JP2001034075A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an image forming device capable of suppressing a transfer current leak from a transfer support to the periphery while obtaining a high quality image which suppresses retransfer. SOLUTION: The transfer belt 8 of the image forming device is an endless polyimide belt having 1014 Ωcm volume resistivity and 80 μm thickness and comprises an upper layer 41 halving 1014 Ω/sq. surface resistivity and a lower layer 42 having 1011 Ω/sq. surface resistivity. The image forming device has a transfer belt charging means 21 which charges the face of the transfer belt 8 opposite to the toner image carrying face, namely the rear face, to the same polarity as that of a toner and which is located on the upper stream side of the traveling direction of the transfer belt 8 with respect to a transfer charger 6a.

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 electrostatic transfer process, such as an electrostatic copying machine and a printer, and includes a transfer carrier such as a transfer material carrier.
The present invention is applied to a color image forming apparatus that obtains a color image by superimposing and transferring a developer image formed on an image carrier on a transfer carrier.

[0002]

2. Description of the Related Art Hitherto, many image forming apparatuses have been proposed which include a step (multiple transfer) of sequentially superimposing and transferring toner images as a plurality of developer images on the same transfer material. FIG. 9 is a side view showing an example of such an image forming apparatus.

In such a multi-transfer image forming apparatus, a toner image transferred to a transfer material carried on a transfer carrier in a certain image forming section passes through a photosensitive member as an image carrier of the next image forming section. In this case, there is a disadvantage that the image is retransferred to the photoconductor.

In particular, when two or more colors of toner as a developer are superimposed to reproduce another color, a toner image transferred later is transferred later than a toner image transferred earlier. Is remarkably re-transferred to the photoreceptor which performs the following.

In the image forming apparatus shown in FIG. 9, toner images are formed, for example, in yellow (Y), magenta (M), cyan (C), and black (B) in image forming sections Pa, Pb, Pc, and Pd.
It is assumed that the red (R) color is reproduced by superimposing the yellow color and the magenta color when the image is transferred in the color order of k).

Then, a transfer belt 8 as a transfer carrier is provided.
When the magenta toner image, which is later transferred and becomes an upper layer on the transfer material 1 carried by the photoconductor 1, passes below the photosensitive drums 2c and 2d as downstream photosensitive members, these photosensitive drums 2c and 2d The red image re-transferred to 2d and obtained on the transfer material 1 has a yellowish color partially or entirely.

Similarly, when the green (G) color is reproduced by superimposing the yellow and cyan colors, the cyan toner image in the upper layer is re-transferred to the downstream photosensitive drum 2d, and the resulting green The image is partially or entirely yellowish.

The retransfer is largely caused by abnormal discharge. Generally, the transfer paper which is most frequently used as a transfer material has a volume resistivity of 10 ⁸ Ωc.
m, and the dried transfer paper has a volume resistivity of 10 ¹³ Ωcm
It changes greatly.

Therefore, the photosensitive drum 2 is placed on the dried transfer paper.
To sufficiently transfer the upper toner image, a high transfer voltage is applied between the transfer charging unit and the photosensitive drum 2.

Here, since the paper fiber has a dense part and a rough part inside the transfer paper, the paper fiber becomes a dielectric layer in the dense part of the paper fiber, and is abnormal even when a high voltage is applied. Although it is difficult to generate a discharge, a rough portion of the paper fiber (a portion having a void) has an air layer which is an insulating layer, so that dielectric breakdown occurs and abnormal discharge occurs. These local abnormal discharges cause the retransfer described above.

Conventionally, proposals have been made to cope with a change in color due to retransfer by making the color order of transfer less noticeable or by optimizing the color reproduction parameters of the input signal and the output signal of the image. Have been.

Further, as another prior art, a transfer carrier has a multilayer structure having a high-resistance layer and a low-resistance layer, as disclosed in Japanese Patent Application Laid-Open No. 9-34269, against abnormal discharge that causes retransfer. A technique for suppressing abnormal discharge has been proposed.

[0013]

However, in a multi-transfer image forming apparatus using a transfer carrier having a multilayer structure including a low-resistance layer such as a transfer material carrier such as a transfer belt or an intermediate transfer body, a transfer charging means is used. In the case of sequentially performing multiple transfer, the surface potential of the transfer carrier gradually increases, and a high transfer voltage is applied particularly when the transfer of the fourth color is performed. The potential difference between the member adjacent to the body increases and the transfer current leaks along the low resistance layer in the transfer carrier, resulting in inefficiency. In a high-humidity environment, the transfer material also has a low resistance, and thus is more susceptible to the influence.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art. It is an object of the present invention to obtain a high-quality image in which retransfer and the like are suppressed while transferring the image from the transfer carrier to the surroundings. An object of the present invention is to provide an image forming apparatus capable of suppressing transfer current leakage.

[0015]

According to the present invention, there is provided an image carrier having a surface on which a developer image is formed, and a developer image formed on the surface of the image carrier. An image forming apparatus comprising: a transfer charging unit for transferring, and a transfer carrier on which a developer image is transferred by the transfer charging unit, or is transferred to a transfer material that is carrying the transfer charging unit. The surface resistivity of the upper layer that carries the developer image or the transfer material is provided in a two-layer structure that is higher than the surface resistivity of the lower layer on the opposite side, and the traveling direction of the transfer carrier is more upstream than the transfer charging unit. A transfer carrier charging means for charging a lower layer of the transfer carrier to the same polarity as the developer is provided.

The transfer carrier has an upper layer having a surface resistivity of 1
0 ¹³ Ω / □ or more, and the surface resistivity of the lower layer is 10 ¹⁰ to 1
It is preferably 0 ¹³ Ω / □.

The transfer carrier has an overall volume resistivity of 1
0 ¹³ Ωcm or more, and the total thickness is preferably 150 μm or less.

Therefore, the transfer current can easily move in the lower layer of the transfer carrier, and the transfer range is extended, the transfer time is lengthened, and the transfer efficiency is increased. Since the potential of the transfer carrier is lowered by the carrier charging means, image defects such as retransfer due to abnormal discharge do not occur.

Also, by lowering the surface potential of the transfer carrier by the transfer carrier charging means and reducing the potential difference between the transfer carrier and the adjacent member, it is possible to prevent the transfer current from flowing through the transfer carrier into the adjacent member. Can be prevented.

[0020]

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the dimensions of the components described in this embodiment,
The materials, shapes, relative arrangements, and the like are not intended to limit the scope of the present invention only to them unless otherwise specified.

(First Embodiment) A first embodiment will be described with reference to FIGS. FIG. 1 is a schematic configuration diagram of a transfer belt as a transfer carrier according to a first embodiment of the present invention, and FIG. 2 is a schematic configuration diagram of an image forming apparatus provided with the transfer belt.

In FIG. 2, the image forming apparatus of the present embodiment is a color multiple transfer image forming apparatus, and includes first, second, third, and fourth image forming units Pa, Pb, P
c, Pd are provided side by side, and cyan, magenta, yellow, and black toner images are sequentially formed through processes of a latent image, development, and transfer.

Each of the image forming units Pa, Pb, Pc, and Pd includes a photosensitive member as a dedicated image carrier, in this example, electrophotographic photosensitive drums 2a, 2b, 2c, and 2d. , 2c and 2d are formed with toner images of respective colors.

A transfer belt 8 as a transfer carrier is installed adjacent to each of the photosensitive drums 2a, 2b, 2c and 2d, and the toner images of the respective colors formed on the photosensitive drums 2a, 2b, 2c and 2d are transferred. The image is transferred onto the transfer material 1 carried and conveyed on the belt 8.

Further, the transfer material 1 onto which the toner images of each color have been transferred is separated from the transfer belt 8 by the separation charger 9 and is heated and pressed by the fixing unit 10 to fix the toner images. It is discharged outside the device.

On the outer periphery of the photosensitive drums 2a, 2b, 2c, 2d, drum chargers 3a, 3b, 3c, 3d,
Exposure devices 4a, 4b, 4c, 4d, developing devices 5a, 5b,
5c, 5d, transfer chargers 6a, 6 as transfer charging means
b, 6c, 6d and cleaners 7a, 7b, 7c, 7
The light source device (not shown) and the polygon mirror 11 are further provided above the device.

The developing units 5a, 5b, 5c, and 5d are filled with a predetermined amount of cyan, magenta, yellow, and black toners, respectively, as a developer by a supply device (not shown).

Next, the operation of the entire image forming apparatus will be described. First, the laser light emitted from the light source device is rotated and scanned by the polygon mirror 11, the light beam of the scanning light is deflected by the reflection mirror, and collected on the generatrix of the photosensitive drums 2a, 2b, 2c and 2d by the fθ lens. By performing light exposure, a latent image corresponding to an image signal is formed on the photosensitive drums 2a, 2b, 2c, and 2d.

The developing devices 5a, 5b, 5c, and 5d develop the latent images on the photosensitive drums 2a, 2b, 2c, and 2d, respectively, to form cyan, magenta, yellow, and black toner images. Visualize.

On the other hand, the transfer material 1 is accommodated in a transfer material cassette 30, and is supplied onto the transfer belt 8 through a plurality of transport rollers and registration rollers 13 from the transfer material cassette 30.
Are sequentially sent to the transfer units facing the photosensitive drums 2a, 2b, 2c, and 2d.

As the transfer belt 8, an endless shape belt or a seamless (seamless) belt is used.

Then, the transfer belt 8 is rotated in a direction indicated by an arrow X by a driving roller 14, and the transfer material 1 is sent out from the registration roller 13 to the transfer belt 8, and the transfer material 1 is transferred.
Is conveyed toward the transfer section of the first image forming section Pa.

At the same time, an image writing signal is turned on, and an image is formed on the photosensitive drum 2a of the first image forming unit Pa at a certain timing based on the signal.

The transfer charger 6a applies an electric field or electric charge at the lower transfer portion of the photosensitive drum 2a,
The first color toner image formed on the photosensitive drum 2a is transferred onto the transfer material 1.

By this transfer, the transfer material 1 is firmly held on the transfer belt 8 by electrostatic attraction, and the second image forming portion P
It is conveyed after b.

Here, the transfer chargers 6a, 6b, 6c, 6
As d, a contact charger using a transfer charging member such as a blade, a roller, or a brush is used. The contact charger has advantages such as ozonelessness, resistance to temperature and humidity environment fluctuation, and high image quality.

Further, the second to fourth image forming units Pb
Image formation and transfer at Pd are performed in the same manner as in the first image forming unit Pa.

Next, the transfer material 1 onto which the four color toner images have been transferred is neutralized by the separating charger 9 to attenuate the electrostatic attraction force on the downstream side of the transfer belt 8 in the conveying direction, thereby reducing the transfer belt 8. From the end of.

Particularly, in a low-humidity environment, the transfer material 1 also dries and the electric resistance increases, so that the electrostatic attraction force with the transfer belt 8 increases, and the effect of the separation charger 9 increases.

Normally, the separation charger 9 charges the transfer material 1 in a state where the toner image is not fixed, so that a non-contact charger is used.

Further, a charge eliminating roller 16 is provided to face the drive roller 14, and the charge on the transfer belt 8 from which the transfer material 1 has been separated is eliminated by the charge eliminating roller 16.

Then, the transfer material 1 detached from the transfer belt 8 is conveyed to a fixing device 10, where the color mixture of the toner image and the fixation to the transfer material 1 are performed by fixing to form a full-color copy image, and the paper is discharged. It is discharged to the tray 15.

Next, the transfer belt 8, which is a feature of the present invention, will be described. As shown in FIG. 1, the transfer belt 8 includes two layers, an upper layer 41 and a lower layer 42. In the present embodiment, the upper layer 41 has a surface resistivity of 10 ¹⁴ Ω / □, the lower layer 42 has a surface resistivity of 10 ¹¹ Ω / □, the volume resistivity of the entire transfer belt 8 is 10 ¹⁴ Ωcm, and the thickness thereof is 80 μm. A polyimide endless belt was used.

Here, the upper layer 41 and the lower layer 4 of the transfer belt 8
The resistance adjustment of No. 2 is performed by changing the amount of addition in the upper layer 41 and the lower layer 42 when carbon as a conductive material is added to the polyimide of the base layer.

In selecting the resistance value of the transfer belt 8,
In order for the transfer belt 8 to obtain a good transfer image, if the surface resistivity ρs of the transfer belt 8 is too low, the electric field applied during the transfer depends on the surrounding electric potential state, and becomes unstable, Leaks and becomes inefficient.

Then, the transfer chargers 6a, 6b, 6c, 6
The transfer electric field amount contributing to the transfer differs between the case where the transfer electric field leaks from the case d and the case where the transfer electric field does not leak, so that a density step corresponding to the transfer state of the transfer material 1 occurs.

On the other hand, when the surface resistivity ρs of the transfer belt 8 is high, the amount of charge decay decreases. Thus, for example, FIG.
Then, when the transfer charging is repeatedly performed on the transfer belt 8 from the first image forming unit Pa to the fourth image forming unit Pd, the final fourth image forming unit Pd has a large charge on the transfer belt 8. Power is required. If constant current control is performed at the time of transfer, a larger high voltage is required if the surface resistivity ρs is large.

In this case, not only power consumption and apparatus cost, but also a discharge phenomenon at the time of transfer is liable to occur, and a good transfer image cannot be obtained, and the latitude of the optimum applied electric field is reduced.

In the image forming apparatus of the present embodiment, if the surface resistivity of the lower layer, which is the charging side of the transfer belt, is set to 10 ⁹ Ω / □ or less, a density step occurs due to transfer electric field leakage, and the surface resistivity of the lower layer Was 10 ¹⁴ Ω / □ or more, the occurrence of image defects due to abnormal discharge during transfer was observed.

FIG. 3 is a view showing the vicinity of the transfer charger 6a of the first image forming section Pa. A transfer current flows from the transfer charger 6a, and the toner image formed on the photosensitive drum 2a is transferred onto the transfer material 1.

At this time, the transfer charger 6a of the transfer belt 8
Since the surface resistivity of the lower layer 42 provided on the side is low resistance,
Since the transfer current easily moves on the surface of the transfer belt 8, the transfer nip where the transfer is performed is widened, and the transfer time is increased, so that the transfer efficiency is improved.

Therefore, the transfer can be favorably performed with a lower transfer current, so that only a low transfer voltage needs to be applied, and image defects such as retransfer due to abnormal discharge hardly occur. Further, since the volume resistivity of the entire transfer belt 8 is high, the scattering of the toner image transferred onto the transfer material 1 due to the attenuation of the charge amount is reduced.

In the image forming apparatus of the present embodiment, when the volume resistivity of the transfer belt is set to 10 ¹² Ωcm or less, an image defect due to scattering occurs. In the case of a transfer belt having a thickness of 150 μm or more, the electrostatic capacity was small, and an image defect due to abnormal discharge occurred.

FIG. 4 shows first to fourth image forming units Pa, P
FIG. 5 is a diagram showing transfer voltages applied to b, Pc, and Pd. A graph 61 is for a polyimide single-layer belt having a volume resistivity of 10 ¹⁴ Ωcm and a surface resistivity of 10 ¹⁴ Ω / □ on both sides, and a graph 62 is for the transfer belt 8 of the present embodiment used this time. The surface resistivity of the upper layer 41 is 10 ¹⁴ Ω /
□, the surface resistivity of the lower layer 42 is 10 ¹¹ Ω / □, and the volume resistivity of the entire transfer belt 8 is 10 ¹⁴ Ωcm.
It is a layer belt.

As shown in FIG. 4, the transfer voltage applied to the transfer belt 8 may be low, and the above effect can be recognized.

On the other hand, a transfer belt as a transfer carrier charging means for charging the surface opposite to the toner image bearing surface of the transfer belt 8, that is, the back surface, to the same polarity as the toner upstream of the transfer charger 6 a in the running direction of the transfer belt 8. A charging unit 21 is provided.

As shown in FIG. 5, the transfer belt charging means 21 is provided with a conductive roller 5 so as to sandwich the transfer belt 8.
And a high voltage power supply 53 to a conductive roller 51 provided on the surface of the transfer belt opposite to the toner image carrying surface.
A more negative DC bias is applied. Further, the toner image bearing surface of the transfer belt 8 opposite thereto, that is, the conductive roller 52 provided on the surface is grounded.

FIG. 6 shows the first to first embodiments in the image forming apparatus of FIG.
12 is a graph showing the potential of the transfer belt 8 when passing through a fourth image forming unit Pa, Pb, Pc, Pd. Graph 7 shows the transfer belt 8 with two layers of polyimide used this time.
Reference numeral 1 denotes a case where the transfer belt charging unit 21 was not used, and a graph 72 denotes a case where the transfer belt charging unit 21 applied a DC bias of −2 kV.

First to fourth image forming units Pa, Pb, P
As the toner image is sequentially transferred at c and Pd, a higher transfer voltage is required toward the image forming unit at the rear, so that
When the potential of the transfer belt 8 is removed by the charge removing roller 16 and the potential of the transfer belt 8 on the side of the transfer charger 6a immediately before the first image forming unit Pa is set to 0 kV, the transfer charging is performed in the fourth image forming unit Pd. A considerably high transfer voltage is applied to the device 6d.

In this case, for example, there is a problem that a potential difference between the driving member 14 and the like, which is a grounded member, becomes large, a transfer current flows into the drive roller 14 side, and the image density is reduced due to a shortage of the transfer current. Occurs.

On the other hand, when a negative voltage is applied to the transfer charger 6a side of the transfer belt 8 by the transfer belt charging means 21 and the potential is set to be negative, the transfer belt 8 is also applied to the fourth image forming portion Pd. Does not increase, and the potential difference between the transfer belt 8 and the member adjacent to the transfer belt does not increase, so that the transfer current does not flow into the member adjacent to the transfer belt, and a good image can be obtained.

FIG. 7 shows the first to fourth image forming units P
FIG. 3 is a diagram showing transfer voltages applied to a, Pb, Pc, and Pd. The graph 81 indicates that the transfer belt charging unit 21 was not used, and the graph 82 indicates −2 k by the transfer belt charging unit 21 for the polyimide two-layer transfer belt 8 used in the present process.
This is when a DC bias of V is applied.

As shown in FIG. 7, the present embodiment uses the transfer belt 8 and further uses the transfer belt charging means 21 to further reduce the transfer voltage applied in the fourth image forming portion Pd. Therefore, image defects such as retransfer due to abnormal discharge can be further suppressed. In addition, since the surface resistivity of the transfer belt 8 on the side where the transfer belt charging means 21 is provided is low, the nip for charging is widened as in the case of the transfer chargers 6a, 6b, 6c and 6d, and the efficiency is improved. Thus, the transfer belt 8 can be reversely charged.

As described above, the structure of the transfer belt 8 is made up of two layers having different surface resistivity, the resistance value is optimized, and the surface potential of the transfer belt 8 is lowered by the transfer belt charging means 21 so that the transfer belt 8 can be regenerated by abnormal discharge. Image defects such as transfer can be suppressed.

The transfer belt charging means 21 lowers the surface potential of the transfer belt 8 and reduces the potential difference between the transfer belt 8 and a member adjacent to the transfer belt 8, thereby transferring the transfer current through the low-resistance layer in the transfer belt 8. Can be prevented from leaking.

The transfer belt 8 of the present embodiment is
Surface resistivity 10 of layer 41¹⁴Ω / □, surface resistance of lower layer 42
Rate 10¹¹Ω / □, but the surface resistivity of the lower layer was 10^Ten~
10 ¹³Ω / □, upper layer has higher surface resistivity than lower layer
10¹³If it is Ω / □ or more, the above effect will be better
Achievable.

The transfer belt 8 of this embodiment has a volume resistivity of 10 ¹⁴ Ωcm and a thickness of 80 μm of the entire transfer belt 8.
m, but the volume resistivity of the entire transfer belt is 10 ¹³ Ωcm
As described above, a thickness of 150 μm or less is a preferable mode in which the above-described effects can be achieved.

(Second Embodiment) Next, a second embodiment will be described with reference to FIG. In this embodiment, the first
Is different from the embodiment in that the two-layer belt of the present invention is used for an intermediate transfer belt, which is an intermediate transfer member, instead of a transfer belt carrying a transfer material.

Since other structures and operations are the same as those of the first embodiment, the same components are denoted by the same reference numerals and description thereof will be omitted.

In FIG. 8, the photosensitive drums 2a, 2b, 2
c and 2d are provided. In addition to the intermediate transfer belt (primary transfer belt) 8a for the photosensitive drums 2a, 2b, 2c and 2d as a first transfer carrier, a second transfer carrier is provided. Transfer material transfer belt (secondary transfer belt) 8b
The transfer carrier of the present invention similar to FIG. 1 was used for the primary transfer belt 8a.

On the primary transfer belt 8a, the photosensitive members 2a to 2a
The toner image of each color of 2d is once superimposed and transferred, and then the transferred toner image is transferred to the transfer material 1 on the secondary transfer belt 8b.
The method of batch transfer is adopted. Further, a transfer belt charging unit 21 is provided upstream of the first image forming unit Pa in the traveling direction of the primary transfer belt 8a.

In the image forming apparatus having the above configuration,
As in the first embodiment, no image defect such as retransfer due to abnormal discharge occurred, and no image defect caused by a transfer current flowing to a member adjacent to the primary transfer belt 8a.

A good image can be obtained even when the transfer carrier as shown in FIG. 1 is used for the secondary transfer belt 8b.

[0074]

As described above, according to the present invention, the transfer carrier has a two-layer structure in which the surface resistivity of the upper layer carrying the developer image or the transfer material is higher than the surface resistivity of the lower layer on the opposite side. In addition to the provision of the transfer carrier charging means for charging the lower layer of the transfer carrier to have the same polarity as the developer, the transfer current is applied to the lower layer of the transfer carrier upstream of the transfer charging means in the direction of travel of the transfer carrier. Because it is easy to move, the transfer range is expanded, the transfer time is lengthened, and the transfer efficiency increases,
Since transfer can be performed with a low transfer current and the potential of the transfer carrier is lowered by the transfer carrier charging means, image defects such as retransfer due to abnormal discharge do not occur, and a high-quality image can be obtained.

Further, by lowering the surface potential of the transfer carrier by the transfer carrier charging means and reducing the potential difference between the transfer carrier and the adjacent member, it is possible to prevent the transfer current from leaking to the adjacent member through the transfer carrier. Can be prevented.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a transfer belt according to a first embodiment.

FIG. 2 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment.

FIG. 3 is a diagram illustrating the vicinity of a transfer charger in a first image forming unit.

FIG. 4 is a diagram illustrating transfer voltages applied to first to fourth image forming units when a single-layer belt and the two-layer belt of the present invention are used.

FIG. 5 is a configuration diagram illustrating a transfer carrier charging unit.

FIG. 6 is a graph showing the potential of the transfer belt when passing through the first to fourth image forming units.

FIG. 7 is a diagram illustrating transfer voltages applied to first to fourth image forming units depending on whether or not charging is performed by a transfer carrier charging unit.

FIG. 8 is a schematic configuration diagram illustrating an image forming apparatus according to a second embodiment.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transfer material 2a, 2b, 2c, 2d Photosensitive drum 3a, 3b, 3c, 3d Drum charger 4a, 4b, 4c, 4d Exposure device 5a, 5b, 5c, 5d Developing device 6a, 6b, 6c, 6d Transfer charger 7a, 7b, 7c, 7d Cleaner 8 Transfer belt 8a First transfer belt 8b Second transfer belt 21 Transfer belt charging means 41 Upper layer 42 Lower layer 51, 52 Conductive roller 53 DC bias power supply

 ────────────────────────────────────────────────── ─── Continued from the front page (72) Inventor Haruhiko Omata 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 2H032 AA05 BA18 BA23

Claims (3)

[Claims] An image bearing member having a surface on which a developer image is formed; transfer charging means for transferring the developer image formed on the image bearing member surface; Or a transfer carrier to be transferred to a transfer material carrying the transfer material, wherein the transfer carrier has a surface resistivity of an upper layer carrying a developer image or a transfer material opposite to that of the transfer material. The transfer carrier is provided in a two-layer structure higher than the surface resistivity of the lower layer on the side of the transfer carrier, and the lower layer of the transfer carrier is charged to the same polarity as the developer upstream of the transfer charger in the traveling direction of the transfer carrier. An image forming apparatus comprising: 2. The transfer carrier has an upper layer having a surface resistivity of 1
0 ¹³ Ω / □ or more, and the surface resistivity of the lower layer is 10 ¹⁰ to 1
0 ¹³ Ω / □ image forming apparatus according to claim 1, characterized in that a. 3. The transfer carrier has an overall volume resistivity of 1
The image forming apparatus according to claim 1, wherein the thickness is not less than 0 ¹³ Ωcm and the total thickness is not more than 150 μm.