JPH0241030B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a developing device using a one-component developer. Various methods are known or proposed for developing devices that use this type of one-component developer.

Among these, the jumping development method is known as unique. In this method, a one-component developer is uniformly applied as a thin layer onto a developer holding means, and then this developer holding means and a latent image holding means are opposed to each other with a small gap being maintained. Then, the electrostatic attraction of the latent image causes the developer to fly from the developer holding means onto the electrostatic latent image holding means to perform development. (Special Publication No. 41-9475, U.S. Patent No.
2839400) According to this method, not only is the developer not attracted to the non-image area where there is no latent image, but also the developer does not come into contact with the non-image area, so that good development is carried out without any fogging. Furthermore, since no carrier particles are used, there is no fluctuation in the mixing ratio of the developer and there is no deterioration of the carrier particles. Good effects can be obtained.

In addition, the applicant has also proposed a development method different from this jumping development method in Japanese Patent Application Laid-open No. 54-43037 and Japanese Patent Application Laid-Open No.
He proposed a completely new developing method as described in No. 55-18659.

In the former development method, a one-component magnetic developer, a developer holding means (non-magnetic), and a magnetic field generating means are arranged in this order, and a uniform thin layer of developer is formed on the developer holding means by the magnetic force of the magnetic field generating means. The latent image holding means are arranged to face each other with a small gap apart so that the surface layer of the thin developer layer does not come into contact with the surface on which the electrostatic latent image is formed. Development is performed by stretching the developer facing the image area due to the electrostatic attraction of the latent image. Also in this case, since development is carried out in a state where the developer does not come into contact with the non-image area, a developed image completely free of fog can be obtained.

The latter developing method has the same configuration as the former, but an AC bias voltage (a DC voltage and an AC voltage may be superimposed) is applied as a developing bias voltage to the latent image holding means, and the electrostatic latent image is Development is performed by changing the gap between the holding means and the developer holding means over time. According to this development method, in the early stage of development, the developer reaches the non-image areas of the electrostatic latent image to develop the halftone area, and over time, the developer reaches only the image area. As a result, compared to the former developing method, it is possible to achieve the effect of developing with better halftone reproducibility and fog-free development.

This development method, in which a thin layer of one-component developer is placed opposite the latent image surface, has extremely superior effects in terms of development performance, image reproducibility, developer life, etc. compared to conventional methods. can get.

However, in actual development, the characteristics of the photoreceptor and developer used as latent image holding means change depending on the humidity in the environment, making it difficult to always perform development under optimal conditions.

For example, cadmium sulfide (CdS) as a photoconductive layer
The dependence of the surface potential V on humidity with respect to the exposure amount E of a photoreceptor in which an insulating layer of acrylic resin is formed on the photoconductive layer is shown in FIG. In the figure, curve 1a shows the characteristics at normal humidity, curve 1b shows the characteristics at low humidity, and curve 1c shows the characteristics at high humidity. These curves 1a,
As is clear from 1b and 1c, when the humidity is high, the surface potential does not drop rapidly even if the amount of exposure increases (fogging tends to occur). The surface potential also decreases when the exposure amount is small (decreased development density). Further, when the humidity is low, the upper and lower limits of the surface potential increase and the slope γ of the curve increases, contrary to when the humidity is high. In other words, the image becomes a high-contrast image.

These developments are caused by a decrease in electrical resistance due to moisture absorption of the photoreceptor, especially the insulating layer or the filler between the insulating layer and the photoconductor layer, but the problem of easy fogging at high humidity is due to the decrease in the DC voltage component of the developing bias. This can be solved by raising it.

Furthermore, regarding the decrease in developed image density at high humidity, the developed image density is saturated even when the surface potential is relatively low, and as will be explained later, the saturation value of developed image density also decreases at high humidity. This problem can be solved by selecting and using a developer that does not reduce the resistance value. In general, fog will not occur if the DC voltage of the developing bias is varied by the same amount as the potential variation of the photoreceptor due to humidity in a region with a large amount of exposure. On the other hand, when an insulating developer mainly composed of magnetite and polyethylene is used and a stainless steel developing sleeve is used as a developer holding means, the surface potential of the developer is V.
The humidity dependence of the developed density is shown in FIG. In FIG. 2, curve 2a shows the characteristics at normal humidity, curve 2b shows the characteristics at low humidity, and curve 2c shows the characteristics at high humidity. That is, when the humidity is high, the saturation concentration also decreases compared to when the humidity is normal. Further, when the humidity is low, the concentration is lower at the same surface potential than when the humidity is normal. However, for a surface potential considerably higher than the surface potential level actually used, a developed density equivalent to that at normal humidity can be obtained.

Among these types of development, development at high humidity is because the amount of charge on the developer decreases due to a decrease in its electrical resistance, and development and transfer to the transfer member are not performed sufficiently. This problem can be solved by selecting a developer made of a material that has low hygroscopicity and maintains a sufficient amount of charge even under high humidity conditions.

On the other hand, as a result of experiments and studies regarding the development at low humidity, the following was clarified.

(1) The amount of charge on the lowest layer of the developer layer, that is, the developer in contact with the sleeve surface increases as the humidity decreases.

(2) Distance of developer from the surface of the developing sleeve (μm)
The dependence of the amount of charge (μc/g) on humidity is shown in FIG. In FIG. 3, curve 3a is the characteristic at normal humidity, curve 3b is the characteristic at low humidity, and curve 3c is the characteristic at high humidity.As is clear from the drawing, at low humidity, the charge amount distribution of the developer on the developing sleeve is The bottom layer is large and the top layer is small. This charging phenomenon occurs when the humidity is low, and the developer on the developing sleeve sticks to the developing sleeve due to a large mirroring force.
It is also presumed that this is because the developer in the upper layer is less likely to be triboelectrically charged due to contact with the developing sleeve, resulting in a smaller amount of charge.

For this kind of development, we thought that by applying a relatively large alternating current electric field between the developing sleeve and the photoreceptor, the highly charged developer stuck to the developing sleeve could also contribute to the developing action, and we conducted an experiment and found that the results were positive. I was able to get good results. The experimental results are shown in FIG. This figure shows the humidity dependence of the image density corresponding to a surface potential of 500V on the photoreceptor with respect to the AC amplitude of the developing bias.Curve 4a is the characteristic at normal humidity, curve 4b is the characteristic at low humidity, and curve 4c is the characteristic at high humidity. It is.

The experimental conditions were as follows:
A one-component insulating developer mainly composed of magnetite and polyethylene coated in a thin layer of about 100 μm is placed facing the photoreceptor with a gap of 300 μm.
During this time, an alternating current electric field with a frequency of 100 to 1500 Hz was applied as a developing bias.

As is clear from Figure 4, when the humidity is low, the developed density largely depends on the alternating current amplitude of the developing bias.
It was found that it does not depend on frequency between 100 and 1500Hz. That is, by increasing the alternating current amplitude of the developing bias, it is possible to improve the decrease in the developed density.

However, it is known that increasing the AC amplitude of the developing bias causes an increase in so-called fog, and at normal humidity, an AC amplitude of about 1000 V _pp is appropriate for a bias frequency of 400 Hz.

However, as shown in Figure 1 above, when the humidity is low, E
-V curve becomes like curve 1b, the slope γ of the curve becomes large, the latitude of the applied potential becomes wide, and fogging becomes relatively difficult.

Therefore, the present invention changes the AC amplitude of the developing bias power supply in response to changes in the humidity environment. Particularly when the humidity is low, the AC amplitude of the developing bias is changed to 500 V _pp from normal humidity.
The purpose is to improve the decrease in developed density due to changes in the humidity environment by increasing the degree of development, thereby making it possible to obtain sufficient developed density without background fog.

Hereinafter, the present invention will be described in detail with reference to specific examples.

