JPH0336230B2 - - Google Patents

Description

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

[Prior art]

FIG. 1 is a process diagram of copying in a conventional two-color copying apparatus.

First, the surface of the photoreceptor 1 is uniformly positively charged by direct current corona discharge (a), and the red area R, white area W, and black area are
The light image of the original document 2 made of BK is exposed through the red filter 3 (b), and a latent image corresponding to the black part BK is formed on the photoreceptor 1. Next, this latent image is developed with black toner 4 to obtain a black toner image (c). With this black toner image supported on the surface of the photoreceptor 1, it is positively charged again by direct current corona discharge (d). Subsequently, the optical image of the original 2 is exposed again through the cyan filter 5 (e), and latent images corresponding to the black portion BK and the red portion R are formed on the photoreceptor 1. This latent image is developed with red toner 6 to obtain a red toner image (f). Corona discharge is applied to make the polarity of each toner formed on the photoreceptor 1 the same.
(g), each toner image is simultaneously transferred onto transfer paper 7 (h), and a two-color image is obtained through a fixing process.

FIGS. 2 and 3 are diagrams showing changes in the surface potential of the photoreceptor in the process shown in FIG. 1. Second
In the figures and FIG. 3, a to f correspond to the steps shown in FIG. 1, and in FIG.
Curves 1 and 3-2 indicate the surface potentials of the photoreceptor corresponding to the black area BK and the red area R, respectively.

[Problems with conventional technology]

Step of developing the first electrostatic latent image (Fig. 1c)
Then, development is performed using negatively charged black toner to obtain a black toner image, but in this state, the black toner has a negative polarity with respect to the positively charged photoreceptor surface, and its static electricity It is adsorbed by force. However, when the entire surface of the photoreceptor carrying the black toner image on its surface is positively charged again in the next secondary charging step (Fig. 1d), some of the black toner becomes positively charged, and this state In this case, a repulsive force acts between the black toner and the positively charged photoreceptor surface, and the black toner becomes unstable in terms of adsorption to the photoreceptor surface. Further, as shown in FIG. 2(d), the surface potential of the photoreceptor in the black corresponding portion is higher than the photoreceptor surface potential in the red and white corresponding portions. Therefore, in the step of developing the second electrostatic latent image (FIG. 1 f), the adsorption force between the second color (red) magnetic brush and the positive black toner works, and the black toner on the photoreceptor is Not only does scraping occur, but in that case, the negatively charged red toner is attracted, so the red color becomes strongly conspicuous in the black toner image, and furthermore, the scraped black toner becomes a secondary If the red toner image gets mixed into the developing device, the red toner image also has the disadvantage that as development is repeated, the black color gradually becomes stronger and more conspicuous.

In addition, a process of applying corona discharge is necessary to make the polarity of the black toner and red toner on the photoconductor the same before transfer, and at that time, it is difficult to match the polarity of the black toner below the red toner. .

[Purpose of the invention]

In view of the above-mentioned drawbacks of the conventional art, the present invention has been developed to provide a two-color toner image that can provide clear two-color toner images for a long period of time.
An object of the present invention is to provide a color image forming apparatus.

[Key points of the invention]

In order to achieve the above object, the present invention performs the first charging, exposure, and development steps on the photoreceptor, and then converts the polarity of the charge applied to the photoreceptor by the charging. A second charging, exposure, and development process is performed to sequentially form two-color images of different colors on the photoreceptor, and then the two-color images are transferred all at once onto a transfer medium. The two-color image forming apparatus is characterized in that the second charging is performed by alternating current corona discharge.

[Embodiments of the invention]

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

FIG. 4 is a block diagram of a two-color image forming apparatus showing an embodiment of the present invention.

The photoreceptor 1 is formed into a drum shape and can be rotated clockwise.A DC charger 8, an AC charger 9, an image exposure section 10, a black developer 11, and a red developer are provided on the outer periphery of the photoreceptor 1. 12, transfer device 13, separator 1
4, a static eliminator 15, a cleaning section 16, and an eraser section 17 are provided.

Examples of the configuration of the AC charger 9 are shown in FIGS. 5 and 6. In FIG. 5, a capacitor 19 is connected in series with the AC power supply 20, and a predetermined potential is applied to the shield of the AC charger 9 by a shield bias 18. The function of the capacitor 19 is to cut out the DC component since AC discharge itself generates a lot of negative current, and to allow only the AC component to pass, so that the absolute values of the positive current and negative current are approximately equal. It is something. In this embodiment, AC power supply 2
0 was 300 Hz and 5.0 Kv, the capacitor 19 was 0.1 μF, and the voltage applied to the shield bias 18 was +600 V.
FIG. 6 shows another configuration example of the AC charger 9, in which a grid wire 21 is provided and a shield bias voltage is applied thereto.

FIG. 7 shows the difference between the AC charging characteristics and the DC charging characteristics.

In the case of DC charging (7-2), the surface potential of the photoreceptor before being charged increases over the entire range of 0 to 1000 V after charging, whereas in the case of AC charging (7-1),
The surface potential of the photoreceptor over the entire range of 0 to 1000 V before being charged converges to approximately +500 V after being charged. This is because in the case of DC charging, it receives only positive polarity corona, whereas in the case of AC charging, in the area where the potential is higher than +500V before being charged, it receives negative polarity corona discharge, and below +500V. This is thought to be due to positive corona discharge occurring in the region where the potential is low.

Next, each step of two-color image formation will be explained based on FIGS. 4 and 8. FIG. 8 is a model diagram showing changes in the surface potential of the photoreceptor in each step of two-color image formation.

First, the surface of the photoreceptor 1 is uniformly charged with a DC charger 8 to obtain a high surface potential of +1000V (FIG. 8A). Next, the image exposure section 10 exposes the optical image of the original through the red filter 3. The manuscript is in red R,
It consists of a black part BK and a white part W. Black part BK
In the corresponding portions, the potential decreases by an amount corresponding to dark attenuation, and in the red portion R and white portion W corresponding portions, the potential decreases by an amount equivalent to light attenuation (FIG. 8B). Next, in the black developing device 11,
Black development is performed at a high potential development bias of +600V. At this time, there is almost no change in the photoreceptor surface potential (FIG. 8C). Negatively charged black toner adheres only to the black BK corresponding area where the surface potential is +600V or more.

Next, the red developing device 12 and transfer device 1 are in an inoperable state.
3, separator 14, static eliminator 15, cleaning section 1
6. Passes through the eraser section 17 and is charged 2 by the AC charger 9.
Next, perform charging.

As mentioned above, due to the characteristics of AC charging, the black part
The potential of the BK corresponding part drops from +900V to +500V,
+400V to + for the red part R and white part W corresponding parts
The voltage increases to 500V, and approximately +500V is uniformly obtained (Fig. 8D). At this time, the black part corresponding to BK receives negative polarity corona discharge and the surface potential decreases, but the attached black toner itself also receives negative polarity corona discharge, so the polarity of the black toner does not change and the photoconductor 1 and the adsorption power of black toner does not decrease.

Subsequently, the image exposure section 10 exposes the light image of the document again through the cyan filter 5. In this case, light passes through only the white portion W, and the surface potential almost disappears due to light attenuation (FIG. 8E). Next, red developer 1
2, lower development bias +200V than when developing black
is applied to perform red development with negative polarity red toner. At this time, there is almost no change in the photoreceptor surface potential of the red area R and the black area BK (Fig. 8F).
However, in the black area BK corresponding area, the black toner adheres to the surface of the photoreceptor 1 while maintaining its negative charge, so there is almost no adhesion of red toner to the black toner image area, and the red toner is attached to the red area. It adheres only to R compatible parts. Furthermore, the black toner is not scraped off by the magnetic brush of the red developing device 12.

Incidentally, at this time, the black developing device 11 is kept in a non-operating state.

The polarities of the black toner and red toner thus formed on the surface of the photoreceptor 1 are both negative, and there is no need to perform polarity matching before transfer. Therefore, after the red color development is completed, the black toner image and the red toner image on the photoreceptor 1 are transferred by the transfer device 13 onto the transfer paper 7 fed by a paper feeding means (not shown), and then transferred by the separator 14. The paper is separated, fixed, and then ejected. After the transfer is completed, the surface of the photoreceptor 1 is subjected to charge removal and cleaning by a charge remover 15, a cleaning section 16, and an eraser section 17, as necessary.

FIG. 9 is a curve diagram of the change in photoreceptor surface potential shown in FIG. 8. Curve 9-1 shows the photoreceptor surface potential of the black part corresponding to BK, curve 9-2 shows the photoreceptor surface potential of the red part corresponding to R, and (A) to (F) show the process steps (A) to (F) shown in FIG. ) is supported.

〔Effect of the invention〕

As described in detail above, according to the two-color image forming apparatus of the present invention, scraping of toner can be prevented and clear two-color toner images can be obtained for a long period of time. Furthermore, since there is no need for polarity adjustment before transfer, there is an effect that image disturbance does not occur.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the process of copying in a conventional two-color copying device, Figure 2 is a diagram showing changes in the surface potential of the photoreceptor in the process shown in Figure 1, and Figure 3 is a diagram showing the changes in the surface potential of the photoreceptor in the process shown in Figure 1. Curve diagram of photoconductor surface potential change,
FIG. 4 is a block diagram of a two-color image forming apparatus showing an embodiment of the present invention, FIGS. 5 and 6 are block diagrams of an AC charger, and FIG. 7 is a diagram showing AC charging characteristics and DC charging characteristics. , FIG. 8 is a diagram showing changes in the surface potential of the photoreceptor in the two-color image forming process, and FIG. 9 is a curve diagram of changes in the surface potential of the photoreceptor in the two-color image forming process. 1... Photoreceptor, 3... Red filter, 5... Cyan filter, 7... Transfer paper, 8... DC charger,
9... AC charger, 11... Black developer, 12...
...Red developer, 18...Shield bias, 19
... Capacitor, 20 ... AC power supply.

Claims (1)

[Scope of Claims] 1. After performing the first charging, exposure, and development steps on the photoreceptor, a second method that does not change the polarity of the charge applied to the photoreceptor by the charging. performing a second charging, exposure, and development process to sequentially form two-color images of different colors on the photoreceptor;
A two-color image forming apparatus that subsequently transfers the two-color image onto a transfer medium at once, wherein the second charging is performed by alternating current corona discharge. 2. The absolute value of the charging potential due to the first charging is greater than the absolute value of the charging potential due to the second charging using AC corona discharge, and the developing bias potential during the first development is is larger than the development bias potential in the second development.
2. Two-color image forming apparatus as described in 2.