JPH0943931A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device in which clear high-speed and multi-color printing can be realized in a compact device by increasing the rotation number of developing rolls in developing machines in the second development and on to be larger than a standard /rotation number. SOLUTION: A first developing machine 4 applied to the black color and a second developing machine applied to chromatic colors are disposed around a photoreceptor 1. In the case where the number of printing pages having a printing rate of an image for the second color and on at a specified printing rate or less exceeds a specified page number, a rotation number of developing rolls of applicable developing means 4, 5 for the second development and on is increased by 10-40%, or more favorably by 15-30%, the standard rotation number by a rotation number control means 46. This means that the rotation number of the developing roll 40 is increased to increase quantity of insulation toner, while discharge of mixed toner included in development toner can be simultaneously accelerated. In the case where the number of the printing pages of a specified printing density or less exceeds the specified page number, therefore, mixing of colors is not increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic recording apparatus, and more particularly to an image forming apparatus suitable for forming a multicolor image on a photoconductor and performing multicolor printing.

[0002]

2. Description of the Related Art A conventional electrophotographic image forming apparatus will be described with reference to a two-color recording apparatus that performs two-color printing while the photosensitive member makes two revolutions and one revolution.

Two-color printing is performed while the photoconductor rotates two times.
The pass two-color recording apparatus has a charging unit, an exposing unit, a first developing machine and a second developing machine corresponding to each color arranged around a photoconductor, creates a charge image by charging and exposing, and then loses the charge. Reversal development in which toner of the same polarity as that of the photoconductor is adhered to the developed part in the developing part (As for the developing method, high potential part with electric charge is developed by regular development using toner of the opposite polarity to that of the photoconductor May be performed for each rotation of the photoconductor, and these image forming steps are performed during two revolutions of the photoconductor.
The toner images of two colors are formed on the photoconductor repeatedly, and the toner images are transferred onto the paper at the transfer section, and then the toner is thermally fused by the fixing device and fixed to the paper.

A one-pass two-color image forming apparatus for performing two-color printing while the photosensitive member makes one rotation has a structure similar to that of the two-rotation two-color printing system described above, for example, a tri-level two-color developing method. Can be used to perform two-color printing. In the tri-level two-color developing method, a charging means and an exposing means for exposing the photosensitive body to the first color and the second color image signals by exposing the photosensitive body to the positive weak exposure and the other to the negative strong exposure are arranged around the photosensitive body. A three-value charge latent image is formed by a charging and exposing process using a toner, and the high potential part is opposite in polarity to the photoconductor in the first developing device and the second developing device using toners having different charging polarities. Of the toner of the same polarity as that of the photoconductor is developed by reversal development with the regular development using the above toner, and two-color toner images are formed on the photoconductor before transfer. Use the charging device to set the polarity of positive and negative toner to one of the polarities, transfer it to the paper by the transfer device, and then heat the toner by the fixing device to fix it on the paper. Two-color printing was performed with one rotation.

However, the following drawbacks have occurred when performing high-speed printing using these methods. That is, in the case where the development is performed by the contact development in the image forming units for the second and subsequent colors, even if the toner image formed in the former stage is scraped off only slightly in the process of the latter stage development, the latter stage image is not The color mixture in which the toner is mixed becomes remarkable as the number of printed pages increases, and there are cases in which clear multicolor printing becomes difficult.

In order to prevent such color mixture, there is a method of removing the mixed toner by providing a separation step (separation mode) separately from the development step of the printed image.
No. 8, JP-A-4-20981 and JP-A-5-224514. Japanese Patent Application Laid-Open No. 4-20981 discloses that when the first color toner and the second color toner have the same polarity, when the first color toner is mixed in the second developer, the polarity of the first color toner is reversed and is different from the second color toner. It is configured to have polarity, measures the amount of the first developer consumed (estimates the amount mixed), and based on that, separates mode different from the normal developing process, that is, only the second developing machine is rotated. A mixing step is provided to remove the mixed toner.

