JPH10268669A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of obtaining a necessary transfer nip width for a satisfactory transfer image even without a backup member by bringing an intermediate transfer belt into electrostatically close contact with a photoreceptive belt, on the downstream side from a transfer nip, by means of charges held on the intermediate transfer belt. SOLUTION: The intermediate transfer belt 10 is stretched so as to be movable to the position where it comes into contact with the surface of the photoreceptive belt 1, which is on the adjacent downstream side, in the direction of the movement of the belt, from contact area with the photoreceptive belt 1 and intermediate transfer belt 10 in the state of being not charged. The intermediate transfer belt 10 is made of a material having such electric resistance that the charges applied by a bias brush 13 are kept on the intermediate transfer belt 10 and downstream for the close contact.

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 technique for transferring a visible image on an image carrier to a transfer body in such an image forming apparatus. is there.

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus of this type, at least one of an image carrier and a transfer member partially contacting the image carrier is constituted by a belt material wound around a plurality of support rollers. 2. Description of the Related Art There is known an image forming apparatus including a charge applying unit that applies a charge for forming a transfer electric field between the image carrier and the transfer body to the transfer body. In this image forming apparatus, a transfer electric field is formed by a charge applied to a transfer body in a contact area between the image carrier and the transfer body, and a visible image is transferred from the image carrier to a transfer material side.

Such a transfer configuration is also applied to a method in which a visual image is formed on an image carrier a plurality of times, and each visual image is transferred by being superimposed on an intermediate transfer member as a transfer member. In recent years, an image forming apparatus using this intermediate transfer member is advantageous in that it is smaller in size and less restricted in the type of transfer material onto which a visualized image is finally transferred. They tend to be used as devices.

[0004]

However, an image in which at least one of the above-described conventional image bearing member and a transfer member partially contacting the image bearing member is constituted by a belt material wound around a plurality of supporting rollers. In the forming apparatus, the width (hereinafter, referred to as “transfer nip width”) of a contact area where the developed image is transferred from the image carrier to the transfer body in a transfer body surface moving direction of a contact area where the transfer body is in contact with the image carrier is increased. It was difficult to do. When the transfer nip width is small, the visual image formed on the image carrier substantially passes through the transfer nip area where the transfer step is performed at the linear velocity of the image carrier and the transfer body employed in the normal process. The time required for the transfer is insufficient, and there is a possibility that a local transfer failure such as a hollow portion or an insect bite in a character image or an edge portion of a solid image may occur.

The local transfer failure due to the insufficient transfer nip width can be solved by using an image carrier and a belt material wrapped around a plurality of supporting rollers as the transfer body to reduce the size of the apparatus. This may also occur when the portion wound around the supporting roller is brought into contact with the bridge between the supporting rollers of the transfer body.

As a method of increasing the transfer nip width, a method in which the transfer member is urged toward the image carrier by a backup member such as a backup roller when the transfer member is formed of a belt material is considered. The cost is increased by the provision of the members.

The present invention has been made in view of the above problems, and an object of the present invention is to secure a transfer nip width necessary for obtaining a good transfer image even when a backup member which causes an increase in cost is not provided. An object of the present invention is to provide an image forming apparatus.

[0008]

According to a first aspect of the present invention, at least one of an image carrier and a transfer member partially contacting the image carrier is provided on a plurality of support rollers. And a charge applying means for applying a charge for forming a transfer electric field between the image carrier and the transfer body to the transfer body, and a visible image on the image carrier is provided. In the image forming apparatus for transferring to the transfer body, one of the image carrier and the transfer body, which is constituted by the belt material, is not charged with the image carrier and the charge in the transfer body surface moving direction. Is movably mounted to a position adjacent to the other surface on the downstream side of the contact area with the transfer body, and the transfer body is charged with the charge provided by the charge providing means on the adjacent downstream side. A material with electrical resistance that can be held by the body It is characterized in that the form. Here, when each of the image carrier and the transfer body is configured by a belt material laid over a plurality of support rollers,
At least one of the image carrier and the transfer member may be movably mounted on the adjacent downstream side. In the case where one of the image carrier and the transfer member is formed of a belt material provided on a plurality of support rollers, the one formed of the belt material may be provided so as to be movable on the adjacent downstream side. .

In the image forming apparatus according to the first aspect of the invention, when a charge for forming a transfer electric field is applied to the transfer body by the charge applying means, the image carrier and the image carrier in the transfer direction of the surface of the transfer body are not charged. On the downstream side adjacent to the contact area with the transfer member, a part of the applied charge remains held by the transfer member that is moving away from the image carrier.
An electrostatic attraction is generated between the transfer body holding the electric charges and the image carrier on the adjacent downstream side. This electrostatic attraction is
It is considered that this is mainly caused by the Coulomb force between the charge held on the image carrier side of the transfer body and the charge induced on the surface of the image carrier on the transfer body side. Due to the electrostatic attraction, one of the image carrier and the transfer member, which is made of a belt material, is brought into close contact with the other surface on the adjacent downstream side, and the transfer of the visible image from the image carrier to the transfer member is performed. The contact area extends to the adjacent downstream side, resulting in a large transfer nip width.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, each of the image carrier and the transfer member is constituted by a belt material spanned by a plurality of support rollers. The transfer member is brought into contact with the stretched portion between one support roller and the other, and the angle between the linearly moving portions on the adjacent downstream side of the image carrier and the transfer member is set to the adjacent angle. An angle is set such that the one can move to a position where the one contacts the other surface on the downstream side.

In the image forming apparatus according to the second aspect, each of the image carrier and the transfer body is advantageous in terms of miniaturization.
It is composed of a belt material laid on a plurality of support rollers. Then, when a charge for forming a transfer electric field is applied to the transfer body by the charge applying unit, a part of the applied charge is held by the transfer body that is moving away from the image carrier on the adjacent downstream side. Will remain. An electrostatic attraction is generated between the transfer body holding the electric charges and the image carrier on the adjacent downstream side. Due to this electrostatic attraction, the adjacent downstream side of the one stretched part with which the other is in contact,
The contact area, in which the visible image is transferred from the image carrier to the transfer body, extends to the adjacent downstream side, in close contact with the other surface,
As a result, the transfer nip width increases.

According to a third aspect of the present invention, in the image forming apparatus of the second aspect, a portion supported by the other support roller is brought into contact with the one stretched portion, and is supported by the other support roller. The other bending rigidity and the tension are set so that the radius of curvature of the portion having the radius is larger than the radius of curvature of the outer peripheral surface of the support roller.

In the image forming apparatus according to the third aspect, the other supporting roller is set to a predetermined magnitude by setting the bending rigidity and tension of the other supporting roller in contact with the stretched portion between the one supporting roller. Is made larger than the radius of curvature of the outer peripheral surface of the support roller. Thereby, on the adjacent downstream side, the other part is partially separated from the outer peripheral surface of the support roller and swells so as to approach the one side, so that the one moving to the other side can easily adhere to the other surface. .

According to a fourth aspect of the present invention, in the image forming apparatus of the first aspect, of the image carrier and the transfer body, the other is in contact with a stretched portion between one of the support rollers made of the belt material. The contact position between the one and the other is set upstream of the center in the surface moving direction of the stretched portion.

In the image forming apparatus according to the present invention, of the image bearing member and the transfer member, one of the supporting rollers formed of a belt material is located on the upstream side of the center in the surface moving direction of the stretching portion, and the other is on the other side. By contacting, the adjacent downstream side of the one stretched portion is more likely to bend as compared with the case where it is brought into contact with the center or the downstream side of the stretched portion, and one of the stretched portions is more likely to adhere to the other surface. Become.

According to a fifth aspect of the present invention, in the image forming apparatus according to the first aspect, the surface of the image carrier and the surface of the transfer body move in the same direction in the contact area. One of the movably installed linear speeds is smaller than the other linear speed.

