JPH08234535A - Color image forming device - Google Patents

Abstract

PURPOSE: To excellent set transfer efficiency regardless of the light and shade of an image density, in a device having one exposure means and plural developing units arranged around a photoreceptor drum, overlappedly forming a color toner image on the photoreceptor drum and then, transferring the color toner images to a paper sheet all together. CONSTITUTION: When the color image forming device is constituted in such a manner that an electrifier 2 and an image exposure member 4 are arranged with respect to the photoreceptor drum 1 and plural different color developing units 6-9 are arranged in a developing part 5, a pretransfer discharger 15 is arranged in a part immediately in front of an image transfer part, to adjust the surface potential of the photoreceptor. Light discharge by the pretransfer discharger 15 is executed with a light quantity smaller than that for making the difference between the absolute values of an image part surface potential VI and a nonimage part surface potential VH, that is, a value of |VI|-|VH| maximum, in the intensity of light for irradiating by the pretransfer destaticizer 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus, and in particular, after repeatedly forming a toner image of a plurality of colors by repeatedly charging, exposing and developing a photoreceptor,
The present invention relates to an improvement of a color image forming apparatus of a type in which transfer is performed at once on a transfer material.

[0002]

2. Description of the Related Art As a color image forming apparatus for making a color copy, for example, an apparatus as disclosed in Japanese Patent Laid-Open No. 5-333662 is known. In the conventional example, a plurality of photosensitive drums are arranged along a belt device that conveys a sheet as a transfer material, and means for forming toner images of different colors are arranged on the plurality of photosensitive drums. In addition, a means is used in which a color toner image is sequentially transferred from each photoconductor drum to a conveyed sheet to form a color copy. In the color image forming apparatus as described above, since only a single color toner image is formed on each photoconductor drum, the operations of forming a color toner image on the photoconductor drum and transferring the image to paper are This can be performed in the same manner as an apparatus using a general black toner.

In addition to the conventional example described above, Japanese Patent Laid-Open No.
In the example disclosed in Japanese Unexamined Patent Publication No. 337747, one photoconductor drum is arranged corresponding to a drum that electrostatically supports a sheet, and a plurality of developing devices of different colors are arranged with respect to the photoconductor drum. And constitutes a color image forming apparatus. Then, the image information of the original is color-separated, an image corresponding to each color is formed on the photoconductor drum, and each time a color toner image is formed, the operation of transferring to the paper is repeated, and the color toner of a plurality of colors is formed on the paper. The images are transferred in an overlapping manner. Therefore, even in the conventional example,
Since the color toner image formed on the photosensitive drum at one time is of a single color, the charging, exposing and developing operations can be performed in the same manner as the operation using only black toner.

Apart from the color image forming apparatus, in the example disclosed in Japanese Patent Laid-Open No. 59-121348, the basic method of the color image forming apparatus is to charge, expose, and develop a photosensitive member only for necessary colors. An apparatus using a system in which color toner images of the required number of colors are formed in a state of being laminated on each other by repeating the same and the color toner images of the number of color colors are collectively transferred to a sheet is described. There is. Among the color electrophotographic methods described above,
Only the developing devices are arranged for the required number of colors, one charging device and one image exposing device are arranged, and each time the photoconductor is rotated, charging and exposure are performed corresponding to the selected color, and the developing devices perform color Since the apparatus of the type that develops a toner image has a small number of components, the apparatus can be downsized and the cost can be reduced.

Moreover, since color toner images of a plurality of colors are formed on a single photoconductor in a superposed state, there is little positional deviation of the images between the colors, and as a result, in principle,
It has an excellent feature that it can provide a high-quality color image with less color shift. Incidentally, in the case of the color electrophotographic apparatus as described above, the same applies to the prior art described below, but when the plurality of color toner images are superposed on the photoconductor, the photoconductor In the same area, image exposure for the second and subsequent colors is performed, and the color toner image for the second color is layered as it is on the color toner image for the first color.

As described above, an image forming apparatus for forming a color image on a photosensitive drum is constructed, for example, as shown in FIG. In the example shown in FIG. 8, the charging device 2 of the scorotron is arranged with respect to the photoconductor drum 1 to provide a means for uniformly charging the surface of the photoconductor drum, and the image exposure member 4 such as a semiconductor laser. An electrostatic latent image is formed by writing an image with. Further, the yellow developing device 6, the magenta developing device 7, the cyan developing device 8, and the black developing device 9 are arranged in the developing unit 5, and according to the image of each color written by the image exposing member 4. , One of the developing devices 6 to 9 is selectively operated to form a color toner image. A sheet serving as a transfer material for transferring a color toner image is conveyed by a sheet conveying device from a sheet feeding unit (not shown), runs while being supported by a conveying belt 14, and discharges the color toner by the transfer corotron 10. The image is transferred onto a sheet and conveyed toward the fixing device. Further, the toner remaining on the surface of the photoconductor drum 1 after the color toner images are transferred onto the paper in a superposed manner is weakened in adhesion to the photoconductor drum due to discharge of the pre-cleaning static eliminator 11, and the cleaning device 12 To be cleaned.

