JP2001324884A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably obtain a high-quality image without an image defect in the range from a multicolor high density part consisting of multilayered toner to a monochrome halftone part. SOLUTION: The intermediate transfer type image forming device is equipped with a contact transfer member 7 pressed and arranged on the side of the toner image T carrying surface of an intermediate transfer body 4 in a secondary transfer means 6, plural electrode members 8 (for example, 8a and 8b) opposed to the member 7 while putting the transfer body 4 in between and arranged in contact with the transfer body 4, and plural bias applying means 9 (for example, 9a and 9b) for applying transfer bias to the members 8 (8a and 8b) so as to form transfer electric field oriented toward the member 7 through the members 8 (8a and 8b).

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 using an intermediate transfer member, such as an electrophotographic copying machine or a laser printer, and more particularly to a latent image carrier such as a photosensitive drum via an intermediate transfer member. The present invention relates to an improvement in an image forming apparatus that transfers a toner image formed thereon onto a recording material such as paper.

[0002]

2. Description of the Related Art Conventional intermediate transfer type image forming apparatuses include, for example, yellow (Y), magenta (M), and cyan (C) around an image carrier such as a photosensitive drum.
And a developing device for each color component of black (K), and a belt-like intermediate transfer member (intermediate transfer belt) is disposed opposite to the image carrier, and the image carrier is rotated every one rotation of the image carrier. After an unfixed toner image of each color component formed on the intermediate transfer belt is sequentially primary-transferred, a composite primary transfer image superimposed on the intermediate transfer belt is secondarily transferred to a sheet as a recording material to obtain a desired image. Is formed on paper. Here, as the secondary transfer device, for example, a transfer roll disposed in contact with the intermediate transfer belt, a backup roll disposed opposite to the transfer roll across the intermediate transfer belt, these transfer rolls and backup rolls And a power supply for applying a bias for forming an electric field for transferring the toner image on the intermediate transfer belt onto the sheet. According to this type, since the composite toner image that has already been subjected to multiple transfer on the intermediate transfer belt is collectively transferred to paper, unstable factors during image formation can be eliminated, and the image during multiple transfer can be eliminated. This has the advantage that irregularities and color shifts can be effectively prevented.

[0003]

However, the above-described image forming apparatus has the following problems. That is, in the secondary transfer, the multi-layered toner superimposed on the intermediate transfer belt is collectively transferred to the paper,
The transfer cannot be performed unless a strong transfer electric field is formed as compared with the transfer of a conventional monochrome (single color) image.

However, when the electric field is increased to transfer the multi-layered toner, there is a technical problem that a discharge is generated in a single-color halftone portion or the like where the amount of the toner is small, thereby lowering the transfer rate and causing image quality defects. Was done. In order to solve such a technical problem, for example, Japanese Patent Laid-Open No.
No. 82 proposes a technique in which the charge amount of the toner on the intermediate transfer belt before the secondary transfer is controlled so as to eliminate the difference in the charge amount between the multilayer portion and the single color portion. In order to control the amount of charge, a non-contact charger must be newly added, and there has been a technical problem that the device is increased in size and ozone is generated from the non-contact charger.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned technical problems, and is intended to stably provide a high-quality image free from image defects from a multicolor high-density portion composed of a multi-layered toner to a monochromatic halftone portion. The present invention provides an image forming apparatus that can be obtained by performing the above steps.

[0006]

That is, the present invention relates to FIG.
(A) As shown in (b), one or more image carriers 1
A latent image forming means 2 for forming an electrostatic latent image on the image carrier 1, a plurality of developing devices 3 for developing the electrostatic latent image on the image carrier 1 with a corresponding color toner, An intermediate transfer member 4 opposed to the image carrier 1 and a primary transfer unit 5 for transferring the toner image T on the image carrier 1 onto the intermediate transfer member 4
And a secondary transfer unit 6 for collectively transferring the toner image T on the intermediate transfer member 4 to the recording material K, wherein the secondary transfer unit 6 includes a toner image on the intermediate transfer member 4. A contact transfer member 7 pressed and arranged on the T-carrying surface side; and a plurality of electrode members 8 opposed to the contact transfer member 7 with the intermediate transfer member 4 interposed therebetween and arranged in contact with the intermediate transfer member 4
(For example, 8a, 8b) and each of the electrode members 8 (8a, 8b).
b) a plurality of bias applying means 9 (for example, 9a, 9b) for applying a transfer bias to each electrode member 8 (8a, 8b) so as to form a transfer electric field toward the contact transfer member 7 via the contact transfer member 7;
b).

In such technical means, the image forming apparatus to which the present invention is applied is, for example, an embodiment in which a plurality of developing devices 3 are arranged on one image carrier 1 as shown in FIG. Needless to say, for example, a configuration in which a developing device 3 is provided on each of a plurality of image carriers 1 (a so-called tandem type) and a configuration in which these are combined are also included.

The image carrier 1 may be a photoconductor, a dielectric, or the like, as long as it can form and carry an electrostatic latent image by the latent image forming means 2. Shape and belt shape. The latent image forming means 2 may be appropriately selected as long as it forms an electrostatic latent image, such as a method through a charging and exposure process or a method including a latent image writing process by an ion flow. .

Further, as long as the developing device 3 can develop the electrostatic latent image formed on the image carrier 1, it is appropriately selected from one-component development, two-component development, contact development, and non-contact development. Any of these various developing systems may be mixed.

The intermediate transfer member 4 includes a toner image T
As long as it can carry and transport
A belt or the like may be selected as appropriate. Here, at the time of primary transfer, a phenomenon (so-called blur) in which the toner image T on the image carrier 1 flies onto the intermediate transfer body 4 at a position near the contact portion between the image carrier 1 and the intermediate transfer body 4 is prevented. From the viewpoint, the surface resistivity is preferably 10 ¹⁰ Ω / □ or more, and from the viewpoint of ensuring the self-static elimination performance of the intermediate transfer member 4, the surface resistivity is 10 ¹⁴ Ω / □. / □
The following is preferred.

Further, if the primary transfer means 5 transfers the toner image T on the image carrier 1 onto the intermediate transfer body 4,
The transfer system may be appropriately selected, and may be a contact transfer system or a non-contact transfer system.

On the other hand, the secondary transfer means 6 collectively transfers the toner image T on the intermediate transfer member 4 to the recording material K.
In the invention of the present application, the contact transfer member 7 pressed and arranged on the toner image T carrying surface side of the intermediate transfer member 4,
And a plurality of electrode members 8 (for example, 8a, 8a,
8b) and a plurality of bias applying means for applying a transfer bias to each electrode member 8 (8a, 8b) so as to form a transfer electric field toward the contact transfer member 7 via each electrode member 8 (8a, 8b). 9 (for example, 9a and 9b), and therefore a contact transfer method is premised.

Here, the electrode member 8 is connected to the intermediate transfer member 4.
Any number may be used as long as it is opposed to the contact transfer member 7 and is arranged to be in contact with the intermediate transfer member 4 with the interposition therebetween. FIG. 1B illustrates a case where two electrode members 8 (8a, 8b) are provided.

The plurality of electrode members 8 (8a, 8a,
The arrangement position of 8b) may be appropriately selected from a portion opposed to the contact transfer member 7 with the intermediate transfer member 4 interposed therebetween, but the position of the intermediate transfer member 4 and the contact transfer member 7 during the secondary transfer may be selected. In order to prevent the phenomenon that the toner image T on the intermediate transfer body 4 flies (so-called blur) on the upstream side of the contact portion (transfer nip N), the transfer electric field on the upstream side of the transfer nip N is weakened. From such a viewpoint, it is preferable that, of the plurality of electrode members 8, the electrode member 8a on the most upstream side in the moving direction of the intermediate transfer member 4 is disposed in the transfer nip N.

In order to prevent the above-mentioned blur, it is preferable to prevent discharge on the upstream side of the transfer nip N. From such a viewpoint, the intermediate transfer of the plurality of electrode members 8 may be prevented. It is preferable that the transfer bias applied to the electrode member 8a on the most upstream side in the moving direction of the body 4 is set smaller than the transfer bias applied to the electrode member 8b disposed downstream of the electrode member 8a.

On the other hand, at the time of the secondary transfer, the toner image T transferred to the recording material K at the downstream side of the transfer nip N between the intermediate transfer member 4 and the contact transfer member 7 returns to the intermediate transfer member 4 side (so-called re-transfer). In order to prevent transfer, it is preferable to increase the transfer electric field on the downstream side of the transfer nip N. From such a viewpoint, the plurality of electrode members 8
Of these, it is preferable that the electrode member 8b on the most downstream side in the movement direction of the intermediate transfer member 4 is disposed outside the transfer nip N.

From the same viewpoint, the transfer bias applied to the electrode member 8b of the plurality of electrode members 8 which is the most downstream in the moving direction of the intermediate transfer member 4 is higher than that of the electrode member 8b. Is preferably set to be larger than the transfer bias applied to the electrode member 8a disposed at the position.

Further, from the same viewpoint, the pressing force of the electrode member 8b of the plurality of electrode members 8 at the most downstream side in the moving direction of the intermediate transfer member 4 against the intermediate transfer member 4 is reduced. It is preferable that the pressure is set to be larger than the pressing force of the electrode member 8a disposed on the upstream side of the electrode member 8b.

The shape of the electrode member 8 may be appropriately selected, for example, a roll shape, a brush shape, a film shape, a blade shape, or a combination thereof, but a stable transfer electric field downstream of the transfer nip N can be selected. From the viewpoint of forming the electrode member 8b, the electrode member 8b of the plurality of electrode members 8 which is the most downstream in the moving direction of the intermediate transfer body 4
Preferably has a roll shape.

The secondary transfer bias applied by the bias applying means 9 via the electrode member 8 may be of a fixed magnitude, or may be of a certain size, and may include the environment, the type of the recording material K, and the image forming mode ( Control may be performed by the control means according to single color / multiple colors, single side / double side). Here, in the latter embodiment, the control means is configured to control the electrode members 8
Although the transfer bias applied to the transfer members a and 8b may be individually controlled, in the image forming apparatus according to the present invention, the electrode on the most downstream side in the moving direction of the intermediate transfer member 4 among the plurality of electrode members 8 is used. Since a substantial transfer electric field is formed by the member 8b, from the viewpoint of simplification of the device configuration, the control means is provided on the most downstream side in the moving direction of the intermediate transfer member 4 among the plurality of electrode members 8. Electrode member 8
It is preferable to control only the transfer bias applied to b.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the accompanying drawings. First Embodiment FIG. 2 shows a first embodiment of an image forming apparatus to which the present invention is applied. In the figure, reference numeral 11 denotes a photosensitive drum as an image carrier, which is 160 mm / sec in the direction of arrow A.
It rotates at the speed. As the photosensitive drum 11 rotates, a charging device 12 is provided on the surface thereof.
An electrostatic latent image corresponding to the image information is formed by a known electrophotographic process such as the exposure device 13. Reference numeral 14
A mirror provided to irradiate the photosensitive drum 11 with a beam from the exposure device 13. Around the photoconductor drum 11, developing devices 15 (specifically, 15Y, 15M, 15C, 15Y, 15M, 15C, and 15K) corresponding to each color of yellow (Y), magenta (M), cyan (C), and black (K) are provided.
15K), and the electrostatic latent image formed on the photosensitive drum 11 is developed by the corresponding developing device 15 to form a toner image T. In the present embodiment,
The photosensitive drum 11 is configured to be negatively charged,
The development is performed by a reversal development method. Therefore, all the toners used are charged negatively. still,
Reference numeral 18 denotes a drum cleaner for removing residual toner on the photosensitive drum 11.

Reference numeral 20 denotes an intermediate transfer belt as an intermediate transfer member disposed so as to contact the surface of the photosensitive drum 11, and includes a drive roll 21, a walk correction roll 22, a tension roll 23, and a secondary transfer. Counter electrode 2
4 and rotates at substantially the same speed as the photosensitive drum 11 in the direction of arrow B. In the present embodiment,
As the intermediate transfer belt 20, PI (polyimide), P
VdF (polyvinylidene fluoride) and PC (polycarbonate) as main components, and CB (carbon black)
Was used to adjust the resistance. When the resistance of the intermediate transfer belt 20 thus prepared was measured, the surface resistivity was 10 ⁹ to 10 ¹⁵ Ω / □. The surface resistivity was measured using an HR probe (manufactured by Mitsubishi Yuka Co., inner electrode diameter 16 mm, ring electrode inner diameter 30) using an ultra-high resistance / micro ammeter (R8340A manufactured by Advantest).
mm) is connected, 100 V is applied between the ring electrodes, and a current value 30 seconds after the start of energization is measured and calculated by the following formula. ρs = 10 × V / I (Ω / □) where V is an applied voltage value (V) and I is a current value (A).

The surface resistivity of the intermediate transfer belt 20 is 10 ¹⁰
If lower than Ω / □, the intermediate transfer belt 2
0 spreads out of the transfer nip through the resistance of the intermediate transfer belt 20 so that the photosensitive drum 1
Since the toner image T was transferred from the photosensitive drum 11 to the intermediate transfer belt 20 before the toner image 1 and the intermediate transfer belt 20 came into contact with each other, a good image was not obtained because the toner scattered extremely. On the other hand, if the surface resistivity of the intermediate transfer belt 20 exceeds 10 ¹⁴ Ω / □, the charging potential of the intermediate transfer belt 20 becomes too high, and spark discharge occurs between the intermediate transfer belt 20 and peripheral members, thereby causing image defects. There has occurred. Furthermore,
When the surface resistivity is lower than 10 ¹⁴ Ω / □, it is difficult for the intermediate transfer belt 20 to be charged up, and a neutralizing device for the intermediate transfer belt 20 becomes unnecessary. For this reason, in the present embodiment, the surface resistivity is 10 ¹⁰ to 10
^The intermediate transfer belt 20 of ¹⁴ Ω / □ is used. When the thickness of the intermediate transfer belt 20 is 50 μm or less, the mechanical strength is insufficient, and the belt may be broken or broken. Therefore, a belt having a thickness of more than 50 μm is used.

Further, at a portion (primary transfer position) of the intermediate transfer belt 20 facing the photosensitive drum 11, a primary transfer device 16 (primary transfer roll in the present embodiment) is disposed on the back side of the intermediate transfer belt 20. A primary transfer bias is applied to the primary transfer roll 16 by a primary transfer bias power supply 17 so that the toner image T on the photosensitive drum 11 is applied to the intermediate transfer belt 20. It is designed to be electrostatically attracted.

A secondary transfer device 40 is provided at a secondary transfer position of the intermediate transfer belt 20 facing the conveyance path of the sheet P. In the present embodiment, the toner image T on the intermediate transfer belt 20 is provided. A secondary transfer roll 25 serving as a contact transfer member that is disposed in pressure contact with the supporting surface side, and a secondary transfer opposing electrode 24 (a member that stretches the intermediate transfer belt 20) that is disposed on the back side of the intermediate transfer belt 20 ).

Here, as shown in FIGS. 2 and 3A, the secondary transfer counter electrode 24 is made of an insulating resin having a semi-cylindrical surface with a radius of 14 mm as a base material. The first electrode 31 and the second electrode 33 were embedded at a predetermined distance from each other. As this insulating resin, a highly slidable POM (polyacetal) is used. As the first electrode 31 and the second electrode 33, a SUS alloy thin plate having a thickness of 0.1 mm is used. The intermediate transfer belt 20 protrudes so as to surely contact the intermediate transfer belt 20. In addition, as shown in FIG.
Of the first and second electrodes 33, the first electrode 31 disposed on the upstream side in the rotation direction B of the intermediate transfer belt 20 is a contact portion between the intermediate transfer belt 20 and the secondary transfer roll 25 (hereinafter referred to as a transfer nip). N), the second electrode 33 disposed downstream of the intermediate transfer belt 20 in the rotation direction B is disposed downstream of the transfer nip N.

Further, as shown in FIG. 2, the first electrode 31 is provided with a first secondary transfer bias power source 32 as a bias applying means, and the second electrode 33 is provided with a second secondary transfer bias power supply as a bias applying means. A power supply 34 is connected to each of the first electrode 31 and the second electrode 33. By applying a secondary transfer bias having the same polarity as the charged polarity of the toner, the toner image T on the intermediate transfer belt 20 is recorded. The sheet is collectively transferred to a sheet P as a material.

On the other hand, the secondary transfer roll 25 is made of silicon rubber, CB (carbon black) mixed with SUS (stainless steel) alloy, the resistance of which is adjusted around the core metal, and EPDM.
(Ethylene propylene diene rubber) or the like was used. The core diameter of the secondary transfer roll is 15 mm, and the thickness of the rubber material is 7 mm. When the resistance of the secondary transfer roll 25 thus prepared was measured, the resistance was 10 ⁶ to 10 ¹⁰ Ω. Note that this resistance value is determined by placing the secondary transfer roll 25 on a metal plate,
A load of 2.5 kg is applied to both ends of the secondary transfer roll 25, and the secondary transfer roll 25 is pressed against a metal plate with a total load of 5 kg, and a DC high-voltage power supply (m.
A constant current of 30 μA is passed between the metal core of the secondary transfer roll 25 and the metal plate using the ODEL 610c), and the applied voltage 30 seconds after the start of energization is measured by a voltmeter built in the DC voltage power supply. It is.

[0029] In the image forming apparatus according to the present embodiment, the resistance of the secondary transfer roll 25 is greater than 10 ⁸ Omega, since the secondary transfer voltage becomes considerably higher (or 6kV), 10 ⁸ Ω or more The secondary transfer roll 25 having the above resistance could not be used.

Further, a belt cleaner 41 for removing residual toner on the intermediate transfer belt 20 is provided downstream of the secondary transfer device 40. The secondary transfer roll 25 and the belt cleaner 41 are disposed so as to be able to contact and separate from the intermediate transfer belt 20, and when a color image including a plurality of color toner images T is formed, the final color Until the previous toner image T passes through the secondary transfer roll 25 and the belt cleaner 41, the toner image T is separated from the intermediate transfer belt 20.

Further, the paper transport system sends out the paper P from the paper tray 50 by a feed roll 51, temporarily stops the positioning by a registration roll (registration roll) 52, and then, via a chute 53 at a predetermined timing. The paper P is sent to the next transfer position,
The sheet P after the secondary transfer is guided to a conveying belt 55 via a sheet conveying guide 54, and the fixing device 5
7. In FIG. 2,
Reference numeral 56 denotes a paper guide provided between the transport belt 55 and the fixing device 57.

Next, an image forming process of the image forming apparatus according to the present embodiment will be described. When a start switch (not shown) is turned on, a predetermined image forming process is executed. First, after the surface of the photosensitive drum 11 is charged by the charging device 12, a latent image is written by the beam from the exposure device 13. At this time, if the electrostatic latent image written on the photosensitive drum 11 corresponds to the yellow image information, the electrostatic latent image is developed by the developing device 15Y corresponding to the yellow toner. 11
A yellow toner image T is formed thereon. The toner image T formed on the photosensitive drum 11 is transferred to the intermediate transfer belt 20 by a primary transfer bias applied to the primary transfer roll 16 at a primary transfer position where the photosensitive drum 11 contacts the intermediate transfer belt 20. Is done. On the other hand, the toner remaining on the photosensitive drum 11 after the primary transfer is removed by the drum cleaner 18.

At this time, when forming a monochrome image,
The toner image T primarily transferred to the intermediate transfer belt 20 is secondarily transferred to the sheet P immediately. When a color image in which toner images of a plurality of colors are overlapped is formed, the toner image T on the photosensitive drum 11 is formed. The process of forming the toner image T and the primary transfer of the toner image T are repeated for the number of colors. For example, when a full-color image is formed by superimposing four-color toner images T, yellow, magenta, cyan, and black toner images T are formed on the photosensitive drum 11 every one rotation thereof. The image T is sequentially primary-transferred to the intermediate transfer belt 20. On the other hand, the intermediate transfer belt 20
Rotates at the same cycle as the photosensitive drum 11 while holding the yellow toner image T that has been primarily transferred first, and the magenta, cyan, and cyan inks are placed at a predetermined position on the intermediate transfer belt 20 every one rotation thereof. The black toner image T is overlaid and transferred.

The toner image T primarily transferred to the intermediate transfer belt 20 in this manner is conveyed to a secondary transfer position as the intermediate transfer belt 20 rotates. On the other hand, the paper P is carried out from the paper tray 50 by the feed roll 51 to the position of the registration roll 52, and thereafter, the registration roll 5
At 2 at a predetermined timing to the secondary transfer position,
The secondary transfer roll 25 nips the sheet P with respect to the secondary transfer counter electrode 24. Then, at the secondary transfer position, the transfer electric field is applied between the secondary transfer roll 25 and the secondary transfer opposing electrode 24 by the transfer bias supplied to the first electrode 31 and the second electrode 33 of the secondary transfer opposing electrode 24. Is formed, and the toner image T carried on the intermediate transfer belt 20 is electrostatically transferred to the paper P. Thereafter, the secondary-transferred paper P is transported to the fixing device 57 via the paper transport guide 54, the transport belt 55, and the paper guide 56, and the toner image T on the paper P is fixed. On the other hand, the residual toner adhered to the intermediate transfer belt 20 after the secondary transfer is removed by the belt cleaner 41.

The present inventor forms an image using the image forming apparatus according to the present embodiment and the image forming apparatus according to the comparative embodiment shown in FIG. 4, and collects and evaluates the obtained image. Was. Here, the image forming apparatus according to the comparative embodiment has the configuration shown in FIG.
As shown in the figure, a backup roll 71 is provided in place of the secondary transfer counter electrode 24,
A metal power supply roll 72 is disposed in contact with the power supply roller 1, and a secondary transfer bias having the same polarity as that of the toner is applied to the power supply roll 72 from a secondary transfer bias power supply 73.

In each of the image forming apparatuses, while changing the secondary transfer voltage, a single color / two-color superposition (hereinafter referred to as a secondary color as required) / three-color superposition (hereinafter referred to as necessary) having an area ratio of 100%. A tertiary color).
The evaluation was performed based on the secondary transfer rate and the presence / absence of image quality defects when copying on A3 size plain paper, and the presence / absence of image quality defects when a single-color halftone toner image T having an area ratio of 30% was formed. Was. However, the secondary transfer voltage here is the second electrode 33 in the image forming apparatus according to the present embodiment.
, And the voltage applied to the backup roll 71 in the image forming apparatus according to the comparative embodiment. Then, the voltage applied to the first electrode 31 in the image forming apparatus according to the present embodiment was kept constant at -1.8 kV. Here, the toner weight per unit area of the single color / secondary color / tertiary color toner image T is 0.4 / 0.8 / 1.
2 (mg / cm ² ).

FIG. 5 shows the relationship between the secondary transfer voltage (second electrode applied voltage) and the secondary transfer rate in the image forming apparatus according to this embodiment, and FIG. 6 shows the image formation according to the comparative embodiment. 4 shows a relationship between a secondary transfer voltage (applying voltage to a backup roll) and a secondary transfer rate in the apparatus. In the image forming apparatus according to the comparative example, as shown in FIG. 6, when the secondary transfer voltage is increased in order to satisfactorily transfer the secondary color and the tertiary color,
When the voltage exceeds -2.0 kV, the secondary transfer rate starts to decrease due to the discharge, and it is difficult to set the transfer conditions for obtaining a good secondary transfer rate from a single color to a tertiary color. On the other hand, in the image forming apparatus according to the present embodiment, as shown in FIG. 5, the secondary transfer voltage (the voltage applied to the second electrode) is -3.0 to -4.5.
A good secondary transfer rate from a single color to a tertiary color was obtained over a wide range of kV. However, the first electrode 31
When the voltage applied to the second electrode 33 is set to −2.0 kV or more, the transfer rate of a single color deteriorates regardless of the voltage applied to the second electrode 33.

The following can be considered as a cause of this.
That is, in the image forming apparatus according to the comparative example, FIG.
As shown in (a) and (b), the transfer electric field formed in the transfer nip N also spreads to the upstream side of the transfer nip N, so that the paper P (not shown) and the intermediate transfer belt 20 in this area. A transfer electric field of a certain magnitude is also formed in the minute gap generated between them. Therefore, when a strong electric field is formed in the transfer nip N in order to transfer a large amount of toner such as a secondary color or a tertiary color, an electric field equal to or greater than the Paschen limit is applied to the minute gap, and discharge occurs in the area. Due to this discharge, the toner on the intermediate transfer belt 20 is charged to a polarity opposite to the charging polarity of the main body, and is not transferred to the sheet P (not shown) in the secondary transfer portion, resulting in white spots.

On the other hand, in the image forming apparatus according to the present embodiment, as shown in FIG. 3B, the voltage applied to the first electrode 31 provided on the upstream side in the transfer nip N is discharged. A small amount of toner of a single color can be transferred by keeping it low enough not to cause such a problem, and by increasing the voltage applied to the second electrode 33 provided downstream of the transfer nip N,
An electric field strong enough to transfer a large amount of toner such as a secondary color and a tertiary color can be generated in the transfer nip N.
As a result, it is possible to obtain a very high secondary transfer rate from a single color to a tertiary color at the same time without causing discharge.

Further, in the secondary transfer device 40 of the image forming apparatus according to the comparative embodiment, the transfer electric field formed in the transfer nip N also spreads to the downstream side of the transfer nip N. When the sheet P (not shown) is peeled from the intermediate transfer belt 20 in this region, the portion of the secondary color or the tertiary color is particularly weakened.
This results in reverse transcription (retransfer) to 0.

On the other hand, in the image forming apparatus according to the present embodiment, by positioning the second electrode 33 downstream of the transfer nip N, a strong electric field is formed even downstream of the transfer nip N. Paper P in this area
(Not shown) can be prevented from being reversely transferred when peeled off from the intermediate transfer belt 20.

In the image forming apparatus according to the present embodiment, the voltage applied to the second electrode 33 is controlled in accordance with, for example, environmental information such as temperature and humidity, the type of paper P, an image forming mode, and the like. By doing so, it is possible to stably obtain a high secondary transfer rate irrespective of fluctuations in conditions. At this time,
The voltage applied to the first electrode 31 may be kept constant.

Embodiment 2 This embodiment is substantially the same as Embodiment 1 except that FIG.
As shown in FIG. 7, the configuration of the secondary transfer device 40 is different. Note that, of the components of the image forming apparatus according to the present embodiment, the same components as those of the image forming apparatus according to the first embodiment are denoted by the same reference numerals as those in the first embodiment, and detailed descriptions thereof will be given here. Description is omitted.

In this embodiment, the secondary transfer device 40
A secondary transfer roll 25 as a contact transfer member that is arranged in pressure contact with the toner image T-bearing surface of the intermediate transfer belt 20;
The back side of the intermediate transfer belt 20 and the intermediate transfer belt 2
It is provided with a film 63 as a first electrode disposed on the upstream side in the rotation direction 0 and a backup roll 61 as a second electrode disposed on the downstream side.

Here, the film 63 has a thickness of 20
0 μm PET (polyethylene terephthalate)
I (polyimide), PVdF (polyvinylidene fluoride), PC (polycarbonate) is mixed with CB (carbon black) and the resistance is adjusted to 108 Ωcm or less. The conductive film side is arranged so as to be in contact with the intermediate transfer belt 20.

Further, as the backup roll 61,
Insulating silicon rubber, EPDM (ethylene propylene diene rubber) around SUS (stainless) alloy core
A rubber material such as silicon rubber, EPDM, etc., whose surface resistance is adjusted by mixing CB (carbon black) around it, and a PFA (particle) whose surface resistance is adjusted by mixing CB, etc. Fluoroalkoxy), PT
A resin coated with a resin such as FE (polytetrafluoroethylene) as a resistance layer was used. The core of the backup roll 61 is 15 mm in diameter, and the thickness of the insulating rubber material is 6 m.
m. The thickness of the resistance layer covering the periphery of the insulating rubber is 0.5 to 2 mm in the case of a rubber material and 50 in the case of a resin.
１００100 μm. When the resistance of the backup roll 61 thus prepared was measured, the surface resistivity was 10 ⁵ to 10 ¹⁰ Ω / □. The surface resistivity is determined by an ultra-high resistance / microammeter (Advantest R
8340A), SUS (stainless steel) with an outer diameter of 10 mm
Two alloy shafts were connected as electrodes, these electrodes were brought into contact with the backup roll 61 with a load of 600 g each, and a voltage of 1000 V was applied between the two shafts,
This is a value obtained by measuring the current value 30 seconds after the start of energization and calculating by the following formula. ρs = (V × l) / (i × d) (Ω / □) where V is the applied voltage (V), i is the current value (A), l is the roll surface length (mm), and d is the electrode Distance (mm). The surface resistivity of the backup roll 61 is 10
^If it exceeds ⁹ Ω / □, the secondary transfer voltage becomes extremely high (6 k
V or more), the backup roll 61 having a surface resistivity of 10 ¹⁰ Ω / □ or more could not be used.

The supply of the transfer voltage to the backup roll 61 was performed by arranging the power supply roll 62 in contact with the outer peripheral portion of the backup roll 61. As the power supply roll 62, a SUS (stainless) alloy roll having an outer diameter of 14 mm was used.

The film 63 is connected to a first secondary transfer bias power supply 32 for applying a secondary transfer voltage in the same manner as in the first embodiment. Similarly, a second secondary transfer bias power supply 34 for applying a transfer voltage is connected, and a voltage having the same polarity as the charged polarity of the toner is applied during the secondary transfer.

The present inventor performed the same evaluation as in the first embodiment using the image forming apparatus according to the present embodiment. still,
In this evaluation, the voltage applied to the film 63 in the image forming apparatus according to the present embodiment was kept constant at -1.8 kV.

FIG. 9 shows the results. As shown in the figure, in the image forming apparatus according to the present embodiment, the secondary transfer voltage (applied voltage of the backup roll) is good in the range of -3.0 to -3.5 kV and good secondary transfer from single color to tertiary color. It is possible to get a rate.

In this embodiment, since the backup roll 61 is used as the electrode member, the rotation of the intermediate transfer belt 20 becomes smoother, and the wear on the inner surface of the intermediate transfer belt 20 is reduced.

Further, by using the backup roll 61, the pressing force against the intermediate transfer belt 20 can be increased, so that a gap is formed between the sheet P (not shown) and the intermediate transfer belt 20 in this area. Such a situation can be prevented, and a stronger and more stable transfer electric field can be formed.

Further, since the backup roll 61 is used, even if the pressing force is increased, the intermediate transfer belt 20 is not damaged, and the transfer nip N
A transfer electric field can be formed downstream of the device.

[0054]

As described above, according to the present invention,
From a multi-color high-density portion composed of a multi-layered toner to a single-color half-tone portion, a high-quality image without image quality defects can be stably obtained.

[Brief description of the drawings]

FIGS. 1A and 1B are explanatory diagrams showing an outline of an image forming apparatus according to the present invention.

FIG. 2 is a schematic configuration diagram illustrating Embodiment 1 of the image forming apparatus to which the present invention has been applied.

FIG. 3A is an enlarged view of a transfer nip of the image forming apparatus according to the first embodiment, and FIG. 3B is a schematic diagram illustrating a transfer electric field formed near the transfer nip.

FIG. 4 is a schematic configuration diagram of an image forming apparatus according to a comparative embodiment.

FIG. 5 is a graph showing a correlation between a secondary transfer voltage and a secondary transfer rate in the image forming apparatus according to the first embodiment.

FIG. 6 is a graph illustrating a correlation between a secondary transfer voltage and a secondary transfer rate in an image forming apparatus according to a comparative embodiment.

FIG. 7A is an enlarged view of a transfer nip of an image forming apparatus according to a comparative example, and FIG. 7B is a schematic diagram illustrating a transfer electric field formed near the transfer nip.

FIG. 8 is a schematic configuration diagram illustrating Embodiment 2 of an image forming apparatus to which the present invention has been applied.

FIG. 9 is a graph showing a correlation between a secondary transfer voltage and a secondary transfer rate in the image forming apparatus according to the second embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Image carrier, 2 ... Latent image forming means, 3 ... Developing device, 4 ...
Intermediate transfer member, 5: primary transfer means, 6: secondary transfer means, 7
... contact transfer member, 8 (8a, 8b) ... electrode member, 9 (9
a, 9b) ... bias applying means

Claims (9)

[Claims] 1. An image carrier, one or more image carriers, a latent image forming means for forming an electrostatic latent image on the image carrier, and developing the electrostatic latent image on the image carrier with a corresponding color toner A plurality of developing devices, an intermediate transfer member arranged to face the image carrier, a primary transfer means for transferring a toner image on the image carrier onto the intermediate transfer member, and a toner on the intermediate transfer member An image forming apparatus comprising: a secondary transfer unit that transfers an image to a recording material; wherein the secondary transfer unit is a contact transfer member that is pressed and arranged on a toner image bearing surface side of the intermediate transfer body; A plurality of electrode members facing the contact transfer member with the body interposed therebetween and arranged in contact with the intermediate transfer member; and forming a transfer electric field toward the contact transfer member via each of the electrode members. Multiple bias applying means for applying a transfer bias An image forming apparatus comprising the and. 2. The image forming apparatus according to claim 1, wherein the intermediate transfer body has a surface resistivity of 10 ¹⁰ to 10 ¹⁴ Ω / □.
An image forming apparatus, characterized in that: 3. The image forming apparatus according to claim 1, wherein, of the plurality of electrode members, an electrode member on the most upstream side in the moving direction of the intermediate transfer member is arranged in a transfer nip. Image forming apparatus. 4. The image forming apparatus according to claim 1, wherein, of the plurality of electrode members, an electrode member on the most downstream side in the moving direction of the intermediate transfer member is disposed outside a transfer nip. Image forming apparatus. 5. The image forming apparatus according to claim 1, wherein a transfer bias applied to an electrode member on the most upstream side in the moving direction of the intermediate transfer member among the plurality of electrode members is downstream of the electrode member. Wherein the transfer bias is set to be smaller than a transfer bias applied to an electrode member disposed in the image forming apparatus. 6. The image forming apparatus according to claim 1, wherein a transfer bias applied to a most downstream electrode member of the plurality of electrode members in the moving direction of the intermediate transfer member is upstream of the electrode member. Wherein the transfer bias is set to be larger than the transfer bias applied to the electrode member disposed in the image forming apparatus. 7. The image forming apparatus according to claim 1, wherein, of the plurality of electrode members, a pressing force of an electrode member on the most downstream side in the moving direction of the intermediate transfer member against the intermediate transfer member is the electrode member. An image forming apparatus, wherein the pressure is set to be larger than a pressure contact force of an electrode member disposed on a more upstream side. 8. The image forming apparatus according to claim 1, wherein, of the plurality of electrode members, an electrode member on the most downstream side in the moving direction of the intermediate transfer member has a roll shape. Image forming device. 9. The image forming apparatus according to claim 1, further comprising a control unit configured to control a transfer bias applied to an electrode member of the plurality of electrode members that is the most downstream in the moving direction of the intermediate transfer member. An image forming apparatus comprising: