JP2002214931A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of surely preventing an after image caused by polarization remaining in the high resistance layer of an intermediate transfer belt having the high resistance layer advantageous in terms of the prevention of transfer scattering before primary transfer even in the case of using such an intermediate transfer belt. SOLUTION: This image forming device is provided with a polarization uniformizing means for uniformizing the polarization remaining in the high resistance layer 501b of the intermediate transfer belt 501 having the high resistance layer whose volume resistivity is >=1010 Ωcm while keeping the polarity of the polarization before starting the primary transfer of a toner image to the belt 501 after starting the surface movement of the belt 501 in the case of starting image forming operation by using the belt 501. A secondary transfer device 600 is used also as the polarization uniformizing means, and pre- processing bias for uniformizing the polarization is controlled to be applied to a secondary transfer bias roller 605 coming into contact with the belt 501 before starting the primary transfer.

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 such as a copying machine, a printer, and a facsimile, and more particularly, to a latent image carrier, and a latent image forming means for forming a latent image on the latent image carrier. Developing means for developing the latent image on the latent image carrier into a toner image, an intermediate transfer member on which the toner image on the latent image carrier is transferred, and a toner image on the latent image carrier on the intermediate transfer member The present invention relates to an image forming apparatus provided with a primary transfer unit for transferring and a secondary transfer unit for transferring a toner image on an intermediate transfer member to a transfer material.

[0002]

2. Description of the Related Art Conventionally, as this type of image forming apparatus,
2. Description of the Related Art An image forming apparatus such as an intermediate transfer type color copying machine or a color laser printer that forms a plurality of color toner images by superimposing them on an intermediate transfer body and collectively transfers the toner images onto a transfer material is known. As a material of the intermediate transfer member, a polymer material having predetermined mechanical characteristics and static electricity characteristics is usually used. In an image forming apparatus employing such an intermediate transfer method, there is a problem that "transfer dust" in which toner scatters around a toner image primarily transferred from a latent image carrier onto an intermediate transfer body is likely to occur.

[0003]

As a measure for preventing the occurrence of transfer dust, it is conceivable to use an intermediate transfer member having a high-resistance layer having a volume resistivity of 10 ¹⁰ Ωcm or more. When such an intermediate transfer member is used, the potential of the latent image carrier can be transferred together with the toner image at the time of primary transfer from the latent image carrier to the intermediate transfer member, and the latent image carrier can be held on the intermediate transfer member. By the action of the transferred potential, the toner can be prevented from scattering from the toner image transferred to the intermediate transfer member.

However, the above volume resistivity is 10 ¹⁰ Ωc.
In an image forming apparatus using an intermediate transfer member having a high resistance layer of m or more, when the same image is continuously formed in a large amount, a residual image in which a black portion of the previous page becomes whitish in a low image density (ID) image of the next page. May occur. The inventors of the present invention evaluated and analyzed the cause of this afterimage. As a result, even if static elimination was performed from the outside of the intermediate transfer member, electric charges remained inside the intermediate transfer member, and the distribution of the residual charges was changed as shown on the next page. It was found that an afterimage was generated in the image. It has been found that this residual charge easily occurs in the high-resistance layer inside the intermediate transfer member formed in a multilayer structure.

[0005] It has been difficult to remove the residual charge inside the intermediate transfer member by a static eliminator that applies a DC voltage having a polarity opposite to that of a conventional transfer bias to the intermediate transfer member.
It is considered that if the magnitude of the DC voltage applied to the intermediate transfer member is increased, the residual charges inside the intermediate transfer member can be eliminated to some extent, but there is a possibility that the surface of the intermediate transfer member may be seriously damaged. Was. It is considered effective to remove the residual charge inside the intermediate transfer member by using an AC voltage having a large amplitude. However, a DC voltage (current: several μA to several tens μA) is used. Since the current becomes as large as about 1 mA as compared with the case where there is, the surface of the intermediate transfer member may be damaged significantly, and the cost may be increased. In particular, in the case of an intermediate transfer body having a multilayer structure having a high resistance layer inside, the above-described conventional static elimination means only changes the potential of the surface layer, and it is impossible to directly apply a static elimination bias to the internal high resistance layer. Is difficult to remove, and there is a possibility that the surface layer may be seriously damaged.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and has as its object to provide an intermediate transfer member before the primary transfer even when an intermediate transfer member having a high resistance layer which is advantageous in preventing transfer dust is used. An object of the present invention is to provide an image forming apparatus capable of reliably preventing an afterimage due to polarization remaining in a high resistance layer of a transfer body.

[0007]

In order to achieve the above object, the present invention is directed to a latent image carrier, a latent image forming means for forming a latent image on the latent image carrier, and a latent image forming device. Developing means for developing the latent image on the carrier to form a toner image, an intermediate transfer body capable of moving the surface to which the toner image on the latent image carrier is transferred,
Transferring the toner image on the latent image carrier to the intermediate transfer body 1
An image forming apparatus including a secondary transfer unit and a secondary transfer unit that transfers a toner image on the intermediate transfer member to a transfer material;
As the intermediate transfer body, one having a high resistivity layer having a volume resistivity of 10 ¹⁰ Ωcm or more is used. After the surface movement of the intermediate transfer body is started at the start of the image forming operation, the toner image on the intermediate transfer body is transferred. Before the first transfer starts,
A polarization homogenizing means for homogenizing the polarization remaining in the high resistance layer of the intermediate transfer body while maintaining its polarity is provided. In the image forming apparatus according to the first aspect, after the surface movement of the intermediate transfer body is started at the start of the image forming operation, before the primary transfer of the toner image to the intermediate transfer body is started, the intermediate of the polarization uniforming unit is used. The polarization remaining in the high-resistance layer of the transfer body is made uniform. Then, by performing this polarization uniformization while maintaining the polarization polarity, compared to the case where the polarization remaining in the high-resistance layer is eliminated or the polarity of the polarization is reversed, the polarization is uniformed. The change in polarization is small, and the polarization remaining in the high-resistance layer can be more quickly and easily made uniform.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the polarization uniformizing means applies a pretreatment bias, which is controlled at a constant current, to the opposing member facing the intermediate transfer member. And a pre-processing bias applying means for applying. In the image forming apparatus according to the second aspect, a pretreatment bias is applied to the polarization uniformizing opposing member facing the intermediate transfer member to apply a predetermined amount of electric charge to the intermediate transfer member to increase the height of the intermediate transfer member. The polarization in the resistance layer can be made uniform. Here, since the pre-processing bias is controlled at a constant current, even if the electric resistance of the intermediate transfer member varies, a predetermined amount of charge is applied to the intermediate transfer member, and the polarization in the high-resistance layer of the intermediate transfer member is controlled. Can be made uniform.

According to a third aspect of the present invention, in the image forming apparatus of the second aspect, the secondary transfer means includes a secondary transfer member facing the intermediate transfer member and a secondary transfer bias controlled at a constant current. And a secondary transfer bias applying means for applying a secondary transfer bias to the secondary transfer member, wherein the current set value of the pre-processing bias is set to be equal to or larger than the current set value of the secondary transfer bias. Things. In the image forming apparatus according to the third aspect, since the current set value of the pre-processing bias is set to be equal to or larger than the current set value of the secondary transfer bias, the direction of the polarization change in the high-resistance layer of the intermediate transfer body is one direction. Become.

According to a fourth aspect of the present invention, in the image forming apparatus of the second aspect, the intermediate transfer member is constituted by an endless rotating member, and the application time of the pre-processing bias is set to one rotation of the intermediate transfer member. It is characterized by being an integral multiple of time. In the image forming apparatus according to the fourth aspect, since the application time of the pre-processing bias is an integral multiple of the rotation time of the intermediate transfer body, the distribution of polarization in the high-resistance layer of the intermediate transfer body does not have a step.

According to a fifth aspect of the present invention, in the image forming apparatus of the second aspect, the humidity detecting means for detecting the humidity and the preprocessing so as to change the preprocessing bias based on the detection result of the humidity detecting means. And control means for controlling the bias applying means. The image forming apparatus according to claim 5, wherein the pre-processing bias is changed based on the detection result of the humidity detecting means, so that a desired secondary transfer characteristic can be obtained even when the humidity changes. The polarization remaining in the high-resistance layer can be made a predetermined magnitude.

According to a sixth aspect of the present invention, in the image forming apparatus of the second aspect, a toner image is formed on the first surface so that the toner image on the intermediate transfer body can be secondarily transferred to both surfaces of the transfer material. A reversal transport unit for reversing the next-transferred transfer material and transporting it to a secondary transfer position such that the second surface faces the intermediate transfer body, and performs secondary transfer on the first surface of the transfer material It is characterized in that the preprocessing bias is made different between the time and the time of the secondary transfer to the second surface. The electrical resistance and the capacitance of the transfer material for secondary transfer to the first surface and the transfer material for secondary transfer to the second surface by inverting the transfer material after the secondary transfer to the first surface are different. Are different. Therefore, in the image forming apparatus according to the sixth aspect, the pre-processing bias is different between when the secondary transfer is performed on the first surface of the transfer material and when the secondary transfer is performed on the second surface. Accordingly, even when the secondary transfer is performed on any one of the first surface and the second surface of the transfer material, a desired 2
The magnitude of polarization of the high-resistance layer of the intermediate transfer member can be adjusted so as to obtain the next transfer characteristic.

According to a seventh aspect of the present invention, there is provided an image processing apparatus comprising:
In the image forming apparatus of the fourth, fifth or sixth aspect, the secondary transfer means is also used as the polarization uniformizing means. In the image forming apparatus according to claim 7,
Since the secondary transfer means is also used as the polarization equalizing means, the cost and size of the apparatus can be reduced.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to an electrophotographic color copying machine (hereinafter, referred to as a color copying machine) as an image forming apparatus will be described. First, the schematic configuration and operation of the color copying machine according to the present embodiment will be described with reference to FIG. This color copier includes a color image reading device (hereinafter, referred to as a color scanner) 1,
Color image recording device (hereinafter referred to as color printer)
2, a paper feed bank 3 and the like.

The color scanner 1 converts the image of the document 4 on the contact glass 121 into an illumination lamp 122, mirror groups 123a, 123b, 123c, and a lens 124.
And form an image on the color sensor 125 via. Then, the color image information of the document 4 is stored in, for example, Red, Green.
The image data is read for each of n, Blue (hereinafter, R, G, B) color separation lights and converted into an electric image signal. The color sensor 125 of this example is composed of R, G, and B color separation means and a photoelectric conversion element such as a CCD, and photoelectrically converts a three-color image of the document 4 separated by the color separation means. Reading by the element at the same time. Then, based on the intensity levels of the R, G, B color separation image signals obtained by the color scanner 1, a color conversion process is performed by an image processing unit (not shown), and Black (hereinafter, referred to as Bk), Cyan
(Hereinafter, referred to as C), Magenta (hereinafter, referred to as M), and Yellow (hereinafter, referred to as Y) color image data are obtained.

The operation of the color scanner 1 for obtaining the color image data of Bk, C, M and Y is as follows. Upon receiving a scanner start signal that is timed with the operation of the color printer 2 described later, an optical system including an illumination lamp 122 and mirror groups 123a, 123b, 123c scans the document 4 in the left direction of the arrow, and performs once. To obtain four color image data. This is stored in a memory (not shown) and pulled out, thereby sequentially obtaining four color image data. Then, each time the color printer 2 sequentially visualizes and superimposes the images to form a final four-color full-color image.

The color printer 2 includes a photosensitive drum 200 as a latent image carrier and a writing optical unit 22.
0, a revolver developing unit 230 as a developing unit, an intermediate transfer unit 500, a secondary transfer device 600, a fixing device 270, and the like. The photosensitive drum 200 rotates in the counterclockwise direction indicated by the arrow, and around the photosensitive drum 200, a photosensitive member cleaning device 201, a neutralization lamp 202, and a charger 20 are provided.
3, a potential sensor 204, a selected developing unit of a revolver developing unit 230 as a developing unit, a development density pattern detector 205, an intermediate transfer unit 500, a secondary transfer device 600 as a secondary transfer unit, and the like. ing. A latent image forming unit that forms a latent image on the photosensitive drum 200 includes:
It is configured using the charger 203 and the writing optical unit 220.

The writing optical unit 220
Converts the color image data from the color scanner 1 into an optical signal, performs optical writing corresponding to the image of the document 4, and forms an electrostatic latent image on the photosensitive drum 200. The writing optical unit 220 includes a semiconductor laser 221 as a light source, a laser emission drive control unit (not shown), a polygon mirror 222 and a rotation motor 223, an f / θ lens 224, a reflection mirror 225, and the like.

The revolver developing unit 230
Is composed of a Bk developing unit 231K, a C developing unit 231C, an M developing unit 231M, a Y developing unit 231Y, and a revolver rotation drive unit which rotates each developing unit in a counterclockwise direction indicated by an arrow. A developing bias in which an AC voltage Vac is superimposed on a negative DC voltage Vdc is applied to each developing sleeve by a developing bias power supply (not shown), and the developing sleeve is set at a predetermined potential with respect to the metal base layer of the photosensitive drum 200. Biased.

In the standby state of the main body of the color copying machine, the revolver developing unit 230 has the Bk developing unit 231K set at 30 degrees before the developing position. From Bk
The reading of the color image data starts, and the optical writing by the laser beam and the formation of the electrostatic latent image start based on the color image data (hereinafter, the electrostatic latent image based on the Bk image data is referred to as a Bk latent image; C, M, Y). The same applies to This B
Before the front end of the electrostatic latent image reaches the Bk development position to enable development from the front end of the k electrostatic latent image, the Bk developing device 231K
Is moved to the developing position, the Bk developing sleeve is started to rotate, and the Bk electrostatic latent image is developed with Bk toner. Thereafter, the developing operation of the Bk electrostatic latent image area is continued, but when the trailing end of the electrostatic latent image passes the Bk developing position, the revolver developing is performed until the developing device of the next color comes to the developing position immediately. The unit 230 rotates. This is completed at least before the leading end of the electrostatic latent image based on the next image data arrives.

As shown in FIG. 2, the intermediate transfer unit 500 includes an intermediate transfer belt 501 which is an intermediate transfer member stretched over a plurality of rollers. Around the intermediate transfer belt 501, a secondary transfer bias roller 605 as a secondary transfer member (secondary transfer charge applying unit) of the secondary transfer device 600, a belt cleaning blade 504 as an intermediate transfer body cleaning unit, a lubrication unit A lubricant application brush 505 or the like, which is an agent application means, is provided so as to face the same.

A position detecting mark is provided on the outer peripheral surface or the inner peripheral surface of the intermediate transfer belt 501. However, on the outer peripheral surface side of the intermediate transfer belt 501, it is necessary to devise a method of providing a position detection mark so as to avoid the pass area of the belt cleaning blade 504, and it may be difficult to arrange the mark. A position detection mark is provided on the inner peripheral surface side of the intermediate transfer belt 501. An optical sensor 514 as a mark detecting sensor is provided with an intermediate transfer belt 501.
Is provided between the bias roller 507 and the driving roller 508 which are bridged.

The intermediate transfer belt 501 includes a primary transfer bias roller 507, a belt drive roller 508, a belt tension roller 509, a secondary transfer opposed roller 510, and a cleaning opposed roller as primary transfer members constituting primary transfer means. 511 and a feedback current detection roller 512. Each roller is formed of a conductive material, and each roller other than the primary transfer bias roller 507 is grounded. The primary transfer bias roller 507 has a primary transfer power source 801 controlled by a constant current or a constant voltage.
As a result, a primary transfer bias controlled to a predetermined current or voltage in accordance with the number of superimposed toner images is applied. In particular, the present embodiment employs constant current control in order to apply a constant primary transfer bias regardless of the electrical resistance of the intermediate transfer belt 501. Then, the current flowing from the primary transfer bias roller 507 to the feedback current detection roller 512 via the intermediate transfer belt 501 is controlled to be a constant value (for example, 22 μA).

The intermediate transfer belt 501 is driven in the direction of the arrow by a belt drive roller 508 that is driven to rotate in the direction of the arrow by a drive motor (not shown). The intermediate transfer belt 501 is made of a semiconductor or an insulator and has a multilayer structure as described later. Further, the intermediate transfer belt is set to be larger than the maximum paper-passable size in order to overlap the toner images formed on the photosensitive drum 200.

The secondary transfer bias roller 605 is stretched on the secondary transfer opposing roller 510 by a contact / separation mechanism (not shown) as a contact / separation unit controlled via a secondary transfer contact / separation clutch by a sequence controller described later. The bridged portion can contact and separate from the surface of the intermediate transfer belt 501. The contact / separation unit may be configured using a secondary transfer contact / separation solenoid. Also, the secondary transfer bias roller 605
The recording paper P is disposed so as to sandwich the recording paper P between the intermediate transfer belt 501 and the portion stretched by the secondary transfer facing roller 510. Is applied.
The current value of the secondary transfer bias can be monitored by a sequence control unit described later. The above 2
As the secondary transfer bias roller 605, for example, a bias roller (φ30 mm) having a medium-resistance polymer film (electric resistance: 10 ⁴ to 10 ⁸ Ω) on its surface can be used.
As the secondary transfer opposing roller 510, for example, a ground roller (φ40 m) having a conductive polymer film on the surface is used.
m) can be used.

The registration roller 610 feeds the transfer paper P as a transfer material at a predetermined timing between the secondary transfer bias roller 605 and the intermediate transfer belt 501 stretched over the secondary transfer opposing roller 510. A cleaning blade 608, which is a cleaning unit, is in contact with the secondary transfer bias roller 605. The cleaning blade 608 is for removing the adhered matter attached to the surface of the secondary transfer bias roller 605 for cleaning.

In a color copying machine having such a configuration,
When the image forming cycle is started, the photosensitive drum 200
Is rotated in a counterclockwise direction indicated by an arrow by a drive motor (not shown), and Bk toner image formation, C toner image formation, M toner image formation, and Y toner image formation are performed on the photosensitive drum 200. The intermediate transfer belt 501 is rotated clockwise by an arrow by a belt driving roller 508. The primary transfer of the Bk toner image, the C toner image, the M toner image, and the Y toner image is performed by the transfer bias by the voltage applied to the primary transfer bias roller 507 with the rotation of the intermediate transfer belt 501, Finally Bk, C, M, Y
Are formed on the intermediate transfer belt 501 in this order.

For example, the formation of the Bk toner image is performed as follows. In FIG. 2, the charging charger 203 includes:
The surface of the photosensitive drum 200 is uniformly charged to a predetermined potential with a negative charge by corona discharge. The timing is determined based on the belt mark detection signal, and a writing optical unit (not shown) performs raster exposure using laser light based on the Bk color image signal. When this raster image is exposed, in the exposed portion of the surface of the photosensitive drum 200 that is initially uniformly charged, the charge proportional to the amount of exposure light disappears, and a Bk electrostatic latent image is formed. In this Bk electrostatic latent image,
The negatively charged Bk on the developing roller of the Bk developing device 231K
Due to the contact of the toner, the toner does not adhere to the portion of the photosensitive drum 200 where the charge remains, and the toner is attracted to the portion having no charge, that is, the exposed portion, and Bk similar to the electrostatic latent image is obtained. A toner image is formed.

The Bk toner image formed on the photosensitive drum 200 in this way is primarily transferred onto the surface of the intermediate transfer belt 501 which is rotating at a constant speed while being in contact with the photosensitive drum 200. The photosensitive drum 20 after this primary transfer
A small amount of untransferred residual toner remaining on the surface of
The photoconductor drum 200 is cleaned by the photoconductor cleaning device 201 in preparation for reuse. This photosensitive drum 200
On the side, the process proceeds to the Y image forming process after the Bk image forming process, and the reading of the Y image data by the color scanner starts at a predetermined timing.
An electrostatic latent image is formed.

Then, after the trailing end of the previous Bk electrostatic latent image has passed and before the leading end of the T electrostatic latent image has reached, the rotating operation of the revolver developing unit 230 is performed, and the Y developing machine is rotated. 231Y is set at the development position, and the Y electrostatic latent image is developed with the Y toner. Thereafter, the development of the Y electrostatic latent image area is continued, but when the rear end of the Y electrostatic latent image passes, the revolver developing unit rotates as in the case of the Bk developing machine 231K, and the next operation is performed. Is moved to the developing position. This is also completed before the leading end of the next C electrostatic latent image reaches the developing position. In the image forming processes of C and M, the operations of reading the color image data, forming the electrostatic latent image, and developing are the same as those in the processes of Bk and Y described above, and thus the description is omitted.

The Bk, Y, C, and M toner images sequentially formed on the photosensitive drum 200 in this manner are sequentially aligned on the same surface of the intermediate transfer belt 501 and primary-transferred. As a result, a toner image in which a maximum of four colors are superimposed on the intermediate transfer belt 501 is formed. on the other hand,
At the time when the image forming operation is started, the transfer paper P is fed from a paper feed unit such as a transfer paper cassette or a manual feed tray, and stands by at the nip of the registration roller 610.

The front end of the toner image on the intermediate transfer belt 501 is applied to a secondary transfer portion where a nip is formed by the intermediate transfer belt 501 stretched around the secondary transfer opposing roller 510 and the secondary transfer bias roller 605. When the transfer paper P is moved, the registration roller 610 is driven so that the leading end of the transfer paper P coincides with the leading end of the toner image.
And the toner image are registered.

As described above, when the transfer paper P passes through the secondary transfer section, four colors on the intermediate transfer belt 501 are transferred by the transfer bias caused by the voltage applied to the secondary transfer bias roller 605 by the secondary transfer power supply 802. The superimposed toner image is collectively transferred (secondarily transferred) onto the transfer paper P. The transfer paper P is discharged and separated by a separation unit disposed downstream of the secondary transfer unit, and then sent to the fixing device 270 by the belt conveyance devices 210 and 211 (see FIG. 1). The transfer paper P is transferred to the fixing roller 27 of the fixing device 270.
After the toner image is fused and fixed at the nip portion of 1, 272,
The sheet is sent out of the apparatus main body by the discharge roller 212 and is stacked face up on a copy tray (not shown). The separating means is provided with a discharging needle 611 as a discharging and separating member disposed so that the leading end faces the transfer paper P passing through the secondary transfer unit, and a separating bias for separating the transfer paper P from the intermediate transfer belt 501. Separation bias power supply 8 applied to static elimination needle 611
03 is used.

On the other hand, the photosensitive drum 2 after the belt transfer
The surface of No. 00 is cleaned by the photoconductor cleaning device 201 and is uniformly discharged by a discharging lamp (not shown). Further, the residual toner remaining on the surface of the intermediate transfer belt 501 after the secondary transfer of the toner image to the transfer paper P is cleaned by a belt cleaning blade 504 pressed against the intermediate transfer belt 501 by a separation mechanism (not shown). .

Here, at the time of repeat copying, the operation of the color scanner and the image formation on the photosensitive drum 200 are performed as follows.
After the fourth sheet (M) image forming step of the first sheet, the process proceeds to the second sheet first color (Bk) image forming step at a predetermined timing. Further, after the batch transfer process of the first four-color superimposed toner image onto the transfer paper, the second Bk toner image is formed on the surface of the intermediate transfer belt 501 by the belt cleaning blade 504. The primary transfer is performed. Thereafter, the operation is the same as that of the first sheet.
The above is the copy mode for obtaining a four-color full-color copy. However, in the three-color copy mode and the two-color copy mode, the same operation as described above is performed for the designated color and the number of times. In the case of the single color copy mode,
Until the predetermined number of sheets is completed, the revolver developing unit 2
Only the developing device of the predetermined color 30 is in the developing operation state, and the belt cleaning blade 504 is moved to the intermediate transfer belt 501.
The copying operation is performed with the position kept pressed.

The above is the copy mode for obtaining a four-color full-color copy. However, in the three-color copy mode and the two-color copy mode, the same operation as described above is performed for the designated color and the number of times. Become. In the case of the single-color copy mode, only the developing device of the predetermined color of the revolver developing unit 230 is set in the developing operation state until the predetermined number of copies is completed, and the belt cleaning blade 504 is pressed against the intermediate transfer belt 501. The copy operation is performed in the state in which the copy operation is performed.

Next, a more specific configuration of the intermediate transfer belt 501 used in the above-described color copying machine and uniformization of residual charges (polarization) in the intermediate transfer belt 501 before the primary transfer will be described in detail. FIG. 3 shows the intermediate transfer belt 5.
It is an exploded sectional view for explaining the sectional structure of No. 01. The intermediate transfer belt 501 has a thickness of 150 μm and a width of 368.
mm, and a belt material having a multilayer structure with an inner peripheral length of 565 mm. The intermediate transfer belt 501 has a surface layer (high-resistance layer) 501a on the toner carrying surface side, an intermediate layer (high-resistance layer) 501b inside, and an inner base layer 501c. Each layer 501a, 501b, 501c is made of PVD
F (polyvinylidene fluoride) is used as a base material, and is prepared by appropriately dispersing additives such as a conductive agent. The thickness and the volume resistivity ρv of the surface layer 501a are about 1 μm and 10 ¹⁰ to 10 ¹⁶ Ωcm, respectively, and the thickness and the volume resistivity ρv of the intermediate layer 501b are about 7 μm, respectively.
5 μm and 10 ¹⁰ to 10 ¹⁶ Ωcm, and the thickness and the volume resistivity ρv of the base layer 501c are about 75 μm, respectively.
m and 10 ⁸ to 10 ¹¹ Ωcm. The electric resistance of the intermediate transfer belt 501 is adjusted by the dispersion amount and thickness of the conductive agent in each layer. Note that the intermediate transfer belt 501 is
The material and the shape are not limited, and any material having a high-resistance layer having a volume resistivity of at least 10 ¹⁰ Ωcm may be used.

When the three-layer intermediate transfer belt 501 including the high resistance layer is used, the photosensitive drum 2
00, when the toner image is transferred,
A part of the electrostatic latent image (potential image) on the intermediate transfer belt 50
1 to the surface. The ferroelectric material such as PVDF used for the intermediate transfer belt 501 is about 50 MV / m at room temperature.
When an external electric field equal to or greater than (= coercive electric field value) is applied, the polarizer spontaneously polarizes in the opposite direction, saturates and stabilizes. Here, the surface layer 501a of the intermediate transfer belt 501 is about 1 μm thick, and when a voltage of 100 V or more is applied, the internal electric field becomes larger than the coercive electric field value. I do. By having such a surface layer 501a, when the toner image is transferred from the photosensitive drum 200, the potential contrast of the previous electrostatic latent image is temporarily erased, and the potential contrast of the new electrostatic latent image is intermediately transferred. It can be held on the surface of the belt 501. This potential contrast is important for reducing the above-mentioned “transfer dust” and preventing the occurrence of an afterimage.

However, when the same toner image is continuously transferred many times onto the intermediate transfer belt 501 as in the case of continuous copying of the same original, the surface layer 50 of the intermediate transfer belt 501 is
The intermediate layer 501b inside 1a is polarized and fixed. This is because the intensity of the external electric field acting on the intermediate layer 501b during the primary transfer does not reach the coercive electric field value, so that the inside of the intermediate layer 501b is gradually polarized under the external electric field of the primary transfer. That's why. Since the intensity of the electric field generated in the intermediate layer 501b by the external electric field is smaller than the electric field generated in the surface layer 501a, it takes much longer time to eliminate or invert the polarization of the intermediate layer 501b than in the case of the surface layer 501a. (For example, Tami and Furukawa, Basics of Ferroelectric Materials, Journal of the Institute of Electrostatics, Vol. 13, No. 2,
No. (1989) pp. 74-81, Odashima, Piezoelectricity and Ferroelectricity of Poly (vinylidene fluoride), Journal of the Japan Society of Applied Physics, No.5
0 No. 12 (1981) pp. 79-83).
Therefore, even if a neutralization bias composed of a DC voltage is applied to the intermediate transfer belt, the intermediate layer 501b cannot be easily neutralized. In addition, even if a static elimination bias including a DC voltage of the opposite polarity is applied, the time constant of the polarization change of the surface layer 501a is short,
The surface layer 501a is polarized in a short time, and the intermediate layer 501a is polarized.
No net neutralization bias is applied to b. When a considerably large discharging bias is applied to the intermediate transfer belt, the intermediate layer 5
Although a predetermined voltage is applied up to 01b, a problem is likely to occur on the surface layer 501a side.

FIG. 4 shows the configuration of an experimental apparatus for evaluating the residual potential in a belt made of PVDF. In this experimental apparatus, a belt 901 is placed on a grounded conductive base 900, and a probe electrode 902 is brought into contact with the probe electrode 902 from the upper surface instead of the bias roller. This probe electrode 9
In 02, the switch 903 is operated to apply a bias composed of a DC voltage from the high voltage power supply 904. First, a bias V0 for initial saturation polarization (= + 250
(V or +500 V) to saturate the polarization. Next, the switch 903 is operated to float the belt 901, and the attenuation of the surface potential is measured by the probe electrode 9.
02 is recorded with an electrometer (not shown) and a pen recorder (not shown). Usually, this decay of the surface potential is very time consuming. Next, static elimination bias V1 [V] (= 0 V or −25)
0V) is applied only for the time Δt. This application time Δt
Is set between 0.1 and 10 seconds. Next, the belt 901
Was floated again, the surface potential decay status was recorded, and the surface potential after a certain period of time (here, the time corresponding to one rotation of the intermediate transfer belt = 6 seconds) had elapsed was plotted.
FIG. 5 shows the application time (log
12 is a graph plotting the relationship between T) and the surface potential when a certain time (= 6 seconds) has elapsed after the application of the bias V1 for static elimination. The values of the bias V0 for initial saturation polarization and the bias V1 for static elimination in the data of each symbol in FIG. 5 are as shown in Table 1 below.

[Table 1]

According to the results shown in FIG. 5, for example, when the bias for initial saturation polarization V0 = + 250 V is applied in advance and the charge is removed with the bias for charge removal V1 = -250 V, the belt surface potential after the charge removal actually becomes -250 V. To get 10
It can be seen that a neutralization time of 2 seconds is required. It can also be seen that the higher the absolute value of the initial saturation polarization bias V0 (the higher the initial belt surface potential itself), or the higher the potential contrast (= the higher the latent image contrast), the longer the neutralization time. This result also indicates that it is difficult to apply a DC voltage to eliminate the belt surface potential to 0 V after the polarization of the ferroelectric belt is saturated.

As described above, even if the potential of the intermediate transfer belt surface becomes apparently 0 V, the polarization corresponding to the potential contrast of the previous toner image remains in the intermediate layer 501b. Note that even when a material other than PVDF is used for the surface layer, a tunnel effect can be obtained if the surface layer is thin, so that the potential in the thickness direction is also easily attenuated. Therefore, in the intermediate transfer belt using the high resistance layer as the intermediate layer, it is difficult to remove the charge by the DC voltage. Therefore, in order to reliably remove the charge of the entire composite layer as in the present embodiment, it is necessary to remove the charge by using an AC bias having a large amplitude. However, this leads to an increase in the cost of the power supply and the intermediate transfer belt is easily damaged. Or banding occurs on the image. In the case of using discharge by an AC bias, post-treatment such as ozone is required.

