JP2008233219A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to prevent soiling in a second charging device in outputting an image through the use of one developing device located on the upstream side out of two developing devices, and also, to suppress uneven discharge to the image, consequently, to form the image with stable image density. <P>SOLUTION: The image forming apparatus 30 includes one photoreceptor 1, two charging devices 2 and 5, and two developing devices 4 and 7, wherein a two-color image is formed by a transfer device 9 that transfers a toner image on an image carrier to a transfer material while letting the transfer material pass through and applying an electrical field. Even in the case of forming only the first toner image, a voltage is applied on a charging wire 8 of the second charging device 2, and the absolute value of the apply voltage on the charging wire of the second charging device is set equal to or more than the surface potential of a non-image part. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus for an electrostatic copying process, such as a copying machine, a facsimile machine, and a printer.

  Conventionally, in an image forming apparatus using an electrophotographic process such as a copying machine, a printer, and a facsimile, the surface of a photosensitive drum is often uniformly charged by a charging device using a corona discharge method such as a scorotron or a corotron. . Thereafter, a latent image portion is formed by an exposure device, an image is formed by a developing device, and an image is formed by a transfer and fixing device. However, due to the use over time, the charging wire in the charging device may be contaminated with toner, paper powder, etc. along with the flow of airflow in the apparatus. Thus, if the charging wire is contaminated, the drum cannot be uniformly charged, streaks are generated on the image, and the life of the charging device is also reduced. In Patent Document 1 and Patent Document 2, the attachment of contaminants to the charging wire is reduced by providing an airflow suction port or exhaust port in the case portion of the charging device or by providing an adsorption filter.

  In Patent Document 1, etc., contamination of the charging wire is suppressed by changing the flow of the airflow. For example, in a two-color image forming apparatus using a one-drum method, an image is obtained only by a developing machine located on the upstream side. In the single color mode for forming the toner, when the developed toner image passes through the downstream charging device, a potential difference may occur, and the toner may fly to the downstream charging device. When the potential difference occurs in this way, toner adhesion to the charging wire cannot be suppressed even if the flow of the airflow is changed. For this reason, when an image is formed in the single color mode, it is necessary to apply a voltage to the charging wire on the downstream side to ensure a potential difference that prevents toner from flying from the toner image to the charging wire.

JP 11-052685 A JP 2001-166651 A

  Accordingly, the present invention has been made in view of the above-described problems, and its problem is to prevent the second charging device from being soiled when an image is output using one of the upstream sides of the two developing devices. In addition, the present invention provides an image forming apparatus capable of suppressing discharge unevenness on an image and performing image formation with a stable image density.

The features of the present invention, which is a means for solving the above problems, are listed below.
The image forming apparatus of the present invention includes an image carrier, a first charging device for charging the image carrier, and a first latent image for exposing the image carrier to form an electrostatic latent image. An image forming means; a first developing device for developing the electrostatic latent image on the image carrier to form a first toner image; and further, charging the image carrier to form a second toner image. A second charging device, a second latent image forming means for exposing the image carrier to form an electrostatic latent image, and developing the electrostatic latent image on the image carrier to produce a second In a second developing device that forms a toner image and an image forming device that forms a two-color image with a transfer device that passes a transfer material and applies an electric field to transfer the toner image on the image carrier onto the transfer material. Even when only image formation is performed, a voltage is applied to the charging wire of the second charging device.
The image forming apparatus of the present invention further sets the absolute value of the voltage applied to the charging wire of the second charging device to be equal to or higher than the surface potential of the non-image portion.
The image forming apparatus according to the present invention further includes a temperature / humidity sensor, and changes the voltage applied to the charging wire of the second charging device in accordance with the measured environmental conditions.
The image forming apparatus of the present invention further includes an ammeter for measuring the drum current, and controls the voltage applied to the charging wire of the second charging device so that the measured current value becomes 0V. And
The image forming apparatus of the present invention further includes a photoreceptor surface potential detecting unit provided downstream from the first charging device, and the surface potential measured by the photoreceptor surface potential detecting unit is used as the second charging wire. The lower limit value of the applied voltage is used.
In addition, the image forming apparatus of the present invention further includes a temperature / humidity sensor and a photoreceptor surface potential detecting means provided downstream from the first charging device, and according to environmental conditions and a surface potential measured from an electrometer. The second charging wire application voltage is controlled.
The image forming apparatus of the present invention is further characterized in that the position where the photoconductor surface potential is detected by the photoconductor surface potential detecting means is immediately before the second charging device.

