JP2003241543A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transfer roller maintaining stable transfer ability to environment and durability fluctuation or the fluctuation of applied voltage, and an image forming apparatus adopting the transfer roller. <P>SOLUTION: In the image forming apparatus having an image carrier on which a toner image in accordance with image information is formed, and the transfer roller for transferring the toner image formed on the image carrier to recording material, the transfer roller consists of at least two or more layers, and at least the outermost layer is an electron conduction layer including an electron conducting agent and is an elastic layer formed of a foaming body. Furthermore, at least one layer other than the outermost layer is an ion conduction layer including an ion conducting agent whose resistance is adjusted to a value smaller than the volume resistivity of the outermost layer and is an elastic layer formed of a foamed body. <P>COPYRIGHT: (C)2003,JPO

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 which employs an electrophotographic system or an electrostatic recording system using a transfer roller.

[0002]

2. Description of the Related Art FIG. 9 shows a schematic diagram for explaining a conventional example.

An endless intermediate transfer belt 51 running in the direction of arrow X is arranged in the main body of the image forming apparatus. The intermediate transfer belt 51 is made of a conductive or dielectric resin such as polycarbonate, polyethylene terephthalate resin film, or polyvinylidene fluoride resin film. The recording material P taken out from the paper feed cassette 8 is supplied to the secondary transfer portion via the registration roller 82, and is further conveyed to the left in the figure.

Above the intermediate transfer belt 51, four image forming portions Pa, Pb, Pc and Pd having substantially the same structure are arranged in series. The configuration of the image forming unit Pa will be described as an example.

The image forming portion Pa is provided with a drum-shaped electrophotographic photosensitive member (hereinafter referred to as "photosensitive drum") 1a rotatably arranged. Around the photosensitive drum 1a, process devices such as a primary charger 2a and a developing device 4a are arranged. The other image forming units Pb, Pc, and Pd have the same configuration as the image forming unit Pa, and only the photosensitive drums 1b, 1c, and 1d are shown in FIG. The difference between the image forming units 1a, 1b, 1c, and 1d is that each forms a toner image of each color of magenta, cyan, yellow, and black.

Magenta toner, cyan toner, yellow toner, and black toner are respectively stored in the developing devices arranged in the respective image forming sections.

An image signal based on the magenta component color of the original is projected onto the photosensitive drum 1a via a polygon mirror (not shown) or the like to form an electrostatic latent image, on which magenta toner is supplied from the developing device to cause a static image. The latent image becomes a magenta toner image. As the toner image is rotated by the photosensitive drum 1a,
When the photosensitive drum 1a and the intermediate transfer body 51 come into contact with each other at the transfer site, the first transfer member 53a transfers the magenta toner image to the intermediate transfer body 51 by the first transfer bias applied.

Intermediate transfer member 5 carrying a magenta toner image
When No. 1 is conveyed to the image forming portion Pb, by this time, in the image forming portion Pb, the cyan toner image formed on the photosensitive drum 1b by the same method as described above is transferred onto the magenta toner image. To be done.

Similarly, as the recording material P advances to the image forming portions Pc and Pd, the yellow toner image and the black toner image are superposed and transferred on the above-mentioned toner images at the respective transfer portions, and until then. Then, the recording material P taken out from the paper feed cassette 8 reaches the second transfer portion, and the above-described four color toner images are transferred onto the recording material P by the transfer bias applied to the second transfer member 57. To be done.

The recording material P is conveyed from the second transfer section to the fixing section. The fixing unit has a step of fixing the toner on the recording material by heat and pressure. Further, in order to improve the releasability between the recording material P and the fixing roller 71, a mechanism for coating the surface of the fixing roller 71 with a releasing oil (for example, silicone oil) is provided. This oil also adheres. The recording material on which the toner image has been fixed is discharged to the discharge tray. In the case of automatically forming a double-sided image, an image can be formed on the back side by passing through a recording material reversing path (not shown) and repeating the above-described series of image forming processes.

Many of the first and second transfer members of such an image forming apparatus adopt a roller shape from the viewpoint of durability or cost. In particular, the toner image is transferred from the photosensitive drums 1a to 1d to the intermediate transfer body 51, or from the intermediate transfer body 51 to the recording material P.
In the step of transferring to, a conductive member has been used because it requires a step of passing an electric current.

Further, the volume resistivity is 1E + 5Ω around the cylindrical core metal so that a sufficient transfer current flows in the recording material portion.
-Rolls covered with rubber are often used, whose resistance is adjusted to m to 1E + 9Ωm. As a means for adjusting the resistance, conductive carbon black, metal powder, or metal oxide or the like is dispersed in rubber "electroconductive type" and epichlorohydrin rubber, tetracyanoethylene and its dielectric, benzoquinone and its dielectric, An "ionic conductive type" in which an inorganic ionic substance such as lithium perchlorate, sodium perchlorate or calcium perchlorate, a cationic surfactant, an amphoteric ionic surfactant or the like is kneaded with rubber can be mentioned.

In the present invention, the following are known as required characteristics of the transfer roller. First, the volume resistivity of the surface layer of the transfer roller, which is in contact with the back surface of the recording material, affects the transferability for transferring the toner image from the image carrier. If the volume resistivity of the surface layer of the transfer roller is small, the current tends to flow away from the recording material, so that the transfer current may not sufficiently flow to the recording material portion, which may cause transfer failure. Further, if the volume resistivity of the surface layer of the transfer roller is too high, leakage of the surface layer may occur, which may cause transfer failure. In addition, a large transfer high-voltage power supply is required, resulting in cost increase and increase in space because of the required creepage.

Regarding the influence of the transfer roller surface layer and the transfer roller dirt on the back surface of the recording material, when the same level of dirt is adhered to the transfer roller surface, the transfer roller surface is superior to the surface layer having excellent toner releasability. Foams have been found to be superior. Therefore, in the case of the surface layer having excellent toner releasability, a sufficient cleaning mechanism is required, which also causes an increase in cost and space.

Further, it has been found that the hardness factor of the transfer roller has a large factor with respect to the "blank" image in which the central portion of characters or fine lines is not transferred. Further, since the transfer roller also has a role of conveying the recording material, a sufficient nip is required to grip the recording material. Therefore, low hardness is required to secure a sufficient nip.

[0016]

However, the conventional techniques have the following problems.

That is, as described in the conventional example, the methods of adjusting the resistance of a conductive roller used for a transfer roller or the like include a method of mixing an ionic conductive agent and a method of dispersing an electronic conductive agent. As a typical example, in the case of a rubber roller containing an ionic conductive agent, for example, the resistance changes as shown in FIG. 2, and the resistance becomes too high in a low humidity and low temperature environment to obtain a constant transfer electric field. In some cases, the transfer voltage required for the process becomes large and a dot-shaped missing image or the like is observed.
In a high-temperature and high-humidity environment, the resistance drops too much, the transfer current easily flows to the area other than the back surface of the paper, and the transfer charge cannot be sufficiently supplied to the back surface of the paper. There may be cases where image dropouts, polka dot images, etc. occur.

In Japanese Patent Laid-Open No. 2000-137391,
A transfer roller having a plurality of layers and a base layer of which is made of electronically conductive rubber has been proposed. Further, in the case of the rubber roller in which the electronic conductive agent is dispersed, since the voltage characteristic as shown in FIG. 3 is exhibited, the resistance decreases as the voltage increases,
When a certain voltage or more is applied, leakage may occur. Further, there is a problem that resistance unevenness due to uneven dispersion of the electronic conductive agent in the rubber is larger than that of the ion conductive type.

In Japanese Patent Laid-Open No. 2000-179539,
For the above problems, as a conductive roller that can provide a stable resistance value with respect to changes in applied voltage and environmental changes,
A conductive roll formed of a plurality of layers of an electron conductive layer and an ion conductive layer has been proposed.

However, Japanese Patent Laid-Open No. 2000-17953
The following problem cannot be solved with the configuration of ionic conductive layer resistance> electronic conductive layer resistance disclosed in Japanese Patent No. The resistance of the transfer roller is dominated by the resistance of the ionic conductive layer, and it is difficult to reduce resistance fluctuation due to environmental fluctuations. Therefore, in a high temperature and high humidity environment, the resistance value of the roller becomes small, and it is not possible to prevent the current from flowing into areas other than the recording material, and it is not possible to prevent defective transfer.

In a low-temperature and low-humidity environment, the resistance value of the ionic conductive layer becomes large, so that the transfer voltage becomes large and an abnormal discharge image as represented by white spots is generated.
In addition, the cost of the transfer high-voltage power source also increases. Further, the resistance of the ionic conductive layer increases with the durability of the applied current, so that the durability variation of the resistance cannot be ignored.

From the above, it is impossible to solve the problems that the cost of the transfer high-voltage power source increases, the size of the transfer high voltage source increases, and the resistance value fluctuates due to fluctuation factors such as durability and environment. Japanese Patent Laid-Open No. 2000-181251 also proposes a combination of ionic conductivity and electronic conductivity in a layer structure.

However, if an ion conductive material is used for the surface layer, it is easily affected by the temperature and humidity of the atmosphere, and it is difficult to keep the resistance value of the transfer roller surface in contact with the back surface of the recording material stable. Also, if the surface layer that needs to keep the volume resistivity relatively high is of the ion conductive type and the inner layer that can keep the volume resistivity relatively low is the electron conductive type, the overall resistance of the transfer roller is reduced. In order to achieve this, it is necessary to realize an electronic conductive layer having a low resistance, and therefore a large amount of a conductive agent must be dispersed, and the conductive agent acts as a filler, and there is a problem that high hardness cannot be avoided. .

Further, even when the surface layer is made of an electronically conductive solid layer, a measure for increasing the film pressure can be considered in consideration of leaks and the like when the transfer high pressure becomes large. It will be difficult to avoid.

The present invention has been made in view of the above problems, and its object is to adopt a transfer roller and a transfer roller which maintain stable transferability against environmental and durability variations and applied voltage variations. An object is to provide an image forming apparatus.

[0026]

In order to achieve the above-mentioned object, the present invention provides an image carrier for forming a toner image according to image information, and a toner image formed on the image carrier with a recording material. In an image forming apparatus having a transfer roller for transferring to, the transfer roller has a structure of at least two layers, and at least the outermost layer is an electronic conductive layer containing an electronic conductive agent, and an elastic layer formed of foam. At least one layer other than the outermost layer is an ion conductive layer containing an ionic conductive agent whose resistance is adjusted to a value smaller than the volume resistivity of the outermost layer, and is an elastic layer formed of a foam. It is characterized by

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

<Embodiment 1> FIG. 1 is a sectional view of a main portion of an image forming apparatus according to the present invention.

An endless intermediate transfer belt 51 running in the direction of the arrow X is arranged in the main body of the image forming apparatus. In this embodiment, the intermediate transfer belt 51 is made of conductive polyimide. The recording material P taken out from the paper feed cassette 8 is supplied to the secondary transfer portion via the registration roller 82, and is further conveyed to the left in the figure.

Above the intermediate transfer belt 51, four image forming portions Pa, Pb, Pc and Pd having substantially the same structure are arranged in series. The configuration of the image forming unit Pa will be described as an example.

The image forming portion Pa is provided with a drum-shaped electrophotographic photosensitive member (hereinafter referred to as "photosensitive drum") 1a rotatably arranged. Around the photosensitive drum 1a, process devices such as a primary charger 2a and a developing device 4a are arranged. The other image forming units Pb, Pc, and Pd have the same configuration as the image forming unit Pa, and only the photosensitive drums 1b, 1c, and 1d are shown in FIG. The difference between the image forming units 1a, 1b, 1c, and 1d is that each forms a toner image of each color of magenta, cyan, yellow, and black.

It is assumed that magenta toner, cyan toner, yellow toner, and black toner are stored in the developing devices arranged in the respective image forming portions.

An image signal based on the magenta component color of the original is projected onto the photosensitive drum 1a via a polygon mirror (not shown) or the like to form an electrostatic latent image, on which magenta toner is supplied from the developing device to be statically formed. The latent image becomes a magenta toner image. As the toner image is rotated by the photosensitive drum 1a,
When the photosensitive drum 1a and the intermediate transfer body 51 come into contact with each other at the transfer site, the magenta toner image is transferred to the intermediate transfer belt 51 by the primary transfer bias applied by the primary transfer roller 53a. When the intermediate transfer member 51 carrying the magenta toner image is conveyed to the image forming portion Pb, by this time, the cyan toner image formed on the photosensitive drum 1b in the image forming portion Pb by the same method as described above. Are transferred onto the magenta toner image.

Similarly, as the recording material P advances to the image forming portions Pc and Pd, the yellow toner image and the black toner image are superposed and transferred on the above-mentioned toner images at the respective transfer portions. Then, the recording material P taken out from the paper feed cassette 8 reaches the secondary transfer portion, and the recording bias is applied to the secondary transfer roller 57 by the transfer bias described above.
The color toner image is transferred onto the recording material P.

The recording material P is conveyed from the second transfer section to the fixing section. In the fixing section, the toner is applied to the recording material P by heat and pressure.
It has a step of fixing it on top. Further, in order to improve the releasability between the recording material P and the fixing roller 71, a mechanism is provided for coating the surface of the fixing roller with a releasable oil (for example, silicone oil). Oil adheres. The recording material P on which the toner image is fixed is ejected to the paper ejection tray. When automatically forming a double-sided image,
By passing through a recording material reversing path (not shown) and repeating the above-described image forming process, it is possible to form an image on the back surface.

In this image forming apparatus, in this embodiment, a sponge roller having a two-layer structure is used as the secondary transfer roller 57. Outer diameter φ24mm, core metal φ12mm,
The surface layer is formed of a sponge layer in which conductive carbon black is dispersed in EPDM (ethylene propylene diene rubber) having a thickness of 2 mm and the volume resistivity is adjusted to 3E + 7 Ω · m. The inner elastic layer is formed of an ion conductive foam of an alloy of epichlorohydrin rubber and NBR (nitrile rubber) having a thickness of 4 mm, and is formed of a sponge layer adjusted to have a volume resistivity of 5E + 5Ω · m. . The overall roller hardness was 28 ° with Asker C under a load of 500 g.

The pressure between the transfer roller and the intermediate transfer belt satisfies the transferability of thick paper, secondary color or tertiary color, or transferability to a recording material having a large surface roughness. 3.3
It is set to E + 4Pa.

The volume resistivity of the paper is about 1E + 9 Ω · m in the case of CLC80 (manufactured by Nippon Paper Industries Co., Ltd .; basis weight 80 g / m ² ) which is our recommended paper for full color image forming apparatus. Since the thickness is about 85 μm, when the nip width at the secondary transfer nip of the image forming apparatus according to the present embodiment is 5 mm, the actual resistance Rp of the paper is obtained from the following equation: Rp = (1E + 9 × 85E-6 ) / (5E-3 × 297)
E-3) = 5.7E + 7 (Ω).

Therefore, in order to reduce the influence of the resistance of the recording material, it is necessary to make it larger than the resistance value of this recording material. In this embodiment, the thickness is 2 mm and the volume resistivity is 5E.
Since it is +7 Ω · m, the nip width is 5 mm, and the thrust rubber length of the secondary transfer roller is 320 mm, the actual resistance of the surface layer portion is obtained from the following formula: Rs = (5E + 7 × 2E-3) / (5E-3 × 320E
-3) = 6.3E + 7 (Ω), which is set to be equal to or higher than Rp, so that a transfer failure due to a large transfer current flowing to a portion other than the recording material does not occur. is there.

Next, the charge amount per unit mass of the toner used in this embodiment is about 30 μC / g in a room temperature and low humidity environment (23 ° C., 5%). Since the maximum amount of toner formed on the intermediate transfer belt is 12 g / m ² , the maximum amount of charge per unit area formed on the intermediate transfer belt is 360 μC / m ² . The amount of charge of the toner in the secondary transfer nip is 5 mm for the secondary transfer nip width and 30 for the maximum image width.
When 0 mm is multiplied by the maximum charge amount per unit area, it becomes about 0.53 μC.

The nip passage time is obtained from the process speed. In this embodiment, the process speed is 13
Since it is 0 mm / s, it takes about 0.038 seconds to pass through the nip portion.

