JPH1078714A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent fogging of toner when a paper sheet does not pass through and to reduce the sticking of the toner to a transfer roller by switching bias supply, so as to make the absolute value of the surface potential of an image carrier when a recording material does not pass through, in a transferring nip part higher than that when the recording material passes through. SOLUTION: A bias power source part 24 is provided with a changeover switch 27 as a switching means for selecting a DC bias, constant-current AC power source 28 connected to DC power source 25 or 26 with the changeover switch 27, etc. In this device, the bias power source is switched to make the absolute value of the surface potential of the photoreceptor drum 1 when the recording material does not pass through, in the transferring nip part higher than that when the recording material passes through. In other words, the changeover switch 27 is switched to select the DC power source 25 or 26 of the bias power source part 24, so that a DC voltage power source to an electrifying roller 6 is made variable to switch the surface potential of the drum 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for cleaning a contact-type transfer member by applying a voltage having a polarity different from the voltage applied during transfer to the contact-type transfer member.

[0002]

2. Description of the Related Art A conventional image forming apparatus is configured, for example, as shown in FIG. That is, in FIG.
Is an organic photoreceptor (OP
C) A rotating drum type electrophotographic photosensitive member (hereinafter, referred to as a “photosensitive drum”) having a layer applied thereon, and the photosensitive drum 1 has an outer diameter of 3 mm.
It is driven to rotate at 0 mm and at a process speed of 100 mm / sec.

The photosensitive drum 1 is provided with a charging device 2 for uniformly charging the photosensitive drum 1 around the photosensitive drum 1 in the rotation direction.
An exposure device (not shown) that irradiates exposure light (laser light) L to form an electrostatic latent image on the photosensitive drum 1, and a developing device that visualizes the electrostatic latent image formed on the photosensitive drum 1 3, a transfer device 4 for transferring a toner image formed on the photosensitive drum 1 to a transfer sheet P as a recording material, and a cleaning device 5;

The charging device 2 includes a charging roller 6 as a contact charging member, and a charging bias power supply 7 for applying a charging bias to the charging roller 6.

The developing device 3 includes a developing container 8 for storing a toner as a developer and a photosensitive drum 1 for transferring the toner.
The image forming apparatus includes a developing roller 9 as a developing member for visualizing the electrostatic latent image above, and a developing bias power supply 10 for applying a developing bias to the developing roller 9.

The transfer device 4 includes a transfer roller 11 as a contact transfer member having an outer diameter of 20 mm,
A transfer bias power supply 12 for applying a transfer bias of about 000 to 6000 V, a transfer roller cleaning bias power supply 23 for applying a transfer roller cleaning bias to the transfer roller 11 when paper is not passed, and a transfer bias power supply for applying a bias to the transfer roller 11 And a changeover switch 39 for selecting either the power supply 12 or the transfer roller cleaning bias power supply 23. Then, the transfer roller 11 is provided with a conductive elastic layer 11b on a metal core 11a.
And the resistance value of the elastic layer 11b is 10 ⁶ to 1
It is formed to about 0 ¹⁰ Ωcm.

The cleaning device 5 includes a cleaning blade 13 that scrapes off transfer residual toner from the photosensitive drum 1.
And a collection container 14 for storing the waste toner scraped off by the cleaning blade 13.

The transfer paper P on which the toner image has been transferred is curvature-separated due to the small diameter of the photosensitive drum 1, and the charge is removed by a charge removing needle unit 15 which is a separation assisting means. And the photosensitive drum 1 is separated from the photosensitive drum 1 by weakening the electrostatic attraction force.

The static elimination needle unit 15 includes a static elimination needle 15a and an insulating member 15b, and the static elimination needle 15a is electrically grounded.

The transfer paper P that has been separated is conveyed on a conveyance guide 16 made of insulating resin and conveyed to a fixing device 17.

The fixing device 17 is formed by spraying or coating PFA particles on an aluminum cylindrical cored bar 18a by spray coating or dispersion, and then baking and coating.
Roller 18 having a release layer 18b formed thereon, a halogen heater 19 for heating the same from the inside, and transfer paper P
And a fixing inlet guide 21 for guiding the transfer paper P to a fixing nip between the fixing roller 18 and the pressing roller 20.
The transfer paper P on which the toner image has been transferred passes through the fixing nip, so that the toner is fixed on the paper surface.

When a bias is applied to the charging roller 6 by a bias power supply 7, the photosensitive drum 1
The image is uniformly charged to a negative dark potential (V _D ) at 50 V and then image-exposed by exposure light L to form an electrostatic latent image in which the exposed portion has a bright potential (V _L ). V _{L in} this case
Is about -200V.

The electrostatic latent image on the photosensitive drum 1 is
Is developed. That is, when a bias is applied to the developing roller 9 by the developing bias 10, the negatively charged toner on the developing roller 9 changes the electrostatic latent image on the photosensitive drum 1 to the _VL portion on the photosensitive drum 1. To a visible image.

The toner image on the photosensitive drum 1 is transferred to a transfer sheet P conveyed along a transfer guide 22 at a transfer nip N between the photosensitive drum 1 and the transfer roller 11.

The toner adhered as dirt to the transfer roller 11 at the time of transfer is transferred to the transfer roller 11 by the transfer roller cleaning bias power supply 23 when the paper is not passed during the pre-rotation or the post-rotation. When the bias is applied, the photosensitive drum 1 is transferred to the photosensitive drum 1 and collected by the cleaning device 5, whereby the surface of the photosensitive drum 1 is cleaned.

FIG. 10 is a sequence diagram showing the relationship between the surface potential of the photosensitive drum, the developing bias, and the transfer bias of the conventional image forming apparatus. In this case, the surface potential of the photosensitive drum 1 and the developing device 3 The relationship between the bias and the transfer bias of the transfer device 4 will be described with reference to the circumferential position of the photosensitive drum 1.

The photosensitive drum 1 is by the charging roller 6 from the front when the rotation is uniformly charged to V _D potential of -650 V, to form an electrostatic latent image by exposing in accordance with image information the uniformly charged portion.

The photosensitive drum 1 is rotated from the time before the charging position reaches the transfer nip N by the timing when the transfer paper P reaches the transfer nip N. The charging roller 6 charges the surface V _D to −650 V until the charging position reaches the transfer nip N in accordance with the timing of passing through. In this case, V of the image portion of the electrostatic latent image formed by the exposure light L
_L becomes -200V.

The developing roller 9 turns the electrostatic latent image on the photosensitive drum 1 into a visible image in synchronization with the timing at which the transfer paper P reaches the transfer nip N, A developing bias of a DC voltage of -500 V is applied until the toner image reaches the transfer nip N in accordance with the timing at which the transfer paper P passes through the transfer nip N.

The transfer roller 11 is supplied with a cleaning voltage of 1500 V from the transfer roller cleaning bias power supply 23 when the surface potential of the photosensitive drum 1 is 0 V during the pre-rotation and post-rotation, and when the paper is passed, the transfer roller 11 is rotated.
Transfer bias V _T is applied, the sheet interval it bias voltage lower V _T0 is applied than to. Transfer bias V _T In this case, voltage to ensure transfer current required for sheet passing, which varies depending on the resistance of the transfer roller 11, A
A voltage for securing a current of about 10 μA is applied in order to correspond to the transfer paper P of 3 size or letter size. To prevent transfer memory to the photosensitive drum 1, _VT0 applies a voltage for securing the same transfer current as at the time of paper passing.

In the above example, the process speed is 100
In the case of the image forming apparatus of mm / sec, when the process speed is 45 mm / sec or less, or in the case of manual paper feeding in which the paper size is uncertain, the bias relationship may be as shown in FIG. .

FIG. 11 is a sequence diagram in which the transfer bias is used as the cleaning bias even between sheets. This is used when the process speed is slow and the rise of the high-voltage power supply is sufficiently fast with respect to the paper passing speed, or when the manual paper feeding mode for small-size paper is likely to be uncertain with respect to the toner image position. In some cases, this method is used to clean toner adhered to the transfer roller 11 as dirt.

[0023]

However, the above-mentioned prior art has the following problems.

1. The developing step of the electrostatic latent image, during pre-rotation, causes so-called fog occurs that the toner adheres to the V _D portion of the non-sheet passing portion, such as during the sheet interval and the post-rotation, fog non-sheet passing The toner on the transfer roller 11 adheres to the transfer roller 11 and thereby stains the rear surface of the transfer paper P.

In the fixing step, the toner transferred from the transfer roller 11 to the rear surface of the transfer paper P is
, The releasability of the pressure roller 20 is significantly reduced, and a problem such as paper jam due to winding of the transfer paper P may occur.

2. Even when the transfer bias is used as the cleaning bias between the sheets, if the toner printed on the non-sheet passing area adheres to the transfer roller 11 due to the mismatch between the toner image position and the transfer timing of the transfer sheet P, the transfer bias may be changed between the sheets. It is difficult to sufficiently clean the transfer paper 11, and the toner remains on the transfer roller 11, and the back surface of the transfer paper P becomes dirty in the next rotation.

3. When a paper jam occurs due to a failure or abrasion of the paper transport system, the transfer paper P remaining in the apparatus is removed, and the toner image on the photosensitive drum 1 is directly transferred to the transfer roller 11 to restart the apparatus. I will touch it. In order to prevent the toner from adhering to the transfer roller 11 at this time, a cleaning bias is applied to the transfer roller 11 during the previous rotation. However, when the printing rate is large like a graphic image, the cleaning bias is electrically applied. The action of repelling the toner becomes insufficient, and the toner is transferred to the transfer roller 11 to cause back surface contamination.

4. Further, in the case of an image forming apparatus using a one-component toner as a developer, the toner rubs against each other,
In some cases, reversal toner is generated.

Since the reversal toner has a polarity opposite to the polarity of the cleaning bias applied by the transfer roller 11, if the reversal toner adheres and deposits on the transfer roller 11, it cannot be cleaned by the cleaning bias. This causes stains on the back surface and toner adhesion to the pressure roller 20.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and it is intended to prevent fogging toner when paper is not passed, reduce toner adhesion to a transfer roller, and apply It is an object of the present invention to provide an image forming apparatus which prevents toner from adhering and paper jam due to the toner.

[0031]

In order to achieve the above object, the invention according to the first aspect of the present invention relates to a charging device for uniformly charging an image carrier, and a charging device for charging the image carrier. An exposure device for exposing an image carrier to form an electrostatic latent image; a developing device for visualizing the electrostatic latent image with a developer to form a toner image on the image carrier; And a transfer device for transferring the toner image to a recording material at a transfer nip portion, wherein the charging device applies a charging bias to the image bearing member and applies a charging bias to the charging member. A bias power supply unit, the bias power supply unit configured to determine a surface potential of the image carrier when the recording material is not passed through the transfer nip unit, and to control the image potential when the recording material is passed through the transfer nip unit. From the surface potential of the carrier And setting the even larger absolute value.

According to a second aspect of the present invention, there is provided a charging device for uniformly charging an image carrier, and exposing the image carrier charged by the charging device to form an electrostatic latent image. And a transfer device for transferring the toner image on the image carrier to a recording material at a transfer nip portion by forming a toner image on the image carrier by visualizing the toner image with a developer. A transfer device that transfers the toner image onto a recording material, and applies a voltage having the same polarity as the charge polarity of the developer during cleaning of the contact transfer member; And a transfer bias power supply unit for applying a voltage having a polarity opposite to the charge polarity of the developer at the time of transfer of the toner image to the toner image transfer device. surface And a charging bias power supply unit for making the surface potential of the recording material different from that of the transfer nip when the recording material is passed, so as to increase the potential difference between the cleaning bias of the contact transfer member and the surface potential of the image carrier. It is characterized by the following.

According to the third aspect of the present invention, the charging device is configured to charge the image carrier when the cleaning bias is applied, more than the charging potential of the image carrier when the recording material passes through the transfer nip portion. The potential was reduced.

According to the fourth aspect of the present invention, the charging device is configured such that a polarity of a charging potential with respect to the image carrier when the recording material passes through the transfer nip portion and a voltage with respect to the image carrier when the cleaning bias is applied. The polarity of the charging potential was reversed.

According to the fifth aspect of the present invention, the developing device has a developing member to which a developing bias power is applied.
When a cleaning bias is applied to the contact type transfer member, the developing bias power supply applies a voltage to the developing member to prevent transfer of the developer to the image carrier.

According to a sixth aspect of the present invention, there is provided a charging device for uniformly charging an image carrier, an exposure device for exposing the image carrier charged by the charging device to form an electrostatic latent image, and A developing device for forming a toner image on the image carrier by visualizing an electrostatic latent image with a developer, and a transfer device for transferring the toner image on the image carrier from a transfer nip portion to a recording material Wherein the transfer device applies a voltage having the same polarity as a charged polarity of a developer during cleaning of the contact-type transfer member and the contact-type transfer member for transferring the toner image onto a recording material. A transfer bias power supply unit for applying a voltage having a polarity opposite to that of the developer when transferring the toner image onto the material, and alternately applying a positive and negative polarity voltage to the contact transfer member before and after image formation. With Wherein the image bearing member and applying alternating voltages reverse polarity of the voltage applied to the contact type transfer member.

According to the seventh aspect of the present invention, the voltage applied to the contact type transfer member when cleaning the contact type transfer member is a voltage for transferring the toner image on the image carrier to the recording material. Lower.

According to the present invention, the voltage applied to the charging member for charging the image carrier and the voltage applied to the developing member of the developing device have the same polarity when cleaning the contact transfer member. I do.

[Operation] According to the configuration of the first aspect, the surface potential of the image carrier when the recording material is not passed through the transfer nip portion is changed to the image potential when the recording material is passed through the transfer nip portion. By switching the bias power supply so that the absolute value is larger than the surface potential of the carrier, the surface potential of the image carrier during non-sheet passing is made larger than at the time of sheet passing, and the fog margin with the developing bias is increased. To prevent the toner from adhering to the non-image forming area.

According to a second aspect of the present invention, the transfer device applies a voltage having the same polarity as the charge polarity of the developer when the cleaning bias is applied to the contact-type transfer member, and simultaneously applies a voltage to the recording material. During the transfer of the toner image to the developer, a voltage having a polarity opposite to the charge polarity of the developer is applied. The charging device may be configured to make a surface potential of the image carrier when a cleaning bias is applied to the contact type transfer member different from a charging potential when the recording material passes through the transfer nip portion. The potential difference between the cleaning bias of the transfer member and the surface potential of the image carrier is increased.

According to a sixth aspect of the present invention, the transfer device applies a voltage having the same polarity as the charged polarity of the developer when cleaning the contact-type transfer member, and the toner image on the recording material. Is applied, a voltage having a polarity opposite to that of the developer is applied. Before and after image formation, positive and negative voltages are alternately applied to the contact transfer member, and a voltage having a polarity opposite to the voltage applied to the contact transfer member is alternately applied to the image carrier. .

[0042]

Embodiments of the present invention will be described below in detail with reference to the drawings. <First Embodiment> FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to a first embodiment of the present invention, and FIG.
FIG. 5 is a sequence diagram showing the relationship between the surface potential of the photosensitive drum and the bias potentials of charging, development, and transfer. In FIG. 1, the same or equivalent components as those in FIG. 9 are denoted by the same reference numerals, and redundant description will be omitted.

In FIG. 1, reference numeral 24 denotes a bias power supply unit for the charging roller 6;
DC power supply 25 that generates a DC bias of
A DC power supply 26 for generating a DC bias, a changeover switch 27 as a switching means for selecting one of these DC biases, and a constant current AC connected to the DC power supply 25 or the DC power supply 26 via the changeover switch 27. Power supply 2
8 is provided. The constant current AC power supply 28
A sine wave AC bias of 000 Hz and 1400 μA (rms value) is generated.

In this embodiment, the changeover switch 27 is switched so that the DC power supplies 25 and 26 of the bias power supply section 24 are switched.
Is selected, and the surface potential of the photosensitive drum 1 is switched by varying the DC voltage applied to the charging roller 6.

