JPH067281B2 - Color image forming method - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a color image forming method used in a copying machine or the like, and more specifically, a latent image is formed by sequentially forming and developing an electrostatic latent image for different colors. The present invention relates to a color image forming method in which a multicolor toner image is formed on an image carrier and the toner image is transferred onto a toner image carrier at one time to obtain a color image.

[Technical background of the invention and its problems]

There have been various ideas for forming a color image on plain paper by applying the contact electrophotographic technique, but only a few have been put to practical use at present. Among the methods that have been successfully put into practical use, all of the methods capable of full-color recording use a method in which paper is wound around a transfer drum and the toner image is sequentially transferred for each color on the transfer drum. On the other hand, increasing the size, complexity, and price of the entire device is an unavoidable problem. In addition, a two-color color process using a special photoreceptor having a multilayer structure (Japanese Patent Publication No. 55-7798).
No.), a two-color printer using a laser beam (Hoshi et al .: "two-color printing semiconductor laser printer", Proceedings of the Institute of Image Electronics Engineers 81,02,2,1981), etc. There are problems in manufacturing and reliability associated with the complexity of the structure of the photoconductor, and the latter has a problem such as a slow recording speed, and neither method is easy to apply to full-color recording. There are drawbacks.

On the other hand, a series of image forming processes consisting of charging the entire surface of the photosensitive drum, image exposure through a color filter, and developing are sequentially performed for each color to form a multicolor toner image on the photosensitive drum. A method of transferring this onto plain paper at once has been devised (Japanese Patent Laid-Open No. 55-69167, etc.). According to this method, the transfer drum is unnecessary, so that the various problems described above can be solved, and the application to full-color recording is easy.

However, this method has a serious problem that the toner image formed on the photosensitive drum is disturbed in the subsequent image forming process for another color. In fact, when the inventor tried an experiment on this method, it was confirmed that the toner image of the color formed earlier is particularly disturbed in the image exposing step, developing step and charge eliminating step in the subsequent image forming process. . Disturbance in the developing process occurs when a so-called contact type developing method is used, in which a magnetic brush or the like is brought into sliding contact with a photosensitive drum. It can be avoided by applying the mold development method.

On the other hand, the disturbance of the toner image in the image exposure step appears as a phenomenon in which a line image or an edge of an image portion is bleeding, or a phenomenon in which toner is scattered from the surface of the photosensitive drum and the image disappears. As a result of experiments, the problem is that the toner particles forming the toner image have an excessive charge in the subsequent charging step. To reduce the amount of light that reaches the surface of the photosensitive drum by the toner particles that absorb the irradiation light during the exposure process. It was found that it was due to two points.

This problem will be described in more detail as follows.
In the color image forming method described in Japanese Patent Laid-Open No. 55-69167, etc., a series of image formation is performed by charging the entire surface of the photoconductor with a corona charger, exposing the image through a color filter, and developing with color toner. A toner image of the first color is formed on the photoconductor by the process, and then a similar image forming process is performed for the remaining colors to form a multicolor toner image, which is then transferred onto plain paper at once. ,
It is fixed and a color image is obtained.

Here, in the image forming process for the second and subsequent colors, the toner image previously formed on the photoconductor is uniformly charged by the corona charger and is charged with an extremely large electrostatic charge. Immediately after passing through the corona charger, the surface potential of the region which adheres the toner image surface potential of the area not attached toner image as V _0, denoted V 0 ₊ ΔV. ΔV is an increase in surface potential derived from the electrostatic charge acquired by the toner. The photoconductor and the toner image are subjected to image exposure after this charging, but the exposure pattern at this time is not always the same as the pattern of the toner image already formed.
That is, the area where the toner adheres may become a non-image portion (exposure portion) in the next exposure pattern. In such a case, the surface potential V _P of the non-toner adhering portion is rapidly attenuated because the charged electric charge on the photoconductor escapes through the photoconductor upon exposure, but the surface potential V _P of the toner adhering portion is reduced.
_T shows slow attenuation because the toner blocks the light and the light does not reach the surface of the photoconductor sufficiently.

