JPH04218076A - Contact electrostatic charging method - Google Patents

Abstract

PURPOSE:To uniformly execute the processing of electrostatic charge without irregularity in a contact electrostatic charging method that the electrostatic charge is executed by impressing a voltage on an electorical conductive member which is made to abut on the surface of a body to be electrostatically charged. CONSTITUTION:The electrical conductive member 2 is provided with a surface area A which abuts on the surface of the body to be electrostatically charged 1 and a surface area B which continues to the area A and which is gradually separated from the surface of the body 1 according as it is faced to the downstream side of the surface moving direction of the body 1. By impressing the pulsating voltage having an inter-peak voltage which is double or more of the voltage at the starting time of the electrostatic charge on the conductive member 2, a vibrating electric field is formed between the separated area B of the conductive member 2 and the surface of the body 1 and the electrostatic charge is executed.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contact charging method. More specifically, the present invention relates to an improvement in a contact charging method in which a conductive member to which a voltage is applied from the outside is brought into contact with a charged object to perform charging (including neutralization).

0002

2. Description of the Related Art For convenience, a description will be given of an example of charging processing (including charge removal processing) of a photoreceptor in an electrophotographic apparatus, which is an image forming apparatus.

As is well known, electrophotography includes the step of uniformly charging the surface of a photoreceptor to a predetermined potential. As the charging processing means, almost all electrophotographic apparatuses currently in practical use utilize a corona discharger whose main components are a wire electrode and a shield electrode. However, the charging system using the corona discharger has the following problems.

1) In order to obtain a surface potential of 500 to 700 V on the high voltage applied photoreceptor, 4
It is necessary to apply a high voltage of ~8KV to the wire, and in order to prevent leakage to the electrode and main body, the distance between the wire and the electrode must be kept large, which requires the discharger itself to be large in size, and the highly insulated cable is required. It is essential to use

2) Most of the discharge current from the wire with low charging efficiency flows to the shield electrode, and the corona current that flows to the photoreceptor side, which is the body to be charged, is only a few percent of the total discharge current.

3) Generation of corona discharge products Corona discharge generates ozone, etc., which causes oxidation of device components and ozone deterioration of the photoreceptor surface, causing image blurring (this phenomenon is particularly noticeable in high humidity environments). In consideration of the influence of ozone on the human body, an ozone absorption/decomposition filter and a fan as means for generating airflow to the filter are required.

4) Wire contamination In order to increase the discharge efficiency, a discharge wire with a large curvature (generally a diameter of 60μ to 100μ is used) is used, but the high electric field formed on the wire surface causes the device to The wire surface gets dirty by collecting minute dust inside. Wire contamination tends to cause uneven discharge, which appears as image unevenness. Therefore, it is necessary to clean the wires and the inside of the discharger quite frequently.

Therefore, recently, consideration has been given to using contact charging means instead of using a corona discharger which has many problems as described above.

Specifically, 1K is applied to the surface of the photoreceptor, which is the object to be charged.
Electric charges are directly injected onto the surface of the photoreceptor by bringing into contact with a conductive member (conductive potential maintaining member) that is a charging member such as a conductive fiber bristle brush or a conductive elastic roller to which a direct current voltage of about V is externally applied. The surface of the photoreceptor is charged to a predetermined potential.

0010

[Problems to be Solved by the Invention] However, in reality, even if the surface of the photoreceptor, which is the object to be charged, is charged by the above-mentioned contact charging means, each part of the surface of the photoreceptor is not uniformly charged, and uneven charging occurs. arise. This is thought to be because the charging member to which a voltage is applied and the surface of the photoreceptor with which it is in contact are microscopically uneven, making it difficult to obtain an ideal contact surface. Even if an image forming process lower than photoimage exposure is applied to the photoreceptor surface with spotty charging unevenness, the output image will be a spotty black dot image corresponding to the spotty charging unevenness, and a high-quality image cannot be obtained.

The present invention improves the charging means so that each part of the surface to be charged is uniformly charged, and instead of using the problematic corona discharger as described above, it can be used in an image forming apparatus such as an electrophotographic apparatus. It is an object of the present invention to enable the present invention to be used without problems as a means for uniformly charging a photoreceptor.

0012

[Means for Solving the Problems] The present invention provides a contact charging method in which charging is performed by applying a voltage to a conductive member brought into contact with the surface of the charged object, in which the conductive member is brought into contact with the surface of the charged object. A surface area that is in contact with the surface area and a surface area that gradually separates from the surface of the object to be charged as it goes downstream in the surface movement direction of the object to be charged are provided, and a voltage of at least twice the charging start voltage is applied to the conductive member. A contact charging method characterized in that a pulsating voltage having a peak-to-peak voltage is applied to form an oscillating electric field between the separated surface area of the member and the surface of the object to be charged, thereby performing charging.

