JP3245783B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image bearing member (electrophotographic photosensitive member) as a member to be charged, such as an image forming apparatus such as an electrophotographic apparatus (copier, optical printer, etc.) and an electrostatic recording apparatus. The present invention relates to an image forming apparatus which performs image formation by applying a transfer type (indirect type) or direct type image forming process including a step of charging a surface of a body or an electrostatic recording dielectric.

[0002]

2. Description of the Related Art In an image forming apparatus as described above, a corona discharge device has been widely used as a device for charging a surface of an image carrier as a member to be charged.

A corona discharge device is effective as a means for uniformly charging a surface of a charged body such as an image carrier to a predetermined potential. However, there is a problem that a high-voltage power supply is required and undesired ozone is generated due to corona discharge.

In such a corona discharge device, a contact-type charging device for charging the surface of the charged object by bringing the charging member to which the voltage is applied into contact with the surface of the charged object as described above can reduce the power supply voltage. Because of its advantages such as low ozone generation, for example, in an image forming apparatus, charging of an image carrier such as a photoreceptor or a dielectric, or other surface to be charged, in place of a corona discharge device Attention has been paid to this as a means, and research into its practical use is underway.

[0005] For example, the present applicant has previously proposed (Japanese Patent Application No.
98419,298420, Japanese Patent Application No. 62-51492, Japanese Patent Application No. 62-230333
As described above, an oscillating electric field (alternating electric field, time) having a peak-to-peak voltage that is at least twice the charging start voltage of the member to be charged when a DC voltage is applied to the charging member in the contact charging device. An electric field (voltage) whose voltage value changes periodically between the charging member and the member to be charged, and further by using a charging member provided with a high-resistance layer on its surface, Uniform charging, prevention of leakage due to pinholes, scratches, etc. on the surface of the member to be charged such as a photoreceptor can be achieved.

Further, a conductive member (conductive potential maintaining member) such as a conductive fiber bristle brush or a conductive elastic roller as a charging member is brought into contact with the member to be charged, and a DC voltage is externally applied to the member to be charged. In some cases, electric charges are directly injected into the surface to charge the surface of the member to be charged to a predetermined potential.

FIG. 13 is a cross-sectional view of a schematic structure of an example of a contact-type charging device.

Reference numeral 1 denotes a member to be charged. In this embodiment, the photosensitive member is a rotating drum type electrophotographic photosensitive member (hereinafter, referred to as a photosensitive member). The photoreceptor 1 of this example includes a conductive base layer 1b such as aluminum,
The photoconductive layer 1a formed on the outer surface thereof is used as a basic constituent layer.

Reference numeral 2 denotes a charging member. This example is a roller type (hereinafter, referred to as a charging roller). The charging roller 2 has a central core 2c and a conductive layer 2b formed on the outer periphery thereof.
And a resistance layer 2a formed on the outer periphery thereof.

The charging roller 2 has both ends of a cored bar 2c rotatably supported by bearing members (not shown), and is disposed in parallel with the drum-type photosensitive member 1 so as to be brought into contact with the surface of the photosensitive member 1 by pressing means (not shown). The photoconductor 1 is pressed against the photoconductor 1 with a predetermined pressing force, and is rotated by the rotation of the photoconductor 1. It is also possible to attach a gear or the like and transmit a driving force from a motor to perform a forced rotation drive.

Reference numeral 3 denotes a power supply for applying a bias to the charging roller 2. The power source 3 is electrically connected to the metal core 2 c of the charging roller 2.
Is applied with a predetermined bias. As the bias, only a DC voltage can be applied, but it is preferable to apply an oscillating voltage obtained by superimposing a DC voltage on an AC voltage as described above.

When the photosensitive member 1 to be charged is driven to rotate, the outer peripheral surface of the photosensitive member 1 is fixed to a predetermined position by a charging roller 2 as a charging member pressed against the photosensitive member 1 and applied with a bias voltage. Charged to polarity and potential.

In addition to the charging roller 2 serving as the above-mentioned charging means, an exposing means, a developing means,
Necessary image forming process equipment such as transfer means / cleaning means, image fixing means and the like are provided to constitute an image forming mechanism, and image formation is executed. It is.

[0014]

In the above-described image forming apparatus, as the number of times of image formation increases, the outer peripheral surface of the photosensitive member is scraped by a cleaning blade of a cleaning means, a developer, or the like.

The charging characteristics change due to a change in equivalent capacity due to a decrease in the thickness (layer thickness, film thickness) of the photoreceptor. In particular, when the charging unit applies a contact type DC voltage, the capacitance of the photoconductor 1 is greatly affected.

That is, when the number of times of image forming use increases and the film thickness of the photosensitive member decreases, the DC current flowing through the charging roller 2 increases, and the surface potential on the outer peripheral surface of the photosensitive member increases. It is generally good that the surface potential can be ensured even if the thickness of the photoreceptor decreases, but the sensitivity of the photoreceptor decreases as the thickness decreases, so that the surface potential corresponding to a white original, that is, the bright portion potential is reduced. The potential does not drop sufficiently.