FIG. 7 is an experimental result showing this state. Convex curve on in the figure represents the time course of the potential difference V _T -V _P, namely, the toner adhered portion and a non-toner adhesion portion. The potential of the toner having a large electrostatic charge due to corona charging with respect to the non-toner adhesion portion follows a path like a broken line when passing through the exposure position. At this time, the toner receives a Coulomb force toward a region having a lower potential, that is, a non-toner adhesion portion, and when the electric field exceeds a certain value, the toner scatters due to this Coulomb force. This phenomenon is particularly remarkable in the edge area of the toner adhesion portion, and when the toner image forms a line image with a large spatial frequency, or when the light absorption rate of the toner is extremely large, the electric field becomes even stronger. May be discharged from the surface of the photoconductor toward the outer space, and the image may disappear. Such disturbance of the toner image is an essential problem that determines whether or not the above-described color image forming method is put to practical use, and a solution thereof has been desired.

[Object of the Invention]

An object of the present invention is to obtain a multi-color toner image by sequentially performing a series of image forming processes including at least an electrostatic latent image forming process and a developing process in order to form a multicolor toner image. Another object of the present invention is to provide a color image forming method capable of forming a good color image by preventing the formed toner image from being disturbed.

[Structure of Invention]

In order to achieve the above object, the present invention includes a step of forming an electrostatic latent image on a latent image holding member including at least a photoconductor and a step of developing the electrostatic latent image with color toner. A multi-color toner image is formed on the latent image holding member by sequentially performing a series of image forming processes including each color, and the toner image is transferred at once to the toner image carrier to obtain a color image. In the forming method, the step of forming the electrostatic latent image is characterized in that the electrostatic latent image is formed on the latent image holding member by simultaneously performing charging and image exposure at the same position.

〔The invention's effect〕

According to the present invention, by performing charging and image exposure for forming an electrostatic latent image on the latent image carrier at the same time and at the same position, the charging and the image exposure in the image forming process for the second and subsequent colors are performed. It is possible to prevent a large electric field from being generated between the toner adhering portion and the non-toner adhering portion in the toner image formed in the image forming process. Therefore, the toner image already formed in the image forming process for the second and subsequent colors is not disturbed, and it is possible to form a good color image without blurring of the image.

Example of Invention

FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus to which a color image forming method according to the present invention is applied. In the figure, a photosensitive drum 1 as a latent image carrier having a selenium-based photoconductor layer on its surface rotates in the direction of arrow 2. The corona charger 3 is composed of a corona wire 4 and metal side plates 5 arranged on both sides of the corona wire 4, and has a structure capable of performing image exposure by introducing reflected light 6 from a full color original (not shown) from above simultaneously with charging. ing. The reflected light 6 moves in the direction of the arrow 10 and is introduced into the corona charger 3 via any of the color filters 7, 8 and 9 of different colors that are sequentially selected. As a result, an electrostatic latent image corresponding to each color is formed on the photoconductor 1. Surface potential V of photoreceptor 1
_It is assumed that _P changes as shown in FIG. 2 depending on the irradiation light amount.

In this embodiment, the metal side plate 5 of the corona charger 3 is used.
The distance between the corona wire 4 and the photosensitive drum 1 is 11 mm, the diameter of the corona wire 4 is 80 μm, and the peripheral speed of rotation of the photosensitive drum 1 is 100 mm / sec. Due to the fact that image exposure is performed simultaneously, a DC voltage of +8 kV, which is slightly higher than the applied voltage (usually about +6 kV) in the conventional technique, was applied. The opening width of the corona charger 3 on the side of the photosensitive drum 1 is 12 mm.
And the exposure width matches this opening width. The opening width and the exposure width of the corona charger 3 refer to the dimensions of the opening and the exposure area in the rotation direction 2 of the photosensitive drum 1, respectively.

