the increase in the exposure potential due to the toner charge and the decrease in resolution due to the dielectric properties of the toner layer, the document "Color Electrophotographic Process by Color Overlapping Development" (Electrophotographic Society Journal, Vol. 30, No. 1, 199).
It is pointed out in 1).

On the other hand, there is a method (for example, Japanese Patent Laid-Open No. 63-88573) in which the charging potential of the photoconductor is increased in each image forming cycle corresponding to each color. In this method, for example, as shown in Table 1, the charging potential, the potential of the image portion, and the developing bias potential are sequentially increased for each image forming cycle corresponding to each color, and the image portion and the non-image portion in the toner adhering portion are sequentially increased. The purpose is to avoid a situation in which the contrast potential of (1) decreases in each image forming cycle corresponding to each color.

[0005]

[Table 1]

Another conventional example, for example, Japanese Patent Laid-Open No. 3-20
According to the technique disclosed in Japanese Patent No. 2869, by charging the photosensitive member at a potential equal to or higher than the charging potential required for development, and then discharging the charge to the photosensitive member's charging potential, the polarity of the toner on the photosensitive member is reversed. The purpose of the present invention is to prevent the toner from scattering due to the reduction of the contrast potential due to the color overlapping.

On the other hand, with respect to the toner shielding effect, there is a method of changing the exposure conditions at the time of image formation for the second and subsequent colors from the exposure conditions before that. Specifically, a method in which the exposure intensity and the exposure time per pixel are sequentially made stronger or longer in each image forming cycle corresponding to each color (for example, JP-A-63-65460 and JP-A-63).
No. 66579), or a method in which the beam diameter for exposure scanning is successively increased for each image forming cycle corresponding to each color (for example, JP-A-63-654).
59).

[0008]

However, the following technical problems remain in the above-mentioned conventional technical means. That is, JP-A-63-885
According to the method disclosed in Japanese Patent Laid-Open No. 73-73, as summarized in Table 1 described above, the contrast potential (latent image contrast potential) between the image portion and the non-image portion can be large even after the second color (in this conventional example, 750 V, which is the same as the color), the development contrast potential, which has a greater effect on the actual image density, is not constant for each color and cannot be said to be effective for stable color reproduction of the composite color. Further, the actual toner layer potential changes greatly depending on the weight of the developing toner per unit area. According to experiments, the weight of developing toner required for a high quality color image is 0.
In the range of 5 to 0.8 mg / cm ² , it becomes 60 to 120 V (absolute value), and when developing the third color, a maximum difference of 240 V occurs between the image part and the non-image part, especially the gray reproduction of the toner multiple part. It was confirmed that there was a problem in doing.

Further, in the method disclosed in Japanese Patent Application Laid-Open No. 3-202869, the charge of the surface layer toner on the photosensitive member is reversed, so that the toner flies backward to the developing device of the next color (so-called scavenging phenomenon). There are challenges. This phenomenon is remarkable when an AC bias component is used as the developing device, and is particularly fatal in non-contact development because the AC bias component cannot be omitted for high image quality development.
Furthermore, in this conventional example, two scorotrons and two sets of high-voltage power supplies are required, and there is a technical problem that the charging device is inevitably increased in size and cost.

A method of changing the exposure condition for each image forming cycle corresponding to each color (for example, Japanese Patent Laid-Open No. 63-6546).
No. 0, JP-A-63-66579, etc.) can be an effective measure against the toner shielding effect, but there is a difference in the weight of the developing toner between the toner multiplex portion and the single color portion,
There remains a technical problem that there is a problem in performing accurate color reproduction.

The present invention has been made in order to solve the above technical problems, and in particular, avoids an increase in the exposure potential due to the toner charge and a shielding effect of the toner layer, while achieving a high quality color image. The present invention is intended for a color image forming apparatus that enables faithful gray reproduction of toner multiple portions important for an image.

