JPH0882971A - Multicolor image forming device - Google Patents

Abstract

PURPOSE: To form 1st and 2nd toner images with a desired density and to obtain a multicolor image having a specified density and color tone at a color mixture part by correcting 1st or 2nd exposing condition or image data according to image information. CONSTITUTION: A back exposure device 17 is provided on the back surface side of a photoreceptive belt (belt photoreceptor) 1 at a position on the downstream side from a 1st developing device 4 and the upstream side from a 2nd electrifier 5 with respect to the turning direction of the belt 1. Furthermore, a 2nd image exposure means 6 between the 2nd electrifier (re-electrifier) 5 and a 2nd developing device 7 is provided on the back surface side of the belt 1. The back exposure device 17 is provided with an E.L. or an LED element and the belt 1 transmits light from the back surface. In the case where 1st exposure 3a and 2nd exposure 6a are performed to the same picture element, at least either of the 1st and the 2nd image signals is corrected in accordance with the image signal.

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 such as an electrophotographic apparatus, and more particularly to an image forming apparatus for multicolor printing.

[0002]

2. Description of the Related Art Conventionally, a two-color electrophotographic apparatus is known in which two image forming means including a charging device, an exposing means and a developing means are arranged around an image carrier. An example of a conventional two-color electrophotographic apparatus is shown in FIG.

In this two-color electrophotographic apparatus, as shown in FIG. 12, a first charging device is provided around the photosensitive belt 1 as an image carrier which rotates in the direction of arrow a from the upstream side in the rotational direction. 2, a first exposing unit 3, a first developing unit 4, a second charging unit (recharging unit) 5, a second exposing unit 6, and a second developing unit 7. Further, a toner charge amount adjusting charger 12, a transfer charger 8 and a separation charger 30 are provided on the downstream side of the second developing device 7.
A belt cleaner 32 is arranged on the upstream side of the first charger 2. In addition, a transfer material 9 is provided near the transfer charger 8.
The fixing device 10 is installed at a position where the registration roller 34 for supplying the toner is supplied to the separation charging device 30 via the conveying means 36.

First, the photosensitive belt 1 is charged by the first charger 2.
Surface is uniformly charged to, for example, −600 V, and the first image light 3a is applied to the photosensitive drum 1 by the first exposure unit 3 to set the surface potential of the exposed portion on the photosensitive belt 1 to, for example, −10.
A first latent image with 0 V is formed. This first latent image is converted into an alternating current of, for example, 2000 Hz and 1500 Vpp by -500 V by the first developing device 4 having black toner.
Reversal development under the application of a development bias that superimposes the direct current of
The first latent image is visualized as a black toner image.

Next, the photosensitive belt 1 is recharged by the second charger 5 to raise the potential of the dark portion to -700V and the potential of the first toner image to, for example, -520V. Then, the second exposure means 6 irradiates the second exposure 6a, and the photosensitive belt 1
A second latent image having an exposed portion potential of, for example, -80 V is formed on the surface of the. The second latent image is thus formed, and a developing bias in which a DC of −420V is superimposed on an AC of 2000Hz, 2500Vpp is applied by the second developing device 7 having a color toner such as magenta toner. When reversal development is performed, only the second latent image portion can be developed as a color toner image without developing the first toner image portion.

After the two-color image of the black toner image and the color toner image is formed on the photosensitive belt 1 as described above, the charger 1
After being charged by 2 to optimize the charge amount of those toners, they are collectively transferred by the action of the transfer charger 8 onto the transfer material 9 supplied in synchronization with the image formation by the registration roller 34. Then, it is conveyed to the fixing device 10 by the conveyance belt 36, and the two-color toner images are fixed on the transfer material 9, and then 2
The color print is discharged outside the image forming apparatus. On the other hand, the photosensitive belt 1 is subjected to the next image forming process after the residual toner is removed by the cleaning device 32.

However, in the conventional two-color image forming apparatus, it has been difficult to place different color toners such as black and magenta on the same pixel, and thus on the same position on the photosensitive belt. The reason is that the second latent image is not sufficiently formed because it is difficult for the second exposure to pass through the first toner image.

[0008]

[Patent Document 1] Japanese Patent Laid-Open No. 2-2
As disclosed in 19074, a method has been considered in which an image bearing member having a photosensitive member formed on a transparent substrate is used, and the photosensitive member is subjected to second exposure from the back surface. This makes it possible to place toners of different colors on the same pixel (herein referred to as dot mixing). Therefore, a full-color original can be reproduced in a pseudo-color space of two colors by using a copying machine having gradation and capable of color-separating the original. This method of dot color mixing has a higher copying speed and a lower cost than a full-color copying machine, and the image expressing power of the two-color copying machine is richer than that of conventional two-color copying machines.

However, the potential of the second image after exposure differs depending on the density of the first toner image, which makes it difficult to form the second toner image as intended.

An object of the present invention is to provide a multicolor image forming apparatus capable of forming the first and second toner images in desired densities and obtaining a predetermined tint in the color mixing portion. .

[0011]

The above object can be achieved by a multicolor image forming apparatus according to the present invention. In summary, the present invention provides a first charging unit that uniformly charges the image carrier from the upstream side in the rotation direction around the image carrier and a first charging unit on the image carrier.
First exposure means for forming a first latent image by first exposure based on the image signal of No. 1, first developing means for developing the first latent image, and uniformly charging the image carrier. Second charging means, second exposing means for forming a second latent image on the image carrier by second exposure based on the second image signal, and second developing for developing the second latent image. A multicolor image forming apparatus comprising: a transfer unit configured to transfer a multicolor image of at least two colors formed on the image carrier onto a transfer material at a time. And the second exposure is performed on the same pixel, the multicolor image forming apparatus is characterized in that at least one of the first and second image signals is corrected in accordance with the image signal.

According to the present invention, the second exposing means is provided on the back surface of the image carrier. In one aspect of the present invention, at least as the second image density increases or the first image density decreases, at least the second image density increases.
The contrast of the latent image is smaller than that of the monochrome region, and in another aspect, conversely, at least as the first image density increases or the second image density decreases, at least the first image density increases. The contrast of the latent image becomes smaller than that of the monochrome region.

[0013]

【Example】

Embodiment 1 FIG. 1 is a schematic sectional view showing an image forming apparatus according to an embodiment of the present invention. In the two-color image forming apparatus according to the present exemplary embodiment, the first direction with respect to the rotation direction of the photosensitive belt (belt photosensitive member) 1.
An exposure device 17 is provided on the back side of the photosensitive belt 1 at a position downstream of the developing device 4 and upstream of the second charger 5, and the second charger (recharger) 5 and the second charger 5 are provided. Developing device 7
The second image exposing unit 6 between the two is provided on the back side of the photosensitive belt 1, which is different from the conventional image forming apparatus shown in FIG. The back exposure device 17 is equipped with an EL or LED element. The photosensitive belt 1 can transmit light from the back surface. The other structure of this embodiment is basically the same as that of the conventional image forming apparatus shown in FIG.
The same reference numerals as those given to the same denote the same members.

The image forming process in this embodiment will be described with reference to the potential diagram shown in FIG. First, the photosensitive belt 1 is, for example, -600 V (dark portion potential) by the first charger 2.
Is uniformly charged (FIG. 2 (a)), and the image exposure 3a is performed by the first image exposure means 3.

The first image exposure 3a is a laser beam which is modulated by the first image signal using the semiconductor laser 15 of the image exposure means 3 as a light source. The laser beam is polarized by a multifaceted surface 13 rotating at a constant rotation speed by a motor 14, reflected by a folding mirror 16 via an imaging lens, and then raster-scanned on the photosensitive belt 1 to image the surface potential of the exposed portion as an image. Attenuating according to the signal level, a first latent image is formed. This first latent image is, for example, − in the maximum density portion.
It is attenuated to a potential of 100 V (FIG. 2 (b)).

The first latent image is developed by the first developing device 4 using a two-component developer consisting of negatively charged black toner and magnetic particles such as ferrite. For example, a developing bias in which a DC voltage (Vdc1) of −500 V is superimposed on an AC voltage of 2000 Hz and 1500 Vpp is applied to the developing device 4, and the first latent image is reversely developed to obtain a first toner image ( FIG. 2C). At this time, the electric potential of the first toner image is lowered by about −50 V to about −150 V at the maximum density portion due to the toner charge.

After the first latent image is developed by the first developing device 4, the photosensitive belt 1 is entirely exposed from the back surface by the exposure device 17 to remove the charge, and the potential of the maximum density portion of the first toner image ( The potential difference between the toner layer potential of the maximum density portion) and the dark portion potential of the photosensitive belt 1 is set to be small, for example, −100 V and −200 V (FIG. 2D).

Next, the photosensitive belt 1 is charged by the second charger 5.
And re-charge the potential of the maximum density portion of the first toner image to -67.
0V and the dark part potential are set to -700V (FIG.2 (e)). That is, the dark part potential is set to be slightly lower than the toner layer potential of the maximum density part, and the toner layer potential of the maximum density part is set to such an extent that the contrast of the second latent image can be obtained.

Next, image exposure 6a is performed by the second image exposure means 6 to form a second electrostatic latent image on the photosensitive belt 1. The second image exposure 6a is the first image exposure 3a.
Similarly to the above, the semiconductor laser 25 of the image exposure means 6 is a laser beam modulated by a second image signal using the semiconductor laser 25 as a light source, and the second laser beam is rotated by a motor 24.
And the folding mirror 16 is polarized through the imaging lens.
After the reflection, the photoconductor is raster-scanned from the back surface of the photosensitive belt 1 to form a second latent image on the photosensitive belt 1.

FIG. 2 (f) shows an example of the latent image potential when the second image signal has an intermediate density and is not corrected.
Area A is an area in which only the second image exposure is performed, area B
Is a region where the first and second image exposures are performed, and region C is a region where the second image exposure is performed. From this, it can be seen that the latent image potentials in the areas B and C are different for the same second image signal. An LED element or the like can be used for the first and second exposure devices 3 and 6.

The second latent image thus formed is not in contact with the second developing device 7 by using, for example, a two-component developer having magenta toner and a carrier so that the carrier does not come into contact with the photosensitive belt 1 during the developing operation. Depending on the developing method, for example, 200
When reversal development is performed under application of a developing bias in which a direct current voltage (Vdc2) of −570 V is superimposed on an alternating current voltage of 0 Hz and 2500 Vpp, only the second latent image portion is developed without developing the first toner image portion. Then, a magenta toner image is formed (FIG. 2F).

After that, the charger 1 for adjusting the toner charge amount
By applying a bias in which an AC voltage is superimposed on a DC voltage to 2 to optimize the toner charge amounts of the black toner image and the magenta toner image of the two-color image on the photosensitive belt 1 by the charger 12, Transfer is performed by the transfer charger 8 on the supplied transfer material 9. Next, the transfer material 9 is separated from the charger 30.
Then, it is peeled from the photosensitive belt 1 due to the curvature of the photosensitive belt 1, sent to the fixing device 10 to fix the two-color image on the transfer material 9, and then discharged as a two-color color print out of the image forming apparatus. On the other hand, the photosensitive belt 1 has a cleaning device 11
After the residual toner is removed by the above method, it is subjected to the next image forming process.

In the above, before the recharging of the photosensitive belt 1 by the second charger 5, it is not always necessary to remove the charge of the photosensitive belt 1 by the whole surface exposure by the exposure device 17. The second
The developing device 7 may use a non-contact developing method using a non-magnetic one-component developer.

The present invention will be described in more detail. FIG. 3 is an explanatory diagram showing the characteristics of the photosensitive member of the photosensitive belt used in this example. In this embodiment, the photosensitive belt 1 uses an organic photoconductor. FIG. 3 shows the relationship between the potential of the dark portion and the potential of the exposed portion of the photoconductor and the potential of the primary charger when the light is irradiated with a constant amount of light after the primary charging of the photosensitive belt. The solid line in FIG. 3 represents the dark part potential, and the dotted line and the one-dot chain line represent the exposed part potential. Two light amounts of the light irradiation are tried, and the dotted line indicates the case where the light amount is small and the dashed line indicates the large amount. From FIG.
It can be seen that the dark part potential of the photoconductor changes according to the primary charger grid potential, and the higher the dark part potential of the photoconductor, the larger the potential difference from the exposed part.