Further, in Japanese Patent Laid-Open No. 2-141768, a non-magnetic toner is used for the first color toner and a magnetic toner is used for the second color toner so that the development start potential difference is different. By reducing the bias voltage of the second developing machine, only the second developing machine is rotationally operated so that only the mixed toner of the first color is discharged to the photoconductor.

Further, in Japanese Unexamined Patent Publication No. 5-224514, toners of different polarities are used for the first color toner and the second color toner, the prediction of the mixed amount is estimated from the fog of the background portion, and the second development is performed in the separation mode. The number of rotations of the machine is increased to about twice as much as that in normal development, and the mixed toner is discharged.

Further, a method of providing an electrode roll for electrostatic separation has been proposed in Japanese Patent Laid-Open No. 63-43180.

Further, there is disclosed a method of removing the mixed toner by utilizing a difference in development start characteristics of the first color toner and the second color toner mixed with each other by providing a separation member adjacent to the printing area on the photosensitive member. Has been proposed to.

[0011]

However, in the method of providing the separation mode, the printing speed becomes slower by that amount, and
The method of providing the separation electrode and the method of providing the photosensitive separator (
The above method tended to increase the size of the developing machine and the photoreceptor.

Further, in the development of the second and subsequent colors, when the contact development without scraping the toner image formed in the preceding stage is carried out,
The peripheral speed ratio χ (≡ developing roll peripheral speed / photoconductor peripheral speed) is lower than in the normal case, and the supply amount of the developer is reduced. Therefore, depending on printing conditions, the image density may be reduced or the toner charge amount may be low. When trying to secure the image density by setting to, the toner scattering tends to increase, and there are cases in which clear multicolor printing becomes difficult.

The present invention eliminates the above-mentioned drawbacks, and a first object of the present invention is to provide an image forming apparatus capable of realizing clear high-speed multicolor printing with a small apparatus.

Further, when the chargeability of the toner changes with changes in environmental conditions, background fog and image density decrease tend to occur, and the multicolor print quality tends to change.

A second object of the present invention is to provide an image forming apparatus capable of ensuring stable multicolor printing quality without an increase in color mixture, background fog, and reduction in image density even if environmental conditions change. Is.

[0016]

A first object of the present invention is to sequentially form a plurality of charge latent images by a plurality of charging means and a plurality of exposing means for performing image exposure based on an image signal, and a plurality of developing means. To develop toners of multiple colors in sequence, to form toner images of multiple colors on the photoconductor, and then collectively transfer to a transfer material,
In an image forming apparatus for fixing, a print rate measuring means for measuring a print density of an image (defined by a black area / (black area + white area). Hereinafter, abbreviated as print rate) and printing with a predetermined print rate. A printed page recognition unit for recognizing the number of pages and a rotation number control unit for changing the rotation number of the developing roller of the developing unit are provided, and at least the printing rate of the images of the second and subsequent colors is the predetermined printing rate or less. When the number of pages exceeds a predetermined number of pages, the rotation speed control means causes the rotation speed of the developing roll of the corresponding developing means after the second development to be a standard rotation speed of 10 to 40.
%, More preferably 15 to 30%.

In addition to the first method described above, the second object is to provide a temperature / humidity detecting means for detecting temperature / humidity in the image forming apparatus, and on the basis of the temperature / humidity detecting means, a control voltage for the charger is obtained. Alternatively, it is achieved by controlling the developing bias.

[0018]

According to the present invention, the charge amount of the mixed toner is equal to or lower than that of the second color toner even when a large number of pages are printed with a low printing rate in which the color mixture tends to increase in the printing conditions. Moreover, since the polarities are set to be the same, the number of rotations of the developing roll can be increased to increase the amount of developing toner, and at the same time, the discharge of the mixed toner contained in the developing toner can be promoted. Therefore, even when the number of printed pages having a predetermined print density or less exceeds the predetermined number of pages, color mixing does not increase.

