JPH08234522A - Color image forming device - Google Patents

Abstract

PURPOSE: To make the transfer efficiency of the multicolor toner image maximum and to provide a color image free from defective transfer and excellent in image quality by electrostatically recharging a multicolor toner image so that toner charge amount may have a value in a specified range by a second discharging means after discharging the multicolor toner image by a first discharging means after a final developing process and before a transferring process. CONSTITUTION: An electrifier 2 constituted of a scorotron, an image exposing means 3, a developing device 4 where the toner of yellow, magenta, cyan and black are separately arranged, an upstream side pre-transfer electrifier constituted of a corotron as the first discharging means 5 and a downstream side pre- transfer electrifier constituted of the scorotron as a second discharging means 6 or the like are successively arrayed in the periphery of a photoreceptor drum 1. By this constitution, the multicolor toner image formed by superposing developing on the photoreceptor is discharged by the first discharging means 5 where AC voltage is applied after the final developing process and before the transferring process and next, is recharged by the second discharging means 5 where DC voltage is applied so as to make the toner charge amount 18 to 25μc/g.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital xerographic color image forming apparatus, and more particularly, a plurality of toner images of different colors are superposed on a photosensitive member by repeating the steps of charging, exposing and developing a plurality of times. The present invention relates to an improvement in a color image forming apparatus of a type in which the image is collectively transferred to a transfer material after being formed.

[0002]

2. Description of the Related Art Heretofore, as a color image forming apparatus of this type of digital xerography system, for example, Japanese Patent Laid-Open No. 59-59 has been disclosed.
There is one disclosed in Japanese Patent Laid-Open No. 121348. That is, in this publication, toner images of the required number of colors are formed on the photoconductor in a laminated state by repeating charging, image exposure and development on the photoconductor for the required number of colors, and There is described a color image forming apparatus which employs a basic color electrophotographic system in which toner images of a required number of colors are collectively transferred onto a recording medium.

Of these color image forming apparatuses, in particular, one charging device and one image exposing device are arranged so as to be used for forming toner images of respective colors, and developing devices are successively connected around the photosensitive member by the required number of colors. In the configuration arranged in such a manner, it is possible to reduce the size and cost of the device due to the small number of components,
Moreover, since toner images of a plurality of colors are formed on a single photoconductor in a superposed state, there is little positional deviation between the toner images of the respective colors, and in principle a high-quality color image with less color deviation can be formed. It has excellent characteristics. Note that such a color image forming apparatus is the same in all of the prior arts described below, but more specifically, when superimposing a plurality of toner images on the photoconductor, a second color image is formed in the same area of the photoconductor. Subsequent image exposure is performed so that the toner images for the second and subsequent colors are directly laminated on the toner image for the first color.

However, in the color image forming apparatus of the color electrophotographic system as described above, in the portion where the toner image of the photoconductor is present, three or more kinds of toner are overlapped due to the color configuration, and the toner of the second and subsequent colors is used. The uniform recharging in the charging process in the image forming process causes the following problems in the transfer process. That is, a high development amount portion in which a plurality of toners are overlapped by three or more kinds and a low development amount portion represented by a single color or highlight area are mixed on the photoconductor, and the charge amount of the toner varies depending on the number of times of recharging. Since the conditions of the multicolor toner image transferred due to the difference are not uniform, there is a problem that the transfer cannot be performed with the best transfer efficiency.

Therefore, as a color image forming technique for solving such a problem, for example, after toner images of a plurality of colors are formed on a photoconductor, the toner images of the plurality of colors are charged by a charger. Has been proposed to introduce a step of adjusting the toner charge amount of the toner image in the final developing step which is not recharged to the toner charge amount of the recharged toner image (Japanese Patent Laid-Open No. 63-81493).
Issue). That is, in this color image forming technology, by introducing the above steps, the charge amount of each color toner image before transfer is made nearly constant, and the transfer problem due to the difference in toner charge amount of each color toner image is reduced. It is what

However, in the above-mentioned color image forming technique, there still remains a difference in the toner charge amount depending on the number of times of recharging, and there is a problem that the high development amount portion and the low development amount portion are mixed. In particular, in order to avoid the problem that the high development amount portion and the low development amount portion are mixed, when the transfer conditions are set on the basis of the high development amount portion, the transfer efficiency of the low development amount portion decreases and the image Roughness and white spots occur, and conversely, when the transfer conditions are set based on the low development amount portion, high transfer efficiency cannot be obtained at the high development amount portion, color change and transfer unevenness occur. Therefore, it is difficult to prevent these problems.

Therefore, in the past, a new color image forming apparatus has been proposed (Japanese Patent Laid-Open No. 5-28).
9550 publication). That is, in this publication, a first pre-transfer step charger to which an AC voltage is applied and a second pre-transfer step charger to which a DC voltage is applied are provided, and a photoreceptor and a toner are charged after the final development and before the transfer step. There is described a color image forming apparatus in which charge removal and charging are separately performed so that the potential of a photoconductor under each color toner layer and the charge amount of each color toner are made uniform to obtain good transfer performance.

[0008]

[Problems to be Solved by the Invention] However, the above 2
The color image forming device equipped with two pre-transfer charging devices
It merely discloses that the functions of charge removal and charging of the photoconductor and toner before the transfer process are separated, and thereby the photoconductor potential and the charge amount of each color toner are made uniform, and how much is actually applied to a multicolor toner image. It has not been disclosed that the transfer (electrostatic transfer) efficiency is most improved when the electric charge is given, and therefore, there is a problem that this technology cannot be put to practical use as it is.

The present invention has been made in view of the above problems, and an object of the present invention is to eliminate a charge of a multicolor toner image with an AC voltage (electric field) after the final development process and before the transfer process. An object of the present invention is to provide a color image forming apparatus which can maximize the transfer efficiency of a multicolor toner image when recharged with a DC voltage (electric field) and can obtain a high-quality color image without transfer defects.

[0010]

A color image forming apparatus of the present invention comprises at least a charging device, an image exposing means, a plurality of developing devices and a transferring device, which are arranged around a photosensitive member. At least an AC voltage is applied in a color image forming apparatus that forms a multicolor toner image by overdevelopment on a photoconductor by repeating each step of development a plurality of times and then collectively transfers the multicolor toner image to a transfer material. The multi-color toner image is provided with a first discharging means and a second discharging means to which at least a DC voltage is applied. It is characterized in that it is recharged so that the charge amount becomes 18 to 25 μC / g.

In this technical means, it is important to recharge by the second discharging means so that the toner charge amount becomes 18 to 25 μC / g in order to maximize the transfer efficiency. When recharging is performed so that the toner charge amount becomes less than 18 μC / g, void discharge occurs in a toner image portion having a low development amount such as a monochromatic image or a halo image, and image defects or white spots occur. On the contrary, if recharging is performed to exceed 25 μC / g, color change or transfer unevenness occurs in the transfer of the toner image portion having a high development amount.

As the first discharging means and the second discharging means in the above technical means, two dedicated pre-transfer chargers are provided between the developing device and the transfer device, and the upstream side thereof is the first discharge device. In addition to being able to use the downstream side as the second discharging means as means, when the existing component is also used, the transfer material peeling charger or the pre-cleaning charger is used as the first discharging means, and the charger of the photoreceptor is used. Can also be used as the second discharging means. If you also use existing components,
A combination in which the transfer material peeling charger is also used as the first discharging means and the photoreceptor charger is also used as the second discharging means is preferable.

[0013]

According to the above technical means, the multicolor toner image formed on the photosensitive member by the overlapping development is discharged by the first discharging means after the final developing step and before the transferring step, and then the second discharging means. The toner charge amount is 18 to 25 μC /
Since the toner is recharged so as to be g, the transfer efficiency is maximized without the occurrence of void discharge in the low density toner image portion and the transfer failure in the high density toner image portion.

Here, it can be presumed that such an action is obtained for the following reasons. That is, the electrostatic transfer can generally regulate the transferability by the toner charge amount and the transfer electric field E, and is published in the Institute of Electronics and Communication Engineers paper '77 / 4 vol J60-C No. 4, the transfer efficiency (ratio of the transferred amount to the developed amount) is finally expressed by the ratio of the surface charge density of the toner layer and the surface charge density given to the transfer material. From this theory, the transfer material has 100% when the surface charge density exceeds the surface charge density of the toner layer
However, it is known that void discharge occurs when the surface charge density of the transfer material becomes too large and the transfer electric field E becomes too large. From this,
In the electrostatic transfer in which a high density toner image portion and a low density toner image portion coexist as in the present invention, the toner charge amount of the transferred multicolor toner image is suppressed to be low as a whole. It is presumed that a high transfer efficiency is obtained in the high density toner image portion without generating void discharge in the low density toner image portion, and the transfer efficiency is maximized when viewed as a whole.

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a schematic view showing a color image forming apparatus according to the first embodiment of the present invention. In FIG. 1, reference numeral 1 is a photosensitive drum as an organic photosensitive member having an insulating surface coating layer, and around the photosensitive drum 1, a charger 2 made of a scorotron, an image exposing means 3 such as a semiconductor laser, and a yellow member. , A developing device 4 (Y, M, C, B) in which magenta, cyan, and black toners are separately contained, an upstream pre-transfer charger including a corotron as the first discharging device 5, and a second discharging device 6. The downstream pre-transfer charger including the scorotron, the transfer charger 7 of the corotron, the pre-cleaning discharger 8, and the cleaner 9 using the cleaning blade are sequentially arranged. Further, 10 in the figure is an AC power supply for applying an AC voltage to the first discharging means (charger 5),
Reference numeral 11 is a DC power source for applying a DC voltage to the second discharging means (charging device 6), 12 is a recording paper as a recording material, and 13 is a dielectric transfer belt for adsorbing and conveying the recording paper 12 and passing it through a transfer portion. is there.

In this embodiment, a DC voltage of 6 kV is applied to the corona discharge wire W of the charger 2 and a grid voltage of -70 is applied to the grid G thereof.
A direct current bias of 0 V is applied, and a direct current developing bias of -600 V is applied to each developing device 4. Further, an AC voltage of AC _pp 10 kV is applied to the discharge wire from the AC power supply 10 to the upstream pre-transfer charger 5, and the downstream pre-transfer charger 6 is connected to the discharge electrode W from the DC power supply 11.
A DC voltage of −500V is applied to the grid G of 4.5 kV.

Next, the operation of the apparatus of this embodiment will be described.

First, the photosensitive drum 1 is rotated by a predetermined amount.
In the cycle, the first color yellow toner image is formed. That is, after the surface of the photosensitive drum 1 is discharged by the pre-cleaning discharger 8 and cleaned by the cleaner 9, it is uniformly charged (to −700 V) by the charger 2, and then the surface of the charged photosensitive drum 1 is charged. The image exposure means 3 performs the first image exposure to form a latent image of the first color. The surface voltage of the photoconductor after this solid exposure is -4.
It is 00V. Then, the latent image is developed with yellow toner by the developing device 4Y to visualize it.

Subsequently, in the second cycle, a magenta toner image of the second color is formed. That is, the surface of the photosensitive drum 1 on which the toner image of the first color is formed is uniformly charged by the charger 2 under the same condition, and then the second image is formed by the image exposing unit 3.
Image exposure is performed twice to form a latent image of the second color, and the latent image is developed with magenta toner by the developing device 4M to be visualized. In this image exposure process, the exposure intensity is increased so that the potential of the photoreceptor after exposure is −400V even when the exposure is performed from the yellow toner. After that, the same steps as above are repeated to form the cyan toner image of the third color and the black toner image of the fourth color so as to be sequentially superposed.