Therefore, in the color copying machine according to the present embodiment, after the rotation of the intermediate transfer belt 501 is started to start the image forming operation, the intermediate transfer belt 50 is started.
Before the primary transfer of the toner image to the first transfer is started, the polarization remaining in the intermediate layer (high resistance layer) 501b of the intermediate transfer belt 501 is made uniform while maintaining its polarity. By making the polarization of the intermediate layer 501b uniform, the potential contrast remaining on the intermediate transfer belt 501 is reduced, and the occurrence of the afterimage is prevented. In particular, the intermediate transfer belt 5
By applying a pretreatment bias in a direction to saturate the polarization of the intermediate layer 501b of No. 01, the potential contrast is reduced more quickly and easily.

In the present embodiment, the secondary transfer device 600 is also used as a polarization uniformizing means for uniformizing the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 while maintaining its polarity. FIG. 6 is a block diagram of a main part of a control system for controlling the secondary transfer device so as to apply the preprocessing bias at the predetermined timing. The sequence control unit 850 as a control unit includes a CPU 85
1, a RAM 852, a ROM 853, an I / O interface 854, and the like, and are connected via the I / O interface 854 to the secondary transfer contact / separation clutch 855, the secondary transfer power supply 802, and the like.

FIG. 7 is a time chart showing an example of an image forming operation including the preprocessing bias application control. The time chart of FIG. 7 is an example of single-color one-sided printing, in which two types of single-color toner images are formed within one circumference of the intermediate transfer belt 501, and are continuously formed on different A3-size transfer papers P. It is formed. On the second transfer sheet P, a halftone image having a low image density (ID) is formed. In the time chart of FIG. 7, when the image forming operation is started by pressing the copy button or the like, the belt cleaning contact / separation clutch is turned on after the rotation drive of the photosensitive drum 200 and the intermediate transfer belt 501 is started. Then, cleaning of the surface of the intermediate transfer belt 501 by the belt cleaning blade 504 is started. Next, when the optical sensor 514 detects a position detection mark on the intermediate transfer belt 501, the secondary transfer contact / separation clutch 855 is turned on, and the secondary transfer bias roller 605 comes into contact with the intermediate transfer belt 501. The secondary transfer power source 802 is controlled together with the contact of the secondary transfer bias roller 605, and the pre-processing bias (for example, +70 μA) controlled at a constant current is applied to the secondary transfer bias roller 605. Of the intermediate layer 501b is started. Further, FGATE signals corresponding to respective images are continuously output at a timing when a predetermined time has elapsed from the detection of the position detection mark. Next, at the timing when at least one rotation time of the intermediate transfer belt 501 has elapsed from the start of the application of the pre-processing bias, a regular secondary transfer bias (for example, +40 μm) is used.
A), and the secondary transfer of the toner image corresponding to each image from the intermediate transfer belt 501 to the transfer paper P is continuously performed.

As described above, according to the present embodiment, after the driving of the intermediate transfer belt 501 is started and before the primary transfer of the toner image to the intermediate transfer belt 501 is started, the secondary transfer contacting the intermediate transfer belt 501 is started. Applying the preprocessing bias to the bias roller 605 to equalize the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501, thereby eliminating the potential contrast remaining in the intermediate layer 501b in the previous image forming operation. Can be. Therefore, the intermediate layer 501b of the intermediate transfer belt 501 in the previous image forming operation
Generation of an afterimage due to the polarization remaining in the substrate can be prevented. In particular, according to this embodiment, the polarization is made uniform while maintaining the polarity of the polarization of the intermediate layer 501b. Therefore, the polarization remaining in the intermediate layer 501b can be eliminated or the polarity of the polarization can be inverted. The polarization remaining in the intermediate layer 501b can be more quickly and easily uniformized as compared with the case where the uniformization is performed once.

Table 2 shows the result of evaluating the afterimage rank when changing the value of the preprocessing bias at two places on the image. As a comparative example, a case where no pretreatment bias is applied is also shown. The afterimage rank has five levels, and the larger the number, the smaller the degree of afterimage and the higher the image quality. From the results in Table 2, it can be seen that by setting the preprocessing bias to 40 μA or more, good image quality with an afterimage rank of 3.5 or more can be obtained.

[0048]

[Table 2]

Further, according to the present embodiment, prior to the primary transfer to the image of the first page, the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 is made uniform to a certain size. An afterimage due to polarization remaining in the intermediate layer 501b at the time of the secondary transfer of the image of the first page,
It hardly occurs on the image of the second page formed by transferring the toner image to a different position on the intermediate transfer belt. The mechanism for preventing occurrence of the afterimage can be considered as follows. FIGS. 8A, 8B and 8C respectively show primary transfer of the first page image, secondary transfer of the first page image, and second page image during the image forming operation shown in FIG. FIG. 4 is an explanatory diagram of primary transfer of an image. 8, for convenience of explanation, the cross section of the intermediate transfer belt 501 is shown with each layer separated. Arrows P1a, P2a, and P3a indicate the direction and magnitude of polarization of the surface layer 501a,
Arrows P1b, P2b, and P3b indicate the direction and magnitude of polarization of the intermediate layer 501b. In the primary transfer of the image of the first page shown in FIG.
A relatively large negative true charge due to the background potential VD of the photoreceptor drum 200 is present in the background portion (upper side in the drawing) of the surface layer 501a of the first photoconductor. Further, the toner portion T on the front side (upper side in the figure) of the surface layer 501a has relatively few negative true charges due to the image portion potential VL of the photosensitive drum 200. Here, a positive true charge is injected in advance from the surface layer side of the intermediate transfer belt 501 by application of the pretreatment bias, and a downward polarization in the figure occurs in the intermediate layer 501b before the primary transfer. Therefore, even if a primary transfer bias (positive polarity) is applied to the primary transfer bias roller 507 in the primary transfer section, the potential difference applied to the intermediate layer 501b is reduced, and the upward polarization P1b in the figure is smaller than in the prior art. Has become. In the secondary transfer of the image of the first page shown in FIG. 8B, the polarization of the surface layer 501a is quickly reversed and the upward polarization P1b of the intermediate layer 501b due to the primary transfer is reduced. The middle layer 50
1b is also inverted, and a relatively small polarization P
2b occurs. In the primary transfer of the image of the second page shown in FIG. 8C, the positive transfer provided on the front side (upper side in the figure) of the intermediate layer 501b in the secondary transfer remains, so that the primary transfer is performed. The polarization of the intermediate layer 501b due to the primary transfer bias (positive polarity) applied to the bias roller 507 is reduced. As described above, although the polarization corresponding to the image of the first page remains in the intermediate layer 501b, the magnitude of the polarization is smaller than that in the related art, so that the image of the second page has a low image density. Also, the influence on the primary transfer is small, and the transfer rate does not change. Therefore, the image of the first page does not appear as an afterimage in the image of the second page.

