JPH07199575A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent the defect in an image caused by the soiling of an electrifier by making first/second electrifying means operative/inactive respectively, at the time of making first/second developing means inactive/operative respectively. CONSTITUTION:When a monochrome image is formed or the image of only red is first formed, the red image is formed by a first electrifier 3, a first exposure L1 and a first developing unit 4 in the same way as that at the time of forming the image of two colors. At this time, a second electrifier 6 and a developing unit 7 are not required to operate and energized by the electricity, that is, in a normal state. Moreover, at the time of forming the image of only black, the image is formed by the first electrifier 3, a second exposure L2 and the second developing unit 7. In other words, the first developing unit 4 and the second electrifier 6 are in the normal state at this time. Therefore, at the time of forming the monochrome image, the second electrifier 6 easy to cause soiling by the scattering of toner is not energized, that is, the image is formed without the second electrifier 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus having a plurality of charging means and a plurality of developing means around an image carrier.

[0002]

BACKGROUND ART Conventionally, two different kinds of latent images (for example, a format frame and character information, or a combination of black character information and a red mark, a seal imprint, etc.) are formed based on various electrophotographic methods, and these are formed. Various methods for developing an image by developing with developers of the same or different colors have been proposed.

FIG. 2 illustrates an example of an image forming process. (I) to (V) show respective image forming steps. In each step, the charge state of the photoconductor is on the upper side and the charge state is on the lower side. The surface potentials of the photoconductors are shown schematically.

In the step (I), the photoconductor A having the Se photoconductive layer a ₁ provided on the conductive layer a _{2 is used} to obtain about +6 by the corona discharger C _1.
Charge to 00V.

In the step (II), a first exposure L ₁ based on image information based on optical information such as a format is performed, and a _first exposure L ₁ is performed.
To form an electrostatic latent image of. At this time, for example, the surface potential is attenuated to about +50 V in the exposed light portion.

In the step (III), for example, a red non-magnetic one-component insulating toner T ₁ having a positive (+) polarity is coated on the developing sleeve to, for example, the Japanese Patent Application Laid-Open No. 54-54 previously proposed by the applicant. The developing method as described in JP-A-43036 (hereinafter abbreviated as jumping development) is performed to reverse-develop the first scanning exposure portion. In this first development, a bias voltage of DC + 500V is superimposed on the alternating voltage and applied to the developing sleeve for development.

In the step (IV), a corona discharge having the same polarity as the primary charging is performed on the surface of the photoconductor A by the charger C ₂ . A large number of grid wires are stretched around the opening of the shield of the charger C ₂ at intervals of about 1 mm in parallel with the corona discharge wires, and a bias voltage of +600 V is applied to the grid wires. The distance between the grid wire and the surface of the photoconductor is about 1 mm. Due to this secondary charging, the surface potential of the photosensitive member in the portion where the red toner T ₁ is applied and the unexposed portion are both about +600 which is almost the same as the grid bias voltage.
It is charged to V.

Next, in the (V) step, a second scanning exposure L ₂ is performed to form a second latent image, and in the (VI) step, for example, a black magnetic insulating toner T ₂ having a positive polarity is used. The developing sleeve is coated, jumping development is performed, the second latent image is reversely developed, and scanning exposure L ₂
Black toner T ₂ is applied to the area. At the time of the second development, a reversal development is performed by applying a bias voltage of DC + 500V to the developing sleeve by superimposing it on the alternating voltage.

During the second development, the surface potential of the photoreceptor in the red toner developing area and the unexposed area becomes about +580 V due to dark decay, but the potential of the red toner developing area becomes a bias to repel the black toner T ₂ . Prevents black toner from mixing with the red toner portion. Also, a black toner image with high contrast can be obtained.

In this way, an image of two colors, red and black, is formed on the photoconductor A, and the two-color image is formed by transferring the image on the paper at the transfer portion.

[0011]

By the way, as a matter of course, an image for forming an image is not limited to two colors, but there are many cases of a single color. At this time, if the image formation is performed according to the above procedure, the chargers / developers which are not actually used operate for all image formation, which is not preferable in consideration of the life of the developers and the chargers. Therefore,
For example, when forming an image of only black, the operations of the first charger C ₁ , the first exposure L ₁ and the first developing device are stopped, and the second charger C ₂ and the second exposure are stopped. An image is formed by L ₂ and the second developing device. However, since the second charging device C ₂ is located downstream of the first developing device, it is easily affected by toner scattering from the first developing device, and the discharge wire is easily contaminated by repeated discharge. turn into. Therefore, in the above-mentioned apparatus, there is a problem that an image defect is likely to occur due to the stain of the second charger C ₂ in the image formation by only the two-color developing device on the downstream side. Further, when the black developing device, which is frequently used, is arranged on the downstream side as in the example of FIG. 2, the tendency is particularly remarkable.

[0012]

In order to solve the above problems, the present invention is directed to a movable image carrier, a first charging unit for charging the image carrier, and a first charging unit for charging the image carrier. And a first developing unit for developing the latent image formed on the image carrier, and the first developing unit with respect to the moving direction of the image carrier.
In the image forming apparatus having the second charging unit and the second developing unit arranged on the downstream side of the developing unit, the first developing unit is deactivated and the second developing unit is activated. The first charging means is activated and the second charging means is deactivated.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. The principle of image formation is the same as that shown in FIG. 2 and its explanation is omitted here.

FIG. 1 is a sectional view of an image forming apparatus which is an embodiment of the present invention.

In FIG. 1, 1 is a Se photosensitive drum which is a movable image carrier, 3 is a charger which is a first charging means, L ₁ is a first image exposure, and 4 is a first developing means. A developing device, 6 is a charging device which is a _second charging device, L ₂ is a second image exposing device, and 7 is a developing device which is a second developing device.

With respect to the moving direction of the photosensitive drum 1, the first charging device, the first developing device, the second charging device, and the second developing device are arranged around the photosensitive drum 1 in this order.

First, the procedure for forming a two-color image will be described. Figure 1
In, the surface of the photosensitive drum 1 is charged to about + 600V by the first charger 3. Then, the photosensitive drum 1 is subjected to the first scanning exposure L ₁ by the laser beam based on the format information. This scanning exposure L ₁ is performed by a laser light source, a light modulator, a rotating mirror, and an imaging lens (not shown) in response to an electric signal.

The first electrostatic latent image thus obtained is subjected to reversal development by the first developing device 4 having the red nonmagnetic one-component insulating toner T ₁ having a positive polarity. The distance between the developing sleeve 4 _{1 of} the first developing device 4 and the photosensitive drum 1 is about 3
00 μm, and a bias voltage is applied to the sleeve 4 ₁ by a power source. This bias voltage is an alternating voltage, and the AC component is 1,500 Hz · 1,500 V.
pp (530 Vr.ms), +500 as DC component
V is superimposed.

Next, the second charger 6 is attached to the surface of the photosensitive drum 1.
Corona discharge by. A bias voltage of +6.0 KV is applied to the corona discharge wire of the second charger 6, and a bias voltage of +600 V is applied to the grid wire of the shield opening of the second charger 6. By this second charging, both the developing portion and the unexposed portion of the red toner T ₁ are charged to about + 600V.

Then, scanning exposure L ₂ corresponding to an electric signal is performed on the surface of the photosensitive drum 1 by a laser oscillator or the like (not shown) to form a second latent image based on black print information.

Next, this second latent image is subjected to jumping development and reversal development by a second developing device 7 having a one-component magnetic toner T ₂ having a black magnetic positive polarity. Second
The distance between the developing sleeve 7 ₁ of the developing device 7 and the photosensitive drum 1 is about 300 μm, and a bias voltage of the same condition as the first developing is applied to the sleeve 7 ₁ by a power source.

Due to the action of the alternating electric field at the time of the second development, the black toner reciprocates between the sleeve and the drum, and the scanning exposure L
A bright area potential corresponding to ₂ is sufficiently developed. On the other hand, unnecessary black toner does not adhere to the red toner image portion, and the red toner is not scraped off by the black toner. Of course, the black toner does not adhere to the portion that has not received the scanning exposure L ₂ .

Next, the case of forming a monochromatic image will be described. First, in the case of forming an image of only red,
In this case, a red image is formed by the first charging device 3, the first exposure L ₁ and the first developing device 4 as in the case of a red image when forming a two-color image. At this time, the second charger 6
The developing device 7 does not need to operate and is not energized. That is, it is in a non-operating state.

When a black-only image is formed, the image is formed by the first charging device 3, the second exposure L ₂ and the second developing device 7. That is, at this time, the first developing device 4 and the second charging device 6 are inactive. Here, if the setting of the first charger 3 is the same as in the case of forming a two-color image, the potential of the dark portion becomes about 580 V due to the sensitivity characteristic of the drum (so-called dark attenuation), and thus the first
The output setting of the charging device 3 is set to be stronger than in the case of two colors, that is, when the first developing device 4 is operating, and is set to about 620 V immediately after charging. By this correction, it is possible to set the dark portion potential at the time of development to about 600V.

As described above, by using the first charging device 3 when forming a black image which is frequently used, the operations of the first developing device 4 and the second charging device 5 become unnecessary.

Therefore, when a monochromatic image is formed, it is possible to form an image without energizing the second charger 6, which is liable to be contaminated by toner scattering, that is, without using it. Conversely speaking, it becomes possible to minimize the contamination of the second charger 6 which is apt to be stained. In particular, as in this embodiment, when the black developing device necessary for forming the black monochromatic image which is expected to be used most frequently is arranged as the second developing device 7,
The effect becomes remarkable.

Although the developer color combinations in the above embodiments are red and black, other colors can be combined as desired. Further, although the combination of the developers in the above embodiment is two colors, the same is true when the arrangement of three or more colors is performed, and the first charger arranged on the most upstream side at the time of forming a monochromatic image, That is, the purpose is achieved by using a charging device that is not easily affected by toner scattering from the developing device. Further, in the present invention, since it is sufficient for the optical information to remove the potential on the photoconductor, as means for applying the photoinformation to the photoconductor, LED elements are arrayed in addition to the modulated lasers and each LED is responded to a control signal. It is possible to use a flashing light, a combination of a uniform light source and a liquid crystal shutter, or optical information from a document table or microfilm.

[0028]

As described above, according to the present invention, it is possible to prevent image defects due to dirt on the charger and maintain stable operation.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an image forming apparatus that is an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an image forming principle.

[Explanation of symbols]

 1 Photoreceptor Drum 3 First Charger 4 First Developing Device 6 Second Charging Device 7 Second Developing Device

Claims (2)

[Claims] 1. A movable image carrier, a first charging unit for charging the image carrier, and a latent image formed on the image carrier charged by the first charging unit is developed. An image forming apparatus comprising: a first developing unit; and a second charging unit and a second developing unit arranged downstream of the first developing unit with respect to the moving direction of the image carrier. An image forming apparatus, wherein when the first developing means is deactivated and the second developing means is activated, the first charging means is activated and the second charging means is deactivated. 2. The output of the first charging means is the first charging means.
2. The image forming apparatus according to claim 1, wherein the second developing means in which the first developing means is deactivated is larger than the second developing means in operating.