JPH0415949B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a one-component developing device used in an electrophotographic copying machine or an electrostatic recording device.

Broadly speaking, two types of one-component developing methods have been proposed.

That is, a toner having a relatively low resistance is used as the toner, and a latent image electric field induces charges of opposite polarity to the latent image to the toner on the latent image surface to perform development. A method called the induction development method uses a relatively high-resistance toner, and the toner is charged with a polarity opposite to that of the latent image in advance, and this toner is brought into contact with or close to the surface of the latent image to develop it. A method called a true charge development method has been proposed.

The dielectric development method has advantages such as a simple device, but has the disadvantage that it is difficult to transfer onto plain paper because the toner has low resistance.

Although the true charge development method has the advantage of having the possibility of obtaining high image quality comparable to that of the two-component development method, it has been unable to be widely commercialized due to the following problems. In other words, in order to pre-charge the toner with one polarity, if the toner layer is thick, it is difficult to uniformly charge the entire toner.
It has been experimentally confirmed that it is necessary to make the toner layer thin, and in order to provide sufficient true charge for development, it is desirable to have a toner layer of 4 to 5 layers or less with a normal toner particle size of about 50μ or less. Therefore, when developing the toner in such a thin layer, it is necessary to place the sleeve very close to the latent image surface from the viewpoint of line image reproducibility.

A developing device is conventionally known in which a conductive brush is formed on the surface of a conductive sleeve, and the flexibility of the bristles of the brush is used to keep the tips of the bristles in constant contact with the surface of the photoreceptor. In the developing device, all the bristles of the brush are at the same potential, so the result is the same as if the developing electrode, such as the sleeve, were brought very close to the surface of the photoreceptor, and line images are well reproduced, but compared to line images. Since the electric field due to the solid image increases rapidly, appropriate gradation cannot be obtained. When trying to obtain the gradation of a solid image, the reproducibility of a line image is insufficient, and it has been difficult for conventional devices to properly reproduce both images. To overcome this difficulty, it is conceivable to provide a dielectric layer on the sleeve surface to widen the substantial development gap between the sleeve and the photoreceptor surface, thereby reducing the electric field of the solid image. In this case, the electric field of the line image does not change much. However, it is known that when a thick dielectric layer is provided on the surface of the sleeve, charges are accumulated on the surface of the dielectric layer due to frictional charging due to friction with the toner, etc., and this appears as uneven development.

Generally, when copying with an electrophotographic copying machine, it is desirable for solid images such as photographs to reproduce the background and gradation of the image as they are, and for line images such as characters and line drawings, for example, thin lines drawn with a pencil. However, it is desirable to be able to reproduce it clearly. Looking at the relationship between the density of the original (OD) and the density of the copy (ID), in a solid image, as shown by curve S in Figure 1, the image is changed from a certain value of OD to prevent staining of the background. Starting with reproduction, the process proceeds at an angle of approximately 45° to reach saturation density, and in the case of line images, the first
As shown by curve L in the figure, it is required that the curve quickly rises from a small value of OD and reaches the saturation concentration. In the figure, the vertical axis is ID,
The horizontal axis shows OD.

Furthermore, in the exposure optical system of a copying machine, a relationship is obtained between the spatial frequency of the image and the transfer function (MTF) of the image gradation transmitted from the original to the photoreceptor, as shown by the curve in Figure 2. When the frequency is 0 lines/mm, that is, a solid image, the MTF is 1, and the original image is sent to the photoreceptor with the same contrast.However, as the spatial frequency increases, that is, as the lines become thinner, the MTF decreases. , because the horizontal image on the photoreceptor tends to be a line with weak contrast, the density of the latent image on the photoreceptor changes even if the image has the same density on the original, and the potential of the solid image is higher than that of the line image. Become. For this reason, it is required to develop the latent image of the line image to be dense. on the vertical axis of the figure
MTF, the horizontal axis shows spatial frequency lines/mm.

As shown in FIG. 3, in a developing region where a developing electrode 1, for example a developing sleeve, and a photoreceptor 2 are opposed to each other at a distance P _G , the potential of the developing electrode 1 is set to 200 V, for example, and the dielectric constant ε of the photoreceptor 2 is 3.0. , assuming the condition that the thickness d = 20μ, various charge patterns were created on the photoreceptor surface, and the magnitude of the electric field on the photoreceptor surface was determined by computer simulation, as shown in Figure 4. An example has been obtained. In the figure, the horizontal axis shows the distance P _G between the photoreceptor and the developing electrode, and the vertical axis shows the developing electric field V/m. The solid line is the curve for a black solid image, and the broken line is the curve OD = 0.2 Spatial frequency = 5 Represents the curve for a low contrast line image of books/mm. The charged potential Vs of the black solid image is 800V, and the background potential Vs is 200V.