FIGS. 9A, 9B, and 9C respectively show the primary transfer of the first page image and the first page of the comparative example in which the polarization of the intermediate layer 501b is not made uniform by applying the pretreatment bias. FIG. 4 is an explanatory diagram of secondary transfer of an eye image and primary transfer of a two-page image. As shown in FIG. 9A, in the case of the present comparative example, the downward polarization in the drawing due to the application of the pre-processing bias is not generated in the intermediate layer 501b of the intermediate transfer belt 501, so that the image of the first page In the first transfer, a stronger polarization P1b ′ is generated in the intermediate layer 501b than in the present embodiment. For this reason, FIG.
In the secondary transfer of the image of the first page shown in FIG.
Although the polarization P2a 'of 1a is inverted, the polarization P2b' of the intermediate layer 501b is not inverted but slightly decreases, but remains upward in the figure. In the primary transfer of the image of the second page shown in FIG.
, The polarization P3b ′ of the intermediate layer 501b is further strengthened by the primary transfer bias (positive polarity) applied to. Therefore,
Due to the strong polarization P3b 'corresponding to the image of the first page remaining in the intermediate layer 501b, the image of the first page appears as an afterimage on the image of the second page.

Further, in this embodiment, since the pre-processed bias which is controlled by the constant current is applied, even if the electric resistance of the intermediate transfer belt 501 changes, the intermediate layer 501b of the intermediate transfer belt is changed. The polarization can be uniformized to a predetermined strength.

Further, in the present embodiment, since the secondary transfer device 600 is also used as the polarization uniformizing means, the cost and size of the device can be reduced.

In the above embodiment, the current value of the pre-processing bias is preferably set to be equal to or larger than the current value of the secondary transfer bias. By setting the current value of the pre-processing bias in this way, the effect of injecting electric charge into the intermediate transfer belt 501 can be enhanced, and the polarization of the intermediate layer 501b can be made more uniform.

In the above embodiment, if the pre-processing bias is applied, for example, for 1.5 rotations of the intermediate transfer belt, a step occurs in the polarization of the intermediate layer 501b, and a band-shaped abnormal image is generated on the output image. Would. Therefore,
The application of the pre-processing bias is performed by one of the intermediate transfer belts 501.
It is desirable to perform the process only by an integral multiple of the circumferential rotation time.

In the above embodiment, the secondary transfer device 6
00 is also used as the polarization uniformizing means, but as shown in FIG. 10, a secondary transfer opposing roller 510 is disposed downstream of the secondary transfer position by the secondary transfer device 600 with the intermediate transfer belt 501 interposed therebetween. A pre-processing bias roller 960 may be provided as an opposing member that opposes the above, and a pre-processing bias controlled to a predetermined current value from a power supply 961 may be applied to the pre-processing bias roller 960. In this case, a low-cost roller with a simple configuration can be used instead of the relatively expensive secondary transfer bias roller. In the case of this configuration, it is desirable to use a roller having a medium resistance in order to prevent current concentration when the film of the intermediate transfer belt 501 is defective. The type of conductivity provided to provide the roller with a medium resistance may be electronic conductivity or ionic conductivity. The pretreatment bias roller 960 can be moved toward and away from the intermediate transfer belt 501 by a contact / separation mechanism (not shown). The contact timing may be set in the same manner as the contact timing of the belt cleaning blade 504.

In the above-described embodiment, the characteristics of the material of the intermediate transfer belt 501 are such that the electrical resistance changes by one digit due to the influence of humidity. Therefore, in a low-temperature and low-humidity environment, the current value of the pretreatment bias that is appropriate under a normal temperature and normal humidity environment may be too high. Therefore, a humidity sensor 970 (see FIG. 1) as a humidity detecting means for detecting the absolute humidity in the apparatus so that the primary transfer can be always performed irrespective of a change in the humidity where the apparatus is installed. May be provided, and the current value of the pre-processing bias may be controlled to be switched according to the absolute humidity detected by the humidity sensor 970. For example, when a low-temperature and low-humidity environment in which the value of the absolute humidity is lower than a preset reference value is set, control is performed so that the current value of the preprocessing bias is switched to a lower value. More specifically, the current value of the pretreatment bias in a normal temperature and normal humidity environment is set to 70 μA, and the current value of the pretreatment bias in a low temperature and low humidity environment where the absolute humidity is less than 4.7 g / m ³ is set to 50 μA. I do.

Further, in the color copying machine of the above-described embodiment, the transfer paper P on which an image is formed on the first surface and which has passed through the fixing device 270 is temporarily stored on a reversing unit 207 in the device.
The paper is fed by the paper feed roller 208 at a predetermined timing, and an image can be formed on a second surface opposite to the first surface. When such double-sided copying is performed, the electrical characteristics (capacitance and electrical resistance) of the unfixed transfer paper P on which an image is transferred to the first first surface
And the electrical characteristics of the transfer paper P that has passed through the fixing process once the image is transferred to the next second surface. On the other hand, the potential difference applied to the transfer paper P due to the secondary transfer bias being divided in the secondary transfer section, that is, the strength of the electric field applied to the transfer paper P differs depending on the electrical characteristics of the transfer paper P. Therefore, the same secondary transfer bias is applied to the unfixed transfer paper P on which the image is transferred to the first surface and the transfer paper P which has passed through the fixing process once and the image is transferred to the next second surface. Is applied, the strength of the electric field received by the transfer paper P is different, and there is a possibility that predetermined secondary transfer cannot be performed. Therefore, the current value of the preprocessing bias is made different between the transfer paper P when the image is transferred to the first surface and the transfer paper P when the image is transferred to the next second surface. By doing so, good secondary transfer can always be performed. More specifically, for example, for an unfixed transfer sheet P on which an image is to be transferred to the first surface, the current value of the preprocessing bias is set to 70 μA, and the image is transferred to the next second surface. The transfer paper P that has passed through the fixing step once is set to 30 μA or less.

In the above embodiment, the case of a color copying machine has been described. However, the present invention can be applied to other image forming apparatuses such as a monochrome copying machine, a printer, and a facsimile.

[0059]

According to the first to seventh aspects of the present invention, compared to the case where the polarization remaining in the high-resistance layer is eliminated or the polarity of the polarization is reversed to make the high-resistance layer uniform, The remaining polarization can be homogenized more quickly and easily. Therefore, even when an intermediate transfer member having a high-resistance layer that is advantageous in preventing transfer dust is used, it is possible to reliably prevent afterimages due to polarization remaining in the high-resistance layer of the intermediate transfer member before the primary transfer. There is an effect that can be.

In particular, according to the second aspect of the present invention, even when the electric resistance of the intermediate transfer member varies, there is an effect that the polarization of the high-resistance layer of the intermediate transfer member can be made uniform.

According to the third aspect of the present invention, since the direction of the change in the polarization of the high-resistance layer of the intermediate transfer member is one, the uniformization of the polarization can be performed more efficiently. There is.

In particular, according to the fourth aspect of the present invention, there is no step in the distribution of polarization of the high resistance layer of the intermediate transfer member.
There is an effect that an afterimage due to polarization remaining in the high resistance layer of the intermediate transfer member can be more reliably prevented.

In particular, according to the fifth aspect of the present invention, even when there is a change in humidity, the polarization of the high-resistance layer of the intermediate transfer member can be set to a predetermined magnitude to obtain a desired secondary transfer characteristic. There is an effect that can be.

According to the present invention, when images are formed on both surfaces of the transfer material, the height of the intermediate transfer member is set for each of the first surface and the second surface of the transfer material. By adjusting the magnitude of the polarization of the resistance layer, there is an effect that desired secondary transfer characteristics can be obtained on both the first surface and the second surface of the transfer material.

In particular, according to the invention of claim 7, there is an effect that the cost and size of the device can be reduced.

[Brief description of the drawings]

FIG. 1 is a front view showing a schematic configuration of a color copying machine according to an embodiment of the present invention.

FIG. 2 is a front view showing a main part of the color copying machine.

FIG. 3 is an explanatory diagram of a cross-sectional structure of an intermediate transfer belt used in the color copying machine.

FIG. 4 is a configuration diagram of an experimental device for evaluating a residual potential in a belt made of PVDF.

FIG. 5 shows the relationship between the application time (logT) of the static elimination bias V1 measured by the same experimental apparatus and the surface potential when a certain time (= 6 seconds) has elapsed after the application of the static elimination bias V1. Graph.

FIG. 6 is a block diagram showing a main part of a control system of the color copying machine according to the embodiment.

FIG. 7 is a time chart showing an example of an image forming operation including preprocessing bias application control in the color copying machine.

FIGS. 8A to 8C are diagrams illustrating a mechanism for preventing occurrence of an afterimage when a preprocessing bias is applied.

FIGS. 9A to 9C are explanatory diagrams of an afterimage generation mechanism in a comparative example.

FIG. 10 is a front view showing a main part of a color copying machine according to another embodiment.

DESCRIPTION OF SYMBOLS 200 Photoconductor drum 203 Charging charger 220 Writing optical unit 230 Revolver developing unit 500 Intermediate transfer unit 501 Intermediate transfer belt 501a Surface layer 501b Intermediate layer 501c Base layer 504 Belt cleaning blade 507 Primary transfer bias roller 508 Belt drive roller 600 Secondary transfer device 605 Secondary transfer bias roller 802 Secondary transfer power supply 850 Sequence controller 855 Secondary transfer contact / separation clutch P Transfer paper T Toner image

Continued on the front page (72) Inventor Shigeru Watanabe 1-3-6 Nakamagome, Ota-ku, Tokyo F-term in Ricoh Co., Ltd. (Reference) 2H027 DA01 DA14 DA38 DE01 DE07 EA03 EA08 EA15 EC06 EC09 ED15 ED24 EE01 FA13 JC03 2H028 BA06 BA09 BA14 BB04 2H032 AA05 AA15 BA09 BA23 BA26 CA02 CA13 CA14 CA15 3F054 AA01 AC02 AC03 AC05 BA02 BB07 BC02 BJ04

Claims (7)

[Claims] A latent image carrier, a latent image forming means for forming a latent image on the latent image carrier, a developing means for developing the latent image on the latent image carrier into a toner image, A surface-transferable intermediate transfer member onto which the toner image on the latent image carrier is transferred, a primary transfer means for transferring the toner image on the latent image carrier to the intermediate transfer member, An image forming apparatus comprising: a secondary transfer means for transferring a toner image onto a transfer material; wherein an intermediate transfer member having a high-resistance layer having a volume resistivity of 10 ¹⁰ Ωcm or more is used as an intermediate transfer member. After the surface transfer of the intermediate transfer member is started and before the primary transfer of the toner image to the intermediate transfer member is started, the polarization remaining in the high-resistance layer of the intermediate transfer member is maintained at the same polarity. Image forming apparatus provided with a polarization homogenizing means for homogenizing while keeping the same 2. The image forming apparatus according to claim 1, wherein said polarization equalizing means includes an opposing member opposing said intermediate transfer member, and a preprocessing bias for applying a preprocessing bias controlled at a constant current to said opposing member. An image forming apparatus comprising an application unit. 3. The image forming apparatus according to claim 2, wherein said secondary transfer means applies a secondary transfer member facing said intermediate transfer member and a secondary transfer bias controlled at a constant current to said secondary transfer member. An image forming apparatus configured to use a secondary transfer bias applying unit to apply the current, wherein a current set value of the pre-processing bias is set to be equal to or larger than a current set value of the secondary transfer bias. 4. The image forming apparatus according to claim 2, wherein the intermediate transfer member is constituted by an endless rotating member, and the application time of the preprocessing bias is an integral multiple of one rotation time of the intermediate transfer member. An image forming apparatus, comprising: 5. An image forming apparatus according to claim 2, wherein said humidity detecting means for detecting humidity and said preprocessing bias applying means are controlled to change said preprocessing bias based on the detection result of said humidity detecting means. An image forming apparatus comprising: 6. The image forming apparatus according to claim 2, wherein the toner image on the intermediate transfer body is secondarily transferred to the first surface so that the toner image can be secondarily transferred to both surfaces of the transfer material. Reversing and conveying means for inverting the material and conveying the material to a secondary transfer position such that the second surface faces the intermediate transfer member, wherein a second transfer is performed on the first surface of the transfer material and a second transfer is performed. 2 on the surface
An image forming apparatus, wherein the pre-processing bias is made different at the time of next transfer. 7. An image forming apparatus according to claim 1, wherein said secondary transfer means is also used as said polarization uniformizing means.