JP2000250324A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device making phenomenon of the void surely prevented, by a contact part formed in a shape of making satisfactory area contact obtainable. SOLUTION: This image forming device is constituted, so that an intermediate transfer body drum 8 is provided with a conductive cylinder 8a of the rigid body constituted in a cylinder shape, a conductive elastic body 8b provided in a state of being wound on a periphery of the conductive cylinder and a transfer film 8c provided with non-contracting property being wound around the periphery of the conductive elastic body 8b, and the device is to set the young's modulus ratio of the conductive elastic body 8b on the intermediate transfer body drum 8 below 1000 g/cm2, to set flexible rigidity of the transfer film 8c on the intermediate transfer body drum 8 below 2.5 g/cm2, and to set an equivalent radial in the contact part between the intermediate transfer body drum 8 and the photoreceptor drum 1, or between the photoreceptor drum 1 and the transfer roller 9 beyond 0.49 cm.

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 color image forming apparatus using an intermediate transfer drum or a transfer roller in addition to a photosensitive drum.

[0002]

2. Description of the Related Art As a conventional image forming apparatus, for example,
There is a transfer device disclosed in Japanese Patent Application Laid-Open No. Hei 1-106085. In the transfer device of the invention according to this patent, an image carrier used as a photosensitive drum, and a rotatable drum body used as a transfer drum for attaching and transferring a transfer paper to an outer peripheral surface are provided. An elastic layer is formed on the outer peripheral surface of the drum body of the drum, and a conductive layer having a film-like resistance value of 10 ¹² Ω · cm or less is formed on the outer peripheral surface of the elastic layer. Is connected to voltage applying means for applying a bias voltage.

[0003] A transfer paper is wound around the outer peripheral surface of the outermost conductive film layer of such a transfer drum, affixed and locked, and then the transfer paper is used as an image carrier as a photosensitive drum. Transfer of the toner image formed on the surface of the image bearing member by contacting the surface of the image bearing member with a contact pressure of 300 g / cm ² or less, thereby obtaining a good transfer image with no hollow on the transfer paper. It can be done.

[0004] The hollowing out is a phenomenon in which a part of the toner adheres to and remains on the photosensitive member at the transfer source of the toner, while a part of the toner is peeled off from the transfer paper at the transfer destination.
This hollow phenomenon occurs when the contact pressure between the photosensitive drum and the transfer paper, that is, the transfer roller pressing the transfer paper and the photosensitive drum is too high.

[0005]

However, in the above-mentioned conventional image forming apparatus, even if the contact pressure between the transfer roller pressing the transfer paper and the photosensitive drum is set to a predetermined value, the transfer is performed. If the bending stiffness of the film-like conductor layer of the outermost layer of the roller is too large relative to the Young's modulus of the lower elastic layer of the conductor layer, the conductor layer and the photosensitive drum, The state of surface contact with which sufficient adhesion can be obtained with the transfer paper is not obtained, and the state is close to a line contact with a very narrow contact width. When such a phenomenon occurs, even if the contact pressure is set to a predetermined value, the contact area becomes very small and the contact pressure per unit area increases, that is, the contact pressure locally increases. As a result, there is a problem that the above-described hollowing easily occurs.

[0006] In view of the above problems, the present invention provides a surface contact state in which the contact portion has sufficient adhesion,
It is an object of the present invention to provide an image forming apparatus capable of reliably preventing a hollow phenomenon.

[0007]

In order to solve the above problems, an image forming apparatus according to the present invention has the following configuration. (1) A photoreceptor drum for forming an electrostatic latent image and a toner image, an intermediate transfer drum for multiplex primary transfer of a plurality of color toner images from the photoreceptor drum, and A secondary transfer device having a transfer roller for performing a secondary transfer from the intermediate transfer drum to a transfer material, wherein the intermediate transfer drum is a rigid conductive cylinder formed in a cylindrical shape. A conductive elastic body wound around the outer peripheral surface of the conductive cylinder, and a non-shrinkable transfer film wound around the outer peripheral surface of the conductive elastic body. Then, the Young's modulus of the conductive elastic body of the intermediate transfer drum is set to 1000 g / cm ² or less, and the bending stiffness of the transfer film of the intermediate transfer drum is set to 2.5 g · cm.
Set ² or less, set the equivalent radius at the contact portion between the photosensitive drum and the intermediate transfer drum than 0.49Cm.

(2) A photosensitive drum for forming an electrostatic latent image and a toner image, an intermediate transfer drum for multiplex primary transfer of a plurality of color toner images from the photosensitive drum, and the primary transfer drum A secondary transfer device having a transfer roller for secondary transfer of the toner image from the intermediate transfer drum to the material to be transferred, wherein the intermediate transfer drum has a rigid body formed in a cylindrical shape. A conductive cylinder, a conductive elastic body provided by being wound on the outer peripheral surface of the conductive cylinder, and a non-shrinkable transfer film provided by being wound on the outer peripheral surface of the conductive elastic body. It is configured to have the Young's modulus of the conductive elastic body of the intermediate transfer drum is set to 1000 g / cm ² or less, the intermediate bending rigidity of the transfer film of the transfer drum to 2.5 g · cm ² or less Set, sets the equivalent radius at the contact portion between the transfer roller and the intermediate transfer drum than 0.49Cm.

According to the image forming apparatus having such a configuration,
The Young's modulus of the conductive elastic body of the intermediate transfer drum is 1000
g / cm ² or less, the bending rigidity of the transfer film of the intermediate transfer drum is set to 2.5 g · cm ² or less, and the intermediate transfer drum and the photosensitive drum, or the intermediate transfer drum and the transfer roller The equivalent radius at the contact between
The contact area between the intermediate transfer drum and the photosensitive drum or between the intermediate transfer drum and the transfer roller or the transfer paper is in a state of surface contact where sufficient adhesion is obtained because the distance is set to 9 cm or more. , It is possible to reliably prevent the phenomenon of hollowing out.

[0010]

Embodiments of the present invention will be specifically described below with reference to the drawings. 1 to 1
FIG. 2 is a diagram referred to for describing a color image forming apparatus according to the first embodiment of the image forming apparatus according to the present invention.

FIG. 1 is a configuration diagram of a color image forming apparatus according to a first embodiment of the present invention. The color image forming apparatus includes a photosensitive drum 1, a charging device 2 such as a scorotron charger, a laser beam unit 3, four developing devices 4a to 4d for each color, a pre-transfer neutralizing lamp 5, a cleaning device 6, Static elimination lamp 7, intermediate transfer drum 8, transfer roller 9, separator 10, pair of registration rollers 12, 1
3 are provided.

The diameter of the photosensitive drum 1 is 20 to 1
The diameter of the transfer film 8c of the intermediate transfer drum 8 is set to 70 to 240 mm, and the diameter of the transfer roller 9 is set to 10 to 30 mm.

In the color image forming apparatus shown in FIG.
The photosensitive surface of the photosensitive drum 1 is charged by a charging device 2 such as a scorotron charger, and the charged photosensitive surface of the photosensitive drum 1 is exposed to the surface charge by a laser light unit 3 based on image information. Thus, an electrostatic latent image is formed.

This electrostatic latent image is stored in four developing devices 4a to 4d.
Thus, for example, magenta (red), yellow (yellow), cyan (blue) color toner and black (black) toner are sequentially developed, and each time the development is performed, the color toner image is formed on the intermediate transfer drum 8. Primary transfer on the surface in order,
As a result, four color toner images are transferred onto the intermediate transfer drum 8 in a multiplex manner.

The first color is developed on the photosensitive drum 1 by one of the charging by the charging device 2, the exposure by the laser beam unit 3, and the developing devices 4a to 4d.
After the remaining charge in the non-image area other than the first color toner image is removed by the pre-transfer charge removing lamp 5, the photosensitive drum 1
Is primarily transferred onto the intermediate transfer drum 8.

For the second color, the residual toner of the first color is cleaned by the cleaning means 6, and after the charge is removed by the main discharge lamp 7, the photosensitive drum 1 is charged again by the charging device 2, and the photosensitive drum 1 is exposed by the laser light unit 3. And the developing device 4a to 4d, the residual charge in the non-image area other than the second color toner image is removed by the pre-transfer discharging lamp 5, and then the photosensitive drum 1 to the intermediate transfer drum 8 The primary transfer is performed.

Similarly to the second color, the third color and the fourth color are sequentially primary-transferred from the photosensitive drum 1 to the intermediate transfer drum 8 so that the toner image of each color is finally formed on the intermediate transfer drum 8. Is transcribed multiple times.

The intermediate transfer drum 8 on which the color toner image has been multi-transferred onto the circumferential surface from the photosensitive drum 1 is transferred between a transfer roller 9 (secondary transfer device) and a transfer sheet 15 (coated). Transfer material) is secondarily transferred to the transfer material).

The transfer paper 15 on which the color toner image has been secondarily transferred passes between the transfer roller 9 and the intermediate transfer drum 8, and is then charged off by the separator 10 and separated from the intermediate transfer drum 8. Then, the sheet is fed in the direction of arrow F and fixed as a full-color image by a fixing device (not shown).

The cross-sectional structure of the intermediate transfer drum 8 has a cylindrical aluminum base (a rigid body) and a conductive cylinder base 8a (conductive cylinder) on the innermost diameter side. A sponge-shaped conductive elastic body 8b having conductivity is provided concentrically wound on the outer peripheral surface of the conductive elastic body 8b. Further, on the outer peripheral surface of the conductive elastic body 8b, a seamless and non-stretchable A transfer film 8c formed of a resin film is provided by being wound concentrically.

FIG. 2 is a view showing a contact state of a contact portion between the photosensitive drum 1 and the transfer film 8c of the intermediate transfer drum 8 where the primary transfer is performed. FIG. 5 is a graph showing the relationship between the maximum contact pressure (maximum nip pressure) and the void in the primary transfer portion. Poor quality is rank 4
The above is a level at which no hollowing occurs, and the rank 3 is a level at which even a slight hollowing does not cause a problem as an image. Therefore, the maximum contact pressure is 3
If it is set to be equal to or less than 00 g / cm ² , the level of the hollow becomes higher than or equal to rank 3, and it is understood that the hollow is not a problem as the image.

The transfer film 8 has a maximum contact pressure.
Smaller bending stiffness of c, as shown in FIG. 2, the transfer film 8c can be bent with a radius of curvature r ₁ of the photosensitive drum 1, the photosensitive drum 1 and the transfer film 8c is "face a large width 2c "I can contact you. In this case, FIG.
Since the contact width 2c is sufficiently large as shown in FIG. 5, the contact pressure P is dispersed and becomes smaller per unit area.

Since the pressing stress applied to the toner between the photosensitive drum 1 and the transfer film 8c is small, the physical adhesion at the interface between the toner and the photosensitive drum 1 or at the interface between the toner and the transfer film 8c is reduced. It is small, and the phenomenon of hollowing out hardly occurs.

On the other hand, the transfer film 8
If the bending stiffness of c of the film is large, as shown in FIG. 3, it is impossible to bend the transfer film 8c until it matches the radius of curvature r ₁ of the photosensitive drum 1, a transfer film 8c and the photosensitive drum 1 Is very close to a "line" contact.

In this case, since the contact width is narrow, the contact pressure cannot be dispersed and becomes large per unit area. For this reason, the pressing stress applied to the toner between the photosensitive drum 1 and the transfer film 8c locally increases, so that the physical adhesion at the interface between the toner and the photosensitive drum 1 or the interface between the toner and the transfer film 8c. Becomes large, and the phenomenon of hollowing out easily occurs. Further, when the phenomenon of hollowing occurs in this way, the transfer paper 15 is transferred from the intermediate transfer drum 8 by the transfer roller 9
The efficiency of the secondary transfer to the ink also decreases.

FIG. 6 shows a layer structure of a primary transfer portion between the photosensitive drum 1 and the intermediate transfer drum 8. The contact state that determines the pressing stress on the toner layer T at the contact portion between the photosensitive drum 1 and the intermediate transfer drum 8 depends on the softness of the photosensitive drum 1 and the intermediate transfer drum 8. In the case where the photosensitive member is in the form of a drum as shown in FIG. 1, it is almost assumed that the photosensitive member is a rigid member. Therefore, the contact state is determined by the Young's modulus E ₂ of the conductive elastic member 8 b of the intermediate transfer drum 8 and the bending rigidity of the transfer film 8 c. It will depend to a B _4.

The bending stiffness B ₄ is given by the following equation. B ₄ = E ₄ · I ₄ where E ₄ : Young's modulus of the transfer film 8c of the intermediate transfer drum 8 I ₄ : Secondary moment of area of the transfer film 8c of the intermediate transfer drum 8

The table in FIG. 11 shows the contact modes in the primary transfer section according to the above values. In the table in the figure,
E ₁ is the Young's modulus of the photosensitive drum 1, E ₂ is the Young's modulus of the conductive elastic body 8b of the intermediate transfer drum 8, B ₄ is bending stiffness of the transfer film 8c, H has a large Young's modulus or bending stiffness In the case, S is small when Young's modulus or bending stiffness is small, DF
During the interval, a contact state between the photosensitive drum 1 and the intermediate transfer drum 8 is shown, L represents a line contact state, and A represents a surface contact state.

As can be seen from this table, in order for the primary transfer portion to make surface contact (A) as shown in FIG.
bending stiffness B ₄ of b Young's modulus E ₂ and the transfer film 8c must be sufficiently small value.

FIG. 7 shows a layer structure of a secondary transfer portion between the transfer roller 9 and the intermediate transfer drum 8. The contact state that determines the pressing stress on the toner layer T at the contact portion between the transfer roller 9 and the intermediate transfer drum 8 is determined by the transfer sheet 1
5. It depends on the softness of the transfer roller 9 and the intermediate transfer drum 8. That is, the intermediate transfer member electroconductive elastic 8b Young's modulus E ₂ of the drum 8, a transfer film 8c bending stiffness B _4, transfer the bending stiffness B ₅ of the sheet 15, and the transfer roller 9
Depends on the Young's modulus E ₃ .

The table in FIG. 12 shows the contact modes in the secondary transfer section according to the above values. In the table in the figure,
E ₂ is the Young's modulus of the conductive elastic body 8b of the intermediate transfer drum 8, B ₄ is bending stiffness of the transfer film 8c, B ₅ bending stiffness of the transfer sheet 15, E ₃ is the Young's modulus of the transfer roller 9, H
Is the case where the Young's modulus or the bending stiffness is large, S is the case where the Young's modulus or the bending stiffness is small,
Contact state between the transfer paper 15 and the transfer paper 15, and the transfer paper 15
L indicates a line contact state, and A indicates a surface contact state.

[0032] As can be seen from the table, between the F-P is a surface contact state (A) is a conductive elastic member 8b of the Young's modulus E ₂ of the intermediate transfer drum 8, the transfer of bending stiffness B ₄ both the film 8c is sufficiently soft, a transfer sheet 15 between the P-R is a surface contact state (a) is bent stiffness B ₅
When both of the transfer roller 9 of Young's modulus E ₃ must be sufficiently soft.

[0033] However, since the transfer sheet 15 can not be regulated in advance, after all, if the Young's modulus of the conductive elastic body 8b E ₂ and were reduced both bending stiffness B ₄ transfer film 8c (Table No.13~ 16) The surface contact state (A) can be reliably established between the FP.

FIG. 8 shows the relationship between the Young's modulus of the conductive elastic body 8b of the intermediate transfer drum 8 and the maximum contact pressure (maximum nip pressure) between the transfer film 8c and the photosensitive drum 1 in the primary transfer section. FIG. The plots of the black square, circle, triangle, and rhombus shapes are represented by the equivalent radius R shown in the following equation.
₁ shows each case of 0.44, 0.78, 1.33, and 4. R ₁ = r ₁ · r ₂ / (r ₁ + r ₂ ) where r ₁ is the radius of the photosensitive drum 1 and r ₂ is the radius of the intermediate transfer drum 8.

Such an equivalent radius R ₁ is a value introduced for the sake of convenience in describing a contact state when a pair of drums make contact. In order to form an arbitrary contact width, the above r
_Although there are countless combinations of ₁ and r ₂ continuously, by using the equivalent radius, one equivalent radius can be made to correspond to one contact width.

In the embodiment of the present invention, when the equivalent radius R ₁ is obtained from the actual specification, it is in the range of 0.44 to 4,
In FIG. 8 described above, in order to prevent the phenomenon of voids,
As described in FIG. 5, the maximum contact pressure is 300 g /
cm ² or less, the Young's modulus of the conductive elastic body 8b of the intermediate transfer drum 8 is a value when the equivalent radius R ₁ is 300 g / cm ^{2 in} the graph of 4, that is, 1
It can be seen that it has to be less than 000 g / cm ² .

FIG. 9 shows the equivalent radius at the primary transfer portion,
Bending stiffness B _C of transfer film 8c of intermediate transfer drum 8
6 is a graph showing the relationship between the upper limit and the upper limit. As the equivalent radius increases, the contact width increases, so that the bending moment of the transfer film 8c increases. Therefore, if the equivalent radius is increased, even the transfer film 8c having a large Young's modulus can bend along the outer peripheral surface of the photosensitive drum 1, and the transfer film 8c adheres to the outer peripheral surface of the photosensitive drum 1 (surface contact). FIG. 9 illustrates what can be done.

The minimum achievable bending stiffness of the transfer film 8c is, for example, a soft material such as fluorine, because of the physical properties and moldability of the material.
Considering the thickness on the order of μm, the limit is 0.13 g · cm ^2. Therefore, from FIG. 9, it can be seen that if the equivalent radius is set to 0.49 cm or more, the primary transfer portion can be brought into surface contact. I understand.

FIG. 10 shows the Young's modulus of the conductive elastic body 8b of the intermediate transfer drum 8 and the bending stiffness B _C of the transfer film 8c.
6 is a graph showing a relationship with the graph. That is, the transfer to the film 8c bending stiffness B _C, the back side of the conductive elastic body 8b of the rear side has no Young's modulus greater than a predetermined value, conductive elastic body 8b is transfer film 8c Therefore, the transfer film 8c cannot be brought into close contact with the outer peripheral surface of the photosensitive drum 1 by applying a repulsive force.

For this reason, in order for the transfer film 8c to be in close contact with the outer peripheral surface of the photosensitive drum 1, the bending stiffness B _C of the transfer film 8c and the Young's modulus of the conductive elastic body 8b must have a fixed relationship. Instead, this is shown as a graph in the graph of FIG. If the Young's modulus of the conductive elastic body 8b as described above to try to 1000 g / cm ² or less, the bending stiffness B _C of the transfer film 8c is 2.5 g · cm ²
By setting as follows, the surface contact between the intermediate transfer drum 8 and the photosensitive drum 1 can be achieved.

As described above, according to the present embodiment, it is possible to optimize the physical property values of the respective component materials.
Since the contact portion between the intermediate transfer drum and the photosensitive drum, or the intermediate transfer drum and the transfer roller, or the transfer paper can be brought into a state of surface contact where sufficient adhesion can be obtained, hollowing out occurs. Phenomenon can be reliably prevented. For this reason, according to the present embodiment, the quality of the image of the color image forming apparatus can be improved, and the durability thereof can be improved.

Although the primary transfer section between the photosensitive drum 1 and the intermediate transfer drum 8 has been mainly described in the above embodiment, the secondary transfer section between the intermediate transfer drum 8 and the transfer roller 9 has been described. The same effect can be obtained in the transfer section. In addition, the same considerations and experiments were carried out for the secondary transfer unit. In the image forming apparatus using the intermediate transfer drum 8, the hollow portion in the secondary transfer unit is mainly caused by the contact state in the primary transfer unit. The results showed that the contact state in the primary transfer section was more important than the contact state in the secondary transfer section.

Further, in the above embodiment, the primary transfer unit and the secondary transfer unit of the image forming apparatus using the intermediate transfer drum have been described. In the case where one of them has rigidity and the other has a film on an elastic body, the present invention can be applied to a transfer unit in a monochrome image forming apparatus.

Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments, and may be variously modified based on the technical idea of the present invention. Can be changed.

[0045]

As described above, according to the image forming apparatus of the present invention, since the physical properties of each component material are optimized, the intermediate transfer drum and the photosensitive drum, or the intermediate transfer member can be used. The contact portion between the drum and the transfer roller or the transfer paper can be brought into a state of surface contact with which sufficient adhesion can be obtained, so that the phenomenon of hollowing out can be reliably prevented.

According to the present invention, for the same reason,
It is also possible to prevent a decrease in color reproducibility due to reverse transfer at the primary and secondary transfer contact portions.

According to the above-described embodiment, the surface layer of the intermediate transfer drum 8 is covered with the resin film 8c.
The contact pressure applied to the toner layer, the photosensitive drum 1, the transfer film 8c, the transfer roller 9 and the transfer paper 15 has been optimized, so that the toner is evacuated and the photosensitive drum 1, the transfer film 8c and the transfer roller 9 are worn. And the like, and an effect that a highly durable color image forming apparatus can be realized.

It is important that the transfer film 8c is made of a resin film instead of an elastomer as an element that can increase the durability.
Since the cleaning property is improved, the degree of freedom in designing the cleaning mechanism and the size of the cleaning mechanism is increased.

Further, according to the above embodiment, the intermediate transfer drum 8 is softened to have a function of forming a surface contact, so that the intermediate transfer drum 8 is not affected by the hardness of the secondary transfer roller 9 at all, and each component is not affected. This has the effect of increasing the margin in design.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a color image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a contact state between a photosensitive drum and an intermediate transfer drum in a primary transfer section of a color image forming apparatus.

FIG. 3 is a diagram illustrating another contact state of the photosensitive drum and the intermediate transfer drum in the primary transfer section of the color image forming apparatus.

FIG. 4 is a graph showing a distribution state of a contact pressure P when a photosensitive drum and an intermediate transfer drum come into surface contact.

FIG. 5 is a graph showing the relationship between the maximum contact pressure between the photosensitive drum and the intermediate transfer drum and the level of hollow holes.

FIG. 6 is a diagram illustrating a layer configuration of a contact portion between a photosensitive drum and an intermediate transfer drum.

FIG. 7 is a diagram illustrating a layer configuration of a contact portion between an intermediate transfer drum and a transfer roller.

FIG. 8 is a graph showing the relationship between the maximum contact pressure at the primary transfer portion for each of various equivalent radii and the Young's modulus of the conductive elastic body of the intermediate transfer drum.

FIG. 9 is a graph showing a relationship between an equivalent radius in a primary transfer portion and an upper limit value of a bending stiffness of a transfer film.

FIG. 10 is a graph showing the relationship between the bending stiffness B _C of the transfer film of the intermediate transfer drum and the Young's modulus of the conductive elastic body in the primary transfer unit.

FIG. 11 is a diagram illustrating a table illustrating changes in a contact state according to various conditions in a primary transfer unit.

FIG. 12 is a table showing a change in a contact state according to various conditions in a secondary transfer unit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photoconductor drum 2 charging device 3 laser light unit 4 a-4 d developing device 5 pre-transfer static elimination lamp 6 cleaning device 7 main static elimination lamp 8 intermediate transfer drum 8 a cylinder base 8 b conductive elastic body 8 c transfer film 9 transfer roller 10 separator 12, 13 registration roller 13a core metal 15 transfer paper 15a surface on which image is formed 15b back surface of surface on which image is formed 10 separation charger 10a shield case 10b discharge electrode 12, 13 registration roller 13a core metal 15 transfer paper 15a image formation Back surface 15b Back side of image forming side 17, 19, 21, 22 Support rollers 17a, 19a Core metal member 17b Support member 18 Transport device 24 Support member 24a Concave sliding surface 24b Sliding member

Claims (4)

[Claims] 1. A photosensitive drum for forming an electrostatic latent image and a toner image, an intermediate transfer drum for multiplex primary transfer of a plurality of color toner images from the photosensitive drum, and the primary transferred toner A secondary transfer device having a transfer roller for secondarily transferring an image from the intermediate transfer drum to a transfer material, wherein the intermediate transfer drum is formed of a rigid conductive material having a cylindrical shape. A conductive cylinder, a conductive elastic body wound around the outer peripheral surface of the conductive cylinder, and a non-shrinkable transfer film wound around the outer peripheral surface of the conductive elastic body. The Young's modulus of the conductive elastic body of the intermediate transfer drum is set to 10
200 g / cm ² is set lower than the bending stiffness of the transfer film of the intermediate transfer drum 2.
The image forming apparatus is set to 5 g · cm ² or less, and an equivalent radius at a contact portion between the intermediate transfer drum and the photosensitive drum is set to 0.49 cm or more. 2. A photosensitive drum for forming an electrostatic latent image and a toner image, an intermediate transfer drum for multiplex primary transfer of a plurality of color toner images from the photosensitive drum, and the primary transferred toner A secondary transfer device having a transfer roller for secondarily transferring an image from the intermediate transfer drum to a transfer material, wherein the intermediate transfer drum is formed of a rigid conductive material having a cylindrical shape. A conductive cylinder, a conductive elastic body wound around the outer peripheral surface of the conductive cylinder, and a non-shrinkable transfer film wound around the outer peripheral surface of the conductive elastic body. The Young's modulus of the conductive elastic body of the intermediate transfer drum is set to 10
200 g / cm ² is set lower than the bending stiffness of the transfer film of the intermediate transfer drum 2.
5 g · cm ² is set below, the image forming apparatus characterized by setting the equivalent radius than 0.49cm at the contact portion between the intermediate transfer drum and the transfer roller. 3. The image forming apparatus according to claim 1, wherein a contact pressure between the intermediate transfer drum and the photosensitive drum is set to 300 g / cm ² or less. 4. The image forming apparatus according to claim 2, wherein a contact pressure between the intermediate transfer drum and the transfer roller is set to 300 g / cm ² or less.