JPH0623899B2 - Image forming device - Google Patents

Description

The present invention relates to a transfer device to which an electrophotographic method or an electrostatic recording method is applied.

Generally, a device that transfers a visible image (hereinafter, a toner image) formed by charged particles formed on an electrostatic latent image carrier (hereinafter, an image carrier) onto a transfer material by an electric field by corona discharge is stable. In order to obtain the above-mentioned transfer efficiency and to transfer without causing image disturbance after the transfer, a sufficient current for transfer (discharge current flowing from the transfer corona discharger to the transfer material) is required.
Then, in order to obtain this sufficient transfer current, the shape of the transfer corona discharger and an appropriate transfer corona discharge voltage are determined according to the distance between the image carrier and the transfer corona discharger.

By the way, in an apparatus that holds the leading edge of the transfer material with a gripper and conveys it, the distance between the transfer corona discharger and the image carrier must be kept such that the gripper can pass between both the discharger and the image carrier. I have to. Therefore, in order to obtain a sufficient transfer current, the transfer corona discharge voltage must be increased.

Moreover, the gripper usually has durability, operability, manufacturing cost,
Metals are often used because of their ease of processing. Therefore, when the gripper comes to the transfer portion, if the transfer corona discharge voltage is large in relation to the distance between the gripper and the discharge wire of the transfer corona discharger, the wire leaks toward the gripper. In practice, the distance between the gripper and the transfer corona discharger discharge wire is sufficiently smaller than the distance between the image carrier and the transfer corona discharger discharge wire.Therefore, the upper limit of the transfer corona discharge voltage is the gripper and the transfer corona discharge wire. It is decided by the relationship of the distance with. Therefore, in a system such as a color copying machine, in which multiple transfer using three color toners is performed by holding a transfer material with a gripper and applying a transfer corona discharge from the back surface of the transfer material, only a transfer current is used for transfer. However, when it is also used to electrostatically attract a transfer material by a dielectric sheet or a mesh screen, the transfer current required for good transfer is
The transfer corona discharge voltage must be increased because it is higher than that of a black-and-white copying machine that transfers only normal black toner having the same image forming speed. Therefore, the risk of leakage to the gripper increases, and a transfer corona discharge voltage at which the corona wire does not leak to the gripper is easily affected by environmental changes such as humidity, and good transfer cannot be performed.

An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional example, to obtain a stable transfer efficiency, and to obtain a transfer without image disturbance after the transfer. Especially, in a multiple transfer system such as a color copying machine,
It is an object of the present invention to provide a transfer device which produces a good multi-transfer image with little color misregistration, no voids (the central portion of an image is not transferred and is lost).

Hereinafter, the present invention will be described in detail with reference to examples.

FIG. 1 is an embodiment of the present invention and is a sectional view showing a transfer portion in a color copying machine. In the figure, the transfer material is fed to the transfer guide 9 by the registration roller 10 at a constant timing and held in the gripper 3 of the transfer drum 2.
Thereafter, the transfer material is pressed by the transfer material pressing plate 7 onto the insulating mesh screen 2a of the transfer drum, and the transfer portion 13
Then, due to corona discharge from the transfer discharger 6, the toner image on the photoconductor 1 is transferred onto the mesh screen 2a by electrostatic attraction. The transfer material is held on the transfer drum 2 to transfer the toner image of the first color, and then the transfer process is repeated twice, and the transfer material is cyan, magenta and magenta three times in total.
Multiple transfer of colors is performed. After the completion of the multi-transfer of the three colors, the transfer material is separated from the transfer drum 2 by the separating claws 12 and, at the same time, the charge is discharged together with the mesh screen 2a by the pair of discharge discharging devices. FIG. 2 shows a perspective view of the transfer drum 2.

The principle of the present invention will be described with reference to FIGS. Third
The figure shows the transfer state. In the transfer state of FIG. 3a, when the corona discharge current for transfer flows to the shield of the transfer discharger 6, the mesh screen and the transfer material, the discharge load of the transfer high voltage becomes maximum. It's time. As an example, as shown in FIG. 6, in a multiple transfer system using a mesh screen having an image forming speed of 150 mm / sec (aperture ratio of 60%), the first color (yellow toner) and the third color (cyan toner) are transferred. In the highly efficient and stable area (transfer efficiency of about 90%), the transfer current is 30 to 40 μA toward the transfer material.
It is when it is flowing. In FIG. 3, the distance between the wire of the transfer corona discharger and the photosensitive member is 16 mm at the shortest. The transfer high voltage required to pass a transfer current of 30 to 40 μA through this distance is about 8 kV. On the other hand, the distance between the lower end of the gripper 3 and the wire of the transfer corona discharger is only 7 mm, and when the transfer high voltage is 8 kV, there is a very high risk of leakage to the gripper 2a having the conductive surface on the opening side of the transfer discharger. . Therefore, it is desirable to lower the transfer high voltage when the gripper 3 approaches, and to increase the high pressure at other places during the transfer.