As a result, the surface potential contrast between the black document and the white document becomes narrow, and if an attempt is made to obtain a sufficient development contrast during development, a sufficient reverse contrast with respect to the potential of the white image cannot be obtained. There was an obstacle that resulted in a "fogging" image when developed thinly with the developer.

[0018] The voltage is applied to the developing bias and the exposure lamp voltage (=
Even if the adjustment is made so as not to overlap with the exposure amount of the light image irradiation), it is necessary to secure a sufficiently wide adjustment width.
The adjustment range is wide, which has been a factor in increasing the cost of power supplies and the like.

Furthermore, in an image forming apparatus configured to calculate an appropriate image forming condition by automatic control, it is difficult to optimize and adjust the appropriate image forming condition because the surface potential of the photoreceptor changes. When the number exceeds a certain number of times, there was a tendency to gradually generate a fog image.

In order to avoid this phenomenon, a surface potential sensor or the like for detecting the surface potential of the photoreceptor is required, which greatly increases the cost and complexity and size of the apparatus. This has been a major obstacle in developing an inexpensive image forming apparatus.

Further, the resistance value of the resistance layer 2a of the charging member 2 tends to fluctuate due to factors such as environmental humidity and progress of durability.

In particular, when the resistance value of the charging member becomes higher than a predetermined value due to the deterioration due to conduction due to durability, the voltage is not sufficiently applied to the member to be charged due to the voltage drop by the charging member. , The surface potential is lowered, and image defects occur.

As described above, the surface potential of the member to be charged increases when the thickness of the member to be charged decreases. However, this is the case when the resistance of the charging member is sufficiently low. Even if the film thickness of the body decreases, the voltage is not sufficiently applied due to the voltage drop in the charging member,
The surface potential of the member to be charged is reduced, and image defects occur.

This will now be described more specifically. FIG. 14 shows a schematic configuration of an example of the image forming apparatus.

Reference numeral 1 denotes an image bearing member as a member to be charged. In this embodiment, a drum-shaped member having a conductive base layer 1b made of aluminum or the like and a photoconductive layer 1a formed on the outer peripheral surface thereof as a basic constituent layer. It is an electrophotographic photosensitive member. It is driven to rotate around the support shaft 1d clockwise in the drawing at a predetermined peripheral speed (process speed).

Reference numeral 2 denotes a contact charging member which comes into contact with the surface of the photoreceptor 1 and performs a primary charging process on the photoreceptor surface to a predetermined polarity and potential uniformly. This example is a roller type (charging roller)
And a conductive core 2 formed on the outer periphery of the central core 2c.
b, and two layers of resistance layers 2a ₂ sequentially formed on the outer periphery thereof.
2a ₁ and both ends of the cored bar 2c are rotatably supported by bearing members (not shown) so as to be arranged in parallel with the drum-shaped photosensitive member 1 and pressed against the surface of the photosensitive member 1 by pressing means (not shown). Then, the photosensitive member 1 is pressed with a predetermined pressing force, and is rotated by the rotation of the photosensitive member 1.

When a predetermined direct current (DC) bias is applied from the power source 3 to the metal core 2c via the sliding contact 3a, the peripheral surface of the rotary photosensitive member 1 is contact-charged to a predetermined polarity and potential. (Primary charging).

The surface of the photoreceptor 1 which has been uniformly charged by the charging member 2 is then subjected to exposure L of target image information (image slit exposure of a document image, laser beam scanning exposure, etc.) by the exposure means 10, An electrostatic latent image corresponding to the target image information is formed on the peripheral surface.

The exposing means 10 in the apparatus of the present embodiment is a well-known document table fixed-optical system moving type document image forming slit exposing means.

In the exposure means 10, reference numeral 20 denotes a fixed platen glass, O denotes a document placed and set on the platen glass with the image surface facing downward, 21 a document pressing plate, and 22 a document illumination lamp (exposure lamp). Lamp), 23 is a slit plate, 24-
26 is a moving first to third mirror, 27 is an imaging lens, 28
Is a fixed mirror.

The ramp 22, slit plate 23, and moving first mirror 24 move the lower surface of the platen glass 20 from one end to the other at a predetermined speed V, and move the second and third mirrors 25,2.
6 is driven to move at a speed of V / 2,
The upper downward document surface is scanned from one end side to the other end side, and the original image is subjected to image forming slit exposure L on the surface of the rotating photoconductor 1.

The latent image formed on one surface of the photoreceptor is then developed by developing means 1
1, the toner image is sequentially visualized as a toner image.
The toner image is then transferred from a paper supply unit (not shown) by a transfer unit 12 to a transfer unit between the photosensitive member 1 and the transfer unit 12 at an appropriate timing in synchronization with the rotation of the photoconductor 1. It is sequentially transferred to the surface of the transfer material 14.

