JPH0476569A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent soiling of an electrifying member and lowering of outputted image quality due to it by changing over and controlling to a pressure released state while drive of an image carrier is stopped and changing over to a pressurized state after a surface part of the image carrier which was faced to a cleaning means at a starting point after the surface movement drive of the image carrier is started is passed the electrified member position which is in a pressure released state. CONSTITUTION:When a printing start signal is inputted to a control system, the device is put into a specified pre rotational period by having rotational drive of the photosensitive drum 1 started, while the electrifying roll 2 is brought into a state of pressurized contact with the photosensitive drum 1 by an SL device 15 being changed over from OFF to ON after DELTAt seconds from the start of the rotation of the photosensitive drum 1, impressing of bias voltage against the electrifying roll is carried out. When printing is finished, the photosensitive drum 1 is put into a specified post rotational period, when this period is completed, the rotational drive of the photosensitive drum 1 is stopped, the SL device 15 is changed over to OFF, and impressing of the bias voltage against the electrifying roll 2 is also turned OFF.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is applicable to image forming apparatuses such as transfer-type electrophotographic devices (copiers, laser printers, etc.), electrostatic recording devices, etc. Image formation is performed by applying an image forming process that includes a step of charging (including removing static electricity) the surface of an image carrier (electrophotographic photoreceptor, electrostatic recording dielectric material, etc.) as a charged object to be driven. The present invention relates to an image forming apparatus in which an image bearing member is cleaned by a cleaning means and then turned over for use in image formation.

The image formed on the surface of the image carrier is transferred to a recording material (transfer paper), and after the image transfer, the image carrier is cleaned by a cleaning means to remove residual developer, etc., and the image carrier is turned over to form an image. In addition to transfer-type image forming devices used for
After displaying or reading an image formed on the surface of an image carrier (such as a display device, reader, etc.), the formed image is removed from the surface of the image carrier by a cleaning means without being transferred, and the image carrier is turned over to form an image. It also includes image forming apparatuses configured to provide such services.

More specifically, the present invention relates to an image forming apparatus that uses a contact-type charging device as a charging processing means for an image bearing member, which charges the surface of the image bearing member by bringing a charging member to which a bias voltage is applied into contact with a charged member.

(Prior Art) In the image forming apparatus as described above, a corona discharge device has conventionally been widely used as a device for charging the surface of an image carrier.

A corona discharge device is effective as a means for uniformly charging the surface of an object to be charged, such as an image carrier, to a predetermined potential. but,
It requires a high-voltage power supply and has problems such as the generation of undesirable ozone due to corona discharge.

In contrast to such corona discharge devices, contact-type charging devices, which charge the surface of a charged object by bringing a conductive charging member to which a positive or negative voltage is applied, into contact with the surface of the charged object, can reduce the voltage of the power source. Because of its advantages such as low ozone generation, it can be used instead of a corona discharge device in image forming apparatuses to charge the surface of image carriers such as photoreceptors, dielectrics, and other objects to be charged. It has attracted attention as a processing means device,
Research on its practical application is underway.

For example, the present applicant first proposed (Patent Application No. 62-51492
No. 62-230334, etc.), an oscillating voltage ( Alternating voltage/alternating voltage/pulsating voltage: sine wave/
Applying a superimposed voltage of a DC voltage and a voltage whose voltage value changes periodically over time, such as a rectangular wave or a triangular wave (hereinafter referred to as AC voltage or AC component), to the charging member, or providing a high resistance layer on the surface layer. By using such a charging member, it is possible to improve the uniformity of charging of the object to be charged, and to prevent leakage due to holes, scratches, etc. on the surface of the object to be charged, such as a photoreceptor.

FIG. 4 shows a schematic configuration of an example of a laser printer using an electrophotographic process (Carlson process) as a contact type charging device as a primary charging means for an image carrier.

Reference numeral 1 denotes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as photosensitive drum) as an image carrier, which is rotated at a predetermined circumferential speed (process speed) in the clockwise direction of the arrow.

2 is a charging member that uniformly charges the surface of the photosensitive drum to a predetermined polarity and potential; in this example, a roller-type member (hereinafter referred to as
(referred to as a charging roller). The charging roller 2 is a conductive elastic roller that is pressed against the surface of the photosensitive drum 1 at a constant pressure with a predetermined pressing force by a pressure means (not shown) in a direction substantially parallel to the drum axis. Rotates as a result of rotational drive.

In this example, a superimposed voltage of a predetermined AC voltage and a predetermined DC voltage is applied to the charging roller 2 by an AC power supply 7 and a DC power supply 8, so that the surface of the rotating photosensitive drum 1 comes into contact with a predetermined polarity and potential. It is uniformly charged using the formula.

The charged surface of the photosensitive drum 1 is scanned and exposed 3 with a laser beam emitted from a laser beam scanner (not shown) and modulated in accordance with time-series electrical digital pixel signals of target image information. An electrostatic latent image of target image information is formed on one surface of the photosensitive drum, and then the electrostatic latent image is subjected to reversal development (toner is applied to the exposed area of the photosensitive drum surface) by the reversal developing device 4 which applies a developing bias to the developing sleeve 9. (the latent image is developed) and developed as a toner image.

Next, the toner image on the surface of the photosensitive drum 1 is transferred to a transfer sheet (recording material) that is fed from a paper feeding section (not shown) to between the photosensitive drum 1 and the transfer charger 5 (transfer section) at an appropriate timing.
The images are sequentially transferred onto the P surface.

The transfer paper P, which has undergone the toner image transfer through the transfer section, is sequentially separated from the surface of the photosensitive drum, introduced into an image fixing device (not shown), and outputted as an image-formed product (print).

After the image has been transferred, the surface of the photosensitive drum is cleaned by a cleaning device 6 to remove residual contaminants such as toner remaining after transfer and paper dust from the transfer paper, and is then turned over and used for image formation.

The cleaning device 6 of this example is of a blade type, and a cleaning blade 11 made of elastic rubber or the like whose tip end portion is in contact with the surface of the photosensitive drum is used to contaminate the surface of the photosensitive drum with residual toner, etc. Things are wiped and cleaned.

FIG. 5 is a timing chart of the operation sequence of the image forming process equipment of this printer.

That is, when the print start signal is input, the photosensitive tram 1
The rotational drive of the photosensitive drum 1 is started (activated), and the charging roller 2 enters a driven rotation state as the rotational drive of the photosensitive drum 1 starts.
Furthermore, by starting the application of a bias voltage to the charging roller 2, a pre-rotation step for equalizing the potential on the surface of the photosensitive drum 1 is executed.

When the predetermined pre-rotation process period ends, the printing process begins, and image exposure (laser beam scanning exposure) 3, development bias application to the development sleeve 9 of the development device 4, and bias voltage application to the transfer charger 5 are sequentially performed in a predetermined manner. The printing process is executed by starting at the timing of .

The charging member 2 is not limited to the roller type described above, but may also be a blade type,
It can be of any suitable shape or form, such as a bat type, brush type, lot type, block type, belt type, or web type.

(Problems to be Solved by the Invention) One of the problems with image forming apparatuses that use a contact-type charging device as a charging processing means for an image carrier, such as the printer in the above example, is that the charging member is contaminated. This may be due to the occurrence of image defects.

In the printer described above, the causes of dirt on the charging member include (1) toner (developer) accumulated on the edge of the cleaning blade 11 of the cleaning device 6 and the photosensitive drum 1;
Contaminant substances 10 such as scraps of scraps and transfer paper dust pass through the cleaning plate and are taken out due to the start-up shock and blade turning-up phenomenon when the rotation of the photosensitive drum 1 is started; When the rotation is stopped, dirt such as toner and dust 10a adheres to the photosensitive drum surface area A between the cleaning blade 11 of the cleaning device 6 and the charging roller 2 due to some reason, and the dirt substance 10 is taken out. When the photosensitive drum 1 rotates, the attached dirt 10a is carried to the position of the charging roller 2 which is in pressure contact with the photosensitive drum 1, and the circumferential surface of the charging roller 2 is completely or locally soiled. become a state.

If the charging roller 2 is soiled, the charging process on the surface of the image carrier becomes uneven, resulting in image defects such as black dots/black lines, white dots/white lines, areas after the image, washed out areas, and fogged areas in the output image. Cause.

In addition, the dirt on the charging roller 2 is rubbed against the surface of the photosensitive drum and firmly adheres to the surface of the photosensitive drum, causing so-called toner fusion areas that inhibit uniform charging of the surface of the photosensitive drum and reducing the quality of the formed image. It will also happen.

The above-mentioned problems are also common to charging members or other forms of charging blades, charging pads, charging brushes, charging wires, etc. other than charging rollers.

The present invention is an image forming apparatus which is also a contact-type charging device and is used as a charging processing means for the surface of an image carrier, and at least prevents the charging member from being contaminated due to the causes of (1) and (3) above and the deterioration of the output image quality caused by this. An object of the present invention is to provide an image forming apparatus devised to do so.

(Means for Solving the Problems) The present invention forms an image by applying an image forming process that includes a step of charging the surface of the image carrier to an image carrier that is driven to move the surface of the image carrier. is an image forming apparatus that uses a cleaning means to clean the image bearing member and turn the edges over for image formation, and the charging processing means for the image bearing member brings a charging member to which a bias voltage has been applied into pressure contact with the image bearing member using a pressure means. This is a contact-type charging device that performs charging by charging the image carrier, and the pressure means has two states: a pressurized state in which the charging member is brought into pressure contact with the surface of the image carrier with a predetermined pressure, and a pressurized state in which the pressure is released. At least when the surface movement drive of the image bearing member is stopped, it is controlled to be switched to the pressure release state, and after the surface movement drive of the image bearing member is started, it faces the cleaning means at the time of activation. An image forming apparatus characterized in that the surface portion of the image carrier is switched to a pressurized state after passing through a position of a charging member in a pressurized state.

Further, in the image forming apparatus of the present invention, when the charging member is substantially in contact with the image bearing member when the pressing means for the charging member is in a pressure-released state, the plane movement drive of the image bearing member is activated. The image forming apparatus is characterized in that a direct current bias is applied to the charging member from the time when the charging member is switched to the pressurizing state until the pressurizing means for the charging member is switched to the pressurizing state.

(Function) That is, when the surface movement drive of the image bearing member is started, dirt substances may be taken out from the cleaning means part due to a startup shock or due to a plate turning phenomenon when the image bearing member surface cleaning means is a cleaning blade. Even if the image carrier is carried to the position of the charging member by the movement of the activated image carrier surface, or for some reason when the surface movement of the image carrier is stopped, the image carrier surface area between the cleaning means and the charging member may be moved. Even if dirt adheres and is carried to the position of the charging member by the surface movement drive of the activated image carrier, the image carrier surface area where the dirt substance or attached dirt is present will not pass the position of the charging member. Until this happens, the pressing means for the charging member is controlled to be switched to a pressure release state, and the charging member is not in contact with the image carrier surface, or even if it is in contact, the positive predetermined pressing force is released. Since the charging member is in a light contact state, contaminants and adhered dirt on the surface area of the image carrier do not adhere to the charging member. That is, the surface of the charging member is prevented from being contaminated.

After the area of the surface of the image carrier where contaminants and attached dirt exist passes the position of the charging member, the pressurizing means is switched to a pressurizing state, thereby applying a predetermined pressing force to the charging member or the surface of the image carrier. A contact charging process is performed on the surface of the image carrier by bringing the charging member into pressure contact and applying a predetermined bias voltage to the charging member.

Contaminants or adhered stains on the surface area of the image bearing member that have passed through the position of the charging member without contaminating the charging member are transferred to the cleaning means as the surface of the image bearing member is subsequently moved. As a result, the charging member is removed from the surface of the image carrier, and the charging member comes into pressure contact with the clean surface of the image carrier.

Therefore, in an image forming apparatus using a contact type charging device as a charging processing means for the surface of an image carrier, at least the above-mentioned
It is possible to prevent contamination of the charging member due to causes such as (1) and (2) and a decrease in output image quality caused by this.

Further, when the charging member is substantially in contact with the image carrier in the state where the pressure is released from the pressure means for the charging member, the pressure means for the charging member may be applied after the surface movement drive of the image carrier is started. A DC bias is applied to the charging member until it is controlled to switch to the pressure state.If the DC bias is set to the same polarity as the developer, dirt substances with the same polarity as the developer or If the polarity of the bias is opposite to that of the developer, it will be actively repelled by electric field force or electrostatic force in the direction of not adhering to the charging member or in the direction of detaching from the charging member, which will more effectively prevent contamination of the charging member. be done.

(Example) <Example 1> FIGS. 1(A), (B), and (C) are explanatory diagrams of an example apparatus.

FIG. 1(A) shows a pressurized state in which the charging roller 2 as a charging member is pressed against the surface of the photosensitive drum 1 as an image carrier with a predetermined pressing force when the pressing means is turned on. Figure (B) shows a pressurized state in which the charging roller 2 is separated from the surface of the photosensitive tram 1 by turning off the pressing means.

The roller 2 of this example has 10
A conductive elastic roller portion 2b made of conductive urethane rubber having a resistance value of about 6 to 10<7>[Omega] is integrally formed concentrically.

15 is an electromagnetic solenoid plunger (
(hereinafter referred to as the SL device), and the advance/retreat plunger 15a
It is mounted and supported by a stationary member (not shown) in a downward position.

Reference numeral 14 designates a gutter-shaped housing with an open lower surface that encloses the charging roller 2, and the longitudinal center portion of its upper wall is connected to and supported by the lower end of the downward reciprocating plunger 15a of the SL device 15.

The upper ends of coil springs 16 are fixedly provided on the inner surface of the upper wall of both longitudinal ends of the housing 14 via electrode plates 17, and a bearing (holder) 13 is mounted and supported on the lower end of each spring 16. The core metal shaft 2 of the charging roller 2 is attached to the bearing 13.
The charging roller 2 is housed in the housing 14 with both ends of the charging roller 2 rotatably supported by bearings.

Reference numerals 18 and 18 designate guide members that restrict the moving direction of the housing 14 in the vertical direction.

The SL device 15 is turned on (energized) by a control circuit (not shown).
Controlled off (de-energized).

When off, the advance/retreat plunger 15a is held in the retracted position by a pull-up spring (not shown) inside the solenoid, so that the housing 14
- The entire charging roller assembly consisting of the electrode 17, spring 16, bearing 13, and charging roller 2 is lifted upward, and the charging roller 2 is separated from the surface of the photosensitive drum 1 without contact α as shown in FIG. 1(B). is maintained in a pressurized state.

When it is on, the advancing/retracting plunger 15a moves downward against the pull-up spring inside the solenoid, the charging roller assembly is pushed down as a whole, the charging roller 2 comes into contact with the surface of the photosensitive drum 1, and the housing 14 is moved by the spring. 16 is compressed and pushed down between the bearing 13 and the charging roller 2 is brought into pressure contact with the surface of the photosensitive tram 1 with a predetermined pressing force due to the compression reaction force of the spring 16.
It is maintained in the pressurized state shown in Figure (A).

In this example, a pressing force of about 500 g is applied to each end of the core metal shaft 2a of the charging roller 2.

B is a nip portion formed by pressure contact of the charging roller 2 with the photosensitive drum 1.

In the pressurized state shown in FIG. 1A, when the photosensitive drum 1 is driven to rotate, the charging roller 2 is driven to rotate around the bearing 13. As shown in FIG. By applying a predetermined bias voltage to the charging roller 2, the surface of the photosensitive drum is charged to a predetermined polarity and potential in a contact manner.

In this example, an AC power supply 7 and a DC power supply 8 apply a superimposed voltage of AC voltage: approximately 1800 vPP, frequency 500 Hz, DC voltage: -1200 V to electrodes 17, springs 16 (conductive) bearings 13 (
By applying power through the core metal shaft 2a (same), a good charging state of the photosensitive drum surface was obtained.

Around the photosensitive drum 1 shown in FIGS. 1(A) and 1(B), a developing device 4, a transfer charger 5, etc. are arranged as in the printer shown in FIG. 4, but these are omitted from the drawings.

FIG. 1(C) shows the ON/OFF state of the SL device 15 as a pressurizing means.
A timing chart of off control is shown. In this example, 2
This is for continuous printing.

1) When the device power (main power switch) is turned on,
The rotation of the photosensitive tram 1 is started, and the apparatus enters a predetermined warming period (pre-multi-rotation period), during which heating of the fixing device and the like are performed.

The SL device 15 as a pressurizing means is off (pressurizing release state)
However, Δ is turned on (pressure state) seconds after the rotation of the photosensitive drum 1 is started, and the charging roller 2 is brought into pressure contact with the photosensitive drum 1, and at the same time, the bias against the charging roller 2 is A voltage is applied.

2) When the predetermined warm-up period ends, the rotation of the photosensitive drum is stopped, the SL device 15 is turned off, and the application of bias voltage to the charging roller is turned off, until the next print start signal is manually input to the control system. The device is kept in a standby state.

3) When the print start signal is manually input to the control system, the rotation of the photosensitive drum 1 is started and the device enters a predetermined pre-rotation period, and the SL device 15 is turned off after Δ seconds from the start of rotation of the photosensitive tram 1. When the charging roller 2 is turned on, the charging roller 2 is brought into pressure contact with the photosensitive drum 1, and a bias voltage is applied to the charging roller.

4) When the predetermined pre-rotation period ends, the first and second sheets are printed in sequence.

5) When the second (last) sheet is printed, the photosensitive tram 1 enters a predetermined post-rotation period, and when this period ends, the rotational drive of the photosensitive drum 1 is stopped and the SL device 15 is turned off. The bias voltage application to the charging roller 2 is also turned off, and the apparatus is maintained in a standby state until the next print start signal is input again.

During the device warming period and the pre-rotation period, the time Δ from the start of rotation of the photosensitive drum 1 to the turning on of the SL device 15 is set to seconds, and from the contact portion of the cleaning blade 11 with the photosensitive drum 1 to the charging roller 2 and the photosensitive drum. If the distance along the drum surface to the downstream end B of the nip portion B with the drum 1 is J2 (mm), and the rotational circumferential speed of the photosensitive drum 1 is VP (mm 7 seconds), then Δt≧j2/VP As described above, when the rotation of the photosensitive drum l starts, the dirt particles 10 accumulated on the edge of the cleaning blade 11 due to the start-up shock and blade turning-up occur.
Even if the photosensitive drum 1 passes through the blade and is taken out, or dirt adheres to the photosensitive drum surface area between the plate 11 and the charging roller 2 while the photosensitive tram 1 is stopped rotating,
These contaminants 10 and adhered contaminants 10a pass through the position of the charging roller 2 which is held in a pressure-released state during the period of Δ seconds after the rotation of the photosensitive tram 1 starts, and the charging roller Since the charging roller 2 is then brought into pressure contact with the surface of the photosensitive drum 1, the charging roller 2 is prevented from being soiled with the above-mentioned dirt substance 10 and the attached dirt 10a, and the soiled charging roller 2 is also brought into contact with the photosensitive drum 1. Contaminated areas such as toner fused areas are prevented from forming on the surface.

The dirt substance 10 and the attached dirt 10a that have passed the position of the charging roller 2 without being attached to the charging roller reach the cleaning device 6 by the subsequent rotation of the photosensitive tram 1, and are removed by the cleaning plate 11.

By keeping the charging roller 2 in a pressure-released state while the photosensitive tram 1 is not rotating, deformation of the charging roller 2 and the photosensitive drum 1 can be prevented. photosensitive drum 1 due to seepage of solvent)
Problems with localized contamination of surfaces can also be prevented. During the warming period of the device, the SL device 15 may be kept off and the charging roller 2 may be kept in the pressurized state.

<Embodiment 2> When the pressure of the charging roller 2 on the photosensitive drum 1 is released, it is not necessary to completely separate the charging roller 2 from the surface of the photosensitive drum 1 as shown in FIG. 1(B).

In Example 1, when the charging roller 2 is in a pressurized state, a pressing force of about 500 g is applied to each end of the charging roller 1, and a total pressure of about I kg. You can just cancel it.

FIGS. 2(A) and 2(B) show an example, in which the charging roller housing 14 is fixed in place, and a horizontally long insulating plate 20 is attached to the lower end of the downward plunger 15a of the SL device 15. Electrode 17, spring 16,
A bearing 13 is supported, and the core metal shaft 2a of the charging roller 2 is supported by the bearing 13. When the SL device 15 is on, the plunger 15a protrudes downward as shown in FIG. 2(A). The spring 16 is compressed to bring the charging roller 2 into pressure contact with the surface of the photosensitive tram with a predetermined pressure.

When the charging roller 2 is off, the back pressure of the spring 16 is released and the charging roller 2 is in light contact with the photosensitive tram 1 (FIG. 2(B)). Even if this increases, the nip part B will almost disappear, and the adsorption force of the dirt substance 10 and attached dirt 10a to the photosensitive drum 1 surface exceeds the adsorption force to the charging roller 2, so in reality, it will not stick to the charging roller 2. It doesn't come.

If the charging roller 2 shown in FIGS. 1 and 2 is disposed below the photosensitive drum 1 and facing upward, the charging roller 2 can be applied with its own weight from the surface of the photosensitive drum 1 by simply releasing the back pressure of the spring 16. It is more preferable to release the pressure.

The control of pressurization and pressurization release may be the same as the timing chart of FIG. 1(C) described above.

<Example 3> As in Example 2, in which the charging roller 2 is in contact with the surface of the photosensitive drum in the pressurized state, the charging roller 2 is rotated (driven rotation or forced rotation may be used). , by applying an appropriate DC bias voltage,
The electric field of the applied DC bias or static electricity causes the charging roller 2 to become contaminated with dirt 10 and adhered dirt 10 on the photosensitive drum 1 side.
The charging roller 2 can be cleaned by actively preventing dirt from adhering to the charging roller 2 and transferring the dirt already attached to the charging roller 2 to the photosensitive tram 1 side.

In the image forming apparatus, developer (toner) is the main contaminant material 10, 10a, so by applying a DC bias with the same polarity as the charging characteristic of the developer, the developer is repelled and charged on the charging roller 2. It is effective to prevent adhesion to.

The printer of this embodiment uses a reversal development method as a development method, and in this case, the toner, which is the developer, has the same charging characteristic as the photosensitive drum 1, that is, negative polarity.

Therefore, in the timing chart of FIG. 1(C), Δ
It is sufficient to apply a negative DC bias to the charging roller 2 during the period t.

Experimentally, we determined the upper limit of the bias value that would not damage the photosensitive tram 1, and found that no particular problem occurs up to about -1.5 KV, but when the bias value is about -2 to -3 V, the image It was found that this caused defects. Also -50
A bias value of about 0 V or less did not have sufficient effect.

Although this depends on the resistance value of the charging roller 2,
In Example 1, the charging bias voltage was 1 during charging.
800 V pp(7) AC bias and -1200
Since a DC bias of V is applied, a sufficient effect can be obtained by turning off only the AC bias to prevent contamination.

Actually, when only the AC bias of the AC power supply 7 was turned off and the DC bias 8 of the DC power supply 8 was turned on during the period Δt shown in FIG. It does not adhere to the charging roller 2, and since the AC bias is turned off, it does not strongly adhere to the photosensitive drum 1, so it passes through the position of the charging roller 2 and the subsequent rotation of the photosensitive drum 1 cleans the cleaning device. 6 and was scraped off by the cleaning blade 11 and collected, and the charging roller 2 did not damage the photosensitive drum 1 through the toner (stain substance 10 and attached dirt 10a).

<Embodiment 4> In Embodiment 3, the case where the charging roller 2 is mainly prevented from being contaminated by the developer has been described, but there are cases where stains of opposite polarity to the developer adhere for some other reason. At this time, it is effective to apply a DC bias having a polarity opposite to that of the developer to the charging roller 2.

FIG. 3 shows a bias application timing chart in this case.

That is, from the start of rotation of the photosensitive tram 1, △1. seconds (Δt1
≧p/vP) A bias of -1200V is applied to the charging roller 2 as a DC bias ■ with the same polarity as the developer, and then +1200V as a DC bias ■ with the opposite polarity to the developer.
A bias of Δt is applied to the charging roller 2 for 2 seconds. This Δt2 seconds is the time required for at least one revolution of the charging roller 2.

As a result, the adhesion of the dirt substance 10 and adhesion dirt 10a having the same polarity as the developer (minus polarity) from the photosensitive drum 1 side to the charging roller 2 side is achieved during the first Δt 1 second, and the adhesion of the dirt substance 10 and the adhering dirt 10a from the surface of the photosensitive drum 1 to the charging roller 2 side is prevented. Dirt with the same polarity as the developer attached to the photosensitive drum moves to the first side of the photosensitive drum and is cleaned, and the next Δt2
In seconds, dirt substances having a polarity opposite to that of the developer (positive polarity) adhering to the charging roller 2 are transferred to the surface of the photosensitive drum 1 and cleaned.

After drying and Δt2 seconds have elapsed, the pressure means 15 is turned on to bring the charging roller 2 into pressure contact with the photosensitive drum 1, and at the same time, a normal bias, AC+DC bias, is applied.

As a result, it is possible to completely prevent defective output images due to dirt on the charging roller.

The charging member is not limited to the roller type in each embodiment, but may also be a blade type or
Similar effects can be obtained by applying the present invention to brush type, rod type, butt type, etc.

(Effects of the Invention) As described above, according to the present invention, in an image forming apparatus which is a contact type charging device and is used as a charging processing means for an image bearing member, the contact charging member is contaminated, and the image quality of the output image due to the staining. By preventing the decline, the intended purpose is well achieved.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the device of the first embodiment, and FIG.
is a charging roller pressure state diagram, the same figure (B) is a pressure release state diagram, and the same figure (C) is a control timing chart of pressure application and pressure release. FIG. 2 is an explanatory diagram of the second embodiment device, and FIG.
(B) is a diagram showing a state in which the charging roller is pressed, and (B) is a diagram in which the pressure is released. FIG. 3 is a control timing chart of pressurization/pressurization release and bias application of the fourth embodiment device. FIG. 4 is a schematic configuration diagram of an example of a printer using a contact-type charging device. FIG. 5 is a control timing chart. Reference numeral 1 denotes a photosensitive drum as an image carrier, 2 a charging roller as a contact charging member, and 15 an electromagnetic solenoid plunger as a pressure means. Prisoner N Turn on the device Print

Claims (2)

[Claims] (1) Image formation is performed by applying an image forming process that includes a step of charging the surface of the image bearing member to an image bearing member that is driven to move the surface, and the image bearing member is cleaned by a cleaning means and images are repeatedly formed. The image forming apparatus is a contact-type charging device in which the charging member for the image carrier is charged by bringing a charging member to which a bias voltage is applied into pressure contact with the image carrier using a pressure member, The pressurizing means can be freely switched between a pressurizing state in which the charging member is brought into pressure contact with the surface of the image carrier with a predetermined pressurizing force, and a pressurizing state in which the pressurizing force is released. While the surface movement drive of the body is stopped, the pressure is released. After the surface movement drive of the image carrier is started, the surface portion of the image carrier that was facing the cleaning means at the time of activation is in the pressure release state. An image forming apparatus characterized in that the image forming apparatus is controlled to be switched to a pressurized state after passing through a charging member position. (2) When the charging member is substantially in contact with the image bearing member in the pressurizing state of the charging member, the charging member is pressed after the surface movement drive of the image bearing member is activated. 2. The image forming apparatus according to claim 1, wherein a DC bias is applied to the charging member until the charging member is controlled to be switched to the pressurized state.