In the first image forming process, the reflected light 6 from the full-color original is color-separated by the blue filter 7 and is irradiated onto the surface of the photosensitive drum 1 via the corona charger 3. The electrostatic latent image thus formed on the surface of the photosensitive drum 1 is visualized by being developed by the first developing device 11 with yellow toner which is a complementary color of blue. Immediately after the development, the toner is negatively charged and the photosensitive drum 1 is positively charged, but these charges are discharged by the AC corona discharge of the discharging charger 18, and the first image forming process is completed.

Hereinafter, in the second image forming process, after the electrostatic latent image is formed by the charging by the corona charger 3 and the image exposure through the green filter 8, the electrostatic latent image is developed by the second developing device 12 with the magenta toner. In the third image forming process, after the electrostatic latent image is formed by the charging by the corona charger 3 and the image exposure through the red filter 9, the third developing device 13 develops it with cyan toner. Done. As the developing devices 11 to 13, the one-component non-magnetic type developing device disclosed in JP-A-59-45468 can be used.

The full-color toner image thus formed on the photosensitive drum 1 is transferred onto the plain paper 15 at once by the transfer charger 14 and further fixed by the fixing device 16 to complete the entire image forming process. After that, the residual toner on the photosensitive drum 1 is removed by the cleaning device 17.

The full-color image obtained by this method was of high quality with little image bleeding, and there was no toner scattering particularly at the exposure position. The reason why the image blur is eliminated can be explained as follows.

That is, image bleeding in the prior art is caused by a large electric field generated between the toner adhering portion and the non-toner adhering portion when the charged toner image is irradiated with light as described above, and the toner scatters. was there. On the other hand, according to the method of simultaneously performing charging and image exposure as in the present invention, it is possible to suppress the electric field between the toner adhering portion and the non-toner adhering portion to a small value.

FIG. 2 shows an experimental result for explaining this effect. For a photosensitive drum having a black toner image on the surface thereof, which gives an optical density of 1.4 after transfer and fixing, It is a graph in which the change in the surface potential V _T of the photosensitive drum 1 when charging and image exposure are simultaneously performed under the conditions is measured and the potential difference V _T −V _P between the toner adhering portion and the non-toner adhering portion is plotted. As can be seen, the potential difference V _T -V _P is much smaller than the values shown in Figure 7.
The reason why the potential difference V _T −V _P becomes small in this way is that the image exposure is performed at the same time as the charging, so that the charging of the photosensitive drum 1 and the toner particularly at the toner adhering portion is particularly higher than that when the charging is performed first. This is because the surface potential V _T is suppressed to a lower value. Since the potential difference V _T -V _P with the toner adhesion portion and a non-toner adhesion portion is kept small as the electric field generated between them is small, a phenomenon resulting toner scattering is suppressed.

Further, FIG. 3 shows the results of measuring the relationship between the charging / exposure time and the surface potential of the photosensitive drum 1 when the photosensitive drum 1 is rotated at four different speeds in the configuration of the above embodiment. is there. The charging / exposure time on the horizontal axis is the time required for one point on the photosensitive drum 1 to pass through the position of the corona charger 3, that is, the charging / exposure area. It represents the change over time of the surface potential as it passes through the exposed area. The third from the experimental results in Figure, it is clear that the surface potential V _T -V _P with the toner adhesion portion and a non-toner adhesion portion is kept small.

