JPH0352058B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a charging method and device for charging an object to be charged, and an electrophotographic apparatus equipped with this device.

[Conventional technology]

For convenience, a description will be given of an example of charging processing (including static elimination processing) of a photoreceptor in an electrophotographic apparatus, which is an image forming apparatus.

As is well known, electrophotography includes a process 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) High voltage application In order to obtain a surface potential of 500 to 700 V on the photoconductor, it is necessary to apply a high voltage of 4 to 8 KV to the wire, and to prevent leakage to the electrode and main body, it is necessary to apply a high voltage of 4 to 8 KV to the wire. In order to maintain a large distance between the electrodes, the discharger itself becomes large in size, and it is essential to use highly insulated cables.

(2) Low charging efficiency Most of the discharge current from the wire 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.
Image blurring is likely to occur due to oxidation of device components and ozone deterioration on the surface of the photoreceptor (this development is particularly noticeable in high humidity environments), and in consideration of the effects of ozone on the human body, ozone absorption/decomposition filters and filters are used. A fan is required to generate airflow.

(4) Wire contamination A discharge wire with a large curvature (generally a diameter of 60μ to 100μ is used) is used to increase discharge efficiency, but the high electric field formed on the wire surface can cause damage to the device. 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, a conductive member (conductive potential fiber member), which is a charging member, such as a conductive fiber bristle brush or a conductive elastic roller, to which a direct current voltage of about 1 KV is applied externally, is brought into contact with the surface of the photoreceptor, which is the object to be charged. By doing so, charges are directly injected onto the surface of the photoreceptor, and the surface of the photoreceptor is charged to a predetermined potential.

[Problem that the invention seeks to solve]

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 photoreceptor surface is not uniformly charged, resulting in spot-like charging unevenness. This consists of a charging member to which a voltage is applied,
This is thought to be because it is difficult to obtain an ideal contact surface with the surface of the photoreceptor with which it is brought into contact due to the microscopic irregularities on both surfaces. Even if an image forming process lower than optical image exposure is applied to the photoreceptor surface with this spotty charging unevenness, the output image will be a spotty black dot image corresponding to the spotty charging unevenness, and a high-quality image will not 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, for example, the photoreceptor in an image forming apparatus such as an electrophotographic apparatus is used. The purpose is to enable it to be used without problems as a uniform charging means.

[Means for solving problems]

The present invention relates to a charging method and device, and an electrophotographic apparatus equipped with this device, which are characterized by the following configuration.

In a charging method of charging an object to be charged with a charging member, the charging member has a region where the distance from the surface of the object to be charged increases, and the charging member has a peak-to-peak voltage value that is at least twice the charging start voltage value for the object to be charged. A charging method characterized in that an oscillating electric field is formed between the surface of the object to be charged and the region of the charging member by applying a voltage between the object to be charged and the charging member.

A charging device that charges an object to be charged includes a charging member having an area where the distance from the object to be charged increases and a region in contact with the object to be charged, and a charging member between the object to be charged and the charging member. A charging device comprising: a voltage applying means for applying a voltage having a peak-to-peak voltage that is at least twice the charging start voltage value for the body; and forming an oscillating electric field between the body to be charged and the charging member. Device.

In an electrophotographic apparatus equipped with a charging member that charges a photoreceptor, the charging member includes an area where the distance from the photoreceptor increases and an area that contacts the photoreceptor, and between the photoreceptor and the charging member, Voltage application means for applying a voltage having a peak-to-peak voltage value that is twice or more the charging start voltage value to the photoreceptor, and forming an oscillating electric field between the photoreceptor and the charging member. Electrophotographic equipment.

[Effect]

When the object to be charged is charged under the above conditions, the surface of the object to be charged is actually always stably and uniformly charged with a uniform predetermined potential at each part without causing uneven charging such as spots, as shown in the examples described later. It was confirmed as shown in

〔Example〕

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

Reference numeral 2 denotes a conductive roller as a charging member which is brought into contact with the surface of the photosensitive drum 1 with a predetermined pressure, and rotates in the direction of the arrow as the photosensitive drum 1 rotates. 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 area that gradually separates from the object to be charged as it goes downstream in the surface movement direction of the photosensitive drum. This is the surface area where 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 has an elastic rubber layer 2b such as EPDM/NBR on a metal core 2a as shown in FIG. 10 ⁵ Ω), a foamed urethane rubber layer 2 with carbon dispersed in a metal core 2a as shown in Fig. 2b.
A material coated with d can be used.

The conductive roller 2 may be a non-rotating roller or a pad member. FIGS. 2c and 2d each show an example constructed as a pad member.

A In the case of general charging means (DC voltage application) In the above case, the photoconductor layer 1b of the photoconductor drum 1
The azo pigment is used as a CGL layer (carrier generation layer), and on top of that, a mixture of hydrazone and resin is used as a CTL layer (carrier transport layer), which is laminated to a thickness of 19μ as a negative polarity organic semiconductor layer (OPC layer). , rotate this OPC photosensitive drum 1,
A conductive roller 2 is brought into contact with the surface of the conductive roller 2, and a DC voltage V _DC is applied to the conductive roller 2.
Contact charging of the OPC photosensitive drum 1 is performed, and the charged
The relationship between the surface potential V of the OPC photosensitive drum 1 and the DC voltage V _DC applied to the conductive roller 2 was measured.

The graph in FIG. 8 shows the measurement results. Charging has a threshold value for the applied DC voltage V _DC , and charging starts from approximately -560V, and when a voltage higher than the charging start voltage value (560V) is applied, the obtained surface potential V slopes upward on the graph. A linear relationship of 1 was obtained. 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 applied voltage 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 there is a relationship of Vc = Va - _VTH. .

The above equation can be derived using Paschen's law.

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

Vg=(Va−Vc)Z/Ls/Ks+Z...(1) Va: Applied voltage value Vc: Value of photoreceptor layer surface potential Z: Gap Ls: Photoreceptor layer thickness Ks: Photoreceptor layer relative permittivity On the other hand, According to Patsien's law, the discharge development in the gap Z has a discharge breakdown voltage of Z = 8 μ or more.
Vb can be approximated by the following linear equation (2).

Vb=312+6.2Z ……(2) If you write equations (1) and (2) on a graph, it will look like the graph in Figure 10. The horizontal axis shows the gap distance Z, the vertical axis shows the gap breakdown voltage, the downward convex curve is the Patsien curve, and the upward convex curve shows the characteristics of the gap voltage Vg with (Va−Vc) as a parameter. show.

Discharge occurs when Patsien's curve intersects with the curve . That is, (Va−Vc−312−6.2×Ls/Ks) ² =4×6.2× 312×Ls/KsVc=Va−√7737.6× +312+6.2×Ls/Ks) ……(3) (Vc=Va− V _TH ) By substituting the dielectric constant 3 and CTL thickness 19 μ of the OPC photoreceptor layer 1b used in the previous experiment into the right side of equation (3), Vc = Va−573 is obtained, and the experimental formula obtained earlier almost matches.

Patsien's law relates to discharge development in voids, but even in the charging process using the dielectric roller 2, a small amount of ozone is generated in the immediate vicinity of the charging area (10% compared to corona discharge).
^-2 to 10 ^-3 ), and it is thought that charging is somehow related to discharge development.

The graph in FIG. 11 shows the charged a-Si after passing through the conductive roller 2 when the photosensitive layer 1b of the photosensitive drum 1 is replaced with the OPC layer in the above example and is replaced with an amorphous silicon (a-Si) layer. Photosensitive drum 1
The relationship between the surface potential of the conductive roller 2 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 before the charging process. Charging started from V _TH ≈440V, and thereafter a linear relationship similar to the graph for the OPC photosensitive drum shown in FIG. 8 was obtained.

The a-
Substituting Ks=12 and Ls=20μ for the Si photosensitive drum
V _TH =432V was obtained, which is almost in agreement with the experimental results.

When a DC voltage is applied to the conductive roller 2,
With the characteristics described above, a charging potential can be obtained on the surface of the photoreceptor, but as mentioned above, when the electrostatic charge pattern is visualized using a known developing method, spot-like unevenness, that is, uneven charging occurs. That's right.

B In the case of the contact charging means of the present invention (pulsating current voltage application) OPC photosensitive drum used in the above section A and a-
Regarding the Si photosensitive drum, direct current is applied to the conductive roller 2.
The relationship between _{the photoreceptor} charging potential and the peak-to-peak voltage when the photosensitive drum is subjected to contact charging by applying a pulsating voltage (V _DC + V _AC ) obtained by superimposing AC V _AC with a peak _- to-peak voltage of V PP on V DC is shown below. Each was measured. FIGS. 3 and 4 are graphs of the measurement results. In the region where V _PP is small, the value of the charged potential increases linearly in proportion to V _PP , and when it exceeds a certain value, it almost saturates to the DC component of the pulsating voltage component V _DC value, _and takes a constant value.

The above inflection point for the V _PP /binary change in the photoconductor charging voltage is approximately 1100 V as shown in the graph of Figure 3 for the OPC photosensitive drum, and approximately 1100 V as shown in the graph of Figure 4 for the a-Si photosensitive drum. about 900V
These values are approximately twice the value of V _TH when direct current is applied, which was determined in Section A above.

This relationship is based on the frequency and DC component of the applied voltage.
Even if V _DC is changed, the saturation point of the charging potential only shifts due to the change in V _DC value, and the position of the inflection point with respect to the change in V _PP remains constant, and It does not depend on the speed (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 V _PP is small, that is, there is a slope of 1 between V _PP /2 and the charging potential.
In the region where there is a linear relationship between The charging potential was constant and 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 V _TH 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 of pulsating voltage
The relationship between V _PP and the charging start voltage value V _TH , that is, V _PP ≧2V _TH, has been experimentally confirmed as described above, but it can be theoretically considered as follows.

That is, the inflection point in the relationship between the charging potential and the change in V _PP 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, assuming that the voltage waveform is a sine wave of V _PP superimposed on the V _DC direct current component, Vmax and Vmin are expressed as Vmax = V _DC + 1/2V _PP , Vmin = V _DC - 1/2V _PP when applying a pulsating voltage. It will be done.

When a voltage of Vmax is applied, the photoreceptor is charged to a surface potential of V=V _DC +1/2V _PP -V _TH according to the above equation (3).

After this, when the voltage applied to the charging member with respect to the above-mentioned surface potential reaches the minimum value among the voltage values, that is, V _nio , and the difference exceeds the charging start voltage value V _TH , the excess charge on the photoreceptor is removed. Reverse transfer 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 V _TH is because the charge transfer is determined by the gap voltage between the two. They can be considered directionally equivalent.

Therefore, in order for reverse charge transition to occur, (V _DC + 1/2V _PP - V _TH ) - (V _DC - 1/2V _PP ) ≧V _TH , that is, V _PP ≧2V _TH , and the above empirical formula is satisfied. The result is consistent with .

In other words, even if, for example, excessive charges are locally applied to the photoreceptor, resulting in a high potential, it is evened out by the above-mentioned charge reverse transfer.

The formation of an oscillating electric field due to the aforementioned voltage between the charging member and the photoreceptor causes charge transfer/reverse transfer between the two, but the charge transfer process is determined by the value _{of V TH ;} If a charge transfer occurs when a potential difference between . As can be seen from the figure, the vibration has an amplitude of (V _PP /2−V _TH ).

Charging start voltage value is the charging start voltage value when the charging member is placed facing the charged object at the use position, and a DC voltage is applied between the charged object and the charging member to start charging the charged object. This is the DC voltage value applied between the body and the charging member.

Regarding V _TH , by definition it is the potential difference at the closest distance where charge transfer occurs, and it is originally dependent on distance. _TH must also become large. The position of the Patsien curve shown in FIG. 10 also shows an increasing development of 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 rotational direction of the photoreceptor, that is, the distance between the charging member and the photoreceptor increases as shown in FIGS. 1 and 2. In this configuration, the photoreceptor potential, which was oscillating in a rectangular waveform with an amplitude (V _PP /2-V _TH ) shown in FIG. 6, becomes 0 as V _TH increases during the separation process. converges to.