The transfer means 12 of this embodiment is a transfer roller, and the toner image on the photoconductor 1 side is transferred to the front side of the transfer material 14 by performing charging of a polarity opposite to that of the toner from the back of the transfer material 14. Go.

The transfer material 14 to which the toner image has been transferred is separated from the surface of the photoreceptor 1 to form a fixing roller (heat roller) 1.
The sheet is conveyed to a fixing device 16 composed of a roller 6a and a pressure roller 16b, receives an image, and is output as an image formed product. Alternatively, in the case of forming an image on the back surface, the sheet is conveyed to a re-conveying unit to the transfer unit.

The surface of the photoreceptor 1 after the image transfer is cleaned by a cleaning means 13 to remove adhered contaminants such as untransferred toner and the like.
It is repeatedly provided for image formation.

The roller-type charging member 2 may be driven or rotated by the surface of the photosensitive member 1 to be charged, or may be non-rotating. Alternatively, the motor may be positively driven to rotate in the forward or reverse direction at a predetermined peripheral speed.

The charging member 2 may be a blade type, a block type, a rod type,
It can be configured in a form such as a belt type.

FIG. 15A is a schematic cross-sectional view of an example of a blade type. In this case, the direction of the blade-shaped charging member 2 abutting on the surface of the photoconductor 1 may be either the forward direction or the reverse direction of the surface movement direction of the surface of the photoconductor 1. (B) is a schematic cross-sectional view of an example of a block or rod.

In each type of charging member 2, reference numeral 2c denotes a conductive core member, 2b denotes a conductive layer, and 2a denotes a resistance layer.

The block-shaped or rod-shaped roller type is a rotatable roller type, and does not require a power supply sliding contact 3a required for applying a bias voltage to the core member 2c. A lead wire leading to the power supply 3 can be directly connected to the member 2c,
In addition to the advantage that electric noise that may be generated from the power supply sliding contact 3a is eliminated, it is possible to save space and to use a cleaning blade that also serves as a cleaning member surface.

Next, a method of performing optimal charging using the DC power supply 3 will be described. First, a charging mechanism when a DC voltage is applied to the charging roller 2 from a DC power supply will be described.

As the photosensitive member 1, an OPC photosensitive drum having a negative polarity was used. Specifically, azo pigment is used as a photoconductor layer by CG.
A negative organic semiconductor layer (OPC) in which an L layer (carrier generation layer) and a mixture of hydrazone and a resin are laminated thereon as a CTL layer (carrier transport layer) to a thickness of 24 μm.
The OPC photosensitive member 1 is rotated and driven, a charging roller 2 is brought into contact with the surface of the OPC photosensitive member 1, a DC voltage _VDC is applied to the charging roller 2, and the OPC photosensitive member 1 is brought into contact with the OPC photosensitive member 1 in a dark place to perform charging. The relationship between the surface potential V _D of the charged OPC photosensitive member 1 after passing through the charging roller 2 and the DC voltage V _DC applied to the charging roller 2 was measured.

The 24 μm linear graph of FIG. 16A shows the measurement results. Charging with respect to the applied DC voltage _VDC has a threshold value for each drum (photoconductor) film thickness, charging starts from a specific voltage, and is obtained when a voltage having an absolute value equal to or higher than the charging start voltage is applied. the surface potential V _D is a linear relationship of the graph on the slope 1 were obtained.

Here, the charging start voltage is defined as follows. That is, when only a DC voltage is applied to the charging member with respect to the image carrier having a potential of 0 and the voltage is gradually increased, a graph of the surface potential of the photosensitive member as the image carrier with respect to the applied DC voltage is obtained. I will write. At this time, the DC potential is set to 1
The measurement is performed every 00V, and the point when the surface potential appears with respect to the surface potential 0 is set as the first point, and 10 points are set every 100V. From these 10 points, a straight line is drawn by the least squares method in statistics, and the value of the applied DC voltage when the surface potential is 0 on this straight line is defined as the charging start voltage. The straight line in the graph of FIG. 16 is created by the above least squares method.

That is, assuming that the DC applied voltage to the charging roller 2 is V _DC , the surface potential obtained on the surface of the OPC photosensitive drum 1 is V _D , and the charging start voltage is V _TH , V _D = V _DC −V _HT ‥ ‥‥ (1)

The above equation (1) can be derived using Paschen's law.

FIG. 17 shows an equivalent circuit in which a microscopic space Z is formed at the contact portion between the charging roller 2 and the OPC photosensitive member layer and between them. When the total resistance _{R r} of the charging roller 2 is small,
Voltage drop I caused by current _ID flowing through photoconductor layer 1a
_{D R r} can be ignored is sufficiently small compared to the V _DC. First, ignoring R _r , the voltage Vg applied to the space Z is represented by the following equation.

Vg = V _DC · Z / (L _S / K _S + Z) ‥‥‥ (2) V _DC : applied voltage Z: void L _S : thickness of photoconductor layer K _S : relative permittivity of photoconductor layer The discharge phenomenon in the gap Z is based on Paschen's law, and when Z = 8 μ or more, the discharge breakdown voltage Vb can be approximated by the following linear equations (3) and (4).

