JPH0792617B2 - Contact charging device - Google Patents

Description

Detailed Description of the Invention a. OBJECT OF THE INVENTION [Field of Industrial Application] The present invention relates to a contact charging device. More specifically, the present invention relates to an improvement of an apparatus for charging (including removing electricity) by bringing a charging member applied with a voltage from the outside into contact with a body to be charged.

[Conventional technology]

Hereinafter, for convenience, a contact charging device for a photosensitive member, which is a member to be charged, in an electrophotographic copying machine will be described as an example.

Conventionally, in this type of contact charging device, a charging member (conductive potential maintaining member) such as a conductive fiber bristle brush or a conductive elastic roller to which a DC voltage containing an AC component of about 1 KV is externally applied is provided on the surface of the photoconductor. By bringing them into contact with each other, an electric charge is introduced onto the surface of the photoconductor to be charged to a predetermined potential.

FIG. 8 is a schematic diagram showing an example of a conventional contact charging device of this type.

In the figure, 1 is a photoconductor, 2 is a charging roller as a charging member, and 3 is a power source.

The charging roller 2 has EPDM / NBR around the conductive metal core rod 2a.
A single layer structure in which a rubber layer 2d such as foamed urethane rubber is formed. Conventionally, the volume resistivity of this roller 2 is 10 ¹ Ωm
I used the thing of the degree. This corresponds to a resistance of 1.89 × 10 ² Ω in the nip portion (in this document, the contact area between the charging roller 2 and the photoconductor 1; in this example, the width is 1 mm and the length is 220 mm). However, the charging roller 2 has an outer diameter of 12 mm and the thickness of the rubber layer 2d.
It is the case of 3 mm.

[Problems to be solved by the invention]

However, in actuality, when the surface of the photosensitive member, which is the member to be charged, is charged by the charging roller as described above, if there is a pinhole on the surface of the photosensitive member, that portion is not normally charged.

That is, when the charging roller and the pinhole on the surface of the photoconductor face each other, spark discharge is generated from the charging roller toward the pinhole, and a large current flows. As a result, the output of the power source applied to the charging roller is lowered, and the charging of the surface of the photosensitive member at that portion is not normally performed. In that case, the electric potential does not properly spread over the entire length in the axial direction on the photoconductor, and the image turns black due to the reversal phenomenon (generally called black obi) and appears white in the normal phenomenon (generally called white obi).
A phenomenon appears.

The present invention has been made in view of the above points, and prevents a charging failure due to damage to the surface of an object to be charged such as the photoconductor in the contact charging device, and a contact charging device having stable charging performance. The purpose is to provide.

B. Configuration of the Invention [Means for Solving the Problems] The present invention has a charging member that contacts a member to be charged and charges the member to be charged, and a power supply that supplies electric power to the charging member. In the contact charging device, wherein the charging member includes a conductive substrate and a resistance layer, the volume resistivity of the resistance layer is ρ,
When the thickness of the resistance layer is 1, the voltage of the power source is E, and the capacity of the power source is P, However, it is characterized in that A = (1 × 10 ⁻³ ) ² [m ² ] is satisfied.

[Action]

With the above configuration, the charging member has a layer having a resistance and a capacitance such that the final surface potential of the charging member is lower than the spark discharge start potential and is equal to or higher than the charge start potential. Even if the defective portion (pinhole) and the charging member face each other, spark discharge does not occur, and the charged body can be uniformly and stably charged.

〔Example〕

Hereinafter, the present invention will be specifically described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of a contact charging device showing an embodiment of the present invention.

In the figure, 1 is a part of a drum-shaped electrophotographic photosensitive member as a member to be charged, and a photosensitive layer (photoconductive semiconductor material layer such as organic semiconductor, amorphous silicon or selenium) 1b is provided on the outer peripheral surface of a drum substrate 1a. It is formed, and is surface-moved and driven at a predetermined speed in the direction of arrow a. The above-mentioned photoreceptor is not limited to the drum shape, but may be a belt shape or a sheet shape.

2 is the above-mentioned drum-shaped photoreceptor (hereinafter referred to as a photosensitive drum) 1
The charging roller is a charging member that is brought into contact with the surface with a predetermined pressure, and is driven to rotate in the direction of the arrow as the photosensitive drum 1 rotates.