[0013]

[Operation] When a charged object is charged under the above conditions, the surface of the charged object is actually always stably and uniformly charged with a uniform predetermined potential at each part without causing uneven charging such as spots. This was confirmed as shown in the Examples below.

[0014]

[Example] In FIG. 1, numeral 1 is a part of an electrophotographic photosensitive drum as an object to be charged, and a photosensitive layer 1b (a photoconductive material such as an organic semiconductor, amorphous silicon, selenium, etc.) is formed on the outer peripheral surface of a drum base 1a. It is formed by forming a semiconductor material layer).
It is driven to move in plane at a predetermined speed in the direction of arrow a.

2 is a conductive roller as a charging member that is brought into contact with the surface of the photosensitive drum with a predetermined pressure;
As the photosensitive drum 1 rotates, it rotates in the direction of the arrow. In this conductive roller 2, A is a surface area that comes into contact with one surface of the photosensitive drum, which is an object to be charged, and B is a surface that gradually separates from the object to be charged as it goes downstream in the surface movement direction of the photosensitive drum. It is an area. In this example, the separation surface area B is constituted by the curved surface of the roller 2 and the curved surface of the photosensitive drum. 3 is a power source that applies voltage to this conductive roller.

Specifically, the conductive roller 2 is shown in FIG.
As shown in a), a two-layer coating structure in which a metal core rod 2a is provided with an elastic rubber layer 2b such as EPDM/NBR, and a urethane rubber layer 2c (resistance ~ 105 Ω) in which carbon is dispersed is further provided on the circumferential surface of the elastic rubber layer 2b. As shown in FIG. 2(b), a metal core rod 2a coated with a foamed urethane rubber layer 2d in which carbon is dispersed can be used. The conductive roller 2 may be a non-rotating roller or a pad member. FIGS. 2(c) and 2(d) each show an example configured as a pad member.

A. In the case of general charging method (DC voltage applied)
In the above, the photosensitive layer 1b of the photosensitive drum 1 has a CGL layer (carrier generation layer) made of azo pigment, and a CTL layer (carrier transport layer) made of a mixture of hydrazone and resin thereon.
A negative polarity organic semiconductor layer (O
PC layer), this OPC photosensitive drum 1 is driven to rotate, a conductive roller 2 is brought into contact with the surface of the OPC photosensitive drum 1, and a DC voltage VDC is applied to the conductive roller 2 to conduct contact charging of the OPC photosensitive drum 1 in a dark place. The surface potential V of the charged OPC photosensitive drum 1 after passing through the conductive roller 2,
The relationship with the DC voltage VDC applied to the conductive roller 2 was measured.

The graph in FIG. 8 shows the measurement results. The charging has a threshold value with respect to the applied DC voltage VDC,
Charging starts from about -560V, and the charging start voltage value (
When a voltage of 560 V) or higher was applied, the obtained surface potential V had a linear relationship with a slope of 1 on the graph. Almost the same results were obtained in terms of environmental characteristics (for example, high temperature and high humidity environments, and low temperature and low humidity environments).

That is, if the value of the DC voltage applied to the conductive roller 2 is Va, the value of the charging potential obtained on the surface of the OPC photosensitive drum is Vc, and the charging start voltage value is VTH, then V
There is a relationship of c=Va-VTH.

The above formula is Paschen
It can be derived using the law of

As shown in the model diagram of FIG. 9, the conductive roller 2
The voltage Vg applied to the microscopic gap Z between and the OPC photoreceptor layer 1b is expressed by the following equation (1).

[0022]

[Equation 1] Va: Applied voltage value Vc: Value of photoreceptor layer surface potential Z: Gap Ls: Photoreceptor layer thickness Ks: Photoreceptor layer relative dielectric constant On the other hand, the discharge phenomenon in the air gap Z is determined by Paschen's law, Z= At 8μ or more, the discharge breakdown voltage Vb is expressed by the following linear formula (
2) can be approximated.

[0023]Vb=312+6.2Z...
...(2)(1)・(2) When the formula is written on a graph, it is shown in Figure 1.
It will look like the graph of 0. The horizontal axis shows the gap distance Z, the vertical axis shows the gap breakdown voltage, the downward convex curve ■ is the Paschen curve, and the upward convex curves ■, ■, and ■ are the gap voltage with (Va-Vc) as a parameter. The characteristics of Vg are shown. Discharge occurs when Paschen's curve ■ and curves ■ to ■ have intersections, and at the point where the discharge starts, Vg = Vb
The discriminant becomes 0 in the quadratic expression of Z. That is,

[0024]

[Equation 2] (3) On the right side of the equation is the OPC photoreceptor layer 1 used in the previous experiment.
By substituting the dielectric constant of b of 3 and the CTL thickness of 19 μ, V
c=Va -573 was obtained, which is almost in agreement with the experimental formula obtained earlier.