In the image forming apparatus constructed as described above, the image information of the original is separated for each color, the image corresponding to yellow is first written by the image exposing member 4, and the yellow developing device 6 is operated. A yellow image is formed on the photosensitive drum. Next, the image corresponding to the magenta of the second color is written and the development is performed by the magenta developing device 7, and the color toner image is developed in a state of being superposed on the surface of the photosensitive drum. Such an operation is sequentially repeated for each color, and the color toner images of all the colors are overlapped,
An image is formed on the photosensitive drum, and the superimposed color toner image is transferred to a sheet by discharging the transfer corotron 10.

[0008]

However, in the color electrophotographic system as described above, a portion having a high development amount of an image density in which three color toner images are superposed and a single color or a highlight region are representative. There is a problem that it is not possible to transfer an image in which a portion having a low image density and a developing amount is mixed with high transfer efficiency. That is, when the transfer condition is set for a high development amount, the transfer efficiency of the portion with a low development amount is reduced, and the image is discolored or white spots occur. However, it was difficult to set a high transfer efficiency in a good state in a portion having a high development amount, and it was difficult to prevent the occurrence of color distortion or transfer unevenness (conventional example in Table 1).

That is, in the image forming apparatus as shown in FIG. 8, as shown in the graph of FIG. 9, when the image density is low (low developing amount), the transfer efficiency is relatively good. However, the image density is high (high development amount)
Then, there arises a problem that the transfer efficiency is greatly reduced. For example, in an image forming apparatus using a general black toner, a developing weight as an image density has a value of about 0.7 mg / square cm, whereas an image on which color toner images are superposed has a value of 1.5 mg / cm 2. The value is more than square cm. However, as shown in the graph, when the image density is high, the transfer efficiency is significantly reduced, and therefore, in a color image in which a portion having a large amount of toner per unit area and a portion having a small amount of toner are mixed. However, there arises a problem that it is difficult to transfer an image at a uniform ratio with respect to the entire image.

Therefore, in order to solve the above-mentioned problem, prior to the operation of transferring the color toner image onto the paper, the charge amount per unit toner amount is substantially equal to the image carried on the photosensitive drum. It is considered that one method is to perform pre-transfer light elimination so that the same charge amount is obtained. However, when the above-mentioned photo static elimination is performed, a new problem occurs in which remarkable toner scattering occurs around characters and line images (Table 1 Prior art example with PTL).

[0011]

It is an object of the present invention to solve the problems of the color image forming apparatus using the color electrophotographic system as described above.
Before the image is transferred onto the paper, the difference between the absolute value | VI | of the toner-developed image portion surface potential VI and the absolute value | VH | of the non-image portion surface potential VH, | VI |-| VH | A device that can improve transfer efficiency when a color toner image is transferred onto a sheet by applying light elimination with a light amount smaller than the maximum light amount or a small light amount close to the maximum light amount is provided. The purpose is to do.

[0012]

The present invention is directed to charging, exposing,
The present invention relates to a color image forming apparatus that forms a multicolor toner image of a plurality of colors on a photoconductor by repeating development and transfers the multicolor toner images to a transfer material at once. In the present invention, the absolute value | VI | of the image portion surface potential VI and the non-image portion surface potential VH, which are toner-developed after the formation of the multicolor toner image, and the absolute value | VH of the non-image portion surface potential VH.
There is provided a means for effecting optical neutralization with a light amount smaller than the light amount that maximizes the difference ||, | VI | − | VH |. Further, in the present invention, the amount of light at the time of the light elimination is the absolute value | VI | of the toner-developed image portion surface potential VI.
And the difference between the absolute value | VH | of the surface potential VH of the non-image portion is desirably a light amount that is close to the light amount that maximizes | VI |-| VH |. .

