JPH04365Y2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a developing device used in electronic copying machines, printers, etc., and particularly to a developing device suitable for two-color printing.

(Prior art) In order to obtain a two-color copy from a two-color original image, electrostatic latent images with different polarities corresponding to each color are sequentially developed using two-color toner charged with different polarities. Two-color color electrophotographic copying machines are known.

One of these is that one color has a positive potential with respect to the white background.
For the other color, two developing machines are installed in the moving direction of the photosensitive drum on which an electrostatic latent image with a negative potential is formed, and the first developing machine develops the first color (for example, black), and the second developing machine develops the first color (for example, black). There is a method of developing a second color (for example, red) with a developing machine. In this method, the first and second developing machines contain a developer such that the toner is charged with a polarity opposite to that of the latent image. That is, the first toner and the second toner are each charged with opposite polarities.

However, the problem with this developing method is that
This means that the toner from one developing machine gets mixed into the other developing machine. If toner of a different color is mixed into the developing machine, disadvantages such as deterioration of the quality of the developed image and accelerated deterioration of the developer occur.

As a means to prevent color mixing of the developer due to mixed toner as described above, there is a method in which a metal roll to which a voltage is applied is installed in the developing machine, only different colored toners are attached to this roll, and this is removed and separated using a blade. Proposed.

(Problem to be solved by the invention) However, even in this method, when an iron oxide powder carrier is used as the second carrier, the bias voltage applied to the bias roll leaks to the developing roll. It was hot.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a developing device for an electronic copying machine in which the bias voltage applied to the bias roll does not leak to the developing roll.

(Means for Solving the Problem) The above object is to form positive and negative electrostatic latent images on a photoconductor, and to apply a first developing device to the electrostatic latent image having a first polarity on the photoconductor. Developed with toner 1,
In the developing device, the electrostatic latent image having the second polarity on the photoreceptor is developed with a second toner by a second developing machine, the second developing machine facing the developing roll. This can be achieved by having a rotating bias roll and applying an alternating voltage having a polarity opposite to that of the first toner or an alternating voltage obtained by superimposing a direct current voltage to the bias roll.

(Function) Bias leak can be prevented from occurring by applying an alternating voltage that cuts off the bias voltage before charge is injected from the developing roll to the bias roll.

(Example) The present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic diagram of a two-color electronic copying machine according to the present invention.

Around the photoreceptor drum 1, there are a primary charger 11, a secondary charger 12, an exposure section 13, a first developer 14, a second developer 15, a pre-transfer charger 16, a transfer charger 17, and a charge-eliminating charger. container 18, cleaning device 19,
and a static elimination lamp 20 are arranged as shown.

The second developing device 15 includes a developing roll 3, an agitating roll 4, and a regulating plate 10, and further includes a bias roll 6, a blade 7, and a spiral roll 8, which will be described later. Note that the first developing device 14 has the same configuration as a well-known one, so a description thereof will be omitted.

Here, the first developing machine 14, the second developing machine 15
It is assumed that a magnetic brush developing device is used and a two-component developer consisting of toner and carrier is used. For example, the first developer 14 uses black toner, the second developer
It is assumed that the developing device 15 uses red toner.

Primary charger 11, secondary charger 12, and exposure section 1
3, a two-color electrostatic latent image is formed on the photoreceptor drum 1, for example, a latent image with a positive potential for the black portion of the document and a latent image of negative potential for the red portion of the document. Ru. For these electrostatic latent images, black (positive potential) on the photosensitive drum is generated in the first developing device 14.
The latent image is developed with black toner, and the red (negative potential) latent image is developed in the second developing device 15 with red toner having a polarity opposite to that of the black toner. During development in the second developing machine 15, the black toner image comes into contact with the red magnetic brush, and a portion of the black toner is scraped off and mixed into the second developing machine 15.

In order to remove the mixed black toner, a metallic bias roll 6 is provided in the second developing machine so as to face the developing roll. The black toner adhering to the bias roll is peeled off by a blade 7 and collected by a spiral roll 8.

In a developing machine having such a configuration, when an iron oxide powder carrier is used as the second developer, when a DC voltage is applied to the bias roll, bias leak occurs at a voltage exceeding a certain value.

Now consider the cause of bias leak.

The conductivity of the iron oxide powder carrier used now can be determined by the thickness of the oxide film on the carrier surface. It is known that its electric field-conductivity characteristics exhibit a nonlinear curve as shown in FIG.

On the other hand, as shown in FIG. 3, the carrier chain extending from the developing roll contacts the bias roll to form a nip N. The time of contact with the nip N is determined by the number of rotations of the developing roll, the distance between the developing roll and the bias roll, etc.

