JP2002268409A - Transfer roll - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transfer roll by which scattering of toner and a transfer memory can be prevented even if the allowable range of a resistance value is wide. SOLUTION: A transfer roll 50 is rotated around a rotation central line 50a extending in an arrow C direction. The transfer roll 50 has a cylindrical electrically conductive core bar 52 which serves as an electric supply electrode. A sponge layer 54 surrounds the circumferential surface of the electrically conductive core bar 52. The sponge layer 54 has a cylindrical shape. The sponge layer 54 is not directly in contact with the circumferential surface of the electrically conductive core bar 52, but comes in contact with it through an intermediate resistive layer 56. The sponge layer 54 consists of a carbon dispersed EPDM foam which has a thickness of 5.2 mm and volume resistivity of about 10<9> Ωcm. The intermediate resistive layer 56 consists of urethane having volume resistivity adjusted to 10<15> Ωcm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transfer roller for transferring a developed image onto a recording medium while rotating the recording medium (transfer material) sandwiched between the recording medium and an image carrier.

[0002]

2. Description of the Related Art Conventionally, an electrostatic latent image is formed on an image carrier such as a photosensitive drum, and a developer is supplied to the electrostatic latent image to form a developed image (toner image). 2. Description of the Related Art An electrophotographic image forming apparatus that transfers an image to a recording medium to form an image is used. In such an image forming apparatus, a toner image is transferred to a recording medium by a corona transfer method or a roller transfer method.

The above-described corona transfer method is a method in which transfer is performed by using a corona charger which is arranged in a non-contact and opposed manner on the surface of an image carrier. In this method, since the corona charger does not contact the image carrier, the image carrier is hardly damaged. In addition, there is an advantage that a developed image can be transferred regardless of the type of recording medium.

However, in this corona transfer system, ozone and NOx are often generated due to corona discharge, and a device for removing these is required. Further, the configuration itself of the corona charger is complicated, and in addition, there is a problem that a configuration for preventing a recording medium from rushing (entering) into the corona charger is required.

In order to solve such a problem, in recent years,
The roller transfer method tends to be used. In the roller transfer method, a transfer roller including a metal core (conductive shaft) also serving as a power supply electrode and an elastic member surrounding (surrounding) the outer peripheral surface of the conductive shaft is used. As an elastic member,
Semiconductive rubber or semiconductive sponge is used.

In this roller transfer method, since the applied voltage can be made smaller than in the case of the corona charger, there is an advantage that generation of ozone and NOx can be reduced. Further, since the transfer roller directly contacts the recording medium to transfer the toner image on the image carrier to the recording medium, there is an advantage that the recording medium can be stably conveyed.

[0007]

However, the above-mentioned roller transfer method has the following problems, so that a laser beam printer (hereinafter, referred to as "LB") having a high process speed is used.
P ") or a copier.

When manufacturing the transfer roller, an extra portion of the elastic member is cut in order to make the long elastic member a desired length. At the time of this cutting, extra stress acts on both ends in the longitudinal direction of the elastic member corresponding to the portion to be cut, and the crush amount (compression amount) of both ends becomes larger than the crush amount of the central portion. Further, the semiconductive sponge is manufactured by foaming and sulphidation, but at the time of the foaming and sulphidation, there is no restriction on the foaming at both ends in the longitudinal direction unlike the central part in the longitudinal direction. Therefore, the foaming ratio tends to be larger at both ends in the longitudinal direction than at the center in the longitudinal direction. As a result, the resistance at both ends in the longitudinal direction of the transfer roller tends to be higher than that in the central part in the longitudinal direction.

In the roller transfer method, both ends in the longitudinal direction of a metal core also serving as a power supply electrode are supported and pressed (pressed) toward an image carrier. Thereby, the transfer roller and the image carrier are brought into close contact with each other, and a charging nip portion is formed between the two. However, only the longitudinal end portions of the core are pressed, and the central portion in the longitudinal direction is not pressed, so the core is bent,
As a result, a difference occurs in the pressing force between both ends in the longitudinal direction of the transfer roller and the central portion in the longitudinal direction. For this reason, the amount of crushing of the elastic member at both ends in the longitudinal direction of the transfer roller is larger than that at the central portion in the longitudinal direction. Typically,
As a characteristic of the semiconductive sponge, when it is crushed (compressed), the resistance of that part becomes high. Therefore, the resistance at both ends in the longitudinal direction of the transfer roller is larger than the resistance at the central part in the longitudinal direction.

As described above, in the transfer roller, the resistance at both ends in the longitudinal direction tends to be larger than the resistance at the central portion in the longitudinal direction. For this reason, the constant current control is performed during non-sheet passing when the recording medium does not exist in the transfer area, the voltage at this time is held, and the constant voltage control is performed based on the held voltage during sheet passing (hereinafter, referred to as control). "ATVC"
) Or constant current control, the transfer current at both ends in the longitudinal direction of the transfer roller is smaller than that at the center in the longitudinal direction. For this reason, at both ends in the longitudinal direction of the transfer roller, the charge applied from the transfer roller to the back surface of the recording medium decreases. As a result, the force for attracting toner when transferring the toner image on the image carrier to the recording medium becomes weak,
The toner transferred to the recording medium may be scattered. This phenomenon occurs remarkably on the second side during double-sided printing when the potential of the recording medium increases.

For this reason, a transfer bias is set so that a necessary current can be sufficiently secured at both ends in the longitudinal direction of the transfer roller. When such a transfer bias is applied, the transfer current flowing in the longitudinal central portion of the transfer roller tends to be larger than necessary. For this reason, a transfer memory is formed on the image carrier between the recording media where the transfer roller is in direct contact with the image carrier, and a transfer memory trace remains on the recording medium. In order to avoid such scattering of toner and traces of transfer memory, the allowable range of the resistance value of the transfer roller becomes very narrow, and the yield in producing the transfer roller is reduced, leading to an increase in cost.

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a transfer roller which can prevent toner scattering and transfer memory even when the allowable range of the resistance value is wide.

[0013]

A transfer roller according to the present invention for achieving the above object sandwiches a recording medium between the transfer roller and an image carrier and transports the recording medium in a predetermined transport direction. The developed image is transferred to the recording medium by applying a voltage having a polarity opposite to the polarity of the formed developed image to the recording medium, and is rotated about a conductive axis extending in a direction perpendicular to the transport direction. (1)
An elastic member surrounding the outer peripheral surface of the conductive shaft and having a uniform volume resistivity when pressed by the image carrier is provided.

Here, the elastic member has (2) a length in the orthogonal direction shorter than the conductive shaft and a portion of the outer circumferential surface of the conductive shaft inside the both ends in the orthogonal direction. It may be a two-layer structure including a surrounding intermediate layer having a predetermined volume resistivity and (3) a sponge layer surrounding the outer peripheral surface of the intermediate layer.

[0015] The sponge layer may also surround (4) an outer peripheral surface of the conductive shaft deviating from the surrounding of the intermediate layer.

Further, (5) the intermediate layer has a volume resistivity in the range of 10 ⁷ Ω · cm or more and 10 ¹⁵ Ω · cm or less, and the orthogonal direction length of the intermediate layer is The sponge layer may be shorter than the length in the direction and located inside both ends of the sponge layer in the orthogonal direction.

Further, the intermediate layer has (6) a volume resistivity in the range of 10 ⁷ Ω · cm to 10 ¹⁵ Ω · cm, and the resistance distribution at the center in the longitudinal direction is It may be higher than the resistance distribution at both ends.

(7) The sponge layer may be formed of a rubber layer.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a transfer roller according to the present invention will be described with reference to the drawings.

Referring to FIG. 1, an image forming apparatus incorporating the transfer roller of the present invention will be described.

FIG. 1 is a schematic diagram showing a schematic configuration of a laser beam printer (LBP) as an example of an image forming apparatus in which the transfer roller of the present invention is incorporated.

The laser beam printer 10 includes an electrophotographic photosensitive member (hereinafter, referred to as a “photosensitive drum”, which is an example of an image carrier) 12 formed in a drum shape. The photosensitive drum 12 is formed by stacking a photosensitive layer 12b made of OPC (organic optical semiconductor) or the like on an outer peripheral surface of a cylindrical conductive base 12a made of aluminum or the like. The photosensitive drum 12 is rotated in the direction of arrow A by a drive motor (not shown). In the vicinity of the outer peripheral surface of the photosensitive drum 12, a charging roller (charging member) 16 connected to a charging power supply 14 and an exposure device (a laser beam irradiating laser beam 18 on the surface of the photosensitive drum 12) are arranged in this order substantially along the rotation direction. Not shown),
A developing device 22 connected to a developing power supply 20, a transfer roller 40 connected to a transfer power supply 24, a cleaning device 28 for cleaning the surface of the photosensitive drum 12, and the like are arranged.

A recording medium 30 such as recording paper is stored in a cassette tray (not shown) or the like, and is guided by a transport guide 32 and transported from the direction of arrow B toward the transfer roller 40. The recording medium 30 that has passed the transfer roller 40 is guided by the transport guide 34 and reaches the fixing roller 36.

The procedure for forming an image on the recording medium 30 will be described.

A charging roller 16 is provided on the outer peripheral surface of the photosensitive drum 12 rotating in the direction of arrow A at a predetermined process speed.
Are pressed and rotated by rotation, and the outer peripheral surface of the photosensitive drum 12 is uniformly charged to a predetermined charging potential by the charging roller 16. The charged outer peripheral surface is irradiated with a laser beam 18 carrying image information. By this irradiation, an electrostatic latent image is formed on the outer peripheral surface. A developer (toner) is supplied from the developing device 14 to the electrostatic latent image,
As a result, a toner image (developed image) is formed on the outer peripheral surface of the photosensitive drum 12.

This toner image is transferred onto the recording medium 30 that has been conveyed between the photosensitive drum 12 and the transfer roller 40 (transfer nip). Recording medium 3 onto which toner image has been transferred
“0” is guided by the conveyance guide 34 and reaches the fixing device 36. In the fixing device 36, the toner image on the surface of the recording medium 30 is heated and pressed to be fixed. The recording medium 30 on which the toner image has been fixed is discharged from the laser beam printer 10. Incidentally, on the outer peripheral surface of the photosensitive drum 12 after the toner image is transferred to the recording medium 30, the toner may remain without being transferred to the recording medium 30. Such residual toner (residual toner) is supplied to the cleaning device 2
8 removed. Thus, preparation for forming the next image is completed.

The transfer roller 40 will be described with reference to FIG.

FIG. 2 is a longitudinal sectional view showing one end of the transfer roller in the upper half of the rotation center line and in the longitudinal direction. The other end in the longitudinal direction (not shown) has the same structure.

The transfer roller 40 rotates about a rotation center line 40a extending in a direction of an arrow C (an example of a direction perpendicular to the present invention, which is a longitudinal direction). The transfer roller 40 has a cylindrical conductive core 42 (conductive shaft) also serving as a power supply electrode. The sponge layer 44 surrounds (surrounds) the outer peripheral surface of the conductive cored bar 42. The sponge layer 44 has a cylindrical shape. The sponge layer 44 is formed by foaming and vulcanizing EPDM, urethane rubber, hydrin rubber, silicone rubber, NBR, or the like in which carbon is dispersed.

The outer peripheral surface of the conductive mandrel 42 and the sponge layer 44
A cylindrical medium resistance layer 46 (an example of an intermediate layer according to the present invention) is formed between the inner layer and the inner peripheral surface. The middle resistance layer 46 has a length in the longitudinal direction (length in the direction of arrow C) shorter than the conductive cored bar 42, and the outer peripheral surface of the outer peripheral surface of the conductive cored bar 42 inside both ends in the longitudinal direction. The surface is in direct contact with and surrounds the outer peripheral surface. Therefore, the sponge layer 44 surrounds the outer peripheral surface of the medium resistance layer 46. In addition, the sponge layer 44 is in direct contact with and surrounds the outer peripheral surface of the conductive cored bar 42 that is not surrounded (outside the outer periphery) by the middle resistance layer 46. The transfer roller 40 will be described in detail as a first embodiment described later.