In a solid black image, charges are uniformly distributed over a wide area on the surface of the photoreceptor, and an almost parallel electric field is formed between the photoreceptor and the developing electrode, so the distance is
As P _G becomes smaller, the developing electric field increases rapidly. However, in the case of a line image, there is a local charge on the surface of the photoreceptor, and the lines of electric force emitted from this are not only directed toward the developing electrode, but also the electric field of the photoreceptor. Since there is a considerable component toward the background area where no latent image is formed, which is adjacent to the charged area, the potential hardly changes as the distance P _G changes.

For example, if you are trying to reproduce a low-contrast line image with OD = 0.2 and a spatial frequency of 5 lines/mm at about 1/2 the saturation density of a solid black image, the distance P _G between the photoreceptor and the developing electrode is From Figure 4, it is necessary to select a value of about 0.2 mm. However, when the toner layer is as thin as several tens of microns as described above, under these conditions a large gap will occur between the surface of the toner layer and the photoreceptor.

When the relationship between the distance from the photoreceptor surface and the change in electric field magnitude was determined, the results shown in FIG. 5 were obtained. In the figure, the horizontal axis represents the spatial frequency /mm,
A group of curves is shown in which the developing electric field V/m is plotted on the vertical axis and the distance μ from the photoreceptor surface is used as a parameter. Fifth
In the figure, the distance between the photoreceptor surface and the developing electrode P _G =
An example is shown for 0.5mm. From the curve group, in the vicinity of the photoreceptor (for example, 5μ), the line image (for example, 5 lines/mm) is different from the solid image (0 lines/mm).
mm) is emphasized, but as the distance from the photoreceptor surface increases, the electric field of the line image decreases rapidly. Therefore, if development is performed with a large gap between the surface of the toner layer and the photoreceptor, the reproducibility of line images will be extremely poor.

One way to solve the above problem was to generate a toner cloud by applying an alternating current magnetic field or an alternating electric field to bring the toner closer to the surface of the photoreceptor, as shown in Figure 6. The toner that reaches the surface of the photoreceptor is only a part of the whole. Therefore, the above problem could not be completely solved. In FIG. 6, 1 is a sleeve, 2 is a photoreceptor, 3 is a magnetic pole, 4 is an AC power source, and 5 is a doctor blade.

It has also been considered to provide a dielectric layer on the surface of the sleeve and bring the toner layer itself close to the photoreceptor surface, which has solved the above problem to a large extent, but the surface of this dielectric layer becomes charged due to friction with the toner, etc. This resulted in uneven development because it was used before the static charge had been completely removed. It is quite difficult to completely eliminate static electricity from such a dielectric layer, which has the drawback of complicating the device.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a developing device capable of solving the above-mentioned drawbacks and properly reproducing solid images and line images with a thin layer of toner having a true charge.

To achieve this objective, the present invention provides a toner carrier disposed adjacent to an electrostatic latent image carrier, and a toner layer having a thickness thinner than the distance between the latent image carrier and the toner carrier. a developing device for developing an electrostatic latent image formed on the electrostatic latent image carrier by the toner carried on the toner carrier; a dielectric layer provided on the surface of the carrier; an electrode provided on at least the surface of the dielectric layer and divided into a number of finely divided electrodes each electrically insulated from each other; This has been achieved by a developing device characterized in that it is equipped with a static eliminating means for eliminating the electric charges generated.

According to the present invention, since the electrode of the sleeve is completely floating during development, it has almost no effect on the developing electric field, and the charge accumulated on the sleeve can be easily removed. Uneven development due to imperfections is eliminated, and with this kind of sleeve, even if the toner layer on the sleeve surface is brought quite close to the photoconductor surface, the electric field of the solid image is weakened by the thickness of the dielectric layer, and an appropriate edge effect can be obtained. I started to be able to do it.

The details of the present invention will be explained with reference to embodiments shown in the figures.

In FIGS. 7 and 8, a stainless steel dielectric support 12 containing a multi-pole magnet 11, for example 8 poles, in which N poles and S poles are arranged alternately has a surface of a dielectric support 12 made of stainless steel.
A 300μ thick epoxy resin layer 13 is applied as a dielectric layer by powder coating. Spherical copper powder 14 having a diameter of about 100 μm is adhered to the surface of the dielectric layer using an epoxy adhesive. Spherical copper powder 14 forms a float electrode. After adhering the spherical copper powder, the outer circumferential surface of the dielectric layer 13 is polished so that the surface exposed to the outer surface of the spherical copper powder is substantially flat. The electrodes made of each copper powder are electrically insulated from each other. A magnetic blade 18 (for example, made of 0.1 mm thick SK material) which also serves as a static eliminating means is arranged so as to be in contact with the outer peripheral surface of the dielectric layer 13 .

A toner layer 15 of about 1 mg/cm ² is formed using magnetic toner on the developing sleeve 16 shown in FIG. The toner charge amount at this time is approximately 7 μC/g.
The volume average particle diameter of the toner is approximately 7μ, and the true specific gravity is 1.8.