[0013]

In the electrostatic transfer which is the object of the present invention, the transferability can be defined by the toner charge amount q and the transfer electric field E. IEICE Transactions '77 / 4 Vol
J60-C No. According to 4, the final transfer efficiency, that is, the ratio of the transfer amount to the development amount can be represented by the ratio of the surface charge density of the toner layer and the surface charge density given to the transfer material (paper). Has been described. Further, if the surface charge density of the transfer material exceeds the surface charge density of the toner layer, 100% transfer can be achieved from this theory, that is, high development amount = high toner charge amount q is transferred. Describes that a high transfer electric field E is required, and when the surface charge density of the transfer material becomes too large and the transfer electric field E becomes too large, void discharge occurs.

Therefore, in the case of this system in which a multicolor superposed toner image having a high development amount and a toner image having a low development amount coexist, the transfer electric field E is set to a multicolor superposition having a high development amount. It can be dealt with by increasing the toner image and reducing the toner image having a low development amount. Then, it becomes possible to transfer a multicolor superposed toner image having a high development amount with high efficiency while preventing the occurrence of void discharge in a single color or a highlight region having a low development amount.

On the other hand, if the conventionally known pre-transfer photo-electrification is applied to the above-described over-developing process, the photo-electrification is applied after the multi-color toner image is formed on the photosensitive member. Depending on the presence or absence of a toner image formed on the photoconductor, that is, the static elimination light does not easily reach the photoconductor in a portion having a high development amount, and the static elimination light easily reaches the photoconductor in the highlight region. Surface potential V of developed image area
I and the surface potential VH of the non-image area are made different from each other, and the transfer electric field E
Can be made large for a multi-color superposed toner image having a high development amount, and can be made small for a toner image having a low development amount. However, when the above-mentioned photo static elimination is performed, a new problem occurs in which remarkable toner scattering occurs around characters and line images (Table 1 PTL in the conventional example).
Attached).

In the present invention, the absolute value | VI of the image portion surface potential VI of the photosensitive member on which the toner has been developed after the formation of the multicolor toner image.
| And the absolute value | VH | of the surface potential VH of the non-image portion,
VI |-| VH | is less than the maximum light quantity,
Alternatively, by applying photo-electrification at a value close to the difference in the amount of light, a multicolor superposed toner image having a high amount of development can be prevented from generating void discharge in a single color or a highlight region having a low amount of development. However, it was possible to transfer with high efficiency, and to obtain a high-quality image in which there was no noticeable toner scattering around the character or line image (Table 1 of the present invention).

Therefore, in the present invention, the above | V
The reason why the static elimination effect at a light amount larger than the light amount at which I |-| VH | becomes maximum causes noticeable toner scattering around a character or line image is that the light attenuation speed of the photoconductor is large in a non-image portion. It is conceivable that the value of | VI | − | VH | becomes instantaneously very large in the region, and the potential difference causes remarkable toner scattering.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The color image forming apparatus of the present invention will be described with reference to the illustrated example. The example shown in FIG. 1 shows a configuration of an image forming member corresponding to a photosensitive drum of an image forming apparatus of the present invention and a portion for transferring a color toner image onto a sheet. In the example shown in FIG. 1, the photosensitive drum 1
Is rotated in the clockwise direction in the figure. Around the photosensitive drum 1, a charger 2 to which a voltage is applied by a voltage adjusting means 3 and an image exposure member 4 composed of a semiconductor laser or the like, The developing unit 5 having four developing units is arranged to form a means for forming a color toner image on the photosensitive drum. Further, a transfer corotron 10 for transferring the color toner image formed on the photoconductor drum, a pre-cleaning static eliminator 11 for removing toner remaining on the photoconductor drum, and a cleaning device 12 are arranged.

The photoconductor drum 1 is constructed as an organic photoconductor having an insulating surface coating layer. Further, a dielectric conveying belt 14 is arranged at a position where the color toner image formed on the photoconductor drum is transferred to the paper.
While the sheet as the transfer material is being conveyed by the conveyor belt 14, the discharge action of the transfer corotron 10 is applied from the back surface of the conveyor belt to transfer the color toner image onto the sheet, and the sheet carrying the image is arranged in the downstream portion. And a means for sending it toward the fixing device. Further, a pre-transfer charge eliminator 15 is disposed downstream of the developing unit 5, and an image portion subjected to toner development after forming a multicolor toner image on the pre-transfer charge eliminator 15 will be described later. Surface potential VI
Absolute value | VI | and the absolute value of the non-image portion surface potential VH |
The difference between VH | and | VI |-| VH | is such that the light elimination is performed with a light amount smaller than the maximum light amount.