In a sufficiently distant region where charge transfer and countertransference no longer occur, the photoreceptor surface potential does not depend on the applied voltage value V _PP and stabilizes at approximately the V _DC value.

〔Effect of the invention〕

As explained above, the charging member has a region where the distance from the surface of the charged object increases, and a voltage having a peak-to-peak voltage value that is twice or more of the charging start voltage value for the charged object is applied to the charged object. By applying the voltage between the charged member and the charged member, an oscillating electric field is formed between the surface of the charged member and the region of the charged member, thereby making it possible to uniformly charge the charged member without uneven charging.

Furthermore, as mentioned above, it is thought that charge transfer and reverse transfer occur between the charged body and the charged member.
A desired potential can be obtained with high precision without depending on the potential of the object to be charged 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 development, such as image fluctuations caused by electrostatic latent image fluctuations in electrophotography.

[Brief explanation of drawings]

FIG. 1 is 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. FIGS. 2a and 2b are cross-sectional views of an example of the structure of a conductive roller, and FIGS. 2c and d are cross-sectional views of an example of the structure of a conductive pad member, respectively. FIGS. 3 and 4 are graphs of the relationship between the applied voltage V _PP value and the photoreceptor charging potential V for the OPC photosensitive drum and the a-Si photosensitive drum, respectively. FIG. 5 shows an example of the voltage waveform applied to the conductive roller. FIG. 6 is 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 is a graph showing the relationship between the potential before charging and the potential after charging for the OPC photosensitive drum. FIGS. 8 and 11 are graphs of the relationship between the DC applied voltage V _DC and the photoreceptor charging voltage V for the OPC photosensitive drum and the a-Si photosensitive drum, respectively. FIG. 9 is a model diagram of the gap between the photoreceptor layer and the conductive roller. Figure 10 is a graph showing the relationship between Patsien's curve and air gap voltage. 1 is a photosensitive drum as an object to be charged, 2 is a charging member, and 3 is a voltage application source.

Claims (1)

[Scope of Claims] 1. In a charging method of charging an object to be charged with a charging member, the charging member has a region where the distance from the surface of the object to be charged increases, and the charging start voltage value for the object to be charged is adjusted. An oscillating electric field is formed between the surface of the charged member and the region of the charged member by applying a voltage having a peak-to-peak voltage value of twice or more between the charged member and the charged member. Charging method. 2. The charging method according to claim 1, wherein the voltage is a superimposed voltage of a DC voltage and an AC voltage. 3. A charging device that charges an object to be charged includes a charging member having an area where the distance from the object to be charged increases and a region in contact with the object to be charged, and a charging member between the object to be charged and the charging member. Voltage application means for applying a voltage having a peak-to-peak voltage value that is twice or more the charging start voltage value to the charged body, and forming an oscillating electric field between the charged body and the charged member. charging device. 4. The charging device according to claim 3, wherein the charging member is roller-shaped. 5. The charging device according to claim 3, wherein the charging member is pad-shaped. 6. The charging device according to any one of claims 3 to 5, wherein the voltage is a superimposed voltage of a DC voltage and an AC voltage. 7. In an electrophotographic device equipped with a charging member that charges a photoreceptor, the charging member includes an area where the distance from the photoreceptor increases and an area that contacts the photoreceptor, and a space between the photoreceptor and the charging member. , a voltage applying means for applying a voltage having a peak-to-peak voltage value that is at least twice the charging start voltage value to the photoreceptor, and forming an oscillating electric field between the photoreceptor and the charging member. electrophotographic equipment. 8. The electrophotographic apparatus according to claim 7, wherein the charging member is roller-shaped. 9. The electrophotographic apparatus according to claim 7, wherein the charging member is pad-shaped. 10. The electrophotographic apparatus according to claims 7 to 9, wherein the voltage is a superimposed voltage of a DC voltage and an AC voltage. 11. The electrophotographic apparatus according to claims 7 to 10, wherein the photoreceptor has an organic photoconductor layer. 12 The photoreceptor has an amorphous silicon photoconductor layer, as claimed in claims 7 to 10.
The electrophotographic device described in Section 1.