JPH0635292A - Electrostatic charging device and image forming device - Google Patents

Abstract

PURPOSE:To make electrostatic charging for a body to be electrostatically charged uniform even when the resistance in the peripheral direction of an electrostatic charging member for electrostatically charging the body to be electrostatically charged in contact with the body to be electrostatically charged is varied. CONSTITUTION:The electrostatic charging member 13 provided with a conductive layer 13b and a surface layer 13c is brought into contact with a photosensitive drum 1. A power source 15 is connected to the member 13 so as to impress DC voltage. The capacity C of the power source 15 is made the capacity capable of performing uniform DC voltage control to the resistance fluctuation range of all the peripheral surface of the member 13. Even when the resistance of the member 13 is largely varied, the electrostatic charging for the drum 1 is not largely changed. Therefore, the drum 1 is uniformly electrostatically charged and a faulty image based on the fault of electrostatic charging is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device suitable for being mounted on an image forming apparatus such as a copying machine or a laser beam printer, and an image forming apparatus equipped with the charging device. The present invention relates to a charging device for charging a photosensitive drum by applying a DC voltage from a power source to a charging member brought into contact with the charging member.

[0002]

2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus is
As shown in FIG. 6, a photosensitive drum (member to be charged) 1 that rotates in the direction of arrow R1 is provided, and each device including a non-contact type corona charging device 2 is arranged around the photosensitive drum 1 in order along the rotation direction. It is set up.

The photosensitive drum 1 uniformly charged by the charging device 2 is exposed by the exposing means 3 to form an electrostatic latent image corresponding to image information of a document or the like. The electrostatic latent image on the photosensitive drum 1 becomes a toner image with toner (developer) attached by the next developing device 5. The toner image on the photosensitive drum 1 is transferred by the transfer device 7 onto the transfer material supplied from the paper feed cassette 6. The transfer material on which the toner image has been transferred is then fixed by the fixing device 9 as a permanent image, and is discharged to the outside of the apparatus main body. On the other hand, after the toner image has been transferred, the photosensitive drum 1 is cleaned of residual toner by the cleaning device 10, and subsequently subjected to pre-exposure 11 to prepare for the next image formation.

In the charging process of the series of image forming processes described above, a so-called contact charging device has recently been proposed and put into practical use, in which a charging member is brought into direct contact with the photosensitive drum 1. The contact charging device presses a charging member against the surface of the photosensitive drum 1 and applies a voltage to the charging member to charge the surface of the photosensitive drum 1 to a predetermined polarity and potential. At this time, the voltage applied to the charging member by the power source may be a DC voltage, a superimposed voltage of a DC voltage and an AC voltage, or the like.

Such a contact charging device is capable of uniformly charging an object to be charged such as the photosensitive drum 1 at a low applied voltage as compared with a non-contact type corona charging device, and also generates ozone. There are advantages such as less.

[0006]

However, in the above-mentioned contact charging device in which the AC voltage is superimposed on the DC voltage, the power source tends to be large, and due to the vibration due to the alternating electric field, so-called charging is performed. There is a problem that an unpleasant sound called "charging sound" is generated. These problems can be solved by not superimposing an AC voltage, that is, by applying only a DC voltage. However, in the case where the charging member is formed of a conductive and elastic roller and charging is performed by being rotated by the rotation of the charged member such as the photosensitive drum, if only the DC voltage is applied, the charging member The constant voltage characteristic of the power source tends to be lowered due to the variation of the resistance value of the entire peripheral surface, and a problem such as a charging failure occurring on the charged body corresponding to the rotation cycle of the charging member occurs.

Therefore, according to the present invention, the power source is provided with a power source having a capacity capable of performing uniform DC constant voltage control over the resistance variation range of the entire peripheral surface of the charging member, so that the charging noise is low and It is an object of the present invention to provide a charging device and an image forming apparatus in which a constant voltage property in the circumferential direction of a charging member is ensured and a charging failure is eliminated.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and a charging device applies a direct current voltage to a charging member brought into contact with a member to be charged by a power source, In a charging device for charging an object to be charged, the charging member is a rotating body having at least a conductive layer and a surface layer in contact with the object to be charged, and the power source is a resistance of the entire circumferential surface of the charging member. It is characterized by having a capacity capable of performing uniform DC constant voltage control with respect to a fluctuation range.

In this case, when the capacity of the power source is C and the ratio of the upper limit value to the lower limit value of the resistance variation range of the entire circumferential surface of the charging member is R, the capacity C of the power source is R <α. (C × 100) α is when R is 1 ≦ R ≦ 5, α = 1 When R is 5 <R, 0.25 ≦ α <1 where C is the capacity of the power supply expressed in μF It is preferable that the conditions of and are satisfied.

The charging member may be in contact with the member to be charged along the generatrix of the member to be charged and rotated by the member to be charged.

The image forming apparatus includes a member to be charged, a charging device described above for charging the member to be charged, an exposing unit for exposing the member to be charged to form an electrostatic latent image, A developing device for forming a toner image by attaching toner to the electrostatic latent image, and a transfer device for transferring the toner image onto a transfer material are provided.

[0012]

According to the above construction, by ensuring a large capacity of the power supply that allows uniform DC constant voltage control over the resistance fluctuation range of the entire peripheral surface of the charging member, the entire peripheral surface of the charging member can be secured. Can be set to a substantially uniform voltage,
This eliminates uneven charging in the circumferential direction of the body to be charged due to the charging member.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 schematically shows a charging device according to the present invention and an image forming apparatus equipped with this charging device. The charging device 12 includes a charging member 13 and a power supply 15 as main members. The charging member 13 has a cored bar 13a penetrating the central portion thereof, and the periphery thereof is covered with a conductive layer 13b. The conductive layer 13b is formed of a low-hardness member, for example, resin such as polyurethane, polyester, or pyrivinyl alcohol, or rubber such as EPDM or NBR. In addition, in these members, as a conductive pigment,
Carbon black, carbon graphite, titanium oxide, zinc oxide, etc. are dispersed appropriately. Conductive layer 13b
Has its outer peripheral surface covered with a surface layer 13c.
The surface layer 13c is formed so that the resistance value of the portion corresponding to the image forming area portion becomes medium (10 ⁶ to 10 ¹⁰ Ω). The member forming the surface layer 13c is, for example,
Polyamide as resin, polyimide, polyurethane,
A conductive material similar to that used in the conductive layer 13b, such as carbon black, carbon graphite, titanium oxide, or zinc oxide, is appropriately added to a base material such as polyester, silicon, or Teflon to adjust the overall resistance to medium resistance. To do. When the resistance value of the surface layer 13c is 1 × 10 ⁶ Ω or less,
When the photosensitive drum, which will be described later, is damaged, the withstand voltage is not exerted, which is not preferable. On the contrary, when the photosensitive drum has a resistance of 1 × 10 ¹⁰ Ω or more, no current flows and there is a risk of defective charging.
The entire charging member 13 is formed in a cylindrical shape and is rotatably supported by the main body of the image forming apparatus (not shown), and the core metal bar 13 a at the center is connected to the DC power supply 15.

When the capacity of the DC power supply 15 is C and the ratio of the upper limit value to the lower limit value of the resistance variation range of the entire peripheral surface of the charging member 13 is R (upper limit value / lower limit value), R <α (C × 100) α is when R is 1 ≦ R ≦ 5, α = 1 When R is 5 <R, 0.25 ≦ α <1 where C is the capacity of the power supply 15 in μF Make sure that the value of and the condition of are satisfied. A method of measuring the resistance value of the charging member 13 is shown in FIG. The roller-shaped charging member 13 is brought into contact with the metal drum 16, and the charging member 13 is rotated while applying a predetermined DC voltage 17 to the charging member 13. At this time, the current flowing through the metal drum 16 is measured by an ammeter 19
Then, the resistance value of the charging member 13 is calculated based on the measured value of the current and the DC voltage 17. The resistance value of the charging member 13 thus obtained,
That is, the current value before the charging member 13 is attached to the photosensitive drum 1 is shown in FIG. The current does not change significantly over time.

By using the DC power source 15 having the capacity C as described above, the charging device 12 can secure the constant voltage characteristic at the time of application even when the resistance of the entire peripheral surface of the charging member 13 changes. it can. That is, the capacity C of the DC power supply 15
Is set sufficiently large so that uniform DC constant voltage control can be performed with respect to the resistance variation range of the entire circumferential surface of the charging member 13.

As shown in FIG. 1, the charging member 13 is pressed against the photosensitive drum 1 as the member to be charged. The photosensitive drum 1 is formed in a cylindrical shape similar to the charging member 13, and a photosensitive surface is formed around the photosensitive member 1, and is further supported by an image forming apparatus main body (not shown) so as to be rotatable about the shaft 1a in the arrow R1 direction. Has been done. The shaft 1a of the photosensitive drum 1 and the cored bar 13a of the charging member 13 are arranged in parallel to each other. When the elastic charging member 13 is pressed against the photosensitive drum 1, a part of the outer periphery of the charging member 13 is cored. It is compressed along 13a, and an elongated contact surface S along the core metal 13a and the shaft 1a is formed between them. That is, the contact surface S of both is formed along each generatrix. The photosensitive drum 1 is indicated by an arrow R by a drive mechanism not shown.
The charging member 13 that is rotationally driven in one direction and does not have a positive rotation mechanism causes the contact surface S to move when the photosensitive drum 1 rotates.
It is designed to be driven to rotate via.

An image forming apparatus provided with the above-mentioned charging device 12 will be briefly described. Around the photosensitive drum 1, the surface of the photosensitive drum 1 after charging is sequentially arranged along the rotation direction thereof, for example, image information of an original. Exposure device 3 for exposing the electrostatic latent image to form a toner image, a developing device 5 for adhering toner to the electrostatic latent image to form a toner image, and a photosensitive drum 1 on the transfer material P supplied from a paper feed cassette 6. A transfer charger 7 for transferring the upper toner image, a cleaning device 10 for removing the residual toner on the photosensitive drum 1 after transfer, and a photosensitive drum 1 for the next charging.
A pre-exposure device 11 for removing the upper charge is provided.
Further, a fixing device 9 for fixing the toner image transferred on the transfer material P to the transfer material P is disposed downstream of the transfer charger 7.

Now, the charging member 13 is attached to the photosensitive drum 1.
If the charging member 13 is left for a long time with the
The contact surface S on the side is deformed, and the resistance of the contact surface S greatly changes with respect to the resistance of the entire circumference of the charging member 13. FIG. 3 shows the measured current value in the circumferential direction of the charging member 13 after leaving the charging member 13 on the photosensitive drum 1 for 3 months. When the capacity C of the DC power supply 15 is smaller than that of the charging member 13, the charging member 13 is inferior in constant voltage, so that the charging of the photosensitive drum 1 is not sufficient and an image defect due to the charging failure occurs. Occur.

Therefore, even when the resistance of the charging member 13 changes by providing the power source 15 having a sufficient capacity C for the resistance variation range of the entire peripheral surface of the charging member 13 as described above. , So that the photosensitive drum 1 is uniformly charged. As a result, defective charging in the circumferential direction of the photosensitive drum 1 is eliminated, and the final image does not become defective.

Specific examples will be shown below. <Example 1> As the conductive layer 13b, EPDM in which carbon black was dispersed was used, and tin oxide was dispersed in a resin, which is a polyamide resin (EF-30T, Teikoku Kagaku), so as to have a resistance value of 10 ⁸ Ω. A coating layer is formed by a dipping method, heated and dried at 150 ° C. for 30 minutes,
A roller-shaped charging member, that is, the conductive roller 13 was prepared.

The following evaluations were performed using this conductive roller 13. The results are shown graphically in FIG. (1) The resistance of the conductive roller 13 was measured by the measuring method shown in FIG. 4, and the ratio (upper limit value / lower limit value) R of the resistance value was calculated. (2) The conductive roller 13 is mounted on the image forming apparatus shown in FIG. 1 for adjustment, and the capacity C of the power supply 15 for the applied voltage is changed to perform image formation, so that the conductive roller 13 prevents charging failure. The images were evaluated by visual observation as follows.

The capacity C of the used power supply 15 is 0.02 μm.
F and 0.04 μF.

⊚: Image is good, no problem ◯: Minor poor charging image is generated (practical use possible) Δ: Partial charging defect image is generated (practical lower limit) x: Poor charging image is generated (practical use) <Example 2> The same evaluation as in Example 1 was carried out except that the above-mentioned conductive roller 13 was mounted on the photosensitive drum 1 and left for 2 weeks in an environment of 15 ° C. and 30%, and the conductive roller 13 was used. <Example 3> The same evaluations as in Example 1 were carried out except that the above-mentioned conductive roller 13 was mounted on the photosensitive drum 1 and left standing for 2 weeks in an environment of 32 ° C and 90%. I went. <Embodiment 4> The same as Embodiment 1 except that the above-mentioned conductive roller 13 is mounted on the photosensitive drum 1 and left for 3 months under the environment of 15 ° C. and 30%. An evaluation was made. <Embodiment 5> The same as Embodiment 1 except that the above-mentioned conductive roller 13 is attached to the photosensitive drum 1 and left for 3 months under the environment of 32 ° C. and 90%, and the conductive roller 13 is used. An evaluation was made. <Embodiment 6> The conductive roller 13 is sandwiched between two flat acrylic resin plates, and the strain amount at two locations of the conductive roller 13 is adjusted to be ⅓ of the entire circumference. Then, the conductive roller 13 is forcibly distorted by leaving it in a dryer at 70 ° C. for 24 hours, and the capacity of the power source used for image formation is 0.04, 0.08, 0. The same evaluation as in Example 1 was performed except that the value was set to 10 μF.

FIG. 5 is a summary of the above first to sixth embodiments. From these experimental results, the charging member 1
The ratio (upper limit value / lower limit value) of the upper limit value to the lower limit value of the resistance fluctuation range of the entire peripheral surface of 3 is R, and the capacity of the power supply 15 is C
R and C above (values in μF)
By mounting a power source 15 having a capacity C that satisfies the following equation, even if the circumferential resistance of the charging member 13 fluctuates, the photosensitive drum 1 cannot be practically supported. It can be charged well.

R <α (C × 100) ………………………………………… α is when R is 1 ≦ R ≦ 5 α = 1 When R is 5 <R 0.25 ≦ α <1 The above equation is obtained from the above-described first to sixth embodiments. That is, in the first to fifth embodiments, R
Since 1 ≦ R ≦ 5, the formula is R <C × 100.
When the capacitance C that satisfies the above equation is selected, that is, the capacitance C
When 100 is compared with R and C is larger,
The evaluation is ◎ or ○ in all cases.

On the other hand, also in the case of Example 6 where 5 <R, the formula is satisfied when the capacity C is 0, and the evaluation is ◎.
Becomes

[0028]

As described above, according to the present invention,
By providing a power source having a capacity capable of uniform DC voltage control over the resistance variation range of the entire circumferential surface of the charging member, it is possible to prevent a charging failure of a non-charged body due to a rapid resistance change of the charging member, and For example, a change in resistance due to a long-term contact of the charging member with the non-charged body can be similarly similarly dealt with, and therefore, the non-charged body can be suitably charged over a long period of time, and the change over time can be prevented. It is possible to provide a few stable and high-quality images.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view showing a charging device and an outline thereof according to the present invention.

FIG. 2 is a diagram showing a relationship between time and current value of the charging device before mounting.

FIG. 3 is a diagram showing the relationship between time and current value of the charging device after mounting.

FIG. 4 is a schematic diagram showing a method for measuring the resistance of a charging member.

FIG. 5 is a diagram showing image evaluation in each example.

FIG. 6 is a vertical sectional view showing an outline of a conventional image forming apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Charged object (photosensitive drum) 3 Exposure means 5 Developing device 7 Transfer device 12 Charging device 13 Charging member (conductive roller) 13b Conductive layer 13c Surface layer 15 Power supply P Transfer material

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Yanagita 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (4)

[Claims] 1. A charging device for charging a charged member, which is brought into contact with a charged member, by applying a direct current voltage from a power source to the charged member, wherein the charging member includes at least a conductive layer and the charged member. And a surface layer in contact with the charging member, wherein the power source has a capacity capable of performing uniform DC constant voltage control with respect to a resistance variation range of the entire peripheral surface of the charging member. apparatus. 2. When the capacity of the power supply is C and the ratio of the upper limit value to the lower limit value of the resistance variation range of the entire circumferential surface of the charging member is R, the capacity C of the power supply is R <α ( C × 100) α is when R is 1 ≦ R ≦ 5, α = 1 When R is 5 <R, 0.25 ≦ α <1 where C is when the capacity of the power supply is expressed in μF. 2. The charging device according to claim 1, wherein the numerical conditions are satisfied. 3. The charging device according to claim 1, wherein the charging member is in contact with the charged body along a generatrix of the charged body and is driven and rotated by the charged body. 4. A charged body, a charging device according to claim 1 for charging the charged body, and an exposing unit for exposing the charged body to form an electrostatic latent image. An image forming apparatus comprising: a developing device that adheres toner to the electrostatic latent image to form a toner image; and a transfer device that transfers the toner image onto a transfer material.