In the apparatus of the present invention constructed as described above, first, as a first cycle, the pre-cleaning charge eliminator 11 is used to remove the charge, and then the cleaning device 12 is used for cleaning. Next, a DC bias -70 is applied from the voltage adjusting means 3 to the grid of the charger 2.
By applying 0 V, the surface of the photosensitive drum is uniformly charged. After that, the image exposure member 4 writes the image of the first color to form an electrostatic latent image on the photosensitive layer on the surface of the photoconductor drum, and the toner is supplied from the yellow developing device 6 to the yellow color toner. Form an image. As described above, when forming a yellow color toner image, the photosensitive member voltage after exposure is −400V, and the developing bias of −600V is applied to the yellow developing device 6 to apply the toner to the photosensitive member surface. Try to attach it.

Then, in the second cycle, the first cycle
Under the same conditions as in the case of the cycle, the operation of forming an electrostatic latent image by discharging the charger 2 to charge the photosensitive drum and writing an image corresponding to a magenta image from the image exposure member 4 I do. Then, a developing bias of -600 V is applied to the magenta developing unit 7 of the developing section 5 to form a magenta color toner image in a state of being superimposed on the yellow image. At this time, the exposure intensity is increased so that the photoconductor potential after exposure by the image exposure member 4 becomes −400 V by the exposure from above the yellow toner. The above-described steps are repeated for each of cyan and black, and development is performed by the cyan developing device 8 and the black developing device 9 in correspondence with the respective color image information, and four color toners are used. An image is formed on the surface of the photosensitive drum in an overlapping manner.

As described above, after the color toner images of four colors are overlaid and before the transfer onto the sheet, light is emitted from the pre-transfer charge eliminator 15 to control the surface potential of the photoconductor. In this state, the color toner image is transferred to the paper transported by the transport belt 14 by discharging the transfer corotron 10. The intensity of the light emitted from the pre-transfer charge eliminator 15 is set as shown in FIGS. The state shown in FIG. 2 shows the surface potential of the photosensitive drum in the state where the black toner image is transferred after the yellow, magenta, and cyan colors are transferred in an overlapping manner. Since it is charged to 700 V, only the portion where the black image is formed is formed as the portion of low potential. When light is emitted from the pre-transfer static eliminator 15 to the surface of the photoconductor drum in the above-described state, as shown in FIG. Does not reach the surface of the photoconductor so much that the potential does not rise, and the surface potential of the photoconductor becomes high in the portion where the toner is thinly formed. The intensity of the light emitted from the pre-transfer charge eliminator 15 is, for example, the toner-developed image portion surface potential VI.
Absolute value | VI | and the absolute value of the non-image portion surface potential VH |
VH | difference, when | VI | − | VH | = 300 V, the light amount that maximizes the difference (300 in terms of voltage)
V) The light elimination is performed with a smaller amount of light.

As described above, when light is emitted from the pre-transfer charge eliminator 15, as shown in FIG. 3, in the highlight region, the potential of the photoconductor becomes low, and the value is close to 0 V, but the three colors are used. The potential at the high density portion of the multiple developed image is stored at a value close to the charging potential. Due to the above action, as shown in FIG. 4, by applying the transfer electric field E, a large action is exerted on a color toner image having a high developing amount, and a small action is exerted on a toner image having a low developing amount. it can. Therefore, it is possible to apply a transfer bias capable of highly efficiently transferring a multicolor superposed color toner image having a high development amount to a single color or a highlight region having a low development amount while preventing the occurrence of void discharge. .

By virtue of the configuration described above, FIG.
As shown in the graph of the conventional example, in the case where the pre-transfer charge eliminator is not provided, the maximum transfer amount of 1. is obtained when the color toner images of three colors of yellow, magenta, and cyan are superposed.
5 mg / square cm, the transfer efficiency at this time was 75%, but the transfer efficiency can be improved as shown in the graph of FIG. That is, in the example shown in the graph of FIG. 5, the maximum transfer amount was 1.8 mg / square cm, and the transfer efficiency at this time was 90%, which was a great improvement. Of course, it is possible to eliminate the occurrence of image blurring and white spots in the low developing amount portion, and the transfer current is 30 μA.
When the transfer corotron is operated by setting to, the transfer efficiency corresponding to the image density (developing amount) can be obtained as shown in the graph of FIG.

Further, as shown in the embodiment of the present invention, when the means for exposing the color toner image before transfer by using the pre-transfer charge eliminator is constructed, the potential of the portion of the surface of the photoconductor having a high development amount is increased. And the potential of the portion of low development amount is shown in FIG.
As shown in the graph. In the example shown in the graph, under the condition that the developing amount is 1.96 mg / cm 2, the vertical axis represents the surface potential and the horizontal axis represents the voltage applied to the pre-transfer charge eliminator, which is indicated by the solid line. The voltage applied to the pre-transfer charge eliminator can be selected according to the value corresponding to the difference between VH and VI. Further, in addition to the graph of FIG. 6, as shown in the graph of FIG. 7, the relationship between the surface potential difference shown on the vertical axis and the development amount shown on the horizontal axis changes while maintaining a substantially linear proportional relationship. It is known to do.

