JPS6259305B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrophotographic method, and in particular,
The present invention relates to an electrophotographic method in which a photoreceptor having a basic structure of a conductive layer, a photoconductive layer, and an insulating layer is repeatedly used for image formation.

Conventionally, a photoreceptor having a basic structure of a conductive layer, a photoconductive layer, and an insulating layer is used, and for example, a primary charging process in which the surface of the photoreceptor is uniformly charged with a predetermined polarity, and a photoimage in which the surface of the photoreceptor is exposed to a light image. exposure process, a secondary charging process in which a corona having a polarity component opposite to that of the primary charging, i.e., a reverse polarity DC corona, an asymmetrical AC corona, or an AC charge removal corona is applied almost simultaneously with the light image exposure, and then a whole surface exposure process in which the entire surface of the photoreceptor is exposed. Electrophotography, in which an electrostatic latent image is formed on a photoreceptor through an exposure process, and this electrostatic latent image is developed and transferred to obtain a reproduction of the above-mentioned optical image, is known for its ability to produce high-contrast images. It is widely used in practical equipment.

Even in the case of electrophotographic methods that cannot obtain high-contrast images such as those mentioned above, in those that use a photoreceptor whose basic structure is a conductive layer, a photoconductive layer, and an insulating layer, the surface of the photoreceptor is physically and chemically damaged. It is known to be strong and extremely effective for repeated use.
However, in an electrophotographic method using such a photoreceptor, it has been discovered that when the following process of negative/positive reversal development is repeated, the contrast of the reproduced image is significantly reduced.

That is, as shown in FIG. 1a, a conductive layer a ₁ , a photoconductive layer
For example, if the photoconductive layer a ₂ is of N type such as a CdS-binder system, the surface of the photoreceptor A having the basic composition of a ₂ and an insulating layer a ₃ is primarily charged with a positive (+) polar corona,
Next, at the same time as light image exposure (L in bright places, D in dark places), direct current or AC having a polarity component opposite to that of the primary charge is applied.
It is secondarily charged with corona (FIG. 1b), and then exposed to light over the entire surface to obtain an electrostatic latent image on the surface of the photoreceptor in which the bright areas L are at a negative potential and the dark areas D are at a positive potential. (Figure 1c) After that,
In order to perform negative/positive reversal development, development is performed using a dry or wet developer T having developer toner particles charged with positive (+) polarity. (FIG. 1d) Then, a transfer material P is placed on the surface of the photoreceptor A on which the developed images have been formed, and a negative (-) polar corona is applied from the back side to effect transfer.
(Fig. 1e) After that, cleaning is performed. In this way, the photoreceptor used for image formation is prepared for reuse by having the developer remaining on its surface removed.

The contrast of the electrostatic latent image formed on the photoreceptor in this way decreases each time the photoreceptor is used repeatedly, and according to the inventors' experimental study, this phenomenon is as follows. It can be considered.

In the transfer step of the above image forming process, the surface of the photoreceptor having the N-type photoconductive layer is negatively charged (-) through the transfer material, so that near the interface between the conductive layer _a1 and the photoconductive layer _a2 is positively (+) charged. ) An electric field is formed which induces a charge to sandwich the photoconductive layer.

Normally, by positively charging the surface of the photoreceptor, electrons that are injected from the conductive layer side into the vicinity of the photoconductive layer and the insulating layer and are drawn together are ejected from the photoconductive layer to the conductive layer by this electric field, causing photoconductivity. Positive space charges (ie, holes) remain within the layer.

These space charges (holes) enter the trap and gradually increase in number as time passes.

In particular, in the case of binder-based photoconductive layers such as ZnO or CdS, where there are many barriers within the layer, the positive space charges (holes) enter deep traps and are difficult to recombine. It becomes.

Such a trapped positive space charge (hole)
In the vicinity of , the injected electrons are trapped when the subsequent primary charging is applied, and the movement of the injected electrons toward the vicinity of the interface between the photoconductive layer and the insulating layer is also inhibited.

For this reason, electron injection into the vicinity of the interface between the photoconductive layer and the insulating layer is insufficient, resulting in a decrease in contrast in the subsequent image forming process.

Moreover, since the number of such trapped space charges differs depending on the exposure position in the previous image forming step, they also appear as memory in subsequent images.

