JPH11352747A - Two-color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an image formed at high-potential and low-potential electro static latent image parts distinctly recordable by irradiating the surface of a photoreceptor with light so that the potential of the photoreceptor may be eliminated, superposing AC voltage on DC voltage and impressing the obtained voltage to a pretransfer electrifying means. SOLUTION: When the area of the photoreceptor where a 1st image and a 2nd image are held reaches a position opposed to a pretransfer discharging device 6, the light radiated from the device 6 acts to attenuate charge to about 0 V. Thus, the current for electrification hardly flows in a background part, and toner at the background part is hardly electrified, so that fogging is hardly increased. Continuously, when the area of the photoreceptor, where the 1st image and the 2nd image are held, reaches the position opposed to a pretransfer electrifier 7, the photoreceptor is electrified to about -400 V by corona generated by the voltage obtained by superposing the AC voltage on the DC voltage impressed to the electrifier 7. Therefore, the surface potential of the photoreceptor is uniformly discharged or electrified to proper potential at a comparatively low level in a short time, and excess current to the toner is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoconductor having a high potential,
The present invention relates to a two-color image forming apparatus that forms an electrostatic latent image formed by combining an intermediate potential and a low potential, and visualizes a high potential electrostatic latent image portion and a low potential electrostatic latent image portion to form an image. is there.

[0002]

2. Description of the Related Art An electrostatic latent image is formed by combining a high potential, a middle potential and a low potential on a photoreceptor, and a high potential electrostatic latent image portion and a low potential electrostatic latent image portion are visualized. The two-color image forming apparatus for forming the image is a known technique disclosed in JP-A-48-37148 or U.S. Pat. No. 4,078,929.

In this type of two-color image forming apparatus, generally, before transferring two types of developed images having different charging polarities formed on a photoreceptor to a transfer target such as paper, each of the developed images is transferred by a pre-transfer charger. The polarity of the developed image is made uniform, and then the image is transferred to a transfer target by a transfer device.

[0004]

In general, a corona discharger is used as a pre-transfer charger of a two-color image forming apparatus. Conventionally, a corona discharger is, for example, -6000 V to -70 V.
A DC voltage of 00 V was applied to charge the developed image formed on the photoreceptor before transfer. Here, when two types of developed images having different charging polarities formed on the photoreceptor are charged by the pre-transfer charger, a toner having a positive (+) polarity (positively charged toner) is charged to a negative (-) polarity. At the same time, the toner having a negative charge polarity (negatively charged toner) receives a further negative charge, so that the toner becomes excessively charged.

For this reason, the two types of developed images after the pre-transfer charging are charged in a state where the charging polarity is temporarily adjusted to the negative polarity, but a large difference occurs in the amount of charge, and the two types of developed images Was not able to be transferred to the transfer-receiving body with the same transfer efficiency. Further, there has been a problem that the developer adhered to a region serving as the background portion of the image is transferred to the transfer target together with the two types of developed images, and an unsightly image is formed.

SUMMARY OF THE INVENTION It is an object of the present invention to clearly record an image formed at a high-potential electrostatic latent image portion and an image formed at a low-potential electrostatic latent image portion, and to have no background fog. Another object of the present invention is to provide a two-color image forming apparatus capable of realizing high quality image formation. Another object of the present invention is to transfer an image formed on a high-potential electrostatic latent image portion and an image formed on a low-potential electrostatic latent image portion with the same transfer efficiency as a transfer target. An object of the present invention is to provide a two-color image forming apparatus capable of transferring.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a latent image forming means for forming an electrostatic latent image on a photosensitive member by combining a high potential, an intermediate potential and a low potential; A first developing unit that develops the image by normal development, a second developing unit that develops the low-potential electrostatic latent image portion by reversal development,
A pre-transfer charging unit for applying a charge of a predetermined polarity to the first and second developed images formed on the photoreceptor by the first and second developing units; and a first charging unit, the polarity of which is adjusted by the pre-transfer charging unit. And a transfer means for transferring the second developed image to a transfer-receiving member.
A pre-transfer discharging unit disposed between the developing unit and the pre-transfer charging unit for irradiating the surface of the photoreceptor with light to eliminate the potential of the photoreceptor; And means for applying a voltage applied thereto.

[0008]

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

First, the overall structure of a two-color image forming apparatus to which the present invention is applied will be described with reference to FIG.

In FIG. 1, reference numeral 1 denotes a photosensitive drum, and the photosensitive drum 1 is rotatably supported at a constant speed in a direction indicated by an arrow a. The photoconductor provided on the surface of the photoconductor drum 1 has a thickness of 20 μm to 40 μm.
μm organic photoconductive photoreceptor (OPC) is used. In the present embodiment, a negative charge of a two-layer structure including a charge generation layer and a charge transport layer provided on the charge generation layer is used. Type organic photoconductive photoreceptor is used.

Around the photosensitive drum 1, a charger 2, an exposure scanning device 3, a first developing device 4, a second developing device 5, a pre-transfer static eliminator 6, a pre-transfer charger 7, a transfer device 8, A pre-cleaning static eliminator 9 and a cleaning device 10 are arranged.

Reference numeral 11 denotes a charger power supply, which is a power supply for applying a voltage to a corona wire 2a provided in the charger 2 to generate corona discharge.

Reference numeral 12 denotes a power supply for the grid electrode, which is a power supply for applying a voltage to the grid electrode 2b provided on the charger 2.

Reference numeral 13 denotes a power supply for developing bias voltage, which is a power supply for applying a developing bias voltage to the developing roller 4a provided in the first developing device 4.

Reference numeral 14 also indicates a developing bias voltage power supply, which is a power supply for applying a developing bias voltage to the developing roller 5a provided in the second developing device 5.

Reference numeral 15 denotes a power supply for the pre-transfer charger, which is a power supply for applying a voltage to the corona wire 7a provided in the pre-transfer charger 7 to generate corona discharge.

Reference numeral 16 denotes a power supply for the transfer device, which is a power supply for applying a voltage to the corona wire 8a provided in the transfer device 8 to generate a corona discharge.

As the developer used in the developing device 4 and the developing device 5, a two-component developer mainly composed of a toner and a magnetic carrier is used. In the developing device 4, a positively charged color toner is used. A charged black toner is used. The particle diameter of the positively charged color toner and the negatively charged black toner is about 5 μm to 12 μm, preferably 7 μm to 10 μm in terms of volume average particle diameter.

Reference numeral 17 denotes an object to be transferred represented by paper. In the present embodiment, an image forming apparatus using so-called continuous paper, which is long and continuous as the transfer object, is exemplified. However, the present invention is not limited to this, and A4, B4, The image forming apparatus may use a sheet cut into A3 size or the like.

Next, an image forming operation in the two-color image forming apparatus having the above configuration will be described with reference to FIG.

When the photosensitive drum 1 starts rotating in the direction of arrow a, the surface of the photosensitive drum 1 facing the charger 2 becomes
As shown in FIG. 2A, the charging device 2 is uniformly charged by the charging device 2 and maintains a high potential (VH) of -800 V, for example.
The photoconductor region charged to a high potential (VH) is exposed by a laser beam emitted from the exposure scanning device 3, and an electrostatic latent image is recorded. The electrostatic latent image in this case is composed of, for example, a combination of a high potential (VH) of -800 V, an intermediate potential (VM) of -400 V, and a low potential (VL) of -50 V as shown in FIG. Of the electrostatic latent image, the high-potential (VH) electrostatic latent image portion is obtained substantially without being irradiated by a laser beam, and has a potential equivalent to the potential at the time of charging. ing. Further, the electrostatic latent image portion having the intermediate potential (VM) is formed by being irradiated with a laser beam having a laser light amount (PM) of about half the exposure amount of the photoconductor. Further, the low-potential (VL) electrostatic latent image portion is formed by irradiation with a laser beam having a laser light amount (PH) of about 3 to 4 times the half-reduction exposure amount of the photoconductor.

When the electrostatic latent image reaches a position facing the first developing device 4, as shown in FIG. 2C, the developing bias voltage (VBH) applied to the developing roller 4a is changed to the static bias voltage. Of the electrostatic latent image, acts on the electrostatic latent image portion including the high potential (VH), by the electric field effect between the high potential (VH) and the developing bias voltage (VBH), causes a so-called well-known regular development phenomenon, High potential
The positively charged toner having the polarity opposite to that of the photoreceptor is attached to the electrostatic latent image portion of (VH), and a first image is formed on the photoreceptor drum 1 by, for example, a red color toner. The developing bias voltage (VBH) is set to about -550V. In addition, the positively charged toner adheres to the high potential (VH) electrostatic latent image portion, so that the potential after development is reduced to the surface potential (V
H ′), it changes to about −600V.