With respect to the developing sleeve 16, that is, the toner carrier formed as described above, the photoconductor is adjusted so that the development gap, which is the distance between the surface of the toner layer 15 and the surface of the photoconductor, that is, the surface of the electrostatic latent image carrier, is 0.05 mm. 1
7 and development was carried out at a linear speed ratio of 1:1, a good image with a suitable edge effect was obtained.

As a second embodiment, the developing gap between the photoreceptor and the surface of the toner layer 15 of the developing sleeve 16 is set to 0.15 mm.
When the image was developed at a linear velocity ratio of 1:3 between the photoreceptor 17 and the developing sleeve 16, an image with almost the same edge effect and even better signal-to-noise ratio was obtained.

In the second embodiment, since the developing gap becomes large, the proportion of toner close to the surface of the photoreceptor decreases, but this is compensated for by increasing the linear velocity ratio of the developing sleeve. That is, it is thought that the image quality is improved by making it difficult for unevenness of the toner layer on the sleeve surface to appear in the image.

Similar effects can be obtained by increasing the thickness of the dielectric layer in the first embodiment and increasing the linear velocity ratio of the photoreceptor sleeve.

If the developing gap is made even smaller, the electrodes on the sleeve surface will have the effect of emphasizing the electric field of the line image.

As shown in FIGS. 9 and 10, a magnetic material 19 such as spherical iron powder with a diameter of about 100 μm is applied to the circumferential surface of a stainless steel non-magnetic sleeve 12' using an epoxy adhesive 20.
It is also possible to use a developing sleeve 16' formed by uniformly dispersing and adhering and polishing the outer peripheral surface.
A thin layer 15' of magnetic toner of about 1 mg/cm ² is formed on the surface of the developing sleeve 16', and is placed facing the photoreceptor 17 with a developing gap of 1.2 mm, with a linear velocity ratio of 1:2 between the photoreceptor and the developing sleeve. When developed, both the solid image and the line image were well reproduced. The toner had a charge amount of about 10 μC/g, a volume average particle size of about 7 μ, and a true specific gravity of 1.8. The reason why the effect of greatly improving the developing efficiency was obtained is that the magnetic material 19 is dispersed on the surface as shown in FIG. In order to form a magnetic field, when the toner moves between the magnetic bodies due to the rotation of the magnet 11, the magnetic toner 15 receives a repelling force from the sleeve, and the toner jumps.
This is thought to be due to an increase in the amount of toner reaching the surface of the photoreceptor.

When compared with the case where a developing sleeve made of only a stainless steel sleeve was used under the same conditions as in the above example using the developing sleeve shown in Fig. 9, in this case a solid image could be reproduced to some extent, but the line width
Line images of about 0.1mm are hardly reproduced,
It has become clear that the developing sleeve shown in FIG. 9 exhibits excellent effects.

By forming a magnetic material pattern made of the magnetic material 19 and adhesive 20 in FIG. 10 on the dielectric layer 13 shown in FIG. 7, the effect of the island-shaped float electrode already proposed by the present inventors You can also get

According to the present invention, it has become possible to properly reproduce both solid images and line images.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the image density characteristic curves of a solid image and a line image, Fig. 2 is a diagram showing changes in the transfer function with respect to the spatial frequency of the image of the exposure optical system of the copying machine,
FIG. 3 is an explanatory diagram schematically showing the developing area, and FIG. 4 is a diagram showing the relationship between the distance between the photoreceptor and the developing electrode and the magnitude of the developing electric field on the surface of the photoreceptor for black solid images and low contrast line images. , FIG. 5 is a diagram showing the relationship between the spatial frequency of an image and a developing electric field, FIG. 6 is a schematic illustration of an example of a conventional developing device, FIG. 7 is an explanatory diagram of a developing sleeve according to the present invention, and FIG. 8 is the seventh
FIG. 9 is an explanatory diagram of another embodiment of the present invention, and FIG. 10 is a partially enlarged diagram of FIG. 11... Magnet, 12... Conductive support, 13...
...Dielectric layer, 14... Electrode, 15... Toner, 1
6, 16'... Toner carrier, 17... Electrostatic latent image carrier, 19... Magnetic material, 20... Adhesive.

Claims (1)

[Scope of Claims] 1. A toner carrier disposed adjacent to the electrostatic latent image carrier, and a toner layer formed on the toner carrier with a thickness thinner than the distance between the latent image carrier and the latent image carrier. a toner layer thickness regulating member for developing an electrostatic latent image formed on the electrostatic latent image carrier by the toner carried on the toner carrier; A dielectric layer provided on the surface of the body; an electrode provided on at least the surface of the dielectric layer and divided into a number of finely divided electrodes each electrically insulated from each other; A developing device characterized by being provided with a static eliminating means for eliminating electric charge.