JPS6368864A - Image forming device - Google Patents

Abstract

PURPOSE:To preclude the color mixing and fogging of toner by carrying out bias exposure between a developing means and a developing means as the next stage, and setting the quantity of image exposure by an image exposing means larger than the quantity of exposure of a latent image forming process as a front stage. CONSTITUTION:The bias exposure is performed prior to a 2nd development to equalize the potential of a no-image part to the potential of a 1st latent image part. Consequently, there is no potential difference between the surface potential of a photosensitive drum 1 and a 1st latent image part potential, so toner of a 2nd color sticks on the 1st latent image part to cause color mixture and the developing bias voltage of a 2nd developing device 12 may have a normal value, so no reverse fogging is caused. Further, the quantity of light of a 2nd image exposure 11 is set larger than the quantity of light of a 1st image exposure 8, so even when the potential of the no-image part is held almost as high as a 1st visual image potential prior to a 2nd development, development contrast can be equalized between the 1st and a 2nd developing processes.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an electrophotographic image forming apparatus, and particularly to a multicolor image forming apparatus.

(Prior Art) Conventionally, there is a multicolor image forming apparatus of this type as shown in FIG. That is, around the photosensitive drum 20, there are three image forming means PI, P, each forming images of different colors.

P3, a transfer charger 21, and a cleaning means 22 are provided. The first image forming means P1 includes a charging section 23
, an image exposure section 24 , and a development section 25 .
The third image forming means P 2 + P 3 each include an image exposing section and a developing section.

Then, the charging section 23 of the first image forming means P1 charges the surface of the photosensitive drum 1 to a predetermined potential VD as shown in FIG.
The image is exposed to light to form a negative electrostatic latent image as shown in FIG. 5(b). Next, this electrostatic latent image is developed by the first developing section 25 using the first color toner Ta as shown in FIG. 5(C). Thereafter, in the same manner, a second image is exposed by the second image exposure unit in the second image forming means P2 to form a negative electrostatic latent image as shown in FIG. 5(d), This electrostatic latent image is developed by a second developing section with toner Th of the second color, as shown in FIG. 5(e). Furthermore, the second image forming process P3
Image exposure and third development of tJS3 are repeated in the same manner as in the case of image formation. Finally, the three-color toner image developed as described above is transferred onto the transfer paper 26 by the transfer charger 21, and then fixed.

The multicolor image forming apparatus repeats the process of uniformly charging the surface of the photoreceptor drum 20, sequentially forming and developing latent images of the same polarity, and it is possible to use different color toners of the same polarity. It has the advantage of good transferability to the transfer paper 26.

(Problems to be Solved by the Invention) However, in the case of such prior art, the photosensitive drum 20
Since all the latent images formed thereon have the same polarity, the following problems arise. That is, even after the first development is completed, the first latent image has a potential difference ΔVD with respect to the base potential VD, as shown in FIG. 5(d). There is a problem that the latent image is developed. This causes so-called color mixture on the image, in which the first latent image developed with the first color toner Ta is developed with the second color toner Tb.

In order to solve the above problem, it may be possible to set the developing bias to the second developing section to a value that does not develop the first latent image, but in this case, the developing bias value and t ground potential V
A new problem arises in that the potential difference with D becomes too large and a fogging phenomenon called inversion fogging occurs on the underlying layer.

In addition, the development contrast (the difference between the development bias voltage and the exposed part potential V1) differs between the first image forming process and the second image forming process, which causes image density, line width of line images, etc. There was a problem in that a difference occurred between the images and the image quality deteriorated.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and its purpose is to form a plurality of latent images of the same polarity on an image carrier to obtain a multicolor image. An object of the present invention is to provide an image forming apparatus that can prevent toner color mixing and fogging phenomenon even when the toner is mixed, and that can obtain high-quality images without variations in image density, line width, etc. of images of each color. .

(Means for Solving the Problems) In order to achieve the above object, the present invention includes a latent image forming step using an image exposure means that exposes an image area after uniformly charging an image carrier. , in an image forming apparatus that forms a multi-color image by repeating a developing step using a developing means for reversing the latent image a plurality of times, bias exposure is performed between the developing means and the next developing means, First, the image exposure amount by the image exposure means is set to be larger than the exposure amount in the preceding latent image forming step.

(Function) In the present invention, by performing bias exposure between the developing means and the next developing stage F, the potential of the non-image area is lowered in advance, and the already developed image is transferred to the next developing means. In addition, the image exposure section by the image exposure means is set to have a higher exposure amount than the previous latent image forming step, and the potential difference between the development bias voltage applied to the development means and the latent image is be approximately equal in each latent image.

(Example) The present invention will be explained below based on illustrated embodiments.