In the image forming apparatus of the present invention, one of the image carrier and the transfer member, which is movably installed, is set to have a lower linear speed than the other linear speed. Due to the frictional force between the two, one of the reduced linear velocities bends on the adjacent downstream side, so that the one is more likely to adhere to the other surface.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a color image forming apparatus will be described below. First, a schematic configuration and operation of the entire color image forming apparatus according to the present embodiment will be described. FIG. 1 is a perspective view illustrating a schematic configuration of a color image forming apparatus according to the present embodiment. In FIG. 1, a flexible belt-shaped photoreceptor (hereinafter, referred to as a “photoreceptor belt”) 1 as an image carrier is provided between support rollers 2 and 3, and driven by the support rollers 2 and 3. It is rotated in the direction of the middle arrow A (clockwise). Around the photoreceptor belt 1, a charger 4 as a charging unit for uniformly charging the surface of the photoreceptor belt, a laser writing unit 5 as an exposure device for forming an electrostatic latent image, yellow, magenta, and cyan A rotary type color developing unit (hereinafter referred to as a "revolver type" or "rotary type") integrally provided with three developing units 6Y, 6M and 6Y each containing a color developer. And a black developing device 7 as a developing device containing a black developer separate from the color developing device unit 6. The photoreceptor belt 1
For example, a substrate obtained by applying an organic photosensitive layer (OPC) on a substrate made of Ni electroforming can be used.

The laser writing unit 5 contains a semiconductor laser (not shown), a polygon mirror 5B rotated by a drive motor 5A, an f-θ lens 5C, and a mirror 5D in a unit case 5E as a holding housing.
It is built into the lower part of the device body. Paper cassette 17
A bottom plate (not shown) is provided between and the laser writing unit 5. A laser writing unit 5 is incorporated on the bottom plate, and side plates (not shown) of the apparatus main body provided on the left and right (rear side and front side in FIG. 1) in the roller axis direction to support each unit correspond to the bottom plate. Has been assembled.

The charging position by the charging charger 4, the irradiation position of the light beam L from the laser writing unit 5, and the cleaning position by the photosensitive member cleaning blade 15A are lower than the two support rollers 2 and 3. Of the photosensitive belt 1 around the support roller 2.

Each of the color developing unit 6 and the black developing device 7 has a developing roller as a developer carrying member which comes close to or in contact with the photosensitive belt 1 at a predetermined position. Has a function of visualizing by a non-contact development or a contact development method. The color developing unit 6 has a rotation axis perpendicular to the plane of FIG. 1 so that any one of the developing units 6Y, 6M, and 6Y synchronizes with the development timing of the corresponding color and moves to a developable position. The rotation about the center is controlled. By the rotation of the color developing unit 6 by 120 degrees, the developing device facing the developing position is switched. The color developing unit 6
Is operated, the rotation of the cam 45 is controlled such that the black developing device 7 moves to a position separated from the photosensitive belt 1.

The upper support roller 3 of the photosensitive belt 1
An intermediate transfer belt 10 as a transfer body is provided so as to come into contact with the wrapped portion. The intermediate transfer belt 10 is erected and inclined between support rollers 11 and 12 as support rollers.
Is rotated and conveyed in the direction of arrow B (counterclockwise) in the figure. Of the two stretched portions between the support rollers of the inclined intermediate transfer belt 10, the stretched portion on the photosensitive belt 1 side (lower left side in the figure) is supported by the upper support roller 3 of the photosensitive belt 1. It is in contact with the supported portion (the portion wound around the support roller 3). Then, the first visible image on the photoreceptor belt 1 is transferred to the intermediate transfer belt 13 by a conductive bias brush 13 as a primary transfer member, which is a component of a charge applying unit provided inside the intermediate transfer belt 10. The image is transferred onto the transfer belt 10 (primary transfer). The bias brush 13 is supplied to a predetermined bias voltage (−0 to 0 in this embodiment) from a power supply, which is another component of the charge applying unit.
(Approximately +1000 V), and the leading end thereof is in contact with the back surface of the intermediate transfer belt 10 under predetermined conditions. A transfer roller 14 as a secondary transfer member is provided at a portion where the intermediate transfer belt 10 is wrapped around the support roller 11 so as to be able to contact and separate from the intermediate transfer belt 10. Is transferred onto a transfer sheet as a transfer material (secondary transfer). Here, the distance on the intermediate transfer belt 10 from the primary transfer position to the secondary transfer position is set to be equal to or longer than the maximum image output length.

Cleaning device 15 for photoreceptor belt
Is a cleaning blade 15A that constantly contacts the photoconductor belt 1 and cleans the toner on the photoconductor belt 1;
A waste toner collecting container 15B and the like are provided. The cleaning device 16 for the intermediate transfer belt includes a cleaning blade 16A for cleaning the surface of the intermediate transfer belt 10, an auger 16B for conveying the toner, and the like.
The cleaning blade 16A for the intermediate transfer belt is kept at a position separated from the surface of the intermediate transfer belt 10 when a toner image is transferred from the photoreceptor belt 1 during image formation by a blade contact / separation arm (not shown). Only at the time of cleaning after the secondary transfer, as shown in the drawing, it is brought into contact with the surface of the intermediate transfer belt 10 to scrape off the residual toner after transferring the toner image onto the transfer paper.

In the above-described color image forming apparatus,
After the photoconductor belt 1 is uniformly charged by the charging charger 4, the laser beam (laser beam) L modulated based on image information is scanned by the laser writing unit 5 to be exposed, and the surface is electrostatically charged. A latent image is formed. Here, the image information used for the exposure is monochromatic image information obtained by decomposing a desired full-color image into yellow, magenta, cyan, and black color information. Laser light L generated by a semiconductor laser (not shown) based on this image information
Is rotationally scanned by a polygon mirror 5B rotated at high speed by a drive motor 5A, passes through an f-θ lens 5C,
The optical path is adjusted by the reflection mirror 5D.

The electrostatic latent image formed on the photoreceptor belt 1 is developed by a color developing unit 6 with a predetermined yellow toner, a magenta toner and a cyan toner, respectively. The electrostatic latent image formed based on the black color information is developed by the black developing device 7 with black toner. In this manner, the cleaning, latent image formation, and development of the photoreceptor belt 1 are repeated, and respective color images (toner images) are sequentially formed on the photoreceptor belt 1.

The single-color images (toner images) sequentially formed on the photosensitive belt 1 rotating in the direction of arrow A are yellow,
By a predetermined transfer bias applied to the bias brush 13 for each of the single colors of magenta, cyan, and black, transfer is sequentially performed at the same position on the intermediate transfer belt 10 that rotates in the direction of arrow B in synchronization with the photosensitive belt 1. Is done.

The yellow, magenta, cyan, and black color toner images superimposed on the intermediate transfer belt 10 are fed from a paper feed table (paper feed cassette)
Conveying roller pair 19A, 19B, registration roller pair 20
After passing through A and 20B, the image is conveyed to the secondary transfer unit at a timing such that the position of the image on the transfer paper becomes a regular position. On the transfer paper 17A conveyed to the secondary transfer section,
The images superimposed on the intermediate transfer belt 10 are collectively transferred by the transfer roller 14. After the transfer, the image is fixed by the fixing device 50 to complete a full-color image. The transfer paper 17A on which the full-color image has been formed is discharged to a discharge stack 52 via a discharge roller pair 51.

The waste toner scraped off from the intermediate transfer belt 10 by the cleaning blade 16A is
The toner is conveyed toward the front of the drawing by an auger 16B provided in the cleaning device 16, and further conveyed to a waste toner collecting container 15B by a conveyance unit (not shown) provided on the front side surface of the process cartridge 31. When a predetermined amount or more of the waste toner is stored in the waste toner collecting container 15B, the container is appropriately replaced.

The photosensitive belt 1, the charging charger 4,
The intermediate transfer belt 10, the cleaning device 16, the registration roller 20B, the transport guide, and the like are integrally configured as a process cartridge, and are configured to be replaceable when the life is over. The waste toner collecting container 15B
Is removably incorporated in the process cartridge.

In addition to replacing the process cartridge,
The color developing unit 6, the black developing device 7, etc. are replaced at the end of their service life. In order to facilitate the replacement and the handling of the jammed transfer paper, the front frame 8 of a part of the main body has a support shaft 9A.
Is rotated about the center of rotation and can be opened. (Hereinafter, margin)

An electrical component / control unit 100 is accommodated on the left side of the apparatus main body in FIG. Above the fan 70, a fan 70 is provided to exhaust air to prevent an excessive rise in temperature in the apparatus. In addition, a relatively small-sized paper feeding device 80 is provided on the right side of the apparatus main body in FIG.

In the color image forming apparatus having the above structure, from the viewpoint of miniaturization, as an image carrier and a transfer body onto which a visible image (toner image) is transferred from the image carrier, the image carrier is provided on two support rollers. Although the photoreceptor belt 1 and the intermediate transfer belt 10 made of a belt material are used, it is difficult to secure a sufficiently large transfer nip width between the photoreceptor belt 1 and the intermediate transfer belt 10 in such a configuration. .

When a plurality of color toner images are superimposed on the intermediate transfer belt 10 as in the color image forming apparatus of this embodiment, for example, the maximum toner amount of the first color toner image to be transferred to the intermediate transfer belt 10 is Assuming 100%, the toner amount on the intermediate transfer belt 10 to which the toner images of four colors can be transferred is 0 to 400%. Assuming that image processing for converting the overlapped portion of the color images C, M, and Y into a black image K is performed, a toner image of as much as 0 to 300% is arbitrarily overlapped on the intermediate transfer belt 10. In such an apparatus, local transfer failure of the toner of the first to third colors, which is called hollowing or worming, may occur in a character image or an edge of a solid image. For transferring the toner image from the photosensitive belt 1 to the intermediate transfer belt 10, it is necessary to set a sufficiently long transfer nip passage time for the toner on the photosensitive belt 1 to pass through the transfer nip portion. . Accordingly, the transfer nip passage time is defined by the transfer nip width and the process speed of the apparatus. In order to obtain a good transfer image when the process speed of the apparatus is more restricted, the transfer nip width must be sufficient. Needs to be set large.
In order to increase the transfer nip width, a method of forcibly pressing the intermediate transfer belt against the photoreceptor belt with a backup roller can be considered. However, the cost is reduced by providing a backup member such as a backup roller.

Therefore, in the present embodiment, the intermediate transfer belt 10 is moved in the state where no charge is applied from the photosensitive belt 1 and the bias brush 13 in the direction of movement of the intermediate transfer belt surface (the two transfer belts in FIG. 2). (Shown by a dashed-dotted line) on the downstream side adjacent to the contact area with the photosensitive belt 1 so as to be movable to a position where the intermediate transfer belt 10 contacts the surface of the photoreceptor belt 1. It is made of a material having an electric resistance to be held on the adjacent downstream side.

More specifically, as shown in FIG. 2, the winding portion of the photosensitive belt 1 around the support roller 3 is brought into contact with a stretchable portion between the support rollers 11 and 12 of the intermediate transfer belt 10 which is easily bent. ing. The contact area between the photoreceptor belt 1 and the intermediate transfer belt 10 (shown by a two-dot chain line in FIG. 2) to which no charge is applied from the bias brush 13 in the surface movement direction of the intermediate transfer belt 10 The angle between the photosensitive belt 1 and the intermediate transfer belt 10 on the adjacent downstream side is set to an angle at which the intermediate transfer belt 10 can move to a position where the intermediate transfer belt 10 contacts the surface of the photosensitive belt 1 on the adjacent downstream side. Further, the intermediate transfer belt 10 is
It is made of a medium-resistance belt made of a material in which a conductive agent such as carbon is dispersed in a polymer material such as E or PC. The volume resistivity of the intermediate resistance intermediate transfer belt 10 is
10 ⁶ to 10 ¹² Ωcm, and more preferably 10 ⁸ to 1
0 ¹⁰ Ωcm. The volume resistivity is 0.15
It is a value when 100 V is applied to a belt having a thickness of mm.

FIG. 3 schematically shows the behavior of the electric charge (positive electric charge) applied from the bias brush 13. In the present embodiment, as described above, the intermediate transfer belt 10 is formed of a medium-resistance material such as carbon dispersion, but the electric resistance of this kind of medium-resistance material has an electric field dependency, and a bias brush having a strong electric field. 13, the resistance tends to be low, and the other parts tend to be high. In FIG.
The charge applied from the bias brush 13 moves to the photosensitive belt 1 side immediately below the bias brush 13, but the region (the region indicated by C in the figure) until the above-described separation discharge on the adjacent downstream side starts to occur. (Hereinafter referred to as “C region”), the electric resistance is relatively high, and the intermediate transfer belt 10 is held mainly on the surface of the photoconductor belt side. Actually, when the surface potential of the intermediate transfer belt 10 in the C region was measured by an experiment, it was several tens V or less on the surface on the photosensitive belt 1 side.
The opposite surface (the upper surface in the figure) reached several hundred volts. An electrostatic attraction is generated between the intermediate transfer belt 10 and the photosensitive drum 1 where the charges are held in the C region. It is considered that this electrostatic attraction is mainly generated by the Coulomb force between the charge held on the intermediate transfer belt 10 and the charge induced on the photosensitive drum 1 side. Due to the electrostatic attraction P, the intermediate transfer belt 10 is brought into close contact with the photoreceptor belt 1 at the adjacent downstream portion, and the transfer nip width Wn
Becomes larger.

Symbols α and Wn in FIG. 2 indicate a winding angle of the photosensitive belt 1 around the support roller 3 and a transfer nip width in the primary transfer portion, respectively. Further, a symbol Itb in FIG. 2 indicates a transfer bias current supplied to the bias brush 13 from the power supply 130 which is a component of the charge applying means.

As described above, according to the present embodiment, the photosensitive belt 1 is located on the adjacent downstream side (region C in FIG. 3) by the electrostatic attraction P between the photosensitive belt 1 and the intermediate transfer belt 10. Since the intermediate transfer belt 10 and the intermediate transfer belt 10 are in close contact with each other, the transfer nip width Wn can be increased, so that the transfer nip width necessary for obtaining a good transfer image is secured even when a backup member or the like that increases costs is not provided. it can.

More specifically, the intermediate transfer belt 10 has a relative dielectric constant εt = 11 and a thickness t = 150 μm, and the photosensitive layer of the photosensitive belt 1 has a relative dielectric constant εp = 3.
When the one having a thickness t of 20 to 30 μm was used, the transfer nip width could be made 3 to 4 mm.

In the above embodiment, the support roller 3 of the photoreceptor belt 1 is supported on the upstream side of the center in the surface moving direction of the stretched portion between the support rollers 11 and 12 of the intermediate transfer belt 10. Parts are in contact.
According to the mechanical model of the free beam at both ends (see the Mechanical Engineering Handbook, Chapter 2, p. 29, edited by the Japan Society of Mechanical Engineers), the amount of deflection is the span (in the apparatus shown in FIG. Is proportional to the third power of the distance). Accordingly, by bringing the photosensitive belt 1 into contact with the above-described position of the stretched portion of the intermediate transfer belt 10, the adjacent downstream side of the stretched portion of the intermediate transfer belt 10 is brought into contact with the center or the downstream side of the stretched portion. The intermediate transfer belt 10 is more likely to be in close contact with the surface of the photoreceptor belt than in the case where the transfer belt is used. In the example of FIG. 1, 1/3 of the span between the support rollers 11 and 12 is used.
The photoreceptor belt 1 is brought into contact with a position separated from the support roller 11 by a shorter distance.

In the above embodiment, as shown in FIG. 4, the radius of curvature of the portion of the photosensitive belt 1 supported by the support roller 3 is set to be larger than the radius of curvature of the outer peripheral surface of the support roller 3. Alternatively, the bending rigidity and the tension of the photoreceptor belt 1 may be set.

The equations used for setting the bending stiffness and tension of the photosensitive belt 1 can be derived as follows. The model of the thin circular tube to which the load P is applied is applied to the photoreceptor belt 1 laid over the support rollers 2 and 3 of the above-described embodiment, and the maximum bending moment M at the winding portion of the photoreceptor belt 1 around the support roller 3 is calculated. When it is obtained, it becomes as shown in Expression 1. Here, Rp in Equation 1 is the radius of curvature of the photoreceptor belt 1. Although the deformation of the photoreceptor belt 1 is very large, the bending moment at the portion where the photoreceptor belt 1 is wound around the support roller 3 (in the vicinity of the load application position) is considered in the same manner as when the thin-walled tube is slightly deformed. May be.

## EQU1 ## M = −P · Rp / π

The formula 1 of the bending moment M and the belt tension T = P / 2 are converted into a formula of material mechanics (by Chimochenko:
“Material mechanics (1st volume)”, pp. 367-369), the radius of curvature R of the photoreceptor belt 1 is obtained.
Equation 3 can be derived which shows the relationship among p, the longitudinal elastic modulus E of the photoreceptor belt 1, the second moment of area I, and the belt tension T.

1 / r = −M / (E · I)

## EQU3 ## Rp ² = EIπ / 2T

According to the above equation (3), the longitudinal elastic modulus E or the second moment of area I of the photosensitive belt 1 is increased to increase the bending rigidity of the photosensitive belt 1 and the belt tension T to reduce the photosensitive belt. It is understood that the curvature radius Rp of No. 1 can be increased.

By setting the bending rigidity and the tension of the photosensitive belt 1 in this manner, the photosensitive belt 1 is partially separated from the outer peripheral surface of the support roller 3 toward the intermediate transfer belt 10 on the adjacent downstream side. Therefore, the intermediate transfer belt 10 moving toward the photoreceptor belt 1 is easily brought into close contact with the surface of the photoreceptor belt 1.

In the above-described embodiment, the friction between the photosensitive belt 1 and the intermediate transfer belt 10 when the electrostatic attraction P is acting on the intermediate transfer belt 10 as shown in FIG. The intermediate transfer belt 1 is moved so that a frictional force acts on the transfer belt 10 in the direction indicated by the symbol F in the figure.
The linear velocity of 0 may be lower than the linear velocity of the photoreceptor belt 1 as long as the transfer image does not deteriorate. By setting the linear velocity in this manner, the intermediate transfer belt 10 easily bends in the adjacent downstream portion, and the intermediate transfer belt 10 easily adheres to the surface of the photoreceptor belt. Actually, it is preferable to set the linear velocity difference between the intermediate transfer belt 10 and the photosensitive belt 1 to about 0.1 to 0.5% so that a good transfer image is obtained.

In the above embodiment, as shown in FIG. 6, an auxiliary roller 90 may be provided at the adjacent downstream portion. In the case of this configuration, when the photoreceptor belt 1 and the intermediate transfer belt 10 cannot be installed close to each other at the adjacent downstream portion due to other layout restrictions or the like, the intermediate transfer belt 10 is further attached to the auxiliary roller. By rotating the belt 90, the photosensitive belt 1 and the intermediate transfer belt 10 can be brought closer to each other at the adjacent downstream portion, and the transfer nip width can be increased by the electrostatic attraction. In order to further increase the electrostatic attraction, a bias may be applied to the auxiliary roller 90.

In the above embodiment, the case where both the image carrier and the transfer member onto which a visible image is transferred from the image carrier is constituted by a belt material has been described. The present invention is also applicable to a structure in which one of the bodies is constituted by a belt material, and one of the members is constructed so as to be movable on the adjacent downstream side to a position in contact with the other surface.

In the above embodiment, the photosensitive belt 1
Although the description has been given of the case where the supporting portion of the intermediate transfer belt 10 is brought into contact with the supporting portion of the intermediate transfer belt 10, the supporting portion of the intermediate transfer belt 10 contacts the extending portion of the photosensitive belt 1. The present invention is also applicable to a case in which such a configuration is provided.

In the above embodiment, the case where the photoreceptor belt 1 is used as the image carrier and the intermediate transfer belt 10 is used as the transfer member has been described. However, the present invention relates to, for example, a photoreceptor belt and two intermediate transfer belts. A transfer portion between the two intermediate transfer belts, that is, one intermediate transfer belt on which a visible image is transferred from the photoconductor is used as the image carrier, and a developed image to be transferred to a final output transfer material is carried. The transfer member between the two intermediate transfer belts using the other intermediate transfer belt as the transfer member can also be applied.

In the above-described embodiment, the case where the bias brush 13 is used as the transfer member constituting the charge applying means for applying the transfer charge to the intermediate transfer belt has been described. The present invention can also be applied to a device using a transfer member having another structure such as a roller type.

[0052]

According to the first to fifth aspects of the present invention, the transfer nip width is obtained by electrostatically contacting the image carrier and the transfer body on the downstream side adjacent to the center of the contact area between the image carrier and the transfer body. Therefore, there is an effect that a transfer nip width necessary for obtaining a good transfer image can be secured even when a backup member which causes an increase in cost is not provided.

In particular, according to the second and third aspects of the present invention, there is provided an image forming apparatus which is advantageous for miniaturization, in which each of the image bearing member and the transfer member is constituted by a belt material laid on a plurality of supporting rollers, and thus the cost is reduced. Even when a backup member for providing a high height is not provided, there is an effect that a transfer nip width necessary for obtaining a good transfer image can be secured.

In particular, according to the third, fourth and fifth aspects of the present invention, there is an effect that the image carrier and the transfer member are easily brought into close contact with each other, and the transfer nip width can be easily increased.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a color image forming apparatus according to an embodiment.

FIG. 2 is an enlarged view of a primary transfer section of the color image forming apparatus.

FIG. 3 is a schematic view showing the behavior of electric charges in the same next transfer section.

FIG. 4 is an enlarged view of a primary transfer section according to a modification.

FIG. 5 is an enlarged view of a primary transfer section according to another modification.

FIG. 6 is an enlarged view of a primary transfer section according to still another modification.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor belt 2, 3 Photoconductor belt support roller 10 Intermediate transfer belt 11, 12 Intermediate transfer belt support roller 13 Bias brush 130 Power supply

Claims (5)

[Claims] At least one of an image carrier and a transfer member that is in partial contact with the image carrier is constituted by a belt material spanned over a plurality of support rollers, and a belt member is provided between the image carrier and the transfer member. An image forming apparatus for transferring a visible image on the image carrier to the transfer member, the image carrier comprising: a charge applying means for applying a charge for forming a transfer electric field to the transfer member; One of the above-mentioned belt members is brought into contact with the other surface on the downstream side adjacent to the contact area between the image carrier and the transfer body in a state where the charge is not applied in the transfer body surface movement direction. The transfer member is constructed so as to be movable to a position where the transfer member is formed of a material having an electric resistance such that the charge applied by the charge applying means is held by the transfer member on the adjacent downstream side. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein each of said image bearing member and said transfer member is constituted by a belt material laid over a plurality of support rollers. The other is brought into contact with the stretched portion between the support rollers, and the angle formed by the linearly moving portions on the adjacent downstream side of the image carrier and the transfer body is one side on the adjacent downstream side. An image forming apparatus, wherein the angle is set such that it can move to a position where it contacts the other surface. 3. The image forming apparatus according to claim 2, wherein a portion supported by the other support roller is brought into contact with the one stretched portion, and a curvature of the portion supported by the other support roller is provided. An image forming apparatus, wherein the other bending rigidity and tension are set so that a radius is larger than a radius of curvature of an outer peripheral surface of the support roller. 4. The image forming apparatus according to claim 1, wherein the other of the image bearing member and the transfer member is brought into contact with a stretched portion between one of the support rollers made of the belt material, and An image forming apparatus, wherein a contact position with the other is set upstream of a center in a surface moving direction of the stretching portion. 5. The image forming apparatus according to claim 1, wherein the surface of the image carrier and the surface of the transfer body move in the same direction in the contact area. An image forming apparatus, wherein one of the linear speeds is lower than the other.