[0012]

That is, the present invention provides:
As shown in FIG. 1, an image carrier 1 having a photosensitive medium on its surface.
A charging means 2 for charging the image carrier 1 for each color component image forming cycle, and an electrostatic latent image corresponding to each color component image on the image carrier 1 for each color component image forming cycle. An exposure unit 3 that is formed so that at least a part thereof overlaps with the electrostatic latent image corresponding to the image, and a multicolor that develops the electrostatic latent image corresponding to each color component image with each color component toner for each color component image forming cycle. In a color image forming apparatus provided with a developing unit 4 and a batch transfer unit 5 for batch-transferring a plurality of toner images of respective color components, which are formed on the image carrier 1 and at least partially overlapped with each other, to an image receptor 6. The batch transfer means 5 has a transfer efficiency so that the toner weight per unit area of each color component toner image in the black toner image formed by overlapping the plurality of color component toner images is transferred to substantially the same level. control Characterized in that the addition of the transfer efficiency control means 7 that.

In such technical means, the present invention is effective for a type in which a black toner image is formed by superposing a plurality of color component toner images, and in this case, as a representative example of the plurality of color component toners. Is a combination of yellow, magenta and cyan.

The charging means 2 and the exposing means 3 may be arranged around the image carrier 1 for the number of image forming cycles corresponding to each color, but the apparatus is compact and inexpensive. From the viewpoint of realization, the image carrier 1 is rotated a plurality of times by the number of image forming cycles corresponding to each color, and the charging unit 2 and the exposing unit 3 are commonly or partially used for each image forming cycle corresponding to each color. It is preferable to do so. Further, the multicolor developing means 4 may be one in which a developing unit corresponding to each color is individually provided around the image carrier 1, or a developing unit for each color is rotatably provided. You may make it arrange | position the developing unit of the color corresponding to the developing part which opposes selectively. The developing method may be either wet type or dry type, but from the viewpoint of achieving high image quality, it is preferable to use a toner having a particle diameter of 7 μm or less. Furthermore, the batch transfer means 5 is not limited to the one for electrostatically transferring each color toner image on the image carrier 1, but may be a thermal transfer or pressure transfer system as long as transfer efficiency can be controlled. But it doesn't matter.

Further, the toner developed by the first color developing unit becomes the lowermost layer of the toner image due to color superposition and is originally difficult to transfer. Therefore, the weight of the toner developed by the first color developing unit is changed to the second and subsequent colors. It is preferable to set it larger than the weight of the developing toner. In order to variably set the developing toner weight in this way, for example, gradation correction of the image data sent to the exposing means 3 at the time of latent image formation may be performed, or the first color developing unit of the multicolor developing means 4 may be used. Can be appropriately selected, such as increasing the development contrast, increasing the rotation speed of the developing roll, increasing the toner density of the developer, and reducing the adhesive force between the toner and the carrier.

Further, as the transfer efficiency control means 7, at least the toner weight per unit area of each color component toner image in the black toner image formed by superimposing a plurality of color component toner images is transferred to substantially the same level. However, from the viewpoint of maintaining a better gray balance, when the developing toner weight of the black toner image due to the above-mentioned color superposition is 1.5 mg / cm ² or more, the transfer of the lowermost layer color of the black toner image is performed. It is preferable that the toner weight be maximized, and that the transfer efficiency of the black toner image due to color superposition be reduced as the developing toner weight of the black toner image due to color superposition becomes saturated. It is preferable that the transfer efficiency is 75% or more with respect to the maximum value of the developing toner weight of the toner image.

[0017]

According to the technical means as described above, the toner images of the respective color components are formed on the image carrier 1 at least partially overlapped with each other before the transfer step. In such a state, the transfer efficiency control unit 7 controls the transfer efficiency of the toner image by the transfer unit 5, and the unit of each color component toner image in the black toner image formed by the color superposition of the plurality of color component toner images. Since the toner weight per area is transferred to substantially the same level, the transferred black toner image has a good gray balance.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on the embodiments shown in the accompanying drawings. Example 1 FIG. 2 shows Example 1 of an electrophotographic color image forming apparatus to which the present invention is applied. In the figure, reference numeral 21 is a photoconductor having a photoconductive thin layer (photosensitive medium) such as an organic photoconductive layer on its surface and rotating in the direction of the arrow, 22 is a charger for uniformly charging the photoconductor 21, and 23 is a charger. A laser exposure device for irradiating a laser beam whose pulse width is modulated based on multi-tone image data of each color component (yellow, magenta, cyan, black in this embodiment), 24 (specifically 24a to 24a
d) is a color developing device equipped with yellow, magenta, cyan, and black color toners, and 25 is a photoconductor 21.
A transfer device for transferring the toner images of the respective color components, 26 is a static eliminator for removing residual charges on the photoconductor 21, 27 is a cleaning device having a blade which is separated during image formation and comes into contact with the photoconductor 21 during cleaning, 28 Is a discharge lamp for removing static charges on the photoconductor 21, and 29 is a transfer device 25 for recording paper 2
This is a fixing device that melts and fixes the toner image transferred onto the image forming apparatus.