Next, another embodiment of the present invention will be described. In the embodiment shown in FIG. 1, the corona charger 3 is composed only of the corona wire 4 and the metal side plate 5, and the width of the image exposure from above is the opening width of the corona charger 3 on the side of the photosensitive drum 1. Match. This coincidence is extremely important for forming a latent image, and when it is not set in this way, for example, when the opening width of the latter is larger than the image exposure width of the former, the photosensitive drum is also exposed. Since the corona ions fall on the surface of No. 1, the contrast and sharpness of the latent image are reduced. However, a high degree of mechanical precision is required to match the opening width of the corona charger 3 with the image exposure width. Particularly, the side plate 5 of the corona charger 3 and the photosensitive drum 1 are required.
There is a difficulty that this gap must be set to an extremely small value in order to prevent leakage of corona ions from the gap between and to the outside of the exposure area.

The embodiment shown in FIG. 4 solves this problem and is characterized in that a grid wire 21 is installed at the open end of the corona charger 3. When a ground voltage or a negative voltage (voltage having a reverse polarity to the voltage applied to the corona wire 4) is appropriately applied to the grid wire 21, the corona ions that have reached the vicinity of the grid wire 21 to which the voltage is applied are in the grid. It is absorbed through the wire and does not reach the surface of the photosensitive drum 1. By controlling the substantial opening width of the corona charger 3 in this manner, it is possible to easily form a good latent image. In this case, the exposure width may be wider than the substantial opening width of the corona charger 3, but there is no problem.

Further, as shown in FIG. 5, if the grid wire 22 is installed over the entire opening width of the corona charger 3 and the voltage applied to the grid wire 22 is appropriately selected, the substantial opening width of the corona charger 3 can be set to a wide range. Therefore, the latent image can be formed with higher quality.

Further, as shown in FIG. 6, by providing a transparent ion shield member 23 at the opening end of the corona charger 3,
It is also possible to more easily define the relationship between the substantial opening width of the corona charger 3 and the exposure width. Ion shield member 23
Is preferably electrically conductive, but the same effect can be obtained even if it is made of an insulating material such as a polyester film.

The present invention is not limited to the above-mentioned embodiment, and for example, in the above-mentioned embodiment, it is explained that the image exposure is performed by the reflected light from the original in consideration of the application to the color copying machine.
A so-called printer can also be configured by performing image exposure with an optical scanning system such as a laser beam. In addition, the present invention can be variously modified without departing from the scope of the invention.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus for explaining a color image forming method according to an embodiment of the present invention, and FIG. 2 is an image forming process for the second and subsequent colors of the color image forming method according to the present invention. FIG. 3 is a diagram showing the relationship between the exposure amount at the time of charging / exposure and the surface potentials of the toner adhering portion and the non-toner adhering portion on the photosensitive drum and the potential difference thereof. FIG. Showing the relationship between the surface potential of the toner portion and the non-toner-attached portion and its potential difference
4 to 6 are cross-sectional views of an essential part of an image forming apparatus for explaining another embodiment of the present invention, and FIG. 7 is a diagram showing the charging / charging process in the image forming process for the second and subsequent colors in the conventional color image forming method. FIG. 6 is a diagram showing a relationship between an exposure amount at the time of exposure, a surface potential of a toner adhering portion and a non-toner adhering portion on a photosensitive drum, and a potential difference thereof. 1 ... Photosensitive drum (latent image holder), 3 ... Corona charger, 4
... Corona wire, 5 ... Metal side plate, 6 ... Reflected light from full color original, 7,8,9 ... Filter, 11,1
2, 13 ... Developing device, 14 ... Transfer charger, 15 ... Plain paper (toner image carrier), 16 ... Fixing device, 17 ... Cleaning device, 18 ... Charge removing device, 21, 22 ... Grid, 23 ... Ion shield member.

Claims (1)

[Claims] 1. A series of image forming processes including a step of forming an electrostatic latent image on a latent image holding member including at least a photoconductor and a step of developing the electrostatic latent image with color toner. In the color image forming method, a multicolor toner image is formed on the latent image holding member by sequentially performing each color, and the toner images are transferred onto the toner image carrier at one time to obtain a color image. A color image forming method, wherein the latent image forming step forms an electrostatic latent image on the latent image holding member by simultaneously performing charging and image exposure at the same position.