Therefore, in the present invention, even if there is a difference in image density, that is, a difference in the amount of development formed on the surface of the photosensitive drum, the toner-developed image portion surface potential VI.
Absolute value | VI | and the absolute value of the non-image portion surface potential VH |
It is possible to transfer an image by irradiating light with a light amount smaller than the difference between VH | and | VI |-| VH |. Further, the same effect can be obtained even when light irradiation by the pre-transfer charge eliminator is performed with a small light amount that is significantly different from the difference | VI | − | VH |.
However, in the present invention, the above |
It was also found that a more preferable effect can be obtained when the pre-transfer charge eliminator is operated with a small light amount that is close to the difference of VI |-| VH |. Further, since the voltage applied to the pre-transfer charge eliminator as described above can be expressed as the amount of light, the voltage applied to the pre-transfer charge eliminator can be set in accordance with the emission characteristics of the lamp. Will be things.

As described above, in the embodiment of the present invention, the pre-transfer static eliminator is arranged between the developing section and the transfer corotron, but the member for performing the function of the pre-transfer static eliminator is described. It is also possible to use a static eliminator arranged downstream of the cleaning device. Then, after the four color toner images are transferred in an overlapping manner, the photosensitive drum is rotated again, the surface potential of the photosensitive member is adjusted by using a static eliminator, and then the transfer corotron is operated to color the paper. A mechanism for transferring the toner image can also be configured.

[0028]

[Table 1]

[0029]

The color image forming apparatus of the present invention comprises the above-mentioned apparatus, and by repeatedly charging, exposing and developing the photosensitive drum, a color toner image of a plurality of colors is obtained. The transfer efficiency of images can be improved by irradiating light by the pre-transfer charge eliminator immediately before the color toner images are transferred to a sheet by overlapping. Then, in the image portion formed on the surface of the photoconductor drum, a multi-color superposed color toner image having a high development amount can be obtained while preventing the occurrence of void discharge in a single color or a highlight region having a low development amount. It is possible to obtain a high-quality image that is efficiently transferred and has no noticeable toner scattering around the character or line image. In the present invention, in the toner-developed photoreceptor, the absolute value | VI | of the image portion surface potential VI and the non-image portion surface potential VH
Of the absolute value of | VH | and the amount of light less than the maximum value of | VI |-| VH | Become.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram of a surface potential when a fourth color toner image is formed on a photosensitive drum.

FIG. 3 is an explanatory diagram of the state of the surface potential of the photosensitive drum at the time when light is emitted by the pre-transfer charge eliminator.

FIG. 4 is an explanatory diagram showing a state in which a color toner image is transferred to a sheet by applying a potential by a transfer corotron.

FIG. 5 is a graph showing the dependency of transfer efficiency on developing weight in the apparatus of the present invention.

FIG. 6 is a graph showing the relationship between the surface potential of the photosensitive drum and the voltage applied to the pre-transfer charge eliminator.

FIG. 7 is a graph showing the relationship between the development amount and the surface potential difference.

FIG. 8 is an explanatory diagram showing a configuration of a conventional image forming apparatus.

FIG. 9 is a graph showing the dependency of transfer efficiency on developing weight in a conventional apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 photoconductor drum, 2 charger, 4 image exposure member, 5 developing section, 10 transfer corotron, 11
Static eliminator before cleaning, 12 cleaning device, 14 conveyor belt, 15 static eliminator before transfer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takuto Tanaka 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd. (72) Inventor Toru Teshigawara 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (2)

[Claims] 1. A color image forming apparatus for forming a multicolor toner image of a plurality of colors on a photoconductor by repeating charging, exposure and development, and transferring the multicolor toner images to a transfer material at once. , The absolute value | VI | of the toner image-developed surface potential VI and the non-image surface potential VH
Of the absolute value of | VH |, | VI |-| VH | is used to perform light neutralization with a light amount smaller than the maximum light amount. 2. The amount of light at the time of the photo-removing is | VI | −, which is the difference between the absolute value | VI | of the toner-developed image portion surface potential VI and the non-image portion surface potential VH.
The color image forming apparatus according to claim 1, wherein | VH | has a small light amount that is close to the maximum light amount.