JPS5852583B2 - Self-bias developing device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a bias developing device, and more particularly to a self-bias developing device that develops an electrostatic latent image by inducing a potential in a developing electrode that corresponds to the potential of the latent image.

Conventionally, bias development has been performed using a development electrode in order to avoid the so-called edge effect, in which a large amount of developer adheres only to the edges of an image.

However, there are various types of originals that are the subject, such as the size of the image area within the original, whether there are many line images or many continuous tone images, etc., and the electrostatic latent image created from these originals Since the conditions of each are different, an appropriate image could not be formed under the same development conditions.

In addition, in devices that form an electrostatic latent image by projecting and exposing the original image onto the surface of a photoreceptor, the potential of the electrostatic latent image is slightly affected by the degree of fatigue of the photoreceptor, surrounding environmental conditions, etc., and the developed image The background may become smudged or the image density may become low.

Therefore, the operator must adjust the image forming conditions such as the exposure light amount, but in reality, the operator creates a trial image for this purpose and repeats the adjustment based on this. The resulting image (for example, the image 1 of an electrophotographic copying machine is a copy image formed on copy paper) is completely wasted.

In order to eliminate this drawback, several devices have been proposed in which a potential corresponding to the potential of the electrostatic latent image is introduced to the developing electrode.

Typical examples include a self-bias developing device using a float developing electrode and a forced bias developing device using an auto-bias method.

The former uses a so-called floating developing electrode that is electrically insulated from others, and the average potential of the electrostatic latent image facing the developing electrode is induced in the developing electrode.

This method has the advantage of being able to perform bias development very easily, but since the potential induced in the development electrode is the average potential of the latent image, the background area is large (and the original has a small image area). Conversely, when using an original with many image areas and few background areas, the potential difference between the potential induced in the developing electrode and the potential of the background area or image area of the electrostatic latent image becomes small, resulting in This may cause stains or lower image density.

In order to alleviate this drawback, a method in which the developing electrode is connected to a power source via a one-way rectifier has also been proposed.

This is effective to some extent when the background potential is high due to fatigue of the photoreceptor, but when the background potential is low, the density of the image area becomes lower than necessary, causing the original to look like thin pencil writing. In some cases, the image may almost disappear.

Also, regarding the part where the latent image has a higher potential than the power supply voltage,
There is no effect of connecting to a power source.

The latter auto-bias method, as described in Japanese Patent Application No. 4952010 and Japanese Patent Application No. 49-67714, detects the potential corresponding to the background of the electrostatic latent image, performs arithmetic processing on this, and determines the optimal By applying a bias potential of 1 to the developing electrode, an extremely reliable effect can be obtained.

However, there is a drawback that the configurations of the potential detection device and the arithmetic processing device are complicated and expensive.

SUMMARY OF THE INVENTION An object of the present invention is to provide a self-bias developing device which has a simple structure like a float developing electrode type and has reliable operation like an auto bias type.

Hereinafter, the present invention will be explained with reference to the drawings.

FIG. 1 is a configuration diagram showing each part of an electrophotographic copying machine to which the present invention is applied, in which a photosensitive drum 1 having a photoconductive insulating layer on its surface is supported by a shaft 2 and rotates at a constant speed in the direction of the arrow. Then, first, the surface of the corona discharger 3 is uniformly charged.

The polarity of the corona discharger 3 is appropriately selected depending on the characteristics of the photoconductive insulating layer on the surface of the photoreceptor.

Next, the image of the document to be copied is projected by the exposure optical system 4 onto the uniformly charged photoreceptor surface, and the charge on the photoreceptor surface is selectively dissipated due to the image beam, and the image becomes static. A latent image is formed.

The electrostatic latent image thus formed on the surface of the photoreceptor is made into a visible image by coming into contact with toner particles in the developing section 5.

The developing section 5 includes a liquid tank 7 storing a developer 6 in which toner particles are dispersed, a pump (not shown) for supplying the developer to the developing electrode 8, and other members.

The developing electrode 8 is connected via a switch 9 and a resistor R2 to an electrode plate 10 provided inside the corona discharger 3 and a varistor 11 whose one end is grounded.

A high-voltage discharge current is taken out from the electrode plate 10 and is made into a constant voltage by a varistor 11 7, which is used as a constant voltage power source for applying a bias to the developing electrode 8.

Of course, a normal bias power source may be used instead of such a power source.

After development, excess developer adhering to the surface of the photoreceptor is scraped off by the squeeze roller 12, resulting in a clear developed image.