FIG. 1 shows an embodiment of an image forming apparatus according to the present invention. In the figure, l is a photosensitive drum, 2 is a primary charger that uniformly charges the photosensitive drum 1, 8 is a first image exposure that exposes the first image area with a laser beam, and 9 is a first image exposure device. A first developing device 10 reversely develops the first latent image formed by the image exposure 8 with a colorant of the first color 11, and a developing bias voltage VDCI is applied to the developing roller of the first developing device 9. A power source 11 is a second image exposure device 1 for exposing a second image area by a laser beam.
2 is a second developing device that reversely develops the second latent image formed by the second image exposure 11 with a second color toner; 13 is a developing roller of the second developing device 12 that is supplied with a developing bias voltage VDC2; 6 is a transfer paper, 5 is a transfer charger that transfers the toner image on the photoreceptor drum 1 onto the transfer paper 6, 14 is a separation charger that separates the transfer paper 6 from the photoreceptor drum 1, and 16 is a transfer charger. A fixing device fixes the toner image on the transfer paper 6, a cleaner 7 removes toner remaining on the photoreceptor drum th, and a static elimination lamp 15 removes residual charges on the surface of the photoreceptor drum 1.

Note that 17 indicates a light source for bias exposure disposed immediately after the first developing device 9.

Next, the image forming process will be explained step by step.

First, the photoreceptor drum 1 is uniformly charged by the primary charger 2, and the surface potential of the photoreceptor drum 1 is set as shown in FIG.
) as shown in VDI. For the sake of explanation, here
It is assumed that the photoreceptor is a P-type semiconductor and that v[11 is positively charged. After uniform charging, first image exposure 8 exposes the first image area on the photoreceptor drum 1 with a laser beam (negative exposure)
A first latent image of potential Vll is formed on the photosensitive drum l as shown in FIG. 2(b). Next, this first latent image is transferred to a first color 1-1 having a positive charge by a first developing device 9.
Reverse development is performed using toner Ta. At this time, the developing bias voltage VDCI applied to the developing roller of the developing device 9 is set to a voltage slightly lower than the base potential VDI.

Since the first color toner Ta is deposited on the first latent image VLI, the potential of the photosensitive drum after development rises to VLI' as shown in FIG. 2(C).

Next, the light source 17 for bias exposure exposes the entire surface of the photoreceptor drum l with a weak amount of light. At this time, the potential VDI of the non-image portion is the same as that of the first color toner T after the first development.
Bias exposure is performed so that the potential VLI' of the first latent image portion to which a is attached is approximately equal to the potential VLI' (FIG. 2(d)).

As a result, the potential VDI of the photoreceptor drum 1 is changed to the potential VL
VD2 is approximately equal to I'.

Thereafter, a second image exposure 11 is performed to perform negative exposure to expose an image area on the photoreceptor drum 1. The second image exposure 11 is set to be larger than the amount of light of the first image exposure 8. That is, the second image exposure 11 is performed using a laser beam similarly to the first image exposure 8, but a light source such as a semiconductor laser that emits a laser beam has a light emission amount E as shown in FIG. changes depending on the supply current i. Therefore, by setting the supply current i to the light source to il for the first image exposure 8 and 12 (i2>i+) for the second image exposure 11 as shown in FIG.
As shown in FIG. 2(e), the potential V1.2 of the image exposure section is lower than the potential V L I of the first image exposure section.

Thereafter, the second latent image is subjected to reversal development using the second color toner Tb having a positive charge by the second developing device 12.

At this time, the developing bias voltage VDC2 applied to the developing roller of the second developing device 12 is as follows.

The voltage is set to be slightly lower than the non-image portion potential VD2.

Through the above steps, toner images of the first color and the second color [1] can be formed on the photoreceptor drum l (Fig. 2(f)).
). Thereafter, the toner images of the first and second colors are transferred to a transfer paper 6'' by a transfer device 5, and a separation charger 1
4 separates the transfer paper 6 from the surface of the photosensitive drum 1.

The toner image on the transfer paper 6 is fixed onto the transfer paper 6 by a fixing device 16 . In addition, the surface of the photosensitive drum 1 after transfer is
While being cleaned by the cleaner 7, residual charges are eliminated by the static elimination lamp 15, and the step (2) is completed.

As described above, in this example, bias exposure was performed prior to the second development to make the potential of the non-image area approximately equal to the potential of the first latent image area. Therefore, in the second development, the surface potential VD2 of the photoreceptor drum l and the first latent image portion potential ■[
Since there is no potential difference between the second color toner Tb
will not adhere to the first latent image area and cause color mixture. Further, the developing bias voltage of the second developing device 12 is set to the normal &
Since only i is sufficient, no inversion fog occurs.