Further, since the number of rotations of the developing roll is increased within a range where the first color image is not scraped, it is not necessary to provide a separation mode other than the developing step, and as a result, the printing speed is not lowered. Furthermore, since it is not necessary to newly provide an additional mechanism such as a separation roll or a separation body, the size of the entire apparatus does not increase.

Further, in the present invention, the control voltage of the recharging device and the second developing bias voltage are controlled on the basis of the temperature and humidity in the image forming apparatus. Even if it changes, the second developing bias voltage can be changed to keep the image density of the second color substantially constant, and
Since the electrical scraping of the first-color toner image and the mixing of the second-color toner into the first-color toner image can be maintained substantially constant based on the potential difference between the second-color developing bias voltage and the potential of the second-color toner image, the multicolor print quality varies. There is nothing to do.

[0021]

【Example】

(1) First Embodiment An example of an electrophotographic image forming apparatus of the present invention is shown in FIG.
This will be described with reference to FIG.

In this example, a two-color recording apparatus performs two-pass two-color printing while the photosensitive member 1 makes two revolutions.

In FIG. 1, a charging unit 2, an exposing unit 3, a first developing unit 4 corresponding to black and R, around a photoconductor 1 are provided.
A second color corresponding to a chromatic color in which a desired color is selected from G and B colors
The developing device 5 is arranged, a first charge image is created by charging and exposure, and then the portion where the charge is lost is adhered to the first developing device 4 by a black toner having the same polarity as that of the photoconductor 1 by reversal development. (As a developing method, a high potential portion having an electric charge may be developed by regular development using a toner having a polarity opposite to that of the photoconductor). Next, the charging unit 2 uniformly charges the surface of the photoconductor, and then the exposure unit 3 forms a second electrostatic latent image, and the second developing device 5 forms a chromatic color having the same polarity as that of the photoconductor 1. Inverse development is performed to attach toner. As described above, the image forming process is repeated twice while the photoconductor 1 rotates twice to form two color toner images on the photoconductor 1, and the pre-transfer charger 6 charges the two color toner images. Was adjusted and was applied to the transfer device 8 by applying a bias voltage to the transfer device 8, and then the toner was thermally melted and fixed to the paper by a fixing device (not shown). The toner remaining on the photoconductor 1 after the completion of the transfer is cleaned by the cleaner 10.

A surface electrometer 11 for detecting the surface potential is arranged around the photoconductor 1 in the middle of the first and second developing machines 4, 5.

The second developing device 5 used in the present invention uses the two-component contact developing which is composed of toner and carrier as the second color developer, and the developing sleeve forming the surface of the developing roll 40 is oriented in the same direction as the photoreceptor 1. And the DC bias Vd ₂ is applied as the second developing bias. The developing roll 40 constitutes and fixes a built-in magnet roll so that two magnetic poles having the same polarity are located in a portion facing the photoconductor 1. The developer is regulated to a predetermined thickness by a doctor plate, and is conveyed to a portion facing the photoconductor 1, that is, a developing portion, and is developed with toner.

Next, the second developing condition will be described. The developing gap was 700 to 1000 μm, the doctor gap was 600 to 800 μm, the peripheral speed of the developing sleeve was 1.0 to 1.6 times the peripheral speed of the photoconductor 1, and the second developing condition was examined. As the carrier, a carrier having a resistance equal to or higher than that of the first color carrier (about 10 ^{9 to} 10 ¹⁴ Ωcm), for example, a resin carrier or a coated ferrite carrier was used. The resin carrier has a bulk density of 1.
Spherical and amorphous resin carriers having 0 to 1.6 g / cm ³ and saturation magnetization of 60 to 80 emu / g were used, and the mixing ratio with the toner was adjusted to 4 to 15% by weight. When a ferrite carrier is used, the bulk density is 2.2 to 2.7 g / cm ³ ,
Use a substantially spherical magnet with a saturation magnetization of 20 to 70 emu / g,
The mixing ratio with the toner was adjusted to 2 to 5% by weight. The charge amount of the second color toner is 450 to 550 V, which is 1 in terms of the contrast potential (difference between the bright portion and the dark portion) of the electrostatic latent image of the second color.
Contrast potential of color electrostatic latent image (550 to 600
Since it is smaller than V), the charge amount of the second color toner is set to about −7 to −12 μc / g so that the image density can be secured.