FIG. 4 shows the voltage applied to the transfer discharger 6 when the transfer high voltage is controlled by a constant current, and points a, b and c in the figure correspond to those in FIG. 3 a, b and c, respectively. is doing. Since the discharge load is maximum at the point a in FIG. 4 showing the transfer process, the applied voltage due to the constant current becomes high. On the other hand, when the state shown by the points b and c is reached, a conductive member (connecting portion 4
And since the gripper 3) approaches, the discharge load becomes small,
As a result, the applied high voltage based on the constant current can be low.

Therefore, in FIG.
The high voltage of kV was applied because the gripper 3 was discharged by the discharger 6
As shown in FIG. 4, the high voltage of the discharger 6 gradually decreases, and finally the applied voltage decreases to about 6 kV. In this way, when the transfer high voltage is controlled by a constant current regardless of the presence or absence of the conductive member (the connecting portion 4 and the gripper 3) in the transfer portion, when the conductive member such as the gripper 3 approaches the transfer discharger 6, the automatic transfer is performed. Since the high voltage applied to the transfer discharger is reduced, the risk of leakage from the transfer discharger to the conductive member such as the gripper is eliminated.

FIG. 5 shows a block diagram of an example of a circuit for constant current and high voltage control of the present invention. The current amplifying circuit of FIG. 5 is a circuit for directly controlling the operation of the primary side oscillating circuit, and the method such as changing the oscillating voltage, the oscillating frequency and the duty ratio of the oscillating wave (PWM modulation) is applied. The differential amplifier circuit generates a voltage from which the current amplifier circuit can output and control the current in a predetermined range. Similarly, a control voltage V is input to the differential amplifier circuit so that the voltage range output from the differential amplifier circuit falls within the above range. On the secondary side of FIG. 5, a positive current is obtained as a high voltage output.
The current is supplied to the discharger 6 via OUT. On the other hand, a current equivalent to this current flows from the ground side toward the resistor R2. By selecting the value of the resistor R2 so that the voltage applied to the resistor R2 can be compared with the control voltage V, the current that is currently flowing is input to the differential amplifier circuit as the voltage Vf. The differential amplifier circuit compares the current setting voltages V and Vf, superimposes the deviation amount on the voltage V as a correction amount, and outputs the control voltage to the differential amplifier circuit. In the high-voltage control circuit unit, the response time t to the current change depends on the circuit.
However, it is generally on the order of 10 ² mmsec. Therefore, if only the gripper 3 suddenly appears on the transfer discharger side from the mesh screen part as the transfer drum 2 rotates, the response of the high-voltage control circuit cannot catch up.
There is a risk of leakage between the discharger and the gripper without lowering the applied high voltage. This is the same as in the case of the constant voltage characteristic because the transfer high-voltage power supply cannot sufficiently exhibit the current characteristic. Therefore, the response of the high-voltage control circuit catches up near the gripper 3, and the connecting portion 4 having a conductive surface on the discharge opening side of a certain length in the rotational direction so that the transfer high-voltage power source works as a constant current power source or a similar effect. It is necessary to provide a member for obtaining Generally, among the response time τ of the high-voltage control circuit, the image forming speed P, and the length l in the transfer direction of the shield member of the transfer discharger, the following relationship is necessary because the existence of the gripper does not cause leakage.

τl / P Further, by setting a constant current, the transfer current flowing to the transfer material side becomes stable against environmental changes and the like, so that good transfer efficiency can always be obtained.

In addition to the multi-transfer system such as a color copying machine, in an image forming apparatus that conveys a transfer material using a gripper, when the image forming speed becomes high, the current required for transfer increases, so that the high voltage of the transfer discharger is increased. Need to raise. At this time, the critical voltage at which leakage occurs in the gripper mainly depends on the distance between the gripper and the transfer corona discharge wire and the transfer high voltage. Therefore, even when the image forming speed becomes high, there is a case where the leak critical voltage in the gripper portion must be exceeded in order to obtain the necessary current for transfer. Even in this case, by controlling the transfer high voltage to a constant current for the above reason, good transfer can be performed without the risk of leakage due to the gripper.

A similar effect is that the transfer corona discharge power supply is provided with limiter means so as not to flow above a certain current value, and the upper limit of the discharge current value is set so that the limiter means works when the gripper part approaches the transfer discharger. Can be obtained by This action is shown in FIGS. The points a, b, and c in FIGS. 7 and 8 showing the voltage applied to the transfer discharger in this transfer step correspond to the states in FIGS. 3 a, b, and c, respectively. Shows the state of fluctuation of the current in each step of the transfer process. On the other hand, FIG. 8 shows the state of the fluctuation of the high voltage of the transfer step corresponding to the case, in which the high voltage power supply is controlled so that the limiter means is activated at 400 μA. The power supply at this time has a constant voltage characteristic. FIG. 9 shows a block diagram of a circuit for controlling the discharger 6 with a constant voltage. The high voltage DC transformer includes a primary coil and a secondary coil, and supplies a positive current as a high voltage output to the discharger 6 via OUT. The current amplification circuit is a circuit for directly controlling the operation of the primary side oscillation circuit of the high voltage DC transformer, and is provided with oscillation voltage control and at least one of oscillation frequency control and oscillation wave duty ratio control (PWM control). .
When the impedance of the discharger decreases, a large amount of current flows in the discharger, and the output voltage of the high voltage DC transformer tries to change. However, the potential corresponding to the output potential of the high-voltage DC transformer is detected, and the differential amplifier circuit compares it with the reference voltage V so that the output potential of the high-voltage DC transformer becomes constant, that is, the discharger is subjected to constant voltage control. Then, the oscillation frequency control or the PWM control circuit is controlled.

Further, in the constant voltage circuit, a large amount of current tries to flow into the discharger when the impedance of the discharger is lowered, but the limiter circuit which is the limiter means prevents the current from flowing beyond a certain current value in the discharger. That is, the high-voltage DC transformer detects the feedback current from the ground so that it does not flow in the discharger at a certain current value or more, and limits the current amplification amount of the current amplification circuit by the limiter circuit so that the feedback current does not exceed the predetermined limit current value. is doing.

In this way, even with a normal constant voltage power supply, by setting the limiter level to the current value flowing in the conductive member before the gripper,
In the vicinity of the gripper part, the same action as a constant current source is shown, and the transfer voltage can be applied with a constant voltage that would leak in the gripper if the transfer high voltage is a normal limiter level without the risk of leakage to the gripper. The current can be increased.

As described above, in the image forming apparatus that conveys the transfer material using the gripper, the transfer high voltage is controlled to a constant current,
By providing a conductive member on the gripper or a member in the vicinity thereof, stable and good transfer efficiency can be obtained without the risk of leakage in the gripper. Further, even if the corona discharge efficiency changes due to changes in the external environment such as humidity, the discharge current is always constant because the discharge current is always constant, so environmental stability can be increased. Also, the same effect as described above can be obtained by setting the limit current by causing the limiter means provided at the transfer high voltage to work when the gripper of the transfer drum approaches.

[Brief description of drawings]

FIG. 1 is a sectional view of a transfer portion in a color copying machine to which the present invention is applicable, FIG. 2 is a perspective view of the transfer drum shown in FIG. 1, and FIGS. 3a to 3c are sectional views of the transfer portion. Figures 4 are graphs showing changes in transfer high voltage according to the transfer process of Figures 3a, 3b, 5c, 5 is a block diagram of the transfer high voltage control system, and 6 is transfer current and transfer. Graph showing efficiency relationship,
FIGS. 7 and 8 are graphs showing changes in transfer current and transfer high voltage according to FIGS. 3A, 3B and 3C, and FIG. 9 is a block diagram of the transfer high voltage control system. In the drawing, 1 is a photoconductor, 2 is a transfer drum, 3 is a gripper, 4 is an opening cylinder connecting portion, and 6 is a transfer discharger.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takao Aoki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) Reference JP-A-57-27286 (JP, A) JP-A-54 -19751 (JP, A)

Claims (2)

[Claims] 1. An image carrier, a transfer charging unit that transfers an image formed on the image carrier to a transfer material at a transfer unit, and a transfer material carrier unit that carries and conveys the transfer material to the transfer unit. In the image forming apparatus provided with the transfer material carrying means, the transfer material carrying means has a dielectric transfer material carrying member carrying the transfer material, and a conductive support member supporting the transfer material carrying member, and the support in the transfer section is provided. An image forming apparatus comprising: a control unit that controls the transfer charging unit with a constant current regardless of the presence or absence of a member. 2. An image carrier, a transfer charging unit that transfers an image formed on the image carrier to a transfer material at a transfer unit, and a transfer material carrier unit that carries and conveys the transfer material to the transfer unit. In the image forming apparatus provided with the transfer material carrying means, the transfer material carrying means has a dielectric transfer material carrying member carrying the transfer material, and a conductive support member supporting the transfer material carrying member, and the support in the transfer section is provided. An image forming apparatus comprising: a control unit that controls the transfer charging unit with a constant voltage regardless of the presence or absence of a member; and a limiter unit that restricts a current of a constant current value or more from flowing through the transfer charging unit. .