This developed image is superimposed on the transfer paper 15 guided by the guide plate 14 in the transfer section 13, and the corona discharger 16 also receives an electric charge sufficient to sufficiently tighten the toner particles forming the developed image. , is transferred from the surface of the photoreceptor to the transfer paper 15.

The transfer paper 15 on which the image has been transferred is discharged by a discharge roller 17.

Toner that remains on the surface of the photoreceptor without being transferred to the transfer paper is
Cleaning roller 18 and cleaning blade 1
9, the photoreceptor can be reused.

In this embodiment, when the switch 9 is opened, the developing electrode 8 is in a floating state, and when the switch 9 is closed, the developing electrode 8 is connected to the bias power supply.

The present invention relates to when the switch 9 is closed, and an electrical equivalent circuit of the developing device at this time is shown in FIG.

Vs is the potential of the DC power supply equivalent to the average latent image potential of the photoreceptor, R1 is the resistance of the developer interposed between the photoreceptor surface and the developing electrode 8, vD is the developing electrode potential, and R2 is the developing electrode An external resistor Va connected between the developing electrode 8 and the power source 20 corresponds to a bias constant potential applied to the developing electrode 8, respectively.

In practice, a resistor Ra is connected in parallel to the power supply 20 and in series to the resistor R2 to match the power supply output impedance.

The relationship between the developing electrode potential VD and the average latent image potential Vs of the photoreceptor with respect to background stains on copies is shown in FIG.

In this figure, a straight line indicated by a dashed-dotted line 21 indicates the relationship when the developing electrode potential VD and the average latent image potential Vs are equal, and the occurrence of background stains on the developed image when the entire electrostatic latent image has a uniform potential. It branches out areas.

That is, in a range where the developing electrode potential ■D is greater than the average latent image potential Vs, an electric field is generated that attracts toner particles from the photoreceptor surface to the developing electrode, so that background staining of the developed image does not occur. Development electrode potential ■.

In a range where V is smaller than the average latent image potential Vs, an electric field is generated that attracts toner particles from the developing electrode to the surface of the photoreceptor, resulting in background staining of the developed image.

This straight line 21 is a theoretical branch line regarding background stains, but it coincides with the characteristic curve in the float developing electrode method,
It has been confirmed that the results match well with actual phenomena.

On the other hand, the developing electrode potential (■) according to the present invention.

The relationship between Vs and the average latent image potential Vs is shown by a solid straight line 22, which is expressed by the following equation.

Now, consider a case where the image area of the electrostatic latent image is extremely small and the average latent image potential Vs is smaller than the bias constant potential Va.

The average latent image potential Vs at this time is a low potential that is extremely close to the background potential of the electrostatic latent image, so in the case of the float developing electrode method, almost no electric field is generated to prevent toner from adhering to the background area. Also, background stains are likely to occur in the developed image.

On the other hand, according to the present invention, as shown in FIG.
2 is located above the straight line 21, and the developing electrode potential vD is larger than the average latent image potential Vs, so an electric field is generated that prevents toner from adhering to the background of the electrostatic latent image, and background staining of the developed image is prevented. .

Next, the image area of the electrostatic latent image is large, and the average latent image potential Vs
Also consider the case where is larger than the bias constant potential Va.

The average latent image potential Vs at this time is relatively close to the image area potential of the electrostatic latent image. According to the float developing electrode method, background staining of the developed image can be prevented, but the image density becomes low. Put it away.

On the other hand, according to the present invention, the straight line 22 is located below the straight line 21 as shown in FIG. 3, and the average latent image potential Vs
A lower potential is induced at the developer electrode.

Therefore, there is no reduction in image density as in the float electrode method.

Furthermore, since the absolute value of the developing electrode potential vD itself is increased as the average latent image potential Vs increases, the increase in the average latent image potential Vs is due to the electrostatic potential caused by the use of colored originals, fatigue of the photoreceptor, etc. Even if it is caused by an increase in the background potential of the image, background staining of the developed image is prevented.

The straight line 23 in FIG. 3 corresponds to the bias constant potential V in FIG.
This is a straight line when a is set to zero.

As is clear from the figure, the straight line 23 has the same slope as the straight line 22, and intersects the straight line 21 at the point where the bias constant potential Va=O.

Furthermore, if the magnitude of the resistance R2 is changed, the slopes of the straight lines 22 and 23 will change.

Therefore, the bias constant potential Va and the resistance R2 may be appropriately set depending on the usage conditions of the apparatus, the type of photoreceptor and developer, or the quality of the image to be obtained.

