JP4408514B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention has a plurality of image forming units that develop an electrostatic latent image formed on an image carrier with a developer and transfer the obtained image onto a transfer material, and can output a multicolor image. Relates to the device.
[0002]
[Prior art]
A conventional image forming apparatus that performs multicolor image formation will be briefly described.
[0003]
In recent years, a color copying machine having a plurality of image forming units as shown in FIG.
[0004]
In an image forming apparatus using a single image forming unit having a plurality of developing means, the photosensitive drum of the unit must be rotated by the number of colors, and the output speed of a color image cannot be increased. The image forming apparatus having a plurality of units has an advantage that the same output speed as that in the case of a single color can be maintained. The operation will be described below.
[0005]
In FIG. 9, first, the original G is set on the original platen 100 with the surface to be copied facing down, and then the copy button is pressed to start copying. An original illumination lamp, a short focus lens array, and a CCD sensor are integrated into a unit 9 to scan the original while irradiating the original, and the reflected light from the original surface of the illumination scanning light is imaged by the short focus lens array. Is incident on the CCD sensor. The analog signal of the image information obtained by the CCD sensor is converted into a digital signal by performing known image processing, and sent to the printer unit.
[0006]
The printer unit receives the image signal and forms an electrostatic latent image on the photosensitive drum 1 of each unit as follows. The photosensitive drum 1 is driven to rotate at a predetermined peripheral speed around a central support shaft, and is subjected to a positive or negative uniform charging process by a charger 3 during the rotation process. The electrostatic latent image corresponding to the original image is formed on the surface of the photosensitive drum 1 by scanning the light of the solid-state laser element of the laser exposure device 2 that emits light ON and OFF correspondingly to the surface of the photosensitive drum 1 with a rotating polygon mirror. It is formed sequentially. Next, the electrostatic latent image is developed by a developing device 4 containing a two-component developer having toner and a magnetic carrier, and a toner image is formed on the photosensitive drum 1.
[0007]
In order to perform such operations in the printer unit simultaneously for each color, a laser scanner and an image forming unit are provided for each color, and a toner image is simultaneously formed on the photosensitive drum 1 and conveyed by the transfer belt 15. A multi-color image can be obtained on the transfer material by superimposing the transfer material onto the transfer material by the transfer charger 7 and performing multiple transfer. Thereafter, the transfer material is electrostatically separated from the transfer belt 15 and sent to the fixing device 6, where the toner image is thermally fixed and output as a full-color image.
[0008]
On the other hand, the surface of the photosensitive drum 1 after the transfer of the toner image is removed by adhering contaminants such as transfer residual toner by a cleaner 5 and, if necessary, an optical memory for image exposure is removed by exposure by the pre-exposure means 8. The photosensitive drum 1 is repeatedly used for image formation.
[0009]
As the photosensitive drum 1 used in the electrophotographic image forming apparatus as described above, an organic photosensitive member, an amorphous silicon photosensitive member (a-Si photosensitive member), or the like is often used. Of these, organic photoreceptors are used in copiers and printers, regardless of color or black and white. However, a-Si photoreceptors have high surface hardness and high sensitivity to semiconductor lasers, and are repeatedly used. Since almost no deterioration is observed, it is used as a photosensitive member for electrophotography such as a monochrome high-speed copying machine or a laser beam printer (LBP), but has not yet been commercialized in a color electrophotographic image forming apparatus.
[0010]
[Problems to be solved by the invention]
As described above, there are various reasons why the a-Si-based photoconductor is not used in the color electrophotographic image forming apparatus. One factor is that the potential unevenness is more likely to occur compared to the organic photoconductor. That is, it was not adopted in a color image forming apparatus having severe conditions such as the halftone density.
[0011]
One of the causes of the above potential unevenness is due to a method for manufacturing an a-Si type photoreceptor. The a-Si photosensitive member is formed by plasma-forming the raw material gas with high frequency or microwave, solidifying it, and depositing it on a cylinder such as aluminum to form a photosensitive layer. If the plasma is not uniform, the film thickness is increased in the circumferential direction. And unevenness of potential of about 20 V occurred in the circumferential direction in the developing portion. This is because the difference in electrostatic capacity can be caused by the unevenness of the film thickness, the difference in the chargeability of the photosensitive layer and the difference in the development amount due to the toner, and before the optical memory in the previous image formation was erased. The potential attenuation that occurs between charging and developing due to exposure occurs when there is a difference in film thickness, and the potential unevenness in the developing portion is further increased.
[0012]
In the case of an a-Si photoconductor, the potential attenuation after charging is much larger even in a dark state than in the case of an organic photoconductor, and further, the potential attenuation due to the optical memory for image exposure is increased. Pre-exposure to erase the optical memory is required before charging. For this reason, the potential attenuation between the charging position and the developing position becomes very large, and a potential attenuation of about 100 to 200 V occurs. At this time, due to the above-described film thickness unevenness, potential unevenness of about 10 to 20 V has occurred in the circumferential direction of the photoreceptor.
[0013]
When such potential unevenness occurs, the a-Si photoconductor having a large electrostatic capacity is affected more because the contrast is smaller than that of the organic photoconductor, and density unevenness becomes remarkable.
[0014]
In this way, when a photosensitive member with uneven potential is used in an image forming apparatus having a plurality of image forming units, unevenness occurs for each color photosensitive drum. For example, when a gray halftone is output, conveyance on a transfer material is performed. The color mixing state changed at a position along the direction (the same direction as the circumferential direction of the photosensitive drum), and the color was shifted accordingly.
[0015]
Density unevenness in the case of a black and white image is also a problem, but in the case of a black and white image, the potential unevenness is allowed to some extent because there is not much halftone and there is no color information, but for color images, halftone is used. In addition to the many cases, the level of tolerance for the color shift is more severe than that for the density shift.
[0016]
An object of the present invention is to control the density unevenness due to the potential unevenness even if the photoreceptors of the plurality of image forming units have the potential unevenness in the circumferential direction, thereby minimizing the color variation on the transfer material. An object of the present invention is to provide an image forming apparatus capable of obtaining a color image.
[0017]
[Means for Solving the Problems]
The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention Each A photoconductor including amorphous silicon, a charging unit, an image exposure unit, a developing unit, and a transfer unit 4 colors of yellow, magenta, cyan and black Image forming unit With The above Each image formation In an image forming apparatus for forming a multicolor image in which a plurality of toner images are superimposed on the transfer material by charging, image exposure and development on each photoconductor of the unit, and transferring the obtained toner image to the transfer material.
Position information regarding the circumferential state of each photoconductor Position information regarding the charging potential at the development position during non-image exposure of the photoreceptor And between each of the photoconductors, For the three-color photoconductors for yellow, magenta, and cyan, the transfer material is arranged so that the portion where the charging potential in the one cycle of the photoconductor becomes the maximum value or the minimum value coincides on the transfer material. The relationship between the transfer position and the position in the circumferential direction of the photoconductor is controlled, and for the photoconductor for black, the portion where the charging potential in the one cycle of the photoconductor becomes the maximum value or the minimum value is the three colors. To reverse the photoconductor, An image forming apparatus that controls a relationship between a transfer position of the transfer material and a position in the circumferential direction of the photoconductor.
[0019]
The charger may be a magnetic brush charger or a charger in which conductive particles are applied to a charging roller. The image forming apparatus may include an intermediate transfer body that transfers the toner image to the transfer material before transferring the toner image, and instead of the relationship between the transfer position of the transfer material and the circumferential upper position of the photoconductor. The relationship between the transfer position of the intermediate transfer member and the upper position in the circumferential direction of the photosensitive member is controlled.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments according to the present invention will be described in more detail with reference to the drawings.
[0021]
Example 1
FIG. 1 is a cross-sectional view showing an embodiment of the image forming apparatus of the present invention.
[0022]
This image forming apparatus includes yellow (Y), magenta (M), and cyan (C) image forming units along a transfer belt 15 serving as a transfer material conveying unit, and can form a three-color full-color image. It is configured.
[0023]
In this embodiment, an a-Si photosensitive member is used as the photosensitive drum (drum type photosensitive member) 10 of each image forming unit. This will be described later.
[0024]
A corona charger system was used for charging the photosensitive drum 10. The surface of the photosensitive drum 10 was charged by applying about 5 to 6 kV to the primary wire of the corona charger 3 and controlling the primary wire current to 1 mA at a constant current. The charging potential was controlled by the grid bias of the charger 3.
[0025]
Image exposure was performed on the charged surface of each photosensitive drum 10 by a laser exposure device 2 serving as a latent image forming unit, and electrostatic latent images of respective colors were written. Next, the electrostatic latent images of the respective colors are developed by the respective developing devices 4 and visualized as toner images of yellow, magenta, and cyan, and then carried on the transfer belt 15 and conveyed. Please The image was transferred onto a transfer material (not shown) while being superimposed by the transfer charger 7. Thereafter, the transfer material was conveyed to the fixing device 6 and the toner image was thermally fixed to output an image.
[0026]
Transfer residual toner remains on the surface of each photosensitive drum 10 after the toner image is transferred. The transfer residual toner is scraped off by the cleaning blade of the cleaning unit 5 and stored in the container of the cleaning unit 5. Next, the photosensitive drum 10 is repeatedly used for image formation after optical memory by image exposure is removed by pre-exposure by the pre-exposure means 8. In this embodiment, the wavelength of the pre-exposure emitted from the pre-exposure unit 8 is 680 nm.
[0027]
FIG. 2 is a cross-sectional view schematically showing a layer structure of an a-Si (amorphous silicon) type photoconductor having a positive charging characteristic used as the photosensitive drum 10 in the present invention.
[0028]
This a-Si photoconductor is formed by forming a photosensitive layer 12 and a surface layer 13 in this order on the surface of a cylindrical conductive support (cylinder) 11 such as Al. The photosensitive layer 12 includes a charge injection blocking layer 12a and a photoconductive layer 12b. The charge injection blocking layer 12a is for blocking injection of charges from the conductive support 11 to the photoconductive layer 12b, and is provided as necessary. The photoconductive layer 12b is made of an amorphous material containing at least silicon and exhibits photoconductivity. The surface layer 13 contains silicon and carbon (hydrogen and / or halogen if necessary) and has a capability of holding a latent image in an electrophotographic apparatus.
[0029]
As described in the prior art, the a-Si type photoconductor is formed by plasmaizing the raw material gas with high frequency or microwave, solidifying it, and depositing it on a cylinder such as aluminum, and forming a photosensitive layer. If the plasma is not uniform, film thickness unevenness occurs in the circumferential direction, and conventionally, a potential unevenness of about 20 V has occurred in the circumferential direction of the photoconductor in the developing unit. This is because there is a difference in electrostatic capacity due to unevenness in film thickness, a difference in charging ability of the photosensitive layer, and a charge-development between pre-exposure performed to erase the optical memory in the previous image formation. The potential decay that occurs occurs when there is a difference in film thickness, and is caused by further increasing potential unevenness in the developing portion.
[0030]
The above-mentioned optical memory will be described. When an a-Si photosensitive member is charged and image exposure is performed, a photocarrier is generated, and the surface potential of the photosensitive member is attenuated. At this time, the a-Si photoconductor has many tangling bonds (unbonded hands), which become localized levels and capture a part of the optical carrier, thereby improving the traveling property of the optical carrier. Or reduce the recombination probability of the optical carrier. Therefore, a part of the optical carrier generated by the image exposure is released from the localized level at the same time when an electric field is applied to the a-Si photosensitive member during charging in the next image forming process, and the a-Si type is released. A difference occurs in the surface potential of the photoreceptor between the exposed portion and the non-exposed portion in the previous image forming process, and this finally becomes an optical memory.
[0031]
Therefore, it is common to erase the optical memory by performing uniform exposure in the pre-exposure process so that the optical carrier latent in the a-Si photoconductor is excessive and uniform over the entire surface. is there. At this time, the amount of pre-exposure emitted from the pre-exposure means 8 is increased, or the wavelength of the pre-exposure is brought closer to the spectral sensitivity peak (about 680 to 700 nm) of the a-Si photoconductor, thereby more effectively optical memory ( (Ghost) can be erased.
[0032]
However, if there is film thickness unevenness in the a-Si photosensitive member as described above, the electric field applied between the photoconductive layers is different, so that there is a difference in the release of the photocarrier from the localized level, and the thin film thickness portion. The more the potential is attenuated. For this reason, even if the charging portion can be uniformly charged, potential unevenness occurs in the developing portion. Also, the charging ability is disadvantageous because the smaller the film thickness is, the larger the electrostatic capacity becomes, which is disadvantageous. When the charging ability is lowered, the charging unevenness in the developing portion becomes more remarkable.
[0033]
As shown in FIG. 3, the potential unevenness due to the film thickness unevenness often has a maximum value and a minimum value in the counter electrode position within one cycle of the a-Si photosensitive drum. This is due to the plasma distribution and varies depending on the method for producing the photosensitive layer. Depending on the production method, several peaks may appear in one cycle.
[0034]
In this embodiment, a photoconductor having a film thickness unevenness that causes a pattern unevenness as shown in FIG. 3 is taken as a representative example, and a control method for dealing with it will be described below. However, the present invention is limited to this. Do not mean.
[0035]
First, a surface potential meter (not shown) is installed at the circumferential development position of the a-Si photosensitive drum 10 of each image forming unit, and circumferential potential unevenness is measured for each color at the development position while rotating the photosensitive drum. Then, position information regarding potential unevenness in the circumferential direction of the photosensitive drum 10 is obtained.
[0036]
At the time of measurement, only the charging step and the pre-exposure step are performed without performing the development step, and the portion where the potential of each photosensitive drum is maximized and the portion where the potential is minimized are specified from the potential measurement information at the circumferential position.
[0037]
On the other hand, the gear flange for rotational driving attached to the photosensitive drum 10 is attached in a state where the maximum value and the minimum value of the potential obtained in the above-described measurement can be detected. Due to the relationship with the rotational drive shaft engaged therewith, the photosensitive drums of each color take into account the distance between the units of each color so that the maximum and minimum potentials of the photosensitive drums of each color overlap on the transfer material. Adjust and attach.
[0038]
The photosensitive drum may be driven either independently or independently with each motor at each station, or when all stations are driven with one motor. However, in the case of independent driving by each motor, it is necessary to control the driving by providing means for detecting the position of the driving shaft (photosensitive drum) in the rotational direction, and all the stations are controlled by one motor. In the case of driving, it is necessary to manage the accuracy of the drive gears of each station so that the positional relationship of the photosensitive drums of each station is not changed by continuous use. In order to make the maximum value or the minimum value of the potentials of the photosensitive drums of the respective colors overlap on the transfer material, it is preferable that the distance between the photosensitive drums between the stations is an integral multiple of the circumferential length of the photosensitive drum. The distance between the photosensitive drums is not particularly limited as long as the positional relationship in the rotation direction of the photosensitive drums is controlled.
[0039]
At this time, although the effect of the present invention can be obtained even if there is a level difference between the colors of the photosensitive drum 10, it is desirable that the photosensitive drums have the same maximum and minimum potential unevenness values for each color. Is preferably used.
[0040]
According to the present embodiment, the position control method of the potential unevenness moves the photosensitive drum 10 in the circumferential direction based on the position information related to the potential unevenness, and moves two or more photosensitive drums in the circumferential direction if necessary. Thus, the circumferential position of the potential unevenness between the photosensitive drums 10 and the transfer position on the transfer material coincide with each other, that is, the maximum value or the minimum value of the potential of the unimaged exposed portion at the development position in one cycle of the photosensitive drum. It consists of adjusting the circumferential position of the photosensitive drum so that the value portion matches with each color on the transfer material.