The charging roller 2 is, for example, a metal core rod (conductive base) 2a.
A conductive elastic rubber layer (hereinafter referred to as the inner layer) 2b such as EPOM / NBR / foamed urethane rubber, which has been processed to have a volume resistivity of about 10 Ωm, and a mylar having a volume resistivity of about 10 ¹⁴ Ωm on the peripheral surface of・ Nylon resistance layer (hereinafter referred to as outer layer) 2c
A two-layer structure in which and are coated is used. However, two or more layers may be provided. In this case, the sum of the resistance and the electrostatic capacity of each layer will be described as the resistance and the electrostatic capacity of the charging roller 2.

A power source 3 applies a voltage to the charging roller 2, and an alternating current voltage (AC) is superimposed on a direct current voltage (DC). Incidentally, in this embodiment, the AC component is regarded as being superimposed in order to make the surface potential of the photosensitive drum uniform, and as a model of the potential change of each part, there is a transient phenomenon when DC is applied to the charging roller. The description will be made assuming that they are equivalent.

a) Countermeasure against charging failure First, the countermeasure against charging failure that occurs when the pinhole on the photosensitive drum faces the charging roller was examined using a single-layer charging roller as shown in FIG.

When the pinhole on the photosensitive drum faces the charging roller, the photosensitive drum cannot be charged normally because the spark discharge from the charging roller toward the pinhole causes a large current to flow. .

As a result, the flowing current exceeds the capacity of the power source, the power source output is reduced, and the photosensitive drum is not charged.

Therefore, as a measure against spark discharge, it is sufficient to provide the power supply with a power supply capacity that does not cause a decrease in the power output even if a larger current than normal flows when the charging roller and the pinhole face each other.

Therefore, if the power capacity is P, the resistance of the charging roller in the pinhole is R, and the power voltage is E, Any power supply capacity that satisfies the relationship

Furthermore, from the definition of volume resistivity, However, since ρ is the volume resistivity of the charging roller, l is the wall thickness of the charging roller, and A is the area of the pinhole, If ρl is satisfied, the charging abnormality due to spark discharge does not occur. Here, for example, A = (1 × 10 ⁻³ ) ² [m ² ], E is
DC component -750V, AC component Vpp = 1500V, P is 10W, Get the condition.

Here, if the wall thickness l of the charging roller is 3.0 mm, for example, ρ
The range of is ρ> 2.81 × 10 ¹ Ωm (5).

b) Difficult to pass alternating current In the single-layer charging roller as shown in FIG. 8, however, a problem arises in that the applied alternating current is difficult to pass for uniform charging.

In other words, allow a sufficient margin for the resistance at the nip part in consideration of the decrease in resistance due to water absorption of the charging roller in a high humidity environment.
It will be about 1.14 × 10 ¹⁰ Ω.

However, the voltage applied to the charging roller with this resistance is AC
At 1600Vpp, the power supply only draws 0.05μA.

As a result, the effect of leveling the potential of the photosensitive drum by the alternating current could not be expected, and uneven charging occurred on the photosensitive drum.

c) Pass an alternating current Next, a charging roller 2 having a two-layer structure as shown in FIG. 1 is used. The inner layer 2b of the roller 2 is made of 10 ¹ Ωm material, and the outer layer 2c is made of a nip portion. A material having a resistance (sum of inner layer and outer layer) equal to the resistance value of the single layer (1.14 × 10 ¹⁰ Ω) was selected. Then, although the outer layer 2c has a large volume resistivity, it also has a capacitance.

As a result, the alternating current flowing through the charging roller was 0.6 mA, and the uneven charging on the photosensitive drum was eliminated by the potential leveling effect.

d) Charging ability of charging roller Furthermore, measures against spark discharge for larger pinholes,
Further, considering the effect of reducing uneven charging due to the flow of a larger alternating current, more appropriate values are required for the resistance and capacitance of the nip portion of the charging roller.

Therefore, various materials for the outer layer 2c portion of the charging roller were studied.
However, depending on the material, the surface potential of the charging roller gradually decreases with the rotation of the roller, and eventually the desired potential cannot be charged on the photosensitive drum.

Therefore, the change in the surface potential of the charging roller 2 was calculated and the appropriate range of resistance and capacitance of the outer layer 2c of the charging roller 2 was calculated.

e) Change in surface potential of charging roller First, a change in voltage for one rotation of the charging roller was obtained using an equivalent circuit. Next, the voltage on the surface of the charging roller when the number of revolutions was set to infinite was obtained. Further, the surface potential of the charging roller that can be charged on the photosensitive drum was obtained by an experiment, and the resistance and the capacitance range of the outer layer 2c of the charging roller satisfying the conditions were obtained.

FIG. 2 shows an equivalent circuit formed by the charging roller 2 and the photosensitive drum 1 in FIG.

In the figure, C ₁ · R ₁ is the electrostatic capacity and resistance of the photosensitive drum 1 in the nip portion, C ₂ · R ₂ is the electrostatic capacity and resistance of the charging roller 2 in the nip portion, R ₃ is the charging resistance, and E ₂ is Charging start voltage, E ₁ is applied voltage, V ₁ and V ₂ are photosensitive drum 1 and charging roller 2, respectively
The voltage S is a switch that indicates that a certain portion on the peripheral surface of the charging roller is turned on when it enters the nip portion and is turned off when it leaves the nip portion.

FIG. 3 shows changes in the surface potential of the charging roller 2 derived from the equivalent circuit of FIG.

In the figure, t ₁ is the time required for the charging roller to pass through the nip portion, and t ₂ is the time required for the charging roller to rotate once.

Here, the surface potential of the charging roller at t = 0 is as follows.

However, V ₂₀ is the initial value of the charging roller.

The voltage of the charging roller at the nip is The charging roller voltage until it leaves the nip and rushes into the nip again is It is expressed as.

As the charging roller rotates, the surface voltage of the charging roller changes while satisfying the above expressions (6), (7), and (8).

Further, the electric potential of the charging roller when the charging roller leaves the nip portion is shown by the alternate long and short dash line in FIG.

Here, when the number of rotations of the charging roller is infinite, the final surface voltage Vn → ∞ of the charging roller when leaving the nip portion is shown as follows.

In FIG. 3, (6) to (9) clearly indicate the portions represented by the above formulas (6) to (9). Furthermore, the two-dot chain line shows the change in the surface potential of the photosensitive drum. This is always lower than the surface potential of the charging roller by E ₂ (charging start voltage).

FIG. 3 shows the case where Vn → ∞ is larger than the charging start voltage E ₂ . That is, in this case, the resistance and the electrostatic capacity of the outer layer 2C of the charging roller have sufficient charging ability.

f) Find the range of R ₂ · C ₂ .

Next, the range of the electrostatic capacitance C ₂ and the resistance R _{2 of the} outer layer 2c of the charging roller at which the surface potential of the charging roller becomes larger than the charging start voltage E ₂ when the charging roller is rotated an infinite number of times by using the formula (9). I asked. As conditions, applied voltage E ₁ = −1300 V, resistance at the nip part of the photosensitive drum R ₁ = 4.00 × 10 ¹² Ω, capacitance C ₁ = 3.
It was set to 10 × 10 ^-10 F.

The charging start voltage E ₂ was obtained by actual measurement by the following method.
That is, the relationship between the applied voltage E ₁ and the photosensitive member surface voltage V ₁ was first obtained by using a charging roller in which the resistance R ₂ of the charging roller is almost zero.

FIG. 4 shows the results, in which the vertical axis represents the photoreceptor surface voltage V ₁ and the horizontal axis represents the applied voltage E ₁ , and the applied voltage E ₁ is varied to obtain the corresponding photoreceptor surface voltage V ₁ Is a plot of. OPC was used as the photoconductor.

The following relationship is derived from this figure.

V ₁ = E ₁ −E ₂ = E ₁ − (− 560) (10) Therefore, E ₂ = −560V was experimentally obtained. From this result, the following can be said. That is, the charging has a threshold value with respect to the applied DC voltage E ₁ , and the charging starts from about −560 V (however, this voltage depends on the capacity of the polar photosensitive member of the applied power source), and the charging start voltage For the above voltage application, the obtained surface potential V ₁ has a linear relationship with a slope of 1 on the graph.

In addition, this characteristic is environmentally similar (for example, high temperature / high humidity, low temperature / low humidity environment), and almost the same result is obtained.

The range of R ₂ · C ₂ where the surface potential of the charging roller becomes equal to or higher than the charging start voltage E ₂ when the charging roller is rotated infinitely under the above conditions can be derived from the equation (9). The calculation result is shown in FIG.

In Fig. 5, the ○ mark indicates that R ₁ -V ₂ is larger than the charging start voltage R ₂
C ₂ .

The cross indicates R ₂ C ₂ where E ₁ −V ₂ is smaller than the charging start voltage.

As is clear from the figure, if R ₂ C ₂ satisfies the following range,
It can be said that the surface potential of the charging roller is higher than the charging start potential.