Another example of the transfer roller will be described with reference to FIG.

FIG. 3 is a longitudinal sectional view showing a transfer roller of another example.

The transfer roller 50 rotates about a rotation center line 50a extending in the direction of arrow C. The transfer roller 50 is formed of a cylindrical conductive core 5 serving also as a power supply electrode.
2 (conductive shaft). The sponge layer 54 surrounds (surrounds) the outer peripheral surface of the conductive cored bar 52.
The sponge layer 54 is cylindrical. Sponge layer 54
Are not in direct contact with the outer peripheral surface of the conductive cored bar 52 but are in contact via the medium resistance layer 56. Sponge layer 5
No. 4 is obtained by foaming and vulcanizing EPDM, urethane rubber, hydrin rubber, silicone rubber, NBR and the like in which carbon is dispersed.

The outer peripheral surface of the conductive core 52 and the sponge layer 54
A cylindrical medium resistance layer 56 (an example of an intermediate layer according to the present invention) is formed between the inner layer and the inner peripheral surface. The medium resistance layer 56 has a length in the longitudinal direction (length in the direction of arrow C) shorter than the conductive cored bar 52, and has an outer peripheral surface of the outer peripheral surface of the conductive cored bar 52 which is inner than both ends in the longitudinal direction. The surface is in direct contact with and surrounds the outer peripheral surface. Therefore, the sponge layer 54 surrounds the outer peripheral surface of the medium resistance layer 56. The middle resistance layer 56 has a central portion 5 in the longitudinal direction.
Reference numeral 6a denotes two layers which are stacked, and both ends 56b in the longitudinal direction are one layer. The transfer roller 50 will be described in detail as a second embodiment described later.

Here, in order to compare the transfer roller of the present invention with a conventional transfer roller, a conventional transfer roller will be described with reference to FIG.

FIG. 4 is a longitudinal sectional view showing one end in the longitudinal direction, which is an upper half of the rotation center line in the conventional transfer roller. The other end in the longitudinal direction (not shown) has the same structure.

The transfer roller 60 is the transfer roller 4 of the present invention.
0, 50, the rotation center line 60 extending in the direction of arrow C
It rotates around a. Transfer roller 60
Has a cylindrical conductive core metal 62 also serving as a power supply electrode. A foamed EPDM (elastic layer) 64 in which carbon is dispersed directly surrounds (surrounds) the outer peripheral surface of the conductive core metal 62. Therefore, unlike the transfer rollers 40 and 50, the transfer roller 60 does not include the medium resistance layers 46 and 56.

The result of measuring the resistance distribution in the longitudinal direction of the transfer roller 60 will be described. First, this measuring method will be described with reference to FIG.

FIG. 5 is a schematic diagram showing a method for measuring the resistance distribution.

To measure the resistance distribution, the transfer roller 6
0 is connected to the high-voltage power supply 80 (TOREK 6).
10C), a bias E (2000 V constant voltage) was applied. In addition, the outer peripheral surface of the transfer roller 60 is moved along its longitudinal direction by eight stainless steel electrode rollers 71 to 78.
By a predetermined load. Each adjacent electrode roller 7
An insulator 82 is inserted between 1 to 78, respectively. The width of each of the electrode rollers 71 to 78 is about 33 mm, and the width of the insulator is about 5 mm. The resistance value R was calculated from the amount of current flowing into each of the electrode rollers 71 to 78 by the following equation. R = 2000 / I

With respect to the result of measuring the above resistance distribution,
This will be described with reference to FIG.

FIG. 6 is a graph showing the resistance distribution. The numbers on the horizontal axis correspond to the respective electrode rollers.

When measuring the resistance distribution, the conductive core 6
2 were pressurized with a pressing force F at both ends in the longitudinal direction. here,
This pressing force F is 100 gf, 250 gf, 500 gf,
The resistance distribution was measured by changing to 850 gf.

When the pressing force F is increased from 100 gf to 850 gf, the resistance value at the longitudinal center of the transfer roller 60 does not change much. In contrast, the transfer roller 6
The resistance value at both ends in the longitudinal direction of 0 greatly changes, and the resistance value increases as the pressing force F increases. This increase is particularly remarkable at both ends in the longitudinal direction of the transfer roller 60 (portions corresponding to the electrode rollers 71 and 78). The resistance hardly increases in the portions (portions corresponding to the electrode rollers 73, 74, 75, 76) apart from the longitudinal end portions 71, 78 by about 60 mm or more.

Here, the cause of the increase in the resistance value is as follows.
Consider with reference to FIG.

FIG. 7 is a schematic diagram showing a state in which the conductive core is bent when the pressing force F changes. In this figure, the same components as those shown in FIGS. 1 and 4 are denoted by the same reference numerals.

When both ends in the longitudinal direction of the transfer roller 60 are pressed toward the photosensitive drum 12 by the pressing force F, the conductive core metal 62 is supported with the end 66 where the photosensitive drum 12 and the transfer roller 60 contact each other as a fulcrum. Bend. For this reason, the crush amount (compression amount) of the transfer roller 60 is larger at both ends in the longitudinal direction of the transfer roller 60 than in the central portion in the longitudinal direction. in this case,
As the pressing force F increases, the amount of deflection of the conductive core metal 62 also increases, and the amount of crushing of both ends in the longitudinal direction of the transfer roller 60 also increases. Therefore, as shown in FIG. 6, the resistance at both ends in the longitudinal direction of the transfer roller 60 is larger than the resistance at the central part in the longitudinal direction. As a result, the current flowing into the photosensitive drum 12 from both ends in the longitudinal direction of the transfer roller 60 is
The current flowing from the central portion in the longitudinal direction is smaller.

As described above, the current flowing to both ends in the longitudinal direction of the transfer roller 60 becomes small due to the collapse of the elastic layer 62 due to the pressurization. Therefore, in the transfer rollers 40 and 50, the resistance at the central portion in the longitudinal direction is increased in advance.

For this reason, in the transfer roller 40, as described with reference to FIG. 2, a medium resistance layer 46 is provided between the outer peripheral surface of the conductive core 42 and the inner peripheral surface of the sponge layer 44. .
As described above, the middle resistance layer 46 is formed of the conductive core metal 42.
And has a shorter length in the longitudinal direction (length in the direction of arrow C), and directly contacts the outer peripheral surface of the outer peripheral surface of the conductive metal core 42 at both ends in the longitudinal direction to surround the outer peripheral surface. are doing. In addition, the sponge layer 44 is in direct contact with and surrounds the outer peripheral surface of the conductive cored bar 42 that is not surrounded (outside the outer periphery) by the middle resistance layer 46. In this way, by previously setting the resistance distribution in the longitudinal direction of the transfer roller 40 to be high at the center in the longitudinal direction and to be low at both ends in the longitudinal direction, the transfer roller 40 The increased resistance at both ends in the longitudinal direction can be offset.

In the transfer roller 50, as described with reference to FIG. 3, a medium resistance layer 56 is provided between the outer peripheral surface of the conductive core 52 and the inner peripheral surface of the sponge layer 54. The medium resistance layer 56 has a length in the longitudinal direction (length in the direction of arrow C) shorter than that of the conductive core 52, and also includes the conductive core 52.
The outer peripheral surface is directly in contact with and surrounds the outer peripheral surface inside both ends in the longitudinal direction. Medium resistance layer 56
The central portion 56a in the longitudinal direction is overlapped with two layers, and both end portions 56b in the longitudinal direction are one layer. In this way, by previously setting the resistance distribution in the longitudinal direction of the transfer roller 50 to be high at the central portion in the longitudinal direction and to be low at both ends in the longitudinal direction, the transfer roller 50 is The increased resistance at both ends in the longitudinal direction can be offset.

As described above, when the transfer rollers 40 and 50 are pressed against the photosensitive drum 12 (see FIG. 1), the transfer current at both ends in the longitudinal direction can be secured to be equal to the transfer current at the center in the longitudinal direction. . For this reason, it is possible to prevent the toner from scattering at the time of transfer.

When the conventional transfer roller 60 is used, as described above, the resistance at both ends in the longitudinal direction of the sponge layer 64 is increased due to the above-described pressing force, and the resistance in the longitudinal direction of the transfer roller 60 is increased. Are not uniform and unevenness occurs. For this reason, the transfer current required for the transfer is larger than that in the case where the transfer rollers 40 and 50 are used so that sufficient transfer current can be secured at both ends in the longitudinal direction of the transfer roller 60. Therefore, there is a problem of a transfer memory trace between papers where the transfer roller 60 directly contacts (contacts) the photosensitive drum 12, and it is difficult to use the transfer roller 60 on the low resistance side, and the allowable range of the total resistance is narrow. . However, by increasing the resistance at the center in the longitudinal direction as in the case of the transfer rollers 40 and 50, when a pressing force is applied to both ends in the longitudinal direction of the transfer rollers 40 and 50, the longitudinal direction of the transfer rollers 40 and 50 is reduced. , A substantially uniform resistance distribution is achieved, and a reduction in transfer current can be achieved. As a result, the transfer rollers 40, 50
Is reduced, the yield in producing the transfer rollers 40 and 50 is improved, and the cost can be reduced.

An embodiment in which a developed image is transferred to a recording medium by using the transfer roller 40 (see FIG. 2) and the transfer roller 50 (see FIG. 3) will be described. (Example 1)

The transfer roller 40 was incorporated into a process cartridge type LBP capable of forming an image on 24 recording media per minute. A specific configuration of the transfer roller 40 shown in FIG. 2 will be described.

The conductive core 42 is formed of a nickel-plated steel rod having a diameter of 8 mm. Also, the sponge layer 44
Is made of carbon-dispersed foamed EPDM having a thickness of 5.2 mm and a volume resistivity of about 10 ⁹ Ω · cm. The medium resistance layer 46 exists only in the central portion in the longitudinal direction of the sponge layer 44, which is 30 mm inward from both ends in the longitudinal direction, and has a thickness of 2 mm.
It is composed of urethane adjusted to 00 μm and volume resistivity of 10 ¹⁵ Ω · cm.

In the transfer roller 40, of the portion excluding the conductive core 42 (the portion in which the sponge layer 44 and the medium resistance layer 46 are combined), 30 inward from both ends 44a in the longitudinal direction.
The portion up to mm is composed of only the sponge layer 44 (one layer), and the other part is composed of the sponge layer 44, the medium resistance layer 46, and (two layers). Accordingly, of the outer peripheral surface of the conductive cored bar 42, both ends 4 in the longitudinal direction of the sponge layer 44 are formed.
The portion from 4a to the inner side up to 30 mm is the sponge layer 44.
It is surrounded only (surrounded). A portion of the outer peripheral surface of the conductive metal core 42 that is more than 30 mm inside is covered with a sponge layer 44 from above surrounded by a medium resistance layer 46.
It will be surrounded by.

The sponge layer 44 has an outer diameter of 18.
5 mm and the length in the longitudinal direction is 300 mm. As shown in FIG. 8, the transfer roller 40 having such a configuration is
The photosensitive drum 12 was driven and rotated at a constant speed in the direction of arrow A. At this time, both ends in the longitudinal direction of the conductive cored bar 42 were pressed with a pressing force F of 550 gf.

Table 1 shows a comparison between the transferred image obtained by transferring the developed image to the recording medium as described above and the transferred image obtained by using the conventional transfer roller 60 (see FIG. 4). . The transfer current was 5.0 μA, and the transfer bias was controlled by performing ATVC.

[Table 1]

When the transfer is performed using the transfer roller 40, the volume resistivity of the transfer roller 40 becomes uniform, and the current distribution becomes uniform. Is prevented from scattering. As a result, a high-quality image (transfer image) is formed on the recording medium. On the other hand, when the transfer is performed using the conventional transfer roller 60, as described with reference to FIG.
Since the volume resistivity increases at both ends in the longitudinal direction of
In this part, the current distribution is low. As a result, as shown in Table 1, toner scattering occurs at both ends in the longitudinal direction, and the quality of the transferred image transferred to the recording medium is reduced. (Example 2)

The transfer roller 50 was incorporated in a process cartridge type LBP capable of forming an image on 24 recording media per minute. The specific configuration of the transfer roller 50 shown in FIG. 3 will be described.

