JP2000275954A - Developing roller and its production as well as integral type image forming unit using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a developing roller which is excellent in surface durability, is free of a development defect by rotation unevenness and drive gear damage, is fixed between shafts and is optimum for use at a prescribed contact width. SOLUTION: The developing roller is constituted by concentrically forming rubber layers having an electrical conductivity in >=2 layers on the outer periphery of the revolving shaft 2 and forming the innermost layer 3 in contact with the revolving shaft of foamed rubber of 0.05 to 0.5 mm in a circle-equivalent average diameter of expansion cells and 5 to 25 in JIS A hardness. Cylindrical rubber stock is foamed by a cylindrical metal mold so as not to include the expansion cells of >=1 mm in diameter, The outermost layer is formed of thermoplastic polyurethane and silicone-acryl copolymer resins and fluororesin base particles are incorporated therein, by which the effect is made higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a developing roller used in an electrophotographic apparatus or an electrophotographic apparatus, a method of manufacturing the same, and an image forming unit incorporating the same, and in particular, a photosensitive member and peripheral devices such as a developing apparatus. The present invention relates to a developing roller used in an integrated image forming unit (referred to as an all-in-one cartridge, a process cartridge, or the like) in which is integrated, a manufacturing method thereof, and an integrated image forming unit.

[0002]

2. Description of the Related Art Conventionally, as an image forming process in an electrophotographic apparatus, as shown in FIG.
A charging unit, a developing unit, a transfer unit, a cleaning unit, and the like have been individually unitized and arranged. In FIG. 4, the photosensitive member 42 is uniformly charged by the charging roller 44 at the charging portion while being rotated by the driving means such as gears in the direction of the arrow, and is exposed at the exposure portion by the copy object or the transmission information to thereby form the electrostatic member. A latent image is formed. In the developing section, a toner image is formed on the photoreceptor by toner supplied from the developing roller 41, and is transferred to a recording paper 47 or the like supplied between the transfer section and the transfer roller 46 in the transfer section. It is fixed in. After the transfer of the toner image, the photoconductor 42
The toner remaining on the surface is removed by the cleaning blade 48 or the like, the charge is removed, and the toner is transferred again to the charging unit, where it is used for the next copy. The toner is put into the toner box 50,
The toner is stirred by the stirring blades 51 and supplied to the developing roller 41 via the supply roller 52. Excess toner on the developing roller 41 is regulated by the toner regulating blade 43 to have a uniform thickness.

In recent years, in electrophotographic printers, facsimile machines, and copiers, there has been a keen need for a compact developing device in order to apply to personal courses, desktop publishing, and other applications and to cope with colorization. Attention is paid to the development method in which a developing roller having a conductive elastic layer formed on the outer periphery of a rotating shaft using a magnetic one-component toner is pressed against the photoreceptor because development can be achieved by reducing the diameter of the developing roller and simplifying the device. I'm taking a bath. In particular, in the case of full color, four developing devices for three primary colors and black are required, so that the effect of downsizing is great. In the development of the above-mentioned applications, there has been a strong demand for simplification of maintenance, and various devices that have been conventionally arranged around the photoreceptor have been integrated into one unit. There is a rapidly growing trend to simplify maintenance by replacing each time.

[0004]

When the members around the photoreceptor are integrated, it is important to make the contact state between the photoreceptor and the developing roller uniform and stable in order to form a stable toner image. For this reason, it has come to be carried out that both are fixed at a fixed distance between the rotating shafts. Conventionally,
In a developing roller provided with a conductive elastic layer of a rubber solid body on the outer periphery of a rotating shaft used in this type of developing device, when the distance between the shafts is fixed and the required contact width with the photoconductor is obtained, the As a result, the rotation torque is greatly increased, the rotation of the photosensitive member and the developing roller becomes unstable, image unevenness called jitter occurs, and the resin gear which is a driving member of the photosensitive member and the developing roller jumps. There was a problem. Also, in order to obtain the necessary contact width with the photoreceptor and to suppress a large increase in rotational torque, a large amount of a softening agent (or plasticizer) should be added to the rubber solid body to make it ultra-low in hardness. And contamination of the photoreceptor due to oozing out of the softener from the developing roller has become a problem more than before.

To cope with these problems, a small amount of a softening agent can be used to reduce photoreceptor contamination, easily deform,
In addition, a developing roller using a foamed rubber having excellent deformation restoring property for the conductive elastic layer has also been used. However, if a developing roller using foamed rubber has a foamed cell having a large cell diameter, deformation (depending on hardness) in a minute area in contact with the photoreceptor and variation in electric resistance occur, resulting in a particularly high resolution. When a toner having a small particle size (conventionally, 10 to 12 .mu.m becomes 6 to 8 .mu.m or less) is used in correspondence with imaging, there has been a problem that development becomes non-uniform. SUMMARY OF THE INVENTION The present invention has been made in order to improve the problem of the conventional developing roller. Even if a necessary contact width with the photoconductor is secured, the drive gear tooth jump due to an increase in the driving torque of the photoconductor and the developing roller may occur. A developing roller capable of reducing the contamination of the photoreceptor without causing rotation unevenness and capable of performing uniform development even with a small particle size toner, a method of manufacturing the same, and an integrated image forming device incorporating the roller The unit is provided.

[0006]

According to the present invention, there is provided a developing roller having a structure in which a conductive rubber layer is formed concentrically on the outer periphery of a rotating shaft, wherein the rubber layer comprises at least two layers, and the innermost layer is Made of foamed rubber with JIS-A hardness of 5 to 25 degrees, and the average equivalent circle diameter of the foamed rubber foam cell is 0.05 to
The developing roller is 0.5 mm. The above-mentioned rubber layer is usually two layers of an inner foamed rubber layer and an outer toner carrying layer. However, the surface skin of the inner foamed rubber layer is regarded as one layer, or a special layer is formed between the inner foamed rubber layer and the outer foamed layer. In some cases, an intermediate layer having a special function is interposed, so that the structure has two or more layers.

According to the present invention, when the developing roller has a necessary contact width (nip width) with the photosensitive member, the driving torque of the photosensitive drum and the developing roller is rarely increased, and image disturbance due to uneven rotation is caused. (Generally called jitter) hardly occurs. If the JIS-A hardness of the innermost layer of the rubber layer exceeds 25 degrees, the torque increases when trying to obtain the required contact width by fixing the distance between the photosensitive drum and the shaft, and jitter due to uneven rotation is likely to occur. . And when this is less than 5 degrees,
Outer diameter accuracy becomes poor, and it becomes difficult to form a uniform toner layer on the developing roller. The JIS-A hardness of the innermost foamed rubber layer referred to in the present invention is a hardness measured by applying a JIS-A type hardness tester with a load of 1 kg to the outer periphery of the innermost foamed rubber layer provided on the outer layer of the rotating shaft. .

Further, the reason why the innermost layer of the rubber layer is limited to the foamed rubber is that if a JIS-A hardness in the above range is to be ensured without foaming, a large amount of a softening agent must be used, and physical properties deteriorate. In contrast, foamed rubber can achieve JIS-A hardness in the above range with a small amount of softener, while the softening agent oozes more and may contaminate the photosensitive drum. This is because the level becomes negligible.

Furthermore, the reason why the average value of the equivalent circle diameter of the foamed cells of the foamed rubber is 0.05 to 0.5 mm is that as the average equivalent circle diameter of the foamed cells becomes larger than 0.5 mm, (6 to 8 mm)
This is because when the toner having a small particle diameter (μm) is used, the unevenness of the image becomes severe, and when it is smaller than 0.05 mm, the JIS-A hardness of the foamed rubber layer exceeds 25 degrees, which causes the above-mentioned inconvenience. Preferably it is 0.1 to 0.3 mm. The average value of the equivalent circle diameter of the foam cell referred to in the present invention is obtained by cutting the rubber layer having the conductivity of the developing roller approximately three equally in the length direction and cutting the rubber layer vertically in the axial direction. Then, the area of the opening of the foam cell was read by a computer and converted into a circle diameter, and the average of three cut surfaces per cut surface and three cut surfaces was obtained.

According to the first aspect of the present invention, if the innermost foamed rubber layer includes a foam cell having an equivalent circle diameter of 1 mm or more, even if the average equivalent circle diameter is 0.05 to 0.5 mm, the minuteness of the average diameter is 0.05 to 0.5 mm. The hardness is slightly changed in the portion, and the uniformity of development tends to be disturbed, which is not preferable. Further, the developing roller of the present invention,
The outermost layer in contact with the photoconductor is made of a thermoplastic polyurethane. This is preferred because a coating film that is strong and excellent in wear resistance is easily formed, and the inner foamed rubber layer easily follows the deformation caused by the pressure contact with the luminous body. Further, the thermoplastic polyurethane is characterized by adding and containing a silicone acrylic copolymer resin, a fluororesin-based fine particle, or both.
This eliminates the occurrence of pinholes when forming the outermost layer,
It is preferable because the releasability from the photoreceptor is improved and the slip stick property is improved.

[0011] The developing roller of the present invention is a material rubber obtained by kneading a compounding agent, and forms a cylindrical material whose inner diameter is equal to or slightly larger than the outer diameter of the rotating shaft and whose outer diameter is smaller than the outer diameter of the roller.
Insert the rotating shaft through the cylindrical material into a mold consisting of a cylindrical middle mold having an inner diameter substantially equal to the roller outer diameter, and a seal mold that fits on both ends and hold the rotating shaft. After fixing both ends of the shaft to a seal mold, and heating the entire mold, the cylindrical material is foamed and vulcanized simultaneously with foaming to form an innermost foamed rubber layer. As a result, unlike in the case of using the split mold, no protrusion (burr) occurs at the roller portion at the roller joint, and it is possible to prevent variations in electrical resistance in the circumferential direction due to the burr, and it is easy to obtain a uniform thin toner layer. Become. Usually, an outer coating layer is formed on the foamed rubber layer obtained as described above by an immersion method or the like to form a developing roller.

An image forming unit incorporating the developing roller of the present invention and integrating a developing device and other peripheral devices of the photosensitive member with the photosensitive member has a structure in which the photosensitive member and the developing roller are fixed at a distance between the axes, and furthermore, the contact width between the two. Is set to 0.1 to 7.0 mm. As a result, the photoconductor is surely and uniformly contacted, and there is no rotation unevenness due to an increase in the rotation torque.
Since the slip stick with the photoreceptor hardly occurs, the outermost layer is hardly torn.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments. FIG. 1 is a perspective view of a developing roller 1 according to the present invention. The developing roller 1 is formed by forming a conductive foamed rubber layer 3 concentrically on the outer periphery of a rotating shaft 2 and covering the outer periphery with an outer surface layer 4. The material rubber forming the foamed rubber layer 3 is a base rubber, a liquid rubber of the same material, a softener such as dioctyl phthalate (DOP) or rapeseed oil, a conductivity-imparting agent such as carbon black, and a filler such as zinc oxide. Table 1 below describes the agent, vulcanizing agent, foaming agent, etc.
, And kneaded. In the present invention, the size of the foaming cells of the foamed rubber layer 3 is determined by changing the type of the foaming agent, for example, from a carbamide type to a hydrazine type, or by using a mixture of both, or by changing the particle type of the foaming agent. Further, it is adjusted by changing the inner diameter of the cylindrical medium mold and changing the expansion ratio.

The base rubber used in the present invention is chlorobrene rubber, taking into consideration that ozone is generated around the photosensitive drum and the developing roller is used in an ozone atmosphere.
Acrylonitrile butadiene rubber, ethylene propylene rubber, polyurethane rubber and the like are preferable. As the softening agent, liquid rubber of the same rubber type as the above base rubber is preferably used in addition to DOP and rapeseed oil. The thermoplastic polyurethane forming the outermost layer and the foamed rubber of the inner layer have similar solubility parameters (SP values) in a solvent, but the outermost layer is hardly peeled off. From this point, acrylonitrile butadiene rubber and chloroprene rubber are used as the base rubber. Rubber is preferred.

Next, the material rubber is passed through an extruder to obtain a tube having an inner diameter equal to or slightly larger than the outer diameter of the rotating shaft 2 and an outer diameter smaller than the final outer diameter of the developing roller 1. The roll material 11 is formed. The rotating shaft 2 is inserted through the inside of the shaft, and is loaded into a molding die. The molding die includes a cylindrical middle die 12 having an inner diameter substantially equal to the final outer diameter of the developing roller 1, and disk-shaped seal dies 13, 13 '. FIG. 2 shows a cylindrical roll material 11 in a mold.
FIG. 4 is a cross-sectional view of a state where the device is loaded. Cylindrical roll material 11
Is heated and the rubber material in the mold is vulcanized and foamed. After cooling, the molded body is removed from the mold, and a foamed rubber layer 3 having conductivity is formed concentrically with the rotating shaft 2. Next, a thermoplastic polyurethane resin having a JIS-A hardness of about 80 degrees is dissolved in a solvent such as tetrahydrofuran (THF), and a conductivity imparting agent such as carbon black, a silicone acrylic copolymer resin and fluorine-based fine particles are added and mixed. To prepare a coating solution for forming a surface layer. Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF) as fluororesin-based fine powder
Are used. The roller having the foamed rubber layer 3 is immersed in this coating liquid, pulled up, and dried to form the outermost layer 4, thereby obtaining the developing roller 1 of the present invention.

FIG. 3 is a sectional view of an integrated image forming unit 21 incorporating the developing roller 1. Photosensitive drum 22
Is driven to rotate in the direction of the arrow. A charging roller 24 contacts the photosensitive drum 22 and is uniformly charged. When the charged photosensitive drum 22 is exposed or data-in, an electrostatic latent image is formed on the outer periphery of the photosensitive drum 22 and is developed by the toner supplied from the developing roller 1, and the toner image is transferred to the photosensitive drum 22 and the transfer roller 26. Is transferred to a recording material 27 such as paper supplied between the printer and the printer, and the transferred image is fixed by a fixing unit (not shown). After the transfer, the photosensitive drum 22
To remove the remaining toner, and supply it to the next image formation.
Normally, a non-magnetic one-component toner is used for the integrated image forming unit 21 of the present invention. The toner is stored in a toner box 30 formed integrally with the unit 21 and supplied to a supply roll 32.
Is supplied to the developing roller 1. The toner supplied to the developing roller 1 is thinned by the toner layer thickness regulating blade 23 and supplied to the photosensitive drum. In the present unit 21, the photosensitive drum 22 and the developing roller 1 are incorporated with their rotating shafts parallel to each other and with the contact width a between 0.1 and 7.0 mm being fixed between the shafts. In order to incorporate the photosensitive drum 22 and the developing roller 25 with a predetermined nip width and fixed between the shafts, it is necessary to accurately open both shaft support holes provided in the frame or to the photosensitive drum 22 side or the developing roller 25 side. Adjustment rollers for keeping the distance between the shafts constant may be provided at both ends in the axial direction, and may be fixed to the shaft support portion at a position where the rollers contact the other side.

[0017]

Example 1 100 parts by weight of medium-high nitrile NBR with a nitrile content of 33% as base rubber, 40 parts by weight of HAF carbon, 5 parts by weight of zinc oxide, 5 parts by weight of softener (DOP), vulcanizing agent, and P as a foaming agent , P'-oxybisbenzylsulfonyl hydrazine (foaming agent-1 in Table 1) and other compounding agents were compounded as shown in Table 1, and kneaded with an open roll to obtain a raw rubber.
This material rubber was kneaded with a screw type extruder and simultaneously extruded into a tube having an outer diameter of 12 mm and an inner diameter of 7.5 mm, and cut into a length of about 275 mm to obtain a cylindrical roll material. Outer diameter 7m
m, length 320mm, both ends are supported, and a D-shaped rotating shaft made of stainless steel for fitting one end with a driving gear is coated with an adhesive, and is prepared and pushed through a cylindrical roll material. .
Separately, insert the roll material that pushed the rotating shaft into a medium size consisting of a steel cylinder with an inner diameter of 16 mm, pass both ends of the rotating shaft through the holes in the center of the seal type, and then taper-fit the seal type to both ends of the medium size Combined to complete the patterning. By heating the entire mold to 150 ° C., the roll material was foamed and heated. After cooling, the molded body was taken out of the mold, and a conductive foamed rubber layer was formed on the outer periphery of the rotating shaft. The hardness of the foamed rubber layer measured using a JIS-A hardness meter was 18 degrees. In addition, the computer read a photograph in which the foam cell was cut 50 times at a point perpendicular to the axial direction at a point 15 mm from the center and both ends in the axial direction of the foam rubber, and the computer read the average diameter of the circle equivalent diameter calculated from the area. Was 0.24 mm, and the maximum cell diameter was 0.45 mm. JIS-A Caprolactone ester-based thermoplastic polyurethane having a hardness of 80 degrees is dissolved in about 4 times the volume of THF solvent, carbon black and silicone acrylic copolymer resin are added and dispersed in the amount shown in Table 1, and immersion for forming outer layer is performed. The liquid was adjusted. The foamed rubber molded article was immersed in the immersion liquid, pulled up, dried, and the solvent was blown off to form an outer layer. Cut both ends of the rubber layer about 2mm
A developing roller of the present invention having a length of 16 mm and a length of two rubber layers of 270 mm was obtained. The JIS-A hardness of the obtained developing roller was 20 degrees. A resin-made inter-shaft adjusting roller is attached to both end shafts of the obtained developing roller, and the nip width is set to 2.0 mm in an integrated image forming unit in which a photoconductor, a developing device, a cleaning device, and the like are integrated. Then, the shaft support was fixed. This integrated image forming unit was attached to a laser beam printer, and an initial image was output, an image was output after 30,000 sheets were run, and a visual check was performed for damage to the drive gear, contamination of the photoconductor, and peeling of the outermost layer of the developing roller, and the results were displayed. 1 is shown.

[0018]

[Example 2] In the same manner as in Example 1 except that the outer diameter of the cylindrical roll material was increased to 12.8 mm, foaming with a JIS-A hardness of 20 and an average equivalent circle diameter of foam cells of 0.10 mm and a maximum cell diameter of 0.24 mm was performed. A rubber layer was formed, an outer layer was formed by immersion, and a developing roller having a JIS-A hardness of 22 was obtained. The developing roller was incorporated into an integrated image forming unit with a nip width of 2 mm with the center fixed, and image evaluation and durability evaluation were performed. The results are shown in Table 1.

[0019]

Example 3 Foaming agent-1 used in Example 1 as a foaming agent
JIS-A hardness using a cylindrical medium mold in the same manner as in Example 1 except that 2 parts by weight of a 1: 1 mixture of azodicarbamide having an average particle size of 4 to 8 μm (blowing agent-2) was used. Form a foamed rubber layer with an average equivalent cell diameter of 0.40 mm and a maximum cell diameter of 0.70 mm at 18, and form an outer layer by immersion to obtain a developing roller with JIS-A hardness of 20. It was assembled with a nip width of 2 mm with the shaft fixed, and image evaluation and durability evaluation were performed. Table 1 shows the results.

[0020]

Comparative Example 1 A cylindrical medium mold was used in the same manner as in Example 1 except that 2 parts by weight of azodicarbamide having an average particle size of 4 to 8 μm was used as a foaming agent. Form a foamed rubber layer with an average diameter of 0.63 mm and a maximum cell diameter of 0.72 mm, and form an outer layer by immersion to obtain a developing roller with JIS-A hardness of 20.
, And image evaluation and durability evaluation were performed. Table of results
Figure 1 shows.

[0021]

Comparative Example 2 A cylindrical medium mold was used in the same manner as in Example 1, except that 2 parts by weight of azodicarbamide having an average particle size of 15 to 20 μm was used as a foaming agent. Form a foamed rubber layer with an average diameter of 0.50 mm and a maximum cell diameter of 1.12 mm, and form an outer layer by immersion to obtain a developing roller with JIS-A hardness of 19.
, And image evaluation and durability evaluation were performed. Table 1 shows the results.

[0022]

[Comparative Example 3] A gap (cavity) formed by fixing a rotating shaft to a cylindrical medium mold and a seal mold having an inner diameter substantially equal to the outer diameter of the developing roller is filled with kneaded rubber containing no foaming agent and heated. Forms an inner layer of non-foamed rubber with JIS-A hardness of 51, forms an outer layer by immersion to obtain a developing roller with JIS-A hardness of 52, and incorporates it into an integrated image forming unit with a nip width of 2.0 mm with fixed shaft between shafts , Image evaluation and durability evaluation. Table 1 shows the results.

[0023]

[Table 1] Blowing agent-1: P, P'-oxybisbenzenesulfonylhydrazine Blowing agent-2: azodicarbamide (average particle size 4 to 8μ)
m) Blowing agent-3: azodicarbanamide (average particle size 15 to 2)
0 μm)

Evaluation Results As is evident from Table 1, the integrated image forming units incorporating the developing rollers of Examples 1 to 3 provided good images with little jitter and image unevenness, and even after running 30,000 sheets. The durability of the driving roller is not broken, and the outer layer of the developing roller in pressure contact with the resin gear and the photosensitive member is not damaged. On the other hand, in the case of the comparative example, the image quality was poor from the beginning, the image quality deteriorated by running, and the driving gear and the outer layer of the developing roller were damaged.

[0025]

According to the developing roller of the present invention, the innermost layer is formed of foamed rubber having an average equivalent circle diameter of foam cells of 0.05 to 0.5 mm and a JIS A hardness of 5 to 25. Even if the photosensitive member is brought into contact with the photosensitive member with the required contact width, deformation and restoration can be easily performed, and a rise in rotational torque is small, so that stable development without image defects due to rotational unevenness and gear damage can be performed. Also,
If the cell diameter of the foamed rubber does not include a cell having a diameter equivalent to a circle of 1 mm or more, the deformation and the variation in electric resistance at a minute portion of the contact surface with the photoreceptor can be neglected. Even when a toner having an average particle size of about 8 μm is used, development does not become uneven. Further, by forming the outermost layer from thermoplastic polyurethane, the durability of the outermost layer is improved. By including silicone-acrylic copolymer resin or fluororesin-based fine particles in this thermoplastic polyurethane, the formation of the outermost layer is stabilized, and the surface roughness required for carrying and transporting the toner in a thin layer can be secured. The inner layer of the foamed rubber of the developing roller uses a cylindrical mold and does not generate burrs on the mold surface in the circumferential direction, so that variations in electrical resistance in the circumferential direction can be reduced. Accordingly, when this developing roller is used in an integrated image forming unit having a contact width with the photoconductor of 0.1 to 7 mm and a fixed inter-axis distance with the photoconductor,
It is possible to provide a simple maintenance unit having a developing unit having no image defect and having excellent durability.

[Brief description of the drawings]

FIG. 1 is a perspective view of a developing roller of the present invention.

FIG. 2 A state in which a cylindrical roll material is loaded in a mold.

FIG. 3 is an integrated image forming unit incorporating a developing roller.

FIG. 4 shows a conventional non-integrated image forming process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1; Development roller 2; Rotating shaft 3; Conductive foam rubber (innermost layer) 4: Outer layer rubber 11; Cylindrical roll material 12; Cylindrical middle mold 13; Disc-shaped seal type 21; integral image forming unit 22; photosensitive drum 23; toner layer thickness regulating blade 26; transfer roller 27; recording material 28; cleaning blade 30; Box 31; Developing roller 32; Supply roller 41; Developing roller 42; Photoreceptor 43; Toner regulating blade 44; Charging roller 46; Transfer roller 47; Recording paper 48; -Ning blade 50; toner box 51; stirring member 52; supply roller

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H035 AA15 2H071 BA03 BA13 DA08 2H077 AC04 AD06 AD13 BA07 BA09 CA13 EA15 FA13 FA22 FA25 GA04 3J103 AA02 AA14 AA23 BA41 EA02 EA11 EA20 FA12 FA15 FA16 FA19 GA02 GA52 HA03 HA12 HA18 HA41 HA43 HA47 HA53

Claims (6)

[Claims] 1. A developing roller having a configuration in which a rubber layer having conductivity is provided concentrically on the outer periphery of a rotating shaft, wherein the rubber layer is composed of at least two layers, and the innermost layer in contact with the rotating shaft is formed of foam cells. Average circle equivalent diameter is 0.05 to 0.5 mm, JIS
-A developing roller comprising a foamed rubber having an A hardness of 5 to 25. 2. The developing roller according to claim 1, wherein the foamed rubber does not include a foamed cell having an equivalent circle diameter of 1 mm or more. 3. The developing roller according to claim 1, wherein the outermost layer of the conductive rubber layer is made of thermoplastic polyurethane. 4. The developing roller according to claim 3, wherein the thermoplastic polyurethane layer contains silicone-acrylic copolymer resin and / or fluororesin-based fine particles. 5. The method for manufacturing a developing roller according to claim 1, wherein an inner diameter of the developing roller is adjusted by mixing and dispersing a compounding agent containing a base rubber, a softening agent, and a foaming agent. Form a cylindrical material whose outer diameter is the same as or slightly larger than the outer diameter of the rotating shaft and whose outer diameter is slightly smaller than the final outer diameter of the roller,
Insert the cylindrical material into a mold consisting of a cylindrical middle mold having an inner diameter substantially equal to the final outer diameter of the roller and a seal mold that fits at both ends to hold the rotating shaft. Manufacturing a developing roller characterized in that a cylindrical material is foamed and vulcanized at the same time as foaming and vulcanization at the same time by fixing and loading the material in a seal mold and heating the entire mold to form an innermost layer of conductive rubber. Method. 6. An integrated image forming unit having at least a developing unit integrally provided around a photoreceptor, wherein a contact width between the photoreceptor and the developing roller is fixed by fixing a distance between shafts of the photoreceptor and the developing roller. 8. An integrated image forming unit, wherein the developing roller is a developing roller according to claim 1.