FIG. 4 is an explanatory diagram showing the latent image potential after recharging, that is, the surface potential of the photoconductor and the toner layer potential of the first toner image. Image signal of first toner image (first image signal)
Has density data in 8-bit hexadecimal system. The shaded portion in the figure shows the voltage of the toner layer, and the upper and lower ends in the height direction of the shaded portion correspond to the surface potential of the photoconductor and the toner layer potential of the first toner image, respectively. . It can be seen from FIG. 4 that the larger the density data of the first image, that is, the higher the density of the first toner image, the smaller the voltage held by the photoconductor (according to FIG. 2E), the first image signal “ In the case of FF ″, the toner layer potential is −670V and the photoconductor potential is −520V). This is because the formed first toner image becomes thicker as the density becomes higher, and the ratio of the potential of the toner layer to the capacity division increases.

As described with reference to FIG. 3, when the voltage of the photosensitive member decreases, the amount of voltage drop due to light irradiation (the potential difference between the dark portion and the exposed portion) decreases. Therefore, the toner image potential after the second exposure becomes high in the portion where the potential of the photoconductor becomes small and the density of the first toner image is high.

An example of the photoconductor potential and the first toner image potential with respect to the first image signal when exposed with a constant second image signal or a constant amount of light after recharging is shown in FIG. From this, it is found that the second toner image potential differs depending on the first image signal, and therefore the density of the second image also differs. In order to obtain the second image having a constant density, the first toner image potential after the second exposure should be constant. That is, it is understood that the halftone dot portion shown in FIG. 5 may be corrected.

When the second image signal is "0", the correction voltage for the light amount after recharging (that is, the second image input signal) when the first image signal is "80" is shown in FIG. 2 (e). Since the condition of is not changed, it is 0V and the second image signal is "
It becomes the maximum when FF ″. The situation is shown in FIG. 6. From this, it is understood that the correction voltage is different when the second image input signal is different.

As described above, in FIG. 3, the potentials of the exposed portion due to different light amounts when the potential of the primary charger grid is changed are shown by the dotted lines and the alternate long and short dash lines, and the potential of the dark portion is shown by the solid line. As can be seen from FIG. 3, changes in the potential difference between the dark part and the exposed part due to changes in the photoconductor dark part potential are different under different light amounts. This is the reason why the correction voltage is different when the second image signal is different.

In order to correct the second image signal, the correction voltage is converted into an image signal, and as shown in FIG. 7, the second image signal A before correction is converted into the second image signal according to the first image signal. Signal B
Instead of. In FIG. 7, the solid line represents the conversion when the first image signal is “FF”, and the broken line represents the conversion when the first image signal is “FF”.
The conversion in the case of 0 "is shown. Ideally, it is better to change the correction value for all the first image signals, but it becomes complicated, so for example, from" 80 "to" 8F "of the first image signal Even if the same correction as "88" of the first image signal is carried out, there is no problem in practical use, and therefore the first image signal may be divided into a plurality of regions, and in the present embodiment, the first image signal is "F".
Although the correction is performed with reference to the case of F ″, any signal value of the first image signal may be used as the reference. Further, the correction may be performed only when the colors are mixed.

Although the case where the photosensitive member is the photosensitive belt has been described above, the same applies to the photosensitive drum. Further, the color order of the first development and the second development and the development method are not limited.
Further, in the present embodiment, the case of two colors has been described, but the same applies to the case of three or more colors.

Embodiment 2 Another embodiment of the present invention will be described. In the present embodiment, since the relative dielectric constant is larger than that of the organic photoconductor such as a photoconductor using amorphous silicon, when the ratio of the voltage division with the toner image is small, or the charging potential is low. The case where the latent image potential of 2 cannot be sufficiently obtained will be described. The image forming process of this embodiment is the same as that of the first embodiment, and therefore its explanation is omitted.

In FIG. 8, the solid line represents the potential of the photosensitive member for the first image signal after recharging, and the broken line represents the potential of the toner image. Figure 9
Regarding the potential after the second image exposure with respect to the first image signal, the potential of the first toner image when the second image signal is “40” is shaded, and the potential of the first toner image when the potential is “C0” The potential is indicated by a halftone dot. From FIG. 9, the second image signal is "C
When it becomes a large value such as "0", the first image signal becomes "B".
It can be seen that the latent image contrast becomes smaller in the area larger than 0 ″. That is, the tint of the first color becomes stronger in the mixed color area.

FIG. 10 shows the first toner image voltage with respect to the first image signal before recharging. The second image signal is "C0"
In the case of, the area to be corrected is the shaded area. By correcting the first image signal so that the voltage to be corrected becomes 1/2, the decrease in density of the first toner image and the lack of density with respect to the intended density of the second toner image become substantially the same, and the intention of the mixed color portion is increased. The desired color can be obtained.

FIG. 11 shows the first image signal A before correction as the first image signal A.
An example of correcting the image signal B has been shown. The solid line 1 is the case where the second image signal is "0" to "60", and the broken line 2 is "8".
0 ", the broken line 3 is" C0 ", and the broken line 4 is" FF ". Since this correction is performed at a high density of the color mixture portion, there is no practical problem in terms of density.

[0036]

As described above, according to the present invention, the first or second exposure condition or the image data is corrected according to the image information, so that the first and second toner images are desired. It was possible to obtain a multicolor image having a predetermined density and tint in the mixed color portion by forming the image with a high density.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a potential diagram illustrating an image forming process in the image forming apparatus of FIG.

FIG. 3 is an explanatory diagram showing characteristics of a photosensitive member of a photosensitive belt in the image forming apparatus of FIG.

FIG. 4 is an explanatory diagram showing a surface potential of a photoconductor after recharging.

FIG. 5 is an explanatory diagram showing an example of a surface potential and a correction voltage of the photoconductor after the second exposure.

FIG. 6 is an explanatory diagram showing an example of a correction voltage for a second image signal.

FIG. 7 is an explanatory diagram showing a correction example of a second image signal.

FIG. 8 is an explanatory diagram showing an example of surface potential after recharging of a photoconductor according to another embodiment of the present invention.

FIG. 9 is an explanatory diagram showing an example of the surface potential of the photoconductor after the second exposure.

FIG. 10 is an explanatory diagram showing an example of toner layer voltage before recharging.

FIG. 11 is an explanatory diagram showing a correction example of a first image signal.

FIG. 12 is a schematic sectional view showing a conventional image forming apparatus.

[Explanation of symbols]

 1 Photosensitive Belt 2 First Charger 3a First Image Exposure 4 First Developer 5 Second Charger (Recharger) 6a Second Image Exposure 7 Second Developer 17 Back Exposure Device

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location G03G 15/04 H04N 1/29 G

Claims (4)

[Claims] 1. A first charging device for uniformly charging the image carrier from the upstream side in the rotation direction around the image carrier, and a first exposure based on a first image signal on the image carrier. A first exposure unit that forms a first latent image, a first developing unit that develops the first latent image, and a second unit that uniformly charges the image carrier.
Charging means, second exposing means for forming a second latent image on the image carrier by the second exposure based on the second image signal, and second developing means for developing the second latent image. And at least 2 formed on the image carrier.
A multicolor image forming apparatus comprising: a transfer unit that collectively transfers multicolor images of different colors onto a transfer material.
The first and second exposures are performed on the same pixel.
A multicolor image forming apparatus, wherein correction is applied to at least one of the image signals according to the image signal. 2. The image forming apparatus according to claim 1, wherein the second exposing unit is provided on the back surface of the image carrier. 3. The method according to claim 2, wherein as at least the second image density increases or the first image density decreases, at least the contrast of the second latent image decreases as compared with a monochrome region. Color image forming apparatus. 4. The method according to claim 2, wherein at least the contrast of the first latent image becomes smaller than that of the single color region as the first image density increases or the second image density decreases. Color image forming apparatus.