Subsequently, when the photosensitive area holding the electrostatic latent image and the first image reaches a position facing the second developing device 5, as shown in FIG. The applied developing bias voltage (VBL) acts on the electrostatic latent image portion including the low potential (VL) in the electrostatic latent image, and the low potential (V
L) and the electric field effect between the developing bias voltage (VBL)
A so-called reversal development phenomenon occurs, and a negatively charged toner having the same polarity as that of the photoreceptor is attached to the low potential (VL) electrostatic latent image portion.
An image is formed. The developing bias voltage (VBL) is set to about -200V.

Here, a typical value of each potential is | VH |
00V to 1000V, | VM | is about 1/2 of | VH |, | VL
Is 10 V to 100 V, and the difference | VH-VM | and | VM-VL | between the developing bias potential and the intermediate potential is 100 V to 2
50V.

Since the first image and the second image formed on the photosensitive drum 1 are formed by toners having different charging polarities, the first image and the second image are transferred before the transfer to the transfer target 17. It is necessary to match the polarity with the image.

In the present invention, when the photosensitive area holding the first image and the second image reaches the position facing the pre-transfer static eliminator 6, the pre-transfer static eliminator 6 is moved from the pre-transfer static eliminator 6 as shown in FIG. The emitted light acts to attenuate the charge to approximately 0V. In FIG. 2E, the reason that the residual potential exists in the black toner region with respect to the color toner region is that the black toner absorbs light more easily than the color toner, and the light transmittance to the photoconductor is higher than that of the color toner. Is considered to be inferior.

Subsequently, when the photosensitive area holding the first image and the second image reaches a position facing the pre-transfer charger 7, an AC voltage is superimposed on the DC voltage applied to the pre-transfer charger 7. The charged corona is charged to about -400V as shown in FIG. 2 (f). The charging potential after the pre-transfer charging step is not limited to -400 V. When | VH | is 800 V, it is about 600 V or less, preferably 1 V or less.
An effect can be obtained by defining the range of 50V to 400V. In the present invention, regulating the potential of the photoreceptor after the charging step before transfer to the intermediate potential (VM) or less is presumed that the current for charging hardly flows into the background portion, and the background portion toner is charged. This is because it is considered that fogging is unlikely to increase due to the difficulty of wetting.

The reason why the AC voltage is superimposed on the pre-transfer charger 7 is that the surface potential of the photoreceptor is made uniform or uniform in a short time to an appropriate potential of a relatively low level. Overcharging can be prevented. The magnitude of the DC voltage applied to the pre-transfer charger 7 is 500 V to 3000 V, and the AC voltage is 300 V
0V-6000V, frequency is 200Hz-2000Hz
It is.

As described above, in the pre-transfer charging step, fogging increases if the toner is excessively charged, toner particles are scattered around the image area, sharpness is reduced, and transfer efficiency of the image area is reduced. There is. On the other hand, if the charging is too low, the transfer efficiency of the image area may be reduced, or the image may not be transferred.

The reason why the pre-transfer charging step is performed after the pre-transfer charge elimination step is that if the pre-transfer charging step is performed after the latent image charge is completely erased, the holding power for the toner is reduced. This is because if the toner is charged in a small amount and the charge amount of the toner changes, a toner scattering phenomenon occurs to cause image deterioration.

Therefore, the pre-transfer static eliminator 6 is connected to the pre-transfer charger 7.
It is preferable to provide an organic photoconductive photoreceptor (OP) having a photoresponse speed of about 0.1 seconds to about 0.3 seconds.
It is desirable to adopt C).

Further, since the light elimination of the toner image portion is performed through the toner, it is preferable to use a light source having a wavelength component that transmits the toner. However, in a corona in which AC is superimposed, the latent image charge is transferred through the toner. Even for a toner having a neutralizing effect and poor transparency, the charging polarity of the toner can be made uniform without being excessively charged.

When the images having the same polarity arrive at a position facing the transfer unit 8, the transfer unit 8 applies a charge of the opposite polarity to the toner from the back of the transfer body 17 by the action of the transfer unit 8.
The image is transferred from the photosensitive drum 1 to the transfer target 17.

After passing through the transfer unit 8, the photosensitive drum 1 is neutralized by the pre-cleaning neutralizing unit 9. Thereafter, the untransferred toner remaining on the photosensitive drum 1 is removed from the surface of the photosensitive drum 1 by the cleaning device 10. Removed. By repeating the above process, two-color image recording is continuously performed. On the other hand, the transfer member 17 holding the first image and the second image
Is transported by a heat roller fixing device 18 composed of a heating roller and a pressure roller, and the toner forming the first image and the second image is fixed on the transfer receiving body 17 by the heating and pressing action of the fixing device 18. .

Next, a specific example of a two-color image forming apparatus to which the present invention is applied will be described. As an image forming apparatus, a laser beam printer having a printing speed of 180 mm / sec was used. VH = −800 V, VM = −400 V, VL = −50 V using a negatively charged organic photoconductive photoreceptor having a thickness of 25 μm and having a two-layer structure.
Is formed, VBH = −550 V, VBL = −20
At 0 V, a color (red) toner image was formed by the first developing device 4 and a black toner image was formed by the second developing device 5. Next, the pre-transfer static eliminator 6 irradiates red light of about 10 times the half-reduced light amount, and immediately thereafter, the pre-transfer charger 7 uniformly charges the toner to a negative polarity. The pre-transfer static eliminator 6 and the pre-transfer charger 7 are provided close to each other. The time required to pass through the charger is about 1 second, and the photoresponse time (90% potential decay time) of the photoconductor is about 1.2. Seconds. As the voltage applied to the pre-transfer charger 7, a DC voltage of -100
A 500 Hz AC voltage of 4000 V from 0 V to -2000 V
The experiment was performed by changing the voltage between V and 7000V.

As a result, when the background charged potential of the photoreceptor before transfer is -150 V or less, the transfer of a color image is insufficient, and at -600 V or more, the reflectance of fog is 1.5% to 2%.
%, And there was a granular fog, which was not practical. The charging potential of the background portion of the photoreceptor is -600 V to -150.
The fog decreased as the voltage was decreased to V, and decreased to a level that was about 0.5% smaller than the reflectance of the fog when the alternating current was not superimposed in the pre-transfer charging.

The condition under which fogging can be suppressed without lowering the image density of the two-color toner image is as follows.
V, an alternating voltage of 500 Hz, 6000 V, and the charging potential of the photoreceptor in the background before transfer was 200 V. At this time, the fog was about 1% in reflectivity, and there was no granular fog. .

An experiment was conducted by applying only a DC voltage as in the prior art without superimposing an AC. As a result, the background potential of the photosensitive member capable of uniformly transferring a two-color toner image was −650 V to −650 V.
900 V, and the fog reflectance at this time was 2% or more, and the granular fog was remarkably large. In addition, when an AC superimposed voltage is applied to the pre-transfer charger 7 after the latent image charge has been attenuated in the pre-transfer static eliminator 6, although the effect of fog reduction is obtained, the image lacks sharpness. became.

[0039]

As described above, according to the present invention, an image formed on a high-potential electrostatic latent image portion and an image formed on a low-potential electrostatic latent image portion are clearly recorded. With
It is possible to provide a two-color image forming apparatus capable of realizing high quality image formation without fogging in the background portion. An image formed on a high-potential electrostatic latent image portion;
It is possible to provide a two-color image forming apparatus capable of transferring an image formed on a low-potential electrostatic latent image portion to a transfer-receiving body at the same transfer efficiency.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a two-color image forming apparatus of the present invention.

FIG. 2 is an explanatory view showing an image forming step in the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Photoreceptor drum, 2 ... Charger, 3 ... Exposure scanning device, 4
... first developing device, 5 ... second developing device, 6 ... static eliminator before transfer, 7 ... charger before transfer, 8 ... transfer device, 9 ... static eliminator before cleaning, 10 ... cleaning device, 17 ... transfer Body, 18 ... fixing device.

Claims (1)

[Claims] A latent image forming means for forming an electrostatic latent image on the photosensitive member by combining a high potential, an intermediate potential and a low potential; and a developing means for developing the high potential electrostatic latent image portion by regular development. (1) developing means, second developing means for developing the low potential electrostatic latent image portion by reversal development, and first and second developed images formed on a photoreceptor by the first and second developing means An image forming apparatus comprising: a pre-transfer charging unit that imparts a charge of a predetermined polarity to the transfer unit; and a transfer unit that transfers the first and second developed images, the polarities of which are adjusted by the pre-transfer charging unit, to a transfer target. A pre-transfer discharging means disposed between the second developing means and the pre-transfer charging means for irradiating the photoreceptor surface with light to discharge the photoreceptor potential; Means for applying a voltage obtained by superimposing a voltage. Two-color image forming apparatus according to symptoms.