In the developing roll 40 of the second developing machine 5,
The homopolarization formed in the developing section will be further described. Two homopolar magnets are embedded in the developing unit of the magnet roll, and the first magnetic force distribution of the homopolar magnets is generated.
The magnetic force peak B1 and the second magnetic force peak B2 are 800 to 1
300 G (Gauss), the angle between the first and second magnetic force peaks (abbreviated as magnetic pole angle θs) is 25 to 40 degrees, Δ
When B (= B1−B ₀ ) is set to be in the range of 500 to 800 G, the magnetic brush for the photoconductor is about 300 mm for the effective length (the axial length of the portion that forms the magnetic brush and conveys the developer). It was found that the rubbing force can be reduced to 20 gf or less, 10 gf or less in a desirable combination, and 5 gf or less in a more desirable combination. Regarding the image density, an organic photoconductor (OPC) is used as a photoconductor, and a second color electrostatic latent image having a contrast potential of about 450 V is formed on the photoconductor having a peripheral speed of 100 to 300 mm / sec. Of the peripheral speed of the photosensitive member 1 to 1.0 to 1.6 times,
When the second developing bias of from 350 V to 350 V was applied and the reversal development was performed, the image density of 1.3 to 1.4 (OD) was secured.

The first color toner mixed in the second color developer is set so that the polarity is maintained and the charge amount is about -5 to -10 μc / g when frictionally charged with the second color carrier. The second-color toner and the second-color image area can be developed.

Further, the developing roll 40 of the second developing machine 5
The motor 47 for rotating the (developing sleeve) is driven by the rotation speed control means 46 so that the rotation speed can be changed.

The two-color print signal 41 is sent to the memory circuit 42.
The information necessary for determining the print density of the image or the print page is sent to the determination unit 44. The determination unit 44 is
Consisting of a print density measuring means for measuring the print density of the image and a print page recognizing means for recognizing the number of print pages of a predetermined print density,
The number of printed pages having a predetermined print density or less is transmitted to the control unit 45. The control unit 45 sends a control signal to the rotation speed control means 46 to increase the rotation speed of the developing roll 40 when the number of printed pages having a predetermined printing density or less exceeds the predetermined number of pages, and controls the rotation speed of the developing roll 40. Control.

The result of two-color printing in such a configuration will be described below.

First, FIG. 9 shows the color mixture ratio and the image density of the second-color toner image in the case of conventional two-color printing without control. The color mixture ratio of the second color toner image is 1.5 to 2.5 (K
P) once showed a saturation value, but when the image density decreased from 4 to 5 (KP), the color mixture tended to increase again.
That is, when the image density of the second color toner image becomes low,
Since the discharge amount of the mixed first-color toner is reduced, it is determined that the color mixing ratio showing the saturation value has once again increased.

The saturated color mixture ratio is as shown in FIG.
Depending on the printing rate (printing density) of the second color image, the saturation color mixing rate tended to increase as the printing rate decreased.

Next, a printing rate measuring means for measuring the printing rate of the second color image, a printed page recognizing means for recognizing the number of printed pages having a predetermined printing rate, and a rotation for changing the number of rotations of the developing roll of the second developing means. The number of rotations of the developing roll of the second developing means is controlled by the rotation number control means when the number of printed pages in which the printing rate of the second color image is less than the predetermined printing rate exceeds the predetermined number of pages. A case in which control is performed to increase the maximum number of rotations by 10 to 40% over the standard rotation speed will be described.