Vb = 312 + 6.2Z (when Vb> 0) ‥‥‥ (3) Vb = − (312 + 6.2Z) (when Vb <0) ‥‥‥ (4) Since Vb <0, (2) When the equation (4) is written on a graph, the graph becomes as shown in FIG. The horizontal axis is the gap distance Z,
The vertical axis indicates the gap breakdown voltage, the downward convex curve indicates the Paschen's curve, the upward convex curve indicates the characteristic of the gap voltage Vg using Z as a parameter.

Discharge occurs when the Paschen's curve and the curve 点 have an intersection, and at the point where the discharge starts, the discriminant of the quadratic equation for Z obtained as Vg = Vb is zero. Since this time is the discharge start limit, _VDC = _VTH .

Although Paschen's law relates to the discharge phenomenon in the air gap, even in the charging process using the charging roller 2, a small amount of ozone is generated in the immediate vicinity of the charging section (10 ^-2 compared to corona discharge). To 10 ^-3 ), and it is considered that the charging by the charging roller is related to the discharge phenomenon. In order to control the V _D by V _DC is _{therefore, V DC = V R + V} TH ‥‥‥ (5) V R: using the target surface potential, V by set the potential target value V _R (5) formula _TH
It is added to seek V _D can be approximated to V _R.

Here, as can be seen from equation (5), the threshold voltage V _TH is determined by D = L _S / K _S ‥‥‥ (6). The rate K _S changes under the influence of the temperature, humidity, and the like around the photoreceptor, and the thickness L _S of the photoreceptor layer changes in a direction decreasing due to durability.

Therefore, the surface potential V
_D changes with the change of the threshold voltage _VTH .
In other words, if the values of K _S and L _S are known, a DC voltage value V _DC for setting the surface potential V _D to an appropriate value can be obtained.

[0054] Here, the capacitance C _P is 19, which is formed by the photosensitive drum 1 and the charging roller 2 (a) · (b)
As shown in the figure, the contact area is formed by the nip n of the contact portion between the two, and when the contact area at the nip portion is S _P , from an equivalent circuit, C _P = S _P × K _S / L _S = S / D ‥‥‥ (7).

That is, C _P ∝1 / D. Therefore, if C _P is determined, an appropriate DC voltage _VDC can be determined by the equation (5).

[0056] In the present embodiment, instead of specifying the C _P of the drum (photosensitive member), a charge transport layer of simplified manner drum (CT
The change in charging characteristics due to the discharge impedance is changed by the thickness of the layer) (the aforementioned L _s) is measured, taking a method for correcting the estimated applied voltage changes in the C _P of the photoreceptor.

FIG. 16A shows the charging roller 2.
The relationship between the voltage applied to and the drum surface potential was measured for each drum CT layer thickness. Similarly, the DC current amount at that time is shown in FIG.

As can be seen from this figure, it can be seen that the charging characteristics, the voltage-current characteristics and the discharge starting voltage change depending on the thickness of the drum CT layer.

FIGS. 20A and 20B show this characteristic as a drum surface potential and a DC current with respect to the drum CT layer thickness when an arbitrary constant voltage is applied. The relationship between the drum surface potential and the DC current according to the CT layer thickness can be read. DC current amount and the drum surface potential (black potential V _D and white potential V _L) as CT layer thickness decreases is seen to rise. In other words, it is understood that capable of estimating a surface potential corresponding to the drum C _P by measuring the DC current amount during specific constant voltage application.

[0060] Figure 21, even if C _P change due drum CT layer thickness change from the above relation, a diagram relating to the correction voltage output of the sensing current amount and the time for controlling the drum surface potential. Correction is made so that the voltage output decreases as the amount of detected current increases.

However, the charging characteristics described above are satisfied when the total resistance Rr of the charging member is sufficiently small.

When the resistance of the charging member is high, a voltage drop occurs with respect to the applied voltage at the charging member, and the potential of the surface of the charging member (the surface facing the member to be charged) becomes a low value, as shown in FIG. The appearance of the graph changes.

The relationship between the surface potential (V _D ) of the charged photoconductor after passing the charging member and a DC voltage (V _DC ) applied to the charging member is described below. Was measured.

The graph of FIG. 22 shows the measurement results. Graph a is a graph in the case where the total resistance of the charging member is sufficiently low. The charging has a charging start voltage α with respect to the applied DC voltage _VDC , starts charging from α, and an absolute value equal to or higher than the charging start voltage. the surface potential V _D obtained photoreceptor for voltage application linear relationship graph on slope 1 were obtained (the α as described above depending on such film thickness of the photosensitive member).

However, if the resistance of the charging member is high,
As shown in the graph b, the charging start voltage β
Shifts to the high voltage side, charging starts from β, and when a voltage having an absolute value equal to or higher than the charging start voltage is applied, the surface potential V _D obtained on the photoconductor is a value smaller than the slope 1 on the graph. A linear relationship was obtained (the slope was sleeping).

Of course, α and β vary depending on the environment to be used, the charging member, the prescription of the photoreceptor, and the accompanying physical properties.

The surface potential of the photoreceptor can be expressed by the current flowing through the photoreceptor since the surface potential is proportional to the current value.

FIG. 23 shows the relationship between the resistance value of the charging member and the current value flowing through the photosensitive member. Naturally, the values of a and b vary depending on the prescription of the charging member and the photoreceptor, the film thickness, and the physical properties associated therewith. According to FIG. 23, if the resistance of the charging member becomes larger than the value of a, a voltage drop occurs at the charging member even with the same applied voltage, and the voltage required to apply a predetermined potential to the photoconductor is not applied, and the potential is secured. Failure to do so results in poor charging, resulting in poor image quality.

However, if the resistance value is equal to or less than a, a predetermined potential can be secured on the surface of the photoreceptor with a constant applied voltage at any resistance value.

This means that if the resistance of the charging member is smaller than a, the current flowing through the photoreceptor is less affected by the charging member as determined by the photoreceptor. This means that the current flowing through the photoconductor is determined by the member. That is, if the resistance of the charging member is sufficiently lower than the value of a, the surface potential of the photoconductor can be secured.

However, the lower limit of the resistance value is determined by factors that prevent poor charging due to the concentration of current when a pinhole or the like occurs in the photosensitive member.

Therefore, the resistance of the charging member cannot be reduced to a sufficiently low value. FIG. 24 shows a graph of the applied voltage with respect to the resistance value of the charging member to keep the surface potential constant.

When the resistance of the charging member rises, a voltage drop occurs in the charging member, and it is understood that a constant surface potential cannot be maintained unless the applied voltage is increased accordingly.

As described above, the voltage applied to the charging member is reduced with respect to the decrease in the thickness of the photosensitive member due to the increase in the number of times of image formation. When the resistance is high due to deterioration due to energization and becomes larger than the value of a, contradictory voltage control is required, in which the voltage applied to the charging member must be increased to secure the surface potential of the photoconductor. is there.

As described above, in order to obtain a stable image over a long period of time, it is necessary to detect the thickness of the photosensitive member and the resistance value of the charging member. Accordingly, the present invention relates to an image forming apparatus of this type, in which the number of image formations is increased and the thickness of the charged body (photoreceptor) is reduced, and the capacity of the charged body is changed. It is an object of the present invention to make it possible to apply the optimum correction application voltage at that time to the charging member even if the occurrence of the occurrence occurs, so that there is no shortage of charging and sufficient image density and image quality can always be maintained. I do.

[0076]

SUMMARY OF THE INVENTION The present invention is an image forming apparatus having the following configuration.

(1) An image forming apparatus for performing image formation by applying an image forming process including a step of charging a surface of a member to be charged, wherein the charging means for the member to be charged includes:
A contact-type charging device that charges a charging member surface by bringing a charging member to which a voltage is applied into contact with a charging member, and has a separate member that can contact the charging member separately from the charging member, When the charging member corresponds to the non-image forming area of the member to be charged, the charging member and the another member are controlled by a DC constant voltage, and the amount of DC current by both members at that time is detected, and the DC current of both members is detected. The difference in the flow rate is detected, and when the charging member corresponds to the image forming area of the member to be charged, the charging member is controlled at a constant DC voltage with a DC voltage corresponding to the DC current amount of the detected difference. An image forming apparatus comprising:

(2) Only when the fixing roller temperature of the fixing device is lower than the specific temperature and the image forming apparatus is ready for operation, the charging is performed when the charging member corresponds to the non-image forming area of the member to be charged. DC constant voltage control of the member and the separate member,
The DC current amount of the two members at that time is detected, and the difference between the DC current amounts of the two members is detected. When the charging member corresponds to the image forming area of the member to be charged, the difference between the detected DC current amounts is calculated. The image forming apparatus according to (1), wherein the charging member is controlled at a constant DC voltage with a corresponding DC voltage.

(3) A member other than the charging member to be brought into contact with the member to be charged can be separated from the member to be charged when the charging member corresponds to the image forming area of the member to be charged. The image forming apparatus according to (1) or (2), wherein

[0080]

FIG. 1 is a conceptual diagram of the present invention.

The member to be charged 1 is a rotating drum type photosensitive member in an electrophotographic apparatus, and is a conductive base layer 1 made of aluminum or the like.
b and the photoconductive layer 1a formed on the outer peripheral surface thereof are used as basic constituent layers.

The charging member 2 is a roller type (charging roller)
And a conductive metal layer 2b formed on the outer periphery of the core metal 2c.
And a resistance layer 2a further formed on the outer periphery thereof.

Reference numeral 100 denotes another member which is in contact with the photosensitive member 1 as a member to be charged separately from the charging roller 2 as the charging member. Reference numeral 110 denotes a unit such as a solenoid for bringing the separate member 100 into and out of contact with the photoconductor 1, and is controlled by a predetermined sequence of a control system so that the separate member 100 comes into contact with the photoconductor 1 in a predetermined manner. , And the state is separated and kept in a non-contact state.

The separate member 100 may be completely the same as the charging member 2 or may be another member. In this example, a member completely equivalent to the charging roller 2 as a charging member was used as the separate member 100. At the time of current detection described later, the charging member 2
The photosensitive member 1 is brought into contact with the photosensitive member 1 closer to the charging member 2 on the downstream side in the rotation direction of the photosensitive member.

In this embodiment, the charging member 2 and the separate member 1 are used.
Although the bias application power supply 3 is provided for each of the power supplies 00, the same power supply may be connected by a relay to apply the voltage in a switching manner.

First, a constant DC voltage is applied to the charging member 2 for a detection current, and the DC current Ic at that time is detected.

Next, another member 100 contacting the photosensitive member 1
A DC voltage similar to that described above is applied, and the DC current amount Is at that time is detected.

The order of current detection by the charging member 2 and the separate member 100 may be either one.

The resistance of the separate member 100 operates only in the morning compared to the charging member 2, that is, the energization time is very short compared to the charging member 2. Since the value is maintained at a value (not more than a and not less than c), the DC current amount shown in FIG.

On the other hand, if the resistance of the charging member 2 is sufficiently low, a DC current equivalent to that of another member 100 flows through the charging member 2. However, if the resistance becomes higher due to deterioration of energization or the like, a voltage drop occurs in the charging member 2 as shown in FIG. 22, and it becomes difficult for DC current to flow.

That is, the amount of DC current at the time of the separate member 100 detects the film thickness of the photoconductor 1, and the difference between the amount of current between the separate member 100 and the charging member detects the resistance value of the charging member. .

If the difference between the current amounts of the charging member 2 and the separate member 100 is 0, the resistance value of the charging member 2 is sufficiently low, and therefore the film thickness of the photosensitive member 1 is reduced as shown in FIG. The applied voltage may be determined according to the corresponding DC current amount.

However, when there is a difference between the DC current amounts of the two 2.100, the charging member 2 is not applied at the applied voltage shown in FIG.
As a result, a voltage drop occurs, and a sufficient potential cannot be obtained on the photoconductor 1.

Therefore, as shown in FIG. 2, by applying a value obtained by adding a correction voltage corresponding to the difference in the amount of current to an applied voltage corresponding to the film thickness of the photoconductor shown in FIG. 21, The surface potential is independent of the thickness of the photoconductor and the resistance of the charging member.
It is kept well.

The DC voltage applied to the charging member 2 used for detecting the amount of DC current to the charging member 2 and the separate member 100 must be larger than the value of α shown in FIG. Preferably, the value is larger than the value of β.

This is because, when a voltage lower than the value of α is applied, no current flows to the member 1 to be charged, and the resistance of the charging member 2 and the thickness of the photosensitive member 1 cannot be detected. is there.

It is preferable that the separate member 100 be in a non-contact state in the image forming area.
This is because if the separate member 100 is kept in contact with the member 1 to be charged, the resistance value of the separate member 100 changes due to dirt or the like.

It is also effective to make the detection timing almost once a day, only in the morning, for stabilizing the image density.

For example, if the power of the image forming apparatus is turned off even for a short time to clear a paper jam, the current is detected again when the power is turned on again, and the correction voltage is updated. That is, the correction voltage may differ depending on the detection accuracy of the detection current before and after the power is turned off.

If the correction voltage is slightly different in a short time, the user of the apparatus has a considerable sense of discomfort, and the density adjustment value must be reset when forming an image.

On the other hand, in order to improve the operation performance of the image forming apparatus, the constant voltage application, the current detection, and the correction constant voltage control are performed only when the apparatus is started up in a usable state. The corrected constant voltage is maintained.

Further, by performing the detection only in the morning, the number of times that the separate member 100 for current detection is used is considerably reduced as compared with the charging member. Can be accurately detected.

As a method of judging the first in the morning, the one that was effective in the results of the practical test is that the temperature detected by the fixing roller 16a of the fixing device 16 (FIG. 14) is lower than the specific temperature when the power of the image forming apparatus is turned on. This is the first method in the morning. Here, it was most effective to set the specific temperature between 30 ° C. and 130 ° C., especially about 100 ° C.

The shape of the separate current detecting member 100 does not need to be the same as that of the charging member 2, and may be the same as that of the charging member shown in the conventional example.

Further, it is not necessary to contact the photosensitive member 1 as the member to be charged over the entire area in the longitudinal direction. However, in this case, the detected DC current has a different discharge area because the discharge area of the charging member 2 is different from that of the separate member 100.
Therefore, the amount of current must be corrected using a correction coefficient that keeps the discharge area constant, and a comparison must be made.

Further, the separate member 100 does not need to have the same material composition as the charging member 2, but may be in the range of the resistance value (not more than a and not less than c) described above.

As a method of performing the optimum exposure using the optimum correction lamp voltage, since the sensitivity of the photosensitive member corresponds to the film thickness, the film thickness of the photosensitive member 1 can be detected from the DC current amount by the separate member 100 during non-image formation. The image exposure lamp voltage is corrected by applying voltage correction at the time of image formation application voltage according to the current amount (FIG. 25).

According to this, as the thickness of the photoreceptor decreases, the amount of detection current when the non-image portion constant voltage is applied increases, and the voltage applied to the image portion is corrected by voltage reduction and ramp voltage increase according to the increase. In order to add the exposure amount increase correction, the charging process and the image formation in the optimum state are always performed. FIG. 26 shows the above sequence diagram.

[0109]

【Example】

<Example 1> (FIGS. 1 to 8) a) is a charging member 2 charging roller, as shown the layer structure model in Figure 1, on a core metal 2c, 10 ⁴ ~10 ⁵ Ωc such as EPDM
m provided with a conductive rubber layer 2b and a resistance layer 2a provided thereon.

[0110] resistance layer 2a, first, ^{107 to 109} in the order of Ωcm to form a resistance layer 2a ₂ made of such hydrin, further Toresin thereon (trade name; Teikoku Chemical Co.)
The blocking layer 2a ₁ of 10 ⁷ to 10 ¹⁰ [Omega] cm made of nylon-based material equal to a two-layer structure layer provided as a surface layer.

The hardness of the charging roller 2 is 50 ° to 70 ° as measured by Asker-C.

The charging roller 2 is applied to the photosensitive member 1 at a total pressure of 16
Then, the photosensitive member 1 is rotated by the rotation of the photosensitive member by applying a voltage, and the photosensitive member 1 is charged by applying a voltage.

B) Separate member 100 The same member as the charging roller 2 described above.

This separate member 100 was disposed close to the charging roller 2 and in contact with the photosensitive member 1 similarly to the charging roller 2 on the downstream side of the charging roller 2 in the rotation direction of the photoconductor.

C) Photoconductor 1 A photoconductor of a copying machine NP-2020 manufactured by Canon Inc.

FIG. 2 shows the applied voltage for the amount of DC current flowing through the separate member 100 (correction applied voltage for reducing the thickness of the photosensitive member 1), and FIG. 3 shows the correction voltage based on the difference in the amount of current between the charging member 2 and the separate member 100. Indicated. FIG. 4 shows the lamp correction voltage value with respect to the DC current amount flowing through the separate member 100.

Using a machine provided with the above-mentioned correction voltage graph and a control circuit for operating the same, a temperature of 23 ° C. and 5% R
The durability test was performed under two environments of H and an environment of 23 ° C. and 60% RH.

FIG. 5 shows the change in the thickness of the photosensitive member in this durability test, and FIG. 6 shows the change in the resistance of the charging member 2 under the above two environments.

The method of measuring the resistance of the charging member 2 was set as shown in FIG. 7, and the resistance was calculated from the current value when 300 V was applied. 4 is an ammeter and 5 is an arithmetic circuit.

According to the resistance values measured by the above method, the value of a in FIG. 23 was 2 × 10 ⁷ Ω, and the value of c was 8 × 10 ⁴ Ω.

FIG. 8 shows the results of changes in the photosensitive member surface potential and the applied voltage due to the durability in Example 1.

Example 2 (FIG. 9) The same evaluation as in Example 1 was performed except that the control was only applied voltage correction (according to FIG. 2) according to the detection current of the charging member 2.

The results are shown in FIG.

Example 3 (FIG. 10) The same evaluation as in Example 1 was performed except that the control was performed only for the correction of the applied voltage according to the detection current of the separate member 100.

FIG. 10 shows the result.

Example 4 (FIG. 11) The same evaluation as in Example 1 was performed except that the lighting voltage of the lamp was not controlled.

FIG. 11 shows the result.

Example 5 (FIG. 12) The same evaluation as in Example 1 was performed, except that the control was not performed and the durability was performed at the primary applied voltage and the lighting voltage at the initial stage of the durability.

The results are shown in FIG.

[0130]

As described above, as the number of times of image formation increases and the thickness of the charged body (photosensitive body) decreases, the capacity of the charged body changes, and the resistance of the charging member changes due to deterioration of energization. However, the voltage-current characteristic according to the capacity with respect to the thickness of the member to be charged, and the voltage-current with respect to the resistance value of the charging member.
By detecting the current characteristics, it is possible to apply the optimum correction application voltage at that time to the charging member.

As a method for this, a non-image forming member controls a charging member and a member other than the charging member with a DC constant voltage control, and applies a voltage correction to an image forming applied voltage value in accordance with the detected current amount to perform a constant voltage control. do.

According to this, as the thickness of the member to be charged decreases, the amount of detection current when a non-image portion constant voltage is applied by another member increases, and the voltage decrease correction is applied to the image portion application voltage value according to the increase. , Charging processing and image formation in an optimal state are always performed.

When the resistance value of the charging member increases, the amount of current detected when the non-image portion constant voltage is applied by the charging member decreases, and the capacitance of the photosensitive member is changed to the image portion application voltage value according to the decrease. Since a voltage due to an increase in the resistance of the charging member is added to the voltage value corrected for the reduced voltage due to the change, the charging process and image formation in an optimal state are always performed.

By performing the above-mentioned control, sufficient image density and image quality can always be provided without insufficient charging.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of the present invention.

FIG. 2 is a graph showing a relationship between a detected current (amount of DC current flowing through another member 100) and a corrected voltage output value.

FIG. 3 is a corrected voltage graph based on a difference in current amount between the charging member 2 and another member 100.

FIG. 4 is a graph of a lamp correction voltage value with respect to a DC current amount flowing through another member 100;

FIG. 5: Transition of photoconductor thickness due to durability

FIG. 6: Change in resistance of a charging member due to durability

FIG. 7 is a diagram showing a procedure for measuring a resistance value of a charging member.

FIG. 8 is a graph showing the results of Example 1 (changes in photoconductor surface potential and applied voltage due to durability).

FIG. 9 is a graph showing the results of Example 2 (changes in photoconductor surface potential and applied voltage due to durability).

FIG. 10 is a graph showing the results of Example 3 (changes in photoconductor surface potential and applied voltage due to durability).

FIG. 11 is a graph showing the results of Example 4 (changes in photoconductor surface potential and applied voltage due to durability).

FIG. 12 is a graph showing the results of Example 5 (changes in photoconductor surface potential and applied voltage due to durability).

FIG. 13 is a schematic view of an example of a contact charging device.

FIG. 14 is a schematic diagram illustrating an example of an image forming apparatus.

FIGS. 15 (a) and (b) are cross-sectional model views of another embodiment of the charging member, respectively.

16 (a) and (b) are charging characteristic graphs, respectively.

FIG. 17 is an equivalent circuit diagram in which a microscopic space is formed between a photoconductor, a charging roller, and a contact portion between the two.

FIG. 18 is a graph showing a relationship between a gap gap and a gap breakdown voltage.

19A is a diagram illustrating a contact nip portion between a photoconductor and a charging roller, and FIG. 19B is an equivalent circuit diagram.

20 (a) and (b) are graphs showing the charging ability film thickness dependence.

FIG. 21 is a graph showing a relationship between a detection current (a DC current flowing through the charging member 2) and a correction voltage output value.

FIG. 22 is a graph showing the relationship between the voltage applied to the charging member and the surface potential of the charged member.

FIG. 23 is a graph showing the relationship between the resistance value of the charging member and the current to be charged.

FIG. 24 is a graph showing a relationship between a resistance value of the charging member and a DC voltage value applied to the charging member.

25 is a graph of a lamp correction voltage with respect to a DC current amount flowing through the charging member 2. FIG.

FIG. 26 is a sequence diagram.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 To-be-charged body (image carrier, photoreceptor) 2 Charging member (charging roller) 3 Bias application power supply 100 Separate member 110 Contacting / separating means

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-223513 (JP, A) JP-A-5-307287 (JP, A) JP-A-4-9883 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G03G 15/02 G03G 15/06 101 G03G 15/16 103 G03G 21/10

Claims (3)

(57) [Claims] 1. An image forming apparatus for forming an image by applying an image forming process including a step of charging a surface of a member to be charged to a surface of the member to be charged. A contact-type charging device for charging the surface of the member to be charged by bringing the charged member into contact with the member to be charged, further comprising a separate member capable of contacting the member to be charged separately from the charging member; Is a direct current constant voltage control of the charging member and the separate member when corresponding to the non-image forming area of the member to be charged, the amount of DC current by the two members at that time is detected, and the difference between the DC current amounts of the two members is detected. Is detected, and when the charging member corresponds to the image forming area of the member to be charged, the charging member is controlled at a constant DC voltage with a DC voltage corresponding to the DC current amount of the detected difference. Image forming apparatus. 2. Only when the temperature of the fixing roller of the fixing device is equal to or lower than a specific temperature and the image forming apparatus is ready for operation.
When the charging member corresponds to the non-image forming area of the member to be charged, the charging member and the separate member are controlled by a DC constant voltage, and the DC current amounts of both members at that time are detected, and the DC current amounts of both members are detected. Is detected, and when the charging member corresponds to the image forming area of the member to be charged, the charging member is controlled at a constant DC voltage with a DC voltage corresponding to the detected difference in the amount of DC current. The image forming apparatus according to claim 1, wherein: 3. A method according to claim 1, wherein a member other than the charging member to be brought into contact with the member to be charged can contact and separate from the member to be charged when the charging member corresponds to an image forming area of the member to be charged. The image forming apparatus according to claim 1, wherein: