JP2002214939A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of surely preventing an after image caused at the next image forming time due to 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 after finishing image forming operation 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 a polarity at the secondary transferring time before stopping the surface movement of the belt 501 after finishing the secondary transfer of a toner image from the belt 501 in the case of finishing the image forming operation by using the belt 501. A secondary transfer device 600 is used also as the polarization uniformizing means, and post-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 after finishing the secondary 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 a toner image is formed many times in the same area on the surface of the intermediate transfer member, the latent image carrier is placed on the high resistance layer of the intermediate transfer member. The history of the potential contrast image corresponding to the presence or absence of the upper toner and the toner on the transfer material remains. Since the potential contrast image of the high resistance layer is difficult to be neutralized and is easily held until the next image formation, forming an image with a low image density (ID) such as a highlight image corresponds to the potential contrast image. An afterimage may occur.

[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 having a high-resistance layer which is advantageous in preventing transfer dust. An object of the present invention is to provide an image forming apparatus capable of reliably preventing an afterimage at the time of next image formation 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 member, one having a high-resistance layer having a volume resistivity of 10 ¹⁰ Ωcm or more is used. After secondary transfer of the toner image from the intermediate transfer member is completed at the end of the image forming operation, the intermediate transfer member is Before the surface movement of the intermediate transfer member is stopped, a polarization homogenizing means for homogenizing the polarization remaining in the high resistance layer of the intermediate transfer body while maintaining the polarity at the time of the secondary transfer is provided. Is what you do. The image forming apparatus according to claim 1, wherein after the secondary transfer of the toner image from the intermediate transfer body is completed at the end of the image forming operation, before the surface movement of the intermediate transfer body is stopped, the polarization uniformizing means is used. Thereby, the polarization remaining in the high-resistance layer of the intermediate transfer member is made uniform.
When the polarization is homogenized while maintaining the polarity of the polarization at the time of the secondary transfer, the polarization remaining in the high-resistance layer is eliminated or the polarization is reversed. As compared with the above, the change in polarization is small, and the polarization remaining in the high-resistance layer can be more quickly and easily uniformized.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the polarization equalizing means applies a post-processing bias, which is controlled at a constant current, to the opposing member facing the intermediate transfer member. And a post-processing bias applying means for applying the bias voltage. In the image forming apparatus according to the second aspect, by applying a post-processing bias to the opposing member for polarization uniformization facing the intermediate transfer member, a fixed amount of charge is applied 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 post-processing bias is controlled at a constant current, even if the electric resistance of the intermediate transfer member varies, a certain 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 post-processing bias is set to 60% or more of the current set value of the secondary transfer bias. It is a feature. Claim 3
In the image forming apparatus, the current set value of the post-processing bias is
By setting the current value of the normal secondary transfer bias to 60% or more, the polarization in the high-resistance layer of the intermediate transfer member can be made uniform.

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 post-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 post-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 post-processing is performed based on the number of superimpositions of the toner images superimposed on the same area of the intermediate transfer body in a series of image forming operations. And a control means for controlling the post-processing bias application means so as to switch on / off of the application of the bias. The afterimage is more likely to occur as the number of superpositions of the toner image superimposed on the same area of the intermediate transfer member increases. Therefore, in the image forming apparatus according to the fifth aspect, the number of times of superimposing the toner images is determined by a predetermined number of times based on the number of times of superimposing the toner images superimposed on the same area of the intermediate transfer body in a series of image forming operations It can be controlled so that the application of the post-processing bias is turned on when the number is larger than the above. With this control, it is possible to prevent the occurrence of the afterimage and to avoid the useless application of the post-processing bias.