Moreover, these values can also be switched arbitrarily.

It should be noted that the straight lines 21 and 23 are point symmetrical with respect to the origin, and therefore it is easy to understand that even if the signs of the respective potentials are reversed, they operate in exactly the same way.

In the above embodiment, if the average latent image potential Vs increases, the developing electrode potential becomes ■.

will rise accordingly.

However, there is a limit to how much the average latent image potential Vs can increase due to an increase in the background potential of the electrostatic latent image, and the average latent image potential Vs
The reason why s increases above a certain level is usually due to the fact that the image area of the electrostatic latent image is large.

In this case, the developing electrode potential ■. There is no need to make the developing electrode potential so high, and if the average latent image potential Vs is above a certain level, the developing electrode potential (■) is set.

Figure 4 shows its configuration. In the above embodiment, a constant voltage element 24 is added with one end connected to the connection point between the resistor R2 and the developing electrode 8. , the other end is connected to ground, and an upper limit bias constant potential 1 is set there.

FIG. 5 is a diagram showing the relationship between the developing electrode potential VD and the average latent image potential Vs of the photoreceptor in this embodiment using a solid line 25.

Generally, for objects with few images, it is safe to regard the average latent image potential as the background potential.In such cases, if the average latent image potential Vs is smaller than the upper limit bias constant potential 1, background staining will occur. Therefore, the upper limit bias potential V1 may be determined according to the normal background potential.

As shown in FIG. 5, by selecting the upper limit bias constant potential v1 to be smaller than the bias constant potential Va, it is possible to prevent the image density from decreasing even when there are many images.

Of course, the value of the upper limit bias potential V, may be determined according to the conditions of the apparatus, and the upper limit bias potential (2) may be set to a value greater than the constant bias potential Va.

Furthermore, in order to effectively use the present invention, if the developing electrode is completely electrically insulated by wrapping it in a highly insulating resin,
Particularly effective.

The value of the resistor R2 is preferably larger than the value of the resistor R1. In the example of the liquid development method described above, the resistor R1 is kept at about 200 MΩ, the resistor R2 is kept at about 600 MΩ, and the bias constant potential is set at about +400 ■. In experiments using a selenium photoreceptor, we were able to obtain various types of copies without background smudges or decreases in image density.

Needless to say, the present invention is not only applicable to liquid development methods, but also can obtain similar effects in dry development methods.

As described above, according to the present invention, by connecting a resistor to the developing electrode and forcibly applying a constant bias potential, the bias potential can be changed according to the latent image, and background stains can also be reduced in image density. You can even get a copy.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the entire structure of an electrophotographic copying machine to which the present invention is applied, FIG. 2 is an electrical equivalent circuit diagram of a developing device to which the present invention is applied, and FIG. 3 is a diagram to which the present invention is applied. FIG. 4 is a graph showing the relationship between the developing electrode potential and the average latent image potential of the photoreceptor in the developed developing device, and FIG. 5 is a partial configuration diagram showing another embodiment of the developing device to which the present invention is applied. 4 is a graph showing the relationship between the developing electrode potential and the average latent image potential of the photoreceptor in the developing device shown in FIG. 4. FIG. R1: resistance of the developer interposed between the photoreceptor surface and the developing electrode, R2: resistance connected between the developing electrode and the power source,
Va: bias constant potential, vD: developing electrode potential, ■s: average latent image potential of the photoreceptor.

Claims (1)

[Scope of Claims] 1. A developing electrode potential corresponding to the average surface potential of an electrostatic latent image surface facing the developing electrode is determined by the resistance of the developer interposed between the electrostatic latent image surface and the developing electrode, and In a self-bias developing device that develops the electrostatic latent image by inducing a distribution between the bias resistor interposed between the developing electrode and ground, the electrostatic latent image is distributed between the bias resistor and ground. A self-bias developing device characterized in that a power supply device is connected to obtain a necessary developing electrode potential when the average surface potential of the latent image surface is low. 2 The development electrode potential corresponding to the average surface potential of the electrostatic latent image surface facing the development electrode is determined by the resistance of the developer interposed between the electrostatic latent image surface and the development electrode, and between the development electrode and ground. By inducing a distribution between the bias resistor and the bias resistor interposed between the two. In the self-bias developing device for developing the electrostatic latent image, a power supply device is provided between the bias resistor and ground for obtaining a necessary developing electrode potential when the average surface potential of the electrostatic latent image surface is low. A self-bias developing device characterized in that the developing electrode is connected to the developing electrode and grounded via a constant voltage element.