More specifically, as the charging device 21, a scorotron charging device using two or more discharge wires is used so that stable and uniform charging can be performed even during image formation for the second and subsequent colors.

The details of the laser exposure device 22 used in this embodiment are shown in FIG. In FIG. 1, a laser exposure device 22 includes a semiconductor laser 31 that is modulated based on exposure data 36 from a laser control circuit (not shown), a collimator lens 32 that collimates a beam from the semiconductor laser 31, and a collimator. A polygon mirror 33 that deflects and scans a beam that has passed through the lens 32 along an axial scanning line of the photoconductor 21 and a beam 35 that is deflected and scanned by the polygon mirror 33 is imaged along the scanning line position of the photoconductor 21. And an image forming lens 34.

Further, in this embodiment, as shown in FIG. 2, the exposure data 36 sent to the laser exposure device 22 is constituted by the image processing device 40. This image processing device 4
A specific configuration example of 0 is shown in FIG. In the figure, a gradation signal based on an image signal from an image reading device or a computer is input to a conversion table 41 as 8-bit gradation information of R, G, B or L ^* a ^* b ^* , and this conversion table It is converted into Y, M and C image data by 41. A conversion matrix may be used instead of a table for this conversion. Then, after these data are converted into Y, M, C, K by the undercolor removal processing unit 42,
The MTF correction processing unit 43 performs MTF correction. Next, the gradation conversion table 44 performs a predetermined gradation conversion on the image signal of each color. The gradation conversion table 44 in this embodiment has the density characteristics as shown in FIG. 5, and is set so that the output data of cyan becomes larger than that of yellow and magenta. Although black data is not shown in the drawing, gradation conversion of any one of yellow, magenta, and cyan or gradation conversion other than that may be performed.

Further, the image signals of the respective colors subjected to the gradation conversion reach the screen generating device 45. The screen generation device 45 uses the gradation information (256 gradations) of the image signal of each color.
For example, a pulse width that is proportional to
A line screen of 0 lines / inch (corresponding to an 8-bit resolution of 0 to 255 steps in this embodiment) is generated, and this is input to the laser control circuit 46 as exposure data.
Incidentally, regarding the specific configuration of the screen generating device 45, for example, a known method using a triangular wave (for example, Japanese Patent Laid-Open No. 62-3
No. 9972) is adopted, and the number of lines of the parallel line screen can be switched by variably setting the pitch of the triangular wave.

The color developing device 24 (24a-24
Details of d) are shown in FIG. In the figure, the color developing device 24 is a two-component developer 2 including a toner and a carrier.
A developing roll 245 that has a housing 244 in which 43 is accommodated, and that conveys the developer 243 by adsorbing the developer 243 on its surface and rotating the housing 244.
And a developer regulating member 246 that regulates the amount of the developer 243 attached to the surface of the developing roll 245, and a screw auger 247 that rotates to stir and convey the developer 243 and supply the developer 243 to the developing roll 245. It has and. Further, the developing roll 245 includes a magnet roll 248 fixed so as not to rotate, and is provided with a developing sleeve 249 rotatably supported around the magnet roll 248. In this embodiment, the diameter of the developing sleeve 249 is 16 mm, the distance between the developing sleeve 249 and the photoconductor 21 is 0.5 mm, the thickness of the developer 243 layer on the developing sleeve 249 is 0.3 mm, and the peripheral speed is 200 mm. / S, the developer 243 on the developing roll 245 is used for development in a non-contact state with the photoconductor 21, and the toner image already developed on the photoconductor 21 at the time of overlapping development. Is adopted to prevent disturbance in the subsequent developing process.