FIG. 12 shows the basic data, and when the printing rate k2 of the second color image is small (k2 / k1≈0.3) and the peripheral speed ratio χ is the standard peripheral speed ratio χ ₀ , the color mixture ratio is 4 to 4. Under the printing condition of 6%, the rotational speed of the developing roll of the second color developing machine was changed to find the relationship between the peripheral speed ratio χ and the saturated color mixing ratio. By increasing the peripheral speed ratio χ, that is, the number of rotations of the developing roll to 20 to 40% of the standard number of rotations, there are variations depending on the image density and the amount of charge, and 4 to 6
It was found that the color mixture ratio, which had been%, was reduced to 1/2 to 3/4.

When contact development (hereinafter abbreviated as low-rubbing development) in which the toner image formed in the previous stage is not scraped is performed in the development of the second and subsequent colors, the peripheral speed ratio χ (= developing roll peripheral speed / It is necessary to set the photoconductor peripheral speed) to χ = 1 to 1.5, which is lower than that in the normal case (χ = 2 to 3: monochromatic image forming apparatus), but the image density is as shown in FIG. Therefore, the toner charge amount tends to be set to a low level. Therefore, depending on the printing conditions, the toner scattering tends to increase.

FIG. 6 is a graph showing the relationship between the peripheral speed ratio χ of the second color developing machine, the color mixture ratio η and the toner scattering. When the printing rate is large and the peripheral speed ratio χ is large, the toner scattering is large. It turned out to be. Furthermore, the relationship between the scraping of the first color image and the peripheral speed ratio χ was investigated. As shown in FIG. 8, in the range in which the rotation speed of the developing roll was increased by about 30% of the standard rotation speed, the first color image was Scraping was found to be at an acceptable level with no significant increase.

In the present invention, based on the above experimental results, as shown in FIG. 7, in the region where the toner scattering is small, the peripheral speed ratio χ is small (for example, the standard peripheral ratio) in the condition A where the printing rate k2 is large. (Equal to the speed ratio χ ₀ ), and under the condition B in which the printing rate k 2 is small, the peripheral speed ratio χ is controlled to be increased.

As a result, by performing this control, the color mixture ratio η is lowered to about 3% in the area where the printing rate k2 of the second color image is small and the color mixture rate is relatively large (η ≧ 5%). I was able to.

Therefore, in the present invention, there is an effect that the color mixing ratio can be kept low even if the printing ratio changes, and toner scattering does not occur. Further, since the number of rotations of the developing roll is increased within the range where the first color image is not scraped, there is an advantage that it is not necessary to provide a discharging process (or a separating process) separately from the developing process.

(2) Second Embodiment In a configuration similar to that of the first embodiment described above, the determining unit 44 includes a print ratio measuring unit for measuring the print ratio of an image and the number of pages printed with a predetermined print ratio. Consists of a print page recognition unit that recognizes
Determine the specific K ₂ / K ₁ of printing rate K ₁ of the first color image and the printing ratio K ₂ of the second-color image, the control unit 45 and the print page number and the print ratio
To communicate. In the control unit 45, when the ratio of K ₂ / K ₁ is less than a predetermined value and the number of printed pages exceeds a predetermined number of pages, a control signal for increasing the number of rotations of the developing roller 40 of the second developing means is used. The rotation speed of the developing roller 40 is controlled by sending it to the control means 46.

The saturation color mixing ratio, from the accumulation and analysis of experimental data, as shown in FIG. 11, depending on the first color print density K ₂ print density K ₁ and second color image of the image, than the thick line in the left It was found that the area had a high color mixture ratio.

Therefore, in such a configuration, the control system becomes a little more complicated than in the first embodiment, but the color mixture ratio and the print ratio K ₁ and the second color of the first color image as shown in FIG. 11 are obtained. The relationship with the printing rate K ₂ of the image of
Since the control can be performed based on that, there is an advantage that the color mixing ratio can be reduced more reliably.

When toner images of three or more colors are multi-developed on the photoconductor, the sum K of the printing ratios of the toner images of plural colors formed on the photoconductor in the developing process up to the preceding stage.
b (= ΣKi; sum from i = 1 to n−1, i is the color order) and 2
Ratio Kn / K with the print rate Kn (n ≧ 2) of the image after the color
When the number of printed pages for which b is a predetermined value or less exceeds the predetermined number of pages, the number of rotations of the developing roll of the corresponding developing means after the second development may be increased.

