JPH0576633B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a two-color image forming apparatus using electrophotographic technology.

[Prior art and its problems]

To explain the conventional two-color image forming process with reference to FIG. 1, first, a bipolar photoreceptor is uniformly charged with a predetermined polarity, for example, a negative polarity.
(a). Next, a black part B and a red part R are placed on the white background part W.
When the light image of the two-color image of the original OR is exposed through a red filter, a first electrostatic latent image corresponding to the black part B of the original OR is formed on the photoreceptor (b), and then a black When the first phenomenon using toner BT is performed, a black image is formed (c). After that, secondary charging with the opposite polarity to the primary charging is applied (d), and this time, when the light image of the original OR is exposed again through the cyan filter, the original OR
A second electrostatic latent image corresponding to the red part R is formed.
(e), followed by a second development using red toner RT to form a red image (f). A two-color image is obtained by transferring both the red and black images formed on the photoreceptor to a sheet of paper and further fixing the images.

However, in the above process, if environmental changes such as temperature and humidity or dirt on the wire of the corona discharger for primary charging occur, the surface potential of the photoreceptor obtained after the primary charging will change. The surface potential of the photoreceptor changes accordingly, causing not only the black image density but also the red image density to fluctuate and become unstable.

For example, if the wire of the corona discharger for primary charging becomes dirty and the surface potential of the photoreceptor after primary charging is low [Figure 2 a'], the potential of the black part remains low even after image exposure, and the potential of the white part With the potential even lower, the first
An electrostatic latent image is formed (Fig. 2b'). Then, a black image is formed in the first development [FIG. 2 C'].
Next, when secondary charging is performed, not only the white background and red areas but also the black areas are highly charged to a positive polarity (FIG. 2 d'). When this state is exposed to light through a cyan filter, the red area remains at a high potential, but even the white background and black areas have a potential higher than the development bias of the second development [Figure 2 e' ]. If the second development with a fixed bias is carried out in this state, not only the red part but also the white background part and the black part will be developed in red [FIG. 2 f']. Therefore, when both the red and black images are transferred to paper and fixed, it becomes a two-color image that is very unsightly.

[Purpose of the invention]

In view of the above-mentioned conventional drawbacks, the present invention has been developed to stably develop the second color even if the primary charging potential fluctuates and the potential of the first image area fluctuates, thereby producing two clear colors without color mixing. An object of the present invention is to provide a two-color image forming apparatus that can obtain an image.

[Key points of the invention]

In order to achieve the above object, the present invention provides a primary charge on a bipolar photoreceptor and a secondary charge of opposite polarity to the primary charge.
Next, by performing charging and performing predetermined first and second exposures corresponding to each color of the two-color original, a first latent image and a second latent image of mutually different polarities are formed, and the first latent image is The second latent image is visualized by first development, and the second latent image is developed by second development under application of a predetermined development bias voltage to form a two-color image.
In the color image forming apparatus, one of the bipolar photoreceptors
As the absolute value of the surface potential due to secondary charging becomes larger, the absolute value of the bias voltage for the second development is decreased, and as the absolute value of the surface potential becomes smaller, the absolute value of the bias voltage for the second development is decreased. The invention is characterized in that it includes means for variably controlling to increase the value.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 3 is a block diagram of a two-color image forming apparatus showing an embodiment of the present invention.

The photoreceptor 1 is rotatably provided in the direction of the arrow shown in the figure, and is, for example, a bipolar organic photoconductive photoreceptor.

Around the photoreceptor 1, a corona discharger 2 for primary charging, a corona discharger 3 for secondary charging, a surface electrometer 6,
First developer 10, second developer 11, polarity matcher 1
2, a transfer device 13, a separator 14, a cleaner 16, and a static elimination lamp 15 are arranged in this order.

The variable bias 9 of the second developing device 11 is set to a predetermined magnitude by the action of the control circuit 7 in accordance with the detection result of the potential of the surface electrometer 6. Surface electrometer 6
Any device that measures surface potential, such as a vibrating capacitance electrometer, can be used. In the above configuration, first, the primary charging corona discharger 2 performs primary charging of a predetermined polarity, for example, negative polarity, and the surface electrometer 6 detects the potential.According to the detection result, the control circuit 7 While setting the variable bias 9 to a predetermined value by the function of , image exposure of the document (having a red part and a black part on a white background part) is performed through the red filter 4. Next, black development is performed in the first developing device 10 to which a bias 8 is applied.
Next, the secondary charging corona discharger 3 performs secondary charging with a positive polarity opposite to the primary charging, and this time image exposure is performed again via the cyan filter 5. Red color development is carried out in the second developing device 11 to which a bias of 50.degree. Next, the polarity is matched by a polarity matcher 12 that converts the polarity of the toner, and then transferred to paper (not shown) by a transfer device 13. The paper is separated from the photoreceptor 1 by a separator 14, and then fixed. (not shown) to finally obtain a two-color image on the paper. On the other hand, residual toner on the photoreceptor 1 is cleaned by a cleaner 16, and residual potential is erased by a static elimination lamp 15 in preparation for the next image formation.

FIG. 4 shows the relationship between the primary charging potential and the appropriate second developing bias voltage setting value in the above embodiment. As shown in the figure, the relationship between the primary charging potential (shown as an absolute value) and the appropriate second developing bias voltage is inversely proportional. That is, when the primary charging potential is high, the second developing bias is lowered, and when the primary charging potential is low, the second developing bias is increased, thereby stably performing the second development. Therefore, color mixing due to adhesion of red toner to areas other than red areas during the second development does not occur, and a stable and clear two-color image can be obtained.

In addition, since the bias applied during the second development is variably set in this way, there is sufficient time from surface potential measurement to bias control, and there is no need to measure potential in advance during preliminary rotation as in normal potential control. , no time loss occurs.

In addition, as another process to which the present invention is applied, there is also a process in which a bipolar photoreceptor is used and after latent images of positive and negative polarities are formed, sequential development is performed.

〔Effect of the invention〕

As explained in detail above, the two-color image forming apparatus of the present invention is capable of changing the primary charging potential due to fluctuations in the primary charging potential.
Even if the potential of the image area fluctuates, the second color can be developed stably, so a clear two-color image without color mixing can be formed.

[Brief explanation of the drawing]

1 and 2 are block diagrams showing changes in photoreceptor surface potential in a conventional two-color image forming process, FIG. 3 is a block diagram of a two-color image forming apparatus showing an embodiment of the present invention, and FIG. FIG. 2 is a characteristic diagram showing the relationship between the primary charging potential and the second developing bias voltage in one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Photoreceptor, 2...Corona discharger for primary charging, 3...Corona discharger for secondary charging, 6...Surface electrometer, 9...Variable bias, 10...First developing device, 11 ...Second developing device, 13...Transfer device, 14
... Separator, 15 ... Static elimination lamp, 16 ... Cleaner.

Claims (1)

[Scope of Claims] 1. A bipolar photoconductor is subjected to primary charging and secondary charging with a polarity opposite to the primary charging, and predetermined first and second exposures corresponding to each color of a two-color original are applied. By doing so, a first latent image and a second latent image of mutually different polarities are formed, and the first latent image is visualized by first development,
A two-color image forming apparatus that forms a two-color image by developing the second latent image under application of a predetermined developing bias voltage to form a two-color image, comprising: As the absolute value of the surface potential due to charging increases, the absolute value of the bias voltage for the second development is decreased, and as the absolute value of the surface potential decreases, the absolute value of the bias voltage for the second development is decreased. A two-color image forming apparatus characterized by comprising means for variably controlling the size of the image.