The conductive core 52 is formed of a nickel-plated steel rod having a diameter of 8 mm. In addition, the sponge layer 54
Is made of carbon-dispersed foamed EPDM having a thickness of 5.2 mm and a volume resistivity of about 10 ⁹ Ω · cm. The medium resistance layer 56 is made of urethane adjusted to have a volume resistivity of 10 ¹⁵ Ω · cm. The medium resistance layer 56 has a thickness of 1 in a range of 30 mm inward from both ends 56c in the longitudinal direction.
00 μm, thickness 300 μm inside 30 mm
m. Thus, the medium resistance layer 5
By changing the thickness of No. 6 in the longitudinal direction, the resistance distribution in the longitudinal direction was low at both ends in the longitudinal direction and high at the central portion in the longitudinal direction.

In the transfer roller 50, the portion excluding the conductive core 52 (the portion where the sponge layer 54 and the medium resistance layer 56 are combined) is composed of the sponge layer 54, the medium resistance layer 56, and (two layers). ing. Therefore, the sponge layer 54 and the medium resistance layer 56 are laminated on the outer peripheral surface of the conductive core metal 52 that is inner than 30 mm from both ends 52a in the longitudinal direction. That is, of the outer peripheral surfaces of the conductive core 52, the outer peripheral surface inside of 30 mm from both ends 52 a in the longitudinal direction is directly in contact with and surrounds the intermediate resistance layer 56.
Is surrounded by a sponge layer 54 in direct contact.

The sponge layer 54 has an outer diameter of 18.
5 mm and the length in the longitudinal direction is 300 mm. As shown in FIG.
The photosensitive drum 12 was driven and rotated at a constant speed in the direction of arrow A. At this time, both ends in the longitudinal direction of the conductive cored bar 42 were pressed with a pressing force F of 550 gf.

Table 2 shows a comparison between the transferred image obtained by transferring the developed image to the recording medium as described above and the transferred image obtained by using the conventional transfer roller 60 (see FIG. 4). . The transfer current was 5.0 μA, and the transfer bias was controlled by performing ATVC.

[Table 2]

When the transfer is performed using the transfer roller 50, the volume resistivity of the transfer roller 50 becomes uniform and the current distribution becomes uniform. Is prevented from scattering. As a result, a high-quality image (transfer image) is formed on the recording medium. On the other hand, when the transfer is performed using the conventional transfer roller 60, as described with reference to FIG.
Since the volume resistivity increases at both ends in the longitudinal direction of
In this part, the current distribution is low. As a result, as shown in Table 2, toner scattering occurs at both ends in the longitudinal direction, and the quality of the transferred image transferred to the recording medium is reduced.

[0066]

As described above, according to the transfer roller of the present invention, since the elastic member has a uniform volume resistivity even if the allowable range of the resistance value is wide, the elastic member has a uniform volume resistivity. Direction), the current distribution becomes uniform. Therefore, a uniform voltage is applied to any part of the recording medium to generate a uniform charge. As a result, scattering of toner is prevented in portions of the recording medium corresponding to both ends in the axial direction of the transfer roller. Further, since an unnecessarily large current does not pass through the elastic member, transfer memory is also prevented. For this reason,
A transfer image without a transfer memory trace is formed on the recording medium.

Here, the elastic member has a length in the orthogonal direction shorter than the conductive shaft, and surrounds an outer circumferential surface of the outer circumferential surface of the conductive shaft inside both ends in the orthogonal direction. In the case of a two-layer structure including an intermediate layer having a predetermined volume resistivity and a sponge layer surrounding the outer peripheral surface of the intermediate layer, the volume resistivity at both ends in the orthogonal direction (axial direction) of the sponge layer is When the volume resistivity at the central portion in the axial direction is higher, the volume resistivity at the central portion in the axial direction of the intermediate layer is set to be larger than the volume resistivity at both ends in the axial direction. Thereby, since the elastic member has a uniform volume resistivity in any part, the current distribution in the axial direction (longitudinal direction) is also uniform.