From the above, the transfer current amount necessary for the necessary transfer can be calculated as 0.53 / 0.038 = 13.9 (μA).

However, the toner is attracted to the intermediate transfer belt due to the mirroring force and the like, and the transfer cannot be sufficiently performed only by the charge amount of the toner. In this embodiment, when the transfer efficiency with respect to the transfer current was measured, the result was as shown in FIG. 4 (A4 side, the above-mentioned image formation on our recommended color paper). Here, the transfer current value at which the transfer efficiency is saturated is 20 μA. Here, the transfer current utilization efficiency is defined as follows.

Transfer Current Utilization Efficiency = (Maximum Charge Amount of Toner in Transfer Nip) / (Maximum Transfer Efficiency Transfer Current Value) Therefore, the transfer current utilization efficiency in this embodiment is 13.9 / 20 = 69.5. %. Toner, external additive of toner, surface property of image carrier,
It is considered that the transfer current utilization efficiency changes depending on the environment, durability, etc., but it is known that it may change from about 40% to about 75% depending on the conditions.

In this embodiment, conductive polyimide is used as the intermediate transfer belt. Specifically, since the volume resistivity is 5E + 8Ω · m and the thickness is 75 μm, the actual resistance of the intermediate transfer belt at the transfer nip portion is Is 2.3E + 7Ω = (1E + 9 × 75E-6) / (5E−
3 * 320E-3).

From the above, the transfer voltage when the transfer current is 22 μA is 2800 (Ω) because the combined resistance at the transfer nip portion is 1.4E + 8 = (5.7 + 6.3 + 2.3) × 1E + 7 (Ω). V) = 1.4E + 8 (Ω) × 20E-6
(A)

However, Table 1 shows the first comparative example.

In order to obtain the same effect with a single layer, for example, when the secondary transfer roller is a single layer roller having a volume resistivity of 5E + 7Ω · m and a thickness of 6 mm, Rs = (5E + 7 × 6E) -3) / (5E-3 x 320E
-3) = 1.8E + 8 (Ω), and the combined resistance at the transfer nip portion is 2E + 8Ω, so the transfer voltage when a transfer current of 20 μA is 5400 (V) = 2.7E + 8 (Ω) × 20E. -6
Despite exhibiting the same transferability as (A), a large voltage was required, and a blank image started to occur.
Further, a transfer roller having a core metal outer diameter of φ20 and a surface layer thickness of 2 mm was also examined under the same conditions as above. The hardness of the transfer roller was measured by the same measurement method as above, using Asker C 57 °.
Became. The transfer voltage is 20 μA, which is 28 μm in this embodiment.
It was almost the same as 00V, and white spots did not occur,
A hollow has occurred.

[0049]

[Table 1] As a second comparative example, when the ion conductive layer is adopted as the outermost layer, the same roller is adopted in the change from a low humidity environment (23 ° C, 5%) to a high temperature and high humidity environment (30 ° C, 80%). Even then, it varies from 1E + 8Ω · m to 5E + 6Ω · m.
Depending on the environment, if the resistance value changes, the appropriate transfer current value also changes (the transfer current amount flowing into the portion other than the recording material changes), which may cause a transfer failure or the like. In addition, the ionic conductive layer tends to increase in resistance with durability. Therefore, the problem that the proper transfer current value or transfer voltage value changes due to the durability still occurs.

As a third comparative example, in a roller having a three-layer structure, an ion conductive layer (sponge layer) provided on the outer periphery of the core metal, an electron conductive layer (solid layer) having conductive carbon dispersed on the outer periphery thereof, and a surface layer As the coating, a conductive fluororesin in which carbon black is dispersed is coated. The one having a solid layer had a higher hardness depending on its film thickness, and like the above-mentioned comparative example, the hollow defects were aggravated. Further, since the contact area with the intermediate transfer belt was increased, the fine abnormal discharge image disappeared, but large white spots were locally generated, and the white spot image became visible. On the other hand, if the surface layer is a sponge,
Even if a whiteout image actually occurs, minute whiteouts are uniformly generated on the entire surface, which has the effect of making it very inconspicuous.

As a fourth comparative example, the volume resistivity of the surface layer is 5E + 4Ω · m, 1E + 5Ω · m, 1E + 6Ω · m, 1
E + 8Ω · m, 5E + 8Ω · m, and 1E + 9Ω · m were examined.

[0052]

[Table 2] When the volume resistivity of the surface layer exceeds 5E + 8Ω · m, the applied voltage exceeds 10 kV, and abnormal discharge occurs on the upstream side of transfer. Also, with 1E + 5Ω · m or less, a transfer current flows in at the edge of the recording material and a transfer current flows in a portion other than the toner portion, causing transfer failure or toner forming a patch image to fly to the non-image portion. A patch splattered.

The volume resistivity of the roller surface layer is 5E + 8Ω · m
When the thickness is 1 mm, the transfer voltage is 10 kV or less and plain paper can be used.
For P sheets, etc., it may exceed 10 kV. In addition, when the thickness is reduced, surface leakage may occur.

As a fifth comparative example, the roller surface layer is not a sponge layer, but a solid layer having a thickness of 2 mm and a volume resistivity of 5
When a roller with resistance adjusted to E + 7 was examined, a load of 5
The hardness of 00g Asker C became 52 °. When this roller was used to perform transfer under the same conditions as above,
A hollow has occurred.

In order to satisfy the stable transferability (including back stain etc.) at low cost, while maintaining the low hardness, the resistance of the surface layer is stabilized against the variable factors such as environment and durability,
In addition, it is necessary to keep the transfer high voltage low.

It has been shown that the structure of this embodiment, in which a low-resistance ionic conductive sponge layer is formed inside and an electron-conductive medium resistance sponge layer is the outermost layer, is effective.

<Embodiment 2> In the present embodiment, FIG.
A case will be described in which a roller having a plurality of layers is used as the transfer roller when the image carrier as shown in FIG.

As in the first embodiment, the photosensitive drum 21 is charged by the charging roller 23. After that, an image signal of an original or the like is transferred to the photosensitive drum 21 via a polygon (not shown).
An electrostatic latent image is formed by being projected onto the surface, and toner is supplied from the developing device 24 to form a toner image. Almost the same as the front end of the toner image comes to the transfer portion, the recording material P is supplied so that the front end becomes the transfer portion.

While the recording material P is nipped and conveyed between the photosensitive drum 21 and the transfer roller 22, the toner image formed on the photosensitive drum 21 is applied onto the recording material P by applying a desired voltage to the transfer roller 22. Transfer to. The toner image on the recording material P is fixed on the recording material P by heat and pressure.

The toner remaining on the photosensitive drum 21 is cleaned by the cleaning blade 25.

Also in such an image forming apparatus, the same effect can be obtained by adopting the same roller as the transfer roller 22 in the first embodiment.

<Third Embodiment> In the present embodiment, the transfer roller can be made stronger due to the leakage of the surface layer by using the three-layer structure. 3 layer structure, from the core metal surface layer,
Volume resistivity of 5E + 5Ω ・ m (normal temperature and humidity environment (23 ° C,
(60%), the resistance of the ion conductive layer is adjusted to 5E + 6Ω · m (normal temperature and humidity environment (23 ° C, 60%).
%) The ion-conducting layer resistance-adjusted to (measured under), ρ3 =
An electronically conductive layer whose resistance is adjusted to 5E + 7Ω · m (measured in a normal temperature and normal humidity environment), with a layer pressure of 4 mm, 1 mm,
It was set to 1 mm. Each layer of this roller is set in high temperature and high humidity environment (30
At 80 ° C), the systematic resistivity is 8E + 4Ω · m, 7E + 5Ω · m, 3E + 7Ω from the layer close to the core metal.
・ M, and the ratio of the volume resistivity of the intermediate layer to the volume resistivity of the surface layer does not exceed two digits.

The volume resistivity of each layer measured in a low humidity environment (23 ° C., 5%) is 1 from the layer close to the core metal.
E + 6Ω · m, 8E + 6Ω · m, 6E + 7Ω · m, the ion conductive layer does not exceed the resistance of the electron conductive layer, the volume resistivity of the ion conductive layer is 1E + 7Ω · m or less, and the transfer voltage also fluctuates. small.

By doing so, leakage of the surface layer can be prevented. When the volume resistivity of the surface layer is different from the volume resistivity of the lower layer by two digits or more, most of the transfer voltage is applied to the surface layer, so that leakage easily occurs. Further, the transfer voltage does not increase even in a low humidity environment, and stable image formation without a blank image can be performed.

<Fourth Embodiment> FIG. 8 shows a schematic view of a transfer roller for explaining the present embodiment.

In the present embodiment, the transfer roller rubber portion 91
The covering members 92 are attached to both ends of the. The rubber part is
This is the same as that adopted in the first embodiment. At the end,
An insulating resin polyacetal (PO) is used as the covering member 92.
M) is adopted. By doing so, there is an effect that the environmental fluctuation of the ion conductive layer can be further suppressed.

The covering member may also be used as a positioning roller that determines the position of the transfer roller. Furthermore, the sliding member such as the POM may not be used. A member having a high μ may be adopted and the transfer roller may also be used as an auxiliary member for following the image carrier.

(Method of Measuring Volume Resistivity of Roller) FIG. 7 shows a schematic view for explaining the method of measuring the volume resistivity of the roller.

Load of 1000 g on the metal roller 41 of φ30
Then, pressure is applied from both ends to bring the transfer roller 42 into contact. The metal roller 41 is rotated at a speed of 20 rpm, and the transfer roller 42 is driven by this rotation. Here, a bias of 2 kV is applied to the cored bar of the transfer roller 42, and the current value flowing into the metal roller 41 is monitored. The current value at that time is I
(A), assuming that the rubber length of the roller is L, the core metal diameter r2, and the roller outer diameter r1, the volume resistivity ρv of the transfer roller 42.
Is determined by ρv = 2πL × 2000 / I × ln (r1 / r2).

[0070]

As is apparent from the above description, according to the present invention, a transfer roller having a plurality of layers is used, and the surface layer is either a medium resistance electronic conductive layer or a lower layer is a low resistance ionic conductive layer. By using the sponge layer as well, it is possible to obtain a transfer roller that satisfies the stable transferability against environmental changes and durability changes while maintaining low hardness of the roller, and an image forming apparatus including the transfer roller.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a low volume efficiency environmental change of a single layer roller using an ion conductive material.

FIG. 3 is a diagram showing applied voltage fluctuation of volume resistivity of a single-layer roller using an electronic conductive material.

FIG. 4 is a diagram showing a relationship between a transfer current and a transfer efficiency under a low humidity environment.

FIG. 5 is a sectional view of a main part of the image forming apparatus according to the second embodiment of the present invention.

FIG. 6 is a schematic view of a transfer roller having a three-layer structure.

FIG. 7 is a schematic diagram illustrating a method for measuring a volume resistivity of a transfer roller.

FIG. 8 is a schematic view of a transfer roller having an end covering member.

FIG. 9 is a schematic diagram of an image forming apparatus for explaining a conventional example.

[Explanation of symbols]

21, 1a-1d photosensitive drum 22 Transfer roller 51 Intermediate transfer belt 53a to 53d Primary transfer roller 57 Secondary transfer roller

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Claims (4)

[Claims] 1. An image forming apparatus having an image carrier for forming a toner image according to image information and a transfer roller for transferring the toner image formed on the image carrier onto a recording material. Is composed of at least two layers, at least the outermost layer is an electronically conductive layer containing an electronically conductive agent, and is an elastic layer formed of a foam, and at least one layer other than the outermost layer is the outermost layer. An image forming apparatus comprising an ionic conductive layer containing an ionic conductive agent whose resistance is adjusted to a value smaller than the volume resistivity of the above, and an elastic layer formed of foam. 2. The volume resistivity of the electronic conductive layer is 1E +
The image forming apparatus according to claim 1, wherein the resistance is adjusted to 6 to 1E + 8Ω · m. 3. The volume resistivity of layers other than the outermost layer is 1E + 7.
3. The image forming apparatus according to claim 1, wherein Ω · m or less, and at least one layer other than the outermost layer is an ion conductive layer containing an ion conductive agent. 4. The image forming apparatus according to claim 1, further comprising a covering member that covers both ends of the transfer roller.