The multicolor toner image thus formed by using the four color toners is first discharged by the upstream pre-transfer charger 5 after the final development process by the developing device 4B. At this time, the surface potential of the photosensitive drum is approximately −
It became 250V. Subsequently, the discharged multicolor toner image is recharged by the downstream pre-transfer charger 6. The toner charge amount after the recharging was about 18 μC / g. Finally, this multicolor toner image is collectively transferred by the transfer charger 7 onto the transfer paper 12 conveyed by the transfer belt 13. The transfer at this time was performed by applying a DC voltage to the transfer charger 7 so that the total current was 200 μA.

In this embodiment, a monochromatic image of each color toner is formed on the surface of the photosensitive drum 1, and a red image in which magenta toner is overlaid on yellow toner and a blue image in which cyan toner is overlaid on magenta toner are formed as multicolor toner images. Each patch image consisting of an image, a green image in which cyan toner is overlaid on yellow toner, and a black image in which magenta toner and cyan toner are overlaid in this order on yellow toner is formed at each density level from low density to high density. , And the transfer efficiency and the maximum transfer amount at that time were investigated. As a result, the transfer efficiency was 92% and the maximum transfer amount was 1.84 g / cm ² particularly in the image patch in which the three toners of yellow, magenta and cyan were superposed. In addition, the transferred image obtained at this time did not cause image roughness, white spots, or the like in the toner image portion having a low development amount. Incidentally, when a DC voltage of -1100 V was applied to the grid of the downstream pre-transfer charger 6 in this example, the transfer efficiency of the same image patch was 75%, and the maximum transfer amount was 1.
It was 5 g / cm ² .

FIG. 2 shows a color image forming apparatus according to the second embodiment of the present invention. In this embodiment, the upstream pre-transfer charger 5 and the downstream pre-transfer charger 6 and the transfer belt 13 in the first embodiment are not provided, but a transfer material peeling charger 15 made of a corotron is provided. It has the same configuration as that of the first embodiment except that it also serves as one discharging means (5) and the charger 2 also serves as the second discharging means (6). In addition, in this embodiment, the transfer material peeling charger 15 is AC _pp 1 when the AC power source 10 removes electricity.
Applying an AC voltage of 0 kV and peeling A
An AC voltage of C _pp 12 kV is applied, respectively, the charger 2
-5 when recharging the grid G from the DC power supply 11
It is set to apply a DC voltage of 00V and a DC voltage of -700V at the time of charging.

To explain the operation of the apparatus of this embodiment, first, the toner images of yellow, magenta, cyan, and black are overlaid and developed on the photosensitive drum 1 in the same manner as in the first embodiment. Form a multicolor toner image. Then, after the development by the developing device 4B is completed, the photosensitive drum 1
Is further circulated once, during which the multicolor toner image is discharged by the transfer material peeling charger 15 and subsequently recharged by the charger 2, and then the transfer paper 12 is fed by a conveying means (not shown). Then, the transfer charger 7 collectively transfers the image. Here, the surface potential of the photosensitive member is about −2 when the above charge is removed.
It becomes 00V, and the toner charge amount by the above recharging is about 18
It became μC / g. In addition, the above transfer is performed by the transfer charger 7.
Was applied by applying a DC voltage so that the total current was 200 μA.

Then, when various image patches were formed by using the apparatus of this embodiment and the transfer efficiency was examined in the same manner as in the first embodiment, the transfer efficiencies of the single-color, two-color and three-color image patches were found. Was 90% or more. Also in this example, the obtained transferred image was free from image roughness, white spots and the like in the toner image portion having a low development amount.

FIG. 3 shows a color image forming apparatus according to the third embodiment of the present invention. The apparatus of this embodiment has the same configuration as that of the second embodiment except that the pre-cleaning discharger 8 is also used as the first discharge means (5) in place of the transfer material peeling charger 15. Also, in this example,
An AC voltage of AC _pp 8kV is applied, respectively, in the cleaning pretreatment is applied with an AC voltage of AC _pp 9 kV during neutralization from the AC power supply 10 to the pre-cleaning discharger 8, the grid G from the DC power supply 11 to the charger 2 -500V when recharged, -7 when charged
It is set to apply a DC voltage of 00V, respectively.

In order to explain the operation of the apparatus of this embodiment, first, the toner images of yellow, magenta, cyan and black are superposed and developed on the photosensitive drum 1 in this order in the same manner as in the second embodiment. Form a multicolor toner image. After the final development by the developing device 4B is completed, the photosensitive drum 1 is rotated once more, during which the above-mentioned multicolor toner image is discharged by the pre-cleaning discharge device 8 and then recharged by the charging device 2. , Is collectively transferred by the transfer charging device 7 to the transfer paper 12 which is fed by a conveying means (not shown). Here, the surface potential of the photosensitive member was about −200 V when the charge was removed, and the toner charge amount by the recharging was about 18 μC / g. The above transfer was performed by applying a DC voltage to the transfer charger 7 so that the total current was 200 μA.

Then, various image patches were formed by using the apparatus of this embodiment and the transfer efficiency was examined in the same manner as in the first embodiment. As a result, the transfer efficiencies of the image patches of single color, two colors and three colors were found. Was 90% or more. Also in this example, the obtained transferred image was free from image roughness, white spots and the like in the toner image portion having a low development amount.

FIG. 4 shows the DC voltage applied to the downstream pre-transfer charger 6 and the charger 2 (grid G thereof) used as the second discharging means in the first to third embodiments, and the second discharging means thereof. FIG. 6 is a characteristic diagram showing a relationship with a toner charge amount per unit weight of multicolor toner when recharged by. From the result of FIG. 4, in order to recharge the multicolor toner image by the second discharging means so that the toner charge amount falls within the range of 18 to 25 .mu.C / g, the grid G thereof has about -500.
It can be seen that it is sufficient to apply a DC voltage of -700V.

FIG. 5 shows a DC voltage applied to the downstream pre-transfer charger 6 and the charger 2 (grid G) used as the second discharging means in the first to third embodiments, and the second discharging means. FIG. 6 is a characteristic diagram showing a relationship with the transfer efficiency of the multicolor toner after being recharged by. The conditions relating to the toner image formation and the transfer process at this time are as described in each example.

The transfer efficiency at this time is expressed by the transfer ratio of the yellow toner which is the lowermost layer when the process black toner image formed by sequentially superposing yellow, magenta and cyan is transferred. The transfer rate of the yellow toner is measured by measuring the reflectance of the process black toner image after it has been transferred and fixed on the recording paper by applying a yellow filter and extracting only the yellow component. did. Therefore, in FIG. 5, the transfer efficiency on the vertical axis is represented by the above reflectance. In FIG. 5, the range in which the transfer efficiency is good is 1.85.
This is the case.

From the results shown in FIG. 5, it can be seen that for the process black toner image, good transfer efficiency can be obtained when a DC voltage of about -700 V or less is applied to the second discharging means. This toner image is for, and it can be seen that the transfer efficiency is good when the voltage applied by the pre-transfer charger 9 on the downstream side is −700 V or less.

Further, in the case of the process black toner image (three-layer toner image), the transfer efficiency is improved when the voltage applied to the second discharging means is in the range of about -300V to -700V as shown in FIG. Good, but in the case of other one-layer or two-layer toner images, the voltage applied to the second discharging means is about -300V.
In the range of to -500V, the surface charge density of the transfer material becomes relatively large, and the void discharge described above occurs and the toner image flies. Therefore, the transfer efficiency is improved irrespective of the type (number of layers) of the toner image because the voltage applied to the second discharging means is about -500V to -700 as a whole.
It is in the range of V. Then, in the range where the voltage applied to the second discharging means is about -500V to -700V, the multicolor toner image is recharged so that the toner charge amount is 18 to 25 .mu.C / g as shown in FIG. Corresponds to the range. Therefore, from the above, the toner charge amount of the multicolor toner image is 1
It can be seen that the best transfer efficiency is obtained when recharged so as to be in the range of 8 to 25 μC / g.