(3) Third Embodiment In addition to the first or second embodiment described above, a temperature / humidity sensor is arranged and the detection value of the temperature / humidity sensor is taken into consideration.
The degree to which the number of revolutions of 0 is increased is changed.

In the first or second embodiment, even if the toner density of the second developing machine is controlled so as to be kept substantially constant even if the temperature and humidity conditions change, the toner charge amount is high under high humidity. The temperature is low and tends to be high under low humidity, and accordingly, toner scattering occurs, and the image density and the color mixture ratio tend to change.
On the other hand, in such a configuration, as compared with the first or second embodiment, the degree of increase in the rotation speed of the developing roller 40 is further reduced under high humidity where the toner charge amount is low, and the toner Under low humidity where the charge amount becomes high, the developing roll 4
Since the degree of increasing the number of rotations of 0 can be set to be larger, it is possible to prevent the toner scattering and the tendency that the image density and the color mixture ratio change depending on the environmental conditions.

(4) Fourth Embodiment In a two-pass two-color image forming apparatus, as shown in FIG.
In addition to the third embodiment described above, the control voltage of the scorotron recharger and the developing bias of the second developing machine are changed based on the detection value of the temperature and humidity sensor.
FIG. 3 is a potential distribution diagram of the two-pass two-color image forming apparatus of FIG. Via the first color toner potential after recharging, Vd ₂ developing bias of the second developing device, the difference ΔVa the first color toner potential Via a second developing bias Vd _2, .DELTA.Vs the background portion potential after recharging V _This is the difference between ₀₂ and the second developing bias Vd ₂ .

In this example, Δ which is a factor of electrical scraping
Since the control voltage of the scorotron recharging device and the developing bias Vd ₂ of the second developing device can be changed so as to keep Va substantially constant, depending on the printing rate and environmental conditions,
There is an advantage that the developing bias of the second developing machine can be adjusted. That is, since the second developing bias can be set high under the condition of the printing rate in which the color mixing ratio is high or in the condition of low humidity in which the charge amount of the toner is high, a higher image density than that in the third embodiment is obtained. Development roll 4 that can be maintained stable
There is an effect that the degree of increasing the rotation speed of 0 can be made smaller.

(5) Fifth Embodiment In the first to fourth embodiments described above, the two-pass two-color system in which two-color printing is performed while the photosensitive member makes two revolutions has been described as an example.
The fifth embodiment is an example in which the present invention is applied to a one-pass two-color system in which two-color printing is performed while the photosensitive member makes one rotation, as shown in FIG. 5 is different from FIG. 1 in that the first developing device and the second developing device are arranged in this order along the rotation direction of the photoconductor 1, and the toner in the first developing device and the toner in the second developing device are arranged. This is the point that the polarities are different. Although not described in detail here, the electrostatic latent image on the photoconductor is also formed by using a known method called a three-level latent image so that the charging level becomes three levels. ing.

The first development 4 and the second development 5 are performed using toners having different charging polarities. To effectively perform this,
It is desirable that the absolute value of the charge amount of the first color toner mixed in the second developing device 5 does not increase in the second developing device 5,
(A) When the charged polarity of the mixed first color toner is retained in the second developing device, it is discharged to the image portion and the background portion of the first color toner, and (B) the charged polarity of the mixed first color toner is second.
When it is reversed in the developing machine, it tends to be discharged to the second color toner image portion. Therefore, in the latter case (B), it is effective to perform the same control as in the above-described first to fourth embodiments. That is, for example, as shown in FIG. 4, a printing rate measuring means for measuring the printing rate of the second color image, a printed page recognizing means for recognizing the number of printed pages having a predetermined printing rate, and a developing roll of the second developing means. Rotation speed control means for changing the rotation speed is provided to rotate the developing roll of the second developing means by the rotation speed control means when the number of printed pages having a predetermined printing rate or less in the second color image exceeds the predetermined number of pages. By controlling to increase the number, it is possible to suppress the color mixture to a low level.

(6) Sixth Embodiment In the former case (A), the higher the printing rate of the first-color toner image, the more the scraping of the first-color toner image increases, so that the color-mixing rate tends to increase. Furthermore, the higher the print rate of the first color toner image and the higher the print rate of the second color toner image,
It was found that the color mixture ratio tends to increase because the discharged area becomes narrow. Therefore, (a) a printing rate measuring means for measuring the printing rate, a printed page recognizing means for recognizing the number of printed pages having a predetermined printing rate, and a rotation speed control means for changing the rotation speed of the developing roll of the second developing means. A method of increasing the rotation speed of the developing roll of the second developing means by the rotation speed control means when the number of printed pages in which the printing rates of the first and second colors are equal to or higher than the predetermined printing rate exceeds the predetermined number of pages. Then, it was effective in suppressing color mixture. Further, depending on the combination of the first color developer and the second color developer, there was a case where the fog on the background portion increased as the color mixture increased. However, (b) the charge amount of the first color toner increased. 7 at first development
To 10 μc / g and the first color toner and 2
When each developer was adjusted so as to have a small charge amount (3 to 6 μc / g or less) when combined with a color carrier, it was effective in reducing fog to the background portion. In addition to the above (b), (c) a printing rate measuring means for measuring a printing rate of an image, a printed page recognizing means for recognizing the number of printed pages having a predetermined printing rate, and a paper gap of the photoconductor. A means for forming an electric field between the photoconductor and the developing roll of the second developing means is provided in the non-printing area, and the number of printed pages for which the printing rates of the first and second colors are equal to or higher than the predetermined printing rate exceeds the predetermined number of pages. In this case, an electric field (electric field based on the potential difference ΔVs between the background portion potential and the developing bias potential of the second developing means) that electrostatically discharges the different color toner mixed in the developer of the developing means to the background portion is set as the standard electric field ( 13), an electric field that is 1.3 to 2.0 times higher than the standard electric field (see FIG. 14) is formed in the non-printing area. Fog could be reduced. The color mixing may be suppressed by using the methods (a), (b), and (c) described above in combination.

[0052]

According to the present invention, color mixture can be suppressed, and the tendency of image density reduction and toner scattering can be prevented, so that clear high-speed multicolor printing can be realized with a compact apparatus. A device can be provided. Further, even if environmental conditions change, there is an effect that stable multicolor printing quality can be secured without increase in color mixture, background fog, and reduction in image density over a long period of time. Further, there is an advantage that when performing the color mixture suppression control, it is not necessary to provide a separation process different from the development process.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a two-color image forming apparatus according to first and second embodiments of the present invention.

FIG. 2 is a schematic configuration diagram of a two-color image forming apparatus according to third and fourth embodiments of the present invention.

FIG. 3 is an explanatory diagram of a potential distribution according to third and fourth embodiments of the present invention.

FIG. 4 is a schematic configuration diagram of a two-color image forming apparatus according to a fifth embodiment of the present invention.

FIG. 5 is an explanatory diagram of developing characteristics.

FIG. 6 is an explanatory diagram of color mixing and toner scattering characteristics.

FIG. 7 is an explanatory diagram of the developing roll peripheral speed control of the present invention.

FIG. 8 is an explanatory diagram of a first color image scraping characteristic.

FIG. 9 is an explanatory view of color mixing characteristics.

FIG. 10 is an explanatory view of color mixing characteristics.

FIG. 11 is an explanatory diagram of color mixing characteristics.

FIG. 12 is an explanatory diagram of color mixing characteristics.

FIG. 13 is an explanatory diagram of a potential distribution of a 1-pass 2-color image forming apparatus.

FIG. 14 is an explanatory diagram relating to a mixed color discharge potential difference of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Photosensitive member, 2 ... Charging means, 3 ... Exposure means, 4 ... First developing machine, 5 ... Second developing machine, 6 ... Pre-transfer charger, 8 ... Transfer device, 10 ... Cleaner, 11 ... Surface electrometer , 40 ... Developing roll, 41 ... Two-color printing signal, 42 ... Memory circuit, 43
... exposure control unit, 44 ... determination unit, 45 ... two-color image control unit,
46 ... Revolution speed control means, 47 ... Drive motor, 48 ... Control voltage power supply, 49 ... Development bias power supply, 50 ... Temperature / humidity sensor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Motoshige Kurobane 2-6-2 Otemachi, Chiyoda-ku, Tokyo Inside Nititsu Koki Co., Ltd.

Claims (7)

[Claims] 1. A photosensitive member comprising a plurality of charging means and a plurality of exposing means for performing image exposure based on an image signal to sequentially form a plurality of charge latent images, and a plurality of developing machines for sequentially developing a plurality of color toners. In an image forming apparatus in which toner images of a plurality of colors are formed on a transfer material, which are collectively transferred to a transfer material and fixed, a print ratio measuring unit for measuring a print ratio of an image and a print page number having a predetermined print ratio are recognized. A print page recognition means and a rotation speed control means for changing the rotation speed of the developing roll of the developing machine are provided, and at least a predetermined number of print pages have a print rate of the image of the second color or later after the predetermined print rate. When the rotational speed exceeds the standard rotational speed, the rotational speed of the developing roller of the corresponding developing machine after the second development is 10 to 40 than the standard rotational speed.
An image forming apparatus characterized by increasing the percentage. 2. The sum Kb (= ΣKi; sum of i = 1 to n−1, i of print ratios of toner images of a plurality of colors formed on the photoconductor in the developing process up to the preceding stage. Are in color order)
And the print ratio Kn (n ≧ 2) of the second and subsequent images Kn
An image forming apparatus which increases the number of rotations of a developing roll of a corresponding developing machine after the second development when the number of printed pages whose / Kb is a predetermined value or less exceeds a predetermined number of pages. 3. The temperature and humidity sensor according to claim 1 and 2, wherein the rotation speed control means adjusts the degree of increase in the rotation speed of the developing roll based on the detected value of the temperature and humidity. An image forming apparatus characterized by. 4. The temperature and humidity sensor according to claim 1 and claim 2, wherein at least the second and subsequent charging steps are performed by a charger equipped with a control electrode, and the detected value of the temperature and humidity is taken into consideration. An image forming apparatus characterized in that a degree of increase in the number of rotations of the developing roll, a control voltage of each charger and a developing bias are set. 5. The method according to claim 1 or 2, wherein the plurality of developing machines form toner images of different colors with different charging polarities, and when the first color toner is mixed in the second developing machine.
An image forming apparatus characterized in that the charging polarity of the second color toner is the same as the charging polarity of the second color toner. 6. A plurality of charging means and a plurality of exposing means for performing image exposure based on an image signal to sequentially form a plurality of charge latent images, and a first and a second developing device for forming a plurality of colors having different charging polarities. In an image forming apparatus in which toner is sequentially developed to form toner images of a plurality of colors on a photoconductor, and then collectively transferred to a transfer material and fixed, a printing rate measuring unit for measuring an image printing rate and a predetermined printing rate And a means for forming an electric field between the non-printed area of the photoconductor and the developing roll of the second developing device, the printing rate of the first and second color images When the number of printed pages whose print rate exceeds the specified print rate exceeds the specified number of pages, the discharge is 1.3 to 2.0 times higher than the standard electric field based on the background potential and the bias voltage of the second developing machine. An image formed by forming an electric field in a non-printed area of the photoreceptor. Forming apparatus. 7. A predetermined page is a printed page having a second color image print rate of not more than a predetermined print rate, further comprising: a rotation speed control means for changing a rotation speed of a developing roll of the developing machine. When the number exceeds the number,
An image forming apparatus, characterized in that the number of revolutions of a developing roll of a developing machine is increased by 10 to 40% over a standard number of revolutions.