JPS6333223Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a developing device for a dry type electrophotographic copying machine.

In a dry electrophotographic copying machine that uses a charged carrier with toner adsorbed as a developer, in order to prevent the carrier from adhering to the non-image area of the photoreceptor, when the non-image area passes through the developing device, To,
Switching the developing bias to a lower value is practiced. Furthermore, in order to prevent toner from adhering to the non-image area, the developing bias is switched to a high value when the non-image area passes through the developing device. On the other hand, there are developing devices that use a plurality of magnetic brushes in order to increase the developing ability.

FIG. 1 shows an example of a copying machine equipped with the plurality of magnetic brushes. 1 is a drum-shaped photoreceptor, 1a is an image forming part, and 1b is a non-image part. The photoreceptor 1 rotates as shown by arrow A and receives the action of the developing device 2 . The developing device 2 includes two magnetic brushes 3 and 4 at positions facing the circumferential surface of the photoreceptor 1 . Both magnetic brushes 3 and 4 rotate in the directions of the arrows, and contact the circumferential surface of the photoconductor 1 with the developer that is adsorbed in the form of spikes on the circumferential surface of the brushes. Attach it to the statue. The supply of developer 6 to both magnetic brushes 3 and 4 is as follows:
This is done by rotating the pumping roller 5 in the direction of the arrow. Both magnetic brushes 3 and 4 also serve as developing electrodes, and a developing bias is applied by applying a DC voltage to them.

By the way, in a copying machine equipped with the two magnetic brushes, if one attempts to simply switch the developing bias, the result will be as shown in FIG. That is, a DC power source 8 is connected to the magnetic brushes 3 and 4 via a changeover switch 7, and when the boundary between the image forming section 1a and the non-image section 1b passes through the position of the magnetic brushes 3 and 4, the changeover switch 7 Operate to switch the developing bias. In the state shown in Fig. a where the image forming section 1a faces both the magnetic brushes 3 and 4, the changeover switch 7 is switched to the DC power supply 8 side, and a developing bias is supplied to both the magnetic brushes 3 and 4. has been done. Even in the state shown in Figure b, where only the upstream magnetic brush 3 faces the non-image area 1b, the development of the image forming area 1a is not completed, so the development bias cannot be cut off, and the changeover switch 7 is set to direct current. The power source remains switched to the 8 side. The changeover switch 7 is switched to the ground side to cut off the developing bias after both magnetic brushes 3 and 4 come to face the non-image area 1b, as shown in Figure c. When the magnetic brush 3 on the upstream side is in the state shown in Fig. d, facing the next image forming section 1a, the changeover switch 7 must be switched to the DC power supply 8 side again to start supplying the developing bias.

As is clear from the above explanation, during the period when the developing bias is switched to a low value, both magnetic brushes 3,
4 are facing the non-image area 1b, it is only during that period that carriers can be prevented from adhering to the non-image area 1b, and a reliable effect cannot be expected.

The present invention aims to solve the above-mentioned problems, and is characterized in that the switching period of the voltage applied to the plurality of bias electrodes is shifted for each electrode.

FIG. 3 shows an example in which the present invention is implemented in a copying machine using two magnetic brushes 3 and 4 as bias electrodes. It is assumed that both magnetic brushes 3 and 4 are arranged with an interval l, and that the photoreceptor 1 travels in the direction of arrow A at a speed v. The magnetic brush 3 on the upstream side is selectively connected to a DC power source 8 and ground via a changeover switch 9, and the magnetic brush 4 on the downstream side is connected to the same DC power source 8 through a changeover switch 10.
and ground. When both the magnetic brushes 3 and 4 are facing the image forming section 1a as shown in FIG.
are both switched to the DC power source 8 side, and a developing bias is applied to both magnetic brushes 3 and 4. In the state shown in Figure b, where only the upstream magnetic brush 3 faces the non-image area 1b, the changeover switch 9
only is switched to the ground side. Therefore, the magnetic brush 4 continues developing, but the magnetic brush 3 stops developing and becomes a low bias state to prevent the carrier from adhering thereto. When both magnetic brushes 3 and 4 are in the state shown in FIG. c, facing the non-image area 1b, the changeover switch 10 is also switched to the ground side. When the state shown in FIG. d is reached, in which only the upstream magnetic brush 3 begins to face the next image forming section 1a, only the changeover switch 9 is switched to the DC power supply 8 side, and development begins. When the magnetic brush 4 on the downstream side also faces the image forming section 1a as shown in Fig. e, both the changeover switches 9 and 10 are switched to the DC power source 8.
Return to the side. The timing for operating the selector switch 9 can be obtained by synchronizing the rotation of the photoreceptor 1, and the timing for operating the selector switch 10 may be delayed by a time l/v.

Note that the above embodiment uses two magnetic brushes as developing electrodes and is intended to prevent carrier adhesion. It is also possible to switch the developing bias to a high value for the purpose of preventing toner from adhering to the area. The present invention can also be applied to an auto-bias method in which the latent image potential of the image forming section is detected and the developing bias is controlled according to the detected value.

As described above, the present invention provides a changeover switch between each of a plurality of developing electrodes and a power source, and shifts the phase of change in the applied developing bias according to the position of each electrode by switching the changeover switch. This makes it possible to align the positions of action of each electrode on the photoreceptor, thereby ensuring the effect of switching the developing bias.

[Brief explanation of the drawing]

FIG. 1 is a vertical sectional view of the main parts of an electrophotographic copying machine, FIG. 2 is an explanatory diagram of the operation of a conventional developing device, and FIG. 3 is an explanatory diagram of the operation of an embodiment of the present invention. 1...Photoreceptor, 2...Developing device, 3, 4...Magnetic brush (also serving as developing electrode), 8...DC power supply, 9,
10... Changeover switch.

Claims (1)

[Scope of utility model registration request] In a developing device that includes a plurality of developing electrodes arranged sequentially in the traveling direction of the photoreceptor and has a variable developing bias, a changeover switch is provided between each development electrode and a power source, and switching of the changeover switch is controlled by an image. 1. A developing device for a dry type electrophotographic copying machine, characterized in that the development is performed corresponding to the non-image areas, and the phase of change in the applied developing bias is shifted depending on the positions thereof.