1) When C ₂ ^{≥10 -2.1} (F) R ₂ ≤10 ¹⁵ (Ω) 2) When 10 ^-2.1 (F)> C ₂ ≥10 ^-9 (F) R ₂ ≤10
¹² (Ω) 3) When C ₂ <10 ^-9 (F), R ₂ ≦ 10 ⁴ / C _{2 From the} above formula (4), volume resistivity ρ when l = 100μ.
Is ρ> 8.44 × 10 ⁻² × (100 × 10 ⁻⁶ ) ⁻¹ = 8.44 × 10 ² [Ωm].

Further, the resistance R ₂ of the nip portion when the outer layer 2c has the volume resistivity ρ satisfying the above condition is obtained as follows.

Therefore, based on the above results and the results of 1), 2), and 3) above, the range of R ₂ finally becomes as follows.

4) When C ₂ ≧ 10 ^−2.1 (F) 3.87 × 10 ² (Ω) <R ₂ ≦ 10
¹⁵ (Ω) 5) 10 ^-2.1 (F)> C ₂ ≧ 10 ^-9 (F) 3.87 × 10
² (Ω) <R ₂ ≤ 10 ¹² (Ω) 6) When C ₂ <10 ^-9 (F) 3.87 × 10 ² (Ω) <R ₂ ≤ 10 ⁴ /
C ₂ Next, as the outer layer 2c of the charging roller 2, a cellophane having a thickness of 25 μ (R ₂ = 1.14 × 10
^{Using 10} Ω, C ₂ = 3.89 × 10 ⁻¹⁰ ), the following results were obtained.

When the electric potential of the photosensitive drum 1 was measured by applying E ₁ = −1300V, it was −380V, which was in good agreement with the surface potential −378V of the photosensitive drum obtained from the above equation (9), and this model was proved to be correct. It was

Further, even if a pinhole was formed on the photosensitive drum, no leak occurred at all and a stable image without uneven charging was obtained.

Although the charging member is formed in a roller shape in the above embodiment, the outer layer 2c of the charging roller 2 may be formed in a belt shape as shown in FIG. 6, for example. With this configuration,
The residual charge in the outer layer 2c is erased for a long time, and recovery of the charging ability is promoted. As a result, the resistance of the material of the outer layer 2c can be selected to be larger, and there is an advantage that the range of selection of the material is widened. Further, by forming the belt shape, the charging width is widened and more reliable charging becomes possible.

Further, as shown in FIG. 7, the inner layer 2b of the charging roller may have a hollow honeycomb shape or a sponge shape. In this case, a wide nip width can be easily obtained, and the same effect as in the case of forming a belt can be obtained.

In the present embodiment, the polarity of the external power source applied to the charging roller has been described as minus, but it can be similarly described when it is plus.

C. As described above, according to the present invention, the volumetric low efficiency of the resistance layer of the charging member is ρ, the thickness of this resistance layer is l, the voltage of the power source for supplying power to the charging member is E, and the power source If the capacity is P However, the following effects can be obtained by satisfying A = (1 × 10 ⁻³ ) ² [m ² ].

By setting the resistance and electrostatic capacity of the charging member to the above conditions, even if there is a defect such as a pinhole in the charged body, the charging failure does not occur in that portion, and the charged body can be It can be stably charged.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a contact charging device showing an embodiment of the present invention, FIG. 2 is an equivalent circuit diagram thereof, FIG. 3 is a graph showing changes in surface potential of a charging member (charging roller), and FIG. FIG. 5 is a graph showing the relationship between the applied voltage and the photosensitive member surface voltage, and FIG. 5 is a graph showing the resistance and the electrostatic capacitance when the surface potential of the charging roller rotates indefinitely and when it does not exceed the charging start voltage. FIGS. 6 and 7 are schematic diagrams of a contact charging device showing another embodiment of the present invention, and FIG. 8 is a diagram showing the same as the conventional example. 1 is a member to be charged (photosensitive drum), 2 is a charging member (charging roller), and 3 is a power source.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masanobu Saito 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Junji Araya 3-30-2 Shimomaruko, Ota-ku, Tokyo Kya Non non corporation

Claims (1)

[Claims] 1. A charging member for contacting a charged member to charge the charged member, and a power supply for supplying electric power to the charging member, the charging member including a conductive substrate and a resistance layer. In the contact charging device, the volume resistivity of the resistance layer is ρ, the thickness of the resistance layer is 1,
When the voltage of the power source is E and the capacity of the power source is P, However, a contact charging device characterized by satisfying A = (1 × 10 ⁻³ ) ² [m ² ].