  As described above, in the image forming apparatus according to the present invention, when an image is formed by only one drum system using two charging devices, a charging device located upstream of the two developing devices, and the developing device, the image passes. Since the bias is applied to the charging wire on the downstream side, the toner adhesion from the toner image on the drum to the charging wire is suppressed and the charging device is not contaminated. Therefore, image deterioration due to charging failure can be suppressed. .

  The best mode for carrying out the present invention will be described below with reference to the drawings. Note that it is easy for a person skilled in the art to make other embodiments by changing or correcting the present invention within the scope of the claims, and these changes and modifications are included in the scope of the claims. The following description is an example of the best mode of the present invention, and does not limit the scope of the claims.

FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. As shown in FIG. 1, this image forming apparatus 30 forms a two-color digital image of red and black by reversal development. A first development is formed around the photoreceptor 1 on which an electrostatic latent image is formed. A first charging device 2 that uniformly charges the surface of the photoconductor, a first exposure device 3 that exposes an original and forms an electrostatic latent image for developing a first toner on the photoconductor, A first developing device 4 for developing the latent image with the first toner and forming a toner image is provided.
Further, in preparation for the second development, a second charging device 5, a second exposure device 6, and a second developing device 7 are provided for forming a toner image with the second toner as in the first development. .
Further, a bias is applied by a transfer voltage applying device 14 for supplying a recording paper for transferring a recording paper for transferring a toner image formed on the photosensitive member 1 to give a charge opposite to the toner charge, and from the transfer belt. A transfer device 9 for transferring a toner image on the photoconductor to a recording paper by passing a transfer current to the photoconductor, a separation claw 10a for preventing the transfer paper, which has become poorly separated at the time of transfer, from being caught in a drum, a toner image transfer A cleaning device 11 that removes toner remaining on the photoreceptor later and an erasing light source 12 that is a charge eliminating device that removes the charge on the surface of the photoreceptor in preparation for the next image formation are sequentially arranged.
The separation claw 10a is rotatably attached to a shaft 10b supported by the casing, and is in contact with the surface of the photoreceptor 1 with a predetermined pressure by the elastic force of a spring (not shown).

The first developing device 4 in the image forming apparatus 30 uses magnetic two-component development including toner and carrier. A DC bias is applied to the first developing roller, and the toner is transferred to the photosensitive member 1 by a magnet brush. A magnetic brush DC bias developing method is used in which contact is made for development.
The second developing device 7 uses a non-magnetic one-component developer using a color pigment, and the toner layer formed on the second developing roller maintains a predetermined distance from the peripheral surface of the photoreceptor 1. So that it is supported. Development is performed by a jumping method in which a DC bias is applied to the second developing roller and toner is caused to fly to the electrostatic latent image portion of the photoreceptor 1. In this manner, the development by the non-contact development method in the second developing device 7 is advantageous for reasons such as the life of the photoconductor and the developing roller and the superposition development being possible.

When the image forming apparatus having the above-described configuration is used to form red and black images, the surface of the photoreceptor 1 is uniformly charged to about −700 V from the first charging application device 20 by the first charging device 2, Due to the intensity of light emitted from one exposure device 3, the charge on the photoreceptor 1 is erased to about −50 V, and an electrostatic latent image is formed on the surface of the photoreceptor 1. The electrostatic latent image formed on the photosensitive member 1 moves to a position facing the first developing device 4 by the rotation of the photosensitive member 1. A DC bias of −550 V is applied to the first developing roller at the time of image formation, and contact development with a black developer using a two-component mag brush is performed while moving in the circumferential direction at a linear velocity ratio of about 2.0. Done.
After development by the first developing device 4, the photosensitive member 1 is sequentially conveyed to a position facing the second charging device 5 and the second exposure device 6 by rotation of the photosensitive member 1. After the image is formed, it moves to a portion facing the second developing device 7. The toner in the second developing device flies by the developing bias voltage applied to the second developing roller and adheres to the surface of the photoreceptor 1.
At this time, the bias applied to the second developing roller is a DC bias, whereby the second development can be performed without disturbing the toner image by the first development, and the first development and the second development. The two red toner images are superimposed to form a two-color toner image.

Then, the toner image formed in this manner is transferred onto the transfer paper that is conveyed from the paper supply tray (not shown) by the registration rollers through the guide plate at a predetermined timing. Transfer is performed by the action of the applying device 14.
The transfer device 9 includes a transfer belt, and a driving roller 17, a driven roller 15, and a bias roller 16 inside the belt. The bias roller 16 is connected to the transfer voltage applying device 14 and is controlled so that the current flowing to the photosensitive member 1 side is constant. The transfer paper onto which the toner image has been transferred is separated in the tangential direction of the photoconductor by the weight of the transfer paper (if not separated, it is separated by the separation claw 10a). Thereafter, the image is conveyed by a transfer belt and fixed by a fixing unit (not shown) to form a two-color image. After the transfer, the residual toner on the surface of the photosensitive member 1 is removed by the cleaning device 11, the residual charge is erased by the static eliminating device 12, and the initial state is restored.

  FIG. 2 is a diagram showing processes from charging, exposure to development in the black color of the image forming apparatus. As in the case of forming a two-color image, first, the surface of the photoconductor is uniformly charged to about −700 V by the first charging device 1, and the charge of the image portion on the photoconductor 1 is charged by the first exposure device 3. Erase to about -50V. Thereafter, the first developing device 4 develops the toner image 9. Further, since the toner charge amount on the toner image is as small as −20 to −40 μC / g, the surface potential on the image area photoreceptor after development is almost unchanged from −50V. While maintaining the surface potential of the toner image at about −50V, the toner image passes over the second charging device 5, the second exposure device 6, and the second developing device 7 provided downstream. However, when the toner image passes over the second charging device, if no bias is applied to the second charging wire 8 with respect to the surface potential of the toner image of −50 V, the negative toner flies to the second charging wire. And may get dirty.

  In FIG. 1, the image signal read from the document by the document reading unit 23 is stored in the image memory control unit 22. At this time, it is determined whether the mode is set by the user on the operation unit screen, or the mode is black single color, red single color, or red-black two-color mode. In this image forming apparatus, when it is determined that the black monochrome mode is performed by the first charging device 2, the exposure device 3, and the developing device 4 positioned on the most upstream side, the second charging is performed by a signal from the controller 25 immediately after the black toner image is formed. A bias is applied to the second charging wire 8 in the apparatus 5. As a result, when the toner image passes through the second charging device 5, a potential difference that does not cause the toner to fly from the toner image to the second charging wire 8 is maintained, toner adhesion to the second charging wire 8 is suppressed, and the charging device is not contaminated. Therefore, it is possible to suppress image deterioration due to charging failure.

  FIG. 3 is a diagram illustrating the behavior of the toner when the black toner color image passes over the second charge. FIG. 4 is a diagram illustrating an example of an image to be formed. Here, for example, an image as shown in FIG. 4 is input. In this case, after the first charging device 2 is charged to about −700 V, the image portion is exposed to about −50 V by the first exposure device 3. Thereafter, a toner image is formed with negative polarity toner in the first developing device 4. When the toner image passes over the second charging device 5 and no voltage is applied to the second charging wire 8, the toner in the toner image is drawn toward the second charging wire 8 (see FIG. 3). reference). In this case, the toner adheres mostly from the toner image portion, but there is a considerable amount of background toner also in the non-image portion, and the toner adheres to the second charging wire 8 also from the non-image portion. For this reason, it is necessary to set the voltage (hereinafter simply referred to as “second charging wire applied voltage”) to be applied to the second charging wire 8 in the case of black monochrome to be equal to or higher than the surface potential of the non-image area.

FIG. 5 is a graph showing the relationship between the second charging wire applied voltage and the stripe rank.
From FIG. 5, the discharge is not generated up to the applied voltage value of −2.0 KV regardless of the environment, and therefore the streak rank is 3.5 or more. Less than 5. Further, when the applied voltage is increased, the discharge light is stabilized, so that no streak due to the discharge occurs. However, when the discharge becomes stronger as the discharge becomes stable, the toner charge amount of the toner image changes, and the problem that the ID decreases due to the decrease in transfer efficiency is also solved. Accordingly, since the surface potential of the background portion is controlled at −700 V, the black charging mode is controlled by controlling the second charging wire applied voltage between −0.7 KV to −2.0 KV in the black monochrome mode. When the toner image passes through the second charging device 5, it is possible to prevent toner adhesion from the background portion or the toner image to the second charging wire. As a result, when the image formed on the upstream side passes, the applied voltage to the downstream second charging wire 8 is set in a range that does not discharge, so that an abnormal image such as discharge unevenness is generated. Therefore, toner adhesion to the second charging wire 8 can be prevented, and the image can be kept good over time.

  FIG. 6 is a graph showing the relationship between the second charging wire applied voltage and the drum current value. FIG. 6 shows that when the voltage applied to the second charging wire is increased, discharge occurs and the value of the current flowing through the photoreceptor 1 increases. Further, the resistance value of the charging wire varies depending on the temperature and humidity, and the resistance decreases in a high temperature and high humidity environment, and the resistance increases in a low temperature and low humidity environment. That is, even if the same voltage is applied depending on the environment, the discharge level is different and the current flowing through the photoreceptor 1 is also different. When a large amount of current flows through the photosensitive member 1, the toner image is disturbed by the discharge, so that streaks are generated. The discharge start voltage is -2.0 KV in a high temperature and high humidity environment, and -3.5 KV in a low temperature and low humidity environment. Further, in order to sufficiently prevent the toner from adhering to the second charging wire 8 in the case of black monochromatic color, it is more difficult to adhere the toner image potential and the potential difference between the charging wires, and the bias applied to the second charging wire 8 is reduced. Should be larger. For this reason, the temperature and humidity are detected by the environment sensor 28 provided inside the machine, and the applied voltage value is set to the discharge start limit voltage value according to the environment. The temperature / humidity detection is performed before entering a normal print sequence, for example, at the start-up after the main unit power is turned on, after a certain operation stop, or after a continuous operation. As described above, by controlling the application of the optimum voltage value to the second charging wire 8 according to the environment, toner adhesion can be more effectively prevented. Thus, when the image formed on the upstream side passes, the voltage applied to the second charging wire 8 on the downstream side is controlled with an optimum value according to the temperature and humidity, so that the second charging wire is more optimal. The toner can be prevented from adhering to 8, and the image can be kept good over time.

  First, the mode of the output image is determined. In the black monochrome mode, before starting the copying operation, a preset voltage value is applied to the second charging wire 8 in the second charging device 5 of FIG. Is measured with an ammeter 26. When the measured value is not 0 V, control to lower the applied voltage of the second charging wire 8 is performed by a signal from the controller 25. The measurement is repeated until the measured current value reaches 0V, and the normal black monochrome mode copy operation is started with the applied voltage value at 0V. In this way, by using the second charging wire applied voltage value such that the drum current value becomes 0 V in the case of black single color, toner adhesion to the second charging wire 8 can be suppressed, and the influence of discharge on the image can also be suppressed. it can. When the image formed on the upstream side passes, the maximum voltage at which the drum current does not flow is applied to the second charging wire 8 on the downstream side, so that the potential difference between the second charging wire 8 and the surface potential is maximized. Toner adhesion can be prevented, and the image can be kept good over time.

  The image forming apparatus 30 is charged to about −700 V by the first charging device 2, but actually, the resistance of the charging wire changes in the environment, and the surface potential on the photoreceptor 1 also changes accordingly. Further, when the image forming apparatus 30 has been used for a long period of time, the photoreceptor 1 becomes fatigued over time, and it becomes difficult to hold charges, so the surface potential on the photoreceptor 1 is lowered. For this reason, the potential difference between the surface potential on the second charging wire 8 and the photoconductor 1 also changes, and control in accordance with the surface potential on the photoconductor 1 is required to optimally suppress toner adhesion. First, the mode of the output image is determined, and in the case of the black monochrome mode, the photosensitive member 1 is rotated and the first charging device 2 is turned on from the controller before starting the normal copy operation. Thereafter, the surface potential on the photosensitive member 1 is measured by a surface potential meter 27 provided downstream of the first charging device 2. The measured surface potential is stored in the memory unit in the controller 25, and is then used as the lower limit value of the voltage applied to the second charging wire 8 during normal copying in the case of black single color. By providing the surface potential thus measured at the lower limit value, it is possible to optimally prevent the dirt toner from adhering to the second charging wire 8. When the image formed on the upstream side passes, the toner image on the photoreceptor 1 is set so that the lower limit of the voltage applied to the second charging wire 8 on the downstream side is the same as the measured background portion charging potential. In addition to the toner adhesion from the toner, the toner adhesion due to the scumming toner can be suppressed more optimally.

  From FIG. 6, even when the same voltage is applied according to the environment, the discharge level is different, and the current flowing through the photoreceptor 1 is also different. For this reason, the value of the voltage applied to the second charging wire is changed according to the environment to prevent the toner from adhering to the charging wire. Since it is mainly the discharge start voltage that changes in the environment, only the upper limit value of the second charging wire applied voltage is determined. As for the lower limit value, the surface potential measured by the surface potential meter 27 provided in the image forming apparatus 30 is set as the lower limit value of the second charging applied voltage. That is, the absolute value of the applied voltage is set to be equal to or higher than the surface potential of the non-image part. By controlling in this way, a more optimal value of the applied voltage of the second charging wire can be obtained by taking into consideration the discharge start voltage from the second charging wire 8 due to environmental changes and the drum surface potential change with time of use and over time. Can prevent toner adhesion. When the image formed on the upstream side passes, the upper limit value of the voltage applied to the second charging wire 8 on the downstream side is set to an optimum value according to temperature and humidity, and the lower limit value is measured. Since the potential is the same as that of the electric potential, toner adhesion from the toner image on the photosensitive member 1 and toner adhesion due to background toner can be suppressed.

  In general, the surface potential of the charged photoreceptor 1 is attenuated with time, so that the measured value varies depending on the time from charging until the surface potential is measured. As a result, it is desirable to measure the surface potential of the photosensitive member immediately before the second charging device 5 as much as possible with respect to toner adhesion to the second charging wire 8. By attaching the surface potential measurement position immediately before the second charging device, toner adhesion can be more optimally suppressed. Since the position of the surface potential meter is disposed immediately before the second charging device 5, an accurate surface potential value can be measured, and adhesion due to dirt toner can be suppressed more effectively.

1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a diagram illustrating a process from charging, exposure, and development when the image forming apparatus is in a single black color. It is a figure which shows the behavior of the toner at the time of the toner image at the time of black monochromatic passing on the 2nd charge. It is a figure which shows the example of the image formed. It is the graph which showed the relationship between a 2nd charging wire applied voltage and a stripe rank. It is a graph which shows the relationship between a 2nd charging wire application voltage and a drum electric current value.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photoconductor 2 1st charging device 3 1st exposure device 4 1st developing device 5 2nd charging device 6 2nd exposure device 7 2nd developing device 8 charging wire 9 transfer device 11 cleaning device 12 static elimination device 30 image forming apparatus

Claims (7)

One image carrier,
A first charging device for charging the image carrier;
First latent image forming means for exposing the image carrier to form an electrostatic latent image;
A first developing device that develops the electrostatic latent image on the image carrier into a first toner image; and
A second charging device for charging the image carrier to form a second toner image;
Second latent image forming means for exposing the image carrier to form an electrostatic latent image;
A second developing device that develops the electrostatic latent image on the image carrier into a second toner image;
In an image forming apparatus that forms a two-color image with a transfer device that passes a transfer material and applies an electric field to transfer the toner image on the image carrier onto the transfer material.
An image forming apparatus, wherein voltage is applied to a charging wire of a second charging device even when only the first toner image is formed. The image forming apparatus according to claim 1,
An image forming apparatus, wherein an absolute value of a voltage applied to a charging wire of the second charging device is set to be equal to or higher than a surface potential of a non-image portion. The image forming apparatus according to claim 1,
The image forming apparatus includes a temperature and humidity sensor,
An image forming apparatus, wherein the voltage applied to the charging wire of the second charging device is changed according to the measured environmental conditions. The image forming apparatus according to claim 1,
The image forming apparatus includes an ammeter that measures a drum current,
An image forming apparatus, wherein the voltage applied to the charging wire of the second charging device is controlled so that the measured current value becomes 0V. The image forming apparatus according to claim 1,
The image forming apparatus includes a photoreceptor surface potential detection unit provided downstream from the first charging device,
An image forming apparatus, wherein the surface potential measured by the photoreceptor surface potential detecting means is set as a lower limit value of the second charging wire applied voltage. The image forming apparatus according to claim 1,
The image forming apparatus has a temperature / humidity sensor and a photoreceptor surface potential detecting means provided downstream of the first charging device, and the second charging wire applied voltage according to the environmental conditions and the surface potential measured from an electrometer. An image forming apparatus characterized by controlling the above. The image forming apparatus according to claim 5 or 6,
In the image forming apparatus, the position where the photoconductor surface potential is detected by the photoconductor surface potential detecting unit is set immediately before the second charging device.