JPH05341620A - Contact electrostatic charging device - Google Patents

Abstract

PURPOSE:To suppress the produced amount of ozone and the increase of the cost of a power source device as muck as possible and to obtain the excellent uniformity of electrostatic charging and environmental stability in the contact electrostatic charging device of a copying machine or a laser printer, etc., which makes an electrostatic charging member impressed with voltage from an outside abut on a body to be electrostatically charged to be electrostatically charged. CONSTITUTION:An electrostatic charging roller 1 is allowed to be in pressurized contact with a photosensitive drum 3 by specified pressing force, and supported to be rotated by following the rotation of the drum 3. Specified DC voltage is impressed on the metallic core material 2 of the roller 1 by a voltage impressing means 5a. The outermost layer of the roller 1 is coated with a coating layer having <=5mum surface roughness and <=15mum film thickness. When the voltage is impressed on the roller 1, electricity is discharged in a minute gap part 9 being about 100mum between the roller 1 and the drum 3, thereby uniformly electrostatically charging the drum 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device for supplying a predetermined surface potential to a photosensitive member in an image forming apparatus using an electrophotographic method such as a copying machine and a laser printer, and more particularly to a charging device for contacting the surface of the photosensitive member. The present invention relates to a contact charging device that applies a voltage from a member to a photoconductor.

[0002]

2. Description of the Related Art As a charging device for applying a predetermined charging potential to the surface of a photosensitive member in a device for forming an image by an electrophotographic method, there has been one using corona discharge by a single wire electrode. However, in corona discharge, the charging efficiency on the surface of the photoconductor is low, and an extremely large voltage (-5 to 6 kV) as compared with the charging potential (-700 V) must be applied to the wire electrode, which leads to an increase in the size of the power supply device. There was a problem that caused an increase in cost.

Further, ozone is generated by this corona discharge, which poses a problem of image deterioration and adverse effects on the human body.

Therefore, recently, it has been considered to use the contact charging means without using the corona discharger which has many problems as described above. This is to charge the surface of the photoconductor to a predetermined potential by contacting the surface of the photoconductor, which is the member to be charged, with a charging member such as a conductive elastic roller to which a voltage is applied from the outside.

Such a contact type charging device will be described with reference to FIG. The conductive elastic roller 1 (hereinafter, referred to as a charging roller) has a conductive rubber layer 7 in which conductive particles such as carbon are dispersed in a metal core material 2, and the rubber is provided outside the conductive rubber layer 7. In order to prevent the photosensitive drum 3 from being damaged or contaminated by the layer 7, a coat layer 6 of nylon, urethane or the like is coated. The charging roller 1 is 500 g on the photosensitive drum 3 by a spring (not shown).
It is pressed by a load of f and is supported by a bearing (not shown) so as to rotate following the rotation of the photosensitive drum 3. A voltage application means 5a is connected to the metal core material 2 and a constant DC voltage is applied (DC application method).

As a result of analyzing the charging mechanism by the charging roller, charging is not performed at the contact portion L (hereinafter referred to as a nip portion) between the roller and the drum, but as shown in FIG. It was found that the discharge phenomenon (air discharge) occurred in the minute gap portion 9 of about 100 μm on the side. This air discharge is the voltage applied to the minute gap,
That is, it occurs when the potential difference between the roller and the drum is equal to or greater than the threshold value (see FIG. 8). On the outlet side of the nip,
The photoconductor drum is already charged and the roller
Since the potential difference between the drums is equal to the threshold value, no air discharge occurs.

FIG. 8 shows the charging characteristics of this roller charging. The applied voltage is the threshold (-550V) with respect to the charging potential.
And below that there is little charging. The charging potential increases linearly when the applied voltage is equal to or higher than the threshold value.

The photosensitive drum having such a charging characteristic is -7
When charged to 00 V, the required applied voltage Vc is Vc = -700 + (-550) =-1.25 kV, which is far more efficient than corona discharge.
Further, since the discharge is generated in the minute gap portion 9 of about 100 μm, the amount of ozone generated is extremely small as compared with the corona discharge.

[0009]

However, when a DC voltage is applied to the charging roller in this way to charge it,
There was a drawback that it was easily affected by changes in the operating environment such as temperature and humidity. In addition, environmental conditions are high temperature and high humidity (HH: 35
℃, 90%), normal temperature and normal humidity (NN: 20 ℃, 55
%) And low temperature and low humidity (LL: 5 ° C., 15%), the volume resistance of the coat layer becomes high under the low temperature and low humidity (LL) environment as shown in FIG.
The constant voltage control using the DC voltage has a problem that the charging potential is lowered.

Therefore, as disclosed in Japanese Patent Laid-Open No. 63-149669 and Japanese Patent Laid-Open No. 1-267667, an AC voltage having a peak-to-peak voltage equal to or more than twice the threshold voltage is applied to the DC voltage as the applied voltage. Using the superimposed voltage,
There has been proposed a method (AC superposition method) in which uniform DC voltage is controlled by constant voltage control and AC voltage is controlled by constant current to obtain uniform and stable charging against environmental changes (see FIG. 10).

In the AC superposition method, an AC voltage having a peak-to-peak voltage that is at least twice the threshold value is superposed on a DC voltage (voltage supply means 5b) and applied to the charging roller. Due to this voltage, discharge from the roller to the drum (charging process) and reverse discharge from the drum to the roller (reverse charging process) in the minute gap portions 9 and 9'of about 100 μm on both sides of the nip portion.
Occurs. By repeating this charging and reverse charging process, local uneven charging becomes uniform, and as the roller 1 and the photosensitive drum 3 are separated from each other at the exit of the nip portion, the oscillating electric field is attenuated and the charging potential becomes a DC voltage value. Converge. Further, since the AC voltage is controlled by a constant current, the AC voltage value automatically increases even if the resistance of the coating layer increases in the LL state, so that a constant charging potential can always be obtained.

However, in this AC superposition system, the discharge from the roller to the drum and the reverse discharge from the drum to the roller are constantly repeated, and the discharge and the reverse discharge are similarly generated not only on the inlet side of the nip portion but also on the outlet side of the nip portion. Because it is repeated
As shown in FIG. 11, the amount of ozone generated is about 6 times as much as that of the DC application method, and the original meaning of the contact charging method aiming at ozoneless is lost. Further, there is a problem in that the application control becomes complicated and the cost of the voltage applying means 5b increases.

An object of the present invention is to provide a contact charging device which has a stable charging potential and a small amount of ozone generated.

[0014]

According to the present invention, there is provided a member to be charged, a charging member in contact with the member to be charged, and a power source section for applying a DC voltage to the charging member. The conductive substrate to which the DC voltage is applied is coated with a resistance layer consisting of two layers, and the outermost layer has a surface roughness of 5 μm.
It is characterized by being coated to a thickness of m or less and a thickness of 15 μm or less.

Further, the present invention is characterized in that the outermost layer is made of urethane.

[0016]

In the present invention, a DC voltage is applied to the charging member that contacts the member to be charged. As a result, the body to be charged is charged with a certain potential. The outermost layer of the charging member has a surface roughness of 5 μm or less and a thickness of 15 μm or less.

FIG. 3 shows the relationship between the surface roughness of the outermost layer of the resistance layer and the charging uniformity of the charging member. This figure shows an example in which an electrophotographic photosensitive member is used as the member to be charged and a charging roller having a conductive rubber layer and a coat layer (outermost layer) is used as the charging member. The applied voltage is -1.25 for DC application.
kV, AC superposition method is -700V (DC) + 2kVp-p,
It is 500 Hz (AC). To evaluate the charge uniformity, expose the image of the halftone test chart on the photoconductor,
The image quality of the copy sample was used. ○, △, in the figure
×: ○: image quality is good and it can be said that uniform charging is performed, Δ: image is slightly disturbed and there is a part where charging is not uniform, ×: image is disturbed and charging is poor, respectively. Shows. From this figure, it can be seen that even in the DC application method, if the surface roughness of the outermost layer is 5 μm or less, charging uniformity equivalent to that in the AC superposition method can be obtained.

Further, FIG. 4 shows the relationship between the thickness of the outermost layer and the resistance value of the charging member under each environment. This figure shows the case where nylon is used as the outermost layer and the case where urethane is used. Environmental conditions are high temperature and high humidity (HH: 35
℃, 90%), normal temperature and normal humidity (NN: 20 ℃, 55
%) And low temperature and low humidity (LL: 5 ° C., 15%). From this figure, it was found that the resistance value under LL was higher than that under NN, and this tendency was more remarkable as the outermost layer was thicker.

The electrical operation of the charging mechanism consisting of the charging member, the member to be charged and the power source is shown in the equivalent circuit of FIG.

[0020]

[Equation 1]

It can be represented by Where R ₁ is the resistance of the body to be charged (drum resistance), R _{2 is} the resistance of the charging member (roller resistance), Va is the applied voltage, Vth is the threshold voltage, V
₀ : initial potential of charged body (drum initial potential), C ₁ : drum capacitance, α = (R ₁ + R ₂ ) / (C ₁ R ₁ R ₂ ).
Is. General usage conditions for each variable (R ₁ = 4
× 10 ¹² Ω, Va = -1.25 kV, Vth = -550
V, V ₀ = 0V, C ₁ = 1.64 × 10 ^-10 F) is set, and the relationship between the photoconductor potential and the roller resistance (resistance value of the charging member) under this condition is calculated from the equation (1). When it asks, it becomes like FIG. From the figure, it can be seen that the charging potential starts to decrease when the roller resistance exceeds 10 ⁷ Ω. Therefore, from the results of FIG. 4, if the thickness of the outermost layer is 15 μm or less, the resistance of the charging member can be suppressed to 10 ⁷ Ω or less even in the LL environment, and a stable charging potential can be obtained.

Further, in FIG. 7, urethane is used as the outermost layer.
It is a charging characteristic under each environment by a charging member coated with a thickness of 0 to 12 μm. From the figure, it can be seen that the charging characteristics in LL are improved as compared with the conventional roller coated with nylon in a thickness of 10 to 30 μm (FIG. 9).

As described above, according to the present invention, even if the DC voltage is controlled, the uneven charging is small, and the stable charging can be performed against the environmental change. If the thickness of the outermost layer is too thin, durability will be a problem, so the thickness of the outermost layer is most preferably about 10 μm.

[0024]

1 is a schematic diagram showing the structure of a contact charging device according to an embodiment of the present invention. The photosensitive drum 3 is rotatably supported inside the image forming apparatus via a shaft 4. The photoconductor drum 3 is generally formed by forming a photoconductive layer such as OPC on the peripheral surface of a base made of a conductive material such as aluminum, and the base is grounded via a shaft 4. The charging roller 1 is in contact with a part of the peripheral surface of the photosensitive drum 3 over substantially the entire axial direction. Charging roller 1
Has a conductive rubber layer 7 (thickness 3 mm) in which conductive particles such as carbon are dispersed in a metal core material 2 (diameter 6 mm). The rubber layer 7 is exposed outside the conductive rubber layer 7. To prevent the body drum 3 from being damaged or contaminated,
A coat layer 6 made of urethane is coated. The film thickness of the coat layer 6 is about 10 μm, and the surface roughness thereof is suppressed to 5 μm or less.

The charging roller 1 is rotatably supported via a metal core material 2. A voltage applying means 5 is connected to the metal core material 2. The voltage applying means 5 applies a constant voltage of -1.25 kV to the charging roller 1 (metal core material 2). The metal core 2 is pressed against the photosensitive drum 3 with a load of 500 gf by a spring (not shown), and when the metal core 2 comes into contact with the photosensitive drum 3, elastic deformation occurs in a predetermined range to form a nip portion L. ing. Nip part L
The friction causes the charging roller 1 to rotate (direction of arrow B) following the rotation of the photosensitive drum 3 (direction of arrow A).

From the voltage applying means 5 to the charging roller 1-1.
When a DC voltage of 25 kV is applied and the photosensitive drum 3 is rotated in the A direction, as shown in FIG. 2, a discharge phenomenon (air discharge) occurs in a minute gap portion 9 of about 100 μm on the nip inlet side. , -1250V-(-550V) =-70
The surface of the photoconductor drum 3 is charged to the voltage of 0V. The coat layer 6 has a surface roughness of 5 μm or less and a thickness of 15 μm.
Since it is set to m or less, the charging potential is kept uniform as described above, and the fluctuation of the charging potential can be minimized even when the environment changes.

On the exit side of the nip portion, the photosensitive drum has already been charged, and the potential difference between the roller and the drum is equal to the threshold value, so air discharge does not occur.
Little ozone is generated.

The present invention is not limited to the charging roller, and the present invention can be similarly implemented by a charging blade or the like.

[0029]

As described above, according to the present invention, since only the DC voltage is applied to the charging member, the amount of ozone generated is extremely small and the cost of the power supply device is low. Furthermore, since the outermost layer of the charging member is coated to have a surface roughness of 5 μm or less and a thickness of 15 μm or less, there is little unevenness in charging even with DC voltage control, and stable charging is possible against environmental changes. is there.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a contact charging device according to this embodiment.

FIG. 2 is a diagram illustrating a DC application type charging mechanism of the present invention.

FIG. 3 is a diagram showing the relationship between the surface roughness of the coat layer and the charging uniformity.

FIG. 4 is a diagram showing the relationship between the thickness of the coat layer and the roller resistance under each environment.

FIG. 5 is a diagram showing an equivalent electric circuit model of contact charging.

FIG. 6 is a diagram showing the relationship between the resistance of the charging member and the charging potential.

FIG. 7 is a diagram showing charging characteristics of the charging member according to the present invention under various environments.

FIG. 8 is a diagram showing a relationship between a roller charging (contact) applied voltage and a charging voltage.

FIG. 9 is a diagram showing charging characteristics of a conventional charging roller under various environments.

FIG. 10 is a diagram illustrating an AC superposition type charging mechanism.

FIG. 11 is a diagram showing the amount of ozone generated by DC charging and AC charging.

[Explanation of symbols]

 1-Charging Roller 2-Coat Layer 3-Photosensitive Drum 4-Voltage Applying Means

Front page continued (72) Inventor Koji Shinkawa 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Prefecture Sharp Corporation (72) Inventor Hiroshi Ishii 22-22, Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture (72) Inventor, Shogo Yokota 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture

Claims (2)

[Claims] 1. A charged body, a charging member that is in contact with the charged body, and a power supply unit that applies a DC voltage to the charging member. The charging member is a conductive member to which the DC voltage is applied. 1. A contact charging device comprising a conductive substrate coated with a resistance layer consisting of two layers, the outermost layer having a surface roughness of 5 μm or less and a thickness of 15 μm or less. 2. The contact charging device according to claim 1, wherein the outermost layer is formed of urethane.