JP2001092197A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the adhesion of a magnetic carrier to a photosensitive drum in a developing section when starting image forming operation or when ending the image forming operation with an image forming device having a developing device using a two-component developer containing the magnetic carrier. SOLUTION: When the photosensitive drum 1 is electrostatically charged to a prescribed surface potential by an electrostatic charging roller 2 at the start of the image forming operation, the developing bias to be impressed on a developing sleeve 21 is continuously or stepwise increased as the absolute value in a prescribed polarity cooperatively with the continuous or stepwise increase as the absolute value in the prescribed polarity in such a manner that the surface potential V1 of the photosensitive drum 1 and the potential V2 of the developing sleeve 21 in the developing section attain |V1|-|V2|=0 to 300 (V), by which the adhesion of the magnetic carrier to the photosensitive drum 1 may be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, etc., which forms an image by an electrophotographic method.

[0002]

2. Description of the Related Art Conventionally, various developing devices have been proposed and practically used as a developing device used in an electrophotographic image forming apparatus, but a two-component developing device is a typical developing device. . The two-component developing apparatus performs development by carrying toner on a developing area using a magnetic carrier, and usually performs a developing process by bringing a developer into contact with a photosensitive drum at a developing position.

FIG. 6 is a schematic sectional view showing a conventional two-component developing device (hereinafter simply referred to as a developing device).

The developing device 3 includes a developing sleeve 21 which is disposed at an opening of a developing container 20 so as to face the photosensitive drum 1 as an image carrier and is rotatable in the direction of an arrow b (counterclockwise).
A magnet roller 22 fixedly arranged in the developing sleeve 21, a regulating blade 23 for regulating the layer thickness of the developer t carried on the developing sleeve 21, and a developer t in the developing container 20.
Screws 24 and 25 for stirring the toner, a toner supply unit 26 for replenishing toner in the developing container 20, and the like are provided. The developer t is composed of a non-magnetic toner and a magnetic carrier.

Here, a description will be given of a developing step of visualizing the electrostatic latent image formed on the photosensitive drum 1 by the two-component magnetic brush method using the developing device 3 and a circulation system of the developer t.

First, the developer t in the developing container 20 is charged with N3 of the magnet roller 22 as the developing sleeve 21 rotates.
S2 pole of magnet roller 22 → N
In the process of being conveyed with the one pole, a thin layer is formed on the developing sleeve 21 by being regulated by the regulating blade 23.
When the developer t having a thin layer is conveyed to the S1 pole (main development pole) of the magnet roller 22, the magnetic force forms a spike (magnetic brush). The electrostatic latent image on the photosensitive drum 1 is reversely developed with the developer t formed in the spike shape.

After that, the developer t on the developing sleeve 21 is
Is returned into the developing container 20 by the repulsive magnetic field of the N3 and N2 poles of the magnet roller 22, and the stirring screw 2
Stirred by 4, 25. In the developing step, a DC bias and an AC bias are applied to the developing sleeve 21 from a developing bias power supply (not shown).

In general, in the two-component developing method, when an AC bias is applied, the developing efficiency is increased, and the quality of an image is high, but on the contrary, fogging tends to occur.

As a method of forming an electrostatic latent image, a photosensitive drum is scanned and exposed by a laser beam modulated in accordance with an image signal to be recorded, and a dot distribution shape, that is, a latent image on a dot is converted to an image. There is known a method of forming an electrostatic latent image distributed in a predetermined manner. Among them, the so-called pulse width modulation (PWM) method, which modulates the width (cascade length) of the driving pulse current of laser light in accordance with the density of a recorded image, can obtain a high recording density (high resolution). And high gradation can be obtained.

In the image forming method using the two-component developing method as described above, a carrier having a low magnetic permeability or a carrier having a small magnetization value in a magnetic field of 1000 Gauss is used for further improving the image quality and extending the life. A developing method has been proposed in, for example, JP-A-6-19222.

By using a low magnetic permeability carrier, the carrier is weakly restrained on the developing sleeve. For this reason,
The image quality can be improved by weakening the rubbing force of the magnetic brush on the toner image developed with respect to the electrostatic latent image on the photosensitive drum 1 in the developing section.
The packing pressure of the developer pool in the developing container 20 on the upstream side in the rotation direction of the third developing sleeve 21 is reduced, and the deterioration of the toner is reduced, so that the life can be extended.

At this time, assuming that the value of magnetization in a magnetic field of 1000 Gauss is used as a measure of the magnetization or magnetic permeability of the carrier, 200
When a low magnetic permeability carrier having a magnetization of emu / cm ³ or less is used, the above-described toner deterioration is particularly reduced.

At this time, the carrier (developer) must have a magnetization of ³⁰ emu / cm ³ or more at least from the top of the stable coat on the developing sleeve 21.

[0014]

However, when a carrier having a low magnetic permeability is used, especially when the photosensitive drum 1 is used.
Carrier easily adheres to the surface. The carrier adhesion is caused by the fact that the magnetization of the carrier is small, and the potential difference between the fogging potential (the DC bias applied to the developing sleeve 21 and the surface potential (non-image part potential) of the photosensitive drum 1) in the non-image part on the photosensitive drum 1 ) Is generated because the electrostatic force due to) exceeds the magnetic force. The fog removal potential is an electric field in a direction in which fog toner is pulled back from the photosensitive drum 1 to the developing sleeve 21, and a carrier having a polarity opposite to that of the toner is a field in a direction in which the toner is attracted in the direction of the photosensitive drum 1.

The cause of carrier adhesion to the photosensitive drum 1 is as follows.
In addition to the case where the electrostatic force becomes stronger than the magnetic force as described above, there may be a case where charges are injected into the carrier or a case where the carriers adhere to the photosensitive drum 1 by a mirror image force due to the injected charges. The charge injection into the carrier is mainly due to the AC component of the developing bias.

Therefore, in order to realize high image quality and long life, the carrier adhesion of the above-mentioned problem using a low magnetic permeability carrier can be greatly reduced by increasing the resistance of the carrier. (For example, see
-227225, JP-A-8-160671, etc.). In this case, the specific resistance of the carrier is 5 × 10
^It is effective to use a magnetic carrier of 10 ¹² Ωcm or more at an electric field strength of ⁴ V / m.

During image formation, the surface potential (non-image portion potential) of the photosensitive drum 1 and the developing sleeve 21
Since the potential difference (fogging potential) between the DC bias applied to the photosensitive drum 1 is relatively small, it was possible to prevent the carrier from adhering to the photosensitive drum 1 by increasing the resistance of the carrier as described above.

However, in the conventional image forming operation,
Prior to the start of the image forming operation, the photosensitive drum 1 is first charged to a predetermined surface potential, and then a developing bias is applied to prevent the toner from being developed. At the end of the image forming operation, the application of the developing bias is turned off,
Subsequently, the charging of the photosensitive drum 1 is turned off.

That is, the photosensitive drum 1 is charged to a predetermined surface potential, and a region where no developing bias is applied exists on the surface of the photosensitive drum 1. As a result, the difference between the surface potential of the photosensitive drum and the potential of the developing sleeve becomes large in this region.
It was not easy to completely prevent the carrier from adhering.

When the specific resistance of the carrier is increased,
Since the counter electrode effect on the charges on the photosensitive drum 1 is reduced, which enhances edge enhancement and causes a reduction in image quality, a carrier having as low resistance as possible is more preferable.

Accordingly, the present invention provides an image forming apparatus provided with a developing device using a two-component developer containing a magnetic carrier, when the photosensitive drum is charged to a predetermined surface potential prior to the start of the image forming operation. Alternatively, it is another object of the present invention to provide an image forming apparatus capable of completely preventing a carrier from adhering to a photosensitive drum at the end of an image forming operation and stably obtaining a high-quality image.

[0022]

According to a first aspect of the present invention, a rotatable image carrier, charging means for uniformly charging the surface of the image carrier, and charging means are provided. A charging bias applying unit for applying a charging bias, an exposing unit for forming an electrostatic latent image according to image information on the surface of the image carrier after charging, and a developer containing at least a non-magnetic toner and a magnetic carrier. A developer carrier having a rotatable developer carrier carried on the surface thereof, and the developer carried on the developer carrier being brought into contact with the image carrier and the developer carrier in a developing section in contact with or in proximity to the developer carrier. An image forming apparatus comprising: developing means for forming a developer image by attaching to the electrostatic latent image on the image carrier; and developing bias applying means for applying a developing bias to the developer carrier. From the bias applying means Control means for controlling the charging bias applied to the charging means, and for controlling the developing bias applied to the developer carrier from the developing bias applying means, wherein the control means, at the start of the image forming operation, When the image bearing member is charged to a predetermined surface potential by the charging means, the surface potential of the image bearing member in the developing unit is set to V
1. Assuming that the potential of the developer carrying member is V2, | V1 |
-| V2 | = 0 to 300 (V), the charging bias applied from the charging bias applying unit to the charging unit continuously or stepwise increases in absolute value with a predetermined polarity. The control is performed such that the developing bias applied from the developing bias applying means to the developer carrier is continuously or stepwise increased in absolute value with a predetermined polarity.

According to a second aspect of the present invention, there is provided a rotatable image carrier, charging means for uniformly charging the surface of the image carrier, charging bias applying means for applying a charging bias to the charging means, Exposure means for forming an electrostatic latent image in accordance with image information on the surface of the charged image carrier, and a rotatable developer carrier for carrying on the surface a developer containing at least a non-magnetic toner and a magnetic carrier Having the developer carried on the developer carrier, the developing unit where the image carrier and the developer carrier are in contact with or in close proximity to the electrostatic latent image on the image carrier. An image forming apparatus comprising: a developing unit that forms a developer image by attaching the developing unit; and a developing bias applying unit that applies a developing bias to the developer carrier.
Control means for controlling a charging bias applied from the charging bias applying means to the charging means, and controlling a developing bias applied to the developer carrier from the developing bias applying means, wherein the control means When the surface potential of the image carrier in the developing unit is V1 and the potential of the developer carrier is V2 at the end of the forming operation, | V
The charging bias applied from the charging bias applying unit to the charging unit decreases as the absolute value continuously or stepwise with a predetermined polarity so that 1 | − | V2 | = 0 to 300 (V). Simultaneously, the control is performed such that the developing bias applied from the developing bias applying means to the developer bearing member is continuously or stepwise reduced in absolute value with a predetermined polarity.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the illustrated embodiments.

<First Embodiment> FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to a first embodiment of the present invention. The image forming apparatus according to the present embodiment is an electrophotographic laser beam printer, and uses a two-component developing device as a developing device. The same members as those of the above-described conventional developing device are denoted by the same reference numerals and described.

The present image forming apparatus includes a photosensitive drum 1 as an image carrier. Around the photosensitive drum 1, a charging roller 2, a developing device 3, a transfer roller 4, and a cleaning device 5 are provided. An exposure device (laser beam scanner) 6 is disposed above the photosensitive drum 1, and a fixing device 7 is disposed downstream of the transfer nip N between the photosensitive drum 1 and the transfer roller 4 in the transfer material transport direction. Has been established.

In the present embodiment, the photosensitive drum 1 is a negatively charged organic photosensitive member, and has a photosensitive member layer (not shown) on a 30 mm-diameter aluminum drum base (not shown). Arrow a at the peripheral speed (for example, 100 mm / sec)
It is driven to rotate in the direction (clockwise), and receives a uniform negative polarity charge by the charging roller 2 that comes into contact during the rotation process.

A charging roller 2 serving as a charging means is rotatably in contact with the surface of the photosensitive drum 1 and uniformly charges the photosensitive drum 1 to a predetermined polarity and potential by a charging bias applied from a charging bias power supply 8.

The exposure device 6 includes a laser driver (not shown)
It has a laser diode, polygon mirror, etc.
Laser light modulated according to the time-series electric digital image signal of the image information input to the laser driver is output from the laser diode, scans the laser light with a polygon mirror that rotates at high speed, and passes through the reflection mirror 6a. By subjecting the surface of the photosensitive drum 1 to image exposure L, an electrostatic latent image corresponding to image information is formed.

The developing device 3 is a two-component developing device that performs development using a two-component developer containing a non-magnetic toner and a magnetic carrier, and is disposed at the opening of the developing container 20 so as to face the photosensitive drum 1. A developing sleeve 21 rotatable in the direction of the arrow b (counterclockwise), a magnet roller 22 fixed and arranged in the developing sleeve 21, a regulating blade 23 for regulating the layer thickness of the developer t carried on the developing sleeve 21, and developing. A stirring screw 24 for stirring the developer in the container 20;
25 and the like. The developing process of developing the image by the two-component magnetic brush method of the developing device 3 and the circulation system of the developer t are the same as those of the above-described conventional developing device.
The toner is adhered to the electrostatic latent image on the photosensitive drum 1 in the developing section where the photosensitive drum 1 and the developing sleeve 21 are almost in contact with each other by the developing sleeve 21 to which the developing bias has been applied, and is visualized as a toner image. I do.

In the present embodiment, the carrier having the above magnetism is 30 emu in a magnetic field of 1000 gauss.
A magnetic carrier having a magnetization of / cm ^{3 to} 200 emu / cm ³ and a specific resistance of 10 ¹² Ωcm or more at an electric field strength of 5 × 10 ⁴ V / m was used.

The fixing device 7 includes a rotatable fixing roller 7a.
And a pressure roller 7b. The toner image transferred to the surface of the transfer material P is heated while the transfer material P is nipped and conveyed at a fixing nip portion M between the fixing roller 7a and the pressure roller 7b. Press and heat fix.

A controller (CPU) 10 is connected to the charging bias power supply 8 and the developing bias power supply 9. The controller 10 includes a charging bias applied from the charging bias power supply 8 to the charging roller 2 and a developing bias power supply. From 9, the developing bias applied to the developing sleeve 21 is controlled. In the present embodiment, the charging roller 2 and the developing sleeve 2
The charging roller 2 and the developing sleeve 2 are rotated by the rotation of the photosensitive drum 1 by driving means (not shown).
1 is started or stopped in synchronization with the start or stop of the photosensitive drum 1.

Next, an image forming operation by the image forming apparatus will be described.

During image formation, the photosensitive drum 1 is driven by a driving means (not shown) at a predetermined peripheral speed (for example, 10
0 mm / sec), and the surface is uniformly charged (−700 V in the present embodiment) by the charging roller 2.

Then, an image exposure L is given by the exposure device 6 on the charged photosensitive drum 1, and an electrostatic latent image corresponding to the input image information is formed. At this time, the non-image portion potential (dark portion potential) which is the surface potential on the photosensitive drum 1 is
In this embodiment, the potential of the image portion on the photosensitive drum 1 (bright portion potential) is -700 V, and the potential of the image portion on the photosensitive drum 1 is -200 to -3
00V.

The charged polarity (negative polarity) of the photosensitive drum 1 is applied to the electrostatic latent image formed on the photosensitive drum 1 by the developing unit.
And the same developing bias (−500 in the present embodiment).
The toner charged to the same polarity as the charged polarity (negative polarity) of the photosensitive drum 1 is adhered by the developing sleeve 21 of the developing device 3 to which V) is applied, and is visualized as a toner image. In this embodiment, in order to increase the development efficiency, a bias in which a direct current (DC) component and an alternating current (AC) component are superimposed is applied as the development bias. For this reason, the photosensitive drum 1
The upper electrostatic latent image is reversely developed by the non-magnetic toner negatively charged by stirring with the carrier in advance by the developing sleeve 21 as described above.
Formed on the surface.

When the toner image on the photosensitive drum 1 reaches the transfer nip N between the photosensitive drum 1 and the transfer roller 4, the transfer material P such as paper in the paper supply cassette 11 is picked up at this timing. The paper is supplied one by one by a paper supply roller (not shown) and a paper supply guide 1.
3. The sheet is conveyed to the transfer nip N via a registration roller (not shown).

The transfer roller 4 to which a transfer bias having a polarity opposite to that of the toner (positive polarity) is applied to the transfer material P conveyed to the transfer nip N, the static electricity generated between the photosensitive drum 1 and the transfer roller 4. The toner image on the photosensitive drum 1 is transferred by the electric power. Then, the transfer material P on which the toner image has been transferred is conveyed to the fixing device 7 via the conveyance guide 14 and the like, and the toner image is heated on the transfer material P at the fixing nip M between the fixing roller 7a and the pressure roller 7b. After pressing and heat fixing, the sheet is discharged to the outside by the discharge roller 15, and a series of image forming operations is completed.

Further, the transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer of the toner image is removed and collected by the cleaning blade 5a of the cleaning device 5.

Next, a case will be described in which the operation sequence for preventing the (magnetic) carrier of the two-component developer from adhering to the photosensitive drum 1 is applied to the image forming apparatus according to the present embodiment. I do.

In this embodiment, at the start of the image forming operation, the charging bias applied to the charging roller 2 from the charging bias power supply 8 can be increased continuously or stepwise under the control of the controller 10. Further, the developing bias applied from the developing bias power supply 9 to the developing sleeve 21 can be increased continuously or stepwise under the control of the control device 10 in conjunction with the increase in the surface potential of the photosensitive drum 1 in the developing section. Is configured.

When the image forming operation is completed, the charging bias applied from the charging bias power supply 8 to the charging roller 2 can be reduced continuously or stepwise under the control of the control device 10. Photosensitive drum 1
The developing bias applied from the developing bias power supply 9 to the developing sleeve 21 can be reduced continuously or stepwise under the control of the control device 10 in conjunction with the decrease in the surface potential of the developing device.

FIG. 2 is a diagram showing an operation sequence for preventing the carrier from adhering to the photosensitive drum 1 in the present embodiment.

First, an operation sequence for preventing the (magnetic) carrier of the two-component developer from adhering to the photosensitive drum 1 at the start of the image forming operation will be described.

At time t1 in FIG. 2, the charging roller 2 and the developing sleeve 21 are started at the same time when the start of the photosensitive drum 1 is started (ON). Thereafter, at time t2 after 100 msec (time T1) when the rotation of the photosensitive drum 1 is stabilized, the charging bias is turned on. At time t4, control is performed so that the increased charging bias reaches a predetermined potential at time T2. ing. In the present embodiment, the time T2 is set to 300 ms.
ec and the charging bias (photosensitive drum surface potential) is controlled to reach -700V.

At time t3 (t3 <t4), the surface potential of the photosensitive drum 1 in the developing section is increased,
Increase the developing bias (DC component). Control is performed so that the increased developing bias reaches a predetermined potential at time t5. Here, the time T3 between the times t1 and t3 is, in the present embodiment, a time (25) between the charging roller 2 and the developing unit.
0 msec), the surface potential of the photosensitive drum 1 becomes approximately -150
The time is set to 320 msec, which is the sum of the time required to reach V (70 msec). In the present embodiment, the time T4 between time t3 and t5 is set to 230 msec.
And the developing bias (developing sleeve potential) is -50.
It is controlled to reach 0V.

Then, at time t5, after the surface potential of the photosensitive drum 1 and the potential of the developing bias reach predetermined potentials,
In this embodiment, the AC component of the developing bias is superimposed after 100 msec.

The inventor of the present invention has intensively studied the relationship between the photosensitive drum surface potential V1 and the developing sleeve potential V2 at the time of starting the above-described image forming operation. By setting the relationship with V2 so as to satisfy | V1 |-| V2 | = 0 to 300 V, 2 to the photosensitive drum 1 is satisfied.
It has been found that the (magnetic) carrier adhesion of the component developer can be completely prevented.

Therefore, in the present embodiment, as shown in FIG. 3, the surface potential (photosensitive drum surface potential V1) of the photosensitive drum 1 in the developing section at the start of the image forming operation is -700V, and the developing sleeve 21 Potential (developing sleeve potential V2) is set to −500 V, and | V1 | −
| V2 | = 0 to 300V. In the present embodiment, | V1 | − | V2 | = 2
00V.

Next, an operation sequence for preventing the (magnetic) carrier of the two-component developer from adhering to the photosensitive drum 1 at the end of the image forming operation will be described.

The above-described image forming operation is performed between time t5 and time t6, and the charging bias is turned off at time t6.
Is started, and at time t8, the charging bias is set to 0 V (OF
F). Here, from time t6 to t8
The time T5 between them is set to 300 msec in the present embodiment. Further, in the present embodiment, the AC component of the developing bias is 100 msec when the charging bias is turned off.
It is turned off before.

Then, at time t7 (t7 <t8), the developing bias is started to be turned off, and at time t9, the developing bias is controlled to be 0 V (OFF). here,
In the present embodiment, time T6 between time t7 and t9 is
It was set to 230 msec. In the present embodiment, the time T7 from the time t6 to the time t7 (the time from when the charging bias is turned off to when the developing bias is turned off) is set to 250 msec.

At time t10, the rotation of the charging roller 2 and the developing sleeve 21 is stopped (OFF) at the same time as the rotation of the photosensitive drum 1 is stopped (OFF).

The inventor of the present invention has intensively studied the relationship between the photosensitive drum surface potential V1 and the developing sleeve potential V2 at the end of the above-described image forming operation. By setting the relationship with V2 so as to satisfy | V1 |-| V2 | = 0 to 300 V, 2 to the photosensitive drum 1 is satisfied.
It has been found that the (magnetic) carrier adhesion of the component developer can be completely prevented.

Therefore, in the present embodiment, as shown in FIG. 4, the surface potential (photosensitive drum surface potential V1) of the photosensitive drum 1 in the developing section at the end of the above-described image forming operation is -700V, the developing sleeve 21 Potential (developing sleeve potential V2) is set to −500 V, and | V1 | −
| V2 | = 0 to 300V. In the present embodiment, | V1 | − | V2 | = 2
00V.

As described above, in this embodiment, at the start of the image forming operation and at the end of the image forming operation, the surface potential of the photosensitive drum 1 (photosensitive drum surface potential V1) in the developing section and the potential of the developing sleeve 21 (developing potential). By controlling the sleeve potential V2) so as to satisfy the condition of | V1 | − | V2 | = 0 to 300 V, the adhesion of the (magnetic) carrier of the two-component developer to the photosensitive drum 1 can be completely prevented. Therefore, a high-quality image could be stably obtained.

The above-described continuous or stepwise increase and decrease control of the charging bias and the developing bias may be digital or analog. In the case of digital control, of course, in order to sufficiently obtain the effect of the present invention, one step and two steps are not enough, and several steps are required. In the present embodiment, the charging bias and the developing bias are controlled to reach the predetermined potential in 16 steps.

<Embodiment 2> This embodiment is also described using the image forming apparatus of Embodiment 1 shown in FIG. The image forming operation is the same as in the first embodiment,
The description of this embodiment is omitted. In the present embodiment, a driving unit (not shown) for the developing sleeve 21 is provided separately from a driving unit (not shown) for the photosensitive drum 1.
The start or stop of the developing sleeve 21 can be performed independently of the start or stop of the photosensitive drum 1. The start or stop of the charging roller 2 is performed in synchronization with the start or stop of the photosensitive drum 1 as in the first embodiment.

Hereinafter, the photosensitive drum 1 according to the present embodiment will be described.
Operation sequence to prevent carrier adhesion to
This will be described with reference to FIG.

First, an operation sequence for preventing the (magnetic) carrier of the two-component developer from adhering to the photosensitive drum 1 at the start of the image forming operation will be described.

At time t1 in FIG. 5, the charging roller 2 is started at the same time as the command to start (ON) the start of the photosensitive drum 1. Thereafter, the rotation of the photosensitive drum 1 is stabilized for 100 ms.
ec (time T1), at time t2, the charging bias becomes O
N, and at time t4, the increased charging bias is applied for time T
2 so as to reach a predetermined potential. In the present embodiment, the time T2 is set to 300 msec, and the charging bias (photosensitive drum surface potential) is controlled to reach -700V.

At time t3 (t3 <t4), the surface potential of the photosensitive drum 1 in the developing section is increased,
Increase the developing bias (DC component). Control is performed so that the increased developing bias reaches a predetermined potential at time t5. Here, the time T3 between the times t1 and t3 is, in the present embodiment, a time (25) between the charging roller 2 and the developing unit.
0 msec), the surface potential of the photosensitive drum 1 becomes approximately -150
The time is set to 320 msec, which is the sum of the time required to reach V (70 msec). In the present embodiment, the time T4 between time t3 and t5 is set to 230 msec.
And the developing bias (developing sleeve potential) is -50.
It is controlled to reach 0V.

Then, at time t5, after the surface potential of the photosensitive drum 1 and the potential of the developing bias reach predetermined potentials, the developing sleeve 21 is activated (ON) at time t6. In the present embodiment, time T5 between time t5 and t6 is 5
It is set to 0 msec. And the developing sleeve 2
The AC component of the developing bias is superimposed at the same time as or after a certain period of time of the start of the first operation. It is more preferable to superimpose the AC component of the developing bias after the rotation of the developing sleeve 21 is stabilized. In the present embodiment, the developing sleeve 21
100 msec after the start of the operation, the AC component of the developing bias was superimposed.

In this embodiment, as in the first embodiment, the surface potential of the photosensitive drum 1 (photosensitive drum surface potential V1) in the developing section at the start of the image forming operation is -700 V, and The potential of the sleeve 21 (developing sleeve potential V2) was set to -500V, and control was performed so as to satisfy the condition of | V1 |-| V2 | = 0 to 300V. In the present embodiment, | V1 |-|
V2 | = 200V.

Next, an operation sequence for preventing the (magnetic) carrier of the two-component developer from adhering to the photosensitive drum 1 at the end of the image forming operation will be described.

The above-described image forming operation is performed between time t6 and time t7, the rotation of the developing sleeve 21 is stopped (OFF) at time t7, and then the charging bias is turned off at time t8. At time t10, control is performed so that the charging bias becomes 0 V (OFF). In the present embodiment, time T6 between time t7 and t8 is set to 50 msec, and time T7 between time t8 and t10 is set to 300 ms.
ec.

At the same time as the stop of the developing sleeve 21,
Although the AC component of the developing bias may be turned off, it is more preferable to turn off the AC component of the developing bias when the developing sleeve 21 is rotating stably. In the present embodiment, the AC component of the developing bias is turned off 100 msec before the stop of the developing sleeve 21.

At time t9 (t9 <t10),
The turning off of the developing bias is started, and control is performed so that the developing bias becomes 0 V (OFF) at time t11. In the present embodiment, the time T8 between time t9 and t11 is set to 23
It was set to 0 msec. In the present embodiment, the time T9 between the time t8 and the time t9 (the time from when the charging bias is turned OFF to when the developing bias is turned OFF) is set to 250 msec.

At time t12, the rotation of the charging roller 2 is stopped (OFF) at the same time as the rotation of the photosensitive drum 1 is stopped (OFF).

In this embodiment, as in the first embodiment, the surface potential (photosensitive drum surface potential V1) of the photosensitive drum 1 in the developing section at the end of the above-described image forming operation is -700 V, and The potential of the sleeve 21 (developing sleeve potential V2) was set to -500V, and control was performed so as to satisfy the condition of | V1 |-| V2 | = 0 to 300V. In the present embodiment, | V1 |-|
V2 | = 200V.

As described above, also in this embodiment, at the start of the image forming operation and at the end of the image forming operation, the surface potential of the photosensitive drum 1 in the developing section (the photosensitive drum surface potential V)
1) and the potential of the developing sleeve 21 (developing sleeve potential V
2) is controlled so as to satisfy the condition of | V1 | − | V2 | = 0 to 300 V, whereby 2
Since the (magnetic) carrier adhesion of the component developer could be completely prevented, a high quality image could be stably obtained.

In this embodiment, the charging bias and the developing bias are controlled digitally in 16 steps so as to reach a predetermined potential.

[0074]

As described above, according to the present invention, when the image carrier is charged to a predetermined surface potential at the start of the image forming operation, or at the end of the image forming operation, the image carrier in the developing section is charged. Potential V1 of the developer carrying member and potential V2 of the developer carrying member
Is set to | V1 |-| V2 | = 0 to 300 (V), the adhesion of the magnetic carrier to the image carrier can be prevented, so that a high-quality image can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing an operation sequence for preventing carrier adhesion to a photosensitive drum in a developing unit at the start and end of the image forming operation according to the first embodiment.

FIG. 3 is a diagram illustrating a state of a photosensitive drum surface potential and a developing sleeve potential in a developing unit at the start of image formation according to the first embodiment.

FIG. 4 is a diagram illustrating a state of a photosensitive drum surface potential and a developing sleeve potential in a developing unit at the end of image formation according to the first embodiment.

FIG. 5 is a diagram showing an operation sequence for preventing a carrier from adhering to a photosensitive drum in a developing unit at the start and end of an image forming operation according to the second embodiment.

FIG. 6 is a schematic sectional view showing a two-component developing device in a conventional example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photosensitive drum (image carrier) 2 charging roller (charging unit) 3 developing device (developing unit) 4 transfer roller 5 cleaning device 6 exposure device (exposure unit) 7 fixing device 8 charging bias power supply (charging bias applying unit) 9 development Bias power supply (developing bias applying means) 10 Controller (control means) 20 Developing container 21 Developing sleeve (Developer carrier) 22 Magnet roller 23 Regulator blade 24, 25 Stirring screw

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G03G 21/14 G03G 21/00 372 (72) Inventor Hiroaki Ogata 3-30-2 Shimomaruko, Ota-ku, Tokyo F term in Canon Inc. (reference) 2H003 BB11 CC05 DD03 2H005 BA00 EA01 EA02 FA01 2H027 DA02 EA01 EA04 EA05 ED03 ED09 2H073 BA02 BA13 BA23 CA03 2H077 AD02 AD06 AD35 AD36 BA03 CA11 DB08 DB14 DB25 EA03

Claims (8)

[Claims] A rotatable image carrier, a charging unit for uniformly charging the surface of the image carrier, a charging bias applying unit for applying a charging bias to the charging unit, and the image carrier after charging. Exposing means for forming an electrostatic latent image corresponding to image information on a body surface, and a rotatable developer carrier for carrying a developer containing at least a non-magnetic toner and a magnetic carrier on the surface; The developer carried on the developer carrier is adhered to the electrostatic latent image on the image carrier at a developing section where the image carrier and the developer carrier are in contact with or close to each other to form a developer image And a developing bias applying unit for applying a developing bias to the developer carrier, wherein the charging bias applied from the charging bias applying unit to the charging unit is controlled, and The development Control means for controlling a developing bias applied from the bias applying means to the developer carrier, wherein the control means sets the image carrier to a predetermined surface potential by the charging means at the start of an image forming operation. When charging, the surface potential of the image carrier in the developing unit is set to V
1. Assuming that the potential of the developer carrying member is V2, | V1 |
-| V2 | = 0 to 300 (V), the charging bias applied from the charging bias applying unit to the charging unit continuously or stepwise increases in absolute value with a predetermined polarity. Interlocking with the developing bias, the developing bias applied to the developer carrier from the developing bias applying means is controlled to increase continuously or stepwise as an absolute value with a predetermined polarity, image forming. apparatus. 2. The magnetic carrier according to claim 1, wherein said magnetic carrier is ³⁰ emu / cm ^{3 to} 200 emu / cm in a magnetic field of 1000 gauss.
The image forming apparatus according to claim 1, wherein the image forming apparatus has a magnetization of ³ and a specific resistance of 10 ¹² Ωcm or more at an electric field intensity of 5 × 10 ⁴ V / m. 3. The image forming apparatus according to claim 1, wherein the developer carrier is rotated after a surface potential of the image carrier and a potential of the developer carrier reach predetermined potentials. apparatus. 4. The image forming apparatus according to claim 1, wherein the developing bias applying unit applies a developing bias in which an AC component is superimposed on a DC component to the developer carrier. 5. A rotatable image carrier, a charging unit for uniformly charging the surface of the image carrier, a charging bias applying unit for applying a charging bias to the charging unit, and the image carrier after charging. Exposing means for forming an electrostatic latent image corresponding to image information on a body surface, and a rotatable developer carrier for carrying a developer containing at least a non-magnetic toner and a magnetic carrier on the surface; The developer carried on the developer carrier is adhered to the electrostatic latent image on the image carrier at a developing section where the image carrier and the developer carrier are in contact with or close to each other to form a developer image And a developing bias applying unit for applying a developing bias to the developer carrier, wherein the charging bias applied from the charging bias applying unit to the charging unit is controlled, and The development Control means for controlling a developing bias applied from the bias applying means to the developer carrier, wherein the control means sets the surface potential of the image carrier in the developing section to V1 at the end of the image forming operation. Assuming that the potential of the developer carrying member is V2, | V1 |-| V2 | = 0
The charging bias applied from the charging bias applying means to the charging means is continuously or stepwise reduced in absolute value at a predetermined polarity so that the absolute value decreases to 300 V. An image forming apparatus, wherein control is performed such that a developing bias applied to the developer carrier from a developing bias applying unit is continuously or stepwise reduced as an absolute value with a predetermined polarity. 6. The magnetic carrier according to claim 1, wherein said magnetic carrier has a magnetic field of 1000 gauss and has a magnetic field of 30 emu / cm ^{3 to} 200 emu / cm.
The image forming apparatus according to claim 5, wherein the image forming apparatus has a magnetization of ³ and a specific resistance of 10 ¹² Ωcm or more at an electric field intensity of 5 × 10 ⁴ V / m. 7. The rotation of the developer carrier is stopped before the charging bias applied to the charging unit from the charging bias control unit decreases continuously or stepwise as an absolute value with a predetermined polarity. The image forming apparatus according to claim 5, wherein: 8. The image forming apparatus according to claim 5, wherein the developing bias applying unit applies a developing bias in which an AC component is superimposed on a DC component to the developer carrier.