Further, in this embodiment, in order to form a thin developer 243 layer having a thickness of 0.3 mm on the developing sleeve 49, a magnetic powder dispersion type having an average particle diameter of 40 μm and a saturation magnetization of 50 emu / g is used. Polymer carriers are used. In addition, the carrier is used to charge the toner to a negative polarity,
30 wt% of styrene-acrylic copolymer, 68 wt% of magnetite, and 2 wt% of nigrosine were kneaded and pulverized to obtain a carrier having a positive charging property.

On the other hand, regarding the toner, the yellow toner is 90 wt% polyester and 10 wt% benzine derivative.
% Was kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. The magenta toner was kneaded and pulverized with 90 wt% of polyester and 10 wt% of polytungstophosphoric acid to obtain colored particles having an average particle size of 7 μm. Cyan toner is polyester 90w
t% and 10 wt% of copper phthalocyanine were kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. For the black toner, 80 wt% of styrene-acryl copolymer, 10 wt% of styrene-butadiene rubber, and 10 wt% of carbon black were kneaded and pulverized to obtain colored particles having an average particle size of 7 μm. Further, regarding these toners, 5 wt% of hydrophobic silica was externally added to the colored particles to obtain negatively charged toner. When the charge amount Q of the toner was measured for these developers, it was about -15 μC / g.

Further, the developing bias applied to each developing device 24 is superposed with AC components so as not to disturb the toner image formed on the photoconductor 21 and to obtain a desired high image quality. DC voltage is -690V as DC component (VDC) and frequency is 6kHz as AC component (VAC).
z and Vp-p = 1,6 kV were applied.

FIG. 7 shows the developing characteristics (characteristics showing the relationship between the developing contrast potential and the developing toner weight) of each of the developing devices 24 thus constructed. In FIG. 7, the developing toner weight measurement condition is obtained by transferring the toner image directly onto the photoconductor 21 by the tape transfer method and measuring the weight before and after the transfer by a precision balance.

In this embodiment, a transfer corotron is used as the transfer device 25, and the corona wire 251 of this transfer corotron is provided with a current variable device 50 for variably setting the transfer total current. In the example,
The total transfer current applied to the corona wire 251 is set to 200 microamps.

The transfer efficiency characteristics at this time are shown in FIG.
In the figure, the horizontal axis represents the toner weight (mg / cm ² ) of the black toner image (hereinafter referred to as process black as a result of color superposition) of the yellow, magenta, and cyan color component toner images, and the vertical axis represents the vertical axis. The transfer efficiency is shown in FIG.
0 is a plot of transfer efficiency against the weight of process black developing toner. As shown in FIG. 10, the transfer efficiency changes with the developing toner weight of the black toner image due to color superposition, but in this embodiment, the maximum developing toner weight of the black toner image is 1.
The total transfer current was adjusted so that a transfer efficiency of about 90% was obtained at 7 mg / cm ² .

Next, the image forming process of the color image forming apparatus according to this embodiment will be described. First, the charge is removed sufficiently by the charge removing lamp 28, or the photoconductor 21 in the initial state is uniformly charged to −750 V by the charger 22. The charging potential and the charging polarity itself can be set to a voltage value different from this value depending on the developing efficiency, the photoconductor used and other conditions.

Next, the laser exposure device 23 forms a first-color electrostatic latent image on the photoconductor 21. That is, the first color image data 00 to FF (8-bit resolution 0 to 255
Laser control circuit 4 using a parallel line screen (for example, 200 lines / inch) generated according to steps).
The laser 31 connected to 6 is turned on, the photoconductor 21 in the laser-irradiated portion is discharged, and a potential contrast is generated between the laser-irradiated portion and the non-irradiated portion. The potential contrast generated on the photoconductor 21 becomes an electrostatic latent image. The electrostatic latent image has a smooth edge shape due to the Gaussian distribution of the laser light and the limit of the frequency response of the laser control circuit 46. As described above, the electrostatic latent image of the first color is formed, and the potential corresponding to the maximum input coverage of the portion other than the edge of the electrostatic latent image thus formed is -250V.
Met.

Next, the electrostatic latent image is reversely developed by the yellow developing device 24a with the negatively charged first color yellow toner. As a result, the yellow toner was attached substantially along the line screen shape of the electrostatic latent image. At this time, from the developing characteristics of the yellow developing device 24a (see FIG. 7), the developing contrast potential 440V [-250V (latent image potential)-
The maximum developing toner weight of about 0.7 mg / cm ² was obtained for (−690 V (developing DC bias)).

Thereafter, as a second color image forming cycle, the photoreceptor 21 including the yellow toner layer is charged by the charger 22.
From the top, it was charged to -750V. Next, the laser exposure device 23 formed an electrostatic latent image of the second color (magenta). At this time, the gradation conversion output of the magenta image data is the same as that of the yellow as shown in FIG. 5, but the transmittance of the yellow toner by the laser beam is attenuated according to the weight of the developing toner as in the experimental data of FIG. 9 and the rise of the toner layer potential in FIG. 9, the latent image potential of the magenta image becomes larger than the latent image potential of the yellow image, and as a result, the development contrast potential becomes small. 100% input coverage of the electrostatic latent image excluding the edges
Potential of −320 V on the yellow toner layer.

After that, the magenta developing device 24b reversely develops the electrostatic latent image of the second color with magenta toner with the same development parameters as the first color to form a magenta toner image. In the present embodiment, yellow is used as the first color and magenta is used as the second color, but conversely, magenta may be used as the first color and yellow as the second color, or a combination of other colors may be used. However, when a latent image is formed by an infrared semiconductor laser, it is desirable to combine yellow or magenta for the first color and the second color in view of the spectral transmittance.

FIG. 11 is a schematic cross-sectional view showing the overlap of the toner images formed on the photoconductor 21 at the time when the magenta developing process is completed. In the figure, the magenta toner image 72 in the secondary color area (color overlapping area on the yellow toner image 71) has a development contrast potential of 370 from the yellow.
By reducing the voltage to V [−320 V (latent image potential) − (− 690 V (developing DC bias))], a small developing toner weight (about 0.5 mg / cm ² ) was obtained (see FIG. 7). Further, the development contrast potential of the magenta toner image 72 in the monochrome region is 440V [-250V (latent image potential)-
(-690V (DC bias for development))], and the same development toner weight as that of yellow was obtained.

After that, as a third color (cyan) image forming cycle, the charging device 22 charged the surface of the photoconductor 21 including the yellow + magenta toner layer to -750V. Next, the laser exposure device 23 formed an electrostatic latent image of the third color. At this time, since the gradation conversion output of the cyan image data is larger than yellow and magenta as shown in FIG. 5, attenuation of transmittance due to yellow and magenta toner (see FIG. 8) and influence of toner layer potential (see FIG. 9) , The latent image potential of the cyan image became almost the same as that of magenta. The potential of the portion other than the edge of the electrostatic latent image thus formed is -320 V in the secondary color area (on the yellow toner and magenta toner layers), and the single color area (on the photoconductor 21).
Is -250V.

After that, the cyan developing device 24c reverse-develos the electrostatic latent image of the third color with cyan toner with the same development parameters as the first color to form a cyan toner image. FIG. 12 shows a schematic cross-sectional view of the overlapping of the toner images formed on the photoconductor 21 at this time. In the figure, the cyan toner image 73 in the tertiary color area (the color toner area on the yellow toner image 71 + the magenta toner image 72) has a developing contrast potential of 370V [−320V (latent image potential) − (− 690V).
(Development DC bias)]], the developing toner weight (about 0.5 mg / cm ² ) is almost the same as that of the second color. Further, the development contrast potential of the cyan toner image 73 in the monochrome region is 440 V [-250 (latent image potential)-(-
690 V (developing DC bias)]], and the same developing toner weight as that of yellow and magenta was obtained.

After that, the image forming cycle using different color toners (black toner in this embodiment) may be repeated if necessary.

Next, the toner images of the respective colors formed on the photoconductor 21 are collectively transferred onto the recording paper 20 by the transfer device 25. In this embodiment, the maximum toner weight of the black toner image (process black) due to color superposition is 1.7.
(Yellow toner image 0.7, magenta toner image 0.5, cyan toner image 0.5) mg / cm ² , and as shown in FIG. 10, the maximum toner weight of this process black is 1.7 mg / cm 2. ^The transfer total current (parameter) applied to the corona wire 251 of the transfer corotron, which is the transfer device 25, is set to 20 so that the transfer efficiency of about 90% of ² can be obtained.
Adjusted to 0 microamps.

As a result, the weight of the transfer toner of the process black is 1.53 mg / cm ² [1.7 × 0.9 mg at maximum].
/ Cm ² ]. At this time, since the magenta toner image and the cyan toner image formed on the yellow toner image are almost transferred, the weight of the transferred process black yellow transfer toner is 0.53 [1.53-0.5-
0.5] mg / cm ² . Therefore, the transferred toner images of the respective colors have almost the same transfer toner weights for yellow, magenta, and cyan, and process black having a good gray balance was obtained.

Further, an AC voltage [12 kVp-p, 600 Hz] is applied to a peeling device (peeling corotron in this embodiment) for peeling the paper, which is not shown, and the recording paper 20
Peeling from the photoconductor 21 was promoted. The peeled recording paper 20 passes through the fixing device 29, and the unfixed toner image on the recording paper 20 is fixed, whereby a color image having excellent halftone reproduction is obtained.

The toner remaining on the photoconductor 21 and the history of charging are shown in the static eliminating device 26, the cleaning device 27 and the static eliminating lamp 28.
Then, the image forming cycle is completed by continuously removing the charge.

As described above, according to the present embodiment, as a result, stable gradation reproduction can be obtained without causing defects such as pseudo contours, and excellent color reproduction can be obtained for both single colors and multiple colors. I understood.

Example 2 The basic structure of the color image forming apparatus according to this example is substantially the same as that of Example 1, except that the latent image forming conditions and the developing conditions are partially different from those of Example 1. , And the transfer device 2
The transfer condition 5 is adjusted more finely.

An image forming process of the color image forming apparatus according to this embodiment will be described. First, the static elimination lamp 2
8, or the photoconductor 21 in the initial state is uniformly charged to −750V by the charger 22. Next, the laser exposure device 23 formed an electrostatic latent image of the first color (yellow). The potential corresponding to the maximum input coverage of the portion except the edge of the electrostatic latent image thus formed was -150V. Next, the yellow developing device 2
The electrostatic latent image was reverse-developed with the first color yellow toner in which 4a was negatively charged. As a result, the yellow toner was attached substantially along the line screen shape of the electrostatic latent image. At this time, the developing characteristics of the yellow developing device 24a (see FIG.
From the developing contrast potential of 540 V [−150
V (latent image potential)-(-690V (developing DC bias))], the maximum developing toner weight is about 0.75 mg /
cm ² was obtained.

Thereafter, as a second color image forming cycle, the photoconductor 21 including the yellow toner layer is charged by the charger 22.
From the top, it was charged to -750V. Next, the laser exposure device 23 formed an electrostatic latent image of the second color (magenta). At this time, the gradation conversion output of the magenta image data is the same as that of the yellow as shown in FIG. 5, but the transmittance of the yellow toner by the laser beam is attenuated according to the weight of the developing toner as in the experimental data of FIG. 9 and the rise of the toner layer potential in FIG. 9, the latent image potential of the magenta image becomes larger than the latent image potential of the yellow image, and as a result, the development contrast potential becomes small. 100% input coverage of the electrostatic latent image excluding the edges
Potential of −220 V on the yellow toner layer.

After that, the magenta developing device 24b reversely develops the electrostatic latent image of the second color with magenta toner with the same development parameter as that of the first color to form a magenta toner image. FIG. 11 shows a schematic cross-sectional view of the overlapping of the toner images formed on the photoconductor 21 at this time. In the figure, the magenta toner image 72 in the secondary color area (color overlapping area on the yellow toner image 71) has a development contrast potential of 4 from yellow.
By reducing the voltage to 70 V [-220 V (latent image potential)-(-690 V (developing DC bias))], a small developing toner weight (about 0.7 mg / cm ² ) was obtained (FIG. 7).
reference). Further, the development contrast potential of the magenta toner image 72 in the monochrome region is 540V [-150V (latent image potential).
-(-690V (DC bias for development))], and the same developing toner weight as that of yellow was obtained.

After that, as a third color (cyan) image forming cycle, the charging device 22 charges the photosensitive member 21 including the yellow + magenta toner layer to -750V. Next, the laser exposure device 23 formed an electrostatic latent image of the third color. At this time, since the gradation conversion output of the cyan image data is larger than yellow and magenta as shown in FIG. 5, attenuation of transmittance due to yellow and magenta toner (see FIG. 8) and influence of toner layer potential (see FIG. 9) , The latent image potential of the cyan image became almost the same as that of magenta. The potential of the portion other than the edge of the electrostatic latent image thus formed is −220 V in the secondary color area (on the yellow toner and magenta toner layers), and the single color area (on the photoconductor 21).
Is -150V.

After that, the cyan developing device 24c reversely develops the electrostatic latent image of the third color with cyan toner with the same development parameters as the first color to form a cyan toner image. FIG. 12 shows a schematic cross-sectional view of the overlapping of the toner images formed on the photoconductor 21 at this time. In the figure, the cyan toner image 73 in the tertiary color area (the color toner area on the yellow toner image 71 + the magenta toner image 72) has a development contrast potential of 470V [-220V (latent image potential)-(-690V
(Developing DC bias)]], the developing toner weight (about 0.7 mg / cm ² ) is almost the same as that of the second color. Further, the development contrast potential of the cyan toner image 73 in the monochrome region is 540V [-150V (latent image potential)-
(-690 V (development DC bias))], and the same developing toner weight as that of yellow and magenta was obtained.

After that, the image forming cycle using different color toners (black toner in this embodiment) may be repeated if necessary.

Next, the toner images formed on the photoconductor 21 are collectively transferred onto the recording paper 20 by the transfer device 25. FIG. 13 is a plot of the toner weight of process black for three conditions of the total transfer current applied to the corona wire of the transfer device (transfer corotron in this embodiment). Conditions 1, 2, 3 (total transfer current is 25 each
It is understood that the transfer efficiency of the high developing toner weight portion decreases as the total transfer current decreases in the order of 0, 200, 150 μA).

Further, focusing on the yellow transfer toner weight in the process black, the yellow transfer toner weight is plotted against the process black developing toner weight in FIG. However, in the case of this embodiment, 80% of the weight of the toner remaining on the photoconductor 21 is yellow, but
Even if 100% of the weight of the residual toner is yellow as described above, a large difference does not occur, and it may experimentally change depending on the image structure of the toner image on the photoconductor 21.

According to FIG. 14, the yellow transfer toner weight increases with the process black developing toner weight,
It is understood that the process black has a curve that peaks at a given developed toner weight. It was confirmed that when this peak is at the maximum toner weight of process black of 1.5 mg / cm ² or more, a process black having a sufficiently small L ^* (close enough to black) can be obtained and a good gray balance can be obtained. It was

Further, by approximating the maximum developing toner weight of the process black before reaching the peak,
It is possible to linearly change the yellow toner transfer toner weight from highlight (low density area) to shadow (high density area), and it is proved that this method is extremely effective in ensuring gray balance. . For example, under the condition 2, the maximum developing toner weight of process black is 1.
At 75 mg / cm ² , as shown in FIG. 13, the transfer efficiency is about 90%, so that the maximum is about 1.58 [1.
A transfer toner weight of 75 × 0.9] mg / cm ² was obtained. At this time, since the yellow transfer toner weight in the transfer toner image is 0.52 mg / cm ² (see FIG. 14), the total transfer toner weight of magenta and cyan in the transfer toner image is 1.06 [1 .58-0.52] mg / c
m ² , and the weight of the magenta or cyan transfer toner is about 0.53 mg / cm ² . As a result, the transferred images had almost the same transfer toner weights for yellow, magenta, and cyan, and the gray balance of the process black was extremely good.

Table 2 shows the relationship between the transfer efficiency and the gray balance at the maximum value of the developing toner weight of process black.

[0056]

[Table 2]

According to this Table 2, when the transfer efficiency is less than 75%, sufficiently low L ^* (close enough to black as process black) and sufficiently low C ^* (√ (a ^{* 2} +
b ^{* 2} )) (color difference of 10 or less is desirable) was not obtained, and from the experiment, satisfactory results were obtained when it was 75% or more.

Further, a recording paper peeling step similar to that of the first embodiment,
By passing through the fixing step, a color image having good halftone reproduction is obtained on the recording paper 20, and the main image forming cycle is completed after passing through the cleaning step and charge removing step similar to those in the first embodiment. As described above, also in the present embodiment, as a result, it has been confirmed that stable gradation reproduction can be obtained without causing defects such as pseudo contours, and excellent color reproduction is obtained for both single colors and multiple colors. It was

In the first and second embodiments, for example, the gradation conversion table 44 of the image processing device 40 is used to compare the weight of the developing toner of the developing device 24a for the first color with the weight of the developing toner of the second and subsequent colors. Although it is made large, it goes without saying that the transfer condition of the transfer device 25 may be adjusted especially for the type in which such gradation conversion is not performed.

[0060]

As described above, according to the present invention, the transfer toner weights of the respective color component toner images in the black toner image formed by the color superposition of the respective color component toner images are controlled to be substantially equal. It is possible to surely realize excellent gray balance and stable multiple color reproduction, and as a result, it is possible to provide a high-quality color image forming apparatus that is excellent in both color reproduction and gradation reproduction.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a schematic configuration of a color image forming apparatus according to the present invention.

FIG. 2 is an explanatory diagram illustrating a color image forming apparatus according to a first embodiment. Laser exposure system configuration diagram

FIG. 3 is an explanatory diagram showing a specific configuration of a laser exposure apparatus used in the first embodiment.

FIG. 4 is an explanatory diagram showing an image processing apparatus used in the first embodiment.

FIG. 5 is an explanatory diagram showing the contents of a gradation conversion table used in the first embodiment.

FIG. 6 is an explanatory diagram showing details of a developing device used in Embodiment 1.

7 is a graph showing the developing characteristics of the developing device used in Example 1. FIG.

FIG. 8 is a graph showing the transmittance of each developing toner used in Example 1.

FIG. 9 is a graph showing the relationship between the developing toner weight and the toner layer potential according to the first exemplary embodiment.

FIG. 10 is a graph showing the relationship between the process black developing toner weight and the transfer efficiency of the transfer device according to the first embodiment.

FIG. 11 is an explanatory diagram showing an overlapping state of toner images formed on the photoconductor at the end of the developing process for the second color.

FIG. 12 is an explanatory diagram showing an overlapping state of toner images formed on the photoconductor at the end of the developing process for the third color.

FIG. 13 is a graph showing the relationship between the process black developing toner weight and the transfer efficiency of the transfer device according to the second embodiment for each of the three conditions.

FIG. 14 is a graph showing the relationship between the process black developing toner weight and the yellow transfer toner weight of the transfer device according to the second embodiment.
It is a graph which shows for every one condition.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Image carrier, 2 ... Charging means, 3 ... Exposure means, 4 ... Multicolor developing means, 5 ... Batch transfer means, 6 ... Image receptor, 7 ... Transfer efficiency control means

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuki Yamauchi 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. (72) Inventor Takuto Tanaka 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd.

Claims (3)

[Claims] 1. An image carrier having a photosensitive medium on its surface (1)
A charging means (2) for charging the image carrier (1) in each color component image forming cycle, and an electrostatic latent image corresponding to each color component image on the image carrier (1) in each color component image forming cycle. An exposure unit (3) for forming an image so that at least a part thereof overlaps with an electrostatic latent image corresponding to another color component image; and an electrostatic latent image corresponding to each color component image for each color component image forming cycle. Multicolor developing means for developing with toner of each color component (4)
And a plurality of toner images of respective color components, which are formed on the image carrier (1) and at least a part of which are superposed and arranged, on the image receptor (6).
In the color image forming apparatus provided with a batch transfer unit (5) for batch transfer to each of the color component toners in the black toner image formed by color superposition of a plurality of color component toner images in the batch transfer unit (5). A color image forming apparatus characterized in that a transfer efficiency control means (7) for controlling transfer efficiency is added so that the toner weight per unit area of an image is transferred to substantially the same level. 2. The color image forming apparatus according to claim 1, wherein the transfer efficiency control means (7) has a developing toner weight of the black toner image by the color superposition of 1.5 mg / cm ^2.
As described above, the transfer toner weight of the lowermost layer color of the black toner image is maximized, and the transfer efficiency of the black toner image due to color superposition is controlled to decrease as the developing toner weight of the black toner image due to color superposition becomes saturated. A color image forming apparatus characterized by the above. 3. The color image forming apparatus according to claim 1, wherein the transfer efficiency control means (7) sets the transfer efficiency to 75% or more with respect to the maximum value of the developing toner weight of the black toner image due to the color superposition. And a color image forming apparatus.