The color image forming apparatus of the present invention may have a monochromatic image forming mode in addition to the color image forming mode. This monochrome image forming mode is yellow,
It refers to a process mode in which an image is formed with a toner of any one of magenta, cyan, and black.

For example, in the case of forming a black image with black toner in the monochromatic image forming mode, after the latent image for the black image is formed, the developing process with the yellow, magenta and cyan toners is not performed, and the black toner developing device 4B is used. Is developed to form a black toner image, which is then transferred to a transfer material to form a black image. In this case, the transfer is performed by applying a DC voltage to the transfer charger 7 so that the total current is 200 μA, and the transfer material is peeled off by applying an AC voltage of AC _pp 10 kV to the transfer material peeling charger 15. It is performed by applying.

The image forming speed in the monochromatic image forming mode is one print per one rotation of the photosensitive drum in contrast to one print per five rotations of the photosensitive drum in the color image forming mode.
Because it is a print, it will be extremely fast. In this single-color image forming mode, the static elimination by the first discharging means and the recharging by the second discharging means are not essential. This is because, in the process of forming a monochromatic image, toner having various charge amounts is not formed on the photoconductor because there is no recharging step, and since there is no overlapping development, there is little difference in the amount of development. Because.

[0037]

As described above, according to the present invention,
The multicolor toner image formed on the photoconductor by overlapping development
After the final development step and before the transfer step, the charge is removed by the first discharging means and then recharged by the second discharging means so that the toner charge amount becomes 18 to 25 μC / g, so that the transfer efficiency becomes maximum, and as a result, It is possible to obtain a high-quality color image without transfer defects.

When the transfer material peeling charger is also used as the first discharging means and the photoconductor charger is also used as the second discharging means, the existing equipment (components) can be effectively used, and the apparatus can be effectively used. Can be simplified and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a color image forming apparatus according to a first embodiment.

FIG. 2 is a schematic view showing a color image forming apparatus according to a second embodiment.

FIG. 3 is a schematic view showing a color image forming apparatus according to a third embodiment.

FIG. 4 is a characteristic diagram showing the relationship between the DC voltage applied to the second discharging means and the toner charge amount per unit weight of the multicolor toner.

FIG. 5 is a characteristic diagram showing the relationship between the DC voltage applied to the second discharging means and the transfer efficiency.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Photoconductor, 2 ... Charging device, 3 ... Image exposure means, 4 ... Developing device, 5 ... 1st discharge means (upstream side pre-transfer charger, transfer material peeling charger 15), 6 ... 2nd discharge means ( Downstream pre-transfer charger, charger 2), 7 ... Transfer device.

 ─────────────────────────────────────────────────── (72) Inventor Yasuki Yamauchi 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd. (72) Inventor Toshiaki Sagara 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (2)

[Claims] 1. A photosensitive member comprising at least a charging device, an image exposing means, a plurality of developing devices and a transferring device, which are arranged around the photosensitive member, and the charging, image exposing and developing processes are repeated a plurality of times. In a color image forming apparatus in which a multicolor toner image is formed by superimposing development on a transfer material and then the multicolor toner image is transferred onto a transfer material at a time, at least a first discharging means to which an AC voltage is applied and at least a DC voltage to which a DC voltage is applied. Two discharge means, and the multicolor toner image is discharged by the first discharge means after the final development step and before the transfer step, and then recharged by the second discharge means so that the toner charge amount becomes 18 to 25 μC / g. A color image forming apparatus that is electrically charged. 2. The color image forming apparatus according to claim 1, wherein the first discharging means is a transfer material peeling charger, and the second discharging means is a photoreceptor charger.