Paschen's law is related to the discharge phenomenon in a gap, and even in the charging process using the conductive roller 2, a small amount of ozone is generated in the immediate vicinity of the charging part (10-10% compared to corona discharge). 2 to 10-3) were observed, and it is considered that charging is somehow related to the discharge phenomenon.

The graph in FIG. 11 shows the charged a- The relationship between the surface potential of the Si photosensitive drum 1 and the DC voltage applied to the conductive roller 2 is measured.

In order to minimize the dark decay factor, experiments were conducted without exposure prior to the charging process. Charging started from VTH≈440V, and thereafter a linear relationship similar to the graph for the OPC photosensitive drum shown in FIG. 8 was obtained.

[0028] In Ks and Ls obtained by the above formula (3),
Ks=12 and Ls= of the a-Si photosensitive drum used
By substituting 20μ, VTH=432V is obtained, which almost agrees with the experimental results.

When a DC voltage is applied to the conductive roller 2, a charged potential is obtained on the surface of the photoreceptor with the above-mentioned characteristics, but when the electrostatic charge pattern is visualized using a known developing method, a spot-like pattern appears. As mentioned above, uneven charging occurs.

B. In the case of the contact charging method of the present invention (pulsating current voltage application), the OPC photosensitive drum used in the above section A and a-S
i Regarding the photosensitive drum, the peak-to-peak voltage when the photosensitive drum is subjected to contact charging by applying to the conductive roller 2 a pulsating voltage (VDC+VAC), which is a superimposition of AC VAC having a peak-to-peak voltage of VP-P on DC VDC. The relationship between the photoreceptor charging potential was measured. 3 and 4 are graphs of the respective measurement results. In the region where VP-P is small, the value of the charged potential increases linearly in proportion to VP-P, and when it exceeds a certain value, it is almost saturated at the DC component VDC value in the pulsating voltage component, and the VP-P changes. takes a constant value for

The above inflection point for the VP-P/binary change in the photoreceptor charging potential is shown in FIG. 3 in the case of an OPC photoreceptor drum.
As shown in the graph of , it is approximately 1100V, and in the case of an a-Si photosensitive drum, it is approximately 900V as shown in the graph of Figure 4, which is approximately twice the VTH when DC is applied, which was determined in section A above. Become.

This relationship shows that even if the frequency of the applied voltage and the DC component VDC are changed, the saturation point of the charging potential only shifts due to the change in the VDC value, and the position of the inflection point with respect to the change in VP-P remains constant. , and does not depend on the speed of the conductive roller 2 relative to the photoreceptor 1 (for example, stopping, rotating, reversing).

When the charged surface of the photoreceptor obtained by applying a pulsating voltage is developed in this way, when the value of VP-P is small, that is, there is a linear relationship with a slope of 1 between VP-P/2 and the charging potential. In areas where this relationship exists, spot-like unevenness occurs, similar to when only direct current was applied to the conductive roller 2 described above, but in areas where a peak-to-peak voltage above the inflection point was applied, the charging potential When the charge was constant, the obtained microscopic image was uniform, indicating that charging was uniform and uniform.

That is, in order to obtain charging uniformity, it is necessary to apply an oscillating voltage having a peak-to-peak voltage that is at least twice the charging start voltage VTH when direct current is applied, which is determined by various characteristics of the photoreceptor. The charging potential obtained at that time depends on the DC component of the applied voltage.

Uniformity of charging and peak-to-peak voltage V of pulsating voltage
Relationship between P-P and charging start voltage value VTH, VP-P
Although ≧2VTH has been experimentally verified as described above, it can be theoretically considered as follows. That is, V
The inflection point in the relationship between the charging potential and the P--P change is considered to be the point at which charge reverse transfer from the photoreceptor to the charging member starts under the electric field between the photoreceptor and the charging member.

FIG. 5 shows the voltage applied to the charging member. For the sake of explanation, if we assume a voltage waveform in which a sine wave of VP-P is superimposed on the VDC DC component, Vm when a pulsating voltage is applied.
ax・Vmin is

[0037]

[Math 3] It is expressed as

When a voltage of Vmax is applied, the photoreceptor is

[0039]

[Math 4] charged to a surface potential of

After that, when the voltage value applied to the charging member with respect to the above-mentioned surface potential reaches the minimum value among the voltage values, that is, Vmin, and the difference exceeds the charging start voltage value VTH, an excessive charge on the photoreceptor is generated. is reversely transferred to the charging member side.

The fact that the charge transfer and reverse transfer between the charging member and the photoreceptor are both carried out with a threshold value of VTH is because the charge transfer is determined by the gap voltage between the two. They can be considered directionally equivalent.

Therefore, in order for charge reverse transfer to occur,

[0043]

[Math 5] Therefore, a result consistent with the above-mentioned experimental formula is obtained.

In other words, even if, for example, an excessive charge is locally placed on the photoreceptor and the potential becomes high, it is made uniform by the above-mentioned reverse transfer of charge.

[0045] By forming an oscillating electric field between the charging member and the photoreceptor due to the voltage described above, charge transfer and transfer occurs between the two.
Reverse transition occurs, but the charge transfer process is determined by the value of VTH.In other words, charge transfer occurs when a potential difference greater than VTH occurs over a certain distance.In the area where the charging member and the photoreceptor are close, The potential oscillates in a shape similar to a rectangular wave as shown in FIG. As you can see from the figure, the amplitude is

[0046]

[Equation 6] What is the charging start voltage value? When the charging member is placed opposite the charged object at the use position and a DC voltage is applied between the charged object and the charging member, the charging of the charged object starts. is the DC voltage value applied between the object to be charged and the charging member.

Regarding VTH, by definition it is a potential difference at the closest distance where charge transfer occurs, and it originally depends on the distance, and if the gap between the charging member and the photoreceptor is large, charge transfer occurs. The required VTH must also be large. The position of the Paschen curve shown in FIG. 10 also shows an increase in the gap breakdown voltage as the distance increases.

Therefore, the charging member 2 and the photoreceptor 1 are configured to gradually move away from each other in the downstream direction of rotation of the photoreceptor, that is, the distance between the charging member and the photoreceptor increases as shown in FIGS. 1 and 2. In the configuration, the amplitude shown in Figure 6

0
049]

[Equation 7] The amplitude of the photoreceptor potential converges to 0 as VTH increases during the separation process.

In a sufficiently distant region where charge transfer/reverse transfer does not occur, the photoreceptor surface potential is V within the applied voltage value.
It does not depend on P-P and stabilizes at approximately the VDC value.

[0051]

As described above, according to the present invention, it is possible to uniformly charge the surface of a charged object by a contact charging method without charging unevenness.

Furthermore, as mentioned above, it is thought that charge transfer/reverse transfer occurs between the charged body and the charging member, and it is possible to set the desired potential with high precision without depending on the potential of the charged body before charging. (See the graph in FIG. 7). In other words, it has an effect similar to that of a grid used in a corona discharger, and enables a stable charging process without phenomena such as image fluctuations caused by electrostatic latent image fluctuations in electrophotography.

[Brief explanation of the drawing]

[Fig. 1] A diagram showing a part of a photosensitive drum as an object to be charged and a voltage-applying conductive roller for contact charging that is in contact with the surface of the photosensitive drum.

[Figure 2] (a) and (b) are cross-sectional views of an example of the configuration of a conductive roller, and (c) and (d) are cross-sectional views of an example of the configuration of a conductive pad member, respectively.

[Figure 3] Relationship graph between applied voltage VP-P value and photoconductor charging potential V for OPC photoconductor drum and a-Si photoconductor drum

[Figure 4] Relationship graph between applied voltage VP-P value and photoconductor charging potential V for OPC photoconductor drum and a-Si photoconductor drum

[Figure 5] Example of applied voltage waveform to conductive roller

[Figure 6]
A graph showing the oscillation state of the photoreceptor potential in a region where the conductive member and the photoreceptor are close to each other.

[Figure 7] Relationship graph between pre-charging potential and post-charging potential for OPC photosensitive drum

[Figure 8] Relationship graph between DC applied voltage VDC and photoconductor charging potential V for OPC photoconductor drum and a-Si photoconductor drum

[Figure 9] Model diagram of the air gap between the photoreceptor layer and the conductive roller

[Figure 10] Relationship graph between Paschen's curve and air gap voltage

[Figure 11] Relationship graph between DC applied voltage VDC and photoconductor charging potential V for OPC photoconductor drum and a-Si photoconductor drum

[Explanation of symbols]

1 Photosensitive drum as a charged object 2　Charging member 3 Voltage application source

Claims (1)

[Claims] 1. In a contact charging method in which charging is performed by applying a voltage to a conductive member brought into contact with the surface of the charged object, the conductive member has a surface area that contacts the surface of the charged object, and pulsating current voltage having a peak-to-peak voltage of twice or more the charging start voltage for the electrically conductive member; A contact charging method characterized in that charging is performed by forming an oscillating electric field between the separated surface area of the member and the surface of the object to be charged by applying .