In FIG. 3, 5a indicates a carrier, 5b indicates mixed toner, and 5C indicates captured toner. Further, although the developing roll and the bias roll are shown as being in contact with each other in FIG. 1, they are actually arranged with a slight interval between them.

In this state, when a voltage is applied to the bias roll, charges are injected from the developing roll to the bias roll.

The injection speed at this time is not uniquely determined, but is thought to be influenced in a complex manner by the conductivity of the carrier, the applied voltage, the distance between the developing roll and the bias roll, etc.

Now, when we apply DC voltage to the bias roll,
A current according to the conductivity as shown in FIG. 2 flows toward the developing roll. Furthermore, the amount of mixed toner removed is determined by the magnitude of the electric field at this time. (The higher the electric field, the more toner is removed.) As the applied voltage is gradually increased, the current increases rapidly, and finally exceeds the capacity of the power supply, the current supply is stopped, and the mixed toner is removed. It disappears.

This is the concept behind the bias leak phenomenon to the developing roll.

Let me explain in a little more detail.

Normally, as the developing roll sleeve of the developing machine rotates, a carrier chain with toner attached (red/black toner) moves around the developing machine as shown by the arrow. Therefore, a portion of the carrier chain comes into contact with a bias roll located close to the developing roll, forming a nip N. In this case, the contact time is determined depending on the distance between the developer roll and the bias roll, the rotational speed of the developer roll sleeve, etc. That is, when looking at only one carrier chain, it is in contact with the bias roll only for a certain time _t1 . On the other hand, a conductive carrier such as an iron oxide powder carrier is not completely free of resistance, but exhibits an electric field-dependent conductivity as shown in FIG. This conductivity and electric field determine the time t ₂ during which charge is injected from the developing roll to the bias roll.

Now, when t ₁ > t _{2 ,} bias leak may occur, and when t ₁ < t ₂ , bias leak will not occur. Therefore, bias leakage can be prevented by turning off the power before charge injection is completed. Therefore, an alternating voltage as shown in FIG. 5 is applied. Considering the peak value and pulse width, t ₁
Appropriately select the conductivity of the carrier, the nip time (rotation speed of the developing roll and bias roll), and the distance between the developing roll and bias roll so that < _t2 .

Here, the mixed toner adheres to the bias roll selectively from the developer due to the electric field caused by the peak voltage (the electric field applied between the developing roll and the bias roll), making it possible to apply a bias without causing breakdown.

The present invention is characterized in that an alternating voltage or an alternating voltage with direct current superimposed thereon is used as the voltage applied to the bias roll.

With this configuration, the bias leakage that conventionally occurred with the same applied voltage is improved, and furthermore, the repetition frequency of the applied alternating voltage causes slight vibrations in the vicinity of the toner to be adhered, which prevents adhesion from other toners. Since the toner to be removed is suitably separated, it is possible to obtain far superior separation and removal effects compared to conventional devices.

As an example, FIGS. 4 and 5 show alternating voltage waveforms in which alternating voltage and direct current applied to the bias roll used in the present invention are superimposed. In the figure, pulse width: 0.1 msec, pulse interval: 0.3 msec., and peak voltage: 200V.

(Bias leakage occurred when DC bias was applied under the same conditions.) (Effects of the invention) As described above, the invention uses an alternating voltage or an alternating voltage with a DC voltage superimposed as the voltage applied to the bias roll. As a result, it is possible to obtain a far superior effect of separating and removing different color toners than in the past.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of the developing device of a copying machine according to the present invention, Fig. 2 shows the electric field-conductivity characteristics of the conductive carrier, and Fig. 3 explains the state in which the carrier chain is generated. This is a diagram for
FIGS. 4 and 5 are diagrams showing examples of applied voltages. Symbols in the figure: 1... Photosensitive drum, 3... Developing roll, 5a... Carrier, 5b... Mixed toner, 5c... Captured toner, 6... Bias roll, 7... Blade, 8... Spiral roll, 11...primary charger, 12...secondary charger,
13... Exposure section, 14... First developing machine, 15...
...Second developer, 16...Pre-transfer charger, 17...
...Transfer charger, 18...Removal charger, 19...Cleaning device, 20...Static elimination lamp.

Claims (1)

[Scope of utility model registration request] Positive and negative electrostatic latent images are formed on a photoconductor, the electrostatic latent image with a first polarity on the photoconductor is developed with a first toner by a first developing device, and a second toner on the photoconductor is developed with a first toner. In the developing device, a second developing device develops an electrostatic latent image having a polarity with a second toner. A developing device characterized in that an alternating voltage having a polarity opposite to that of the first toner or an alternating voltage obtained by superimposing a direct current voltage is applied to the bias roll.