[0041]
In this example, the position control of the potential unevenness was performed so that the circumferential positional relationship of the potential unevenness of the photosensitive drums of the respective colors coincided on the transfer material, and a gray full-tone halftone chart was formed and output. The color unevenness of the image was evaluated. For comparison, image formation was similarly performed using the photosensitive drum without irregularly controlling the position of the potential unevenness with the photosensitive drum irregularly attached.
[0042]
The evaluation was made on the 10-point side taken on the image at regular intervals in the transfer material conveyance direction using a chart with a reflection density of about 0.3 in a gray image generated by superimposing yellow, magenta, and cyan. At a fixed point, L ^* A ^* B ^* The chromaticity value of the space was measured, and the measurement was performed with the maximum value of the color difference (chromaticity difference) δ between the points. Here, as a measuring device, measurement is performed using a Spectrolino of Gretag Macbeth.
[0043]
As a result, in the case of the comparative example, the maximum value of the color difference δ of the output image is 10 or more, but in this example, the maximum value of the color difference δ of the output image can be kept within 5.
[0044]
This is because, when an image is output without performing the potential control of the potential unevenness of the photosensitive drum of each color, since the position of the unevenness of each color is different in one image, the color mixture state at each point changes, and accordingly the color This is because the taste has shifted. On the other hand, as in this embodiment, when the positional relationship of the potential unevenness of the photosensitive drums of the respective colors is made to coincide on the transfer material by the position control of the potential unevenness, in one image. This is because the positions of unevenness of the respective colors coincide with each other, so that unevenness in density occurs, but the color mixture ratio is almost uniform, and as a result, the variation in color can be minimized.
[0045]
Example 2
FIG. 4 is a sectional view showing another embodiment of the image forming apparatus of the present invention.
[0046]
In the first embodiment, a corona charger is used as the charging means. In this embodiment, a magnetic brush charger 20 made of magnetic particles is used. The other configurations of the present embodiment are basically the same as those of the first embodiment, and the same reference numerals as those in FIG. 1 in FIG. 4 denote the same members.
[0047]
Conventionally, a corona charger has been put to practical use as a charging means for an a-Si type photosensitive member. However, since the relative dielectric constant of the a-Si type photosensitive member is 11 to 12, which is larger than that of an organic photosensitive member, As the electrostatic capacity increases, the charging ability decreases, and the image flow due to the flow of the latent image due to the discharge tends to occur.
[0048]
On the other hand, contact charging means using a conductive roller, a fur brush roller, a magnet roller carrying magnetic particles, or the like as a charging member, when the a-Si photoconductor is charged, the surface of the a-Si photoconductor is 10. ⁹ -10 ¹⁴ Since it is formed of a layer made of a material of Ωcm, it is possible to obtain a charging potential on the surface of the photosensitive member that is almost equal to the direct current component of the charging bias applied to the contact charging member.
[0049]
Such a charging method is called injection charging because charging is performed by directly injecting a charge into a photoreceptor without using discharge. According to this injection charging, since the charging to the photoconductor does not use a discharge phenomenon as in the case of charging by a corona charger, a complete ozone-less and blackout power consumption type charging becomes possible. It is coming. Further, the charging ability can be prevented from being lowered and the image can be prevented from flowing, and the charged potential can be easily controlled because the charging is performed in the vicinity of the applied voltage.
[0050]
The magnetic brush charger 20 magnetically constrains conductive magnetic particles on a support sleeve containing a magnet, forms a magnetic brush on the magnetic particles, and rotates the magnetic brush by the rotation of the sleeve while rotating the photoconductor. The charging is started by applying a voltage to the sleeve in contact with the sleeve.
[0051]
FIG. 5 is a sectional view of the magnetic brush charger used in this embodiment. In the magnetic brush charger 20, a nonmagnetic support sleeve 22 having an outer diameter of 16 mm, which includes a fixed magnet roller 21 in a non-rotating manner, is rotatably installed in a casing 25 whose lower end is open. The magnetic particles 23 are attached and held by the magnetic force of the magnet roller 21. The magnetic particles 23 are conveyed toward the photosensitive drum 10 by the rotation of the sleeve 22, and a magnetic brush is formed on the magnetic particles 23 whose layer thickness is regulated by the regulating blade 24 by the magnetic field of the magnet roller 21. Let
[0052]
In this embodiment, the nip width at which the magnetic brush of the magnetic particles 23 comes into contact with the photosensitive drum 10 is adjusted to about 6 mm. The sleeve 22 was rotated in the counter direction with respect to the photosensitive drum 10, and the rotational speed was 150 mm / second with respect to the rotational speed of the photosensitive drum 10 being 100 mm / second. When a charging bias is applied to the support sleeve 22, charges are injected from the magnetic particles 23 on the sleeve 22 onto the surface of the photosensitive drum 10, and the surface of the photosensitive drum 10 is charged to a charging potential close to the DC component of the charging bias.
[0053]
In general, the magnetic particles 23 have an average particle diameter of 10 to 100 μm and a saturation magnetization of 20 to 250 emu / cm. ^Three , Resistance 10 ² -10 ^Ten The resistance is preferably 10 Ωcm, considering that the photosensitive drum 10 has an insulation defect such as a pinhole. ⁶ It is preferable to use a Ωcm or more. The resistance value of magnetic particles has a bottom area of 228 cm. ² After putting 2 g of magnetic particles in a metal cell of 6.6 kg / cm ² And measured by applying a voltage of 100V.
[0054]
In order to improve the charging performance of the magnetic brush, it is better to use magnetic particles with as low a resistance as possible. In this example, the average particle size is 25 μm, the saturation magnetization is 200 emu / cm. ^Three , Resistance 5 × 10 ⁶ Ωcm magnetic particles were used. These magnetic particles are obtained by adjusting the resistance by oxidizing / reducing the ferrite surface.
[0055]
In this embodiment, as in the first embodiment, a surface potential meter (not shown) is installed at the circumferential development position of the a-Si photosensitive drum 10 of each image forming unit, and the circumferential direction is developed at the development position while rotating the photosensitive drum. Is measured for each color to obtain position information regarding the potential unevenness in the circumferential direction of the photosensitive drum 10, and the portion where the potential of each photosensitive drum is maximized from the obtained potential measurement information at the circumferential development position. And identify the smallest part.
[0056]
Further, the rotational drive gear flange attached to the photosensitive drum 10 is attached in a state where the maximum value and the minimum value of the potential obtained in the above-described measurement can be detected, and the rotational drive engaged therewith. From the relationship with the axis, the photosensitive drums of each color are adjusted and mounted on the transfer material so that the maximum value and the minimum value of the photosensitive drums of each color overlap on the transfer material in consideration of the distance between the units.
[0057]
In this example, as in Example 1, the position control of potential unevenness was performed to form an image of a gray halftone chart, and the color unevenness of the output image was evaluated. As a comparative example, as in the case of Example 1, image formation was performed even when position control for potential unevenness was not performed.
[0058]
The evaluation method is the same as in Example 1, and a gray image generated by superimposing yellow, magenta, and cyan is used with a chart in which the reflection density is about 0.3. 10 side fixed points taken in ^* A ^* B ^* The chromaticity value of the space was measured, and the measurement was performed with the maximum value of the color difference (chromaticity difference) δ between the points.
[0059]
As a result, in the present example, the maximum value of the color difference δ was 10 or more in the comparative example in which the three-color full-color image was output without controlling the positional relationship of the potential unevenness of the photosensitive drum of each color. Control was performed so that the positional relationship in the circumferential direction of potential unevenness of the photosensitive drums of the respective colors coincided on the transfer material, and the image was output, so that the maximum value of the color difference δ of the image could be kept within 5.
[0060]
As described above, the present invention can also be applied to an image forming apparatus that charges a-Si photosensitive drums using a magnetic brush charger, and similarly exhibits excellent effects.
[0061]
Example 3
6 is a sectional view showing another embodiment of the image forming apparatus of the present invention, and FIG. 7 is a sectional view showing a charger used in the image forming apparatus of FIG.
[0062]
In Example 1, a corona charger was used as the charging means, and in Example 2, a magnetic brush charger was used. In this example, a contact charger 30 as shown in FIG. 7 was used. The other configurations of the present embodiment are basically the same as those of the first embodiment, and the same reference numerals as those in FIG. 1 in FIG. 6 denote the same members.
[0063]
The charging device 30 includes a charging roller 31 that is rotatably disposed in a casing 34 that is partially open at a lower end, and the charging application particle 33 is applied to the charging roller 31 by the particle applying unit 32, and the charging promoting particle 33 is applied. The roller 31 is brought into contact with the surface of the photosensitive drum 10, and the surface of the photosensitive drum 10 is charged by a charging bias applied to the charging roller 31.
[0064]
The charging roller 31 is provided with a middle resistance layer of an elastic foam on a cored bar. , In this example, the medium resistance layer was formed into a roller shape on the cored bar by prescribing materials from a resin such as urethane, for example, conductive particles such as carbon black, and a sulfiding agent, a foaming agent, and the like. . Thereafter, the surface of the intermediate resistance layer was polished as necessary to form the charging roller 31 as an elastic conductive roller having a diameter of 12 mm.
[0065]
The roller resistance of the charging roller 31 thus prepared was measured as 100 kΩ. In the measurement, the charging roller is pressure-bonded to an aluminum drum having an outer diameter of 30 mm so that a load of 1 kg is applied to the core bar protruding from both ends of the charging roller, and 100 V is applied between the core metal and the aluminum drum in this state. I went there.
[0066]
It is important that the charging roller 31 functions as an electrode. In other words, the charging roller 31 has elasticity and not only can obtain a sufficient contact state with the photosensitive drum 10 as a member to be charged, but also sufficient to charge the moving photosensitive drum 10 by charging (charge injection). It is necessary to have a low resistance. On the other hand, when a defective portion such as a pinhole exists in the photosensitive drum 10, it is necessary to prevent voltage leakage. From these, in order to obtain sufficient charging property and leakage resistance for the charging roller 31, 10 ^Four -10 ⁷ It preferably has a resistance of Ω.
[0067]
The material of the middle resistance layer of the charging roller 31 is not limited to an elastic foam, but may be an elastic material such as EPDM, urethane, silicone rubber, IR, carbon black, metal oxide, etc. for resistance adjustment. A rubber material in which a conductive substance is dispersed is mentioned. In particular, the resistance can be adjusted using an ion conductive material without dispersing the conductive substance.
[0068]
Charging is possible by contacting the charging roller 31 as described above with the a-Si photosensitive drum 10 and applying a voltage. However, as in this embodiment, the charge accelerating particles 33 are provided on the surface of the charging roller 31. When applied, the contact with the photosensitive drum 10 can be improved and the frictional force can be greatly reduced. At the contact nip between the charging roller 31 and the photosensitive drum 10, the photosensitive drum 10 is charged with the charge accelerating particles 33 interposed therebetween.
[0069]
As a result, the charging roller 31 can contact the photosensitive drum 10 with a speed difference, and at the same time, charges can be injected densely into the photosensitive drum 10 via the charge accelerating particles 33, and the charging efficiency is higher than in the case of only the charging roller 31. And a potential substantially equal to the DC voltage applied to the charging roller 31 can be applied to the photosensitive drum 10.
[0070]
In this embodiment, the charge promoting particles 33 have a specific resistance of 10 ⁶ Conductive zinc oxide particles having an Ωcm and an average particle diameter of 3 μm were used.
[0071]
As the material for the charge accelerating particles 33, various conductive particles such as conductive inorganic particles such as metal oxides and mixtures of these with organic particles can be used. Charge promotion Particulate The resistance is 10 with a specific resistance in order to transfer charges through the particles. ^Ten Ωcm or less is preferable. Charge promotion Particulate The specific resistance is measured by the tablet method, and the bottom area is 2.26 cm. ² About 0.5g of charge in the cylinder Particles Then, a voltage of 100 V was applied in a state where the upper and lower electrodes of the cylinder were pressurized with 15 kg, the resistance value of the pellet-like particles was measured, and the resistance value was normalized to obtain the specific resistance value.
[0072]
The average particle diameter of the charge accelerating particles 33 is preferably 50 μm or less in order to obtain good uniform charging. The lower limit value of the average particle diameter is 10 nm, as long as particles can be stably obtained. In the present invention, the particle diameter in the case where the particles are constituted as an aggregate is defined as the average particle diameter of the aggregate. For the measurement of the particle size, 100 or more particles were extracted from observation with an optical microscope or electron microscope, the volume particle size distribution was calculated with the maximum chord length in the horizontal direction, and the particle size was determined with the 50% average particle size.
[0073]
The particle applying means 32 is in contact with the charging roller 31 by rotating the solidified charge accelerating particles 33 against the rotating roller 32a by a spring 32b and sliding the roller 32a by a small amount. Further, the charge accelerating particles 33 adhering to the surface of the roller 32a are uniformly applied onto the charging roller 31, and the charging roller 31 makes contact with the photosensitive drum 10 (charging nip). This is for supplying the charge promoting particles 33 uniformly.
[0074]
In this embodiment, the charging roller 31 is rotated with a speed difference with respect to the photosensitive drum 10. For this reason, the vicinity of the contact nip of the charging roller 31 made of an elastic body is greatly deformed as compared with the conventional case, and the charge accelerating particles attached to the surface of the charging roller 31 are easily transferred to the photosensitive drum 10. Surface charge promoting particles are reduced. The decrease in the charge promoting particles 33 on the charging roller 31 is compensated by the application of the charge promoting particles 33 by the coating unit 32.
[0075]
The operation of the charging device 30 in this embodiment will be described. First, the charge accelerating particles 33 are applied to the surface of the charging roller 31 by the particle applying means 32. The applied charge accelerating particles are conveyed to a charging portion (contact nip) facing the photosensitive drum 10 by the rotation of the charging roller 31. The charging roller 31 is driven at a constant speed so that the contact portion moves in the direction opposite to the photosensitive drum 10, and applies a charging bias to the core metal of the charging roller 31. At the contact nip between the charging roller 31 and the photosensitive drum 10, charges are injected into the surface of the a-Si-based photosensitive drum 10 through the charge accelerating particles 33 that rub the surface of the photosensitive drum 10 without gaps, and the surface of the photosensitive drum 10. Is charged to a potential substantially equal to the applied voltage.
[0076]
In this embodiment, as in the previous embodiments, the potential unevenness in the circumferential direction of each photosensitive drum 10 is measured at the development position by a surface potential meter, and the rotational flange of the photosensitive drum and the rotation driving shaft thereof are measured. From the relationship, the circumferential position of the photosensitive drum was adjusted and controlled so that the circumferential positional relation of the potential unevenness of each photosensitive drum coincided on the transfer material. Then, a three-color full-color image was formed, and the color unevenness of the output image was evaluated. For comparison, image formation was similarly performed using a photosensitive drum without performing potential unevenness position control. Similarly, the evaluation method is 10 fixed points taken on the image at equal intervals in the transfer material conveyance direction. ^* A ^* B ^* The determination is based on the maximum value of the color difference (chromaticity difference) δ between the points by measuring the chromaticity value of the space.
[0077]
As a result, in the present example, the maximum value of the color difference δ was 10 or more in the comparative example in which the three-color full-color image was output without controlling the positional relationship of the potential unevenness of the photosensitive drum of each color. Control was performed so that the positional relationship in the circumferential direction of potential unevenness of the photosensitive drums of the respective colors coincided on the transfer material, and the image was output, so that the maximum value of the color difference δ of the image could be kept within 5.
[0078]
As described above, the present invention can also be applied to an image forming apparatus that charges a-Si type photosensitive drums using a contact charger in which a charge accelerating particle and a charging roller are combined. There is an effect.
[0079]
Example 4
FIG. 8 is a sectional view showing still another embodiment of the image forming apparatus of the present invention.
[0080]
In the first to third embodiments, the description has been given of the case where the present invention is applied to an image forming apparatus that includes yellow, magenta, and cyan three-color image forming units and performs three-color full-color image formation. As shown, the image forming apparatus includes four color image forming units of yellow (Y), magenta (M), cyan (C), and black (K), and is applicable to an image forming apparatus that forms a full color image of four colors. it can. In FIG. 8, the same reference numerals as those in FIG. 1 denote the same members.
[0081]
Such four-color full-color image forming apparatuses have become the mainstream of color image forming apparatuses since the image forming apparatuses have become digital. Advantages of having a black unit include sharpening of black characters and reduction of color toner by UCR (Under Color Removal).
[0082]
UCR is a technology that is generally used in full-color image forming apparatuses that currently use black developer, and replaces yellow, magenta, and cyan with black to improve black reproducibility and toner. This is intended to reduce the amount. Replacing all common parts of yellow, magenta, cyan, and black with black is called 100% UCR, and replacing 50% of common parts with black is called 50% UCR. The current general full-color image forming apparatus does not perform 100% UCR, and reproduces black and gray using yellow, magenta, cyan, and black.
[0083]
In the present embodiment, a method was used that can realize a reduction in density variation as well as a reduction in color variation caused by uneven charging of the a-Si photosensitive drum 10 using the UCR.
[0084]
Specifically, for the yellow, magenta, and cyan photosensitive drums 10, as in the first to third embodiments, the maximum or minimum value portion of the potential of the unimaged exposed portion at the development position in one cycle of the photosensitive drum is The positional relationship of the photosensitive drums is controlled so that the colors on the transfer material coincide with each other, and the black photosensitive drums are arranged in the reverse order of the above cycle, that is, the positions where the black minimum value and the maximum value become yellow. , Magenta and cyan were controlled so that the portions corresponding to the maximum value and the minimum value coincided on the transfer material.
[0085]
In this way, in the case of a black unit, by applying potential unevenness in reverse, for example, when an image is output with 50% UCR of gray, a gray made with three colors of yellow, magenta, and cyan, and a gray made with black Are almost equivalent, and density unevenness can be reduced. In addition, the color can be considerably improved by making the combined balance of the three colors yellow, magenta, and cyan as uniform as possible.
[0086]
In this embodiment, control is performed so that the positional relationship of the potential unevenness of the three color photosensitive drums coincides on the transfer material, and the positional relationship of the potential unevenness of the black photosensitive drum has a reverse period (reverse phase). Then, four color full-color images were formed, and color irregularities of the output images were evaluated. For comparison, image formation was performed in the same manner without controlling the positional relationship of potential unevenness of the photosensitive drum. As before, 10 side fixed points taken on the image at equal intervals in the transfer material conveyance direction, L ^* A ^* B ^* The chromaticity value of the space was measured, and the image was evaluated by the maximum value of the color difference (chromaticity difference) δ between the points.
[0087]
As a result, the maximum value of the color difference δ is 10 or more in the case of the comparative example in which the image is output without controlling the positional relationship of the potential unevenness on the photosensitive drums of four colors of yellow, magenta, cyan, and black. As in this embodiment, the positional relationship of the potential unevenness of the photosensitive drums of the three colors is controlled, and the positional relationship of the potential unevenness of the black photosensitive drum is controlled so as to be in the reverse cycle. Is output, the maximum value of the color difference δ can be kept within 5 and density unevenness can be reduced to a level that is hardly recognized.
[0088]
This is because, when an image is output without performing the position control of the potential unevenness of the position of the photosensitive drum of each color, the color mixture state at each point changes because the position of the unevenness of each color is different in one image. This is because the color has shifted accordingly. In contrast, in this embodiment, since the positional relationship in the circumferential direction of the potential unevenness of the photosensitive drums of each color of yellow, magenta, and cyan is adjusted to match on the transfer material, the image is output. Since the color mixture ratio is almost uniform, the variation in color tone can be minimized, and for black, the potential unevenness is overlapped in the reverse cycle and the image is output. Counteract unevenness Doing This is because density unevenness could be reduced.
[0089]
In the above-described embodiments, the case where the charging unit is a corona charger, a magnetic brush charger, or a charger in which a charge accelerating particle is combined with a charging roller is used.
[0090]
Preferably, it has a sufficient contact point with respect to the a-Si photosensitive member, such as a magnetic brush charger as used in Examples 2 and 3, and a charger in which a charge accelerating particle is combined with a charging roller. It is desirable to use an injection charging method in which charging is performed by directly injecting the toner into the photoreceptor. If this injection charging is used, a discharge phenomenon in which charging to the photoreceptor is performed using a corona charger is not used, and thus complete ozone-less and low power consumption type charging is possible. In addition, the charging ability can be prevented from being lowered and the image can be prevented from flowing, and the potential can be easily controlled since the charging is performed in the vicinity of the applied voltage. In addition, it has been confirmed that the use of injection charging is improved against the influence of the optical memory, which is the subject of the present invention, and the above-described potential unevenness tends to be improved.
[0091]
However, the present invention provides a sufficient effect even in the case of the corona charger used in Example 1, and is not limited to the case where injection charging is used. Even when a brush charger or the like is used, the effect of the present invention is not affected and a sufficient effect can be obtained.
[0092]
Although the positively charged a-Si photosensitive drum has been described as the photosensitive member, a negatively charged a-Si photosensitive drum can also be used, and it has been confirmed that the same effect can be obtained. In other words, the present invention can be applied when the circumferential charging unevenness occurs in the a-Si photosensitive drum regardless of the charging polarity and layer structure of the photosensitive drum, and a sufficient effect is exhibited.
[0093]
In the embodiments, the cause of uneven charging in the circumferential direction has been described by taking the film thickness unevenness of the photosensitive layer as an example. However, charging unevenness may occur due to the film characteristics or the influence of the base such as the drum substrate. The present invention can be applied to all cases where a plurality of image forming units using an a-Si type photosensitive member having uneven charging in the circumferential direction is provided, regardless of the cause of uneven charging in the circumferential direction.
[0094]
In each of the above embodiments, an image forming apparatus of a type in which a plurality of image forming units are arranged along a transfer belt and a toner image of each color on the photoreceptor is directly transferred to a recording material has been described. The present invention is not limited to this, and a plurality of image forming units are arranged along the intermediate transfer member (intermediate transfer belt), and the toner images of the respective colors on these photosensitive members are transferred onto the intermediate transfer member in a superimposed manner (1 (Secondary transfer) and then an image forming apparatus of a type (secondary transfer) in which the toner images of the respective colors superimposed on the recording material sent to the intermediate transfer member are collectively transferred. In this case, instead of the relationship between the transfer position of the transfer material and the circumferential position of the photosensitive member, the relationship between the transfer position of the intermediate transfer member and the circumferential position of the photosensitive member is controlled. if Well, the same effect can be achieved.
[0095]
【The invention's effect】
As described above, according to the present invention, the position relating to the charging potential at the development position at the time of non-image exposure of the photoconductor for the photoconductor including amorphous silicon installed in each of the image forming units of yellow, magenta, and cyan. The portion having the maximum value or the minimum value of the charged potential at the developing position in one cycle of the photosensitive member between the photosensitive members on the transfer material (if there is an intermediate transfer member) The relationship between the transfer position of the transfer material (intermediate transfer body) and the position in the circumferential direction of the photoconductor is controlled so that the transfer material is consistent with each other. It is possible to obtain a multicolor image with minimal color variation. When the image forming units are provided for four colors of yellow, magenta, cyan, and black, the three color photoconductors for yellow, magenta, and cyan are controlled in the same manner as described above, so that the photosensitive material for black is used. For the body, the transfer position of the transfer material (intermediate transfer body) is such that the portion where the charging potential at the development position becomes the maximum value or the minimum value in one cycle of the photoreceptor is opposite to that of the three-color photoreceptor. Since the relationship between the positions in the circumferential direction of the photoconductor is controlled, it is possible to obtain a multicolor image in which density unevenness is minimized as well as color variation.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an embodiment of an image forming apparatus of the present invention.
FIG. 2 is a cross-sectional view showing a layer structure of an a-Si type photoreceptor used in the present invention.
FIG. 3 is a potential diagram showing unevenness in charging potential of an a-Si photosensitive drum.
FIG. 4 is a cross-sectional view showing another embodiment of the image forming apparatus of the present invention.
5 is a cross-sectional view showing a magnetic brush charger used in the image forming apparatus of FIG.
FIG. 6 is a cross-sectional view showing still another embodiment of the image forming apparatus of the present invention.
7 is a cross-sectional view showing a charger in which charging promoting particles are combined with a charging roller used in the image forming apparatus of FIG. 6;
FIG. 8 is a cross-sectional view showing still another embodiment of the image forming apparatus of the present invention.
FIG. 9 is a cross-sectional view showing a conventional image forming apparatus.
[Explanation of symbols]
2 Laser exposure equipment
3 Corona charger
4 Developer
7 Transfer charger
8 Pre-exposure means
10 a-Si photosensitive drum
15 Transfer belt
20 Magnetic brush charger
30 Charger with electrostatic charge particles

Claims (3)

A photosensitive member, each comprising amorphous silicon, a charging unit, comprising an image exposure means, a developing means, a yellow and a transfer unit, magenta, cyan, a 4-color image forming unit for black, the image forming units In an image forming apparatus for forming a multicolor image in which a plurality of toner images are superimposed on the transfer material by charging, image exposure and development on each of the photosensitive members, and transfer of the obtained toner image to the transfer material,
Position information relating to the state of each photoconductor in the circumferential direction includes position information relating to a charging potential at a developing position during non-image exposure of the photoconductor, and for each of the photoconductors , for yellow, magenta, and cyan For the three-color photoconductors, the transfer position of the transfer material and the circumference of the photoconductor are such that the portion where the charging potential in the one cycle of the photoconductor becomes the maximum value or the minimum value coincides on the transfer material. The relationship of the position in the direction is controlled, and for the black photoconductor, the portion where the charging potential in the one cycle of the photoconductor is the maximum value or the minimum value is opposite to the three-color photoconductor. an image forming apparatus characterized by controlling the relationship between the circumferential direction on the position of the photosensitive member and the transfer position of the transfer material. The image forming apparatus according to claim 1 , wherein the charger is a magnetic brush charger or a charger in which conductive particles are applied to a charging roller. Before transferring the toner image onto the transfer material, an intermediate transfer member for transferring the toner image is provided. Instead of the relationship between the transfer position of the transfer material and the circumferential upper position of the photoconductor, the transfer position of the intermediate transfer member 3. The image forming apparatus according to claim 1, wherein the relationship between the position of the photosensitive member and the upper position in the circumferential direction of the photosensitive member is controlled.

Images