Embodiment 1 FIG. 5 is a schematic diagram of the developing device of the present invention.
is a humidity sensor made of metal oxide ceramic consisting of magnesium spinel (MgCr ₂ O ₄ ) and rutile (TiO ₂ ), and detects humidity changes by converting them into resistance value changes of 10 ⁴ to ^{10 8} Ω. The temperature characteristics of this humidity sensor are corrected by a temperature sensing element.

Reference numeral 2 denotes a power source that generates a developing bias voltage according to the change in resistance value of the humidity sensor. The output voltage of the power supply 2 corresponding to the humidity is as shown in FIG. In FIG. 6, 6a is an AC component, and 6b is a DC component.

In Figure 5, the toner thickness on the developing sleeve 3 is approximately
100μm, distance between developing sleeve 3 and photoreceptor 4
300μm, a photosensitive drum with an insulating layer made of acrylic resin on CdS was used, and the circumferential speed was 150mm/
This is an example of rotation at sec, and the alternating current portion of the developing bias was 400 Hz.

In this way, the developing bias changes depending on the environment.
In the configuration of the present invention that changes as shown in the figure, when the development characteristics were measured under various humidity environments, it was possible to always obtain high-quality developed images with sufficient image density and no fogging.

Example 2 Using the same developing device as in Example 1, the AC frequency of the developing bias power supply was fixed at 400 Hz, and the DC frequency was changed to 250 V AC at high humidity according to the output of the humidity sensor 1.
When we made it possible to adjust it to three levels: 900V _pp , normal humidity, 200V DC, 1000V AC, 150V DC and 1500V _pp low humidity, we achieved an example with only a slight exposure correction under each humidity environment _. Almost the same results as 1 were obtained.

Example 3 As a developing bias power supply, a rectangular wave (duty 50%) whose effective value DC component is the same as the voltage characteristics shown in Figure 6 and a sine wave with only one side distorted by DC voltage Vdc as shown in Figure 7 are used. When used, results equivalent to those of Example 1 were obtained.

As described above, the present invention changes the AC amplitude of the developing bias power supply in accordance with changes in the humidity environment, and increases the AC amplitude especially when the humidity is low. Therefore, it is possible to improve the decrease in developer density due to changes in the humidity environment. can. In addition, in response to potential fluctuations due to the humidity of the photoreceptor in areas with high exposure, such as those corresponding to white originals, the DC component of the developing bias is changed by approximately the same amount as the potential fluctuations, so that the developer By selecting the material, it is possible to improve the decrease in developed density at times of high humidity, and at the same time, it is possible to obtain developed images with sufficient density without fogging in environments of low to high humidity. be.

In FIG. 5, reference numeral 5 indicates a developer container, 6 indicates a developer stored in the container, and 7 indicates a blade for applying the developer 6 to the developing sleeve 3 in a uniform layer thickness.

[Brief explanation of the drawing]

FIG. 1 is a characteristic diagram showing the humidity dependence of the surface potential on the exposure amount of the photoreceptor, FIG. 2 is a characteristic diagram showing the humidity dependence of the developer density on the surface potential of the developer,
Fig. 3 is a characteristic diagram showing the humidity dependence of the charge amount on the distance of the developer from the developing sleeve surface, Fig. 4 is a characteristic diagram showing the humidity dependence of image density on the alternating current amplitude of the developing bias, and Fig. 5 is a characteristic diagram showing the humidity dependence of the image density on the AC amplitude of the developing bias. A schematic diagram of the developing device of the present invention, FIG. 6 is a characteristic diagram of the output voltage corresponding to the humidity of the power source for developing bias, and FIG. 7 is a waveform diagram of the developing bias. 1 is a humidity sensor, 2 is a power supply for developing bias, 3
is a developing sleeve, and 4 is a photoreceptor.

Claims (1)

[Claims] 1. A latent image holding member and a developer holding member that uniformly holds a one-component developer are arranged facing each other with a gap between them, and an AC electric field is applied between them from a development bias power source that superimposes a DC voltage on an AC voltage. The developing device is provided with a humidity detector, and control is performed so that when the humidity is high, the DC voltage is increased and the amplitude of the AC voltage is decreased compared to when the humidity is low, according to the output of the humidity detector. A developing device.