Next, the operation will be described with reference to FIG.

First, at the time of pre-rotation, the bias power supply 24 for applying a bias to the charging roller 6 is in an off state. However, in order to sufficiently raise the surface potential of the photosensitive drum 1 in the image forming area, w is shown in FIG. The bias power supply unit 24 for applying the charging roller 6 from a position immediately before the image forming area shown in FIG.
₍ Indicated as _D ) is applied to switch the changeover switch 27 to connect the DC power supply 26 and the constant current AC power supply 28 to turn them on. As a result, the charged potential _VDK of the photosensitive drum 1 becomes about -900V.

When the portion corresponding to the image forming area is charged by the photosensitive drum 1, the changeover switch 27 is switched so that −650 V is applied as a DC component of the bias power supply 24 applied to the charging roller 6. By connecting a DC power supply 25 and a constant current AC power supply 28, the photosensitive drum 1
Is charged to a normal _VD potential and is irradiated with exposure light L to form an electrostatic latent image. Further, the sheet interval, the photosensitive drum 1 also post-rotation, the charge potential V _DK connects the DC power source 26 so as to be -900V and the constant current AC source 28.

[0048] That is, only the image forming region, the surface potential of the photosensitive drum 1 is charged so that V _D potential of -650V regular, other non-image formation region of, V of -900V
_The charging potential is increased so as to _{reach the DK} potential.

By operating as described above, the potential difference between the surface potential of the photosensitive drum 1 and the developing roller 9 when paper is not passed can be made a large potential difference to which fog toner is less likely to adhere. , And the toner adhesion to the transfer roller 11 can be suppressed.

As a result, it is possible to prevent the back surface of the transfer paper P from being stained during continuous paper feeding, and at the same time to prevent toner from adhering to the pressure roller 20, and to prevent paper jams caused by toner adhesion to the pressure roller 20. Can be prevented. <Second Embodiment> Next, a second embodiment of the present invention will be described with reference to FIG.

In the figure, a bias power supply unit 29 of the charging roller 6 has a DC power supply 30 for generating a voltage of -200 V.
, A DC power supply 25 for generating a voltage of -650 V, and a changeover switch 2 for selecting any one of these DC biases.
7 and a constant-current AC power supply 28 connected to either the DC power supply 25 or the DC power supply 30 via the changeover switch 27.

Reference numeral 31 denotes a developing bias power supply. The developing bias power supply 31 includes an AC bias power supply 32 for performing stable development, a DC power supply 33 for generating a potential _VDC on the developing roller 9, and an AC power supply. A changeover switch 34 for switching whether to superimpose the DC power supply 33 on the bias power supply 32. And an AC bias power supply 32
Generates a rectangular wave alternating bias of 2000 Hz and a peak-to-peak voltage Vpp of 1600 V.
Generates a DC bias of 0V. 3, the same or equivalent components as those in FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted.

The operation of this embodiment will be described with reference to FIG.

In this embodiment, the transfer roller 11
The purpose of the present invention is to enhance the effect of cleaning when applying a cleaning bias to.

The charging potential of the photosensitive drum 1 during printing, the developing bias of the developing roller 9 and the transfer bias of the transfer roller 11 are the same as in the first embodiment.

On the other hand, in order to enhance the effect of the cleaning bias of the transfer roller 11, the DC power source 2
5 is connected to a DC power supply 30.
It is connected, to reduce the absolute value of the surface potential of the photosensitive drum 1 from -650 V (shown as V _D in FIG. 4) to -200 V (shown as V _L in Figure 4). Then, the potential difference between the transfer roller 11 and the photosensitive drum 1 is increased, so that the toner on the transfer roller 11 is easily transferred to the photosensitive drum 1. Further, in order to prevent the toner from transferring from the developing roller 9 to the photosensitive drum 1, the DC component of the developing bias is set to 0V.
And

By doing so, the effect of cleaning the transfer roller 11 between the paper sheets is improved, and even in an image forming apparatus having a manual paper feed mode in which the image and the transfer paper size are likely to be inconsistent, the transfer is performed. The contamination of the roller 11 with the toner can be prevented.

That is, it is possible to prevent the back surface of the transfer sheet P from being stained and the stain on the pressure roller 20 due to the stain and the paper jam caused by the stain. <Third Embodiment> Next, a third embodiment of the present invention will be described with reference to FIG.

In the figure, a bias power supply unit 35 of the charging roller 6 includes a DC power supply 36 for generating a bias of +500 V, a DC power supply 25 for generating a voltage of -650 V,
A changeover switch 27 for selecting one of these DC biases, and a DC power supply 25
Or a constant-current AC power supply 28 connected to one of the DC power supplies 36. In FIG. 5,
The same or equivalent components as those in FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted.

The operation of this embodiment will be described with reference to FIG.

Normally, the cleaning of the transfer roller 11 is performed by pre-rotation and post-rotation.

Although not shown in FIG. 6, the transfer roller cleaning bias power supply 23 is applied to the transfer roller 11 even when the process parts and the conveyance roller are driven to rotate for forced ejection after a paper jam. To apply a cleaning bias to clean the transfer roller 11.

When the cleaning bias is applied to the transfer roller 11 as described above, the cleaning effect increases as the potential difference between the surface potential of the photosensitive drum 1 and the cleaning bias applied to the transfer roller 11 increases. Therefore, the cleaning bias during the application, the charging roller 6 a bias DC component of which is applied to switch from direct-current power source 25 to the DC power supply 36 is the + 500V, and approximately + 500V the surface potential of the photosensitive drum 1 (shown in Figure 6 as V _P) The cleaning bias of the transfer roller 11 is set to -150.
By setting the voltage to 0 V, a potential difference of approximately 2000 V can be generated during that time, and more excellent cleaning of the transfer roller 11 can be performed.

As a result, it is possible to prevent the transfer paper P after the paper jam processing from being stained by the toner on the back surface thereof, and to prevent the toner from adhering to the pressure roller 20 and the occurrence of transfer paper transfer failure.

The surface potential of the photosensitive drum 1 is set to +500
When the voltage is set to V, the developing device 3 is required to be separated from the photosensitive drum 1 or a DC bias of +500 V is applied to the developing bias power supply unit 10 to prevent the toner from adhering to the photosensitive drum 1 from the developing roller 9. It is. <Fourth Embodiment> Next, a fourth embodiment of the present invention will be described with reference to FIG.

In this embodiment, a transfer device 4 of an image forming apparatus having a developing device 3 using a one-component developer will be described.

In the figure, the developing bias power supply 37
Is an AC bias power supply 32 for performing stable development.
When a DC power source 33 for generating a DC potential V _DC of -500V to the developing roller 9, the DC power source 38 for generating a voltage of + 200V, DC potential -500V or + 200
A changeover switch 34 for switching between superimposing an AC bias power supply 32 on V and applying only an AC bias to the developing roller 9. That is, the DC component of the developing bias is supplied to the DC power
-500V, DC power supply 38 + 200V, or 0V
Can be switched to either. The AC bias power supply 32 has a frequency of 2000 Hz and a peak-to-peak voltage Vpp of 1600.
An alternating bias of a V square wave is generated. In FIG. 7, the same or equivalent components as those in FIG. 3 are denoted by the same reference numerals, and redundant description will be omitted.

The operation of this embodiment will be described with reference to FIG.

When a one-component toner is used as the developer, some toner is charged to a polarity opposite to the original charging polarity due to friction between the toners in the developing container 8. For example, there is a type in which a minimal amount of toner is positively charged even if the toner is negatively charged.

In this embodiment, since toner adhering to the transfer roller 11 as dirt is charged to both polarities, it has already been proposed to alternately apply positive and negative voltages as a cleaning bias for the transfer roller 11. However, it is characterized in that the effect is further enhanced and sufficient cleaning is performed.

In FIG. 11, the transfer roller 11 is rotated during the pre-rotation.
Applying a cleaning bias to, but the voltage to be applied this time is alternately applied to the sheet between the bias V _TO is lower than -1500V and the transfer bias V _T.

Since the effect of cleaning the transfer roller 11 increases as the potential difference with the photosensitive drum 1 increases, the surface potential of the photosensitive drum 1 becomes +200 V (−200 V in FIG. 8) when −1500 V is applied to the transfer roller 11. ₍ Indicated as _P ), and the paper gap bias V _TO is applied to the transfer roller 11.
Is applied, the surface potential of the photosensitive drum 1 becomes −650.
V (shown as V _{D in} FIG. 8).

Further, the surface potential of the photosensitive drum 1 is set to +200
The toner from the developing roller 9 to the photosensitive drum 1 when the V is prevented from metastasis, the portion of the surface potential of the photosensitive drum 1 is + 200V (shown as V _P in FIG. 8), to the developing roller 9 + 200V (Displayed as _{VDCP in} Fig. 8)
Is applied.

With the above-described configuration, even in the transfer device of the image forming apparatus having the developing system having the reversal toner, the transfer roller 11 can be prevented from being stained by the toner.
It is possible to prevent stains on the back surface of the transfer paper P, stains on the pressure roller 20, and paper jam caused by the pressure roller 20.

[0075]

As described above, according to the first aspect of the present invention, the surface potential of the image carrier when the recording material is not passed through the transfer nip portion is determined by the recording material passing through the transfer nip portion. By switching the bias power by the switching means so that the surface potential of the image carrier at the time of paper becomes larger than the surface potential of the image carrier at the absolute value, the surface potential of the image carrier in the non-paper passing portion is made larger than that of the paper portion so that the developing bias and To prevent the toner from adhering to the non-image area.

According to the second aspect of the present invention, when the contact type transfer member for transferring the toner image on the image carrier onto the recording material is cleaned, the transfer device has the same polarity as the charged polarity of the developer. In addition to applying a voltage, a voltage having a polarity opposite to the charging polarity of the developer is applied during transfer of the toner image onto the recording material. The charging device may be configured to cause a surface potential of the image carrier when a cleaning voltage is applied to the contact transfer member to be different from a charging potential when the recording material is passed through the transfer nip portion. The potential difference between the cleaning bias and the surface potential of the image carrier is increased.

According to a sixth aspect of the present invention, when the contact type transfer member for transferring the toner image on the image carrier onto the recording material is cleaned, the transfer device has the same polarity as the charged polarity of the developer. And at the time of transfer of the toner image onto the recording material, a voltage having a polarity opposite to the charging polarity of the developer is applied. Before and after the image formation, positive and negative voltages are alternately applied to the contact transfer member, and a voltage having a reverse polarity to the voltage applied to the contact transfer member is alternately applied to the image carrier. I do.

That is, each of the above-described inventions prevents the toner from adhering to the contact-type transfer member as dirt.
As a result, it is possible to prevent the back surface of the recording material from being stained and the toner from accumulating on the pressure roller and causing paper jam.

[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 sequence diagram illustrating the relationship between the surface potential of the photosensitive drum and each bias according to the first embodiment.

FIG. 3 is a schematic configuration diagram illustrating an image forming apparatus according to a second embodiment of the present invention.

FIG. 4 is a sequence diagram illustrating the relationship between the surface potential of a photosensitive drum and each bias according to the second embodiment.

FIG. 5 is a schematic configuration diagram illustrating an image forming apparatus according to a third embodiment of the present invention.

FIG. 6 is a sequence diagram illustrating a relationship between a surface potential of a photosensitive drum and each bias according to a third embodiment.

FIG. 7 is a schematic configuration diagram illustrating an image forming apparatus according to a fourth embodiment of the present invention.

FIG. 8 is a sequence diagram illustrating a relationship between a surface potential of a photosensitive drum and respective biases according to a fourth embodiment.

FIG. 9 is a schematic configuration diagram illustrating an example of a conventional image forming apparatus.

FIG. 10 is a sequence diagram illustrating the relationship between the surface potential of a photosensitive drum and each bias in a conventional image forming apparatus.