The present invention has been made in view of the above points, and provides an electrophotographic method that makes it possible to repeatedly form images with good contrast using a photoreceptor whose basic composition is a conductive layer, a photoconductive layer, and an insulating layer. It is related to.

That is, an object of the present invention is to prevent the influence of previous images from adversely affecting subsequent image formation in repeated image formation on a photoreceptor whose basic structure is a conductive layer, a photoconductive layer, and an insulating layer. An object of the present invention is to provide an electrophotographic method and apparatus that enable formation of images with good contrast.

The method of the present invention is a process in which a photoreceptor having a basic structure of a conductive layer, a photoconductive layer, and an insulating layer is repeatedly used. A process of forming an electrostatic latent image according to optical information having a density, a process of applying a developer to the bright part of the electrostatic latent image formed on the photoreceptor to develop it, and a surface of the photoreceptor carrying the developer. and a step of applying a discharge having a component of opposite polarity to the primary charging to the back surface of the transfer material while the light history of the exposure step exists. be.

Hereinafter, details of the present invention will be explained using specific examples with reference to the drawings.

FIGS. 2a to 2f are schematic diagrams illustrating a specific electrophotographic method according to the present invention.

Portions common to those in FIG. 1 described above are indicated by the same symbols.

The process shown in FIG. 2a includes a conductive layer a ₁ , a photoconductive layer a ₂ ,
This is a primary corona discharge step in which the surface of the photoreceptor A, which has an insulating layer _A3 as its basic structure, is uniformly charged by corona discharge of a predetermined polarity. The corona discharge polarity is selected depending on the polarity of the photoconductive layer, for example, positive (+) polarity in the case of N type such as CdS, and negative (-) polarity in the case of P type such as Se. The figure shows the case where the photoconductive layer is of N type.

The process shown in FIG. 2b involves exposing the uniformly charged photoreceptor surface to light information having a bright and dark density (for example, a scanned light image of an original document), and at the same time applying a voltage having a polarity component opposite to that of the primary corona discharge. For example, applying
This is a process in which corona discharge is performed by applying a DC voltage, an AC voltage, or a combination of both of the polarity opposite to the primary charging.

The figure shows a case where a negative (-) polarity voltage with the opposite polarity to the primary voltage is applied.

The step shown in FIG. 2c is a full-surface exposure step in which a change in surface potential in bright and dark areas is quickly formed in accordance with the charge distribution formed in response to the above-mentioned photoimage exposure.

Through each of the above steps, a high contrast electrostatic latent image is formed on the surface of the photoreceptor.

Next, in FIG. 2d, the dark area D is developed so as to obtain a so-called negative/positive reversal image in which the bright area L of the optical image is developed.
This is a step of developing with a dry or wet developer T having toner particles of the same polarity as . The above process is
This is the same as in FIGS. 1a to d above.

FIG. 2e shows an exposure step for imparting a light history to the photoconductive layer of the photoreceptor. Due to this exposure, the photoconductive layer becomes low in resistance, and the low resistance state is maintained for a certain period of time as a photohistory of the photoconductive layer.

Fig. 2 f shows that during the transfer process, a polarity opposite to that of the toner particles (that is, in the illustrated example, as shown in Fig. 2, a negative (-) corona discharge with a polarity opposite to that of the primary charge) is generated from the back side of the transfer material. The developed images are transferred to the transfer material. At this time, the negative corona applied to the back surface of the transfer material induces positive charges in the photoreceptor conductive layer. This induced positive charge cannot originally be injected into the N-type photoconductive layer if the N-type photoconductive layer is in a dark state, and it forms an electric field between the corona and the photoconductive layer. However, as mentioned above, since the photoconductive layer is given a photohistory and is in a low resistance state, there are many negative photocarriers, and the negative photocarriers attracted by the positive charges pass through the photoconductive layer and are transferred to the conductive layer. Since the positive charges that have been induced are removed by moving to the interface of the photoconductive layer, there is no possibility of an electric field sandwiching the photoconductive layer. That is, negative (-) due to transfer corona discharge.
When a charge is applied to the surface of the photoreceptor, a positive (+) charge of opposite polarity is induced on the conductive layer side.

However, since the photoconductive layer of the photoreceptor is given a photohistory by exposure, the space charge within the photoconductive layer that tends to fall into a deep trap is released, and the resistance of the photoconductive layer decreases. That is, since electron-hole pairs are generated, the opposite polarity charges induced near the interface between the conductive layer and the photoconductive layer are immediately removed. The risk of the generation of electric fields across the photoconductive layer is therefore eliminated.

Since the generation of such an electric field is prevented, there is no possibility that it will adversely affect the subsequent image formation. The surface of the photoreceptor after the transfer is completed in this state is cleaned if necessary to prepare for subsequent image formation.

FIG. 3 is a side view of a specific example electrophotographic apparatus to which the electrophotographic method of the embodiment of the present invention is applied.

The photosensitive drum 1 uses aluminum foil as a conductive layer, on which a 50 μm photoconductive layer made of CdS powder bound with a binder is coated as a photoconductive layer, and 35 μm Mylar is coated as an insulating layer. be.

The photosensitive drum 1 then rotates clockwise in the figure.

An elastic cleaning blade 2 made of rubber, synthetic resin, etc. and a pre-exposure lamp 3 are arranged near the top of the photoreceptor drum 1 to prepare the surface of the photoreceptor for image formation. A latent image forming means is disposed on the rotating downward side surface of the photosensitive drum 1 for image formation. Specifically, there is a primary corona discharger 4 that applies positive (+) charge to the surface of the photoreceptor, a negative (-) charge that has the opposite polarity to the primary charge, or
They are a secondary corona discharger 5 that performs AC static elimination, and a full-surface exposure lamp 6.

In order to provide a light image on the photoreceptor, an original document table 7, a document table illumination light source 8, a reflection mirror 9,
An in-mirror lens 10 is provided adjacently to provide optical image exposure from the optically open back surface of the secondary corona discharger. It goes without saying that the optical image exposure is not limited to the above-mentioned means, but also CRT, laser beam, etc. can be used.

Next, a developing device 12 for developing the electrostatic latent image formed on the photoreceptor is arranged below the photoreceptor drum 1 . The illustrated developing device is of a liquid developing type. In this embodiment, in order to achieve negative/positive reversal development,
A positively (+) charged developer toner is dispersed in a carrier liquid. Reference numeral 13 denotes a developing tray that brings the developer into contact with the surface of the photoreceptor, and 14 denotes a feed pump that sends the developer to the developing tray. Following the developing device 12, a post-corona discharger 15 for squeezing excess liquid is arranged. The aperture effect may be applied to any charge polarity of the developer toner, but it is particularly preferable to apply positive (+) charging with the same polarity as the developer toner. In this case, not only the aperture effect but also the flow of the developer particles due to the movement of the developer toner can be prevented.

In order to feed the transfer material P onto which the developed images are to be transferred to the transfer position, a paper feed roller 16, a paper feed guide 17, a timing roller 18, etc. are arranged.

A transfer corona discharger 19 is arranged above the aperture corona discharger for applying a negative (-) polar corona discharge to the back side of the transfer material sent to the transfer position so as to attract the developed images.

A post exposure source 21 using a tungsten lamp or the like is arranged between the transfer corona discharger 19 and the post corona discharger 15.

Due to this exposure from the post-exposure source 21, the photoconductor carrying the developed images has a low resistance, and due to the light history that maintains this state, even if the photoconductor receives a continuous transfer corona discharge from the back side of the transfer material, the photoconductive layer No electric field is generated between the two.

Further, a separation roller 20 or the like shown in the drawing is arranged as separation means at the upper end along the photosensitive drum 1 to separate the transfer material.

On the other hand, a hot plate fixing device 22 for fixing the transferred image on the separated transfer material, a delivery roller 23 for ejecting the fixed transfer material to the outside of the machine, and the like are arranged adjacent to each other.

As mentioned above, in an image forming apparatus that performs negative/positive reversal development, negative (-) charge may be generated on the photoreceptor due to transfer corona discharge, which may generate an electric field across the photoconductive layer. However, by arranging the post-exposure source 21 as described above, the generation of such an electric field is eliminated and the cause of fatigue of the photoreceptor is eliminated.

Therefore, even when images are formed continuously, there is no fear that the contrast will deteriorate and good image reproduction can be achieved.

In addition, when a reverse electric field sandwiching the photoconductive layer is generated on the surface of the photoreceptor after development, other than the transfer corona, it is effective to carry out the post-exposure as described above before the step in which the electric field may be generated. Of course.

Examples will be shown below to further facilitate understanding of the present invention.

〔Example〕

A photosensitive material obtained by mixing 10 parts by weight of vinyl chloride with 100 parts by weight of cadmium sulfide activated with a small amount of copper (Cu) and indium (In) is applied to a thickness of approximately 50μ on the surface of the polished aluminum drum. A photoconductive layer was formed. The surface thereof was coated with Mylar (trade name) having a thickness of about 35 μm as a transparent insulating layer to obtain a photoreceptor.

Image formation was performed using the photoreceptor in the apparatus shown in the third figure.

At this time, +6.5KV DC voltage is applied to the primary corona discharger, and ±
An alternating current voltage of 7.2KV was applied. Further, a DC voltage of +7.5 KV was applied to the aperture post-corona discharger, and a DC voltage of -5.0 KV was applied to the transfer corona discharger. Then, the photoreceptor drum was rotated at a circumferential speed of 59 mm/secv to repeatedly form images on the photoreceptor. Post-exposure was performed at a light intensity of 66 μW/cm ² on the surface of the photoreceptor for an irradiation time of 0.2 seconds about 0.3 seconds before corona transfer. That is, the photoreceptor exposed to the transfer corona has a light history of 0.3 seconds. At this time, due to repeated image formation, the change in dark area potential was 0.2 or less even at the 500th time.
Further, no reverse electric field memory was generated due to corona transfer.

In the above configuration device, the post-exposure light intensity was set to 66μ.
W/cm ² , 132μW/cm ² , 395μW/cm ² , 1320μW/
Figure 4 shows the change in dark potential when the light history condition is changed for each case of cm ² (irradiation time 0.2 seconds in both cases).

The horizontal axis is the light history time (unit: sec), and the vertical axis is
The rate of change of the dark potential at the 500th time, that is, the difference between the dark potential Vd ¹ at the 1st time and the dark potential Vd ⁵⁰⁰ at the 500th time.
It is the value of (Vd ¹ − Vd ⁵⁰⁰ )/Vd ¹ divided by Vd ¹ .

Figure 5 shows that the light history time is 0.2sec,
This is the rate of change in post-exposure light intensity and dark area potential in each case of 0.4 sec, 0.6 sec, 1.0 sec, and 2.0 sec.
In both cases, the irradiation time was 0.2 seconds.

In the figure, the horizontal axis is post-exposure intensity (μW/cm ² )
As in Figure 4, the vertical axis is 500% from the initial dark potential.
Rate of change in dark potential up to the first cycle (Vd ¹ − Vd ⁵⁰⁰ )/Vd ¹
is the value of As is clear from the figure, sufficient contrast can be obtained in repeated image formation by providing a predetermined light history with sufficient light intensity.

As detailed in the specific examples above, the present invention is an excellent method that prevents the generation of an electric field (reverse electric field) that sandwiches the photoconductive layer, thereby enabling consistently good image formation even when the photoreceptor is repeatedly used. It is something.

[Brief explanation of the drawing]

1A to 1E are schematic diagrams showing an example of the electrophotographic method according to the present invention, FIGS. FIG. 3 is a side view of an embodiment device based on the present invention. FIG. 4 is a correlation diagram between light history time and dark-place potential change rate when images are repeatedly formed in the apparatus according to the embodiment of the invention, and FIG. 5 is a correlation diagram between post-exposure light intensity and dark-place potential change rate. In the figure, 1: photoreceptor drum, 2: elastic cleaning blade, 3: pre-exposure lamp, 4: primary corona discharger, 5: secondary corona discharger, 6: full-surface exposure lamp, 21: post-exposure source.

Claims (1)

[Claims] 1. In an electrophotographic method that repeatedly uses a photoreceptor whose basic composition is a conductive layer, a photoconductive layer, and an insulating layer,
A process of primarily charging the photoconductor, a process of forming an electrostatic latent image on the photoconductor after the primary charging according to optical information having a brightness and darkness density, and a process of optically brightening the electrostatic latent image formed on the photoconductor. a step of applying a developer to the surface of the photoreceptor for development, an exposure step of imparting a light history for a predetermined time to the surface of the photoreceptor carrying the developed image, and a component having a polarity opposite to that of the primary charge during the light history of the exposure step. An electrophotographic method comprising the step of applying electric discharge to the back surface of a transfer material.