JP2007033835A - Electrostatic charge controller and electrostatic charge control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charge controller without generating image quality defect even when the film thickness of a body to be charged varies. <P>SOLUTION: The electrostatic charge controller includes a first charge roll 3 for charging a photoreceptor 2 to target potential, a second charge roll 4 for previously charging the photoreceptor 2 to reverse polarity to the target potential, and a control part 14 for controlling charge potential charged to the reverse polarity on the second charge roll 4 so that direct current made to flow in the first charge roll 3 is a prescribed value or more when being charged by the first charge roll 3. Thus, even when the film thickness of the photoreceptor 2 varies, discharge current is sufficiently secured not to generate the image quality defect. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a charge control device and a charge control method for charging a photosensitive member by applying a DC voltage by a contact or proximity charging method using discharge as a charging principle.

  2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus has advantages such as low ozone and low power. Therefore, a voltage is applied to a contact charging device, that is, a charging member in contact with a charged body, to be charged. An apparatus using a method of charging is in practical use.

  In a contact charging device, a conductive charging roll as a charging member is brought into pressure contact with a member to be charged, and a voltage is applied thereto to charge the member to be charged. Specifically, charging is performed by discharging from the charging member to the member to be charged. Therefore, charging is started by applying a voltage equal to or higher than a certain threshold voltage at which discharge starts to occur between the charging member and the member to be charged to the charging member. Thereafter, the photoreceptor surface potential increases linearly with a slope of 1 with respect to the applied voltage. This threshold voltage is defined as the charging start voltage Vth.

  That is, in order to obtain the photoreceptor surface potential Vd required for electrophotography, it is necessary to apply a DC voltage of Vd + Vth to the charging member. A contact charging method in which only the DC voltage is applied to the contact charging member to charge the object to be charged is referred to as a DC contact charging method.

  However, the surface of the photosensitive member as a member to be charged is scraped by a cleaning blade, a developer, or the like as the number of image formations increases. Therefore, the thickness (film thickness) of the photoreceptor is reduced. The photosensitive member and the charging member in contact therewith have a function equivalent to a capacity. When the film thickness of the photoconductor changes, the surface potential of the photoconductor does not become the same even when the same voltage is applied.

  In Patent Document 1, a DC voltage Vdc is applied to a charging roll brought into contact with or close to the photoconductor, and a decrease in the film thickness of the photoconductor is detected from the amount of DC current flowing through the charging roll at this time. Then, even if the film thickness of the photoconductor is reduced, correction is performed so that the DC voltage is decreased as the DC current amount is increased so that a substantially constant surface potential of the photoconductor is always secured.

Japanese Patent No. 3397339

  However, even if the surface potential of the photosensitive member is controlled to be constant, white spots are generated due to poor charging (non-uniform charging), and a stable and uniform image cannot be obtained. When the film thickness of the photoconductor is increased in order to extend the life of the photoconductor, the generation of white spots becomes noticeable particularly in a low temperature and low humidity environment. This will be described with reference to FIG. FIG. 1A shows the relationship between the surface potential of the photoconductor when the film thickness of the photoconductor is 20 μm and 30 μm and the DC voltage (DC voltage) applied to the charging roll. Also, FIG. 1B shows the relationship between the direct current (DC current) flowing through the charging roll and the direct current voltage (DC voltage) applied to the charging roll when the film thickness of the photoconductor is 20 μm and 30 μm. ing.

  It is assumed that the place where the film thickness of the photoreceptor is usually 20 μm is made 30 μm by increasing the film thickness. As shown in FIG. 1A, Vdc applied to set the target potential when the film thickness is 20 μm is Va, and Vdc applied to set the target potential when the film thickness is 30 μm is Vb. It is. Further, Va <Vb as shown in FIG. This is because the charging start voltage increases as the capacitance value of the photoreceptor decreases due to the increase in thickness.

  As shown in FIG. 1B, when the applied voltage Vdc is Va, the current supplied from the charging roll is Ia, and when the applied voltage Vdc is Vb, the current supplied from the charging roll is Let Ib. As shown in FIG. 1B, Ia> Ib. As is clear from FIG. 1B, the DC current supplied from the charging roll, that is, the discharge current decreases as the thickness of the photosensitive member increases. As a result, the charged state on the surface of the photoreceptor becomes non-uniform and white spots are generated.

  Even if the voltage Vdc applied to the charging roll is set large (for example, Vc in FIG. 1A) in order to increase the discharge current, that is, the amount of direct current supplied from the charging roll, the surface potential itself of the photoreceptor increases. As a result (Vh in FIG. 1A), it cannot be controlled to the target potential, causing image quality degradation.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a charge control device and a charge control method in which image quality defects do not occur even when the film thickness of a charged body varies.

  In order to achieve such an object, the charging control device of the present invention includes a first charging member that charges a member to be charged to a target potential, a second charging member that precharges the member to be charged to a polarity opposite to the target potential, And a control means for controlling a charging potential to be charged with a reverse polarity by the second charging member so that a direct current flowing through the first charging member becomes a predetermined value or more when charged by the first charging member. It is said. Therefore, even if the film thickness of the charged body fluctuates, a sufficient discharge current is ensured and image quality defects do not occur.

  In the charging control device having the above-described configuration, the control unit may change a charging potential to be charged with a reverse polarity by the second charging member according to a film thickness of the object to be charged.

  In the charging control device having the above configuration, the predetermined value may be a minimum current value in a range in which no image quality defect occurs.

  In the charging control device having the above configuration, the second charging member may also serve as a transfer member.

  In the charging control method of the present invention, when the object to be charged is charged to the target potential by the first charging member, the second charging member has a reverse polarity so that the direct current flowing through the first charging member becomes a predetermined value or more. Determining a charging potential to be charged, precharging the object to be charged to a polarity opposite to the target potential so as to be the determined charging potential, and charging the object to be charged to a target potential. Have.

  The present invention does not cause image quality defects even when the film thickness of the member to be charged varies.

  Preferred embodiments of the present invention will be described with reference to the accompanying drawings.

  First, the configuration of the present embodiment will be described with reference to FIG. The photoconductor 2 as an image carrier is a cylindrical OPC photoconductor, and is driven to rotate at a predetermined process speed (circumferential speed) in the clockwise direction indicated by an arrow about a central axis perpendicular to the paper surface.

  Around the photosensitive member 2, a first charging roll 3 and a second charging roll 4 as charging members are disposed so as to be in contact with the photosensitive member 2. The first charging roll 3 rotates following the rotation of the photosensitive member 2, and a predetermined voltage is applied from the first DC power source 12, so that the peripheral surface of the rotating photosensitive member 2 is uniformly set to a predetermined polarity and potential. Charged (negatively charged). Further, the second charging roll 4 rotates following the rotation of the photosensitive member 2, and a predetermined voltage is applied from the second DC power source 13, so that the peripheral surface of the photosensitive member 2 has a polarity opposite to that of the first DC power source 12. In the same way, pre-charge (positive charge).

  Next, an image-modulated laser beam output from a ROS (Raster Optical Scanner) 5 is irradiated (scanning exposure) to the charged surface of the rotating photoconductor 2, and the potential of the exposed portion is attenuated to form an electrostatic latent image. It is formed.

  When the latent image arrives at the development site facing the developing device 6 as the photoconductor 2 rotates, a negatively charged toner is supplied from the developing device 6 and a toner image is formed by reversal development.

  A conductive transfer roll 7 is disposed in pressure contact with the photoconductor 2 on the downstream side of the developing device 6 when viewed in the rotation direction of the photoconductor 2, and a nip portion between the photoconductor 2 and the transfer roll 7 serves as a transfer site. Forming.

  When the toner image formed on the surface of the photoconductor 2 reaches the transfer site as the photoconductor 2 rotates, the paper is supplied to the transfer position at the same time, and a predetermined voltage is applied to the transfer roll 7 at the same time. As a result, the toner image is transferred from the surface of the photoreceptor 2 to the paper.

  The sheet that has received the toner image transfer at the transfer position is conveyed to the fixing device 8 where the toner image is fixed and discharged outside the apparatus.

  On the other hand, the untransferred toner remaining on the surface of the photoreceptor 2 is scraped off by the cleaning blade 9 so that the surface of the photoreceptor 2 is cleaned to prepare for the next image formation.

  Further, as shown in FIG. 2, the present embodiment includes a charging control device 10 having a direct current detection unit 11, a first DC power supply 12, a second DC power supply unit 13, and a control unit 14. The direct current detector 11 measures the direct current flowing from the first DC power source 12 to the first charging roll 3. That is, the discharge current to the photosensitive member 2 is measured by measuring the direct current flowing into the first charging roll 3.

  The control unit 14 controls the charging potential to be charged with the reverse polarity by the second charging roll 4 so that the direct current flowing through the first charging roll 3 becomes a predetermined value or more during charging by the first charging roll 3.

  In this embodiment having the above configuration, by using two charging rolls, white spots are prevented by securing a discharge current of a certain level or more while controlling the surface potential of the photosensitive member to a target voltage by DC charging.

  First, the applied voltage of the first charging roll 3 is set based on the theoretical formula “Vsur = Vdc−Vth” (Vsur: target surface potential, Vdc: applied voltage, Vth: charging start voltage).

  In order to secure a predetermined amount of discharge current that flows when the photosensitive member 2 is charged so as not to generate a white spot, the photosensitive member 2 is precharged to a reverse polarity by the second charging roll 4. This will be described with reference to FIG. FIG. 3A shows the relationship between the photosensitive member surface potential when the film thickness of the photosensitive member 2 is 20 μm and 30 μm and the direct current voltage (DC voltage) applied to the first charging roll 3. FIG. 3B shows a direct current (DC current) flowing through the first charging roll 3 when the film thickness of the photoconductor is 20 μm and 30 μm, and a direct current voltage (DC voltage) applied to the first charging roll 3. The relationship is shown.

By precharging the photosensitive member 2 to the opposite polarity with the second charging roll 4 (-Vch2 shown in FIG. 3A), the charging start voltage (FIG. 3A) at which the photosensitive member 2 starts to be charged is obtained. Vthb ′) shown can be lowered. That is, as shown in FIG. 3A, the charging start voltage Vthb when not charged with a reverse polarity is greater than the charging start voltage Vthb ′ when charged with a reverse polarity. As a result, a sufficient discharge current (Ia shown in FIG. 3B) can be ensured until the photosensitive member 2 is charged to the target potential by the first charging roll 3, and the occurrence of white spots is prevented. Degradation of image quality can be prevented.
Depending on the film thickness of the photoconductor 2, a line indicating the relationship between the applied voltage Vdc of the charging roll 3 shown in FIG. 3A and the surface potential of the photoconductor, and an applied voltage Vdc shown in FIG. A straight line indicating a relationship with a direct current flowing through the first charging roll 3 is determined. When the control unit 14 measures the film thickness of the photoconductor 2, the control unit 14 can determine how much the second charging roll 4 should be charged with the opposite polarity from the measured film thickness. For example, for the photosensitive member having a film thickness of 30 μm shown in FIG. 3B, the reverse polarity necessary to allow the direct current Ia to flow when the DC voltage Vb is applied to the first charging roll 3. The charging potential is determined. These data are measured in advance, for example, and recorded in a nonvolatile memory (not shown) in the control unit 14. Alternatively, the photoconductor may be regarded as the capacity of the film thickness d, and may be calculated from the following approximate expression “Ia = d * (Vsur−Vch2) / k” (Vsur: target surface potential, Vch2: reverse polarity) Voltage, d: film thickness, k: constant).

  In addition, there are various methods for measuring the film thickness of the photoconductor 2. As an example, data indicating the relationship between the saturation current that flows when the surface voltage of the photoconductor 2 is saturated and the photoconductor film in advance. The thickness of the photoconductor can be estimated from the current when the voltage is applied to the photoconductor 2 and the photoconductor 2 is actually saturated.

  The reverse polarity applied voltage from the second charging roll 4 is set so that at least a direct current at which the white spot disappears flows, so that the direct current is independent of any value while always setting the surface sensitive potential to the target potential. To prevent white spots.

The operation procedure of the control unit 14 will be described with reference to the flowchart shown in FIG.
When starting the charging control of the photosensitive member, the control unit 14 first starts measuring the photosensitive member film thickness (step S1). In the measurement of the photoconductor film thickness, for example, as described above, the film thickness of the photoconductor can be estimated from the current value when the surface potential of the photoconductor 2 is saturated.

  Next, in accordance with the measured thickness of the photosensitive member, a potential to be charged with a reverse polarity by the second charging roll 4 is set (step S2). For example, as shown in FIG. 3B, when the film thickness of the photosensitive member is 30 μm, the reverse polarity potential necessary for flowing the target direct current Ia is set. The control unit 14 controls the second DC power supply 13 to apply a voltage having a reverse polarity to the second charging roll 4 so that the surface potential of the photosensitive member 2 becomes a set reverse polarity potential (step S3). Next, the control unit 14 controls the first DC power supply 12 to apply a voltage to the first charging roll 3 so that the surface potential of the photoreceptor 2 becomes a target potential. That is, by applying the DC voltage Vb shown in FIG. 3B, the DC current Ia flows to the first charging roll 3 and a sufficient discharge current can be secured (step S4).

  In this way, in this embodiment, even if the film thickness of the photoconductor varies, a sufficient discharge current can be passed and image quality defects do not occur.

  The embodiment described above is a preferred embodiment of the present invention. However, the present invention is not limited to this, and various modifications can be made without departing from the scope of the present invention. For example, in the above-described embodiment, the second charging roll 4 is provided separately to charge the surface potential of the photosensitive member to the reverse polarity. However, the transfer roller 7 is used to charge the surface potential of the photosensitive member to the reverse polarity. Also good.

It is a figure for demonstrating the conventional charge control method. 1 is a diagram illustrating a configuration of an image forming apparatus. It is a figure for demonstrating the charge control method of this invention. It is a flowchart which shows the control procedure of a control part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Photoconductor 3 1st charging roll 4 2nd charging roll 5 ROS 6 Developing device 7 Transfer roll 8 Fixing device 9 Cleaning blade 10 Charging control device 11 DC current detection part 12 1st DC power supply 13 2nd DC power supply 14 Control Part

Claims (5)

A first charging member that charges a member to be charged to a target potential;
A second charging member for precharging the object to be charged to a polarity opposite to the target potential;
Control means for controlling a charging potential to be charged with a reverse polarity by the second charging member so that a direct current flowing through the first charging member becomes a predetermined value or more when charged by the first charging member. A charge control device characterized by the above.   The charging control apparatus according to claim 1, wherein the control unit changes a charging potential to be charged with a reverse polarity by the second charging member in accordance with a film thickness of the object to be charged.   3. The charging control apparatus according to claim 1, wherein the predetermined value is a minimum current value in a range where no image quality defect occurs.   The charging control apparatus according to claim 1, wherein the second charging member also serves as a transfer member. A step of determining a charging potential to be charged to a reverse polarity by the second charging member so that a direct current flowing through the first charging member becomes equal to or greater than a predetermined value when the object to be charged is charged to a target potential by the first charging member; When,
Precharging the object to be charged to a polarity opposite to the target potential so as to be the determined charging potential;
Charging the object to be charged at a target potential.

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