JPH07281509A - Electrophotographic method - Google Patents

Abstract

PURPOSE:To provide an electrophotographic method whereby a change in the residual potential of a photoreceptor is supressed, the contrast potential of the photoreceptor is maintained and stable, satisfactory image quality can be obtained for a long time. CONSTITUTION:In the electrophotographic method for repeating image formation by an arrangement of an electrifier 3, image exposure device 4, developing device 5, transfer device 6, separation device 7, destaticization device 8 and a cleaner 9 in that order around the organic photoreceptor 1, electrifying units A2 and B3 are provided as the electrifier for the photoreceptor and arranged away from each other, one of them for use in negative electrification and the other for use in positive electrification, and image formation is carried out by causing electrification so that the polarity of the electrifying unit near the destaticization 8 is the same as the photoreceptor polarlity which is used in the operation at the time of latent-image formation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic method for effectively reducing the residual potential of a photoconductor and its level in an image forming apparatus, stabilizing the repeating characteristics, and maintaining good image quality.

[0002]

2. Description of the Related Art In recent image forming apparatuses such as electrophotographic copying machines and laser beam printers, most of them are organic photoconductors. When used in the image forming apparatus not only for the inorganic type but also for the organic type photoconductor, the non-image part is attenuated by the light when the image is exposed, but it does not completely drop to 0 volt during one copying cycle and is several tens of volts. A potential of several hundreds of volts remains (this is generally called residual potential), and a kind of fatigue phenomenon is observed which is further increased when this copying method is repeated. This residual potential occurs because holes and electrons generated by light irradiation during image exposure remain inside the photosensitive layer without being sufficiently erased during the copying cycle, and holes and electrons move slowly. The higher the potential, the greater the accumulation.

If the photosensitive layer is thickened to shorten the life due to abrasion, the residual potential is likely to increase further. Factors that reduce the movement of holes and electrons are due to impurity elements in the photosensitive layer, residual solvent, structural defects, etc., and holes or electrons are trapped in these, and are generated over time from the initial stage or external factors. Like As the film thickness increases, the electric field per unit film thickness decreases and the number of structural defects also increases, so the residual potential is considered to increase. When such a photoconductor is used, in extreme cases, the image density may decrease due to the increase of residual potential, the low density part may disappear, and background stain may occur, and if the accumulation is low but the residual potential is high, There is a possibility that a phenomenon such as an image loss in the low image density portion of the document and a decrease in image density may occur.

Several examples of methods for improving the fatigue phenomenon caused by the photoreceptor itself or external factors have been proposed. The following example uses an a-Si photosensitive member having no protective layer. In JP-A-60-142355, photo-erasing is performed before cleaning, and AC charging is performed before main charging.
The trapped photocarriers are released to improve the fatigue phenomenon. Japanese Unexamined Patent Publication No. 60-156069 is to improve the fatigue phenomenon by performing AC charging (roller charging) at the same time as heating before main charging, or static elimination while applying + DC voltage. JP-A-60-203964 describes the above 15
The roller of 6069 is replaced with a corona discharger. Japanese Unexamined Patent Publication No. 61-165764 improves the fatigue phenomenon by performing pre-charging having the same polarity as that of charging before charging or discharging. All of these cases are effective methods, but when the residual potential is originally high, there is almost no effect of lowering the potential level, and when the photosensitive layer is thick, continuous copying produces residual potential. It is possible that the improvement of accumulation of water is insufficient.

[0005]

The present invention ensures the contrast potential of the photoconductor by further suppressing such fluctuations in the residual potential and lowering the potential level of the photoconductor having a higher residual potential than the initial stage. The object of the present invention is to provide an electrophotographic method capable of obtaining stable and high quality image quality over a long period of time.

[0006]

The constitution of the present invention for solving the above-mentioned problems is an electrophotographic method as described in the scope of claims. That is, in the electrophotographic method in which each device for charging, image exposure, development, transfer, separation, cleaning and charge removal is sequentially arranged around the organic photoconductor as an electrification device for the photoconductor. , Two charging devices are provided, and these charging devices are arranged apart from each other, one of them is for negative charging, the other is for positive charging, and the polarity of the charging device closer to the static eliminator is This is an electrophotographic method in which an image is formed by charging so as to have an operating polarity when forming a latent image. In particular, after charging the surface potential of the photoconductor after operation of the charging device, which is closer to the static eliminator, to an absolute value larger than the surface potential of the photoconductor before latent image formation by 200 to 800 volts, Image formation is preferably performed by setting a predetermined surface potential with a charging device. The present invention will be specifically described below with reference to the drawings.

FIG. 1 is a schematic diagram of the copying method of the present invention, and FIG. 2 is a structural diagram of a photoconductor used. In the following description, the charger on the side of the static elimination lamp 10 in FIG. 1 is the charger A and the charger on the image exposure side is the charger B. The copying method shown in FIG. 1 is a function-separated type photoconductor (hole-moving type one charging system) having the structure shown in FIG.
First, the charger A2 is negatively charged, then the charger B3 is positively charged to set the charging potential required for image formation.
Here, the photosensitive member is a function-separated organic photosensitive member having a four-layer structure including a negatively-charged conductive support (CB), an undercoat layer (UL), a charge generation layer (CGL) and a charge transport layer (CTL). Although the body (OPC photoconductor) is used, there is no particular limitation, and the photoconductor configuration to be set (CGL and CTL are reversed 4
Layer structure, 3 layer structure that has the functions of CGL and CTL)
It can be used if the polarity of charging is changed according to. When the negative charging type photoreceptor shown in FIG. 2 is used, the charging is performed by the charger A2.
To a voltage higher than the charging potential required for image formation by 200 to 800 volts, and then a positive current is applied to the charger B3 to generate a surface potential required for image formation (usually at about -600 to -1200 volts). ) Is set.

The residual potential observed when the photoreceptor having the structure as shown in FIG. 2 is used with a negative charge is the charge transport layer (C
TL), holes trapped in the charge generation layer (CGL), electrons trapped in the charge generation layer (CGL), and holes / electrons that are moving. It is sufficient to apply a negative voltage to, but considering the removal efficiency, a higher voltage is preferable. The voltage that can be applied to the photoconductor is limited by the withstand voltage of the photoconductor, but it is usually adopted.
The allowable charging potential for a photosensitive layer having a thickness of -40 μm is about −
Since it is about 2000 V, it is desirable to set it within the range of 200 to 800 V higher than the potential required for image formation in consideration of image quality. Also, considering the movement time of holes and electrons in the photosensitive layer, it is effective to separate the charger A and the charger B within a possible range, although it is related to the copying speed of the photoreceptor. .

When the photoreceptor using the hole-transporting charge transport material is negatively charged by the charger A2, the higher the surface potential thereof, the more holes that are moving in the photosensitive layer and trapped in the shallow trap level. It is considered that the formation of space charges is reduced because the electrons are easily attracted by the charges of the surface layer or the support and disappear. As a result, the light is smoothly attenuated, the potential level can be reduced, and the rising rate of the residual potential can be suppressed to a low level, so that good image quality can be maintained for a long period of time. In order to reduce the surface potential charged to about -1500 V to about -800 V, a positive voltage is applied to the discharge wire and the grid (control grid) of the charger B3 to adjust the surface potential. Therefore, the charger B3 is preferably a scorotron type charger having a control grid. The charging method shown in FIG. 1 is a corona discharge method. However, when negative discharge is performed at a high pressure, a large amount of harmful substances such as ozone and nitrogen oxides are generated, which adversely affects the photoconductor to a considerable extent. It is also possible to switch to a roller charging system that produces less product. When a positively charged photoreceptor is used, the polarity applied to the charger is the opposite of that described above, and positive charging is followed by negative charging to obtain the positive charging potential necessary for image formation.

After the surface potential required for image formation is set, a latent image is formed on the photoconductor in an analog or digital image exposure process 4, and a developing device 5 visualizes it with toner. Next, after being transferred to the copy paper 13 by the transfer device 6, the copy paper 13 is separated from the photoconductor 1 by the separator 7. Next, the photoconductor 1 on which the toner remains is cleaned by the cleaning device 9,
The entire surface is exposed by the static elimination lamp. On the other hand, copy paper 1
The toner image transferred to No. 3 is fixed by the fixing device 11 to form a hard copy, and a series of copying steps are completed. Figure 2
Shows the function-separated type photoreceptor structure for negative charging.
The carriers are holes. In the case of a photoreceptor that operates by positive charging, a charge generation layer (CGL) and a charge transport layer (CTL) are of opposite type, or a single layer type photosensitive layer configuration in which a charge transport material and a charge generation material are mixed. There is a photoconductor. In the case of a photoconductor for positive charging, the charging process may be positive charging and negative charging in sequence as described above. Of course, the other devices also need to change their polarities accordingly. FIG. 3 is a schematic diagram showing a conventional copying method in which the charging step is composed of one charger 21.

[0011]

EXAMPLES Examples and comparative examples will be described below. Example 1 A resin liquid in which fine particles of TiO ₂ (manufactured by Ishihara Sangyo Co., Ltd.) were dispersed in a polyamide resin was applied onto a cylinder made of aluminum having a diameter of 80 mm, a length of 340 mm and a thickness of 1 mm by a dipping method, followed by heating and drying to obtain a subbing of about 2 μm. The layers (UL) were made. Then, a resin solution in which a trisazo pigment is dispersed in a polyester resin is applied so as to form a charge generation layer (CGL) having a thickness of 0.15 μm, and dried by heating. Further, a stilbene compound is added onto the charge generation layer with a polycarbonate resin (Panlite, C
-1400, manufactured by Teijin Chemicals Co., Ltd.) and then dried by heating to form a charge transport layer (CTL), and a functional separation type organic photoreceptor having a thickness of the photosensitive layer of about 25 μm.
It was used as a sample. The photoconductor thus obtained was set in an experimental apparatus, and the in-machine potential was measured to evaluate the photoconductor.

As a test machine, a digital copying machine with a modified charging system (Ricoh's copying machine Imagio 420) was prepared, a negative charging device was set in the exposure section of the discharge lamp, and a high voltage power supply for the main charging device was connected to the conventional copying machine. A positive external high-voltage power supply was connected to the discharge part and the grid of the main charger. In addition, this time, the interval between the charger A and the charger B was set to 0.36 seconds between the centers of the chargers in terms of time. As a confirmation method, first, -13
After adjusting the charger A so that the charging potential is 00 or -1700 V, +700 V is applied to the grid of the charger B so that the surface potential (charging potential) during image formation is -7.
It was set to 50 to -760 volts. Using a gray scale as a document, the surface potential of 5,000 consecutive sheets was measured, and image evaluation was performed periodically. The result is shown in Figure 4.
Shown in.

Comparative Example 1 Using the same photoconductor as in Example 1, the characteristics were evaluated by an experimental machine (Imagio 420 machine manufactured by Ricoh) using the conventional copying method shown in FIG. The results are shown in Fig. 4. Figure 4 shows the transition of the electric potential
As shown in, the electric potential of the image area of the charging method using one ordinary charger starts at -200 V (normal method
In contrast to the solid line), when negative electrification is given at -1300 V (broken line), starting from -140 V, and at -1700 V (dashed line), starting from -110 V. I was able to lower it. Moreover, the repeat characteristics were also improved. This makes it possible to increase the contrast potential between the image portion and the non-image portion, so that the reproducibility of a document having a lower image density can be improved and the initial image quality can be maintained for a long time. Means there is.

The image quality obtained by the method of the present invention has a resolution of 5 or less.
It was as clear as 4.5 to 5.6 lines / mm between 000 sheets, and the reproducibility and uniformity of halftone images and character images were good, and there was no problem in practical use. On the other hand, in the case of the image by the ordinary method, the reproducibility was slightly reduced in the character image and the halftone image in the low density area after the completion of 5000 sheets, and the effect due to the difference in the method was confirmed. Further, the charging potential by the charger A is changed, and the discharge current of the charger B is adjusted so that the charging potential at the time of image formation becomes −730 to −750 V. 10
The reduction amount of the potential level in the image area after passing 0 sheets was confirmed. Results are shown in FIG. It can be seen that the reduction amount of the image portion potential changes depending on the charged potential amount by the charger A, and the higher the potential, the higher the image portion potential level (the image portion potential level can be lowered).

Example 2 and Comparative Example 2 In order to confirm the effect of CTL on thickening the film, a UL of about 2 μm and then about 0.15 μm were sequentially deposited on an aluminum cylinder according to the manufacturing method shown in Example 1. C
GL and CTL with a film thickness of 35 μm or 42 μm on it
Were laminated to prepare an organic photoconductor. The thus prepared photoconductor was set in the experimental machine (modified Imagio 420 machine) shown in the copying method of FIG. 1 and the normal type experimental machine shown in FIG. As a method of confirming the effect, in the method of the present invention, the charging potential of the charging device A is kept constant at -1400 V, and the current value of the charging device B is set so that the charging potential during latent image formation is about -750 V. Comparative Example Then, the discharge current of the charger shown in FIG. 3 was adjusted so that the charging potential was about −750 V, and continuous copying was performed on 5,000 sheets each, and the transition of the charging potential was confirmed. Results are shown in FIG.

In the method of the present invention in which the charging device A and the charging device B are used to charge the photosensitive member before the latent image is formed, the increase in the image portion potential is suppressed even when the CTL is thickened, and the image portion potential level is also reduced. It was also confirmed that sufficient practical potential transition was achieved even when the thickness of CTL was increased to increase the durability of the photoconductor. In the method using the ordinary charging method, it is observed that the electric potential rise in the image area becomes large due to the thickening of the CTL film thickness, and that tendency is larger at 42 μm than at 35 μm, which may cause a problem in image quality. Is recognized.

Examples 3 to 7 and Comparative Example 3 As the sample, the same product as the CTL 42 μm photosensitive member produced in Example 2 was used. As an effect confirmation method, in the embodiment, the surface potential of the charger A is changed between -920 and -2150 volts, and the current of the charger B is adjusted so that the surface potential before latent image formation becomes -700 volts. After adjustment, the potential level of the image area and its potential increase were continuously copied on 5,000 sheets, and the improvement effect was confirmed. Further, in the comparative example, the conventional method shown in FIG. 3 was used, the charging potential before latent image formation was set to −700 V, and the transition of the potential and the images at the initial stage and after 5,000 sheets were confirmed. The potential conditions are shown in Table 1.

[0018]

[Table 1]

The results are shown in Table 2.

[0020]

[Table 2]

Since the sensitivity is improved by increasing the thickness of the photosensitive layer, the potential level in the image area is lower than that of the sample of Comparative Example 1, but residual potential is likely to be accumulated, so that a character image of a low-density original is particularly likely to be generated. As a result, the density is reduced, and the image quality is degraded, such as broken characters. However, if the charging method according to the present invention is used, the potential level is corrected, and stable image quality can be maintained. However, the charger A is -215
If an excessively high voltage of 0 volt is charged, there will be almost no problem in the initial image, but gradually it will become noticeable in the halftone image, and black and white spots of 0.1 mm or less will gradually become noticeable. Clear image deterioration was confirmed. Therefore, the amount of charge by the charger A needs to be kept within a range that does not cause image deterioration.

[0022]

As described above, according to the present invention,
Since the residual potential can be made lower than only the conventional negative charging step, the contrast potential can be increased,
Further, even if the photosensitive layer thickness is increased in consideration of the decrease in charging property due to abrasion, the potential can be stabilized repeatedly, so that it is possible to maintain the initial image quality stably for a long time. It is possible in comparison.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a copying method of the present invention,

FIG. 2 is a configuration diagram of a photoconductor,

FIG. 3 is a schematic view of a conventional copying method,

FIG. 4 is a graph showing the repeatability of in-machine potential using the copying method of the present invention,

FIG. 5 is a graph showing the relationship between the charging potential of the charger A and the image area level reduction amount;

FIG. 6 is a graph showing the repeatability of the in-machine potential by a photosensitive layer thick film photoreceptor.

[Explanation of symbols]

 1 Photoconductor 2 Charger A 3 Charger B 4 Image exposure 5 Developing device 6 Transfer device 8 Separator 8 Static eliminator 9 Cleaning device 10 Static erasing lamp 11 Fixing device 12 Copy paper tray 13 Copy paper 21 Main charger

Claims (2)

[Claims] 1. An electrophotographic method in which charging, image exposure, development, transfer, separation, cleaning and charge removal devices are sequentially arranged around an organic photoconductor to repeatedly perform image formation. As a charging device, two charging devices are provided, and these charging devices are arranged apart from each other. One of them is for negative charging, the other is for positive charging, and the polarity of the charging device closer to the static eliminator is set. An electrophotographic method characterized in that an image is formed by charging the photosensitive member so as to have an operating polarity when forming a latent image. 2. The surface potential of the photoconductor after the operation of the charging device, which is closer to the static eliminator, is charged so that the absolute value thereof becomes larger than the surface potential of the photoconductor before the latent image is formed by 200 to 800 volts, The electrophotographic method according to claim 1, wherein an image is formed by setting a predetermined surface potential with a second charging device.