According to a sixth aspect of the present invention, in the image forming apparatus of the second aspect, based on the number of transfer materials on which the same image is formed in a series of image forming operations, A control means is provided for controlling the post-processing bias application means so as to switch on / off of the application of the processing bias. The afterimage is more likely to occur as the number of times that the same toner image is primarily transferred to the same area of the intermediate transfer member in a series of image forming operations increases. The number of times of the primary transfer of the same toner image corresponds to the number of transfer materials on which the same image is formed in a series of image forming operations. Therefore, in the image forming apparatus according to the sixth aspect, based on the number of transfer materials on which the same image is formed in a series of image forming operations, when the number of transfer materials is larger than a predetermined number, a post-processing bias is set. It can be controlled to turn on the application. With this control, it is possible to prevent the occurrence of the afterimage and to avoid the useless application of the post-processing bias.

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. Also, the intermediate transfer belt is
In order to overlap the toner images formed on the photosensitive drum 200, the size is set to be larger than the maximum size that can be passed.

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 at the end of image formation will be described in detail. FIG. 3 is an exploded cross-sectional view for explaining a cross-sectional structure of the intermediate transfer belt 501. The intermediate transfer belt 501 is made of a multi-layered belt material having a thickness of 150 μm, a width of 368 mm, and an inner peripheral length of 565 mm, and is rotated at a moving speed of, for example, 245 mm / sec. 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 composed of P
VDF (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 a toner image is continuously transferred many times to the same area on the intermediate transfer belt 501 as in the case of full-color copy or continuous copy, the intermediate transfer belt 5
The intermediate layer 501b inside the surface layer 501a of No. 01 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 intermediate layer
This is because the inside of 1b is gradually polarized. The intensity of the electric field generated in the intermediate layer 501b by the external electric field is
01a is smaller than the electric field generated in the intermediate layer 50a.
In order to eliminate or reverse the polarization of 1b, the surface layer 5
It takes much longer time than in the case of 01a (for example, Tami and Furukawa, "Basics of Ferroelectric Materials," Journal of the Electrostatic Society of Japan, Vol. 13, No. 2 (1989), pp. 74-81; Piezoelectricity and Ferroelectricity of Vinylidene ”Journal of the Japan Society of Applied Physics, Vol. 50, No. 12, (1981), pp. 79-8
3). 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 when a static elimination bias composed of a DC voltage of the opposite polarity is applied, the surface layer 501a is polarized in a short time because the time constant of the polarization change of the surface layer 501a is short, and a net static elimination bias is not applied to the intermediate layer 501b. . When a considerably large neutralization bias is applied to the intermediate transfer belt, a predetermined voltage is applied to the intermediate layer 501b, but at that time, 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 this embodiment, after the secondary transfer of the toner image from the intermediate transfer belt 501 is completed at the end of the image forming operation, and before the rotation of the intermediate transfer belt 501 is stopped. Further, 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 uniformizing the polarization of the intermediate layer 501b, the intermediate transfer belt 50
The potential contrast remaining at 1 is reduced to prevent the occurrence of an afterimage at the time of the next image forming operation. In particular, by applying a post-processing bias in a direction that saturates the polarization of the intermediate layer 501b of the intermediate transfer belt 501, 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 post-processing 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 the image forming operation including the post-processing bias application control. The time chart of FIG. 7 is an example of single-color one-sided printing. A single-color toner image is formed within one circumference of the intermediate transfer belt 501, and transfer paper P of different A3 size is formed.
Formed continuously. 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,
An FGATE signal corresponding to each image is continuously output at a timing when a predetermined time has elapsed from the detection of the position detection mark. Further, at a timing when a predetermined time has passed from the detection of the position detection mark, the secondary transfer contact / separation clutch 85
5 is turned on, and the secondary transfer bias roller 605 is brought into contact with the intermediate transfer belt 501, so that the secondary transfer can be performed. Then, the secondary transfer power supply 802 is controlled, and a regular secondary transfer bias (for example, +50 μA) controlled with a constant current is applied to the secondary transfer bias roller 605.
Secondary transfer of a toner image corresponding to each image is continuously performed from the intermediate transfer belt 501 to the transfer paper P. Next, the bias applied to the next transfer bias roller 605 is changed to a constant current controlled post-processing bias (for example, +3).
0 μA), and a uniforming process for equalizing the polarization remaining in the intermediate layer 501b in the intermediate transfer belt 501 is started. The post-processing bias is at least one rotation time of the intermediate transfer belt 501 (for example, 6 to 10 rotations).
Second).

