JPH0926681A - Contact electrifying method and image forming method using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent the dielectric breakdown of a photoreceptor, to prevent the inflow of a high current to a defect part even when the defect such as a pin hole, etc., is caused on the surface of the photoreceptor, and to stably uniformly electrify the photoreceptor over a long term by setting voltage impressed on the photoreceptor equal to or under electrification start voltage. SOLUTION: An electrifying roller 1 is provided with a high-electric resistance layer 3 on the surface of a conductive base plate 2. The photoreceptor 5 is provided with a transparent conductive layer 6 on a transparent base plate 6 and a positive electrification type photoreceptive layer 7 formed on the layer 6. By rotating the roller 1 in contact with the layer 7 of the moving photoreceptor 5 and performing photoirradiation from the base plate 6 side of the photoreceptor 5 while impressing the negative voltage on the photoreceptor 5 through the base plate 2 by a DC power source 10, the surface of the photoreceptor 5 is electrified to a specified positive potential. The resistance value of the layer 3 and the DC voltage impressed on the roller 1 are set within the range where a potential difference in the thickness direction of the layer 7 at a dark time is equal to or under the electrification start voltage of the layer 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for charging a photosensitive member in electrophotography and an image forming method using the charging method.

[0002]

2. Description of the Related Art In an electrophotographic image forming method, the surface of a photoconductor is uniformly charged, and image exposure is performed to form an electrostatic latent image corresponding to an original image on the surface of the photoconductor. An image is formed by developing and transferring the latent image. In such an image forming method, charging (main charging) of the surface of the photoconductor is generally performed by corona charging, but this method inevitably suffers from the problem of environmental pollution such as generation of ozone. Recently, in order to avoid generation of ozone, a conductive rubber roller is used as a charging roller, a direct current power source is connected to the rubber roller, the rubber roller is brought into frictional contact with the surface of the photoconductor, and the photosensitive material is exposed through the rubber roller. A method has been proposed in which a main body surface of a photoconductor is charged by applying a bias voltage to the body (JP-A-63-149669 and JP-A-1-267667).

[0003]

However, the above-mentioned charging method using the conductive rubber roller has various problems that must be solved. For example, in this charging, the bias voltage applied to the photoconductor via the rubber roller has a certain threshold value. This threshold value is called a charging start voltage, and the surface of the photoconductor cannot be charged to a target potential unless a voltage larger than this value is applied. By the way, this threshold value is about 600 V for a normal organic photoconductor and about this level for other photoconductors.
Therefore, in order to form an electrostatic image necessary for image formation, it is necessary to apply a considerably high voltage to the rubber roller, and as a result, dielectric breakdown of the photosensitive layer is caused or the surface of the photosensitive member is damaged. If there is a defect such as a pinhole, the conductive roller is in direct contact with the pinhole, so a large current flows into that part, which causes the output of the power supply and defective charging, resulting in a defective portion of the photoconductor. Inevitably, the problem that clear white stripes or the like occur in the image in response to the above. Further, since the charging is performed by discharging in a minute gap by applying a high voltage equal to or higher than the charging start voltage of the photoconductor, the ozone generation amount cannot be completely reduced to zero and a small amount of ozone is generated.

Further, in the above-mentioned conventional contact charging method, since the charging is performed by discharging the conductive roller in a minute space by bringing the conductive roller into contact with the surface of the photosensitive member, in order to uniformly charge the conductive roller and the photosensitive member. The contact with the surface must be kept uniform. Therefore, the inconvenience that the charging ability is lowered due to deterioration of the surface of the photoconductor cannot be avoided.

Therefore, the object of the present invention is to effectively prevent the dielectric breakdown of the photoconductor, and to prevent a large current from flowing into the defective portion even when there is a defect such as a pinhole on the surface of the photoconductor. In addition, the present invention provides a novel contact charging method that does not generate ozone at all and can stably and uniformly charge a photoconductor for a long period of time without being affected by deterioration of the photoconductor surface. It is in. Another object of the present invention is to provide an image forming method utilizing the above contact charging method.

[0006]

According to the present invention, there are provided a contact charging roller having a high electric resistance layer provided on a conductive substrate, and a photoconductor having a photosensitive layer provided on a transparent conductive substrate. By rotating the contact charging roller in contact with the photosensitive layer of the moving photosensitive member and applying a negative DC voltage to the roller, by irradiating light from the transparent conductive substrate side of the photosensitive member, A contact charging method is provided, which comprises charging the surface of the photosensitive layer to a positive potential.

According to the present invention, the surface of the photosensitive layer is charged to a positive potential by the above charging method, then imagewise exposure is performed to form an electrostatic latent image, and the latent image is developed. An image forming method is provided.

Further, according to the present invention, contact charging is performed by using a contact charging roller having a high electric resistance layer provided on a conductive substrate and a photoconductor having a photosensitive layer provided on a transparent conductive substrate. The roller is rotated while being in contact with the photosensitive layer of the moving photosensitive member, and a negative DC voltage is applied to the roller, and at the same time, light is irradiated from the transparent conductive substrate side of the photosensitive member in accordance with an image signal to be positive. An image forming method is provided, which comprises forming an electrostatic latent image having an electric potential of 1 and developing the latent image.

In the above-described contact charging method of the present invention, the resistance value of the high electric resistance layer and the DC voltage applied to the contact charging roller are such that the potential difference in the thickness direction of the photosensitive layer at dark starts charging of the photosensitive layer. It is necessary to set the voltage within the range below the voltage.In the image forming method using this contact charging method, the resistance value of the high electric resistance layer and the DC voltage applied to the contact charging roller are In addition to the above conditions, it is necessary to set the potential difference in the thickness direction of the photosensitive layer in the dark to be 400 V or more. In the method of the present invention, a positive charging type photoconductor is used as the photoconductor.

In the present invention, the charging start voltage of the photosensitive layer refers to when the conductive roller connected to the DC power source is brought into contact with the photosensitive layer and the power source voltage (applied voltage) is increased from 0V. , As the potential difference in the thickness direction of the photosensitive layer when the surface of the photosensitive layer starts to be charged. That is, when the surface potential of the photosensitive layer is measured for each applied voltage, a curve as shown in FIG. 1 is obtained. From this curve, the applied voltage at the start of charging can be known, but the electric resistance of the photosensitive layer in the dark is significantly larger than the electric resistance of the conductive roller, so the electric resistance of the conductive roller is ignored. be able to. Therefore, since the power supply voltage, that is, the applied voltage is substantially equal to the potential difference in the thickness direction of the photosensitive layer, the charging start voltage in the curve of FIG. 1 corresponds to the charging start voltage of the photosensitive layer. Although the charging start voltage of the photosensitive layer is slightly different depending on the type and thickness of the photosensitive layer, it is generally about 600 V as described above. In the present invention, since the voltage applied to the photoconductor is equal to or lower than the charging start voltage, no discharge is generated and it is possible to completely prevent generation of ozone.

[0011]

In FIG. 2 for explaining the principle of the contact charging method of the present invention, the charging roller 1 used in the method of the present invention has a high electric resistance layer 3 on the surface of a conductive substrate 2. On the other hand, the photoconductor 5 used is one in which a transparent conductive layer 6 such as ITO is provided on a transparent substrate 6 such as glass, and a positive charging type photosensitive layer 7 is formed on the transparent conductive layer 6. is there.

In the contact charging method of the present invention, the charging roller 1 is brought into contact with the photosensitive layer 7 of the moving photosensitive member 5 and rotated, and a negative voltage is applied to the photosensitive member via the conductive substrate 2 by the DC power supply 10. By applying light from the back surface of the photoconductor 5, that is, from the transparent substrate 6 side, the surface of the photoconductor 5 (the surface of the photosensitive layer 7) is charged to a predetermined positive potential.

That is, as shown in FIG. 2, when light is irradiated from the transparent substrate 6 side, + and − charges are generated in the photosensitive layer 7. However, since a negative voltage is applied to the charging roller 1, a negative electric charge is removed through the transparent conductive layer 6 by the electric field formed between the charging roller 1 and the photoconductor 5, and The positive charges move to the surface of the photosensitive layer 7. Therefore, the photoconductor 5 is positively charged.

In the charging method of the present invention, the DC voltage applied to the charging roller 1 and the resistance value of the high electric resistance layer 3 are such that the potential difference in the thickness direction of the photosensitive layer 7 in the dark is the charging start voltage of the photosensitive layer. It is necessary to set as follows.

That is, when the contact charging is performed according to the present invention, an electric circuit including the DC power source 10, the charging roller 1 and the photosensitive layer 7 is formed as shown in FIG. 3 is V ₁ , the dark potential difference of the photosensitive layer 7 is V ₂ , the electric resistance of the high electrical resistance layer 3 is R ₁ , and the dark resistance of the photosensitive layer 7 is R _2. And the power supply voltage of the DC power supply 10 is V
Is represented by the following formula (1): I = V / (R ₁ + R ₂ ) (1).

Therefore, the dark potential difference V ₂ in the photosensitive layer 7 is
The following formula (2): V ₂ = IR ₂ = V · [R ₂ / (R ₁ + R ₂ )] (2) Since the charging start voltage is V ₀ , the following formula (2) gives The condition described above is expressed by the following equation (3). V ・ [R ₂ / (R ₁ + R ₂ )] ≦ V ₀ (3)

However, in the above formula (3), R ₂ and V ₀ are values determined by the type and thickness of the photosensitive layer. Therefore, in order to satisfy the condition of the formula (3), the type of the photoconductor or the like should be selected. Accordingly, the applied voltage V and the electric resistance R ₁ of the high electric resistance layer 3 may be set in combination.

In the present invention, when the condition of the formula (3) is not satisfied, that is, the dark potential difference V ₂ in the photosensitive layer 7 is
Is set higher than the charging start voltage V _0, the surface of the photosensitive layer 7 is negatively charged by the injection of electric charge through the high electric resistance layer 3, and therefore the surface of the photosensitive layer 7 is irradiated with light. The transferred positive charge is neutralized, and the desired positive charge cannot be performed.

Further, the contact charging method of the present invention can be applied to the main charging and neutralization of the photosensitive member in the image forming cycle in the electrophotographic method, but particularly when it is applied to the main charging, the photosensitive layer 7 is used. It is preferable to set the dark potential difference V ₂ at 400 V or more.

That is, the light resistance R ₂ ′ of the photosensitive layer 7 upon light irradiation is significantly lower than the dark resistance R ₂ , and therefore the light potential difference V ₂ ′ of the photosensitive layer 7 is also dark. Remarkably lower than the time potential difference V ₂ . Incidentally, the charging potential of the photosensitive layer surface to be charged by the contact charging method is equivalent to the 'difference between the (V ₂ -V _2' dark potential V ₂ and bright when the potential difference V _2), bright-field potential difference V ₂ The dark potential difference V _{2 which} is much larger than that of 'is dominant in the image density. Therefore, when the dark potential difference V ₂ is lower than 400 V, even if the photosensitive layer 7 is positively charged, its surface potential is low and it becomes difficult to effectively form an image.

A photosensitive member having a positively chargeable organic photosensitive layer having a thickness of 30 μm (dark specific resistance; 10 × 10 ¹³ Ωcm, light specific resistance; 10 ¹⁰ Ωcm, charging start voltage: 600 V) was used under the above conditions. It is used as an example and the case where the power supply voltage (V) is set to -1000V is shown. That is, in this case,
When the dark potential difference V ₂ of the photosensitive layer is calculated for each resistance value of the high electric resistance layer of the charging roller, the curve shown in FIG. 3 is obtained.
The bright specific resistance of the photosensitive layer is a value when the photosensitive layer is irradiated with light of sufficient intensity to have a minimum specific resistance. This figure 3
As is apparent from the above, it is understood that when the power supply voltage (V) is −1000 V, the resistance value of the high electric resistance layer may be set to 3 × 10 ^{10 to} 7 × 10 ¹⁰ Ω.

Further, using the above-mentioned photoconductor, the applied voltage (power supply voltage) when the dark potential difference V ₂ of the photoconductive layer becomes 600 V (corresponding to the charging start voltage) with respect to the resistance value of the high electric resistance layer. The plotted diagram is shown in FIG. As is clear from FIG. 4, for example, when the applied voltage is set to −2600V, the resistance value of the high electrical resistance layer needs to be at least 10 ¹¹ Ω or less, and when set to −800V. Needs to be 10 ¹⁰ Ω or less.

When the photoconductor is charged by the above-mentioned contact charging method of the present invention, there is a remarkable advantage that it is hardly affected by the surface characteristics of the photosensitive layer. That is, at the time of charging, it is sufficient if a constant electric field is formed on the photosensitive layer. For example, the surface of the photosensitive layer is dirty, or the surface of the photosensitive layer is worn due to execution of an image forming process for a long time. In this case, the desired charging can be stably performed. Further, in such a contact charging method, the bright potential V ₂ 'of the photosensitive layer can be adjusted also by the intensity of the light irradiated during the contact charging.

Image formation using the contact charging method of the present invention can be carried out by a so-called analog system or a digital system. In the case of the analog method, after the entire surface of the photosensitive layer is positively charged by the above method, image exposure is performed to form an electrostatic image on the surface of the photosensitive layer, and the electrostatic image is developed by a known method. do it. In the case of the digital method, the light irradiation at the time of the contact charging is performed by the image signal from the computer using the LED array,
There is a remarkable advantage that the surface of the photosensitive layer is charged and the electrostatic image is formed at the same time. After the electrostatic image is formed, the latent image may be developed as described above. Further, in the case of this method, since the background portion is not irradiated with light and is not substantially charged, it is possible to effectively prevent toner adhesion to the background portion, that is, fogging. Furthermore, since the charging potential can be adjusted by the intensity of the light irradiated during contact charging, this digital system is advantageous in that an image with high gradation can be obtained by changing the exposure intensity for each bit. is there.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION

(Charging Roller) Charging roller 1 used for contact charging of the present invention
As described above with reference to FIG. 2, the roller-shaped conductive substrate 2 has the high electric resistance layer 3 on the surface thereof. As the conductive substrate, various metals such as aluminum are generally used, but if the volume resistance is 10 ⁻² Ωcm or less, resins or rubbers mixed with various conductive agents can also be used.

As such a resin, various thermoplastic elastomers such as polyester elastomers, polyamide elastomers, polyurethane elastomers, soft vinyl chloride resins, styrene-butadiene-styrene block copolymer elastomers and acrylic elastomers are preferable. , Nylon 6, Nylon 6,6, Nylon 6
-Nylon 6,6 copolymer, Nylon 6,6 -Nylon 6,10
Copolymers, polyamides such as alkoxymethylated nylon such as methoxymethylated nylon, copolyamides or modified products thereof, silicone resins, acetal resins such as polyvinyl butyral, polyvinyl acetate, ethylene-
Vinyl acetate copolymers, ionomers and the like can also be used. Examples of the rubber include natural rubber, butadiene rubber, styrene rubber, butadiene-styrene rubber, nitrile-butadiene rubber, ethylene-propylene copolymer rubber, ethylene-
Propylene-non-conjugated diene copolymer rubber, chloroprene rubber, butyl rubber, silicone rubber, urethane rubber, acrylic rubber and the like are preferable. Further, a metal foil such as nickel, aluminum, copper, brass or tin can also be used.

Further, as a conductive agent to be mixed with the above-mentioned resin or rubber to adjust the volume resistance value, conductive carbon black or a metal such as silver, gold, copper, brass, nickel, aluminum or stainless steel is used. Powder or a powdered conductive agent such as tin oxide-based conductive agent can be used. In addition, a nonionic, anionic, cationic, amphoteric-based organic conductive agent or an organic tin-based conductive agent can be used. You can also Generally, higher conductivity is obtained when these conductive agent particles form a chain structure in resin or rubber, but in this case, a dot-shaped high potential portion is generated when a voltage is applied. , There is a risk of uneven charging. Therefore, these conductive agents are preferably uniformly and finely dispersed in the resin or rubber, and for this purpose, it is important to sufficiently knead the conductive agent-containing resin or rubber.
Further, in order to effectively disperse the conductive agent uniformly, it is also possible to partially use an acid-modified resin or rubber obtained by copolymerizing an ethylenically unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic anhydride. It is valid.

As the high-electric resistance layer 3 provided on the conductive substrate 2, a resin or rubber adjusted to have an appropriate resistance value by the blending of the conductive agent and the layer thickness is used.
The types of the resin or rubber and the conductive agent may be the same as those described above, but in addition to these, a fluorine-based resin or rubber such as polyvinylidene fluoride (PVD) may be used.
F), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (PTFE.HFP), perfluoroalkoxy fluororesin and the like are preferably used. In particular, when these fluorine-based resins or rubbers are used, they are inactive and have a small friction coefficient, so that there is a great merit in terms of the life of the photoconductor as the member to be charged and the life of the charging roller.

The surface of the high electric resistance layer 3 is preferably as smooth as possible, and for example, the average roughness according to JIS B0601 is preferably 5 μm or less, and particularly preferably 1 μm or less.

The resistance value of the high electric resistance layer 3 is set so as to satisfy the condition of the above-mentioned formula (3) according to the applied voltage, as understood from the example of FIG. However, it is generally about 10 ^{9 to} 10 ¹¹ Ω, particularly about 10 ^{10 to} 10 ¹¹ Ω.

(Photoreceptor) As shown in FIG. 2, a photoreceptor to which the contact charging method of the present invention is applied has a transparent conductive layer 6 such as ITO provided on a transparent substrate 5 such as glass or a transparent film. And a photosensitive layer 7 capable of being positively charged is further formed thereon. The thicknesses of the transparent substrate 5 and the transparent conductive layer 6 are not particularly limited as long as the photosensitive layer 6 on the transparent substrate 5 and the transparent conductive layer 6 exhibits conductivity by the above-mentioned light irradiation.

The positively chargeable photosensitive layer 7 is known per se, and includes a positively chargeable organic photosensitive layer, an a-Si photosensitive layer, an a-SeTe photosensitive layer and the like. Yes, but generally from a cost perspective,
A positively chargeable organic photosensitive layer is suitable. The positively chargeable organic photosensitive layer may be a so-called single layer type in which a charge generating agent and a charge transporting agent are dispersed in a single layer resin, or a charge generating layer in which a charge generating agent is dispersed. It may be a laminated type in which a charge transport layer in which a charge transport agent is dispersed is formed. The thickness of these photosensitive layers 7 varies depending on the type, but is usually 10 to 50 μm.

(Contact Charging) The contact charging using the charging roller 1 and the photoconductor 2 described above is performed by bringing the charging roller into contact with the photosensitive layer 7 of the moving photoconductor 2 as shown in FIG. It is rotated by applying a negative DC voltage from the DC power supply 10 to the photosensitive layer 7 via the charging roller 1 and, at the same time, irradiating light from the back surface of the photosensitive layer 7 (that is, the transparent substrate 5 side).

In this case, the output voltage of the DC power supply 10, that is, the applied voltage, is the potential difference V in the thickness direction of the photosensitive layer 7 in the dark.
₂ is set in accordance with the resistance value of the high electric resistance layer 3 of the charging roller 1 so that the charging start voltage of the photosensitive layer 7 becomes equal to or lower than the charging start voltage. When this contact charging is used for main charging in the image forming process, the applied voltage is adjusted so that the potential difference V ₂ is 400V or more.

In this case, since the specific resistance of the photosensitive layer 7 changes between the dark specific resistance value and the light specific resistance value depending on the intensity of the irradiation light, the charging potential is adjusted by the light intensity in this range. You can also

(Image Forming Apparatus) The contact charging method of the present invention described above can be used for main charging in an image forming cycle, and can be applied to both so-called analog type image forming and digital type image forming.

FIG. 5 is a diagram showing an example of an apparatus for applying the contact charging method to the main charging to form an image by an analog method. In FIG. 5, a contact charging roller 21, an optical system 22 for image exposure, a developing device 23 having a developing sleeve 23a therein, a transfer unit 24, and a charge removing unit are provided around the rotating photosensitive drum 20 along the rotation direction thereof. The light source 25 and the cleaning device 26 are arranged in this order. A charging light source 27 such as a fluorescent lamp is disposed inside the photoconductor drum 20 so as to face the contact charging roller 21. Further, a fixing device 28 is provided adjacent to the photosensitive drum 20.

Photoreceptor drum 20 and contact charging roller 21
The structure of is as described in FIG. In this case, the photosensitive layer used to form the photosensitive drum 20 is usually one having sensitivity to short wavelengths. The contact charging roller 21 is provided in contact with the photosensitive drum 20 so that it can be driven to rotate, and a DC power source 30 is connected to the roller 21 so that a negative voltage is applied. There is.

In such an image forming apparatus, the contact charging roller 2 is first applied while applying a predetermined negative voltage from the DC power source 30 and irradiating the charging light source 27 with light.
The entire surface of the photosensitive drum 20 is positively charged by the contact charging using 1. Then, an electrostatic image is formed on the surface of the photoconductor drum 20 by image exposure by the optical system 22. The formed electrostatic image is filled in the developing device 23,
It is developed by the developer carried by the developing sleeve 23a to form a toner image. This toner image is transferred onto a predetermined recording paper 31 by the transfer device 24, and the recording paper 31 having the transferred toner image is introduced into the fixing device 28.
The transfer toner image is fixed by heat, pressure, or the like. On the other hand, after the transfer is completed, the residual charge is erased from the surface of the photoconductor drum 20 by irradiation with light from the static elimination light source 25, and the cleaning device 26 further removes the residual toner and paper dust from the surface. One cycle is completed.

FIG. 6 is a diagram showing an example of an apparatus for applying the above-mentioned contact charging method to the main charging to perform digital image formation. In FIG. 6, the optical system 22 for image exposure is removed from the apparatus of FIG. 5, and an LED array is used as the charging light source 27. The photosensitive layer used for forming the photoconductor drum 20 is, for example, one having sensitivity to long-wavelength light for laser printers.

In the image forming apparatus shown in FIG. 6, light is emitted from the LED array 27 in accordance with an image signal, so that only the image forming portion of the surface of the photosensitive drum 20 is positively charged. Therefore, since the electrostatic image is formed at the same time when the surface of the photoconductor drum 20 is charged, it is not necessary to perform image exposure in a separate step, and after charging, the steps of development, transfer, fixing, charge removal, cleaning, etc. are directly performed. This is performed in the same manner as the device of FIG.

Particularly in the image formation using the apparatus shown in FIG.
When the light is emitted from the LED array 27, the exposure intensity is changed for each bit, so that an image with high gradation can be obtained. In this case, since the entire surface of the photoconductor drum 20 is not charged and only the image forming portion is charged, toner adhesion to a portion other than the image forming portion, that is, fogging is effectively prevented.

In the image formation in the apparatus shown in FIGS. 5 and 6, the developer is a one-component magnetic developer made of magnetic toner or a two-component magnetic developer made of non-magnetic toner and magnetic carrier. A known developer such as a developer is used, and the electrostatic image formed on the photosensitive drum 20 is developed by being conveyed by a developing sleeve 23a in the form of a magnetic brush. The development with the magnetic brush may be contact development or non-contact development. Further, in the usual case, the charge polarity of the toner has to be changed between when the image is formed by analog and when the image is formed by digital. However, when the charging method of the present invention is adopted, analog and digital are used. In any case, it can be dealt with by using a negatively charged toner.

Further, in the above-mentioned image forming method, the charging potential of the surface of the photosensitive drum 20 charged by the contact development is 6
00V or less. That is, generally, in the image formation using the conventionally known positive charging type organic photoconductor, the main charging potential of the photoconductor is set to about 700 to 800 V, but when the image formation is performed according to the present invention, Since so-called low potential development is performed, development conditions are appropriately changed so that a good image is formed.

As an example, the following means are adopted. (1) Toner having a high dielectric constant of 4.0 or more is used as the toner to increase the charge amount of the toner. (2) The relative frequency difference between the developing sleeve 23a and the photoconductor drum 20 is set to be large, or the interval between the developing sleeve 23a and the photoconductor drum 20 is set to be a small interval, and the contact frequency between the developer and the photoconductor drum 20 is set. Make it higher (3) When a two-component magnetic developer is used, the toner concentration is set relatively high.

In the image forming apparatus shown in FIGS. 5 and 6, the static elimination light source 25 is provided on the upstream side of the cleaning device 26. The static elimination light source 25 is not included in the cleaning device 26.
May be provided on the downstream side, or may be provided on both the upstream side and the downstream side.

The image forming method utilizing the contact charging method of the present invention is effectively applied to image formation by various electrophotographic methods such as copying machines, facsimiles and laser printers.

[0048]

EXAMPLES Experimental Example 1 The following were used as a charging roller, a photosensitive drum and a charging light source. (Charging roller) The resistance in the film thickness direction is 5 × 10 ¹⁰ Ω on the surface of the conductive rubber roller (10 mm diameter, volume resistance 10 ² Ωcm).
Highly insulating Teflon was coated so as to obtain a charging roller. (Photoreceptor) On the surface of an ITO glass tube having an inner diameter of 78 mm, the following positively chargeable organic photosensitive layer having sensitivity to short wavelength was formed, and this was used as a photosensitive drum. Physical properties of photosensitive layer: Charging start voltage: 600 V Dark specific resistance: 10 ¹³ Ωcm Light specific resistance: 10 ¹⁰ Ωcm Thickness: 30 μm (charging light source) A fluorescent lamp was used.

A copying machine having the structure shown in FIG. 5 was assembled using the charging roller, the photosensitive drum and the charging light source.
In this copying machine, an image was formed under the following developing conditions while applying a voltage of -1000 V to the charging roller and turning on the fluorescent lamp so that the illuminance on the glass surface on the back surface of the photosensitive drum would be 300 Lux. It was

(Development conditions) Developer: Two-component magnetic developer carrier; Ferrite carrier (average particle size 80 μm) Toner (negatively charged type); Dielectric constant 4.0, average particle size 10 μm Toner concentration; 4.5 wt. % Development sleeve: Diameter; φ30mm Rotation direction; Forward direction (rotation directions are opposite to each other) Peripheral speed; 350mm / sec Drum-sleeve spacing; 0.8mm Photoconductor drum peripheral speed: 150mm / sec Development bias voltage: + 50V

In this image formation, the surface potential of the photosensitive drum immediately after contact charging by the charging roller was measured and found to be in the range of +530 to + 570V. The obtained image had an image quality equivalent to that of a normal corona charging.

Further, the same image forming process as above was executed without turning on the fluorescent lamp, and the surface potential of the photosensitive drum immediately after contact charging was measured.
, The surface potential required for image formation could not be obtained.

Experimental Example 2 The following were used as the charging roller, the photosensitive drum and the light source for charging. (Charging roller) The same one as in Experimental Example 1 was used. (Photoreceptor) On the surface of an ITO glass tube having an inner diameter of 78 mm, the following positively chargeable organic photosensitive layer having a sensitivity to long-wavelength light for a laser printer was formed and used as a photosensitive drum. Physical properties of photosensitive layer: Charging start voltage: 600 V Dark specific resistance: 3 × 10 ¹³ Ωcm Light specific resistance: 9 × 10 ⁹ Ωcm Thickness: 30 μm (charging light source) An LED array (750 nm) was used.

A copying machine having the structure shown in FIG. 6 was assembled using the above charging roller, photosensitive drum and charging light source.
In this copying machine, a voltage of -1000 V was applied to the charging roller, and image formation was performed under the same development conditions as in Experimental Example 1 while image exposure was performed with a light intensity of 300 μW / cm ² by the LED array. A line image was obtained.

[0055]

According to the contact charging method of the present invention, since the voltage applied to the photosensitive member is equal to or lower than the charging start voltage, discharge does not occur during charging and ozone is not generated at all. Further, the photosensitive member can be positively charged with almost no influence of the surface condition of the photosensitive member and without applying a high bias voltage. Therefore, even if the surface characteristics of the photoconductor are deteriorated, stable charging is performed, and since high voltage is not applied to the photoconductor, problems such as dielectric breakdown of the photoconductor do not occur. This charging method is particularly advantageously applied to main charging when performing an image forming process by electrophotography, but when used for digital image formation, electrostatic images are formed at the same time as charging, Moreover, since the light source for forming the latent image is housed inside the photosensitive drum, the device can be made compact. Further, since the surface of the photoconductor corresponding to the background portion is not charged, it is also advantageous in suppressing fog.

[Brief description of drawings]

FIG. 1 is a diagram for calculating a charging start voltage of a photosensitive layer.

FIG. 2 is an explanatory diagram for explaining the principle of the contact charging method of the present invention.

FIG. 3 is a diagram showing a relationship between a dark potential difference of a photosensitive layer and a resistance value of a high electric resistance layer of a charging roller when a power supply voltage is set to −1000V.

FIG. 4 is a diagram showing a relationship between a power supply voltage and a resistance value of a high electric resistance layer of a charging roller when a dark potential difference of a photosensitive layer is set to 600V.

FIG. 5 is a diagram showing an example of an apparatus for forming an image in an analog manner by using the contact charging method of the present invention.

FIG. 6 is a diagram showing an example of an apparatus for digitally forming an image using the contact charging method of the present invention.

Claims (6)

[Claims] 1. A photosensitive member comprising a contact charging roller having a high electric resistance layer provided on a conductive substrate and a photoconductor having a photosensitive layer provided on a transparent conductive substrate, wherein the contact charging roller is moved. The surface of the photosensitive layer is charged to a positive potential by irradiating light from the transparent conductive substrate side of the photosensitive body while rotating the photosensitive body while contacting with the photosensitive layer and applying a negative DC voltage to the roller. A contact charging method characterized by the above. 2. The resistance value of the high electric resistance layer and the DC voltage applied to the contact charging roller are set such that the potential difference in the thickness direction of the photosensitive layer in the dark is within a range equal to or lower than the charging start voltage of the photosensitive layer. The contact charging method according to claim 1, wherein 3. The contact charging method according to claim 1, wherein the conductive substrate of the contact charging roller is formed of a conductive material having a volume resistance lower than 10 ² Ωcm. 4. The surface of the photosensitive layer is charged to a positive potential by the charging method according to claim 1, imagewise exposure is performed to form an electrostatic latent image, and the latent image is developed. Image forming method. 5. A photosensitive member which uses a contact charging roller having a high electric resistance layer provided on a conductive substrate and a photoconductor having a photosensitive layer provided on a transparent conductive substrate, and which moves the contact charging roller. While rotating in contact with the photosensitive layer of the body and applying a negative DC voltage to the roller, at the same time, light is irradiated from the transparent conductive substrate side of the photoreceptor in accordance with an image signal to generate a static potential having a positive potential. Forming a latent image,
An image forming method characterized by developing this latent image. 6. The resistance value of the high electric resistance layer and the DC voltage applied to the contact charging roller are such that the potential difference in the thickness direction of the photosensitive layer at dark is 400 V or more and less than or equal to the charging start voltage of the photosensitive layer. The contact charging method according to claim 4, wherein the contact charging method is set to fall within the range.