In the case where the sponge layer also surrounds the outer peripheral surface of the conductive shaft that is deviated from the surroundings of the intermediate layer, the elastic member is provided at any part as described above. Since the volume resistivity can be made uniform, the current distribution in the axial direction (orthogonal direction) can be made uniform.

Further, the intermediate layer has a volume resistivity in the range of 10 ⁷ Ω · cm or more and 10 ¹⁵ Ω · cm or less, and the orthogonal direction length is greater than the orthogonal direction length of the sponge layer. If the sponge layer is short and located inside both ends of the sponge layer in the orthogonal direction, the current distribution in the axial direction (orthogonal direction) of the elastic member can be made more uniform.

Further, the intermediate layer has a volume resistivity in the range of 10 ⁷ Ω · cm or more and 10 ¹⁵ Ω · cm or less, and the resistance distribution at the center in the longitudinal direction has a When the resistance distribution is higher than the resistance distribution, the current distribution in the axial direction (perpendicular direction) of the elastic member can be made more uniform.

Further, when the sponge layer is made of a rubber layer, the current distribution in the axial direction (longitudinal direction) of the elastic member can be more reliably made uniform as described above.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a schematic configuration of a laser beam printer (LBP) as an example of an image forming apparatus in which a transfer roller of the present invention is incorporated.

FIG. 2 is a longitudinal cross-sectional view showing an upper half of a transfer center line of a transfer roller of the present invention and showing one end in a longitudinal direction.

FIG. 3 is a longitudinal sectional view illustrating a transfer roller of another example.

FIG. 4 is a longitudinal sectional view showing one end in the longitudinal direction of the conventional transfer roller, which is in an upper half of a rotation center line.

FIG. 5 is a schematic diagram showing a method for measuring a resistance distribution.

FIG. 6 is a graph showing a resistance distribution, in which the numbers on the horizontal axis correspond to the respective electrode rollers.

FIG. 7 is a schematic diagram showing a state in which a conductive core is bent when a pressing force F changes.

FIG. 8 is a schematic diagram illustrating a transfer roller pressed against a photosensitive drum.

[Explanation of symbols]

 Reference Signs List 10 laser beam printer 12 photosensitive drum 40, 50 transfer roller 42, 52 conductive core 44, 54 sponge layer

Claims (6)

[Claims] 1. A recording medium is sandwiched between an image carrier and conveyed in a predetermined conveying direction, and a voltage having a polarity opposite to a polarity of a developed image formed on the image carrier is applied to the recording medium. A transfer roller configured to transfer the developed image to a recording medium by applying a voltage, the transfer roller rotating about a conductive axis extending in a direction perpendicular to the transport direction, the outer periphery of the conductive axis being surrounded by the image; A transfer roller comprising an elastic member having a uniform volume resistivity when pressed against a carrier. 2. The method according to claim 1, wherein the elastic member has a length in the orthogonal direction shorter than the conductive shaft and surrounds an outer circumferential surface of the outer circumferential surface of the conductive shaft inside both ends in the orthogonal direction. An intermediate layer having a volume resistivity of: and a sponge layer surrounding the outer peripheral surface of the intermediate layer.
The transfer roller according to claim 1, wherein the transfer roller is a layer. 3. The transfer roller according to claim 2, wherein the sponge layer also surrounds an outer peripheral surface of the conductive shaft that is deviated from a surrounding of the intermediate layer. 4. The intermediate layer has a volume resistivity of 10 ⁷ Ω · cm or more and 10 ¹⁵ Ω · cm.
Within the following range, the orthogonal direction length is shorter than the orthogonal direction length of the sponge layer, and is located inside both ends of the sponge layer in the orthogonal direction. The transfer roller according to claim 2. 5. The intermediate layer has a volume resistivity of 10 ⁷ Ω · cm or more and 10 ¹⁵ Ω · cm.
4. The transfer roller according to claim 2, wherein the resistance distribution at the center in the longitudinal direction is higher than the resistance distribution at both ends in the longitudinal direction within the following range. 5. 6. The transfer roller according to claim 2, wherein the sponge layer is made of a rubber layer.