As described above, according to the present embodiment, after the secondary transfer of the toner image from the intermediate transfer belt 501 is completed, and before the rotation drive of the intermediate transfer belt 501 is stopped, the intermediate transfer belt 501 is brought into contact with the intermediate transfer belt 501. Secondary transfer bias roller 60
5, the post-processing bias is applied to the intermediate transfer belt 50.
By making the polarization remaining in the first intermediate layer 501b uniform while maintaining the polarity at the time of the secondary transfer, the potential contrast due to the polarization remaining in the intermediate layer 501b can be eliminated in a series of image forming operations. it can. Therefore, in the next image formation, the intermediate layer 5 of the intermediate transfer belt 501 is used.
The occurrence of an afterimage due to the polarization remaining in 01b can be prevented. In particular, when performing four-color full-color copying, the toner image formation and the primary transfer are performed by detecting the position detection marks on the intermediate transfer belt 501 for each color toner image. A toner image of each color of the same size is transferred to the area. Therefore, the potential contrast due to the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 tends to increase. However, it is necessary to apply the post-processing bias to uniform the polarization remaining in the intermediate layer 501b as in the present embodiment. Accordingly, occurrence of an afterimage in the next image formation can be prevented.

According to the present embodiment, the intermediate layer 50
Since the polarization of 1b is uniformized while maintaining the polarity by the immediately preceding application of the secondary transfer bias, the intermediate layer 501 is used.
The polarization remaining in the intermediate layer 501b can be homogenized more quickly and easily than in the case where the polarization remaining in the intermediate layer 501b is eliminated or the polarity of the polarization is inverted to make the polarization uniform. .

Further, according to the present embodiment, the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 at the end of the image formation is made uniform to a certain size.
The residual image due to the polarization remaining in the intermediate layer 501b at the time of the secondary transfer of the image of the first page during the next image forming operation becomes the second page formed by transferring the toner image to a different position on the intermediate transfer belt 501. Is less likely to occur on the image. The mechanism for preventing the occurrence of an afterimage in this case can be considered as follows. FIGS. 8A, 8B and 8C respectively show the primary transfer of the image of the first page and the secondary transfer of the image of the first page during the next image forming operation after the post-processing bias is applied. FIG. 4 is an explanatory diagram of transfer and primary transfer of a two-page 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 surface layer 501.
a indicates the direction and magnitude of polarization of a, and 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. 8A, the background portion (upper side in the figure) of the surface layer 501a of the intermediate transfer belt 501 is relatively moved by the background potential VD of the photosensitive drum 200. There are many negative true charges. 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, by applying the post-processing bias, positive true charges are previously injected from the surface layer side of the intermediate transfer belt 501, and a downward polarization in the figure occurs in the intermediate layer 501b before the primary transfer. Therefore, the primary transfer bias roller 507
Even if the next transfer bias (positive polarity) is applied, the potential difference applied to the intermediate layer 501b is small, and the upward polarization P1b in the figure is smaller than in the related art. FIG. 8B
In the secondary transfer of the image of the first page shown in FIG.
The polarization of the intermediate layer 501b is also inverted because the polarization of the intermediate layer 501b is inverted quickly because the polarization P1b of the intermediate layer 501b due to the primary transfer is reduced as well as the polarization of the intermediate layer 501b is reduced. P2b occurs.
In the primary transfer of the image of the second page shown in FIG. 8C, the front side (upper side in the figure) of the intermediate layer 501b in the secondary transfer.
, The polarization of the intermediate layer 501b due to the primary transfer bias (positive polarity) applied to the primary transfer 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. 9 (a), 9 (b) and 9 (c) are images of the first page in the next image forming operation in the comparative example in which the polarization of the intermediate layer 501b is not made uniform by the application of the post-processing bias. FIG. 3 is an explanatory diagram of the primary transfer of the first page, the secondary transfer of the image of the first page, and the primary transfer of the image of the second page. As shown in FIG. 9A, in the case of the comparative example, since the downward polarization in the figure due to the application of the post-processing bias is not generated in the intermediate layer 501b of the intermediate transfer belt 501,
In the primary transfer of the image of the first page, a stronger polarization P1b 'occurs in the intermediate layer 501b than in the present embodiment. Therefore, in the secondary transfer of the image of the first page shown in FIG. 9B, the polarization P2a ′ of the surface layer 501a is inverted,
The polarization P2b 'of the intermediate layer 501b is slightly inverted without being inverted, but remains upward in the figure. In the primary transfer of the image of the second page shown in FIG. 9C, the polarization P3b 'of the intermediate layer 501b is further strengthened by the primary transfer bias (positive polarity) applied to the primary transfer bias roller 507. Therefore, the first page image appears as an afterimage on the second page image due to the strong polarization P3b ′ corresponding to the first page image remaining in the intermediate layer 501b.

Further, in the present embodiment, a constant current controlled bias is applied as the post-processing bias, so that even if the electrical resistance of the intermediate transfer belt 501 changes, the intermediate layer 501b of the intermediate transfer belt is not changed. The polarization can be uniformized to a predetermined strength.

Further, in this 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, it is preferable that the current value of the post-processing bias is set to 60% or more of the secondary transfer bias. By setting the current value of the post-processing bias in this manner, the effect of injecting electric charge into the intermediate transfer belt 501 is reliably exhibited, and
The polarization of 1b can be made more uniform.
Table 2 shows the results of evaluating the afterimage rank at the time of forming the next image when the current value of the post-processing bias applied after forming the full-color image is changed. As a comparative example, a case where no post-processing 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. The current value of the normal secondary transfer bias is 50
μA. From the results shown in Table 2, by setting the post-processing bias to 30 μA or more, that is, 60% or more of the current value of the normal secondary transfer bias, good image quality with an afterimage rank of 3.5 or more can be obtained. You can see that.

[0053]

[Table 2]

In the above embodiment, when the post-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-like image is formed on the output image by the next image forming operation. An abnormal image is generated. Therefore, the application of the post-processing bias is desirably performed by an integral multiple of the rotation time of the intermediate transfer belt 501 for one rotation.

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 post-processing bias roller 960 may be provided as an opposing member that opposes the above, and a post-processing bias controlled to a predetermined current value from a power supply 961 may be applied to the post-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 post-processing bias roller 960 can be moved toward and away from the intermediate transfer belt 501 by a contacting / separating 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 embodiment, the potential contrast due to the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 at the end of the image forming operation is the same as that of the toner superimposed on the same area of the intermediate transfer belt 501 in a series of image forming operations. It increases according to the number of superpositions of the images. Therefore, the intermediate transfer belt 50 in a series of image forming operations
The control unit 850 may control the on / off of the application of the post-processing bias on the basis of the number of times of superimposition of the toner images superimposed on the same one area.
For example, in a monochrome image forming mode such as black-and-white copying, the potential contrast due to the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 is small.
The post-processing bias after the end of the next transfer is controlled so as to turn off the application. On the other hand, when the secondary transfer to the transfer paper is performed after performing the primary transfer of the toner image onto the intermediate transfer belt 501 a plurality of times, such as in the two-color copy mode or the four-color full-color copy mode, the intermediate transfer is performed. Intermediate layer 5 of transfer belt 501
Since the potential contrast due to the polarization remaining in 01b tends to increase, the post-processing bias after the completion of the secondary transfer is controlled to turn on the application. By controlling the application of the post-processing bias in this manner, the occurrence of an afterimage in the next image formation can be prevented, and the unnecessary application of the post-processing bias can be avoided, so that the productivity of copying is not reduced.

In the above embodiment, the potential contrast due to the polarization remaining on the intermediate layer 501b of the intermediate transfer belt 501 at the end of the image forming operation depends on the number of transfer sheets on which the same image is formed in a series of image forming operations. Tend to be larger. Therefore, the number of transfer sheets P on which the same image is formed in a series of image forming operations is determined by the control unit 850.
The control unit 850 controls the on / off of the application of the post-processing bias based on the counting result.
May be controlled. For example, when the number of transfer papers P on which the same color image is formed is four or less, the potential contrast due to the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 is relatively small. The bias is controlled to turn off the application. on the other hand,
When the number of transfer papers P on which the same color image is formed is five or more, the potential contrast due to the polarization remaining in the intermediate layer 501b of the intermediate transfer belt 501 tends to increase. The processing bias is controlled to turn on the application. As described above, even when the post-processing bias application is controlled, the occurrence of an afterimage in the next image formation can be prevented, and the unnecessary application of the post-processing bias can be avoided, so that the productivity of the copy is not reduced.

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 with the case where the polarization remaining in the high resistance layer is eliminated or the polarity of the polarization is reversed to make the polarization uniform, the image formation is completed. The polarization remaining in the high-resistance layer of the intermediate transfer member can be more quickly and easily uniformized. Therefore, even when an intermediate transfer member having a high-resistance layer that is advantageous in preventing transfer dust is used, afterimages due to polarization remaining in the high-resistance layer of the intermediate transfer member after the image forming operation is completed can be reliably performed. There is an effect that can be prevented.

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.

In particular, according to the third aspect of the present invention, there is an effect that the polarization of the high resistance layer of the intermediate transfer member can be made uniform.

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.

According to the fifth and sixth aspects of the present invention, the occurrence of the afterimage can be prevented.
By avoiding the useless application of the post-processing bias, there is an effect that the productivity of the transfer material on which the image is formed does not decrease.

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 post-processing bias application control in the color copying machine.

FIGS. 8A to 8C are explanatory diagrams of a mechanism for preventing occurrence of an afterimage in a next image forming operation when a post-processing 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.

[Explanation of symbols]

 Reference Signs List 200 photosensitive 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

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shigeru Watanabe 1-3-6 Nakamagome, Ota-ku, Tokyo F-term in Ricoh Co., Ltd. (Reference) 2H027 DA21 DA32 DC04 DE02 DE07 DE09 EA03 EB04 EC06 ED24 EE02 EF09 2H032 AA05 AA15 BA09 BA23 BA26 BA30 CA02 CA13

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 unit for transferring a toner image onto a transfer material; wherein the intermediate transfer member has a high resistivity layer having a volume resistivity of 10 ¹⁰ Ωcm or more. After the secondary transfer of the toner image from the intermediate transfer member is completed and before the surface movement of the intermediate transfer member is stopped, the polarization remaining in the high-resistance layer of the intermediate transfer member is subjected to secondary transfer. An image type characterized by providing a polarization homogenizing means for homogenizing while maintaining the polarity at the time. Apparatus. 2. An image forming apparatus according to claim 1, wherein said polarization equalizing means includes an opposing member opposing said intermediate transfer member, and a post-processing bias for applying a post-processing 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. Image forming means for applying a secondary transfer bias, the current setting value of the post-processing bias being set to 60% or more of the current setting value of the secondary transfer bias. apparatus. 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 post-processing bias is an integral multiple of one rotation time of the intermediate transfer member. An image forming apparatus, comprising: 5. The image forming apparatus according to claim 2, wherein the application of the post-processing bias is turned on based on the number of superpositions of the toner images superimposed on the same area of the intermediate transfer body in a series of image forming operations. An image forming apparatus comprising: a control unit that controls the post-processing bias application unit so as to switch on / off. 6. The image forming apparatus according to claim 2, wherein the application of the post-processing bias is switched on / off based on the number of transfer materials on which the same image has been formed in a series of image forming operations. An image forming apparatus comprising a control unit for controlling a post-processing bias application unit. 7. An image forming apparatus according to claim 1, wherein said secondary transfer means is also used as said polarization uniformizing means.