FIG. 11 is a sequence diagram illustrating another example of the relationship between the surface potential of the conventional photosensitive drum and each bias.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Image carrier (photosensitive drum) 2 Charging device 3 Developing device 4 Transfer device 6 Charging member (charging roller) 11 Contact transfer member (transfer roller) 12 Transfer bias power supply 23 Transfer cleaning bias power supply 24 Charging bias power supply unit 27 Switching means ( Selector switch) 29 charging bias power supply unit 35 charging bias power supply unit P Recording material (transfer paper)

Claims (8)

[Claims] A charging device for uniformly charging the image carrier; an exposure device for exposing the image carrier charged by the charging device to form an electrostatic latent image; and a developer for charging the electrostatic latent image. An image forming apparatus comprising: a developing device that forms a toner image on the image carrier by visualizing the toner image on the image carrier; and a transfer device that transfers the toner image on the image carrier to a recording material at a transfer nip portion. The charging device has a charging member for charging the image carrier, and a bias power supply unit for applying a charging bias to the charging member. The bias power supply unit is configured to prevent the recording material from passing through the transfer nip unit. The surface potential of the image carrier at the time of printing is set to an absolute value greater than the surface potential of the image carrier at the time of passing the recording material through the transfer nip portion. A charging device for uniformly charging the image carrier, an exposure device for exposing the image carrier charged by the charging device to form an electrostatic latent image, and a developer for charging the electrostatic latent image. An image forming apparatus comprising: a developing device that forms a toner image on the image carrier by visualizing the toner image on the image carrier; and a transfer device that transfers the toner image on the image carrier to a recording material at a transfer nip portion. A transfer device configured to transfer the toner image onto a recording material; a transfer device configured to apply a voltage having a polarity opposite to a charge polarity of a developer during transfer of the toner image onto the recording material; And a transfer bias power supply for applying a voltage having the same polarity as the charge polarity of the developer at the time of cleaning, and the charging device is provided with a surface of the image carrier at the time of applying a cleaning bias to the contact-type transfer member. A charging bias power supply unit for causing a potential and a surface potential of the recording material to pass through the transfer nip portion when the recording material is passed to increase a potential difference between a cleaning bias of the contact transfer member and a surface potential of the image carrier; An image forming apparatus, characterized in that: 3. The charging device according to claim 1, wherein a charging potential of the image carrier when the cleaning bias is applied is smaller than a charging potential of the image carrier when the recording material passes through the transfer nip portion. The image forming apparatus according to claim 2, wherein 4. The charging device according to claim 1, wherein the polarity of the charging potential with respect to the image carrier when the recording material passes through the transfer nip portion is opposite to the polarity of the charging potential with respect to the image carrier when the cleaning bias is applied. The image forming apparatus according to claim 2, wherein: 5. The developing device has a developing member to which a developing bias power supply is applied, and when the cleaning bias is applied to the contact-type transfer member, the developing bias power supply causes transfer of the developer to the image carrier. The image forming apparatus according to claim 2, wherein a voltage to be prevented is applied to the developing member. 6. A charging device for uniformly charging an image carrier, an exposure device for exposing the image carrier charged by the charging device to form an electrostatic latent image, and a developer for charging the electrostatic latent image. An image forming apparatus comprising: a developing device that forms a toner image on the image carrier by visualizing the toner image on the image carrier; and a transfer device that transfers the toner image on the image carrier to a recording material at a transfer nip portion. A transfer device that transfers the toner image onto a recording material, and applies a voltage having the same polarity as a charged polarity of a developer during cleaning of the contact type transfer member; A transfer bias power supply unit for applying a voltage having a polarity opposite to that of the developer at the time of transfer, and before and after image formation, a positive and negative polarity voltage is alternately applied to the contact-type transfer member, Carrier Applies alternating voltage and reverse polarity of the voltage applied to the contact type transfer member, that the image forming apparatus according to claim. 7. The method according to claim 1, wherein a voltage applied to the contact-type transfer member during cleaning of the contact-type transfer member is lower than a voltage for transferring a toner image on the image carrier onto a recording material. The image forming apparatus according to claim 6. 8. The cleaning method according to claim 1, wherein a voltage applied to a charging member for charging the image carrier and a voltage applied to a developing member of the developing device have the same polarity when cleaning the contact transfer member. The image forming apparatus according to claim 6.