In addition, in this embodiment, the light amount of the second image exposure 11 is
Since the light amount of the image exposure 8 is set to be large, even if the potential of the non-image area is made approximately equal to the first developing potential prior to the second development, the development contrast (development bias voltage and exposed area potential 1) can be made equal between the first and second developing steps, and there is no difference in image density, line width of line images, etc. between the two, and the image quality can be improved. Next, this example will be explained using specific numerical values.

P-type organic photoreceptor drum (hereinafter referred to as OPC drum)
Positive charging VDI of approximately 700V is uniformly applied to the top. next,
A laser beam emitted from a semiconductor laser causes
Negative exposure corresponding to the black image signal is performed. At this time, the potential VLI of the exposed portion becomes approximately 150V. Next, reversal development is performed by applying a developing bias voltage of about 550 .mu.m using positively charged black toner. After that, turn the bean lamp 3
Bias exposure is performed using a light source 17 in which io light sources 17 are arranged along the axial direction of the photoreceptor drum at intervals of 0 mm and whose light intensity is adjusted using an ND filter. This bias exposure causes the first
The potential (approximately 700V) of the non-image area that was not exposed during image exposure is changed to approximately 600V. After that, the second
Negative exposure corresponding to the red image signal is performed using a semiconductor laser at a predetermined light amount that is greater than the first negative exposure light amount. At this time, the potential of the exposed area was about 100V. Thereafter, reversal development is carried out using a second developing device using positively charged red toner at a developing bias voltage of approximately 500V. In this way, it was possible to obtain a high-quality image of two colors, black and red, without color mixture, positive fog, reverse fog, and density unevenness.

Note that in the above embodiment, the first. Although the case where the second image exposure is performed by a laser beam has been described, the LE
It goes without saying that exposure using a light emitting diode (D), exposure using a liquid crystal shutter to turn light from a light source into 0N10FF, exposure using light from an original or film, exposure using transmitted light, etc. may also be used.

Further, in the illustrated embodiment, a case has been described in which the light source 17 for bias exposure is arranged between the first developing device 9 and the second image exposure 11; The developing device 12 may be placed anywhere between them.

Furthermore, bias exposure can be done using small lamps, LEDs, fluorescent lights,
Any laser beam can be used as long as it can provide substantially constant exposure in the axial direction of the photoreceptor drum. At this time, in the case of an apparatus that uses a semiconductor laser or the like as an image exposure means, a bias radio wave is added to the input radio wave of the semiconductor laser, and a small amount of laser beam is emitted even when the image signal is OFF, thereby performing bias exposure. may be done. In this case, a light source for bias exposure is not required, making it possible to reduce costs and downsize the apparatus.

Further, in the above embodiment, the case where a two-color image is formed has been described, but it goes without saying that the invention can be similarly applied to a case where three or more colors are formed.

Furthermore, the present invention can also be applied to a device in which the photosensitive drum is charged again after a series of image forming steps are completed to perform the next image forming step. That is, the present invention repeats a charging process in which the photosensitive drum is uniformly charged by a charging device, an image exposure process in which an image area is exposed by an image exposure device, and a development process in which reversal development is performed by a developing device. It includes a device that forms images in multiple colors.

(Effects of the Invention) The present invention has the above-described structure and operation, and can prevent toner color mixing and fogging, and can form high-quality multicolor images with uniform image density, line width, etc. can do.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing a first embodiment of an image forming apparatus according to the present invention, FIGS. 2(a) to (f) are explanatory diagrams showing the image forming process, and FIG. 3 shows image exposure characteristics. Graph shown, 4th
The figure is a schematic configuration diagram showing a conventional image forming apparatus, and FIG.
) to (e) are explanatory views showing image forming steps. Explanation of symbols

Claims (2)

[Claims] (1) After uniformly charging the image carrier, a latent image forming step using an image exposure means to expose the image area and a developing step using a developing means to reversely develop the latent image are repeated multiple times to produce multiple colors. In an image forming apparatus that forms an image, bias exposure is performed between the developing means and the next developing means, and the image exposure amount by the image exposing means is made lower than the exposure amount of the previous latent image forming step. An image forming apparatus characterized by having many settings. (2) The charging process of uniformly charging the image bearing member by the charging means, the image exposure process of exposing the image area by the image exposing means, and the developing process of performing reversal development by the developing means are repeated multiple times to produce multiple colors. In an image forming apparatus that forms an image, bias exposure is performed between the developing means and the next developing means, and the amount of image exposure by the image exposing means is set to be larger than the amount of exposure in the previous image exposure step. An image forming apparatus characterized by: