JPH08272214A - Image forming device - Google Patents

Abstract

PURPOSE: To obtain high quality printing without disturbing a toner image, even if a photoreceptor having a high dielectric constant and low volume resistance and a printing process in which the surface potential decay of the photoreceptor becomes great are used by providing a reelectrifying mechanism part between a developing part and a transfer part. CONSTITUTION: A reelectrifying unit 13 is installed before a transfer, to reelectrify the toner image on a photoreceptor drum 1. In other words, the photoreceptor developed between a developing mechanism part 2 and a transfer mechanism part 4 is reelectrified to the same polarity as that of primary electrification, to adjust a contrast potential before a transfer unit (the photoreceptor surface potential difference between toner sticking and unsticking parts). When the contrast potential before the transfer unit is smaller than 300V, the shutting in of the toner image by electrostatic force becomes weak and the toner image becomes irregular due to the contact with a paper sheet at the time of transferring, etc., to reduce resolution. For reelectrifying, not normal DC corona electrification but corona electrification by the superimposing of an AC on a DC is used to prevent the overelectrification of toner.

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 a copying machine or a printer using an electrostatic application method.

[0002]

2. Description of the Related Art The reversal developing method is one of the most well known developing methods used in printers and the like. The photoconductors used in this electrostatic application method are pure Se-based photoconductors, SeTe
System photoconductors, As ₂ Se ₃ system photoconductors, OPCs, amorphous silicon photoconductors, and the like are known. In recent years, printers (especially line printers) have been required to have higher-speed printing capability and higher image quality and higher definition as the amount of information to be processed has increased.

[0003]

Here, in high-speed printing, friction with a paper or a developer causes a great wear of the photoconductor, and the OPC (Vickers hardness;
Hv = 10-20) or pure Se photoconductor (Hv = 30), S
For eTe photoreceptor (Hv = 30 to 40) in the wear life is small, the surface hardness of the multi-heard As ₂ Se ₃ photosensitive member (Hv = 1
50) and an amorphous silicon photoconductor (Hv = 120
0) is frequently used. However, the dielectric constant of the film is about 10 for As ₂ Se ₃ photoconductors and amorphous silicon photoconductors.
Since it is large as described above, the ability to retain surface charges is inferior to other pure Se-based photoreceptors, SeTe-based photoreceptors and OPCs.
As a result, there is a problem in that the latent image pattern is disturbed at the transfer portion (a sufficient contrast cannot be maintained), and image quality deterioration such as resolution deterioration easily occurs.

Particularly, in recent years, it has been strongly desired to employ a semiconductor laser or an LED as an exposure light source in order to reduce the size and cost of the exposure light source unit. Semiconductor laser and LE
When D is used as an exposure light source, red light having a light wavelength of about 630 nm or more is usually used because of its light output.
Here, since the long-wavelength light (red light) has a long penetration distance into the photoconductor, a memory is likely to occur, and in order to prevent the memory, light having a similar wavelength is used as the neutralization light. As a result, the light fatigue of the photoconductor is increased, and the charge retention of the photoconductor is further reduced. Further, when a low-resistance developer is used as the developer, the charges on the surface of the photoconductor leak to the developer, and the latent image is disturbed.

Therefore, an object of the present invention is to use a photoconductor having a low charge holding ability because of its large dielectric constant, and to use long-wavelength light as an exposure light source which causes a large damage to the photoconductor, and a low resistance developer. An object of the present invention is to provide an image forming apparatus capable of realizing stable print quality without causing deterioration in image quality such as deterioration in resolution even when the above is used.

[0006]

SUMMARY OF THE INVENTION The above object is to provide an image forming method in which a photoconductor is exposed to light based on image information to form a latent image on the photoconductor surface, and then development is performed by a reversal development method. This is achieved by having a recharging mechanism section between the transfer section and the transfer section.

[0007]

According to the present invention, the photosensitive member developed between the developing mechanism section and the transfer mechanism section is recharged with the same polarity as the primary charge, and the contrast potential (toner-attached portion and non-attached portion) before the transfer device is recharged. It is solved by adjusting the photosensitive surface potential difference of the part. Here, when the contrast potential before the transfer device is less than 300 V, the confinement of the toner image due to the electrostatic force becomes weak, and the toner image is disturbed due to contact with the paper at the time of transfer and the resolution is lowered. When a high resistance developer is used, the toner may be overcharged even if recharged, and a sufficient pre-transfer contrast potential may not be obtained.
In such a case, overcharging of the toner can be prevented by using corona charging in which AC is superimposed on DC instead of normal DC corona charging for recharging. In this case, the same effect can be obtained by providing the AC corona charger separately from the DC corona charger.

Further, according to the present invention, the image is formed on the photosensitive drum by the electrostatic application method in the image forming section. As a specific electrostatic application method, a relatively uniform charge is retained on the surface of the photoconductor by a charging method using corona discharge. Next, the image to be formed is drawn on the surface of the photoconductor by the exposure light source. At that time, the surface charge of the photoconductor in the portion irradiated with light is canceled by the electrons (or holes) generated by the photoelectric effect in the photosensitive layer, and an electrostatic latent image is formed on the photoconductor surface after exposure. It is formed. After that, the electrostatic latent image is made into a visible image by electrostatic toner adhesion in the developing section. This visible image is transferred to the paper at the transfer section thereafter. The toner and the electrostatic latent image left on the surface of the photoconductor are removed by a subsequent charge removal and cleaning process, and the photoconductor is prepared for charging for the next printing.

In recent years, OPC, which is excellent in manufacturing cost, is becoming the mainstream of the photosensitive material used in the electrostatic application system. However, in some high-speed printers such as line printers and high-speed copying machines, the As ₂ Se ₃ photoconductor or the amorphous silicon photoconductor is mainly used. This is because in the high-speed printing process of about 100 ppm or more, the friction of the paper and the developer on the photoconductor is larger than that in the low-speed process, and therefore the photoconductor is required to have excellent abrasion resistance. However, the wear resistance of As ₂ Se ₃ photoconductors and amorphous silicon photoconductors is higher than that of OPC and SeTe type photoconductors (As ₂ Se ₃ photoconductor ≈10, amorphous silicon photoconductors). ≒ 12), and its volume resistance is OPC
1 to 2 orders of magnitude smaller than those of SeTe photoconductors (As ₂ S
e ₃ photoconductor ≈1.4 × 10 ¹¹ and amorphous silicon photoconductor ≈1.0 × 10 ¹¹ ), so the charge retention capacity of the photoconductor is low (the surface charge of the photoconductor easily leaks), and the dark decay of the surface potential. Is a big problem. In other words, the contrast potential of the electrostatic latent image formed in the exposure section is reduced by the time the development process reaches the transfer section until the electrostatic latent image is formed. As a result, the electric field that traps the toner electrostatically adhered to the low-potential portion of the electrostatic latent image becomes small, and the toner latent image is disturbed.
In other words, the resolution of the image on the transfer portion is likely to decrease.

This phenomenon also occurs when a low resistance developer is used as the developer. It is considered that the reason is that the surface resistance of the photoconductor leaks during the developing process because the resistance value of the developer is small. Furthermore, a red light such as a semiconductor laser or LED is used as a latent image writing light source or a static elimination light source.
When (long-wavelength light) is used, the charge holding power of the photoconductor is further reduced. This is because long-wavelength light has a long penetration distance into the photoconductor and the position where photocarriers generated in the photosensitive layer are also deep, so the photocarriers generated remain in the photoconductor during a short process time such as high-speed printing. This is because it is easy to do.

As one of the measures for preventing the deterioration of resolution, it is conceivable to make the initial contrast potential at the time of exposure sufficiently large, but As has a small volume resistance of the photosensitive layer and a small charging ability.
_{Using a 2} Se ₃ photoconductor or an amorphous silicon photoconductor in a high-speed process and providing a large contrast potential increases the load on the charging process. In addition, problems such as pressure resistance of the photoconductor itself occur.

Therefore, in the present invention, the charging mechanism is provided between the developing mechanism section and the transfer mechanism section for the purpose of adjusting the latent image contrast in front of the transfer mechanism section. By recharging the surface of the photoconductor, a sufficient contrast potential is maintained in the transfer mechanism section, thereby preventing the disturbance of the toner image and the deterioration of the image resolution. Here, by recharging after the developing mechanism section, the charging of the surface of the photoconductor in the portion where the toner adheres becomes smaller than that in the portion where the toner does not adhere. This is because the electric charge supplied to the toner surface easily leaks to the photoconductor from which the surface charge has been removed by the exposure, and as a result of the load, the contrast potential becomes large and the disturbance of the toner image is prevented. However, when a high resistance material is used for the toner, the toner may be overcharged due to recharging, and the contrast potential before transfer may not be increased. In that case, it is possible to prevent the toner from being overcharged by using corona charging in which AC is superimposed on DC in the recharging device. The AC frequency at this time is 500-500
0 Hz is desirable.

[0013]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is an example of a schematic view of an image forming apparatus according to the present invention. FIG. 2 shows the surface potential of the photosensitive drum in each printing process. In the figure, reference numeral 1 is a photosensitive drum which is an image carrier, and around the photosensitive drum 1, a charging device 2, a developing device 3, a transfer device 4, an AC static eliminator 5,
Erase lamp 6, cleaning brush, blade,
Process devices for image formation such as a cleaning device 7 including a blower are provided. Further, a paper feed retractor 8 is provided below the transfer device 4, and a paper discharge retractor 9 is provided above the transfer device 4. In addition, the photosensitive drum 1
A scanner unit 10 including an exposure light source of any one of a semiconductor laser, an LED, and a gas laser, a polygon mirror, a lens, and the like is disposed on the upper right side of FIG.

When the scanner unit 10 exposes the photosensitive drum 1 uniformly charged by the charger 2 with the image light A, an electrostatic latent image is formed on the photosensitive drum 1.
The electrostatic latent image is developed by the developing device 3 as the photosensitive drum 1 rotates.
The toner is supplied by the developing device 3 and is visualized as a toner image. This toner image on the photosensitive drum 1 is transferred onto the paper 11 by the transfer device 4. Here, the paper 11 is conveyed toward the transfer unit 4 and the photoconductor drum 1 by the paper feed retractor 8. The sheet 11 after transfer is sent to a fixing device (not shown) by the paper ejecting retractor 9, and the toner image is fixed as a permanent image by the fixing device.

Further, the photosensitive drum 1 after the transfer is
After the surface of the erase lamp 6 is neutralized, the residual toner is cleaned by the cleaning device 7 to prepare for the next image formation. Reference numerals 14, 15 and 16 are surface potential sensors. 14 is provided immediately after the exposure, 15 is provided immediately after the developing device, and 16 is provided immediately before the transfer to detect the surface potential value of the photosensitive drum.

In this embodiment, in the apparatus shown in FIG. 1, the writing exposure light source in the scanner unit 10 is In
GaAlP / GaAs semiconductor laser (wavelength 680n
m) is used, and the exposure light amount is about 6 mW on the surface of the photoconductor drum.
Was set. The photoconductor drum 1 is an As ₂ Se ₃ photoconductor (shape: outer diameter φ2
62 mm × length L430 mm, film thickness: 60 μm, relative dielectric constant: 10) was used. The rotation speed of the photosensitive drum 1 is 60 r
pm.

Image formation according to the present invention was performed as follows. First, by applying a voltage of about +7.5 kV to the charger 2, the surface potential of about +800 V is charged on the photosensitive drum 1. After that, image exposure is performed by the scanner unit 10 to form a latent image on the surface of the photosensitive drum 1. In this embodiment, the laser exposure spot diameter is about 80 μm and the resolution is 400 dpi. The developing conditions were such that a two-component developer was used as the developer, the toner 12 was a styrene acrylic toner having a particle diameter of φ11 μm, and the bias voltage was set to about 400V. The toner image visualized by the developing device 3 is transferred to the sheet 11 by the transfer device 4. Transfer voltage is about -6.0
It was set to kV. The untransferred residual toner is discharged by the AC static eliminator 5 (AC frequency: 1 kHz, applied voltage: 5 kV), and the electrostatic latent image on the photoconductor is erased by the erase lamp 6 (1
Approximately 600n wavelength through a red filter on a 5W white fluorescent lamp
m, light quantity: 250 μW / cm ² of red light).
After that, the surface of the photoconductor drum 1 is cleaned by the cleaning device 7 to prepare for the next image formation.

FIG. 2 shows As ₂ S in the above printing process.
e ₃ shows the change in surface potential of the photosensitive drum. As can be seen from the change in potential indicated by the solid line in the figure, the surface potential of the photoreceptor after charging decreases exponentially. In particular, since red light is used for the writing exposure light source and the erase lamp 6 in this embodiment,
The attenuation width is large, and it is about 0.5 between the charger 2 and the transfer device 4.
A potential drop of 400 V or more occurs in seconds. The contrast potential of 750 V or more immediately after the exposure is about 300 V immediately before the transfer device. That is, as shown in FIG. 3, the height of the electrostatic wall that prevents the toner image from collapsing is drastically lowered. Therefore, when the toner image is transferred to the paper 11, the toner 12 is likely to scatter due to friction with the paper or the like, resulting in deterioration of image quality such as deterioration of resolution.

Therefore, the recharging device 1 according to the present invention before transfer is used.
3, the toner image on the photosensitive drum 1 is recharged by the recharging device. Here, the charging method is a specific contact charging method. In this example, the scorotron charging method was used. The charging conditions were a corona wire applied voltage: +3.5 kV and a grid mesh applied voltage: +750 V. As a result, a toner image on the drum before the transfer device and a contrast potential in a non-region were obtained at about 700 V, and the line width of one dot line of the printed sample was 120 μm or less, and high definition image quality was obtained.

(Embodiment 2) The printing process conditions are set as in Embodiment 1.
And the grid mesh application voltage of the recharging device 13 was set to + 400V. At that time, there is a toner image in front of the transfer device,
The contrast potential of the non-region was 300 V, and the print sample had a line width of 1 dot line of 140 μm or less, and high definition image quality was obtained.

(Embodiment 3) A red LED having a wavelength of 720 nm is used as a writing light source used in the scanner unit 10, and an amorphous silicon photosensitive member (shape: outer diameter φ262 mm × length L430 mm, film thickness :) is used for the photosensitive drum 1.
A similar printing experiment was conducted using 75 μm and a relative dielectric constant of 12). The voltage applied to the grid mesh of the recharger 13 was + 700V. At that time, the contrast potential of the toner image before the transfer device and the non-region were 650 V, and the line width of one dot line of the printed sample was 140 μm or less, and high definition image quality was obtained.

(Embodiment 4) The printing process conditions are set as in Embodiment 1.
And the recharger 13 has AC: 3 kV (frequency:
DC: +2 kV was superimposed on 1 kHz) to perform corona charging. At that time, the contrast potential of the toner image before the transfer device and the non-region was 700V, and the line width of one dot line of the printed sample was 140 μm or less, and high-definition image quality was obtained.

[0023]

As described above, in the image forming apparatus of the present invention, a toner image is used even when a photoconductor having a large dielectric constant and a small volume resistance or a printing process in which the surface potential attenuation of the photoconductor becomes large. It is possible to realize high-quality printing without disturbing.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram showing changes in the surface potential of the photosensitive drum in each printing process.

FIG. 3 is an explanatory diagram showing a relationship between a contrast potential and the presence / absence of a toner image.

[Explanation of symbols]

1 is a photoconductor drum, 2 is a charger, 3 is a developing device, 4 is a transfer device, 5 is an AC neutralizer, 6 is an erase lamp, 7 is a cleaning device, 11 is paper, 12 is toner, and 13 is a recharger. Is.

Claims (5)

[Claims] 1. An image forming method in which a photosensitive member is exposed to light based on image information to form a latent image on the surface of the photosensitive member, and then development is carried out by a reversal developing method. An image forming apparatus having a mechanism section. 2. The image forming apparatus according to claim 1, wherein the base material of the photoconductor is arsenic triselenide or amorphous silicon. 3. The image forming apparatus according to claim 1, wherein a difference in surface potential between the toner adhering portion and the non-adhering portion immediately before the transfer portion is 300 V or more. 4. The image forming apparatus according to claim 1, wherein a red light source having a light wavelength of 600 nm or more is used as a light source for writing the latent image or a charge eliminating light source. 5. The image forming apparatus according to claim 1, wherein AC corona charging DC corona charging is used for the recharging mechanism section.